The most commonly used over current relays are IDMT Inverse definite mean time over current relays, DT definite time over current relays and Instantaneous time over current relays. They are designed to sense the over-current. For low amount of fault current relay takes more time to operate, for higher current the relay trips fastly. The relay operates according to its characteristics curve. The operating time decreases as the current flow through the circuit increases. Different types of Inverse time relays are Inverse, very inverse and extreme inverse.
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Over Current Relay and Its Characteristics Protection schemes can be divided into two major groups: a Unit schemes, and b Non-unit schemes. Any deviation from this must indicate an abnormal current path.
In these schemes, the effects of any disturbance or operating condition outside the area of interest are totally ignored and the protection must be designed to be stable above the maximum possible fault current that could flow through the protected area. As well as protecting their own designated areas, the protective zones can overlap into other areas. While this can be very beneficial for backup purposes, there can be a tendency for too great an area to be isolated if a fault is detected by different non unit schemes.
The most simple of these schemes measures current and incorporates an inverse time characteristic into the protection operation to allow protection nearer to the fault to operate first. Over-current relays can be used to protect practically any power system elements, i. For feeder protection, there would be more than one over-current relay to protect different sections of the feeder.
These over-current relays need to coordinate with each other such that the relay nearest fault operates first. Use time, current and a combination of both time and current are three ways to discriminate adjacent over-current relays. Hence fast fault clearance is always desirable on short circuits. Primary requirement of Over-current protection is that the protection should not operate for starting currents, permissible over-current, and current surges. To achieve this, the time delay is provided.
Over-current Relay Ratings: In order for an over-current protective device to operate properly, over-current protective device ratings must be properly selected.
These ratings include voltage, ampere and interrupting rating. Current limiting can be considered as another over-current protective device rating, although not all over-current protective devices are required to have this characteristic Voltage Rating: The voltage rating of the over-current protective device must be at least equal to or greater than the circuit voltage.
The over-current protective device rating can be higher than the system voltage but never lower. The time of operation of such Relay is approximately ms. Instantaneous Over-current relay is employed where the impedance between the source and the Relay is small as compared with the impedance of the section to be provided.
The more the fault current the lesser will be the time of operation of the Relay. Normally it has more inverse characteristic near the pick-up value which in turn means that if fault current is equal to pick-up value then the relay will take infinite time to operate. From the picture, it is clear that there is some definite time after which the Relay will operate. It is also clear that the time of operation at Pick-up value is nearly very high and as the fault current increases the time of operation decreases maintaining some definite time.
This happens so as the CT saturation occurs at a later stage but as soon as CT saturation occur there will not be any flux change and hence the current output of CT will become zero and hence the time of operation will nearly become constant.
Over Current Relay and Its Characteristics
Difference Between IDMT DT and Instantaneous Relays