The overcurrent relays, even though simplest of all types of electromechanical relays, are the most difficult static relays. This is because the induction disc characteristics of the overcurrent relays (inverse characteristics) are not amenable to simple mathematical analysis. The first static relays developed were the high speed differential relays and the distance relays.
Fault current level detectors are termed overcurrent relays. They are more complicated in static form as compared to their electromagnetic counterparts.
However, static overcurrent relays offer several advantages over the electromagnetic form:
1. Low CT burden – The VA consumption of static overcurrent relays is quite low (7 mVA to 100 mVA) as compared with that of electromagnetic relays (1VA to 3VA) so that smaller CTs are required. The performance of CT under short-circuit condition is also improved.
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2. Compact in size – The size of a single three-phase overcurrent relay may be about one-fourth of three electromagnetic relays. Hence less panel space is required in case of static overcurrent relays.
3. Possibility of Instantaneous Reset. In case of static overcurrent relays instantaneous reset is possible. This is due to the absence of moving parts, which facilitates the application of automatic reclosing of breakers.
4. No over-reaching tendencies and more accurate time-current characteristic.
5. Less maintenance, long life and not affected by shock and vibration.
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The technique of overcurrent relaying is still widely used as a means of fault detection on distribution system and on transmission lines supplied from one end. In the case of transmission lines supplied from both ends, it is employed with directional relays. Overcurrent relays are also employed in conjunction with distance relays to provide backup protection.
The protective relays are either single actuating quantity relays such as overcurrent, under-voltage, earth fault relay or double actuating quantity relays such as distance relays, differential relays.
Types of Static Overcurrent Relays:
Static Instantaneous and Definite Time Overcurrent Relays:
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The block diagram of an instantaneous overcurrent relay is shown in Fig. 4.9. The same construction may be used for under-voltage, overvoltage or earth fault relays too.
The secondaries of the line CTs are connected to a summation circuit (not shown in the figure). The output of this summation CT is fed to an auxiliary CT, whose output is rectified, smoothened and supplied to the measuring unit (level detector). The measuring unit determines whether the quantity has attained the threshold value (set value) or not. When the input to measuring unit is less than the threshold value, the output of the level detector is zero. For an overcurrent relay-
For Iinput < Ithreshold ; Ioutput = 0
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For Iinput > Ithreshold ; Ioutput = Present
In an actual relay, Ithreshold can be adjusted.
After operation of the measuring unit, the output is amplified by the amplifier. The amplified output is given to the output circuit to cause trip/alarm.
If time delay is desired, a timing circuit is introduced before the level detector.
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Smoothening circuit and filters are introduced in the output of the bridge rectifier.
Static overcurrent relay is made in the form of a single unit in which diodes, transistors, resistors, capacitors etc., are arranged on printed board and are bolted with epoxy resin.
The general equation for time characteristic is given as –
Int = K …(4.5)
Where, I is the relay current, t is the time of operation and n is the characteristic index of relay and K is constant.
In conventional electromagnetic relays, n may vary between 2 and 8. The characteristic becomes a straight line parallel to current axis for n = 0. Such a characteristic is known as definite time characteristic of overcurrent relay.
With n = 1; It = K. The characteristic becomes inverse characteristic.
With higher value of n, the characteristic becomes more and more inverse. With n = 7 or 8, the characteristic becomes extremely inverse.
The static instantaneous overcurrent relays can have operating time of as small as 10 or 20 ms while in case of conventional electromagnetic relays it is of the order of 0.1 second.
Definite time overcurrent relays are used for wide variations of systems conditions, as backup relays for differential and distance protection, and differential protection of transformers to avoid mal-operation during magnetization inrush currents. Inverse-time relays are used where the source impedance is much smaller than the line impedance. Extremely inverse overcurrent relays are used for the fuse coordination and thermal protection of transformers and induction motors.
