P1_c7.doc

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CHAPTER SEVENBASIC DIFFERENTIAL PROTECTION1.0Introduction

1.1Differential Protection is a form of protection whereby a relay operates when the vector difference of two or more similar electrical quantities exceeds a predetermined amount.

1.2This relay is called a Differential Relay and may take on a variety of forms depending upon the equipment being protected.

1.3Almost any type of relay when connected in a certain way can be made to operate as a Differential relay. In other words it is not so much as the construction of the relay, but the manner in which the relay is connected in a circuit that makes it a differential relay.

2.0Basic Differential Relaying2.1The basic scheme of differential relaying is explained with reference to the diagram below:-

2.2The protected equipment may be a length of a circuit or a winding of a generator, or a portion of the bus etc. A C.T is connected at either end of the protected equipment. The secondaries of the C.Ts are interconnected as shown with a relay which may be an over-current relay.

2.3Now suppose current flows through the protected equipment to an external load or to a fault at X. If the two C.Ts have an identical ratio, and are properly connected with respect to polarity, then the secondary induced currents i1 and i2 will merely circulate between the two C.Ts and no current flows through the relay.

2.4However if an internal fault should develop in the protected equipment or between the primaries of the two C.Ts then the currents I1 and I2 will be different. The current I1 will be the sum of current I2 and fault current If. Accordingly the C.T secondary currents will be different and the difference in the (i1-i2) flows through the relay causing the relay to operate.

2.5This relay which operates on the vector difference of the current entering and leaving protected equipment is called a differential relay and the scheme of protection as Differential Protection.

3.0Types of Differential Protection or RelayingThere are two basic types of Differential Relaying namely:-

a) Current Differential Relayingb) Voltage Differential Relaying

3.1Current Differential RelayingThis is also called the current balance method or circulating current method of Differential Relaying. The principle of this method has already been described in paragraph 2.0 above. Most differential relay applications are of the Current Differential Method and the method is applied for the protection of Generators, Transformers, Motors and Bus bars.

3.2Voltage Differential RelayingThis is called voltage balance method or opposed voltage relaying. The two C.Ts at either end of the protected equipment are cross connected with the relay in series as shown below:

Let Z be the impedance of the relay coil.

Then voltage drop produced due to secondary current i1 in Z:V1=i1 Z.

Similarly the voltage dropV2=i2 Z

When current i1 and i2 are equal the voltage drop V1 and V2 are equal and opposed and the relay does not operate.

The relay operates when the vector difference in the voltage drop exceeds the pick up value of the relay. Opposed voltage method of Differential Relaying is generally employed for the protection of transmission lines and feeders in A.C. wire Pilot Relaying.

4.0Biased Differential Protection or Percentage differential Protection4.1This is the most extensively used form of differential protection. It is essentially the same scheme as described in paragraph 2.0 above except that restraining coils are introduced in the C.T secondary circuit as shown below:-The differential current to operate this relay is a variable quantity owning to the effect of the restraining coil. The differential current in the operating coil is proportional to (I1 I2).

4.2The development of the percentage differential relay was necessitated to take care of the following:-

a) Although C.Ts of identical ratios are used, their performance during through faults when the C.Ts are saturated cannot be ensured to be the same. The errors introduced may be different with the result an operating current will flow.

Example:

I1=I2=2100 A

i1=2100 x 5 (3 x 2100 x 5)

300 100 300

=35 1.05

=33.95 A

i2 =2100 x 5 + (6 x 2100 x 5)

300

100300

=35 + 2.1=37.1 A

Difference in currents =i2i1=37.133.95

=3.15 A

This difference in current may be sufficient to cause operation in the 5A relay unless the pick up value is greater than 3.15 A.

It should also be noted that no two C.Ts however identical they may be in so far as their secondary currents are concerned will give exactly the same secondary current for the same primary current. These discrepancies may be traced to manufacturing variations and to differences in secondary loading caused by unequal length of leads between C.T and relay, unequal burden of meters and instruments connected in one or both secondaries.b) Another example of an on load tap changing Transformer is taken to study the effects on the differential circuit.

At normal tap of 330KV/132KV

Primary full load current=80 x 106 _____3 x 330 x 103=140 A

Primary C.T. Ratio

=140/1C.T. secondary current of primaryi1=140 x 1_140

=1.0 A

Secondary full load current

=80 x 106 _____3 x 132 x 103=350 A

Secondary C.T. Ratio=350/1

C.T. secondary current of secondaryi2=350 x 1__350

=1.0A

Since i1 = i2 no current flows in the differential relay and therefore the relay does not operate. Let us now assume that the primary incoming voltage is 310 KV and the OLTC gear is operated to raise the secondary voltage to say 140 KV.

Then primary full load current=80 x 106 _____3 x 310 x 103=149 A

C.T secondary current of primary i1=149.00 x 1 140

i1=1.06 A

Secondary full load current

=80 x 106 _____3 x 140 x 103=330 A

C.T secondary current of secondaryi2=330 x 1__350

=0.94 A

Now the difference in the currents between (i1 and i2)

=(1.06 0.94) A=0.120 A

This current of 0.120A flowing in a 1 A relay may cause operation of the differential relay unless the pick up value of the relay is raised to be beyond 0.120 Amps. It is also not practical to keep on raising the pick up value whenever a tap changing operation is carried out.

4.3Thus to obviate all these practical difficulties, the percentage differential relay was developed. The current flowing through the restraining coil or windings is called the Through Current and the current flowing through the operating winding is called the Spill Current.

4.4This spill current necessary to operate the relay expressed as a percentage of the through current is called the percentage bias.

% bias=Spill current for relay operation x 100

Through fault current causing it

4.5Bias is provided on both the restraining windings by a plug setting bridge in electromagnetic relays and in static relays by a rotary switch. Bias settings are usually from 10% to 80% in multiples of 10% or 20% to 80% in multiples of 20% or sometimes in multiples of 15%.

4.6The number of turns on both the restraining windings are always the same so that in effect it can be considered as one winding, with the operating coil or winding connected at its mid point.

Let N be the turns in the restraining winding.

Then the restraining torque produced is:=I1 N+I2 N 2

2

=(I1 + I2) N

2

Or Restraining Torque is I1 + I2 2

4.7The operating characteristic of such a relay is as shown below:

4.8It can be seen that (I1 + I2)/2 is the average of the two currents I1 and I2. Specifically the term through current is used to designate I2 as it is this current which flows in the circuit from one end to the other and also causes the relay operation. Hence the characteristic is also plotted with I2 as abscissa instead of (I1 + I2)/2.

4.9The operating characteristic is a straight line indicating that the spill or operating current is a fixed percentage of the through current. Hence the name Percentage Differential

5.0ApplicationPercentage Differential relays are almost always used as the primary protection device for generators, transformers, motors and other costly electrical apparatus in industry.

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