According to the requirement of grid-connected, the magnitude and phase of the output voltage at the end of the line must be consistent with the magnitude and phase of the large grid voltage in the process of grid-connected photovoltaic microgrid. When the output voltage of photovoltaic micro-grid does not meet the grid-connected conditions, UPFC can be used to compensate the output voltage of the micro-grid to achieve the goal of grid-connected. Nowadays, in the process of construction and development of smart grid, more and more new energy generation will be connected to large power grid by distributed way, while large power grid is directly oriented to power terminal users, so the demand for voltage quality is higher. Therefore, a new energy distributed grid-connected flexible interface device for distribution network is developed.
It has far-reaching research value [1]. As the most flexible and powerful device in the family of flexible transmission devices, UPFC is used to realize flexible transmission control of photovoltaic micro-grid interconnection, which is more convenient and has lower voltage requirements for the output of micro-grid [2]. Therefore, a unified power flow controller (UPFC) based grid-connected method for photovoltaic power generation system is proposed in this paper.
Unified Power Flow Controller (UPFC) consists of two voltage source converters (VSC1 and VSC2) and a DC capacitor separating them. VSC1 is a parallel-side converter. Its main functions are to stabilize the DC-side capacitor voltage and to control the AC bus voltage at the input end of UPFC. The schematic diagram of UPFC is shown in Figure 1. U1 and U2 are the node voltage of the large grid side and the terminal voltage of the microgrid line respectively. idc2 is the current input to VSC2, and U is the voltage input to VSC1. By adjusting the phase difference between the input voltage Ush and U1, VSC1 can achieve the balance of active power between VSC1 and VSC2, thus maintaining the stability of DC side voltage. Formula (1) and Formula (2) show that by controlling the amplitude of SH and the phase pecking of SH relative to bus voltage 1, the active and reactive power exchange between VSC1 and the system can be adjusted, so as to maintain the stability of capacitive voltage on the DC side and control the bus voltage of the system. According to the condition of grid connection, the sum of voltage se vector injected by output voltage 2 of microgrid and series converter to line is equal to voltage 1 of large grid.
The magnitude and phase of the output voltage 2 of the microgrid may be inconsistent with the magnitude and phase of the large grid 1. This requires se to adjust its amplitude and phase. Firstly, the realization process of phase shifting is analyzed. When? Peck 1 > 0, advance phase shifting, then use = 2sinU2,? Zse=? Z1 , when the opposite is lagging phase shifting, then use = – 2sinU2,? Zse=? Z1 – ( ). From the above analysis, it can be concluded that the magnitude and phase of Se compensated by the series converter to the end of the microgrid can be determined according to the output voltage 2 of the microgrid. According to the operation characteristics of UPFC, the functions of two converters in UPFC are different. Double-loop decoupling control strategy is adopted for VSC1 of parallel converter and cross-decoupling control strategy is adopted for VSC2 of series converter. Figure 2 shows the control chart of the parallel side. In the figure I is the reference value of the current active component; I is the reference value of the current reactive component; I and I are the actual value of the current active component and the reactive component. Fig. 3 shows that Ush passes through PLL, obtains its phase angle and then coordinate transformation, obtains the components Ushd and Ushq of voltage in dq coordinate system. By the same method, the actual value I of the active component and Ishq of the reactive component of the current are obtained. By comparing the given value I of the active current component with the actual value i, the input signal with the most difference PI is obtained, and then the voltage U SHD is added to get the U’shd through comprehensive action. The process of obtaining U’shq is similar. The input signal of SVPWM is obtained by Park-1 transformation, and the functions of stabilizing capacitor terminal voltage and controlling bus voltage are realized. The control chart of the series side is shown in Figure 3. I is the line current; Used is the compensation voltage; Used and Useq are the DQ axis components of the compensation voltage; PL and QL are the active and reactive power of the line; P and Q are the indicative values of the active and reactive power of the line respectively. The difference signal is used as the input signal of PI. The difference signal between the actual value and the instruction value of the d-axis component of the compensation voltage is added to the PI controller. Then the control quantity U’sed of the reactive power is obtained by double-loop decoupling. Similarly, the active power control quantity U’seq can be obtained. Thus the functions of voltage compensation and power flow regulation of VSC2 are realized. In order to verify the effectiveness of the control strategy, the simulation model of UPFC is built by using MATLAB/SIMULINK. In order to save simulation time, the micro power supply is equivalent to the voltage source. The simulation parameters are as follows: the maximum terminal voltage of large power grid is 35 kV, the maximum terminal voltage of photovoltaic microgrid is 28 kV, capacitance C = 5 mF, Lsh = 60 mH, Lr = 500 mH. The simulation results are as follows. Figure 4 shows the A-phase voltage and line A-phase current at the end of the microgrid. From the figure, it can be concluded that the voltage amplitude before 0.3s is 28KV, and after 0.4s is 35KV, which is the same as the voltage amplitude at the end of the large grid, and the grid connection is realized. Fig. 5 shows the magnitude of the compensation voltage Used, which stabilizes at 7000V after 0.4s. According to the given magnitude difference between the large and micro grids, it can be seen that the magnitude difference is 7000V. Fig. 6 shows the exchange amount between the series converter and the active power of the system. The compensated active power is negative.
VSC2 absorbs active power from the system.
The capacitor charges and stores energy. The DC capacitor voltage will rise slightly. Figure 7 shows the active power exchanged between shunt converter and line. By comparing with Figure 6, it can be seen that the active power absorbed and injected by UPFC from the system is balanced. Without considering the loss of UPFC itself, the power absorbed and injected from the circuit is equal. In this paper, a method of using UPFC to connect to the grid is proposed. The integrated control function of UPFC is used to compensate the voltage at the end of the microgrid line, so that it can meet the conditions of connecting to the grid. Through simulation analysis, it can be concluded that the system is relatively stable in the process of grid-connected, thermostatic element and can achieve smooth grid-connected.