SVG impedance remodeling-based VSC-HVDC grid-connected stability improvement method and system under weak power grid

Abstract

The invention discloses a VSC-HVDC grid-connected stability improvement method and system under a weak power grid based on SVG impedance remodeling, and the method can adjust the actual reference value of the dq-axis current of a current control inner ring through introducing a damping factor on the premise of not adding an oscillation detection module and a sensor, equivalently changes the output impedance of a static var generator, and improves the stability of the VSC-HVDC grid-connected stability. And therefore, the SVG low-frequency-band negative damping characteristic can be improved, and the grid-connected system damping can be According to the method, the grid-connected point voltage oscillation when the VSC-HVDC grid-connected system containing the SVG interacts with a weak power grid can be effectively suppressed, the stability of the grid-connected system is enhanced, and the stability margin of the system is improved.

Description

Technical Field

[0001] The present invention belongs to the field of high voltage direct current transmission systems, and relates to a method and system for improving the grid-connected stability of VSC-HVDC in a weak power grid based on SVG impedance reshaping. Background Art

[0002] In recent years, the development of photovoltaic, wind power and other new energy power generation equipment connected to the grid through voltage source converter-based high voltage direct current (VSC-HVDC) has attracted more and more attention. In order to balance the reactive power of the transmission line, static var generators (SVG) and other reactive compensation devices are often installed on the AC side bus of the receiving end of the long-distance VSC-HVDC system. When SVG is operated in parallel with the new energy grid-connected system, it can significantly improve the frequency stability of the system. However, studies have shown that the new energy grid-connected system containing SVG has stability problems such as voltage instability when interacting with weak power grids, especially in the low frequency band of the coupled system.

[0003] In terms of improving stability, existing methods mainly start from three aspects: parameter optimization, device topology improvement, and control strategy improvement. Among them, parameter optimization is difficult to fundamentally solve the stability problem, which may cause practical problems such as cost increase, and the amount of work to improve the device topology in actual engineering is huge. In contrast, improving the control strategy can fundamentally improve stability and the amount of work is relatively small, which is easy to apply in practice, but it may add oscillation detection modules and sensors to the original equipment, which will also increase the application cost. Therefore, it is urgent to study more effective control strategies for SVG to improve the stability of coupled systems. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide a method and system for improving the grid-connected stability of VSC-HVDC in a weak power grid based on SVG impedance reshaping in view of the deficiencies in the prior art.

[0005] The technical solution adopted by the present invention is:

[0006] A method for improving the grid-connected stability of VSC-HVDC in a weak power grid based on SVG impedance reshaping, characterized in that the method comprises the following steps:

[0007] Step 1: In the SVG outer loop DC voltage control link, based on the actual value of the SVG DC side voltage v dc Get the theoretical reference value i of the d-axis current dref ;

[0008] Step 2: Obtain the theoretical reference value of the d-axis current i dref Multiplying by the damping factor δ, we get the actual reference value i of the d-axis current dr ;

[0009] Step 3: The known theoretical reference value of q-axis current i qref Multiplying by the damping factor δ, we get the actual reference value i of the q-axis current qr ;

[0010] Step 4: obtain the actual reference value of the d-axis current i dr The SVG grid-connected current d-axis component i d Subtract the value obtained from the voltage outer loop controller transfer function G i (s) to obtain the control value of the d-axis current;

[0011] The actual reference value of the q-axis current i qr The q-axis component i of the SVG grid-connected current q Subtract the value obtained from the voltage outer loop controller transfer function G i (s) to obtain the control value of the q-axis current;

[0012] Among them, G i The expression of (s) is:

[0013]

[0014] Among them, k ip is the current loop proportionality coefficient, k ii is the current loop integral coefficient, s is the complex frequency;

[0015] Step 5: Convert the SVG grid-connected current q-axis component i q , the system reference angular frequency ω and the inductance L of the SVG AC side filter circuit eq The product of the three is equal to the d-axis component v of the grid-connected point voltage. d The sum is subtracted from the d-axis current control value, and then divided by the SVG DC side voltage given value v dcref , get the d-axis modulation signal m d ;

[0016] The SVG grid-connected current d-axis component i d , the system reference angular frequency ω and the inductance L of the SVG AC side filter circuit eq Multiply the three and get the product value ωL eq i d , the q-axis component v of the grid-connected voltage q Subtract the product value ωL eq i dThe difference is then subtracted from the control value of the q-axis current and divided by the given value of the SVG DC side voltage v dcref , and get the q-axis modulation signal m q ;

[0017] Step 6: modulate the signal m according to the d axis d and the q-axis modulation signal m q An SVPWM wave is generated to drive the SVG, so as to achieve impedance reshaping control which is equivalent to improving the damping characteristics of the SVG.

