Abstract : Addressing the current challenges of low equipment utilization and severe harmonic pollution in grid-connected photovoltaic (PV) systems, this paper explores a unified control system integrating PV grid-connected systems and Active Power Filter (APF). The system structure and control strategy for implementing this unified control are analyzed, enabling the system to not only perform PV grid-connected power generation but also compensate for system harmonics. Harmonic detection methods based on instantaneous reactive power theory and the synthesis method of the grid-connected current command signal in a PV grid-connected system with APF functionality are analyzed. A dual closed-loop PI control system (voltage and current) is employed to achieve steady-state error-free tracking control of the active fundamental signal in the grid-connected command signal and to compensate for specific harmonics. Finally, a simulation of the PV grid-connected system with APF functionality is performed using MATLAB/Simulink, verifying the correctness and effectiveness of the system structure, parameter design, and control strategy.
1 Introduction
Distributed photovoltaic (PV) grid-connected power generation has become one of the main ways to utilize solar energy. However, PV inverters only operate when the output capacity of the PV panels reaches a certain value. When the sunlight intensity is low or at night, the entire system must be disconnected from the grid, significantly reducing equipment utilization. With the acceleration of China's industrialization, non-linear electrical equipment is widely used, and the reactive power and harmonic currents generated by these loads are increasingly harmful to the public power grid. By integrating the topology and control methods of PV grid-connected power generation systems and active power filters ( APFs), the unified control of both achieves the integration of PV power generation and APF. The grid-connected power generation function of the PV grid-connected system is combined with the harmonic compensation function of the APF, giving it the dual functions of PV grid-connected power generation and harmonic compensation. This improves the power quality of the grid, saves on equipment investment costs, and increases the utilization rate of the PV grid-connected system. During the day, the inverter realizes both PV grid-connected power generation and APF function; during low sunlight intensity or at night, it can continue to operate as an APF. This not only improves the utilization rate of the equipment, but also improves the power supply quality of the power grid, avoiding the control difficulties caused by frequent switching of photovoltaic grid-connected systems.
2 System Circuit Structure
Since the basic structure and key control technologies of photovoltaic grid-connected systems and APF (Automatic Power Filter) are similar, some scholars have already studied unified control technologies for the two. Extending the functionality of the original photovoltaic grid-connected system to include APF capabilities can effectively suppress harmonics, achieve integrated functionality, improve system utilization, and eliminate the need for additional investment.
Figure 1. Block diagram of a photovoltaic grid-connected power generation system with APF function.
During the day, the system can perform maximum power point tracking (MPPT) of solar energy, integrating the active power from photovoltaic (PV) power into the grid. During cloudy or rainy weather, or at night when there is no sunlight, the active power output is zero. The system can be directly used as an active power filter to suppress harmonic pollution in the power system and improve the power quality of the grid. When the PV array is operating, but the output solar active power is low, the remaining capacity of the grid-connected system can be used to control the inverter to operate simultaneously for PV grid connection and harmonic compensation. If the PV grid-connected system cannot provide sufficient capacity for harmonic compensation, appropriate control strategies can be employed to coordinate and ensure the safe and stable operation of the system.
3 System Control
Figure 2 shows the control structure block diagram of a photovoltaic grid-connected system with APF (Active Power Factor) function. For the photovoltaic grid-connected system, the dual closed-loop control based on coordinate transformation theory includes an outer voltage loop and an inner current loop. Ensuring the stability of the DC bus voltage is crucial, as its stability determines the normal operation of the converter. The outer voltage loop regulates the output to generate the reference amplitude for the inner current loop. The inner current loop tracks the grid-connected current output of the inverter, ensuring the grid current is close to a sine wave, thus meeting the grid-connection standards for harmonic content and power factor. The grid-connection command current is a superposition of the photovoltaic grid-connection active power command signal and the harmonic compensation command signal. The control system can simultaneously achieve both photovoltaic grid-connection and active filtering functions.
Figure 2 Control diagram of a photovoltaic grid-connected system with APF function
3.1 Harmonic Current Detection
Figure 3. Detection process of power grid harmonic current using the ip-iq method.
