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Methods of improving stability

when the maximum power limit of various power-angle curves is raised, the accelerating area decreases and decelerating area increases for a given clearing angle. Consequently, δ0 is decreased and δm 1s increased. This means that by increasing Pmax the rotor can swing through a larger Pmax increases the critical clearing time and improves stability.

The steady-state power limit is given by

Pmax=EV/X

It can be seen from this expression that P max can be increased by increasing either or both V and E and reducing the transfer reactance. The following methods are available for reducing the transfer reactance:

1. Operate Transmission Lines in Parallel

By operating transmission lines in parallel reactance decreases and power increases.

2. Use of Double-Circuit Lines

The impedance of a double-circuit line is less than that of a single-circuit line. A double-circuit line doubles the transmission capability. An additional advantage is that the continuity of supply is maintained over one line with reduced capacity when the other line is out of service for maintenance or repair. But the provision of additional line can hardly be justified by stability consideration alone.

3. Use of Bundled Conductors

Bundling of conductors reduces to a considerable extent the line reactance and so increases the power limit of the line.

4. Series Compensation of the Lines

The inductive reactance of a line can be reduced by connecting static capacitors in with the line.It is to be noted that any measure to increase the steady-state limit Pmax will improve the transient stability limit. The use of generators of high inertia and low reactance improves the transient stability, but generators with these characteristics are costly. In practice, only those methods are used which are economical.

5. High-Speed Excitation Systems

High-Speed excitation helps to maintain synchronism during a fault  by quickly increasing the excitation voltage. High-speed governors help by quickly adjusting  the generator inputs.

6. Fast Switching

Rapid isolation of faults is the principal way of improving transient stability. The fault should be cleared as fast as possible. It so the time required for fault removal is the sum of relay response time plus the circuit breaker operating time. Therefore, high speed relaying and circuit breaking are commonly used to improve stability during fault conditions. It has now become possible to isolate the fault in less than two cycles (that is. 0.04 s for 50 Hz system). System stability can be further improved by making circuit-breaker reclosure automatic, as many faults do not re-establish themselves after restoration of supply. The time interval between removal and reclosure should be reduced keeping in mind that the line must remain de-energized for a certain minimum time in order that the line insulation should recover fully.

7.Breaking Resistors

In this method an artificial electric load in the form of shunt resistors is temporarily connected at or near the generator bus. Such resistors partially compensate the reduction of load on a generator following a fault. The acceleration of the generator rotor is therefore, reduced. For this reason, these resistors are called braking resistors. This method is also known as dynamic braking. A control scheme connects the through circuit breakers. The scheme also amounts of resistance to be connected and the duration of its connection. the   braking resistors are connected immediately following the fault and remain in the   circuit for few cycles. They are disconnected at the moment of enclosure when the system voltage has recovered.

8. Single-Pole Switching

Majority of the line faults are single line-to-ground (LG) faults. In single-pole switching (also called independent pole operation), the three phases of the circuit breaker are closed or opened independently of each other. In the event of an LG fault, the circuit breaker pole corresponding to the faulty line is opened and the remaining two healthy phases continue to transfer power. Since most of the faults are transitory, this phase can be reclosed after it has been open for a predetermined time. The System should not be operated for long periods with one phase open. Therefore, provision should be made to trip the whole line if one phase remains open predetermined time .

• A dc. tie line provides a loose coupling between two ac. Systems to be interconnected.
• A d.c  link may be interconnect two a.c systems at different frequencies.
• There is no transfer of fault energy from one a.c. system to another if they are interconnected by a d.С. tie line.