Alternating current voltage control method for offshore flexible direct current system

Abstract

The invention discloses an alternating-current voltage control method for an offshore flexible direct-current system, and belongs to the technical field of offshore wind power flexible direct-current power transmission. The method aims at the problem of intermediate frequency oscillation of the flexible direct current power transmission system caused by alternating current voltage outer loop control. Comprising the steps that after voltage and current of a grid-connected point are detected, alternating-current power disturbance of the flexible direct-current power transmission system is calculated, the alternating-current power disturbance sequentially passes through a phase lead correction link and a proportional controller, and then a compensation item is introduced to the input end of an alternating-current voltage loop. New feedback gain is equivalently increased, the equivalent output admittance of the system is remodeled, and the stability of AC voltage control of the flexible DC system is improved. The problem of intermediate frequency oscillation of the flexible DC system caused by AC voltage outer loop control can be solved, and the dynamic performance of AC voltage loop control of the system is improved while the stability is ensured.

Description

Technical Field

[0001] The invention relates to an AC voltage control method for an offshore flexible DC system, and belongs to the technical field of offshore wind power flexible DC transmission. Background Art

[0002] With the development of science and technology, flexible direct current transmission technology is one of the important solutions for new energy transmission technology. Compared with traditional direct current transmission technology, flexible direct current transmission has shown obvious advantages. It can directly convert direct current voltage into alternating current voltage with precise control of amplitude and phase, without relying on the voltage of the receiving power grid to complete the commutation process, and is not restricted by the capacity of the receiving power grid system. How to improve the stability of flexible direct current transmission technology is the focus of current research.

[0003] In flexible DC transmission technology, in order to improve the voltage stability of the grid connection point, the system can output reactive power to the grid. There are two main modes for reactive power control, one is based on reactive power outer loop control technology, and the other is based on AC voltage outer loop control technology. Among them, reactive power outer loop control technology can achieve precise regulation of reactive power and improve the power factor of the system; AC voltage outer loop control technology can achieve precise regulation of output voltage and improve the voltage stability of the system. Regardless of the control method, its dynamic characteristics have a great influence on the stability of the grid-connected converter system.

[0004] The stability of the control system is not only affected by the actual operating conditions, but also related to the control loop parameters within the system. The control bandwidth based on the AC voltage control technology has an important impact on the stability of the system. In particular, when the control bandwidth of the AC voltage outer loop increases, the converter system is prone to medium-frequency oscillation. The traditional method for suppressing this oscillation is mainly to reduce the control bandwidth of the AC voltage loop, but this method will slow down the control response, resulting in an increase in the delay of the outer control loop and poor dynamic performance.

[0005] Therefore, in order to take into account both faster system dynamic response capability and stronger system stability, it is necessary to propose a method to improve the AC voltage control stability of the flexible DC system. Summary of the invention

[0006] In view of the medium frequency oscillation problem caused by AC voltage outer loop control in a flexible direct current transmission system, the present invention provides an AC voltage control method for an offshore flexible direct current system.

[0007] The present invention provides an AC voltage control method for an offshore flexible direct current system, comprising:

[0008] Collect the three-phase output current i of the flexible direct current transmission system adc And three-phase AC voltage u adc, combined with the grid connection angle θ, coordinate transformation is performed to obtain the actual value of the dq axis current i dq and the actual value of the dq axis AC voltage u dq The grid connection angle θ is based on the q-axis voltage component u of the adjacent previous cycle q Calculated;

[0009] According to the closed-loop control strategy of the converter module, the actual value of the dq axis current i dq and the actual value of the dq axis AC voltage u dq Calculating an AC power disturbance Δp caused by an intermediate frequency oscillation caused by an AC voltage loop control; performing phase advance correction and proportional control according to the AC power disturbance Δp, and calculating a compensation term m;

[0010] The DC voltage loop is used to set the DC voltage value v on the DC side of the converter module. dcref And the actual value of DC voltage v dc Calculate the given value of the d-axis current i dref ;

[0011] The AC voltage loop is used to set the AC voltage value v according to the grid connection point. pccref , Actual value of AC voltage at the grid connection point v pcc The q-axis current given value i is calculated by the compensation term m qref ;

[0012] The current loop is based on the actual value of the dq axis current i dq , Actual value of dq axis AC voltage u dq , d-axis current given value i dref and q-axis current given value i qref Calculate the given value v of the dq axis voltage dqref ;

[0013] Based on the dq axis voltage given value v dqref The pulse width modulation is performed on the grid-connected angle θ to obtain the voltage control signal of the converter module.

