Distribution Grid Power Electronic Devices as Mitigation Options for Circuits with High PV Penetration
As the penetration of variable Distributed Generation (DG) is increasing due to incentives and mandates, such as the Renewables Portfolio Standard (RPS) target in California of 33% by 2020, a number of challenges to utilities have arisen. The existing electric grid was designed and created to safely and reliably distribute power from a few centralized power generation sources through highly monitored and controlled transmission lines to distributed loads typically supplied via radial distribution feeders. Some DG such as small hydro and bio-gas generators are relatively constant and predictable in output. However, most of the recent growth in DG has been to exploit renewable resources, such as solar and wind energy, which are inherently intermittent. This intermittency can cause wide and unpredictable variability of the net power flow over the distribution system. This variability, along with the frequently wide dispersal of the generation by numerous small sources, can cause a number of voltage and power quality problems, and inhibits the utility’s operational control over the voltage and loading conditions in their service territory. At the same time, DG, in particular solar photovoltaic (PV) due to its recent popularity resulting in large-scale deployment of PV installations in many distribution systems, offers opportunities for utilities that, if leveraged, can help optimize grid performance.
DG Might Cause Issues on Distribution Feeders
The detrimental impact DG potentially has when installed in large numbers on an unprepared grid includes:
- overvoltages causing voltages on distribution feeders that are outside permissible limits and/or tripping of PV inverters;
- voltage sags and swells
- flicker caused by the variability of DG-generated power and inrush currents during start-up of wind turbine generators;
- reverse fault current flow causing existing protection schemes to be uncoordinated or possibly non-functional;
- increased wear on utility equipment such as tap-changers of voltage regulators and capacitor switches;
- unintentional islanding;
- and real and reactive power phase imbalances.
Many of these issues have already been observed as evident from the results of utility surveys we conducted and also from our experience with utilities we support with mitigating issues that have arisen on their high DG penetration distribution feeders. Utilities are experiencing more frequent operation of voltage regulators on their high-PV penetration feeders leading to greater maintenance requirements and PV inverters disconnect frequently due to overvoltage conditions. Simply adding DG to the system without preparing for the consequences will almost certainly result in an escalation of the DG-caused issues that already exist on the feeder, and in the emergence of additional issues once even higher DG penetration levels are reached.