The directional power relay is not suitable under short circuit conditions because as short circuit occurs the system voltage falls to a low value resulting in insufficient torque to cause relay operations. This difficulty is overcome in the directional over current relay, which is independent of system voltage and power factor.
Constructional details: – Figure shows the constructional details of a typical induction type directional over current relay. It consists of two relay elements mounted on a common case viz. (i) directional element and (ii) non-directional element.
(i) Directional element: It is essentially a directional power relay, which operates when power flows in a specific direction. The potential of this element is connected through a potential transformer (PT.) to the system voltage. The current coil of the element is energized through a CT by the circuit current. This winding is carried over the upper magnet of the non-directional element. The trip contacts (1 and 2) of the directional element are connected in series with secondary circuit of the over current element. The latter element cannot start to operate until its secondary circuit is completed. In other words, the directional element must first operate (ie. contacts 1 and 2 should close) in order to operate the over current element.
(ii) Non-directional element: – It is an over current element similar in all respects to a non-directional over current relay. The spindle of the disc of this element carries a moving contact which closes the fixed contact after the operation of directional element. Plug setting bridge is provided for current setting. The tappings are provided on the upper magnet of over current element and are connected to the bridge.
Operation:-Under normal operating conditions, power flows in the normal direction in the circuit operated by the relay. Therefore, directional power relay does not operate, thereby keeping the (lower element) un-energized. However, when a short circuit occurs, there is a tendency for the current or power to flow in the reverse direction. The disc of the upper element rotates to bridge the fixed contacts 1 and 2. This completes the circuit for over current element. The disc of this element rotates and the moving contact attached to closes the trip circuit. This operates the circuit breaker which isolates the faulty section.
The step of relay operates when the, power in the circuit flows, in a specific direction. A directional power relay is so designed that it obtains its operating torque by the interaction of magnetic field derived from both voltage and current source of the circuit it protects. The direction of torque depends upon the current relative to voltage.
Constructional Details:- Figure shows the essential pails of a typicalinduction type directional power relay. It consists of an aluminum disc, which is free to rotate in between the poles of two electromagnet. The upper electromagnet carries a winding called potential coil on the central limb, which is connected through a potential transformer (PT.) to the circuit voltage source. The lower electromagnet has a separate winding called current coil connected to the secondary of CT. in the line to be protected.. The current coil is provided with a number of tappings connected to the plug setting bridge. This permits to have any desired current setting. The restraining torque is provided by a spiral spring. The spindle of the disc carries a moving contact which bridges two fixed contacts when the disc has rotated through a preset angle. By adjusting this angle, desired time setting can be obtained.
Operation:- The flux Ф1 due to current in the potential coil will be nearly 90° lagging behind the applied voltage V. The flux Ф2 due to current coil will be nearly in phase with the operating current I, as in the vector diagram. The interaction of fluxes Ф1 and Ф2 with the eddy currents induced in the disc produces a driving torque given by:
T α Ф1 Ф2 sin α.
Ф1 α V, Ф2 α I and α.= 90 – θ
α V I sin (90 – θ)
α V I cos θ
α Power in the circuit
It is clear, that the direction of driving torque on the disc depends on the direction of power flow in the circuit to which the relay is associated. When the power in the circuit flows in the normal direction the driving torque and the restraining torque help each other to turn away the moving contact from the fixed contacts. Thus the relay remains in operative. But with reversal of current in the circuit the direction of driving torque on the disc reverses. When the reversed driving torque is large enough, the disc rotates in reverse direction, and then the moving contact closes the trip circuit
The over load inverse time relay is shown in fig 26. It consists of an upper electromagnet that has been provided with two windings one primary and the other secondary. Primary is connected to a current transformer in the line which is under protection and is provided with eight tappings. These tappings are connected to a plug setting bridge by which the number of turns to be used can be adjusted in order to have the desired current setting. The second winding called secondary is energized by the induction effect and is wound over the central limb of the upper magnet as well as it is spread over the two limbs of the lower magnet. By this method, the leakage flux from the upper magnet entering the disc have been displaced in phase from the flux entering the disc from the lower magnet. The deflecting torque is produced on the disc in the fashion as already explained. The spindle of the disc carries a moving contact which bridges two fixed contacts after the disc has rotated through a certain angle which has been set before. Any setting for this angle is possible varying from 0 to 360°. The variation of this angle imparts to the relay, various time settings.
