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