CHAPTER 4 PROTECTIVE DEVICES COORDINATION

1 TM 5-811-144-1 CHAPTER 4 PROTECTIVE DEVICES series DEVICES from the load to the source areWhere there are two or more series PROTECTIVE de-vices between the fault point and the power supply,these DEVICES must be coordinated to insure that thedevice nearest the fault point will operate first. Theother upstream DEVICES must be designed to operatein sequence to provide back-up protection, if anydevice fails to respond. This is called selectivecoordination. To meet this requirement, protectivedevices must be rated or set to operate on minimumovercurrent, in minimum time, and still be selectivewith other DEVICES on the system. When the aboveobjectives are fulfilled, maximum protection toequipment, production, and personnel will beaccomplished. As will be seen later in this CHAPTER ,protection and COORDINATION are often in directopposition with each other. Protection may have tobe sacrificed for COORDINATION , and vice versa.

2 It isthe responsibility of the electrical engineer to designfor optimum COORDINATION and protection. This issometimes more art than COORDINATION studyA COORDINATION study consists of the selection orsetting of all series PROTECTIVE DEVICES from the loadupstream to the power supply. In selecting orsetting these PROTECTIVE DEVICES , a comparison ismade of the operating times of all the DEVICES inresponse to various levels of overcurrent. The ob-jective, of course, is to design a selectively coordi-nated electrical power system. A new or revisedcoordination study should be made when the avail-able short-circuit current from the power supply isincreased; when new large loads are added or ex-isting equipment is replaced with larger equipment;when a fault shuts down a large part of the system;or when PROTECTIVE DEVICES are characteristic curves. Time isplotted on the vertical axis and current is plotted onthe horizontal axis of all time-current characteristiccurves.

3 Log-log type graph paper is used to covera wide range of times and currents. Characteristiccurves are arranged so that the area below and tothe left of the curves indicate points of "nooperation, and the area above and to the right ofthe curves indicate points of "operation." The pro-cedure involved in applying characteristic curves toa COORDINATION study is to select or set the variousprotective DEVICES so that the characteristic curveslocated on a composite time-current graph from leftto right with no overlapping of curves. The result isa set of coordinated curves on one composite time-current required for the COORDINATION following data is required for a coordinationstudy.(1)Single-line diagram of the system understudy.(2)System voltage levels.(3)Incoming power supply data.(a)Impedance and MVA data.(b)X/R ratio.(c)Existing protection including relay devicenumbers and settings, CT ratios, and time-currentcharacteristic curves.

4 (d)Generator ratings and impedance data.(e)Transformer ratings and impedance data.(4)Data on system under study.(a)Transformer ratings and impedance data.(b)Motor ratings and impedance data.(c) PROTECTIVE DEVICES ratings including mo-mentary and interrupting duty as applicable.(d)Time-current characteristic curves forprotective DEVICES .(e)CT ratios, excitation curves, and windingresistance.(f)Thermal (It) curves for cables and rotat-2ing machines.(g)Conductor sizes and approximate lengths.(5)Short-circuit and load current data.(a)Maximum and minimum momentary (firstcycle) short-circuit currents at major buses.(b)Maximum and minimum interrupting duty(5 cycles and above) short-circuit currents at majorbuses. The exact value of ground-fault current(especially arcing ground-fault current) is im-possible to calculate. Methods are available for es-timating ground-fault current.

5 The application ofNEMA damage curves for ground-fault current isillustrated in appendix G.(c)Estimated maximum and minimum arcingand bolted ground-fault currents at major buses.(d)Maximum load 5-811-144-2(e)Motor starting currents and starting times.(2)When coordinating inverse time overcurrent(f)Transformer protection procedure. The followingprocedure should be followed when conducting acoordination study:(1)Select a convenient voltage base and con-vert all ampere values to this common base. Nor-mally, the lowest system voltage will be chosen, butthis may not always be the case.(2)Indicate short-circuit currents on the hori-zontal axis of the log-log graph.(3)Indicate largest (or worst case) load ampa-ities on the horizontal axis. This is usually a motorand should include FLA and LRA values.(c)Safety factor for CT satu- seconds(4)Specify protection points. These includeration, setting errors, con-magnetizing inrush point and NFPA 70 limits forcertain large transformers.

