2021 Question B-Three-Phase Rectifier-PowerOnP03 Team

1. Foreword: Title B of this competition is three-phase rectifier circuit. This solution uses a three-phase rectifier circuit cascaded with Buck converter, and uses STM32F407VET6 microcontroller as the controller of the rectifier and Buck converter to realize three-phase AC-DC conversion. The rectifier controller uses a digital phase-locked loop for phase locking, and uses dq transformation and PI control algorithm to control the voltage amplitude and phase stability of the rectifier. This circuit can work stably under rated working conditions, with stable output voltage, load regulation rate and voltage regulation rate both less than 0.1%. The overall efficiency of the machine at the national test site can reach 95.9%, the input side power factor can reach 0.998, and it can Adjustable within the range of ±0.70~1.00. At the same time, the system has overvoltage and overcurrent protection and friendly human-computer interaction functions.

2. Team introduction: Our team has two level 18 and one level 20 team members. The team leader responsible for the main circuit is a superb welder and is good at designing inductors, selecting capacitors, and the layout and line connections of the entire circuit board. He is responsible for programming and joint debugging. The programmers are proficient in C language, have a complete program framework, are familiar with STM32 configuration, and can quickly code efficient codes. The hardware engineers in charge of the modules are good at the production of various measurement modules, auxiliary power modules, and drive circuit modules, and can provide the team with reliable module circuits. .

 

Switch tube selection:
The switch tube's withstand voltage and conduction current are the rectifier output voltage and current, which are 50V and 2A respectively. There should be a margin when selecting the switch tube. At the same time, in order to reduce the loss of the system, the switching loss and conduction loss of the switch tube need to be comprehensively considered. Infineon's IRF540NPbF was finally selected. Its maximum withstand voltage is 100V, it can conduct 7.5A current, and its on-resistance is 44mΩ. The input capacitance is 1960pF and the output capacitance is 250pF.

Filter parameter design
(1) Input AC side inductance parameter calculation
The value of the system's AC inductance not only affects the dynamic and static performance of the system, but also affects other factors such as the input current waveform. Increasing the inductance value can suppress the harmonics of the AC side current, but it will affect the rapidity of current tracking. So a suitable inductor value should satisfy two conditions. First, the rapidity of current tracking should be ensured. Second, the harmonic size of the inductor current should be ensured within the allowable range. Based on the above two conditions, the value range expression of the AC side inductance can be calculated as:

The DC side output voltage Uo is 50V, and the switching frequency fs of the switching tube is 48kHz. Calculate the current ripple according to the input current harmonic distortion rate of 2%, Uipp is the peak-to-peak value of the AC side voltage, and Im is the peak value of the input current reference current. Putting the above parameters into the above formula, we can get the inductance value 214uH≤L1≤568uH. Select the AC side inductor L1=290uH, and use an iron-silicon-aluminum magnetic powder core and 2 strands of parallel-wound 0.7mm enameled wire to wind the inductor.

(2) Calculation of DC capacitance parameters measured at rectifier output

The process of charging and discharging is always repeated on the DC capacitor. In order to control the pulsation size of the bus voltage within a certain range, the value of the DC side capacitor cannot be too small. Considering the response speed of the bus voltage, the value of the DC capacitor cannot be too large. The reference range of DC side capacitance is as follows:

Among them, △Um*=0.01 is the bus voltage ripple rate, RL=18Ω, which is the equivalent resistance obtained from the rectifier output power. The rise time tr of the output DC voltage from the initial value to the rated voltage is 40ms. Putting the above parameters into the above formula, we can get the capacitance value 1670μF<C<4324μF, and the actual DC capacitance is 2200uF.

(3) Calculation of output DC side inductance parameters.
The DC input voltage of Buck is 50V, the output voltage is set to 36V, and the calculated duty cycle D=0.72. The Buck inductance calculation formula is as follows:

Among them, the duty cycle D=0.72, the output DC voltage Uo=36V, the inductor current Id=2A, the inductor current ripple rate rd=0.3, the modulation wave frequency fc=20kHz, put the above parameters into the above formula, the inductor value L2 can be obtained =605uH. In practice, a margin is left and the inductor value is 980uH. The inductor is wound using an iron-silicon-aluminum magnetic powder core and 2 parallel-wound 0.7mm enameled wires.

The upper end of the circuit board in the picture is the input terminal, and the lower end is the output terminal. The upper left corner of the picture is the button keyboard, which is used to set the power factor. The microcontroller serves as the main controller to complete the control of the system and takes power from the front-stage output bus as an auxiliary power supply. Provides power to the measurement module, microcontroller and drive module.

