(1) Five classical types of busbar protection systems are discussed: system protection, frame-earth protection, differential protection, phase comparison protection, and directional blocking protection. System protection and phase comparison protection are only suitable for small substations, while frame-earth and differential protection are discussed in more detail. (2) Frame-earth protection measures fault current flowing from the switchgear frame to earth. Differential protection compares currents flowing into and out of the busbar and trips if they are not equal. (3) Modern digital differential algorithms aim to improve filtering, response time, restraint techniques, and transient blocking compared to classical schemes.

1. 1
Professor: Saady Abdel Hamid
}‫توفيقي‬ ‫وما‬‫ﺇ‬‫و‬ ‫توكلت‬ ‫عليه‬ ‫باهلل‬ ‫ال‬‫ﺇ‬‫أنيب‬‫ليه‬{
‫ميحرلا نمحرلا هللا مسب‬
Lecture 2

2. Classical TYPES OF PROTECTION SYSTEM
A number of busbar protection systems have been devised:
1- System protection used to cover busbars
2-Frame-earth protection
3- Differential protection
4- Phase comparison protection
5- Directional blocking protection
Of these, (1) is suitable for small substations only, while
(4) and (5) are obsolete. Detailed discussion of types (2)
and (3) occupies most of this chapter.

3. 1-System protection used to cover busbars
In systems where over-current or distance protection
systems are present, busbars will be protected.
It should be noted that over-current protection will
only be applied to relatively simple distribution
systems, or as a back-up protection, which gives a
considerable time delay, whereas distance protection
provides cover for busbar faults in its second and
possibly subsequent zones.
In Any case, the protection acquired
is slow and unsuitable.

4. 2- Frame-Earth Protection
This method has been extensively used in the past.
Various schemes are available for this type of
protection each having a certain capability.
Many of them are still in existence and each can
provide good service for a particular situation.
However, the need to insulate the switchboard frame
and provide cable gland insulation and the
availability of alternative schemes using numerical
relays, has contributed to a decline in use of
frame leakage systems.

5. Frame-Earth Protection (Single-Busbar)
This protection scheme is basically an earth fault
system which simply measures the fault current
flowing from the switchgear frame to earth. A CT is
mounted on the earthing conductor and is used to
energize a simple instantaneous relay as shown in
Figure. Meanwhile, no other earth connections of any
type, including incidental connections to structural
steelwork are allowed. This guarantees that:
1. The principal earth connection and current
transformer are not shunted, thereby raising the
effective setting. An increased effective setting gives
rise to the possibility of relay maloperation. This risk
is small in practice.

6. 2. Earth current flowing to a fault elsewhere on
the system cannot flow into or out of the
switchgear frame via two earth connections, as
this might lead to a spurious operation.
Careful construction of the system is of most
importance in this case, as the switchgear must
be insulated from ground, usually by standing it
on concrete and the foundation bolts must not
touch the steel reinforcement.

8. Frame Leakage Protection

9. Note:- The switchgear must be insulated as a
whole, usually by standing it on concrete. Care
must be taken that the foundation bolts do not
touch the steel reinforcement; sufficient
concrete must be cut away at each hole to
permit grouting-in with no risk of touching
metalwork.
The insulation to earth finally achieved will not
be high, a value of 10 ohms being satisfactory.
(very important)

11. Under external fault conditions
the current I1 flows through the frame-
leakage current transformer. If the
insulation resistance is too low, sufficient
current may flow to operate the frame-
leakage relay.
The earth resistance between the
earthing electrode and true earth is
seldom greater than 1Ω , So
.

12. so with 10Ω insulation resistance the
current I1 is limited to 10% of the total
earth fault current I1 and I2.
For this reason, the recommended
minimum setting for the scheme is
about 30% of the minimum earth fault
current.