The speed of rotation of the disc is dependent upon the torque which in turn is dependent on the current setting, when the load current increases from this setting it will increase the speed of rotation of the disc resulting into decrease of operation time. Thus the time current characteristics of the relay observe inverse-Square law. The definite minimum time characteristics of the relay are obtained by the use of a saturated upper magnet. This ensures that there is no further increase in f1ux when the current has reached a certain value and any further increase of current will not affect the relay operation. This results in a flattened current time characteristic and the relay obtains its name asInverse definite minimum time lag (I.D.M.T.) relay
The current time characteristics of the relay have been illustrated in Fig. 27. It represents the time required to close the trip contacts for different values of over current. Its horizontal scale is marked in terms of current-setting multipliers i.e. number of times the relay current is in excess of current setting
Distance relays are characterized by having two input quantities proportional to the voltage and current at a particular point in the power system, referred to as the relaying point. Ideal static distance relays have characteristics independent of actual magnitudes of voltage and current but dependent only on their ratio and phase angle between them. The versatile family of distance relays includes impedance relays, reactance relays and mho relays. The measurement of impedance, reactance or admittance is done by comparing input current and voltage. Hence distance relays have voltage and current as input quantities. In a static distance relay it is necessary that the two input quantities are similar i.e., voltage/voltage or current/current because they are not electrically separate as they are in case of electro—magnetic relays (in an impedance relay magnets are energized by voltage and currents). A practical static distance protection scheme includes a starting, measuring and timing elements made up of solid-state devices. The output unit is usually a moving coil relay. The starting element is usually an over current relay. The output is given to the measuring element. Phase comparators are employed in the measuring devices. The measuring device determines whether the fault is within the protected zone or not. A tripping signal is initiated in case the fault is within the protected zone. In case the fault is outside protected zone, the timer unit starts which initiates zone-wise protection.
A block diagram of a distance relay based on current comparison principle is given in fig 25. The line PT secondary is connected to auxiliary PT and the output of auxiliary PT is converted into current and this current is compared with the output of the auxiliary CT.
Static distance relays do not have any moving part so they operate much faster (operating time of the order of some milli-seconds) and without risk of incorrect tripping as compared to electro-magnetic relays. Static distance relays are accurate over a wider range of fault currents and line lengths and require much lower burden as compared to their counterparts in electro-magnetic relays. Static distance relays are compact in size and have better stability under power swing conditions. Static distance relays are extensively used for protection of medium and long transmission lines, parallel feeders and unit back-up protection as well as inter-connected and T-connected lines.
The differential relay measures the phasor difference between two similar electrical quantities(voltage-voltage or current-current). The block diagram for such a relay is shown in fig 24. Inputs I and II are supplied to the comparator. The output of the comparator (phase difference of inputs I and II) is amplified and used to operate the relay.
The static differential relays are most commonly used for the protection of generators and transformers for any type of internal faults (two-and three-phase faults, earth faults with solidly grounded neutral or low resistance grounded neutral inter turn faults).
These relays are advantageous over electromagnetic differential relays as they are very compact, highly sensitive for internal faults and have absolute stability for heavy through faults, extremely short tripping times (20-50 ms) regardless magnitude of auxiliary voltage, accurate and absolutely stable tripping characteristic even for asymmetrical faults as each phase can have its own relay, low VA burden, inrush current proof characteristic even during high starting currents, inrush currents. The selection of auxiliary voltage is also easy. A permanent magnet moving coil relay is usually employed as tripping device.
The difference of the currents in the operating coil and restraining coil is fed to the output element for the relay operation. They relay operates when Ko no Io > Kr nr Ir + Kt where no and nr are the number of turns on the operating and restraining coils respectively and Ko and Kr the design constants and Kt the spring control torque constant.At the threshold of operation Kt = Ko no Iomin . The differential current schemes do not react to the peak currents caused by overloads or swings, also due to dissimilarity in CTs, inrush-magnetizing current in transformer protection.
The over-current relays, even though simplest of all types of electro-mechanical relays, are the most difficult static relays. Static over current relays are of two types:
(i) Instantaneous over-current relays and
(ii) Time over-current relay.
STATIC INSTANTANEOUS OVER-CURRENT RELAY
The block diagram of an instantaneous over-current relay is shown in fig 21. The same construction may be used for under-voltage, over-voltage and earth fault relays too.
The secondaries of the line CT’s are connected to a summation circuit (not shown in the fig). 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 over-current relay
For I input < I threshold; Ioutput = 0
For I input > I threshold; Ioutput = Present
In an actual relay I threshold can be adjusted.
After operation of the measuring unit, the amplifier amplifies the output. 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. Smoothing circuit and filters are introduced in the output of the bridge rectifier. Static over-current 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.
STATIC OVER-CURRENT TIME RELAY
The block diagram of static over current time relay is shown in fig 22.
