How Inverters Work

What Goes on Inside the Magic Box

One of the most incredible things about photovoltaic power is its simplicity. It is almost completely solid state, from the photovoltaic cell to the electricity delivered to the consumer. Whether the application is a solar calculator with a PV array of less than 1 W or a 100 MW grid-connected PV power generation plant, all that is required between the solar array and the load are electronic and electrical components. Compared to other sources of energy humankind has harnessed to make electricity, PV is the most scalable and modular. Larger PV systems require more electrical bussing, fusing and wiring, but the most complex component between the solar array and the load is the electronic component that converts and processes the electricity: the inverter.

In the case of grid-tied PV, the inverter is the only piece of electronics needed between the array and the grid. Off-grid PV applications use an additional dc to dc converter between the array and batteries and an inverter with a built-in charger. In this article we discuss how inverters work, including string, or single-phase, and central, 3-phase inverters; explore major inverter functions, key components, designs, controls, protections and communication; and theorize about future inverter technology.

KEY INVERTER FUNCTIONS

Four major functions or features are common to all transformer-based, grid-tied inverters:

  • Inversion
  • Maximum power point tracking
  • Grid disconnection
  • Integration and packaging

Inversion. The method by which dc power from the PV array is converted to ac power is known as inversion. Other than for use in small off-grid systems and small solar gadgets, using straight dc power from a PV array, module or cell is not very practical. Although many things in our homes and businesses use dc power, large loads and our electrical power infrastructure are based on ac power. This dates back to the early days of Edison versus Tesla when ac won out over dc as a means of electrical power distribution.

An important reason that ac won out is because it can be stepped up and travel long distances with low losses and with minimal material. This could change in the distant future if more of our energy is produced, stored and consumed by means of dc power. Today, the technology exists to boost dc electricity to high voltages for long distance transfer, but it is very complex and costly. For the foreseeable future, ac will carry electricity between our power plants, cities, homes and businesses.

In an inverter, dc power from the PV array is inverted to ac power via a set of solid state switches—MOSFETs or IGBTs—that essentially flip the dc power back and forth, creating ac power. Diagram 1 shows basic H-bridge operation in a single-phase inverter.

Maximum power point tracking. The method an inverter uses to remain on the ever-moving maximum power point (MPP) of a PV array is called maximum power point tracking (MPPT). PV modules have a characteristic I-V curve that includes a short-circuit current value (Isc) at 0 Vdc, an open-circuit voltage (Voc) value at 0 A and a "knee" at the point the MPP is found - the location on the I-V curve where the voltage multiplied by the current yields the highest value, the maximum power. Diagram 2 shows the MPP for a module at full sun in a variety of temperature conditions. As cell temperature increases, voltage decreases. Module performance is also irradiance dependent. When the sun is brighter, module current is higher; and when there is less light, module current is lower. Since sunlight intensity and cell temperature vary substantially throughout the day and the year, array MPP current and voltage also move significantly, greatly affecting inverter and system design.

The terms full sun or one sun are ways to describe the irradiance conditions at STC (1000 W/m2). Sunlight intensity varies from nothing to full sun or a little more than one sun in some locations and conditions. This means that PV output current can vary from zero to full array rating or more. Inverters need to work with arrays at their lowest voltages, which occur under load on the hottest days, as well as at their highest voltages, which occur at unloaded open circuit array conditions on the coldest days. In some climates, temperatures can vary by 100°F or more, and PV cell temperatures can vary by 150°F. This means array voltage can vary by ratios of nearly 2:1. A string of 22 Evergreen ES-A-210 modules, for example, will reach a Voc of 597 Vdc with a cell temperature of -30°C (-22°F). The MPP voltage (Vmp) can get as low as 315 Vdc in an ambient temperature of 50°C (122°F). In most cases, the maximum power point voltage operates over a 25% variation. However, this number is lower in regions with more consistent year-round temperatures, such as San Diego, California, and is higher in regions where temperature varies more, such as the Midwest and Northeast. Finding the array’s MPP and remaining on it, even as it moves around, is one of the most important grid-direct solar inverter functions.

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