The high-power grid-connected inverter consists of a power switch inverter bridge, DC and AC EMC filters, current sensors, grid-connected converters, AC and DC filters, and multiple control switches.
The system hardware circuit adopts the design method of integrating IGBT module, DC energy storage capacitor, absorption capacitor, drive unit, control unit, sensor and other main circuit components into one module. The module has the advantages of independent function, compact structure and convenient power expansion. On this basis, only need to connect circuit breakers, contactors, filter inductors and capacitors, transformers, control power supply, pre-charging unit, touch screen, cooling fan and other peripheral circuit devices, you can design a complete hardware circuit system.
The laboratory has built two three-phase grid-connected platforms, one is a two-level power module that uses Mitsubishi’s IPM, and the main control chip TMS320F2406 uses hardware phase lock. The other is a three-level power module that uses Infineon’s flat-shaped 75AIGBT module, the main control chip is TMS320F28335, and software phase lock is used.
(1) Sampling circuit design
The voltage parameters that need to be collected are grid voltage, inverter output voltage, and DC bus voltage. The sampling circuit is the same as that of a string-type single-phase grid-connected inverter. The only difference is that the number of samples is different. In the string-type single-phase grid-connected inverter, the number of collected AC voltage and AC current is one channel respectively. In the string-type three-phase grid-connected inverter, the number of collected AC voltage and AC current are respectively Two channels and the third channel are calculated based on the vector sum of the three-phase AC voltage and AC current as zero.
(2) Drive circuit design
The IGBT selected is Infineon’s FF200R12KT4, and the absorption capacitance parameter is 3μF/1200V DC. After analysis and selection, the IGBT drive module uses TX-DA962D6. This drive module has fault alarms, supports multiple input signal levels, and is unified The advantages of output enable terminal control, input power polarity protection, etc. TXDA962D6 drive module is equipped with a DC/DC power supply PD203 for every two drives, n channels of DC/DC power input are combined together, and the input DC voltage range is 12~ 30v, this experimental system uses +15V DC power supply. The ENA end of the input signal connection socket is the input signal enable end.
(3) Analysis of experimental results
The experiment parameters are as follows: DC side capacitance 2200μF/900v; filter inductance 3mH/35A.
The control parameters are as follows. dq axis PI regulator parameters: Kpd=Kpq=0.05, Kid=Kiq=8: MPPT link PI regulator parameters: Kpmppt=10.Kimppt=8.
The three-phase output voltage and current waveforms and the harmonic analysis of the current Ia when connected to the grid are shown in Figure 1 and Figure 2. When the photovoltaic grid-connected system is operating normally, the current output by the inverter is at the same frequency and phase as the grid voltage, and the power factor is 1.
When the output power is 15kw, the measured THID values of the three-phase currents Ia., Ib, and Ic are 2.57%, 2.59%, and 2.56% respectively, and the total distortion rate of the three-phase current is below 3%, which meets the national standards for integration into the power grid. . Experiments show that the total current distortion rate tends to decrease when the output power increases.
In Figure 3, signal channel 2 is the AC component of the midpoint potential difference. In the steady state, the fluctuation amplitude of the midpoint potential does not exceed 5V. It can be seen that the DC component of the midpoint voltage is very good after the midpoint potential balance control strategy is adopted. The suppression and the dynamic performance of the exchange have been well improved.
