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ISLANDING OF GRID-CONNECTED AC MODULE INVERTERS Achim Woyte, Ronnie Belmans, K.U.Leuven, ESAT-ELEN Kard. Mercierlaan 94, B-3001 Leuven, Belgium Johan Nijs, IMEC v.z.w. and K.U.Leuven, ESAT-INSYS Kapeldreef 75, B-3001 Leuven, Belgium ABSTRACT behavior under different load conditions. The applied test circuit is shown in Figure 1. A major safety issue about grid-connected photovoltaics is to avoid non-intentional operation in PV array resonant domestic inverter S public grid islanding mode, the grid being tripped. simulator circuit load This paper presents detailed measurements on the = P, Q ∆P, ∆Q islanding behavior of four module inverters with a == maximum rated power of 200 W. Although applying active ~ L R anti-islanding measures each inverter could be forced into { islanding. It could be observed experimentally what QL = QC recently has been shown analytically, that some methods = 100 VAr against islanding fail if inverters are loaded with Fig. 1. Test circuit for islanding protection, as proposed considerable parallel capacitance. As most distribution by Häberlin [2]. grids contain a considerable capacitance, those methods are to be improved. One of the inverters failed totally what In principle, every self-commutated inverter is able to illustrates the need for standardized type approvals. operate in islanding mode. If no particular control The outcomes show where to put accents in the algorithm for islanding prevention is implemented, the development and implementation of efficient protection load conditions under which islanding occurs, depend algorithms. only on the inverter's frequency and voltage limits. Assuming constant active and reactive power output before and after grid tripping, voltage and frequency in islanding operation can be determined from the power INTRODUCTION balance, yielding equation (1) and (2). A photovoltaic AC module is a set of one or two PV 2 panels and a small so-called module inverter mounted on ∆P Vgrid = 1− 2 (1) the panel's backside. AC modules are considered as P Visland being one option for wide market dissemination of grid- connected photovoltaics. AC modules render the DC installation superfluous. That makes them suited for being sold as a "plug-and-play" product ready for connection to ω island ∆P ∆Q ⋅ − any electric outlet by the consumer. ω grid P Q As every decentralized production unit being æω2 ö Q ω connected to the public grid, the PV AC module has to = ç island − 1÷ ⋅ C + island − 1 (2) ç ω2 ÷ Q ω grid comply with common safety standards. A major issue is è grid ø to avoid non-intentional operation in islanding mode with the grid being tripped at fault conditions or for In these equations P and Q indicate the inverter maintenance purposes. operating point. QC is the reactive power supplied by the ISLANDING PHENOMENON AND TESTS capacitor of the resonant circuit. ∆P and ∆Q are the active and reactive power, supplied to the grid before grid Investigations carried out at K.U.Leuven in 1997 [1] tripping. When inductive power is supplied to the grid, ∆Q have shown that small so-called "module inverters" are in is positive. ∆P and ∆Q can be adjusted by tuning the general more sensitive to islanding than larger units. domestic load. For a given capacitance and inverter Recently four module inverters that are currently available power, a so-called non-detective zone (NDZ) can be on the European market, ranging from 90 to 200 W rated determined in the ∆P-∆Q-domain where an inverter with power, have been examined with regard to their islanding predefined voltage and frequency limits will operate in islanding mode [3]. In Figure 2 the NDZs of a 200 W low, but realistic value, in order to represent a low-voltage inverter are indicated for different combinations of P, Q grid, all higher order current harmonics in the inverter and QC. output current virtually become short-circuited. As a consequence, the voltage remains approximately 60 sinusoidal. P Q Qc 200 W, 50 VAr, 100 VAr Upper voltage limit 40 200 W, 50 VAr, 50 VAr The cosϕ is given by the chopping fraction as 100 W, 50 VAr, 100 VAr calculeted from equation (4): 20 100 W, 25 VAr, 100 VAr P to Grid in W Upper frequency limit π 0 cosϕ = ⋅ cf (4) 2 -20 Lower frequency limit Since the cosϕ is predetermined, and P is enforced -40 by the PV array, the reactive power can also be assumed constant. This means that for considerable values of QC, Lower voltage limit -60 AFD has no impact neither on the size, nor on the -30 -20 -10 0 10 20 30 location of the NDZ in the ∆P-∆Q-domain. The NDZ is still ∆ Qind to Grid in VAr determined as described by means of (1) and (2). Fig. 2. Calculated NDZ of a 200 W inverter at different power levels and for different load capacitances with fixed 1.5 voltage and frequency limits. Grid voltage Vgrid Amplitude normalized on RMS value 1 APPLIED ANTI-ISLANDING MEASURES 0.5 Inverter current Iinv In order to improve the islanding protection by voltage and frequency monitoring, several active and 0 tZ passive methods are available [4]. In Europe, the active frequency drift method (AFD) is often applied. Of course -0.5 in practice, every manufacturer implements his own -1 particular protection algorithm in a slightly different Inverter current fundamental Iinv,f manner. -1.5 0.0 0.2 0.4 0.6 0.8 1.0 Inverters with AFD generate a slightly distorted t ⋅ fgrid current waveform (Fig. 3). In this example, the first Fig. 3. Grid voltage and current waveform of a PV current half cycle is shorter than half of the