With the rapid expansion of distributed photovoltaic (PV) systems, voltage stability issues have become increasingly prominent due to the inherent variability of solar irradiance and the complex grid integration of numerous small-scale solar inverters. As a researcher focusing on distributed PV grid integration, I explore a source-grid coordinated voltage control strategy to mitigate voltage fluctuations and ensure stable operation. This approach leverages the reactive power capability of solar inverters, combined with an Automatic Voltage Control (AVC) system, to achieve real-time voltage regulation at both the point of common coupling (PCC) and the solar inverter terminals. The strategy addresses challenges such as communication delays and data volume in distributed systems through hierarchical control, enhancing grid resilience. In this article, I analyze the voltage fluctuation mechanisms of solar inverters, evaluate their reactive power regulation capacity, and propose a coordinated control framework validated through simulations. The integration of formulas, tables, and empirical data underscores the effectiveness of this method in maintaining voltage stability under varying conditions.
Introduction to Voltage Stability in Distributed PV Systems
Distributed solar inverters are pivotal in converting DC power from PV panels to AC power for grid integration. However, fluctuations in solar irradiance cause unpredictable power output variations, leading to voltage instability, especially in remote areas where power transmission distances are long. The proliferation of distributed solar inverters, characterized by their small capacity and widespread distribution, exacerbates voltage quality issues at the PCC. Traditional voltage control methods often fall short due to the decentralized nature of these systems. Thus, implementing an AVC system that coordinates reactive power devices, including solar inverters, capacitors, and Static Var Compensators (SVCs), is essential. This article delves into a source-grid coordinated strategy that optimizes reactive power dispatch from solar inverters, ensuring voltage stability through real-time data collection and command generation. The goal is to establish a closed-loop voltage control mechanism that adapts to dynamic grid conditions.
Voltage Fluctuation Mechanism of Solar Inverters
The voltage stability of solar inverters is influenced by grid parameters, line impedances, and power output variations. To understand this, consider the equivalent impedance model of a distributed solar inverter connected to the grid. The PCC voltage ($U_{PCC}$) can be approximated using the voltage drop formula, neglecting transverse components, as follows:
$$U_{PCC} \approx U + \frac{P_1 R_1 + Q_1 X_1}{U}$$
Here, $U$ is the grid voltage, $P_1$ and $Q_1$ represent the active and reactive power at the PCC, and $R_1$ and $X_1$ are the resistance and reactance of the line. The total active power ($P_l$) and reactive power ($Q_l$) account for losses in collectors and transformers, expressed as:
$$P_l = \sum P_i – \Delta P_i – \Delta P_{Ti} – \Delta P_T$$
$$Q_l = \sum Q_i – \Delta Q_i – \Delta Q_{Ti} – \Delta Q_T + Q_C$$
Where $\Delta P_i$ and $\Delta Q_i$ denote losses in distribution lines, and $Q_C$ is the reactive power from compensation devices. Substituting these into the voltage equation yields:
$$U_{PCC} \approx U + \frac{\left[\sum P_i – \left(\frac{\sum P_i}{U}\right)^2 R_{eq}\right] R_1 + \left[\sum Q_i – \left(\frac{\sum P_i}{U}\right)^2 X_{eq} + Q_C\right] X_1}{U}$$
This equation indicates that increasing active power from solar inverters initially raises $U_{PCC}$ until a threshold ($P_0$), beyond which voltage declines. Reactive power support from solar inverters can mitigate this effect. Similarly, the terminal voltage ($U_i$) of a solar inverter depends on upstream voltages and line parameters:
$$U_i \approx U_{i2} + \frac{P_i R_{Ti} + Q_i X_{Ti}}{U_{i2}}$$
Where $U_{i2}$ is the voltage at the previous node, calculated recursively. This spatial-temporal distribution highlights the vulnerability of end-of-line solar inverters to voltage deviations, necessitating reactive power optimization.
Reactive Power Regulation Capacity of Solar Inverters
Solar inverters possess inherent reactive power capabilities constrained by their apparent power rating. The maximum reactive power ($Q_{MAX_{PV}}$) a solar inverter can provide or absorb is given by:
$$Q_{MAX_{PV}} = \pm \sqrt{S_{INV}^2 – P_{PV}^2}$$
Here, $S_{INV}$ is the inverter’s apparent power capacity, and $P_{PV}$ is the active power output. This relationship underscores the importance of utilizing solar inverters as dynamic reactive power sources. For instance, during periods of low active power generation, solar inverters can enhance voltage stability by injecting or absorbing reactive power. The following table summarizes key parameters affecting reactive power capacity in a typical distributed PV system:
| Parameter | Symbol | Typical Value |
|---|---|---|
| Inverter Apparent Power | $S_{INV}$ | 50 kVA |
| Active Power Output | $P_{PV}$ | 30 kW |
| Maximum Reactive Power | $Q_{MAX_{PV}}$ | ±40 kvar |
This capacity allows solar inverters to participate in voltage control, reducing reliance on external devices like SVCs. However, the reactive power output must be coordinated to avoid overloading the solar inverters.
