IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 18, NO. 1, MARCH 2003 157

Testing the Islanding Protection Function ofPhotovoltaic Inverters

Achim Woyte, Ronnie Belmans, Senior Member, IEEE, and Johan Nijs, Senior Member, IEEE

Abstract—A major safety issue in grid-connected photovoltaicsis to avoid nonintentional operation in islanding mode when thegrid is being tripped. Worst-case conditions under which islandingcan occur are specified analytically. The circuit that is commonlyused for testing is described. The issue of appropriate test condi-tions with regard to reactive-power injection to the grid is discussedand the stabilizing impact of rotating machines and resonant cir-cuits is evaluated in detail. Islanding test results for small invertersare presented. They confirm that very simple islanding protectionmethods that are commonly used, are likely to fail, if inverters areloaded with considerable capacitance. The obtained results sup-port the assessment of the islanding protection function. They em-phasize important points when defining new certification proce-dures for upcoming guidelines and standards.

Index Terms—Certification, inverters, islanding, photovoltaicpower systems, protection, reactive power, safety, standardization,testing.

I. INTRODUCTION

I N A grid-connected photovoltaic (PV) system, the PV gen-erator is connected to the public low-voltage grid via an in-

verter. As every distributed generation unit being connected tothe public grid, the PV system has to comply with commonsafety standards. A major issue is to avoid nonintentional op-eration in islanding mode when the grid is being tripped at faultconditions or for maintenance purposes [1].

A self-commutated inverter can detect grid outage by meansof frequency and voltage monitoring. As a consequence of thedisturbed power balance in the grid section that becomes iso-lated from the main power supply, frequency and voltage obtainnew values, mostly being situated beyond the allowed limits.However, if the local load matches the power delivered by thelocal PV generator, the power balance can be in equilibrium inthe isolated grid section. Frequency and voltage remain constantand the inverter keeps on operating. The isolated grid section,still being energized by PV or other embedded generation, isthen referred to as an island.

In order to overcome such a potentially dangerous situation,most inverter manufacturers implement additional islandingprevention measures beside frequency and voltage monitoring.The proper functioning of such islanding protection algorithms

Manuscript received December 7, 2001. This research was supported in partby IMEC vzw., Leuven, and by the European Commission under Contract ERK5CT199900014.

The authors are with the Department of Electrical Engineering,Katholieke Universiteit Leuven, B-3001 Leuven, Belgium (e-mail:achim.woyte@esat.kuleuven.ac.be).

J. Nijs is also with the IMEC vzw., B-3001 Leuven, Belgium.Digital Object Identifier 10.1109/TEC.2002.808410

Fig. 1. Test circuit for islanding protection, as proposed by Häberlin [2].

is usually checked by a laboratory test procedure according tolocal guidelines and standards.

The purpose of this work is to assess different test proceduresfor the islanding protection function as they are applied in dif-ferent countries. Parameters of typical control algorithms for PVinverters that can have an influence on their islanding behaviorare identified. The impact of the applied test circuit on the ef-ficiency of the islanding detection scheme is studied. Finally,laboratory tests are carried out with four small inverters in orderto study their islanding behavior under typical test conditions.

II. CONDITIONS FORISLANDING

In principle, every self-commutated inverter can operate inislanding mode. In the laboratory, this can be shown by usingthe test circuit from Fig. 1. Resonant circuit and testing loadtogether represent the disconnected grid section.

If no particular control algorithm for islanding prevention isimplemented, the load conditions under which islanding occurs,depend only on the inverter’s frequency and voltage limits. As-suming constant active- and reactive-power output before andafter grid tripping, voltage and frequency during islanding canbe determined from the power balance

(1)

(2)

In these equations, and indicate the inverter operatingpoint. is the reactive power supplied by the capacitor ofthe resonant circuit. and are active- and reactive-powermismatch, describing the power that is supplied to the grid be-fore tripping. When inductive reactive power is supplied to thegrid, is positive.

and can be adjusted by tuning the testing load. For agiven capacitance and inverter power, a so-called nondetectivezone (NDZ) can be determined in the -domain where

0885-8969/03$17.00 © 2003 IEEE

158 IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 18, NO. 1, MARCH 2003

Fig. 2. Calculated NDZ of a 200-W inverter at different power levels and fordifferent resonant circuits with fixed voltage and frequency limits.

an inverter with predefined voltage and frequency limits oper-ates in islanding mode [1], [3]. In Fig. 2, the NDZs of a 200-Winverter are indicated for different operating points, being deter-mined by combinations of , , and .

Theoretical examinations on the probability of noninten-tional islanding show that matched load may occur with highPV grid penetration (i.e., installed PV power normalized to theload of the distribution grid [4], [5]). Probability of occurrenceand duration of nonintentional islanding in real low-voltagegrids are still under examination by the International EnergyAgency (IEA) in task V of the photovoltaic power systemsprogram (PVPS) [6]–[8].

