How do solar PV Inverters work?

What is the basic scientific principle based on which solar PV inverters work?

What are the main components of the solar PV inverters and what are their main roles and the principles on which each of these components work?

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 inthe 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.

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.

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.

Grid disconnection - As required by UL 1741 and IEEE 1547, all grid-tied inverters must disconnect from the grid if the ac line voltage or frequency goes above or below limits prescribed in the standard. The inverter must also shut down if it detects an island, meaning that the grid is no longer present. In either case, the inverter may not interconnect and export power until the inverter records the proper utility voltage and frequency for a period of 5 minutes. These protections eliminate the chance that a PV system will inject voltage or current into disconnected utility wires or switchgear and cause a hazard to utility personnel. If an inverter remained on or came back on before the utility was reliably reconnected, the PV system could backfeed a utility transformer. This could create utility pole or medium voltage potentials, which could be many thousands of volts. A significant battery of tests is performed on every grid-tied inverter to make certain that this situation can never occur.

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