The time scale of transient voltage stability due to OLTC operation is

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Examoogle · Accepted Answer

2 min

Explanation: The time scale of transient voltage stability influenced by On-Load Tap Changers (OLTC) typically ranges from 1 to 3 minutes, with 2 minutes often cited as the representative value in power system stability studies. This delay is intentionally designed into OLTC controllers to prevent hunting and ensure the system is reacting to sustained voltage deviations rather than temporary transients.

Working Principle: When a voltage drop occurs due to a system disturbance, the OLTC detects the deviation after an intentional time delay (typically 30s to 60s per tap or a combined response time). As the OLTC moves to increase the secondary voltage, it draws more reactive power from the transmission network, which may further depress the transmission bus voltage. If the load is voltage-sensitive (e.g., induction motors), this feedback loop can lead to voltage instability if the system cannot support the additional reactive demand within this timeframe.

Key Formulas: $V_{sec} = V_{prim} 	imes \frac{N_2}{N_1}$ — Ideal transformer turns ratio relationship governing tap adjustment; $Q = V^2 B$ — Relationship showing reactive power demand sensitivity to voltage levels

Key Points: OLTCs operate on a medium-term time scale (seconds to minutes).; The inherent time delay is to avoid unnecessary switching during short-duration disturbances.; OLTC-driven instability is a major component of 'Load-Area' voltage instability.; Continuous tap-changing without coordination leads to voltage collapse.

Advantages: Maintains constant secondary voltage despite primary side fluctuations.; Improves distribution network power quality.

Disadvantages: Can contribute to voltage instability if the network lacks reactive reserve.; Mechanical wear of moving contacts over long operational life.

Applications: Distribution transformers; HVDC converter transformers; Transmission substations

Additional Info: Standard industrial OLTC controllers have adjustable delays, often set between 30 and 120 seconds.. Option B (2 min) is the widely accepted standard value for these stability studies in academic literature.

Answer: B is correct — The standard time scale for transient voltage stability analysis associated with OLTC operation is approximately 2 minutes.

Exam Tip: Always remember that OLTCs act on a slower timescale than generator excitation control but faster than human intervention or AGC (Automatic Generation Control).

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