With more and more solar coming onto the grid, solar plants are expected to provide greater grid management services. Much of this responsibility falls on the inverter. To tell us why and more, Managing Editor Kathie Zipp spoke with Soonwook Hong, Power Systems Engineering Manager at inverter manufacturer Solectria Renewables. Here’s a recap of his answers below. To hear more, listen to the full interview.
Kathie: Soonwook, as I understand, in the past when there were fewer solar installations connected to the grid, interconnection was pretty simple. I But as more and more solar is coming online and being connected to the grid, grid operators are asking these plants to have more sophisticated grid control. Why is this necessary? And what exactly are they asking from solar plants?
Soonwook: There are two major aspects to why special grid control functions are required: Power flow and voltage. The distribution system was designed to deliver power from generation, transmission, down to distribution feeder. All equipment protections were configured with this type of infrastructure in mind. As the aggregated solar installation grows, the power can flow from the distribution feeder to the transmission network or other distribution networks, which is called backfeeding. This conflicts with the protection coordination in utility infrastructure. The inverter can be used to prevent this by limiting the maximum power generation.
Also, when a distributed generation (DG) source generates power, the DG terminal voltage is increased to deliver power to the load. This voltage rise needs to be maintained so that it will not cause any damage to the equipment connected to the feeder. Inverters can control the output voltage by providing the reactive power. The inverter can also be used to regulate the voltage fluctuations caused by load changes or weather. Other than the functions described, voltage and frequency ride through functions are used to increase the distribution line stability. Slew rate control is used to increase power quality.
Kathie: Why does most of this responsibility fall on the inverter? In the past, the inverter’s main job has been to convert DC to AC, but what other tasks will inverters increasing be expected to do?
Soonwook: Inverter technology has been used in the industry since the 1960s to control voltage, current, frequency and power. So far, we have not needed to use those existing capabilities because the DG installation size was not large and the functions were not needed. As PV installation capacity increases, the influence of PV plants on the grid is significant so these functions are being discussed as options or requirements. Other tasks for inverters in PV projects includes coordinated operation with energy storage systems, microgrids and islanded operation. These are not as popular yet, but there’s great potential for these to be used in near future.
Kathie: I’ve heard inverter manufacturers say that their inverters already have these “smart” features and are able to handle these tasks. Is this true, do inverter manufacturers have any concerns about more demands on their products?
Soonwook: Yes, Kathie. Some of the functions are available with software changes only, and at Solectria, we have provided functions like power factor control or power curtailment to some large scale projects. Most of the functions discussed in the industry can be achieved with relatively minor hardware changes. Regarding concerns on more demand, there are a couple that I would like to point out.
1) To provide the grid supporting functions, there is a possibility that inverters may need to limit power generation and provide reactive power instead. The business model for how to compensate the site owner for the loss of generation or how to compensate for the generation of the reactive power have not been resolved yet. I know these are being discussed in smart grid consortium.
2) To provide grid supporting functions, the inverter operating profile will be changed. The inverters may need to operate more hours a day or at high loading conditions, which make the internal components run hotter and reduces their life. So, the inverter cost and reliability model can be dependent on the functions to be used.
Kathie: Can you explain how guidelines and standards come into play?
Soonwook: Yes, at first, it is not easy to meet all requirements defined in the standard. Still, the industry standard is a great infrastructure that everybody benefits from. When the industry uses a standard product, the manufacturers can use the same processes and materials to make equipment, the customers can purchase the inverters at reduced prices and the operators can use a universal interface to control them. It is a must to promote DGs on the grid.
The European grid codes and case studies have been widely used to develop the IEEE and U.S. industry standards. In the case of Hawaii, the penetration level is high and the grid size is relatively limited, so they came up with their own requirement. Also, the California Rule 21 consortium is leading the standard development effort and more utility companies join the consortium. I believe these efforts will benefit the industry and develop a standard that everyone can use for at least the North American market.
Still, there are 3000 utility companies in U.S. and each has different policies. We expect the industry standard committee will continue to develop a requirement considering different utility requirements and operating policies.
The hosting capacity is defined to be the size of possible DG installation for a feeder. Because of the voltage and backfeeding issues, the maximum installation size is limited by the utility company. However, this hosting capacity can be increased by using these special grid supporting functions. So, you can consider these functions to be the way to install more DGs in the distribution feeders.