Power self-adaptive virtual impedance oscillation suppression control method for cascaded dynamic voltage restorer

Abstract

The invention discloses a control method for power adaptive virtual impedance oscillation suppression of a cascaded dynamic voltage restorer, and relates to the technical field of power electronics. According to an instruction that a superior system outputs a three-phase voltage to the dynamic voltage restorer, a corresponding current instruction signal is obtained according to a voltage controller and coordinate transformation, and a direct-current bus oscillation component of a load converter parallel input virtual impedance in the equivalent cascade type dynamic voltage restorer is superposed to serve as a reference input signal of the current controller; therefore, the low-frequency-band negative real number input impedance of the load converter is weakened. Compared with the prior art, the input impedance can be adaptively adjusted according to the load power, the small signal stability of the cascade type dynamic voltage restorer under the condition that the load power changes is greatly improved, the influence of virtual impedance on the dynamic performance of the load converter can be avoided, the response speed to the load change is improved, and the stability of the load converter is improved. And meanwhile, the controller is low in order, few in control parameters and easy to digitally control.

Description

Technical Field

[0001] The present invention belongs to the field of impedance suppression, and particularly relates to a power adaptive virtual impedance oscillation suppression control method for a cascaded dynamic voltage restorer. Background Art

[0002] With the large-scale access of renewable energy such as wind power and solar power generation to the power grid, their characteristics of intermittency, large volatility, and strong uncertainty have led to a large fluctuation in power quality. In the power system, 80% of the power quality problems are voltage sags, which are the key points and difficulties in the current research on power quality governance. Therefore, the cascaded dynamic voltage restorer has emerged, with a strong compensation ability for grid sags. However, due to the constant power load characteristic of the load converter in the cascaded dynamic voltage restorer, the cascaded dynamic voltage restorer will be affected by the load power fluctuation, resulting in serious system oscillation, causing the system to lose stability and endangering electrical equipment. Summary of the Invention

[0003] Object of the Invention. The object of the present invention is to provide a control method for power adaptive virtual impedance oscillation suppression of a cascaded dynamic voltage restorer to solve the system oscillation problem of the cascaded dynamic voltage restorer under the condition of load power fluctuation.

[0004] Technical Solution. To achieve the above object, the present invention adopts a power adaptive virtual impedance oscillation suppression control method for a cascaded dynamic voltage restorer, and the method includes:

[0005] Equivalent the load converter to a Norton equivalent circuit model, and sample the grid connection point voltage, filter inductor current, and DC bus voltage of the cascaded dynamic voltage restorer respectively;

[0006] Subtract the grid connection point voltage from the AC side filter capacitor voltage command value given by the upper-level system, and the difference is obtained through the voltage controller to get the command value of the current controller;

[0007] Adopt a 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, and equivalently connect an input virtual impedance in parallel on the DC side of the load converter in the cascaded dynamic voltage restorer;

[0008] Subtract the command value of the current controller from the filter inductor current, and the obtained difference is superimposed with the virtual current input feedforward and input to the current controller to obtain the modulation voltage;

[0009] Subtract the DC bus voltage from the reference bus voltage, and cycle through the typical frequency values to change the center frequency of the virtual impedance controller until the difference is less than the set threshold;

[0010] Adjust the input impedance of the DC side of the load converter within the control bandwidth of the virtual impedance controller adaptively according to the load power.

[0011] Further, the load converter is equivalent to a Norton equivalent circuit model, and its low-frequency band is 0 to 1 kHz. The simplified expression of the input impedance of the DC side of the load converter within this frequency band is as follows:

[0012]

[0013] In the formula, Z in,cl (s), V dc , P CPL , Y in , Y vir (s), and s are respectively the input impedance of the DC side of the load converter within 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 DC side of the load converter before adding the virtual impedance, the admittance of the parallel input virtual impedance on the DC side, and the differential operator.

