If positive, negative and zero sequence reactances of an element of power system are 0.3 pu, 0.3 pu and 0.8 pu

Question

If positive, negative and zero sequence reactances of an element of power system are 0.3 pu, 0.3 pu and 0.8 pu respectively, then element would be

Examoogle · Accepted Answer

Option D: Transmission line

Explanation & Concepts:
Given: Positive sequence reactance X1 = 0.3 pu, negative sequence reactance X2 = 0.3 pu, and zero sequence reactance X0 = 0.8 pu.

Formula Used: $$X_1 = X_2 
eq X_0$$

Steps: Analyze Sequence Reactances: In static power system elements like transmission lines and cables, the positive and negative sequence impedances are equal due to the symmetry of the physical path ($X_1 = X_2$).. Evaluate Zero Sequence Reactance: For transmission lines, the zero-sequence path includes return currents through earth or shield wires, which significantly increases the zero-sequence impedance compared to positive and negative sequence paths ($X_0 > X_1$).. Comparison with Other Elements: For synchronous machines, $X_2$ is often less than $X_1$ and $X_0$ is significantly smaller than $X_1$, unlike the given values. The condition $X_1 = X_2

Answer: D is correct because transmission lines are static, passive, and symmetrical components where the positive and negative sequence reactances are identical, while the zero sequence reactance is higher due to return path effects.

Exam Tip: Remember that for all static network components (lines, transformers, reactors), $X_1 = X_2$ holds true, whereas for rotating machines (generators/motors), $X_1 
eq X_2$ because of the non-symmetrical nature of the rotor flux paths.

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Option A: Synchronous generator

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Option B: Synchronous motor

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Option C: Static load

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