T&D World Magazine

EPRI Assesses Smart Inverters to Provide Optimized Volt/Var Support for PV Integration

The Electric Power Research Institute has launched a collaborative initiative to develop standard functions and a standard language for grid integration of smart solar and storage inverters. To move this forward, EPRI is conducting research that is analyzing the effects of advanced voltage control strategies on real distribution systems. The specific focus of this analysis is on the use of smart inverters to provide optimized volt/var support. Preliminary results of this research indicate that a utility could incorporate up to twice the amount of photovoltaic (PV) power into a distribution circuit.

In mid 2009, EPRI worked with the Department of Energy, Sandia National Laboratories, and the Solar Electric Power Association to launch a collaborative initiative to develop standard functions and a standard language for grid integration of smart solar and storage inverters. The initiative was designed around an open process and a large interest group became engaged, including inverter manufacturers, system integrators, research organizations and utilities.

A phased approach was selected, and the project participants identified a set of high priority functions to address in the first phase. These included intelligent autonomous volt/var management and multiple mechanisms for storage management. Mapping to standard communication protocols are being done in coordination with NIST.

EPRI is currently carrying forward the work of this initiative by analyzing the effects of advanced voltage control strategies on real distribution systems. The specific focus of this analysis is on the use of smart inverters to provide optimized volt/var support. Drawing from a database of distribution circuits throughout the U.S., EPRI is analyzing the impact volt/var control can have when implemented in conjunction with grid-tied solar PV. Full three-phase electrical representations of these circuits, from the substation to each customer meter have been modeled in OpenDSS, a comprehensive electrical power system simulation tool for electric utility distribution systems. This allows EPRI to evaluate a wide range of circuit characteristics and solar PV deployment scenarios.

Specific distribution circuits were selected for closer analysis based on their inability to accept significant levels of distributed generation. In these cases, the maximum allowable amount of distributed resources was limited by its impact on system voltage. Increasing numbers of PV installations were then incorporated into each circuit model until a voltage criteria was violated. Additional PV beyond that point would result in unacceptable voltages. The test was repeated with the same number of PV installations, but with each PV inverter supporting autonomous volt/var control.

Preliminary results indicate that for circuits that limit the amount of distributed PV due to voltage constraints, higher levels of PV could be installed without voltage violations occurring when autonomous volt/var control was used. With just this one advanced function, an additional 25 – 100 percent PV power was found to be incorporated into a distribution circuit.

Going forward, these preliminary results of smart inverter performance will need to be verified in various distribution feeder configurations and compared against other options such as capacitor banks to understand the unknowns about their capabilities.

EPRI intends to further investigate the impact volt/var control has on voltage quality in terms of compensating for secondary high voltages, primary voltage variations due to customer and system load, as well as impact on distribution system efficiency and coordination with conservation voltage reduction efforts.

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