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Fundamentals of Bus Bar Protection GE Multilin Outline • Bus arrangements • Bus components • Bus protection techniques • CT Saturation • Application Considerations: High impedance bus differential relaying Low impedance bus differential relaying Special topics 2 GE Consumer & Industrial Multilin 22-May-17 Single bus - single breaker ZONE 1 1 2 3 ---- n-1 n • Distribution and lower transmission voltage levels • No operating flexibility • Fault on the bus trips all circuit breakers 3 GE Consumer & Industrial Multilin 22-May-17 Multiple bus sections - single breaker with bus tie ZONE 1 ZONE 2 • Distribution and lower transmission voltage levels • Limited operating flexibility 4 GE Consumer & Industrial Multilin 22-May-17 Double bus - single breaker with bus tie 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 5 GE Consumer & Industrial Multilin 22-May-17 Main and transfer buses MAIN BUS ZONE 1 TRANFER BUS • Increased operating flexibility • A bus fault requires tripping all breakers • Transfer bus for breaker maintenance 6 GE Consumer & Industrial Multilin 22-May-17 Double bus – single breaker w/ transfer bus ZONE 1 ZONE 2 • Very high operating flexibility • Transfer bus for breaker maintenance 7 GE Consumer & Industrial Multilin 22-May-17 Double bus - double breaker 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 8 GE Consumer & Industrial Multilin 22-May-17 Breaker-and-a-half bus 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 9 GE Consumer & Industrial Multilin 22-May-17 Ring bus L1 L2 TB1 B1 B2 TB1 L3 L4 • Higher voltage levels • High operating flexibility with minimum breakers • Separate bus protection not required at line positions 10 GE Consumer & Industrial Multilin 22-May-17 Bus components breakers BUS 1 BUS 2 ISO 1 ISO 2 Low Voltage circuit breakers CB 1 ISO 3 BYPASS SF6, EHV & HV - 11 GE Consumer & Industrial Multilin Synchropuff 22-May-17 Disconnect switches & auxiliary contacts BUS 1 BUS 1 BUS 2 ISO 2 ISOLATOR 1 ISO 1 + 7B 7A ISOLATOR 1 OPEN F1a F1c F1b Contact Input F1a On Contact Input F1c On - CB 1 ISO 3 BYPASS ISOLATOR 1 BUS 1 + 7B 7A ISOLATOR 1 CLOSED F1a F1c F1b - Contact Input F1a On Contact Input F1c On 12 GE Consumer & Industrial Multilin 22-May-17 Current Transformers BUS 1 BUS 2 ISO 1 ISO 2 Gas (SF6) insulated current transformer Oil insulated current transformer (35kV up to 800kV) CB 1 ISO 3 BYPASS Bushing type (medium voltage switchgear) 13 GE Consumer & Industrial Multilin 22-May-17 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 subtransmission voltage levels) • system-wide stability problems (transmission voltage levels) With both dependability and security important, preference is always given to security 14 GE Consumer & Industrial Multilin 22-May-17 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 15 GE Consumer & Industrial Multilin 22-May-17 Interlocking Schemes BLOCK 50 50 50 50 50 50 • Blocking scheme typically used • Short coordination time required • Care must be taken with possible saturation of feeder CTs • Blocking signal could be sent over communications ports (peer-to-peer) • This technique is limited to simple one-incomer distribution buses 16 GE Consumer & Industrial Multilin 22-May-17 Overcurrent (unrestrained) Differential • • 51 • • • 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 17 GE Consumer & Industrial Multilin 22-May-17 Linear Couplers ZC = 2 – 20 - typical coil impedance (5V per 1000Amps => 0.005 @ 60Hz ) 40 V 10 V 10 V 0V 20 V 0V 59 External Fault If = 8000 A 2000 A 2000 A 0A 4000 A 18 GE Consumer & Industrial Multilin 22-May-17 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 Internal Bus Fault 40 V 0V 0A 10 V 2000 A 10 V 2000 A 0V 0A 59 20 V 4000 A 19 GE Consumer & Industrial Multilin 22-May-17 Linear Couplers • • • • Fast, secure