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Bus Bar Protection

Bus Protection

� The word bus is derived from the Latin

word omnibus which means common

for all.

� It may be noted that under the normal

power flow condition the sum of

incoming currents is equal to the sum

of outgoing currents,

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Bus Protection

� A fault at the busbar is quite rare but can cause enormous damage.

� When protective relays operate to isolate the busbar from the system, there is a large

disruption to- the loads.

� The switchyards, substations and kiosks are well shielded.

� The causes of faults experienced on busbars are: weakening of insulation because of

ageing, corrosion because of salty water, breakdown of insulation because of over

voltages, foreign objects, and so on.

� Because of the low probability of busbar faults, for many years, it was considered

unnecessary to provide explicit protection to busbars. However, as the system voltage

went on increasing and short-circuit capacities went on building up, it was no. longer

advisable to leave busbars unprotected on a primary basis.

Differential Protection of Busbars

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Differential Protection of Busbars

� The CT ratios of all the CTs are equal and are based on the primary current of that feeder

which carries the maximum current

External and Internal Fault

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External and Internal Fault

CT Behavior

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� At low values of primary current, voltage E to be induced by the secondary winding is quite

low.

� The working flux in the CT is also very low. Thus the magnetizing current requirement is

correspondingly low.

� If the primary current increases, initially, the secondary current also increases proportionately.

This causes the secondary induced voltage to increase as well.

� Increased secondary voltage can only be met with an increase in the working flux of the CT.

� As the flux increases, the transformer needs to draw a higher magnetizing current.

� Because of the nonlinear nature of the B-H curve for the CT, as the knee of the excitation

characteristics is passed, any further increase in flux demand causes a disproportionately large

increase in the magnetizing current requirement of the CT.

CT Operation Beyond Knee Point

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Circuit Model of Saturated CT

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External Fault

High Impedance Differential Schemes

� One CT gets

saturated and the

other two are

operating normally

� for simplicity, all

leakage

reactances have

been neglected.

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High Impedance Differential Schemes

� The OC relay needs to be restrained from tripping on external faults

(with one CT completely saturated).

� It can be accomplished by connecting a high resistance (known as

the stabilizing resistance) in series with the OC relay.

� The stabilizing resistance should be of such a value that under the worst

case of maximum external fault and full saturation of one CT, the

current through the OC relay is less than its pick-up value.

� Such schemes are known as high impedance busbar differential

schemes.

Finding Stabilizing Resistance Value

� Let the pick-up value of the OC relay be ���� The value of the resistance associated with the saturated CT be ��� � � �

��. � Assume that the OC relay is not connected. Find the voltage across it will be

��� ���� � ��� � ��

� � ��� �� ���� � ��

� � ��� � ��� ��� Under this condition, the current through the relay be less than or equal to its pick-up value Ipu

����� � ������

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� Assuming n feeders and further assuming that the magnetizing current of each unsaturated CT is same and equal to I0 and noting that the transformation ratio of all the CTs is same and equal to N, the general expression:

��� � � ���

���

���� �� � 1�� ��

Where the subscript k varies from 1 to n - 1, n being the total number of feeders terminating on the bus.\

� Ignoring I0, ∑ "#$

������ is the maximum external fault current up to which the

differential scheme remains stable.

�%,�'�,(�' �� �����

Minimum Internal Fault

� Determine the minimum internal fault current �%,)���*��+,�*)(�*, that can be detected by the high impedance busbar differential scheme.

��� � �%,()�,��-������%,()�,��- �

��������

�������� � ���

�%,()�,��- � ���

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� Minimum Internal Fault Current

�%,()�,��- � ��� � ��� � ��� � ��

� � ��� � �.� � ��.

� �� � �� � �.� � �I01�I02 � I03

Assuming I01 � I02 � I03 � �4� �%,()�,�*)(

� � 3I4���� ��%,()�,�*)( � 3��4�%,()�,�*)( � ���� � 3��4

� For the general case of n feeders terminating on the busbar, the minimum internal fault current

that can be detected by the high impedance busbar differential scheme will be given by

�%,()�,�*)( � ����� � ��4

Stability Ratio

� The stability ratio S of the high impedance busbar differential scheme is defined as the ratio of

maximum external fault current for which the scheme remains stable to the minimum internal

fault current for which it operates.

� � �%,�'��*��+,(�')(�(�%,)���*��+,()�)(�(

� The higher the value of stability ratio S, the better is the quality of differential protection.

� Stability ratios of a few tens are common in EHV busbar differential schemes.