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LESSON 34
BUSBAR PROTECTION
OUTLINE OF THE LESSON
• INTRODUCTION
• BUSBAR FAULTS
• PROTECTION REQUIREMENTS
• TYPES OF PROTECTION SYSTEMS
– FRAME EARTH PROTECTION
– DIFFERENTIAL PROTECTION
INTRODUCTION
• The protection scheme for a
power system should cover the
whole system against all
probable type s of fault.
• Unrestricted forms of line protection
such as overcurrent and distance
systems,meet this requirement,
although faults in the Busbar zone
are cleared only after some time
delay.
• If unit protection is applied to
feeder and plant the busbars are
not inherently protectected
• Busbars have been left without
specific protection for one or more of
the following reasons:
– The busbars and switchgear have high
degree of reliability ,to the point of
being regarded as intrinsically safe.
• It was feared that accidental operation of
busbar protection might cause wide
spread dislocation of the power
system ,which ,if not quickly
cleared ,would cause more loss than
would the very infrequent actual bus faults
• It was hoped that system protection
or back-up protection would provide
sufficient bus protection
BUSBAR FAULTS
• Majority of bus faults involve one
phase and earth, but faults arise
from many causes and a significant
number are inter-phase clear of
earth.
• With fully phase -segregated metal
clad gear ,only earth faults are
possible ,and a protective scheme
need have earth fault sensitivity only.
• For outdoor busbars , protection
schemes ability to respond to inter-
phase faults clear of earth is an
advantage
BUSBAR FAULT STATISTICS
• For a particular system in which total
faults equal to 129 , the distribution is :
– PHASE TO EARTH 87
– TWOPHASE TO EARTH 15
– THREE PHASES TO EARTH 19
– THREE PHASE 6
– UNKNOWN 2
PROTECTION EQUIREMENTS
• Although not basically different from
other circuit protections,the key
position of busbars intensifies the
emphasis put on the essential
requirements of speed and stability
SPEED
• High speed operation is required for
limiting the damage due to fault.
• Removal of bus faults in less time
than could be achieved by back –up
protection ,with the object of
maintaining system stability.
• Most of the modern busbar
protection are based on low
impedance biased or high impedance
unbiased differential protection
systems.The operating time is of the
order of one cycle .
STABILITY• The stability of bus protection is of
paramount importance
• In order to maintain the high order of
integrity needed for bus protection,it is
an almost invariable practice to make
tripping depend on two independent
measurements of fault quantities .
• Security of both stability and
operation is obtained by providing
three independent discriminating
channels,the trip outlets of which are
connected in series and in pairs as
shown below:
TWO OUT OF THREE PRINCIPLE
FIG: Two out of three principle
• Three channels x,y and z from three
independent trip circuits ,x+ y,y+z,and
z+x. This means that any kind of
failure can occur in any one channel
without prejudging either stability or
ability to operate for the scheme as a
whole.
TYPES OF PROTECTION
SCHEMES
• System protection used to cover
bus bars
• Frame –earth protection
• Differential protection
SYSTEM PROTECTION
• A system protection that includes
overcurrent or distance systems will
inherently give protection cover to the
busbars.
• Overcurrent protection will only be
applied to relatively simple distribution
systems,or as a back-up protection set
to give considerable time
delay.Distance protection will provide
cover with its second zone.
• In both cases,therefore ,the busbar
protection so obtained is slow
FRAME-EARTH ROTECTION
• This is purely an earth fault system,and ,in principle ,involves simply measuring the fault current flowing from the switchgear frame to earth.To this end a current transformer is mounted on the earthing conductor and is used to energize a simple instantaneous relay.
FRAME –EARTH PROTECTION
DIFFERENTIAL PROTECTION
• BASIC PRINCIPLE
– Basic Principle
– Summation scheme
BASIC PRINCIPLE
• The merz-price principle is applicable
to multi-terminal zone such as a
busbar.The principle is the direct
application of kirchhoff’s first
law.Usually the circulating current
arrangement is used.
BASIC CIRCULATING CURRENT
SCHEME
Fig Basic Circulating Current Scheme
SUMMATION SCHEME
• By using current transformers of
different ratios, for example
300/1,400/1and 500/1in each three
phase group,connected in parallel and
interconnected between groups ,by
two bus wires only a response is
obtained to all types of fault.
• An alternate arrangement to that
above is obtained by using current
transformers of equal ratio but cross
-connecting two of them and taking a
centre tap from the third.
RECOMMENDATIONS ON BUSBAR
PROTECTION
(Reference: Manual on
PROTECTION of GENERATOR,
GENERATOR TRANSFORMER, and
220 KV and 400KV NETWORKS,
Publication No. 274(revised) CBIP
New Delhi).
1. Busbar protection must be
provided in all new 400 kV and 200
kV substations as well as
generating station switchyards.
2. It shall be of 3-phase type and
operate selectively for each bus
bar section.
3. It shall operate on differential
principle and provide independent
zones of protection for each bus.
4. It shall be stable for through fault
conditions up to maximum 40KA fault
level.
5. It shall incorporate continuous
supervision for C.T. secondaries
against any possible open circuits. In
case of detection of any open circuit of
C.T. secondaries, after a time delay, the
affected zone of protection shall be
rendered inoperative and an alarm will
be initiated
6. It shall include D.C. supply
supervision.
7. Include adequate number of high
speed tripping relays.