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High_Low_Impedance_BusBar_Protection.ppt

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This document discusses various techniques for bus protection, including high impedance and low impedance differential relaying. It describes different bus configurations and components. High impedance differential protection requires dedicated current transformers with matching ratios. Low impedance relaying samples currents digitally and uses additional algorithms to improve security during faults. It can model reconfigurable buses dynamically without switching hardware. CT saturation is also addressed and various methods for securing the differential element during saturation are presented.

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Fundamentals of Bus Bar Protection GE Multilin
2 GE Consumer & Industrial Multilin 29-Oct-22 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 GE Consumer & Industrial Multilin 29-Oct-22 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
4 GE Consumer & Industrial Multilin 29-Oct-22 ZONE 1 ZONE 2 • Distribution and lower transmission voltage levels • Limited operating flexibility Multiple bus sections - single breaker with bus tie
5 GE Consumer & Industrial Multilin 29-Oct-22 ZONE 1 ZONE 2 • 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
6 GE Consumer & Industrial Multilin 29-Oct-22 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
7 GE Consumer & Industrial Multilin 29-Oct-22 ZONE 1 ZONE 2 • Very high operating flexibility • Transfer bus for breaker maintenance Double bus – single breaker w/ transfer bus
8 GE Consumer & Industrial Multilin 29-Oct-22 ZONE 1 ZONE 2 • High operating flexibility • Line protection covers bus section between two CTs • Fault on a bus does not disturb the power to circuits Double bus - double breaker
9 GE Consumer & Industrial Multilin 29-Oct-22 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
10 GE Consumer & Industrial Multilin 29-Oct-22 • Higher voltage levels • High operating flexibility with minimum breakers • Separate bus protection not required at line positions B1 B2 TB1 L1 L2 L3 L4 TB1 Ring bus
11 GE Consumer & Industrial Multilin 29-Oct-22 Bus components breakers SF6, EHV & HV - Synchropuff Low Voltage circuit breakers BUS 2 CB 1 BUS 1 ISO 1 ISO 2 ISO 3 BYPASS
12 GE Consumer & Industrial Multilin 29-Oct-22 - + F1a F1c Contact Input F1a On Contact Input F1c On F1b ISOLATOR 1 ISOLATOR 1 OPEN 7B 7A BUS 1 - + F1a F1c Contact Input F1a On Contact Input F1c On F1b ISOLATOR 1 ISOLATOR 1 CLOSED 7B 7A BUS 1 Disconnect switches & auxiliary contacts BUS 2 CB 1 BUS 1 ISO 1 ISO 2 ISO 3 BYPASS
13 GE Consumer & Industrial Multilin 29-Oct-22 BUS 2 CB 1 BUS 1 ISO 1 ISO 2 ISO 3 BYPASS Current Transformers Oil insulated current transformer (35kV up to 800kV) Gas (SF6) insulated current transformer Bushing type (medium voltage switchgear)
14 GE Consumer & Industrial Multilin 29-Oct-22 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 GE Consumer & Industrial Multilin 29-Oct-22 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 GE Consumer & Industrial Multilin 29-Oct-22 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 50 50 50 50 50 50 BLOCK
17 GE Consumer & Industrial Multilin 29-Oct-22 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 51
18 GE Consumer & Industrial Multilin 29-Oct-22 59 Linear Couplers ZC = 2  – 20  - typical coil impedance (5V per 1000Amps => 0.005 @ 60Hz ) If = 8000 A 40 V 10 V 10 V 0 V 20 V 2000 A 2000 A 4000 A 0 A 0 V External Fault
19 GE Consumer & Industrial Multilin 29-Oct-22 59 Linear Couplers Esec= Iprim*Xm - secondary voltage on relay terminals IR= Iprim*Xm /(ZR+ZC) – minimum operating current where, Iprim – primary current in each circuit Xm – liner coupler mutual reactance (5V per 1000Amps => 0.005 @ 60Hz ) ZR – relay tap impedance ZC – sum of all linear coupler self impedances If = 8000 A 0 A 0 V 10 V 10 V 0 V 20 V 40 V 2000 A 2000 A 4000 A 0 A Internal Bus Fault
20 GE Consumer & Industrial Multilin 29-Oct-22 • 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) Linear Couplers
