Smart Grid Inverters to Support Photovoltaics in Distribution Systems

With increasing penetration levels of solar PV, the need for inverter technology to provide grid support has become more and more critical. Since 2009, EPRI and industry partners have been working to establish a common set of functions that can provide such capability. The development of a common set of functions is complete, however, very little work to date has addressed the impact these common functions will have on grid performance, especially at the distribution level. At the same time, EPRI has established a method for determining the PV hosting capacity of distribution feeders using model analysis and simulation. Present work combines these two efforts to provide a better understanding of the impact of grid supportive inverters on distribution feeders through hosting capacity. This paper summarizes analysis to determine the additional amount of PV that distribution feeders can accommodate with the use of smart inverters. INTRODUCTION The analysis in this paper focuses on the impact of three different smart inverter functions[1]power factor, voltvar, volt-watton four feeders in the Northeastern United States. Four feeders were chosen with varying characteristics, and the three different functions were applied to determine the possible increase in hosting capacity utilizing these smart inverter functions. FEEDER CHARACTERISTICS Two of the feeders were relatively short in overall primary circuit distance, and two were relatively long in overall circuit distance. All four feeders were 15 kV class feeders. The primary voltages of the four feeders are 13.2 kV. Two feeders have exclusively 13.2 kV primary voltage. The other two feeders have multiple primary voltages. Feeder N3 has 13.2 kV and 4.16 kV, while the N4 feeder has 13.2 kV wye, and 4.8 kV un-grounded sections. The peak loads of the four feeders range from 9 MW for Feeders N1 and N2 down to 7 MW for feeder N3. The lengths of the feeders vary from 9 total circuit miles for feeder N2 up to 138 total circuit miles for feeder N4. Two of the feeders are relatively short (N1 and N2), while the other two (N3 and N4) are relatively long 15 kV class feeders. The maximum short circuit impedances of feeders N1 and N2 are 6.7 Ω and 2.7 Ω, respectively. Feeders N3 and N4 which are much longer feeders have higher maximum short circuit impedances of 19.3 Ω and 27.6 Ω. Feeder N1 has a substation three-phase load tap changer (LTC) that supplies a 13.2 kV (line-line) nominal feeder. Its’ set-point is 123V (on a 120V base) and it has a bandwidth of 3V. It is configured with line drop compensation with a potential transformer (PT) ratio of 63.5V:1V and a current transformer (CT) primary rating of 1200 amperes. The line drop compensation R value is set to 3V and the X value is set to 5V. There are three fixed feeder capacitors providing 2700 kvar total compensation. Feeder N2 has an LTC that supplies a circuit whose nominal voltage is 13.2 kV (line-line). The set-point of the LTC is 124.5V (on a 120V base). It has a bandwidth of 3V. There are two feeder capacitors, totaling 900 kvar. One capacitor is switched and the other is fixed. The capacitor that is switched, is switched on current with voltage over-ride. Feeder N3 has three single-phase regulators at the head of the feeder. The voltage setpoint is 122V with a 2V bandwidth. Each regulator has line drop compensation enabled with R=2V and X=0V. There are 5 line voltage regulators that vary between a single-phase regulator and three single-phase regulators. Four out of the 5 line voltage regulators have line drop compensation enabled, with various settings. The fifth voltage regulator does not have line drop compensation enabled. There are three feeder capacitors in-service with 2400 kvar total compensation. Two banks are fixed, and the largest bank (1200 kvar) is voltage controlled with 120/126V on/off settings. 23rd International Conference on Electricity Distribution Lyon, 15-18 June 2015