DC control method and system for improving transient stability of grid-connected converters

Abstract

The present invention discloses a DC control method and system for improving the transient stability of a grid-connected converter. The method includes: when any one of a power angle over-limit fault judgment module and a voltage over-limit fault judgment module determines that a voltage drop or phase jump fault has occurred in the power grid, the angular velocity control signal of the synchronization control link is processed by a DC port transient energy correction module, and the DC voltage reference value is raised for temporary unbalanced power storage and for inertia correction; the voltage value of the DC port capacitor is compared with the DC voltage reference value, and processed by a steady-state DC voltage control module to obtain a DC voltage control reference value; the output data of the converter at the common coupling point is used to obtain a synchronization control signal according to the synchronization control link; based on the synchronization control signal, an internal potential reference value is obtained according to the internal potential control link, and then a converter drive signal is generated. The present invention can effectively solve the stability problem of the grid-connected converter when a transient fault occurs in the power grid.

Description

Technical Field

[0001] The present invention belongs to the technical field of power system applications, and in particular relates to a direct current control method and system for improving the transient stability of a grid-connected converter. Background Art

[0002] With the rapid development of renewable energy generation technologies, photovoltaic (PV) and wind power, among other renewable energy sources, are being connected to the same AC power grid through power electronics. Together, these renewable energy sources and the AC grid constitute a multi-renewable energy infeed system. The integration of renewable energy via converters significantly alters the dynamic characteristics of modern power grids, posing challenges to grid security and stability. When voltage sags and phase jumps occur in the grid, the DC-side power of the grid-connected converter overflows the AC-side output power. This transient power imbalance creates the risk of transient power angle instability. Summary of the Invention

[0003] The purpose of the present invention is to provide a DC control method and system for improving the transient stability of a grid-connected converter, which can effectively solve the stability problem of the grid-connected converter when a transient fault occurs in the power grid.

[0004] In order to achieve the above object, the solution of the present invention is:

[0005] A DC control method for improving transient stability of a grid-connected converter, comprising:

[0006] When either the power angle limit-exceeding fault judgment module or the voltage limit-exceeding fault judgment module determines that a voltage drop or phase jump fault has occurred in the power grid, the control signal of the synchronous control link is processed by the DC port transient energy correction module to increase the DC voltage reference value for temporary unbalanced power and inertia correction;

[0007] Obtaining a voltage value of the DC port capacitor, comparing it with the DC voltage reference value, and processing it through a steady-state DC voltage control module to obtain a DC voltage control reference value;

[0008] Obtain output data of the converter at the common coupling point and obtain a synchronous control signal according to the synchronous control link;

[0009] Based on the synchronous control signal, an internal potential reference value is obtained according to the internal potential control link;

[0010] A converter driving signal is generated according to the internal potential reference value.

[0011] The DC voltage reference value is raised for temporarily storing unbalanced power and for inertia correction, including:

[0012] Obtain the angular velocity ω of the converter output voltage and compare the angular velocity ω with the rated value ωn The difference is filtered out by the notch filter to remove the power frequency disturbance, and the damping-voltage mapping coefficient k is used. D-V Amplify and obtain the DC voltage rise ΔV for temporary storage of transient unbalanced power Pu ;

[0013] Get the angular velocity change rate of the converter output voltage The rate of change of angular velocity The high frequency oscillation is filtered out by a low-pass filter, and the inertia-voltage mapping coefficient k J-V Amplify and obtain the DC voltage rise ΔV for converter inertia correction J ;

[0014] The sum of the DC voltage increase ΔV for temporarily storing transient unbalanced power and the DC voltage increase ΔV for converter inertia correction is calculated. dc With the fault detection signal S F Multiply and add to the DC voltage reference value V dc_ref , and thereby correct it.

[0015] The design method of the inertia-voltage mapping coefficient is to determine the inertia correction value ΔJ of the converter swing characteristic, and the inertia-voltage mapping coefficient is calculated by the following formula: J-V =ΔJ / k pdc , where k pdc is the proportional coefficient in steady-state DC voltage control;

[0016] The damping-voltage mapping coefficient is designed by determining the damping correction value ΔD of the converter swing characteristic. The damping-voltage mapping coefficient is calculated by the following formula: D-V =(ΔD+k idc ·k J-V ) / k pdc , where k idc is the integral coefficient in steady-state DC voltage control.

[0017] The converter is a grid-type converter; the angular velocity ω of the converter output voltage is obtained, including:

[0018] Get the output voltage u of the converter at the common coupling point PCC and current i PCC , get the active power;

[0019] According to the difference between the active power and the active power reference value, the angular velocity ω of the converter output voltage is obtained through active-phase synchronous control processing; or,

[0020] The converter is a grid-following converter; the angular velocity ω of the converter output voltage is obtained, including:

[0021] Get the output voltage u of the converter at the common coupling point PCC ;

[0022] The output voltage u PCC Perform phase-locked loop processing to obtain the angular velocity ω of the converter output voltage.

[0023] Among them, judging whether a voltage drop or phase jump fault occurs in the power grid includes:

[0024] When it is determined that at least one of the power angle and voltage of the converter exceeds the limit, it is considered that the power grid has a voltage drop or phase jump fault, and the fault detection signal S F The output is 1, otherwise the fault detection signal S F The output is 0.