21 GE Consumer & Industrial Multilin 29-Oct-22 High Impedance Differential • Operating signal created by connecting all CT secondaries in parallel o CTs must all have the same ratio o Must have dedicated CTs • Overvoltage element operates on voltage developed across resistor connected in secondary circuit 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 59
22 GE Consumer & Industrial Multilin 29-Oct-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 51 87 n DIF I I I I     ... 2 1 n RES I I I I     ... 2 1   n RES I I I I ..., , , max 2 1 
23 GE Consumer & Industrial Multilin 29-Oct-22 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
24 GE Consumer & Industrial Multilin 29-Oct-22 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)
25 GE Consumer & Industrial Multilin 29-Oct-22 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 52 DAU 52 DAU 52 DAU CU copper fiber
26 GE Consumer & Industrial Multilin 29-Oct-22 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
27 GE Consumer & Industrial Multilin 29-Oct-22 CT Saturation
28 GE Consumer & Industrial Multilin 29-Oct-22 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
29 GE Consumer & Industrial Multilin 29-Oct-22 CT Saturation Ratio Current CT Current Ratio Current CT Current No DC Offset • Waveform remains fairly symmetrical With DC Offset • Waveform starts off being asymmetrical, then symmetrical in steady state
30 GE Consumer & Industrial Multilin 29-Oct-22 External Fault & Ideal CTs • Fault starts at t0 • Steady-state fault conditions occur at t1 differential restraining t0 t1 Ideal CTs have no saturation or mismatch errors thus produce no differential current
31 GE Consumer & Industrial Multilin 29-Oct-22 External Fault & Actual CTs • Fault starts at t0 • Steady-state fault conditions occur at t1 differential restraining t0 t1 Actual CTs do introduce errors, producing some differential current (without CT saturation)
32 GE Consumer & Industrial Multilin 29-Oct-22 External Fault with CT Saturation • Fault starts at t0, CT begins to saturate at t1 • CT fully saturated at t2 differential restraining t0 t1 t2 CT saturation causes increasing differential current that may enter the differential element operate region.
33 GE Consumer & Industrial Multilin 29-Oct-22 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
34 GE Consumer & Industrial Multilin 29-Oct-22 High-Impedance Bus Differential Considerations
35 GE Consumer & Industrial Multilin 29-Oct-22 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
36 GE Consumer & Industrial Multilin 29-Oct-22 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 GE Consumer & Industrial Multilin 29-Oct-22 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 GE Consumer & Industrial Multilin 29-Oct-22 Electromechanical High Impedance Bus Differential Relays • Single phase relays • High-speed • High impedance voltage sensing • High seismic IOC unit
39 GE Consumer & Industrial Multilin 29-Oct-22 Operating time: 20 – 30ms @ I > 1.5xPKP P -based High-Impedance Bus Differential Protection Relays
40 GE Consumer & Industrial Multilin 29-Oct-22 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
41 GE Consumer & Industrial Multilin 29-Oct-22 High-Impedance Module + Overcurrent Relay
42 GE Consumer & Industrial Multilin 29-Oct-22 • 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
43 GE Consumer & Industrial Multilin 29-Oct-22 Low-Impedance Bus Differential Considerations
44 GE Consumer & Industrial Multilin 29-Oct-22 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 GE Consumer & Industrial Multilin 29-Oct-22 • 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 2-8 Circuit Applications Small Bus Applications
46 GE Consumer & Industrial Multilin 29-Oct-22 • 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 CB 12 CB 11 • Different CT Ratio Capability for Each Circuit • Largest CT Primary is Base in Relay • 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 9-12 Circuit Applications Medium to Large Bus Applications