14. 14
CT CB
Busbar
CT CB
CT CB
CT CB
Busbar protection
Kirchhoff’s law 1
I1 + I2 + I3 + I4 = 0
Protection System
3- Differential protection for Bus-Bar

15. A busbar protection system will trip all
breakers of the connected objects if there
is a fault on the busbar.
The CTs of all connected objects give
signal to the protection system, and it will
be activated if the sum of the currents
flowing to the busbar is not zero.

16. Diff. relay
1000/5 1000/5 1000/5
3.5 A 2.5 A1 A
500 A200 A700 A
SINGLE BUS System Protection
16
Case of using the
same CT ratio
CT ratio should be
taken according to
the highest SC
current in any
feeder
Problem associated with all differential protection
Small current
flow (1 A) can
not accurate it
lies on zone of
residual flux

17. Diff. relay
1000/5 200/5 500/5
3.5 A 5 A5 A
500 A200 A700 A
0.7 A 0.2 A 0.5 A
5/1 5/0.2 5/0.5
SINGLE BUS System Protection
17
Case of using
different CT ratio
Feeder CT ratio
should be taken
according to SC
current in each
feeder
Required auxiliary CT

18. This type of protection is mainly accomplished by
different schemes such as:-
Circulating Current Differential Protection Scheme,
Biased Percentage Differential Protection Scheme,
High Impedance Voltage Scheme,
Moderately High Impedance Scheme and
Protection using Liner Couplers.
Each of this method is discussed
Differential protection for Bus-Bar

19. The main requirement of this scheme is that the CTs
should be of the same ratio or matching CTs are required.
The main drawback of this scheme is its maloperation
because of production of error current due to CT
saturation and also due to transient DC component
1`- Circulating Current
Protection

20. Circulating Current
Protection

21. The main requirement of this scheme is
that the CTs should be of the same ratio or
matching CTs are required.
The main drawback of this scheme is its
maloperation because of production of error
current due to CT saturation and also due to
transient DC component

22. The scheme may consist of a single relay connected to
the bus wires connecting all the current transformers in
parallel, one set per circuit, associated with a particular
zone, as shown in Figure (a). This will give earth fault
protection for the busbar. This arrangement has often been
thought to be adequate.
If the current transformers are connected as a balanced
group for each phase together with a three-element relay, as
shown in Figure (b), additional protection for phase faults
can be obtained.
The phase and earth fault settings are identical, and this
scheme is recommended for its ease of application and good
performance.
Circulating Differential Protection Scheme,

25. 2- Biased Percentage Differential Protection:
In order to avoid the problem of maloperaration of relays
due to CT saturation and transient DC current, biased
differential protection scheme is used. Maximum security
for external faults is obtained when all CTs have the same
ratio.

26. Biased Percentage Differential
• Percent characteristic used
to cope with CT saturation
and other errors
• Restraining signal can be
formed in a number of
ways
• No dedicated CTs needed
• Used for protection of re-
configurable buses
possible

27. The main advantages of this scheme are:-
 high tolerance against substantial CT
saturation,
reduced requirement of dedicated CTs and
its use where comparatively high speed
tripping is required.
The most important limitation of the said
scheme is that the relay may maloperate in
case of a close-in external fault due to
complete saturation of CT.

28. 3- High Impedance Voltage Scheme:
This scheme is used to overcome the problem of
spill current due to CT saturation in case of
external fault.
The effect of saturation is controlled by
keeping CT secondary & lead resistance low
& by adding resistance into relay circuit.
Here, full wave bridge rectifier adds
substantial resistance to that leg of circuit.

29. The series L-C circuit is turned to 50 Hz
fundamental frequency in order to
respond only fundamental component of
current and make over voltage relay
immune to DC offset and harmonics.
This scheme discriminates between
internal and external faults by the relative
magnitudes of the voltage across the
differential junction points.

