An inverter is one of the most important devices in a solar system. It is a device that converts the direct current (DC) generated by the solar panels into alternating current (AC) used by the grid. In direct current, the current flows in one direction at a constant voltage. In alternating current, the current flows in both directions in a circuit as the voltage changes from positive to negative. An inverter is just one type of power electronic device that regulates the flow of power.
Fundamentally, an inverter converts DC to AC by switching the direction of the DC input back and forth very quickly. Thus, the DC input becomes an AC output. filters and other electronic devices can be used to produce a voltage that varies in the form of a clean, repetitive sine wave that can be injected into the grid. A sine wave is the shape or pattern of voltage changes over time, and it is a pattern of power that the grid can use without damaging electrical devices that are designed to operate at a specific frequency and voltage.
The first inverters were created in the 19th century and were mechanical. rotating motor could be used to continuously change the forward or reverse connection of a DC power source. Today, we make electrical switches with transistors, which are solid-state devices with no moving parts. Transistors are made of semiconductor materials such as silicon or gallium arsenide. They control the flow of electricity based on external electrical signals.
If you have a home solar system, your inverter probably has several functions. In addition to converting solar energy into AC power, it can monitor the system and provide a portal for communication with computer networks. Solar-plus-battery storage systems rely on advanced inverters to operate without grid support during power outages (if they are designed to do so).
Toward an Inverter-Based Grid
Historically, electricity has been produced primarily by burning fuel to create steam, which then spins turbine generators. The motion of these generators creates AC power as the devices spin, which also determines the frequency, or the number of times the sine wave repeats. The frequency of the power supply is an important indicator of the health of the grid. For example, if the load is too large (too many devices consuming energy), then energy is being consumed from the grid faster than it can be supplied. As a result, the turbines will slow down and the frequency of the AC power will decrease. Because turbines are large rotating objects, they resist changes in frequency just as all objects resist changes in motion, a property called inertia.
As more solar systems are added to the grid, more inverters are connected to the grid than ever before. Inverter generation can produce energy at any frequency and, because no turbine is involved, does not have the same inertia characteristics as steam generation. Therefore, the transition to a grid with more inverters requires building smarter inverters that can respond to frequency changes and other disruptions that occur during grid operation and help stabilize the grid from these disruptions.
Grid Services and Inverters
Grid operators manage the supply and demand of electricity on the power system by providing a range of grid services. Grid services are activities that grid operators perform to maintain system-wide balance and better manage the transmission of electricity.
When the grid is no longer operating as expected, such as when voltage or frequency deviates, smart inverters can respond in a variety of ways. Generally speaking, the standard for small inverters, such as those connected to a home solar system, is to stay on or "ride out" interruptions when there are small interruptions in voltage or frequency, and they will automatically disconnect from the grid and shut down if the interruption lasts for a long time or is larger than normal. Frequency response is particularly important because frequency drops are associated with unexpected offline generation. In response to frequency changes, inverters are configured to change their power output to restore standard frequency. Inverter-based resources may also respond to operator signals to change their power output as other supply and demand on the power system fluctuate, a grid service called automatic generation control. To provide grid services, the inverter needs to have a source of power it can control. This can be generation, such as a solar panel currently generating power, or storage, such as a battery system that can be used to provide previously stored power.
Another grid service that some advanced inverters can provide is grid forming. Grid-forming inverters can start the grid when a grid fault occurs, a process called a black start. Traditional "grid-following" inverters require an external signal from the grid to determine when to switch in order to produce a sine wave that can be injected into the grid. In these systems, the power from the grid provides the signal that the inverter tries to match. More advanced grid-forming inverters can generate the signal themselves. For example, a small solar panel network might designate one of its inverters to operate in grid-forming mode, and the rest of the inverters follow its lead like dancing partners, forming a stable grid without any turbine-based generation.
Reactive power is one of the most important grid services that inverters can provide. In the electrical grid, voltage (the force that pushes electric charges) is always switching back and forth, as is current (the movement of electric charges). When voltage and current are in sync, electrical energy is maximized. However, sometimes there can be a delay between the two alternating modes for voltage and current, such as when a motor is running. If they are out of sync, part of the power flowing through the circuit cannot be absorbed by the connected equipment, resulting in efficiency losses. More total power is required to produce the same amount of "real" power (the power that the load can absorb). To counteract this, utilities provide reactive power to bring voltage and current back into sync, making electricity easier to consume. This reactive power itself is not used, but rather enables other power to become useful. Modern inverters can both provide and absorb reactive power to help the grid balance this important resource. In addition, because reactive power is difficult to transmit over long distances, distributed energy resources such as rooftop solar are particularly useful sources of reactive power.
