A cascade dynamic voltage restorer power adaptive virtual impedance oscillation suppression control method
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- CHINA UNIV OF MINING & TECH
- Filing Date
- 2025-04-22
- Publication Date
- 2026-07-21
Smart Images
Figure CN120280952B_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of impedance suppression, specifically relating to a power adaptive virtual impedance oscillation suppression control method for a cascaded dynamic voltage restorer. Background Technology
[0002] With the large-scale integration of renewable energy sources such as wind and solar power into the power grid, their intermittent, highly volatile, and unpredictable characteristics lead to significant fluctuations in power quality. In power systems, 80% of power quality problems are voltage sags, making them a key focus and challenge in current power quality management research. Consequently, cascaded dynamic voltage restorers have emerged, possessing superior compensation capabilities for grid sags. However, due to the constant power load characteristics of the load converter in the cascaded dynamic voltage restorer, it is susceptible to load power fluctuations, resulting in severe system oscillations, causing system instability, and damaging electrical equipment. Summary of the Invention
[0003] The purpose of this invention is to provide a control method for power adaptive virtual impedance oscillation suppression in a cascaded dynamic voltage restorer, so as to solve the system oscillation problem of the cascaded dynamic voltage restorer under load power fluctuation conditions.
[0004] To achieve the above objectives, the present invention employs a power adaptive virtual impedance oscillation suppression control method for a cascaded dynamic voltage restorer, the method comprising:
[0005] The load converter is equivalent to the Norton equivalent circuit model, and the grid connection point voltage, filter inductor current, and DC bus voltage of the cascaded dynamic voltage restorer are sampled respectively.
[0006] The difference between the AC side filter capacitor voltage command value given by the upper-level system and the grid connection point voltage is used to obtain the current controller command value through the voltage controller.
[0007] A second-order virtual impedance controller is used to extract the oscillation component of the DC bus voltage as the virtual current input feedforward of the current controller, which is equivalent to parallel input virtual impedance on the DC side of the load converter in the cascaded dynamic voltage restorer.
[0008] The difference between the command value of the current controller and the current of the filter inductor is then added to the virtual current input feedforward and input to the current controller to obtain the modulation voltage.
[0009] The difference between the DC bus voltage and the reference bus voltage is calculated, and the typical frequency values are iterated to change the center frequency of the virtual impedance controller until the difference is less than the set threshold.
[0010] The virtual impedance controller adaptively adjusts the DC-side input impedance of the load converter within the control bandwidth based on the load power.
[0011] Furthermore, the load converter is equivalent to the Norton equivalent circuit model, with a low-frequency range of 0~1kHz. The simplified expression for the DC-side input impedance of the load converter within this frequency range is as follows:
[0012] ;
[0013] In the formula, Z in,cl (s), V dc P CPL Y in Y vir (s) and s are the DC input impedance of the load converter on the DC side, the DC component of the DC bus voltage, the input power of the load converter, the negative input admittance of the load converter on the DC side before adding the virtual impedance, the admittance of the parallel input virtual impedance on the DC side, and the differential operator, respectively.
[0014] Furthermore, based on the difference between the AC side filter capacitor voltage command value given by the upper-level system and the grid connection point voltage, the difference is used by the voltage controller to obtain the current controller command value, including:
[0015] The grid connection point voltage U PCC The grid connection point voltage U is obtained after abc / dq transformation. PCC Components U on the d and q axes d U q Consider the dq decoupling term ωCU d ,ωCU q The command value of the current controller is calculated according to the following formula:
[0016] ;
[0017] In the formula, , , , , , These represent the d- and q-axis components of the current controller's command value and the d- and q-axis components of the AC side filter capacitor voltage command value, respectively. The grid connection point voltage U... PCC Components on the d and q axes; , , , These are the proportional and integral coefficients of the voltage controller, the grid angular frequency, and the filter capacitor, respectively.
[0018] Furthermore, the calculation formula for using the second-order virtual impedance controller to extract the oscillation component of the DC bus voltage as the virtual current input feedforward of the current controller is as follows:
[0019] ;
[0020] In the formula, , , , V bus (s) represent the components of the virtual current input feedforward on the d and q axes, the transfer function of the virtual impedance controller on the d and q axes, and the bus capacitor voltage, respectively; K ip G is the proportional gain of the current controller, k is the adjustment parameter for controlling the virtual impedance compensation level, and G is the proportional gain of the current controller. band (s) is a second-order bandpass filter, f osc The center frequency of the virtual impedance controller. For the quality factor, the virtual impedance controller is only second-order and only requires the control parameter f. osc .