47 GE Consumer & Industrial Multilin 29-Oct-22 Large Bus Applications 87B phase A 87B phase B 87B phase C Logic relay (switch status, optional BF)
48 GE Consumer & Industrial Multilin 29-Oct-22 Large Bus Applications For buses with up to 24 circuits
49 GE Consumer & Industrial Multilin 29-Oct-22 Summing External Currents Not 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 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 CT-1 CT-2 CT-3 CT-4 I 3 = 0 I 2 = 0 I 1 = Error IDIFF = Error IREST = Error Maloperation if Error > PICKUP
50 GE Consumer & Industrial Multilin 29-Oct-22 Definitions of Restraint Signals “maximum of” “geometrical average” “scaled sum of” “sum of” n R i i i i i      ... 3 2 1   n R i i i i n i      ... 1 3 2 1   n R i i i i Max i ,..., , , 3 2 1  n n R i i i i i      ... 3 2 1
51 GE Consumer & Industrial Multilin 29-Oct-22 “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 GE Consumer & Industrial Multilin 29-Oct-22 Bus Differential Adaptive Approach differential restraining Region 1 (low differential currents) Region 2 (high differential currents)
53 GE Consumer & Industrial Multilin 29-Oct-22 Bus Differential Adaptive Logic Diagram DIFL DIR SAT DIFH OR AND OR 87B BIASED OP AND
54 GE Consumer & Industrial Multilin 29-Oct-22 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 Secondary Current of Faulted Circuit (Severe CT Saturation)
55 GE Consumer & Industrial Multilin 29-Oct-22 Phase Comparison Principle Continued… BLOCK OPERATE BLOCK          p D p I I I real          p D p I I I imag Ip ID - Ip External Fault Conditions OPERATE BLOCK BLOCK          p D p I I I real          p D p I I I imag Ip ID - Ip Internal Fault Conditions OPERATE OPERATE
56 GE Consumer & Industrial Multilin 29-Oct-22 CT Saturation • Fault starts at t0, CT begins to saturate at t1 • CT fully saturated at t2 differential restraining t0 t1 t2
57 GE Consumer & Industrial Multilin 29-Oct-22 CT Saturation Detector State Machine NORMAL SAT := 0 EXTERNAL FAULT SAT := 1 EXTERNAL FAULT & CT SATURATION SAT := 1 The differential characteristic entered The differential- restraining trajectory out of the differential characteristic for certain period of time saturation condition The differential current below the first slope for certain period of time
58 GE Consumer & Industrial Multilin 29-Oct-22 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 GE Consumer & Industrial Multilin 29-Oct-22 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) 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
60 GE Consumer & Industrial Multilin 29-Oct-22 The bus differential protection element picks up due to heavy CT saturation The CT saturation flag is set safely before the pickup flag The directional flag is not set The element does not maloperate Despite heavy CT saturation the external fault current is seen in the opposite direction CT Saturation Example – External Fault 0.06 0.07 0.08 0.09 0.1 0.11 0.12 -200 -150 -100 -50 0 50 100 150 200 time, sec current, A ~1 ms
61 GE Consumer & Industrial Multilin 29-Oct-22 The bus differential protection element picks up The saturation flag is not set - no directional decision required The element operates in 10ms The directional flag is set All the fault currents are seen in one direction CT Saturation – Internal Fault Example
62 GE Consumer & Industrial Multilin 29-Oct-22 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 GE Consumer & Industrial Multilin 29-Oct-22 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 GE Consumer & Industrial Multilin 29-Oct-22 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 GE Consumer & Industrial Multilin 29-Oct-22 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
66 GE Consumer & Industrial Multilin 29-Oct-22 Configuring the Bus Differential Zone 1. Configure the physical CT Inputs o CT Primary and Secondary values o Both 5 A and 1 A inputs are supported by the UR hardware o Ratio compensation done automatically for CT ratio differences up to 32:1 2. Configure AC Signal Sources 3. Configure Bus Zone with Dynamic Bus Replica Bus Zone settings defines the boundaries of the Differential Protection and CT Trouble Monitoring.
