Tanque vivo Circuit Breakers Gua de Aplicacin

Editado por ABB AB Productos de Alta Tensin Departamento: Marketing y Ventas Texto: Tomas Roininen, Carl Ejnar Slver, Helge Nordli, Anne Bosma, Per Jonsson, Anders Alfredsson Diseo, 3D e imgenes: Mats Findell, Karl-Ivan Gustavsson SE-771 80 LUDVIKA, Suecia 2 Gua de aplicacin | ABB Interruptores Automticos

Tabla de contenido 1. Introduccin 8

1.1 Qu es un interruptor de circuito? 8

1.2 Por qu necesitamos los interruptores? 9

1.3 Diferentes tipos de conmutacin 9

1.4 Desconectar y disyuntores extrables 9

1.5 Switcher Circuito 11

1.6 Aspectos ambientales 11

Diseos de interruptores 2. tanque vivo y principios de funcionamiento 12

2.1 Desarrollo histrico 12

2.2 Componentes principales 13

2.2.1 Unidad de Breaking 14

2.2.2 Soporte aislante 14

2.2.3 Mecanismo de funcionamiento 14

2.2.4 Estructura de soporte 14

2.3 Componentes adicionales 15

2.3.1 Condensadores de calificacin 15

2.3.2 Resistencias de pre-insercin 15

2.3.3 Armarios de control central 15

2.4 SF 6 Interruptores 16

2.4.1 Gas SF 6 16

2.4.2 Principios de extincin del arco 16

2.4.2.1 Rgimen trmico 17

2.4.2.2 Rgimen dielctrica 18

2.4.3 Diseos interruptor anteriores 19

2.4.4 De SF6 disyuntores 19

2.4.5 SF 6 autogeneracin disyuntores 22

2.4.6 Configuracin de los contactos mviles 25

2.5 Mecanismos de funcionamiento 26

2.5.1 General 26

2.5.2 Mecanismo a resorte 27

2.5.3 Motor Drive 30

2.5.4 Mecanismo neumtico que funciona 31

2.5.5 Mecanismo hidrulico que funciona 31

2.5.6 Mecanismo de resorte que funciona hidrulico 31

2.5.7 Otros tipos de mecanismos de operacin 32

3. tensiones de conmutacin y de red actuales 33

3.1 Las corrientes de cortocircuito 33

3.1.1 Constante de tiempo estandarizado y asimetra 35

3.1.2 Pico corriente admisible 36

3.2 Fallas de terminales 37

3.2.1 Recuperacin de tensin transitoria (TRV) en redes monofsicas 37

3.2.2 TRV en redes trifsicas 40

3.3 Fallas de lnea corta 43

3.4 Tensin Recuperacin Inicial Transitorio (ITRV) 45

3.5 Fuera de la fase condiciones 46

3.6 La conmutacin de corrientes capacitivas 47

3.6.1 Desenergizacin cargas capacitivas 47

ABB Interruptores Automticos | Gua de Aplicacin 3

Tabla de contenido

3.6.2 Tensin de Recuperacin 50

3.6.3 Energizante de bateras de condensadores 51

3.7 Conmutacin de carga inductiva 53

3.7.1 La conmutacin de reactores en derivacin 53

3.7.1.1 Interrupcin de corriente y sobretensiones resultantes 54

3.7.1.2 Reencendidos 57

3.7.1.3 Las sobretensiones y limitacin de sobretensin 58

3.7.2 La conmutacin de transformadores sin carga 59

4. tensiones mecnicas y efectos ambientales 60

4.1 Cargas mecnicas 60

4.1.1 Las cargas estticas 60

4.1.1.1 Peso propio 60

4.1.1.2 Terminal de carga esttica 60

4.1.1.3 Carga de hielo 62

4.1.1.4 Carga del viento 62

4.1.2 Las cargas dinmicas 63

4.1.2.1 Las cargas dinmicas debido a la operacin 63

4.1.2.2 Cargas actuales dinmicas 64

4.1.3 Carga ssmica 65

4.1.3.1 Medidas para aumentar los niveles de resistencia ssmica 67

4.2 Combinacin de cargas 67

4.3 Influencia de las temperaturas ambientales extremas 68

4.4 Propiedades del gas 69

4.4.1 Efecto de la temperatura ambiente 69

4.4.2 El contenido de humedad en gas SF6 70

4.5 Los efectos de sonido de operacin de disyuntor 71

4.5.1 Normas 71

4.5.2 Nivel de ruido como funcin de la distancia 71

5. tensiones trmicas 72

5.1 Lmites trmicos 72

5.1.1 Reduccin de la intensidad nominal debido a la temperatura 72

5.2 Ensayo de calentamiento 73

5.3 El aumento de temperatura en la sobrecarga de corriente 74

5.4 Influencia de la altitud de instalacin 76

6. requisitos de aislamiento 78

6.1 Coordinacin del aislamiento 78

6.2 Las sobretensiones 78

6.2.1 Tensin a frecuencia industrial de corta duracin 78

6.2.2 Choque de rayo 78

6.2.3 Impulso picado 80

6.2.4 Impulso de conmutacin 80

6.3 Niveles de aislamiento 81

6.4 Pruebas dielctricas en interruptores 84

6.4.1 General 84

6.4.2 Prueba de tensin combinada 84

6.4.3 Otras pruebas de tensin 85

4 Gua de Solicitud | ABB Interruptores Automticos

6.4.3.1 RIV (Radio Interferencia Voltaje) pruebas 86

6.4.3.2 Ensayo de descargas parciales 86

6.4.3.3 Prueba de la Contaminacin 86

6.4.3.4 Las pruebas en circuitos de baja tensin 86

6.5 Factor de correccin atmosfrica 87

6.6 Instalacin en altitudes elevadas 88

6.7 Efectos ambientales y formas de aisladores 89

6.7.1 Distancia de fuga y contaminacin 89

6.7.2 Clases ambientales segn IEC 90

6.7.3 Clases ambientales de acuerdo con IEEE 91

6.8 Espacios libres en el aire 92

6.9 El material aislante 93

7. Aplicacin 94

7.1 Lnea de transmisin disyuntores 94

7.1.1 Los fallos en las lneas areas 94

7.1.2 Conmutacin de lneas de transmisin en vaco 95

7.1.2.1 Factor de tensin 95

7.1.2.2 Lnea de corriente de carga 95

7.1.2.3 Reconexin 96

7.1.2.4 Derivacin de lneas de transmisin compensadas 96

7.1.2.5 Compensadas en serie lneas de transmisin 96

7.1.3 Clasificacin 96

7.2 Interruptores de circuito del transformador de potencia 97

7.2.1 La asimetra y la hora dc constante 98

7.2.2 Condiciones de conmutacin sin carga 98

7.2.3 Sincronizacin 99

7.2.4 Clasificacin 99

7.3 Condensador / filtro disyuntores 99

7.3.1 Factores de tensin de recuperacin y de tensin 100

7.3.2 Corriente de arranque 100

7.3.3 Clasificacin y clasificacin 101

7.4 Disyuntores de reactores en derivacin 101

7.4.1 Condiciones de funcionamiento 102

7.4.2 Reencendidos 103

7.4.3 Eliminacin de reencendidos 103

7.4.4 Pruebas de conmutacin de reactores en derivacin 103

7.4.5 Clasificacin 104

7.5 Acopladores de bus 104

7.6 Las aplicaciones especiales 104

7.6.1 Aplicaciones ferroviarias 105

7.6.1.1 Aplicaciones ferroviarias con una frecuencia de potencia inferior a 50 Hz 105

7.6.2 Condensador Series by-pass interruptores 105

7.6.3 Filtros HVDC 106

7.6.4 SVC (compensador esttico Var) 107

7.7 Transformadores de medida y rels en combinacin 107

con interruptores de tanque vivo

7.8 Conmutacin controlada 108

ABB Interruptores Automticos | Gua de aplicacin 5

Tabla de contenido 8. Normas y pruebas 109

8.1 Normas 109

8.1.1 IEC 109

8.1.1.1 Definiciones del tiempo de acuerdo con la norma IEC 110

8.1.2 ANSI / IEEE 111

8.2 Prueba de interruptores de circuito 111

8.3 Los ensayos de tipo 112

8.3.1 Pruebas dielctricas 112

8.3.2 Radio Interferencia de Voltaje (RIV) pruebas 113

8.3.3 Pruebas de aumento de la temperatura 113

8.3.4 Medicin de la resistencia del circuito principal 113

8.3.5 -De corta duracin admisible y pico soportar las pruebas actuales 113

8.3.6. Ensayos mecnicos y medioambientales 114

8.3.6.1 Prueba de funcionamiento mecnico a temperatura ambiente 114

8.3.6.2 Pruebas de baja y alta temperatura / pruebas de estanqueidad 115

8.3.6.3 Prueba de alta temperatura 116

8.3.6.4 Pruebas de humedad 117

8.3.6.5 Pruebas de hielo 117

8.3.6.6 Prueba de carga esttica del terminal 117

8.3.7 Conexin y desconexin pruebas 118

8.3.7.1 Preparacin para pruebas 118

8.3.7.2 Prueba monofsico / trifsico 118

8.3.7.3 Unidad de prueba / test polo completo 119

8.3.7.4 Las pruebas directas 119

8.3.7.5 Prueba sinttico 120

8.3.7.6 Resumen de las funciones de prueba 121

8.3.7.7 Culpa Terminal 121

8.3.7.8 Culpa Short-line 121

8.3.7.9 Fuera de la fase de hacer y deshacer las pruebas 122

8.3.7.10 Conmutacin de la corriente capacitiva 122

8.3.7.11 Pruebas de conmutacin actual de reactores en derivacin 123

8.4 Las pruebas de rutina 123

8.5 Los informes de ensayo 123

8.5.1 Informes Tipo de prueba 124

8.5.2 Informes Tipo de prueba de laboratorios independientes 124

8.5.3 Organizacin STL 124

8.5.4 Organizacin SATS 125

Los costes del ciclo 9. Fiabilidad, mantenimiento y vida 126

9.1 Estadsticas de fallas 126

9.2 Vida elctrica y mecnica 126

9.3 Mantenimiento 128

9.4 Monitorizacin de estado 128

9.5 Costes del ciclo de vida 129

9.6 Aspectos ambientales 129

10 Seleccin de los interruptores automticos 131

6 Gua de aplicacin | ABB Interruptores Automticos

Alcance Este documento da informacin de fondo para la seleccin de la mejor solucin interruptor de circuito posible para cada aplicacin particular. La utilidad direcciones de gua, consultor y los ingenieros de proyecto que especifican y aplican interruptores de alta tensin. Las direcciones de gua viven interruptores de tanque, en general, para tensiones de hasta 800 kV. Se mencionan los requisitos ms habituales en un interruptor de circuito, tales como las capacidades para manejar las tensiones de red, niveles de aislamiento, las fuerzas mecnicas y las condiciones ambientales. La construccin de los polos del interruptor y los mecanismos de funcionamiento se menciona slo brevemente ya que estas partes se describen en la Gua ABB s buyer para Interruptores tipo tanque vivo. ABB Interruptores Automticos | Gua de Aplicacin 7

