In this article we will discuss about induction type directional relays used in an electric circuit.
Induction Type Directional Power Relay:
The non-directional relay can operate for fault flow in either direction. In order to achieve operation for the fault flowing in a specific direction, it is necessary to add a directional element to the non-directional element.
The directional (or reverse) power relay operates when the power through the relay will be reversing i.e. generator supply to the network fails and the power from the other sources in the system try to feed the power to this unit in the reverse direction. In the case of motors such a relay is employed to prevent the motors reversing the direction of rotation.
The principle of operation of this relay is similar to that of an overcurrent (non-directional) induction relay. The difference lies in the fact that in case of overcurrent relay the torque is developed due to interaction of magnetic fields obtained from the current in the circuit through CT, while in case of directional power relay the driving torque is derived from the interaction of the fields produced from both voltage and current sources of the circuit it protects. Since the relay has both voltage and current coils, the relay is essentially a wattmeter and the direction of the torque developed in the relay depends upon the direction of current in relation to the voltage with which it is associated i.e. the relay recognizes the phase difference between voltage and current.
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Constructional Details:
The induction type directional power relay essentially consists of an aluminium disc which is free to rotate in between the poles of two electromagnets. The upper electromagnet has a winding, called the voltage or potential coil, on the middle limb connected to the circuit voltage source through a potential transformer (PT). The lower electromagnet has a separate winding, called the 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 bridge, so as to give the desired current setting. The restraint torque is provided by a spiral spring.
Operation:
The torque developed on the disc suspended between the two magnets is proportional to VI. When the power flows in the normal direction, the torque developed on the disc assisted by the spring tends to turn away the moving contact from the fixed trip circuit contacts. Thus the relay remains inoperative. A reversal of current in the circuit reverses the torque produced on the disc and when this is large enough to overcome the control spring torque, the disc rotates in the reverse direction and the moving contact closes the trip circuit. This causes the operation of the circuit breaker to disconnect the faulty section. The relay can be made very sensitive by having a very light control spring so that a very small reversal of power will cause the relay to operate.
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The relay can be a single phase or a 3-phase having two voltage and two current elements like a 3-phase energy meter.
Operating Characteristics:
Let V be the voltage applied to the relay through PT and I be the relay current through CT. In phasor diagram (Fig. 3.31) I is shown leading the relay voltage V by an angle θ. Here ɸV is the flux due to voltage coil and lags behind the voltage by angle ɸ (about 60° to 70°) and ɸI is the flux due to the current coil and is in phase with current I. The net torque is produced due to the interaction of ɸI and ɸ V.
Torque, therefore, is given as –
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T ∝ ɸV ɸI sin (ɸ + θ) where ɸV ∝ V and ɸI ∝ I
So the torque equation for the relay can be given as –
T = K V I sin (ɸ + θ) … (3.10)
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The torque is maximum when the two fluxes are displaced by 90° i.e. when (ɸ + θ) = 90°. Here dotted line in the phasor diagram represents the desired position of ɸI for maximum torque. Since V is the reference quantity and ɸV has fixed position with respect to V for a particular design, the angle between the dotted line and the reference quantity V is known as the maximum torque angle and let it be denoted by τ.
Zero torque will occur when sin (ɸ + θ) = 0 i.e., (ɸ + θ) = 0° or 180°, this being satisfied when the relay current phasor lies along the chain dotted line which is at right angles to the maximum torque line. The directional element will, therefore, operate provided the current phasor lies within ± 90° of the maximum torque line. If the current phasor is displaced by more than 90° the directional element will restrain. The operating and the non-operating regions are shown in the figure.
It may be seen that –
τ = 90° – ɸ
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or ɸ = 90° – τ …(3.11)
and the torque equation becomes
T = KV I sin (θ + 90° – τ)
= KV I cos (θ – τ) …(3.12)
When the relay is about to start, neglecting the spring constant,
V I cos (θ – τ) = 0
or θ – τ = 90°
or θ = τ + 90° …(3.13)
This is the equation describing the polar characteristic (Fig. 3.32) of the directional relay.
