Control method and system suitable for weak power grid cascade voltage type rectifier

Abstract

The invention discloses a control method and system suitable for a weak power grid cascade voltage type rectifier. The method comprises the following steps: extracting a d-axis voltage component ed and a q-axis voltage component eq on a d-axis and a q-axis according to circuit parameters of the rectifier; performing AC-DC separation on the voltage component ed to obtain a DC component eddc and an AC component edac of the ed, extracting a feature value B of the AC component edac of the ed, and reconstructing the AC component edac and the q-axis voltage component eq of the ed by using the feature value B to obtain a new AC component edacnew and a new q-axis voltage component eqnew of the ed, then adding the new alternating current component edacnew of the ed and the direct current component eddc of the ed to obtain a new d-axis voltage component ednew; the d-axis current loop output idPI is subtracted from the new d-axis voltage component ednew to obtain ud, the q-axis current loop output iqPI is subtracted from the new q-axis voltage component eqnew to obtain uq, and Park inverse transformation is performed on ud and uq to obtain three-phase modulation waves ua, ub and uc. By adopting the technical scheme of the invention, impedance matching with a power grid can be realized, and the stability margin of a cascade system is improved.

Description

Technical Field

[0001] The present invention relates to the field of voltage-type rectifier control, and in particular to a control method and system suitable for a weak-grid cascade voltage-type rectifier. Background Art

[0002] With the rapid popularization of electric vehicles in recent years, car charging technology has been widely studied by the academic community. At present, the mainstream fast charging technologies include DC charging and AC charging. Both charging technologies require the rectifier in the car charger to convert the three-phase AC power of the power grid into DC power, and then charge the car's power battery. In order to provide a stable DC output voltage, the rectifier in the car charger mostly uses a voltage-type rectifier. During the car charging process, the input side of the charger (that is, the input side of the voltage-type rectifier) ​​is usually connected to the low-voltage end of the power grid. Figure 1 As shown in the figure, the rectifier part needs to be connected to the grid through a long transmission line and multiple transformers. At this time, the impedance of the transmission line and the leakage inductance of the transformer cannot be ignored, and the grid exhibits the characteristics of a weak grid. When the rectifier works under weak grid conditions, the parasitic inductance and parasitic capacitance on the input side line will resonate with the rectifier, causing the rectifier's grid-connected current to oscillate at a low frequency. In severe cases, the input current and voltage will be severely distorted, and the rectifier will not be able to work stably. Improving the robustness of the rectifier control method is the key to solving this problem.

[0003] At present, the research on the cascade stability of weak grid and converter is mainly focused on the inverter, while the research on the rectifier is relatively small. The article entitled "Control Strategy Research and Stability Analysis of Three-Phase VIENNA Converter" improves the stability margin of the VIENNA rectifier by introducing the virtual impedance method, and improves the stability of the converter under the cascade condition with weak grid, but the improvement ability of this method is limited and requires a larger input filter capacitor.

[0004] The article entitled "Research on Improving Current Control Performance of Three-Phase Grid-Connected Converters under Weak Grids" proposes a simple voltage feedforward method to offset grid voltage disturbances, suppress the generation of grid-connected converter startup impact current, and improve the current waveform quality and system stability under weak grid input. However, this method is only applicable to grid-connected inverters and cannot be directly applied to AC / DC rectifiers. Summary of the invention

[0005] The purpose of the present invention is to provide a control method suitable for a weak grid cascade voltage type rectifier in view of the defects of the above-mentioned prior art, so as to improve the stability margin of the rectifier and ensure that the rectifier can work reliably under complex input environment.

[0006] In an embodiment of the present invention, a control method for a weak power grid cascade voltage-type rectifier is provided, which includes:

[0007] According to the circuit parameters of the rectifier, the d-axis voltage component e on the dq axis is extracted. d , q-axis voltage component e q , d-axis current component i d , q-axis current component i q and the phase information θ of the three-phase voltage;

[0008] For voltage component e d Perform AC-DC separation to obtain e d The DC component e d_dc and AC component e d_ac , extract e d The AC component e d_ac The eigenvalue B of e is used to d The AC component e d_ac and the q-axis voltage component e q Reconstruct each to get a new e d AC component e dac_new and the new q-axis voltage component e q_new , and then the new e d AC component e dac_new With e d The DC component e d_dc After adding, the new d-axis voltage component e is obtained d_new ;

[0009] The new d-axis voltage component e d_new Subtract the d-axis current loop output i d_PI Get u d , the new q-axis voltage component e q_new Subtract the q-axis current loop output i q_PI Get u q , for u d and u q Perform Park inverse transform to obtain the three-phase modulation wave u a ,u b and u c .