3.2 Method for Synthesizing Current under Grid Connection Command
When a photovoltaic (PV) grid-connected system is operating, the active power output of solar energy varies due to weather and environmental factors. By incorporating active power filtering (APF) functionality, it can compensate for harmonics in nonlinear loads. When the system needs to simultaneously perform PV active power grid connection and harmonic compensation, the grid-connected current must be synthesized into a grid-connected command current. As shown in Figure 2, a PV grid-connected system with APF detects harmonic currents and synthesizes them with the active power command current to obtain the grid-connected command current signal. This signal controls the PV grid-connected system to simultaneously achieve the dual functions of active power grid connection and harmonic compensation. The system unifies the coordinate system and phase information of the harmonic compensation command signal and the active power command signal, then directly adds them to obtain the grid-connected current command signal. Alternatively, the active power command signal and the compensation signal can be synthesized directly in the harmonic detection module, as shown by the dashed line in Figure 3. The active power current is a sinusoidal signal synchronized with the grid voltage. Multiplying the signal obtained from the voltage regulator by a three-phase sinusoidal signal yields the active power command signal:
The synthesized grid connection command current signal is:
The grid connection command signal of a photovoltaic grid-connected system with an APF (Active Power Factor) is a superimposed signal composed of photovoltaic active current and harmonic compensation current. Its relationship can be simply expressed as follows: A current limiter is added to the grid connection command current synthesis stage, and a capacity limitation strategy is used to control the grid connection current. A compensation coefficient k is introduced into the current limiter, and the compensation coefficient k is adjusted according to changes in solar active power or harmonic compensation, thereby ensuring that the grid connection current does not exceed its rated range. The purpose of the current limiter is to adjust the magnitude of the grid connection current using the compensation coefficient, ensuring that its peak value is less than the maximum current allowed to pass through the switching components, and its effective value is less than the rated current. By obtaining the value of the compensation coefficient through the current limiter, the expression for the system's control of the grid connection current using the capacity limitation strategy can be obtained:
3.3 Grid Connection Control
The grid voltage-oriented vector control (PEVDC) employs a dual-closed-loop cascaded control structure: an outer voltage loop and an inner current loop. The primary function of the voltage loop is to control the DC bus voltage; the current loop controls the AC input current based on the current commands provided by the voltage loop, achieving unity power factor operation. Specifically, the PEVDC algorithm first transforms the grid voltage vector from a three-phase stationary coordinate system to a two-phase stationary vertical coordinate system, and then from the two-phase stationary vertical coordinate system to a two-phase synchronous rotating coordinate system. The d-axis of the synchronous rotating coordinate system is oriented according to the grid voltage vector E. The d-axis component of the AC current vector i on the grid-connected converter side is the active current, and the q-axis component is the reactive current. Setting the reference value of this reactive current component to 0 achieves unity power factor operation on the grid side.
The dq components of the AC current in the grid-side converter are mutually coupled, making it difficult to control the dq-axis components of the grid-side current vector independently. Therefore, a feedforward decoupling control strategy is adopted. As shown in Figure 4, the grid-side current dq components are symmetrical, and the controller can use the same parameters; therefore, the controller design can consider only one component. The output of the PI regulator compensates for the voltage drop across the AC-side inductor. The controller uses decoupling terms for the current dq components to cancel the cross-coupling terms of the two components in the actual system, and the feedforward component of the grid voltage cancels the influence of the grid voltage in the actual system.
Figure 4 is a block diagram of the current loop control structure of a photovoltaic grid-connected converter.
The decoupled control structure block diagram is shown in Figure 5. The controlled object is now simplified to an AC-side inductor, and the control quantity is the current flowing through the inductor. The PI controller output is the voltage applied across the inductor; the value of ...
Figure 5 is a simplified block diagram of the current loop control structure of a photovoltaic grid-connected converter.
The reference signal for the current loop consists of two parts: the output of the voltage loop PI controller processed by the MPPT algorithm and the feedforward signal. Initially, the bus voltage is low, and the input deviation of the voltage PI controller is relatively large, with its output accounting for the majority of the current loop reference. Once the voltage PI controller output reaches its limit, the current loop reference continues to increase due to the increase in the feedforward signal as the bus voltage rises, eventually reaching the current loop reference limit. When the system enters steady state, the PI controller output is almost zero, and the feedforward signal component serves as the reference for the current loop. This gives the system excellent immunity to disturbances and grid voltage fluctuations.
4. System Simulation and Analysis
When a system needs to perform both photovoltaic active power grid connection and harmonic compensation generated by nonlinear loads, it needs to achieve the dual functions of photovoltaic grid connection and APF.
Figure 6 Simulation waveforms of photovoltaic grid connection and harmonic compensation
Figure 6 shows the waveforms when the system simultaneously performs photovoltaic grid connection and harmonic compensation. At this time, the waveform i*c of the grid connection current command signal is between a sine wave and the harmonic compensation signal, indicating that the command signal contains both active power components and harmonic compensation signals. The grid voltage and grid current are out of phase and their waveforms are almost sinusoidal, indicating that the grid-connected inverter is injecting photovoltaic active power into the grid while simultaneously compensating for harmonic currents generated by nonlinear loads. This verifies that a photovoltaic grid-connected system with APF function can simultaneously achieve the dual functions of active power grid connection and harmonic compensation.
5. Conclusion
The shortage of traditional energy sources has made the development of new energy sources urgent, with solar energy becoming a new research hotspot due to its green and pollution-free characteristics. Meanwhile, the widespread use of nonlinear loads has led to severe harmonic pollution in power systems. Active power filters (APFs) can effectively suppress power harmonics and are a promising means of solving harmonic pollution. This paper analyzes a photovoltaic grid-connected system and an APF, studying a photovoltaic grid-connected system with APF functionality. Addressing the current problems of low equipment utilization and severe harmonic pollution in photovoltaic grid-connected systems, the feasibility and significance of an integrated design for unified control of the photovoltaic grid-connected system and APF are explored, enabling the system to not only generate photovoltaic power but also compensate for system harmonics. System simulation studies were conducted on the photovoltaic grid-connected system with APF functionality, verifying the feasibility and correctness of the system and control strategy.
