The global energy landscape is undergoing a significant transformation, driven by the imperative to mitigate climate change and ensure energy security. Among renewable energy sources, solar photovoltaic (PV) power generation has experienced exponential growth due to technological advancements and cost reductions. As the penetration level of utility-scale and distributed PV systems increases, their interaction with the power grid becomes critically important for overall system stability. Unlike traditional synchronous generators, solar inverters—the power electronic interfaces between PV arrays and the grid—are sensitive to grid disturbances. A key challenge is ensuring these systems remain connected and supportive during grid faults, particularly voltage sags, a capability known as Low Voltage Ride-Through (LVRT). This article delves into an effective LVRT control strategy for a solar inverter, focusing on a seamless transition to reactive current injection using a simplified single-current-loop control scheme, thereby enhancing grid support during faults without requiring additional hardware.
The core of a grid-connected PV system is the solar inverter. Its primary function is to convert the direct current (DC) power produced by PV modules into grid-compliant alternating current (AC) power. Modern three-phase solar inverter topologies typically consist of a two-stage conversion process. The first stage is a DC-DC boost converter, which steps up the variable DC voltage from the PV array to a stable, higher DC-link voltage. This stage also implements Maximum Power Point Tracking (MPPT) to extract the maximum available power from the solar panels under varying irradiance and temperature conditions. The second stage is a DC-AC voltage source inverter (VSI) that synthesizes three-phase AC voltages from the DC-link and feeds power into the grid through filtering inductors. The control system of the solar inverter is paramount, traditionally employing a cascaded dual-loop control structure for the inverter stage: an outer DC-voltage control loop regulates the DC-link voltage, and an inner fast-acting current control loop dictates the AC current injected into the grid.
Mathematical Modeling and Normal Control of the Solar Inverter
To understand the proposed LVRT strategy, a model of the solar inverter system is essential. The power circuit can be represented in the synchronous rotating reference frame (dq-frame), which simplifies control by transforming AC quantities into DC values. The dynamic equations for the inverter connected to the grid through an L-filter are:
$$
\begin{aligned}
L \frac{di_d}{dt} &= v_{d}^{inv} – e_d – R i_d + \omega L i_q \\
L \frac{di_q}{dt} &= v_{q}^{inv} – e_q – R i_q – \omega L i_d \\
C_{dc} \frac{dV_{dc}}{dt} &= i_{pv} – \frac{3}{2} (S_d i_d + S_q i_q)
\end{aligned}
$$
Where \( i_d \) and \( i_q \) are the d- and q-axis components of the grid current (representing active and reactive current, respectively), \( v_{d}^{inv} \) and \( v_{q}^{inv} \) are the inverter output voltages in the dq-frame, \( e_d \) and \( e_q \) are the grid voltages, \( R \) and \( L \) are the filter resistance and inductance, \( \omega \) is the grid angular frequency, \( C_{dc} \) is the DC-link capacitance, \( V_{dc} \) is the DC-link voltage, \( i_{pv} \) is the current from the PV array/boost stage, and \( S_d, S_q \) are modulation indices.
Under normal grid conditions, the standard control for the solar inverter involves two main parts:
1. Boost Converter with MPPT: The boost converter operates with a duty cycle \( D \) determined by an MPPT algorithm (e.g., Perturb and Observe). The input-output voltage relationship is:
$$
V_{dc} = \frac{V_{pv}}{1-D}
$$
where \( V_{pv} \) is the PV array voltage. The controller adjusts \( D \) to move \( V_{pv} \) towards the voltage at which the PV array delivers maximum power \( P_{mppt} \).
2. Inverter with Dual-Loop Control:
- Outer Voltage Loop: A PI controller regulates the DC-link voltage \( V_{dc} \) to its reference \( V_{dc}^* \). Its output is the reference for the active current component \( i_d^* \).
$$
i_d^* = K_{p}^{dc}(V_{dc}^* – V_{dc}) + K_{i}^{dc} \int (V_{dc}^* – V_{dc}) dt
$$ - Inner Current Loop: Fast PI controllers regulate the d- and q-axis currents. To achieve decoupled control, cross-coupling terms and grid voltage feedforward are added:
$$
\begin{aligned}
v_d^{inv*} &= K_{p}^{id}(i_d^* – i_d) + K_{i}^{id} \int (i_d^* – i_d) dt – \omega L i_q + e_d \\
v_q^{inv*} &= K_{p}^{iq}(i_q^* – i_q) + K_{i}^{iq} \int (i_q^* – i_q) dt + \omega L i_d + e_q
\end{aligned}
$$
Typically, \( i_q^* \) is set to zero for unity power factor operation.
