IEC TS 63217-2021 pdf Utility-interconnected photovoltaic inverters – Test procedure for over voltage ride-through measurements

The inductor L is used to limit the effect of the short circuit for the utility grid that powers the test circuit. The sizing of L shall therefore account for all test sequences to be performed and limit the current taken from the grid to values that do not cause an excessive increment of the grid voltage. Considering an acceptable voltage increment of at most 5 % when performing the test, the minimum reactance value of the inductor L shall be at least 20 × Z Grid , where Z Grid is the grid short-circuit impedance of the utility grid. To ensure that the test is realistic, however, the apparent short-circuit power (S EUT ) available at the EUT connection node N EUT should be at least equal to 3 × P N , where P N is the rated power of the EUT ( minimum value S EUT = 3 × P N , recommended 5 × P N < S EUT < 6 × P N ). This means that during the OVRT tests, the contribution of current through L from the grid remains dominant compared to the current contributed by the EUT. In this way, the output AC current of the EUT does not create a significant voltage increment for the duration of the test relative to the no-load test. The two conditions described above define the minimum and maximum limit of L . The two conditions combined also define the limit criteria for the choice of a grid infrastructure suitable for performing the test with the impedance circuit. If the grid infrastructure cannot meet above requirements, an alternative test circuit utilizing a back-to-back converter is allowed, as shown in Figure 2 and Figure 3 and may be added to reduce the grid short-circuit impedance Z Grid . Generally, the X/R value of inductor L for the RLC-series emulator may be close to the transmission line impedance values for different countries and regions. It is also appropriate that the inductor L should be characterised by an X/R ratio equal to at least 3, in order to reproduce the typical minimum values of X/R found in HV as well as MV power lines.