[0014] According to the method for controlling AC voltage of an offshore flexible DC system of the present invention, the three-phase output current i adc And three-phase AC voltage u adc First, the coordinates are transformed into a two-phase stationary coordinate system, and then the coordinates are transformed into a two-phase rotating coordinate system to obtain the actual value of the dq axis current i dq and the actual value of the dq axis AC voltage u dq .

[0015] According to the AC voltage control method of the offshore flexible DC system of the present invention, the DC voltage loop calculates the d-axis current given value i dref The method is:

[0016] idref =(v dcref -v dc )·G dvc ,

[0017] Where G dvc This is the PI control link of the DC voltage loop:

[0018]

[0019] Where K dvcp K is the DC voltage loop proportional coefficient, dvci is the DC voltage loop integral coefficient, and s is the Laplace operator.

[0020] According to the AC voltage control method for an offshore flexible DC system of the present invention, the AC voltage loop initially calculates the q-axis current given value i qref The method is:

[0021] i qref =(v pccref -v pcc )·G avc ,

[0022] Where G avc This is the PI control link of the AC voltage loop:

[0023]

[0024] Where K avcp K is the AC voltage loop proportional coefficient, avci is the AC voltage integral coefficient.

[0025] According to the AC voltage control method of the offshore flexible DC system of the present invention, the current loop calculates the dq axis voltage given value v dqref The method is:

[0026] v dqref =(i dqref -i dq )·G i ,

[0027] Where i dqref is the given value of dq axis current, G i This is the current loop PI control link:

[0028]

[0029] Where K ip is the current loop proportionality coefficient, K ii is the current loop integration coefficient.

[0030] According to the AC voltage control method of the offshore flexible DC system of the present invention,

[0031] According to the switching cycle average model of the converter module, we can get:

[0032] u dq =K m V dc v dqref ,

[0033] Where K m is the PWM modulation ratio, V dc is the steady-state value of DC voltage;

[0034] Calculate the AC power disturbance Δp as:

[0035] Δp=1.5(Δu d I d +U d Δi d +Δu q I q +U q Δi q ),

[0036] Where Δu d is the small disturbance value of d-axis voltage, I d is the steady-state value of the three-phase output current d axis, U d is the steady-state value of the d-axis voltage, Δi d is the small disturbance value of the d-axis current, Δu q is the small disturbance value of the q-axis voltage, I q is the q-axis steady-state value of the three-phase output current, U q is the steady-state value of the q-axis voltage, Δi q is the small disturbance value of d-axis current; where:

[0037] Δu d =u d -U d , Δu q =u q -U q ;

[0038] Δi d =i d -I d , Δi q =i q -I q .

[0039] According to the AC voltage control method of the offshore flexible DC system of the present invention, the calculation method of the compensation term m is:

[0040]

[0041] Where k a is the proportional controller coefficient, Tn is the time constant of the phase correction link.

[0042] According to the AC voltage control method of the offshore flexible DC system of the present invention, after the compensation term m is introduced, the AC voltage loop calculates the q-axis current given value i qref The method is:

[0043] i qref =(v pccref -v pcc -m)·G avc .

[0044] According to the AC voltage control method of the offshore flexible DC system of the present invention, the order admittance modeling is performed on the converter module including the AC voltage loop, and the key disturbance term Y ac for:

[0045]

[0046] Where T1 is the fundamental period of the system, V p / n Modeling the positive and negative sequence voltage components in sequence admittance.