This dual-loop control ensures stable power transfer, a constant DC-link voltage, and high-quality current injection from the solar inverter to the grid.
Low Voltage Ride-Through (LVRT) Requirements and Challenges
LVRT capability is a grid code requirement that mandates distributed generation resources, including solar inverter-based plants, to remain connected to the grid during and after a specified voltage dip for a defined period. The objective is to prevent widespread disconnection of generation that could exacerbate the grid disturbance. LVRT profiles define the relationship between the depth/duration of the voltage sag and the required connection time. Furthermore, modern grid codes often require the injection of reactive current to support grid voltage recovery during the fault.
| Standard/Region | Zero-Voltage Hold Time | Reactive Current Injection Requirement |
|---|---|---|
| German Grid Code | 150 ms | 100% of rated current at 50% voltage dip |
| Chinese National Standard (GB/T 19964) | 150 ms | At least 1.05 p.u. current for dips below 20%; proportional injection between 20% and 90% voltage. |
| North American (IEEE 1547-2018) | Varies by category | Mandates dynamic voltage support (e.g., 2% reactive current for 1% voltage deviation). |
The primary challenge for a solar inverter during a voltage sag is power imbalance. The PV array continues to generate power \( P_{pv} \) (determined by MPPT), while the power that can be delivered to the grid \( P_{grid} = 1.5 \cdot (e_d i_d + e_q i_q) \) reduces proportionally with the grid voltage. This excess energy charges the DC-link capacitor, causing a rapid rise in \( V_{dc} \), which can trigger overvoltage protection and lead to the solar inverter tripping offline. The proposed strategy addresses this by fundamentally changing the control objectives during faults.
Proposed LVRT Strategy: Transition to Single-Current-Loop Control with Reactive Support
The proposed strategy for the solar inverter is based on two coordinated control mode switches that occur upon detection of a grid voltage dip below a threshold (e.g., 0.9 p.u. of nominal voltage).
1. Inverter Control Switch: From Dual-Loop to Single-Current-Loop.
During normal operation, the DC-link voltage loop is active. During an LVRT event, this outer loop is deactivated. The reference for the active current \( i_d^* \) is no longer generated by the voltage PI controller. Instead, it is calculated based on the priority to keep the total inverter current within its rated limit \( I_{max} \) while meeting the reactive current demand \( i_q^*_{lvrt} \) set by the grid code. The control law becomes:
$$
i_d^* = \sqrt{ I_{max}^2 – (i_q^*_{lvrt})^2 }
$$
The inner current controllers remain active, now receiving these new \( i_d^* \) and \( i_q^*_{lvrt} \) references. This transforms the control from a dual-loop to an effective single-current-loop structure, where the current references are directly dictated by the LVRT logic and current limit, not by the DC-link voltage error. The current control equations remain the same, ensuring fast and accurate tracking of the new current commands.
2. Boost Converter Control Switch: From MPPT to DC-Link Voltage Regulation.
Simultaneously, the boost converter abandons its MPPT algorithm. Its objective shifts from maximizing PV power extraction to actively regulating the PV array voltage \( V_{pv} \) to a constant reference value \( V_{pv}^* \), which is typically set slightly below the open-circuit voltage. A PI controller adjusts the duty cycle \( D \) to achieve this:
$$
D^* = K_{p}^{boost}(V_{pv}^* – V_{pv}) + K_{i}^{boost} \int (V_{pv}^* – V_{pv}) dt
$$
By fixing \( V_{pv} \), the power drawn from the PV array \( P_{pv} \) is reduced to a level that can be managed by the inverter operating under its current limit, thereby preventing DC-link overvoltage. The control modes are summarized below:
| Component | Normal Operation Mode | LVRT Operation Mode | Control Objective in LVRT |
|---|---|---|---|
| Solar Inverter (DC-AC) | Dual-Loop (Voltage & Current) | Single-Current-Loop | Inject reactive current per grid code while limiting total current to \( I_{max} \). |
| Boost Converter (DC-DC) | MPPT (Max Power) | Voltage Regulation | Regulate PV voltage \( V_{pv} \) to a constant setpoint to limit input power. |
Reactive Current Reference Calculation: The reactive current reference \( i_q^*_{lvrt} \) for the solar inverter is generated based on the measured positive-sequence grid voltage magnitude \( V_{grid} \) (in p.u.) and the applicable grid code. A typical piecewise function based on common requirements is:
$$
i_q^*_{lvrt} =
\begin{cases}
1.5 \cdot (0.9 – V_{grid}) \cdot I_n & \text{for } 0.2 \leq V_{grid} \leq 0.9 \\
1.05 \cdot I_n & \text{for } V_{grid} < 0.2 \\
0 & \text{for } V_{grid} \geq 0.9
\end{cases}
$$
where \( I_n \) is the rated current of the solar inverter. This ensures proportional reactive support during moderate dips and maximum support during deep dips.