[0021] Furthermore, the admittance expression for the parallel input virtual impedance of the load converter on the DC side in the equivalent cascaded dynamic voltage restorer is:
[0022] ;
[0023] In the formula, Y vir (s) represents the admittance form of the virtual impedance, I Ldq (s) is the column vector expression of the AC side current of the load converter on the d and q axes.
[0024] Furthermore, the difference between the current controller's command value and the filter inductor current is calculated, and this difference is then superimposed on the virtual current input feedforward and input to the current controller. The formula for calculating the modulation voltage is as follows:
[0025] ;
[0026] In the formula, D d D q K represents the d-axis and q-axis components of the modulation voltage. ii ωL is the integral coefficient of the current controller. f I d ωL f I q U d U q Both are d and q decoupling terms, where ω is the grid angular frequency, and L... f For filter inductance, I d I qThese are the components of the filter inductor current on the d and q axes, respectively.
[0027] Furthermore, the difference between the DC bus voltage and the reference bus voltage is calculated, and the typical frequency values are iterated to change the center frequency of the virtual impedance controller until the difference is less than a set threshold, including:
[0028] Taking 2% of the DC bus voltage as the set threshold, the formula for determining the center frequency of the virtual impedance controller is as follows:
[0029] ;
[0030] In the formula, , , , These are the reference bus voltage, the DC component of the DC-side bus voltage, the difference between the DC bus voltage and the reference bus voltage, and the set threshold.
[0031] Furthermore, the described cyclic traversal of typical frequency values to change the center frequency of the virtual impedance controller includes:
[0032] The center frequency f of the virtual impedance controller osc The modification rule is as follows: Divide the system oscillation frequency band of the cascaded dynamic voltage restorer from 0 to 1000 Hz into 10 bands, and take the center value of each band as its typical frequency. osc Select from 10 typical frequencies in a loop until... Established.
[0033] Furthermore, based on the load power, the DC-side input impedance of the load converter is adaptively adjusted within the control bandwidth of the virtual impedance controller, including:
[0034] The load power variation ignores the active power losses of the line and filter branches, and the AC side load power P Load Always equal to the DC-side input power P of the load converter CPL And U d U q Always controlled by cascaded dynamic voltage restorer , The DC-side input impedance of the load converter in the low-frequency band is calculated using the following formula:
[0035] ;
[0036] In the formula, Z in,cl (s) represents the DC-side input impedance of the load converter in the low-frequency band, V dc P CPLThese are the DC component of the DC-side bus voltage and the input power of the load converter, respectively. k is an adjustment parameter controlling the virtual impedance compensation level, 0 < k < 1, Δf osc The bandwidth of the virtual impedance controller is ω, and the angular frequency is ω.
[0037] In the low-frequency band, Z within the virtual impedance controller bandwidth in,cl The negative real part of (s) is weakened by adjusting the parameter k, while Z outside the bandwidth in,cl (s) is It remains the negative impedance before the addition of virtual impedance, and the low frequency band is 0~1kHz.
[0038] Beneficial effects: Compared with the prior art, the technical solution of the present invention has the following beneficial technical effects:
[0039] 1. The method provided by this invention can effectively suppress system oscillations when a cascaded dynamic voltage restorer supplies power to the load, and can also adaptively adjust the DC-side input impedance of the load converter according to changes in load power to maintain system stability.
[0040] 2. The method provided by this invention, while suppressing system oscillation, only changes the input impedance of the load converter in the cascaded dynamic voltage restorer within the bandwidth of the virtual impedance controller, thus avoiding the impact of virtual impedance on the dynamic performance of the load converter in other frequency bands.
[0041] 3. The method provided by this invention can greatly improve the response speed of the virtual impedance controller to system oscillations, and the controller is only second-order, requiring no power detection circuit and additional sampling current, making it easy to control and implement.