[0014] Further, the process of obtaining the command value of the current controller by taking the difference between the AC side filter capacitor voltage command value given by the upper-level system and the grid connection point voltage, and passing the difference through the voltage controller includes:

[0015] Perform an abc / dq transformation on the grid connection point voltage U PCC to obtain the components U PCC of the grid connection point voltage on the d and q axes, U d , U q ; Considering the dq decoupling terms ωCU d , ωCU q , calculate the command value of the current controller according to the following formula:

[0016]

[0017] In the formula, U d , U q are respectively the components of the command value of the current controller on the d and q axes, the components of the AC side filter capacitor voltage command value on the d and q axes, and the components of the AC side filter capacitor voltage on the d and q axes; K vp , K vi , ω, C f are respectively the proportional coefficient, integral coefficient of the current controller, grid angular frequency, and filter capacitor.

[0018] Further, 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:

[0019]

[0020] Wherein, I vir-d and I vir-q , G vir-d (s), G vir-q (s), V bus (s) are respectively the components of the virtual current input feedforward on the d-axis and q-axis, the transfer functions of the virtual impedance controller on the d-axis and q-axis, and the bus capacitor voltage; K ip is the proportional coefficient of the current controller, k is the adjustment parameter for controlling the virtual impedance compensation strength, G band (s) is a second-order band-pass filter, f osc is the center frequency of the virtual impedance controller, Q is the quality factor. Among them, the virtual impedance controller has only two orders and only needs to control the parameter f osc .

[0021] Furthermore, the admittance expression of the virtual impedance input in parallel at the DC side of the load converter in the cascaded dynamic voltage restorer is:

[0022]

[0023] Wherein, Y vir (s) is the admittance form of the virtual impedance, I Ldq (s) is the column vector expression of the current on the AC side of the load converter on the d-axis and q-axis.

[0024] Furthermore, the command value of the current controller is subtracted from the filter inductor current, and the obtained difference is superimposed with the virtual current input feedforward and input to the current controller. The calculation formula for the modulation voltage is as follows:

[0025]

[0026] Wherein, D d and D q are the d-axis and q-axis components of the modulation voltage, K ii is the integral coefficient of the current controller, ωL f I d , ωL f I q , U d , U q are all d-q decoupling terms. Among them, ω is the grid angular frequency, L f is the filter inductor, I d , I q are respectively the components of the filter inductor current on the d-axis and q-axis.

[0027] Further, the difference between the DC bus voltage and the reference bus voltage is taken, and the typical frequency values are cycled through to change the center frequency of the virtual impedance controller until the difference is less than the set threshold, including:

[0028] Take 2% of the DC bus voltage as the set threshold, and the formula for determining the change of the center frequency of the virtual impedance controller is as follows:

[0029]

[0030] In the formula, V dc , ΔV dc , and threshold 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 respectively.

[0031] Further, the cycling through the typical frequency values to change the center frequency of the virtual impedance controller includes:

[0032] The change rule of the center frequency f osc of the virtual impedance controller: Divide the system oscillation frequency band 0 - 1000 Hz of the cascaded dynamic voltage restorer into 10 frequency bands, take the center value of each frequency band as its respective typical frequency, and f osc is cycled through and selected from the 10 typical frequencies until |ΔV dc | ≤ threshold holds.

[0033] Further, according to the load power, adaptively adjust the DC side input impedance of the load converter within the control bandwidth of the virtual impedance controller, including:

[0034] When the load power changes, ignoring the active power loss of the line and the filtering branch, the AC side load power P Load is always equal to the DC side input power P CPL of the load converter, and U d , U q are always controlled by the cascaded dynamic voltage restorer as Calculate the DC side input impedance of the load converter in the low frequency band according to the following formula:

[0035]

[0036] In the formula, Z in,cl (s) is the DC side input impedance of the load converter in the low frequency band, V dc , P CPL are the DC component of the DC side bus voltage and the input power of the load converter respectively, k is an adjustment parameter for controlling the virtual impedance compensation strength, 0 < k < 1, Δf osc is the bandwidth of the virtual impedance controller, and ω is the angular frequency;

[0037] In the low-frequency band, the negative real part of Z(s) within the bandwidth of the virtual impedance controller is weakened by adjusting the parameter k, while Z(s) outside the bandwidth remains the negative impedance before adding the virtual impedance. The low-frequency band is 0 - 1 kHz. in,cl (s) has its negative real part weakened by adjusting the parameter k, while Z(s) outside the bandwidth remains the negative impedance before adding the virtual impedance. in,cl (s) is still the negative impedance before adding the virtual impedance, and the low-frequency band is 0 - 1 kHz.