1. Introduccin 1.1 Qu es un interruptor de circuito? Un interruptor es un aparato en el sistema elctrico que tiene la capacidad para, en el menor tiempo posible, pase de ser un conductor ideal para una nsula-tor ideal y viceversa. Adems, el interruptor de circuito debe ser capaz de cumplir con los siguientes requisitos: 1. En la posicin cerrada estacionaria, conducir su corriente nominal sin producir aumento de temperatura permisible en cualquiera de sus componentes. 2. En sus posiciones fijas, abrir y cerrar, el interruptor debe ser capaz de soportar cualquier tipo de sobretensiones dentro de su calificacin. 3. El interruptor automtico deber, a su tensin nominal, ser capaz de hacer y romper cualquier corriente pos-sible dentro de su calificacin, sin llegar a ser inadecuados para realizar alguna operacin. Los requisitos de los interruptores automticos de depsito directo pueden ser tan altos como capacidad de interrupcin de 80 kA y 800 kV de tensin nominal. Adems de interruptores automticos de tanque vivo, tambin hay otras construcciones de los polos del interruptor (tanque muerto, GIS). En pocas anteriores, el aceite y aire comprimido eran aislante tpico y extincin de me-dium. Hoy en da estn casi totalmente reemplazados por gas SF6 por razones econmicas y prcticas, y tambin debido a la mayor demanda de calificaciones ms altas. Hay diferentes tipos de mecanismos de operacin, por ejemplo por resorte, mecanismos hidrulico-y neumticos que funcionan, y los motores recientemente controladas digitalmente-han entrado en uso.

Figura 1.1 Interruptor automtico para 145 kV Figura 1.2 Interruptor automtico para 420 kV8 Gua de Solicitud | ABB Interruptores Automticos 1.2 Por qu necesitamos los interruptores? El interruptor es un componente crucial en la subestacin, donde se utiliza para el acoplamiento de barras de distribucin, transformadores, lneas de transmisin, etc La tarea ms importante de un interruptor de circuito es interrumpir corrientes de falla y as proteger los equipos elctricos y electrnicos. La interrupcin y la reconexin posterior deben llevarse a cabo de tal manera que el funcionamiento normal de la red se restablece rpidamente, a fin de mantener la estabilidad del sistema. Adems de la funcin de proteccin, los interruptores de circuito tambin se aplican para la conmutacin intencional como energizante y de-energizante de reactores en derivacin y bancos de condensadores. Para el mantenimiento o reparacin de equipos y lneas de transmisin elctrica, los interruptores de potencia, junto con los seccionadores, interruptores de puesta a tierra o desconectando los interruptores cir-cuit con funcin integrada de desconectar, garantizar la seguridad del personal. 1.3 Diferentes tipos de conmutacin El requisito para cambiar cualquier corriente dentro del circuito breaker calificacin s incluye making-di ferente y condiciones de ruptura: fallas Terminal A ', cortocircuitos en las inmediaciones de o cerca del disyuntor. fallas a corto lnea A ", cortocircuitos a tierra a lo largo de la lnea de transmisin a pocos kilmetros del disyuntor. 'condiciones fuera de fase en la que diferentes partes de la red estn fuera de sincronismo. conmutacin intencional 'de los bancos de capacitores, bancos de reactores en derivacin, transformadores sin carga, lneas de vaco y cables. En este interruptor-cin conexin controlada debe ser mencionado. Esto se describe en conmutacin ABB controlado, buyer s y Gua de Aplicacin. Los diferentes estados de conexin se explican en la Seccin 3, Conmutacin de corriente y tensiones de red. 1.4 Desconectar y disyuntores extrables Se ha mencionado que el interruptor automtico es un elemento importante en el sys-tem, ya sea como un autnomo breaker circuito en una subestacin convencional o como una parte integrada de una instalacin de conmutacin compacta. Las soluciones modernas con Interruptores Seccionadores (DCB) y Retiro-able Circuit Breakers (WCB) hacen posible el desarrollo de nuevos tipos de construcciones de aparamenta. El propsito de la instalacin de conmutacin compacta es simplificar la aparamenta y al mismo tiempo para mejorar la fiabilidad del sistema. ABB Interruptores Automticos | Gua de Aplicacin 9

1. Introduccin Los diferentes tipos de interruptores compactos tienen una cosa en comn la eliminacin de los seccionadores convencionales en el sistema. Seccionadores tienen bsicamente la misma tasa de fracaso como disyuntores, pero necesitan un mantenimiento ms frecuente. El DCB es un disyuntor que cumpla con los requisitos de un interruptor de circuito, as como un seccionador. Esto se describe en la norma IEC 62271-108. Puntuaciones para corriente y la tensin son los mismos que para un disyuntor de circuito, mientras que los niveles de aislamiento cumplen con aquellos para los seccionadores. Desconexin del circuito de quiebre-res suelen combinar con seccionadores de tierra operados a control remoto-y los sistemas de inter-bloqueo para proporcionar una mayor seguridad. A DCB para la tensin nominal de 145 kV se muestra en la Figura 1.3. En el WCB, los polos del interruptor de circuito estn montados en un carro mvil y provistos de contactos adicionales para la funcin de seccionador. El movimiento del carro reemplaza la funcin de cierre / apertura de los seccionadores convencionales. Ver Figura 1.4. A WCB se puede ampliar con un prtico completo y embarrados. Incluso es posible equipar el WCB con transformadores de corriente o interruptores de puesta a tierra. Disponibilidad estudios han demostrado que en subestaciones con DCB o WCB, la disp-dad se mejora considerablemente en el de las soluciones convencionales. Adems de la baja tasa de fallos y los largos perodos entre mantenimiento, otra ventaja es la reduccin sustancial en el espacio.

Figura 1.3 Figura 1.4

Desconexin del disyuntor (DCB) de 145 kV. Interruptor extrable (WCB)

El seccionador de tierra est pintado de rojo y amarillo. de 145 kV con barras y prtico.

10 Gua de Aplicacin | ABB Interruptores Automticos

1.5 Circuito de conmutacin El conmutador de circuito es un encendedor Varianta economy del interruptor de tanque vivo. La construccin es similar y se puede aplicar para la interrupcin de corrientes de cortocircuito, proteccin y conmutacin de de condensadores y reactores-bancos, transformadores, lneas y cables de conformidad con las normas IEC e IEEE / ANSI. El fallo Puntuacin claro-cin es algo menor que la del interruptor de circuito correspondiente, y los tiempos de funcionamiento son ms largos. IEEE C37.016 especifica los requisitos para los interruptores de circuito. 1.6 Aspectos ambientales Los interruptores automticos instalados en todo tipo de entornos y deben ser capaces de con-stand y operar en cualquier tipo de condiciones climticas, tales como temperaturas extremas altas y bajas, humedad elevada, cargas de hielo y altas velocidades de viento. Otros requisitos importantes son la capacidad para soportar la actividad ssmica y de mantener el correcto funcionamiento en reas con alta contaminacin, as como en instalaciones a gran altura. ABB Interruptores Automticos | Gua de aplicacin 11

Diseos de interruptores 2. tanque vivo y principios de funcionamiento 2.1 Desarrollo histrico Los interruptores de circuito de chorro de aire, que utilizan aire comprimido como la extincin de me-dium, tenan la ventaja de la capacidad de interrupcin elevada y tiempos de interrupcin cortas. Sin embargo, las unidades de ruptura (interruptores) tuvieron dielctrica limitada soportar capabil-dad y, como puede verse en la Figura 2.1, un interruptor automtico para 420 kV necesitaba hasta 10 elementos de corte en serie por fase. La presin de extincin del arco requerido de aire elevado, en torno a 2 MPa, lo que significa que el riesgo de fuga era alto. Instalacin, manteni-miento y reparacin eran costosos.

Chorro de aire Petrleo Gas SF 6 Figura 2.1 El desarrollo histrico de ABB de tanque vivo disyuntores Introduccin de los interruptores de circuitos mnimos de aceite alrededor de 1970 fue un gran paso adelante. El nmero de unidades rompiendo se redujo; un disyuntor de 420 kV necesita slo cuatro interruptores en serie por fase. La demanda de energa para-cin pera era relativamente bajo, y mecanismos de resorte cargado podra ser utilizado. Tanto el interruptor de circuito mnimo de aceite y los mecanismos de resorte son prcticamente afectada por la temperatura ambiente. Otra gran ventaja es que todo el mantenimiento, incluso apertura de las unidades de ruptura, puede llevarse a cabo al aire libre. Sin embargo, aunque el mantenimiento es relativamente simple, ciertas operaciones de conmutacin (por ejemplo, conmutacin de pequeas corrientes inductivas) requieren un mantenimiento ms frecuente. SF interruptores de circuito de gas 6 son superiores a estas tecnologas anteriores, ya que requieren sub-sustancialmente menos mantenimiento. Adems, se reducen el nmero de unidades de ruptura. Hasta 300 kV se utiliza un interruptor por fase, y a 550 kV slo se requieren dos interruptores. Todos ABB SF 6 de tanque vivo interruptores pueden ser operados con mecanismos de resorte cargado, de manera que la demanda de energa para la operacin es ahora incluso ms bajo para ciertos interruptores de circuito SF 6 que para el disyuntor de circuito de aceite mnimo correspondiente. Los mecanismos operativos han desarrollado correspondientemente. A principios de Neumtica- y operado hidrulicamente mecanismos eran tpicas, pero hay una tendencia general hacia mecanismos de resortes. 12 Gua de Aplicacin | ABB Interruptores Automticos

2.2 Principales componentes Interruptores de tanque vivo constan de cuatro componentes principales: 'Una o ms unidades de ruptura (interruptores) Soporte aislante ' 'Uno o ms mecanismos de funcionamiento Estructura de soporte '(soporte) Las cifras siguientes muestran las diferentes partes de SF 6 disyuntores.

