Controlled switching

Introduction: In AC system, magnitude of current and voltage signal are always varying in time domain. Whenever opening command is given to a circuit breaker (CB), there may be some current flowing depending on the instant of contact separation. Magnitude of this current will be different for three phases, as they will be passing through different position (angle) at any point.

CB will quench the arc at next current zero. The duration of arc will depend on position of current wave at the moment of separation. As we know, longer the duration of arc, larger the heat generated and larger the contact deterioration. Therefore, best time of opening instant will be just before the current zero to reduce arcing time. Opening at exact current zero will not work due to the reason that the speed of contacts will be slow and recovery voltage across contacts will be larger than dielectric strength gained. This will result in to restrike across contacts and arc will be quenched at next current zero (another 10ms for 50Hz system). Further there may be open time variation due to mechanical reasons in CB for each operation.

Controlled Switching Device (CSD): EHV class CBs, which are electrically ganged, have different operating mechanism for each CB pole. CSD device is used to operate all three CB poles at different times so that arcing in each phase is minimum. 

For proper operation of CSD, CB mechanical opening time for each phase is to be measured and entered in to CSD relay. As shown in image below, delay of 3.3ms (for 50Hz system) is to be provided for achieving opening of all phase just before current zero.

CSD is used only for manual operation due to added delay in opening. Protection trip will be issued to CB directly without and controlled switching.

CSD is used mainly in following applications:

1. Reactor opening: to avoid current chopping
2. Transformer charging: to avoid high inrush current
3. Capacitor charging: to avoid high inrush current
4. Transmission line charging: to avoid switching voltage surges

Floating DC system and DC earth fault relay

Introduction: DC supply is widely used as auxilliary supply for control and protection system. For larger systems 110V or 220V DC supply is used. For reliability both positive and negative terminals of DC supply are isolated from ground. This is system is called floating DC system.  

Floating DC system has advantage that single earth fault will not cause any outage. Second earth fault may blow the fuse or maloperation of protection. 

DC earth fault relay: For identification of earth faults in early stages, sensative DC earth fault relay is used. 

DC earth fault relay is connected to midpoint of two high value resistors (~ 20kΩ). In normal condition as there will be no path for current to earth, there will be no current in relay element. During a DC earth fault in +ve or -ve terminal, current will flow in to earth by completing return path through DC earth fault relay element. Typical setting for DC earth fault relay is ~ 5mA.

This will cause operation of DC earth fault relay and give alarm to operator. Earth fault can be identified and rectified by maintenance staff before second earth fault. As high value resistance is used in earth fault circuit, fault current will be very low and identification of exact feeder and loaction of fault is difficult. Following methods are generally used for identification of location:

1. Switching off feeders one by one.
2. Using DC earth fault locator instrument, which inject low voltage low frequency signal of ~5Hz in DC system. Faulty feeder is identified by measuring leakage current using tuned clamp on meters.