and proven Require dedicated air gap CTs, which may not be used for any other protection Cannot be easily applied to reconfigurable buses The scheme uses a simple voltage detector – it does not provide benefits of a microprocessor-based relay (e.g. oscillography, breaker failure protection, other functions) 20 GE Consumer & Industrial Multilin 22-May-17 High Impedance Differential • Operating signal created by connecting all CT secondaries in parallel CTs must all have the same ratio o Must have dedicated CTs o 59 • 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 21 GE Consumer & Industrial Multilin 22-May-17 Percent Differential • • 87 51 • • I DIF I1 I 2 ... I n I RES I1 I 2 ... I n Percent characteristic used to cope with CT saturation and other errors Restraining signal can be formed in a number of ways No dedicated CTs needed Used for protection of reconfigurable buses possible I RES max I1 , I 2 , ..., I n 22 GE Consumer & Industrial Multilin 22-May-17 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 23 GE Consumer & Industrial Multilin 22-May-17 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) 24 GE Consumer & Industrial Multilin 22-May-17 Low Impedance Differential (Distributed) 52 52 DAU 52 DAU CU copper fiber DAU • 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 25 GE Consumer & Industrial Multilin 22-May-17 Low Impedance Differential (Centralized) 52 52 52 CU • All currents applied to a single central processor • No communications, external sampling synchronisation necessary • Perceived more reliable (less hardware needed) • Well suited to both new and retrofit applications. copper 26 GE Consumer & Industrial Multilin 22-May-17 CT Saturation 27 GE Consumer & Industrial Multilin 22-May-17 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 28 GE Consumer & Industrial Multilin 22-May-17 CT Saturation No DC Offset • Waveform remains fairly symmetrical Ratio Current CT Current With DC Offset Ratio Current CT Current • Waveform starts off being asymmetrical, then symmetrical in steady state 29 GE Consumer & Industrial Multilin 22-May-17 differential External Fault & Ideal CTs t1 t0 restraining • Fault starts at t0 • Steady-state fault conditions occur at t1 Ideal CTs have no saturation or mismatch errors thus produce no differential current 30 GE Consumer & Industrial Multilin 22-May-17 differential External Fault & Actual CTs t1 t0 restraining • Fault starts at t0 • Steady-state fault conditions occur at t1 Actual CTs do introduce errors, producing some differential current (without CT saturation) 31 GE Consumer & Industrial Multilin 22-May-17 External Fault with CT Saturation differential t2 t1 restraining • Fault starts at t0, CT begins to saturate at t1 • CT fully saturated at t2 t0 CT saturation causes increasing differential current that may enter the differential element operate region. 32 GE Consumer & Industrial Multilin 22-May-17 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 33 GE Consumer & Industrial Multilin 22-May-17 High-Impedance Bus Differential Considerations 34 GE Consumer & Industrial Multilin 22-May-17 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 35 GE Consumer & Industrial Multilin 22-May-17 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. 36 GE Consumer & Industrial Multilin 22-May-17 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. 