High voltage ride through control method for equivalent dynamic Crowbar wind storage system with reactive power support

Abstract

The invention discloses a high voltage ride through control method for an equivalent dynamic Crowbar wind storage system with no power support, and the method comprises the steps: enabling an energy storage system ESS to be connected with two ends of a DC bus of a wind power generation system DFIG in parallel through a bidirectional Buck-Boost DC-DC converter, and monitoring the voltage of a grid-connected point of the wind power generation system DFIG; if the voltage does not exceed the preset threshold value, it is judged that the wind power generation system is in a normal operation state, a grid-side converter GSC, a rotor-side converter RSC and an energy storage system ESS adopt a conventional control strategy, otherwise it is judged that the wind power generation system is in an HVRT operation state, the grid-side converter GSC cooperates with the energy storage system ESS to carry out reactive power support, direct-current bus overvoltage in the HVRT period is restrained, and the voltage of the wind power generation system is restrained. A rotor side converter RSC simulates a Crowbar circuit to carry out equivalent dynamic Crowbar circuit control, and stator and rotor currents of a DFIG during an HVRT period are suppressed. According to the invention, when the PCC voltage rises, the grid-side converter GSC cooperates with the energy storage system ESS to carry out reactive power support, and the rotor-side converter RSC simulates a Crowbar circuit, so that the HVRT capability of the DFIG is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of doubly-fed induction wind turbine control, and more specifically, relates to a high voltage ride-through control method for an equivalent dynamic Crowbar wind storage system with no power support. Background Art

[0002] With the continuous increase in wind power penetration, doubly fed induction generators (DFIG) have been widely used in large-scale wind power generation systems. Therefore, countries around the world have put forward more stringent requirements on the stability and reliability of wind turbines. Among them, the sharp increase and decrease of the voltage at the common connection point (PCC) of the wind turbine will cause a large fluctuation in the internal parameters of the wind farm, resulting in large-scale wind turbines being disconnected from the grid, posing a huge threat to the stability of the power grid. At present, a large number of studies have proposed corresponding low voltage ride through (LVRT) strategies for the low voltage problem of PCC of DFIG, but the attention paid to the PCC overvoltage problem is relatively low. The current research status shows that the direct cause of PCC overvoltage is still unclear and is usually caused by indirect causes. In a high-voltage AC transmission system, when a low voltage ride through occurs in the wind turbine system, the reactive compensation device will increase the capacitive reactive power. As the fault is eliminated and the voltage is restored, the delay hysteresis of the compensation device causes it to be unable to be removed in time, resulting in a PCC overvoltage fault.

[0003] Compared with PCC voltage sag, voltage swell will cause the wind turbine to generate higher transient DC flux components, thus causing more serious stator overcurrent, rotor current oscillation, machine-end and DC bus overvoltage and other problems. Therefore, some countries have formulated wind turbine high voltage ride through (HVRT, High voltage ride through) grid guidelines. The guidelines require that during the HVRT period, when the PCC voltage is below the voltage time-curve, the wind turbine is guaranteed to operate without disconnecting from the grid, and a certain amount of reactive power support is provided during the fault process. The current high voltage ride through strategy mainly considers two aspects: changing the control strategy and adding hardware circuits. The following will introduce these two aspects in detail:

[0004] The advantage of changing the control strategy is its economic benefits, without adding additional construction and maintenance costs. At present, some scholars have achieved the absorption of inductive reactive power from the power grid during HVRT and ensured the stability of the DC bus voltage. For example, the concept of reactive current performance factor of the grid-side converter GSC was proposed, and the level of reactive current injection under different overvoltage conditions was determined by using the performance factor within the reactive current operating range. Other scholars have achieved coordinated control of DFIG reactive power by the grid-side converter GSC and the rotor-side converter RSC during HVRT for DFIG overvoltage by calculating the dynamic reactive power demand. However, the above methods do not fully exert the reactive power support capacity of GSC, and they all stay at the research on the grid-side control strategy of the wind turbine, and do not analyze the problem of DFIG stator overcurrent during HVRT. In addition, some scholars focus on suppressing the oscillation of stator, rotor current and flux during HVRT. They respectively adopt RSC virtual impedance and active and reactive coordination model predictive control strategies to improve the dynamic performance of DFIG high voltage ride-through, but none of them study the DC bus overvoltage problem. For example, a current vector hysteresis controller was designed and applied to RSC and GSC during DFIG faults, which solved the problems of HVRT stator overcurrent, rotor current oscillation and DC bus overvoltage. However, the experimental results were not ideal. The DC bus voltage oscillated more than 20% during HVRT, and the fault ride-through capability was still limited. Based on the above strategies, changing the control strategy alone is not enough to bring out the converter's own regulation potential, and the improvement of DFIG's fault ride-through capability is limited. Therefore, many scholars choose to further improve DFIG's high voltage ride-through capability by adding hardware circuits.