The general expression for the operating time of a static time-current relays may be given as –
t = K M/In – Inp …(4.6)
where M is time-multiple setting, I is multiple of tap current; Ip is the multiple of tap current at which pick-up occurs, n is characteristic index of relay, t is time of operation in seconds and K is design constant of the relay.
If the relay picks up at top value current i.e., Ip = 1. then t = K M/In – 1
The static overcurrent time relays can have the following typical characteristics
IDMT standard inverse t = 0.14/I0.02 – 1
Very inverse t = 13.5/I – 1
Extremely inverse t = 80/I2 – 1
In static relays it is advantageous to choose a circuit accommodating a wide range of alternative inverse time characteristics, precise minimum operating levels, definite minimum times and additional high set features, if necessary. The block diagram of a static overcurrent time relay is shown in Fig. 4.11 (a).
The current from the line CT is reduced to 1/1000th by an auxiliary CT, the auxiliary CT has taps on the primary for selecting the desired pick-up and current range and its rectified output is supplied to level detector I (overload level detector) and an R-C timing circuit. When the voltage on the timing capacitor VC attains the threshold value of the level detector II, tripping occurs.
Time delay given by the timing circuit shown in Fig. 4.11 is given as –
TC = RC loge [E/(E – VT )] …(4.7)
Where, VT is the threshold value of the level detector II.
By varying values of R and C the time can be varied without difficulties. The basic R-C circuit can also be arranged in several series-parallel combinations to have different values of TC.
Nonlinear resistors are used to have other time characteristics.
Time delay overcurrent relays are used in overcurrent protection of utility equipment, distribution circuits, protection of transformers, generators, motors etc.
In instantaneous overcurrent relays, the time delay circuit shown in Fig. 4.11 (a) is deleted. Such a circuit would need only one level detector. As there is no moving part, operating time of the order of 0.02 s (1 cycle) can be achieved. Instantaneous overcurrent relays are used for short-circuit protection of large equipment. Instantaneous overcurrent relays are also useful in other protective relay systems.
Overcurrent Ground Protection:
Overcurrent ground fault protection can be obtained with only one overcurrent relay by connecting it across a zero phase sequence current filter. On occurrence of ground fault, the zero sequence current will flow through the relay coil and relay will operate if the relay current exceeds the threshold value.
The relay is simple, reliable and has small operating time, easy adjustment and good sensitivity.
For complete protection against phase and ground faults, generally three CTs and two phase and one ground overcurrent relays are employed in the system
Directional Static Overcurrent Relays:
For obvious reasons of obtaining selectivity overcurrent relays are made directional. Directional relay senses direction of power flow by means of phase angle between V and I. When the phase angle between V and I exceeds certain predetermined value, the directional relay operates with a condition that the current is above the pick-up value. Thus directional relay is a double actuating quantity relay with one input as current I from CT and the other input V from PT.
The induction cup type of electromagnetic relays, even though are very sensitive, have dead zone in their operation. In static directional relays this problem is less serious because static comparators are very sensitive and it is comparatively easier to make a static directional unit reliable down to 1% of system voltage which is well within the minimum fault voltage.
The choice of directional unit depends upon the type of comparator used. The comparator used in directional overcurrent relay may be Hall Effect generator; Rectifier bridge comparator; Instantaneous coincidence comparator or integrating coincidence phase comparator. The Hall crystal as phase comparator has gained popularity only in Russia.
Figure 4.15 represents the static directional relay with two inputs (V and I). The inputs are supplied to the phase comparator. A phase shifter is included in the voltage input circuit, whose output is fed to the phase comparator; so that the output from phase comparator under phase faults/earth fault condition is maximum. The output of phase comparator is fed to the level detector. The output of the level detector is amplified and in case a timer is necessary, the output is applied to the output device through the timer.
Relay will operate when IS < I cos(ɸ – θ).
Where, IS is the magnitude of set current, ɸ is the phase angle between V and I and θ is characteristic angle. For maximum sensitivity of relay ɸ should be equal to θ.
The directional overcurrent relay can be made either instantaneous or time-lag type.