Often referred to as the “brains” of a renewable energy system, an inverter is an electronic device that converts direct current (DC) from batteries or solar modules into alternating current (AC) at the voltage and frequency required to run electrical loads or feed into the grid.
Grid-tie, or utility intertie, inverters convert DC power from photovoltaic (PV) modules directly into AC power to be fed into the utility grid. Storage batteries are not needed, as any power produced that is not consumed by the owner’s electrical loads is fed into the utility grid to be used elsewhere. Due to the high voltages involved, grid-tie inverters should be installed and serviced only by qualified personnel.
All grid-tie PV systems use the utility grid for energy storage. Whenever the PV array is generating more power than the loads are using, excess energy is fed into the grid, turning the meter backward. When the loads require more power than the PV array can supply, the utility makes up the difference. Known as “net metering,” this arrangement is the most efficient and cost-effective for grid-tied applications since there are no batteries to maintain. However, most grid-tie inverters are required by law to shut down during a utility outage per IEEE 1547, which is incorporated into UL 1741. Battery-based grid interactive inverters (See Battery-Based Inverters) are required for back-up power applications.
Most batteryless grid-tie inverters are called “string” inverters because the PV modules must be wired together in series to obtain a higher input voltage. String Inverters are designed to run at voltages up to 600 VDC in residential systems and up to 1,000 VDC for commercial and industrial systems. String wiring is quick and easy to install, and the higher voltage helps to minimize line losses and required wire size. However, in string wiring, maximum power point tracking (MPPT), along with any monitoring output, is performed at the string or array level.
An important trend to note is that most string inverter manufacturers have introduced Transformerless (aka non-isolated) inverters due to the higher efficiency and lower manufacturing costs of that topology. While transformerless inverters have dominated the European market and are arguably even safer than their isolated counterparts, they do impose special “ungrounded system” requirements according to NEC 690.35. This includes the use of PV-Wire for DC connections, including the module outputs, as well as fusing and switching on both output legs. The term ungrounded should not be confused with the equipment ground, which is still required; it means that neither the positive nor negative outputs are connected to ground. Some jurisdictions will also require special circuit labels noting that both conductors are “hot”.
Module Optimizers can be deployed behind each module to provide individual module-level MPPT tracking and monitoring, optimizing the DC output that is connected to a string inverter for very high efficiency. Systems that combine optimizers with low-cost high-efficiency string inverters can simplify system design and maximize safety and energy harvest with minimal impact on cost.
Microinverters are typically mounted behind each solar module. They convert the DC output of each module to AC, replacing the high DC voltages (up to 1,000 VDC) with comparatively lower AC potentials (240 VAC or less) and simplifying system design. The microinverter output connects directly to the breakers in the AC load center using conventional wiring. Since microinverters provide MPPT tracking and monitoring for individual modules, the impact of differences in orientation or shading between modules is reduced. Microinverters are a popular solution for electrical contractors that are new to solar as DC wiring is essentially eliminated.
Three-Phase Inverters are used in larger commercial grid-tie systems, and output at 480 VAC, which is more common in larger buildings. Many of these 10 to 50 kW inverters are available with input volt- age ratings of 1,000 VDC. This higher input voltage enables longer module strings, which can improve design flexibility and eliminate external combiners. Traditional Central Inverters are rarely used anymore for systems under several megawatts in scale.