[0025] The process of judging whether the power angle of the converter exceeds the limit includes:

[0026] Obtain the current power angle δ of the grid-connected converter and its initial power angle δ0 under stable operating conditions, and calculate the difference between the two;

[0027] The absolute value of the difference is compared with the power angle threshold δ th Compare, if the absolute value exceeds the power angle threshold δ th , it is determined that the converter power angle exceeds the limit.

[0028] The process of judging whether the voltage of the converter exceeds the limit includes:

[0029] Get the grid-connected converter voltage amplitude V and its rated value V n The difference between

[0030] The absolute value of the difference is compared with the voltage stability threshold V th If the absolute value exceeds the voltage stability threshold V th , it is determined that the converter voltage exceeds the limit.

[0031] Among them, the converter is a grid-type converter; the voltage value V of the DC port capacitor is obtained dc , and the DC voltage reference value V dc_ref Compare and get the DC voltage control reference value, including,

[0032] Get the voltage value V of the DC port capacitor dc , and the DC voltage reference value V dc_ref After comparison, the active power reference value P is generated ref , as the DC voltage control reference value;

[0033] Obtain the output data of the converter at the common coupling point, and obtain the synchronous control signal according to the synchronous control link, including:

[0034] Get the output voltage u of the converter at the common coupling point PCC and current i PCC , we get the active power P e and reactive power Q e ;

[0035] According to the active power P e and active power reference value P ref The difference between the output voltage and the output voltage is obtained, and the reference value of the output voltage phase of the converter is obtained; according to the reactive power Q e and reactive power reference value Q ref The difference between the phase reference value θ and the amplitude reference value V is obtained; the synchronous control signal includes a phase reference value θ and an amplitude reference value V;

[0036] Based on the synchronous control signal, an internal potential reference value is obtained according to the internal potential control link, including:

[0037] Performing coordinate transformation on the amplitude reference value V using the phase reference value θ to obtain an output voltage reference value of the converter output voltage in a synchronous coordinate system;

[0038] The output voltage u of the converter at the common coupling point PCC The internal potential reference value is compared with the output voltage reference value in the synchronous coordinate system, and the difference between the two is used to obtain the internal potential reference value.

[0039] The converter is a grid-following converter; the voltage value of the DC port capacitor is obtained and compared with the DC voltage reference value V dc_ref Compare and get the DC voltage control reference value, including,

[0040] Get the voltage value V of the DC port capacitor dc , and the DC voltage reference value V dc_ref After comparison, the active current reference value I is generated d_ref , as the DC voltage control reference value;

[0041] Obtain the output data of the converter at the common coupling point, and obtain the synchronous control signal according to the synchronous control link, including:

[0042] Get the output voltage u of the converter at the common coupling point PCC ;

[0043] The output voltage u PCC Perform phase-locked loop processing to obtain the angular velocity ω of the converter output voltage, integrate it to obtain the voltage phase θ, which serves as the synchronization control signal;

[0044] Based on the synchronous control signal, an internal potential reference value is obtained according to the internal potential control link, including:

[0045] Get the output current i of the converter at the common coupling point PCC , the current i PCC Use the voltage phase θ to perform coordinate transformation and obtain the converter output active current I d and reactive current I q ;

[0046] The converter output active current and active current reference value I d_ref Compare the reactive current output by the converter with the preset reactive current reference value I q_ref Perform comparison to obtain a second difference;

[0047] An internal potential reference value is obtained according to the first difference and the second difference.

[0048] A DC control system for improving transient stability of a grid-connected converter, comprising:

[0049] The fault judgment module based on power angle exceeding the limit is configured to judge whether a voltage drop or phase jump fault occurs in the power grid;

[0050] A fault judgment module based on voltage over-limit is configured to judge whether a voltage drop or phase jump fault occurs in the power grid;

[0051] a DC port transient energy correction module configured to, when either the power angle limit-exceeding fault judgment module or the voltage limit-exceeding fault judgment module determines that a voltage drop or phase jump fault has occurred in the power grid, process the control signal of the synchronous control link through the DC port transient energy correction module, and raise the DC voltage reference value for temporarily storing unbalanced power and for inertia correction;

[0052] a steady-state DC voltage control module, configured to obtain a voltage value of the DC port capacitor, compare it with the DC voltage reference value, and obtain a DC voltage control reference value;

[0053] a synchronization module configured to obtain output data of the converter at a common coupling point and obtain a synchronization control signal according to a synchronization control link;

[0054] an internal potential control module configured to obtain an internal potential reference value according to an internal potential control link based on the synchronous control signal; and

[0055] The pulse width modulation module is configured to generate a converter driving signal according to the internal potential reference value.

[0056] After adopting the above scheme, the present invention targets the transient stability problem faced by the new energy field containing the grid-connected converter under the grid fault condition. It detects the occurrence of the grid fault in real time according to the power angle and voltage amplitude of the grid-connected converter synchronization module, and enables the damping correction module and the inertia correction module when the fault is detected, so as to realize the instantaneous storage of the unbalanced energy at the DC port of the grid-connected converter during the transient process, thereby avoiding the transient instability problem and the occurrence of DC voltage over-limit. The present invention does not affect the control structure and performance of the original grid-connected converter synchronization strategy. It only needs to add a DC port compensation link on the basis of the original control system, which is convenient to implement, so that the transient stability of the grid-connected converter is improved without changing its operating characteristics.