period of the inverter applying active frequency drift. grid voltage. The current is controlled to be zero during a fixed phase interval equal to ω⋅tz, and starts its second CONDUCTED TESTS half-cycle at the positive zero crossing of the grid voltage. For the second half-cycle the current of the first half-cycle Test procedure becomes inverted and the control bias for ω⋅tz is measured. For the current fundamental this means a The NDZs of four module-inverters have been phase shift by 0.5⋅ωgrid⋅tz with regard to the grid voltage. recorded at rated power and at 30 % rated power by Hence, in order to maintain a high power factor, tz must applying the test circuit presented in Figure 1. Samples not be chosen too high. The ratio of tz to half of the period have been taken by stepping through the ∆P-∆Q-domain of the grid voltage is referred to as the chopping in steps of 5 % of each inverter's rated power. The fraction (cf): inverter is considered to be in islanding operation if it does not switch off within five seconds after the switch S 2 ⋅ tz has been opened. For each point (∆P,∆Q) at least four cf = = 2 ⋅ f grid ⋅ t z (3) tests have been carried out. If these showed varying Tgrid results three more tests were added in order to achieve a representative sample of seven tests. A point is counted If islanding occurs in a grid section with purely for being located within the NDZ if islanding operation resistive loads, the voltage adopts the shape of the occurred at least two times. distorted current waveform. As a consequence, in order to maintain a constant chopping fraction, the control Figures 4 to 6 show the outcomes for the different algorithm increases the frequency of the output current. inverters. Inside the inner zone indicated by triangles, the The voltage again adopts the current waveform and the inverter remained islanding. Outside the outer zone frequency will drift until the frequency limit is reached. indicated by circles the inverter immediately switched off. However, as it has recently been shown, this method fails The border of its NDZ is thus located in between both for loads with considerable parallel capacitance, as is zones. The theoretical NDZ as calculated from (1) and (2) found in most European cable distribution grids [3]. With from the settings of the frequency and voltage relays is the circuit shown in Figure 1 and QC = 100 VAr, being a indicated by the solid border. Inverter A could infinitely be maintained islanding. As expected, the recorded NDZ is approximately the same as calculated The recorded NDZ of inverter A (Fig. 4) has analytically. approximately the same size as the calculated zone, 16 indicating that the implemented AFD has no effect at 30 % Pr. Inverter A has no internal clock and the 12 Voltage shutdown reference current is calculated from the grid-voltage. As a 8 consequence it does not apply AFD but another 4 algorithm, causing the control loop of grid-voltage and P/Pr in Percent 0 inverter current to become unstable. However, as higher Frequency Frequency order harmonics are short-circuited by the capacitor, the -4 shutdown shutdown voltage remains sinusoidal while voltage and frequency -8 satisfy equations (1) and (2). -12 -16 theory During further tests with this inverter at ∆P/Pr = -8 % no islanding and ∆Q/Pr ranging from –5 to 5 % islanding was also -20 Voltage shutdown islanding observed. However, tests with less than a 5 W step size -24 -12 -8 -4 0 4 8 12 are in practice not feasible for every point, which is why Qind/Pr in Percent for all inverters the 5 % step was chosen. Fig. 5. NDZ for inverter B recorded at P = Pr, 8 Q = 0.18·Pr. Voltage shutdown 4 Frequency Inverter C shutdown 0 The results for inverter C are surprising. For this inverter an external, highly sophisticated protection P/Pr in Percent -4 device is available. As this device is very expensive and in most countries considered for not being interesting in -8 Frequency practice, the tests were conducted without this device. shutdown Nevertheless, islanding could only be observed in very -12 theory few cases for less than 10 % of rated power. In that case no islanding islanding the NDZ is much smaller than calculated by (1) and (2). Voltage shutdown -16 The reasons for the good performance of this inverter are -12 -8 -4 0 4 8 12 not yet clear. It can be supposed that further anti- Qind/Pr in Percent islanding measures and not an AFD are implemented. Fig. 4. NDZ for inverter A recorded at P = 0.3·Pr, Q = 0.1·Pr. Inverter D At rated power the same inverter showed no Figure 6 shows the recorded NDZ of inverter D at islanding at all. In that case, the parallel capacitance 30 % rated power. For significant values of ∆P the voltage becomes less significant due to a lower load resistance. relays trip at the boundaries, calculated from (1). Changes in voltage and current cause instabilities and 12 islanding can be detected by the frequency relays. theory Voltage shutdown no islanding 8 Finally, Figure 4 shows that the measured NDZ is islanding shifted to negative active power when compared to the 4 calculated NDZ. This phenomenon has not yet been P/Pr in Percent Frequency further investigated and may be caused by deviations shutdown 0 from the specified values of the voltage relays. Inverter B -4 Figure 5 shows the recorded NDZ of inverter B at -8 Frequency rated power. Although the frequency relays are set very shutdown Voltage shutdown narrow, in most of the cases the inverter was only shut -12 -24 -20 -16 -12 -8 -4 0 4 8 down due to tripping of the voltage relays. Qind/Pr