67 GE Consumer & Industrial Multilin 29-Oct-22 Dual Percent Differential Characteristic High Breakpoint Low Breakpoint Low Slope High Slope High Set (Unrestrained) Min Pickup
68 GE Consumer & Industrial Multilin 29-Oct-22 Calculating Bus Differential Settings • The following Bus Zone Differential element 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)
69 GE Consumer & Industrial Multilin 29-Oct-22 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 GE Consumer & Industrial Multilin 29-Oct-22 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
71 GE Consumer & Industrial Multilin 29-Oct-22 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 GE Consumer & Industrial Multilin 29-Oct-22 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 GE Consumer & Industrial Multilin 29-Oct-22 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 GE Consumer & Industrial Multilin 29-Oct-22 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 GE Consumer & Industrial Multilin 29-Oct-22 Dual Percent Differential Characteristic High Breakpoint Low Breakpoint Low Slope High Slope High Set (Unrestrained) Min Pickup
76 GE Consumer & Industrial Multilin 29-Oct-22 NORTH BUS SOUTH BUS CT-8 B-5 B-6 CT-5 CT-6 S-5 S-6 B-4 CT-4 S-3 S-4 B-3 CT-3 S-1 S-2 B-2 CT-2 CT-1 B-1 C-1 C-2 C-4 C-3 C-5 CT-7 B-7 Protecting re-configurable buses Reconfigurable Buses
77 GE Consumer & Industrial Multilin 29-Oct-22 NORTH BUS SOUTH BUS CT-7 CT-8 B-7 B-5 B-6 CT-5 CT-6 S-5 S-6 B-4 CT-4 S-3 S-4 B-3 CT-3 S-1 S-2 B-2 CT-2 CT-1 B-1 C-1 C-2 C-4 C-3 C-5 Protecting re-configurable buses Reconfigurable Buses
78 GE Consumer & Industrial Multilin 29-Oct-22 NORTH BUS SOUTH BUS CT-7 CT-8 B-7 B-5 B-6 CT-5 CT-6 S-5 S-6 B-4 CT-4 S-3 S-4 B-3 CT-3 S-1 S-2 B-2 CT-2 CT-1 B-1 C-1 C-2 C-4 C-3 C-5 Protecting re-configurable buses Reconfigurable Buses
79 GE Consumer & Industrial Multilin 29-Oct-22 NORTH BUS SOUTH BUS CT-8 B-5 B-6 CT-5 CT-6 S-5 S-6 B-4 CT-4 S-3 S-4 B-3 CT-3 S-1 S-2 B-2 CT-2 CT-1 B-1 C-1 C-2 C-4 C-3 C-5 CT-7 B-7 Protecting re-configurable buses Reconfigurable Buses
80 GE Consumer & Industrial Multilin 29-Oct-22 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
81 GE Consumer & Industrial Multilin 29-Oct-22 Isolator – Typical Open/Closed Connections
82 GE Consumer & Industrial Multilin 29-Oct-22 Isolator Open Auxiliary Contact Isolator Closed Auxiliary Contact Isolator Position Alarm Block Switching Off On CLOSED No No Off Off LAST VALID After time delay until acknowledged Until Isolator Position is valid On On CLOSED On Off OPEN No No NOTE: Isolator monitoring function may be a built-in feature or user- programmable in low impedance bus differential digital relays Switch Status Logic and Dyanamic Bus Replica
83 GE Consumer & Industrial Multilin 29-Oct-22 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.
84 GE Consumer & Industrial Multilin 29-Oct-22 • 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
85 GE Consumer & Industrial Multilin 29-Oct-22 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) 87B operates Undervoltage condition CT OK
86 GE Consumer & Industrial Multilin 29-Oct-22 Example Architecture for Large Busbars Dual (redundant) fiber 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
87 GE Consumer & Industrial Multilin 29-Oct-22 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 Auxuliary switches wired here; Isolator Monitoring function configured here Example Architecture – Dynamic Bus Replica and Isolator Position
88 GE Consumer & Industrial Multilin 29-Oct-22 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 Example Architecture – BF Initiation & Current Supervision
89 GE Consumer & Industrial Multilin 29-Oct-22 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 command generated here and send to trip appropriate breakers Trip Trip Trip Example Architecture – Breaker Failure Tripping Trip
90 GE Consumer & Industrial Multilin 29-Oct-22 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 GE Consumer & Industrial Multilin 29-Oct-22
High_Low_Impedance_BusBar_Protection.ppt