[0057] Compared with the prior art, the beneficial effects of the present invention are: the control structure of the present invention is simple, the parameters are easy to adjust, it does not affect the converter reference power under normal operation of the power grid, and does not require mode switching between normal operation and fault state of the power grid. It can adaptively control the DC port of the grid-connected converter to temporarily store unbalanced energy at the moment of transient fault in the power grid, providing immediate transient stability support for the grid-connected converter. In the case of transient fault in the power grid, the transient process of the system can be shortened, so that the system can recover to a stable operation state in a short time, and the overshoot of the DC capacitor voltage is reduced, alleviating the overvoltage risk of the DC port. Only the power angle and its derivative, and the voltage amplitude in the synchronization module of the grid-connected converter are used as feedback quantities, which does not rely on time-varying system information such as the voltage drop degree and line impedance, avoids the DC port temporary energy deviation caused by communication delay, improves the rapidity of transient stability support, and reduces the complexity and implementation difficulty of the control strategy. BRIEF DESCRIPTION OF THE DRAWINGS

[0058] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, those skilled in the art can derive other drawings based on these drawings without inventive effort.

[0059] Figure 1 This is a typical system structure and control block diagram of a grid-connected converter according to an embodiment of the present invention;

[0060] Figure 2 is a control block diagram of a grid-connected converter that introduces DC port control in an embodiment of the present invention;

[0061] Figure 3 This is a block diagram of a typical synchronous control link of a grid-connected converter according to an embodiment of the present invention;

[0062] Among them, (a) is the active power droop synchronization control module, (b) is the virtual synchronous machine control module, (c) is the reactive power droop synchronization control module, and (d) is the phase-locked loop synchronization control module;

[0063] Figure 4 Schematic diagram of the control effect of the present invention;

[0064] Among them, (a) represents the DC voltage and its reference value using traditional DC voltage control, (b) represents the DC voltage and its reference value using the present invention, (c) represents the actual value and reference value of the output active power using traditional DC voltage control, (d) represents the actual value and reference value of the output active power using the present invention, (e) represents the output power angle of the converter (converter phase minus grid phase) using traditional DC voltage control, and (f) represents the output power angle of the converter (converter phase minus grid phase) using the present invention;

[0065] In the picture:

[0066] 1. Power grid; 2. Filtering equipment; 3. Grid-type converter;

[0067] 4. DC port; 4-1. DC port capacitance;

[0068] 5. DC voltage control module; 5-1. Steady-state DC voltage control module;

[0069] 5-2. Fault judgment module; 5-2-1. Fault judgment module based on power angle exceeding the limit; 5-2-2. Fault judgment module based on voltage exceeding the limit;

[0070] 5-3 DC port transient energy correction module; 5-3-1, transient power storage module; 5-3-2, inertia correction module; 5-3-3, reference voltage superimposer;

[0071] 6. Synchronization module; 6-1-1-1. Active power droop synchronization control module; 6-1-1-2. Virtual synchronous machine control module; 6-1-2. Reactive power droop synchronization control module; 6-2. Phase-locked loop synchronization control module;

[0072] 7. Internal potential control module; 8. Pulse width modulation module. DETAILED DESCRIPTION

[0073] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. 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 making any creative efforts shall fall within the scope of protection of the present invention.

[0074] The grid-connected converter applicable to the present invention can be referred to Figure 1As shown, it includes a grid-type converter 3, a DC port 4 is provided on its DC side, and a DC port capacitor 4-1 is connected in parallel to the DC port 4, wherein P dc Indicates the active power of the DC side front-stage input, V dc Represents the voltage of the DC side capacitor; the AC side of the grid-type converter 3 is connected to the infinite grid 1 through the filter device 2, wherein the filter device 2 includes a capacitor L f and voltage C f , the connection point after the filtering device becomes the common coupling point, u PCC and i PCC Represent the voltage and current at the common coupling point respectively; the infinite grid 1 is composed of the grid side impedance Z g and the ideal voltage source V g e jθg representation.

[0075] The present invention provides a DC control method for improving transient stability of a grid-connected converter, comprising the following steps:

[0076] Step 1: When either the power angle limit-exceeding fault judgment module or the voltage limit-exceeding fault judgment module determines that a voltage drop or phase jump fault has occurred in the power grid, the control signal of the synchronous control link is processed by the DC port transient energy correction module to increase the DC voltage reference value for temporary unbalanced power and inertia correction.

[0077] Step 2: Obtain the voltage value of the DC port capacitor, compare it with the DC voltage reference value, and obtain a DC voltage control reference value through processing by the steady-state DC voltage control module;

[0078] Step 3: Obtain output data of the converter at the common coupling point and obtain a synchronous control signal according to the synchronous control link;

[0079] Step 4, obtaining an internal potential reference value according to an internal potential control link based on the synchronous control signal;

[0080] Step 5, generating a converter drive signal according to the internal potential reference value;

[0081] Among them, steps 2-5 are the conventional control process of the existing grid-connected converter. The improvement of the present invention is that, without affecting the existing control strategy, a DC port compensation link is added to control the DC port of the grid-connected converter to temporarily store unbalanced energy at the moment of a transient fault in the power grid, thereby providing immediate transient stability support for the grid-connected converter and improving the rapidity of transient stability support.