37 GE Consumer & Industrial Multilin 22-May-17 Electromechanical High Impedance Bus Differential Relays • Single phase relays • High-speed • High impedance voltage sensing • High seismic IOC unit 38 GE Consumer & Industrial Multilin 22-May-17 P -based High-Impedance Bus Differential Protection Relays Operating time: 20 – 30ms @ I > 1.5xPKP 39 GE Consumer & Industrial Multilin 22-May-17 High Impedance Module for Digital Relays RST = 2000 - stabilizing resistor to limit the current through the relay, and force it to the lower impedance CT windings. MOV – Metal Oxide Varistor to limit the voltage to 1900 Volts 86 – latching contact preventing the resistors from overheating after the fault is detected 40 GE Consumer & Industrial Multilin 22-May-17 High-Impedance Module + Overcurrent Relay 41 GE Consumer & Industrial Multilin 22-May-17 High Impedance Bus Protection - Summary • Fast, secure and proven • Requires dedicated CTs, preferably with the same CT ratio and using full tap • Can be applied to small buses • Depending on bus internal and external fault currents, high impedance bus diff may not provide adequate settings for both sensitivity and security • Cannot be easily applied to reconfigurable buses • Require voltage limiting varistor capable of absorbing significant energy • May require auxiliary CTs • Do not provide full benefits of microprocessor-based relay system (e.g. metering, monitoring, oscillography, etc.) 42 GE Consumer & Industrial Multilin 22-May-17 Low-Impedance Bus Differential Considerations 43 GE Consumer & Industrial Multilin 22-May-17 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 44 GE Consumer & Industrial Multilin 22-May-17 Small Bus Applications 2-8 Circuit Applications • Up to 24 Current Inputs • 4 Zones • Zone 1 = Phase A • Zone 2 = Phase B • Zone 3 = Phase C • Zone 4 = Not used • Different CT Ratio Capability for Each Circuit • Largest CT Primary is Base in Relay 45 GE Consumer & Industrial Multilin 22-May-17 Medium to Large Bus Applications 9-12 Circuit Applications • Relay 1 - 24 Current Inputs • 4 Zones • Zone 1 = Phase A (12 currents) • Zone 2 = Phase B (12 currents) • Zone 3 = Not used • Zone 4 = Not used • Relay 2 - 24 Current Inputs • 4 Zones • Zone 1 = Not used • Zone 2 = Not used • Zone 3 = Phase C (12 currents) • Zone 4 = Not used • Different CT Ratio Capability for Each Circuit • Largest CT Primary is Base in Relay CB 11 CB 12 46 GE Consumer & Industrial Multilin 22-May-17 Large Bus Applications 87B phase A 87B phase B 87B phase C Logic relay (switch status, optional BF) 47 GE Consumer & Industrial Multilin 22-May-17 Large Bus Applications For buses with up to 24 circuits 48 GE Consumer & Industrial Multilin 22-May-17 Summing External Currents Not Recommended for Low-Z 87B relays • Relay becomes combination of restrained and unrestrained elements •In order to parallel CTs: CT-1 I1 = Error CT-2 I2 = 0 CT-3 I3 = 0 CT-4 IDIFF = Error IREST = Error Maloperation if Error > PICKUP • 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 49 GE Consumer & Industrial Multilin 22-May-17 Definitions of Restraint Signals iR i1 i2 i3 ... in 1 iR i1 i2 i3 ... in n iR n i1 i2 i3 ... in iR Max i1 , i2 , i3 ,..., in “sum of” “scaled sum of” “geometrical average” “maximum of” 50 GE Consumer & Industrial Multilin 22-May-17 “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) 51 GE Consumer & Industrial Multilin 22-May-17 differential Bus Differential Adaptive Approach Region 2 (high differential currents) Region 1 (low differential currents) restraining 52 GE Consumer & Industrial Multilin 22-May-17 Bus Differential Adaptive Logic Diagram AND DIFL OR OR DIR AND SAT 87B BIASED OP DIFH 53 GE Consumer & Industrial Multilin 22-May-17 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) 54 GE Consumer & Industrial Multilin 22-May-17 Phase Comparison Principle Continued… External Fault Conditions Ip imag ID I p Ip imag ID I p OPERATE BLOCK ID - Ip Internal Fault Conditions Ip Ip real ID I p OPERATE BLOCK ID - Ip Ip real ID I p Ip BLOCK BLOCK OPERATE OPERATE 55 GE Consumer & Industrial Multilin 22-May-17 CT Saturation differential t2 t1 t0 restraining • Fault starts at t0, CT begins to saturate at t1 • CT fully saturated at t2 56 GE Consumer & Industrial Multilin 22-May-17 CT Saturation