[0042] 4. The method provided by this invention is applicable to any source converter in a cascaded dynamic voltage restorer, and the introduced virtual impedance does not affect the source converter. Attached Figure Description
[0043] Figure 1 This is a topology diagram of a cascaded dynamic voltage restorer system in a specific embodiment of the present invention;
[0044] Figure 2 This is a load converter control block diagram of a cascaded dynamic voltage restorer in a specific embodiment of the present invention;
[0045] Figure 3 This is a block diagram of the small-signal control of the load converter in a specific embodiment of the present invention;
[0046] Figure 4 This is a flowchart illustrating the process of changing the center frequency of the virtual impedance controller in a specific embodiment of the present invention.
[0047] Figure 5The Bode plots of DC-side input and output impedances before and after adding virtual impedance are shown in a specific embodiment of the present invention.
[0048] Figure 6 The following is a waveform diagram of the DC bus voltage before and after adding virtual impedance in a specific embodiment of the present invention;
[0049] Figure 7 This is a waveform diagram of the DC bus voltage under load power variation in a specific embodiment of the present invention. Detailed Implementation
[0050] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.
[0051] The embodiments of the present invention will be further described in detail below with reference to the accompanying drawings.
[0052] As mentioned earlier, when a cascaded dynamic voltage restorer supplies power to a load, due to the constant power load characteristics of the load converter, the cascaded dynamic voltage restorer will be affected by load power fluctuations, resulting in severe system oscillations, causing system instability and damaging electrical equipment. Therefore, this invention proposes a control method for suppressing power-adaptive virtual impedance oscillations in a cascaded dynamic voltage restorer, based on the existing design.
[0053] like Figure 1 As shown, the cascaded dynamic voltage restorer consists of three parts: a source converter, a load converter, and an LC filter. The output of the LC filter is connected to the load through the grid connection point. a e b e c These are the three-phase voltages of the power grid, L g S is the power grid impedance. g It is a bidirectional thyristor. When a voltage dip occurs in the mains voltage, S... g When switched to the off state, the load is powered by a cascaded dynamic voltage restorer. The control block diagram of the load converter is shown below. Figure 2 As shown, the load converter control system consists of two parts: voltage and current dual closed-loop control and virtual impedance control.
[0054] Figure 2 middle, , , , , , V bus These are the components of the virtual current input feedforward on the d and q axes, the transfer function of the virtual impedance controller on the d and q axes, the transfer function of the voltage controller, the transfer function of the current controller, and the bus capacitor voltage, respectively. , , , , , , , These represent the components of the AC side filter capacitor voltage command value on the d and q axes, the grid connection point voltage on the d and q axes, the current controller command value on the d and q axes, and the filter inductor current on the d and q axes, respectively; C f L f These are the filter capacitor and filter inductor, respectively, R f ω is the internal resistance of the filter inductor, and ω is the angular frequency of the power grid. Figure 3 Middle, Y in,op (s), Y tr,1,op (s), Y tr,2,op (s), Z o,op (s), G di,1,op (s), G di,2,op (s) are all open-loop transfer functions of the load converter, G m (s) is the transfer function for PWM modulation. See also... Figure 3 The load converter is modeled as an equivalent Norton circuit. According to Mason's formula, its DC-side input impedance in the low-frequency range is expressed as follows:
[0055] ;
[0056] In the formula, Z in,cl (s), V dc P CPL Y in Y vir (s) and s are the DC-side input impedance of the load converter in the low-frequency band, the DC component of the DC-side bus voltage, the input power of the load converter, the negative input admittance of the load converter on the DC side before adding the virtual impedance, the admittance of the parallel input virtual impedance on the DC side, and the differential operator, respectively.
[0057] Combination Figure 1 , 2 As shown in Figures 3 and 4, this invention proposes a control method for power adaptive virtual impedance oscillation suppression in a cascaded dynamic voltage restorer, comprising:
[0058] Step S1: The load converter is equivalent to the Norton equivalent circuit model, and the grid connection point voltage, filter inductor current and DC bus voltage of the cascaded dynamic voltage restorer are sampled respectively.
[0059] In each sampling period, the grid connection point voltage U is sampled respectively. PCC Filter inductor current I abc And will U PCC and I abc Perform an abc / dq transformation to obtain the components on the d and q axes. , , , .
[0060] Step S2: The difference between the AC side filter capacitor voltage command value given by the upper-level system and the grid connection point voltage is calculated, and the difference is used to obtain the current controller command value through the voltage controller.