[0038] Advantageous effects: Compared with the prior art, the technical solution of the present invention has the following advantageous technical effects:

[0039] 1. The method provided by the present invention can effectively suppress the system oscillation when the cascaded dynamic voltage restorer powers the load. At the same time, it can adaptively adjust the DC-side input impedance of the load converter according to the change of the load power to maintain the system stability.

[0040] 2. While suppressing the system oscillation, the method provided by the present invention only changes the input impedance of the load converter in the cascaded dynamic voltage restorer within the bandwidth of the virtual impedance controller, avoiding the influence of the virtual impedance on the dynamic performance of other frequency bands of the load converter.

[0041] 3. The method provided by the present invention can greatly improve the response speed of the virtual impedance controller to the system oscillation, and the controller is only second-order, without the need for a power detection circuit and additional sampled current, which is easy to control and implement.

[0042] 4. The method provided by the present invention is applicable to any source converter in the cascaded dynamic voltage restorer, and the introduced virtual impedance does not affect the source converter. Description of the Drawings

[0043] Figure 1 It is the system topology diagram of the cascaded dynamic voltage restorer in the specific embodiment of the present invention;

[0044] Figure 2 It is the control block diagram of the load converter of the cascaded dynamic voltage restorer in the specific embodiment of the present invention;

[0045] Figure 3 It is the small-signal control block diagram of the load converter in the specific embodiment of the present invention;

[0046] Figure 4 It is the flow chart for changing the center frequency of the virtual impedance controller in the specific embodiment of the present invention;

[0047] Figure 5 It is the bode diagram of the DC-side input and output impedance before and after adding the virtual impedance in the specific embodiment of the present invention;

[0048] Figure 6Waveform diagram of the DC-side bus voltage before and after adding virtual impedance in the specific embodiment of the present invention;

[0049] Figure 7 Waveform diagram of the DC-side bus voltage under the change of load power in the specific embodiment of the present invention. Specific embodiments

[0050] To make the objectives, technical solutions, and advantages of the embodiments of the present application clearer, the following will clearly and completely describe the technical solutions in the embodiments of the present application with reference to the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the scope of protection of the present application.

[0051] The following further elaborates on the embodiments of the present invention with reference to the accompanying drawings.

[0052] As mentioned above, when the cascaded dynamic voltage restorer powers a load, due to the constant-power load characteristic of the load converter, the cascaded dynamic voltage restorer will be affected by load power fluctuations, resulting in severe system oscillations, causing the system to lose stability and endangering electrical equipment. Therefore, based on the cascaded dynamic voltage restorer, the present invention proposes a control method for suppressing oscillations of the cascaded dynamic voltage restorer's power adaptive virtual impedance.

[0053] As Figure 1 shown, the cascaded dynamic voltage restorer consists of three parts: a source converter, a load converter, and an LC filter. The output terminal of the LC filter is connected to the load through a connection point, and e a 、e b 、e c are the three-phase grid voltages respectively, L g is the grid impedance, and S g is a bidirectional thyristor. When the grid voltage experiences a sag, S g switches to the off state, and the load is powered by the cascaded dynamic voltage restorer. Among them, the control block diagram of the load converter is as shown in Figure 2 shown, and the load converter control system consists of two parts: voltage-current double closed-loop control and virtual impedance control.

[0054] Figure 2 In, I vir-d 、I vir-q , G vir-d (s), G vir-q (s), G v (s), G i (s), V busThey are the components of the virtual current input feedforward on the d-axis and q-axis, the transfer functions of the virtual impedance controller on the d-axis and q-axis, the transfer function of the voltage controller, the transfer function of the current controller, and the bus capacitor voltage respectively. U d and U q , I d and I q are the components of the command value of the AC-side filter capacitor voltage on the d-axis and q-axis, the components of the grid connection point voltage on the d-axis and q-axis, the components of the command value of the current controller on the d-axis and q-axis, and the components of the filter inductor current on the d-axis and q-axis respectively; C f and L f are the filter capacitor and the filter inductor respectively, and R f is the internal resistance of the filter inductor, and ω is the grid angular frequency. Figure 3 In, 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 the open-loop transfer functions of the load converter, and G m (s) is the transfer function of PWM modulation. Refer to Figure 3 , the load converter is equivalent to a Norton equivalent circuit model. According to Mason's formula, the expression of the DC-side input impedance in its low-frequency band is 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 DC-side of the load converter before adding the virtual impedance, the admittance of the DC-side parallel input virtual impedance, and the differential operator respectively.

