Fault Analysis MCQ Quiz - Objective Question with Answer for Fault Analysis - Download Free PDF

Ring distribution system

• A ring distribution system is an electrical power distribution network where feeders connect in a closed loop, providing multiple paths for power delivery.
• This design ensures that if one feeder fails, power can continue to flow through an alternative path, maintaining reliability and preventing power outages.
• There are fewer voltage fluctuations at the consumer's terminal.

The most common internal fault in transformers is winding inter-turn short circuits.

Winding inter-turn short circuits

An inter-turn short circuit in a transformer refers to a short between adjacent turns of a winding due to insulation failure. Unlike phase-to-phase or winding-to-core faults, these occur within the same winding, making them harder to detect early.

Causes of Inter-Turn Short Circuit in Transformers:

• Insulation degradation due to thermal aging
• Overvoltages / surges
• Mechanical vibrations or shocks during transport or operation
• Moisture ingress leading to partial discharge
• Manufacturing defects
• High inrush or fault currents causing turn displacement

Circuit breaker

• A circuit breaker is an electrical safety device designed to protect an electrical circuit from damage caused by current more than that which the equipment can safely carry (overcurrent).
• Its basic function is to interrupt current flow to protect equipment and to prevent fire.
• In power systems, protection schemes follow the principle of selective coordination. The circuit breaker closest to the fault (downstream) must trip first to isolate the fault while keeping the rest of the system operational.
• This prevents unnecessary disconnection of large portions of the electrical network.

Statement 1 is True.

Relay

• Relays are electrically operated switches that open and close the circuits by receiving electrical signals from outside sources.
• Relays should operate as quickly as possible to minimize damage to equipment and ensure safety.

​
Statement 2 is False.

Commutation

Commutation refers to the process of turning off a conducting thyristor. There are six major types of commutation methods, classified into two categories:

Category 1: Forced Commutation

• It is used in DC circuits where the current does not naturally reach zero. It requires external components (capacitors, inductors) to force the current to zero.
• Forced commutation is classified into five commutation techniques.
   

Class A (Resonant Commutation)

• Uses an LC circuit to create oscillations that naturally bring the current to zero, turning off the thyristor.
• Completely independent of external circuit components.

Class B (Resonant-Pulse Commutation)

• The thyristor is turned off using an LC resonant circuit discharging when the current reaches zero.

Both Class A and Class B rely on the circuit's natural behavior to turn off the thyristor, making them self-commutation methods.

Class C: Complementary Commutation

• Uses another thyristor (auxiliary SCR) and a capacitor to turn off the main thyristor.
• Used in DC choppers and inverters.

Class D: Auxiliary Commutation

• Similar to Class C but uses an external voltage source to force commutation. Used in high-power inverters and choppers.​

Class E: External Pulse Commutation

• Uses an external commutation pulse from a circuit to turn off the thyristor.
• Used in Cycloconverters and high-power circuits.

Category 2: Class F (Natural Commutation)

• It occurs in AC circuits where the current naturally becomes zero. In AC applications, the supply voltage reverses polarity every half-cycle, automatically bringing the current to zero.
• Example: Line commutation in rectifiers.

Overload in Power System

• An overload occurs when electrical equipment or a circuit carries more current than its rated capacity, but there is no direct fault or connection between phases or ground.
• This condition can cause the wiring to overheat, potentially leading to a fire. Common signs include tripped circuit breakers, flickering lights, and unusual buzzing or sizzling sounds from electrical outlets.
• In an overload condition, the electrical load exceeds the rated capacity, causing a voltage drop but not a complete loss of voltage. Unlike a short circuit, where voltage at the fault point is nearly zero, an overload only reduces the voltage.

Hence, Statement 1 is True.

Short Circuit in the Power System

• A short circuit is an unintentional, low-resistance connection between two points in a circuit, allowing excessive current to flow, bypassing the intended load path. This can be caused by damaged insulation, faulty wiring, or accidental contact, leading to potential hazards like overheating, fires, and equipment damage. 
• In an overload, the current is higher than normal but still within a limited range (e.g., 2-5 times the rated current).
• In a short circuit, the current can be hundreds or thousands of times the normal current, depending on system impedance.
• Therefore, overload current is substantially lower compared to short-circuit current.

Hence, Statement 2 is True.

Electrical faults in three-phase power system mainly classified into two types, namely open and short circuit faults.

Open circuit faults: These faults occur due to the failure of one or more conductors.  The most common causes of these faults include joint failures of cables and overhead lines, and failure of one or more phase of circuit breaker and also due to melting of a fuse or conductor in one or more phases.

Type of open circuit faults:

• Single phase open circuit fault
• Two phase open circuit fault
• Three phase open circuit fault

Open circuit faults are also called as series faults. These are unsymmetrical or unbalanced type of faults except three phase open fault.

Important Points

Shunt faults:

The shunt fault involves short circuit between conductor and ground or short circuit between two or more conductors.