[0005] In terms of improving the HVRT capability of DFIG by adding hardware equipment, some scholars have chosen static synchronous compensator (STATCOM) as a support device for PCC voltage during HVRT. For example, the function, structure, working principle and feedforward decoupling control strategy of distributed STATCOM system are adopted, and the HVRT capability is improved in conjunction with DFIG. Some scholars have proposed a transient equation of STATCOM to improve the high voltage ride-through capability of DFIG, and determined the parameters of STATCOM in combination with grid guidelines, number of wind turbines and grid connection point voltage. Or the Chopper circuit is used to reduce the DC bus voltage during HVRT. Although the above method can provide reactive power support to the grid during HVRT, reduce the PCC voltage and thus stabilize the DC bus voltage, the function of STATCOM is single and there is no systematic solution to the problems existing in DFIG during the HVRT process. A stator-side superconducting fault current limiter (SSFCL) can also be added to the DFIG to cooperate with the Chopper circuit of the DC bus to suppress the DC bus overvoltage and stator overcurrent of the DFIG during HVRT. Compared with the above-mentioned method, this method solves many problems of DFIG during HVRT, but the SSFCL is expensive and does not meet the economic requirements of the power system. In addition, some scholars use Crowbar circuit as a protection device on the rotor side of DFIG to improve the overcurrent protection capability of DFIG, thereby improving its high voltage ride-through capability. For example, the traditional Crowbar circuit is improved, the dynamic resistor is connected in series to the circuit, and a commutation protection device is added to avoid the risk of DFIG losing control under the protection of the Crowbar circuit, but this method cannot provide reactive support and has limited use scenarios. In summary, although adding hardware circuits can further improve the high voltage ride-through capability of DFIG, the construction and maintenance costs of the DFIG power generation system will also increase. The hardware used in the above literature has a single function and does not meet the economic and practical requirements of the power system. Therefore, it is crucial to choose a suitable hardware circuit. Summary of the invention

[0006] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a high voltage ride-through control method for an equivalent dynamic Crowbar wind storage system with no power support. Once the PCC voltage rises, the grid-side converter GSC cooperates with the energy storage system ESS to suppress the DC bus voltage fluctuation and provide reactive power support to the grid, thereby reducing the grid connection point voltage; the rotor-side converter RSC simulates the Crowbar circuit and makes dynamic adjustments according to the grid connection point voltage, thereby suppressing the stator overcurrent by reducing the rotor current and improving the HVRT capability of the DFIG.

[0007] In order to achieve the above-mentioned object of the invention, the high voltage ride-through control method of the equivalent dynamic Crowbar wind storage system with no power support of the present invention comprises the following steps:

[0008] S1: The energy storage system ESS is connected in parallel to both ends of the DC bus of the wind power generation system DFIG through a bidirectional Buck-Boost DC-DC converter;

[0009] S2: Monitor the grid connection point voltage U of the wind power generation system DFIG pcc , and determine the grid voltage U pcc Is it less than the preset voltage threshold U pcc , if yes, go to step S3, otherwise go to step S4;

[0010] S3: Determine that the wind power generation system is in normal operation and use the following method to control it:

[0011] The grid-side converter GSC is based on the d-axis component reference value i of the GSC output current gd_ref and the q-axis component reference value i gq_ref The GSC output voltage is vector controlled, where the d-axis component reference value i of the GSC output current is gd_ref Determine using the following method:

[0012] Get the current DC bus voltage U dc , according to the preset DC bus voltage reference value U dc_ref Calculate the DC bus voltage difference ΔU dc :

[0013] ΔU dc =U dc_ref -U dc

[0014] Then the DC bus voltage difference ΔU dc Input PI controller to get the d-axis component reference value i gd_ref ;

[0015] q-axis component reference value i gq_ref Determine using the following method:

[0016] Get the current grid connection point PCC voltage U pcc , according to the preset grid connection point PCC voltage reference value U pcc_ref Calculate the difference ΔU pcc :

[0017] ΔU pcc =U pcc_ref -U pcc

[0018] Then the difference ΔU pccInput PI controller to get q-axis component reference value i gq_ref ;