Simulation Analysis and Performance Evaluation
A detailed simulation model of a three-phase, two-stage solar inverter system was developed in a professional electromagnetic transients tool to validate the proposed strategy. The system was subjected to a balanced voltage sag where grid voltage drops to 0.5 p.u. for 500 ms.
The key simulation results demonstrate the effectiveness of the control strategy:
- Grid Currents: Upon fault inception, the solar inverter rapidly transitions its control mode. The three-phase AC currents remain sinusoidal and are maintained at or near their pre-fault magnitude. Crucially, they do not exhibit dangerous overcurrent peaks, as the total current is explicitly limited by the single-current-loop logic. The phase angle shifts to provide the required lagging reactive current.
- DC-Link Voltage (\( V_{dc} \)): The DC-link voltage experiences a minor transient swell but is quickly stabilized and maintained within safe limits. This stability is a direct result of the boost converter switching to voltage regulation mode, which reduces the power flow from the PV array to match the limited power output capability of the inverter during the fault.
- Active and Reactive Power (\( P, Q \)): The power output of the solar inverter undergoes a significant change. The active power \( P \) injected into the grid decreases in proportion to the reduced grid voltage and the priority given to reactive current. Simultaneously, the reactive power \( Q \) increases substantially from nearly zero to a positive value, providing the mandated voltage support. The power transition is smooth without severe oscillations.
- Post-Fault Recovery: When grid voltage recovers, the control system seamlessly switches back to normal MPPT and dual-loop control. The solar inverter smoothly ramps its active power output back to the maximum available from the PV array while reducing reactive injection to zero, achieving a stable post-fault steady state.
The simulation confirms that the proposed method allows the solar inverter to fulfill LVRT requirements. The solar inverter remains connected, avoids overcurrent and overvoltage trips, and provides dynamic reactive power support to aid grid voltage recovery—all achieved through a logical reconfiguration of its existing controllers without extra hardware.
Advantages and Implementation Considerations
The primary advantage of this strategy is its simplicity and cost-effectiveness. It leverages the existing hardware and fast control loops of a standard solar inverter. The strategy requires only software modifications to implement the fault detection algorithm, the reactive current reference calculator, and the logic for switching control modes. This makes it highly attractive for retrofitting existing solar inverter fleets and for new designs.
Key implementation considerations include:
- Accurate and Fast Grid Voltage Measurement: Reliable detection of the voltage sag depth and phase angle (for positive-sequence extraction) is critical. This requires robust phase-locked loop (PLL) algorithms capable of tracking during unbalanced and distorted fault conditions.
- Seamless Transition Logic: The switching between control modes must be bumpless to avoid introducing secondary transients. This can be achieved by proper initialization of integral terms in the PI controllers or by using state reset techniques.
- Setting of PV Voltage Reference (\( V_{pv}^* \)): The chosen constant voltage for the boost converter during LVRT must be high enough to significantly reduce PV power but low enough to stay within the operating range and avoid triggering array-level protections.
- Compliance with Local Grid Codes: The reactive current reference calculation block must be configurable to adhere to the specific formulas and timing requirements of the regional grid code where the solar inverter is installed.
Conclusion
As solar power generation becomes a cornerstone of the modern electricity grid, the ability of solar inverters to support grid stability during disturbances is non-negotiable. This article presented a practical and effective low voltage ride-through strategy for a two-stage solar inverter system. The core of the strategy is the coordinated transition from normal maximum power point tracking and dual-loop inverter control to a fault-mode operation where the boost converter stabilizes the DC-link by regulating PV array voltage, and the inverter dedicates its current capacity primarily to injecting grid-code compliant reactive current using a single-current-loop control structure. This approach addresses the fundamental challenge of power imbalance during faults, prevents protection trips, and actively supports grid voltage recovery. Simulation results validate that the strategy ensures stable operation throughout the fault sequence, enabling the solar inverter to reliably meet stringent LVRT requirements. The method’s implementation through software updates to existing control platforms underscores its significant practical value for enhancing the grid resilience contribution of widespread solar PV installations.