High Low Impedance BusBar Protection

Fundamentals of Bus Bar Protection GE Multilin

Outline • Bus arrangements

• • • •

Bus components Bus protection techniques CT Saturation Application Considerations:  High impedance bus differential relaying  Low impedance bus differential relaying  Special topics 2 GE Consumer & Industrial Multilin Oct 31, 2009

Single bus - single breaker

• Distribution and lower transmission voltage levels • No operating flexibility • Fault on the bus trips all circuit breakers 3 GE Consumer & Industrial Multilin Oct 31, 2009

Multiple bus sections - single breaker with bus tie

• Distribution and lower transmission voltage levels • Limited operating flexibility

4 GE Consumer & Industrial Multilin Oct 31, 2009

Double bus - single breaker with bus tie

• Transmission and distribution voltage levels • Breaker maintenance without circuit removal • Fault on a bus disconnects only the circuits being connected to that bus 5 GE Consumer & Industrial Multilin Oct 31, 2009

Main and transfer buses

• Increased operating flexibility • A bus fault requires tripping all breakers • Transfer bus for breaker maintenance


ble bus – single breaker w/ transfer

• Very high operating flexibility • Transfer bus for breaker maintenance 7 GE Consumer & Industrial Multilin Oct 31, 2009

Double bus - double breaker

• High operating flexibility • Line protection covers bus section between two CTs • Fault on a bus does not disturb the power to circuits


Breaker-and-a-half bus

• Used on higher voltage levels • More operating flexibility • Requires more breakers • Middle bus sections covered by line or other equipment protection


Ring bus

L1

L2

TB1

B1

B2

TB1

L3

L4

• Higher voltage levels • High operating flexibility with minimum breakers 10 GE Consumer & Industrial Multilin Oct 31, 2009

• Separate bus protection not required at

Bus components

breakers

BUS 1

BUS 2

ISO 1

ISO 2

Low Voltage circuit breakers CB 1 ISO 3 BYPASS

SF6, EHV & HV Synchropuff


Disconnect switches & auxiliary contacts BUS 1

BUS 2

ISO 1

ISO 2

BUS 1

CB 1

1

ISO 3 BYPASS


Current Transformers BUS 1

BUS 2

ISO 1

ISO 2

Gas (SF6) insulated current transformer

CB 1 ISO 3 BYPASS

Oil insulated current transformer (35kV up to 800kV)

Bushing type (medium voltage switchgear)


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 • Interlocking schemes • Overcurrent (“unrestrained” or “unbiased”) differential • Overcurrent percent (“restrained” or “biased”) differential • Linear couplers • High-impedance bus differential schemes • Low-impedance bus differential schemes


Interlocking Schemes

BLOCK

50

50

50

50

50

50

• Blocking scheme typically used • Short coordination time required • Care must be taken with possible saturation of feeder CTs • Blocking signal could be sent over communications ports (peer-to-peer) • This technique is limited to simple one-incomer distribution buses


Overcurrent (unrestrained) Differential • Differential signal formed by •

51

•

• •

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 Instantaneous differential OC function useful on integrated microprocessorbased relays


Linear Couplers

ZC = 2 Ω – 20 Ω - typical coil impedance (5V per 1000Amps => 0.005Ω @ 60Hz )

40 V

10 V

10 V

0V

20 V

0V

59

External Fault If = 8000 A 2000

2000 A

0

4000


Linear Couplers

Esec = Iprim *Xm - secondary voltage on relay terminals IR= Σ Iprim *Xm /(ZR+Σ ZC) – minimum operating current where, Iprim – primary current in each circuit Xm – liner coupler mutual reactance (5V per 1000Amps => 0.005Ω @ 60Hz ) ZR – relay tap impedance Σ ZC – sum of all linear coupler self impedances If = Internal Bus 8000 A

Fault

40 V 0V

0

10 V

2000

10 V

2000

0V

0

59

20 V

4000


Linear Couplers • 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) 20 GE Consumer & Industrial Multilin Oct 31, 2009

High Impedance Differential • Operating signal created by connecting all CT secondaries in parallel CTs must all have the same ratio o Must have dedicated CTs o

• Overvoltage element operates on voltage developed across resistor connected in secondary circuit 59

o

Requires varistors or AC shorting relays to limit energy during faults

• 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


Percent Differential

I DIF = I1 + I 2 + ... + I n I RES = I1 + I 2 + ... + I n

• Percent characteristic used to cope with CT saturation and other errors • Restraining signal can be formed in a number of ways • No dedicated CTs 87 51 needed • Used for protection of re-configurable buses possible