[0019] The energy storage system ESS is based on the energy storage system current reference value I ESS_ref For the energy storage system current I ESS Control is performed, where the energy storage system current reference value I ESS_ref Determine using the following method:

[0020] Get the current output active power P out , according to the preset reactive power reference value P out_ref Calculate the output energy storage system power reference value P ESS_ref :

[0021] P ESS_ref =P out_ref -P out

[0022] Then the energy storage system current reference value I is calculated using the following formula ESS_ref ;

[0023]

[0024] Among them, U ESS Indicates the current voltage of the energy storage system;

[0025] The rotor-side converter RSC is based on the d-axis component reference value i of the rotor current rd_ref and the q-axis component reference value i rq_ref The rotor voltage is vector controlled, where the d-axis component of the rotor current reference value i rd_ref Determine using the following method:

[0026] Get the current active power P s , according to the preset active power reference value P s_ref Calculate the difference ΔP s :

[0027] ΔP s =P s_ref -P s

[0028] Then the difference ΔP s Input PI controller to get the d-axis component reference value i rd_ref ;

[0029] q-axis component reference value i rq_ref Determine using the following method:

[0030] Get the current reactive power Q s , according to the preset reactive power reference value Q s_ref Calculate the difference ΔQs :

[0031] ΔQ s =Q s_ref -Q s

[0032] Then the difference ΔQ s Input PI controller to get q-axis component reference value i rq_ref ;

[0033] S4: The wind power generation system is judged to be in the HVRT operation state. The grid-side converter GSC cooperates with the energy storage system ESS to provide reactive power support to suppress the DC bus overvoltage during the HVRT period. The specific method is as follows:

[0034] The grid-side converter GSC is based on the d-axis component reference value i of the output current gd_ref and the q-axis component reference value i gq_ref The GSC output voltage is vector controlled, where the d-axis component of the output current is referenced by q-axis component reference value i gq_ref Determine using the following method:

[0035]

[0036] The energy storage system ESS is based on the energy storage system current reference value I ESS_ref Stable DC bus voltage U dc , where the energy storage system current reference value I ESS_ref Determine using the following method:

[0037] Get the current DC bus voltage U dc , according to the preset DC bus voltage reference value U dc_ref Calculate the DC bus voltage difference ΔU dc :

[0038] ΔU dc =U dc_ref -U dc

[0039] Then the DC bus voltage difference ΔU dc Input PI controller to get the output energy storage system power reference value P ESS_ref ;

[0040] Then the energy storage system current reference value I is calculated using the following formula ESS_ref ;

[0041]

[0042] Among them, U ESS Indicates the current voltage of the energy storage system;

[0043] The rotor-side converter RSC simulates the Crowbar circuit to perform equivalent dynamic Crowbar circuit control to suppress the stator and rotor currents of the DFIG during the HVRT period. The specific method is as follows:

[0044] The rotor-side converter RSC is based on the d-axis component reference value i of the rotor current rd_ref and the q-axis component reference value i rq_ref , using vector control method to control the rotor current i r Control is performed, where the d-axis component of the rotor current reference value i rd_ref and the q-axis component reference value i rq_ref Determine using the following method:

[0045] The impedance factor k of the equivalent Crowbar is calculated according to the following formula:

[0046]

[0047] Among them, k dyn represents the equivalent Crowbar circuit dynamic impedance calculated in advance by the DFIG stator current during normal operation, It represents the terminal voltage when the rotor is open circuit in the rotor coordinate system;

[0048] Then the rotor current in the rotor coordinate system is calculated using the following formula

[0049]

[0050] Among them, L s , L r and L m They represent stator inductance, rotor inductance and field inductance respectively. represents the stator flux in the rotor coordinate system, represents the leakage inductance coefficient;

[0051] The rotor current The d-axis component and q-axis component of the d-axis component are respectively used as the d-axis component reference value i rd_ref and the q-axis component reference value i rq_ref .

[0052] The present invention provides a high voltage ride-through control method for an equivalent dynamic Crowbar wind storage system with no power support. The energy storage system ESS is connected in parallel to both ends of the DC bus of the wind power generation system DFIG through a bidirectional Buck-Boost DC-DC converter, and the grid connection point voltage of the wind power generation system DFIG is monitored. If the voltage does not exceed a preset threshold, the wind power generation system is determined to be in a normal operating state. The grid-side converter GSC, the rotor-side converter RSC and the energy storage system ESS adopt conventional control strategies. Otherwise, the wind power generation system is determined to be in a HVRT operating state. The grid-side converter GSC cooperates with the energy storage system ESS to perform reactive power support to suppress overvoltage of the DC bus during HVRT. The rotor-side converter RSC simulates the Crowbar circuit to perform equivalent dynamic Crowbar circuit control to suppress the stator and rotor currents of the DFIG during HVRT.