Figura 2.2. Tripolar circuito operado Figura 2.3. Uno de los polos de un solo polo operado

interruptor con un solo interruptor por polo. interruptor de circuito con dos interruptores por polo.

Componentes del interruptor automtico

1. Unidad de Breaking 5. Mecanismo de viaje 9. Capacitor (si es necesario)

2. Soporte aislante 6. Supervisin de gas (en el lado opuesto) 10. Resistencia de pre-insercin (PIR) (si es necesario)

3. Estructura de soporte 7. Pullrod con tubo protector 11. Terminales primarios

4. Mecanismo de funcionamiento 8. Indicador de posicin

ABB Interruptores Automticos | Gua de aplicacin 13 Diseos de interruptores 2. tanque vivo y principios de funcionamiento 2.2.1 Unidad de Breaking La carcasa aislante est hecho de porcelana o de material compuesto y se llena con presin de gas SF 6. La unidad de interrupcin se somete a potencial, es decir, es live , de ah el trmino live Tanka disyuntor. Un circuito de polo del interruptor puede incluso consistir en dos o ms unidades de ruptura en las series. El nmero de unidades que rompen depende de la tensin del sistema y los-mentos requieren en capacidad de interrupcin. La funcin de la cmara de extincin se describe en 2.4.2 Principles de arco extinction. 2.2.2 Apoyo aislante La funcin principal del aislador de apoyo es suficiente para asegurar el aislamiento de los terminales de alta tensin y la unidad de fractura (s) a tierra. El aislador de soporte es una carcasa hueca hecha de porcelana o de material compuesto y contiene gas SF 6 a la misma presin que el gas en las unidades de ruptura; el aislador de soporte y la unidad (s) de rotura tienen una atmsfera de gas comn. La varilla de traccin aislado, (tambin llamado aislante operativo) que es parte del sistema de la edad de enlace entre el mecanismo operativo y los contactos principales, est montado en el interior del aislador de apoyo. 2.2.3 mecanismo de funcionamiento El mecanismo de funcionamiento, junto con el muelle de viaje, almacena la en-ener- necesario para la operacin de cierre y apertura del interruptor automtico. Situado a potencial de tierra, el mecanismo operativo tambin incluye cableado secundario, que acta como una interfaz al sistema de control y proteccin de un Reda s. Estructura 2.2.4 Apoyo Hay dos versiones de las estructuras de apoyo (o soporte de apoyo) para interruptores de tanque vivo: Tres columnas 'apoya, es decir, cada polo del interruptor de circuito est montado sobre su soporte individual. 'apoyo Polo-haz, es decir, los tres polos montado sobre una viga comn con dos patas de apoyo. Los soportes se hacen generalmente de inmersin en caliente de acero galvanizado. 14 Gua de Aplicacin | ABB Interruptores Automticos

2.3 Componentes adicionales 2.3.1 condensadores de calificacin Para interruptores con dos o ms unidades de ruptura en la serie, el voltaje (tensin nominal, as como las sobretensiones de conmutacin y relmpagos transitorios) por lo general no se distribuyen uniformemente a travs de los interruptores. Con el fin de evitar el alto voltaje tensiones a travs de una de las unidades de ruptura, los condensadores se montan a menudo en paralelo con los interruptores. La capacitancia es generalmente del orden de 900 "1600 pF por unidad de romper. El rendimiento de los interruptores est mejorando poco a poco, y hoy en da los condensadores-cin de posgrado son generalmente no se necesita con tensiones nominales de hasta 420 kV. El disyuntor de circuito ABB del tipo HPL ha sido recientemente verificada por pruebas de tipo para manejar 550 kV sin clasificacin de condensadores. Esto ofrece varias ventajas: fcil trans-puerto y la instalacin; menor masa, lo que mejora la resistencia ssmica capabil-dad; disminuciones en las corrientes de fuga; y un menor riesgo de ferrorresonancia en transformadores de tensin inductivos cercanas. 2.3.2 Resistencias de pre-insercin Resistencias de pre-insercin en la lnea de interruptores de circuito se utilizan ocasionalmente en voltios-rated edades 362 "420 kV y ms a menudo a 550 " 800 kV. Su propsito es reducir la tensin transitorios generados cuando se activa una lnea de transmisin sin carga, o re-energiza despus de un fallo de lnea. Las resistencias son operados por el mismo mecanismo de funcionamiento como los contactos principales. Resistencias de pre-insercin fueron previamente utilizados a veces en los interruptores de las bateras de condensadores, bancos de reactores y transformadores. Para estas aplicaciones, sin embargo, la conmutacin controlada es ahora ampliamente utilizado como un medio poderoso para reducir los transitorios de conmutacin-ing. Modernos SF 6 disyuntores tambin tienen propiedades mejores de conmutacin que viejos tipos de interruptores de circuito. Esto ha hecho generalmente resistencias de pre-insercin super-fluo para estas aplicaciones. Las nuevas tecnologas tambin pueden eliminar la necesidad de resistencias de pre-insercin de los disyuntores de lnea. En muchos casos, la conmutacin controlada puede reemplazar las resistencias y reducir los transitorios de tensin en la misma medida como, o mejor que, las resistencias. Ver ABB Conmutacin controlada, s buyer y Gua de Aplicacin. 2.3.3 Armarios de control central Interruptores automticos para operacin monopolar pueden ser provistos de armarios para el control central. Estos son convenientes para la operacin tripolar local. En algunos casos uno de los armarios de mando es ampliado para manejar la integracin de las funciones de la cabina de control central. Esta solucin se refiere a veces como Master-slave o ICC, Cubculo de Control Integrado. ABB Interruptores Automticos | Gua de aplicacin 15 Diseos de interruptores 2. tanque vivo y principios de funcionamiento 2.4 SF 6 Interruptores 2.4.1 gas SF 6 Interruptores de circuito de alta tensin con gas SF6 como aislante y medio de extincin han estado en uso en todo el mundo durante ms de 30 aos. Este gas es particu-larmente adecuado debido a su alta resistencia dielctrica y conductividad trmica. 2.4.2 Principios de extincin del arco El proceso de interrupcin de la corriente en un disyuntor de circuito de alto voltaje es un complejo mat-ter debido a la interaccin simultnea de varios fenmenos. Cuando los contactos del interruptor de circuito independiente, se establecer un arco elctrico, y la corriente seguir fluyendo a travs del arco. La interrupcin tendr lugar en un instante cuando la corriente alterna llega a cero. Cuando un interruptor automtico se dispara con el fin de interrumpir una corriente de cortocircuito, la separacin de contacto puede comenzar en cualquier parte del bucle de corriente. Despus de que los contactos se han separado mecnicamente, la corriente fluir entre los contactos a travs de un arco elctrico, que consiste en un ncleo de gas extremadamente caliente con una temperatura de 5.000 a 20.000 K. Esta columna de gas est totalmente ionizado (plasma) y tiene una conduc-tividad elctrica comparable a la de carbono. Cuando la corriente se aproxima a cero, el dimetro de arco disminuir, con la seccin transversal aproximadamente proporcional a la corriente. En las proximidades de cero paso de la corriente, el gas se ha enfriado a alrededor de 2.000 K y ya no ser ionizado plasma, ni ser elctricamente conductor. Dos requisitos fsicos (regmenes) estn involucrados: rgimen trmico ': El canal de arco caliente tiene que ser enfriado a una temperatura lo suficientemente baja que deja de ser conductor de la electricidad. 'rgimen dielctrica: Despus de la extincin del arco, el medio de aislamiento entre los contactos debe soportar la tensin de restablecimiento rpido-creciente. Esta tensin de recuperacin tiene un componente transitorio (tensin transitoria de restablecimiento, TRV) causada por el sistema cuando se interrumpe la corriente. Si cualquiera de estos dos requisitos no se cumple, la corriente seguir fluyendo para otro ciclo de medio, hasta que se alcanza la siguiente corriente cero. Es bastante normal para un disyuntor de cir-cuit para interrumpir la corriente de cortocircuito en la segunda o incluso tercera corriente cero despus de la separacin de contactos. 16 Gua de Aplicacin | ABB Interruptores Automticos

a) circuito equivalente simplificado.

b) Curvas de cortocircuito corriente i sc y el voltaje de recuperacin u r t 1 =Contacto separacin t 2 = Extincin del arco S = velocidad de aumento de la tensin de recuperacin (El voltaje del arco de contacto separacin de extincin de arco es baja Figura 2.4 Tensiones en la cmara de extincin a la interrupcin. y se ha hecho caso omiso) 2.4.2.1 rgimen trmico El rgimen trmico es especialmente crtico en un corto-line interrupcin de fallo (vase la seccin 3). Los parmetros del circuito que afectan directamente a este rgimen son la tasa de disminucin de la cur-renta ser interrumpido (di / dt) y la velocidad inicial de aumento de la recuperacin transitoria voltios-edad (du / dt) inmediatamente despus de cero actual. Cuanto mayores sean los valores de cualquiera de estos dos parmetros, el ms grave de la interrupcin. Un alto valor de di / dt resultados en un arco caliente con una gran cantidad de energa almacenada a corriente cero, lo que hace Interrumpir-cin ms difcil. Los valores altos de du / dt se traducir en un aumento de la energa a la corriente post-arco. Existe una cierta inercia en la conductividad elctrica del arco (vase la Figura 2.5). Cuando la corriente se aproxima a cero, todava hay una cierta cantidad de conductividad elctrica que queda en la trayectoria del arco. Esto da lugar a lo que se llama un post arco cur-Renta con amplitud hasta unos pocos A. Sea o no la interrupcin va a tener xito est determinado por una carrera entre el efecto de enfriamiento y la entrada de energa en la trayectoria de arco por la tensin transitoria de restablecimiento. Cuando las escalas de la punta bal-ANCE energa a favor de la entrada de energa, el interruptor de circuito fallarn trmicamente. El rgimen de interrupcin-ther mal para SF 6 interruptores de circuito corresponde al perodo de tiempo de arranque algunos s antes de corriente cero, hasta la extincin de la corriente de arco posterior, despus de unos pocos s corriente cero. ABB Interruptores Automticos | Gua de aplicacin 17