The zone between the dotted line and the line parallel to it corresponds to the spring torque. If the current phasor lies within these lines the torque developed is less than spring torque and hence the relay does not operate. If the current phasor crosses the dotted line the operating torque exceeds the spring torque and hence the relay operates. Relay will not pick up or it will reset for any current phasor lying in the negative torque region.
It may be noted that the system current usually lags behind the system voltage but the relay current is made to lead the relay voltage by inserting resistance or capacitance or a combination of the two in series with the voltage or potential coil.
Such relays are very suitable for protection of parallel feeders. The directional overcurrent relay suffers from the drawback that the feeder voltage falls to a much lower value when a fault occurs resulting into non-operation of the relay. This shortcoming may be overcome by compensating the relay secondary winding on the lower magnet.
The compensating winding ampere-turns on the lower magnet opposes the ampere-turns produced by the current coil. Therefore, turns of current coil will have to be appropriately increased. When the voltage falls due to the fault on the feeder, the resultant ampere-turns provided by the windings on the lower electromagnet jointly increase, compensating the reduced ampere-turns provided by the voltage coil.
Induction Type Directional Overcurrent and Earth-Fault Relay:
The directional power relay, cannot be employed as a directional protective relay under short-circuit conditions because under short-circuit conditions the system voltage drops to a low value and therefore the torque developed in the relay may be insufficient to cause its operation. This difficulty is overcome in the directional overcurrent relay which is designed to be almost independent of system voltage and power factor.
Constructional Details:
It consists of two relay elements, viz.,:
(i) Directional element and
(ii) Non-directional element, mounted in a common case.
Directional element is essentially a directional power relay. The voltage coil of this element is connected to the circuit voltage through a PT while its current coil is energized through a CT by circuit current. This winding is carried over the upper magnet of the non-directional element. The trip contacts of the directional element are connected in series with the secondary circuit of the overcurrent element. Thus overcurrent element cannot start to operate until its secondary circuit is completed i.e., the directional element must operate first in order to operate the overcurrent element.
Non-directional element is an overcurrent element similar in all respects to a non-directional overcurrent relay. The spindle Earth-Fault Relay of the disc of this element carries a moving contact which closes the trip-circuit contacts after the operation of directional element. The tappings are provided over the upper magnet of the overcurrent element and are connected to the bridge, thereby provide facility for current setting.
Under normal operating conditions, power flow is in the normal direction in the circuit protected by the relay. Thus the directional power relay (lower element) does not operate, thereby keeping the overcurrent element (upper element) unenergised. But as soon as there is a reversal of current or power the disc of the reverse power relay (lower element) starts rotating and completes the circuit for overcurrent element. Due to overcurrent a torque is produced in the disc and the action closes the trip circuit, thereby enabling the circuit breaker to operate and isolate the faulty section.
The directional element is made as sensitive as possible to ensure positive operation—even 20% of the power in the reverse direction operates it.
The relay operates only when:
(i) The direction of current is in reverse direction
(ii) Current in the reverse direction exceeds the preset value and
(iii) Excessive current (greater than the present value) persists for duration longer than its time setting.
Directional relays must have the following features:
(i) High speed of operation
(ii) High sensitivity
(iii) Adequate short-time thermal rating
(iv) Ability to operate with low values of voltage
(v) Burden must not be excessive and
(vi) There should be no voltage and current creep i.e., if either the voltage coil alone or the current coil alone is energized with the other one de-energized there should be no movement.
Induction cup units satisfy the above requirements and are, therefore, very popular. Such relays are employed when graded time overload protection is applied to ring mains and interconnected networks, since fault current can flow in either direction.