[0082] Wherein, in the step 1, the DC voltage reference value is raised for temporarily storing unbalanced power and for inertia correction, including:

[0083] Obtain the angular velocity ω of the converter output voltage and compare the angular velocity ω with the rated value ω n The difference is filtered out by the notch filter to remove the power frequency disturbance, and the damping-voltage mapping coefficient k is used. D-V Amplify and obtain the DC voltage rise ΔV for temporary storage of transient unbalanced power Pu ;

[0084] Get the angular velocity change rate of the converter output voltage The rate of change of angular velocity The high frequency oscillation is filtered out by a low-pass filter, and the inertia-voltage mapping coefficient k J-V Amplify and obtain the DC voltage rise ΔV for converter inertia correction J ;

[0085] The sum of the DC voltage increase ΔV for temporarily storing transient unbalanced power and the DC voltage increase ΔV for converter inertia correction is calculated. dc With the fault detection signal S F , superimposed on the DC voltage reference value V dc_ref , and thereby correct it.

[0086] The design method of the inertia-voltage mapping coefficient is to determine the inertia correction value ΔJ of the converter swing characteristic, and the inertia-voltage mapping coefficient is calculated by the following formula: J-V =ΔJ / k pdc , where k pdc is the proportional coefficient in steady-state DC voltage control;

[0087] The damping-voltage mapping coefficient is designed by determining the damping correction value ΔD of the converter swing characteristic. The damping-voltage mapping coefficient is calculated by the following formula: D-V =(ΔD+k idc ·k J-V ) / k pdc , where k idc is the integral coefficient in steady-state DC voltage control.

[0088] The converter is a grid-type converter; the angular velocity ω of the converter output voltage is obtained, including:

[0089] Get the output voltage u of the converter at the common coupling point PCC and current i PCC , get the active power;

[0090] According to the difference between the active power and the active power reference value, the angular velocity ω of the converter output voltage is obtained through active-phase synchronous control processing; or,

[0091] The converter is a grid-following converter; the angular velocity ω of the converter output voltage is obtained, including:

[0092] Get the output voltage u of the converter at the common coupling point PCC ;

[0093] The output voltage u PCC Perform phase-locked loop processing to obtain the angular velocity ω of the converter output voltage.

[0094] Wherein, in said step 1, determining whether a voltage drop or phase jump fault occurs in the power grid includes:

[0095] When it is determined that at least one of the power angle and voltage of the converter exceeds the limit, it is considered that the power grid has a voltage drop or phase jump fault, and the fault detection signal S F The output is 1, otherwise the fault detection signal S F The output is 0.

[0096] The process of judging whether the power angle of the converter exceeds the limit includes:

[0097] Obtain the current power angle δ of the grid-connected converter and its initial power angle δ0 under stable operating conditions, and calculate the difference between the two;

[0098] The absolute value of the difference is compared with the power angle threshold δ th Compare, if the absolute value exceeds the power angle threshold δ th , it is determined that the converter power angle exceeds the limit.

[0099] The process of judging whether the voltage of the converter exceeds the limit includes:

[0100] Get the grid-connected converter voltage amplitude V and its rated value V n The difference between

[0101] The absolute value of the difference is compared with the voltage stability threshold V th If the absolute value exceeds the voltage stability threshold V th , it is determined that the converter voltage exceeds the limit.

[0102] Among them, the active power-phase synchronization control can adopt various schemes, such as the droop control scheme, the virtual synchronous machine control scheme, etc., which are not limited to this embodiment;

[0103] The droop control scheme refers to the difference between the active power and the active power reference value through the active-frequency droop coefficient K. p Amplify to get the angular velocity ω;

[0104] Among them, the virtual synchronous machine control scheme refers to the difference between the active power and the active power reference value through the inertia coefficient J p and the damping coefficient D pThe angular velocity ω is obtained by simulating the frequency response of the synchronous machine.

[0105] Wherein, the converter is a grid-type converter; the specific contents of step 2 include:

[0106] Get the voltage value V of the DC port capacitor dc , and the DC voltage reference value V dc_ref After comparison, the active power reference value P is generated ref , as the DC voltage control reference value;

[0107] The specific contents of step 3 include:

[0108] Get the output voltage u of the converter at the common coupling point PCC and current i PCC , we get the active power P e and reactive power Q e ;

[0109] According to the active power P e and active power reference value P ref The difference between the output voltage and the output voltage is obtained, and the reference value of the output voltage phase of the converter is obtained; according to the reactive power Q e and reactive power reference value Q ref The difference between the phase reference value θ and the amplitude reference value V is obtained; the synchronous control signal includes a phase reference value θ and an amplitude reference value V;

[0110] The specific contents of step 4 include:

[0111] Performing coordinate transformation on the amplitude reference value V using the phase reference value θ to obtain an output voltage reference value of the converter output voltage in a synchronous coordinate system;

[0112] The output voltage u of the converter at the common coupling point PCC The internal potential reference value is compared with the output voltage reference value in the synchronous coordinate system, and the difference between the two is used to obtain the internal potential reference value.