I RES = max ( I1 , I 2 , ..., I n )


Low Impedance Percent Differential • 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 buses o Done digitally with logic to add/remove current inputs from

differential computation o Switching of CT secondary circuits not required

• Low secondary burdens • Additional functionality available o Digital oscillography and monitoring of each circuit connected to bus

zone o Time-stamped event recording o Breaker failure protection


Digital Differential Algorithm Goals

• Improve the main differential algorithm operation o Better filtering o Faster response o Better restraint techniques o 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 o External faults with CT saturation o CT secondary circuit trouble (e.g. short circuits)


Low Impedance Differential (Distributed) • Data Acquisition Units (DAUs) 52

52 DAU

•

52 DAU

DAU

•

• • CU copper

•

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

f ib e r


Low Impedance Differential (Centralized) 52

52

52

CU

• 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.

copper


CT Saturation


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


CT Saturation No DC Offset • Waveform remains fairly symmetrical Ratio Current

CT Current

With DC Offset

Ratio Current

CT Current

• Waveform starts off being asymmetrical, then symmetrical in steady state 29 GE Consumer & Industrial Multilin Oct 31, 2009

differential

External Fault & Ideal CTs

t1 t0

r e s t r a in in g

• Fault starts at t0 • Steady-state fault conditions occur at t1

Ideal CTs have no saturation or mismatch errors thus produce no differential current


differential

External Fault & Actual CTs

t1

t0

r e s t r a in in g

• Fault starts at t0 • Steady-state fault conditions occur at t1

Actual CTs do introduce errors, producing some differential current (without CT saturation)


External Fault with CT Saturation

differential

t2

t1

t0

r e s t r a in in g

• Fault starts at t0, CT begins to saturate at t1 • CT fully saturated at t2

CT saturation causes increasing differential current that may enter the differential element operate region.


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

evolving faults

• Change settings of the percent differential characteristic (usually Slope 2) o Improves security of differential element by increasing the amount of

spurious differential current needed to incorrectly trip o Difficult to explicitly develop settings (Is 60% slope enough? Should it be 75%?)

• Apply directional (phase comparison) supervision o Improves security by requiring all currents flow into the bus zone before

asserting the differential element o Easy to implement and test o Stable even under severe CT saturation during external faults


HighImpedance Bus Differential


High Impedance Voltageoperated Relay

• 59 element set above max possible voltage External Fault developed across relay during external fault causing worst case CT saturation • For internal faults, extremely high voltages (well above 59 element pickup) will develop across relay


High Impedance Voltage Operated Relay Ratio matching with

• ApplicationCTs of high impedance differential relays Multi-ratio with CTs of different ratios but ratio matching taps is possible, but could lead to voltage magnification. • Voltage developed across full winding of tapped CT does not exceed CT rating, terminal blocks, etc.


High Impedance Voltage Operated Relay Ratio matching with

• Use of auxiliary Multi-ratio CTs CTs to obtain correct ratio matching is also possible, but these CTs must be able to deliver enough voltage necessary to produce relay operation for internal faults.


Electromechanical High Impedance Bus Differential Relays • • • •

Single phase relays High-speed High impedance voltage sensing High seismic IOC unit


µ P -based High-Impedance Bus Differential Protection Relays

Operating time: 20 – 30ms @ I > 1.5xPKP


High Impedance Module for Digital Relays

RST = 2000Ω - stabilizing resistor to limit the current through the relay, and force it to the lower impedance CT windings. MOV – Metal Oxide Varistor to limit the voltage to 1900 Volts 86 – latching contact preventing the resistors from overheating after the fault is detected


High-Impedance Module + Overcurrent Relay


High Impedance Bus Protection Summary

• Fast, secure and proven • Requires dedicated CTs, preferably with the same CT ratio and using full tap • Can be applied to small buses • Depending on bus internal and external fault currents, high impedance bus diff may not provide adequate settings for both sensitivity and security • Cannot be easily applied to reconfigurable buses • Require voltage limiting varistor capable of absorbing significant energy • May require auxiliary CTs • Do not provide full benefits of microprocessor-based relay system (e.g. metering, monitoring, oscillography, etc.) 42 GE Consumer & Industrial Multilin Oct 31, 2009