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High_Low_Impedance_BusBar_Protection.ppt

  • 1. Fundamentals of Bus Bar Protection GE Multilin
  • 2. 2 GE Consumer & Industrial Multilin 29-Oct-22 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. 3 GE Consumer & Industrial Multilin 29-Oct-22 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
  • 4. 4 GE Consumer & Industrial Multilin 29-Oct-22 ZONE 1 ZONE 2 • Distribution and lower transmission voltage levels • Limited operating flexibility Multiple bus sections - single breaker with bus tie
  • 5. 5 GE Consumer & Industrial Multilin 29-Oct-22 ZONE 1 ZONE 2 • 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
  • 6. 6 GE Consumer & Industrial Multilin 29-Oct-22 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
  • 7. 7 GE Consumer & Industrial Multilin 29-Oct-22 ZONE 1 ZONE 2 • Very high operating flexibility • Transfer bus for breaker maintenance Double bus – single breaker w/ transfer bus
  • 8. 8 GE Consumer & Industrial Multilin 29-Oct-22 ZONE 1 ZONE 2 • High operating flexibility • Line protection covers bus section between two CTs • Fault on a bus does not disturb the power to circuits Double bus - double breaker
  • 9. 9 GE Consumer & Industrial Multilin 29-Oct-22 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
  • 10. 10 GE Consumer & Industrial Multilin 29-Oct-22 • Higher voltage levels • High operating flexibility with minimum breakers • Separate bus protection not required at line positions B1 B2 TB1 L1 L2 L3 L4 TB1 Ring bus
  • 11. 11 GE Consumer & Industrial Multilin 29-Oct-22 Bus components breakers SF6, EHV & HV - Synchropuff Low Voltage circuit breakers BUS 2 CB 1 BUS 1 ISO 1 ISO 2 ISO 3 BYPASS
  • 12. 12 GE Consumer & Industrial Multilin 29-Oct-22 - + F1a F1c Contact Input F1a On Contact Input F1c On F1b ISOLATOR 1 ISOLATOR 1 OPEN 7B 7A BUS 1 - + F1a F1c Contact Input F1a On Contact Input F1c On F1b ISOLATOR 1 ISOLATOR 1 CLOSED 7B 7A BUS 1 Disconnect switches & auxiliary contacts BUS 2 CB 1 BUS 1 ISO 1 ISO 2 ISO 3 BYPASS
  • 13. 13 GE Consumer & Industrial Multilin 29-Oct-22 BUS 2 CB 1 BUS 1 ISO 1 ISO 2 ISO 3 BYPASS Current Transformers Oil insulated current transformer (35kV up to 800kV) Gas (SF6) insulated current transformer Bushing type (medium voltage switchgear)
  • 14. 14 GE Consumer & Industrial Multilin 29-Oct-22 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. 15 GE Consumer & Industrial Multilin 29-Oct-22 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. 16 GE Consumer & Industrial Multilin 29-Oct-22 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 50 50 50 50 50 50 BLOCK
  • 17. 17 GE Consumer & Industrial Multilin 29-Oct-22 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 51
  • 18. 18 GE Consumer & Industrial Multilin 29-Oct-22 59 Linear Couplers ZC = 2  – 20  - typical coil impedance (5V per 1000Amps => 0.005 @ 60Hz ) If = 8000 A 40 V 10 V 10 V 0 V 20 V 2000 A 2000 A 4000 A 0 A 0 V External Fault
  • 19. 19 GE Consumer & Industrial Multilin 29-Oct-22 59 Linear Couplers Esec= Iprim*Xm - secondary voltage on relay terminals IR= Iprim*Xm /(ZR+ZC) – minimum operating current where, Iprim – primary current in each circuit Xm – liner coupler mutual reactance (5V per 1000Amps => 0.005 @ 60Hz ) ZR – relay tap impedance ZC – sum of all linear coupler self impedances If = 8000 A 0 A 0 V 10 V 10 V 0 V 20 V 40 V 2000 A 2000 A 4000 A 0 A Internal Bus Fault
  • 20. 20 GE Consumer & Industrial Multilin 29-Oct-22 • 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) Linear Couplers
  • 21. 21 GE Consumer & Industrial Multilin 29-Oct-22 High Impedance Differential • Operating signal created by connecting all CT secondaries in parallel o CTs must all have the same ratio o Must have dedicated CTs • Overvoltage element operates on voltage developed across resistor connected in secondary circuit 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 59