[0018] Moreover, in the SVG outer loop DC voltage control link, based on the actual value of the SVG DC side voltage v dc Get the theoretical reference value i of the d-axis current dref The method is:

[0019] Set the SVG DC side voltage to a given value v dcref Subtract the actual value of SVG DC side voltage v dc , and compare the obtained value with the transfer function G of the voltage outer loop controller v (s) to obtain the theoretical reference value of the d-axis current i dref ,

[0020] Among them, G v The expression of (s) is:

[0021]

[0022] Among them, k vp is the voltage loop proportional coefficient, k vi is the voltage loop integral coefficient, and s is the complex frequency.

[0023] Moreover, the expression of the damping factor δ is:

[0024]

[0025] Among them, τ is the filtering time constant, n is the number of control iterations, v dcref is the given value of SVG DC side voltage, v dc is the actual value of the SVG DC side voltage, s is the complex frequency, i dref is the theoretical reference value of d-axis current.

[0026] The present invention provides a VSC-HVDC grid-connected stability improvement system under weak power grid based on SVG impedance reshaping, comprising:

[0027] The voltage outer loop control module is used to control the actual value of the SVG DC side voltage to track the given value and obtain the theoretical reference value of the d-axis current i dref ;

[0028] Damping factor generation module, based on the d-axis current theoretical reference value i dref , SVG DC side voltage set value v dcref and the actual value of SVG DC side voltage v dc , by setting the filtering time constant and controlling the number of iterations, the damping factor δ is generated;

[0029] The damping control module is used to introduce the damping factor δ into the current loop control strategy based on the theoretical reference value i of the d-axis current. dref , q-axis current theoretical reference value i qref , obtain the actual reference value i of the d-axis current dr , q-axis current actual reference value i qr ;

[0030] The current control module is used to control the d-axis current and q-axis current actually output by the SVG to track the actual reference value of the d-axis current i dr , q-axis current actual reference value i qr , and obtain a d-axis output voltage signal and a q-axis output voltage signal for modulation;

[0031] The modulation module obtains the d-axis modulation signal m based on the d-axis output voltage signal and the q-axis output voltage signal for SVG modulation obtained in the current control module. d , q-axis modulation signal m q , and obtain the SVPWM driving signal to drive the actual operation of SVG.

[0032] Moreover, it also includes a damping factor generation module based on the d-axis current theoretical reference value i dref , SVG DC side voltage set value v dcref and the actual value of SVG DC side voltage v dc , by setting the filtering time constant and controlling the number of iterations, the damping factor δ is generated.

[0033] Moreover, the voltage outer loop control module is used to control the actual value of the SVG DC side voltage to track the given value and obtain the d-axis current theoretical reference value by: setting the SVG DC side voltage given value v dcref Subtract the actual value of SVG DC side voltage v dc , and compare the obtained value with the transfer function G of the voltage outer loop controller v (s) multiplied by each other, we get i dref ,

[0034] Among them, G v The expression of (s) is:

[0035]

[0036] Among them, k vpis the voltage loop proportionality coefficient, k vi is the voltage loop integral coefficient, and s is the complex frequency.

[0037] Moreover, the expression of the damping factor δ is:

[0038]

[0039] Among them, τ is the filtering time constant, n is the number of control iterations, v dcref is the given value of SVG DC side voltage, v dc is the actual value of the SVG DC side voltage, s is the complex frequency, i dref is the theoretical reference value of d-axis current.