LowImpedance Bus Differential


µ P-based Low-Impedance Relays

• No need for dedicated CTs • Internal CT ratio mismatch compensation

• Advanced algorithms supplement percent differential protection function making the relay very secure • Dynamic bus replica (bus image) principle is used in protection of reconfigurable bus bars, eliminating the need for switching physically secondary current circuits • Integrated Breaker Failure (BF) function can provide optimal tripping strategy depending on the actual configuration of a bus bar


Small Bus Applications 2-8 Circuit Applications • Up to 24 Current Inputs • 4 Zones • Zone • Zone • Zone • Zone

1 2 3 4

= = = =

Phase A Phase B Phase C Not used

• Different CT Ratio Capability for Each Circuit • Largest CT Primary is Base in Relay


Medium to Large Bus 9-12 Circuit Applications Applications

• Relay 1 - 24 Current Inputs • 4 Zones • Zone 1 = Phase A (12 currents) • Zone 2 = Phase B (12 currents) • Zone 3 = Not used • Zone 4 = Not used

• Relay 2 - 24 Current Inputs • 4 Zones • Zone 1 = Not used • Zone 2 = Not used • Zone 3 = Phase C (12 currents) • Zone 4 = Not used

• Different CT Ratio Capability for Each Circuit • Largest CT Primary is Base in Relay

CB 11

CB 12


Large Bus Applications

87B phase A 87B phase B 87B phase C

Logic relay (switch status, optional BF) 47 GE Consumer & Industrial Multilin Oct 31, 2009

Large Bus Applications For buses with up to 24 circuits


Summing External Currents

Not Recommended for Low-Z 87B relays C T -1

• Relay becomes combination of restrained and unrestrained elements •In order to parallel CTs:

C T -2

I 3 =0

I 2 =0

I 1 = Error

C T -3

C T -4

ID

IF F

IR

ES T

= E rro r = E rro r

M a lo p e r a tio n if E rro r > P IC K U P

• CT performance must be closely matched o Any errors will appear as differential currents • Associated feeders must be radial o No backfeeds possible • Pickup setting must be raised to accommodate any errors


Definitions of Restraint Signals iR = i1 + i2 + i3 + ... + in

“sum of”

1 iR = ( i1 + i2 + i3 + ... + in n iR = n i1 ⋅ i2 ⋅ i3 ⋅ ... ⋅ in

iR = Max ( i1 , i2 , i3 ,..., in

)

“scaled sum of”

“geometrical average”

)

“maximum of” 50 GE Consumer & Industrial Multilin Oct 31, 2009

“Sum Of” vs. “Max Of” Restraint Methods “Sum Of” Approach • More restraint on external faults; less sensitive for internal faults • “Scaled-Sum Of” approach takes into account number of connected circuits and may increase sensitivity • Breakpoint settings for the percent differential characteristic more difficult to set

“Max Of” Approach • Less restraint on external faults; more sensitive for internal faults • Breakpoint settings for the percent differential characteristic easier to set • Better handles situation where one CT may saturate completely (99% slope settings possible)


Bus Differential Adaptive Approach

differential

R e g io n 2 ( h ig h d if f e r e n t ia l c u rre n ts )

R e g io n 1 ( lo w d if f e r e n t ia l c u rre n ts )

r e s t r a in in g 52 GE Consumer & Industrial Multilin Oct 31, 2009

Bus Differential Adaptive Logic Diagram AND

DIFL OR

OR

DIR

AND

SAT

87B BIASED OP

DIFH


Phase Comparison Principle • Internal Faults: All fault (“large”) currents are approximately in phase.

• External Faults: One fault (“large”) current will be out of phase

• No Voltages are required or

Secondary Current of Faulted Circuit (Severe CT Saturation) needed


Phase Comparison Principle Continued… E x te r n a l F a u lt C o n d itio n s

 Ip   imag   ID − I p   

 Ip   imag   ID − I p   

O PERATE

BLO CK ID - Ip

In te r n a l F a u lt C o n d itio n s

Ip

 Ip real   ID − I p 

   

O PERATE

BLOCK

ID - Ip

 Ip   real   ID − I p   

Ip BLO CK

BLOCK O PERATE

O PERATE


CT Saturation

differential

t2

t1

t0

r e s t r a in in g

• Fault starts at t0, CT begins to saturate at t1 • CT fully saturated at t2 56 GE Consumer & Industrial Multilin Oct 31, 2009

