As the global energy crisis intensifies and environmental degradation becomes more pronounced, the development of renewable and clean energy sources to achieve sustainable development has emerged as a top priority. Among these, solar photovoltaic power generation has attracted widespread attention due to its cleanliness and renewability. In photovoltaic systems, the solar inverter serves as the core equipment, and its performance directly impacts the overall efficiency and stability of the entire system. In recent years, mirror virtual resistance technology, as an emerging control strategy, has been extensively studied in the field of power electronics. In this paper, I explore the application of mirror virtual resistance technology in three-phase solar inverters, aiming to provide new insights and methods for improving the performance of photovoltaic power generation systems.
Fundamental Principles and Functions of Three-Phase Solar Inverters
Role of the Inverter in Photovoltaic Systems
The solar inverter plays a crucial role in photovoltaic power generation systems. Its primary function is to convert the direct current (DC) electricity generated by solar panels into alternating current (AC) electricity compatible with the power grid. Specifically, since photovoltaic modules or electrochemical energy storage batteries output DC, while modern power grids transmit AC, the DC must be converted to AC before grid connection. The solar inverter accomplishes this conversion through internal electronic devices and control circuits. Additionally, the solar inverter has power regulation and control functions. It can adjust and control the power of the photovoltaic system based on actual conditions. For example, when sunlight is intense, the solar panels generate more electricity, and the solar inverter can inject the excess energy into the grid. Conversely, in low light or at night, the solar inverter can draw power from the grid to maintain normal operation.
Working Principle of Three-Phase Solar Inverter
The working principle of a three-phase solar inverter can be described as follows. The input circuit provides a stable DC voltage for the main inverter circuit. The main inverter circuit, which is the core of the device, performs the inversion function by turning power electronic switches on and off. Specifically, a three-phase inverter has a relatively complex control method, but the basic principle is similar to that of a single-phase inverter. Six switch devices are used, with two switch devices on each phase line. Each phase sequence operates a pair of switch devices. For example, to obtain the R phase, S1-S2 are turned on; for the Y phase, S3-S4 are turned on; for the B phase, S5-S6 are turned on. This produces the final output waveform. The output circuit corrects, compensates, and regulates the waveform, frequency, voltage, current amplitude, and phase of the AC output from the main inverter circuit. The control circuit provides a series of control pulses to the main inverter circuit to control the switching of the inverter devices, thus completing the inversion function. The auxiliary circuit converts the input voltage into a suitable DC voltage for the control circuit. It includes various detection circuits. Protection circuits include input over-voltage/under-voltage protection, output over-voltage/under-voltage protection, over-current protection, short-circuit protection, anti-islanding protection, etc.
Key Technical Specifications and Performance Parameters of Solar Inverters
In photovoltaic systems, the performance of the solar inverter directly affects the overall system efficiency and stability. Understanding the main technical indicators and performance parameters of the solar inverter is essential for evaluating its performance. Table 1 lists some critical parameters.
| No. | Specification / Parameter | Description |
|---|---|---|
| 1 | Rated Output Capacity | The ability of the solar inverter to supply power to the load, usually expressed in VA or kVA. |
| 2 | Output Voltage Stability | The ability to maintain a stable output voltage, often measured by voltage regulation and load regulation. |
| 3 | Input Voltage Range | The maximum and minimum DC voltage range that the solar inverter can accept. |
| 4 | Conversion Efficiency | The efficiency of converting DC to AC, usually expressed as a percentage. |
| 5 | Maximum Output Power | The maximum continuous power the solar inverter can deliver, usually expressed in kW. |
| 6 | Output Voltage Waveform Distortion | The degree of deviation from an ideal sine wave, usually expressed as a percentage. |
| 7 | Protection Functions | Built-in protections such as over-voltage, under-voltage, over-current, and short-circuit protection. |
The rated output capacity reflects the solar inverter’s ability to drive loads. A higher rated capacity means the solar inverter can handle larger loads. Output voltage stability is crucial; high-performance solar inverters have low voltage regulation and load regulation, maintaining stable output voltages under varying loads. The input voltage range determines the adaptability of the solar inverter to different operating conditions. A wider range enhances flexibility. Conversion efficiency is critical—high-efficiency solar inverters minimize energy losses. The maximum output power must exceed the peak power of the solar panels or wind turbines to ensure normal operation. Output voltage waveform distortion indicates output quality; for sine wave solar inverters, distortion should not exceed 5% (single-phase output distortion should not exceed 10%). Protection functions safeguard the system from abnormal conditions, improving reliability.
