Grid-Forming Inverter

Overview

A grid-forming inverter converts DC power from a battery or solar array into three-phase AC and, unlike a conventional grid-following inverter, it actively establishes the voltage magnitude and frequency of the AC system rather than measuring them from an existing grid. Utility-scale units rated from 1 to 4 MW operate from a 900–1500 V DC bus and deliver 400–690 V AC, which a step-up transformer then raises to medium or high voltage. The inverter behaves like a controlled voltage source behind an impedance, so it can energize a network with no other generation present, ride through faults, and share load with synchronous machines. This capability matters as conventional power stations retire and inverter-based resources supply a larger share of generation, because the system still needs a source that defines frequency and supplies inertia-like response.

The Control and DSP Board is the element that distinguishes a grid-forming machine. It runs a virtual synchronous machine or droop algorithm that computes the voltage waveform the inverter should produce, then commands the Power Module to synthesize it. The remaining subsystems handle power conversion, filtering, protection, and heat removal in the same way as any large inverter.

Grid-forming versus grid-following control

A grid-following inverter uses a phase-locked loop to track the existing grid voltage and injects a commanded current in phase with it. It cannot operate if the grid is absent or weak. A grid-forming inverter instead generates an internal voltage phasor with its own frequency and angle, and the power it exchanges follows from the angle and magnitude difference between that internal voltage and the grid. Active power is governed by a frequency-versus-power droop, typically 2–5 percent, and reactive power by a voltage-versus-reactive droop. When grid frequency falls, the controller increases power output within the limits of the DC source; when frequency rises, it backs off. This mirrors how a synchronous generator responds and lets several grid-forming units share load without fast communication between them. The Sensing and Protection subsystem feeds isolated current and voltage measurements into the control loop at high sample rates so the algorithm can regulate the output and detect faults within a fraction of a cycle.

Power stage

The conversion core is a three-phase two-level or three-level bridge. High-performance units use silicon carbide MOSFET modules in the Power Module, switching at 4–20 kHz, which raises efficiency to roughly 98.5–99.0 percent and shrinks the magnetics; IGBT designs switch at 2–4 kHz and cost less. Each phase leg uses isolated gate drivers with desaturation protection, and film DC link capacitors stabilize the bus and absorb ripple current. Power enters through the DC Input Stage, which carries the 1500 V bus on a laminated busbar, protects it with high-speed fuses and surge devices, and limits capacitor inrush with a pre-charge resistor that a relay bypasses once the bus is charged. An insulation monitor checks resistance to ground before the unit connects.

Output filter and magnetics

Switching produces voltage steps that must be filtered before the current reaches the grid. The LCL Output Filter is an LCL network: an inverter-side inductor, a shunt capacitor, and a grid-side inductor per phase. This third-order filter attenuates switching harmonics far more steeply than a single inductor, which keeps current total harmonic distortion below 3 percent at rated load while using smaller, lighter inductors. The trade is a resonant peak, so passive damping resistors are placed in series with the filter capacitors, or active damping is implemented in firmware. The inductor cores are sized to avoid saturation at the peak fault current the inverter may push during a grid disturbance.

Cooling and protection

Power module losses of tens of kilowatts at full load are removed by the Cooling System, a water-glycol loop that pumps coolant through cold plates clamped to the modules and rejects heat through a liquid-to-air heat exchanger. Coolant enters the plates at 40–55 °C and a flow switch interlocks the inverter so it cannot run dry. On the grid side, the AC Output Stage holds the synchronizing contactor, a molded case breaker for fault clearing, and surge arrestors on each phase. The Cabinet Enclosure is an outdoor-rated steel cabinet with door interlocks, ventilation, main busbars, and an HMI touchscreen, and the Auxiliary Power Supplies supplies feed the gate drivers and controls, with a backup source that holds the controller alive through grid voltage dips.

Control and use

In a battery storage plant, the grid-forming inverter can black-start a network, supply fast frequency response, and form a stable island during an outage, then resynchronize when the wider grid returns. Anti-islanding and over and under frequency protection, handled in the Sensing and Protection relays, govern when the unit disconnects from a utility tie. Commissioning follows IEEE 1547 and IEEE 2800 for interconnection, with droop coefficients, fault current contribution, and ride-through curves configured to the plant operator and the local grid code.