Adv. Protection , Fauclty of Engineer Helwan University. Busbar protection LEC 1 Professor. Saady Abdel Hamid Published by : Eng .Rahaf Waheep

1. 1
Professor: Saady Abdel Hamid
}‫توفيقي‬ ‫وما‬‫ﺇ‬‫و‬ ‫توكلت‬ ‫عليه‬ ‫باهلل‬ ‫ال‬‫ﺇ‬‫ل‬‫أنيب‬‫يه‬{
‫الرحيم‬‫الرحمن‬‫هللا‬ ‫بسم‬

2. Bus bars are conducting bars to which a
number of incoming or out going circuits are
connected.
 Each circuit connected to the bus bar will have
certain electrical component such as circuit
breakers, isolators, earth switches, current
transformers and voltage transformers.

3. These components are connected in a
definite sequence such that a circuit
can be switched off during normal
operation by manual command and
also automatically during abnormal

4. Bus – Bar Faults
Bus zone faults can usually be classified under
one of the following heading:
Insulation failure due to material deterioration.
Flash over caused by prolonged and excessive
overvoltage.
Error in the operation and maintenance of
switchgears especially earth switch.
Foreign objects accidentally falling across busbars.

5. Protection Requirements
1. High speed for prompt fault clearance, to
minimize damage and maintain system
stability. It is fast enough to minimize
damage and maintain system stability
2. Stability during external faults to the
switch gear installation, since failure to
stabilize would cause unnecessarily wide
spread interruption of supply. avoiding
mal-operation in case of an external fault

6. Protection Requirements
3. Capability of complete discrimination
between zones to ensure that the minimum
number of circuit breakers are tripped to
isolate the fault. It supervises all CTs, which
provides actuating quantity to the relay in
case of fault

7. Need for Bus-bar protection
• In its absence fault clearance takes place in
Zone-II of distance relay by remote end
tripping (time delay).
• This means slow and unselective tripping and
wide spread black out
Effect of delayed clearance
 Greater damage at fault point
 Indirect shock to connected equipments
like shafts of Generator and windings of
transformer.
7

9. 1 2 3 n-1 n
ZONE 1
- - - -
• Distribution and lower transmission voltage
levels
• No operating flexibility
• Fault on the bus trips all circuit breakers
1- Single bus - single breaker
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Bus arrangements

10. ZONE 1
ZONE 2
• Distribution and lower transmission voltage levels
• Limited operating flexibility
2- Multiple bus sections - single
breaker with bus tie
10

11. ZONE 1
ZONE 2
• Transmission and distribution voltage levels
• Breaker maintenance without circuit removal
• Fault on a bus disconnects only the circuits being
connected to that bus
3- Double bus - single breaker with
bus Coupler
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12. ZONE 1
MAIN BUS
TRANFER BUS
• Increased operating flexibility
• A bus fault requires tripping all breakers
• Transfer bus for breaker maintenance
3- Main and transfer buses
12

13. ZONE 1
ZONE 2
• Very high operating flexibility
• Transfer bus for breaker maintenance
4- Double bus – single breaker w/ transfer bus
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14. ZONE 1
ZONE 2
• High operating flexibility
• Line protection covers bus section between two
CTs
• Fault on a bus does not disturb the power to
circuits
5- Double bus - double breaker
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15. ZONE 1
ZONE 2
• Used on higher voltage levels
• More operating flexibility
• Requires more breakers
• Middle bus sections covered by line or other
equipment protection
6- Breaker-and-a-half bus
15

16. • Higher voltage levels
• High operating flexibility with minimum breakers
• Separate bus protection not required at line
positions
B1 B2
TB1
L1 L2
L3 L4
TB1
7- Ring bus
16

17. During faults their will be high bus fault currents
due to large number of circuits connected. This
lead to:
 CT saturation often becomes a problem as CTs
may not be sufficiently rated for worst fault
condition case
 large dynamic forces associated with bus faults
require fast clearing times in order to reduce
equipment damage
Bus-bar Protection Challenges

18. False trip by bus protection may create serious
problems:
 service interruption to a large number of
circuits (distribution and sub-transmission
voltage levels)
 system-wide stability problems (transmission
voltage levels)
High operation speed required With both
dependability and security important, preference
is always given to security
Bus-bar Protection Challenges

19. CT Saturation Challenge
Faults in busbars are different in the way
that during an external fault, all of the other
circuits connected to the bus contribute to
that fault.
Therefore the current through the breaker
of the faulty circuit will be significantly
higher than that for any of the other circuits.

