Negative sequence reactance of a transformer is

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Option A: Equal to the positive sequence reactance

Explanation & Concepts:
In a transformer, the positive and negative sequence impedances are identical because a transformer is a static, linear, and passive electromagnetic device. The physical construction and the flux paths for both positive and negative sequence currents remain unchanged, resulting in equal reactance values.

Working Principle: The transformer consists of stationary windings and a magnetic core. When sequence currents flow, they encounter the same leakage reactance regardless of the phase sequence of the currents ($a-b-c$ vs $a-c-b$), because the direction of rotation of the magnetic field does not affect the stationary transformer windings.

Key Formulas: $Z_{1} = Z_{2} = Z_{leakage}$ — where $Z_1$ is positive sequence and $Z_2$ is negative sequence impedance

Key Points: Transformers are static components and do not have rotating parts to influence sequence sequences.; The leakage reactance is purely a function of winding geometry and core saturation, which is identical for all balanced sequence currents.; Zero sequence impedance ($Z_0$) typically differs from $Z_1$ and $Z_2$ depending on the transformer connection (e.g., star/delta).; This property is a fundamental assumption in the construction of Y-bus matrices and fault analysis.

Advantages: Simplifies power system stability studies; Reduces complexity in symmetrical component analysis

Applications: Short circuit level calculations; Symmetrical fault analysis; Relay coordination studies

Additional Info: The negative sequence impedance $Z_2$ is equal to the positive sequence impedance $Z_1$ only for static equipment like transformers and transmission lines.. For rotating machines like motors and generators, $Z_2$ is usually smaller than $Z_1$ due to the relative motion between the stator and rotor fields.. Option B and C are incorrect because the transformer's magnetic circuit symmetry is independent of sequence phase rotation.

Answer: A is correct — The negative sequence reactance of a transformer is equal to the positive sequence reactance because the transformer is a static, stationary component.

Exam Tip: Always remember: Static components (Transformers, Lines) have $Z_1 = Z_2$, whereas Rotating machines (Generators, Motors) have $Z_1 
eq Z_2$.

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Option B: Larger than the positive sequence reactance

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Option C: Smaller than the positive sequence reactance

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Option D: Any of above

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