[0113] Wherein, the converter is a grid-following converter; the specific contents of step 2 include:

[0114] Get the voltage value V of the DC port capacitor dc , and the DC voltage reference value V dc_ref After comparison, the active current reference value I is generated d_ref , as the DC voltage control reference value;

[0115] The specific contents of step 3 include:

[0116] Get the output voltage u of the converter at the common coupling point PCC ;

[0117] The output voltage u PCC Perform phase-locked loop processing to obtain the angular velocity ω of the converter output voltage, integrate it to obtain the voltage phase θ, which serves as the synchronization control signal;

[0118] The specific contents of step 4 include:

[0119] Get the output current i of the converter at the common coupling point PCC , the current i PCC Use the voltage phase θ to perform coordinate transformation and obtain the converter output active current I d and reactive current I q ;

[0120] The converter output active current and active current reference value I d_ref Compare the reactive current output by the converter with the preset reactive current reference value I q_ref Perform comparison to obtain a second difference;

[0121] An internal potential reference value is obtained according to the first difference and the second difference.

[0122] like Figure 1 and Figure 2 As shown, an embodiment of the present invention further provides a DC control system for improving transient stability of a grid-connected converter, comprising:

[0123] The DC voltage control module 5 is used to receive the voltage value V of the DC port capacitor. dc , and the DC voltage reference value V dc_ref After comparison, the active power or active current reference value is generated, and the active power reference value P ref Or active current reference value I d_ref Input to synchronization module 6;

[0124] The DC voltage control module 5 includes: a steady-state DC voltage control module 5-1, a fault judgment module 5-2 and a DC port transient energy correction module 5-3;

[0125] The steady-state DC voltage control module 5-1 receives the voltage value V of the DC port capacitor of the converter dc , and the preset DC voltage reference value V dc_ref After making the difference, the input DC voltage is double-channel zero-static difference regulator outputs the AC side active power reference value P of the converter ref Or active current reference value I d_ref .

[0126] The fault judgment module 5-2 is composed of a fault judgment module 5-2-1 based on power angle exceeding limit and a fault judgment module 5-2-2 based on voltage exceeding limit. The fault judgment module 5-2-1 based on power angle exceeding limit calculates the difference between the power angle δ calculated by the grid-connected converter synchronization module and its initial point δ0 under stable operation conditions, and after absolute processing, it is compared with the power angle threshold δ th Compare, if it exceeds the threshold, it is considered that the system has failed; the fault judgment module 5-2-2 based on voltage limit exceeds the limit and calculates the voltage amplitude V of the grid-connected converter and its rated value V n The difference is then converted to absolute value and compared with the voltage stability threshold. If it exceeds the threshold V th , it is considered that the system has failed. For the grid-connected converter, the output voltage reference value V of the reactive power droop control module in the synchronization module is used. o_ref For fault diagnosis, for grid-following converters, use the d-axis voltage amplitude V in the coordinate transformation module. PCC_d Perform fault diagnosis.

[0127] The fault judgment module 5-2-1 based on power angle exceeding the limit and the fault judgment module 5-2-2 based on voltage exceeding the limit run in parallel. If the detection result of any of the modules is that a fault occurs, it is considered that the system has a fault;

[0128] The DC port transient energy correction module 5-3 is composed of a transient power temporary storage loop 5-3-1, an inertia correction loop 5-3-2 and a reference voltage superimposer 5-3-3; the transient power temporary storage loop 5-3-1 receives the angular velocity ω in the synchronization module 6 and its rated value ω n The difference is passed through the damping-voltage mapping link to obtain the damping correction amount of the grid-connected converter swing characteristic, so as to achieve the temporary storage of the unbalanced power of the AC port at the DC port of the converter, which is equivalent to increasing the unbalanced energy consumed by the damping effect in the transient process. Among them, the damping amplification factor of the damping-voltage mapping link can be set according to the required damping improvement target, such as 1.5 or 2; the inertia correction module 5-3-2 receives the angular velocity change rate in the synchronization module 6 After low-pass filtering, the inertia correction value of the grid-connected converter swing characteristic is obtained through the inertia-voltage mapping link, which accelerates the transient process of the grid-connected converter and reduces the voltage rise of the DC port. Among them, the inertia amplification factor of the inertia-voltage mapping link can be set according to the required inertia improvement target; the reference voltage superimposer 5-3-3 adds the sum of the damping correction value and the inertia correction value (ΔV dc ) 2 With the fault detection signal S F Compared with the DC voltage reference value V in steady state dc_ref The DC voltage reference value for AC / DC power balance under grid fault conditions is obtained through the superimposer.

[0129] Synchronization module 6, used to synchronize the output voltage u of the grid-connected converter at the common coupling point PCC and current i PCC Calculate the amplitude and phase of the converter output voltage or current, and input the amplitude V and phase θ of the converter output voltage or current to the internal potential control module 7;

[0130] Among them, for the grid-type converter, the power calculation module is used to calculate the active power and reactive power output by the converter; the grid-type power synchronization module is used to calculate the phase of the converter output voltage according to the active power value output by the grid-type converter and the active power reference value, and the amplitude of the converter output voltage is calculated according to the reactive power value output by the grid-type converter and the reactive power reference value, and the amplitude and phase of the converter output voltage are input to the output internal potential control module; for the grid-type converter, the grid-type voltage synchronization module is used to calculate the voltage phase and amplitude on the grid side according to the voltage at the common coupling point of the converter, and output to the internal potential control module 7;

[0131] For the grid-type power synchronization module, the active power synchronization module includes droop control, virtual synchronous machine control, matching control, etc. Figure 3 As shown, the active power droop synchronization control module 6-1-1-1 will active power P e and active power reference value P ref The difference between the active power and frequency droop coefficient K p Amplify the angular velocity ω; the virtual synchronous machine control module 6-1-1-2 converts the active power P e and active power reference value P ref The difference is calculated by the inertia coefficient J p and the damping coefficient D p The angular velocity ω is obtained by simulating the frequency response of the synchronous machine. The reactive power droop synchronous control module 6-1-2 uses droop control to output reactive power Q on the AC side of the receiving converter. e Its reference value Q ref The offset is calculated by the reactive droop coefficient K q Adjust the reference value V to generate the output voltage amplitude o_ref .