Diseos de interruptores 2. tanque vivo y principios de funcionamiento

Figura 2.5 en la forma actual de interrupcin (la escala de tiempo est en el rango de microsegundos). 2.4.2.2 rgimen dielctrica Cuando el interruptor de circuito ha pasado con xito el rgimen trmico, la tensin transitoria de restablecimiento (TRV) entre los contactos se eleva rpidamente y alcanzar un valor alto. Por ejemplo, en un interruptor de circuito de una sola unidad 245 kV la distancia entre contactos puede estar estresado por 400 kV o ms, 70 a 200 s despus del cero de corriente. En el rgimen dielctrica la extincin / aislar medio es ms largo elctricamente con-ductos, pero todava tiene una temperatura mucho ms alta que la del ambiente. Esto reduce la tensin soportada capacidad de la apertura de los contactos. La tensin en el interruptor de circuito depende de la tasa de aumento de temperatura y la amplitud de la TRV.

Figura 2.6 rgimen interrupcin dielctrica La capacidad de resistir de la apertura de los contactos debe ser siempre superior a la tensin de recuperacin-tran sient, ver Figura 2.6, de lo contrario se producir un reencendido dielctrica (falla dielctrica). Esto requiere un dielctrico extremadamente alta capacidad de resistencia el gas, que es todava bastante caliente y por lo tanto tiene una baja densidad. 18 Gua de Aplicacin | ABB Interruptores Automticos

2.4.3 diseos interruptor anteriores Los primeros interruptores aplican gas SF 6 tuvieron la cmara de extincin dividido en dos partes separadas con diferentes presiones (interruptor automtico de doble presin), que opera en el mismo principio que los disyuntores de aire hornos. Hoy en da todos los de alta tensin interruptores de SF6 de circuito se han extincin de las cmaras que se aplican principios puffer o auto-hornos. 2.4.4 de SF6 disyuntores En el SF 6 globo, se crea la presin del gas de la explosin de enfriamiento durante la carrera-cin abierta en un cilindro de compresin. En la operacin de apertura, la compresin del gas comenzar al mismo tiempo que los contactos comienzan su movimiento. El gas presionado com-se sopla a travs de una boquilla aislante en la que el arco se est quemando. La figura 2.7.2 muestra la funcin de un interruptor tipo puffer. La boquilla aislante est normalmente hecha de PTFE (Teflon).

Figura 2.7.1 Principales componentes del interruptor del soplador. El color rojo indica la trayectoria de la corriente a travs del interruptor cerrado. ABB Interruptores Automticos | Gua de aplicacin 19

2. Diseos de interruptores tanque vivo y principios de funcionamiento Contacto de despedida

Cerrado Principal La formacin de arcos Arco Abierta Cierre

posicin extincin posicin

A B C D E F Figura 2.7.2 Funcin de un interruptor tipo puffer: A. Posicin cerrada. La corriente es conducida a travs de los contactos principales. B. La separacin de los contactos principales. Los contactos mviles han comenzado a cambiar de posicin, los contactos principales se han separado. La corriente se conmuta a los contactos de arco. La presin se empieza a acumular en el volumen puffer. C. Despus de la separacin de los contactos de arco se establece un arco entre ellos. Presin en el volumen del soplador contina aumentando. D. Extincin del arco. La corriente se aproxima a cero y el gas fro de las explosiones de volumen globo a travs de la boquilla, el enfriamiento del arco y extinguirlo. E. Los contactos son ahora completamente abierta; el movimiento se ha detenido y amortiguado por el mecanismo operativo. F. Durante el cierre de los contactos se cierran y el volumen del soplador se vuelve a llenar con gas fro, por lo que es listo para la siguiente operacin de apertura. Una caracterstica importante del diseo del globo es la dependiente de la corriente acumulacin de presin de extincin. En una operacin sin carga (sin arco) las pres-seguro en el cilindro puffer mximo es normalmente el doble de la presin de llenado. Ver la curva de vaco en la Figura 2.8. 20 Gua de Aplicacin | ABB Interruptores Automticos

Un arco pesado quema entre los bloques de contactos el flujo de gas a travs de la boquilla. Cuando la corriente disminuye hacia cero, el dimetro de arco tambin disminuye, dejando ms y ms toma del rea libre para el flujo de gas. Un flujo de gas total se establece as en la corriente cero, lo que resulta en la mxima refrigeracin cuando sea necesario. El bloqueo de la boquilla (obstruccin de la boquilla) durante el intervalo de alta corriente da una acumulacin de presin-ther de piel en el cilindro soplador que puede ser varias veces la presin mxima sin carga (vase la figura 2.8). En otras palabras: el volumen puffer disminuyendo, obstruccin de la boquilla y de calefaccin del gas por el arco interactuar para crear una alta presin. La alta presin en el globo requiere una fuerza de funcionamiento alta. Por tanto, la energa de la explosin se suministra casi en su totalidad por el mecanismo operativo.

Figura 2.8. La presin en el volumen puffer en funcionamiento en vaco y durante la interrupcin de una corriente de cortocircuito asimtrica de 40 kA Con el fin de extinguir el arco, se requiere una cierta presin de la explosin y se determina por el cambio de la velocidad de la corriente en el cero de corriente (di / dt) y la tasa de aumento de la tensin de recuperacin inmediatamente despus de corriente cero (du / dt ) como se describe en 2.4.2.1. ABB Interruptores Automticos | Gua de aplicacin 21

2. Diseos de interruptores tanque vivo y principios de funcionamiento 2.4.5 SF 6 autogeneracin disyuntores Servicio experiencia ha demostrado que las fallas del disyuntor debido a insuficiente capacidad de inter-rupting son raros. La mayora de las fallas reportadas son de naturaleza mecnica, por lo que se hacen esfuerzos para mejorar la fiabilidad general de los mecanismos de op-erating. Debido al hecho de que los disyuntores puffer requieren alta energa de operacin, los fabricantes se vieron obligados a utilizar mecanismos neumticos, mecanismos hidrulicos o mecanismos de resorte de alta energa. En un disyuntor de circuito del soplador normal, la mayor parte de la presin de la explosin se crea con la energa del mecanismo de operacin. La situacin ideal sera dejar que el arco producir la presin de la explosin. De esta manera, el mecanismo de funcionamiento solamente necesita para entregar energa necesaria para el movimiento del contacto. Sin embargo, esta situacin ideal no puede actualmente no puede llegar a voltajes ms altos. Los problemas surgen cuando se interrumpir corrientes pequeas, ya que slo hay una cantidad limitada de energa disponible para el aumento de presin. Por esta rea-hijo se ha alcanzado un compromiso: un disyuntor de auto-explosin con pre-compresin. Los principios de auto-hornos representan un gran paso adelante en el camino hacia la reduccin de la energa de funcionamiento. The self-blast technology has several designations: auto-puffer, arc-assisted circuit breaker, thermal-assisted circuit breaker or simply self-blast circuit breaker. Because the blast pressure required for interruption of low currents and low cur-rent derivatives (see 2.4.2.1) is moderate, a small pressure rise independent of the current is sufficient. For higher currents, the energy producing the blast pressure is taken from the arc through heating of the gas.

Figure 2.9.1 Main components of the self-blast interrupter. Red color indicates the current path through the closed interrupter. 22 Application Guide | ABB Circuit Breakers

Contact parting

Cerrado Principal Arcing Arco Abierta Cierre

posicin extincin posicin

A B C D E fFigure 2.9.2 Self-blast SF 6 interrupter with pre-compression. The figure shows high current interruption. A. Closed position. The current is conducted through the main contacts. B. Separation of main contacts. The moving contact has started to change position, the main con-tacts have parted. Pressure is starting to build up in the puffer and self-blast volumes. The current is commutated to the arcing contacts. C. After separation of the arcing contacts an arc is established between them. Heat from the arc generates pressure in the self-blast volume, the valve closes when the pressure is higher than in the puffer volume.* D. Arc extinction. The current approaches zero and the gas from the self-blast volume blasts up through the nozzle, cooling the arc and extinguishing it. Excessive pressure in the puffer volume is released through the pressure relief valve. E. The contacts are now fully open; the motion has been damped and stopped by the operating mechanism. F. During closing the contacts close and the puffer volume is refilled with cold gas, making it ready for the next opening operation. At low breaking current the pressure generated by the arc will not be sufficient to close the valve. The self-blast interrupter will then operate as a puffer interrupter.

F ABB Circuit Breakers | Application Guide 23

2. Live tank circuit breaker designs and operating principles As can be seen in Figure 2.9.1, the extinction chamber is divided into two sections: the self-blast volume and the puffer volume. The two sections are separated by the self-blast valve. When high-fault currents are interrupted, the pressure in the self-blast volume gen-erated by the arc will be so high that the valve will close, preventing the gas from escaping into the puffer volume. Instead, the pressurized gas will flow through the nozzle and extinguish the arc. At lower currents, typically a few kA, the arc will not have sufficient energy to gener-ate a pressure high enough to close the valve and the interrupter will function as a puffer interrupter. The pressure in the puffer volume is relatively independent of the current whether the circuit breaker operates as a self-blast interrupter or as a puffer interrupter. It is limited to a moderate level by means of a spring-loaded valve (overpressure valve), which means that the compression energy required from the operating mechanism is limited. Figure 2.10 shows how the energy from the operating mechanism is used. Compared with a conventional puffer circuit breaker of the same rating, the energy requirements of the operating mechanism can be reduced to 50% or less.