[0047] According to the AC voltage control method of the offshore flexible DC system of the present invention, the admittance Y of the compensation term m m for:

[0048] Y m =0.75mG i G avc ,

[0049] Introducing admittance Y in flexible DC system m Offset the key disturbance term Y ac The impact caused by this is to reshape the output admittance of the flexible DC system and suppress the intermediate frequency oscillation caused by the AC voltage loop.

[0050] Beneficial effects of the present invention: After the method of the present invention detects the voltage and current at the grid connection point, it calculates the AC power of the flexible direct current transmission system, and passes it through the phase advance correction link and the proportional controller in sequence to obtain the compensation term. The compensation term is then introduced into the input end of the AC voltage loop. The new feedback gain is equivalently added, the system equivalent output admittance is reshaped, the stability of the AC voltage control of the flexible direct current system is improved, and the stability margin of the system is improved. The method of the present invention solves the problem of medium-frequency oscillation in the flexible direct current system caused by the AC voltage outer loop control, and improves the dynamic performance of the system AC voltage loop control while ensuring stability.

[0051] Compared with the traditional AC voltage outer loop stabilization control method, the method of the present invention has the advantages of fast response and small delay of the control loop, and has better dynamic performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Figure 1 It is a flow chart of the AC voltage control method of the offshore flexible DC system according to the present invention;

[0053] Figure 2 It is a control block diagram of the AC voltage control method of the offshore flexible DC system according to the present invention;

[0054] Figure 3 It is a schematic diagram of the oscillation feedback suppression strategy caused by AC voltage loop control;

[0055] Figure 4 Compare the phase-frequency characteristic curves before and after the flexible direct current system admittance remodeling;

[0056] Figure 5 The waveform diagram of grid-connected current before and after adding compensation item control;

[0057] Figure 6 This is the frequency analysis diagram of the intermediate frequency oscillation caused by the AC voltage loop;

[0058] Figure 7 This is the harmonic frequency analysis diagram of the grid-connected current after adding the compensation item. DETAILED DESCRIPTION

[0059] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0060] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other.

[0061] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments, but they are not intended to limit the present invention.

[0062] Specific implementation method 1. Combination Figures 1 to 3 As shown, the present invention provides an AC voltage control method for an offshore flexible direct current system, comprising:

[0063] Collect the three-phase output current i of the flexible direct current transmission system adc And three-phase AC voltage u adc , combined with the grid connection angle θ, coordinate transformation is performed to obtain the actual value of the dq axis current i dq and the actual value of the dq axis AC voltage u dq The grid connection angle θ is based on the q-axis voltage component u of the adjacent previous cycle q Calculated;

[0064] According to the closed-loop control strategy of the converter module, the actual value of the dq axis current i dq and the actual value of the dq axis AC voltage u dq Calculating an AC power disturbance Δp caused by an intermediate frequency oscillation caused by an AC voltage loop control; performing phase advance correction and proportional control according to the AC power disturbance Δp, and calculating a compensation term m;

[0065] The DC voltage loop is used to set the DC voltage value v on the DC side of the converter module. dcref And the actual value of DC voltage v dc Calculate the given value of the d-axis current i dref ;

[0066] The AC voltage loop is used to set the AC voltage value v according to the grid connection point. pccref , Actual value of AC voltage at the grid connection point v pcc The q-axis current given value i is calculated by the compensation term m qref ;

[0067] The current loop is based on the actual value of the dq axis current i dq , Actual value of dq axis AC voltage u dq , d-axis current given value i dref and q-axis current given value i qref Calculate the given value v of the dq axis voltage dqref ;

[0068] Based on the dq axis voltage given value v dqref The pulse width modulation is performed on the grid-connected angle θ to obtain the voltage control signal of the converter module.

[0069] This implementation method derives the medium frequency oscillation transmission path caused by AC voltage control based on the system control link, and finally obtains the disturbance relationship between AC voltage and AC power; based on the disturbance relationship, a compensation term is obtained, and then the term is introduced into the input end of the AC voltage loop control; the system introduces the compensation term, which is equivalent to adding a new feedback gain, reshaping the output admittance of the flexible DC system, and improving the stability of the AC voltage control of the flexible DC system.