CT Saturation Detector State Machine NORMAL

SAT := 0 The differential current below the first slope for certain period of time

saturation condition EXTERNAL FAULT

SAT := 1 The differential characteristic entered EXTERNAL FAULT & CT SATURATION

The differentialrestraining trajectory out of the differential characteristic for certain period of time

SAT := 1


CT Saturation Detector Operating Principles • The 87B SAT flag WILL NOT be set during internal faults, regardless of whether or not any of the CTs saturate. • The 87B SAT flag WILL be set during external faults, regardless of whether or not any of the CTs saturate. • By design, the 87B SAT flag WILL force the relay to use the additional 87B DIR phase The Saturation Detector NOT Block the comparison for Region WILL 2 Operation of the Differential Element – it will only Force 2-out-of-2 Operation 58 GE Consumer & Industrial Multilin Oct 31, 2009

CT Saturation Detector • The oscillography records on the next two slides were Examples captured from a B30 relay under test on a real-time digital power system simulator • First slide shows an external fault with deep CT saturation (~1.5 msec of good CT performance) o SAT saturation detector flag asserts prior to BIASED PKP bus differential pickup o DIR directional flag does not assert (one current flows out of zone), so even though bus differential picks up, no trip results • Second slide shows an internal fault with mild CT saturation o BIASED PKP and BIASED OP both assert before DIR asserts o CT saturation does not block bus differential • More examples available (COMTRADE files) upon request


CT Saturation Example – External Fault 200 150

current, A

100

~1 ms

50 0 -50 -100 -150 -200 0.06

0.07

0.08

0.09

0.1

0.11

0.12

time, sec T h e b u s d iffe re n tia l p r o te c tio n e le m e n t p ic k s u p d u e to h e a v y C T s a tu ra tio n

The d ir e c tio n a l fla g is n o t s e t

T h e C T s a t u r a tio n fla g is s e t s a fe ly b e fo re th e p ic k u p fla g

T h e e le m e n t does not m a lo p e r a te

Despite heavy CT saturation the external fault current is seen in the opposite direction 60 GE Consumer & Industrial Multilin Oct 31, 2009

CT Saturation – Internal Fault Example

T h e b u s d iffe r e n tia l p r o te c t io n e le m e n t p ic k s u p

T h e s a tu ra tio n fla g is n o t s e t - n o d ire c tio n a l d e c is io n r e q u ire d

A ll t h e f a u lt c u r r e n ts a re s e e n in o n e d ir e c tio n

T h e e le m e n t o p e r a te s in 10m s

The d ire c tio n a l fla g is s e t


Applying Low-Impedance Differential Relays for Busbar Protection Basic Topics • Configure physical CT Inputs • Configure Bus Zone and Dynamic Bus Replica • Calculating Bus Differential Element settings Advanced Topics • Isolator switch monitoring for reconfigurable buses • Differential Zone CT Trouble • Integrated Breaker Failure protection 62 GE Consumer & Industrial Multilin Oct 31, 2009

Configuring CT Inputs • For each connected CT circuit enter Primary rating and select Secondary rating. • Each 3-phase bank of CT inputs must be assigned to a Signal Source that is used to define the Bus Zone and Dynamic Bus Replica

Some relays define 1 p.u. as the maximum primary current of all of the CTs connected in the given Bus Zone 63 GE Consumer & Industrial Multilin Oct 31, 2009

Per-Unit Current Definition Example Current Channel

Primary Secondary

Zone

CT-1

F1

3200 A

1A

1

CT-2

F2

2400 A

5A

1

CT-3

F3

1200 A

1A

1

CT-4

F4

3200 A

1A

2

• For Zone 1, 1 p.u. = 3200 AP F5 1200 A 5A CT-5 • For Zone 2, 1 p.u. = 5000 AP

2

CT-6

F6

5000 A

5A

64

2GE Consumer & Industrial

Multilin Oct 31, 2009

Configuration of Bus Zone • Dynamic Bus Replica associates a status signal with each current in the Bus Differential Zone • Status signal can be any logic operand o Status signals can be developed in programmable logic to provide additional checks or security as required o Status signal can be set to ‘ON’ if current is always in the bus zone or ‘OFF’ if current is never in the bus zone • CT connections/polarities for a particular bus zone must be properly configured in the relay, via either hardwire or software 65 GE Consumer & Industrial Multilin Oct 31, 2009

Configuring the Bus Differential Zone Bus Zone settings defines the boundaries of the Differential Protection and CT Trouble Monitoring.

