Research and design of household inverter power supply system

1 Introduction

The Northwest region of my country has a vast territory and abundant solar and wind energy resources, with an average annual solar radiation intensity of 6000-8400MJ/m2, an average annual solar illumination time of 3000-3200h, and an average wind force of 5-6. The economy in remote areas of the Northwest is underdeveloped, and the residents are very scattered. If these users are provided with city electricity, the cost is too high. Therefore, how to reasonably utilize the existing resources - solar and wind energy has become an effective way to solve these problems.

2 Wind and solar complementary household power supply system

The structural block diagram of the system is shown in Figure 1.

This system can use solar energy and wind energy to charge the battery, convert natural energy into chemical energy and store it in the battery, and then invert the chemical energy into 220V AC power for users; it can also directly invert solar energy and wind energy into 220V AC power for users.

3 System Hardware Circuit

The hardware circuit of this system mainly includes main circuit, isolation and drive circuit and control circuit.

3.1 Main circuit

The topology of the main circuit is shown in Figure 2. As shown in Figure 2, the main circuit mainly includes the battery overcharge protection circuit and the inverter circuit. In the figure, uFP represents the rectified fan output voltage, uSP represents the solar cell output voltage, K is the electromagnetic relay, and GB is the battery pack with a rated voltage of 24V.

3.1.1 Working principle of overcharge protection circuit

When the battery voltage is too high, the voltage at point A will be greater than the reference voltage value Uref (=2.5V) of TL431 , so that TL431 is turned on, point B is clamped to a low level, V1 is turned off, point C is a high level, V3 is turned on, V2 is turned off, point D is a high level, at this time VT14 and VT15 are both turned on, and relay K is activated. According to the characteristics of solar cells and wind turbines, the output voltage of the solar cell is directly short-circuited, and the output voltage of the wind turbine is discharged through the high-power unloading resistor R9; on the contrary, when the battery voltage is too low, VT14 and VT15 are both turned off, and the output voltage of the solar cell and wind turbine charges the battery.

3.1.2 Inverter circuit

A single-phase full-bridge inverter circuit is used, and power MOSFET is used as the switching device of the inverter circuit. Power MOSFET is a multi-conducting unipolar voltage-controlled device with the advantages of fast switching action, large input impedance, small driving power, no secondary breakdown, simple driving circuit, and large safe working area. In particular, due to its positive temperature coefficient, it can automatically balance the current, so in the inverter power supply system with low input voltage and large working current, several power MOSFETs can be connected in parallel to increase the current capacity. In this system, three power MOSFETs are connected in parallel to increase the current capacity by three times. The inverter converts the rectified DC voltage into an SPWM wave of a specific frequency, and then converts it into a standard 220V sinusoidal voltage through inductor and capacitor filtering. The inductor is replaced by the leakage inductance of the secondary of the transformer. This method makes the system structure simple, the noise is low, and the high-order harmonic components in the waveform can be effectively suppressed.

The SPWM control method pre-tables the sine values ​​of 0 to 360 degrees in the EPROM. Since the switch drive signal is generated by comparing a sine wave reference signal with a triangular carrier signal, it is often divided into two cases: unipolar and bipolar. Under the same switching frequency, since the sine wave generated by the bipolar SPWM control has greater harmonic content and switching loss than the unipolar one, this system uses unipolar SPWM control.

3.2 System Isolation and Drive Module

The isolation and driving circuit isolates and amplifies the SPWM signal output by the Intel80C196MC chip to form a circuit that drives the switch action signal of each power device. This system uses the gate isolation driver chip TLP250 produced by Toshiba, which is specially used to drive power MOSFET and IGBT. Its structural block diagram is shown in Figure 3. It is an optocoupler device, but it is different from ordinary optocouplers. Since its output stage is amplified and output by a push-pull circuit, it can not only isolate the primary and secondary sides, but also has driving capabilities, which is particularly suitable for driving medium-power MOSFET and IGBT. At the same time, in engineering applications, in order to reliably prevent the two power devices on the same bridge arm from passing up and down from the hardware, the pins 2 and 3 of the two TLP250s driving the same bridge arm power devices are connected to each other to form an interlocking circuit, thereby effectively preventing the direct fault of the bridge arm power device. The specific circuit is shown in Figure 4.

