High-voltage direct-current power transmission system oscillation suppression method based on MMC negative sequence impedance remodeling

Abstract

The invention discloses a high-voltage direct-current power transmission system oscillation suppression method based on MMC negative-sequence impedance remodeling. The method comprises the steps that equivalent admittance of a new energy unit considering MMC impedance characteristic influence is established; mMC negative sequence impedance remodeling control based on virtual parallel impedance is constructed; and designing control parameters considering impedance coupling degree constraint. According to the invention, the oscillation phenomenon of the system can be effectively suppressed, and safe and stable operation of the high-voltage direct-current power transmission system is ensured.

Description

Technical Field

[0001] The present invention relates to the technical field of power systems, and in particular to a method for suppressing oscillations in a high-voltage direct current transmission system based on MMC negative-sequence impedance reshaping. Background Art

[0002] With the rapid development of new energy, the scale and large-scale characteristics of new energy projects are becoming increasingly prominent. Flexible DC transmission technology has become an important technical approach for connecting new energy to the power grid. However, while benefiting from the flexible control of the high-voltage DC transmission system, the control interaction between the new energy station and the high-voltage DC converter station has made the problem of broadband oscillation increasingly prominent. According to many oscillation accidents that have occurred at home and abroad, the oscillation problem in the high-voltage DC transmission system not only covers multiple frequency bands, but also has new characteristics, which is manifested as the mirror symmetry of the oscillation frequency about the base frequency, which is called mirror frequency coupling.

[0003] The existing control strategies for suppressing the mirror frequency coupling phenomenon are all aimed at single-converter grid-connected systems. For new energy grid-connected systems via high-voltage DC, both sides of the AC collection line are characterized by power electronic controlled power supplies. Compared with the traditional wind turbine connected to the AC system, the impedance coupling characteristics have undergone fundamental changes. It is still unknown whether the above strategy will have a negative impact on the operating characteristics of the interconnected system if it is directly invested in the new energy grid-connected system via high-voltage DC. And even if the above strategy can achieve the expected effect, the complex control transformation of a large number of new energy units in the existing new energy projects will face huge manpower costs and economic costs. Therefore, it is urgent to use the controllability of the high-voltage DC converter station to realize the impedance coupling of the new energy grid-connected system via high-voltage DC to ensure the safe and stable operation of the interconnected system. Summary of the invention

[0004] In view of the risk that the existing frequency coupling suppression strategy has a negative impact on the operating characteristics of the new energy high-voltage direct current grid-connected system, and the need to carry out complex control transformation of a large number of new energy units in the existing new energy projects when they are put into use, which faces huge manpower and economic costs, the present invention proposes an oscillation suppression method for a high-voltage direct current transmission system based on MMC negative-sequence impedance reshaping on the basis of the traditional frequency coupling suppression strategy.

[0005] The present invention discloses a method for suppressing oscillation of a high-voltage direct current transmission system based on MMC negative sequence impedance reshaping, which comprises:

[0006] Step 1: Establish the equivalent admittance of the new energy unit considering the influence of MMC impedance characteristics;

[0007] Step 2: Construct MMC negative sequence impedance reshaping control based on virtual parallel impedance;

[0008] Step 3: Design of control parameters considering impedance coupling constraint.

[0009] Furthermore, the step 1 comprises:

[0010] After the new energy unit is AC interconnected with the MMC, the impedance characteristics of the MMC affect the port characteristics of the new energy unit. It is necessary to consider the impedance coupling relationship between the MMC and the new energy unit to obtain the equivalent admittance model of the new energy unit considering the influence of the MMC impedance characteristics.

