Electrical fault in power system

Electricity is required for various types of loads such as Domestic load, Commercial load, Industrial load, Municipal load, Irrigation load, and Traction load. 

Domestic load: Electrical devices which consume power in a home. Example Ceiling fan, Television, etc.

Commercial load: Electrical devices which consume power in shopping malls, Theatres. (Lighting, Air conditioner, etc.

Industrial Load: Electrical devices which consume power in industries (Electrical motor, induction motor, etc)

Irrigation load; 

To power-up, these loads, electricity needs to travel from generating station to the distribution station, and the infrastructure capable to collect and taps electricity for these loads is called an electrical grid.

During the transmission of electricity, the power network and high voltage equipment face many issues and these issues are classified into different types of faults.

Electrical fault

The abnormal flow of electric current in a power network is considered as the electrical fault or flow of electric current in an undesired path.

Power disturbance or failure in the power network lead to temporary or permanent damage to electrical equipment and power outage to the loads.

Effects of an electrical fault

An electrical fault can result in two types of damages to the electrical equipment

Thermal damage

Thermal is a word related to heat. From the name, we can easily understand that electrical fault produces heat with high temperature to the electrical equipment. 

All insulation classes of the electrical equipment will have a certain limit of thermal withstand capability. Whenever the fault current or voltage exceeds the rated or operating limit then this causes to increase in thermal.

Whenever the heat or temperature exceeds the thermal withstand capability insulation, then it is considered as Thermal damage.

Thermal damage occurs very slowly because it is related to temperature. So it did not require instantaneous tripping. 

Electrodynamic damage

In this case, the magnitude of the current is very high roughly 10 times or 20 times the rated or operating current. This current produces a high amount of repulsive force which can reshape or destruct the whole electrical equipment structurally.

The 10-20 times fault current can damage the electrical equipment very fastly. So it requires an instantaneous trip (without depending on time). 

Instantaneous trip* Whenever the fault current or voltage exceeds the rated current circuit breaker immediately stops the flow of power within a microsecond. 

Reason for an electrical fault

The electrical fault can occur for numerous reasons, some of them are addressed here,

• Insulation failure in the transmission line or electrical equipment
• Flashover of the transmission line or insulators
• The swinging of two conductors or short-circuit
• Lightning surge and switching surges increases the voltage abnormally on electrical equipment lead to electric fault
• Temporary or Permanent damage to towers and conductors lead to electric fault
• Wrong operation or wrong co-ordination in interlock
• The irresponsibility of handling electrical equipment.
• Poor maintenance of power system.

Probability of occurrence of a fault on different elements

The major types of electrical faults can occur on the overhead transmission lines and power transformers.

1. In overhead transmission lines conductor, the percentage of occurrence of an electrical fault is 50% due to weather condition or in other condition.
2. In underground cables, the percentage of occurrence of an electrical fault is 10% because they laid below the ground. So the chances of affection due to weather condition is less.
3. In switchgear including CTs and PT's the percentage of occurrence of an electrical fault is 10% to 15%.
4. In power transformers, the percentage of occurrence of an electrical fault is 15%.

Different types of fault in overhead transmission line

The electrical fault can occur in two conditions, one in the open condition and another in the close condition (short circuit). These two condition faults are categorized into symmetrical and unsymmetrical faults.

The other names for open and close condition,

1. Series fault (open condition fault)
2. Shunt fault (close condition fault)

Series fault

If a fault occurs in an open condition, then it is called a series fault.

Example of series fault

• Explode fuses cause an open circuit
• The unbalance opening of the circuit breaker (A circuit breaker which cannot capable to open all three phases is  considered as the open circuit)

Broken conductors that make open circuits in power networks are considered a series fault.

Characterization of series fault

• Voltage and frequency increase with a decrease in magnitude  current

Shunt fault

If a fault occurs in a closed condition, then it is called a shunt fault.

Example of shunt fault

• The line to line fault 
• Triple phase fault (line to line to line fault)
• The line to ground fault
• Double line to ground fault

The broken conductors touch with other conductors and make a short circuit then this is considered a Shunt fault.

Characteristics of shunt fault

• Current increases with a decrease in voltage and frequency

Categorization of fault (symmetrical and asymmetrical fault)

As with the shunt and series fault mentioned above, an electrical fault can be categorized as a balanced fault and an unbalanced fault.

Technically we can mention it as,

1. Symmetrical fault
2. unsymmetrical fault

Symmetrical fault

The symmetrical fault involves all three phases in a 3 phase network. So it is a balanced fault. The parameters(V, I, f, ф) of the symmetrical fault are equal. All three phases' voltage and current angle displaced 120 degrees equally.

The  different types of symmetrical faults are

1. Line-Line-Line fault ( All the transmission line touch each other or becomes short circuit)
2. Line-Line-Line-Ground fault (All the transmission line touches the ground)

The wrong operation or wrong coordination between circuit breaker and earth switch gives rise to symmetrical faults.

For example:

Let us consider a simple transmission line connecting three buses, the receiving end of the transmission line is connected with the circuit breaker, and the three main buses connected with the disconnect switch.

Whenever the line is energized by closing the circuit breaker, the earthing switch should be in an open position otherwise, it will give rise to the symmetrical fault.

If engineers or workers energize a transmission line without opening the earthing switch contact, cause the symmetrical fault.

Asymmetrical fault

This fault involves one or two phases in a three-phase network unequally.  The parameters (V, I, f, ф) of the asymmetrical fault are unequal. Hence it leads to flow unequal current in the system.

The different types of unsymmetrical faults are

1. Line-Line fault (Two phases [R and Y] of the 3 phase transmission line touches each other)
2. Line-Ground fault (Single phase [ R] of the 3 phase transmission line broken and touches the ground) 
3. Line-Line-Ground fault (Two phases[ R and Y] of the 3 phase transmission line touches the ground)

Dielectric strength of the transmission line decreases therefore, flashover or failure of insulators occurs, this gives rise to the line to ground fault.

Swinging of two conductors or shorting of wires due to strings, tree branches, birds sitting on the transmission line, also give rise to the line to line fault.