The stochastic examinations of matched load conditions donot take into account the effect of additional active antiislandingmeasures. Islanding can be prevented even under matched loadconditions by applying additional active measures. Possible al-gorithms are different types of current pulse [9], [12], [19], [41]and frequency shift methods [3], [10]–[12], [14], [17], [42]. Anissue that still has to be solved is the question of mutual distur-bances of different antiislanding schemes on one section of thedistribution grid [9], [12].

III. I NFLUENCE OFREACTIVE POWER

Earlier studies on islanding of self-commutated inverters forgrid-connected PV assumed an inverter power factor close tounity [2], [3]. The testing load was purely resistive. Reactive-power mismatch was assumed to be negligible. Morerecent measurements found reactive-power output values of upto 20% of active power at full load and 60% of active power at30% of full load [13].

A. Reactive-Power Consumption of the Inverter Capacitance

The main reactive power is introduced by the grid filter beingpart of the grid interface. While the inverter output stage is con-trolled to generate active power only, the subsequent grid filterintroduces a capacitive component proportional to the square ofthe grid voltage. In practice, manufacturers apply more sophisti-cated filter designs in order to ensure the high power factor overa wide power range, requested by most guidelines and standards[24]–[39].

Fig. 3. Grid voltage and current waveform of a PV inverter applying AFD.

B. Reactive Power by Active Antiislanding Techniques

The reactive power introduced by active antiislanding tech-niques depends on the technique applied. Here, the impact ofactive frequency drift (AFD) on the reactive-power balance isillustrated. A detailed discussion of AFD is given in [14].

Inverters with AFD generate a slightly distorted current wave-form (Fig. 3). In this example, the first current half cycle isshorter than half of the period of the grid voltage. The timedifference between both zero crossingsis calledzero timeor dead time. For the second half-cycle, the current of the firsthalf-cycle becomes inverted and the control bias for ismeasured.

The ratio of to half of the period of the grid voltage isreferred to as the chopping fraction

(3)

If islanding occurs with purely resistive loads, voltage andcurrent have the same shape. As a consequence, in order tomaintain a constant chopping fraction, the control algorithm in-creases the frequency of the output current. Again, the voltagefollows and the frequency drifts until the frequency limit is met.However, this method fails for loads with considerable capaci-tance [13], [14], as present in most European cable distributiongrids. With the circuit shown in Fig. 1 and VAr,all higher-order current harmonics virtually become short-cir-cuited. As a consequence, the voltage remains approximatelysinusoidal.

For AFD, the power factor is given by the chopping fractionas calculated from (3)

(4)

Since the power factor of the fundamental is predeterminedby , and is enforced by the PV array, the reactive power canalso be assumed constant. This means that for realistic values of

, AFD has no impact neither on the size, nor on the locationof the NDZ in the -domain.

C. Description of Nondetective Zones

In order to take into account the influence of a resonant load,phase criteria have been proposed for describing nondetective

WOYTE et al.: TESTING THE ISLANDING PROTECTION FUNCTION OF PHOTOVOLTAIC INVERTERS 159

zones substituting the power equations in the -do-main [14]–[16]. The NDZ is defined in a parameter space ofresistance , inductance , and capacitance normalized on res-onant capacitance at grid frequency within the isolatedpart of the grid. However, the positive impact of reactive powerintroduced by active antiislanding measures becomes overesti-mated by this criteria. For determining the worst-case testingload, the reactive-power portion according to (4) is simply ne-glected and only reactive power supplied by the grid filter istaken into account.

In practice, for an islanding test procedure, this definition ofthe NDZ implies that the reactive-power portion originatingfrom AFD is not compensated by the testing load. Reac-tive-power mismatch is not adjusted to zero, implying thatthe tests do not represent a worst-case scenario. Reactive poweroriginating from AFD might therefore be misinterpreted: ahigher chopping fraction would lead to decreasing size of theNDZ not being true when assuming a precisely matched loadand a large parallel capacitance.

Unlike , , and , and directly result fromthe power balance. Therefore, it is better to keep on describingNDZs in the -domain with as an additional parameter,the stored reactive power of the resonant circuit or, alterna-tively, the resonant circuit’s quality factor as to be definedby (5).

IV. STABILIZING ELEMENTS

The introduction of stabilizing factors to the islanding testprocedure renders detection more difficult. First islanding testshad been carried out by applying the circuit from Fig. 1 in theabsence of any stabilizing component. Islanding under thesecircumstances only occurred with pure frequency and voltagemonitoring in the absence of any further antiislanding technique[2], [17], [19]. In existing grids, an island in equilibrium wouldbe stabilized by rotating machines, other embedded generators,and reactive power being stored in the grid’s inductive and ca-pacitive elements. Therefore, a large variety of approaches tointroducing stabilizing elements to the islanding test is beingdiscussed worldwide.