Figure 2.10 Utilization of operating energy at a breaking operation 24 Application Guide | ABB Circuit Breakers

2.4.6 Configuration of the moving contacts For the self-blast circuit breakers, several different principles of moving contact con-figurations exist. Their purpose is to further decrease the amount of energy needed for the operation of the circuit breaker. The single-motion design dominates the market today. This is the simplest type of design, utilizing one moving set of arcing and main contacts. The double-motion design uses a linkage system to move both the puffer with the lower contact system, and the upper arcing contact, in different directions. This means that the speed requirement from the operating mechanism is drastically reduced, since the contact speed will be the relative speed between the upper and lower contacts. The main benefit of the double-motion arrangement is the mini-mized energy need for the contact movement due to lower speed demands. This is visualized by the equation for kinetic energy:

The triple-motion design is based on the double-motion design, but moves the up-per shield at a different speed than that of the upper arcing contact to optimize the distribution of the electrical fields and get a better dielectrical performance.

Figure 2.11 Double motion design ABB Circuit Breakers | Application Guide 25

2. Live tank circuit breaker designs and operating principles 2.5 Operating mechanisms 2.5.1 General The main requirement of the operating mechanism is to open and close the con-tacts of the circuit breaker within a specified time. The operating mechanism shall provide the following consecutive functions: ' Charging and storing of energy ' Release of energy ' Transmission of energy ' Operation of the contacts In addition, an operating mechanism shall provide control and signaling interface to a networks control and protection system. Figures 2.2 and 2.3 show the location of the operating mechanism. A requirement common to most circuit breakers, regardless of the type of operating mechanisms, is to carry out an open-close-open (O - 0.3 s - CO) sequence with no external power supply to the operating mechanism. The circuit breaker shall, after a closing operation, always be able to trip immediately without intentional time delay. For circuit breakers intended for rapid auto-reclosing, the operating duty cycle in accordance with IEC 62271-100 is: O ' 0.3 s ' CO ' 3 min - CO The time of 3 min is the time needed for the operating mechanism to restore its power after a O ' 0.3 s - CO. Modern spring and hydraulic operating mechanisms do not need 3 min to restore their power, as an alternative IEC specifies that the time val-ues 15 s. or 1 min. tambin puede ser utilizado. The dead time of 0.3 s is based on the recovery time of the air surrounding an external arc in the system (ie a short-circuit). Sometimes the operating sequence CO - 15 s - CO is specified. 26 Application Guide | ABB Circuit Breakers

2.5.2 Spring-operated mechanism In the spring mechanism, the energy for open and close operation is stored in springs. When the mechanisms control system receives an open or close command, the energy stored in the spring will be released and transmitted through a system of levers and links and the contacts will move to the open or closed position. In most designs the closing spring has two tasks: to close the contacts, and at the same time to charge the opening spring (or springs). Thus the criteria stated above are fulfilled; the circuit breaker in closed position is always ready to trip. After the O - 0.3 s - CO operation, the closing spring will be recharged by an elec-tric motor, a procedure that lasts 10-20 seconds. The circuit breaker will then be ready for another CO operation. One example of a spring mechanism is shown in Figure 2.12. This type of mecha-nism has a set of parallel helical wound springs with linear motion. The electric motor charges the springs via an endless chain. When the closing latch is released, the energy stored in the springs is transmitted via a rotating cam disc and a system of levers and links to the circuit breaker pole or poles. The trip spring is in this case located outside the operating mechanism. Instead of helical springs, clock springs may be applied. The function is the same as for the helical springs described above. Figure 2.13 shows a system with clock spring for closing operation. The advantage of the spring-operated mechanism is that the system is purely mechanical; there is no risk of leakage of oil or gas, which could jeopardize the reli-ability. A well-balanced latching system provides stable operating times. Furthermore, the spring system is less sensitive to variations in temperature than pneumatic or hydraulic mechanisms are. This ensures stability even at extreme temperatures. The spring mechanism has fewer components than hydraulic and pneumatic mechanisms, which improves its reliability. ABB Circuit Breakers | Application Guide 27

2. Live tank circuit breaker designs and operating principles

3 6 1 2 7 4 5 8 Figure 2.12 Spring operating mechanism with helical wound springs. Trip and close springs in charged position. ABB type BLG.

1. Link gear 5. Closing damper

2. Trip spring 6. Tripping latch with coil

3. Closing latch with coil 7. Opening damper

4. Motor 8. Close springs

28 Application Guide | ABB Circuit Breakers

6 3 1 4 2 5 7 Figure 2.13 Spring operating mechanism with clock spring. ABB type BLK.

1. Trip spring 5. Closing latch with coil

2. Link gear 6. Motor

3. Close spring 7. Opening damper

4. Tripping latch with coil

ABB Circuit Breakers | Application Guide 29

2. Live tank circuit breaker designs and operating principles 2.5.3 Motor Drive One of the most recently-developed operating mechanisms is the electrical motor drive. The motor drive uses a servomotor to perform a smooth and silent operation of the circuit breaker. The operation is actively controlled by a sensor which con-tinuously reads the position of the motor and adjusts the motor current to obtain an optimal travel curve. The energy is stored in a capacitor bank and can be delivered instantaneously to the converter, which transforms dc from the capacitors and feeds the motor with regulated ac. The major advantage of the motor drive is the minimized mechanical system, which reduces the service need to a minimum and makes the technology ideal for applica-tions with frequent operation.

3 2 1

4 AC

DC

Viaje

Cerrar

Motor Status signals

7

Position sensor

6

5

Figure 2.14 Block diagram showing function of the Motor Drive 1. Charger unit that converts supply voltage and feeds the capacitors and internal electronics. 2. Capacitor unit stores the energy for operation. 3. The converter unit transforms dc from the capacitors to ac for the motor. 4. Servomotor that delivers the force to move the contacts, integrated position sensor gives information to the control unit. 5. The I/O unit takes care of signaling between the station control system and the operating mechanism. 6. Control unit that controls and regulates the motion of the contacts. The interlock functions are also handled by this module. 7. Link gear that transforms rotary movement to linear movement. 30 Application Guide | ABB Circuit Breakers

2.5.4 Pneumatic-operated mechanism The pneumatic-operated mechanism uses compressed air as energy storage, and pneumatic cylinders for operation. Solenoid valves allow the compressed air into the actuating cylinder for closing or for opening. The compressed-air tank is replenished by a compressor unit. The use of pneumatic operating mechanisms is decreciente. Due to the high operating pressure, there is always a risk of leakage of air, particularly at low temperatures. There is also a risk of corrosion due to humidity in the compressed air. 2.5.5 Hydraulic-operated mechanism The hydraulic mechanism usually has one operating cylinder with a differential piston. The oil is pressurized by a gas cushion in an accumulator, and the operating cylinder is controlled by a main valve. The hydraulic mechanism has the advantages of high energy and silent operation. However, there are also some disadvantages. There are several critical components which require specialized production facilities. The risk of leakage cannot be ne-glected as the operating pressure is in the range of 30-40 MPa (300-400 bar). It is necessary not only to check the pressure as such but also to supervise the oil level in the accumulator or, in other words, the volume of the gas cushion. Large varia-tions in temperature lead to variations in operating time. Until recently several manufacturers used hydraulic mechanisms for their SF 6 circuit breakers. However, with the introduction of self-blast circuit breakers, the require-ment of high energy for operation is decreasing and the hydraulic mechanisms are losing ground to spring-operated mechanisms. 2.5.6 Hydraulic spring-operated mechanism The hydraulic spring-operated mechanism is an operating mechanism combining hydraulics and springs. Energy is stored in a spring set, that is tensioned hydrauli-cally. A differential piston, powered by oil that is pressurized by the spring package, is used to operate the circuit breaker during opening and closing.

Figure 2.15 Hydraulic spring-operated mechanism. ABB type HMB ABB Circuit Breakers | Application Guide 31

2. Live tank circuit breaker designs and operating principles 2.5.7 Other types of operating mechanisms In addition to the types of operating mechanisms mentioned above, there are other variants, eg a design which basically applies the same technology as the pneu-matic mechanisms but with SF 6 gas instead of air. Another design is the magnetic actuator mechanism, which is applied only for cer-tain medium-voltage circuit breakers. 32 Application Guide | ABB Circuit Breakers

3. Current switching and network stresses 3.1 Short-circuit currents A circuit breaker should be able to interrupt both symmetrical and asymmetrical short-circuit currents. Asymmetrical short-circuit currents consist of a symmetrical component superimposed on a dc component. The dc component will decrease with time. Consider a part of a typical network as shown in Figure 3.1. At normal operation, load currents of the magnitude of approximately 100 amperes will flow in the dif-ferent branches. If a short-circuit occurs somewhere in the system, the currents will change dramatically into short-circuit currents that are two or three orders of magnitude higher than the load currents.

Figure 3.1 Network with short-circuit Figure 3.2 Single-phase equivalent network With a short-circuit in point A, the system may, in a simple approximation, be repre-sented by a single-phase equivalent network as shown in Figure 3.2. The inductance L may be obtained from the short-circuit power (fault level) S k of the network as:

In this equation is the angular frequency (2 f ) and U is the system voltage (phase-to-phase). The resistance R represents the losses in the network. It is normally low compared to the inductive reactance X = L , perhaps 5 to 10%. The voltage source u in Fig-ure 3.2 is the phase-to-neutral voltage. ABB Circuit Breakers | Application Guide 33

3. Current switching and network stresses Figure 3.3 shows the voltages and currents associated with a short-circuit at point A. Up to the short-circuit instant - which has been chosen arbitrarily - the current is low and has been shown as a zero line in the figure. After the short-circuit has oc-curred, the current will approach a steady state symmetrical value i sym . The phase angle will be almost 90 lagging, since the circuit is more or less purely inductive. Since the current in an inductive circuit can not be discontinuous at the short-circuit instant, the total short-circuit current ( i total ) will consist of a symmetrical part i sym and dc component i dc :

Figure 3.3 Current and voltage at short-circuit The initial magnitude I dc of the dc component will be equal to the instantaneous value of the symmetrical short-circuit current i sym at the short-circuit instant and has the opposite polarity. In this way the transition from load to short-circuit current will be continuous. The dc component will follow an exponentially damped curve and can be written as:

donde:

t time from initiation of the short-circuit

the time constant of the network is determined by the losses of the circuit, = L/R and is of the order of 20 - 60 ms in typical networks.

