# Power Systems Questions and Answers – One-Line Diagram and Impedance or Reactance Diagram

This set of Power Systems Multiple Choice Questions & Answers (MCQs) focuses on “One-Line Diagram and Impedance or Reactance Diagram”.

1. What is the main purpose of reactance diagram?
b) Falt analysis
c) Calculation of ratings of Alternators
d) Calculation of ratings of Transformers

Explanation: The main purpose of reactance diagram is fault analysis. Load flow analysis is done by the help of Impedance diagram. All these diagrams are made using symmetrical component analysis method on per phase basis.

2. Which of the following is not neglected during formation of reactance diagram from impedance diagram?
a) Resistance of various power system components
c) Shunt component of Transformers
d) Reactance of alternators

Explanation: Reactance diagram is drawn by the help of symmetrical network of various power system components by neglecting resistance of various components, static loads like induction motors and shunt component of Transformers.

3. Impedance diagram is used for analysis of __________
b) Alternator
c) Fault
d) Transmission Line

Explanation: Impidance diagram is made for the analysis of load flow. Fault analysis is doen by the help of reactance diagram. All these diagrams are made using symmetrical component analysis method on per phase basis.

4. Reactance diagram contains which of the following ?
a) Resistance of Alternator
b) Resistance of transformer winding
c) Induction motor’s equivalent corcuit
d) Inductive reactance of transmission lines

Explanation: Reactance diagram contains only reactance of synchronous machines, transmission line and Transformers. Static loads like induction motors are neglected in diagram.

5. Single line diagram does not represents:
a) Star connection of transformer winding
b) Delta connection of transformer winding
c) Neutral wire of transmission lines
d) Ratings of machines

Explanation: Single line diagram is a representation of balanced power system on per phase basis with neutral eliminated. Neutral wire is not shown in single line diagram. Winding connections (star or neutral) is represented beside the symbol of transformer with its Impidance and rating.
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6. Single line diagram of which of the following power system is possible?
a) Power system with LLG fault
b) Power system with LG fault
c) Power system with LL fault
d) Balanced power system

Explanation: Only balanced power system can be represented by a single line diagram. Single line diagram is drawn on per phase basis. A power system with LLG, LG and LL faults becomes unbalanced and can not be represented in per phase basis.

7. In impedance diagram different power system elements are represented by symbols.
a) True
b) False

Explanation: In reactance diagram different power system elements are represented by their symmetrical networks.

8. A three phase transformer has a nameplate rating of 30 MVA, 230Y/69Y kV with a leakage -reactance of 10% and the transformer connection is wye-wye. Choosing a base of 30MVA and 230 kV on high voltage side, the transformer reactance referred to the low voltage side will be _______(in ohms).
a) 15.87 Ω
b) 157.8 Ω
c) 176.33 Ω
d) 17.67 Ω

Explanation: X Ω = Xpu*Xb(LV) = 0.1*158.87 = 15.87 Ω.

Sanfoundry Global Education & Learning Series – Power Systems.