Fundamentals of Bus Bar Protection

1
Fundamentals ofBus Bar Protection

• GE Multilin

2
Outline

• Bus arrangements
• Bus components
• Bus protection techniques
• CT Saturation
• Application Considerations
• High impedance bus differential relaying
• Low impedance bus differential relaying
• Special topics

3
Single bus - single breaker

• Distribution and lower transmission voltage
  levels
• No operating flexibility
• Fault on the bus trips all circuit breakers

4
Multiple bus sections - single breaker with bus
tie

• Distribution and lower transmission voltage
  levels
• Limited operating flexibility

5
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

6
Main and transfer buses

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

7
Double bus single breaker w/ transfer bus

• Very high operating flexibility
• Transfer bus for breaker maintenance

8
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

9
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

10
Ring bus

• Higher voltage levels
• High operating flexibility with minimum breakers
• Separate bus protection not required at line
  positions

11
Bus components
breakers
12
Disconnect switches auxiliary contacts
13
Current Transformers
14
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

15
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

16
Interlocking Schemes

• 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

17
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
• Instantaneous differential OC function useful on
  integrated microprocessor-based relays

18
Linear Couplers
ZC 2 ? 20 ? - typical coil impedance (5V per
1000Amps gt 0.005? _at_ 60Hz )
59
External Fault
19
Linear Couplers
Esec IprimXm - secondary voltage on relay
terminals IR ?IprimXm /(ZR?ZC) minimum
operating current where, Iprim primary
current in each circuit Xm liner coupler mutual
reactance (5V per 1000Amps gt 0.005? _at_ 60Hz ) ZR
relay tap impedance ?ZC sum of all linear
coupler self impedances
Internal Bus Fault
59
20
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-base
  d relay (e.g. oscillography, breaker failure
  protection, other functions)

21
High Impedance Differential

• Operating signal created by connecting all CT
  secondaries in parallel
• 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

Cannot easily be applied to reconfigurable buses
and offers no advanced functionality
22
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

23
Low Impedance Percent Differential

• Individual currents sampled by protection and
  summated digitally
• CT ratio matching done internally (no auxiliary
  CTs)
• Dedicated CTs not necessary
• Additional algorithms improve security of percent
  differential characteristic during CT saturation
• Dynamic bus replica allows application to
  reconfigurable buses
• Done digitally with logic to add/remove current
  inputs from differential computation
• Switching of CT secondary circuits not required
• Low secondary burdens
• Additional functionality available
• Digital oscillography and monitoring of each
  circuit connected to bus zone
• Time-stamped event recording
• Breaker failure protection

24
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)

25
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

26
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.

27
CT Saturation
28
CT Saturation Concepts

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

29
CT Saturation

• No DC Offset
• Waveform remains fairly symmetrical

• With DC Offset
• Waveform starts off being asymmetrical, then
  symmetrical in steady state

30
External Fault Ideal CTs
t1
t0

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

Ideal CTs have no saturation or mismatch errors
thus produce no differential current
31
External Fault Actual CTs
t1
t0

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

Actual CTs do introduce errors, producing some
differential current (without CT saturation)
32
External Fault with CT Saturation
t2
t1
t0

• 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.
33
Some Methods of Securing Bus Differential

• Block the bus differential for a period of time
  (intentional delay)
• Increases security as bus zone will not trip when
  CT saturation is present
• Prevents high-speed clearance for internal faults
  with CT saturation or evolving faults
• Change settings of the percent differential
  characteristic (usually Slope 2)
• Improves security of differential element by
  increasing the amount of spurious differential
  current needed to incorrectly trip
• Difficult to explicitly develop settings (Is 60
  slope enough? Should it be 75?)
• Apply directional (phase comparison) supervision
• Improves security by requiring all currents flow
  into the bus zone before asserting the
  differential element
• Easy to implement and test
• Stable even under severe CT saturation during
  external faults

34
High-Impedance Bus Differential Considerations
35
High Impedance Voltage-operated RelayExternal
Fault

• 59 element set above max possible voltage
  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

36
High Impedance Voltage Operated Relay Ratio
matching with Multi-ratio CTs

• Application of high impedance differential
  relays 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.

37
High Impedance Voltage Operated Relay Ratio
matching with Multi-ratio CTs

• Use of auxiliary 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.

38
Electromechanical High Impedance Bus Differential
Relays

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

39
?P -based High-Impedance Bus Differential
Protection Relays
Operating time 20 30ms _at_ I gt 1.5xPKP
40
High Impedance Module for Digital Relays
RST 2000? - 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
41
High-Impedance Module Overcurrent Relay
42
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.)

