As the fault current level increase, the distance protection scheme will serve the need for quicker clearing times. For the

protection of very long voltage transmission lines, both differential protection and time graded are not suitable [40]. This type of

protection scheme also used because of the difficulty of the first scheme above with the increasing number of switching station

which creating more stage of protection [12]. The distance protection acts depends on the impedance or distance between the

point where the relay is installed and the point of the fault occurred [40]. This type of scheme provides discrimination protection

without employing the pilot wires. So, it protects the certain length of the line of transmission line by comparing the set value at

the relay with the measured impedance value of the line [40]. Usually, it is applied for the feeder protection of 66 kV and 132

KV and above lines [12]. There are factors that need to be considered if distance protection scheme want to be used:

The distance between point where relays are existing

The number of stations which are in series

The configuration of the system

The other protection which will exist in the line

Consideration of stability of the system

Differential pilot protection protection

This type of protection is a quick response and it consists of pilot wire protection [12]. In this type of protection scheme, each

conductor has its own C.Ts and relays while the secondary of C.T is connected in series with pilot wires. So, when the current

entering one end of a line is equal to the current leaving to the other line meaning induces current in the secondary of C.T are

equal cause the relay to not working. On the other hand, on the occurrence of fault between the two ends [40], there is a

difference between the incoming and outgoing current and then that circulating current is arranged to flow through the pilot

wires and relay which will operates the circuit breaker and disconnect the faulty line [40]. Normally, is applied for the

transformer protection which having capacity of 5-10MVA and above. There are two types of well-known differential protection

systems [40]:

Merz-Price system

This system is applied for the feeder or line protection which utilizes the principle of voltage balance.

Translay system

This system is almost similar to Merz-Price system except that the opposition or balance is between the voltages induced in the

secondary winding wound on the relay magnet and not between the secondary voltages of the line C.T [40].

Restricted earth fault protection

This type of protection is used with the differential protection to provide adequate earth fault sensitivity as a simple overcurrent

earth relay do not meet the requirement and it is added to both winding of the transformer [41] [42].Generally, it is used for the

winding of the transformer connected in star, where its neutral line is either earthed through impedance or solidly earthed [12].

Separate C.T.s can be used for this purpose or the restricted earth-fault protection can be operated from the C.T. associated

with the differential protection. The relay used for this scheme is of high impedance type to make the scheme more stable for

any external faults.

2.8 Relay Coordination Concept

2.8.1 Radial System

The protective relay as primary and backup is important in distribution system. A relay itself will be the primary protection to

protect its own system element and when to backup other relays for fault at a location, it is serve to be a backup relay [12]. The