With the rapid expansion of renewable energy integration, a massive number of power electronic converters are employed to interconnect renewable sources with the power grid. However, as the proportion of renewable generation rises, the share of traditional synchronous generators declines, leading to reduced system inertia and degraded frequency stability. Grid-forming control technology offers an effective solution by enabling power converters to emulate the inertia characteristics of synchronous machines, thereby enhancing dynamic stability. In photovoltaic applications, this inertia emulation is achieved through DC voltage control that leverages the energy stored in the DC-link capacitor of the solar inverter. Conventional grid simulators used for testing typically focus only on AC-side grid characteristics, ignoring the influence of the DC-side power source characteristics. This limitation makes them inadequate for validating the AC-DC coordinated control strategies of grid-forming solar inverters. In this work, we propose an innovative AC-DC dual-port operation condition emulation test platform. By synchronously emulating the AC-side grid voltage, frequency, and impedance characteristics alongside the DC-side photovoltaic generation dynamics, we construct a comprehensive emulation method tailored for grid-connected testing of grid-forming solar inverters.
Introduction
The increasing penetration of renewable energy sources such as solar and wind has led to a paradigm shift in power system operation. Power electronic converters now dominate the interface between generation and the grid. However, the replacement of synchronous machines with inertia-less converters reduces the system’s ability to withstand frequency disturbances. Grid-forming (GFM) control, which mimics the behavior of synchronous generators, has emerged as a promising solution. Among various GFM strategies, the matching control method establishes a direct relationship between the DC-link voltage of a solar inverter and the rotor speed of a synchronous machine. This approach utilizes the energy stored in the DC capacitor to emulate rotor inertia, eliminating the need for power reserve that would otherwise reduce the efficiency of photovoltaic systems. Unlike virtual synchronous generator (VSG) control, which requires active power reserve to provide frequency support, matching control allows the solar inverter to operate at the maximum power point while still contributing to grid inertia. This feature is particularly attractive for photovoltaic systems where the primary energy source is intermittently variable.
For comprehensive performance evaluation of such grid-forming solar inverters, test platforms must replicate both the AC grid environment and the DC-side power source behavior. Traditional grid simulators are limited to imposing voltage sags, frequency deviations, and impedance variations on the AC side, but they neglect the dynamics of the photovoltaic source. In contrast, our proposed platform integrates an AC grid emulator and a DC power source emulator within a single power loop. This design enables simultaneous emulation of grid voltage, impedance, and photovoltaic generation characteristics, providing a realistic test environment for grid-forming solar inverters employing matching control.
The following figure illustrates the conceptual layout of the proposed test platform, which consists of an AC grid emulator and a DC source emulator sharing a common external DC supply.
Grid-Forming Control of Solar Inverters
The matching control method is specifically designed for photovoltaic systems where the input power is constrained by environmental conditions. In a typical grid-forming solar inverter, the DC side is connected to a photovoltaic array through a DC/DC converter, while the AC side interfaces with the grid via a DC/AC inverter. The inverter’s control system regulates both the DC voltage and the AC output power. The matching control law establishes a relationship between the DC voltage \(u_{dc}\) and the angular frequency \(\omega\) of the AC output. This relationship is expressed as:
$$ \omega = \omega_0 – \frac{1}{D_p}(u_{dc}^2 – u_{dcref}^2) $$
where \(\omega_0\) is the nominal angular frequency, \(D_p\) is the active power damping coefficient, and \(u_{dcref}\) is the reference DC voltage. The corresponding reactive power control follows a conventional voltage-reactive power droop characteristic:
$$ U = U_0 – D_q (q – Q_0) $$
Here, \(U_0\) is the nominal voltage amplitude, \(D_q\) is the reactive power-voltage coefficient, \(q\) is the instantaneous reactive power, and \(Q_0\) is the reactive power reference. By adopting this control scheme, the solar inverter can autonomously respond to grid frequency changes by adjusting its active power output through variations in the DC-link voltage. The DC capacitor provides the energy required for inertia support, and the photovoltaic source operates at its maximum power point without the need for reserve margin.