34 Application Guide | ABB Circuit Breakers

3.1.1 Standardized time constant and asymmetry Both IEC and IEEE use a standard value of the time constant, = 45 ms. This value covers most cases. Sometimes specification of the time constant is replaced by specification of the X/R (= L/R ) ratio of the network. This value depends on the network fre-quency. The standard time constant 45 ms corresponds to X/R = 14 at 50 Hz, and X/R = 17 at 60 Hz. Normally the amount of asymmetry of the current at a certain instant of time is given by stating the dc component as a percentage of the (peak value of) sym-metrical current. The dc component used for type testing of a circuit breaker is determined as fol-lows (refer to IEC 62271-100): For a circuit-breaker that is intended to be tripped solely by means of auxiliary power (eg by giving an impulse to a device that releases the circuit-breaker for operation), the percentage dc component shall correspond to a time interval equal to the minimum opening time T op of the circuit-breaker plus one half-cycle of rated frequency T r . (See also Figure 3.4). Ejemplo: What is the dc component required for a circuit breaker having a minimum opening time of 18 ms, used on a 50 Hz system with a time constant of 45 ms? Respuesta: Minimum opening time: T op = 18 ms Rated frequency 50 Hz gives T r = 10 ms T op + T r = 18 + 10 = 28 ms leads to a dc component of 54% (see Figure 3.4) For rated frequency 60 Hz ( T r = 8.3 ms) the dc component is 56%. The rms value of the asymmetrical current (also known as the total current) can be determined using the following formula:

donde:

p the dc component in pu I sym the rms. value of the symmetrical part

ABB Circuit Breakers | Application Guide 35

3. Current switching and network stresses Sometimes it is necessary to use larger values for the time constant than 45 ms. For example, this can be the case close to large generator units, where the network resis-tance is low. For such situations IEC specifies alternative, special case time constants : ' 120 ms for rated voltages up to and including 52 kV ' 60 ms for rated voltages from 72.5 kV up to and including 420 kV ' 75 ms for rated voltages 550 kV and above In order to minimize special testing, etc. it is preferable that these values are applied when the standard value 45 ms cannot be used. Percent dc 100

90 80 70

60 50 40 30 20 10

Time (ms) Figure 3.4 Percentage dc component in relation to time (from IEC 62271-100) 3.1.2 Peak withstand current Due to the dc component, the short-circuit current will be asymmetrical for a certain time after the initiation of the short-circuit. On the one hand, this has to be consid-ered when designing circuit breakers which will be called upon to interrupt such currents. On the other hand, this is important for all the components in the network, since the mechanical stresses due to the currents will be most severe at the maxi-mum peak of the asymmetrical short-circuit current. The highest possible current peak will occur if the short-circuit is initiated at the zero passage of the voltage. The probability of this happening in the network is low, but needs to be considered. Lightning strikes occur at random time instants, and can therefore in the worst case initiate short-circuits when the instantaneous voltage in the system is zero. In contrast, any short-circuits caused by the system voltage itself, eg in the case of damaged or polluted insulation systems, will mainly occur close to the peak value of the voltage in the system. 36 Application Guide | ABB Circuit Breakers

The initial value of the dc component in the worst case would be:

Half a period later (10 ms at 50 Hz) the peak value would occur. Assuming a rated frequency of 50 Hz and a time constant of 45 ms, the peak value would be:

At 60 Hz the peak value would be:

Based on this relation IEC specifies that, in cases with time constant 45 ms, the peak withstand current is 2.5 times the rms value of the symmetrical short-circuit current at 50 Hz. For 60 Hz, both IEEE and IEC specify a multiplying factor of 2.6. For the special case time constants, larger than 45 ms, the multiplying factor is 2.7. A circuit breaker must be able to cope with the maximum peak current in closed position (rated peak withstand current). In addition, it must be able to withstand the same peak current in the event that the short-circuit is initiated by a closing opera-tion of the circuit breaker (rated short-circuit making current). 3.2 Terminal faults Terminal faults are faults located directly at/or in the vicinity of the circuit breaker terminals. In this case, the total short-circuit impedance is equal to the source side impedance. Consequently, the terminal fault is the condition that gives the highest short-circuit current. 3.2.1 Transient Recovery Voltage (TRV) in single-phase networks Consider again the network in Figure 3.1 with a short-circuit at point A and the equivalent network shown in Figure 3.2. In order to understand what happens when the short-circuit is interrupted by the circuit breaker in point B, the same network may be used, only with the addition of a capacitance C , representing the total stray capacitance in the source side network. See Figure 3.5. ABB Circuit Breakers | Application Guide 37

3. Current switching and network stresses

Figure 3.5 Single-phase equivalent Figure 3.6 Current and voltage at interruption network for determination of the TRV Figure 3.6 shows the current and the voltage at interruption of the current. The circuit breaker will only interrupt the current at a current zero. At this instant the supply voltage u of the network is close to its peak value, since the circuit is almost purely inductive. The voltage across the circuit breaker is low (equal to the arc volt-age) as long as the current has not been interrupted. After interruption, this voltage will approach the value of the supply voltage through a transient oscillation with a frequency determined by L and C of the network. The overshoot will typically be of the order of 40 - 60%. (Had the circuit been entirely without losses, R = 0, the overshoot would have been 100%). The voltage across the circuit breaker after interruption is called the recovery volt-age. The first oscillatory part of it is referred to as the Transient Recovery Voltage (TRV), while the later part is called power frequency recovery voltage. The power frequency recovery voltage is equal to the open-circuit voltage of the network at the location of the circuit breaker. Both the Rate of Rise of Recovery Voltage (RRRV) and the peak value of the TRV are important parameters. Together with the magnitude of the current, these param-eters determine the severity of the switching case. The ratio of the peak of the TRV and the peak of the source side voltage is called the amplitude factor. The equivalent network in Figure 3.5 is applicable in situations with short lines. In systems with long lines, it is less reasonable to use an equivalent network with a only a lumped inductance and capacitance, and the recovery voltage will have a more complicated shape. Figure 3.7a, b and c shows three different simplified situations. 38 Application Guide | ABB Circuit Breakers

'' In Figure 3.7a, the network consists of relatively short lines (typical for distribution networks), and the TRV is a damped, single-frequency oscillation. This is the same situ-ation as in Figure 3.6. ' In Figure 3.7b, the network is dominated by an extremely long line. In this case the TRV will have approximately an exponential wave shape. ' Finally, in Figure 3.7c, the line is shorter, and reflected voltage waves from the remote end of the line will add to the TRV wave shape. TRV shapes in different simplified networks

Figure 3.7a

Figure 3.7b

Figure 3.7c ABB Circuit Breakers | Application Guide 39

3. Current switching and network stresses Typical networks with relatively high rated voltage will have TRV wave shapes that are combinations of the single-frequency response of Figure 3.7a and the exponen-tial (with reflection) of Figure 3.7c. Both IEC and IEEE have the same approach for specification of the standard tran-sient recovery voltages: '' For rated voltages below 100 kV, a TRV waveshape as illustrated in Figure 3.7a is assumed. It is described by means of two parameters, u c and t 3 , see Figure 3.8a. '' For rated voltages 100 kV and above, a TRV waveshape that is essentially a com-bination of Figure 3.7a and 3.7c is assumed. It is described by means of four pa-rameters, u 1 , u c , t 1 and t 2 , see Figure 3.8b. This four-parameter method is used for high breaking currents (terminal fault type tests at 100% and 60% of rated break-ing current), while the two-parameter method is used for lower breaking currents.

Figure 3.8a TRV with Figure 3.8b TRV with

2-parameter reference line 4-parameter reference line

3.2.2 TRV in three-phase networks In a three-phase network, several types of short-circuits may occur: single-phase to earth; two-phase, with or without earth connection; and three-phase, with or without earth connection. Three-phase short-circuits are very rare, but lead to the most severe stresses on the circuit breakers. Therefore the TRV volt-age values used for type testing are based on three-phase faults. In the network, the three phases will affect each other, and the first phase to interrupt will experi-ence the most severe TRV stress. To compensate for that, a first-pole-to-clear factor is introduced. Consider the three-phase circuit shown in Figure 3.9. It is a simplified equivalent circuit of a network with effectively earthed neutral, with a three-phase short-circuit. The short-circuit point is isolated, ie has no connection to earth. When the first pole (pole a is assumed to be the first pole) interrupts the fault, symmetry is lost and a pure two-phase short-circuit remains, causing the potential of the short-circuit point to shift. The power frequency recovery voltage ra across the circuit breaker in pole a may be written: ra = a - n 40 Application Guide | ABB Circuit Breakers

Due to the symmetry of the circuit, the potential of the short-circuit point related to earth, n will attain a value halfway between b and c after interruption of the current in pole a. See the vector diagram in Figure 3.10. The absolute value of the power frequency recovery voltage U ra will then be U ra =1.5 U a ie 1.5 times the phase-to-neutral voltage.

