AHSANULLAH UNIVERSITY OF SCIENCE AND TECHNOLGY
DEPARTMENT OF ELECTRICAL AND ELECTRONIC ENGINEERING
SWITCHGEAR AND PROTECTION LAB
Experiment: 5
Experiment name: Performance study of directional relay.
Introduction:
Directional protection responds to the flow of power in a definite direction with
reference to the location of CT’s and PT’s. Directional relay senses direction of power flow
by means of phase angle between voltage (V) and current (I). When this angle exceeds
certain predetermined value, the directional relay operates. A directional relay is a double
actuating quantity relay with one input as current I from CT and the other input as voltage, V
from PT. With the electromagnetic directional O/C relays, discrimination is affected when
the voltage drops down to a low value due to faults close to the location of PT. With static or
digital directional O/C relay can function well up to 1% of the system voltage.
Working principle:
Active power flowing through a part of an electric circuit is P = VI Cosθ, where θ is the
angle between voltage and current.
θ = 90o
Region of +ve power
Region of -ve power
Vr
θ = 180o θ
Ii
θ = 270o
Fig 1: Vector diagram of power.
From the above vector diagram,
For θ< ±90o, Cosθ is positive, hence the real power is positive.
For θ> ±90o, Cosθ is negative, hence the real power is negative.
For θ=90o & θ=270o, Cosθ is zero, hence the real power is zero.
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The direction of power flow can be sensed by sensing the magnitude and sign of power. Here
we used microprocessor based directional relay.
Directional phase fault Protection: (67)
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Directional Earth fault Protection (67N)
Description of the set up
The following figure shows the single line diagram of a power system which feeds 33 KV
consumers of bus 3. The bus 3 is fed from bus 2 at 33 KV through two circuits in parallel.
There are two breakers and associated relays at the two ends of each circuit. These relays are
A & B and C & D as shown in the figure. Of these, A&C are non directional O/C relays
whereas B and D are directional O/C relays.
When the fault current flows through the relays B and D in the direction of arrow as shown in
the figure, the relays operate and trip associated breakers.
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Case I: For a fault at ‘P’ on one of the circuits, the direction current flow is as shown.
Under this fault condition, the non directional relay A and directional relay B operate and trip
the associated breakers to isolate the fault.
Case II: For a fault at ‘Q’ in the other circuit, the direction of current flow is as shown.
Under the fault condition, the non directional relay C and the directional relay D operate and
trip the associated breakers to isolate the faulty circuit.
Case III: For a fault at ‘W’ in the other circuit, the direction of current flow is as shown.
Under the fault condition, the non directional relay C and A operate and trip the associated
breakers to isolate the faulty circuit. In this condition the directional relay should not work.
Apparatus:
1. Circuit breaker.
2. Protected line.
3. CT 300:1 – 6 no’s
4. PT 33KV/110V – 6no’s
5. Microprocessor based directional O/C Relay.
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Circuit Diagram:
Procedure:
1. Switch ON Substation I & II.
2. Switch ON every breaker of the substations.
3. Connect 100amp load of the substation -II.
4. Draw the circuit diagram of this directional protection.
5. Observe the feeder trip circuit carefully.
6. Connect the directional O/C relay I and II with the communication cable to
the computer.
7. Open the relay software. Activate directional protection Iφ>. Set pick up setting
0.1In and angle offset -100 degree for each relay to set correct coordination.
8. Assign output relay (T1 & A1) and fault LED to trip the breaker for directional
line and directional earth fault from Matrix menu.
9. On the scaling menu give CT ratio 300:1 and PT ratio 33kV/110V.
10. Activate directional phase and earth fault Protection.
11. Assign time 0.2Sec DT of the directional relay Iφ>.
12. Assign time 0.2Sec DT of the directional earth fault relay Ioφ>
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13. Now create fault on feeder I, feeder II and BUS II and observe tripping
operation of the associated breaker.
14. Find magnitude of the fault current on the directional fault recording menu.
NOTE:
• TO RESET ALL LATCHES OF VAMP 52 RELAY PRESS TWICE THE
FUNCTION KEY F1.
Reports:
1. In what conditions is it necessary to use directional protection? Explain how close up
faults affects the relay operation.
2. How does directional protection determine the direction of current?
3. What is relay characteristic angle, and what are usual values for this angle?
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