TRANSIENT STABILITY CONSIDERATIONS IN SYSTEM DESIGN

Transient stability is an important consideration that must be dealt with during the design of power systems. In the design process, time-domain simulations are conducted to assess the stability of the system under various conditions and when subjected to various disturbances. Since it is not practical to design a system to be stable under all possible disturbances, design criteria specify the disturbances for which the system must be designed to be stable. The criteria disturbances generally consist of the more statistically probable events, which could cause the loss of any system element and typically include three phase faults cleared in normal time and line-to-ground faults with delayed clearing due to breaker failure. In most cases, stability is assessed for the loss of one element (such as a transformer or transmission circuit) with possibly one element out-of-service in the pre-disturbance system. In system design, therefore, a wide number of disturbances are assessed and if the system is found to be unstable (or marginally stable) a variety of actions can be taken to improve stability. These include the following:

REDUCTION OF TRANSMISSION SYSTEM REACTANCE: This can be achieved by adding additional parallel transmission circuits, providing series compensation on existing circuits, and by using transformers with lower leakage reactances.

HIGH SPEED FAULT CLEARING: In general, two-cycle breakers are used in locations where faults must be removed quickly to maintain stability. As the speed of fault clearing decreases, so does the amount of kinetic energy gained by the generators during the fault.

DYNAMIC BRAKING: Shunt resistors can be switched in following a fault to provide an artificial electrical load. This increases the electrical output of the machines and reduces the rotor acceleration.

REGULATE SHUNT COMPENSATION: By maintaining system voltages around the power system, the flow of synchronizing power between generators is improved.

REACTOR SWITCHING: The internal voltages of generators, and therefore stability, can be increased by connected shunt reactors.

SINGLE POLE SWITCHING AND RE-CLOSING: Most power system faults are of the single-line-to-ground type. However, in most schemes, this type of fault will trip all three phases. If single pole switching is used, only the faulted phase is removed, and power can flow on the remaining two phases thereby greatly reducing the impact of the disturbance. The single-phase is reclosed after the fault is cleared and the fault medium is deionized.

STEAM TURBINE FAST VALVING: Steam valves are rapidly closed and opened to reduce the generator accelerating power in response to a disturbance.

GENERATOR TRIPPING: Perhaps one of the oldest and most common methods of improving transient stability, this approach disconnects selected generators in response to a disturbance that has the effect of reducing the power, which is required to be transferred over critical transmission interfaces.

HIGH SPEED EXCITATION SYSTEMS: As illustrated by the simple examples presented earlier, increasing the internal voltage of a generator has the effect of proving transient stability. This can be achieved by fast acting excitation systems, which can rapidly boost field voltage in response to disturbances.

SPECIAL EXCITATION SYSTEM CONTROLS: It is possible to design special excitation systems that can use discontinuous controls to provide special field boosting during the transient period thereby improving stability.

SPECIAL CONTROL OF HVDC LINKS: The DC power on HVDC links can be rapidly ramped up or down to assist in maintaining generation/load imbalances caused by disturbances. The effect is similar to generation or load tripping.

CONTROLLED SYSTEM SEPARATION AND LOAD SHEDDING: Generally considered a last resort, it is feasible to design system controls that can respond to separate, or island, a power system into areas with balanced generation and load. Some load shedding or generation tripping may also be required in selected islands. In the event of a disturbance, instability can be prevented from propagating and affecting large areas by partitioning the system in this manner. If instability primarily results in generation loss, load shedding alone may be sufficient to control the system.

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