[0010] In the embodiment of the present invention, the circuit parameters of the rectifier include the input three-phase voltage v a 、v b 、v c , three-phase current i a 、i b 、i c And BUS bus voltage v bus .

[0011] In the embodiment of the present invention, the d-axis voltage component e on the dq axis is extracted according to the circuit parameters of the rectifier. d, q-axis voltage component e q , d-axis current component i d , q-axis current component i q And the phase information θ of the three-phase voltage, including:

[0012] For input three-phase voltage v a , v b and v c Perform Park transformation to obtain the d-axis voltage component e on the dq axis d and the q-axis voltage component e q , and e q Input into the phase-locked loop to obtain the three-phase voltage v a , v b and v c The phase information θ is then used to calculate the three-phase current i a ,i b and i c Perform Park transformation to obtain the current component i on the dq axis d and i q .

[0013] In the embodiment of the present invention, the control method for the weak grid cascade voltage type rectifier further comprises: calculating the d-axis current loop output i d_PI and q-axis current loop output i q_PI , the specific process is as follows:

[0014] Calculate the reference voltage V bus_ref With bus voltage v bus The error is then fed into the PI controller to obtain the voltage loop output v bus_PI ;

[0015] The voltage loop output v bus_PI As a reference value, the d-axis current component i d The error signal is obtained by performing the difference, and the d-axis current loop output i is obtained through the PI controller. d_PI , set the q-axis current component reference value to 0, and the q-axis current component i q The error signal is obtained by performing the difference, and the q-axis current loop output i is obtained through the PI controller. q_PI .

[0016] In an embodiment of the present invention, the control method applicable to the weak power grid cascade voltage type rectifier further includes:

[0017] For three-phase modulation wave u a ,u b and u c Modulation is performed to generate drive signals PWMa, PWMb and PWMc for corresponding switch tubes.

[0018] In the embodiment of the present invention, extract e d The AC component e d_ac The eigenvalues ​​B include:

[0019] Using the phase information θ calculated by the phase-locked loop as a reference, calculate e d_ac The absolute value of the characteristic value B in one or more power frequency cycles T is e d_ac The average, effective value or peak value of the absolute value.

[0020] In the embodiment of the present invention, extract e d The AC component e d_ac The expression of the eigenvalue B is as follows:

[0021]

[0022] Wherein, k is the set correction coefficient, and T is the power frequency period of the power grid.

[0023] In the embodiment of the present invention, the characteristic value B is used to d The AC component e d_ac and the q-axis voltage component e q Reconstruct them separately to get new e d AC component e dac_new and the new q-axis voltage component e q_new The expression is as follows:

[0024] e dac_new =e d_ac / B,e q_new =e q / B.

[0025] In an embodiment of the present invention, a control system applicable to a weak grid cascade voltage-type rectifier is further provided. When controlling the weak grid cascade voltage-type rectifier, the control method applicable to the weak grid cascade voltage-type rectifier is adopted.

[0026] Compared with the prior art, the control method and system of the present invention, which is applicable to the weak grid cascade voltage type rectifier, converts the three-phase input voltage to the dq axis, and extracts the d-axis voltage component e d The AC component e d_ac The characteristic value B is used to calculate the d-axis voltage component e d and the q-axis voltage component e q Reconstructed separately, rebuilt a new feedback mechanism, through an additional e d_acThe control loop can adjust the size of the AC component in the feedforward in real time according to the characteristic value B, establish impedance matching between the rectifier and the weak grid input, and significantly improve the stability margin of the rectifier. Even under weak grid conditions, it can still maintain a balanced relationship between the loop and the voltage feedforward, ensuring that the rectifier can work stably under complex input environments. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is the equivalent circuit diagram of the grid cascaded grid-connected converter.

[0028] Figure 2 It is the circuit diagram of a three-phase VIENNA rectifier.

[0029] Figure 3 It is the circuit diagram of a three-phase six-switch rectifier.