6 (5)Indicate PROTECTIVE relay pick-up ranges.(6)Starting with the largest (or worst case)load at the lowest voltage level, plot the curve forthis device on the extreme left side of the log-loggraph. Although the maximum short-circuit currenton the system will establish the upper limit ofcurves plotted to the right of the first and succeed-ing DEVICES , the number of curves plotted on asingle sheet should be limited to about five to avoidconfusion.(7)Using the overlay principle, trace the curvesfor all PROTECTIVE DEVICES on a composite graph, se-lecting ratings or settings that will provide over-current protection and ensure no overlapping time intervals. * When plottingmain relay contact. This eliminates over-travel incoordination curves, certain time intervals must bethe relay so equipped. The time interval often usedmaintained between the curves of various protec-on carefully calibrated systems with high-dropouttive DEVICES in order to ensure correct sequentialinstantaneous relays is of the DEVICES .

7 These intervals are re-quired because relays have overtravel and curvetolerances, certain fuses have damage characteris-tics, and circuit breakers have certain speeds ofoperation. Sometimes these intervals are calledmargins.(1) COORDINATION can be easily achieved withstream relay curve could be as low as secondlow voltage current-limiting fuses that have fastwhere clearing times below 1 second are times. Manufacturer's time current curvesand selectivity ratio guides are used for bothoverload and short-circuit conditions, precludingthe need for calculating time may be decreased to a shorter time as ex-*Reprinted with permission from ANSI/IEEE Standard 242-1986, IEEE Recommended Practice for Protection and Coordi-(7)When coordinating circuit breakersnation of Industrial and Commercial power Systems, copyrightequipped with direct-acting trip units, the charac-1986 by curves should not overlap.

8 In general onlyrelays, the time interval is usually interval is measured between relays in serieseither at the instantaneous setting of the load sidefeeder circuit breaker relay or the maximum short-circuit current, which can flow through bothdevices simultaneously, whichever is the lowervalue of current. The interval consists of thefollowing components:(a)Circuit breaker secondstime (5 cycles).(b)Relay secondstact gap, etc.(3)This safety factor may be decreased by fieldtesting relays to eliminate setting errors. This in-volves calibrating the relays to the coordinationcurves and adjusting time dials to achieve specificoperating times. A margin is widely used infield-tested systems employing very inverse andextremely inverse time overcurrent relays.(4)When solid-state relays are used, overtravelis eliminated and the time may be reduced by theamount included for overtravel.

9 For systems usinginduction disk relays, a decrease of the time intervalmay be made by employing an overcurrent relaywith a special high-dropout instantaneous elementset at approximately the same pickup as the timeelement with its contact wired in series with the(5)When coordinating relays with downstreamfuses, the circuit opening time does not exist for thefuse and the interval may be reduced total clearing time of the fuse should be usedfor COORDINATION purposes. The time marginbetween the fuse total clearing curve and the up-(6)When low-voltage circuit breakers equippedwith direct-acting trip units are coordinated withrelayed circuit breakers, the COORDINATION time in-terval is usually regarded as seconds. This in-plained previously for relay-to-relay 5-811-144-3a slight separation is planned between the and medium-voltage coordinationcharacteristics curves.

10 This lack of a specified timemargin is explained by the incorporation of all thevariables plus the circuit breaker operating times forthese DEVICES within the band of the devicecharacteristic curve.(8)Delta-wye transformers. When protecting adelta-wye transformer, an additional 16 percentcurrent margin over margins mentioned previouslyshould be used between the primary and secondaryprotective device characteristic curves. This helpsmaintain selectivity for secondary phase-to-phasefaults since the per-unit primary current in onephase for this type of fault is 16 percent greaterthan the per-unit secondary current which flows fora secondary three-phase 4-1 shows a single-line diagram (modifiedfor simplicity) of the electrical distribution systemat an Army Ammunition Plant. Two 115kV utilitylines supply the double-ended, main substation,which transforms the voltage down to fordistribution throughout the facility.