[0061] The grid connection point voltage U PCC After abc / dq transformation, the grid connection point voltage U is obtained. PCC Components U on the d and q axes d U q Consider the dq decoupling term ωC f U d ωC f U q The command value of the current controller is calculated according to the following formula:
[0062] ;
[0063] In the formula, , , , , , These represent the dq-axis components of the current controller's command value and the dq-axis components of the AC side filter capacitor voltage command value, respectively. The grid connection point voltage U is also shown. PCC Components on the d and q axes; , , , These are the proportional and integral coefficients of the voltage controller, the grid angular frequency, and the filter capacitor, respectively.
[0064] Step S3: The second-order virtual impedance controller is used to extract the oscillation component of the DC bus voltage as the virtual current input feedforward of the current controller, which is equivalent to parallel input virtual impedance on the DC side of the load converter in the cascaded dynamic voltage restorer.
[0065] By employing a second-order virtual impedance controller to extract the oscillating component of the DC bus voltage as a virtual current input feedforward for the current controller, it is equivalent to connecting a positive real virtual impedance at the oscillation frequency in parallel on the DC side of the load converter. This weakens the negative real part of the original load converter input impedance at the oscillation frequency, thereby satisfying the Nyquist stability criterion, suppressing oscillations caused by load power changes, and improving system stability. Furthermore, the virtual impedance controller is only second-order, making it easy to control and implement.
[0066] Specifically, the calculation formula for the second-order virtual impedance controller to extract the oscillating component of the DC bus voltage as the virtual current input feedforward of the current controller is as follows:
[0067] ;
[0068] In the formula, , , , V bus (s) represent the components of the virtual current input feedforward on the d and q axes, respectively; the transfer function of the virtual impedance controller on the d and q axes; and the bus capacitor voltage; k is the adjustment parameter controlling the virtual impedance compensation level; G band (s) is a second-order bandpass filter, f osc The center frequency of the virtual impedance controller. The quality factor is used. The virtual impedance controller is only second-order and only requires the control parameter f. osc It is easy to control and implement.
[0069] In this embodiment, the admittance expression for the virtual impedance connected in parallel on the DC side of the load converter is as follows:
[0070] ;
[0071] In the formula Y vir (s) represents the admittance form of the virtual impedance, I Ldq (s) is the column vector expression of the AC side current of the load converter on the d and q axes.
[0072] Step S4: Subtract the command value of the current controller from the current of the filter inductor, and then add the difference to the virtual current input feedforward and input it to the current controller to obtain the modulation voltage.
[0073] The d-axis and q-axis components of the current controller's command value are subtracted from the d-axis and q-axis components of the filter inductor current, respectively. This difference is then superimposed with the virtual current input feedforward and fed into the current controller. The output of the current controller is then superimposed with the dq decoupling term ωLI. d ,ωLI q U d U qThe final modulation voltage D of the load converter is obtained. d D q The calculation formula is as follows:
[0074] ;
[0075] In the formula, D d D q ωLI represents the d-axis and q-axis components of the modulation voltage. d ,ωLI q U d U q All are dq decoupling terms, K ii This represents the integral coefficient of the current controller.
[0076] Step S5: Calculate the difference between the DC bus voltage and the reference bus voltage, and iterate through typical frequency values to change the center frequency of the virtual impedance controller until the difference is less than the set threshold.
[0077] Figure 4 This is a flowchart illustrating the process of changing the center frequency of the virtual impedance controller in a specific embodiment of the present invention. Since the virtual impedance controller has a certain control bandwidth, it can effectively suppress system oscillations as long as the actual system oscillation frequency is within that bandwidth. Therefore, in this specific embodiment, by detecting the difference between the DC bus voltage and the rated voltage, the center frequency of the virtual impedance controller is selected cyclically from only 10 typical frequencies until the difference is less than a set threshold. This greatly improves the response speed of the virtual impedance controller to load changes.
[0078] Specifically, taking 2% of the DC bus voltage as the set threshold, the formula for determining the center frequency of the virtual impedance controller is as follows:
[0079] ;
[0080] In the formula, , , , These are the reference bus voltage, the DC component of the DC-side bus voltage, the difference between the DC bus voltage and the reference bus voltage, and the set threshold.