43
Low-Impedance Bus Differential Considerations
44
?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

45
Small Bus Applications
2-8 Circuit Applications

• Up to 24 Current Inputs
• 4 Zones
• Zone 1 Phase A
• Zone 2 Phase B
• Zone 3 Phase C
• Zone 4 Not used

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

46
Medium to Large Bus Applications
9-12 Circuit 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

47
Large Bus Applications
87B phase A
87B phase B
87B phase C
Logic relay (switch status, optional BF)
48
Large Bus ApplicationsFor buses with up to 24
circuits
49
Summing External CurrentsNot Recommended for
Low-Z 87B relays

• Relay becomes combination of restrained and
  unrestrained elements
• In order to parallel CTs
• CT performance must be closely matched
• Any errors will appear as differential currents
• Associated feeders must be radial
• No backfeeds possible
• Pickup setting must be raised to accommodate any
  errors

50
Definitions of Restraint Signals
sum of
scaled sum of
geometrical average
maximum of
51
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)

52
Bus Differential Adaptive Approach
53
Bus Differential Adaptive Logic Diagram
54
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 needed

55
Phase Comparison Principle Continued
56
CT Saturation
t2
t1
t0

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

57
CT Saturation Detector State Machine
58
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 comparison
  for Region 2

The Saturation Detector WILL NOT Block the
Operation of the Differential Element it will
only Force 2-out-of-2 Operation
59
CT Saturation Detector - Examples

• The oscillography records on the next two slides
  were 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)
• SAT saturation detector flag asserts prior to
  BIASED PKP bus differential pickup
• 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
• BIASED PKP and BIASED OP both assert before DIR
  asserts
• CT saturation does not block bus differential
• More examples available (COMTRADE files) upon
  request

60
CT Saturation Example External Fault
61
CT Saturation Internal Fault Example
62
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

63
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
64
Per-Unit Current Definition - Example
Current Channel Primary Secondary Zone
CT-1 F1 3200 A 1 A 1
CT-2 F2 2400 A 5 A 1
CT-3 F3 1200 A 1 A 1
CT-4 F4 3200 A 1 A 2
CT-5 F5 1200 A 5 A 2
CT-6 F6 5000 A 5 A 2

• For Zone 1, 1 p.u. 3200 AP
• For Zone 2, 1 p.u. 5000 AP

65
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
• Status signals can be developed in programmable
  logic to provide additional checks or security as
  required
• 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

66
Configuring the Bus Differential Zone
Bus Zone settings defines the boundaries of the
Differential Protection and CT Trouble Monitoring.

• Configure the physical CT Inputs
• 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 321
• Configure AC Signal Sources
• Configure Bus Zone with Dynamic Bus Replica

67
Dual Percent Differential Characteristic
High Set (Unrestrained)
High Slope
Low Slope
Min Pickup
68
Calculating Bus Differential Settings

• The following Bus Zone Differential element
  parameters need to be set
• Differential Pickup
• Restraint Low Slope
• Restraint Low Break Point
• Restraint High Breakpoint
• Restraint High Slope
• 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)

69
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

70
Calculating Bus Differential Settings Low Slope

• Defines the percent bias for the restraint
  currents from IREST0 to IRESTLow 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

71
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

72
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

73
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

74
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.

75
Dual Percent Differential Characteristic
High Set (Unrestrained)
High Slope
Low Slope
Min Pickup
76
Reconfigurable Buses
Protecting re-configurable buses
77
Reconfigurable Buses
Protecting re-configurable buses
78
Reconfigurable Buses
Protecting re-configurable buses
79
Reconfigurable Buses
Protecting re-configurable buses
80
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
• Both N.O. and N.C. contacts should be used
• Isolator Alarm should be established and
  non-valid combinations (open-open, closed-closed)
  should be sorted out
• Switching operations should be inhibited until
  bus image is recognized with 100 accuracy
• Optionally block 87B operation from Isolator
  Alarm
• Each isolator position signal decides
• Whether or not the associated current is to be
  included in the differential calculations
• Whether or not the associated breaker is to be
  tripped

81
Isolator Typical Open/Closed Connections
82
Switch Status Logic and Dyanamic Bus Replica

• NOTE Isolator monitoring function may be a
  built-in feature or user-programmable in low
  impedance bus differential digital relays

83
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
• Trip the bus zone as the problem with a CT will
  likely evolve into a bus fault anyway
• Do not trip the bus, raise an alarm and try to
  correct the problem manually
• Switch to setting group with 87B minimum pickup
  setting above the maximum load current.

84
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)

85
Differential Zone CT Trouble Strategy 2 Example

• 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)

86
Example Architecture for Large Busbars
87
Example Architecture Dynamic Bus Replica and
Isolator Position
88
Example Architecture BF Initiation Current
Supervision
89
Example Architecture Breaker Failure Tripping
Breaker Fail Op command generated here and send
to trip appropriate breakers
90
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.

91
Questions?
92
Thanks for the time