in Percent Inverter B applies a frequency shift algorithm that can Fig. 6. NDZ for inverter D recorded at P = 0.3·Pr, also be adjusted by means of a bus interface. During the Q = 0.18·Pr. tests, the so-called parameter "phase shift" was set to a maximum frequency drift towards lower frequency values. On the right-hand part of the ∆P-∆Q-domain, Nevertheless, for several load cases as shown in containing capacitive domestic loads (∆Q ≥ 0), the Figure 5, no frequency drift was observed and the inverter frequency relay trips around the calculated boundary. However, on the left-hand side, with inductive domestic with the manufacturer. The implemented algorithm will be loads, the frequency relay trips much later than expected. re-checked. The reactive power for the domestic load inductance is in the latter case supplied by the inverter. At 30 % of rated The tests on the inverters A and B verify the power this can be observed for ∆Q between 0 and -15 %. investigations made in [3]. Their recorded NDZs correspond to the ones that were computed for a situation At rated power a similar behavior can be observed. without further anti-islanding measures. The AFD At load matching and on the right half of the ∆P-∆Q- algorithm implemented in inverter B fails with capacitive domain, no islanding is observed at all. However, for loads. Inverter A remains islanding because the current capacitive loads, the inverter supplies as much reactive voltage control loop does not become unstable as power as required by the domestic load in islanding in intended. order to keep the frequency stable at 51.7 Hz. The inverter thus shows the behavior of an inverter for stand- Inverter C shows excellent results with regard to alone operation i.e. the islanding protection function fails. islanding. The manufacturer has apparently made a good effort in order to prevent from islanding. TEST SUMMARY CONCLUSIONS The tests demonstrated that each of the four module inverters can be forced into islanding under certain load The results show that small inverters are still conditions. Partly, the NDZs correspond with the sensitive to islanding if tested in a worst-case scenario calculated areas, indicating that some of the applied and loaded with parallel capacitance. Thus, in order to algorithms work insufficiently with loads containing a avoid the need for clumsy, oversized and expensive capacitive component. However, the NDZs are not protection equipment for such "plug-and-play" devices, always located around the origin of the ∆P-∆Q-domain as the applied islanding protection algorithms still have to be (1) and (2) imply. improved. From the location of the NDZ and the causes for Algorithms based on instability of voltage and shutdown at its boundaries, one can draw conclusions on frequency while the grid being tripped can play a major the effectiveness of the particular protection algorithm. part. However, those algorithms must be implemented Moreover, estimations are possible to see whether an with care. The particular stability limits should thoroughly NDZ is small enough for being tolerable from the safety be checked by theoretical examination, simulation, and point of view and how likely the occurrence of islanding is worst-case tests. in the field. Table 1 gives a summary of the different REFERENCES inverters' islanding behavior. Type Results and Conclusions [1] H. Van Reusel et al., "Adaptation of the Belgian A − No islanding at rated power: frequency relay trips. Regulation to the Specific Island Behaviour of PV Grid th − At low power (0.3·Pr) islanding occurs in a small Connected Inverters", 14 European Photovoltaic Solar zone with negative ∆P. Frequency limits could be set Energy Conference and Exhibition, Barcelona, Spain, closer. 1997, pp. 2204 – 2206 B − At rated power, islanding for ∆P/Pr between –20 % and 10 % with ∆Q = 0. Voltage limits could be set [2] H. Häberlin, J. Graf, "Islanding of Grid-Connected nd closer. PV Inverters: Test Circuits and some Test Results", 2 − Most shutdowns occur due to exceeding of a voltage World Conference and Exhibition on Photovoltaic Solar limit. Frequency drift is observed very little and esp. Energy Conversion, Vienna, Austria, 1998, pp. 2020 – for negative ∆Q only after a few seconds. 2023 − At low power the inverter is less sensitive to islanding. [3] M.E. Ropp, M. Begovic, A. Rohatgi, "Analysis and C − Islanding occurred only very seldom and only at very Performance Assessment of the Active Frequency Drift low power. This is surprising because the inverter is Method of Islanding Prevention", IEEE Transactions on designed for use with an external islanding Energy Conversion, Vol. 14, No. 3, 1999, pp. 810 – 816 protection device. The voltage limits are wide and no further islanding protection algorithm is documented. [4] M.E. Ropp, M. Begovic, A. Rohatgi, "Prevention of D − At rated power, for 0 < ∆P/Pr < 25 % and ∆Q/Pr < Islanding in Grid-connected Photovoltaic Systems", -5 % the frequency does not rise above 51.5 Hz. The inverter remains islanding and ∆Q is taken over by Progress in Photovoltaics: Research and Application 7, the inverter. 1999, pp. 39 – 59 − The same tendency can be observed at 0.3·Pr, but in this case, at ∆Q/Pr < -15 % the frequency relay trips at 52 Hz. Table 1. Summary of the test results and conclusions. The failing of inverter D has to be interpreted as an infant disease. The results have already been discussed

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posted: | 5/13/2010 |

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