An inverter converts direct current (DC), for example from PV modules or a battery energy storage system (BESS), into alternating current (AC). It generates voltage, frequency and phase angle that match the connected utility grid or local AC system. Bidirectional BESS inverters can also draw energy from the AC system and convert it into DC for storage in the battery.

Inverters at a Glance

• PV inverters convert the direct current generated by PV modules into grid-compliant alternating current and regulate the operating point of the PV array using Maximum Power Point Tracking (MPPT).
• BESS inverters generally operate bidirectionally, enabling both charging and discharging of the battery.
• Grid-following inverters normally require an existing grid signal. For island or backup power operation, at least one suitable component must establish voltage and frequency.
• Inverters can provide reactive power in addition to active power. The possible combination of both quantities is limited by the inverter’s permissible apparent power.

How Does an Inverter Work?

An inverter uses power electronic switching components to generate an alternating voltage from a DC voltage. Control and filter stages ensure that the output parameters comply with the technical requirements of the connected utility grid or island grid. In grid-connected systems, the inverter synchronizes with the existing grid voltage and feeds in alternating current with the appropriate frequency and phase angle.

PV inverters also regulate the electrical operating point of the PV array. MPPT adjusts this operating point so that the connected PV modules provide as much power as possible under the current irradiance and temperature conditions. Voltage and current are determined by the current-voltage characteristic of the PV array. At the same time, the inverter monitors permissible voltage, current, temperature and insulation limits, among other parameters.

In a BESS, the inverter controls the flow of energy between the battery and the AC system. During charging, it converts alternating current into direct current; during discharging, it converts direct current into alternating current. In larger battery storage systems, the term Power Conversion System (PCS) is also frequently used for bidirectional power electronics. Depending on the system architecture, however, a PCS may include additional electrical components and functions.

State of charge, battery temperature and current, voltage and power limits are generally taken into account through coordination between the inverter, the battery management system and the higher-level controller.

Grid-following inverters rely on an existing voltage and frequency. If this grid signal is lost, they normally stop operating. Island and backup power systems therefore require at least one grid-forming component. This component establishes the local AC system and defines its voltage and frequency, allowing suitable inverters and loads to operate within that system.

What Types of Inverters Are Available?

String inverters connect one or more PV strings and are commonly used in PV systems with a decentralized architecture. Multiple independent MPPT trackers can regulate groups of modules with different orientations or operating conditions separately.

Central inverters combine the output of many PV strings in a single device. They are primarily used in larger PV installations where high power levels are converted centrally.

Hybrid inverters can connect a PV system and a BESS within a shared system. Depending on the device architecture, the battery may be connected on the DC side. However, a hybrid inverter is not automatically suitable for island or backup power operation. The required grid-forming functions must be explicitly supported.

Module inverters, also known as microinverters, are assigned to individual PV modules or small groups of modules. They enable decentralized power conversion and individual operating-point control but increase the number of electronic components within the PV system.

Typical Applications of Inverters

Inverters perform essential functions in different energy systems:

• In PV systems, they convert the generated DC power into usable or grid-exportable AC power.
• In a BESS, bidirectional inverters control the charging and discharging of the battery.
• In hybrid systems, suitable devices coordinate power conversion between PV, BESS, loads and the utility grid.
• In grid-connected systems, inverters can provide active and reactive power in accordance with local requirements or external setpoints.

Important Parameters and Technical Limits

Relevant monitoring values may include DC and AC power, voltage, current, grid frequency, energy yield, temperature, insulation values, operating status, warnings and fault messages, depending on the device. These data can help identify yield losses, shutdowns, excessive temperatures, communication failures and other deviations.

The rating in kilowatts (kW) describes the active power of an inverter. Kilovolt-amperes (kVA), by contrast, indicate its apparent power limit. Active power and reactive power jointly determine the required apparent power. In simplified form:

S = √(P² + Q²)

Here, S represents apparent power, P active power and Q reactive power. If an inverter must provide more reactive power, the available active power may be restricted by its apparent power limit. Temperature, DC voltage, grid conditions and device-specific operating limits can also reduce the power that is actually available.

Monitoring or external control requires suitable communication interfaces, a supported device protocol and a reliable data connection. The available measurements, setpoints and fault codes differ between manufacturers and device types. Not every inverter provides all relevant values or supports external setpoint control.

Monitoring and Controlling Inverters with EcoPhi

EcoPhi can integrate inverters from different manufacturers into a common monitoring environment through available interfaces. Depending on the product, device integration and project scope, measurements and operating states can be recorded, visualized, evaluated historically and used for alarms or reports.

If the inverter provides a compatible and approved control interface, EcoPhi can also transmit active and reactive power setpoints and integrate the device into higher-level operating strategies for PV, BESS or hybrid systems. The specific functionality depends on the inverter, communication protocol and available control options. EcoPhi does not replace the inverter’s internal protection functions or any certified plant or grid controller required under the applicable grid code.

Inverters Summarized

An inverter connects DC-based energy sources and storage systems to an AC system. Its functions range from power conversion and MPPT to the provision of active and reactive power. Monitoring and external control require suitable device interfaces and clearly defined operating limits.

Frequently Asked Questions About Inverters

What Is the Difference Between a PV Inverter and a BESS Inverter?

A PV inverter primarily converts the direct current from PV modules into alternating current. A BESS inverter usually operates bidirectionally so that the battery can be both charged and discharged.

Can Every Inverter Continue Operating During a Power Outage?

No. To operate without the utility grid, the local system must include a suitable grid-forming component.

What Do kW and kVA Mean for an Inverter?

Kilowatts describe active power, while kilovolt-amperes indicate apparent power. The apparent power limit determines which combination of active and reactive power the inverter can provide.

Can an EMS Control Every Inverter?

No. Active control through an EMS requires a compatible and approved interface as well as supported setpoint functions. Some devices only allow measurements and status information to be read.