Increasing the penetration of distributed renewable sources, including photovoltaic (PV) sources, poses technical challenges for grid management. The grid has been optimized over decades to rely upon large centralized power plants with well-established feedback controls, but now non-dispatchable, renewable sources are displacing these controllable generators.
By programming autonomous functionality into distributed energy resources—in particular, PV inverters—the aggregated PV resources can act collectively to mitigate grid disturbances. the problem of frequency regulation is the focus of a paper titled â€Evaluation of PV Frequency-Watt Function for Fast Frequency Reserves†by J. Neely, J. Johnson, J. Delhotal, S. Gonzalez, and M. Lave, all with Sandia National Laboratories. Specifically, the use of existing IEC 61850-90-7 grid support functions to improve grid frequency response using a frequency-watt function was investigated. The results of this investigation were presented at last week’s IEEE Applied Power Electronics Conference (APEC 2016).
The proposed approach dampens frequency disturbances associated with variable irradiance conditions as well as contingency events without incorporating expensive energy storage systems or supplemental generation, but it does require some curtailment of power to enable headroom for control action. Thus, this study includes a determination of the trade-offs between reduced energy delivery and dynamic performance. This paper includes simulation results for an island grid and hardware results for a testbed that includes a load, a 225kW diesel generator, and a 24kW inverter.
To demonstrate the ability of frequency-watt (FW) implementation to mitigate frequency (generator speed) deviations, experiments were performed at the Distributed Energy Technology Laboratory (DETL). These experiment configurations included a 225kW diesel generator, a PV array simulator, a 24kW inverter with frequency-watt functionality, a 50kW resistive load, and a 25kW resistive load. Four scenarios were considered: (a) the inverter operating with 100% of capacity with FW enabled, (b) the inverter operating with 100% of capacity with FW disabled, (c) the inverter operating at 50% of capacity with FW enabled, and (d) the inverter disconnected.
The authors concluded, “In this work, use of the frequency-watt function to mitigate frequency deviations in a power system with high penetration of PV power was investigated. Using models of an island grid that have been validated against field-data, the approach is demonstrated in simulation, and the trade-off between renewable energy production and frequency deviation is quantified. The simulations show that using frequency-watt functionality and fixed curtailment both improve the frequency deviations on the grid during normal operation and during fault transients, but the frequency-watt functions provide a greater improvement for a given curtailment level. Furthermore, feasibility of implementing the frequency-watt function in an electrical system experiencing a loss of load fault was demonstrated using a hardware testbed.â€
In the future, the research team plans to investigate the optimal FW settings to provide frequency regulation and contingency reserve capabilities while minimizing the loss of PV power from curtailment. The team also plans to develop specifications and recommendations for limits on inverter response time.