[0053] The present invention has the following beneficial effects:

[0054] 1) The present invention deduces the causes of DC bus voltage and wind turbine stator overcurrent during high voltage ride-through by analyzing the mathematical model of DFIG, the transient processes of grid-side and rotor-side converters, and draws the GSC output voltage and current curves in combination with the energy storage system ESS. The reactive power support strategy of the grid-side converter GSC is proposed to suppress the DC bus overvoltage during HVRT.

[0055] 2) The present invention analyzes the operating status of DFIG in the above control strategy, combines the equivalent circuit diagram of Corwbar and RSC, draws the DFIG rotor side vector diagram, and proposes a control strategy for RSC equivalent dynamic Crowbar circuit, which reduces the stator and rotor currents of DFIG during HVRT. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Figure 1 It is a schematic diagram of the topological structure of DFIG-ESS in the present invention;

[0057] Figure 2 It is a schematic diagram of the surface of the GSC output voltage changing with the current in the present invention;

[0058] Figure 3 It is a schematic diagram of the reactive power support control strategy including ESS in the present invention;

[0059] Figure 4 This is the equivalent circuit diagram of the rotor side of the DFIG power generation system using Crowbar circuit during normal operation and HVRT;

[0060] Figure 5 It is the equivalent circuit diagram of the rotor side without Crowbar circuit;

[0061] Figure 6is the stator and rotor voltage, flux, and current vector diagram;

[0062] Figure 7 is the rotor side equivalent circuit vector diagram;

[0063] Figure 8 is the rotor voltage The value range and different rotor voltage Lower rotor impedance voltage Graph of changes in

[0064] Fig. 9 is the rotor impedance voltage Minimum vector graph;

[0065] Fig.10 It is the overall control block diagram of the DFIG-ESS wind storage power generation system in the high voltage ride-through period of the present invention;

[0066] Fig.11 It is a flow chart of a high voltage ride through control method of an equivalent dynamic Crowbar wind storage system with no power support according to the present invention;

[0067] Fig.12 : is a comparison chart of the network side HVRT strategy experimental results of the present invention and the comparative method in this embodiment;

[0068] Fig.13 : is a comparison chart of the experimental results of the HVRT strategy on the fan side of the present invention and the comparative method in this embodiment;

[0069] Fig.14 3 is a comparison chart of the overall HVRT strategy experimental results of the present invention and the comparative method in this embodiment. DETAILED DESCRIPTION

[0070] The specific implementation of the present invention is described below in conjunction with the accompanying drawings so that those skilled in the art can better understand the present invention. It should be noted that in the following description, when the detailed description of known functions and designs may dilute the main content of the present invention, these descriptions will be omitted here.

[0071] In order to better illustrate the technical solution of the present invention, the topological structure and control method of the energy storage system ESS combined with the DFIG wind power generation system are first briefly described. The energy storage system ESS is widely used in renewable energy output, peak shaving and valley filling, frequency and voltage regulation due to its flexibility and high efficiency. However, the utilization rate of existing ESS in the current power system still needs to be explored. ESS has the characteristics of richer functions, stronger flexibility, and higher adaptability. In addition, wind turbines can directly utilize existing energy storage resources to reduce construction costs. Therefore, the present invention selects a wind storage system DFIG-ESS composed of ESS and DFIG, and cooperates with the rotor-side converter RSC and the grid-side converter GSC to propose a set of systematic collaborative control strategies to enhance the high voltage crossing capability of DFIG.

[0072] Figure 1 The topological structure of the conventional DFIG wind power generation system is shown in FIG. Figure 1 As shown in the yellow part, the DFIG stator is directly connected to the grid, and the rotor is connected to the grid through a back-to-back converter and a filter, wherein the back-to-back converters are connected through a DC bus. The DFIG-ESS topology structure adopted by the present invention directly connects the ESS in parallel at both ends of the DC bus through a bidirectional Buck-Boost DC-DC converter on the basis of the traditional DFIG. When the system is operating normally, the RSC and GSC adopt vector control, and the ESS output power is controlled by the DC-DC converter to automatically suppress the power fluctuation caused by the wind speed change, thereby ensuring the power quality of the power generation system.

