In the current era of heightened emphasis on environmental protection and low-carbon strategies, photovoltaic power generation, as a green energy source, is developing at an accelerating pace, with its share in the power distribution system increasing annually. The integration of large-scale distributed photovoltaic systems is set to颠覆 the traditional power flow distribution patterns of distribution networks, posing significant challenges to the existing distribution system models. For instance, studies have focused on the impact of photovoltaic system access points of different scales on grid harmonics, providing crucial insights into the harmonic characteristics under various connection scenarios. Other research has optimized the grid integration planning of distributed generation from a power system security perspective, considering factors like voltage deviation, harmonics, and three-phase imbalance, which is vital for ensuring stable and safe operation. Additionally, studies have explored the influence of key factors such as distributed photovoltaic access capacity and location on node voltage in distribution networks, offering important guidance for network design and optimization.
This paper proposes a novel strategy that fully considers the critical role of inverters, centering on the participation of solar inverters in grid reactive power voltage control. This strategy aims to address the adverse effects of photovoltaic grid integration on voltage stability and power quality, providing new solutions for stable grid operation. The widespread adoption of solar inverters in modern grids underscores their importance in managing reactive power and maintaining voltage levels.
The operation of solar inverters in large grids involves a voltage source \(U_S\) with equivalent inductance \(L_g\) and resistance \(R_g\), along with a specific inverter circuit structure. In such configurations, when the grid-connected inductor current contains only active components, the inductor current \(I_L\) will align in frequency and phase with the grid voltage \(U_g\). During grid voltage drops, the solar inverter injects inductive reactive current to help stabilize the grid voltage. Conversely, when grid voltage rises, the solar inverter releases capacitive reactive current to maintain stability. When the solar inverter outputs inductive reactive current, from the grid’s perspective, the inductor current direction reverses by 180 degrees, leading the grid voltage. In this case, the solar inverter acts similarly to a capacitor. Typically, grid loads are inductive, meaning grid current lags behind grid voltage most of the time. To address this, a controllable series compensation-based external photovoltaic inverter filtering and reactive power compensation device has been proposed. This approach not only enhances the grid integration performance of photovoltaic systems but also improves power system stability and power quality, offering technical support for the broad application of photovoltaic power generation systems. The calculation is based on the following formula:
$$Q = V \times I \times \sin \phi$$
where \(Q\) represents the reactive power provided by the solar inverter (in kVAR), \(V\) is the grid voltage (in V), \(I\) is the output current of the solar inverter (in A), and \(\phi\) is the phase difference between the solar inverter output current and the grid voltage (in radians). This formula is rooted in the definition of reactive power, where reactive power is the product of current and voltage and the sine of their phase difference. In this context, we assume that the active power output of the solar inverter is in phase with the grid voltage, while reactive power is determined by the phase difference \(\phi\), set by the solar inverter’s control system to meet grid reactive power demands and maintain voltage stability.
Analyzing the impact of losses on solar inverter control, when neglecting reactor and converter losses, the equivalent circuit and phasor diagram of the solar inverter are as shown. By adjusting the amplitude of the AC output voltage \(U_{ab}\) and its phase difference relative to \(U_S\), the voltage across the reactor can be controlled, thereby regulating the phase and amplitude of the current absorbed by the converter and effectively managing the type and amount of reactive power. When the phase difference between the converter output voltage \(U_{ab}\) and the grid current \(I_L\) is 90 degrees, and the phase difference between grid voltage \(U_S\) and current \(I_L\) is slightly less than 90 degrees, it indicates that current \(I_L\) contains an active component. The angle \(\delta\) represents the phase difference between \(U_{ab}\) and \(U_S\). In steady state, the presence of \(\delta\) causes the solar inverter to absorb active power, compensate for losses, and adjust the output voltage via the DC-side capacitor to control reactive output. If \(U_{ab} > U_S\), the solar inverter provides capacitive reactive power; otherwise, it provides inductive reactive power.
Through precise control of the solar inverter’s output voltage \(U_{ab}\), the reactive power integrated into the grid can be directly influenced, including its magnitude and nature. This method, known as direct control, focuses on adjusting the solar inverter output voltage to act directly on the grid connection point, enabling accurate regulation of reactive power output. This approach is widely used in grid management due to its directness and efficiency, particularly in scenarios requiring rapid response and precise control. An alternative method involves indirectly controlling the integrated reactive power by managing \(\delta\) and \(U_{ab}\). This indirect control is more complex, involving fine adjustments to the phase of the solar inverter output voltage and the phase difference between grid voltage and current. Although more intricate, it offers flexible control strategies, especially in dynamic grid conditions.
The implementation of filtering and reactive power compensation devices, as analyzed, utilizes a controllable series compensation-based external solar inverter filtering and reactive power compensation device to effectively absorb reactive power without altering the solar inverter itself. This solves the issue of high terminal voltage and employs innovative composite alloy materials in reactors to ensure low energy consumption and full-band response, while suppressing higher-order harmonic interference. Furthermore, new zero-phase and ground-loop harmonic interference suppression technologies have been developed to effectively control harmonics generated by solar inverters. The use of interference suppressors made with silicon steel cores having a relative permeability exceeding \(1 \times 10^4\), installed on frequency converter ground lines, significantly reduces high-frequency current interference, enhancing system stability and safety. This opens new pathways for the application of solar inverters in modern grids.