[0081] The center frequency f of the virtual impedance controller osc The modification rule is as follows: Divide the system oscillation frequency band from 0 to 1000 Hz into 10 bands, and take the center value of each band as its typical frequency, f osc Select only from 10 typical frequencies in a loop until... Established.
[0082] Further reading Figure 4 , The column vector of the transfer function of the virtual impedance controller on the d and q axes is represented by a backward differential pair. Discretization is performed as shown in the following equation:
[0083] ;
[0084] In the above formula, , Discrete domain virtual impedance controllers Input and output variables on the d-axis or q-axis, The sampling period is This refers to the d-axis or q-axis component of the AC side filter capacitor voltage command value.
[0085] Step S6: Adaptively adjust the DC-side input impedance of the load converter within the control bandwidth of the virtual impedance controller according to the load power.
[0086] When the load power changes, the oscillation amplitude on the actual DC bus voltage also changes, and may even increase with the increase in load power. However, the virtual impedance controller in this specific embodiment of the invention adapts to changes in load power, adjusting the amplitude of the parallel virtual impedance accordingly, thereby allowing the system to reach a new stable state. Furthermore, the virtual impedance controller only adjusts the DC-side input impedance of the load converter within its control bandwidth. In addition, during the adaptive process, the virtual impedance controller does not require additional power detection circuitry or current sensors.
[0087] Specifically, when the load power changes, ignoring the active power losses of the lines and filter branches, the AC side load power P Load Always equal to the DC-side input power P of the load converter CPL And U d U q Always controlled by cascaded dynamic voltage restorer , The admittance expression for the DC-side parallel virtual impedance in the low-frequency band is calculated using the following formula:
[0088] ;
[0089] The expression for the DC-side input impedance of the load converter is as follows:
[0090] ;
[0091] Further results were obtained:
[0092] ;
[0093] In the formula, Z in,cl(s) represents the DC-side input impedance of the load converter in the low-frequency band, V dc P CPL These are the DC component of the DC-side bus voltage and the load converter input power, respectively. k is an adjustment parameter controlling the virtual impedance compensation level, 0 < k < 1, G band (s) is a second-order bandpass filter, f osc The center frequency of the virtual impedance controller, Δf osc Here, ω is the bandwidth of the virtual impedance controller, and k is the angular frequency. The parameter k only needs to be between 0 and 1 to reduce the amplitude of Z. in,cl The low-frequency negative real part impedance of (s) ensures system stability margin while greatly avoiding the impact on the dynamic performance of the load converter.
[0094] In the low-frequency band, Z within the virtual impedance controller bandwidth in,cl The negative real part of (s) is weakened by adjusting the parameter k, and Z outside the bandwidth in,cl (s) is It remains the negative impedance before the addition of virtual impedance and is not affected by virtual impedance. The low frequency band is 0~1kHz.
[0095] In a specific embodiment of the present invention, to verify the effectiveness of the designed virtual impedance controller in maintaining the dynamic performance of the load converter and suppressing oscillations, no virtual impedance was applied during the simulation period of 0.25~0.3s, and a virtual impedance was applied during the simulation period of 0.3~0.35s. The simulation results are as follows. Figure 5 , Figure 6 As shown.
[0096] Figure 5 This is a Bode plot of the DC-side input impedance and output impedance before and after adding a virtual impedance in a specific embodiment of the present invention, where Z... in,cl (s) represents the DC-side input impedance of the load converter before the virtual impedance is added, Z in,cl_new (s) represents the DC-side input impedance of the load converter after adding a virtual impedance; from Figure 5 From this, we can conclude that Z in,cl_new (s), Z in,cl (s) The Bode curves of the two differ only near the oscillation frequency, and almost overlap in the remaining frequency bands. Therefore, the specific embodiments of the present invention ensure that the dynamic performance of the load converter remains almost unchanged in other frequency bands.
[0097] See Figure 6 It can be seen that when no virtual impedance is applied within 0.25~0.3s, the DC bus voltage oscillates; when a virtual impedance is applied within 0.3~0.35s, the DC bus voltage oscillation component disappears rapidly, leaving only the switching ripple, which verifies the speed and effectiveness of the virtual impedance in suppressing oscillation in the specific embodiment of the present invention.