The shunt faults are characterized by:

• Increase in current
• Fall in voltage
• Fall in frequency

Shunt faults are classified as follows:

• Single line to Ground fault (LG fault)
• Line to Line fault (LL fault)
• Double line to Ground fault (LLG fault)
• Three phase faults

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.

​Ia0 = Ia1 = Ia2

I_F = 3Ia0 = 3Ia1 =3 Ia2

\({I_F} = \frac{{3{E_a}}}{{{Z_1} + {Z_2} + {Z_3}}}\)

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

Ia0 =0 and Ia1 = - Ia2

I_F = √3 Ia1 = √3 Ia2

\({I_F} = \frac{{√ 3 {E_a}}}{{{Z_1} + {Z_2}}}\)

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.

Ia0 + Ia1 + Ia2 = 0

Va0 = Va1 = Va2

IF = 3Ia0

a - 3, b - 1, c - 2

Additional Information

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 > LLLG

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 given below.
   

LLLG > LLG > LL > LG

Concept:

Short circuit MVA = \( \frac{{\;\ MVA}_{base}}{{{X_{pu}}}}\)

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 = \( \frac{{\ 0.5}}{{{0.05}}} =10 ~MVA\)

Concept:

In a single line to ground fault, all the sequence components of fault currents are equal.

I_a1 = I_a2 = I_a0

In a double line to ground fault, the sum of all sequence components of fault currents is zero.

I_a1 + I_a2 + I_a0 = 0

Where, Ia1 = Positive sequence component of the current

Ia2 = Negative sequence component of the current

Ia0 = Zero sequence component of the current

Calculation:

Given

I_positive = j 1.5 p.u. 

I_negative = - j 0.5 p.u. 

I_zero = - j 1 p.u.

Ia1 = j 1.5, Ia2 = -j 0.5, Ia0 = -j 1.0

⇒ Ia1 + Ia2 + Ia0 = j 1.5 – j 0.5 – j 1.0 = 0

Therefore, the fault is double line to ground fault.

Concept:

\({Z_{pu}} = {Z_{\rm{\Omega }}}\frac{{{{\left( {MVA} \right)}_b}}}{{{{\left( {k{V_b}} \right)}^2}}}\)

Z_pu = per unit impedance

Z_Ω = Impedance in Ω

(MVA)_b = base MVA

(kV)_b = base voltage

Calculation:

Base MVA = 100 kVA = 0.1 MVA

Base Voltage = 5 kV

Reactance in ohms = 5 Ω

\({Z_{pu}} = 5 \times \frac{{0.1}}{{{5^2}}} = 0.02\)

The correct answer is option 2): (200 A )

Concept:

The short circuit current is given by

I_sc = I × \(100 \over percentage\: of \: reactance\)

Where 

I is the full load current 

Calculation:

Isc = I × \(100 \over percentage\: of \: reactance\)

= 40 × \(100 \over 20\)

 = 200 A 

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

Concept:

The short-circuit MVA is given by:

SCMVA = \( {1\over X_{eq}}\times(MVA)_{base}\)

Calculation:

Given, (MVA)_base = 5 MVA

X_eq = \(20\over 4\) = 5% = 0.05 pu

SCMVA = \( {1\over 0.05}\times5\)

SCMVA = 100 MVA

Positive sequence component: It has three vectors of equal magnitude but displaced in phase from each other by 120° and has the same phase sequence as the original vectors. It specifies that the current is flowing through the source to load.

Negative sequence component: It has three vectors of equal magnitude but displaced in phase from each other by 120° and has the phase sequence opposite to the original vectors. It specifies that the current is flowing from load to source.

Zero sequence component: It has three vectors of equal magnitude and also are in phase with each other. It specifies that the current is flowing from source to ground.

• Positive sequence components exist in both balanced and unbalanced conditions.
• Negative sequence components exist in unbalanced conditions only.
• Zero sequence components exist in the unbalanced conditions involving ground.

In double line fault or line to line fault, there is no ground. Therefore, the zero sequence current is zero.

Concept

The fault current in different types of faults are:

1.) Short circuit fault

\(I_{3\phi}={E_a\over X_1}\)

2.) Single line to ground fault

\(I_{LG}={3E_a\over X_1+X_2+X_0}\)

3.) Line-to-line fault

\(I_{LL}={\sqrt{3}E_a\over X_1+X_2}\)

where, X₁ = Positive sequence reactance

X₂ = Negative sequence reactance

X_o = Zero sequence reactance

E_a = Generator voltage

Generally, the fault current is calculated at no-load condition, hence E_a = 1 pu

Calculation

Given, X1 = 0.12 pu

X2 = 0.12 pu

Xo = 0.06 pu

\(I_{3\phi}={1\over 0.12}=8.33 \space pu\)

\(I_{LG}={3\over 0.12+0.12+0.06}=10\space pu\)

\({I_{3\phi}\over I_{LG}}={8.33\over 10}=0.833\)