2017 Electric Power Competition Question A Microgrid System

• system design

The system consists of four parts: three-phase inverter circuit, drive circuit, feedback circuit and control circuit. The system structure block diagram is shown in Figure 2. The three-phase inverter circuit uses a half-bridge inverter structure and controls the output through the opening and closing of the switch tube. The driver circuit uses the IR2103 driver chip, which can drive the PWM wave output from the microcontroller to turn the switch tube on and off. The feedback circuit is composed of current transformer, voltage transformer and AD637 chip with true effective value detection. The current transformer and voltage transformer can well isolate and reduce the output AC power. The microcontroller reads the output value of AD637 to adjust the degree. adjustment. The control circuit uses the most popular STM32 series microcontroller STM32F103C8T6 as the control core, which includes key control, ADC, OLED display and other functions.  

• circuit design

1. Demonstration and selection of three-phase inverter modules

Solution 1: Three-phase inverter topology based on full-bridge structure. This structure is composed of three independent single-phase inverters. Each module is independent of each other and does not interfere with each other, so the system control is simple. However, it requires the input of three single-phase inverters and is isolated from each other, which increases the complexity of the system.

Option 2: Topology based on half-bridge structure, as shown in Figure 2 below. This structure has fewer switching devices than the three-phase full-bridge inverter, and only requires one DC input. However, the voltage that its switching tube withstands is twice that of the three-phase full-bridge three-phase full-bridge inverter. Therefore, Need to choose high voltage resistant MOSIRF540.

Solution analysis: In order to simplify the system design as much as possible and reduce the hardware complexity, the system adopts the second solution.

2. Feedback circuit module Voltage transformer and current transformer can isolate and scale the output AC power, and can be well used as the input of the true RMS module.

Option 1: Use the true RMS conversion chip AD637 for direct measurement. AD637 can accurately calculate the effective value of various signals and is simple to use, but the calculation time is long.

Option 2: AD sampling calculation, obtain the effective value by taking the root mean square of the discrete sample value. This solution has simple hardware and strong real-time measurement performance, but requires complex algorithms and takes up a lot of resources.

Solution analysis: Since the communication method is used between the microcontroller and the microcontroller, asynchronous communication will lead to asynchronous waveforms. Therefore, it is necessary to reduce the resource occupation of the microcontroller, so scheme one is adopted.

3. Inverter control scheme selection Scheme 1: Use hardware to generate sine waves and triangle waves. The sine wave is used as the fundamental wave and the triangle wave is used as the carrier wave. They are input to the analog op amp comparator for comparison and then the SPWM wave is output.

Option 2: A microcontroller with PWM generation function uses the sine table scanning method to generate SPWM and drive the inverter circuit. The control circuit of this scheme is simple, the amplitude and frequency of the SPWM generated by software are easier to control, and the quality of the generated SPWM wave is better.

Solution analysis: Solution 1: Due to the arbitrariness of the focus of the triangle wave and the sine wave, the pulse centers are not equidistant within one cycle, which increases the complexity of the calculation. Hardware frequency modulation is very difficult and difficult to debug. So choose option two.

4. Filter design The inverter output will contain odd-numbered harmonics of the fundamental wave. We need to filter these harmonics or suppress these harmonic outputs.

The inverter output is used as power supply and the output resistance needs to be small, so an LC passive filter is chosen. The filter parameters are calculated as follows. The LC passive low-pass filter filters out high-order harmonic components and makes the voltage output waveform a sine wave. The SPWM modulation signal of this system is 20K. The required wave for output is up to 100Hz, and filtering is easy to implement.

Filter cutoff frequency: Set the cutoff frequency f=3000Hz, and the CCB capacitor is 10μF. The L=1mH in the circuit is obtained, which meets the requirements of the filter.

• programming

1. Design idea The core of the design is to use PWM modulation in STM32 to control the switching tube on and off; use 72M high-precision TIM2 and TIM3 timers to accurately control PWM on and off; use internal 12-bit AD sampling to obtain the output value and adjust it through the PID algorithm Adjust the PWM modulation degree; an OLED display that is easier for information interaction is added to the core program to make the overall status of the power supply more concise and clear.

2.Program flow chart ***  ***

• Special Note:
• Due to limitations of laboratory power equipment, the indicators in the question have been partially reduced (such as AC 24V effective value, 3A current)