Equipment and Stability Constraints in System Operation

Module 2 : Equipment and Stability Constraints in System Operation

Lecture 8 : Large disturbance Angle stability

Angular Stability and System Operation

Operation under loss of synchronism conditions is not feasible as voltage, torques etc. undergo large fluctuations and may damage equipment.

How does one prevent loss of synchronism due to disturbances?

Some of the methods used to reduce the possibility of loss of synchronism are:

a) Reducing the magnitude of disturbance: Large disturbances are typically due to faults which are cleared by line tripping. Reducing the duration of fault is one method of preventing loss of synchronism. These days, it is possible to clear faults within 3-5 cycles (taking into account relay and circuit breaker opening times).

b) Planning: (for a "stronger" transmission network). This would mean augmenting the transmission network with new transmission lines. Increasing the power transfer capability of transmission lines by compensating transmission reactance with series capacitors and shunt capacitors.

c) Control: The short term or excess capacity of many equipment can be used to enhance stability by appropriate controls. For example, excitation systems can allow for short term boosting of field voltage. Also, capacitors can be switched into the network in series and shunt. The switching may be done by means of circuit breakers or power electronic controls (power electronic controllers also allow for continuous control; this will be discussed in later modules).

d) Operation: During operation, it is ensured that the system is operated such that there exists sufficient "margin" for the system to withstand a credible disturbance (like a fault). This imposes a constraint on the level of power transfers which are permissible -- for larger levels of power flow, the synchronous generators are more likely to lose synchronism after a large disturbance. A system operator continuously monitors a power system and ensures that this is unlikely to happen. If the system is unlikely to withstand a credible disturbance, then preventive control actions like reducing the power transfer levels are taken. This may be achieved by re-scheduling power generated at various generators.