Overcurrent protection
Transmission and distribution systems are exposed to overcurrent flow into their elements. In an electric power system, overcurrent or excess current is a situation where a larger than intended electric current exists through a conductor, leading to excessive generation of heat, and the risk of
fire or damage to equipment. Possible causes for overcurrent include short circuits, excessive load, transformer inrush current, motor starting, incorrect design, or a ground fault. Therefore, for normal system conditions, some tools such as demand - side management, load shedding, and soft
motor starting can be applied to avoid overloads. In addition, distribution systems are equipped with protective relays that initiate action to enable switching equipment to respond only to abnormal system conditions. The relay is connected to the circuit to be protected via CTs and VTs according to the required protection function.
In order for the relay to operate, it needs to be energized. This energy can be provided by battery sets (mostly) or by the monitored circuit itself.
5.1 Overcurrent relays
The basic element in overcurrent protection is an overcurrent relay. The ANSI device number is 50 for an instantaneous overcurrent (IOC) or a Definite Time Overcurrent (DTOC) and 51 for the Inverse Definite Minimum Time. There are three types of operating characteristics of overcurrent relays:
• Definite(Instantaneous)-Current Protection,
• Definite-Time Protection and
• Inverse-Time Protection.
5.2 Definite(instantaneous)-current protection
This relay is referred as definite(instantaneous) overcurrent relay. The relay operates as soon as the current gets higher than a preset value. There is no intentional time delay set. There is always an inherent time delay of the order of a few milliseconds.The relay setting is adjusted based on its location in the network. The relay located furthest from the source, operates for a low current value. Example, when the overcurrent relay is connected to the end of distribution feeder it will operate for a current lower than that connected in beginning of the feeder, especially when the feeder impedance is larger. In the feeder with small impedance, distinguishing between the fault currents at both ends is difficult and leads to poor discrimination and little selectivity at high levels of short-circuit currents. While, when the impedance of feeder is high, the instantaneous protection has advantages of reducing the relay’s operating time for severe faults and avoiding the loss of selectivity.
In this type, two conditions must be satisfied for operation (tripping), current must exceed the setting value and the fault must be continuous for at least a time equal to the time setting of the relay. This relay is created by applying intentional time delay after
crossing pick up value of the current. A definite time overcurrent relay can be adjusted to issue a trip output at definite amount of time after it picks up. Thus, is has a time setting and pick up adjustment. Modern relays may contain more than one stage of protection each stage includes each own current and time setting. The settings of this kind of relay at different locations in the network can be adjusted in such a way that the breaker closest to the fault is tripped in the shortest time and then the other breakers in the direction toward the upstream network are tripped successively with longer time delay. The disadvantage of this type of protection is that it’s difficult to coordinate and requires changes with the addition of load and that the short-circuit fault close to the source may be cleared in a relatively long time in spite of its highest current value
5.4 Inverse-Time Protection
In this type of relays, operating time is inversely changed with the current. So, high current will operate overcurrent relay faster than lower ones. They are available with standard inverse, very inverse and extremely inverse characteristics. Inverse Time relays are also referred to as Inverse Definite Minimum Time (IDMT) relay. The operating time of both overcurrent definite-time relays and overcurrent inverse-time relays must be adjusted in such a way that the relay closer to
the fault trips before any other protection. This is known as time grading. The difference in
operating time of these two relays for the same fault is defined as discrimination margin. The adjustment of definite-time and inverse-time relays can be carried out by determining two settings: time dial setting and pickup setting. The time dial setting adjusts the time delay before the relay operates whenever the fault current reaches a value equal to, or greater than, the relay current setting. The time dial setting is also referred to as the time multiplier setting. The tripping
characteristics for different TMS settings
using the IEC 60225 are shown in the
table to the right. Pickup setting is used to define the pickup current of the relay by which the fault current exceeds its value. It is determined by:
Pickup setting is used to define the pickup current of the relay by which the fault current exceeds its value. It is determined by:
As we can see from the Fig. 6.4.2 the VI curve is much steeper and therefore the operation increases much faster for the same reduction in current compared to the SI curve. Very inverse overcurrent relays are particularly suitable if there is a substantial reduction of fault current as the distance from the power source increases. With EI characteristic, the operation time is approximately inversely proportional to the square of the applied current. This makes it suitable for the protection of distribution feeder circuits in which the feeder is subjected to peak currents on switching in, as would be the case on a power circuit supplying refrigerators, pumps, water heaters and so on, which remain connected even after a prolonged interruption of supply.