Source-Grid Coordinated Voltage Control Strategy
The proposed strategy employs an AVC system to achieve source-grid coordination for voltage stability. The AVC system hierarchically manages distributed solar inverters, addressing communication challenges through local, regional, and central layers. Data from solar inverters, including voltage, current, and power parameters, are collected in real-time. The AVC主站 sets a target voltage for the PCC, and sub-stations calculate voltage drops along lines and transformers to derive reference voltages for individual solar inverters. The control process involves:
- Data Acquisition: Real-time collection of operational data from solar inverters.
- Target Voltage Setting: The AVC主站 defines $U_{bus}$ for the PCC based on grid conditions.
- Voltage Drop Calculation: Using line impedances and power flows, voltage drops are computed:
$$\Delta U_{line1} = \frac{P R_1 + Q X_1}{U_{bus}}$$
$$U_{bus1} = U_{bus} + \Delta U_{line1}$$ - Reference Voltage Derivation: For transformer high-voltage sides:
$$\Delta U_{line2} = \frac{P R_2 + Q X_2}{U_{bus}}$$
$$U_{bus2} = U_{bus} + \Delta U_{line2}$$ - Inverter Command Generation: The reference voltage ($U_{ref}$) for solar inverters is:
$$\Delta U_T = \frac{P R_T + Q X_T}{U_{bus2}}$$
$$U_{ref} = \frac{U_{bus2} + \Delta U_T}{K_T}$$
where $K_T$ is the transformer ratio.
This method ensures that solar inverters receive precise voltage commands, enabling rapid reactive power adjustments. The communication architecture supports this through a layered approach, as illustrated below:
The AVC system’s algorithm includes reactive power estimation, allocation, and security constraints, ensuring that solar inverters operate within safe limits. Compared to conventional equal reactive power distribution, this strategy accounts for line impedances and real-time generation, improving voltage stability.
Case Study and Verification
To validate the strategy, a case study was conducted on a distributed PV plant in Jiangxi Province, China. The plant comprises multiple solar inverters with parameters listed in the table below:
| Component | Parameter | Value |
|---|---|---|
| Solar Inverters | Number of Units | 80 |
| Unit Capacity | 50 kW | |
| Total Capacity | 4 MW | |
| Reactive Power Range | ±2.5 Mvar | |
| Transformers | Rated Capacity | 0.5 MVA |
| Short-Circuit Impedance | 7% | |
| No-Load Loss | 2.4 kW | |
| Collection Lines | Lengths | 1,100 m to 2,800 m |
Simulations were performed over a 24-hour period using actual power output data. The active power profiles of equivalent solar inverters showed significant fluctuations due to irradiance changes. Under the source-grid coordinated control, solar inverters adjusted their reactive power output to maintain voltage stability. For example, the reactive power output ($Q$) of four equivalent solar inverters varied between -2.5 Mvar and 0.5 Mvar, as described by:
$$Q = \pm \sqrt{S_{INV}^2 – P_{PV}^2}$$
Comparative analysis with equal reactive power distribution demonstrated the superiority of the coordinated strategy. The PCC voltage ($U_{PCC}$) remained within 32–40 kV under coordinated control, whereas it deviated significantly under equal distribution. Similarly, solar inverter terminal voltages stabilized around 0.30–0.36 kV, reducing the risk of disconnection. The voltage improvement ($\Delta U$) achieved was quantified as:
$$\Delta U = U_{coordinated} – U_{equal}$$
Where $U_{coordinated}$ and $U_{equal}$ represent voltages under respective strategies. The results confirm that the coordinated approach minimizes voltage perturbations and enhances grid integration of solar inverters.
Conclusion
In summary, the source-grid coordinated voltage control strategy effectively addresses voltage instability in distributed PV systems by leveraging the reactive power capabilities of solar inverters. The AVC system facilitates real-time data processing and command generation, ensuring stable voltages at the PCC and solar inverter terminals. Key findings include: (1) Solar inverters can serve as primary reactive power sources, with their capacity determined by P-Q characteristics; (2) The coordinated strategy accounts for line impedances and power fluctuations, providing accurate voltage references; and (3) This method offers faster convergence compared to optimization-based approaches, meeting real-time control requirements. Future work will focus on extending this strategy to low-voltage integration scenarios and enhancing fault ride-through capabilities. Ultimately, this research contributes to the development of resilient grid interfaces for distributed solar inverters, promoting widespread renewable energy adoption.