31. High Impedance Differential
–Operating signal created by connecting all

32. •CTs must all have the same ratio
•Must have dedicated CTs
–Overvoltage element operates on voltage
developed across resistor connected in
secondary circuit
•Requires varistors or AC shorting relays
to limit energy during faults
–Accuracy dependent on secondary circuit
resistance
•Usually requires larger CT cables to
reduce errors  higher cost

33. High Impedance Voltage Scheme(cont.)
The main merits of the said scheme are
stability against transient DC component due to tuned
circuit
improved CT saturation characteristics because of
stabilizing resistors
Faster operating time.
The disadvantages of this scheme are.
• requirement of dedicated CTs (cost increases),
• maloperation of relay in case when the secondary leakage
reactance is present
• inapplicability of the scheme to re-configurable busbars.

36. 4- Moderately High Impedance Scheme:
This scheme is a combination of high
impedance voltage relay and the
percentage differential relay.
But the prime limitation of this scheme is
that it requires special type of auxiliary
transformer.

37. 5- Protection Using Linear Couplers:
Linear coupler is a special device which
requires low energy relay.
This may create problems with change in
bus configuration.
However, the need of extra equipments in
order to achieve benefits of
microprocessor based relays and high cost
are the main drawbacks of this scheme

38. 5- Protection Using Linear Couplers:
Problem of CT saturation in case of iron core
CT is rectified using linear couplers (air core
mutual reactors) as they use air core.
The secondary of all linear couplers are
connected in series as shown.
The output voltage of linear coupler is
proportional to the derivative of the input
current.
If the voltage sum across relay is zero then
input current is equal to output current at
bus. (normal condition and external faults)

39. 59
Linear Couplers
ZC = 2  – 20  - typical coil impedance
(5V per 1000Amps => 0.005 @ 60Hz )
If = 8000 A
40 V 10 V 10 V 0 V 20 V
2000 A 2000 A 4000 A0 A
0 V
External
Fault
External faults

40. 59
Linear Couplers at internal Fault
Esec= Iprim*Xm - secondary voltage on relay terminals
IR= Iprim*Xm /(ZR+ZC) – minimum operating current
where,
Iprim – primary current in each circuit
Xm – liner coupler mutual reactance (5V per 1000Amps => 0.005 @ 60Hz )
ZR – relay tap impedance
ZC – sum of all linear coupler self impedances
If = 8000 A
0 A
0 V 10 V 10 V 0 V 20 V
40 V
2000 A 2000 A 4000 A0 A
Internal Bus
Fault
Internal fault

41. During an internal fault, all line current
flows toward bus and thus the induced
voltage appears across the relay.

42. • Fast, secure and proven
• Require dedicated air gap CTs, which may
not be used for any other protection
• Cannot be easily applied to reconfigurable
buses
• The scheme uses a simple voltage detector
– it does not provide benefits of a
microprocessor-based relay (e.g.
oscillography, breaker failure protection,
other functions)
Linear Couplers

43. 43
- Voltage transformer on busbar
• Synchronizing
– Phase position
• Over voltage protection
– Ex. No load line
• Under voltage
protection
– Ex. Overloaded line
• Voltage indication
E
C
Synch.
relay
7/19/2016

44. Digital Differential Algorithm Goals
– Improve the main differential algorithm operation
• Better filtering
• Faster response
• Better restraint techniques
• Switching transient blocking
– Provide dynamic bus replica for reconfigurable bus bars
– Dependably detect CT saturation in a fast and reliable
manner, especially for external faults
– Implement additional security to the main differential
algorithm to prevent incorrect operation
• External faults with CT saturation
• CT secondary circuit trouble (e.g. short circuits)

45. P-based Low Impedance Differential
(Distributed)
– Data Acquisition Units (DAUs)
installed in bays
– Central Processing Unit (CPU)
processes all data from DAUs
– Communications between
DAUs and CPU over fiber using
proprietary protocol
– Sampling synchronisation
between DAUs is required
– Perceived less reliable (more
hardware needed)
– Difficult to apply in retrofit
applications

46. P-based Low Impedance Differential
(Centralized)
– All currents applied to a single
central processor
– No communications, external
sampling synchronisation
necessary
– Perceived more reliable (less
hardware needed)
– Well suited to both new and
retrofit applications.