Detector State Machine NORMAL SAT := 0 The differential current below the first slope for certain period of time saturation condition EXTERNAL FAULT SAT := 1 The differential characteristic entered EXTERNAL FAULT & CT SATURATION The differentialrestraining trajectory out of the differential characteristic for certain period of time SAT := 1 57 GE Consumer & Industrial Multilin 22-May-17 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 58 GE Consumer & Industrial Multilin 22-May-17 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 59 GE Consumer & Industrial Multilin 22-May-17 CT Saturation Example – External Fault 200 150 current, A 100 ~1 ms 50 0 -50 -100 -150 -200 0.06 0.07 0.08 0.09 0.1 0.11 0.12 time, sec The bus differential protection element picks up due to heavy CT saturation The directional flag is not set The CT saturation flag is set safely before the pickup flag The element does not maloperate Despite heavy CT saturation the external fault current is seen in the opposite direction 60 GE Consumer & Industrial Multilin 22-May-17 CT Saturation – Internal Fault Example The bus differential protection element picks up The saturation flag is not set - no directional decision required All the fault currents are seen in one direction The element operates in 10ms The directional flag is set 61 GE Consumer & Industrial Multilin 22-May-17 Applying Low-Impedance Differential Relays for Busbar Protection Basic Topics • Configure physical CT Inputs • Configure Bus Zone and Dynamic Bus Replica • Calculating Bus Differential Element settings Advanced Topics • Isolator switch monitoring for reconfigurable buses • Differential Zone CT Trouble • Integrated Breaker Failure protection 62 GE Consumer & Industrial Multilin 22-May-17 Configuring CT Inputs • For each connected CT circuit enter Primary rating and select Secondary rating. • Each 3-phase bank of CT inputs must be assigned to a Signal Source that is used to define the Bus Zone and Dynamic Bus Replica Some relays define 1 p.u. as the maximum primary current of all of the CTs connected in the given Bus Zone 63 GE Consumer & Industrial Multilin 22-May-17 Per-Unit Current Definition - Example Current Channel Primary Secondary Zone CT-1 CT-2 CT-3 CT-4 F1 3200 A 1A 1 F2 2400 A 5A 1 F3 1200 A 1A 1 F4 3200 A 1A 2 CT-5 CT-6 F5 1200 A 5A 2 F6 5000 A 5A 2 • For Zone 1, 1 p.u. = 3200 AP • For Zone 2, 1 p.u. = 5000 AP 64 GE Consumer & Industrial Multilin 22-May-17 Configuration of Bus Zone • Dynamic Bus Replica associates a status signal with each current in the Bus Differential Zone • Status signal can be any logic operand o Status signals can be developed in programmable logic to provide additional checks or security as required o Status signal can be set to ‘ON’ if current is always in the bus zone or ‘OFF’ if current is never in the bus zone • CT connections/polarities for a particular bus zone must be properly configured in the relay, via either hardwire or software 65 GE Consumer & Industrial Multilin 22-May-17 Configuring the Bus Differential Zone Bus Zone settings defines the boundaries of the Differential Protection and CT Trouble Monitoring. 1. Configure the physical CT Inputs o o o CT Primary and Secondary values Both 5 A and 1 A inputs are supported by the UR hardware Ratio compensation done automatically for CT ratio differences up to 32:1 2. Configure AC Signal Sources 3. Configure Bus Zone with Dynamic Bus Replica 66 GE Consumer & Industrial Multilin 22-May-17 Dual Percent Differential Characteristic High Set (Unrestrained) High Slope Low Slope High Breakpoint Min Pickup Low Breakpoint 67 GE Consumer & Industrial Multilin 22-May-17 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) 68 GE Consumer & Industrial Multilin 22-May-17 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 69 GE Consumer & Industrial Multilin 22-May-17 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 70 GE Consumer & Industrial Multilin 22-May-17 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 71 GE Consumer & Industrial Multilin 22-May-17 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 72 GE Consumer & Industrial Multilin 22-May-17 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 73 GE Consumer & Industrial Multilin 22-May-17 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. 