What is the main cause of voltage instability?

Question

Examoogle · Accepted Answer

Loads

Explanation: Voltage instability is primarily caused by the inability of the power system to maintain steady voltages at all buses under normal conditions and after being subjected to a disturbance. This is fundamentally driven by the system's inability to meet the reactive power demand of the loads.

Working Principle: Voltage instability occurs when the load characteristic is such that a change in load power or system configuration leads to a progressive and uncontrollable drop in voltage. As the load demands more reactive power (Q) than the system can supply at a given voltage level, the voltage at the load bus decreases, which in turn causes the load to demand even more reactive power, eventually leading to a voltage collapse.

Key Formulas: $Q_{demand} = f(V)$ — Reactive power demand as a function of voltage; $V_{bus} \approx V_s - \frac{PR + QX}{V_s}$ — Approximate voltage drop at the receiving end

Key Points: Voltage instability is essentially a reactive power support problem.; Induction motors and air conditioners are major contributors to voltage instability due to their constant power load characteristics.; Static Var Compensators (SVC) and STATCOMs are used to provide rapid reactive power support.; The nose point of the P-V curve represents the point of voltage collapse.

Applications: Power system stability analysis; Design of shunt compensation equipment

Additional Info: Voltage collapse often happens in systems heavily loaded with inductive motors.. Option D (Line losses) is a consequence of high current, but voltage instability is specifically the inability of the system to provide necessary reactive power under load conditions.

Answer: C is correct — Voltage instability is primarily triggered by the load's demand for reactive power exceeding the power system's capability to supply it.

Exam Tip: Always remember: Frequency is a global parameter (same everywhere), but Voltage is a local parameter (varies from bus to bus), making voltage stability a local bus-level issue.

Related Questions

Discussion (0)