high low impedance busbar protection

PowerPoint PresentationSpecial topics
No operating flexibility
* GE Consumer & Industrial Multilin *
Limited operating flexibility
Transmission and distribution voltage levels
Breaker maintenance without circuit removal
Fault on a bus disconnects only the circuits being connected to that bus
Double bus - single breaker with bus tie
Transfer bus for breaker maintenance
Main and transfer buses
Very high operating flexibility
Double bus – single breaker w/ transfer bus
Line protection covers bus section between two CTs
Fault on a bus does not disturb the power to circuits
Double bus - double breaker
More operating flexibility
Requires more breakers
Middle bus sections covered by line or other equipment protection
Breaker-and-a-half bus
Separate bus protection not required at line positions
Ring bus
Gas (SF6) insulated current transformer
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
Protection of power system busbars is one of the most critical relaying applications. Busbars are areas in power systems where fault current levels may be very high. In spite of that, some of the circuits connected to the bus may have their Current Transformers (CTs) insufficiently rated. This creates a danger of significant CT saturation and jeopardizes security of the busbar protection system.
A false trip of a distribution bus can cause outages to a large number of customers as numerous feeders and/or sub-transmission lines may get disconnected. A false trip of a transmission busbar may drastically change system topology and jeopardize power system stability. Hence, the requirement of a maximum security of busbar protection.
On the other hand, bus faults generate large fault currents. If not cleared promptly, they endanger the entire substation due to both dynamic forces and thermal effects. Hence, the requirement of high-speed operation of busbar protection.
With both security and dependability being very important for busbar protection, the preference is always given to security.
Overcurrent percent (“restrained” or “biased”) differential
Linear couplers
Power system busbars vary significantly as to the size (number of circuits connected), complexity (number of sections, tie-breakers, disconnectors, etc.) and voltage level (transmission, distribution).
The above technical aspects combined with economic factors yield a number of solutions for busbar protection.
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
A simple protection for distribution busbars can be accomplished as an interlocking scheme. Overcurrent (OC) relays are placed on an incoming circuit and at all outgoing feeders. The feeder OCs are set to sense the fault currents on the feeders. The OC on the incoming circuit is set to trip the busbar unless blocked by any of the feeder OC relays. A short coordination timer is typically required to avoid race conditions.
When using microprocessor-based multi-functional relays it becomes possible to integrate all the required OC functions in one or few relays. This allows not only reducing wiring but also shortening the coordination time and speeding-up operation of the scheme.
Modern relays provide for fast peer-to-peer communications using protocols such as the UCA with the GOOSE mechanism. This allows eliminating wiring and sending the blocking signals over the communications.
The scheme although easy to apply and economical is limited to specific (simple) busbar configurations.
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
Typically a differential current is created externally to a current sensor by summation of all the circuit currents. Preferably the CTs should be of the same ratio. If they are not, a matching CT (or several CTs) is needed. This in turn may increase the burden for the main CTs and make the saturation problem even more serious.
Historically, means to deal with the CT saturation problem include definite time or inverse-time overcurrent characteristics.
Although economical and applicable to distribution busbars, this solution does not match performance of more advanced schemes and should not be applied to transmission-level busbars.
The principle, however, is used as a protection function in an integrated microprocessor-based busbar relay. If this is the case, such unrestrained differential element should be set above the maximum spurious differential current and may give a chance to speed up operation on heavy internal faults as compared to a percent (restrained) bus differential element
(5V per 1000Amps => 0.005 @ 60Hz )
External
Fault
40 V
10 V
10 V
0 V
20 V
2000 A
2000 A
4000 A
0 A
0 V
A linear coupler (air core mutual reactor) produces its output voltage proportional to the derivative of the input current. Because they are using air cores, linear couplers do not saturate.
During internal faults the sum of the busbar currents, and thus their derivatives, is zero. Based on that, a simple busbar protection is thus achieved by connecting the secondary windings of the linear couplers in series (in order to respond to the sum of the primary currents) and attaching a simple voltage sensor.
Disadvantages of this approach are similar to those of the high-impedance scheme
where,
Xm – liner coupler mutual reactance (5V per 1000Amps => 0.005 @ 60Hz )
ZR – relay tap impedance
Internal Bus
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)
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
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
Percent differential relays create a restraining signal in addition to the differential signal and apply a percent (restrained) characteristic. The choices of the restraining signal include “sum”, “average” and “maximum” of the bus currents. The choices of the characteristic include typically single-slope and double-slope characteristics.
This low-impedance approach does not require dedicated CTs, can tolerate substantial CT saturation and provides for high-speed tripping.
Many integrated relays perform CT ratio compensation eliminating the need for matching CTs.
