With the global shift towards renewable energy and the pursuit of carbon neutrality, distributed photovoltaic (PV) systems are experiencing rapid growth in installation capacity. The ability of these systems to provide active support to the grid has become a critical research focus. In scenarios where distributed PV operates without the support of the grid or energy storage, it must transition to an islanding mode without energy storage, employing voltage-controlled strategies to independently maintain voltage stability. The seamless switching between grid-connected and islanding modes for PV inverters is essential to ensure uninterrupted power supply to loads after the loss of grid synchronization. This paper explores multi-mode control strategies for PV inverters without energy storage and proposes a seamless transfer strategy between grid-connected and islanding operations. The strategy ensures stable operation in both modes and smooth transitions, maintaining power balance between the PV inverter and the load. Experimental results validate the effectiveness of the approach, enhancing power quality in distribution networks with high PV penetration.
Distributed PV systems, as a key component of modern power systems, face challenges related to power fluctuations and grid stability. Various types of solar inverter, such as grid-tied, off-grid, and hybrid inverters, play a vital role in managing these challenges. However, in the absence of energy storage, PV inverters must adapt to extreme operating conditions, such as grid faults, by switching control modes. This paper focuses on the multi-modal operation of PV inverters, including grid-connected mode, islanding mode without energy storage, and the transitions between them. The control strategies for these types of solar inverter are designed to ensure reliability and efficiency, leveraging advanced power electronics and control theory.
The topology of the PV inverter system is based on a two-stage structure, comprising a front-end Boost converter and a rear-end three-level neutral point clamped (NPC) inverter. This configuration decouples the DC bus voltage from the PV output power, enabling flexible control. The use of three-level inverters reduces current ripple, switching stress, and losses, making them suitable for high-performance applications. The system connects to the grid via a solid-state switch, allowing for rapid disconnection during faults and seamless reconnection when the grid is restored. This architecture supports the diverse requirements of different types of solar inverter in multi-modal operations.
In grid-connected mode, the PV inverter operates under current-controlled strategies, tracking power references from the grid operator. The active and reactive power outputs are governed by the following equations in the synchronous reference frame:
$$P = \frac{3}{2} u_{md} i_{md} + \frac{3}{2} u_{mq} i_{mq}$$
$$Q = \frac{3}{2} u_{mq} i_{md} – \frac{3}{2} u_{md} i_{mq}$$
Assuming the grid voltage is aligned with the d-axis through phase-locked loop (PLL) control, the q-axis component $u_{mq}$ is approximately zero, simplifying the equations to:
$$P = \frac{3}{2} u_{md} i_{md}$$
$$Q = -\frac{3}{2} u_{md} i_{mq}$$
These equations form the basis for power control in grid-connected operations. The DC bus voltage is regulated by the Boost converter, which adjusts the PV operating point to match the power reference. If the PV cannot meet the power demand due to environmental constraints, it defaults to maximum power point tracking (MPPT). This approach ensures that the PV system contributes to grid stability while maximizing energy harvest. The control structure for grid-connected mode includes outer power loops and inner current loops, as summarized in Table 1.
| Control Component | Strategy | Parameters |
|---|---|---|
| Boost Converter | Voltage and current double-loop control | $u_{dc\_ref}$, $i_{L\_ref}$ |
| Inverter | Current-controlled with power reference | $P_{ref}$, $Q_{ref}$, $i_{dq\_ref}$ |
| PLL | Grid synchronization | $\theta_{PLL}$ |
In islanding mode without energy storage, the PV inverter switches to voltage-controlled operation, maintaining constant voltage and frequency at the AC bus. The power balance between the PV source and the load is achieved by regulating the DC bus voltage. Any imbalance causes DC voltage fluctuations, which are corrected by adjusting the PV operating point through the Boost converter. The control strategy for this mode involves outer voltage loops and inner current loops, as depicted in the following equations for voltage reference generation:
$$u_{mdq\_ref} = f(\omega_{ref}, u_{mdq}^*)$$
where $u_{mdq}^*$ is the voltage command, and $\omega_{ref}$ is the frequency reference. The phase angle $\theta_{ref}$ is generated by integrating $\omega_{ref}$. This ensures stable voltage and frequency support for the load, even in the absence of external sources. The flexibility of these types of solar inverter in adapting to islanding conditions highlights their importance in resilient power systems.