A. Rotating Machines and Resonant Circuit

Induction machines add inertia to a separated grid section,considerably retarding the frequency shutdown. As these ma-chines are present in existing grids, the need was felt to connecta rotating machine in parallel to the test setups [19], [20]. The is-landing test circuit as it is currently requested in Germany wherethe grid is not disconnected but only weakened by introductionof a series resistance is still a relic from these approaches [2],[18], [19], [21].

In other countries, the island was stabilized by a resonant cir-cuit tuned to grid frequency rather than by the inertia of a ro-tating machine. Although physically, the impact of rotating ma-chines is different from the one of a resonant circuit, their effectis comparable as both of them, in the seconds after a grid discon-nection, increase the tendency to maintain the grid frequency,thus preventing the frequency relay from tripping.

Further research, recently carried out in the U.S. has demon-strated that rotating loads not necessarily result in worse dis-connection properties of the islanding detection scheme thanmatched resonant loads with high quality factor[17]. A prac-tical difficulty with the application of rotating machines lies inthe matter of reproducibility of test conditions. In order to createreproducible test conditions, the machine properties, includinginertia and friction, must be specified and test laboratories allover the world would have to apply them. Hence, in Europe, theU.S., and Australia, solely a resonant circuit is applied.

B. Sizing of the Resonant Circuit

Regarding the application of a resonant circuit, it is discussedwhether this resonant circuit should have a specified size in ab-solute terms of reactive power or a specified quality factor, im-plying that reactive power of the resonant circuit is linked to theinverter operating point.

Specifying the absolute size of the circuit, meaning a fixedvalue for in VAr renders islanding detection more difficultfor small inverters and at operating points with partial load [13].The resonant circuit represents reactive power that under realconditions is stored in the grid. The value of 100 VAr as re-quested in Australia [26], Belgium [28], and Germany [37] hasbeen agreed upon after discussion on what would be a reason-ably high value that still could be realized with simple labora-tory equipment. It must therefore be seen rather on a historicaland practical background than on a technical one.

In the U.K. and U.S., a specified quality factor is re-quested. The quality factor of a resonant circuit is defined asthe ratio of reactive power in the circuit to active power con-sumption. At resonant frequency with and matchedactive load ( , according to Fig. 1), the relation betweenquality factor and size of the resonant circuit is

(5)

With being the inverter’s phase angle and assuming nohigher order harmonics, (2) becomes

(6)Equations (1) and (6) describe the NDZ only by relative terms

of power mismatch and . The NDZ is now scal-able to all different inverters and operating points.

In the U.S., is proposed as being a typical qualityfactor for a distribution grid being fully compensated by capac-itance [17]. In the according standard and test procedure [24],[25] therefore, it has to be tested with .

In the U.K., a quality factor above 0.5 has to be applied. Thereasoning is different from the one in the U.S. The resonantcircuit is applied in order to simulate a second PV inverter withinthe disconnected grid section, introducing a tendency to keepthe grid energized at rated frequency. The value hasbeen chosen as this represents the aperiodic limit for a resonantcircuit to perform free oscillations. The quality factor of the totalgrid section does not play a role in this argumentation [16], [22],[23], [29].

160 IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 18, NO. 1, MARCH 2003

Fig. 4. NDZ for inverter A recorded atP = 0:3 � P , Q = 0:1 � P .

Fig. 5. NDZ for inverter D recorded atP = 0:3 � P , Q = 0:18 � P .

The application of a constant quality factor rather than a res-onant circuit of constant size is useful as this provides scala-bility of the test procedure and can also be justified technically.From a practical point of view, it is beneficial to test with nottoo high of a quality factor. For typical single phase inverters ofup to approximately 5 kVA, leads already to consid-erable values of inductance and capacitance. Higher values for

might make testing unreasonably expensive due to the largeequipment that would be required.

V. LABORATORY TESTING OFMODULE INVERTERS

The NDZs of four small, so-calledmoduleinverters of max-imum 200-W rated power are recorded by applying the cir-cuit presented in Fig. 1 with VAr.

The tests demonstrated that each of the four inverters can beforced into islanding under certain load conditions. Partly, theNDZs correspond to the calculated areas, indicating that someof the applied algorithms work insufficiently with loads con-taining a capacitive component. However, the NDZs are not al-ways located around the origin of the -domain as (1)and (2) imply.

Figs. 4 and 5 show the results of two of the measurements. In-side the inner zone indicated by triangles, the inverter remainesislanding. Outside the outer zone indicated by circles, the in-verter immediately switched off. The border of its NDZ is thus

TABLE ISUMMARY OF THE TEST RESULTS AND CONCLUSIONS

located in between both zones. The theoretical NDZ as calcu-lated from (1) and (2) from the settings of the frequency andvoltage relays is indicated by the solid border.