[0011] Furthermore, in step 1, the MMC does not have a control strategy for dq gain asymmetry, and the frequency coupling characteristics of the MMC are only derived from the submodule capacitor voltage and circulating current. After the commutation suppression is put into operation, the frequency coupling phenomenon is suppressed, that is, the non-diagonal elements of the two-dimensional impedance model can be ignored; the port characteristics of the MMC are expressed as:

[0012]

[0013] Where s = jω p ,s -2 =jω p -j2ω1, where ω p =2πf p , f p is the disturbance frequency, represents the MMC AC impedance matrix, Z pp (s) represents the frequency f p The MMC AC side current disturbance Δi MMC (s) for frequency f p The voltage response of the MMC AC side Δv MMC The transfer function of (s), Z nn (s -2 ) indicates the frequency is f p -2f1 MMC AC side current disturbance, Δi MMC (s -2 ) for frequency f p -2f1 MMC AC side voltage response Δv MMC (s -2 ) is the transfer function, f1 is the fundamental frequency.

[0014] Furthermore, when the AC port of the new energy unit presents a two-input-two-output characteristic, the port characteristic of the new energy unit can be expressed as:

[0015]

[0016] Where s = jω p ,s -2 =jω p -j2ω1, where ω p =2πf p , f pis the disturbance frequency, represents the MMC AC impedance matrix, Y pp (s) represents the frequency f p The voltage disturbance Δv on the AC side of the MMC MMC (s) for frequency f p The MMC AC side current response Δi MMC The transfer function of (s), Y nn (s -2 ) indicates the frequency is f p -2f1 MMC AC side voltage disturbance Δv MMC (s -2 ) for frequency f p -2f1 MMC AC side current response Δi MMC (s -2 ) transfer function, f1 is the fundamental frequency; Y pn (s) represents the frequency f p The voltage disturbance of frequency f p -2f1 current response transfer function, Y np (s -2 ) indicates the frequency is f p The voltage disturbance of -2f1 with frequency f p The transfer function of the current response.

[0017] Furthermore, based on the impedance characteristics of the MMC and the new energy unit, the dynamic interaction of each electrical quantity is displayed through the signal flow diagram between the AC voltage disturbance and the current response of the new energy unit, and the equivalent impedance of the new energy unit is obtained;

[0018] Based on the relationship shown in the signal flow graph, the equivalent admittance of the new energy unit is obtained as:

[0019]

[0020] in, is the equivalent admittance of the new energy unit, i p,ωp The frequency is ω p The current response, v pcc,ωp is the voltage disturbance at the PCC point.

[0021] Furthermore, the step 2 comprises:

[0022] A virtual parallel impedance is connected at the PCC to reshape the equivalent impedance on the MMC side, and a bandpass filter is added in the additional control; the dq-axis disturbance component Δv of the PCC point voltage sdq_ref The dq-axis disturbance component Δv of the PCC point voltage sdq After subtraction, the AC voltage is passed through the outer loop controller G u (s) Generates the current control inner loop reference value Δisdq-ref , Δi sdq-ref The dq-axis disturbance component Δi of the PCC point current sdq After subtraction, it passes through the current inner loop controller G i (s) and the dq-axis disturbance component Δv of the PCC point voltage sdq Generate modulation signal Δm sdq ; Modulation signal Δm sdq With modulation gain K PWM The AC voltage is generated by the virtual resistor R vir With virtual inductance L vir After being multiplied, they are fed back to the voltage outer loop.

[0023] Furthermore, the feedback current is added to the current on the parallel inductive reactance, and the current inner loop control equation is updated to:

[0024]

[0025] Among them, G bpf (s) is a bandpass filter, G i (s) is the current loop transfer function, L eq is the equivalent inductance; m vd With m vq is the dq axis modulation signal, v sd With v sq They are the dq axis components of the inverter port voltage, i sd-ref with i sq-ref They are the reference values ​​of the dq axis of the AC current, i sd with i sq They are the dq axis components of the AC current, R is the virtual resistance, L vir is the virtual inductance and ω is the angular frequency.

[0026] Furthermore, the step 3 comprises:

[0027] Calculate the minimum loop gain of the system, and calculate the maximum value of the impedance coupling when the minimum loop gain is less than the specified value;

[0028] Calculate the value range of virtual resistance and virtual inductance that makes the system coupling degree lower than the maximum impedance coupling degree;

[0029] Selecting the minimum value of the virtual resistance and the minimum value of the virtual inductance from the value range of the virtual resistance and the virtual inductance to perform a system dynamic / steady-state characteristic test;

[0030] Determine whether the system steady-state / dynamic characteristic requirements are met; if so, output the values ​​of the virtual resistance and virtual inductance; if not, add the specified values ​​to the values ​​of the virtual resistance and virtual inductance and output them.