Principles and Characteristics of Mirror Virtual Resistance Technology
Definition and Principle
Mirror virtual resistance technology is a control strategy in power electronics. Its core idea is to simulate a virtual resistor with characteristics similar to a physical resistor using control algorithms and power electronic devices. The fundamental definition is:
$$ R = \frac{U}{I} $$
where \( U \) is voltage and \( I \) is current. Another formulation is:
$$ R = \rho \frac{L}{S} $$
where \( \rho \) is resistivity (intrinsic material property), \( L \) is length, and \( S \) is cross-sectional area. In practice, mirror virtual resistance technology is based on control theory of power electronic converters. By continuously measuring current or voltage in the circuit, the control algorithm calculates the required virtual resistance value and adjusts the converter output so that the circuit system perceives an impedance similar to an actual physical resistor. This approach allows regulation and optimization of circuit characteristics without adding physical resistors.
Advantages in Power Electronics
Mirror virtual resistance technology offers significant advantages in power electronics. Since the virtual resistor is implemented via algorithms, its resistance value can be easily adjusted to meet different circuit requirements. This flexibility enables the technology to be applied in various power electronics applications. Compared to physical resistors, mirror virtual resistors require no additional hardware, thus saving cost. Moreover, because there is no need to consider heat dissipation or reliability issues of physical resistors, system complexity and maintenance costs are reduced. By precisely controlling the virtual resistance value, circuit performance can be optimized, improving system efficiency and stability. For example, in grid stability control, mirror virtual resistance technology can effectively suppress harmonics and oscillations, enhancing power quality. Additionally, this technology can be combined with other control strategies to form more complex control systems, expanding its applicability.
Implementation Methods
Implementing mirror virtual resistance technology involves several steps. First, a detailed analysis of the circuit system is performed to understand its working principle, operating characteristics, and aspects to optimize. This helps determine the type and range of virtual resistance needed. Based on the system requirements, a suitable control algorithm is designed. This algorithm must be able to detect current or voltage in real time and compute the virtual resistance value accordingly. Then, the corresponding hardware circuit is designed to receive signals from the control algorithm and drive the power electronic converter to output the required voltage or current. Next, the control algorithm is coded using a programming language or development tool, and the program runs on the hardware circuit, adjusting the virtual resistance value based on the system state. Finally, the entire system is tested and optimized, including tuning the algorithm parameters to ensure accurate simulation of the desired resistance characteristics and achieve the expected optimization effect.
Design of Three-Phase Solar Inverter Based on Mirror Virtual Resistance Technology
System Design Objectives and Requirements
In designing a three-phase solar inverter based on mirror virtual resistance technology, the primary objective is to improve the stability and efficiency of the photovoltaic power generation system. To achieve this, the system must meet several requirements, as illustrated in the following list:
- The solar inverter should efficiently convert DC power from photovoltaic modules into AC power and inject it into the grid, minimizing energy losses.
- The solar inverter must exhibit excellent stability, operating reliably under various conditions and effectively suppressing harmonics and oscillations to improve power quality.
- The system should allow flexible adjustment of output voltage, frequency, and power factor to adapt to different grid requirements and photovoltaic module operating states.
- The solar inverter should have high reliability and long lifespan, operating continuously in harsh environments while reducing maintenance costs and downtime.
These objectives guide the overall design of the solar inverter.