[0132] For the voltage synchronization module of the grid-following converter, the voltage u PCC Input the phase-locked loop synchronous control module 6-2, perform abc / dq coordinate transformation, and convert the q-axis component U of the voltage passing through the common coupling point into PCC_q Perform proportional-integral processing to calculate the phase of the grid side voltage, and calculate the phase of the grid side voltage according to the d-axis component U of the common coupling point voltage. PCC_d Calculate the magnitude of the grid-side voltage.

[0133] An internal potential control module 7 is used to compare the phase and amplitude of the converter output voltage or current with the actual value to generate an internal potential reference value, and input the internal potential reference value into the pulse width modulation module 8;

[0134] Among them, for the grid-type converter, the internal potential control module 7 is composed of a voltage control module and a current control module. The voltage control module compares the converter output voltage reference value with the actual value to generate a current reference value, and inputs the current reference value into the current control module; the current control module is used to compare the current reference value with the actual value to generate an internal potential reference value, and input the voltage reference value before the filter device into the pulse width modulation module; for the grid-type converter, the common coupling point current is subjected to abc / dq coordinate transformation and compared with the dq axis current reference value to generate an internal potential reference value E ref ;

[0135] The pulse width modulation module 8 is used to generate a converter driving signal according to the internal potential reference value.

[0136] The specific working principle of the present invention is:

[0137] When the grid is operating normally, the power angle and voltage in the converter synchronization module 6 are within the threshold value, the fault detection module detects that there is no fault in the grid, the fault detection signal output is zero, the output of the DC port transient energy correction module 5-3 is set to zero, the DC port of the grid-connected converter operates according to the preset DC voltage reference value, and the DC voltage control module does not affect the normal operation and dynamic characteristics of the grid-connected converter;

[0138] When the grid voltage drops or phase jumps, the active power output by the grid-type converter changes suddenly at the moment of the fault, and a large power difference is generated between the output active power of the converter on the AC side and the active power reference value, causing the power angle of the synchronization module 6 to rise rapidly; the q-axis voltage of the grid-type converter rises rapidly at the moment of the fault, causing the power angle of the synchronization module 6 to rise rapidly. At this time, the power angle difference received by the fault detection module 5-2-1 based on the power angle exceeds the threshold, and it is judged that a fault has occurred. Since the power angle characteristic is the direct factor leading to transient instability, the fault detection module based on the power angle exceeding the limit can realize the accurate detection of grid faults; the grid voltage drops When a fault or phase jump fault occurs, the reactive power output by the grid-type converter changes suddenly at the moment of the fault, and a large power difference is generated between the output reactive power of the converter AC measurement and the reactive power reference value, causing the voltage amplitude of the synchronization module 6 to rise rapidly; when a grid voltage drop fault or a phase jump fault occurs, the d-axis voltage in the grid-type converter lock synchronization link 6 changes suddenly at the moment of the fault. At this time, the difference received by the voltage-exceeding fault detection module 5-2-2 exceeds the threshold, and it is determined that a fault has occurred. Due to the fast response characteristics of the reactive synchronization module / coordinate transformation module, the voltage-exceeding fault detection module can realize rapid and timely detection of grid faults;

[0139] When any fault detection module detects a fault, the fault detection signal output is 1, and the DC port transient energy correction module 5-3 starts working. The transient power storage loop 5-3-1 receives the difference between the angular velocity in the synchronization module 6 and its rated value, and obtains the DC voltage rise for transient power storage after damping-voltage mapping. The inertia correction module 5-3-2 receives the angular velocity change rate in the synchronization module 6, performs low-pass filtering, and obtains the DC voltage rise for inertia correction after inertia-voltage mapping. The DC voltage rise for transient power storage and the DC voltage rise for inertia correction are summed and multiplied by the fault detection signal. The DC voltage reference value under the transient fault state is raised by the reference voltage superimposer 5-3-3, changing the dynamic characteristics of the DC port so that the DC port can transiently store and compensate for the converter power difference. The converter unbalanced power reduced by the DC port reduces the power angle overshoot, avoids the potential risk of converter transient power angle instability, and limits and protects the DC voltage at the same time.

[0140] Figure 4The left and right figures show the effects of traditional DC control and improved DC control, respectively, demonstrating that the DC control method proposed in this invention can enable the grid-connected converter to maintain synchronization with the grid during transient faults, avoiding the risk of transient instability. The control method, which does not require mode switching, does not affect the converter's operating point in steady state and can trigger control at the moment a fault occurs, avoiding the risk of instability caused by communication delays. This control method accelerates the converter's transient process, shortens the overshoot of the DC capacitor voltage, and avoids the risk of overvoltage at the DC port.