[0040] Compared with the prior art, the present invention has the following beneficial effects:

[0041] The present invention suppresses voltage oscillation of the new energy grid-connected system by increasing the damping of the new energy grid-connected system containing SVG, that is, introducing a damping factor in the SVG control link, reshaping the actual reference value of the dq axis current, and improving the negative damping characteristics of the SVG in the low frequency band without adding an oscillation detection module and a sensor.

[0042] The present invention effectively improves the damping of a new energy grid-connected system containing SVG, thereby suppressing voltage oscillation at the grid-connected point of the system and improving system stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 This is a structural diagram of a VSC-HVDC grid-connected system including SVG involved in the present invention;

[0044] Figure 2 This is a schematic diagram of the impedance reshaping control strategy applied to SVG involved in the present invention;

[0045] Figure 3 FIG. 1 is an image of the change of SVG output impedance with frequency when the proposed stability improvement method is not introduced in an embodiment of the present invention; (a) is an image of the dd-axis component Z of the SVG output impedance when the proposed stability improvement method is not introduced dd (s) is the curve of the amplitude and phase changing with frequency; (b) is the dq-axis component Z of the SVG output impedance when the proposed stability improvement method is not introduced dq (s) is the curve of the amplitude and phase changing with frequency; (c) is the qd-axis component Z of the SVG output impedance when the proposed stability improvement method is not introduced qd (s) is the curve of the amplitude and phase changing with frequency; (d) is the qq-axis component Z of the SVG output impedance when the proposed stability improvement method is not introduced dd (s) The amplitude and phase curves of the change with frequency;

[0046] Figure 4 Figure 1 is a graph showing the change of SVG output impedance with frequency after the stability improvement method is introduced according to an embodiment of the present invention; (a) is the dd-axis component Z of SVG output impedance after the stability improvement method is introduced dd (s) is the curve of the amplitude and phase changing with frequency; (b) is the dq-axis component Z of the SVG output impedance after the proposed stability improvement method is introduced dq (s) is the curve of the amplitude and phase changing with frequency; (c) is the qd-axis component Z of the SVG output impedance after the proposed stability improvement method is introduced qd (s) is the curve of the amplitude and phase changing with frequency; (d) is the qq-axis component Z of the SVG output impedance after the proposed stability improvement method is introduced dd (s) The amplitude and phase curves of the change with frequency;

[0047] Figure 5 The simulation test results of the grid connection point voltage of the VSC-HVDC grid-connected system containing SVG when the impedance reshaping control strategy is not introduced in an embodiment of the present invention; (a) is the simulation curve of the grid short-circuit ratio SCR with time t within 0-6s, the horizontal axis is time t, and the vertical axis is the size of the system short-circuit ratio SCR; (b) is the effective value of the grid voltage v rms The simulation curve of the time t in 0-6s, the horizontal axis is time, the vertical axis is the effective value of the grid voltage v rms (c) is the grid voltage v within 1.9-2.1s pcc The simulation curve changes with time t, the horizontal axis is time t, and the vertical axis is the grid connection point voltage v pcc (d) is the voltage at the grid point v within 3.9-4.1s. pcc The simulation curve changes with time t, the horizontal axis is time t, and the vertical axis is the grid connection point voltage v pcc The amplitude of

[0048] Figure 6 After the impedance reshaping control strategy is introduced in one embodiment of the present invention, the grid connection point voltage simulation test results of the VSC-HVDC grid-connected system containing SVG are shown in Figure 1. (a) is the simulation curve of the grid short-circuit ratio SCR with time t within 0-6s, with the horizontal axis being time t and the vertical axis being the size of the system short-circuit ratio SCR; (b) is the effective value of the grid voltage v rms The simulation curve of the time t in 0-6s, the horizontal axis is time, the vertical axis is the effective value of the grid voltage v rms (c) is the grid voltage v within 1.9-2.1s pcc The simulation curve changes with time t, the horizontal axis is time t, and the vertical axis is the grid connection point voltage v pcc (d) is the voltage at the grid point v within 3.9-4.1s.pcc The simulation curve changes with time t, the horizontal axis is time t, and the vertical axis is the grid connection point voltage v pcc The amplitude of . DETAILED DESCRIPTION

[0049] The present invention is further described in detail below through specific examples. The following examples are only illustrative and not restrictive, and the protection scope of the present invention cannot be limited thereto.

