Bus Differential Function Settings
Contents
1. f People with Passion Best in Class Technology Power System Focus Easy to Use Monitor and Manage your Power System with Ease B PRO Bus Protection Relay Bus Differential Function Settings Introduction The B PRO Bus Protection Relay provides low impedance differential protection for transmission and distribution buses for up to 6 three phase inputs The B PRO is a per unit based relay meaning all secondary currents entering the relay are converted to per unit values for protection calculations Choosing the correct per unit base for the current permits the development of standard protection settings that work for most bus protection applications The resulting settings method only requires 2 calculations to implement The settings described in this technical note will provide a good balance between dependability and security of the protection sys tem This method also assumes the CTs have burdens within their ratings and that the measured secondary current by the CTs is between 0 2 A and 15 A secondary Bus Differential 87B Protection Function Characteristic The operating characteristic of the 87B function is shown in Figure 1 lOmin is the minimum operating current for fault detection IRs is the setting for the breakpoint between the region of slope S1 and the region of slope S2 IRs is normally used as the upper limit of the load range or the protection zone used to detect bus faults during normal load cond2. Bus Differential Function Settings Easy to Use Settings Philosophy The recommended settings method described in this technical note is simple choose the MVA Base setting of the relay such that the maximum bus transfer load current is equal to the IRs setting in per unit current All other settings are related to the per unit base current The preferred setting for IRs is 2 per unit but there are some applications where a higher value is required This method ensures that IRs is the upper limit setting for the load range provides dependability for normal operating conditions and provides adequate security for through fault events The basic settings philosophy is to make sure operating and restraint current values during normal operating conditions fall in the load range This means the S1 range accommodates the actual operating current accounting for the maximum CT measurement error under any load condition External fault conditions will be in the S2 or High Current range of the characteristic Figure 2 describes recommended settings for most bus differential applica tions 87B Bus Differential Flines Omn 925 Fa St 2 08 u a Hga Current Semng 16 00 F FZ CT Satursten Osiectoa CT Sat Wax Sbor ois Sus Sase UVA 4789 UVA Pi ins set n Syste Parameters Oaa Retress PretGrapn Reset Zos Figure 2 87B settings Settings Method The clearest way to understand the proposed bus differential settings
3. method is through a specific example Figure 3 illustrates normal load conditions Figure 3 Bus transfer load www erlphase com info erlphase com 204 477 0591 PHASE 74 Scurfield Blvd Power Technologies Ltd Winnipeg MB smart solutions that empower you Canada R3Y 1G4 ERL B PRO Bus Protection Relay Bus Differential Function Settings Easy to Use Settings are developed using simple steps 1 Enter Winding CT connection information 2 Determine the maximum bus transfer current 3 Choose a value for IRs 4 Calculate the Bus Base MVA 5 Determine the CT Saturation Block Timer 6 Enter settings in the B PRO 1 Enter Winding CT Connection Information Enter Winding and CT connection information to match the specific bus protection application as in the example of Figure 4 Winding CT Connections LA Oifferente Zones Bua Swe fier tu Sue Oha Wantage w Pri 13406 Tranatormat Cessectnn O Bes ana Tranatecmer Prose oss O ret Tras toemer 3 Phase Capacty UVA Pri ct Cttee 830 CT Tune Onta Current noes ConnectTo Connection toeg Rate Ao 1 Controt External rost 1 rout Sus v y 2Ppe2 t lt Y Y t Iimo Aus 9 G 4 japi Du 8 n 400 400 e2 coocoo S mossi Joogogg at iS 444 emaan Figure 4 Winding CT Connections settings 2 Determine the Maximum Bus Transfer Current The maximum bus transfer current is going to be based on the CT primary ratings F
4. for the bus differential protection is 2400 A 2 1200 A When the secondary current of a nominally 5A CT is as small as 0 2 A there is a possibility that the current may be interpreted as close to zero amps due to CT error and A to D conversion error Assuming the worst case 0 2 amps is interpreted as zero amps by the relay Then for this light load condition the operating current IO is perceived as 320 0 320 primary amps On a 1200 A base this is 0 27 per unit which is above the recommended setting for lOmin namely 0 25 per unit For this case it is recommended to set lOmin to 0 4 per unit in order to prevent possible false tripping of the bus References 1 B PRO Bus Protection Relay User Manual Version 1 1 Revision 2 NxtPhase Corporation Vancouver BC Canada 2000 www erlphase com info erlphase com 204 477 0591 ERL PHASE 74 Scurfield Blvd Power Technologies Ltd Winnipeg MB smart solutions that empower you Canada R3Y 1G4