[0031] Furthermore, the expression of the minimum loop gain is:

[0032]

[0033] Where, the minimum loop gain Tm(s) is the minimum loop gain, Z WFMMC (s) is the AC impedance of WFMMC, Z wf (s) is the impedance of the new energy unit, and s is a complex frequency variable;

[0034] The expression of the impedance coupling degree is:

[0035]

[0036] Where β is the impedance coupling, Z eq To consider the AC equivalent impedance of the grid-connected converter after impedance coupling, Z pp Ignoring the impedance of the grid-connected converter with impedance coupling, β reflects the impedance coupling degree of the interconnected system. The larger β is, the stronger the impedance coupling degree of the interconnected system is.

[0037] Furthermore, the frequency band with the highest impedance coupling degree of the new energy through the high-voltage DC grid-connected system is located in the bandwidth frequency band of the phase-locked loop of the new energy unit. At the same time, the frequency band with the highest impedance coupling degree of the new energy through the high-voltage DC grid-connected system is also the frequency band with the worst system stability within the specified frequency band; the bandwidth of the phase-locked loop of the new energy unit is f pll ; f0±f pll Set the high resonance risk frequency, f0 is the fundamental frequency.

[0038] Due to the adoption of the above technical solution, the present invention has the following advantages:

[0039] 1) The present invention only needs to optimize the control of MMC to achieve impedance coupling suppression of the interconnected system. The control will not increase the extra loss of the system.

[0040] 2) The present invention has clear physical meaning and a simple control structure, and will not affect the dynamic control performance of the AC voltage after being put into use.

[0041] 3) The present invention can significantly suppress the impedance coupling between the new energy unit and the modular multi-level converter through virtual parallel impedance, thereby effectively suppressing the system oscillation phenomenon and ensuring the safe and stable operation of the high-voltage direct current transmission system. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the embodiments of the present invention. For ordinary technicians in this field, other drawings can also be obtained based on these drawings.

[0043] Figure 1 A schematic flow chart of a method for suppressing oscillation of a high-voltage direct current transmission system based on MMC negative-sequence impedance reshaping according to an embodiment of the present invention;

[0044] Figure 2 A signal flow diagram between AC voltage disturbance and current response of a new energy generator set according to an embodiment of the present invention;

[0045] Figure 3 This is a system transfer block diagram after negative sequence impedance control is put into effect according to an embodiment of the present invention. DETAILED DESCRIPTION

[0046] The present invention is further described in conjunction with the accompanying drawings and embodiments, and the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by ordinary technicians in this field should fall within the scope of protection of the embodiments of the present invention.

[0047] The technical terms of this application are explained as follows: MMC is a modular multilevel converter; GSC is a grid-side converter for new energy generators.

[0048] See also Figure 1 The present invention provides an embodiment of a method for suppressing oscillation of a high-voltage direct current transmission system based on MMC negative sequence impedance reshaping, which comprises:

[0049] S1: Establish the equivalent admittance of the new energy unit considering the influence of MMC impedance characteristics;

[0050] In one embodiment, after the new energy unit is AC-connected with the MMC, the impedance characteristics of the MMC will affect the port characteristics of the new energy unit. Therefore, the impedance coupling relationship between the MMC and the new energy unit needs to be considered to obtain an equivalent admittance model of the new energy unit that considers the influence of the MMC impedance characteristics.

