Basics of Solar Integration: Inverters and Grid Services

HOW DO INVERTERS WORK?

One of a solar energy system’s most crucial components is an inverter. It is a device that changes the electricity produced by solar panels from direct current (DC) to alternating current (AC), which is used by the electrical grid. Electricity is kept at a constant voltage in one direction in DC. Electricity in an AC circuit moves in both directions as the voltage swings from positive to negative. An example of a power electronics equipment, which controls the flow of electrical power, is an inverter.

A DC input is quickly switched from one direction to the other by an inverter to convert DC to AC. A DC input consequently becomes an AC output. Additionally, a voltage that fluctuates as a clear, repeating sine wave can be generated and injected into the electrical grid using filters and other electronics. The sine wave is a shape or pattern that the voltage takes over time, and it’s the kind of power pattern that the grid can utilize without harming electrical equipment, which is designed to function at specific frequencies and voltages.

Inverters were originally made mechanically in the 19th century. To alternate between connecting the DC source forward and forward, for instance, a spinning motor would be utilized. Transistors, which are solid-state, non-moving electronics, are used nowadays to create electrical switches. Silicon or gallium arsenide are examples of semiconductor materials used to make transistors. In reaction to external electrical impulses, they regulate the flow of electricity.

a 500 kW Westinghouse “rotary converter,” a pioneering inverter.
a 500 kW Westinghouse “rotary converter,” a pioneering inverter. Wikimedia provided the image.
If your home has a solar system, your inverter probably has a variety of uses. In addition to converting solar energy into AC electricity, the device can monitor itself and serve as a hub for computer networks’ communications. If solar-plus-battery storage systems are built to do so, they rely on cutting-edge inverters to function without assistance from the grid in the event of outages.

TOWARD A GRID BASED ON INVERTERS

In the past, the main method of producing electricity involved burning fuel to produce steam, which spun a turbine generator and turned it into electricity. These generators rotate, producing AC power as they do so. This rotation also determines the frequency, or how frequently the sine wave repeats. Power frequency is a crucial metric for keeping track on the condition of the electrical system. For instance, energy is withdrawn from the grid quicker than it can be supplied if there is a high load—too many devices using energy. The outcome will be a decrease in AC frequency and a slowing of the turbines. Due to their size and mass, the turbines’ inertia causes them to resist frequency changes in the same way that other things do when their motion is altered.

Additional inverters than ever before are being connected to the grid as a result of the addition of more solar systems. Because there is no turbine involved, inverter-based generating can produce energy at any frequency and does not share the same inertial characteristics as steam-based power. Building smarter inverters that can react to frequency shifts and other grid disruptions and assist stabilize the system against them is therefore necessary to make the switch to an electrical grid with more inverters.

SERVICES AND INVERTERS FOR GRID

By offering a variety of grid services, grid operators control the supply and demand of electricity on the electric grid. Grid services are tasks that grid operators carry out to keep the system in balance and improve the management of power transmission.

Smart inverters can react in a number of different ways when the grid ceases to function as planned, such as when there are variations in voltage or frequency. Small inverters, like those connected to a home solar system, are often designed to “ride through” minor voltage or frequency fluctuations. However, if the disturbance persists for an extended period of time or is more severe than usual, the inverter will disconnect from the grid and stop down. Because a drop in frequency is linked to generation being abruptly knocked offline, frequency responsiveness is particularly crucial. Inverters are set up to alter their power output in reaction to a change in frequency in order to return to the normal frequency. Automatic generation control is a grid service that allows inverter-based resources to adjust their power output in response to operator signals as other supply and demand on the electrical system fluctuate. Inverters require sources of electricity that they can manage in order to offer grid services. This might either be generation—like a solar panel that is currently generating electricity—or storage—like a battery system that can be utilized to release energy that has been previously stored.

Grid-forming is a different grid service that some cutting-edge inverters can offer. Grid-forming inverters have the ability to do a “black start” to restart a grid if it goes down. The timing of the switching in conventional “grid-following” inverters must be determined by an external signal from the electrical grid in order to create a sine wave that can be injected into the power grid. In these systems, the inverter attempts to match the signal that is provided by the grid electricity. Grid-forming inverters with more modern technology can produce the signal independently. A network of small solar panels, for instance, may designate one of its inverters to function in grid-formation mode while the others follow along like dancing partners, establishing a stable grid devoid of turbine-based generation.

One of the most crucial grid functions that inverters may offer is reactive power. Voltage, the force that drives electric charge, and current, the movement of electric charge, are constantly switching back and forth on the grid. The synchronization of voltage and current maximizes electrical power. When a motor is running, for example, it’s possible that the voltage and current’s two alternating patterns will occasionally have delays. When connected devices are out of sync, some of the electricity going through the circuit cannot be absorbed, which reduces efficiency. To generate the same amount of “actual” power—the power the loads can withstand—more overall power will be required. Utility companies respond to this by supplying reactive power, which synchronizes the voltage and current and facilitates the use of the electricity. Instead of being used directly, reactive power serves to utilize other forms of power. To assist networks balance this crucial resource, modern inverters can both generate and consume reactive electricity. Additionally, distributed energy resources like rooftop solar are particularly helpful sources of reactive power because it is challenging to transmit reactive power across vast distances.

A RANGE OF INVERTERS

Inverters of many varieties could be put as a component of a solar system. Every solar panel may be connected to a single central inverter in a major utility plant or a medium-sized community solar project. One inverter is connected to a string of solar panels by string inverters. The entire string’s power is converted to AC by the converter. This configuration, while economical, results in less power being produced on the string if any one panel has problems, such as shading. Smaller inverters called microinverters are installed on each panel. A microinverter prevents shade or damage to one panel from affecting the amount of power that may be pulled from the other panels, but they can be more expensive. A system that manages how the solar system communicates with the associated battery storage could help both types of inverters. Solar energy can recharge a battery either directly over DC or after being converted to AC.