[0030] Figure 4 It is a flow chart of a control method applicable to a weak grid cascade voltage-type rectifier according to an embodiment of the present invention.

[0031] Figure 5 yes Figure 4 4. The control block diagram corresponding to step S2 in FIG.

[0032] Figure 6 yes Figure 4 4. The control block diagram corresponding to step S3 in FIG.

[0033] Figure 7 yes Figure 4 The control block diagram corresponding to step S5 in FIG.

[0034] Figure 8 yes Figure 4 A way to implement the eigenvalue part of step S5.

[0035] Fig. 9 yes Figure 4 The control block diagram corresponding to steps S4, S6 and S7.

[0036] Fig.10 This is the experimental waveform diagram of the three-phase VIENNA rectifier with an output power of 6kW under weak grid conditions when using the traditional control method.

[0037] Fig.11 This is an experimental waveform diagram of a three-phase VIENNA rectifier with an output power of 6 kW under weak power grid conditions when the control method proposed in the present invention is adopted.

[0038] Fig.12 This is an experimental waveform diagram of a three-phase VIENNA rectifier with an output power of 40 kW under weak power grid conditions when the control method proposed in the present invention is adopted. DETAILED DESCRIPTION

[0039] The present invention provides a control method for a weak grid cascade voltage type rectifier, the control method is applicable to a three-phase PWM rectifier, including but not limited to the following: Figure 2 The three-phase VIENNA rectifier shown in Figure 3 The control method for the weak grid cascade voltage type rectifier includes:

[0040] According to the circuit parameters of the rectifier, the d-axis voltage component e on the dq axis is extracted. d , q-axis voltage component e q , d-axis current component i d , q-axis current component i q and the phase information θ of the three-phase voltage;

[0041] The d-axis voltage component e d Perform AC-DC separation to obtain e d The DC component e d_dc and AC component e d_ac , extract e d The AC component e d_ac The eigenvalue B of e is used to d The AC component e d_ac and the q-axis voltage component e q Reconstruct them separately to get the new e d AC component e dac_new and the new q-axis voltage component e q_new , and then the new e d AC component e dac_new With e d The DC component e d_dc After adding, the new d-axis voltage component e is obtained d_new ;

[0042] The new d-axis voltage component e d_new Subtract the d-axis current loop output i d_PI Get u d , the new q-axis voltage component e q_new Subtract the q-axis current loop output i q_PI Get u q , for u d and u q Perform Park inverse transform to obtain the three-phase modulation wave u a ,u b and u c .

[0043] In the above method, by extracting the d-axis voltage component e d The AC component e d_acThe characteristic value B is used to calculate the d-axis voltage component e d and the q-axis voltage component e q Reconstructed separately, rebuilt a new feedback mechanism, through an additional e d_ac The control loop can adjust the size of the AC component in the feedforward in real time according to the characteristic value B, establish impedance matching between the rectifier and the weak grid input, and significantly improve the stability margin of the rectifier. Even under weak grid conditions, it can still maintain a balanced relationship between the loop and the voltage feedforward, ensuring that the rectifier can work stably under complex input environments.

[0044] like Figure 4 As shown, as a preferred embodiment of the present invention, the control method applicable to the weak grid cascade voltage-type rectifier includes steps S1-S7, which are described below respectively.

[0045] Step S1: Sample the three-phase input voltage v of the rectifier a , v b and v c , three-phase input current i a ,i b and i c And BUS bus voltage v bus .

[0046] It should be noted that the three-phase input current i a ,i b and i c By three power inductors L a , L b and L c The current is sampled.

[0047] Step S2: The three-phase input voltage v a , v b and v c Perform Park transformation to obtain the voltage component e on the dq axis d and e q , where e q Used for phase-locked loop calculation to obtain the three-phase input voltage v a , v b and v c The phase information θ of i a ,i b and i c Perform Park transformation to get i d and i q , its control block diagram is as follows Figure 5 shown.

[0048] Step S3: Calculate the reference voltage V bus_ref With vbus The error is then fed into the PI controller to obtain the voltage loop output v bus_PI , its control block diagram is as follows Figure 6 shown.

[0049] It should be noted that in order to achieve the stability of the BUS bus voltage, a voltage loop is set to set a reference voltage V bus_ref , to adjust the BUS bus voltage. The output of the voltage loop is v bus_PI It will be used as the reference value of the current loop to adjust the active power.