  • 22. 22 GE Consumer & Industrial Multilin 29-Oct-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 51 87 n DIF I I I I     ... 2 1 n RES I I I I     ... 2 1   n RES I I I I ..., , , max 2 1 
  • 23. 23 GE Consumer & Industrial Multilin 29-Oct-22 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
  • 24. 24 GE Consumer & Industrial Multilin 29-Oct-22 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)
  • 25. 25 GE Consumer & Industrial Multilin 29-Oct-22 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 52 DAU 52 DAU 52 DAU CU copper fiber
  • 26. 26 GE Consumer & Industrial Multilin 29-Oct-22 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
  • 27. 27 GE Consumer & Industrial Multilin 29-Oct-22 CT Saturation
  • 28. 28 GE Consumer & Industrial Multilin 29-Oct-22 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
  • 29. 29 GE Consumer & Industrial Multilin 29-Oct-22 CT Saturation Ratio Current CT Current Ratio Current CT Current No DC Offset • Waveform remains fairly symmetrical With DC Offset • Waveform starts off being asymmetrical, then symmetrical in steady state
  • 30. 30 GE Consumer & Industrial Multilin 29-Oct-22 External Fault & Ideal CTs • Fault starts at t0 • Steady-state fault conditions occur at t1 differential restraining t0 t1 Ideal CTs have no saturation or mismatch errors thus produce no differential current
  • 31. 31 GE Consumer & Industrial Multilin 29-Oct-22 External Fault & Actual CTs • Fault starts at t0 • Steady-state fault conditions occur at t1 differential restraining t0 t1 Actual CTs do introduce errors, producing some differential current (without CT saturation)
  • 32. 32 GE Consumer & Industrial Multilin 29-Oct-22 External Fault with CT Saturation • Fault starts at t0, CT begins to saturate at t1 • CT fully saturated at t2 differential restraining t0 t1 t2 CT saturation causes increasing differential current that may enter the differential element operate region.
  • 33. 33 GE Consumer & Industrial Multilin 29-Oct-22 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
  • 34. 34 GE Consumer & Industrial Multilin 29-Oct-22 High-Impedance Bus Differential Considerations
  • 35. 35 GE Consumer & Industrial Multilin 29-Oct-22 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
  • 36. 36 GE Consumer & Industrial Multilin 29-Oct-22 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. 37 GE Consumer & Industrial Multilin 29-Oct-22 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. 38 GE Consumer & Industrial Multilin 29-Oct-22 Electromechanical High Impedance Bus Differential Relays • Single phase relays • High-speed • High impedance voltage sensing • High seismic IOC unit
  • 39. 39 GE Consumer & Industrial Multilin 29-Oct-22 Operating time: 20 – 30ms @ I > 1.5xPKP P -based High-Impedance Bus Differential Protection Relays
  • 40. 40 GE Consumer & Industrial Multilin 29-Oct-22 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
  • 41. 41 GE Consumer & Industrial Multilin 29-Oct-22 High-Impedance Module + Overcurrent Relay
  • 42. 42 GE Consumer & Industrial Multilin 29-Oct-22 • 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
  • 43. 43 GE Consumer & Industrial Multilin 29-Oct-22 Low-Impedance Bus Differential Considerations
  • 44. 44 GE Consumer & Industrial Multilin 29-Oct-22 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. 45 GE Consumer & Industrial Multilin 29-Oct-22 • 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 2-8 Circuit Applications Small Bus Applications
  • 46. 46 GE Consumer & Industrial Multilin 29-Oct-22 • 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 CB 12 CB 11 • Different CT Ratio Capability for Each Circuit • Largest CT Primary is Base in Relay • 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 9-12 Circuit Applications Medium to Large Bus Applications
  • 47. 47 GE Consumer & Industrial Multilin 29-Oct-22 Large Bus Applications 87B phase A 87B phase B 87B phase C Logic relay (switch status, optional BF)
  • 48. 48 GE Consumer & Industrial Multilin 29-Oct-22 Large Bus Applications For buses with up to 24 circuits