[0070] In this embodiment, the three-phase output current i adc And three-phase AC voltage u adc First, transform the coordinates to the two-phase stationary coordinate system to obtain u α 、u β 、i α 、i β , and then transform the coordinates to the two-phase rotating coordinate system to obtain the actual value of the dq axis current i dq and the actual value of the dq axis AC voltage u dq The voltage and current components after coordinate transformation will participate in the closed-loop control operation of the system.

[0071] Furthermore, according to the closed-loop control strategy of the converter system, the medium-frequency oscillation transmission path caused by the AC voltage control is derived;

[0072] The converter outer loop control consists of a DC voltage loop and an AC voltage loop:

[0073] The DC voltage loop calculates the d-axis current given value i dref The method is:

[0074] i dref =(v dcref -v dc )·G dvc ,

[0075] Where G dvc This is the PI control link of the DC voltage loop:

[0076]

[0077] Where K dvcp is the DC voltage loop proportional coefficient, K dvci is the DC voltage loop integral coefficient, and s is the Laplace operator.

[0078] Initial calculation of the AC voltage loop q-axis current given value i qref The method is:

[0079] i qref =(v pccref -v pcc )·G avc ,

[0080] Where G avc This is the PI control link of the AC voltage loop:

[0081]

[0082] Where K avcp K is the AC voltage loop proportional coefficient, avci is the AC voltage integral coefficient.

[0083] The inner loop of the control system, i.e. the current loop, calculates the dq axis voltage given value v dqref The method is:

[0084] v dqref =(i dqref -i dq )·G i ,

[0085] Where i dqref is the given value of dq axis current, G i This is the current loop PI control link:

[0086]

[0087] Where K ip is the current loop proportionality coefficient, K ii is the current loop integration coefficient.

[0088] Furthermore, according to the switching cycle average model of the converter module, we can obtain:

[0089] u dq =K m V dc v dqref ,

[0090] Where K m is the PWM modulation ratio, V dc is the steady-state value of DC voltage;

[0091] Calculate the AC power disturbance Δp as:

[0092] Δp=1.5(Δu d I d +U d Δi d +Δu q I q +U q Δi q ),

[0093] Where Δu d is the small disturbance value of d-axis voltage, I d is the steady-state value of the three-phase output current d axis, U d is the steady-state value of the d-axis voltage, Δi d is the small disturbance value of the d-axis current, Δu q is the small disturbance value of the q-axis voltage, I q is the q-axis steady-state value of the three-phase output current, U q is the steady-state value of the q-axis voltage, Δi q is the small disturbance value of d-axis current; where:

[0094] Δu d =u d -U d , Δu q =u q -U q ;

[0095] Δi d =i d -I d , Δi q =i q -I q .

[0096] After simplification, the small disturbance relationship between AC power and grid connection point voltage can be derived:

[0097]

[0098] Where L g is the grid-side inductance, ω n is the reference angular frequency, V pcc is the steady-state value of the AC voltage.

[0099] From the above relationship, it can be seen that small disturbances in the grid-connected point voltage not only affect the AC power through the control link, but also small power disturbances will continue to affect the grid-connected point voltage. This shows that the small disturbance transmission path in the control link forms a positive feedback path, affecting the system stability.

[0100] According to the disturbance relationship, the AC power is passed through the phase advance correction link and the proportional controller to obtain the small disturbance compensation term, and then the calculation result is introduced into the AC voltage loop control input terminal;

[0101] In this implementation, the calculation method of the compensation term m is:

[0102]

[0103] Where k a is the proportional controller coefficient, T n is the time constant of the phase correction link. The compensation term coefficient can be selected according to the amplitude-frequency and phase-frequency characteristics of the converter overall admittance model to ensure that the system has a large stability margin.

[0104] After introducing the compensation term m, the AC voltage loop calculates the q-axis current given value i qref The method is:

[0105] i qref =(v pccref -v pcc -m)·G avc .

[0106] Furthermore, the order admittance modeling of the converter module including the AC voltage loop is carried out, and the key disturbance term Y ac for:

[0107]

[0108] Where T1 is the fundamental period of the system, V p / n Modeling the positive and negative sequence voltage components in sequence admittance.

