Читать книгу Smart Solar PV Inverters with Advanced Grid Support Functionalities - Rajiv K. Varma - Страница 29

Solar PV systems tend to raise the voltages due to active power injection in distribution feeders to which they are connected [27, 28]. Voltage rise is prominent especially during low load conditions or when the PV power output exceeds the load during high PV penetration scenarios. The voltage rise depends upon the power generated by the DERs, location of the DERs, and the electrical characteristics, i.e. system short circuit level and X/R ratio of the feeder. Higher voltage excursions are experienced in weak systems having relatively high short circuit impedances. The voltage rise is more when the solar PVs are connected at the end of long feeders, and gets aggravated in presence of shunt capacitors. The steady‐state voltage rise often goes beyond the acceptable voltage limits of the utility. This leads to the utility putting a restriction on the number of solar PVs that can be connected in their grid.

A typical example of voltage rise caused by a solar PV‐based DER located at the far end of a distribution feeder connected to a substation is shown in Figure 1.9 [29]. In the absence of DER, the voltage is much below the ANSI voltage limit of 1.05 pu [30]. The power injection by DER causes the voltage to rise beyond the ANSI limit. If the DER is made to absorb reactive power (through any smart inverter function), the voltage goes below the stipulated limit.

The solutions typically employed by utilities to alleviate such overvoltage conditions are [27]:

1 reducing the use of fixed shunt capacitor banks and instead using switchable capacitor banks which can be switched during high power production from the DERs. It must be understood that while changing capacitor settings is helpful for voltage control, varying capacitor values impacts network resonant conditions which may potentially cause harmonic amplification in the grid. This possibility must be examined beforehand, and solutions implemented, if required.

2 lowering the voltage setpoints on the Load Tap Change (LTC) transformers and line voltage regulators, or altering the control modes of operation of such voltage control equipment.

Conventional transformer taps, voltage regulators, switched capacitors, etc., are used to regulate voltages but may not be adequate under all scenarios. In cases where very rapid voltage control in time frames of 2–3 cycles is required, Flexible AC Transmission system (FACTS) Controllers such as Static VAR systems (SVCs) and STATCOMs or Dynamic VARs (DVARs) are employed for this purpose [31, 32]. These controllers although very effective and are quite expensive.

Figure 1.9 Voltage rise due to active power injected by solar PV based DER in a feeder.

Source: Based on Zandt [29]. Used with permission from EPRI.