200W Solar Photovoltaic Grid-connected Inverter Control Design

The whole process of designing a 200W solar photovoltaic grid-connected inverter can directly convert the DC power generated by the solar panel into 220V/50Hz industrial frequency sinusoidal AC power and output it to the grid.

　　A control system for a low-power photovoltaic grid-connected inverter: The topology of the DC/DC controller adopts a push-pull circuit, which is controlled by the chip SG3525. The circuit effectively prevents magnetic bias. The DC/AC inverter is a full-bridge inverter circuit, which is controlled by DSP. Since the DSP has a relatively high computing speed, the output current of the inverter can track the grid voltage waveform well. The effectiveness of the photovoltaic grid-connected inverter control scheme has been verified in the laboratory. The control system can ensure that the output power factor of the inverter power supply is close to 1 and the output current is a sine waveform.

　　System working principle and control scheme

　　1 Principle of photovoltaic grid-connected inverter circuit

　　The main circuit schematic diagram of the solar photovoltaic grid-connected inverter is shown in Figure 1. In this system, the rated voltage of the solar panel output is 62V DC, which is converted into 400V DC through the DC/DC converter, and then 220V/50Hz AC is obtained after DC/AC inversion. The system ensures that the 220V/50Hz sinusoidal current output by the grid-connected inverter is synchronized with the phase voltage of the power grid.

　　Figure 1 Circuit block diagram

　　2 System Control Scheme

　　Figure 2 is the main circuit topology of the photovoltaic grid-connected inverter. This system consists of a front-stage DC/DC converter and a rear-stage DC/AC inverter. The inverter circuit of the DC/DC converter can be selected in half-bridge, full-bridge, and push-pull types. Considering the low input voltage, if the half-bridge type is used, the switch tube current will increase, while the full-bridge type will make the control complex and the switch tube power consumption increase, so a push-pull circuit is used here. The DC/DC converter consists of a push-pull inverter circuit, a high-frequency transformer, a rectifier circuit, and a filter inductor. It converts the 62V DC voltage output by the solar panel into a 400V DC voltage.

　　Figure 2 Main circuit topology

　　The main circuit of the DC/AC inverter adopts a full-bridge structure, consisting of 4 MOS tubes (with anti-parallel parasitic diodes inside the tube), which converts 400V DC power into 220V/50Hz industrial frequency AC power.

　　2.1 DC/DC Converter Control Scheme

　　Figure 3 Control block diagram of DC/DC converter

　　The control block diagram of the DC/DC converter is shown in Figure 3. The control circuit is based on the integrated circuit SG3525. The two 50kHz drive signals output by SG3525 are added to the gates of the push-pull circuit switch tubes Q1 and Q2 through the gate drive circuit. In order to maintain the stability of the output voltage of the DC/DC converter, the detected output voltage is compared with the command voltage. The error voltage controls the duty cycle of the SG3525 output drive signal after passing through the PI regulator. The control circuit also has the protection function of limiting output overcurrent and overvoltage. When it is detected that the output current of the DC/DC converter is too large, SG3525 will reduce the width of the gate pulse, reduce the output voltage, and thus reduce the output current. When the output voltage is too high, the DC/DC converter will stop working. Since the push-pull circuit is prone to transformer saturation due to DC bias magnetism, the difficulty in designing the push-pull circuit lies in how to prevent the magnetic saturation of the transformer. In this circuit, in addition to paying attention to the symmetry of the circuit, a magnetic saturation detection circuit is also designed. When the current flowing through the two branches of the push-pull circuit is unbalanced, the soft start function of SG3525 will be activated, the DC/DC converter will be restarted, and the transformer will be reset.

　　Figure 4 Bias detection circuit

　　The bias magnetization detection circuit is shown in Figure 4. Only the secondary side of the magnetic ring is drawn in the figure. The two coils on the primary side are connected to the two windings on the primary side of the transformer of the main circuit, and the current flowing through the two coils should be in opposite directions. When the transformer is biased, the current in one direction is abnormally large. Through the detection of the current transformer, a voltage can be generated on the output resistor R1 of the transformer. If the voltage is large enough, the voltage-stabilizing diode D5 can be turned on, and a voltage drop can be generated on the potentiometer. The value of the potentiometer is adjusted to a suitable resistance value, so that the voltage drop on the potentiometer is greater than the threshold voltage of the transistor, so that the transistor is turned on, and the capacitor connected between pin 8 of the chip SG3525 and the ground is discharged, and then the constant current source in the SG3525 charges it, and the SG3525 is restarted, thereby resetting the magnetic core of the transformer.

　　2.2 DC/AC Inverter Control Scheme

　　The DC/AC inverter is the key and difficulty of photovoltaic grid connection, so the following will focus on this part. The DC/AC inverter control block diagram is shown in Figure 5. The core control chip uses TI's TMS320F240. Although the single-chip microcomputer can also realize the pulse width modulation of the grid-connected inverter, the DSP has a stronger real-time processing capability, so it can ensure that the system has a higher switching frequency. From Figure 5, we can clearly see the input and output signals of the system.

　　Figure 5 Control block diagram of DC/AC inverter

　　3 Output power optimization control scheme

　　In static conditions, when the grid-connected inverter is connected to the solar cell, the grid-connected inverter can be equivalent to the load resistance of the solar cell. When the light intensity λ and temperature T change, the terminal voltage output by the solar cell will change accordingly. In order to effectively utilize solar energy, the output of the solar cell should always be at an appropriate operating point. Therefore, the control scheme requires that when the voltage of the solar cell increases, its output power can be increased; otherwise, its output power should be reduced.