[0050] Figure 1 The present invention relates to a VSC-HVDC grid-connected system structure diagram containing SVG, including a weak power grid, a VSC-HVDC AC side filter inductor, a VSC-HVDC inverter station converter, an SVG AC side filter, an SVG converter, and an SVG DC side capacitor.

[0051] Figure 2 This is a schematic diagram of the impedance reshaping control strategy applied to SVG involved in the present invention, and the specific implementation steps include:

[0052] Step 1: In the SVG outer loop DC voltage control link, based on the actual value of the SVG DC side voltage v dc Get the theoretical reference value i of the d-axis current dref , set the SVG DC side voltage to a given value v dcref Subtract the actual value of SVG DC side voltage v dc , and compare the obtained value with the transfer function G of the voltage outer loop controller v (s) to obtain the theoretical reference value of the d-axis current i dref , where G v The expression of (s) is:

[0053]

[0054] Among them, k vp is the voltage loop proportionality coefficient, k vi is the voltage loop integral coefficient, s is the complex frequency;

[0055] Step 2: Set the theoretical reference value of the d-axis current i dref Multiplying by the damping factor δ, we get the actual reference value i of the d-axis current dr , where the expression of the damping factor δ is:

[0056]

[0057] Among them, τ is the filtering time constant, n is the number of control iterations, v dcref is the given value of SVG DC side voltage, v dc is the actual value of the DC side voltage of SVG, s is the complex frequency;

[0058] Step 3: Set the theoretical reference value of the q-axis current i qref Multiplying by the damping factor δ, we get the actual reference value i of the q-axis current qr ;

[0059] Step 4: Set the actual reference value of the d-axis current i dr The SVG grid-connected current d-axis component i d Subtract the value obtained from the voltage outer loop controller transfer function G i (s) to obtain the control value of the d-axis current; the actual reference value of the q-axis current i qr The q-axis component i of the SVG grid-connected current q Subtract the value obtained from the voltage outer loop controller transfer function G i (s) to obtain the control value of the q-axis current; where G i The expression of (s) is:

[0060]

[0061] Among them, k ip is the current loop proportionality coefficient, k ii is the current loop integral coefficient, s is the complex frequency;

[0062] Step 5: Convert the SVG grid-connected current q-axis component i q With ωL eq The product value of the grid-connected voltage d-axis component v d The sum is subtracted from the d-axis current control value, and then divided by the SVG DC side voltage given value v dcref , get the d-axis modulation signal m d ; The q-axis component v of the grid voltage q Subtract the SVG grid-connected current d-axis component i d With ωL eq The product value minus the control value of the q-axis current is divided by the given value of the SVG DC side voltage v dcref , and get the q-axis modulation signal m q ;

[0063] Where ω is the system reference angular frequency, L eq is the inductance of the filter circuit on the AC side of SVG;

[0064] Step 6: modulate the signal m according to the d axis d and the q-axis modulation signal m q An SVPWM wave is generated to drive the SVG, so as to achieve impedance reshaping control which is equivalent to improving the damping characteristics of the SVG.

[0065] Figure 3 and Figure 4 , Figure 5 and Figure 6There are two groups of comparison examples,

[0066] Figure 3 FIG. 1 is an image showing how the SVG output impedance changes with frequency when the stability improvement method is not introduced in an embodiment of the present invention. SVG output impedance dd-axis component Z dd (s) and the qq-axis component Z qq (s) In the low frequency range of 0-200 Hz, the phase curve is in the negative damping range of less than -90°, which can easily cause voltage oscillation at the grid connection point of the coupling system composed of the grid-side converter of the SVG, VSC-HVDC system and the weak power grid.

[0067] Figure 4 This is the image of SVG output impedance changing with frequency after the proposed stability improvement method is introduced. Figure 3 SVG output impedance dd-axis component Z dd (s) and qq axis components Z qq (s) The phase curve that changes with frequency. After the proposed stability improvement method is introduced, the curves of phase changing with frequency are no longer within the negative damping range, indicating that the proposed stability improvement method can improve the negative damping characteristics of the SVG output impedance and help enhance the system stability margin.

