Busbar Protection Scheme, Thesis of Electrical Engineering

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BUSBAR PROTECTION

Dinesh Kumar Sarda A.Manager(MRSS)

CONTENTS

What is a bus bar? Causes of fault Suitable protection Selection of CT ratios Types of faults Overcoming the faults Dot convention or polarity marks Why PS class is preferred over other protection class. Stability ratio Bus Bar Protection drawing of MRSS Why is bus bar protection not in line at MRSS Philosophy of pilot wire supervision relay Conclusion References

CAUSES OF FAULT

• Breakdown of insulation because

of over voltages, foreign objects,

etc

• Weakening of insulation because

of ageing,corrosion,salty water,etc.

Suitable Protection

• (^) Differential protection Why differential protection?
• (^) Terminals of the system are near to each other.
• (^) Hence by installing CT’s on the two sides of the bus, comparison can be made between the current entering it & leaving it. Any difference in current will immediately signal an internal fault.
• (^) The difference in current can be used to excite the coil of a differential relay via CT secondary and thus issue trip commands to CB on both sides of the bus to isolate it.

That means the method of selecting CT ratio on the basis of maximum primary current seen by an individual feeder is not correct.

Correct method of CT ratios selection

TYPES OF FAULTS

Faults may be broadly classified as

External Fault(through fault) & Internal

Fault.

Requirement of a unit protection is that

the differential scheme should respond to

internal faults and should not respond to

external fault.

Internal Fault

The maloperation of bus bar differential

scheme on external faults is caused due

to non ideal behavior of CT carrying

excessive primary current, during fault

conditions.

• when due to external faults one of the CT’s get saturated, the differential relay coil needs to be restrained from tripping.
• We can easily accomplish this by connecting a high resistance known as stabilizing resistance in series with the differential relay coil
• (^) The stabilizing resistance should be of such a value,that under the worst case of maximum external fault and full saturation of CT,the current through differential coil is less than its pick up value and at the same time it should respond to minimum internal fault current.

CORRECTION

DOT CONVENTION

• (^) Dot markings are used to know the

direction of circulating current in the CT

secondary circuit.

• (^) Rule of dot convention says that

“When current enters the dot mark on the

primary side of a CT, the current must

leave the similarly marked dot mark on the

secondary side & vice versa”.

WHY PS CLASS ONLY?

Protection class CT’S such as 5P10,5P20 may produce undesired difference current in the CT secondary due to following reasons 1.Two or more CT’S of class 5P10 may have different accuracy (in this case for 10 times the rated current) 2.Even if the CT’S are identically manufactured, deterioration of core properties over time may differ & they may behave differently over time.

FEATURES OF PS CLASS

1.Here instead of generalizing minimum

saturation level of a CT,the users have to

exactly specify the saturation level of the CT.

This is called as knee point voltage as it

appears as a human knee in CT

magnetization characteristics.

2. This specification will take into account the

maximum through fault current,the actual

lead burden,the relay burden,resistance of

the CT secondary winding & also a factor of

safety.

KNEE POINT CALCULATION

The minimum knee point voltage for a given PS class CT is calculated by: Vkp = KIf(s)(Rct+Rb). Vkp stands for knee point voltage. If(s) stands for maximum through fault current as reflected at the CT secondary terminals=If(s)/CT ratio. Rct stands for CT winding secondary resistance. Rb stands for connected burden(which includes the burden of connecting leads & relay burden. K stands for factor of safety (normally taken as 2). i.e atleast upto 2 times the maximum possible through fault current the CT would not saturate.