　　Figure 6 DSP control scheme

　　The control scheme of DSP is shown in Figure 6. After the reference voltage is compared with the actual voltage of the solar cell, the error is adjusted by PI. The current command (DC value) IREF is multiplied by the sine table value in ROM to obtain the alternating output current command iref. After comparing it with the actual output current value, the error is passed through the proportional (P) link, and the obtained command is inverted. After adding it to the collected AC side voltage Us, the obtained waveform is compared with the triangular wave to generate 4-way PWM modulation signals (the frequency of the triangular wave is 20kHz).

　　4 Detection of AC side voltage Us

　　By filtering and rectifying the synchronous signal of the secondary side of the synchronous transformer, a relatively stable DC power can be obtained and sent to the A/D conversion port of the DSP. Since the final DC voltage has a relatively stable relationship with the grid voltage, it is relatively easy to convert the value of Us.

　　Figure 7 Rectification circuit of Us

　　Because of the common ground problem, a full-wave precision rectifier circuit of an operational amplifier is used, as shown in Figure 7.

　　5. Synchronization of current command

　　When connected to the grid, the inverter outputs a sine wave current with the same frequency and phase as the grid voltage. First, the grid voltage signal is filtered and shaped into a synchronous square wave signal, and then input into the external interrupt port XINT1 of TMS320F240 in order to capture the zero-crossing signal of the grid voltage. As shown in Figure 8, the grid voltage sine wave is shaped into a square wave.

　　Figure 8 Synchronous signal waveform

　　When the DSP detects the rising edge of the zero-crossing signal, it triggers a synchronous interrupt, and uses this time point as a reference to set the starting point of the sine wave signal, that is, the sine table pointer is reset to zero; whenever T1 underflows (PWM real-time control), the sine table pointer is incremented by 1 and takes a value from the sine table. The unit sine wave data of one cycle is divided into 400 points and stored in the memory in the form of a table. Since the synchronous signal is more susceptible to interference from harmonics and spike voltages, a delay can be made after entering the synchronous interrupt to determine whether the external interrupt pin XINT1 is still at a high level. If it is high, the interrupt program is executed, otherwise it jumps out of the interrupt program.

　　From the control scheme of Figure 6, it can be seen that after IREF is multiplied by the data in the sine table, a current given signal of a sine wave with adjustable amplitude is formed. Then, the current given value is compared in real time. After passing through the P link, the obtained signal is inverted and added to the collected AC side grid voltage signal Us. The obtained waveform is compared with the triangular wave to generate a PWM wave to control the on and off of the bridge arm. In short, the requirements of the same frequency and phase of the output current and the grid voltage are achieved through current tracking control.

　　6 Generation of PWM pulse width modulation wave

　　The generation of PWM wave is output by the full comparison unit of TMS320F240, with a frequency of 20kHz. As shown in Figure 6, the generation of modulated pulse is obtained by comparing the current command value with the actual current value, passing through the P link, and comparing the waveform obtained with the triangular wave (frequency of 20kHz). Therefore, the generation time of the pulse of MOS tubes Q3, Q4, Q5, and Q6 (see Figure 2) can be obtained from Figure 8. Referring to the modulation of sine wave and triangular wave, the intersection of the two determines the pulse time of PWM. The waveform actually sampled (actually a step wave) intersects with the triangular wave, and the pulse width is obtained from the intersection. This system samples the waveform at the bottom point of the triangular wave to form a step wave. The pulse width determined by the intersection of the step wave and the triangular wave is symmetrical within a sampling cycle, as shown in Figure 9.

　　Figure 9 Sine pulse width modulation waveform

　　Figure 9 (a) The intersection of sine wave B and the triangle wave determines the turn-on time of Q3; the intersection of sine wave A and the triangle wave determines the turn-on time of Q5.

　　FIG9( b ) is a pulse diagram of Q3 . The pulses of Q3 and Q4 on the same bridge arm are complementary.

　　FIG9( c ) is a pulse diagram of Q5 . The pulses of Q5 and Q6 on the same bridge arm are complementary.

　　7 TMS320F240 software control flow

　　Figure 10 Software Flowchart

　　This part of the software is mainly divided into 4 parts, namely the main program, T1 underflow interrupt, T2 underflow interrupt and synchronous interrupt. The flow chart is shown in Figure 10. T1 underflow interrupt occurs once every 50μs, and the program is mainly used to generate PWM waves; T2 underflow interrupt occurs once every 10ms, and the program is mainly used to generate current instructions; synchronous interrupt occurs approximately once every 20ms (grid voltage cycle).

　　8 System Protection

　　This system is designed with multiple protections such as overvoltage and undervoltage on the DC side, overcurrent and overheating on the AC side. When the output voltage of the solar panel is overvoltage or undervoltage, TMS320F240 sends a signal to SG3525 to block the pulse of DC/DC and stop it from working. When the DC voltage is detected to be back to normal, DC/DC automatically resets and starts working; when AC overcurrent or overheating occurs, the program enters the interrupt service subroutine to block all drive signals. When the fault is eliminated, manually reset and the system restarts.

　　9. Selection of main components

　　The push-pull circuit MOS tube uses IRFP350 (withstand voltage 400V, drain-source rated current 16A). The bridge inverter circuit MOS tube uses IRFPC40 (withstand voltage 600V, drain-source rated current 6.8A). The DC/DC filter inductor L1 uses 1.2mH, and the DC/AC filter inductor L2 uses 33.4mH.