Which of the following is unit type of protection in feeder?

Question

Examoogle · Accepted Answer

Option C: Balanced voltage system

Explanation & Concepts:
A unit protection scheme is one where the protection system responds only to faults occurring within the specific protected zone (the 'unit' or 'feeder'). The Balanced Voltage System (such as the Merz-Price circulating current scheme) acts as a unit protection because it compares the currents at the two ends of the feeder and only operates if there is a difference, implying a fault within the zone.

Working Principle: The principle relies on Kirchhoff's Current Law. Under healthy conditions, the current entering the feeder equals the current leaving it. Current transformers at both ends are connected such that their secondary currents circulate through a pilot wire; if an internal fault occurs, the balance is disrupted, causing current to flow through the relay operating coil.

Key Formulas: $I_{in} = I_{out} \implies I_{relay} = 0$; $I_{in} 
eq I_{out} \implies I_{relay} = |I_{in} - I_{out}|$

Key Points: Unit protection is selective and does not require time grading.; It only operates for faults within the protected zone defined by the CT locations.; Non-unit protection (like distance or overcurrent) relies on time or impedance grading to achieve selectivity.; Balanced voltage systems are inherently sensitive to internal faults regardless of the magnitude of external faults.

Advantages: High degree of selectivity; Instantaneous operation; Independent of fault magnitude (no grading required)

Disadvantages: Requires pilot wires or communication channels between ends; Higher installation cost; Sensitive to CT saturation and pilot wire capacitance

Applications: Transmission line protection (Pilot wire differential protection); Transformer differential protection; Generator stator winding protection

Additional Info: Directional and Distance protections are classified as 'Non-Unit' or 'Graded' protection because they require time/impedance settings to coordinate with other sections of the network.. The 'Balanced Voltage' system is a classic form of differential protection.

Answer: C is correct — Balanced voltage systems are classified as unit protection because they only respond to conditions within the protected feeder section.

Exam Tip: Remember that 'Unit' protection is synonymous with 'Differential' protection; it is absolute and does not need to coordinate with neighboring zones.

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Option A: Directional time graded protection

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Option B: Distance protection

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Option D: None of above

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