Bus bars are the nerve center of a power system where various circuits are connected. Differential protection is suitable for bus bars since terminals are near each other, allowing comparison of current entering and leaving via CTs. Any difference signals an internal fault and causes the relay coil to trip circuit breakers on both sides, isolating the bus. CT ratios for bus differential schemes equal the maximum feeder current divided by 1 or 5 amps. External faults may cause maloperation if a CT saturates, but a stabilizing resistance can restrain the relay. Dot convention defines the direction of current flow in CT secondaries. Only class PS CTs should be used to avoid undesired difference currents. Differential protection is important to protect bus bars

1. TOPIC :-TOPIC :-BUSBARBUSBAR
PROTECTIONPROTECTION
SUBMITTED TO : -SUBMITTED TO : -
MR.MANTOSH KUMARMR.MANTOSH KUMAR PRESENTED BY : -PRESENTED BY : -
RAGHVENDRA KUMARRAGHVENDRA KUMAR
PANDITPANDIT
COURSE :-COURSE :-B.TECH 3B.TECH 3RDRD
YEARYEAR
SECTION :-SECTION :-A2A2
Department of Electrical Engineering

2. What is a bus bar?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.
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3. Cause of FaultCause of Fault
Breakdown of insulation because of over
voltages, foreign objects, etc
Weakening of insulation because of ageing
,corrosion ,salty water,etc.
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4. Suitable ProtectionSuitable 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.
Department of Electrical Engineering

5. Selection of CT RatiosSelection of CT Ratios
 That means the method of selecting CT ratio
on the basis of maximum primary current seen
by an individual feeder is not correct.
CT ratios for all CT’s in bus differential schemes
is = (max out of all feeder currents/1Aor 5A)
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6. Types of FaultsTypes of Faults
Faults may be broadly classified as : -
a) External Fault(through fault) &
b)Internal Fault.
 Requirement of a unit protection is that the
differential scheme should respond to internal
faults and should not respond to external
fault.
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7. Internal FaultInternal Fault
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8. External FaultExternal 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.
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9. CorrectionCorrection
 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.
Department of Electrical Engineering

10. Dot ConventionDot 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”.
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11. Why PS Class Only ?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.
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12. ConclusionConclusion
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.
Department of Electrical Engineering

13. THANK
THANK
YOYO
UU
Department of Electrical Engineering