1. Configure the physical CT Inputs o o o

CT Primary and Secondary values Both 5 A and 1 A inputs are supported by the UR hardware Ratio compensation done automatically for CT ratio differences up to 32:1

1. Configure AC Signal Sources 2. Configure Bus Zone with Dynamic Bus Replica 66 GE Consumer & Industrial Multilin Oct 31, 2009

Dual Percent Differential Characteristic

High Set (Unrestrained)

High Slope Low Slope High Breakpoint

Min Pickup

Low Breakpoint


Calculating Bus Differential • The following Bus Zone Differential element Settings parameters need to be set: o Differential Pickup o Restraint Low Slope o Restraint Low Break Point o Restraint High Breakpoint o Restraint High Slope o Differential High Set (if needed)

• All settings entered in per unit (maximum CT primary in the zone) • Slope settings entered in percent • Low Slope, High Slope and High Breakpoint settings are used by the CT Saturation Detector and define the Region 1 Area (2-out-of-2 operation with Directional) 68 GE Consumer & Industrial Multilin Oct 31, 2009

Calculating Bus Differential Settings – Minimum Pickup • Defines the minimum differential current required for operation of the Bus Zone Differential element • Must be set above maximum leakage current not zoned off in the bus differential zone • May also be set above maximum load conditions for added security in case of CT trouble, but better alternatives exist


Calculating Bus Differential Settings – Low Slope • Defines the percent bias for the restraint currents from IREST =0 to IREST =Low Breakpoint • Setting determines the sensitivity of the differential element for low-current internal faults • Must be set above maximum error introduced by the CTs in their normal linear operating mode • Range: 15% to 100% in 1%. increments


Calculating Bus Differential Settings – Low Breakpoint • Defines the upper limit to restraint currents that will be biased according to the Low Slope setting • Should be set to be above the maximum load but not more than the maximum current where the CTs still operate linearly (including residual flux) • Assumption is that the CTs will be operating linearly (no significant saturation effects up to 80% residual flux) up to the Low Breakpoint setting


Calculating Bus Differential Settings – High Breakpoint • Defines the minimum restraint currents that will be biased according to the High Slope setting • Should be set to be below the minimum current where the weakest CT will saturate with no residual flux • Assumption is that the CTs will be operating linearly (no significant saturation effects up to 80% residual flux) up to the Low Breakpoint setting


Calculating Bus Differential Settings – High Slope • Defines the percent bias for the restraint currents IREST ≥ High Breakpoint • Setting determines the stability of the differential element for high current external faults • Traditionally, should be set high enough to accommodate the spurious differential current resulting from saturation of the CTs during heavy external faults • Setting can be relaxed in favour of sensitivity and speed as the relay detects CT saturation and applies the directional principle to prevent maloperation • Range: 50% to 100% in 1%. increments


Calculating Unrestrained Bus Differential Settings • Defines the minimum differential current for unrestrained operation • Should be set to be above the maximum differential current under worst case CT saturation • Range: 2.00 to 99.99 p.u. in 0.01 p.u. increments • Can be effectively disabled by setting to 99.99 p.u.


Dual Percent Differential Characteristic

High Set (Unrestrained)

High Slope Low Slope High Breakpoint

Min Pickup

Low Breakpoint


Reconfigurable Buses C -3

C -5 NO RTH BU S

S -1

B -1

S -5

S -3

B -5 C T -7

C T -1 C T -2

B -2

C T -3

B -3

C T -4

C T -5

B -4

B -7 C T -6 C T -8 B -6 S -2

S -6

S -4

SO UTH BUS C -1

C -2

C -4

Protecting re-configurable buses



C -5 N O R TH BUS

S -1

B -1

S -5

S -3

B -5

C T -1

C T -2

B -2

C T -3

C T -4

B -3

C T -7

B -4

C T -5 B -7 C T -6 C T -8 B -6

S -2

S -6

S -4

SO U TH BUS C -1

C -2

C -4

Protecting re-configurable buses 77 GE Consumer & Industrial Multilin Oct 31, 2009


