3 Definition Overcurrent Coordination A systematic study of current responsive devices in an electrical power system.
4 ObjectiveTo determine the ratings and settings of fuses, breakers, relay, etc.To isolate the fault or overloads.
5 Criteria Economics Available Measures of Fault Operating Practices Previous Experience
6 Design Open only PD upstream of the fault or overload Provide satisfactory protection for overloadsInterrupt SC as rapidly (instantaneously) as possibleComply with all applicable standards and codesPlot the Time Current Characteristics of different PDs
7 Analysis When: New electrical systems Plant electrical system expansion/retrofitsCoordination failure in an existing plant
8 Protection vs. Coordination Coordination is not an exact scienceCompromise between protection and coordinationReliabilitySpeedPerformanceEconomicsSimplicity
9 Protection Prevent injury to personnel Minimize damage to components Quickly isolate the affected portion of the systemMinimize the magnitude of available short-circuit
10 Spectrum Of Currents Load Current Overcurrent Fault Current Up to 100% of full-load% (mild overload)OvercurrentAbnormal loading condition (Locked-Rotor)Fault CurrentFault conditionTen times the full-load current and higher
11 Coordination Limit the extend and duration of service interruption Selective fault isolationProvide alternate circuits
35 Fault Protection (NEC Art 430-52) Non-Time Delay Fuses300% of FLADual Element (Time-Delay Fuses)175% of FLAInstantaneous Trip Breaker800% of FLA*Inverse Time Breakers250% of FLA*MCPs can be set higher
39 Time-Overcurrent Unit Ampere Tap CalculationAmpere Pickup (P.U.) = CT Ratio x A.T. SettingRelay Current (IR) = Actual Line Current (IL) / CT RatioMultiples of A.T. = IR/A.T. Setting= IL/(CT Ratio x A.T. Setting)ILIRCT51
40 Instantaneous Unit Instantaneous Calculation Ampere Pickup (P.U.) = CT Ratio x IT SettingRelay Current (IR) = Actual Line Current (IL) / CT RatioMultiples of IT = IR/IT Setting= IL/(CT Ratio x IT Setting)ILIRCT50
41 Relay CoordinationTime margins should be maintained between T/C curvesAdjustment should be made for CB opening timeShorter time intervals may be used for solid state relaysUpstream relay should have the same inverse T/C characteristic as the downstream relay (CO-8 to CO-8) or be less inverse (CO-8 upstream to CO-6 downstream)Extremely inverse relays coordinates very well with CLFs
42 Fixed PointsPoints or curves which do not change regardless of protective device settings:Motor starting curvesTransformer damage curves & inrush pointsCable damage curvesSC maximum fault pointsCable ampacities
43 Calculate Relay Setting (Tap, Inst. Tap & Time Dial) Situation4.16 kVDS5 MVACable1-3/C 500 kcmilCU - EPRCBIsc = 30,000 A6 %50/51Relay: IFC 53CT 800:5Calculate Relay Setting (Tap, Inst. Tap & Time Dial)For This System
45 What is ANSI Shift Curve? QuestionWhat is ANSI Shift Curve?
46 AnswerFor delta-delta connected transformers, with line-to-line faults on the secondary side, the curve must be reduced to 87% (shift to the left by a factor of 0.87)For delta-wye connection, with single line-to- ground faults on the secondary side, the curve values must be reduced to 58% (shift to the left by a factor of 0.58)
47 What is meant by Frequent and Infrequent for transformers? QuestionWhat is meant by Frequent and Infrequent for transformers?