This principle became really attractive with the advent of microprocessor-based relays because of the following:
Advanced algorithms supplement the percent differential protection function making the relay very secure.
Protection of re-configurable busbars becomes easier as the dynamic bus replica (bus image) can be accomplished without switching secondary current circuits.
Integrated Breaker Fail (BF) function can provide optimal tripping strategy depending on the actual configuration of a busbar.
Distributed architectures are proposed that place Data Acquisition Units (DAU) in bays and replace current wires by fiber optic communications.
Low Impedance Percent Differential
Dedicated CTs not necessary
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
Better filtering
Faster response
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)
Low Impedance Differential (Distributed)
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
Low Impedance Differential (Centralized)
No communications, external sampling synchronisation necessary
Perceived more reliable (less hardware needed)
Well suited to both new and retrofit applications.
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
Waveform starts off being asymmetrical, then symmetrical in steady state
External Fault & Ideal CTs
Fault starts at t0
t0
t1
Ideal CTs have no saturation or mismatch errors thus produce no differential current
External Fault & Actual CTs
Fault starts at t0
t0
t1
Actual CTs do introduce errors, producing some differential current (without CT saturation)
Fault starts at t0, CT begins to saturate at t1
CT fully saturated at t2
t0
t1
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)
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
High-Impedance Bus Differential Considerations
High Impedance Voltage-operated Relay
External 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
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.
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.
Single phase relays
P -based High-Impedance Bus Differential Protection 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
High Impedance Module for Digital Relays
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.)
High Impedance Bus Protection - Summary
High-impedance protection responds to a voltage across the differential junction points. The CTs are required to have a low secondary leakage impedance (completely distributed windings or toroidal coils). During external faults, even with severe saturation of some of the CTs, the voltage does not rise above certain level, because the other CTs will provide a lower-impedance path as compared with the relay input impedance. The principle has been used for more than half a century because is robust, secure and fast.
The technique, however, is not free from disadvantages. The most important ones are:
The high-impedance approach requires dedicated CTs (significant cost associated).
It cannot be easily applied to re-configurable buses (current switching using bistable auxiliary relays endangers the CTs, jeopardizes security and adds an extra cost).
It requires a voltage limiting varistor capable of absorbing significant energy during busbar faults.
The scheme requires only a simple voltage level sensor. From this perspective the high-impedance protection scheme is not a relay. If BF, event recording, oscillography, communications, and other benefits of microprocessor-based relaying are of interest, then extra equipment is needed (such as a Digital Fault Recorder or dedicated BF relays).
Low-Impedance Bus Differential Considerations
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
The low-impedance approach used to be perceived as less secure when compared with the high-impedance protection. This is no longer true as microprocessor-based relays apply sophisticated algorithms to match the performance of high-impedance schemes, and at the same time, the cost considerations make the high-impedance scheme less attractive. This is particularly relevant for large (cost of extra CTs) and complex (dynamic bus replica) buses that cannot be handled well by high-impedance schemes.
Microprocessor-based low-impedance busbar relays are developed in one of the two architectures:
Distributed
Centralized
4 Zones
2-8 Circuit Applications
Small Bus Applications
4 Zones
Zone 3 = Not used
Zone 4 = Not used
Relay 2 - 24 Current Inputs
4 Zones
Zone 4 = Not used
CB 12
CB 11
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
Definitions of Restraint Signals
“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
CT saturation difficult to detect
More security required Use 2-out-of-2 Operating Mode
Region 2
CT saturation easy to detect
Security required only if CT saturation detected
Dynamically decide if 1-out-of-2 or 2-out-of-2 Operating Mode
DIFL
DIR
SAT
DIFH
OR
AND
OR
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
Secondary Current of Faulted Circuit
(Severe CT Saturation)
Select n fault “Contributors”
A contributor is a feeder carrying a significant amount of current (above load)
A feeder is a contributor if it’s current magnitude is:
Above the high breakpoint setting of the bus differential element
Above a certain portion of the restraint current
Determine the angle between each Contributor and the sum of the remaining n-1 Contributors.
Determine the maximum of the angles and compare with the directional threshold
Threshold is set at 90o
For external faults, the maximum of the angles should be greater than 90o
An angle of more than 90o for an internal fault due to CT saturation is not physically possible.
Fault starts at t0, CT begins to saturate at t1
CT fully saturated at t2
t0
t1
t2
NORMAL
time
The CT saturation condition is declared when the magnitude of the restraining signal becomes larger than the higher breakpoint and at the same time the differential current is below the first slope.

It should be emphasized that the saturation detector has no dedicated settings, but does rely on settings for the main differential characteristic for proper operation.
