OVERCURRENT PROTECTION RELAY

Overcurrent protection relays are the protective relay that operates to issue a trip command when the current value in a circuit exceeds a preset. The preset value of the current which if exceeded, makes the relay operational is known as the pickup value of the overcurrent protection relay. As in most faults, the current flow increases beyond the rated current of a circuit, therefore, the overcurrent protection relay provides the cheapest protection to isolate such electrical faults in the circuit.

Earlier in the history of power system protection in the name of overcurrent protection, fuses were commonly used. However, they are still in use for overcurrent protection in LT lines of distribution networks. Nowadays all relays have turned numerical and digital, electromechanical ones are obsolete.

The most obvious effect of a common electrical fault is a sudden build-up of current. Therefore, the magnitude of the current is utilized as a positive indication of the existence of a fault. It is no wonder, therefore, that overcurrent protection is the most widely used form of protection. It is often the only protection provided in many situations. This type of protection which depends on only the magnitude of the current, without taking any cognizance of its phase angle, is known as non-directional overcurrent protection.

However, many times it is required to discriminate between faults are on front of the breaker or faults behind the breaker. This is possible only if we consider the magnitude of the current along with its phase concerning the voltage at the relay location. In such cases, the protection is known as the directional overcurrent protection. The directional overcurrent protection affords greater selectivity than a non-directional overcurrent protection.

These relays normally operate when a preset value of the parameter is exceeded. The limiting value of the preset is known as the pickup value. These relays are often time delayed so that the transient developed clears on its own.

Classification of overcurrent protection relays

Overcurrent protection relays used in an electrical substation are basically classified according to their time-current characteristics.

Definite-time Overcurrent Relay:

As the name suggests, in a definite time overcurrent relay, the relay is made to operate only after a definite time when the current in the circuit crosses the preset limit. This definite time of the relay is not influenced by the magnitude of the fault current. The definite time therefore is a constant in the relay operation and can be fixed to a time value with a time delay mechanism, which is inherently a part of the relay unit.

Instantaneous Overcurrent Relay

An instantaneous relay operates instantaneously when the current in the circuit exceeds the pick-up value. In this type of relay, the operating time is constant, irrespective of the magnitude of the current. However, there is no intentional time delay as in the case of definite time over current relay. These relays operate in 0.1s or less and are often called a high set or high-speed relays for very fast operation.

Inverse-time Overcurrent Relay

This type of relay operates when the current exceeds the pickup value. However, the operating time of the relay varies inversely with the magnitude of the current above the pickup value. The operating time of the relay is minimum for a high current value above pickup and maximum for a low magnitude of current above pickup.

Inverse Definite Minimum Time Overcurrent (IDMT) Relay

At a lower fault current magnitude, this relay provides an inverse time-current characteristic but when the fault current is high, it shows a definite time characteristic. Generally, the inverse-time characteristic is obtained at a PSM value below 10. However, between 10 to 20, the characteristics curve tends to become a straight line, showing definite time characteristics. These relays find application in the distribution feeders and have both provisions of current and time settings.

Very Inverse-time Overcurrent Relay

The time-current characteristics of a very inverse time-current relay lie between IDMT and an extremely inverse relay. The relay, therefore, shows more inverse nature and thus provides better selectivity than IDMT characteristics. Therefore, it is to be used where IDMT fails to achieve proper selectivity. They are best suited for backup protection of long transmission lines as fault current reduces because of long-distance power sources. These relay provides extremely good protection against earth fault because of their inherently steep characteristics.

The time-current characteristics relation of this relay is given by t=13.5 / Iⁿ-1, where the value of n lies between 1.02 to 2.

Extremely Inverse-time Overcurrent Relay

These relays show the most inverse time-current characteristics. It offers superior selectivity compared to IDMT and very inverse type. These relays can also be time graded with fuse, for which IDMT isn’t suitable. This is because the time-current characteristics of the fuse are governed by I^3.5 x t = K, where I is the current, t is the relay operating time and K is constant which is related to a maximum energy threshold that the protected element can withstand without failure.

This relay can also provide protection against the overheating of machines as the thermal characteristics of electrical machines are given by I² x t = K. Therefore, in alternators protection, power transformers protection, protection of EHV power cables or marine cables. these relays are used.

The following are the important characteristics in accordance with the British Standards.

PLUG SETTING MULTIPLIER OR CURRENT SETTING OF OVERCURRENT PROTECTION RELAY

The pickup value of the overcurrent relay can be set with the plug setting multiplier. Let, the CT secondary output be 5A and the pickup of the relay is set at 5 A. The relay will only operate when the current at the relay exceeds 5 A. Below 5 A current, the relay will not operate. Therefore, a certain amount of flux and ampere-turns are required for the operation of the relay. The current settings or the pickup value of the relay are chosen by altering the number of turns of the current coil by means of a plug in the old electromagnetic type relay. In modern relays, there is no plug, however.

The plug-setting (or, current-setting) is either given directly in amperes or indirectly as percentages of the rated current. An overcurrent relay that is used for phase-to-phase fault protection is usually set at 50% to 200% of the rated current in steps of 25%. Considering the usual current rating of this relay is 5 A. So, it can be set at 2.5 A, 2.625 A, 2.75 A,…, 5 A, …, 10 A. When a relay is set at 2.5 A, it will operate when the current exceeds 2.5 A. When the relay is set at 10 A, it will operate when the current exceeds 10 A.

The relay for protection against earth faults (earth-fault relay) has settings of 20% to 80% of the rated current in steps of 10%. The current rating of an earth-fault relay is usually 1A.

The actual rms value of the current flowing in the relay that is expressed as a multiple of the setting current (pickup current) is known as the plug setting multiplier (PSM).

Hence, PSM can be expressed as

TIME SETTING MULTIPLIER OF RELAY

The time for operation of the relay can be set to any desired value in various steps. In the old induction disc-type relay, the relay operating time can be set by adjusting the angular distance between moving parts of the relay and closing contacts. There are basically 10 steps in which the time can be set. However, in modern numeric relays, there are no moving parts.

The term time setting multiplier (TSM) is used for these steps to set the time. The values of TMS are 0.1, 0.2, 0.3, 0.4…, 0.9, 1. For example, at a particular value of the current or plug setting multiplier (PSM), the operating time is 4 s with TSM = 1. The operating time for the same current with TSM = 0.5 will be 4 × 0.5 = 2 s. The operating time with TSM = 0.2 will be 4 s × 0.2 = 0.8 s.

EXAMPLE OF TSM AND PSM

Let the current rating of an overcurrent relay be 5 A and the relay is with a plug setting of 150% and a time setting multiplier (TSM) of 0.4. The CT ratio is 400/5. For the determination of the operating time of the relay at a fault current of 6000 A at TMS = 1, operating times at various PSM are given as:

Solution:

CT ratio = 400/5 = 80

Relay current setting = 150% of 5 A = 1.5 × 5 A = 7.5 A

From the table above, at a PSM of 10, the operating time is 2.8 s. This time is for TMS = 1.

Therefore, the operating time for TSM of 0.4 is equal to 2.8 × 0.4 = 1.12 s.

Application of Overcurrent protection

• It is used as the primary protection for 33kv feeders.
• It is used as a backup protection in 132KV feeders and above.
• It is used as a backup protection for transformers.
• Also used as backup protection for the busbar.
• Used in the distribution panels to isolate faults.
• Used as primary protection for motors and capacitor banks.

FAQ’s

This article is a part of the Protection System, where other articles related to the protection of electrical equipments are discussed in details.