Figure 3.9 Equivalent circuit, isolated Figure 3.10 Vector diagram three-phase fault in network with effectively earthed neutral The ratio between the power frequency recovery voltage in the first pole to clear and the phase to earth voltage is the first-pole-to-clear factor:

The first-pole-to-clear factor k pp depends on the network conditions. For a three-phase earthed fault in a system with non-effectively earthed neutral, or a three-phase unearthed fault in a network with solidly earthed neutral, k pp is equal to 1.5. This is the highest value k pp can attain. In the quite common case of a three-phase-to-earth fault in a network with earthed neutral, k pp is related to the positive sequence reactance X 1 and zero sequence reactance X 0 of the network as:

The variation of k pp with the system reactance ratio X 1 / X 0 is shown in Figure 3.11. ABB Circuit Breakers | Application Guide 41

3. Current switching and network stresses

Figure 3.11 Variation of k pp with X 1 /X 0 For a normal overhead line, the ratio X 1 /X 0 is generally around 1/3. Hence for a net-work with earthed neutral, in which the reactance is dominated by line reactance, the resulting first-pole-to-clear factor, in the case of an earthed short-circuit, will be of the order of 1.3 (dashed line in Figure 3.11). IEC 62271-100 is based on the assumption that three-phase faults involve earth (available fault statistics show that this is normally the case). Therefore IEC uses first-pole-to-clear factors to reflect normal practice with regard to system earth-ing, with k pp = 1.5 for non-effectively earthed and k pp = 1.3 for effectively earthed neutral systems: Rated voltage U First-pole-to-clear factor ( kpp )

U 72.5 kV 1.5

72.5 < U 170 kV 1.3 or 1.5

U 245 kV 1.3

Table 3.12 First-pole-to-clear factors according to IEC Basically the IEEE standards specify the same first-pole-to-clear factors as IEC. In addition, however, IEEE takes into consideration the small possibility that isolated three-phase faults may occur. Therefore, as an option, a first-pole-to-clear factor k pp = 1.5 is specified for rated voltages of 100 kV and above. 42 Application Guide | ABB Circuit Breakers

3.3 Short-line faults Short-line faults (SLF) are short-circuits that occur on an overhead transmission line within a relatively short distance (some kilometers) from the substation. The fault current is determined not only by the source impedance (network) but also by the impedance of the line between the circuit breaker and the fault location. The TRV in this case is characterized by the propagation of traveling waves along the line, between the circuit breaker and the fault location.

Figure 3.13 Short-line fault in single-phase equivalent circuit Consider Figure 3.13, which shows a short-line fault in a single-phase equivalent circuit. The supply side has been modeled in the same way as in Figure 3.5. On the load side there is now a certain length of overhead line, characterized by its surge impedance Z , between the fault location and the circuit breaker. The TRV at interruption of the current in this circuit may be obtained as the differ-ence between the supply and load side voltages:

Up to the moment of current interruption, u 1 and u 2 will be practically equal. After interruption, u 1 will approach the power frequency voltage of the system in an oscillatory manner in the same way as at a terminal fault. The voltage u 2 on the load side will approach zero through a damped saw-tooth oscillation, associated with voltage waves traveling on the section of line between the circuit breaker and the fault location. As shown in Figure 3.14, the transient recovery voltage u r across the circuit breaker will have the same shape as that of a terminal fault, but with a superimposed saw-tooth oscillation in the initial part, directly after current zero. ABB Circuit Breakers | Application Guide 43

3. Current switching and network stresses

Figure 3.14 The transient recovery voltage for a short-line fault The rate-of-rise of the saw-tooth voltage on the line side, du 2 / dt is directly propor-tional to the rate-of-decay di / dt of the current immediately before current zero and the equivalent surge impedance Z of the line:

where di/dt = 2f , with f = power frequency and = short-line fault peak current. Both IEC and IEEE apply a standard value Z = 450 at all rated voltages. The fact that the fault occurs relatively close to the circuit breaker means that the SLF current is almost as high as the terminal fault current. Since the rate-of-rise of the TRV is more severe than at a corresponding terminal fault, the major stresses on the circuit breaker are in the thermal regime (refer to Section 2.4.2.1). Increasing the current at a short-line fault thus rapidly increases the stresses on the circuit breaker at current zero, since both di / dt and du 2 / dt will be higher, requiring more efficient cooling of the arc region. The most critical conditions will be obtained for a specific line length. With a very short line, the current will be high, but the first peak of the saw-tooth oscillation will be low. Therefore the stresses at current zero will resemble those of a terminal fault. For long line lengths, the current will be de-creased so much that it will again be easier for the circuit breaker to clear. The time t to the first peak of the saw-tooth oscillation is related to the length of the line between the circuit breaker and the fault location. Since the propagation speed of the waves is equal to the speed of light (300 m/s), a typical line length of 1 km 44 Application Guide | ABB Circuit Breakers

will give a time of the order of 6 or 7 s (the time for the wave to travel to the fault location and back again). Theoretically, the reflected waves exhibit themselves as a pure saw-tooth wave shape, as presented in Figure 3.14. However, the relatively high concentration of stray capacitances at the line end in the substation introduced by instrument trans-formers, bushings of adjacent equipment, support insulators, etc. generates an initial time delay in the line side voltage oscillation. IEC specifies SLF requirements for circuit breakers rated 52 kV and above, hav-ing a rated short-circuit breaking current exceeding 12.5 kA. In addition, SLF is a requirement for medium-voltage circuit breakers intended for direct connection to overhead lines (Class S2). IEEE has the same requirements. The test requirements in both standards are based on single-phase-to-earth faults. 3.4 Initial Transient Recovery Voltage (ITRV) A TRV stress similar to that which occurs at a short-line fault may occur, due to the busbar connections on the supply side of the circuit breaker. This TRV stress is referred to as the Initial Transient Recovery Voltage, or ITRV. Due to the relatively short distances involved, the time to the first peak will be short, typically less than 1 s. The surge impedance of the busbar in a station is lower than that observed for overhead lines. Both IEEE and IEC apply a value of Z = 260 for air-insulated substations (AIS). For GIS substations the surge imped-ance is lower, and therefore the ITRV stresses can be neglected. Figure 3.15 shows the origin of the various contributions to the total recovery volt-age for terminal faults and short-line faults. At the source side of the circuit breaker the TRV is generated by the supply network, whereas the substation topology, mainly the busbars, generates the ITRV oscillation. For any fault occurring at the load side of the circuit breaker, both TRV and ITRV will be present in the recovery voltage. For a short-line fault, the total recovery voltage comprises three components: the TRV (network), the ITRV (substation) and the line oscillation.

Figure 3.15 Simplified diagram showing the origin of the ITRV and the TRV for terminal fault (1), and for short-line fault (2) ABB Circuit Breakers | Application Guide 45

3. Current switching and network stresses 3.5 Out-of-phase conditions Two cases in which out-of-phase conditions may occur are shown in Figure 3.16. One case occurs when a generator is accidentally switched on to the network at the wrong phase angle (Figure 3.16a). The other case occurs when different parts of a transmission network lose their synchronism, eg due to a short-circuit some-where in the network (Figure 3.16b). In both cases, an out-of-phase current will flow in the networks and will have to be interrupted by the circuit breaker.

Figure 3.16a and 3.16b Out-of-phase conditions Considering a single phase, and assuming effectively earthed neutral, both net-works in Figure 3.16 may be simplified to the circuit shown in Figure 3.17. The reactances X 1 and X 2 may be obtained from the short-circuit power of the two parts of the network, one on each side of the circuit breaker.

Figure 3.17 Single-phase equivalent network Figure 3.18 Orientation of voltage vectors In normal operation, the two voltage vectors 1 and 2 will be approximately equal in both amplitude and phase angle. Under the most severe out-of-phase conditions, the voltage vectors may be separated 180 electrical degrees. The cor-responding maximum power frequency recovery voltage U r will be U r = 2 U , under the assumption that U 1 = U 2 = U. In a corresponding case with non-effectively earthed neutral, the power frequency recovery voltage may, in the worst case, reach the value U r = 3 U . Both IEC and IEEE specify tests at 2 U for effectively earthed neutral systems, whereas it has been considered sufficient to use 2.5 U for non-effectively earthed neutral systems. Due to the strong damping caused by overhead lines, the am-plitude factor of the TRV for out-of-phase switching is generally lower than the amplitude factor at interruption of terminal fault current. An amplitude factor 1.25 is specified. Overall the peak of the TRV for out-of-phase switching is higher than that for interruption of short-circuit current, and leads to severe dielectric stress on the circuit breaker. 46 Application Guide | ABB Circuit Breakers

The current in this switching case is lower than that observed at interruption of short-circuit current in the network, due to the relatively high impedance of the loop between the two voltage sources (reactance X 1 + X 2 ). Both the IEC and IEEE stan-dards specify that circuit breakers shall be tested at 25% of the rated short-circuit breaking current. Higher current values have been considered highly improbable. 3.6 Switching of capacitive currents Capacitive currents are encountered in the following cases: ' switching of no-load overhead lines ' switching of no-load cables ' switching of capacitor banks ' switching of filter banks Interruption of capacitive currents is generally an easy duty for a circuit breaker, because the currents are normally small, perhaps a few hundred amperes. There is, however, a risk that restrikes will occur, which may lead to undesirable overvoltages in the network. Energizing of capacitive loads may also lead to overvoltages or high currents. 3.6.1 De-energizing of capacitive loads For a single-phase case, the equivalent circuit shown in Figure 3.19 may be used to illustrate the conditions when de-energizing a capacitor bank. Figure 3.20 shows the current and voltage shapes at interruption.

C Capacitive load (capacitor bank) u s Source side voltage

u r Voltage across the circuit breaker L s Source side inductance

u c Voltage across the capacitor bank C s Source side capacitance

Figure 3.19 Single-phase equivalent circuit for capacitive current interruption ABB Circuit Breakers | Application Guide 47

3. Current switching and network stresses With the purely capacitive load, the current will be 90 electrical degrees lead-ing relative to the voltage. This means that when the current is interrupted at a current zero, the voltage will be at its maximum value. After interruption of the current, the supply side voltage u s will be more or less unaffected. There is only a minor decrease in amplitude, associated with the disappearance of the capacitive load. The transition to the new amplitude value is associated with a slight oscil-lation, the frequency of which is determined by L s and C s . From the moment the current is interrupted, the capacitor C is isolated from the rest of the network. In a short time perspective, the voltage u c will therefore remain constant at the value it had at current zero, ie the peak value of the supply voltage. (In a longer time perspective, the capacitor will gradually discharge. There are generally built-in discharge resistors in capacitor units.)

