The operation of OLTC improve voltage stability if

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The reactive power status at load bus improves

Explanation: An On-Load Tap Changer (OLTC) improves voltage stability by regulating the secondary voltage of transformers under varying load conditions. It restores the load bus voltage profile by adjusting the turns ratio to compensate for voltage drops caused by reactive power consumption.

Working Principle: As load increases, the reactive power demand ($Q$) at the load bus rises, leading to increased voltage drop across the transmission lines ($V_{drop} \approx \frac{RP + XQ}{V}$). By changing the transformer tap position, the OLTC alters the voltage ratio, effectively injecting reactive support by raising the local bus voltage, thereby preventing voltage collapse and maintaining system stability.

Key Formulas: $V_{drop} \approx \frac{R \cdot P + X \cdot Q}{V}$ — Approximate voltage drop at the load bus showing dependence on reactive power $Q$; $V_{sec} = \frac{V_{pri}}{a}$ — Relationship between transformer secondary voltage and tap ratio $a$

Key Points: OLTC is primarily used for voltage regulation in distribution and sub-transmission systems.; It maintains a constant voltage at the load bus despite fluctuations in source voltage or load magnitude.; Correcting voltage via tap changes effectively manages the reactive power balance of the local load area.

Advantages: Automatic voltage regulation without disconnecting load; Prevents voltage instability and potential cascading failure

Disadvantages: Mechanical wear and tear due to moving parts; Complexity in coordination with other control devices

Applications: Power transformers in grid substations; Industrial distribution networks with varying motor loads

Additional Info: The OLTC compensates for the voltage drop by increasing the secondary voltage, which helps in meeting the reactive power requirement of the load.. Option B and C are incorrect because OLTC is specifically designed to handle voltage regulation, which is intrinsically tied to reactive power flow, not directly controlling active power dispatch.

Answer: A is correct — The operation of OLTC improves voltage stability by regulating the load bus voltage, which is directly linked to the reactive power status and consumption at that point.

Exam Tip: Always remember that voltage stability is a reactive power problem ($V-Q$ control), whereas frequency stability is an active power problem ($f-P$ control).

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