LCL three-phase PWM rectifier (inverter)

LCL three-phase ｐｗｍ Rectifier control strategy

1. LCL Schematic diagram of filter

LCL The filter structure is shown in the figure .

2.LCL Filter single current variable instantaneous value feedback control

LCL The filter has two control modes of feedback inverter bridge current and network side current . Different output current feedback points , It will have a certain impact on the closed-loop control performance and system operating characteristics .

2.1 1 Feedback inverse bridge current

For three-phase symmetrical systems , The structure of single-phase control loop is shown in the figure , among VSI It is a voltage type inverter bridge ．

From inverter bridge voltage Ui Current to inverter bridge il The transfer function of is

According to design , take LCL Filter parameters ：L1=0.15mH,L2=0.08mH,C =8uF, Rd =0.005Ω; Its frequency characteristics are shown in the figure below ：

G1(s) The frequency characteristic of is more complex , stay 6.29 kHz、7.79 kHz Each exists -66dB and 27.7dB Resonance peak of , The phase changes from the initial -90° lagging , At the first resonant frequency point, it jumps to 90° leading , At the second resonant frequency point, it returns to -90° lagging .6.29 kHz The resonant peak of is caused by L2 and C Caused by parallel resonance , Due to the high impedance of parallel resonance , therefore i1 Strongly attenuated .7.79 kHz The resonant peak of is L1、L2 and C Caused by series resonance ,i1 Be magnified .

7.79 kHz The resonant peak of G1(s) The amplitude frequency characteristic curve exists 3 individual 0 dB Crossing point , The usual phase margin stability judgment method is difficult to apply . actually , In frequency domain analysis of linear systems , Nyquist stability criterion is the fundamental method to judge stability ． As shown in the figure below G1(s) The Nyquist curve of does not surround the critical point (-1,0j), According to Nyquist criterion ,G1(s) Closed loop stability . actually , because G1(s) non-existent -180° Phase angle crossing point of , Therefore, no matter how the open-loop gain and bandwidth are designed , Nyquist curve will never surround the critical point (-1,0j), The control loop is always stable , have “ Infinite ” stability . Of course , Too high gain is unrealistic , The high bandwidth it brings will amplify the switching ripple signal and introduce it into the control loop , Interfere with normal PWM modulation , This leads to multiple switching actions of power devices .

2.2 Feedback grid current

The single-phase control structure of feedback grid current is shown in the figure

From inverter bridge voltage ui Current to the grid i2 The transfer function of is

Still take LCL Filter parameters ：L1=0.15mH,L2=0.08mH,C =8uF, Rd =0.005Ω; Its frequency characteristics are shown in the figure below ： Then its frequency characteristics are shown in the figure below .

and G1(s) similar ,G2(s) Also exist 3 individual 0 dB Crossing point , The difference is G2(s) The phase angle curve of is 7．79 kHz There is -180° Crossing point , The corresponding amplitude gain is 33.1 dB. such ,G2(s) The Nyquist curve of will surround the critical point (-1,0j) , According to Nyquist criterion , The closed loop of the system is unstable , The Nyquist curve is shown in the figure below .

3. Comparison of two current feedback methods

The feedback inverter bridge current control has good stability in theory , And different from intuitive ideas ： o
Side filter inductance 、 The high-frequency shunt circuit of the capacitor has no significant impact on the grid current in the effective frequency band ． by
The influence of the high-frequency component of the feedback signal is reduced , The controller gain is limited , This will affect steady-state performance ,
And the phase shift brought by low-pass filter ink will affect the dynamic response ．
It is difficult to stabilize the feedback network current control by using the traditional correction method , But use the phase lag control in this paper
The control method can carry out stable control , And the steady-state performance is better than the feedback inverter bridge current control ． Feedback grid
Current control can take advantage of the one beat control delay peculiar to digital control , Simplify the control system 、 Conducive to digital reality
present .