[0068] Figure 5 The grid connection point voltage v of the VSC-HVDC grid-connected system including SVG when the stability improvement method is not introduced in an embodiment of the present invention pcc And the effective value of the grid voltage v rms Simulation test results when the short-circuit ratio SCR of the power grid changes. At 2s, the short-circuit ratio SCR of the power grid decreases from 3 to 2, and the grid connection point voltage v pcc The waveform changes only slightly before returning to stability. The effective value of the grid-connected voltage is v rms However, at 4s, the grid short-circuit ratio SCR decreases from 2 to 1, and the grid connection point voltage v pcc And the grid voltage effective value v rms The waveform of the grid-connected voltage changes significantly, and the effective value of the grid-connected voltage v rms It gradually increases to nearly 5000kV, indicating that the grid-connected system has obvious oscillations until it becomes completely unstable.

[0069] Figure 6 After the impedance reshaping control strategy is introduced in an embodiment of the present invention, the grid connection point voltage v of the VSC-HVDC grid-connected system containing SVG is pcc And the effective value of the grid voltage v rms Simulation test results when the short-circuit ratio SCR of the power grid changes. At 2s, the short-circuit ratio SCR of the power grid decreases from 3 to 2, and the grid connection point voltage v pcc And the grid voltage effective value v rmsThe waveform of has not changed significantly, and can be restored to the state before the grid short-circuit ratio SCR changes. At 4s, the grid short-circuit ratio SCR decreases from 2 to 1, and the grid connection point voltage v pcc And the grid voltage effective value v rms After a short change, the grid short circuit ratio can be restored to the state before the SCR change. The results show that the proposed control strategy can enhance the stability of the system, improve the stability margin of the system, and ensure the safe and stable operation of the VSC-HVDC grid-connected system containing SVG under weak power grid.

[0070] In summary, the advantages of the present invention are:

[0071] 1. No need to introduce additional sensors and detection devices, thus achieving the purpose of reducing costs;

[0072] 2. By determining the theoretical reference value of the d-axis current, the different forms of the damping factor introduced are controlled, and the output impedance characteristics of the SVG are flexibly controlled, which is flexible and highly applicable;

[0073] 3. The introduction of the damping factor will not change the equivalent filter inductance of the SVG AC side, so it can be applied to situations with different grid strengths, making it more applicable.

[0074] Although the embodiments and drawings of the present invention are disclosed for illustrative purposes, those skilled in the art will appreciate that various substitutions, changes and modifications are possible without departing from the spirit and scope of the present invention and the appended claims. Therefore, the scope of the present invention is not limited to the contents disclosed in the embodiments and drawings.

Claims

1. A method for improving the grid-connected stability of VSC-HVDC in weak power grid based on SVG impedance reshaping, characterized by: The method comprises the following steps: Step 1: In the SVG outer loop DC voltage control link, based on the actual value of the SVG DC side voltage v dc Get the theoretical reference value i of the d-axis current dref ; Step 2: Obtain the theoretical reference value of the d-axis current i dref Multiplying by the damping factor δ, we get the actual reference value i of the d-axis current dr ; Step 3: The known theoretical reference value of q-axis current i qref Multiplying by the damping factor δ, we get the actual reference value i of the q-axis current qr ; Step 4: obtain the actual reference value of the d-axis current i dr The SVG grid-connected current d-axis component i d Subtract the value obtained from the voltage outer loop controller transfer function G i (s) to obtain the control value of the d-axis current; The actual reference value of the q-axis current i qr The q-axis component i of the SVG grid-connected current q Subtract the value obtained from the voltage outer loop controller transfer function G i (s) to obtain the control value of the q-axis current; Among them, G i The expression of (s) is: Among them, k ip is the current loop proportionality coefficient, k ii is the current loop integral coefficient, s is the complex frequency; Step 5: Convert the SVG grid-connected current q-axis component i q , the system reference angular frequency ω and the inductance L of the SVG AC side filter circuit eq The product of the three is equal to the d-axis component v of the grid-connected point voltage. d The sum is subtracted from the d-axis current control value, and then divided by the SVG DC side voltage given value v dcref , get the d-axis modulation signal m d ; The SVG grid-connected current d-axis component i d , the system reference angular frequency ω and the inductance L of the SVG AC side filter circuit eq Multiply the three and get the product value ωL eq i d , the q-axis component v of the grid-connected voltage q Subtract the product value ωL eq i d The difference is then subtracted from the control value of the q-axis current and divided by the given value of the SVG DC side voltage v dcref , and get the q-axis modulation signal m q ; Step 6: modulate the signal m according to the d axis d and the q-axis modulation signal m q An SVPWM wave is generated to drive the SVG, so as to achieve impedance reshaping control which is equivalent to improving the damping characteristics of the SVG.

