fundamentals of bus bar protection ge multilin. 2 ge consumer & industrial multilin 2-jun-14 outline...
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Fundamentals of Bus Bar Protection GE Multilin Slide 2 2 GE Consumer & Industrial Multilin 2-Jun-14 Outline Bus arrangements Bus components Bus protection techniques CT Saturation Application Considerations: High impedance bus differential relaying Low impedance bus differential relaying Special topics Slide 3 3 GE Consumer & Industrial Multilin 2-Jun-14 Distribution and lower transmission voltage levels No operating flexibility Fault on the bus trips all circuit breakers Single bus - single breaker Slide 4 4 GE Consumer & Industrial Multilin 2-Jun-14 Distribution and lower transmission voltage levels Limited operating flexibility Multiple bus sections - single breaker with bus tie Slide 5 5 GE Consumer & Industrial Multilin 2-Jun-14 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 Slide 6 6 GE Consumer & Industrial Multilin 2-Jun-14 Increased operating flexibility A bus fault requires tripping all breakers Transfer bus for breaker maintenance Main and transfer buses Slide 7 7 GE Consumer & Industrial Multilin 2-Jun-14 Very high operating flexibility Transfer bus for breaker maintenance Double bus single breaker w/ transfer bus Slide 8 8 GE Consumer & Industrial Multilin 2-Jun-14 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 Slide 9 9 GE Consumer & Industrial Multilin 2-Jun-14 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 Slide 10 10 GE Consumer & Industrial Multilin 2-Jun-14 Higher voltage levels High operating flexibility with minimum breakers Separate bus protection not required at line positions Ring bus Slide 11 11 GE Consumer & Industrial Multilin 2-Jun-14 Bus components breakers SF6, EHV & HV - Synchropuff Low Voltage circuit breakers Slide 12 12 GE Consumer & Industrial Multilin 2-Jun-14 Disconnect switches & auxiliary contacts Slide 13 13 GE Consumer & Industrial Multilin 2-Jun-14 Current Transformers Oil insulated current transformer (35kV up to 800kV) Gas (SF6) insulated current transformer Bushing type (medium voltage switchgear) Slide 14 14 GE Consumer & Industrial Multilin 2-Jun-14 Protection Requirements High bus fault currents due to large number of circuits connected: CT saturation often becomes a problem as CTs may not be sufficiently rated for worst fault condition case large dynamic forces associated with bus faults require fast clearing times in order to reduce equipment damage False trip by bus protection may create serious problems: service interruption to a large number of circuits (distribution and sub- transmission voltage levels) system-wide stability problems (transmission voltage levels) With both dependability and security important, preference is always given to security Slide 15 15 GE Consumer & Industrial Multilin 2-Jun-14 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 Slide 16 16 GE Consumer & Industrial Multilin 2-Jun-14 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 Slide 17 17 GE Consumer & Industrial Multilin 2-Jun-14 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 Slide 18 18 GE Consumer & Industrial Multilin 2-Jun-14 59 Linear Couplers Z C = 2 20 - typical coil impedance (5V per 1000Amps => 0.005 @ 60Hz ) If = 8000 A 40 V10 V 0 V20 V 2000 A 4000 A0 A 0 V External Fault Slide 19 19 GE Consumer & Industrial Multilin 2-Jun-14 59 Linear Couplers E sec = I prim *X m - secondary voltage on relay terminals I R = I prim *X m /(Z R + Z C ) minimum operating current where, I prim primary current in each circuit X m liner coupler mutual reactance (5V per 1000Amps => 0.005 @ 60Hz ) Z R relay tap impedance Z C sum of all linear coupler self impedances If = 8000 A 0 A 0 V10 V 0 V20 V 40 V 2000 A 4000 A0 A Internal Bus Fault Slide 20 20 GE Consumer & Industrial Multilin 2-Jun-14 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 Slide 21 21 GE Consumer & Industrial Multilin 2-Jun-14 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 Slide 22 22 GE Consumer & Industrial Multilin 2-Jun-14 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 Slide 23 23 GE Consumer & Industrial Multilin 2-Jun-14 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 Slide 24 24 GE Consumer & Industrial Multilin 2-Jun-14 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) Slide 25 25 GE Consumer & Industrial Multilin 2-Jun-14 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 Slide 26 26 GE Consumer & Industrial Multilin 2-Jun-14 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. Slide 27 27 GE Consumer & Industrial Multilin 2-Jun-14 CT Saturation Slide 28 28 GE Consumer & Industrial Multilin 2-Jun-14 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 Slide 29 29 GE Consumer & Industrial Multilin 2-Jun-14 CT Saturation No DC Offset Waveform remains fairly symmetrical With DC Offset Waveform starts off being asymmetrical, then symmetrical in steady state Slide 30 30 GE Consumer & Industrial Multilin 2-Jun-14 External Fault & Ideal CTs Fault starts at t 0 Steady-state fault conditions occur at t 1 t0t0 t1t1 Ideal CTs have no saturation or mismatch errors thus produce no differential current Slide 31 31 GE Consumer & Industrial Multilin 2-Jun-14 External Fault & Actual CTs Fault starts at t 0 Steady-state fault conditions occur at t 1 t0t0 t1t1 Actual CTs do introduce errors, producing some differential current (without CT saturation) Slide 32 32 GE Consumer & Industrial Multilin 2-Jun-14 External Fault with CT Saturation Fault starts at t 0, CT begins to saturate at t 1 CT fully saturated at t 2 t0t0 t1t1 t2t2 CT saturation causes increasing differential current that may enter the differential element operate region. Slide 33 33 GE Consumer & Industrial Multilin 2-Jun-14 Som