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Examoogle • User • info@examoogle.com • EE-2024-8821
Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
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What is phase difference during symmetrical fault?
90°
120°
150°
180°
120°
Quick Summary: A symmetrical fault, also known as a balanced three-phase fault, is a fault in which all three phases are short-circuited to each other and/or to ground. Because the system remains balanced during the fault, the phase relationship of $120^\circ$ between the three phases of the supply voltage is maintained.
A symmetrical fault, also known as a balanced three-phase fault, is a fault in which all three phases are short-circuited to each other and/or to ground. Because the system remains balanced during the fault, the phase relationship of 120° between the three phases of the supply voltage is maintained.
Ia+Ib+Ic=0 — Sum of phase currents in a symmetrical fault
Va=V∠0°,Vb=V∠−120°,Vc=V∠−240° — Balanced phase voltages
In a balanced three-phase system, the generators and power sources are designed to produce voltages separated by an electrical angle of 120°. During a symmetrical fault, the impedance paths across all three phases are identical. Since the source voltages remain balanced and the fault impedance is equal for each phase, the currents in the three phases maintain their relative 120° phase displacement, even though the magnitude of the fault current increases significantly.
Symmetrical faults involve all three phases equally.
Only positive sequence components exist during a symmetrical fault; negative and zero sequence components are zero.
The fault current is limited primarily by the system impedance (reactance) up to the point of the fault.
The phase difference remains 120° because the system is balanced, unlike unsymmetrical faults where phase balance is disturbed.
Simpler calculations using per-phase equivalent circuits.
Does not require sequence network analysis (as only positive sequence exists).
Results in the highest magnitude of fault current compared to unsymmetrical faults.
Causes severe stress on the electrical apparatus due to high mechanical and thermal forces.
Used for selecting the rupturing capacity of circuit breakers.
Basic study for setting protection relays.
A symmetrical fault is the most severe type of fault in terms of magnitude.
Option D (180) is incorrect as it would imply a single-phase system or a total phase reversal not typical in three-phase faults.
B is correct — In a symmetrical fault, the power system remains balanced; therefore, the 120° displacement between the phases of a three-phase system is preserved.
Always remember that in symmetrical faults, negative and zero sequence impedances are irrelevant because only positive sequence currents flow.