  • 49. 49 GE Consumer & Industrial Multilin 29-Oct-22 Summing External Currents Not 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 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 CT-1 CT-2 CT-3 CT-4 I 3 = 0 I 2 = 0 I 1 = Error IDIFF = Error IREST = Error Maloperation if Error > PICKUP
  • 50. 50 GE Consumer & Industrial Multilin 29-Oct-22 Definitions of Restraint Signals “maximum of” “geometrical average” “scaled sum of” “sum of” n R i i i i i      ... 3 2 1   n R i i i i n i      ... 1 3 2 1   n R i i i i Max i ,..., , , 3 2 1  n n R i i i i i      ... 3 2 1
  • 51. 51 GE Consumer & Industrial Multilin 29-Oct-22 “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. 52 GE Consumer & Industrial Multilin 29-Oct-22 Bus Differential Adaptive Approach differential restraining Region 1 (low differential currents) Region 2 (high differential currents)
  • 53. 53 GE Consumer & Industrial Multilin 29-Oct-22 Bus Differential Adaptive Logic Diagram DIFL DIR SAT DIFH OR AND OR 87B BIASED OP AND
  • 54. 54 GE Consumer & Industrial Multilin 29-Oct-22 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 Secondary Current of Faulted Circuit (Severe CT Saturation)
  • 55. 55 GE Consumer & Industrial Multilin 29-Oct-22 Phase Comparison Principle Continued… BLOCK OPERATE BLOCK          p D p I I I real          p D p I I I imag Ip ID - Ip External Fault Conditions OPERATE BLOCK BLOCK          p D p I I I real          p D p I I I imag Ip ID - Ip Internal Fault Conditions OPERATE OPERATE
  • 56. 56 GE Consumer & Industrial Multilin 29-Oct-22 CT Saturation • Fault starts at t0, CT begins to saturate at t1 • CT fully saturated at t2 differential restraining t0 t1 t2
  • 57. 57 GE Consumer & Industrial Multilin 29-Oct-22 CT Saturation Detector State Machine NORMAL SAT := 0 EXTERNAL FAULT SAT := 1 EXTERNAL FAULT & CT SATURATION SAT := 1 The differential characteristic entered The differential- restraining trajectory out of the differential characteristic for certain period of time saturation condition The differential current below the first slope for certain period of time
  • 58. 58 GE Consumer & Industrial Multilin 29-Oct-22 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. 59 GE Consumer & Industrial Multilin 29-Oct-22 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) 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
  • 60. 60 GE Consumer & Industrial Multilin 29-Oct-22 The bus differential protection element picks up due to heavy CT saturation The CT saturation flag is set safely before the pickup flag The directional flag is not set The element does not maloperate Despite heavy CT saturation the external fault current is seen in the opposite direction CT Saturation Example – External Fault 0.06 0.07 0.08 0.09 0.1 0.11 0.12 -200 -150 -100 -50 0 50 100 150 200 time, sec current, A ~1 ms
  • 61. 61 GE Consumer & Industrial Multilin 29-Oct-22 The bus differential protection element picks up The saturation flag is not set - no directional decision required The element operates in 10ms The directional flag is set All the fault currents are seen in one direction CT Saturation – Internal Fault Example
  • 62. 62 GE Consumer & Industrial Multilin 29-Oct-22 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. 63 GE Consumer & Industrial Multilin 29-Oct-22 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. 64 GE Consumer & Industrial Multilin 29-Oct-22 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. 65 GE Consumer & Industrial Multilin 29-Oct-22 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
  • 66. 66 GE Consumer & Industrial Multilin 29-Oct-22 Configuring the Bus Differential Zone 1. Configure the physical CT Inputs o CT Primary and Secondary values o Both 5 A and 1 A inputs are supported by the UR hardware o Ratio compensation done automatically for CT ratio differences up to 32:1 2. Configure AC Signal Sources 3. Configure Bus Zone with Dynamic Bus Replica Bus Zone settings defines the boundaries of the Differential Protection and CT Trouble Monitoring.
