Kauai is a small island with a daytime peak demand that usually falls in the 60 – 70 MW range. The island has around 4 MW of customer sited distributed PV, in addition to the utility’s first purchase power agreement for a 1.2 MW plant, so on a sunny day PV meets about 7-9% of the island’s peak power needs. This level of penetration is already much higher than that experienced by mainland utilities in the US, but it is below the penetration levels of 15-30% at which concerns arise about grid instability caused by variable output generation sources.
Kauai provides an ideal environment for isolating the impact that PV penetration level has on grid instability, because the island has no wind farms. In fact, it may never have a wind farm, due to the presence of federally endangered seabirds. Solar PV, on the other hand, is well established and rapidly growing; with the potential to meet 20% or more of the island’s peak power needs on sunny days within the next few years.
There are a few concerns utilities have about high penetration PV, but the primary one focuses on ramp rates: picture a sunny day with a few clouds rolling by, which cause a PV system’s output to swing abruptly between its maximum output and a much lower level. The other generation on the system must make up for the variable output before voltage and frequency levels on the system drop too low or go too high. If the other generation cannot respond fast enough, power quality on the grid will be adversely affected as voltages and frequencies fluctuate to unacceptable levels.
As long as the penetration of PV on the grid is low, the utility should have no trouble maintaining power quality as the output from PV systems fluctuate. However, even if overall PV penetration levels in a region are low, it is possible to have local “hot spots” where penetration on a single distribution circuit is very high. In this case utilities have concerns that power quality will suffer on that distribution circuit due to the high penetration of PV. KIUC is testing that hypothesis to the extreme with its 1.2 MW solar farm, by supplying 100% of a distribution circuit with PV during the day.
Now for the good news: as the utility monitors the distribution circuit on sunny days and cloudy days, with the PV system turned on and the PV system turned off, they are seeing very little difference in the voltage levels, harmonics, and overall power quality between the different scenarios. These preliminary results suggest that utilities could go to very high levels of PV penetration in localized areas without causing problems for the grid. KIUC is continuing to monitor the system, but the initial results look very positive for the PV industry.
As PV penetration on the island grows, KIUC will have the opportunity to evaluate the impact of high penetration levels of PV over the whole island grid (as opposed to just a single distribution circuit). The potential for power quality issues is much greater when overall PV penetration levels become high, which is why KIUC has begun investing in battery energy storage systems to mitigate the potential impact from such a high concentration of PV. This initial battery investment with be programmed to control the ramp rate for a planned 3MW PV farm and provide system wide frequency support. Whatever KIUC finds, this will be an important island to keep an eye on as utilities prepare for ever increasing penetration levels of PV.
Source: IDC Energy Blog