C -5 N O R TH BUS

S -1

B -1

S -5

S -3

B -5

C T -1

C T -2

B -2

C T -3

C T -4

B -3

C T -7

B -4

C T -5 B -7 C T -6 C T -8 B -6

S -2

S -6

S -4

SO U TH BUS C -1

C -2

C -4




C -5 NO RTH BU S

S -1

B -1

S -5

S -3

B -5 C T -7

C T -1 C T -2

B -2

C T -3

B -3

C T -4

C T -5

B -4

B -7 C T -6 C T -8 B -6 S -2

S -6

S -4

SO UTH BUS C -1

C -2

C -4



Isolators • Reliable “Isolator Closed” signals are needed for the Dynamic Bus Replica • In simple applications, a single normally closed contact may be sufficient • For maximum safety: o Both N.O. and N.C. contacts should be used o Isolator Alarm should be established and non-valid

combinations (open-open, closed-closed) should be sorted out o Switching operations should be inhibited until bus image is recognized with 100% accuracy o Optionally block 87B operation from Isolator Alarm

• Each isolator position signal decides: o Whether or not the associated current is to be included in the

differential calculations o Whether or not the associated breaker is to be tripped


Isolator – Typical Open/Closed Connections


Switch Status Logic and Dyanamic Bus Replica Isolator Open Auxiliary Contact Off

Isolator Closed Auxiliary Contact On

Isolator Position

Alarm

Block Switching

CLOSED

No

No

Off

Off

LAST VALID

Until Isolator

On

On

CLOSED

On

Off

OPEN

After time delay until acknowledge d No

Position is valid No

NOTE: Isolator monitoring function may be a built-in feature or user-programmable in low impedance bus differential digital relays 82 GE Consumer & Industrial Multilin Oct 31, 2009

Differential Zone CT Trouble • Each Bus Differential Zone may a dedicated CT Trouble Monitor • Definite time delay overcurrent element operating on the zone differential current, based on the configured Dynamic Bus Replica • Three strategies to deal with CT problems: 1. Trip the bus zone as the problem with a CT will likely evolve into a bus fault anyway 2. Do not trip the bus, raise an alarm and try to correct the problem manually 3. Switch to setting group with 87B minimum pickup setting above the maximum load current.


Differential Zone CT Trouble • Strategies 2 and 3 can be accomplished by:  Using undervoltage supervision to ride through the period from the beginning of the problem with a CT until declaring a CT trouble condition  Using an external check zone to supervise the 87B function  Using CT Trouble to prevent the Bus Differential tripping (2)  Using setting groups to increase the pickup value for the 87B function (3)


Differential Zone CT Trouble – Strategy #2 Example 87B operates

Undervoltage condition CT OK

• CT Trouble operand is used to rise an alarm • The 87B trip is inhibited after CT Trouble element operates • The relay may misoperate if an external fault occurs after CT trouble but before the CT trouble condition is declared (double-contingency)


Example Architecture for Large Dual (redundant) fiber Busbars with 3msec delivery time between neighbouring IEDs. Up to 8 relays in the ring

Phase A AC signals and trip contacts

Phase B AC signals and trip contacts

Phase C AC signals and trip contacts

Digital Inputs for isolator monitoring and BF


Example Architecture – Dynamic Bus Replica and Isolator Iso lat Position or n o P iti l Iso

Iso la

os P r ato

Phase A AC signals wired here, bus replica configured here

Phase B AC signals wired here, bus replica configured here to rP os it

ion

os itio n

Phase C AC signals wired here, bus replica configured here

n io t i os P r to

la o s I Auxuliary switches wired here; Isolator Monitoring function configured here


Example Architecture – BF Initiation up& Current Supervision BF v. I e& t a i nit I BF

BF

In iti

at

nt e r r Cu

nit

S

Phase A AC signals wired here, current status monitored here

Phase B AC signals wired here, current status monitored here e

&

Cu rre nt Su p

v.

iat e

&C

urr en t

Su pv .

Phase C AC signals wired here, current status monitored here

BF Breaker Failure elements configured here

iti n I

e at

&

nt e rr u C

v. p Su


Example Architecture – Breaker Trip Failure Tripping F er k a Bre

O ail

p

Trip

Br ea

Bre a

ai

lO

p

Fa

Phase A AC signals wired here, current status monitored here

Phase B AC signals wired here, current status monitored here ke rF

ke r

Trip

Phase C AC signals wired here, current status monitored here

il O

p

Trip

p O l ai

rF e k ea r B Breaker Fail Op command generated here and send to trip appropriate breakers


IEEE 37.234 • “Guide for Protective Relay Applications to Power System Buses” is currently being revised by the K14 Working Group of the IEEE Power System Relaying Committee.



High Low Impedance BusBar Protection

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