[0050] Step S4: The current reference value v outputted in step S3 is bus_PI and the d-axis current component i d The difference is used as the error signal, and the d-axis loop output i is obtained through the PI controller. d_PI ; Set the q-axis current component reference value to 0, which is consistent with the q-axis current component i q The error signal is obtained by performing the difference, and the q-axis loop output i is obtained through the PI controller. q_PI .

[0051] It should be noted that in step S4, two current control loops are set to adjust the active power and reactive power of the converter respectively. d The loop is used to control the active power, i q The loop is used to control reactive power.

[0052] Step S5: The voltage component e obtained by Park transformation in step S2 d Through high / low pass filter, AC-DC separation is performed to obtain e d The DC component e d_dc and AC component e d_ac , and extract the AC component e d_ac The eigenvalue B of e is used to d_ac and e q Reconstruction to obtain the new AC component e dac_new and e q_new , and then the AC component e dac_new With the DC component e d_dc After adding, the new d-axis voltage component e is obtained d_new , its control block diagram is as follows Figure 7 As shown, the eigenvalue B is used to e d_ac and e q Reconstruction to obtain the new AC component e dac_new and e q_new The expression is as follows:

[0053] e dac_new =e d_ac / B,eq_new =e q / B.

[0054] It should be noted that in step S5, the AC component e is extracted d_ac The eigenvalues ​​B include:

[0055] Using the phase information θ calculated by the phase-locked loop in step S2 as a reference, calculate e d_ac The absolute value of the characteristic value B in one or more power frequency cycles T. The characteristic value B can be e d_ac The average value, effective value or peak value of the absolute value reflects the disturbance of the input voltage and the working state of the converter under different weak grid conditions.

[0056] Specifically, Figure 8 As shown, as a specific implementation method, calculate e d_ac The average value of the absolute value is taken as the eigenvalue B. The specific expression is as follows:

[0057]

[0058] Wherein, k is the set correction coefficient, and T is the power frequency period of the power grid.

[0059] Step S6: The d-axis voltage component e obtained in step S5 is converted to d_new Subtract the d-axis loop output i d_PI Get u d , e q_new Subtract the q-axis loop output i q_PI Get u q , for u d and u q Perform Park inverse transform to obtain the three-phase modulation wave u a ,u b and u c .

[0060] Step S7: Three-phase modulation wave u a ,u b and u c Modulation is performed to generate drive signals PWMa, PWMb and PWMc for corresponding switch tubes.

[0061] The control block diagram of step S4, step S6 and step S7 is as follows: Fig. 9 shown.

[0062] like Fig.10As shown in the figure, the experimental waveform when the traditional control method is used, when the three-phase VIENNA rectifier is connected to a weak power grid, the output power is 6kW, and severe low-frequency oscillations will occur on the input voltage and input current, resulting in severe distortion of the input current and voltage, and the rectifier cannot work stably. If the output power is further increased, the low-frequency oscillation will be further aggravated, the BUS voltage ripple will increase, and the power quality of the rectifier output will be seriously affected; it may even generate high-frequency oscillations, which will bring great harm to the stability of the rectifier; Fig.11 As shown in FIG. 1 , the experimental waveform of the three-phase VIENNA rectifier output power of 6 kW when the control method of the present invention is adopted is shown in FIG. Fig.10 In comparison, the input current and input voltage waveforms are significantly improved. Fig.12 This is an experimental waveform of a three-phase VIENNA rectifier with an output power of 40kW under weak power grid conditions when the control method proposed in the present invention is used. The experimental results show that the control method provided by the present invention can ensure that the rectifier works stably under the input environment of a weak power grid.

[0063] In summary, the control method and system for the weak grid cascade voltage type rectifier of the present invention converts the three-phase input voltage into the dq axis, and extracts the d-axis voltage component e d The AC component e d_ac The characteristic value B is used to calculate the d-axis voltage component e d and the q-axis voltage component e q Reconstructed separately, rebuilt a new feedback mechanism, through an additional e d_ac The control loop can adjust the size of the AC component in the feedforward in real time according to the characteristic value B, establish impedance matching between the rectifier and the weak grid input, and significantly improve the stability margin of the rectifier. Even under weak grid conditions, it can still maintain a balanced relationship between the loop and the voltage feedforward, ensuring that the rectifier can work stably under complex input environments.

