ISLANDING OF GRID-CONNECTED AC MODULE INVERTERS

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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