Effects On Post Disturbance Period

EFFECT OF TRANSFORMER TAP CHANGING IN THE POST DISTURBANCE PERIOD:

If distribution tap changer transformer not work properly it lead to the failure of system. So following factors are need to taken care of for tap changing transformers.

1. Operating time of transformer should be precise so voltage can be raised at lower time.

2. Load sharp increase or overshoot should not be happen.

3. Strategic load shedding should be precise.

4. Load overshoot due to tap changing transformer need to be avoided.

EFFECT OF GENERATOR EXCITATION ADJUSTMENT IN THE POST DISTURBANCE PERIOD:

Sudden large reduction in transmission voltage level leads to over-excitation of generators and field forcing.

This can also leads to hampering of voltage stability causing system collapse. Sometime this leads to over excitation of other units leading to the over excited system.

PRACTICAL ASPECTS OF REACTIVE POWER FLOW PROBLEMS LEADING TO VOLTAGE COLLAPSE IN EHV LINES :

In case EHV lines, voltages control is a major problem which is associated with following

categories.

1. Long Transmission Lines :

For long transmission line having voltage uncontrolled buses at receiving end, gives major voltage problems at light load or heavy load condition. If load level is low, receiving end voltage increases while for heavy load, receiving end voltage falls.

2. High Voltage Problem :

In lightly loaded condition due to reactive power loss, voltage across receiving end increases, this is commonly called as Ferranti effect.

3. Radial Transmission Lines :

Parallel EHV lines in a power system network, are basically radial transmission lines. In these networks loss one line results into increase in reactance of system. Here increase reactance causes large reactive power demand. Use on load tap change transformer improves the distribution voltage but no improvement in transmission line voltage.

4. Reactive Power Dispatch Capability :

This problem more related to the power system using local generator to supply reactive power and loss. If system voltage drops, reactive loss increases with decrease in charging capability of the system.

This cause system generator to provide more reactive power. If reactive power demand increase beyond the thermal limit of generator, causes failure of system due to voltage collapse.

By using AVR, generator voltage can be increase, reading to increase of reactive power flow from generator giving unwanted operation. If generator do not have reactive power producing capacity, then system have to face under voltage problem.

5. Shortage Of Local Reactive Power :

For voltage stability reactive power generation must be equal to the reactive power demand.

Due to outage and maintenance of equipment, there may be a condition where we have to face local reactive power shortage causing voltage control problem. So there should be a counter measure to overcome reactive power shortage Examples. SVC'S, STATCOM, etc.