2. 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
3. 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.
4. CAUSES OF FAULT
• Breakdown of insulation because
of over voltages, foreign objects,
etc
• Weakening of insulation because
of ageing,corrosion,salty
water,etc.
5. 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.
9. Selection of CT Ratios
CT ratios for all CT’s in bus
differential schemes is = (max out
of all feeder currents/1Aor 5A)
10. 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.
13. 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.
14. CORRECTION
• 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.
16. 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”.
17. 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.
18. 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.
19. 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.
20. 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.
21. 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)
26. 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
27. • 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.
28. Why BUS BAR
PROTECTION
in MRSS is not in line
currently?
29. 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.