[0057] Combined with Figure 1 , 2 , Figures 3 and 4 show that the control method for suppressing the power adaptive virtual impedance oscillation of the cascaded dynamic voltage restorer proposed by the present invention includes:

[0058] Step S1, equivalent the load converter to a Norton equivalent circuit model, and sample the grid connection point voltage, filter inductor current, and DC bus voltage of the cascaded dynamic voltage restorer respectively;

[0059] In each sampling period, the grid-connected point voltage U PCC and the filter inductor current I abc are sampled respectively, and U PCC and I abc are subjected to abc / dq transformation to obtain their respective components on the d and q axes, U d , U q , I d , I q .

[0060] Step S2: Subtract the command value of the AC-side filter capacitor voltage given by the upper-level system from the grid-connected point voltage, and the difference is used to obtain the command value of the current controller through the voltage controller;

[0061] The grid-connected point voltage U PCC is subjected to abc / dq transformation to obtain the components of the grid-connected point voltage U PCC on the d and q axes, U d , U q ; Considering the dq decoupling terms ωC f U d , ωC f U q , calculate the command value of the current controller according to the following formula:

[0062]

[0063] In the formula, U d , U q are the components of the command value of the current controller on the dq axes, the components of the command value of the AC-side filter capacitor voltage on the dq axes, and the components of the AC-side filter capacitor voltage on the dq axes respectively; K vp , K vi , ω, C f are the proportional coefficient, integral coefficient of the current controller, grid angular frequency, and filter capacitor respectively.

[0064] Step S3: Use a 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, which is equivalent to connecting a virtual impedance in parallel at the DC side of the load converter in the cascaded dynamic voltage restorer;

[0065] Using a 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 can be equivalent to connecting a positive real virtual impedance at the oscillation frequency in parallel at the DC side of the load converter, so as to weaken the negative real part of the original input impedance of the load converter at the oscillation frequency, thereby meeting the Nyquist stability criterion, suppressing the oscillation caused by the change of the load power, and improving the system stability. In addition, the virtual impedance controller is only second-order and is easy to control and implement.

[0066] Specifically, the calculation formula for 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:

[0067]

[0068] In the formula, I vir-d and I vir-q , G vir-d (s), G vir-q (s), V bus (s) are the components of the virtual current input feedforward on the d-axis and q-axis, the transfer functions of the virtual impedance controller on the d-axis and q-axis, and the bus capacitor voltage, respectively; k is an adjustment parameter for controlling the virtual impedance compensation strength, G band (s) is a second-order band-pass filter, f osc is the center frequency of the virtual impedance controller, and Q is the quality factor. Among them, the virtual impedance controller has only two orders and only needs to control the parameter f osc , which is easy to control and implement.

[0069] In this embodiment, the admittance expression of the virtual impedance connected in parallel to the DC side of the load converter is as follows:

[0070]

[0071] In the formula, Y vir (s) is the admittance form of the virtual impedance, and I Ldq (s) is the column vector expression of the load converter AC side current on the d-axis and q-axis.

[0072] Step S4: Subtract the command value of the current controller from the filter inductor current, and then superimpose the obtained difference with the virtual current input feedforward and input it to the current controller to obtain the modulation voltage;

[0073] Subtract the d-axis and q-axis components of the current controller command value from the d-axis and q-axis components of the filter inductor current respectively, then superimpose the virtual current input feedforward and send it to the current controller. The output of the current controller is then superimposed with the dq decoupling terms ωLI d , ωLI q , U d , U q to obtain the final modulation voltage D d , D q of the load converter. The calculation formula is as follows:

[0074]

[0075] In the formula, D d , D q are the d-axis and q-axis components of the modulation voltage, and ωLI d, ωLI q , U d , U q are all dq decoupling terms, and K ii is the integral coefficient of the current controller.

