Test: Fault Analysis - Electrical Engineering (EE) MCQ

Symmetrical Faults:

• In power system, it refers to type of fault which leads to short circuit of three phases. This may be either short circuit of three phases or three phases to ground fault.
• In symmetrical fault, fault currents in phases are symmetrical in the sense that their magnitudes are equal and they are equally displaced by an angle of 120°.
• Thus, a symmetrical fault may be assumed as a normal case but with high value of current in phases.

Three phase fault:

• In these type of faults, all the three phases i.e. R, Y and B are shorted. This can be assumed as the three lines are connected to a delta connected load with zero impedance in each phase.
• The current in lines will be symmetrical but with higher value. The value of fault current i.e. I_R, I_Y and I_B in such fault will only be limited by the source impedance as well as the impedance of line up to the point of fault.

Concept:

Power system fault:

• Open circuit fault
• Short circuit fault

Open circuit fault: The open-circuit fault mainly occurs because of the failure of one or two conductors. The open-circuit fault takes place in series with the line, and because of this, it is also called the series fault. Such types of faults affect the reliability of the system.

Short circuit fault: In this type of fault, the conductors of the different phases come into contact with each other with a power line, power transformer or any other circuit element due to which the large current flow in one or two phases of the system. The short-circuit fault is divided into the symmetrical and unsymmetrical fault.

Explanation:

So, an Open circuit electrical fault is also called a series fault.

Single line-to-ground fault

On a transmission line when one conductor drops to the ground or comes in contact with the neutral conductor, then there is an insulation breakdown between one of the phases and earth, due to which single line to ground fault takes place. Such types of failures may occur in a power system due to many reasons like high-speed wind, falling off a tree, lightning, etc.

Note:

• Line to line fault or unsymmetrical fault occurs when two conductors are short-circuited.

The different type of faults in power systems are:

• Single line to ground fault (LG)
• Line to line fault (LL)
• Double line to ground fault (LLG)
• Three-phase faults (LLL or LLLG)

Frequency of occurrence:

Among the given faults, LG or line to ground fault is most common and occurs frequently.

The order of frequency of occurrence is given below.

LG > LL > LLG > LLL

Severity of faults:

Among the given faults, LLLG or 3 phase faults are most severe. LG or line to ground fault is least severe.

Line to line fault is more severe than line to ground fault while double line to ground fault is one level severe than LL.

The order of Severity of faults is given below.

LLL > LLG > LL > LG

Unsymmetrical Faults:

• Unsymmetrical faults are the faults that lead unequal currents with unequal phase shifts in a three-phase system
• The unsymmetrical fault occurs in a system due to the presence of an open circuit or short circuit of transmission or distribution line
• It can occur either by natural disturbances or by manual errors
• The natural disturbances are heavy wind speed, ice loading on the lines, lightning strikes and other natural disasters

Symmetrical faults:

• A symmetrical fault is a fault where all phases are affected so that the system remains balanced
• A three-phase fault is a symmetrical fault
• As symmetrical faults result in balanced conditions, they may be analyzed using per-phase analysis

Irrespective of the causes, both symmetrical faults, and unsymmetrical faults occur in a 3-phase system.

The amount of current that will flow through a short circuit depends on two values:

• The system voltage
• The connected total impedance of the path from the source to the point of the fault.​
• This impedance normally includes the feeder conductor’s resistance and reactance, any transformers, impedances (going from the point of fault back to the energy source), and any other equipment connected in the path of current flow.

The different type of faults in power systems are:

• Single line to ground fault (LG)
• Line to line fault (LL)
• Double line to ground fault (LLG)
• Three-phase faults (LLL or LLLG)

Frequency of occurrence:

• Among the given faults, LG or line to ground fault is most common and occurs frequently.
• The order of frequency of occurrence is given below.

LG > LL > LLG > LLL

Severity of faults:

• Among the given faults, LLLG or 3 phase faults are most severe. LG or line to ground fault is least severe.
• Line to line fault is more severe than line to ground fault while double line to ground fault is one level severe than LL.
• The order of frequency of occurrence is given below.

LLL > LLG > LL > LG

Concept:

Short circuit MVA =

Where,

MVA_base = Full load or base MVA

X_pu = Per unit reactance

Calculation:

Given that, MVA_base = 500 kVA = 0.5 MVA, X_pu= 5% = 0.05

Since the transformer is connected to an infinite bus, the p.u. the reactance of the circuit will be 0.05 i.e., the p.u. reactance offered by the transformer.

∴ short circuit MVA can be calculated as

Short circuit MVA = 0.5/0.05 = 10 MVA

Unsymmetrical Fault:

The fault gives rise to unsymmetrical current, i.e., current differing in magnitude and phases in the three phases of the power system are known as the unsymmetrical fault. It is also defined as the fault which involves one or two phases such as LG, LL, LLG fault. The unsymmetrical makes the system unbalanced. 

Single Line to Line Ground (SLG): 

• The single line of ground fault occurs when one conductor falls to the ground or contact the neutral conductor. 
• The 70 – 80 percent of the fault in the power system is the single line-to-ground fault.
• All sequence networks are connected in series.

​I_a0 = I_a1 = I_a2

I_F = 3I_a0 = 3I_a1 =3 I_a2 

Line to Line Fault (LL):  

• A line-to-line fault occurs when two conductors are short-circuited. The major cause of this type of fault is the heavy wind.
• The heavy wind swinging the line conductors which may touch together and hence cause short-circuit. 
• The percentage of such types of faults is approximately 15 – 20%.
• Positive and negative sequence connected in series opposition

I_a0 = 0 and I_a1 = - I_a2 

Double Line to line Ground Fault (LLG): 

• In double line-to-ground fault, the two lines come in contact with each other along with the ground. 
• The probability of such types of faults is nearly 10%. 
• All sequence networks are connected in parallel.

Concept:

Short circuit MVA =
Where,

MVA_base = Full load or base MVA

X_pu = Per unit reactance

Calculation:

Given that, MVA_base = 500 kVA = 0.5 MVA, X_pu= 5% = 0.05

Since the transformer is connected to an infinite bus, the p.u. the reactance of the circuit will be 0.05 i.e., the p.u. reactance offered by the transformer.

∴ short circuit MVA can be calculated as Short circuit MVA = 0.5/0.05 = 10 MV A