Directional Overcurrent Relay Connections:
Relay connections must be made so that the currents and voltages applied to the relay during different fault conditions, which may arise on the protected circuit section, afford the relay a positive and sufficiently large operating torque. To achieve this for all types of faults the relays cannot be connected to operate on true watts since for some faults the voltage will be extremely small and also the power factor will be very small which will result in a negligible small torque. To overcome this difficulty, and thus ensure that sufficient torque is available, each relay is supplied with current and voltage.
There are two types of relay connections in use. Directional element connections are conveniently and popularly described in terms of the angle by which unity power factor (UPF) balanced load current flowing in the tripping direction leads the applied voltage applied to the relay voltage coil with due consideration given to the polarity of the relay coils.
The two types of relay connections used are:
(i) The 30°-relay with a maximum torque angle of 0°.
(ii) The 90°-relay with a maximum torque angle of 45°
The relay angle is defined as the angle between the voltage and current supplied to the relay under balanced three phase unity power factor conditions.
In phasor diagram for 0° directional relay with zero maximum torque angle IR, IY, IB and VR, VY, VB represent the phase currents, and phase voltages of a 3-phase balanced system with unity power factor conditions. Phasor VRB representing the system red phase to blue phase voltage lags behind phasors IR and VR by 30°. Let the relay element be supplied with current IR and voltage VRB, in phase relation as shown i.e., IR leads VRB by 30°. The connections are, therefore, referred to as 30° relay.
The angle between the current and voltage supplied to the relay for maximum torque, T is zero so that the position of the relay current phasor for maximum torque will be along VRB. Also since according to Eq. (3.12).
T = KVI cos (θ – τ)
= KVI cos θ for τ = 0
Now for unity power factor condition i.e., system current IR in phase with system voltage, the relay current IR leads the relay voltage VRB by 30° i.e., θ = 30° so that torque developed
T = KVI cos 30° = 0.866 KVI … (3.14)
While if system current IR lags behind the system voltage VR by 30°, the relay current IR will be in phase with relay voltage VRB so that θ = 0° and the torque developed will be maximum and is given as –
T = KVI … (3.15)
On occurrence of fault, IR may lag VR, say by 90°, and in such case θ will be 60° and the torque developed will be 0.5 Tmax.
We thus see that the initial lead of 30° makes the relay more sensitive at low power factors. Such relays are usually satisfactory for plain feeders.
The phasor diagram for the 90°-relay with a maximum torque angle of 45°.
Since the angle between the current and voltage supplied to relay for maximum torque, τ is 45°, the torque developed will be maximum for θ = 45° and is given as –
Tmax = KVI cos (θ – τ) = KVI cos(45° – 45°) = K V I
and torque for 90° connection will be T = KVI cos (90°- 45°) = 0.707 KVI = 0.707 Tmax .
Voltage Source Connection for Providing Residual Voltage:
Directional relay controlling earth faults on all the three phases of a 3-ɸ circuit is energized by residual current and residual voltage. The residual current is obtained by summing the currents in the three phases, employing line CTs or a core balance current transformer. The former is more common. For three line currents IR, IY, IB .
IR+ IY + IB = 0 for healthy conditions
IR+ IY + IB = IRES for earth fault on one phase.
A suitable residual voltage source can be a broken delta secondary winding on three single phase transformers or an equivalent three-phase five-limb voltage transformer. The application of a broken delta to provide a residual voltage is shown in Fig. 3.39. Under normal operating (or healthy) conditions the secondary voltage phasors form a closed triangle and therefore no voltage appears across the relay potential coil, as illustrated in Fig. 3.39 (a).
On occurrence of earth fault on any line say at point F on line B in Fig. 3.39 (b) the voltage phasors are as shown, leaving a residual voltage VRES across the open delta secondary which is the voltage appearing at the voltage coil of the directional element of the relay. The magnitude and phase of this residual voltage will depend on the magnitude of the earth fault current and the impedance of the fault path.
The connections for direction earth fault relay are shown in Fig. 3.40. The torque developed on the relay will be proportional to VRES IRES cos (θ – τ) where τ is the maximum torque angle of the relay and θ is the angle between VRES and IRES.