2. A method for improving the grid-connected stability of VSC-HVDC in a weak power grid based on SVG impedance reshaping according to claim 1, characterized in that: In the SVG outer loop DC voltage control link, based on the actual value of the SVG DC side voltage v dc Get the theoretical reference value i of the d-axis current dref The method is: Set the SVG DC side voltage to a given value v dcref Subtract the actual value of SVG DC side voltage v dc , and compare the obtained value with the transfer function G of the voltage outer loop controller v (s) to obtain the theoretical reference value of the d-axis current i dref , Among them, G v The expression of (s) is: Among them, k vp is the voltage loop proportionality coefficient, k vi is the voltage loop integral coefficient, and s is the complex frequency.

3. The method for improving the grid-connected stability of VSC-HVDC in a weak power grid based on SVG impedance reshaping according to claim 1, characterized in that: The expression of the damping factor δ is: Among them, τ is the filtering time constant, n is the number of control iterations, v dcref is the given value of SVG DC side voltage, v dc is the actual value of the SVG DC side voltage, s is the complex frequency, i dref is the theoretical reference value of d-axis current.

4. A VSC-HVDC grid-connected stability improvement system under weak power grid based on SVG impedance reshaping, characterized by: include: The voltage outer loop control module is used to control the actual value of the SVG DC side voltage to track the given value and obtain the theoretical reference value of the d-axis current i dref ; Damping factor generation module, based on the theoretical reference value of d-axis current i dref , SVG DC side voltage set value v dcref and the actual value of SVG DC side voltage v dc , by setting the filter time constant and controlling the number of iterations, the damping factor is generated; The damping control module is used to introduce the damping factor δ into the current loop control strategy based on the theoretical reference value i of the d-axis current. dref , q-axis current theoretical reference value i qref , obtain the actual reference value i of the d-axis current dr , q-axis current actual reference value i qr ; The current control module is used to control the d-axis current and q-axis current actually output by the SVG to track the actual reference value of the d-axis current i dr , q-axis current actual reference value i qr , and obtain a d-axis output voltage signal and a q-axis output voltage signal for modulation; The modulation module obtains the d-axis modulation signal m based on the d-axis output voltage signal and the q-axis output voltage signal for SVG modulation obtained in the current control module. d , q-axis modulation signal m q , and obtain the SVPWM driving signal to drive the actual operation of SVG.

5. A VSC-HVDC grid-connected stability improvement system based on SVG impedance reshaping in weak power grid according to claim 4, characterized in that: The voltage outer loop control module is used to control the actual value of the SVG DC side voltage to track the given value, and the method for obtaining the theoretical reference value of the d-axis current is: set the given value of the SVG DC side voltage v dcref Subtract the actual value of SVG DC side voltage v dc , and compare the obtained value with the transfer function G of the voltage outer loop controller v (s) multiplied by each other, we get i dref , Among them, G v The expression of (s) is: Among them, k vp is the voltage loop proportionality coefficient, k vi is the voltage loop integral coefficient, and s is the complex frequency.

6. A VSC-HVDC grid-connected stability improvement system based on SVG impedance reshaping in weak power grid according to claim 4, characterized in that: The expression of the damping factor δ is: Among them, τ is the filtering time constant, n is the number of control iterations, v dcref is the given value of SVG DC side voltage, v dc is the actual value of the SVG DC side voltage, s is the complex frequency, i dref is the theoretical reference value of d-axis current.

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