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
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
Despite heavy CT
time, sec
current, A
~1 ms
Two examples of relay operation are presented: an external fault with heavy CT saturation and an internal fault with mild CT saturation.
The protected bus includes six circuits connected to CT banks F1, F5, M1, M5, U1 and U5, respectively. The circuits F1, F5, M1, M5 and U5 are capable of feeding some fault current; the U1 circuit supplies a load. The F1, F5 and U5 circuits are significantly stronger than the F5 and M1 connections.
The M5 circuit contains the weakest (most prone to saturation) CT of the bus. The Figure presents the bus currents and the most important logic signals for the case of an external fault. Despite very fast and severe CT saturation, the B30 remains stable.
CT Saturation – Internal Fault Example
The Figure presents the same signals but for the case of an internal fault. The B30 trips in 10 ms (fast form-C output contact).
Basic Topics
Calculating Bus Differential Element settings
Advanced Topics
Differential Zone CT Trouble
Integrated Breaker Failure protection
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
For B30, CTs are connected in 3-phase sets
For B90, CTs are connected as individual single phases
UR hardware is built to support combinations of both 1 A and 5 A secondaries within the same DSP module
Per-Unit Current Definition - Example
Current Channel
For Zone 2:
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
Configuring the Bus Differential Zone
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 32:1
Configure AC Signal Sources
Configure Bus Zone with Dynamic Bus Replica
Bus Zone settings defines the boundaries of the Differential Protection and CT Trouble Monitoring.
Dual Percent Differential Characteristic
Calculating Bus Differential Settings
The following Bus Zone Differential element parameters need to be set:
Differential Pickup
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)
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 IRESTHigh 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
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
Complex busbars are re-configurable.
In particular a given circuit having a single metering point (CT) and single current interrupting device (CB) may be connected to more than one section depending on positions of isolators. This requires to monitor positions of the isolators in order to determine if a given current shall be included in the differential zone for a given section, and whether a given breaker shall be tripped upon detecting a fault in a given zone.
This calls for “current switching” (AC) and “trip re-direction logic” (DC). The second operation does not impose any technical difficulties, but the first one is not a preferred solution in analog schemes as it may lead to damaging the CTs. Ability to follow the actual busbar configuration is referred to as a “dynamic bus image” or “dynamic bus replica” and is one of the strongest features of microprocessor-based busbar relays.
Protecting re-configurable buses
Here, as an example one zone of protection shall cover the NORTH BUS. The figure shows zone boundaries by indicating metering and current interruption points.
Here, another zone of protection is shown for the SOUTH BUS. The figure shows zone boundaries by indicating metering and current interruption points.
Note, for example that a pair of a metering point CT-2 and a current interruption point B-2 belongs to the SOUTH zone only if the S-2 isolator is closed (S-1 is opened). The CT-2 / B-2 point belongs to the NORTH zone if the S-1 is closed (S-2 must be open).
Ideally, zones shall overlap. This includes protection of the connected circuits as well. In this example, two B30 relays could be used to protect this double-bus arrangement.
In reality, there is a physical area between a metering point (CT) and the associated current interruption point (CB). Depending on the mutual location of the two, certain blind or over-tripping spots may occur. This issue can be resolved using dynamic bus replica, and is addressed later in this material.
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:
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
Isolator – Typical Open/Closed Connections
NOTE: Isolator monitoring function may be a built-in feature or user-programmable in low impedance bus differential digital relays
Isolator Open
Auxiliary Contact
Isolator Closed
Auxiliary Contact
Isolator Position
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.
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)
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)
87B operates
Undervoltage condition
CT OK
trip contacts
trip contacts
trip contacts
Digital Inputs for isolator monitoring and BF
Dual (redundant) fiber with 3msec delivery time between neighbouring IEDs. Up to 8 relays in the ring
Phase A AC signals wired here, bus replica configured here
Phase B AC signals wired here, bus replica configured here
Phase C AC signals wired here, bus replica configured here
Isolator Position
Isolator Position
Isolator Position
Isolator Position
Phase A AC signals wired here, current status monitored here
Phase B AC signals wired here, current status monitored here
Phase C AC signals wired here, current status monitored here
Breaker Failure elements configured here
BF Initiate & Current Supv.
Example Architecture – Breaker Failure Tripping
Breaker Fail Op command generated here and send to trip appropriate breakers
Phase A AC signals wired here, current status monitored here
Phase B AC signals wired here, current status monitored here
Phase C AC signals wired here, current status monitored here
Breaker Fail Op
Breaker Fail Op
Breaker Fail Op
Breaker Fail Op
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.
CT saturation
pickup flag
directional

high low impedance busbar protection

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