20. CT Saturation Challenge
3. When this large current flows through CT
some degree of saturation will occur.
4. A saturated CT will not deliver its
appropriate current to the relay. This may
cause the relay to misinterpret the external
fault for an internal fault. The relay must not
misunderstand this current.

21. Note:-
1. DC component of fault current can
saturate the CT a lot more that AC.
2. The L/R ratio of the power-system
impedance, which determines the decay
of the DC component of fault current.
3. L/R ratio should strongly influence the
selection of the bus protective relaying.

22. CT Saturation Concepts
CT saturation depends on a number of factors
• Physical CT characteristics (size, rating,
winding resistance, saturation voltage)
• Connected CT secondary burden (wires +
relays)
• Primary current magnitude, DC offset
(system X/R)
• Residual flux in CT core

23. CT Saturation Concepts
–Actual CT secondary currents may not
behave in the same manner as the ratio
(scaled primary) current during faults
–End result is spurious differential
current appearing in the summation of
the secondary currents which may
cause differential elements to operate if
additional security is not applied

24. CT Saturation
No DC Offset
• Waveform remains fairly symmetrical

25. CT Saturation
With DC Offset
• Waveform starts off being asymmetrical, then
symmetrical in steady state

26. External Fault & Ideal CTs
– Fault starts at t0
– Steady-state fault conditions occur at t1
t0
t1
Ideal CTs have no saturation or mismatch errors thus produce
no differential current

27. External Fault & Actual CTs
– Fault starts at t0
– Steady-state fault conditions occur at t1
t0
t1
Actual CTs do introduce errors, producing some
differential current (without CT saturation)

28. External Fault with CT Saturation
– Fault starts at t0, CT begins to saturate at t1
– CT fully saturated at t2
t0
t1
t2
CT saturation causes increasing differential current that
may enter the differential element operate region.

29. Some Methods of Securing Bus
Differential
1- Block the bus differential for a period
of time (intentional delay)
•Increases security as bus zone will not
trip when CT saturation is present
•Prevents high-speed clearance for
internal faults with CT saturation or
evolving faults

30. Some Methods of Securing Bus
Differential
2- Change settings of the percent differential
characteristic (usually Slope 2) which is
specifically designed to be insensitive to DC
saturation
• Improves security of differential element
by increasing the amount of spurious
differential current needed to incorrectly
trip
• Difficult to explicitly develop settings (Is
60% slope enough? Should it be 75%?)

31. 3- Eliminating the problem by eliminating iron
in the current transformer (a linear coupler
(LC) system)
4. Using a high impedance differential relay
with a series resonant circuit to limit
sensitivity to CT saturation (KAB relay system)
5. Using a Differential Comparator relay with
moderately high impedance to limit sensitivity
to CT saturation (RED-521)

32. Performance of Busbar Protection Scheme
during CT Saturation and Ratio Mismatch
 The iron core CTs are widely used in the
electric power system for the protection of bus
bar.
 These types of CTs are not linear transducers
due to the characteristics of the iron core.

33.  Different levels of saturation occur in almost
all CTs depending on the magnitude of the
fault current being measured and DC
component.

34.  secondary current I2 is sampled into a
discrete-time sequence of values, i2(t), by
a data-acquisition module.
 If a sample of i2(t) is determined
within a saturation, a compensated
sample will be generated by a
compensation algorithm.

35. Finally, the compensated currents
are supplied to protective relays. The
distorted secondary current can be
compensated based on the detection
of the CT saturation period