[0076] Step S5: Subtract the DC bus voltage from the reference bus voltage, and loop through the 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 the flowchart for 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, as long as the actual system oscillation frequency is within the control bandwidth, the virtual impedance controller can effectively suppress system oscillation. Therefore, in a specific embodiment of the present invention, by detecting the difference between the DC bus voltage and the rated voltage, the center frequency of the virtual impedance controller is only selected by looping through 10 typical frequencies one by one until the difference is less than the set threshold, thus greatly improving the response speed of the virtual impedance controller to load changes.

[0078] Specifically, take 2% of the DC bus voltage as the set threshold, and the formula for determining the change of the center frequency of the virtual impedance controller is as follows:

[0079]

[0080] In the formula, V dc , ΔV dc , and threshold 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, respectively.

[0081] The change rule of the center frequency f osc of the virtual impedance controller is: divide the system oscillation frequency band of 0 - 1000 Hz into 10 frequency bands, take the center value of each frequency band as its respective typical frequency, and f osc is only selected by looping through 10 typical frequencies until |ΔV dc | ≤ threshold holds.

[0082] Further referring to Figure 4 , G vir-dq (s) is the transfer function column vector of the virtual impedance controller on the d and q axes. The backward difference is used to discretize G vir-dq (s), as shown in the following formula:

[0083]

[0084] In the above formula, x i , yi are the input variable and output variable of the discrete-domain virtual impedance controller G vir-dq (z) on the d-axis or q-axis, and T s is the sampling period, is the component of the AC-side filter capacitor voltage command value on the d-axis or q-axis.

[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 will also change, and may even increase due to the increase in load power. The virtual impedance controller in the specific embodiment of the present invention can adapt to the change of load power, and correspondingly adjust the amplitude of the parallel virtual impedance, so as to make the system reach a new stable state; and the virtual impedance controller only adjusts the DC-side input impedance of the load converter within the control bandwidth. In addition, during the adaptation process, the virtual impedance controller does not require an additional power detection circuit and current sensor.

[0087] Specifically, when the load power changes, ignoring the active power loss of the line and the filtering branch, the AC-side load power P Load is always equal to the DC-side input power P CPL of the load converter, and U d , U q are always controlled by the cascaded dynamic voltage restorer as Calculate the admittance expression of the DC-side parallel virtual impedance in the low-frequency band according to the following formula:

[0088]

[0089] Then the expression of the DC-side input impedance of the load converter is as follows:

[0090]

[0091] Furthermore, it can be obtained that:

[0092]

[0093] In the formula, Z in,cl (s) is the DC-side input impedance of the load converter in the low-frequency band, V dc , P CPL are the DC component of the DC-side bus voltage and the input power of the load converter respectively, k is an adjustment parameter for controlling the virtual impedance compensation strength, 0 < k < 1, G band (s) is a second-order band-pass filter, f osc is the center frequency of the virtual impedance controller, and Δf oscis the bandwidth of the virtual impedance controller, and ω is the angular frequency. As long as the parameter k takes values between 0 and 1, the negative real part impedance in the low frequency band of Z in,cl (s) is weakened. While ensuring the system stability margin, the influence on the dynamic performance of the load converter is greatly avoided.

[0094] In the low frequency band, the negative real part of Z in,cl (s) within the bandwidth of the virtual impedance controller is weakened by adjusting the parameter k, and Z in,cl (s) outside the bandwidth is still the negative impedance before adding the virtual impedance and is not affected by the virtual impedance. The low frequency band is 0 - 1 kHz.

[0095] In a specific embodiment of the present invention, in order to verify the effect of the designed virtual impedance controller in maintaining the dynamic performance of the load converter and suppressing oscillations, no virtual impedance is added within the simulation time of 0.25 - 0.3 s, and virtual impedance is added within 0.3 - 0.35 s. The simulation results are as Figure 5 、 Figure 6 shown.

[0096] Figure 5 is the bode diagram of the DC - side input impedance and output impedance before and after adding the virtual impedance in a specific embodiment of the present invention. Among them, Z in,cl (s) is the DC - side input impedance of the load converter before adding the virtual impedance, and Z in,cl_new (s) is the DC - side input impedance of the load converter after adding the virtual impedance; It can be obtained from Figure 5 that the bode curves of Z in,cl_new (s) and Z in,cl (s) are only different near the oscillation frequency, and almost overlap in the remaining frequency bands. Therefore, the specific embodiment of the present invention ensures that the dynamic performance of the load converter in other frequency bands remains almost unchanged.