[0141] Based on the same inventive concept, the present invention also provides a computer device, which includes: one or more processors and a memory for storing one or more computer programs; the program includes program instructions, and the processor is used to execute the program instructions stored in the memory. The processor may be a central processing unit (CPU), or other general-purpose processors, digital signal processors (DSP), application-specific integrated circuits (ASIC), field-programmable gate arrays (FPGA) or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. It is the computing core and control core of the terminal, which is used to implement one or more instructions, specifically for loading and executing one or more instructions in a computer storage medium to implement the above method.

[0142] It should be further explained that, based on the same inventive concept, the present invention also provides a computer storage medium having a computer program stored thereon, which executes the above method when executed by a processor. The storage medium can be any combination of one or more computer-readable media. The computer-readable medium can be a computer-readable signal medium or a computer-readable storage medium. The computer-readable storage medium can be, for example, but not limited to, an electrical, magnetic, optical, electrical, magnetic, infrared, or semiconductor system, device or component, or any combination thereof. More specific examples (a non-exhaustive list) of computer-readable storage media include: an electrical connection with one or more wires, a portable computer disk, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or flash memory), an optical fiber, a portable compact disk read-only memory (CD-ROM), an optical storage device, a magnetic storage device, or any suitable combination thereof. In the present invention, a computer-readable storage medium can be any tangible medium containing or storing a program that can be used by or in conjunction with an instruction execution system, device or component.

[0143] Throughout this specification, references to terms such as "one embodiment," "example," or "specific example" indicate that a specific feature, structure, material, or characteristic described in conjunction with that embodiment or example is included in at least one embodiment or example of the present disclosure. In this specification, schematic representations of these terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

[0144] The above shows and describes the basic principles, main features and advantages of the present disclosure. Those skilled in the art should understand that the present disclosure is not limited to the above embodiments. The above embodiments and descriptions are merely illustrative of the principles of the present disclosure. Various changes and improvements may be made to the present disclosure without departing from the spirit and scope of the present disclosure, and such changes and improvements shall fall within the scope of the present disclosure.

Claims

1. A DC control method for improving transient stability of a grid-connected converter, characterized by: include, When either the power angle limit-exceeding fault judgment module or the voltage limit-exceeding fault judgment module determines that a voltage drop or phase jump fault has occurred in the power grid, the angular velocity control signal of the synchronous control link is processed by the DC port transient energy correction module to increase the DC voltage reference value for temporary unbalanced power and inertia correction. Obtaining a voltage value of the DC port capacitor, comparing it with the DC voltage reference value, and processing it through a steady-state DC voltage control module to obtain a DC voltage control reference value; Obtain output data of the converter at the common coupling point and obtain a synchronous control signal according to the synchronous control link; Based on the synchronous control signal, an internal potential reference value is obtained according to the internal potential control link; generating a converter drive signal according to the internal potential reference value; The DC voltage reference value is raised for temporarily storing unbalanced power and for inertia correction, including: Get the angular velocity of the converter output voltage ω , the angular velocity ω and rated value ω n The difference is filtered out by the notch filter to remove the power frequency disturbance, and the damping-voltage mapping coefficient is used k D-V Amplify and obtain the DC voltage rise Δ for temporarily storing transient unbalanced power V Pu ; Obtain the angular velocity change rate of the converter output voltage, filter out high-frequency oscillations through a low-pass filter, and then use the inertia-voltage mapping coefficient to calculate the angular velocity change rate. k J-V Amplify and obtain the DC voltage rise Δ for converter inertia correction V J ; The sum of the DC voltage rise for temporarily storing transient unbalanced power and the DC voltage rise for converter inertia correction Δ V dc With fault detection signal S F Multiply and add to the DC voltage reference value V dc_ref , and thereby correct it.

2. The method according to claim 1, wherein: The design method of the inertia-voltage mapping coefficient is to determine the inertia correction value Δ of the converter swing characteristic. J , the inertia-voltage mapping coefficient is calculated by the following formula: k J-V =Δ J / k pdc ,in k pdc is the proportional coefficient in steady-state DC voltage control; The design method of the damping-voltage mapping coefficient is to determine the damping correction value Δ of the converter swing characteristic. D , the damping-voltage mapping coefficient is calculated by the following formula: k D-V =(Δ D+k idc ∙k J-V ) / k pdc ,in k idc is the integral coefficient in steady-state DC voltage control.

3. The method according to claim 1, wherein: The converter is a grid-type converter; the angular velocity of the converter output voltage is obtained ω ,include, Get the output voltage of the converter at the common coupling point u PCC and current i PCC , get the active power; According to the difference between the active power and the active power reference value, the angular velocity of the converter output voltage is obtained through active-phase synchronous control processing. ω ; or, The converter is a grid-following converter; Get the angular velocity of the converter output voltage ω ,include, Get the output voltage of the converter at the common coupling point u PCC ; The output voltage u PCC Perform phase-locked loop processing to obtain the angular velocity of the converter output voltage ω .

4. The method according to claim 1, wherein: Determine if a voltage drop or phase jump fault occurs in the power grid, including: When it is determined that at least one of the power angle and voltage of the converter exceeds the limit, it is considered that the power grid has a voltage drop or phase jump fault, and the fault detection signal S F Output is 1, otherwise the fault detection signal S F The output is 0.

