Technology for Safe Integration of Distributed Generation

"Ultimately, we want to see the adoption of smart inverters that can support grid reliability the way more traditional generation sources do. "

At our utility, Southern California Edison (SCE), we’re testing a key safety component of rooftop solar systems: the inverter. Inverters convert the direct current (DC) power produced by photovoltaic (PV) panels into alternating current (AC) power, so it can be fed into the grid or used where it’s generated. Our objective is to ensure that inverters function in a way that doesn’t compromise utility grid safety and reliability.

Ensuring Safe and Reliable Integration of Distributed Resources

Distributed generation can pose potential safety risks for utility workers, first emergency responders, and the public -- if, for example, the generation source fails to de-energize when there is a downed power line. Inverters are designed to prevent this so-called “islanding” from happening.

While it is important that inverters stop the flow of power when there is a downed power line, it is also important that inverters continue to supply power when there is a small or short variation in voltage or frequency. Without this “voltage/frequency ride through” capability, disconnection could worsen abnormal system conditions, creating issues related to stability and reliability. 

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Testing in the Lab and in the Real World

Our utility, SCE, is evaluating how inverters perform under a variety of typical grid conditions, such as during grid faults and emergencies. Our goal is to find out whether they are grid-friendly devices, and, if not, to determine what needs to be done to make it happen. Until now, limited data has been available on how inverters perform under typical grid conditions, with inverter manufacturers focusing primarily on safety testing.

Working with the National Renewable Energy Lab (NREL), Sandia National Laboratory, inverter manufacturers, and others, SCE developed “Solar PV Inverter Test” procedures to evaluate the performance of residential and commercial inverters. These procedures are now in use at our Distributed Energy Resources Lab in Westminster, Calif.

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Real-Life Evaluation at Irvine Smart Grid Demonstration

In addition to our work in the lab, we’re also evaluating inverter performance in the real world. At the Irvine Smart Grid Demonstration project in Irvine, Calif., homes are equipped with photovoltaic panels and inverters.

Sharing Data with National Renewable Energy Lab

The data is being shared with NREL for its High Penetration Photovoltaics project, and will be used to validate models and help refine predictions of circuit behavior with high penetration of distributed resources.

More about NREL

NREL’s High Penetration Photovoltaics Project

A Tighter Integration Into the Grid

In the future, as clean distributed resources play a bigger role in our electricity supply, distributed resources will need to be more tightly integrated with the grid. Even though inverters are on the customer side of the meter, they are components of the Smart Grid that we may depend on in the future to help maintain the reliability of grid.  For example, inverters could supply local “reactive power” (used to create the magnetic fields needed to operate equipment such as electric motors and transformers), which can free up distribution system capacity to serve additional real power demand and defer the need for capacity additions.

Standards to Enable Streamlined Interconnection of Distributed Generation

Our utility is working with other utilities and stakeholders to help revise requirements under the California Public Utility Commission’s Rule 21 and the Institute of Electrical and Electronics Engineers 1547 standard. Incorporating new inverter capabilities into these standards will ensure the continued safety and reliability of the grid, reduce the time and cost of interconnecting distributed generation, and support increased use of PV and other clean, distributed resources. 

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