[0051] MMC usually uses AC voltage control to provide stable AC voltage to new energy units. At this time, MMC does not have a dq gain asymmetric control strategy. The frequency coupling characteristics of MMC only come from the submodule capacitor voltage and circulating current. After the commutation suppression is put into use, the above frequency coupling phenomenon will be greatly suppressed, that is, the non-diagonal elements of the two-dimensional impedance model can be ignored. Therefore, the port characteristics of MMC can be expressed as:

[0052]

[0053] Where s = jω p ,s -2 =jω p -j2ω1, where ω p =2πf p , fp is the disturbance frequency, represents the MMC AC impedance matrix, Z pp (s) is expressed as frequency f p The MMC AC side current disturbance Δi MMC (s) for frequency f p The voltage response of the MMC AC side Δv MMC The transfer function of (s), Z nn (s -2 ) is expressed as frequency f p -2f1 MMC AC side current disturbance, Δi MMC (s -2 ) for frequency f p -2f1 MMC AC side voltage response Δv MMC (s -2 ) transfer function, f1 is the fundamental frequency of 50Hz.

[0054] New energy units usually use a phase-locked loop to track the voltage phase of the common coupling point, and the phase-locked loop usually only realizes phase tracking by controlling the q-axis voltage component, which makes the output of the control link of the new energy unit no longer symmetrical, so that the AC port of the new energy unit presents a two-input-two-output characteristic. Therefore, the port characteristics of the new energy unit can be expressed as:

[0055]

[0056] Where s = jω p ,s -2 =jω p -j2ω1, where ω p =2πf p , f p is the disturbance frequency, represents the MMC AC impedance matrix, Y pp (s) is expressed as frequency f p The voltage disturbance Δv on the AC side of the MMC MMC (s) for frequency f p The MMC AC side current response Δi MMC The transfer function of (s), Y nn (s -2 ) is expressed as frequency f p -2f1 MMC AC side voltage disturbance Δv MMC (s -2 ) for frequency f p -2f1 MMC AC side current response Δi MMC (s -2 ) transfer function, f1 is the base frequency of 50Hz. pn (s) is expressed as frequency fp The voltage disturbance of frequency f p -2f1 current response transfer function, Y np (s -2 ) is expressed as frequency f p The voltage disturbance of -2f1 with frequency f p The transfer function of the current response.

[0057] On the basis of obtaining the impedance characteristics of MMC and new energy units, the dynamic interaction of various electrical quantities can be intuitively displayed through the signal flow diagram between the AC voltage disturbance and current response of the new energy unit, thereby obtaining the equivalent impedance of the new energy unit.

[0058] The signal flow diagram between the AC voltage disturbance and current response of the new energy unit is as follows: Figure 2 As shown, after the voltage disturbance is injected, the angular frequency at PCC is ω p The disturbance voltage Δv pcc,ωp Can be passed through Y pp , Y pn (The angular frequency mark is omitted here, Y pp With Y pn Corresponding to Y in formula (2) pp (s) and Y pn (s -2 )) produces an angular frequency of ω p The current response with angular frequency ω p The coupled current response of -2ω0 is shown in paths ① and ②. Furthermore, these two current responses will simultaneously serve as the current disturbance of the MMC, passing through Z pn , Z nn The post-excitation angular frequency is ω p -2ω0 coupled voltage response Δv c,ωp-2ω0 , and finally through Y np The frequency is ω p The current response i p,ωp It can be seen that Δv pcc,ωp There are two forward paths that can stimulate the current response Δi p,ωp , which are ①③⑤ and ②, which also shows that the relationship between the voltage and current at the GSC AC side port is no longer independent of the GSC, but is affected by the MMC impedance.

[0059] Based on the relationship shown in the signal flow graph, the equivalent admittance of the new energy unit can be obtained as follows:

[0060]

[0061] in, is the equivalent admittance of the new energy unit, i p,ωp The frequency is ω p The current response, vpcc,ωp Y is the voltage disturbance at the PCC point. pp , Z pp The elements are defined in the same way as in equation (1) and equation (2), and the frequency information is omitted.

[0062] S2: Construct MMC negative sequence impedance reshaping control based on virtual parallel impedance;

[0063] In one embodiment, it can be seen from the signal flow diagram between the AC voltage disturbance and current response of the new energy unit and the analytical expression of the AC equivalent admittance of the new energy unit that the key hub of the impedance coupling between the new energy unit and the MMC is the negative sequence impedance Z nn , if the negative sequence impedance Z can be effectively suppressed nn If the size of the coupled voltage disturbance is reduced, the value of the coupled voltage disturbance will be limited, and the impedance coupling of the entire interconnected system can be effectively weakened.

