BUSBAR PROTECTION

Dinesh Kumar Sarda A.Manager(MRSS)

CONTENTSWhat is a bus bar?Causes of faultSuitable protectionSelection of CT ratiosTypes of faultsOvercoming the faultsDot convention or polarity marksWhy PS class is preferred over other protection class.Stability ratioBus Bar Protection drawing of MRSSWhy is bus bar protection not in line at MRSSPhilosophy of pilot wire supervision relayConclusionReferences

What is a bus bar?

• Bus is derived from Latin word

“OMNIBUS” (common for all)

• Nerve centre of the power system

where various

circuits are

connected together.

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.

Wrong method of CT ratios selection

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

Selection of CT Ratios

CT ratios for all CT’s in bus differential schemes is = (max out of all feeder currents/1Aor 5A)

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

External 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

External & Internal Fault 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.

3. Distance of incoming side CT & outgoing side CT may lead to difference in lead lengths, thus imposing additional burden on CT’s, which may in turn shift the saturation levels of the CT’s.

Owing to many other similar factors contributing to maloperation of unit protection scheme when conventional protection class CT’s( such as 5P10) are employed, usage of a special protection class of CT known as

“PS” class became popular.

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 = K*If(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.

Stability Ratio

Ratio of max. ext. fault current for which the scheme remains stable to the min. int. fault current for which it operates

S = (IF,EXT,MAX)/(IF,INT,MIN)

PILOT WIRE SUPERVISION

USE OF METROSIL• In some applications of high impedance

relays, the maximum internal fault current can develop high voltages, that can damage the relay insulation.

• A range of "METROSILS”(nonlinear resistance) of 150 mm dia discs have been developed to limit the voltage to safe levels.

• Choice of “METROSILS” depend on relay setting, voltage & maximum internal fault current

• Correct METROSIL selection ensures that best protection is obtained, while maintaining a metrosil leakage current low enough to have negligible effect on the accuracy of the relay at its maximum setting voltage.

• Single disc METROSILS are suitable for secondary internal fault current up to 50A rms.

• METROSILS with multiple discs connected in parallel are used for large fault currents.

Why BUS BAR PROTECTION

in MRSS is not in line currently?

Conclusion

• Thus it is concluded that the bus bar protection is an important part of the power system, as the system voltage has been increasing and short circuit capacities are building up.

• So, it is not advisable to leave the bus bars unprotected on a primary basis.

REFERENCES

• Fundamentals of power system protection

by Y.G.Paithankar & S.R.Bhide

• WWW.GOOGLE.COM