Hardware Circuit Design
The main circuit is the core of the solar inverter, responsible for converting DC to AC. In the main circuit design, appropriate power electronic switching devices (such as IGBTs or MOSFETs) must be selected, and suitable filter circuits must be designed to eliminate harmonics and noise in the output voltage. Additionally, heat dissipation and protection measures for the main circuit are necessary to ensure stable operation under high temperature and overload conditions. The control circuit serves as the control center of the solar inverter, responsible for real-time detection of circuit parameters such as current and voltage, and adjusting the main circuit’s working state accordingly. In the control circuit design, a high-performance digital signal processor (DSP) or field-programmable gate array (FPGA) is chosen as the control core, along with appropriate interface circuits to receive signals from sensors. The driver circuit connects the control circuit and the main circuit, converting control signals into drive signals for the power electronic switches. Suitable driver chips and isolation devices must be selected to ensure accurate signal transmission and reliable switching. Heat dissipation and protection for the driver circuit are also considered to ensure stable operation under adverse conditions.
Software Algorithm Design
In the three-phase solar inverter based on mirror virtual resistance technology, suitable control strategies must be selected to achieve efficient and stable energy conversion. Common control strategies include voltage-current dual-loop control and maximum power point tracking (MPPT) control. These strategies can be flexibly adjusted according to the output characteristics of photovoltaic modules and grid requirements to optimize the solar inverter’s performance and stability. The control algorithm is the key to implementing the control strategy. It calculates control signals based on real-time detected circuit parameters and sends them to the driver circuit. In implementing the control algorithm, a high-performance algorithm program is written, and real-time performance and robustness are considered. Protection algorithms are crucial for ensuring the safe operation of the solar inverter. They are responsible for promptly cutting off the circuit under abnormal conditions (over-voltage, under-voltage, over-current, short-circuit, etc.) to protect equipment and personnel. Therefore, appropriate protection logic and thresholds must be designed.
To illustrate the mirror virtual resistance implementation, consider a control loop where the virtual resistance \( R_{v} \) is introduced in the current control path. The relationship between the voltage reference \( v_{ref} \) and the measured current \( i \) can be expressed as:
$$ v_{ref} = v_{ref0} – R_{v} \cdot i $$
where \( v_{ref0} \) is the original voltage reference from the MPPT or outer loop. This equation shows that the virtual resistor effectively adds a voltage drop proportional to the current, thereby shaping the output impedance of the solar inverter. The value of \( R_{v} \) can be dynamically adjusted based on grid conditions to enhance damping and stability.
Another important aspect is the design of the filter. The output filter of the solar inverter typically consists of an inductor \( L \) and a capacitor \( C \). With mirror virtual resistance, the equivalent series resistance can be increased virtually to reduce resonance peaks. The transfer function of the LCL filter with virtual resistance can be written as:
$$ G(s) = \frac{1}{L_1 L_2 C s^3 + (L_1 + L_2) R_v C s^2 + (L_1 + L_2) s} $$
where \( L_1 \) and \( L_2 \) are the inverter-side and grid-side inductances, respectively. By selecting appropriate \( R_v \), the damping ratio can be improved without adding physical resistors, thus reducing power losses.
The MPPT algorithm is also integrated. For example, the perturb and observe method updates the duty cycle based on power change:
$$ P_{new} > P_{old} \Rightarrow \text{continue same direction} $$
$$ P_{new} < P_{old} \Rightarrow \text{reverse direction} $$
The mirror virtual resistance can be adjusted in coordination with MPPT to maintain stable operation near the maximum power point.
Conclusion
In summary, the application of mirror virtual resistance technology in three-phase solar inverters offers significant benefits for improving the stability and efficiency of photovoltaic power generation systems. By simulating a virtual resistor through control algorithms, the solar inverter can achieve better harmonic suppression, enhanced damping, and flexible impedance shaping without additional physical components. The design objectives, hardware circuit, and software algorithm presented in this paper provide a comprehensive framework for implementing this technology. As research progresses, the three-phase solar inverter based on mirror virtual resistance technology will play an increasingly important role in the field of photovoltaic power generation. I look forward to more researchers focusing on this area and jointly promoting the development and advancement of photovoltaic technology for a sustainable future.