S
∆P, ∆Q
public grid
LR
{
QL = QC = 100 VAr
Fig. 1. Test circuit for islanding protection, as proposed by Häberlin [2].
In principle, every self-commutated inverter is able to
capacitor of the resonant circuit. ∆P and ∆Q are the active and reactive power, supplied to the grid before grid
tripping. When inductive power is supplied to the grid, ∆Q
islanding mode [3]. In Figure 2 the NDZs of a 200 W inverter are indicated for different combinations of P, Q
and QC.
60
P Q Qc
40
200 W, 50 VAr, 100 VAr 200 W, 50 VAr, 50 VAr
20
30
∆ Qind to Grid in VAr
Fig. 2. Calculated NDZ of a 200 W inverter at different power levels and for different load capacitances with fixed voltage and frequency limits.
particular protection algorithm in a slightly different manner.
Inverters with AFD generate a slightly distorted current waveform (Fig. 3). In this example, the first current half cycle is shorter than half of the period of the grid voltage. The current is controlled to be zero during a
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
As every decentralized production unit being connected to the public grid, the PV AC module has to comply with common safety standards. A major issue is to avoid non-intentional operation in islanding mode with the grid being tripped at fault conditions or for maintenance purposes.
is positive. ∆P and ∆Q can be adjusted by tuning the domestic load. For a given capacitance and inverter power, a so-called non-detective zone (NDZ) can be
ABSTRACT
A major safety issue about grid-connected photovoltaics is to avoid non-intentional operation in islanding mode, the grid being tripped.
This paper presents detailed measurements on the islanding behavior of four module inverters with a maximum rated power of 200 W. Although applying active anti-islanding measures each inverter could be forced into islanding. It could be observed experimentally what recently has been shown analytically, that some methods against islanding fail if inverters are loaded with considerable parallel capacitance. As most distribution grids contain a considerable capacitance, those methods are to be improved. One of the inverters failed totally what illustrates the need for standardized type approvals.
ISLANDING PHENOMENON AND TESTS
Investigations carried out at K.U.Leuven in 1997 [1] have shown that small so-called "module inverters" are in general more sensitive to islanding than larger units. Recently four module inverters that are currently available on the European market, ranging from 90 to 200 W rated power, have been examined with regard to their islanding
Assuming constant active and reactive power output before and after grid tripping, voltage and frequency in islanding operation can be determined from the power balance, yielding equation (1) and (2).
behavior under different load conditions. The applied test circuit is shown in Figure 1.
PV array simulator
inverter
==
= ~
resonant circuit
P, Q
domestic
load
∆P P
=
1
−
Vg2rid Vis2land
(1)
ω island ω grid
⋅
∆P P
−∆Q Q=来自çæ ç èω
2 island
ω
2 grid
−
1÷÷ö ø
⋅
QC Q
+ ω island ω grid
−1
(2)
In these equations P and Q indicate the inverter operating point. QC is the reactive power supplied by the
determined in the ∆P-∆Q-domain where an inverter with predefined voltage and frequency limits will operate in
P to Grid in W Amplitude normalized on RMS value
becomes inverted and the control bias for ω⋅tz is measured. For the current fundamental this means a
phase shift by 0.5⋅ωgrid⋅tz with regard to the grid voltage. Hence, in order to maintain a high power factor, tz must not be chosen too high. The ratio of tz to half of the period of the grid voltage is referred to as the chopping fraction (cf):
operate in islanding mode. If no particular control algorithm for islanding prevention is implemented, the load conditions under which islanding occurs, depend only on the inverter's frequency and voltage limits.
The outcomes show where to put accents in the development and implementation of efficient protection algorithms.