3.3 Control circuit and control chip

The control circuit mainly detects DC current, DC voltage, AC current, AC voltage and other signals to achieve the system's overvoltage, undervoltage, overcurrent, overdischarge, overheating and inverse time protection functions. The control chip uses Intel80C196MC microprocessor.

Intel80C196MC is a true 16-bit microcontroller launched by Intel in 1992. Since a distinctive waveform generator (WG) unit is integrated in this chip, the software and external hardware circuits used to generate SPWM waveforms are greatly simplified. The waveform generator has three independent modules, each of which contains a numerical comparator, a comparison register, a comparison buffer, a signal-free time generator, and a pair of programmable output drive channels. The three-phase waveform has a common carrier frequency and a common dead time, and can be programmed as a triangle wave modulation mode or a sawtooth wave modulation mode. Once started, it only requires intervention when changing the PWM duty cycle, and the rest of the time does not occupy the CPU.

The waveform generator consists of a time base generator, a phase drive channel and a control and protection circuit.

The time base generator establishes the carrier period for the PWM waveform. The 80C196MC determines the length of the carrier period by reading data from the reload register (WG-RELOAD), so the user can change the carrier period value by changing the value of the reload register in the program.

The phase drive channel determines the duty cycle of the PWM waveform. Each phase drive channel has its own phase comparison buffer register (WG-COMPX). Generally, the duty cycle of the PWM waveform is determined by three aspects: the working mode, the reload register and the phase comparison buffer register.

The control circuit includes a control register (WG-CONTROL) and an output register (WG-OUT). At the same time, there is a protection circuit inside the CPU to monitor the EXTINT input terminal in order to handle abnormal situations.

In addition, the no-signal time generator circuit is a very important function of the waveform generator. It can be used to prevent a pair of complementary PWM signals from being valid at the same time, thereby avoiding the direct pass of the upper and lower power tubes of the same bridge arm. At the same time, the user can set the no-signal time arbitrarily by loading a number into the lower 10 bits of the WG-CON register through software.

4 System Control Principle

The system control method adopts a composite control method combining current feedback, voltage feedforward and voltage feedback. The current feedback and voltage feedback use digital PI regulators to achieve a steady-state output without static error. PI control is to form a control quantity by linearly combining the proportion (P) and integral (I) of the sampling time deviation to control the controlled object. The transfer function of the PI controller is:

In the formula: Kp——proportional coefficient;

TI——Integral time constant.

The current feedback is completed when the WG is interrupted. Since the current regulation time is extremely short, the dynamic response speed of the system is greatly improved and the overshoot of the system is effectively suppressed. The current regulation time t is approximately as follows:

t = (20/196) × 5 × 8ms ≈ 4ms

It can be seen from this that the system output voltage can return to normal in less than 1/4 of the fundamental wave period.

5 System Software Design

The system software mainly includes the main program, WG interrupt program, PI adjustment subroutine, etc. The main program tasks are mainly initialization, fault judgment, operation signal judgment and waiting for interruption, etc. The main program block diagram is shown in Figure 5.

6 Conclusion

The 1kW prototype designed according to the above control concept has been tested to have a whole machine efficiency ≥ 85%, an output voltage of 220 (1 ± 4%) V, an output voltage frequency of 50 (1 ± 0.5%) Hz, and the system has perfect protection functions such as overvoltage, undervoltage, overheating, overcurrent, short circuit and inverse time. The voltage waveform when no load is shown in Figure 6. At present, the system has been running in the Kaifeng Yellow River Administration for nearly half a year and is in good condition.