Is the Electrical Grid Keeping Up with Smart Solar Inverters?
Within the fast-growing solar-energy market, solar-inverter suppliers must keep pace with electronic technology advances in order to deliver more efficient and reliable parts at a lower cost. Inverter efficiency indicates the percentage of the available solar power that’s actually converted by the inverter and fed into the utility grid; some smart inverters reach a total efficiency of 98%. To achieve high efficiency, it’s important to design the inverters using the most reliable components from power semiconductors (MOSFETs and/or IGBTs), capacitors (electrolytic capacitors, high-capacity film capacitors), transformers, cooling systems, etc.
An inverter is a highly demanding power electronic device. Recognizing the smart inverter’s vital role in the solar renewable-energy market, ABB announced the creation of a multimillion-dollar facility with a utility-scale solar-inverter testing laboratory earlier this year.
The laboratory features a unique, large climate chamber capable of full power electrical testing in simulated conditions ranging from the arctic tundra to an equatorial rainforest. The chamber also allows for accelerated product testing, which is important when the inverters are typically expected to operate for more than 20 years. The laboratory that opened in Helsinki supports the following: testing and verification of inverters for safe operation; compatibility to the most demanding renewables-specific-grid code requirements; and the measurement and testing of harmonics and grid interactions.
Europe has been working on grid modernization for quite a while and the United States is taking the first steps toward the implementation of a smart grid. Early this year, U.S. Secretary Ernest Moniz announced that the Energy Department’s (DoE) Grid Modernization Initiative would improve the resiliency, reliability, and security of the nation’s electrical power grid.
For example, the DoE announced $18 million in funding for six new projects across the United States. It said that the six new integrated photovoltaic (PV) and energy storage projects will utilize Internet-capable inverters, working with smart buildings, smart appliances, and utility communication and control systems. The results developed under this effort will enable the sustainable and holistic integration of hundreds of gigawatts of additional solar energy onto the electric grid throughout the U.S.
Internet-capable inverters or smart inverters have a digital architecture with bidirectional communications capability and robust software infrastructure to provide the following functionalities to improve the grid:
• Remote ON/OFF: Smart inverters can connect/disconnect from the grid.
• Power factor control: Smart inverters allow rising or sinking the reactive power by setting the ratio of real power to apparent power.
• Reactive power control: Smart inverters set the level of reactive power generation or consumption.
• Volt/reactive power management: Smart inverters provide voltage regulation by modulating reactive power output.
• Volt/real power management: Smart inverters provide voltage regulation by modulating real power output.
• Frequency/watt management: Smart inverters allow distributed energy resources (DER) systems to help with frequency regulation by changing its real power output.
• Low/high voltage and frequency ride-through: Smart inverters can remain connected to the grid and adjust their output to operate at high/low frequencies and under zero-, low-, and high-voltage ride-through conditions.
• Power curtailment: Smart inverters specify an upper limit for inverter active-power output to reduce the array’s power output.
Solar inverters, also known as grid-tie inverters, may be classified as string inverters, central inverters, and micro-inverters (Fig. 1):
• A string inverter is the type most commonly used in applications up to 100 kW, such as home and commercial solar PV systems where a maximum-power-point-tracking (MPPT) system captures the maximum energy from the PV panel.
• Central inverters are designed for applications above 100 kW, such as large arrays installed on buildings, industrial facilities, and field installations.
• A micro-inverter is a string inverter with a MPPT module placed to capture every panel, optimizing each solar panel instead of the entire solar PV system—much like central inverters but at a lower power (typically 300 W).