In civil buildings, combining photovoltaic modules with buildings and utilizing surface space for power generation has broad development prospects. Photovoltaic power generation in civil buildings has the characteristics of “spontaneous self use and grid connection of surplus electricity”. Roof solar inverters are often connected in cluster form in parallel at the point of common coupling (PCC) and then connected to the grid to improve system capacity and power generation efficiency. However, solar inverters often use LCL filters, which have inherent resonance. In addition, the impedance coupling between the solar inverter cluster and the power grid in weak current networks can cause resonance, which can affect system stability and power quality. Therefore, the research on the resonant control method of solar inverters in civil buildings is of great engineering practical significance.
Unlike a single solar inverter, the resonance mechanism of a solar inverter cluster connected to the grid is more complex. It is pointed out that there are two resonance points in the solar inverter cluster grid connected system under weak current grid conditions: the inherent resonance of the LCL filter and the external coupling resonance between the solar inverter cluster and the grid side impedance. A suppression strategy combining state variable feedback active damping method and PCC point passive damping circuit is proposed for solar inverter cluster resonance, but adding hardware circuit will result in additional power loss. To solve the problem of power loss caused by hardware circuits, a virtual admittance strategy based on PCC point parallel connection is proposed to address the resonance caused by impedance coupling between solar inverter clusters and the power grid. However, no specific parameter design method for virtual admittance is provided.
On the basis of previous research, the solar inverter cluster in civil buildings is taken as the research object. Firstly, a topology structure is established, and the resonance mechanism and characteristics of the solar inverter cluster are derived based on a single solar inverter; Secondly, a hierarchical coordinated control strategy is proposed to address the resonance generated by the grid connection of solar inverter clusters; Finally, a model was built using simulation software MATLAB/Simulink platform to verify the effectiveness and correctness of the proposed control method.
1. Grid connected topology structure of solar inverter cluster
The solar inverter cluster system in civil buildings consists of a single solar inverter in parallel. The output current of each solar inverter is collected through a common grid connection point, and flows into the large power grid after passing through the grid impedance. The topology structure of the solar inverter cluster connected to the grid in civil buildings is shown in Figure 1.
In Figure 1, each photovoltaic grid connected system consists of four main components: photovoltaic panels, solar inverters, LCL filters, and the power grid. Among them, PVn (n=1, 2, 3,…, N, N is the number of parallel solar inverters, the same below) is the rooftop photovoltaic panel; Un is the output voltage of the solar inverter; L1n and L2n are the filtering inductance on the solar inverter side and the filtering inductance on the grid side, respectively; Ci is the filtering capacitor; Uc is the voltage across the filtering capacitor; IL1n is the current flowing through the inductor on the side of the solar inverter; Gn is the grid connected current of the i-th solar inverter; Upcc is the PCC point voltage of the solar inverter cluster grid connected system; Lg is the equivalent inductance on the grid side. As the resistive component in the grid impedance is beneficial for system stability, to demonstrate the feasibility of the proposed strategy in the worst-case scenario, only the inductive component of the grid, i.e. Zg=sLg, is considered; IG and ug represent the incoming current and grid voltage.
2. Resonance mechanism and characteristics of solar inverters
2.1 Resonance mechanism and characteristics of a single solar inverter
Based on a single LCL type photovoltaic grid connected solar inverter, the grid connected current feedback control block diagram is shown in Figure 2, where i1 is the grid connected system current reference value; IG1 is the grid connected current of the first solar inverter; Gi (s) is the current controller, and KPWM is the transfer function of the modulated wave to the voltage on the solar inverter side. The expressions for G1 (s), Gc (s), and G2 (s) corresponding to Figure 2 are:
According to Figure 2, the transfer function from the output voltage Ui of a single solar inverter to the grid connected current ig1 can be calculated as:
According to the characteristic equation of the formula, the system has an inherent harmonic characteristic, and the resonant frequency is:
There is a resonant peak at f1, which generates a pair of right half plane closed-loop poles, causing oscillations in the grid connected current and affecting system stability to a certain extent. When the parameters of the LCL filter are fixed, the resonance point generally does not change.
According to Figure 2, the Norton equivalent circuit of each LCL type grid connected solar inverter is shown in Figure 3, which is composed of a controlled source G1i1 and an output admittance Y1 in parallel, and then connected in series with the power grid. The corresponding expressions are:
2.2 Mechanism and characteristics of grid connected resonance of solar inverter clusters
Figure 4 shows the Norton equivalent circuit of N solar inverter clusters when connected to the grid. According to Kirchhoff’s voltage and current law, the output current of the first grid connected solar inverter can be calculated as:
In the case of weak power grid, according to the formula, there is a coupling relationship between the output current of any solar inverter and three parts: the first part is the instruction current of the solar inverter itself, and the strength of the coupling relationship is represented by K (s); The second part is the command current of 2-N other solar inverters, and the coupling strength is represented by H (s); The third part is the grid voltage, and the strength of the coupling relationship is represented by F (s).
The specific expression is as follows:
Assuming that the parameters and control methods of each solar inverter are the same, but the command current of each solar inverter is different, that is, the solar inverter is not in synchronous operation. Based on the instruction current coupling relationship of the solar inverter itself, taking the first grid connected solar inverter as an example, the transfer function from grid connected current ig1 to the output voltage U1 on the solar inverter side can be calculated as:
According to the formula, it can be calculated that there are two resonance frequency points in the solar inverter cluster grid connected system, as shown in the formula:
In the formula, f1 is the inherent resonant frequency of the LCL solar inverter itself; FN is the resonant frequency generated by the impedance coupling between the solar inverter cluster and the power grid. Figure 5 shows the relationship curve between the resonant frequency of a solar inverter cluster and the number of solar inverters. It can be seen that when there is impedance coupling in the power grid, as the number of parallel solar inverters increases, fN gradually shifts towards low frequencies and tends to a certain value, increasing the risk of resonance in the system; And f1 does not change.