  • 67. 67 GE Consumer & Industrial Multilin 29-Oct-22 Dual Percent Differential Characteristic High Breakpoint Low Breakpoint Low Slope High Slope High Set (Unrestrained) Min Pickup
  • 68. 68 GE Consumer & Industrial Multilin 29-Oct-22 Calculating Bus Differential Settings • The following Bus Zone Differential element 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)
  • 69. 69 GE Consumer & Industrial Multilin 29-Oct-22 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. 70 GE Consumer & Industrial Multilin 29-Oct-22 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
  • 71. 71 GE Consumer & Industrial Multilin 29-Oct-22 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. 72 GE Consumer & Industrial Multilin 29-Oct-22 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. 73 GE Consumer & Industrial Multilin 29-Oct-22 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. 74 GE Consumer & Industrial Multilin 29-Oct-22 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. 75 GE Consumer & Industrial Multilin 29-Oct-22 Dual Percent Differential Characteristic High Breakpoint Low Breakpoint Low Slope High Slope High Set (Unrestrained) Min Pickup
  • 76. 76 GE Consumer & Industrial Multilin 29-Oct-22 NORTH BUS SOUTH BUS CT-8 B-5 B-6 CT-5 CT-6 S-5 S-6 B-4 CT-4 S-3 S-4 B-3 CT-3 S-1 S-2 B-2 CT-2 CT-1 B-1 C-1 C-2 C-4 C-3 C-5 CT-7 B-7 Protecting re-configurable buses Reconfigurable Buses
  • 77. 77 GE Consumer & Industrial Multilin 29-Oct-22 NORTH BUS SOUTH BUS CT-7 CT-8 B-7 B-5 B-6 CT-5 CT-6 S-5 S-6 B-4 CT-4 S-3 S-4 B-3 CT-3 S-1 S-2 B-2 CT-2 CT-1 B-1 C-1 C-2 C-4 C-3 C-5 Protecting re-configurable buses Reconfigurable Buses
  • 78. 78 GE Consumer & Industrial Multilin 29-Oct-22 NORTH BUS SOUTH BUS CT-7 CT-8 B-7 B-5 B-6 CT-5 CT-6 S-5 S-6 B-4 CT-4 S-3 S-4 B-3 CT-3 S-1 S-2 B-2 CT-2 CT-1 B-1 C-1 C-2 C-4 C-3 C-5 Protecting re-configurable buses Reconfigurable Buses
  • 79. 79 GE Consumer & Industrial Multilin 29-Oct-22 NORTH BUS SOUTH BUS CT-8 B-5 B-6 CT-5 CT-6 S-5 S-6 B-4 CT-4 S-3 S-4 B-3 CT-3 S-1 S-2 B-2 CT-2 CT-1 B-1 C-1 C-2 C-4 C-3 C-5 CT-7 B-7 Protecting re-configurable buses Reconfigurable Buses
  • 80. 80 GE Consumer & Industrial Multilin 29-Oct-22 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
  • 81. 81 GE Consumer & Industrial Multilin 29-Oct-22 Isolator – Typical Open/Closed Connections
  • 82. 82 GE Consumer & Industrial Multilin 29-Oct-22 Isolator Open Auxiliary Contact Isolator Closed Auxiliary Contact Isolator Position Alarm Block Switching Off On CLOSED No No Off Off LAST VALID After time delay until acknowledged Until Isolator Position is valid On On CLOSED On Off OPEN No No NOTE: Isolator monitoring function may be a built-in feature or user- programmable in low impedance bus differential digital relays Switch Status Logic and Dyanamic Bus Replica
  • 83. 83 GE Consumer & Industrial Multilin 29-Oct-22 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.
  • 84. 84 GE Consumer & Industrial Multilin 29-Oct-22 • 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
  • 85. 85 GE Consumer & Industrial Multilin 29-Oct-22 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) 87B operates Undervoltage condition CT OK
  • 86. 86 GE Consumer & Industrial Multilin 29-Oct-22 Example Architecture for Large Busbars Dual (redundant) fiber 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
  • 87. 87 GE Consumer & Industrial Multilin 29-Oct-22 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 Auxuliary switches wired here; Isolator Monitoring function configured here Example Architecture – Dynamic Bus Replica and Isolator Position
  • 88. 88 GE Consumer & Industrial Multilin 29-Oct-22 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 Example Architecture – BF Initiation & Current Supervision
  • 89. 89 GE Consumer & Industrial Multilin 29-Oct-22 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 command generated here and send to trip appropriate breakers Trip Trip Trip Example Architecture – Breaker Failure Tripping Trip
  • 90. 90 GE Consumer & Industrial Multilin 29-Oct-22 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. 91 GE Consumer & Industrial Multilin 29-Oct-22