Читать книгу Smart Solar PV Inverters with Advanced Grid Support Functionalities - Rajiv K. Varma - Страница 43

Protection systems of power distribution systems comprise overcurrent relays, fuses, and reclosers that are designed to operate for only one direction of power flow, i.e. from the grid to the loads or to the possible fault location [20, 64]. With the high proliferation of solar PV systems, the following challenges are experienced [16, 20,64–69, 70]:

1 The fault current may become bidirectional as it will be fed both from the grid and the different solar PV systems in the distribution system.

2 It is not necessary that the fault current will always increase with the introduction of solar PV systems: it can both increase and decrease depending upon the location of the concerned PV systems with respect to the fault location [71]. Solar PV systems installed upstream of the fault location would increase the fault current while locating them downstream could reduce the fault current level. Hence the protection coordination done earlier without the solar PV systems, i.e. with only unidirectional flow of power will not apply when multiple solar PV systems are connected.

3 The power output from solar PV systems is variable. It can be either zero as in the night or maximum with full solar irradiance. Furthermore, the power output is variable due to varying cloud coverage. These operating conditions lead to changing fault current levels. Hence, variability in solar and wind DER power production can adversely impact the operation of overvoltage and overcurrent protection systems.

4 The conventional power systems were not designed for power flow in reverse direction. The relays and protection systems have to be modified to adapt to the backflow of power. Presence of high amount of solar PV systems results in current flows from multiple sources, which makes protection coordination even more complex and difficult. Protection device settings need to be modified or relays may need to be replaced in distribution systems where reverse power flow is expected [26].