Table 1 summarizes the key parameters of the grid-forming solar inverter used in our simulation study.
| Parameter | Symbol | Value | Unit | Remarks |
|---|---|---|---|---|
| DC-link capacitance | \(C_{dc}\) | 2200 | \(\mu\)F | Based on voltage ripple suppression requirement, Eq. (7) |
| Line inductance | \(L_{line}\) | 10 | mH | Typical low-voltage distribution grid (L/R ≈ 10) |
| Line resistance | \(R_{line}\) | 1 | Ω | Typical distribution grid resistance (L/R ≈ 10) |
| Inertia coefficient | \(J\) | 0.1 | kg·m² | Designed for time constant ≈ 0.5 s |
| Active damping coefficient | \(D_p\) | 5 | N·m·s/rad | Stability criterion from reference |
| LPF time constant | \(T\) | 0.01 | s | Suppress noise while ensuring phase delay < 5° at 50 Hz |
| Rated DC voltage | \(u_{dc}\) | 750 | V | — |
| DC voltage reference | \(u_{dcref}\) | 750 | V | — |
| Rated active power | \(P_n\) | 20 | kW | — |
| AC grid voltage amplitude | \(U_0\) | 311 | V | — |
| AC grid frequency | \(f_0\) | 50 | Hz | — |
The DC capacitance selection follows the energy conservation principle: the energy variation \(\Delta E\) equals 10% of the rated active power, and the voltage variation \(\Delta u_{dc}\) is limited to 5% of the rated value. The required capacitance is calculated as:
$$ C_{dc} = \frac{\Delta E}{0.5 \Delta u_{dc}^2} $$
Substituting \(\Delta E = 0.1 \times P_n \times 1 s = 2000\) J and \(\Delta u_{dc} = 0.05 \times 750 = 37.5\) V gives approximately 2200 μF.
Operation Condition Emulation Platform
System Architecture
Our proposed emulation platform comprises two main components: an AC grid emulator and a DC source emulator. The AC grid emulator is implemented using a DC/AC converter, while the DC source emulator uses a dual-active-bridge (DAB) DC/DC converter. Both converters share a common external DC power supply, forming a power loop with the device under test (DUT), which is the grid-forming solar inverter. In this configuration, the external DC supply only needs to provide the total losses of the three converters, significantly reducing its required power rating. The DAB converter isolates the zero-sequence path through its high-frequency transformer, effectively eliminating zero-sequence currents that could otherwise circulate in the platform.
Table 2 outlines the main components and their roles in the emulation platform.
| Component | Topology | Function | Key Feature |
|---|---|---|---|
| AC grid emulator | Two-level DC/AC | Emulate grid voltage, frequency, and impedance | Virtual impedance + voltage control |
| DC source emulator | Dual active bridge (DAB) | Emulate photovoltaic power source | Power control, zero-sequence isolation |
| External DC supply | — | Provide system losses | Low power rating needed |
| Device under test | Three-phase inverter | Grid-forming solar inverter with matching control | DC voltage and AC output regulation |
DC-Side Emulation Method
The photovoltaic source is modeled as a constant power source in the context of maximum power point tracking. The MPPT controller operates on a timescale of seconds to minutes, so for dynamic testing, the PV output can be considered as a constant power source during short transients. The DAB converter is controlled in power regulation mode. Figure 5 in the original work illustrates the control block: the output power is calculated from the measured voltage and current, and a PI controller adjusts the phase shift angle \(d\) to regulate the power. The control equation is:
$$ d = K_p (P_{ref} – P_{meas}) + K_i \int (P_{ref} – P_{meas}) dt $$
where \(P_{ref}\) is the desired PV power (e.g., 20 kW), and \(K_p\), \(K_i\) are PI gains. The DAB converter’s ability to handle bidirectional power flow is not required in normal operation because the solar inverter only absorbs power from the PV source, but it allows flexibility for testing scenarios involving reactive power or reverse power flow.
AC-Side Emulation Method
The AC grid is emulated as an ideal voltage source in series with a line impedance. The impedance consists of a resistor \(R_{line}\) and an inductor \(L_{line}\). The voltage at the point of common coupling (PCC) is given by:
$$ u_{gref} = u_s – i_g Z_{line}(s) = u_s – i_g (s L_{line} + R_{line}) $$
Direct implementation of the derivative term \(s\) is problematic in digital controllers due to noise amplification. Therefore, we employ a virtual impedance filter that includes a low-pass filter:
$$ u_{gref} = u_s – i_g G_{VI}(s) = u_s – i_g \frac{s L_{line} + R_{line}}{1 + T s} $$
The low-pass filter time constant \(T = 0.01\) s is chosen to attenuate high-frequency noise while maintaining acceptable phase lag at the fundamental frequency (less than 5° at 50 Hz). Both the voltage controller and the current controller in the AC emulator use proportional-resonant (PR) controllers to achieve zero steady-state error at the fundamental frequency. The PR controller transfer function is:
$$ G_{PR}(s) = K_p + \frac{K_r s}{s^2 + \omega_0^2} $$
where \(K_p\) and \(K_r\) are the proportional and resonant gains, respectively, and \(\omega_0 = 2\pi \times 50\) rad/s is the resonant frequency.