[0098] In a specific embodiment of the present invention, to verify the power adaptability of the designed virtual impedance controller, two operating conditions were set: Case 1: load power = 10kW; Case 2: load power = 20kW. The simulation switched from Case 1 to Case 2 at 0.3s, and the results are as follows. Figure 7 As shown. See also Figure 7 Figure (a) shows the DC bus voltage waveform under load power variation without virtual impedance, and Figure (b) shows the DC bus voltage waveform under load power variation with virtual impedance. As can be seen from Figures (a) and (b), without virtual impedance, the oscillating component of the DC bus voltage increases with increasing load power; with virtual impedance, the DC bus voltage exhibits no oscillating component regardless of load power variation, verifying the power adaptability of the designed virtual impedance controller. These simulation results demonstrate the effectiveness of the present invention.
[0099] In summary, this invention, based on the command from the upper-level system to the dynamic voltage restorer to output three-phase voltage, obtains the corresponding current command signal according to the voltage controller and coordinate transformation. This signal is then superimposed with the DC bus oscillation component of the parallel input virtual impedance of the load converter in the equivalent cascaded dynamic voltage restorer, serving as the reference input signal for the current controller. This weakens the low-frequency negative real-value input impedance of the load converter. Compared to existing technologies, this invention can adaptively adjust the input impedance of the load converter according to the load power, significantly improving the small-signal stability of the cascaded dynamic voltage restorer under varying load power conditions. Furthermore, it avoids the influence of virtual impedance on the dynamic performance of the load converter, improving the response speed to load changes. Simultaneously, it features a low controller order, fewer control parameters, and is easily digitized.
[0100] The above description is merely a specific embodiment of the present invention, enabling those skilled in the art to understand or implement the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the present invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.
Claims
1. A power adaptive virtual impedance oscillation suppression control method for a cascaded dynamic voltage restorer, characterized in that, The method includes: The load converter is equivalent to the Norton equivalent circuit model, and the grid connection point voltage, filter inductor current, and DC bus voltage of the cascaded dynamic voltage restorer are sampled respectively. The difference between the AC side filter capacitor voltage command value given by the upper-level system and the grid connection point voltage is used to obtain the current controller command value through the voltage controller. A second-order virtual impedance controller is used to extract the oscillation component of the DC bus voltage as the virtual current input feedforward of the current controller, which is equivalent to parallel input virtual impedance on the DC side of the load converter in the cascaded dynamic voltage restorer. The difference between the command value of the current controller and the current of the filter inductor is then added to the virtual current input feedforward and input to the current controller to obtain the modulation voltage. The difference between the DC bus voltage and the reference bus voltage is calculated, and the typical frequency values are iterated to change the center frequency of the virtual impedance controller until the difference is less than the set threshold. The virtual impedance controller adaptively adjusts the DC-side input impedance of the load converter within the control bandwidth based on the load power.
2. The power adaptive virtual impedance oscillation suppression control method for a cascaded dynamic voltage restorer according to claim 1, characterized in that, The load converter is equivalent to the Norton equivalent circuit model, with a low-frequency range of 0~1kHz. The simplified expression for the DC-side input impedance of the load converter in this frequency range is as follows: ； In the formula, Z in,cl (s), V dc P CPL Y in Y vir (s) and s are the DC-side input impedance of the load converter in the low-frequency band, the DC component of the DC-side bus voltage, the input power of the load converter, the negative input admittance of the load converter on the DC side before adding the virtual impedance, the admittance of the parallel input virtual impedance on the DC side, and the differential operator, respectively.
3. The control method for power adaptive virtual impedance oscillation suppression of the cascaded dynamic voltage restorer according to claim 1, characterized in that, The difference between the AC-side filter capacitor voltage command value given by the upper-level system and the grid connection point voltage is used to obtain the current controller command value through the voltage controller, including: The grid connection point voltage U PCC The grid connection point voltage U is obtained after abc / dq transformation. PCC Components U on the d and q axes d U q Consider the dq decoupling term ωCU d ,ωCU q The command value of the current controller is calculated according to the following formula: ； In the formula, , , , , , These represent the d- and q-axis components of the current controller's command value and the d- and q-axis components of the AC side filter capacitor voltage command value, respectively. The grid connection point voltage U... PCC Components on the d and q axes; , , , These are the proportional and integral coefficients of the voltage controller, the grid angular frequency, and the filter capacitor, respectively.