[0064] MMC positive sequence impedance Z pp It does not participate in the impedance coupling process, and through the analysis of the phase sequence relationship between the positive and negative sequences, in the 0-100Hz frequency band, the MMC positive sequence impedance Z pp With negative sequence impedance Z nn Satisfies the following relationship:

[0065] Z nn (ω p )=Z pp (2ω1-ω p ) (4)

[0066] By connecting a parallel impedance on the AC side of the MMC, the AC impedance amplitude of the MMC can be reduced, thereby making the coupling degree β less than β max , improve system stability. Since it is difficult to add passive inductance in engineering, it can only be considered from the control system. Different from the previous virtual impedance control strategy added to the MMC bridge arm, a virtual parallel impedance will be added at the PCC to reshape the equivalent impedance on the MMC side, and in order to avoid causing the inner loop integrator to saturate and affect the operating characteristics and reduce the impact on the steady-state characteristics of the system, a bandpass filter is added in the additional control. The system transfer block diagram after adding the parallel virtual inductance controller is shown in the figure.

[0067] The dq-axis disturbance component of the PCC voltage ΔV sdq_ref The dq-axis disturbance component of the PCC voltage ΔV sdq After subtraction, the AC voltage is passed through the outer loop controller G u (s) Generates the current control inner loop reference value Δi sdq-ref , Δi sdq-ref The dq-axis disturbance component Δi of the PCC point current sdq After subtraction, it passes through the current inner loop controller G i(s) (including decoupling control) and the dq-axis disturbance component Δv of the PCC point voltage sdq (Voltage feedforward) generates the modulation signal Δm sdq . Modulation signal Δm sdq Further with the modulation gain K PWM The AC voltage is generated by the virtual resistor R vir With virtual inductance L vir Multiply them and feed them back to the voltage outer loop, G bpf (s) is a bandpass filter, which can reduce the impact of negative sequence impedance on other frequency bands. load is the equivalent load on the AC side, R arm With L arm are the equivalent resistance and inductance of the bridge arm, respectively, used to describe the system.

[0068] The feedback current will be added to the current on the parallel inductive reactance, and the current inner loop control equation is updated to:

[0069]

[0070] Among them, G bpf (s) is a bandpass filter, G i (s) is the current loop transfer function, L eq is the equivalent inductance. m vd With m vq is the dq axis modulation signal, v sd With v sq They are the dq axis components of the inverter port voltage, i sd-ref with i sq-ref They are the reference values ​​of the dq axis of the AC current, i sd with i sq They are the dq axis components of the AC current, R is the virtual resistance, L vir is the virtual inductance and ω is the angular frequency.

[0071] S3: Control parameter design considering impedance coupling constraints.

[0072] In one embodiment, in order to quantitatively analyze the impedance coupling degree of the interconnection system, the impedance coupling degree is defined as follows:

[0073]

[0074] Where: Z eq The AC equivalent impedance of the grid-connected converter after impedance coupling is considered, Z pp is the impedance of the grid-connected converter with impedance coupling ignored. β reflects the impedance coupling degree of the interconnected system. The larger β is, the stronger the impedance coupling degree of the interconnected system is.

[0075] The frequency band with the highest impedance coupling degree of the new energy through the high-voltage DC grid-connected system is located in the bandwidth band of the phase-locked loop of the new energy unit. At the same time, this is also the frequency band with the worst system stability in the 0-100Hz frequency band. The bandwidth of the phase-locked loop of the new energy unit is f pll Therefore, f0±f pll Set the high resonance risk frequency, f0 is the fundamental frequency. The principle of maximum coupling is to meet the minimum loop gain |T m |<1, the minimum loop gain expression is:

[0076]

[0077] Among them, the minimum loop gain T m (s) is the minimum loop gain, Z WFMMC (s) is the AC impedance of WFMMC, Z wf (s) is the impedance of the new energy unit, and s is a complex frequency variable.