Simulation Validation
We built a simulation model in PLECS software to validate the proposed emulation method. The simulation parameters are listed in Table 1. Two test scenarios were designed: AC-side frequency variation and DC-side power step change.
Test Scenario 1: AC Grid Frequency Ramp
The grid frequency was increased from 50 Hz to 51 Hz at a rate of 0.4 Hz/s, simulating a severe frequency disturbance. The response of the grid-forming solar inverter under test was recorded. Key quantitative results are summarized in Table 3.
| Parameter | Value | Unit | Note |
|---|---|---|---|
| DC voltage rise | 750 → 810 | V | 8% increase |
| Active power reduction | 20.0 → 17.5 | kW | −12.5% |
| Transient frequency response time | 245 | ms | Time to reach new steady-state |
| Regulation time | 680 | ms | Settling time within ±2% |
| Active power regulation deviation | ±1 | % | Steady-state error |
The simulation confirmed that the solar inverter successfully absorbed the frequency rise by increasing its DC voltage and reducing its output power, thereby providing inertia support. The response metrics align with the requirements of the T/CEEIA 854-2024 standard for bulk-power photovoltaic plants.
Test Scenario 2: DC Power Step Change
The DC input power was stepped down from 20 kW to 15 kW at t = 1 s, emulating a sudden cloud shadow. The dynamic response of the solar inverter is quantified in Table 4.
| Parameter | Value | Unit | Note |
|---|---|---|---|
| DC voltage recovery time | 0.08 | s | To within ±2% of 750 V |
| Active power tracking time | <0.1 | s | Output power matches new input |
| Maximum voltage deviation | ±2 | % | During transient |
The solar inverter demonstrated fast and stable tracking of the reduced input power, maintaining DC voltage regulation within acceptable bounds. The dynamic response time of 0.08 s is well within the typical 0.5 s requirement for grid-forming converters.
Zero-Sequence Current Suppression by DAB
We compared the zero-sequence current in a conventional Buck-Boost topology versus the DAB topology under identical frequency disturbance. The conventional topology exhibited a peak zero-sequence current of 3 A, while the DAB topology reduced it to below 0.1 A, confirming the effectiveness of the transformer isolation. This is critical to prevent unintended circulating currents that could corrupt test results or damage equipment.
Discussion
The proposed AC-DC dual-port emulation platform offers several advantages over conventional test setups. First, it simultaneously emulates both the AC grid and the DC photovoltaic source, enabling realistic evaluation of the AC-DC coordinated control strategies characteristic of grid-forming solar inverters. Second, the shared DC bus architecture minimizes the power rating of the external supply, reducing cost and energy consumption. Third, the use of a DAB converter for the DC source emulator provides inherent zero-sequence isolation, improving system reliability and measurement accuracy.
Table 5 compares the proposed emulation method with traditional approaches.
| Feature | Proposed AC-DC Dual-Port Emulator | Conventional AC Grid Simulator | Real Hardware Testbed |
|---|---|---|---|
| AC grid emulation | Yes (voltage, frequency, impedance) | Yes | Limited to available grid |
| DC source emulation | Yes (photovoltaic dynamics) | No | Requires actual PV panels |
| Flexibility | High (parameters configurable in software) | Low (requires hardware changes) | Low (fixed conditions) |
| Power loss | Low (shared DC bus) | Moderate | High (separate supplies) |
| Zero-sequence isolation | Inherent (DAB transformer) | Not provided | Depends on grid connection |
The flexibility of software-configurable parameters allows emulation of various grid strengths (e.g., short-circuit ratio SCR = 1.2, 3.0, 5.0) and PV power ramp rates (e.g., 0.5 kW/s, 1.0 kW/s) without hardware modifications. This adaptability is essential for certification testing and research on grid-forming solar inverters.
Conclusion
In this work, we have presented a comprehensive operation condition emulation method tailored for grid-connected testing of grid-forming solar inverters. The proposed platform integrates an AC grid emulator and a DC photovoltaic source emulator in a shared power loop, enabling the simultaneous reproduction of grid characteristics and PV generation dynamics. The matching control strategy of the solar inverter is fully supported by this platform, as it requires accurate emulation of both the AC and DC sides. Simulation results validated the effectiveness of the emulation method under frequency ramps and power steps, demonstrating that the solar inverter responds appropriately by adjusting its DC voltage and active power output. The DAB-based DC emulator effectively suppresses zero-sequence currents, enhancing test fidelity. Future work will involve constructing a hardware prototype and conducting hardware-in-the-loop experiments to further verify the method under a wider range of grid conditions, including low short-circuit ratios and harmonic distortions. The proposed emulation platform provides a robust and flexible tool for advancing the development and certification of grid-forming photovoltaic generation systems.