4. The power adaptive virtual impedance oscillation suppression control method for a cascaded dynamic voltage restorer according to claim 3, characterized in that, The calculation formula for using the oscillation component of the DC bus voltage extracted by the second-order virtual impedance controller as the virtual current input feedforward of the current controller is as follows: ； In the formula, , , , V bus (s) represent the components of the virtual current input feedforward on the d and q axes, the transfer function of the virtual impedance controller on the d and q axes, and the bus capacitor voltage, respectively; K ip G is the proportional gain of the current controller, k is the adjustment parameter for controlling the virtual impedance compensation level, and G is the proportional gain of the current controller. band (s) is a second-order bandpass filter, f osc The center frequency of the virtual impedance controller. For the quality factor, the virtual impedance controller is only second-order and only requires controlling the parameter f. osc .
5. The power adaptive virtual impedance oscillation suppression control method for a cascaded dynamic voltage restorer according to claim 4, characterized in that, The admittance expression for the parallel input virtual impedance of the load converter on the DC side in a cascaded dynamic voltage restorer is: ； In the formula, Y vir (s) represents the admittance form of the virtual impedance, I Ldq (s) is the column vector expression of the AC side current of the load converter on the d and q axes.
6. The power adaptive virtual impedance oscillation suppression control method for a cascaded dynamic voltage restorer according to claim 4, characterized in that, The difference between the current controller's command value and the filter inductor current is then added to the virtual current input feedforward and input to the current controller. The formula for calculating the modulation voltage is as follows: ； In the formula, D d D q K represents the d-axis and q-axis components of the modulation voltage. ii ωL is the integral coefficient of the current controller. f I d ,ωL f I q U d U q Both are d and q decoupling terms, where ω is the grid angular frequency, and L... f For filter inductance, I d I q These are the components of the filter inductor current on the d and q axes, respectively.
7. The power adaptive virtual impedance oscillation suppression control method for a cascaded dynamic voltage restorer according to claim 4, characterized in that, The difference between the DC bus voltage and the reference bus voltage is calculated, and the typical frequency values are iterated to change the center frequency of the virtual impedance controller until the difference is less than a set threshold, including: Taking 2% of the DC bus voltage as the set threshold, the formula for determining the center frequency of the virtual impedance controller is as follows: ； In the formula, , , , These are the reference bus voltage, the DC component of the DC-side bus voltage, the difference between the DC bus voltage and the reference bus voltage, and the set threshold.
8. The power adaptive virtual impedance oscillation suppression control method for a cascaded dynamic voltage restorer according to claim 7, characterized in that, The described loop iterating through typical frequency values to change the center frequency of the virtual impedance controller includes: The center frequency f of the virtual impedance controller osc The modification rule is as follows: Divide the system oscillation frequency band of the cascaded dynamic voltage restorer from 0 to 1000 Hz into 10 bands, and take the center value of each band as its typical frequency. osc Select from 10 typical frequencies in a loop until... Established.
9. The power adaptive virtual impedance oscillation suppression control method for a cascaded dynamic voltage restorer according to claim 4, characterized in that, Based on the load power, the DC-side input impedance of the load converter is adaptively adjusted within the control bandwidth of the virtual impedance controller, including: The load power variation ignores the active power losses of the line and filter branches, and the AC side load power P Load Always equal to the DC-side input power P of the load converter CPL And U d U q Always controlled by cascaded dynamic voltage restorer , The DC-side input impedance of the load converter in the low-frequency band is calculated using the following formula: ； In the formula, Z in,cl (s) represents the DC-side input impedance of the load converter in the low-frequency band, V dc P CPL These are the DC component of the DC-side bus voltage and the input power of the load converter, respectively. k is an adjustment parameter controlling the virtual impedance compensation level, 0 < k < 1, Δf osc The bandwidth of the virtual impedance controller is ω, and the angular frequency is ω. In the low-frequency band, Z within the virtual impedance controller bandwidth in,cl The negative real part of (s) is weakened by adjusting the parameter k, while Z outside the bandwidth in,cl (s) is It remains the negative impedance before the addition of virtual impedance, and the low frequency band is 0~1kHz.