[0064] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A control method for a weak grid cascade voltage-type rectifier, characterized in that: include: According to the circuit parameters of the rectifier, the d-axis voltage component e on the dq axis is extracted. d , q-axis voltage component e q , d-axis current component i d , q-axis current component i q and the phase information θ of the three-phase voltage; The d-axis voltage component e d Perform AC-DC separation to obtain e d The DC component e d_dc and AC component e d_ac , extract e d The AC component e d_ac The eigenvalue B of e is used to d The AC component e d_ac and the q-axis voltage component e q Reconstruct them separately to get the new e d AC component e dac_new and the new q-axis voltage component e q_new , and then the new e d AC component e dac_new With e d The DC component e d_dc After adding, the new d-axis voltage component e is obtained d_new ; The new d-axis voltage component e d_new Subtract the d-axis current loop output i d_PI Get u d , the new q-axis voltage component e q_new Subtract the q-axis current loop output i q_PI Get u q , for u d and u q Perform Park inverse transform to obtain the three-phase modulation wave u a ,u b and u c .

2. The control method for a weak grid cascade voltage source rectifier according to claim 1, characterized in that: The circuit parameters of the rectifier include the input three-phase voltage v a 、v b 、v c , three-phase current i a 、i b 、i c And BUS bus voltage v bus .

3. The control method for a weak grid cascade voltage source rectifier according to claim 2, characterized in that: According to the circuit parameters of the rectifier, the d-axis voltage component e on the dq axis is extracted. d , q-axis voltage component e q , d-axis current component i d , q-axis current component i q and the phase information θ of the three-phase voltage, include: For input three-phase voltage v a , v b and v c Perform Park transformation to obtain the d-axis voltage component e on the dq axis d and the q-axis voltage component e q , and e q Input into the phase-locked loop to obtain the three-phase voltage v a , v b and v c The phase information θ is then used to calculate the three-phase current i a ,i b and i c Perform Park transformation to obtain the current component i on the dq axis d and i q .

4. The control method for a weak grid cascade voltage source rectifier according to claim 2, characterized in that: Also includes: Calculate the d-axis current loop output i d_PI and q-axis current loop output i q_PI The specific process includes: Calculate the reference voltage V bus_ref With bus voltage v bus The error is then fed into the PI controller to obtain the voltage loop output v bus_PI ; The voltage loop output v bus_PI As a reference value, the d-axis current component i d The error signal is obtained by performing the difference, and the d-axis current loop output i is obtained through the PI controller. d_PI , set the q-axis current component reference value to 0, and the q-axis current component i q The error signal is obtained by performing the difference, and the q-axis current loop output i is obtained through the PI controller. q_PI .

5. The control method for a weak grid cascade voltage source rectifier according to claim 1, characterized in that: Also includes: For three-phase modulation wave u a ,u b and u c Modulation is performed to generate drive signals PWMa, PWMb and PWMc for corresponding switch tubes.

6. The control method for a weak grid cascade voltage source rectifier according to claim 1, characterized in that: Extraction d The AC component e d_ac The eigenvalues ​​B include: Using the phase information θ calculated by the phase-locked loop as a reference, calculate e d_ac The absolute value of the characteristic value B in one or more power frequency cycles T is e d_ac The average, effective value or peak value of the absolute value.

7. The control method for a weak grid cascade voltage source rectifier according to claim 1, characterized in that: Extract AC component e d_ac The expression of the eigenvalue B is as follows: Wherein, k is the set correction coefficient, and T is the power frequency period of the power grid.

8. The control method for a weak grid cascade voltage source rectifier according to claim 1, characterized in that: Use eigenvalue B to e d The AC component e d_ac and the q-axis voltage component e q Reconstruct each to get a new e d AC component e dac_new and the new q-axis voltage component e q_new The expression is as follows: e dac_new =e d_ac / B,e q_new =e q / B。 9. A control system suitable for a weak grid cascade voltage type rectifier, characterized in that: When controlling the weak-grid cascade voltage-type rectifier, the control system adopts the control method applicable to the weak-grid cascade voltage-type rectifier as described in any one of claims 1 to 7.

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