[0097] Referring to Figure 6 it can be seen that when no virtual impedance is added within 0.25 - 0.3 s, the DC bus voltage oscillates; when virtual impedance is added within 0.3 - 0.35 s, the oscillation component of the DC bus voltage quickly disappears, leaving only the switching ripple, verifying the rapidity and effectiveness of the virtual impedance in suppressing oscillations in the specific embodiment of the present invention.

[0098] In a specific embodiment of the present invention, in order to verify the power adaptability of the designed virtual impedance controller, two working conditions are set: Case1: Load power = 10 kW; Case2: Load power = 20 kW. The simulation switches from Case1 to Case2 at 0.3 s, and the results are as Figure 7 shown. Referring to Figure 7In Figure (a), the DC bus voltage waveform under the change of load power without adding virtual impedance is shown. In Figure (b), the DC bus voltage waveform under the change of load power with the addition of virtual impedance is shown. From Figures (a) and (b), it can be seen that without adding virtual impedance, the oscillation component of the DC bus voltage increases as the load power increases. After adding virtual impedance, regardless of how the load power changes, there is no oscillation component in the DC bus voltage, verifying the power adaptability of the designed virtual impedance controller. The above simulation results prove the effectiveness of the present invention.

[0099] In summary, according to the instructions of the superior system for the output three-phase voltage of the dynamic voltage restorer, the present invention obtains the corresponding current command signal through the voltage controller and coordinate transformation, and superimposes the DC bus oscillation component of the load converter in the equivalent cascaded dynamic voltage restorer with a parallel input virtual impedance as the reference input signal of the current controller to weaken the low-frequency negative real input impedance of the load converter. Compared with the prior art, the present invention can adaptively adjust the input impedance of the load converter according to the load power, greatly improving the small-signal stability of the cascaded dynamic voltage restorer under the condition of load power change, and can avoid the influence of the virtual impedance on the dynamic performance of the load converter, improving the response speed to load changes. At the same time, the controller has a low order, few control parameters, and is easy to be digitally controlled.

[0100] The above are only specific embodiments of the present invention, enabling those skilled in the art to understand or implement the present invention. Various modifications to these embodiments will be obvious to those skilled in the art. The general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to these embodiments shown herein, but will conform to 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 a Norton equivalent circuit model, and the grid connection point voltage, filter inductor current and DC bus voltage of the cascade dynamic voltage restorer are sampled respectively; The voltage command value of the AC side filter capacitor given by the upper system is subtracted from the grid connection point voltage, and the difference is used to obtain the command value of the current controller 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 the virtual impedance connected in parallel to the DC side of the load converter in the cascade dynamic voltage restorer. The command value of the current controller is subtracted from the filter inductor current, and the obtained difference is then fed forward and superimposed with the virtual current input and input to the current controller to obtain a modulation voltage; Subtracting the DC bus voltage from the reference bus voltage, and cycling through typical frequency values ​​to change the center frequency of the virtual impedance controller until the difference is less than a set threshold; The DC side input impedance of the load converter is adaptively adjusted within the control bandwidth of the virtual impedance controller according to the load power.

2. A 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 a Norton equivalent circuit model, and its low frequency band is 0-1kHz. In this frequency band, the DC side input impedance of the load converter is simplified as follows: Wherein, Z in,cl (s), V dc , P CPL , Y in , Y vir (s), and s are respectively 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 DC side of the load converter before adding the virtual impedance, the admittance of the DC side in parallel with the input virtual impedance, and the differential operator.

3. The control method for suppressing the power adaptive virtual impedance oscillation of the cascaded dynamic voltage restorer according to claim 1, characterized in that, The voltage command value of the AC side filter capacitor given by the upper system is subtracted from the grid connection point voltage, and the difference is used to obtain the command value of the current controller through the voltage controller, including: The grid connection point voltage U PCC is transformed through the abc / dq transformation to obtain the grid connection point voltage U PCC on the d and q axes, with components U d and U q ; considering the dq decoupling terms ωCU d and ωCU q , the command value of the current controller is calculated according to the following formula: Wherein, U d and U q are the components of the command value of the current controller on the d-axis and q-axis, the components of the command value of the AC-side filter capacitor voltage on the d-axis and q-axis, and the components of the AC-side filter capacitor voltage on the d-axis and q-axis respectively; K vp and K vi , ω, and C f are the proportional coefficient and integral coefficient of the current controller, the grid angular frequency, and the filter capacitor respectively.

