Prot III Pratc Report2222

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1. IntroductionThis report is about the Practical 3 of the subject of Electrical Protection III. The practical

was conducted in the laboratory of Electrical Protection III on September 23, 2011, under

the assistance and supervision of a Lab Technician.

2. Practical: Relay Settings and Grading of the SimulatorThere were 3 sections for this practical namely Practical 2A, Practical 2B and Practical 2C.

2.11. Objective

3 different relays were be set to operate at various times for a fault at the end of the

simulator line and then to check that they are graded.

2.12. Apparatus:

Connecting cables Analogue simulator Timer CDG36 and 7SJ50 relays

2.13. Methodology

The trip times of the 3 relays were measured for faults at other parts on the simulator

system and these measurements were compared with the trip times for a fault at the end of

the system.

The simulator comprised 3 CDGs relays in a row.

The middle relay was an earth fault relay. The two on the sides were the overcurrent relays

To set the PSM for the middle relay an earth fault was applied on the board( simulator) also

to set the PSM for the overcurrent relays, an overcurrent fault was applied.


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Practical 2A

The Relay was the CDG36 electromecanical overcurrent and earth fault relay. This relay was

connected to the current 1 (CT1) of the simulator. The timer was used to set the relay to

trip after 1 second for a fault at fault point RST5( red-to-yellow-to-blue phases). Thetransformer on the simulator was connected in star-star zero (YY0).

The relay was set and checked to make sure that it tripped after 1 second for an earth fault

on to each phase, a 3 phase fault and a phase to phase fault.

The same procedure was repeated for transformer connections DY11, DD0 and YD1.

Figure 1 shows the connection of the C.Ts on the 3 phrases. All these components are part

of the simulator.

Fig.1. Current Transformer connections for overcurrent and earth fault protection.

After the tests and simulations the following table was drawn for practical 2A only:

Table1. Results.

Transformer

Connection

Type of Fault Trip time(seconds)

DY11 3 phases (RYB) 0.344

2 phases (RB) 1.09

Phase to earth( B-E) 1.57


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DD0 3 phases(RYB) 0.695

2 phases(RY) 1.28

Phase to earth(B-E) NA ( because theres no zero

sequence)YD11 3 phases (RYB) 0.878

2 Phase (RY) 2.0

Phase to earth (B-E) NA (because theres no zero

sequence)

Findings

For transformer connections DD0 and YD11 there was no readings when a phase-to-earth fault was applied. The reason is that there is no neutral point on the secondary

of the transformer for this connection, hence no zero sequence.

For a 3 phase fault applied to all types of transformer connection it was noted thatthe relay trip time was relatively shorter.

For earth faults the relay that tripped was the middle one whereas for overcurrentfault the one that tripped were the two relays on the sides.

Practical 2B

The relay used was the 7SJ50 solid state 3 phase overcurrent and earth fault relay. The relay

was connected to the current transformer 8 (CT 8) on the simulator. Then the timer was

used was used to set the trip time of the relay to trip after 0.7 secs for a fault at fault point

RST5 (red-to-yellow-to-blue phases) with the transformer connected in star-star zero (YY0).

Figure2. Siemens 7SJ50 Tripping Circuit.


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The procedure was repeated also for practical 2A and the following results were obtained:

Table2. For Prac 2B

Transformer

Connection


DD0 3 phases (RYB) 0.5

2 phases (RB) 0.68

Phase to earth( B-E) NA

YD11 3 phases(RYB) 0.63

2 phases(RY) 0.72

Phase to earth(B-E) 0.48

Practical 2C

Another 7SJ50 solid state 3 phase overcurrent and earth fault relay was used. The relay was

connected to CT11 on the simulator and using the time again the relay was set to trip after0.3 secs for a fault at fault point RST5 with the transformer connected in YY0. This Practical

applied for al practical 2A, 2B and 2C.

The following results were obtained:

Table 3. Results for Prac 2A, 2B and 2C.


Position RST5(red, yellow,

blue)

Blue-to-earth 1.3Yellow-to-earth 1,55

Red-to-earth 0.55

Position RST2(

red, yellow,

blue)

3 phases(RYB) 0.35

2 phases(RY) 0.324

Phase to earth(B-E) 0.903


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Position RST1(

red, yellow,

blue)

3 phases (RYB) 0.636

2 Phases (RY) 0.349

Phase to earth (B-E) 0.896

Findings:

Eventually, it was noted that for the different fault position for the phase-to-earthfaults the relay took a bit longer to trip.

Example how to calculate the PSM:

CT Ratio =

Current setting = 125%

Fault current = 60A (Assumed)

IPU = current setting * ICT(RATED SECONDARY) = =1.25A

IF(RELAY COIL) = IF *

PSM =

=

9.6


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Conclusion

From the Practical it can be concluded that the tripping time of relays depend in which

position the relay is placed along a power line. Therefore relays have to correctly graded

depending on the position they are so that they trip accordingly.

Undoubtedly, it can be seen that relays play an extremely important role when it comes to

protecting a part of the power system. They normally placed together with circuit breakers.

Terminology

Pick-up Current: the minimum coil current at which the relay operates

PSM( Plug Setting Multiplier) : the ratio of fault current in the relay coil to the pick-up value.


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Bibliography

Lab technician tips and advice.

Electrical Protection III notesG.F dAlmaine

Principles of power system V.K. Metha 4th

edition

Documents

Prot III Pratc Report2222