Operation of high voltage Transmission line
The operation of transmission line starts as soon as the line is energized for commercial operation.
The control and relay panels on both the ends of lines need proper Relay settings. The entire line bay comprising L.A., CVT, Isolators, Wave Trap, Circuit breaker, C.T. etc. needs proper operation for keeping the line loaded.
Depending upon the bus configuration, the line can be on any one bus and various operations will have to be done on the equipment for keeping the line in service.
The operation also includes logbook entries. Hourly readings of power Import-Export are taken which includes KWH, KVA, KVARH, P.F., Frequency, Ampere, voltage etc.
The lines of 220 kV and above have the facility to provide record of disturbance and logging of events. They are called disturbance recorder and event loggers. They indicate the nature of disturbance and the real time/durations of the disturbance. Disturbance include Transient fault, Line to earth fault, line to line fault, over current, over voltage, switching surge etc.
The details of log book help in assessing the load variation on the line when the power system is healthy and when there are contingencies. The load curve can also be drawn on day to day basis.
The operation of the line also includes giving line clear permit (LCP) during planned shut down and in emergency. Special procedures are laid down for the purpose.
The operation problems frequently encountered in EHV system, could be classified under the following three broad categories:
Reactive power absorption and generation
Dynamic stability
Over-voltage in the power system is a cause of concern particularly during the initial operation of a system when the system short circuit level are low. The over voltages in the power system are mainly caused by (a) switching. (b) fault-clear (c) loss of load or changes in the load.
Switch-gear design normally takes into consideration the over-voltage due to switching and if all aspects of the system in which the switch –gear has to function such as characteristic of the transmission lines, and the reactive power compensation equipment are taken into consideration, while selecting the switch-gear, no problem of over voltage due to switching would normally arise.
The provision of closing-registers is now, more or less, a common feature in most EHV switch gear application, and if the insertion resistance is chosen, so as to ensure that over voltage due to switching within the permissible limits.
During fault clearing, particularly single-phase to ground fault, the switch gears testing for any particular application also takes into account the possible over-voltages.
During loss of load, there is every possibility of raising the voltage in the system.
Engineer-in-charge should take care during all such operations, so that the over voltages in the system should be avoided. During operation of EHV line in normal condition, the Engineer-incharge should always contact the Load Dispatch Center of the system, who will guide him whether the line is safe to switch off at the particular time.
It is likely that if the advice or permission is not taken from Load Dispatch Center, then, by cutting off the load from E.H.V. lines, at any time, without consultation with them may raise the system voltages, and three will be lot of complaints from the consumers.
During fault condition, the Engineer-in-charge should take care before switching on the line that the line which he wanted to close, is safe to charge or not. We can decide after verifying the operation of the relays operated during the fault conditions of the line. If the line is tripped with phase to phase faults, then it is advisable not to charge the line immediately without consultation with the Higher Authorities, and if the line is tripped with the relay indication as earth fault, then, the line can be charged immediately in consultation with receiving-end and sending-end.
If the lines are connected with the power stations, where line is not to be charged without consulting Power Station then with the concurrence of load dispatch center, the line is to be charged. If such lines are charged without the concurrence of the Power Station, then it is likely that the system may collapse due to such wrong operation. And the power station Engineer may not be aware of same.
It is also seen that the light – load are not to be kept in ON position, during low load-day, because such line will increase the voltage in the system, and there is likely that consumers may complain due to high voltages. In such condition, Load Dispatch Center should be contacted, and after getting their advice, out of two lines one line can be switched OFF to avoid such over-voltage problems.
The Engineer-in-charge of the sub-station, should also take care during operation of the line, particularly when the permit to work card is issued to the line staff for maintaining the line or line equipment, He should take care that the works permit is written by the authorized person who has taken line is safe to charge. During operation of the equipment of the line they should also take care that no isolators are open of either line except where the line clear is to be issued. They should remain present during operations of such E.H.V. lines without fail.