[0078] By changing the equivalent capacitance of the submodule, the relationship between the maximum coupling degree and the minimum loop gain at the dangerous resonance point can be obtained (the results obtained by changing the other parameters are the same). vir and L vir The value of makes the impedance coupling degree after the virtual parallel impedance control is lower than the maximum coupling degree, then the R that satisfies the coupling degree constraint (the system impedance coupling degree after the proposed control is lower than the maximum impedance coupling degree) can be obtained. vir and L vir Value range.

[0079] After obtaining R that satisfies the coupling constraint vir and L vir After the value range is determined, it is also necessary to ensure that when the virtual parallel impedance control strategy is put into use to reshape the MMC negative sequence impedance, it will have a small impact on the stable operation of the system. Therefore, it is necessary to compare the characteristics of the AC voltage before and after the control strategy is put into use, and screen out the parameter values ​​of the AC voltage that fluctuate greatly after the strategy is put into use. Then, the changes in the steady-state power step characteristics of the system are further tested to verify the impact of the virtual parallel inductive reactance control strategy on the dynamic characteristics of the system. In order to finally determine R vir and L vir Provide a reference, and finally form the parameter design process of virtual parallel impedance as follows Figure 3 shown.

[0080] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the relevant field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents. Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A method for suppressing oscillation of a high-voltage direct current transmission system based on MMC negative sequence impedance reshaping, characterized in that: include: Step 1: Establish the equivalent admittance of the new energy unit considering the influence of MMC impedance characteristics; Step 2: Construct MMC negative sequence impedance reshaping control based on virtual parallel impedance; Step 3: Design of control parameters considering impedance coupling constraints.

2. The method according to claim 1, characterized in that: The step 1 comprises: After the new energy unit is AC interconnected with the MMC, the impedance characteristics of the MMC affect the port characteristics of the new energy unit. It is necessary to consider the impedance coupling relationship between the MMC and the new energy unit to obtain the equivalent admittance model of the new energy unit considering the influence of the MMC impedance characteristics.

3. The method according to claim 1 or 2, characterized in that: In step 1, the MMC does not have a dq gain asymmetric control strategy, and the frequency coupling characteristics of the MMC are only derived from the submodule capacitor voltage and circulating current. After the commutation suppression is put into operation, the frequency coupling phenomenon is suppressed, that is, the non-diagonal elements of the two-dimensional impedance model can be ignored; the port characteristics of the MMC are expressed as: Where s = jω p ,s -2 =jω p -j2ω1, where ω p =2πf p , f p is the disturbance frequency, represents the MMC AC impedance matrix, Z pp (s) represents the frequency f p The MMC AC side current disturbance Δi MMC (s) for frequency f p The voltage response of the MMC AC side Δv MMC The transfer function of (s), Z nn (s -2 ) indicates the frequency is f p -2f1 MMC AC side current disturbance, Δi MMC (s -2 ) for frequency f p -2f1 MMC AC side voltage response Δv MMC (s -2 ) is the transfer function, f1 is the fundamental frequency.

4. The method according to claim 3, characterized in that When the AC port of the new energy unit presents a two-input-two-output characteristic, the port characteristics of the new energy unit can be expressed as: Where s = jω p ,s -2 =jω p -j2ω1, where ω p =2πf p , f p is the disturbance frequency, represents the MMC AC impedance matrix, Y pp (s) represents the frequency f p The voltage disturbance Δv on the AC side of the MMC MMC (s) for frequency f p The MMC AC side current response Δi MMC (s) transfer function, Y nn (s -2 ) indicates the frequency is f p -2f1 MMC AC side voltage disturbance Δv MMC (s -2 ) for frequency f p -2f1 MMC AC side current response Δi MMC (s -2 ) transfer function, f1 is the fundamental frequency; Y pn (s) represents the frequency f p The voltage disturbance of frequency f p -2f1 current response transfer function, Y np (s -2 ) indicates the frequency is f p The voltage disturbance of -2f1 with frequency f p The transfer function of the current response.

