Unsymmetrical Fault Analysis Questions and Answers

Answer: B

The most dangerous is the Dead short circuit.

A dead short is a bolted, zero-impedance fault between all three phases. It is a symmetrical fault and usually produces the largest fault current, with the greatest thermal and mechanical stress on the equipment, which is why breaker ratings are normally based on it.

Line to line, line to ground and double line to ground faults are unsymmetrical. They are more frequent, line to ground being the most common of all, but they generally draw less current than a dead three-phase short.

Answer: A

The keyed answer is the three phase fault to ground.

A symmetrical three phase fault short-circuits all three phases together, so the full system voltage drives current through only the positive sequence impedance. In most systems this gives the largest fault current, which is why circuit breakers are rated on the three phase fault level.

In unsymmetrical faults the negative (and for ground faults the zero) sequence impedances come into the circuit and usually limit the current. A single line to ground fault can exceed the three phase value only in special cases, such as at the terminals of a solidly grounded generator.

Answer: A

The decreasing order of severity is a-b-c-d: three phase, L-L-G, L-L, L-G.

From the stability point of view, a fault is more severe the more it reduces the power that can be transferred across the system during the fault. A three phase fault brings the transfer close to zero. A double line to ground fault reduces it the most of the unsymmetrical faults, a line to line fault less, and a single line to ground fault, the commonest fault, the least.

The other orders place the single line to ground fault or the line to line fault above the double line to ground fault, which reverses the effect of the fault on power transfer.

Answer: B

The theory is Fortescue's theory.

C.L. Fortescue showed in 1918 that any set of unbalanced three-phase phasors can be resolved into three balanced sets: positive sequence, negative sequence and zero sequence. These symmetrical components are the standard tool for analysing unsymmetrical faults.

Slepian's theory and Cassie's theory are theories of arc interruption in circuit breakers: Slepian's is the race between dielectric strength and restriking voltage, and Cassie's is a model of the arc's conductance. Faraday's work is on electromagnetic induction. None of them deals with symmetrical components.

Answer: C

Zero sequence current flows in both LG and LLG faults.

Zero sequence currents are equal and in phase in all three lines, so they can flow only if there is a return path through the earth or a neutral. Both the single line-to-ground (LG) fault and the double line-to-ground (LLG) fault involve ground, so both carry zero sequence current, provided the system neutral is earthed.

A line-to-line (LL) fault does not involve ground, so it has only positive and negative sequence components. Any option containing the LL fault is therefore wrong.

Answer: A

The most frequent fault is the single line to ground fault.

Most faults on overhead lines begin with one conductor flashing over to an earthed tower or structure, through lightning, a damaged insulator, a bird or a tree branch. Single line to ground faults make up roughly 70 to 80 per cent of all faults.

Line to line faults are next at about 15 per cent, double line to ground (LLG) faults about 10 per cent, and symmetrical three phase faults are the rarest, about 5 per cent, though they are the most severe and are used to rate switchgear.

Answer: B

The positive sequence voltage at the fault becomes zero in a three phase fault.

A bolted three phase fault is symmetrical: all three phases are short-circuited together at the fault, so every phase voltage there is zero. A balanced set of zero voltages has no negative or zero sequence part, and its positive sequence component is zero as well.

In the unsymmetrical faults this does not happen. For a line to ground, line to line or double line to ground fault, the positive sequence voltage at the fault equals the voltage drop across the negative sequence network (and, where it is involved, the zero sequence network), which is not zero.