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LOSS OF M AINS DETECTION FOR EMBEDDED GENERATION BYSYSTEM IMPEDANCE M ONITORINGP.OKane, B.FoxThe Queens University of Belfast, U.K.

Abstract: Embedded generators operating in parallelwith the electricity supply network are required by lawto fi t loss of mains protection. T h ~ s aper presents anew technique for detecting loss of mains based onmeasurement of system impedance and overcomes thedeficiencies of more popular relays. The unitsdecision to trip is based on identification of the changein impedance that occurs at a private generators sitewhen it becomes disconnected from the mains supply.1. INTRODUCTIONSince privatisation of the Electricity Supply Industry in1992. a renewed interest has been shown in theinstallation of small and medium sized synchronousgenerators to operate in parallel with the distributionsystem. This renewed interest has been stimulatedby anumber of technical, commercial and environmentalfactors. These include the high overall thermalefficiencies possible with combined heat and powersystems, the huge savings possible when using existingplant in peak-lopping mode and the continuingcommitment of many governments to new renewableenergy sources such as wind power.By the year 2000, the UK government plans to have5,000 MW of CHP and 1,500MW of renewable energyschemes in place. These and other forms of dspersedgeneration are known colloquially as embeddedgeneration (EG) and their re-introduction into thetransmission system poses a number of technicalproblems. One of the more prevalent is loss of mainsdetection.

In order to coniply with the UK electricity supplyregulations[,21,ll EG plants rated greater than 150 kWand operating in parallel with the utility networkrequire a rudimentary degree of protection, which mustinclude under and over voltage and frequency, and lossof mains. Other forms of protection may also berequired dependmg on the nature of the EG , such asneutral voltage Qsplacement, overcurrent, earth-faultand reverse power protection. ProviQng protectionagainst the loss of mains condtion for privategenerators is one of the more Qfficult aspects ofelectrical system design. Several techniques areavailable to detect the condition, but they fail to activateunder certain operating conditions and are prone tonuisance trips,

2. LOSS OF MAINSThe loss of mains (LOM) phenomenon is also referredto as islanding and occurs when a utility circuit-breakeropens, dsconnecting the utility from the embeddedgenerator but leaving a section of network loadconnected to the generator, as shown in figure 1.

Tripped utility circuit-breaker Inter-tie circuit-breaker/f

.6 NetworkLoad Load EmbeddedgeneratorUtility i /Power sland Industrial site

Figure 1 - lndustnal site op erating while islandedIslanQng is undesirable, creating a hazard to personneland the possibility of out-of-phase reclosure, resultingin a torque transient which can damage the machine.Out-of-phase reclosure occurs since many supplies areautomatically restored within a relatively short periodafter a fault by auto-reclose circuit-breakers which arenot fitted with synchronising equipment. Theirdeployment improves service continuity, since seventyto eighty percent of all utility distribution faults arenon-persistent. Reclose periods can be as short as onesecond and the connected load quite small. In thesecircumstances network load is capable of beingsupported by the embedded generating plant.Loss of mains detection enables the inter-tie circuit-breaker connecting the embedded generator to theutility to be tripped allowing re-synchronization whennetwork conchtions permit. Specialist relayingtechmques are required to detect LOM and their modesof operation can be divided into two fundamentalgroups:active and passive[31.2.1 Passive TechniquesPassive devices function by nioiiitoring various systemparameters and make their decision to trip withoutdxectly interacting with system operation. Typicalpassive techniques include rate of change of frequency(ROCOF) and sudden change of voltage vector orpower factor.

D ev e l opmen t s i n P ow er S y s t em P ro t ec t ion , 25-27th March 1997,Confer ence Publ i cation No . 434, 0 EE, 1997