5. The method according to claim 4, characterized in that On the basis of obtaining the impedance characteristics of MMC and new energy units, the dynamic interaction of various electrical quantities is displayed through the signal flow diagram between the AC voltage disturbance and current response of the new energy unit, and the equivalent impedance of the new energy unit is obtained; Based on the relationship shown in the signal flow graph, the equivalent admittance of the new energy unit is obtained as: in, is the equivalent admittance of the new energy unit, i p,ωp The frequency is ω p The current response, v pcc,ωp is the voltage disturbance at the PCC point.

6. The method according to claim 1, characterized in that The step 2 comprises: A virtual parallel impedance is connected at the PCC to reshape the equivalent impedance on the MMC side, and a bandpass filter is added in the additional control; the dq-axis disturbance component Δv of the PCC point voltage sdq_ref The dq-axis disturbance component Δv of the PCC point voltage sdq After subtraction, the AC voltage is passed through the outer loop controller G u (s) Generates the current control inner loop reference value Δi sdq-ref , Δi sdq-ref The dq-axis disturbance component Δi of the PCC point current sdq After subtraction, it passes through the current inner loop controller G i (s) and the dq-axis disturbance component Δv of the PCC point voltage sdq Generate modulation signal Δm sdq ; Modulation signal Δm sdq With modulation gain K PWM The AC voltage is generated by the virtual resistor R vir With virtual inductance L vir After being multiplied, they are fed back to the voltage outer loop.

7. The method according to claim 6, characterized in that The feedback current is added to the current on the parallel inductive reactance, and the current inner loop control equation is updated to: Among them, G bpf (s) is a bandpass filter, G i (s) is the current loop transfer function, L eq is the equivalent inductance, m vd With m vq is the dq axis modulation signal, v sd With v sq They are the dq axis components of the inverter port voltage, i sd-ref with i sq-ref They are the reference values ​​of the dq axis of the AC current, i sd with i sq They are the dq axis components of the AC current, R is the virtual resistance, L vir is the virtual inductance and ω is the angular frequency.

8. The method according to claim 1, characterized in that The step 3 comprises: Calculate the minimum loop gain of the system, and calculate the maximum value of the impedance coupling when the minimum loop gain is less than the specified value; Calculate the value range of virtual resistance and virtual inductance that makes the system coupling degree lower than the maximum impedance coupling degree; Selecting the minimum value of the virtual resistance and the minimum value of the virtual inductance from the value range of the virtual resistance and the virtual inductance to perform a system dynamic / steady-state characteristic test; Determine whether the system steady-state / dynamic characteristic requirements are met; if so, output the values ​​of the virtual resistance and virtual inductance; if not, add the specified values ​​to the values ​​of the virtual resistance and virtual inductance and output them.

9. The method according to claim 8, characterized in that The expression of the minimum loop gain is: Among them, the minimum loop gain T m (s) is the minimum loop gain, Z WFMMC (s) is the AC impedance of WFMMC, Z wf (s) is the impedance of the new energy unit, and s is a complex frequency variable; The expression of the impedance coupling degree is: Where β is the impedance coupling, Z eq To consider the AC equivalent impedance of the grid-connected converter after impedance coupling, Z pp Ignoring the impedance of the grid-connected converter with impedance coupling, β reflects the impedance coupling degree of the interconnected system. The larger β is, the stronger the impedance coupling degree of the interconnected system is.

10. The method according to claim 8, characterized in that The frequency band with the highest impedance coupling degree of the new energy through the high-voltage DC grid-connected system is located in the bandwidth frequency band of the phase-locked loop of the new energy unit. At the same time, the frequency band with the highest impedance coupling degree of the new energy through the high-voltage DC grid-connected system is also the frequency band with the worst system stability within the specified frequency band; the bandwidth of the phase-locked loop of the new energy unit is f pll ; f0±f pll Set the high resonance risk frequency, f0 is the fundamental frequency.

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