Which of the following is the short term stability in a load driven system?

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Examoogle · Accepted Answer

Short term voltage stability

Explanation: Short-term voltage stability is defined as the ability of a power system to maintain steady voltages at all buses after being subjected to a severe disturbance within a time frame of a few seconds. It is primarily driven by the dynamics of fast-acting load components such as induction motors, electronically controlled loads, and HVDC converters.

Working Principle: During a fault or disturbance, the voltage at the load terminals drops, causing induction motors to decelerate or draw high reactive current. If the system cannot supply the required reactive power quickly enough, the voltage will fail to recover, leading to voltage instability. This is a load-driven phenomenon because the restorative behavior depends on the interaction between the fast load dynamics and the network's transmission capability.

Key Formulas: $Q_{load} = Q_{load,0} + K_{qv} \cdot \Delta V$; $T_e = \frac{3V^2R_r}{s\omega_s(\frac{R_r}{s})^2 + X_{eq}^2}$

Key Points: Short-term stability involves time frames of 0 to 10 seconds.; Driven by fast load dynamics including induction motor stall and thermal protection interaction.; Critical factor is the reactive power imbalance during the disturbance.; Contrast with long-term stability which involves slower control mechanisms like OLTCs and generator excitation.

Advantages: Identifies critical clearing times for induction motor loads.; Enables effective tuning of under-voltage load shedding (UVLS) schemes.

Disadvantages: Computationally intensive due to non-linear differential-algebraic equations.; Requires high-fidelity modeling of consumer load composition.

Applications: System security analysis for industrial complexes.; Grid code compliance for large-scale motor loads and HVDC connections.

Additional Info: Rotor-angle stability is primarily a generator-driven phenomenon relating to the synchronization of machines.. Frequency stability relates to the power-frequency balance across the entire system, not just local load bus voltage.

Answer: B is correct — Short-term voltage stability is fundamentally characterized by the response of load components (such as induction motors) to disturbances over a period of a few seconds.

Exam Tip: Always remember: Short-term stability (0-10s) is dominated by fast motor loads, whereas Long-term stability (minutes) is dominated by secondary controls and tap-changers.

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