The current from the line CT is reduced to 1/1000 th by an auxiliary CT, the auxiliary has taps on the primary for selecting the desired pick-up and current range and its rectified output is supplied to level detector I (over-load 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 22 b is given as Tc = RC log e E/(E – VT) .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
A static relay refers to a relay in which there is no armature or other moving element and response is developed by electronic, magnetic and other components without mechanical motion. The solid-state components used are transistors, diodes, resistors, capacitors and so on. Static circuits accomplish the function of comparison and measurement. A relay using combination of both static and electro-magnetic units is also called a static relay provided that static units accomplish the response.
In static relays, the measurement is performed by electronic, magnetic, optical or other components without mechanical motion. Additional electro-mechanical relay units may be employed in output stage as auxiliary relays. A protective system is formed by static relays and electro-mechanical auxiliary relays.
BASIC STATIC RELAY
The essential components of static relays are shown in fig 20. Rectifier rectifies the relaying quantity i.e., the output from a CT or PT or a transducer. The rectified output is supplied to a measuring unit comprising of comparators, level detectors, filters, logic circuits. The output is actuated when the dynamic input (i.e., the relaying quantity) attains the threshold value. This output of the measuring unit is amplified by amplifier and fed to the output unit device, which is usually an electro-magnetic one. The output unit energizes the trip coil only when relay operates.
In a static relay the measurement is carried out by static circuits consisting of comparators, level detectors, filter etc while in a conventional electro-magnetic relay it is done by comparing operating torque (or force) with restraining torque (or force). The relaying quantity such as voltage/current is rectified and measured. When the quantity under measurement attains certain well-defined value, the output device is triggered and thereby the circuit breaker trip circuit is energized.
Whenever there is an unbalance in circuit, the unbalanced currents will have a negative phase sequence component. A negative phase sequence (or phase unbalance) relay is essentially provided for the protection of generators and motors against unbalanced loading that may arise due to phase-to-phase faults. Such relay has a filter circuit, which is responsive only to the negative sequence components. Since small magnitude over-current can cause dangerous conditions, it becomes necessary to have low setting of such relays. An earth relay can also provide the desired protection but only in case when there is a fault between any phase and earth. For phase-to-phase faults an earth relay cannot provide necessary protection and hence negative phase sequence relay is required.
Fig. 19 a. illustrates the scheme used for negative phase sequence relay. A network consisting of four impedances Z1, Z2, Z3 and Z4 equal magnitude connected in a bridge of formation, which is energized from three CTs. A single pole relay having an inverse-time characteristic connected across the circuit, as illustrated in be figure. Z1 and Z3are non-inductive resistors while Z2and Z4 are composed of both resistance and inductance. The values of Z2 and Z4 are so adjusted that currents flowing through them lag behind those in impedances Z3and Z1, by 60º.
The relay is assumed to have negligible impedance. The current from phase R at junction A is equally divided into two branches, as I1 and I4 but I4will lag behind I1 by 60°.
From fig 19 b, I 1= I4 = I R /⌡3
Similarly the current from phase B divide at junction C into two equal components I3 and I2; I2 lagging behind l3 by 60º.
I 2= I3 = I B/⌡3
Note that I 1 lead I R by 30º while I4lags behind I R by 30º. Similarly I 2 lag behind I B by 30º whereas I3 leads I Bby 30°.
The current through relay operating coil at junction B will be equal to phasor sum of
I 1, I 2 and I Y.
i.e. IRELAY = I 1 + I 2 + I Y
= I R /⌡3 leading I R by 30º + I B/⌡3 lagging behind I B by 30º + I Y
Flow of + ve Sequence Currents – Fig 19 c, represents the phasor diagram when the load is balanced or when there is no negative sequence current. Since the current through the relay I 1 + I 2 + I Y= 0 because I 1 + I 2 = – I Y
So, the relay remains in operative for a balanced system.
Flow of – ve Sequence Currents – Fig 19 d, represents the phasor diagram for negative sequence currents. It is noted that at junction B current I 1 and current I 2are equal but opposite to each other, so they cancel each other and current I Yflows through the relay operating coil. Thus the relay operates due to flow of currentI Y through it. A low setting value well below the normal full-load rating of the machine is provided since comparatively small values of unbalance currents produce a great danger.
Flow of Zero Sequence Currents – The current at junction B of the relay is represented in phasor diagram fig 19 e, from which it is observed that the currents I 1 and I 2 are displaced from each other by 60º, so that the resultant of these currents is in phase with the current in phase Y. Thus a total current of twice the zero sequence current would flow through the relay and would therefore cause its operation.
To make the relay inoperative under the influence of zero sequence current, the CTs are connected in delta as shown in fig 19 f, because then no zero sequence current can flow in the network circuit