4. A power adaptive virtual impedance oscillation suppression control method for a cascaded dynamic voltage restorer according to claim 1, characterized in that, 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: where I vir-d and I vir-q are the components of the virtual current input feedforward on the d and q axes, G vir-d (s) and G vir-q (s) are the transfer functions of the virtual impedance controller on the d and q axes, and V bus (s) is the bus capacitor voltage; K ip is the proportional coefficient of the current controller, k is the adjustment parameter for controlling the virtual impedance compensation strength, G band (s) is a second-order band-pass filter, f osc is the center frequency of the virtual impedance controller, Q is the quality factor. Among them, the virtual impedance controller has only two orders and only needs to control the parameter f osc .

5. A power adaptive virtual impedance oscillation suppression control method for a cascaded dynamic voltage restorer according to claim 4, characterized in that The admittance expression of the DC-side parallel input virtual impedance equivalent to the load converter in the cascade dynamic voltage restorer is: where Y vir (s) is the admittance form of the virtual impedance, and I Ldq (s) is the column vector expression of the load converter AC side current on the d and q axes.

6. A power adaptive virtual impedance oscillation suppression control method for a cascaded dynamic voltage restorer according to claim 4, characterized in that, The command value of the current controller is subtracted from the filter inductor current, and the difference is then added to the virtual current input feedforward and input to the current controller to obtain the modulation voltage calculation formula as follows: Where D d and D q are the d - axis and q - axis components of the modulation voltage, K ii is the integral coefficient of the current controller, ωL f I d , ωL f I q , U d , and U q are all d - q decoupling terms. Among them, ω is the grid angular frequency, L f is the filter inductor, and I d , I q are respectively the components of the filter inductor current on the d - axis and q - axis.

7. A power adaptive virtual impedance oscillation suppression control method for a cascaded dynamic voltage restorer according to claim 4, characterized in that Subtract the DC bus voltage from the reference bus voltage and cycle through typical frequency values ​​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 changing the center frequency of the virtual impedance controller is as follows: Wherein, V dc , ΔV dc , and threshold 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 value, respectively.

8. A power adaptive virtual impedance oscillation suppression control method for a cascaded dynamic voltage restorer according to claim 7, characterized in that The loop iterates through typical frequency values ​​to change the center frequency of the virtual impedance controller, including: Center frequency f of the virtual impedance controller osc Change rule: Divide the system oscillation frequency band of the cascaded dynamic voltage restorer from 0 to 1000 Hz into 10 frequency bands, and take the central value of each frequency band as its respective typical frequency, f osc is cyclically selected from 10 typical frequencies until |ΔV dc | ≤ threshold holds.

9. A power adaptive virtual impedance oscillation suppression control method for a cascaded dynamic voltage restorer according to claim 2, characterized in that, According to 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 ignores the active power losses of the line and the filtering branch, and the AC-side load power P Load is always equal to the input power P CPL of the DC side of the load converter, and U d 、U q are always controlled by the cascaded dynamic voltage restorer to be Calculate the input impedance of the DC side of the load converter in the low-frequency band according to the following formula: where, Z in,cl (s) is the input impedance of the DC side of the load converter in the low-frequency band, V dc , P CPL are respectively the DC component of the DC side bus voltage and the input power of the load converter, k is the adjustment parameter for controlling the virtual impedance compensation strength, 0 < k < 1, Δf osc is the bandwidth of the virtual impedance controller, ω is the angular frequency; In the low-frequency band, the negative real part of Z(s) within the bandwidth of the virtual impedance controller is attenuated by adjusting the parameter k, while Z(s) outside the bandwidth remains the negative impedance before the addition of the virtual impedance. The low-frequency band is 0 to 1 kHz. in,cl (s) has its negative real part weakened by adjusting the parameter k, while Z in,cl (s) outside the bandwidth remains the negative impedance before adding the virtual impedance, and the low-frequency band is 0 - 1 kHz. The low-frequency band is 0 to 1 kHz.

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