5. itions The S2 region is therefore used for protection during through fault events where CT measurement errors can be quite large The High Current Setting is an un re strained differential element Settings are described more completely in the B PRO instruction manual In addition to this operating characteristic the B PRO includes a CT Saturation Detection algorithm that correctly identifies CT saturation due to external faults and blocks the differential element from tripping Operate Restrain Figure 1 87B operating characteristic In addition to this operating characteristic the B PRO includes a CT Saturation Detection algorithm that correctly identifies CT satu ration due to external faults and blocks the differential element from tripping While performing setting calculations for the B PRO the IO operate current is calculated by the equation 10 AET Ager unit The restraint current IR calculation is n 3 ii 14 5 A 2 per unit Our multi function relays and recorders provide smart solutions to your protection monitoring and control needs ERLPhase formerly a division of NxtPhase is a subsidiary of Easun Reyrolle Ltd with over 60 years of power management expertise 16 IR www erlphase com info erlphase com ERL PHASE 204 477 0591 74 Scurfield Blvd Winnipeg MB Canada R3Y 1G4 Power Technologies Ltd smart solutions that empower you B PRO Bus Protection Relay
6. nologies Ltd Winnipeg MB smart solutions that empower you Canada R3Y 1G4 B PRO Bus Protection Relay Bus Differential Function Settings Easy to Use For the configuration of Figure 3 the maximum bus transfer current 4000A and IRs 2 per unit 4000 A Base Current 2000 A per unit Bus Base MVA V3 x138 kV x2000 A 478 MVA Enter this amount as a relay setting in System Parameters as illustrated in Figure 5 System Parameters Bus Base MVA 478 0 MWA Pri Bus Voltage 138 0 ky Pri PT Turns Ratio 2000 0 1 Phase Rotation ABC v Directional Control Alpha 0 0 deg Directional Control Beta 180 0 deg Figure 5 System Parameters settings The maximum relay setting for the Bus Base MVA in the B PRO is 1000 MVA This setting range will be modified to 10 000 MVA in a future product up date If the calculated Bus Base MVA exceeds this value choose the next higher whole number value for IRs 3 per unit in this example and re calcu late the Bus Base MVA 5 Determine the CT Saturation Block Timer The settings method used in this Technical Note assumes the CT Saturation Detector is enabled When an external fault occurs on a feeder it can look like an internal fault if the feeder CTs saturate sufficiently The saturation detection algorithm detects this condition and blocks tripping It does not block tripping for CT saturation during an internal fault Once the CT Sa
7. or the example of Figure 3 there are many load flow configurations possible but it is not possible to exceed an inflow of 4000 Apri without exceeding at least one CT rating Therefore 4000 Apri is the maximum bus trans fer condition for this bus It is possible of course for the maximum bus transfer current to be smaller than the rating limits of the CTs due to other system constraints 3 Choose a Value for IRs The IRs setting determines the break point between the slope S1 and slope S2 The IRs setting is also the upper limit of the load range of the B PRO For this recommended settings method the maximum bus transfer current is equal to IRs in per unit current The preferred setting for IRs is 2 per unit which will work for most applications but IRs can be any whole number 2 or greater This method ensures the IR restraint current of the B PRO for maxi mum bus transfer current is always within the upper limit of the load range 4 Calculate the Bus Base MVA The B PRO does not have an explicit setting for the current base but calculates the current base from the Bus Voltage and Bus Base MVA settings Therefore choosing the base current requires calculating the Bus Base MVA Bus Base MVA 3 x Bus Voltagex Base Current Maximum bus transfer current IRs Maximumbus transfer current Apri IRs A Base Current per unit www erlphase com info erlphase com 204 477 0591 ERL PHASE 74 Scurfield Blvd Power Tech
8. turation Detector has identified a CT saturation condition the 87B function is blocked from tripping until the CT Saturation Max Block timer expires This setting should be greater than the slowest fault clearing time for any of the feeders connected to the bus A typical clearing time is 5 cycles or 2 cycles to recognize the fault and 3 cycles for the circuit breaker to open The minimum setting of 0 10 seconds provides some safety margin 6 Enter Settings in the B PRO The un restrained differential High Current Setting should be set at 5 x IRs For this example 5 x 2 10 per unit lOmin is set at 0 25 per unit other than for applications with widely diverse CT ratios S1 is recommended to be at 25 and S2 at 50 as shown in Figure 2 www erlphase com info erlphase com 204 477 0591 ERL PHASE 74 Scurfield Blvd Power Technologies Ltd Winnipeg MB smart solutions that empower you Canada R3Y 1G4 B PRO Bus Protection Relay Bus Differential Function Settings Easy to Use Widely Diverse CT Ratios Widely diverse CT ratios differ by more than 5 to 1 for example 1200 5 and 8000 5 With widely diverse CT ratios there is a possibility of false tripping under light load conditions Consider a case such as Figure 6 Figure 6 Widely diverse CT ratios Assume that the base current is chosen in the same way described by this Technical Note Thus the maximum bus transfer current through this bus is 2400 A and the base current
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