The LVRT Capability of DGs

The DGs are required to remain connected to the network during the voltage dips or faults, and depending on the requirements, the DGs may be requested to feed-in short-circuit fault currents up to the agreed value (or according to the limits of DGs) in order to support the detection of faults. This is called the low-voltage ride through (LVRT), under-voltage ride through (UVRT) or fault ride through (FRT) requirement. In the grid-connected mode, the LVRT capability of DGs is usually required to maintain the stability of the system, since the disconnection of many DGs even for a fraction of a second may result in large voltage or frequency fluctuations, causing the voltage or frequency instability of the entire system. The synchronous generators are more sensitive to voltage dips, and hence, their LVRT requirements are considerably less stringent in comparison with the nonsynchronous generators, including the converter-based generators, which can remain connected for the extended durations. In the islanded mode of operation, not only the system stability is important, but also, the quick detection and isolation of the fault is equally important. Additionally, the protection coordination or selectivity between the main and backup protection has to be ensured. Figure 24 shows the comparison of di fferent LVRT requirements of nonsynchronous generators, including the converter-based generators, according to the previous German BDEW-2008 standard [32] and the latest European Standard EN 50549-1:2019 adopted as the Finnish National Standard SFS-EN 50549-1:2019 [47]. The DGs are required to remain connected in parallel with the LV or MV networks if the voltage at the connection point is above the voltage-time curves of Figure 24 (red and black curves). Although, the LVRT requirements are expectedly limited to the most stringent curves, however, the network operators may define their own LVRT characteristics. These standards do not define the LVRT requirements for the islanded mode of operation. Therefore, a new LVRT characteristic was proposed in this paper for the islanded mode operation of the nonsynchronous DGs, including the converter-based DGs shown as a green voltage-time curve in Figure 24. According to this new proposed LVRT characteristic, the DGs will remain connected to the islanded MV/LV microgrid for at least 2 s after the voltage dip or the fault and feed-in short-circuit current of at least 1.2 p.u. of the rated current. With the proposed LVRT characteristic, not only the stability of the islanded microgrid will be maintained, but also, a good protection coordination between adaptive IEDs will be ensured. The WTG and BESS (battery energy storage systems) with full-scale converters are capable of providing this requirement. Normally, in the presence of high-speed communication, the standard grid-connected LVRT curves will be used; however, in case of communication failure, the definite-time coordination of IEDs with the proposed LVRT curve will be applied.

**Figure 24.** The low-voltage ride through (LVRT) capability of the nonsynchronous generators, including the converter-based generators: The red and blue voltage-time curves according to [32] and the black voltage-time curves according to [47]; the green voltage-time curve is the new proposed LVRT curve for the islanded mode of operation.
