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Protection
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Bus Bar ProtectionBus Bar Protection
Outline
Bus arrangements Bus components Bus protection techniques
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
Single bus - single breaker
ZONE 1 ZONE 2
Distribution and lower transmission voltage levels Limited operating flexibility
Multiple bus sections - single breaker with bus tie
ZONE 1
ZONE 2
Transmission and distribution voltage levels Fault on a bus disconnects only the circuits
connected to that bus
Double bus - single breaker with bus tie
ZONE 1
MAIN BUS
TRANFER BUS
Increased operating flexibility A bus fault requires tripping all breakers Transfer bus for breaker maintenance
Main and transfer buses
ZONE 1
ZONE 2
Very high operating flexibility Fault on a bus disconnects only the circuits
connected to that bus Transfer bus for breaker maintenance
Double bus single breaker w/ transfer bus
ZONE 1
ZONE 2
High operating flexibility Line protection covers bus section between two CTs
Double bus - double breaker
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
Breaker-and-a-half bus
Higher voltage levels High operating flexibility with minimum breakers
B1 B2
TB1
L1 L2
L3 L4
TB1
Ring bus
Bus components breakers
SF6, EHV & HV - Synchropuff
Low Voltage circuit breakers
BUS 2
CB 1
BUS 1
ISO 1 ISO 2
ISO 3BYPASS
-+
F1aF1c
Contact Input F1a OnContact Input F1c On
F1b
ISO
LATO
R 1
ISOLATOR 1 OPEN
7B 7A
BUS 1
-
+
F1aF1c
Contact Input F1a OnContact Input F1c On
F1b
ISO
LATO
R 1
ISOLATOR 1 CLOSED
7B 7A
BUS 1
Disconnect switches & auxiliary contacts
BUS 2
CB 1
BUS 1
ISO 1 ISO 2
ISO 3BYPASS
BUS 2
CB 1
BUS 1
ISO 1 ISO 2
ISO 3BYPASS
Current Transformers
Oil insulated current transformer (35kV up to 800kV)
Gas (SF6) insulated current transformer
Bushing type (medium voltage switchgear)
Bus Bar Protection Requirements
High bus fault currents due to large number of circuits connected: 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
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)
With both dependability and security important, preference is always given to security
Bus Protection Techniques
Overcurrent (unrestrained or unbiased) differential
Overcurrent percent (restrained or biased) differential
High-impedance bus differential schemes Low-impedance bus differential schemes
Overcurrent (unrestrained) Differential
Differential signal formed by summation of all currents feeding the bus
CT ratio matching may be required
On external faults, saturated CTs yield spurious differential current
Time delay used to cope with CT saturation
51
Relay
AdvantagesSimpleLow costDifferent ratios of CTs can be used with the help of Aux CT
High operating timeAll CTs must have equal ratioLow sensitivityDegree of CT saturation level to be very low
Disadvantages
High Impedance relay
R
Relay
High Impedance Differential Operating signal created by
connecting all CT secondaries in parallel
o CTs must all have the same ratioo Must have dedicated CTs
Overvoltage element operates on voltage developed across resistor connected in secondary circuit
Accuracy dependent on secondary circuit resistance
o Usually requires larger CT cables to reduce errors higher cost
Cannot easily be applied to reconfigurable buses and offers no advanced functionality
59
AdvantagesIt avoids the problem of unequal CT performances.It uses high impedance voltage relay.Highly sensitive and fast.
DisadvantagesAll CTs must have equal Turns ratioAll CTs must have high Knee point voltage preferably twice the nominal valueThe CT must be exclusively used for differential protection.
Percent 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
5187
Low Impedance Percent Differential( Digital relay) Individual currents sampled by protection and summated digitally
o CT ratio matching done internally (no auxiliary CTs)o Dedicated CTs not necessary
Additional algorithms improve security of percent differential characteristic during CT saturation
Dynamic bus replica allows application to reconfigurable buseso Done digitally with logic to add/remove current inputs from differential
computationo Switching of CT secondary circuits not required
Low secondary burdens Additional functionality available
o Time-stamped event recordingo Breaker failure protection
Relay
Low Impedance Differential (Distributed)
Data Acquisition Units (DAUs) installed in bays
Central Processing Unit (CPU) processes all data from DAUs
Communications between DAUsand CPU over fiber using proprietary protocol
Sampling synchronisation between DAUs is required
Perceived less reliable (more hardware needed)
Difficult to apply in retrofit applications
52
DAU
52
DAU
52
DAU
CU
copper
fiber
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.
52 52 52
CU
copper
Bus bar protections
Standard practice400 kV Duplicated main protection per zone (bus) Two prot connected to different CT cores Preferably, both main protections on different principle Two out of two principle(main 1 & 2) for tripping
220 kV / 132 kV One main protection per zone (bus) Single CT core is switched to respective main zone through switching relays Common check zone for all the buses connected to an independent CT core other
than main protection Tripping based on operation of both main and check zone
27
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