OVERCURRENT PROTECTION RELAY
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.
Table of Contents
The most obvious effect of a common electrical fault is a sudden build-up of current. Therefore, it is only natural that 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. In many situations, it may be the only protection provided. 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 take into account, not only the magnitude of the current but also its phase with respect to 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 relays
They can be classified on the time characteristics:
A wide variety of time-current characteristics is available for Overcurrent protection relays. The name assigned to an Overcurrent protection relay indicates its time-current characteristic.
Definite-time Overcurrent Relay:
A definite-time overcurrent protection relay operates after a predetermined time when the current exceeds its pick-up value. The operating time of this relay is constant, irrespective of the magnitude of the current above the pick-up value. The desired definite operating time can be set with the help of an intentional time-delay mechanism provided in the relaying unit.
Instantaneous Overcurrent Relay
An instantaneous relay operates at a definite time when the current exceeds its pick-up value. The operating time is constant, irrespective of the magnitude of the current above the preset. There is no intentional time delay. It operates in 0.1s or less. Sometimes terms like “high set” or “high speed” are used for very fast relays having operating times less than 0.1s.
Inverse-time Overcurrent Relay
An inverse-time overcurrent protection relay operates when the current exceeds its pick-up value. The operating time depends on the magnitude of the operating current. The operating time decreases as the current increases and vice-versa.
Inverse Definite Minimum Time Overcurrent (IDMT) Relay
This type of relay gives an inverse time-current characteristic at lower values of the fault current and a definite-time characteristic at higher values of the fault current. Generally, an inverse-time characteristic is obtained if the value of the plug setting multiplier is below 10. For values of plug setting multiplier between 10 and 20, the characteristic tends to become a straight line, i.e. towards the definite time characteristic. IDMT relays are widely used for the protection of distribution lines. Such relays have a provision for current and time settings.
Very Inverse-time Overcurrent Relay
A very inverse-time Overcurrent protection relay gives more inverse characteristics than of a plain inverse relay or the IDMT relay. Its time-current characteristic lies between an IDMT characteristic and an extremely inverse characteristic. The very inverse characteristic gives better selectivity than the IDMT characteristic. Hence, it can be used where an IDMT relay fails to achieve good selectivity. Its recommended standard time-current characteristic is given by t=13.5 / I-1
The general expression for the time-current characteristic of overcurrent protection relays is given by
The value of n for very inverse characteristics normally lies between 1.02 and 2.
Very inverse time-current relays are recommended for cases where there is a substantial reduction of fault current as the distance from the power source increases. They are particularly effective with ground faults because of their steep characteristic.
Extremely Inverse-time Overcurrent Relay
An extremely inverse time overcurrent protection relay gives a time-current characteristic more inverse than that of the very inverse and IDMT relays, as shown in Fig. 5.2. When IDMT and very inverse relays fail in selectivity, extremely inverse relays are employed. IDMT relays are not suitable to be graded with fuses. Enclosed fuses have time-current characteristics according to the law
I3.5 x t = K
An extremely inverse relay is very suitable for the protection of machines against overheating. The heating characteristics of machines and other apparatus are also governed by the law I2 x t = K.
Hence, this type of relay is used for the protection of alternators, power transformers, earthing transformers, expensive cables, railway trolley wires, etc. The rotors of large alternators may be overheated if an unbalanced load or fault remains for a longer period on the system. In such a case, an extremely inverse relay, in conjunction with a negative sequence network is used. By adjusting the time and current settings, a suitable characteristic of the relay is obtained for a particular machine to be protected.
For definite-time characteristics, the value of n is equal to 0. According to the British Standard, the following are the important characteristics of overcurrent relays.
PLUG SETTING MULTIPLIER OR CURRENT SETTING OF RELAY
The current above which an overcurrent relay should operate can be set. Suppose the CT secondary output is 5A and the relay is set at 5 A. It will then operate if the current exceeds 5 A. Below 5 A, the relay will not operate. There are a number of tapings on the current coil, available for the current setting, as shown. The operation of the relay requires a certain flux and ampere-turns. The current settings of the relay are chosen by altering the number of turns of the current coil by means of a plug PS in Fig.
The plug-setting (current-setting) can either be given directly in amperes or indirectly as percentages of the rated current. An overcurrent relay which is used for phase-to-phase fault protection, can be set at 50% to 200% of the rated current in steps of 25%. The usual current rating of this relay is 5 A. So, it can be set at 2.5 A, 3.75 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 which is used for protection against ground faults (earth-fault relay) has settings 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 current flowing in the relay 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 operating time of the relay can be set at a desired value. In an induction disc-type relay, the angular distance by which the moving part of the relay travels for closing the contacts can be adjusted to get different operating times. There are 10 steps in which time can be set. However, in modern numeric relays, there are no moving parts.
The term time multiplier setting (TMS) is used for these steps of time settings. The values of TMS are 0.1, 0.2, …, 0.9, 1. Suppose that at a particular value of the current or plug setting multiplier (PSM), the operating time is 4 s with TMS = 1. The operating time for the same current with TMS = 0.5 will be 4 × 0.5 = 2 s. The operating time with TMS = 0.2 will be 4 × 0.2 = 0.8 s.
EXAMPLE OF TSM AND PSM
The current rating of an overcurrent relay is 5 A. The relay has a plug setting of 150% and a time multiplier setting (TMS) of 0.4. The CT ratio is 400/5. Determine the operating time of the relay for a fault current of 6000 A. At TMS = 1, operating times at various PSM are given as:
| PSM | 2 | 4 | 5 | 8 | 10 | 20 |
| Operating Time | 10 | 5 | 4 | 3 | 2.8 | 2.4 |
Solution:
CT ratio = 400/5 = 80
Relay current setting = 150% of 5 A = 1.5 × 5 A = 7.5 A
The operating time from the given table at PSM of 10 is 2.8 s. This time is for TMS = 1.
Therefore, the operating time for TMS of 0.4 is equal to 2.8 × 0.4 = 1.12 s.
FAQ’s
What can cause overcurrent in a circuit?
The overcurrent in a circuit can be caused by overloading a circuit, a short circuit, and a ground fault.
What is the difference between an earth fault relay and an overcurrent relay?
The only difference between the two is in the Plug setting Multiplier settings which means only in the threshold value or pickup value of current.