      以下代码为控制中断中的代码，中断频率为20kHz， 首先进行有效值计算用来计算输入电压有效值，做过压保护，然后利用ADC-DMA进行采样，得到输出电压、输入电压及输入电流，并进行dq变换，之后利用PID做控制器，并加入前馈从而实现稳压控制。红色字体为额外说明

        time_count++;
        sum_u_a+=adc_6ch.nowI1*adc_6ch.nowI1;        
        if(time_count==400)计算有效值
            {        
                time_count=0;                
                I_a=sqrt(sum_u_a*0.0025f);//
                sum_u_a=0;//
            }
        if(I_a>1.1f)对参考电压做修正
        {
            U_out.U_ref = 36.26f;
        }
        else
        {
        U_out.U_ref = 36.13f;
        }
            
        ADC_Process_6CH(); 获取采样值

        U_out.U_now = adc_6ch.nowUo;    
        
        U_in.theta = theta;获取锁相环输出值
        I_in.theta = theta;
        
        U_in.sin_value = arm_sin_f32(U_in.theta);
        U_in.cos_value = arm_cos_f32(U_in.theta);
        I_in.sin_value = U_in.sin_value;
        I_in.cos_value = U_in.cos_value;
        
        U_in.ab = adc_6ch.nowU1;
        U_in.bc = adc_6ch.nowU2;
        U_in.ca = - adc_6ch.nowU2 - adc_6ch.nowU1;
        
        //线电压转相电压
        U_in.a =  0.3333333f * U_in.ab - 0.333333f * U_in.ca;
        U_in.b =  0.3333333f * U_in.bc - 0.333333f * U_in.ab;
        U_in.c =  0.3333333f * U_in.ca - 0.333333f * U_in.bc;
        
        abc_to_dqz(U_in.a,U_in.b,U_in.c,U_in.theta,&U_in.d_now,&U_in.q_now,&U_in.z_now);得到电压dq值
        
        I_in.a = - adc_6ch.nowI1;
        I_in.b = - adc_6ch.nowI2;
        I_in.c = adc_6ch.nowI1 +adc_6ch.nowI2;
        
        abc_to_dqz(I_in.a,I_in.b,I_in.c,I_in.theta,&I_in.d_now,&I_in.q_now,&I_in.z_now);

        U_in.d_now = 0.999f*U_in.d_last + 0.001f*U_in.d_now;
        U_in.q_now = 0.999f*U_in.q_last + 0.001f*U_in.q_now;
        
        U_in.d_last = U_in.d_now;
        U_in.q_last = U_in. q_now;    
        
        //Set reference value         
        setPIDTarget(&PID_U, U_out.U_ref); Perform pid closed loop
        I_in.d_ref = LocPIDCalc(&PID_U,U_out.U_now);
        setPIDTarget(&PID_Id, I_in.d_ref);
        
        
        setPIDTarget(&PID_Iq, I_in.q_ref);
        //Perform PID calculation on D and Q, and output the duty cycle under rotating coordinates 
        I_in.d_duty = - LocPIDCalc(&PID_Id,I_in.d_now);
        I_in.q_duty = - LocPIDCalc(&PID_Iq, I_in.q_now);

        //Use feedforward to decouple dq and calculate the midpoint voltage of the board bridge 
        U_in.d_duty = I_in.d_duty + U_in.d_now + 314.15926f*inductance*0.000001f*I_in.q_now;//Calculate the expected midpoint voltage of the half-bridge
        U_in.q_duty = I_in.q_duty + U_in.q_now - 314.15926f*inductance*0.000001f*I_in.d_now; Add feedforward

        arm_sqrt_f32((U_in.d_duty*U_in.d_duty)+(U_in.q_duty*U_in.q_duty),&U_R);

        if(sw.lastPWM_on != sw.PWM_on) clear pid
        {
            sw.lastPWM_on = sw.PWM_on;
            PIDClear(&PID_Id);
            PIDClear(&PID_Iq);
            PIDClear(&PID_U);
        }
        
        if(PLL_Locked == 1&& sw.PWM_on == 1) 
            TIM_CtrlPWMOutputs(TIM1,ENABLE);
        else 
            TIM_CtrlPWMOutputs(TIM1,DISABLE);
    
        if(lastpll != PLL_Locked || sw.PWM_on == 0) clear pid
            {
                lastpll = PLL_Locked;
                PIDClear(&PID_Id);
                PIDClear (&PID_Iq);
                PIDClear(&PID_U);
            }
        if(U_out.U_now>40.f||I_a>3.f)//
            protect_sta=0;
        
        if(duty<2000)
        SVPWMmodulation(U_R,U_out.U_now/0.85f, theta+atanf(U_in.q_duty/U_in.d_duty)); Use SVPWM modulation
        else if(duty>2000)
        SVPWMmodulation(U_R,U_out.U_now/0.72f,theta+atanf(U_in.q_duty/U_in.d_duty));
        
        if (U_in.d_now<24.3f && U_in.d_now>22.5f)
            duty = 1260;
        else duty = 2352;
        
        if(lock == 1 )
            duty = 2352;
        
        TIM_SetCompare2(TIM8,duty);
      TIM_ClearITPendingBit(TIM1,TIM_IT_Update);