reading / British practice / Vol D - 1990 (ocr) ELECTRICAL SYSTEM & EQUIPMENT

SOURCE 1

1 , 0V DC SUPPLY

ICB2 LOC

CB 3 POS

-0--* (4■1%.-ms

CR3 LOG

CB4 POS

a-

FIG. 1.47 Circuit diagram showing '3 out of 4' interlocked circuit-breakers

SOURCE 2

SW.TC:HBO4 PD

SECTION C

:RC I

IRC 2

IRC 3

iRC.4

a

already closed. Check synchronising interlock facilities are fitted to the LV circuit-breaker which prevents it closing if unacceptable voltage mismatch or phase differences exist.

Interconnectors, especially cable interconnectors, when not in service are usually left energised, from one end only, so as to monitor the integrity of the circuit continually. Bringing the interconnector into service, however, must always be accomplished via the circuit-breaker fitted with the check synchronising facilities.

To ensure that this procedure is always followed, operational electrical interlocks are fitted which prevent the circuit-breaker without synchronising facilities being closed if the circuit-breaker with the check synchronising is already closed. Check synchronising interlock facilities operate as described above.

The circuit and loop diagrams for the transformer or interconnector arrangement are shown in Figs 1.48 and 1.52 respectively.

When the above interlock scheme has to operate between local and remote circuit-breakers of such separation that direct wiring between the plant protection interlocks would cause excessive volt drop in the circuit-breakers control circuits, then circuitbreaker repeat relays are used.

•One out of two Operational electrical interlocks are fitted to the circuit-breakers of both feeder circuits connected to the board. These interlocks prevent both feeder circuit-breakers being closed at the same time, which would otherwise result in unacceptable paralleling of the supplies. Either of the two feeders may be selected for duty and closed onto the board, but having accomplished this the interlocks then prevent the remaining feeder circuit-breaker being closed. No synchronising facilities are required as the above interlock prevents the two supplies being connected together. This scheme is shown in Figs 1 .53 and 1.54..

•Two out of three This interlock scheme is designed to prevent the paralleling of supplies without in-

2

y],

INTERCONNECTOR

..--■•■••••■■ HV

r•-•

\

1 , 3 301 TRANSFORI,IER

TRANSFORMER

FIG. 1.48 Circuit diagram showing cabled interconnector and transformer electrical interlock

hibiting the operational flexibility of the system. Operational interlocks are fitted to the circuit-breakers of both feeder circuits and on the bus section circuitbreaker of the board. The system may be operated with both feeders closed onto their respective halves of the board (as long as the bus section circuitbreaker is open), or if maintenance is to be carried out on one of the feeder circuits, both halves of the board can be coupled together via the bus section circuit-breaker and the board fed from the remaining feeder. No synchronising facilities are required as the interlock prevents any paralleling of supplies. This scheme is shown in Figs 1.55 and 1.56.

• Sequence operational interlocks Sequence interlocks, if required, are to ensure the closure of switch-

gear in a correct and predetermined sequence with all out-of-sequence operations inhibited.

If sequence interlocks are specified on the transformer and interconnector circuits, their application must be developed and incorporated into the operational interlocking scheme as shown in Fig 1.57. It should be noted that the sequence interlock does not inhibit the operation of circuit-breaker No 2 in any way, and it is not therefore an alternative to an operational interlock.

8.4 Computer-based systems

As mentioned above, an alternative and more flexible arrangement for system monitoring is to utilise a computer-based scheme. The most recent of these installed by the CEGB has been at Heysham 2. From the description of the electrical auxiliaries system in Section 3.2 of this chapter, it will be apparent that a relay logic scheme for achieving interlocking could have proved to be cumbersome and possibly restrictive in the connections available to the operators. The decision was therefore made to develop and install a computer-based monitoring system. The block diagram for a computer-based fault level monitoring and indication equipment which has been installed is shown in Fig 1.58.

This equipment does not prevent any circuit-breaker from closing. The equipment advises the operator when it is unsafe to close a circuit-breaker by an indication at the point of switchgear control. Dual computer systems operating in a main and standby mode are provided for main unit failure or routine maintenance. The equipment is designed for mean time between failure (MTBF) of 10 years. The computer system allows changes to be made to the number of circuitbreakers being monitored and the plant data used in the computer model without the need to modify the software.

The fault level calculation and graphical display software for the system is based on the UMIST IPSA package (see Chapter 2) with modifications to enable the software to run in real-time, providing colour displays acceptable for use in a central control room (CCR) and switchgear safety status information (LED states).

As the software has been specified to be flexible and allow circuits and switchboards to be added arid removed, it follows that it can be used on future or existing power stations.

A monitor in the CCR can display the connections and switchgear for any of the switchboards included in the system. The display identifies any switchgear which cannot be closed safely at any instant in time and gives the potential increase in fault level if this were to be done. Each switchgear control switch in the CCR is also provided with a red LED, which is illuminated if it is unsafe to close the circuit. A times

record is also provided for each switchgear change of state, for confirmation of operation and post-incident analysis.

The information provided by the computer will be used as an advisory aid by the operator. Although a computerised interlocking system has been installed at the Drax coal-fired power station, it was decided not to provide full interlocking at the Heysham 2 nuclear power station. This decision was taken so that the operators would not be inhibited from reestablishing vital electrical supplies under controlled emergency conditions to ensure reactor safety.

The natural progression of the computer-based systems is towards full interlocking schemes and elec-

trical auxiliaries system remote control. The CEGB are investigating several options to this end which, in the case of the remote control feature, would supersede the need for the full mimic panel in the CCR with discrepancy switch control used at present.

8.5 Maintenance interlocking equipment

Maintenance interlocking is concerned essentially with personnel safety while maintenance is being performed on electrical equipment. Plant damage must also be considered, but is less likely due to the plant being nonoperational during maintenance.