Voltage stability; transient study assumptions

Voltage stability

Voltage stability is the ability of a power system to keep steady, acceptable voltages at all buses after a disturbance (load increase, line outage, etc.), starting from a given operating condition. Instability shows up as a progressive, uncontrollable fall in voltage (voltage collapse). It is mainly caused by a shortage of reactive power to support the load.

Assumptions in transient stability study (classical model)

1. Mechanical input $P_m$ is constant during the transient period, because governor action is slow.
2. Damping and asynchronous (induction) power are neglected.
3. Each machine is represented by a constant voltage $E'$ behind its transient reactance $X_d'$.
4. The rotor angle of a machine equals the angle of $E'$.
5. Transmission line resistance and shunt capacitance are usually neglected.
6. Loads are represented by constant impedances.
7. Rotor speed stays close to synchronous speed, so power in pu ≈ torque in pu.

Factors affecting transient stability

• Inertia constant $H$ of the machines
• Initial loading (pre-fault rotor angle $\delta_0$)
• Type and location of the fault
• Fault clearing time (relay and breaker speed)
• Transfer reactance of the network before, during and after the fault
• Generator internal voltage, that is, the excitation level and the speed of the excitation system
• Use of auto-reclosing, fast valving and braking resistors