Time synchronization

Introduction: After any disturbance in power system, analysis for cause of disturbance requires sequential events from relevant protection and control devices. Therefore, all numerical IEDs (relays and controllers) needs to be time synchronized for proper sequence of events in chronological order. 

Generally IEDs are time synchronized by one of the following method:

1. Pulse Per Minute (PPM)
2. Inter-range instrumentation group timecode-B (IRIG-B)
3. Simple Network Time Protocol (SNTP)
4. Precision Time Protocol (PTP)

PPM: Time of the relay has to be set once manually, only seconds and milliseconds will be reset to 00.000 after receiving pulse from time synchronization equipment. It is simplest way of time synchronization. Large number of relays can be synchronized by connecting in parallel. Its accuracy depends on accuracy of internal clock as after synchronization it has to run on internal clock for one minute untill next pulse is received. Being obsolete system it is rarely used these days.

IRIG-B: It is standard format for transferring time information. Information can contains data for day of year, Hours, Minutes, Seconds and milliseconds.Data is transmitted by moduling on 1KHz sine wave through co-axial cable. Accuracy of microsecond level can be achieved. 

SNTP: It is time synchronization using Local Area Network (LAN) system. GPS equipment works as server and sends time information, which contains complete data Year, Month, day, Hours, Minutes, Seconds and milliseconds. All the IEDs get time information from server and time is synchronized. Its accuracy is low as delay in network components is not compensated. Accuracy of millisecond level can be achieved.

PTP: It is used where more accuracy is required than provided by SNTP. In PTP delay in network components is calculated by system and compensated by modifying time stamps. In this way, accuracy of nanosecond level can be achieved.

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.

Interlocking schemes

Introduction: In power system following switchgears are used for operation purpose:

1. Circuit Breaker (CB): It has capability to close or open electric circuits under load or fault conditions. 
2. Disconnector (or Isolator): It can close or open electric circuits under no load condition.
3. Earth switch: It is used to earth primary conductors for maintenance purposes.

Above description leads to following requirements:

1. CB can be opened or closed on no load or full load or fault condition. Therefore, no check is required for disconnector or earth switch status. However, for safety and security of grid, synchronization check is carried out in grids.
2. Disconnector should be operated only when load current is not flowing, i.e. CB should be open. Further, if earth switch is closed, closing of disconnector may cause fault. Therefore, before closing disconnector, associated earth switch status needs to be checked.
3. Earth switch should be operated only when primary conductor is already de-energized. Therefore, status of associated disconnectors needs to be checked. In case of long transmission lines, that may be charged from remote end, status of remote end switchgear may not be available. In this case presence of voltage can be checked for interlocking.

For example, let's take a simple single bus bar scheme:

• 189A can be operated when: 152 is open, Bus earth switch is open
• 189B can be operated when: 152 is open
• 289A can be operated when: 252 is open, Bus earth switch is open
• 289B can be operated when: 252 is open
• Bus earth switch can be operated when: 189A is open, 289A is open

For one and half breaker scheme:

Disconnector 389A can be operated when:

• CB 352 is open
• Earth switch 389AE and 389BE is open
• Bus-2 Earth switch is open

Disconnector 389B can be operated when:

• CB 352 is open
• Earth switch 389AE and 389BE is open

Disconnector 389L can be operated when:

• CB 352 & CB 252 is open
• Earth switch 389LE is open

ES 389AE can be operated when:

• Disconnector 389A & 389B is open

ES 389BE can be operated when:

• Disconnector 389A & 389B is open

ES 389LE can be operated when:

• Disconnector 389L should be open
• There should be no voltage in line