74 GE Consumer & Industrial Multilin 22-May-17 Dual Percent Differential Characteristic High Set (Unrestrained) High Slope Low Slope High Breakpoint Min Pickup Low Breakpoint 75 GE Consumer & Industrial Multilin 22-May-17 Reconfigurable Buses C-3 C-5 NORTH BUS S-1 B-1 S-5 S-3 B-5 CT-7 CT-1 CT-2 B-2 CT-3 B-3 CT-4 CT-5 B-4 B-7 CT-6 CT-8 B-6 S-2 S-6 S-4 SOUTH BUS C-1 C-2 C-4 Protecting re-configurable buses 76 GE Consumer & Industrial Multilin 22-May-17 Reconfigurable Buses C-3 C-5 NORTH BUS S-1 B-1 CT-1 S-5 S-3 B-5 CT-2 B-2 CT-3 CT-4 B-3 CT-7 B-4 CT-5 B-7 CT-6 CT-8 B-6 S-2 S-6 S-4 SOUTH BUS C-1 C-2 C-4 Protecting re-configurable buses 77 GE Consumer & Industrial Multilin 22-May-17 Reconfigurable Buses C-3 C-5 NORTH BUS S-1 B-1 CT-1 S-5 S-3 B-5 CT-2 B-2 CT-3 CT-4 B-3 CT-7 B-4 CT-5 B-7 CT-6 CT-8 B-6 S-2 S-6 S-4 SOUTH BUS C-1 C-2 C-4 Protecting re-configurable buses 78 GE Consumer & Industrial Multilin 22-May-17 Reconfigurable Buses C-3 C-5 NORTH BUS S-1 B-1 S-5 S-3 B-5 CT-7 CT-1 CT-2 B-2 CT-3 B-3 CT-4 CT-5 B-4 B-7 CT-6 CT-8 B-6 S-2 S-6 S-4 SOUTH BUS C-1 C-2 C-4 Protecting re-configurable buses 79 GE Consumer & Industrial Multilin 22-May-17 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 80 GE Consumer & Industrial Multilin 22-May-17 Isolator – Typical Open/Closed Connections 81 GE Consumer & Industrial Multilin 22-May-17 Switch Status Logic and Dyanamic Bus Replica Isolator Open Auxiliary Contact Isolator Closed Auxiliary Contact Isolator Position Alarm Block Switching Off On CLOSED No No Off Off LAST VALID Until Isolator Position is valid On On CLOSED After time delay until acknowledged On Off OPEN No No NOTE: Isolator monitoring function may be a built-in feature or userprogrammable in low impedance bus differential digital relays 82 GE Consumer & Industrial Multilin 22-May-17 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. 83 GE Consumer & Industrial Multilin 22-May-17 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) 84 GE Consumer & Industrial Multilin 22-May-17 Differential Zone CT Trouble – Strategy #2 Example 87B operates Undervoltage condition CT OK • CT Trouble operand is used to rise an alarm • The 87B trip is inhibited after CT Trouble element operates • The relay may misoperate if an external fault occurs after CT trouble but before the CT trouble condition is declared (double-contingency) 85 GE Consumer & Industrial Multilin 22-May-17 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 86 GE Consumer & Industrial Multilin 22-May-17 Example Architecture – Dynamic Bus Replica and Isolator Position 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 87 GE Consumer & Industrial Multilin 22-May-17 Example Architecture – BF Initiation & Current Supervision 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 88 GE Consumer & Industrial Multilin 22-May-17 Example Architecture – Breaker Failure Trip Tripping Trip Phase A AC signals wired here, current status monitored here Phase B AC signals wired here, current status monitored here Trip Trip Phase C AC signals wired here, current status monitored here Breaker Fail Op command generated here and send to trip appropriate breakers 89 GE Consumer & Industrial Multilin 22-May-17 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. 90 GE Consumer & Industrial Multilin 22-May-17 91 GE Consumer & Industrial Multilin 22-May-17