[0109] The admittance Y of the compensation term m m for:

[0110] Y m =0.75mGi G avc ,

[0111] Introducing admittance Y in flexible DC system m Offset the key disturbance term Y ac The impact caused by this is to reshape the output admittance of the flexible DC system and suppress the intermediate frequency oscillation caused by the AC voltage loop.

[0112] Experimental verification: combined Figure 2 , P dc is the DC side input power, the grid connection angle θ is output through the phase-locked loop, L is the filter inductance, C f is the filter capacitor, Δu dc =v dcref -v dc , Δu pcc =v pccref -v pcc ;

[0113] Figure 3 As shown, G pll is the phase-locked loop transfer function, where G represents Δv pcc Calculation formula: Δv dc is the small disturbance value of DC voltage, C is the DC capacitance; the black straight line in the figure forms a positive feedback path, which is offset by the compensation term m;

[0114] Figure 4 As shown in the figure, it is the phase-frequency characteristic curve of the flexible DC system before and after the admittance reshaping, where the horizontal axis is the frequency and the vertical axis is the phase of the equivalent output admittance of the flexible DC system. The two curves are the phase margins before and after the compensation is added to the system. It can be seen from the figure that the phase margin after compensation is smoother and has a higher stability margin.

[0115] Figure 5 This is the grid-connected current diagram before and after the compensation term is added to the control system. The system has medium-frequency oscillation before 2.9s when the compensation control is not put into operation. After the compensation control is put into operation, the three-phase grid-connected current tends to be stable.

[0116] Figure 6 The figure shows the Fourier analysis of the intermediate frequency oscillation caused by the AC voltage loop. It can be seen that before the compensation control is put into use, there are intermediate frequency oscillations of 130Hz and 230Hz in the system, and it conforms to the frequency coupling phenomenon, and the harmonic distortion rate is 30.4%. In the figure, Frequency represents frequency, Mag represents amplitude, Fundamental represents fundamental component, and the vertical axis represents the percentage of signal amplitude to fundamental component.

[0117] Figure 7The figure shows the harmonic Fourier analysis of the grid-connected current after adding the compensation term. It can be seen that after the compensation control is put into operation, the medium-frequency oscillation of the system is suppressed, and the harmonic distortion rate is 2.56%, which meets the grid-connected requirements. This realizes a method to improve the AC voltage control stability of the flexible DC system.

[0118] Although the present invention is described herein with reference to specific embodiments, it should be understood that these embodiments are merely examples of the principles and applications of the present invention. It should therefore be understood that many modifications may be made to the exemplary embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims. It should be understood that the various dependent claims and features described herein may be combined in a manner different from that described in the original claims. It should also be understood that the features described in conjunction with a single embodiment may be used in other described embodiments.

Claims

1. A method for controlling AC voltage of an offshore flexible direct current system, characterized in that: include, Collect the three-phase output current i of the flexible direct current transmission system adc And three-phase AC voltage u adc , combined with the grid connection angle θ, coordinate transformation is performed to obtain the actual value of the dq axis current i dq and the actual value of the dq axis AC voltage u dq The grid connection angle θ is based on the q-axis voltage component u of the adjacent previous cycle q Calculated; According to the closed-loop control strategy of the converter module, the actual value of the dq axis current i dq and the actual value of the dq axis AC voltage u dq Calculating an AC power disturbance Δp caused by an intermediate frequency oscillation caused by an AC voltage loop control; performing phase advance correction and proportional control according to the AC power disturbance Δp, and calculating a compensation term m; The DC voltage loop is used to set the DC voltage value v on the DC side of the converter module. dcref And the actual value of DC voltage v dc Calculate the given value of the d-axis current i dref ; The AC voltage loop is used to set the AC voltage value v according to the grid connection point. pccref , Actual value of AC voltage at the grid connection point v pcc The q-axis current given value i is calculated by the compensation term m qref ; The current loop is based on the actual value of the dq axis current i dq , Actual value of dq axis AC voltage u dq , d-axis current given value i dref and q-axis current given value i qref Calculate the given value v of the dq axis voltage dqref ; Based on the dq axis voltage given value v dqref The pulse width modulation is performed on the grid-connected angle θ to obtain the voltage control signal of the converter module.

