Short-circuit currents
During voltage dips, especially complete grid failures, all PV and battery inverters connected to the grid may generate currents that are slightly above the maximum current in normal operating conditions. Such currents are relevant for the correct dimensioning of the wiring and the protective devices, both at the system level and the grid level.
Grid operators frequently ask manufacturers of PV and battery inverters to provide maximum values of short-circuit currents. In other cases, the manufacturers are asked to provide characteristic values such as Ik and ip or further electrical values at defined times during a grid failure.
At an international level, the IEC 61400-21 standard describes testing procedures for wind farms that can be easily applied to PV inverters. In some countries, such as Germany (TR3, Technical Guidelines for Power Generating Units, Part 3, FGW e.V.) and Italy (CEI 0-21 and CEI 0-16), there are specific standard requirements for testing the capability of “riding through“ grid-voltage dips.
The instantaneous values of alternating currents and alternating voltages are recorded synchronously with 50 kHz (20 μs). Basic frequencies of the co-system based on measurement of instantaneous voltages and currents are calculated according to IEC 61400-21. All results are measured as half-period RMS values.
The following table shows the test results for the following SMA PV and battery inverters of type Sunny Tripower, Sunny Highpower and Sunny Tripower Storage:
Inverter type | Short-circuit surge current ip (A) | Symmetrical initial short-circuit current Ik‘‘ (A) | Uninterrupted short-circuit current Ik (A) | Maximum current Imax (A) | |
|---|---|---|---|---|---|
FRT "full" | FRT "partial" | ||||
Sunny Highpower | |||||
SHP 100-20 / SHP100-21 | 318.67 | 166.39 | 151.0 | 0 | 144.3 |
Sunny Tripower | |||||
STP 12-50 | 106.84 | 38.3 | 36.6 | 0 | 36.6 |
STP 15-50 | 101.42 | 42.53 | 36.6 | 0 | 36.6 |
STP 20-50 | 95.68 | 43.06 | 36.6 | 0 | 36.6 |
STP 25-50 | 100.01 | 42.73 | 36.6 | 0 | 36.6 |
STP 125-70 | 328.81 | 192.22 | 181.1 | 0 | 181.1 |
Sunny Tripower Storage | |||||
STPS30-20 | 151.1 | 61.0 | 45.6 | 0 | 45.6 |
STPS50-20 | 179.6 | 90.2 | 75.5 | 0 | 75.5 |
Note that the ip values are given as an amplitude, whereas the values for Ik” and Ik are RMS values.
The values for Ik’’ and ip were measured during the certification process (TR3, CEI 0-16) by an accredited test institute and are comparable to the characteristic values defined in DIN EN 60909. These values represent the maximum values of all tests.
Additional information is available under the following link: Technical Information "Short-Circuit Currents" and at www.SMA-Solar.com.