Figure 3.20 Voltage and current shapes at capacitive current interruption The low initial rate-of-rise of the recovery voltage, together with the low current to be interrupted, makes it extremely easy for the circuit breaker to interrupt. Even if a current zero occurs closely after contact separation, the circuit breaker will inter-rupt. Half a cycle after current zero, the recovery voltage has risen to no less than twice the peak value of the supply voltage. The circuit breaker may then not be able to withstand the high value of the recovery voltage across a still relatively small con-tact gap. In this case, dielectric breakdown will occur between the contacts, and current will start to flow again. Figure 3.21 shows current and voltage wave-shapes in a case where voltage breakdown occurs relatively close to the recovery voltage peak. The load side voltage will swing up to a voltage that ideally (without any damping) may reach 3 times the supply voltage peak U p . The oscillation frequency of the current and voltage after the breakdown is determined by L s and C (assuming C >> C s ). The circuit breaker may easily interrupt the current again at one of its current zeros, with the result that the voltage across the capacitor may attain a new constant value, perhaps higher than before. Further breakdowns associated with even higher overvoltages across the load may then occur. If a circuit breaker does not interrupt at any of these high-frequency current zeros, it will in any case interrupt at the next power frequency current zero. 48 Application Guide | ABB Circuit Breakers

Figure 3.21 Voltage and current wave-shapes in the case of a restrike Voltage breakdowns during capacitive current interruption are divided into two categories:

Reignitions: Voltage breakdown during the first 1/4 cycle following current interruption.

Restrikes: Voltage breakdown 1/4 of a cycle or more following current interruption.

Restrikes will lead to overvoltages across the capacitive load (maximum 3 pu for a single restrike, where 1 pu is the peak value of the phase-to-earth voltage), while reignitions will not produce any overvoltages (theoretically max. 1 pu). A properly-designed circuit breaker should not produce restrikes. The phenom-enon is statistical, however, and both IEC and IEEE define two different classes of circuit breakers:

Class C1: Circuit breaker with low probability of restrike during capacitive current breaking as demonstrated by specific type tests. Class C2: Circuit breaker with very low probability of restrike during capacitive current breaking as demonstrated by specific type tests.

The type tests for Class C1 are performed on a circuit breaker with new contacts, and with a number of breaking operations with capacitive current at various arcing times. The corresponding type tests for Class C2 are performed on a circuit breaker that has been previously aged by means of short-circuit switching operations, and consist of a higher number of breaking operations than for Class C1. ABB Circuit Breakers | Application Guide 49

3. Current switching and network stresses 3.6.2 Recovery voltage In a three-phase circuit, the recovery voltage will have a more complicated shape than in a corresponding single-phase situation. It will be most severe in the first-pole-to-clear, and in the majority of cases be higher than in the single-phase case. Figure 3.22 shows as an example the recovery voltage of the first-pole-to-clear when de-energizing a capacitor bank with isolated neutral. The recovery voltage initially has a shape that would lead to a peak value equal to three times the supply voltage peak (dotted line). When the last two poles interrupt 1/4 cycle (90 electrical degrees) after the first, there is, however, a discontinuity in the slope, and the final peak value for the first pole-to-clear is limited to 2.5 times the supply voltage peak.

Figure 3.22 Recovery voltage in the first-pole-to-clear at interruption of capacitor bank with isolated neutral. The IEC and IEEE standards specify capacitive voltage factors for different types of capacitive load. The capacitive voltage factor is used for calculation of the relevant test voltage in a single-phase test circuit intended to simulate the conditions in the first-pole-to-clear of a three-phase network. 50 Application Guide | ABB Circuit Breakers

Standard values for capacitive voltage factors, k c , for normal service conditions are as follows: No-load line switching k c = 1.2 Effectively earthed neutral k c = 1.4 Non-effectively earthed neutral

No-load cable switching k c = 1.0 Screened cables in systems with solidly earthed neutral k c = 1.2 Belted cables in systems with effectively earthed neutral k c = 1.4 In systems with non-effectively earthed neutral

Capacitor bank switching k c = 1.0 Capacitor bank with earthed neutral in systems with solidly earthed neutral k c = 1.4 Capacitor bank with isolated neutral

Figure 3.23a Screened cable Figure 3.23b Belted cable 3.6.3 Energizing of capacitor banks Energizing of capacitive loads is usually associated with transient voltages and currents. This section covers the phenomena associated with energizing of capacitor banks. Given the increasing use of capacitor banks for compensation purposes, it is com-mon that more than one capacitor bank is connected to the same busbar. This has no influence on the conditions at interruption. The current at closing, however, is affected to a high degree. With one or more capacitor banks already connected, there will be an inrush current when closing a circuit breaker to connect an addi-tional bank (so-called back-to-back switching). This inrush current may have a very high amplitude and frequency, and will sometimes have to be limited in order to not harm the circuit breaker, the capacitor banks and/or the network. The single-phase equivalent of a circuit where two capacitor banks are connected to a busbar is shown in Figure 3.24. The inductances L 1 and L 2 represent the stray inductance (or stray inductance plus additional damping inductance). The inductance L s of the supply network will be several orders of magnitude higher than L 1 and L 2 . ABB Circuit Breakers | Application Guide 51

3. Current switching and network stresses

Figure 3.24 Parallel capacitor banks The case of energizing a single capacitor bank is equal to energizing of C 1 when C 2 is not connected in the circuit described in Figure 3.24. The circuit then consists of the source inductance L s in series with the capacitor bank C 1 . The inductance L 1 can be disregarded here, since L s >> L 1 . In this case, the peak of the inrush current i peak and inrush current frequency f i are limited by the source impedance L s . Assuming that bank C 1 is to be connected to the busbar and bank C 2 is not con-nected, the following equations apply. The highest inrush current peak is obtained when energizing the capacitor bank at the peak of the supply voltage:

y

with L s >> L 1 , the frequency of the inrush current, is:

If bank C 1 is connected to the busbar and bank C 2 is to be connected, the inrush current associated with the charging of bank C 2 is supplied by bank C 1 (back-to-back switching). The peak and frequency of the inrush current are now limited by L 1 and L 2 :

con y

The frequency of the inrush current is now:

The same equations may be applied in a three-phase case. The voltage is then the peak value of the phase-to-earth voltage. 52 Application Guide | ABB Circuit Breakers

Typical amplitudes of the inrush currents for back-to-back energizing of capacitor banks are several kA, with frequencies of 2 - 5 kHz. Capacitor banks can normally withstand amplitudes up to 100 times their rated normal current. For circuit breakers, IEC suggests 20 kA peak at 4.25 kHz as standard withstand capability. Controlled switching is an attractive and well-proven means of limiting the inrush current amplitude and associated stresses on the equipment. See ABB Controlled Switching, Buyers and Application Guide 1HSM 9543 22-01en. In addition, the inrush current amplitude and frequency can be decreased by insertion of additional series inductance in the circuit. 3.7 Inductive load switching Inductive load switching will occur in the following cases: ' switching of shunt reactors ' switching of unloaded transformers The currents to be interrupted are low compared to the short-circuit current, and range from a few amperes to some hundred amperes. Therefore, they are often referred to as small inductive currents. After opening of the circuit breaker, the current flows through the arc between the contacts. This arc is stable at high cur-rents, but becomes unstable at low currents in the order of 5 - 10 A, and is usually forced to a premature current zero. This phenomenon of current interruption prior to the natural current zero is usually referred to as current chopping. The resulting chopping overvoltages, and particularly overvoltages due to subsequent reignitions, may be a concern. Controlled switching of the circuit breaker is an efficient way to eliminate the reignition overvoltages. See ABB Controlled Switching, Buyers and Application Guide. 3.7.1 Switching of shunt reactors The most clear-cut case of inductive load switching is the interruption of a reactor current. The reactor may be directly connected to the network (shunt reactor) or through a transformer (eg connected to a tertiary winding). In both cases the load consists of an approximately linear inductance. A three-phase shunt reactor configuration may differ considerably. The following configurations exist: ' a bank of single-phase reactors ' a three-phase unit with a 3-legged core ' a three-phase unit with a 5-legged core ' a three-phase unit of the shell type The earthing conditions also may vary: ' solidly earthed neutral ' earthing through a neutral reactor ' isolated neutral ABB Circuit Breakers | Application Guide 53

3. Current switching and network stresses 3.7.1.1 Current chopping and resulting overvoltages Due to the variety of cases, the treatment of interruption of reactor current is com-plex. For this reason, the relatively simple single-phase circuit shown in Figure 3.25 depicts only one such case. This case is applicable for shunt reactors with directly earthed neutral and negligible interaction between phases.

L s Source inductance L b Connection series inductance

C s Source side capacitance C L Load side capacitance

C p Parallel capacitance L Reactor inductance

Figure 3.25 Single-phase equivalent circuit for shunt reactor switching Assuming that the current is chopped at an instantaneous value I ch , there is a mag-netic energy W m stored in the load inductance at the moment of interruption:

With C s >> C p , which is normally the case, the total capacitance on the reactor side of the circuit breaker C t is approximately

The voltage across the reactor at the instant of interruption is approximately equal to the peak U p of the source side phase-to-earth voltage, and is determined by the system voltage U (assumed equal to the rated voltage of the reactor):

Therefore the capacitance C t will be charged to the energy:

54 Application Guide | ABB Circuit Breakers

After interruption, the energy stored in the inductance and capacitance swings back and forth between the inductance and the capacitance. After some time the energy will decrease due to the losses in the circuit. When all the energy, W c , is stored in the capacitance, the voltage across the reactor will reach its highest value U m . This value is called the suppression peak overvoltage:

Figure 3.26 Voltage across the reactor at interruption ABB Circuit Breakers | Application Guide 55

3. Current switching and network stresses The frequency of the oscillation is of the order of some 1 to 5 kHz at high voltages and is determined by the natural frequency of the reactor load circuit, ie the reac-tor itself and all equipment connected between the circuit breaker and the reactor. It is convenient to express the suppression peak overvoltage U m as an overvolt-age factor k a :

The overvoltage is not only dependent on the type of circuit breaker, but also on the total capacitance C b in parallel with the circuit breaker. It can be shown that for air-blast, oil and SF 6 circuit breakers, the chopping current is given by the relation

where is called chopping number. The capacitance C b is the capacitance seen from the circuit breakers terminals. It is approximately equal to C t provided that C s >> C L , which is normally the case. The chopping number , is a characteristic of the circuit breaker and can be as-sumed to be a constant for different types of circuit breakers, except for vacuum circuit breakers. Ranges of typical chopping numbers are given in Table 3.1. Circuit breaker types Chopping number ()

(AF 0.5 )

Minimum oil 7-10 x 10 4

Air blast 15-25 x 10 4

SF 6 4-17 x 10 4

Table 3.1 Circuit breaker chopping numbers The chopping number applies to circui