5. The method according to claim 4, wherein: Determine if the converter power angle exceeds the limit, including: Get the current power angle of the grid-connected converter δ The initial point of the power angle under stable operating conditions δ 0, calculate the difference between the two; The absolute value of the difference is compared with the power angle threshold δ th Compare, if the absolute value exceeds the power angle threshold δ th , it is determined that the converter power angle exceeds the limit.

6. The method according to claim 4, wherein: Determine if the converter voltage exceeds the limit. include, Get the voltage amplitude of the grid-connected converter V and its rated value V n The difference between The absolute value of the difference is compared with the voltage stability threshold V th If the absolute value exceeds the voltage stability threshold V th , it is determined that the converter voltage exceeds the limit.

7. The method according to claim 1, wherein: The converter is a grid-type converter; obtain the voltage value of the DC port capacitor V dc , and the DC voltage reference value V dc_ref Compare and get the DC voltage control reference value, including, Get the voltage value of the DC port capacitor V dc , and the DC voltage reference value V dc_ref After comparison, the active power reference value is generated P ref , as the DC voltage control reference value; Obtain the output data of the converter at the common coupling point and obtain the synchronous control signal according to the synchronous control link, including: Get the output voltage of the converter at the common coupling point u PCC and current i PCC , get the active power P e and reactive power Q e ; According to active power P e and active power reference value P ref The difference between the two values ​​is used to obtain the output voltage phase reference value of the converter. θ ; According to reactive power Q e and reactive power reference value Q ref The difference between the two values ​​is used to obtain the reference value of the converter output voltage amplitude. V ; The synchronization control signal includes a phase reference value θ and amplitude reference values V ; Based on the synchronous control signal, an internal potential reference value is obtained according to the internal potential control link, including: The amplitude reference value V Using Phase Reference θ Perform coordinate transformation to obtain an output voltage reference value of the converter output voltage in a synchronous coordinate system; The output voltage of the converter at the common coupling point u PCC The internal potential reference value is compared with the output voltage reference value in the synchronous coordinate system, and the difference between the two is used to obtain the internal potential reference value.

8. The method according to claim 1, wherein: The converter is a grid-following converter; the voltage value of the DC port capacitor is obtained and compared with the DC voltage reference value. V dc_ref Compare and get the DC voltage control reference value, including, Get the voltage value of the DC port capacitor V dc , and the DC voltage reference value V dc_ref After comparison, the active current reference value is generated I d_ref , as the DC voltage control reference value; Obtain the output data of the converter at the common coupling point, and obtain the synchronous control signal according to the synchronous control link, including: Get the output voltage of the converter at the common coupling point u PCC ; The output voltage u PCC Perform phase-locked loop processing to obtain the angular velocity of the converter output voltage ω , integrating it to get the voltage phase θ , as a synchronization control signal; Based on the synchronous control signal, an internal potential reference value is obtained according to the internal potential control link, include, Get the output current of the converter at the common coupling point i PCC , the current i PCC Use voltage phase θ Perform coordinate transformation to obtain the converter output active current I d and reactive current I q ; The converter output active current and active current reference value I d_ref Perform comparison to obtain a first difference; The reactive current output by the converter is compared with the preset reactive current reference value. I q_ref Perform comparison to obtain a second difference; An internal potential reference value is obtained according to the first difference and the second difference.

9. A DC control system for improving transient stability of a grid-connected converter, characterized by: include, The fault judgment module based on power angle exceeding the limit is configured to judge whether a voltage drop or phase jump fault occurs in the power grid; A fault judgment module based on voltage over-limit is configured to judge whether a voltage drop or phase jump fault occurs in the power grid; The DC port transient energy correction module is configured to, when either the power angle limit-exceeding fault judgment module or the voltage limit-exceeding fault judgment module determines that a voltage drop or phase jump fault has occurred in the power grid, process the control signal of the synchronous control link through the DC port transient energy correction module, and raise the DC voltage reference value for temporarily storing unbalanced power and for inertia correction; a steady-state DC voltage control module, configured to obtain a voltage value of the DC port capacitor, compare it with the DC voltage reference value, and obtain a DC voltage control reference value; a synchronization module configured to obtain output data of the converter at a common coupling point and obtain a synchronization control signal according to a synchronization control link; an internal potential control module, configured to obtain an internal potential reference value according to an internal potential control link based on the synchronous control signal; as well as, a pulse width modulation module, configured to generate a converter driving signal according to the internal potential reference value; The DC voltage reference value is raised for temporarily storing unbalanced power and for inertia correction, including: Get the angular velocity of the converter output voltage ω , the angular velocity ω and rated value ω n The difference is filtered out by the notch filter to remove the power frequency disturbance, and the damping-voltage mapping coefficient is used k D-V Amplify and obtain the DC voltage rise Δ for temporarily storing transient unbalanced power V Pu ; Obtain the angular velocity change rate of the converter output voltage, filter out high-frequency oscillations through a low-pass filter, and then use the inertia-voltage mapping coefficient to calculate the angular velocity change rate. k J-V Amplify and obtain the DC voltage rise Δ for converter inertia correction V J ; The sum of the DC voltage rise for temporarily storing transient unbalanced power and the DC voltage rise for converter inertia correction Δ V dc With fault detection signal S F Multiply and add to the DC voltage reference value V dc_ref , and thereby correct it.

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