2. The AC voltage control method for an offshore flexible DC system according to claim 1, characterized in that: The three-phase output current i adc And three-phase AC voltage u adc First, the coordinates are transformed into a two-phase stationary coordinate system, and then the coordinates are transformed into a two-phase rotating coordinate system to obtain the actual value of the dq axis current i dq and the actual value of the dq axis AC voltage u dq .

3. The AC voltage control method for an offshore flexible DC system according to claim 1, characterized in that: The DC voltage loop calculates the d-axis current given value i dref The method is: i dref =(v dcref -v dc )·G dvc , Where G dvc This is the PI control link of the DC voltage loop: Where K dvcp is the DC voltage loop proportional coefficient, K dvci is the DC voltage loop integral coefficient, and s is the Laplace operator.

4. The AC voltage control method for an offshore flexible DC system according to claim 3, characterized in that: Initial calculation of the AC voltage loop q-axis current given value i qref The method is: i qref =(v pccref -v pcc )·G avc , Where G avc This is the PI control link of the AC voltage loop: Where K avcp K is the AC voltage loop proportional coefficient, avci is the AC voltage integral coefficient.

5. The AC voltage control method for an offshore flexible DC system according to claim 4, characterized in that: The current loop calculates the dq axis voltage given value v dqref The method is: v dqref =(i dqref -i dq )·G i , Where i dqref is the given value of dq axis current, G i This is the current loop PI control link: Where K ip is the current loop proportionality coefficient, K ii is the current loop integration coefficient.

6. The AC voltage control method for an offshore flexible DC system according to claim 5, characterized in that: According to the switching cycle average model of the converter module, we can get: u dq =K m V dc v dqref , Where K m is the PWM modulation ratio, V dc is the steady-state value of DC voltage; Calculate the AC power disturbance Δp as: Δp=1.5(Δu d I d +U d Δi d +Δu q I q +U q Δi q ), Where Δu d is the small disturbance value of d-axis voltage, I d is the steady-state value of the three-phase output current d axis, U d is the steady-state value of the d-axis voltage, Δi d is the small disturbance value of the d-axis current, Δu q is the small disturbance value of the q-axis voltage, I q is the q-axis steady-state value of the three-phase output current, U q is the steady-state value of the q-axis voltage, Δi q is the small disturbance value of d-axis current; where: Δu d =u d -U d ,Δu q =u q -U q ; Δi d =i d -I d ,Δi q =i q -I q 。 7. The AC voltage control method for an offshore flexible DC system according to claim 6, characterized in that: The calculation method of the compensation term m is: Where k a is the proportional controller coefficient, T n is the time constant of the phase correction link.

8. The AC voltage control method for an offshore flexible DC system according to claim 7, characterized in that: After introducing the compensation term m, the AC voltage loop calculates the q-axis current given value i qref The method is: i qref =(v pccref -v pcc -m)·G avc 。 9. The AC voltage control method for an offshore flexible DC system according to claim 8, characterized in that: The order admittance modeling of the converter module including the AC voltage loop is carried out, and the key disturbance term Y ac for: Where T1 is the fundamental period of the system, V p / n Modeling the positive and negative sequence voltage components in sequence admittance.

10. The AC voltage control method for an offshore flexible DC system according to claim 9, characterized in that: The admittance Y of the compensation term m m for: <h2 style=";text-align:left;direction:ltr">Y<h2 style=";text-align:left;direction:ltr"> m <h2 style=";text-align:left;direction:ltr"> <0.75mG<h2 style=";text-align:left;direction:ltr"> i <h2 style=";text-align:left;direction:ltr"> G<h2 style=";text-align:left;direction:ltr"> avc <h2 style=";text-align:left;direction:ltr"> , Introducing admittance Y in flexible DC system m Offset the key disturbance term Y ac The impact caused by this is to reshape the output admittance of the flexible DC system and suppress the intermediate frequency oscillation caused by the AC voltage loop.

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