The transition from grid-connected to islanding mode is triggered by the detection of grid faults. To achieve smooth switching, the control loops are seamlessly handed over. In grid-connected mode, the current reference $i_{dq\_ref}$ is derived from power references, and the phase $\theta_{ref}$ comes from the PLL. Upon switching, the voltage control loop takes over, initializing its integrators with the pre-switch values to avoid current spikes. The phase generation switches from PLL-based to internal frequency integration, ensuring continuity. This approach minimizes transients and maintains power quality during the transition.
Conversely, when transitioning from islanding to grid-connected mode, pre-synchronization is necessary to align the inverter output with the grid voltage. The pre-synchronization control involves adjusting the voltage magnitude and phase using PI controllers, as shown below:
$$\Delta u_{dq} = K_{p,u} (u_{dq\_PLL} – u_{mdq\_ref}) + K_{i,u} \int (u_{dq\_PLL} – u_{mdq\_ref}) dt$$
$$\Delta \omega = K_{p,\theta} (\theta_{PLL} – \theta_{ref}) + K_{i,\theta} \int (\theta_{PLL} – \theta_{ref}) dt$$
where $\Delta u_{dq}$ and $\Delta \omega$ are adjustment terms for voltage and frequency, respectively. Once synchronization is achieved, the solid-state switch closes, and the inverter reverts to current-controlled operation. This process ensures that the reconnection does not cause overcurrent or voltage disturbances, critical for the reliability of various types of solar inverter in microgrid applications.
Experimental validation was conducted using a test bench with a two-stage PV inverter. The system parameters are listed in Table 2. A DC source with a series resistor emulated the PV characteristics, and the inverter was controlled by a DSP28377 processor. The experiments demonstrated smooth transitions between modes, with stable DC voltage and AC power balance. For instance, during the switch from grid-connected to islanding mode, the PV voltage adjusted from 74 V to 70 V, and the output power increased from 210 W to 360 W to match load demand. The DC bus voltage experienced a minor dip but recovered quickly, confirming the effectiveness of the control strategy.
| Parameter | Value |
|---|---|
| Grid Voltage $u_g$ | 70 V |
| Islanding Voltage Command $u_{md\_ref}$ | 70 V |
| Grid-Connected Power Command $P_{ref}$ | 210 W |
| Islanding Frequency $f_{ref}$ | 50 Hz |
| Load Resistance $R_{load}$ | 20 Ω |
| DC Source Voltage $u_{dc}$ | 70-80 V |
| Switching Frequency $f_k$ | 10 kHz |
| Series Resistance $R_{dc}$ | 2.5 Ω |
The experimental waveforms showed no significant current or voltage spikes during mode transitions, validating the seamless transfer strategy. In grid-connected mode, the harmonic distortion of the output voltage was measured at 2.88%, indicating high power quality. These results underscore the capability of advanced types of solar inverter to maintain stability in dynamic environments. The multi-modal control approach reduces reliance on energy storage, lowering costs and enhancing the applicability of distributed PV systems.
In conclusion, this paper presents a comprehensive multi-mode control and seamless transfer strategy for PV inverters without energy storage. The proposed methods enable stable operation in grid-connected and islanding modes, with smooth transitions that ensure uninterrupted power supply. The integration of voltage and current control strategies, along with pre-synchronization techniques, addresses the challenges of power balance and grid support. Experimental results confirm the practicality of the approach, contributing to the advancement of distributed PV systems. Future work will explore optimization for larger-scale applications and integration with other types of solar inverter to further improve grid resilience.
The development of these control strategies is crucial for the widespread adoption of renewable energy. By enhancing the functionality of various types of solar inverter, we can achieve a more sustainable and reliable power infrastructure. The insights from this study provide a foundation for further research into multi-modal operations and smart grid technologies.