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The ROCOF method is the most widely applied andassumes that for all practical purposes, when loss ofgrid occurs. some difference will exist between theisland load and generator output, resulting in afrequency deviation over time (df/dt).The relay is set so that a rate of change of frequencygreater than x Hz/s will cause operation. The value of xis determined by network characteristics and should beset slightly greater than the largest frequency excursionthat would occur on the system during normaloperating conditions, thus avoiQng nuisance tripping.However, if the load remaining on the island isapproximately equal to the output of the generator atthe time of LQM, the rate of change of frequency maynot be sufficient to operate the relay. A compromisemust be reached between the ability to detect theconQtion and the possibility of an erroneous trip,which adversely affects the relay's performance. Theend result is that passive devices are susceptible tonuisance trips during network faults and grid frequencytransients and can fail to operate when the islandcondition does not produce the required change ingenerator loaQng.Nuisance trips can occur as often as once a month, andgenerally do not cause the RE C a problem since theyisolate the EG plant (which trahtionally offered little interms of system capacity) at a time when the system isweak. However, with increasing numbers and size ofEG plant, many with little or no local load (such aswindfarms), their dlsconnection from the network mayresult in unused capacity and excess load-shedQng.Passive devices remain, however, the preferred optionof utilities since they do not impinge on or upset systemoperation.2.2 Act ive TechniquesActive devices function by Qrectly interacting with thesystem under consideration, and the two main methodsare reactive error export dete~tion'~]nd fault levelmonitoring[51.The reactive error export detector controls theembedded generator excitation current so that itgenerates a known value of reactive current, whichcannot be supported unless the generator is connectedto the mains. It provides a highly reliable means ofdetecting LOM but takes several seconds to operate,which is longer than the reclose period of many auto-reclose breakers. Despite its slow operation, reliabilityenables it to be used as a backup for faster methods.Fault level monitoring provides a very fast response tothe condition with detection possible in half a cycle.Using point-on-wave thyristor switchmg, triggered nearthe voltage zero point, it measures the current througha shunt inductor, enabling calculation of system

impedance and fault level. It provides a very effectivemeans of detection, with the Qsadvantage ofintroducing a small voltage glitch at the zero crossoverp in t .3. NEW METHOD FOR LOM DETECTIONSince utility impedance is considerably smaller than theimpedance of a power island, the impedance of asection of network will increase when that sectionbecomes disconnected from the utility. The change ofimpedance that occurs results in a correspondngchange of system fault level and is suitablydemonstrated in figure 2, whtch shows a typical 2-MVA embedded generator connected through twotransformers and two cables to a site substation with a250-MVA fault level. The component impedances areshown in figure 2b, in p.u. values to a base of 10 MVA.

Figure 2a - Embed ded generator, single line diagram2b Component mpedance diagram

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With the utility circuit-breaker closed, the impedance ofthe EG network at the site substation is approximately0.04p.u., and with the circuit-breaker opened(islanded), the impedance increases to over 1p.u.Any step increase in system impedance, whensynchronised to the utility, greater than an increase thatwould occur under normal operating conditions, isattributed to loss of mains and will cause the unit totrip. The method is independent of generator loadlng,and will not cause nuisance trips during network faultsor grid frequency transients.3.1 An Impedanc e Sensi t ive DeviceSince there is generally a considerable Qfference insystem impedance when synchronised to and operatingin isolation from the network, an impedance measuringdevice need not be particularly accurate. One way ofmeasuring impedance, where accuracy is not a primeconsideration, is by exploiting the characteristics of thevoltage divider. Application of a signal to the voltagedwider circuit shown in figure 3will result in a voltageV,, at the output.

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Variation of the value Z2 will result in a change ofoutput voltage, V,,,. By configuring a circuit with thesystem impedance as Z2, then the variation of thatsystem impedance will result in a change of Vout, romwhch an informed decision can be taken as to whetherthe system is operating as an island or not.

Figure 3 - The voltage dividerThe magnitude of Vou,depends on V,, Z1 and Z2:

7

A small high frequency (hf)signal is used as the inputlo the voltage &vider and coupled to the mains via acoupling capacitor. The coupling capacitor is thusincorporated into the dwider circuit and acts as Z, . Itenables the small hf signal to be superimposed onto themains as shown in figure 4.The hf signal is generally afew volts at a frequency in the low l cHz range.

Figure 4 - Small hf signal superimposed onto the m ains

A response for a typical voltage dvider circuit is shownin figure 5 and shows that when the device is correctlytuned a considerable change in V, can be obtained fora small change in system impedance. V,, is measuredusing a high-pass filter connected across the mains,removing the 50-Hz waveform and returning a valuefor the magnitude of the hf signal, which will vary withchange in system impedance.It is usual to choose component values for Z1 such thatthe ripple on the mains will be negligible whensynchronised to the utility. When LOM occurs Z z willincrease and a small hf ripple will appear,superimposed on the 50-Hz waveform. The detectioncircuit will sense the magnitude of the signal and whenit exceeds a predetermined value the unit will trip.