From the location of the NDZ and the causes for shutdown atits boundaries, conclusions on the effectiveness of the particularprotection algorithm can be drawn. Table I gives a summary ofthe different inverters’ islanding behavior. Comprehensive re-sults of the conducted test series have been presented in [13].

The failing of inverter D has to be interpreted as a prototypeproblem. The results have already been discussed with the man-ufacturer. The implemented algorithm will be rechecked.

The tests on inverters A and B verify the investigations madein [14]. Their recorded NDZs correspond to the ones computedfor a situation without further antiislanding measures. The fre-quency drift algorithm implemented in inverter B fails with con-siderable capacitive loads. Inverter A remains islanding becausethe voltage-control loop does not become unstable as intended.

Inverter C shows excellent results with regard to islanding.The manufacturer has apparently made a good effort in order toprevent islanding. Information about the islanding preventionschemes applied is, however, not publicly available.

VI. CONCLUSIONS

In order to ensure worst-case test conditions independent ofthe antiislanding algorithm, the active- and reactive-power mis-match, and , must be adjusted to zero with regard to thecurrent and voltage fundamentals.

The introduction of stabilizing elements makes the test con-ditions more realistic. For practical reasons, the introduction ofa resonant circuit with a predefined quality factor is favorablerather than a rotating load. With regard to the value for, thepractical boundary conditions for implementation in the labo-ratory should be taken into account. Regarding the low proba-bility for matched load conditions, the choice of a relatively lowquality factor between 0.5 and 1 would be appropriate.

Test results show that small inverters are still sensitive to is-landing if tested in a worst-case scenario and loaded with ca-

WOYTE et al.: TESTING THE ISLANDING PROTECTION FUNCTION OF PHOTOVOLTAIC INVERTERS 161

pacitance. In order to avoid the need for oversized and expen-sive protection equipment, the applied islanding protection al-gorithms still have to be improved. Algorithms based on in-stability of voltage and frequency, while the grid is tripped,can play a major part. However, those algorithms must be im-plemented with care. The particular stability limits should bechecked thoroughly by theoretical examination, simulation, andworst-case tests.

Still, an issue for future antiislanding schemes is the avoid-ance of mutual disturbances by a high number of different typesof inverters in an isolated grid section. Research about this sub-ject is still ongoing.

ACKNOWLEDGMENT

The authors wish to thank J. Appelbaum for his useful crit-icism and comments during the preparation of this paper andL. Conings who carried out part of the measurements described.

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162 IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 18, NO. 1, MARCH 2003

Achim Woyte received the electrical engineeringdegree from the University of Hannover, Germany,in 1997. He spent half a year working and studyingin Venezuela and Italy. He is currently pursuing thePh.D. degree at K.U. Leuven, Leuven, Belgium.

He worked for more than three years in electroheatand high-voltage engineering at University of Han-nover.

His research interests include the grid interfaceand grid interaction of residential photovoltaic sys-tems.

Ronnie Belmans(S’77-M’84-SM’89) received theM.S. degree in electrical engineering in 1979, thePh.D. degree in 1984, and the Special Doctorate in1989 from the K.U. Leuven, Belgium, and the Ha-bilitierung degree from the Rheinisch-Westfälische(RWTH), Aachen, Germany, in 1993.

Currently, he is Full Professor with K.U. Leuven,teaching electrical machines and variable speeddrives. He is Appointed Visiting Professor atImperial College, London, U.K. He is also Presidentof the Union Internationale pour les applications de

Electricité (UIE). He was with the Laboratory for Electrical Machines of theRWTH, and was a Von Humboldt Fellow from October 1988 to September1990. He was Visiting Associate Professor at McMaster University, Hamilton,ON, Canada. During the academic year 1995-1996, he occupied the Chair at theLondon University, U.K., offered by the Anglo-Belgian Society. His researchinterests include variable speed drives, vibrations and noise in electricalmachines, electrical energy systems, and power quality.

Dr. Belmans is a fellow of the IEE U.K.

Johan Nijs (SM’97) received the university degreein electrical engineering, and the Ph.D. degree in ap-plied sciences from the K.U. Leuven, Belgium, in1977 and 1982, respectively.

After having worked at Philips, Leuven, and, K.U.Leuven, Belgium, and I.B.M. Thomas J. Watson Re-search Center, Yorktown Heights, NY, he joined theInteruniversity Micro-Electronics Center (IMEC) inLeuven, in 1984 where he became groupleader andrecently associate vice-president/department directorof the Packaging, MEMS, and Photovoltaics depart-

ment. In 1990, he was also appointed Part-Time Associate Professor at K.U.Leuven.

Dr. Nijs is a full-member of ISES-Belgium.