Carl J. S. Lenox

Initial operating experience of the 1.2-MW La Ola photovoltaic system

The 1.2-MW La Ola photovoltaic (PV) power plant in Lanai, Hawaii, has been in operation since December 2009. The host system is a small island microgrid with peak load of 5 MW. Simulations conducted as part of the interconnection study concluded that unmitigated PV output ramps had the potential to negatively affect system frequency. Based on that study, the PV system was initially allowed to operate with output limited to 50% of nameplate power capacity to reduce the potential for frequency instability due to PV variability. Based on the analysis of historical voltage, frequency, and power output data at 50% output level, the PV system has not significantly affected grid performance. However, it should be noted that the impact of PV variability on active and reactive power output of the nearby diesel generators was not evaluated.

Methods of integrating a high penetration photovoltaic power plant into a micro grid

The island of Lanai is currently one of the highest penetration PV micro grids in the world, with the 1.2 MWAC La Ola Solar Farm operating on a grid with a peak net load of 4.7 MW. This facility interconnects to one of Lanais three 12.47 kV distribution circuits. An initial interconnection requirements study (IRS) determined that several control and performance features are necessary to ensure safe and reliable operation of the island grid. These include power curtailment, power factor control, over/under voltage and frequency ride through, and power ramp rate limiting. While deemed necessary for stable grid operation, many of these features contradict the current IEEE 1547 interconnection requirements governing distributed generators. These controls have been successfully implemented, tested, and operated since January 2009. Currently, the system is producing power in a curtailed mode according to the requirements of a power purchase agreement (PPA).

Assessment of a new simulation approach for estimating PV output variability from satellite imagery

Uncertainty about the impacts of very large PV plants on utility operations is a barrier to building such plants. Output variability is of particular concern due to its potential impact on grid reliability and operations. Quantifying the impacts of proposed plants generally relies on simulating plant performance without the benefit of irradiance measurements at the proposed plant locations. Accordingly, methods for simulating irradiance are of great interest. Sandia National Laboratories (SNL) has developed a relatively simple method to estimate one-minute power output from fleets of large PV plants. The method relies on generating artificial time series of irradiance from hourly satellite irradiance estimates (available nearly everywhere) and one-minute ground measurements of irradiance from analogue sites in the region [1]. When applied to the southern Nevada region, the resulting time series preserve basic statistics for irradiance and for changes in irradiance, and exhibit characteristics evident in measured irradiance from other regions. Here we investigate the dependence of the simulation on the available ground measurements. If ground measurements capture generic irradiance patterns, then the method could be applied to regions other than where the measurements were made. We use ground measurements from southern Nevada and Tennessee to simulate irradiance for a site in Florida, where we use hourly averages of ground-measured irradiance in lieu of satellite estimates. The simulation results compare poorly with ground observations of irradiance at a one-minute time scale. We conclude that our simulation method is not general in the sense that it appears to require irradiance measurements within the region of interest.