Operation will cause the relay to:Trip the inter-tie circuit breaker.Trip the input to the voltage chvider, causing the hfripple to cease, enabling the embedded generator tocontinue its operation with an undstortedwaveform.Vout v system mpedance

volts* T n0 1 0 3 0 5 0 7 09 1 1 1 3 1 5 1 7 1 9Reactance (&I)

Figure 5 - Voltage dvrder responseThe hf signal only appears on the mains during theperiod between LOM and identification of thecondhon. During this time a small ripple will beapparent on the 50-Hz waveform. which attenuateswith &stance from the transmitter. It can be eliminatedtotally, if necessary, with the use of a line trap but itsmomentary presence on the system should beacceptable. Also, since the carrier signal is a lowfrequency sine wave it does not contribute to systemrado-frequency interference.3.2 Power Factor Correct io nIn its present form the device can fail to operate whenlocated near power-factor correction equipment. Shuntcapacitors will appear as a low impedance to the hfsignal and, if they are close to the device, can cause thesystem impedance to appear constantly low, even whileislanded. One method of eliniinating the lowimpedance effect of power factor correction equipmentis to use a line trap.Line traps were first used in the 1930's in carrier pilotrelay systems to confine the carrier power to a protectedsection of transmission line. They typically consist of aparallel induclor/capacitor combination tuned to gve ahigh impedance at the carrier frequency and negligibleimpedance at S O H Z . We can use a line trap in thissituation to 'weight' the impedance of the EG networkat the carrier frequency, making it appear much largerthan it is and removing the apparent low impedanceintroducedby power-factor correction.4. TESTING OF THE DEVICE

Digital simulations were employed to test the validty ofthe technique and a micromachine system used as a

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practical test bed. The micromachine system consistedof a 3-kVA synchronous generator, tied to a busbarthrough a transformer and artificial transmission line.Setting of the new unit requires a knowledge of thesynchronised and island impedances at the particularbusbar to whch the unit will be connected. A bus closeto the low impedance infinite bus should be chosen.This serves to place as many transformers (which act asimpedance buffers) between the embedded generatorand the unit, ensuring the ratio between island andsynchronised impedances is a maximum. The systemsetup is shown in figure 6.

Auho-amp\I0 ncut-breakaO U p h g = TFjgure 6 - System setup

When the system is synchronisedZ2 is small and whencorrectly tuned the ripple is negligble, as indlcated bythe oscilloscope waveform in figure 7a. Islandmgresults in an increase in Zzand a ripple becomesapparent at the output of the high pass filter circuit asshown in figure 7b,enabling the relay to trip.

Figure 7a - Synchronised Figure 7b - IslandedThe unit was subjected to the various range of faultconditions available on the micromachine and in allcases remained stable.5. CONCLUSIONPower companies are continuously striving to ensurethat the presence of embedded generation on theirsystem does not have an adverse effect on the quality ofsupply to its customers. They attempt to solve the lossof mains problem by the application of passivetechniques such as rate of change of frequency. but

these methods cannot, at present. be guaranteed tooperate in the required time under all condltions.Active techniques provide a more reliable approach toidentlfication of loss of mains, but traditionally at theexpense of dlrectly influencing either the quality ofsupply or the performance of the power system. As aresult they have generally not been implemented.The new method presented here provides fastidentification of loss of mains, incorporating theadvantages of active devices with the compatibility ofpassive techniques. It correctly identlfies LOM duringoperating condltions in whch more established formsof protection fail to operate and remains stable duringsystem dlsturbances.In terms of spinning reserve, embedded generation hastradltionally been considered negative reserve. Ths isdue to the probability of erroneous trips from ROCOFrelays during gnd frequency transients, isolating theprivate generator at a time when its capacity isrequired. Since the proposed method remains stableunder all frequency and fault conditions, it presents theopportunity of using embedded generation to contributeto spinning reserve.6. ACKNOWLEDGMENTThe authors wish to express their gratitude to ESwidenbank and D Flym for their assistance with themicromachine system at the Queens University ofBelfast.7. REFERENCES[11 ELECTRICITY ASSOCIATION: Recommendationsfor the connection of embedded generating plant to thedistribution system. Engineering recommendation G59/1,Northern Ireland, 1993.[2] ELECTRICITY ASSOCIATION: Notes of guidance forthe protection of embedded generating plant up to 5 M W foroperation in parallel with public electricity suppliersdishbution systems. Engineering TechnicalReport No. 113,1994.[3 ] Redfem; M A, Usta, 0 and Fielding, G: Protectionagainst loss of utility grid for a dispersed storage andgeneration unit. IEEE Trans. Power Delivery, Vol.8, No.3,July 1993, pp.948-954.[4] Warin, J W: Loss of mains protection. ER A Cod. onCircuit Protection for Industrial and CommercialInstallations,London, 1990.[ 5 ] Cooper, C B: Standbygeneration- problems andprospectivegains from parallel nmning. Power SystemProtection 89, Singapore, 1989.