Central Inverter Trends in Power Plant Applications
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PV projects continue to trend toward larger systems. In 2009 and the first half of 2010, more than 26 MW of projects larger than 500 kW were installed in California under the California Solar Initiative program, which provides rebates for customer-owned generation. For the manufacturers of central inverters, particularly larger inverters, the market for power plants has great potential. Planned projects have been announced ranging from 2 to 500 MW, and even larger projects are in the works.
The Solar Energy Industries Association reports that across the US, 129 MW of utility-scale PV plants are under construction and more than 12.5 GW are in development. Not all of these plants will be installed, but even a fraction of these numbers represents a huge increase in inverter demand. Significant commercial and power plant project development is occurring in Ontario, Canada, and throughout the Southwest and Northeast in the US. This development is also emerging in the South and Midwest.
My focus here is on inverter selection and system design considerations for large projects and power plant applications. I look at product offerings designed especially for use in power plants, where packaged equipment simplifies repetitive installation processes and interconnections are invariably at medium or high voltages. I also look at the factors influencing future inverter development, such as evolving utility requirements, codes and standards; improved safety and protection measures; and reliability and service needs.
PV POWER PLANTS
Here I loosely define a power plant as a PV system with most of these characteristics: The capacity is larger than 1 MW; installation is ground-mounted; the plant owner sells energy directly to the electric utility; and utility interconnections occur at medium or high voltage.
The vast majority of PV systems installed in North America to date are connected behind utility customers’ meters, reducing their metered consumption and load, and possibly exporting through a net-metering agreement. Power plants, however, are typically not interconnected on the load side of a customer service meter. Large plants installed within military or other government or industrial complexes are notable exceptions, such as the 14 MW system at Nellis Air Force Base in Nevada.
Similarly, whereas distributed PV systems interconnect at load-side service voltages, a power plant is more likely to interconnect at distribution or transmission voltages. Medium voltage is associated with distribution system interconnections, generally from 4 to 34.5 kV, while high- voltage subtransmission or transmission level interconnections range from 69 to 500 kV.
Many commercial systems of 1 MW or larger and most, if not all, multi-megawatt power plant systems interconnect with the utility at medium- or high-voltage levels. The cabling and switchgear that collect the output of multiple dispersed inverters or inverter clusters are called the collection system, borrowing from the term used in wind and other conventional power plants. Because of the long distances involved, it is not economical for a collection system to operate at a low voltage such as 480 Vac and transform to medium or high voltage at the point of interconnection (POI) with the utility. At low voltage, the cable ampacity requirements and voltage drop losses are significant. It is therefore better to step up the voltage at the individual inverter clusters or stations and distribute the power on a medium-voltage collection system.
MEGAWATTS IN A BOX
Several inverter manufacturers offer packaged solutions that include two or more inverters, a medium voltage (MV) transformer, the interconnecting switchgear, and a container or skid that houses the equipment. This essentially provides the customer with a plug-and-play solution—not that this should be equated with the ease of setting up a desktop computer. Nevertheless, the field installation and labor requirements are greatly reduced, and any engineering performed in the factory is taken out of the field. Packaged solutions are efficient because internal connections are predesigned and largely preinstalled, pad layouts and foundations are standardized, and equipment is shipped and dropped in place all together.
Broadly speaking, there are two types of packaged inverters with MV transformer configurations. One utilizes standard unipolar inverters, which incorporate a low-voltage transformer for 480 or 208 Vac output, and include an additional transformer to step up to MV levels. The other type directly couples the ac output of the inverters—typically ranging from 200 to 270 Vac—to the transformer, thereby eliminating a transformer stage. The obvious advantage to the latter is the greater overall efficiency since there is only one step-up transformer. There are limitations to this type, however. To provide the galvanic isolation needed between inverters, particularly with grounded dc systems, the transformers have separate secondary windings for each inverter. Practically, this limits the number of inverters to two per transformer. The bipolar Solaron-brand inverters from Advanced Energy (AE) are an exception. These inverters can provide a transformerless output at 480 Vac and be connected in parallel with multiple other inverters on a single transformer. Design considerations for working with AE bipolar inverters are discussed later.