Transmission lines are used to transmit electric power from one place to far distant places where power has to be consumed. We used long towers and conductors to transmit the power. So this is natural or need of the process that transmission conductors must be protected in order to keep working.
Introduction of Distance protection:
Distance protection is mainly used for transmission lines. We use a relay system for tripping in case of fault and then that relay gives a tripping signal to its respective circuit breaker and then that faulty portion gets separated from the healthy portion.
We also use an overcurrent relay for protection but due to some limitations, we prefer to use a distance relay. In over current relay we fixed the pickup current value. Fault current depends on source impedance and voltage value. Due to this reason overcurrent relay is not much efficient as distance relay. We should be independent of the type of fault and the characteristics of the generator.
That’s the reason we go for distance protection. “We have a fixed value of impedance and then we check the impedance between the relay and at the fault point. If the value meets the condition of tripping for a specific impedance value then our breaker will trip for that fault.”
You can have a look on below diagram:
Here we have source side impedance set valueZs and in the circuit we have Relay, CT (Current transformer) and PT (Potential/Voltage transformer) and Bus A and Bus B.
F represents the fault happening on the transmission line. At fault, we have current ad voltage values so that will depends on the type of fault. But the ratio of voltage and current will give us the impedance value from the relay point to fault location. So, it is independent of the source or set value of impedance.
We have a tripping setting mentioned below:
Vr = Relay voltage
Ir = Realy Current
Vr/Ir<Zset then trip the corresponding breaker.
Not automatically it becomes independent from the characteristics of our system.
Zr<Zset then the relay will trip.
Tripping will happen using both the current coil and voltage values.
Zr<Zset as ratio of constants is Zset value
The relay will issue a trip signal for a value less than Zset value. As we are using ratio of values so it will be independent of circuit characteristic values. It is independent of the type of fault, voltage, and source impedance value.
The relay will compare the value obtained current and voltage using CT and VT.
Zones of protection:
In this diagram, all relays are provided for protection purposes in the forward diagram. For forward direction, the relay operates in the transmission line direction and bus bar side of relay behaves as the backward direction of a relay.
Zone-1in the practical concept covers only 80% before the next bus (zone-1), not 100%. This s the actual area covered by the zone-1 relay. The reason is due to DC effect there will be an overreach of the relay and whenever overreach happens, it creates problems. So we skip the 20% of the zone and that 20% will be added in the zone-2 protection relay.
In Zone-2 20% we cover the remaining area of zone-1 and 40% next part from bus bar B (In above diagram) to avoid overreach conditions.
Next, all portions will go into zone-3 and hence our transmission lien is protected using zones of relays easily without any disturbance overreaching.
In Zone-1 there will be instantaneous tripping when a fault occurs and after a specific time delay of relay. Similar case goes for other zones. There will be more time delay in zone 3.