In the field of reactive power compensation technology, traditional static var generator (SVG) compensation devices, as fully controlled active reactive power generators, differ from solar inverter filtering and reactive power compensation devices. The following table compares the two approaches:
| Parameter | SVG Compensation Device | Solar Inverter Reactive Power Compensation Device |
|---|---|---|
| Reliability | Requires protection devices | No additional protection devices needed |
| Energy Consumption | No-load loss, reactive loss | Nighttime lossless |
| Installation | Requires dedicated space and environmental conditions | No special requirements |
| Economy | 1.1 times overload capacity | 1.6 times overload capacity |
From the table, it is evident that compared to traditional SVG compensation devices, solar inverter filtering and reactive power compensation devices demonstrate superiority in multiple aspects. In terms of reliability, solar inverters do not require additional protection devices, making them more stable and reliable in operation. Regarding energy consumption, solar inverters have no energy consumption at night, significantly reducing overall usage compared to SVG devices. For installation, the lack of special requirements for solar inverters makes them more flexible and applicable in a wider range of scenarios. Economically, solar inverters offer higher overload capacity, enhancing their cost-effectiveness. These advantages highlight the potential of solar inverters in improving grid stability and power quality, especially as the deployment of solar energy systems expands.
To further elaborate on the control strategies, the direct control method for solar inverters involves regulating the output voltage magnitude and phase to directly inject or absorb reactive power. This can be mathematically represented by modifying the reactive power formula to account for control parameters. For instance, the relationship between the output voltage of the solar inverter and the grid voltage can be expressed as:
$$U_{ab} = U_S + j I_L X$$
where \(X\) is the reactance of the line. By controlling \(U_{ab}\), the solar inverter can adjust the reactive power flow. The phase difference \(\delta\) plays a crucial role in this process, and its effect on active and reactive power can be described using the power equations:
$$P = \frac{U_S U_{ab} \sin \delta}{X}$$
$$Q = \frac{U_S (U_{ab} \cos \delta – U_S)}{X}$$
These equations show how solar inverters can manage both active and reactive power by tuning \(U_{ab}\) and \(\delta\). In practice, solar inverters use pulse-width modulation (PWM) techniques to generate the desired output voltage waveforms, enabling precise control. The use of advanced materials, such as composite alloys in reactors, enhances the efficiency of solar inverters by reducing core losses and improving thermal stability. This is particularly important for high-frequency operations where harmonics are prevalent.
Another key aspect is the harmonic suppression capability of solar inverters. With the increasing penetration of photovoltaic systems, harmonic distortion can lead to issues like equipment malfunction and increased losses. Solar inverters equipped with filtering devices can mitigate these effects. For example, the external filtering device mentioned earlier utilizes high-permeability materials to attenuate harmonics. The effectiveness of such devices can be quantified using total harmonic distortion (THD) metrics, where lower THD indicates better performance. Solar inverters with integrated filters can achieve THD levels below 5%, complying with grid standards and ensuring reliable operation.
In addition to technical benefits, the economic advantages of solar inverters in reactive power control are significant. The higher overload capacity of solar inverters, as shown in the comparison table, allows for better utilization of infrastructure, reducing the need for additional compensation devices. This translates to lower capital and operational expenses. Moreover, the modular nature of solar inverters facilitates scalability, making them suitable for both small-scale and large-scale applications. As the cost of solar technology continues to decrease, the adoption of solar inverters for grid support functions is expected to grow, contributing to a more sustainable energy landscape.
Looking at real-world applications, solar inverters have been deployed in various grid scenarios to demonstrate their effectiveness. For instance, in distribution networks with high photovoltaic penetration, solar inverters have successfully regulated voltage levels by dynamically adjusting reactive power output. Field studies have shown that solar inverters can respond to voltage fluctuations within milliseconds, providing rapid support during transient events. This responsiveness is crucial for maintaining grid stability, especially in regions with intermittent renewable generation. The integration of solar inverters with energy management systems further enhances their capability, allowing for coordinated control across multiple nodes.
To summarize, this paper has detailed the role and effectiveness of solar inverters in grid reactive power voltage control. The research focuses on how solar inverters can efficiently participate in voltage regulation and reactive power management, highlighting their potential in practical grid applications. Key findings include that the use of controllable series compensation with external solar inverter filtering and reactive power compensation devices enables reactive power absorption without modifying the solar inverter itself. This approach effectively addresses terminal voltage issues, reduces energy consumption through innovative materials, and suppresses higher-order harmonic interference. A comparison with traditional SVG compensation devices reveals differences in reliability, energy consumption, installation, and economy, providing valuable insights for selecting appropriate grid reactive power voltage control solutions. The advancements in solar inverter technology underscore their importance in achieving a stable, efficient, and sustainable power system.
Future work could explore the integration of artificial intelligence and machine learning algorithms into solar inverter control systems for predictive maintenance and optimized performance. Additionally, as grid codes evolve, solar inverters may need to adapt to new requirements, such as providing inertia support or participating in frequency regulation. Continued research into materials and control strategies will further enhance the capabilities of solar inverters, solidifying their position as a cornerstone of modern power systems.