Power System Protection

Fault Vs Abnormal condition:

Abnormal condition in a power system, as the word itself states, is a non-desirable state in a power system.
Best example is 'Overload' in a power system, where the load current is above the nominal rating of the system it is designed for.

Whereas Fault is an abnormal condition which is detrimental to the power system , and will be dangerous if not eliminated.
Best example is a short-circuit, whereby two phase conductors touch each other between the load and the generator - so that a very high current flows in the circuit.

Transformer getting fire due to fault

Image from: www.seeclab.com

Sequence analysis:


Balanced system is where in a 3-phase power system - the current flowing in all three phases are equal.

Unbalanced system is where in a 3-phase power system - the current flowing in all three phases are unequal.


Any unbalanced system can be represented by balanced systems containing 3 elements. Those are,

1. +ve sequence
2. -ve sequence
3. zero sequence

Vector addition of the above 3 elements will result in the former unbalanced system.

Protection Equipments:

Fuse:

1. Semi-enclosed
-found in domestic installations, rewirable.

rewirable fuse holders

2. Cartridge type
-found in electrical apparatus such as UPS etc, not re-wirable.

cartridge fuse








3. High Rupturing Capacity (HRC)
-normally found in the secondary (LV) side of transformers. This has high current capacity before breaking.

Knife-edged Low volatge HRC fuse links








Drop-down-lift-operate (DDLO) fuse:

These are sometimes called fusible cutouts.

This is a type of expulsion fuse, which is normally found in the HV side of transformers. When high current flows in the primary side (HV) of transformers - the fusing element (a special metal wire) will melt so that the connection will be cut-off due to gravity.

a DDLO in opened position









Miniature circuit breaker (MCB)

This is the  protective device seen in modern homes, replacing older fuses. available in low current versions.
for e.g. 6A, 10 A, 16A etc. These MCBs normally have two tripping phenomena. one is magnetic coil used for instantaneous tripping ,and thermal bimetallic strips used for inverse-time overcurrent tripping.

These are available in different classes - considering the load current characteristics such as high inrush (starting) currents. High starting currents are caused when loads such as fluorescent lamps and motors are switched on.

single pole , three pole , four pole versions are available depending on the number of wires.






Moulded case circuit breaker (MCCB)
These are high current versions of domestic MCBs. Normally found in factories, utility bulk supply (3-phase) entry points.

3-phase currents in the orders of even 500A can be handled by these MCCBs.

Earth leakage circuit breaker (ELCB)

This is the older version  of RCCBs. These devices detect the leakage current to earth and trip if that current exceeds a threshold, for e.g. 30mA, 100mA.

It should be noted that ELCB is a (residual)-voltage-operated device.

Residual current Devices (RCD)/ Residual current circuit breaker (RCCB)

This is a modern version of an ELCB, which works by comparing the residual current (resultant) produced - by means of checking the current difference between live and neutral wires .

It should be noted that RCCB is a (residual)-current-operated device.

Protective Relays:


1. Overcurrent (O/C) relays
2. Overload  (O/L)  relays
3. Earthfault  (E/F) relays
4. Under frequency  (U/F) relays
5. Overvoltage (O/V) relays
6. Distance relays
7. Differential relays
8. Reverse power (R/P) relays
9. Bucholz  relays
10. Directional relays
11. Overspeed  (O/S) relays


Arcing Horns/ gaps:
These are sometimes employed in transformer HV terminals or between the terminals of an HV insulator, to protect them from lightning surges. This works on the simple principle of HV rod-gap breakdown.

High Voltage Circuit breakers:
This was earlier explained in the post - "Grid sub stations"

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