[0073] The space vector equations of voltage and flux of DFIG can be expressed by stator and rotor currents as follows:

[0074]

[0075] Among them, the subscripts r and s represent the rotor and stator respectively, and the superscripts r and s indicate that the vector is established in the rotor or stator coordinate system, that is, They represent the stator voltage, stator current and stator flux in the stator coordinate system respectively, represents the rotor current in the stator coordinate system, represents the stator current in the rotor coordinate system, They represent the rotor voltage, rotor current and rotor flux in the rotor coordinate system respectively, R s , R r , L s , L r and L m They represent stator resistance, rotor resistance, stator inductance, rotor inductance and excitation inductance respectively.

[0076] When DFIG is in normal operation, a vector control strategy based on d-axis grid voltage orientation is adopted. The GSC transient voltage equation in the synchronous rotating coordinate system is shown in equation (3):

[0077]

[0078] where u gd 、u gq represents the d-axis and q-axis components of the GSC output voltage; L f , R f Respectively represent the inductance and resistance of the filter between GSC and the grid; ω1, U g Respectively represent the grid frequency and voltage; i gd 、i gq They represent the d-axis and q-axis components of the output current of the power grid GSC respectively. f >>R f , so the output voltage U of GSC gout It can be simplified to formula (4):

[0079]

[0080] According to the vector control strategy adopted by GSC, the output voltage U gout The PCC voltage will be continuously tracked, so when the grid voltage suddenly rises, the PCC overvoltage, U gout According to the space vector modulation theory, the GSC output voltage and the DC bus voltage U dc The relationship should satisfy formula (5):

[0081]

[0082] in U gout After the increase, the formula (5) cannot be satisfied, and GSC cannot convert U dc The DC bus is designed to operate in a normal DFIG power generation system, which will cause U dc Unstable, unbalanced power transmission between GSC and RSC, causing the entire power generation system to lose control, and eventually large-scale DFIG disconnection from the grid, affecting the stable operation of the power system.

[0083] In order to facilitate the analysis of DFIG, the variables are usually converted to the stator side. Assuming that the PCC voltage surge accident occurs at time t=0, the increase is p, then the DFIG stator voltage at this time is It can be expressed by formula (6):

[0084]

[0085] Among them, U preis the DFIG stator peak voltage before the fault; ω s is the stator voltage frequency, and t represents the time. s The voltage drop generated on the stator is significantly smaller than the grid voltage (about 1% for megawatt motors), so the stator flux vector can be simplified to However, the stator flux is a transient quantity and cannot change suddenly. Its transient process can be expressed as formula (7):

[0086]

[0087] Among them, τ s =L s / R s Represents the stator flux time constant.

[0088] Like other differential equations, the expression of the solution of equation (7) can be divided into two parts: the free component With steady-state component The free component depends on the initial state of the equation, that is, the transient component caused by voltage change; the steady-state component is only related to the input of the equation, that is, the stator voltage. Therefore, the stator flux can be expressed as equation (8):

[0089]

[0090] in, is the initial value of the free component of magnetic flux.

[0091] Since the stator flux changes continuously, equation (10) can be derived from equation (9):

[0092]

[0093] Substituting equations (9) and (10) into equation (7), the transient equation of stator flux during high voltage ride-through is obtained as shown in equation (11):

[0094]

[0095] From equations (1) and (2), the rotor voltage equation can be derived as equation (12):

[0096]

[0097] in, Represents the leakage inductance coefficient.

[0098] The rotor voltage consists of two parts. The first one corresponds to the rotor electromotive force generated by the stator flux, which is the terminal voltage when the rotor is open circuit. The latter term represents the rotor impedance voltage After transforming equation (11) to the rotor side and substituting it into the rotor electromotive force equation, the rotor electromotive force change equation during high voltage ride-through can be obtained as equation (13):

[0099]

[0100] When the stator flux is oriented, the relationship between current and flux can be expressed as formula (14):

[0101]

[0102] The rotor current i can be calculated by (14): s Equation (15):

[0103]

[0104] Since the high voltage ride-through grid guidelines require that the wind turbine does not disconnect from the grid for a long time (taking China's grid guidelines as an example, when the PCC voltage is below 1.2pu, it is necessary to ensure that the wind turbine does not disconnect from the grid within 10 seconds), the steady-state components of the rotor electromotive force and stator current dominate during the high voltage ride-through period, so the stator flux will gradually increase to (1+p) times of the normal situation over time, so the stator current will also increase when the rotor current remains unchanged. Excessive current will cause electromagnetic interference, temperature rise, etc. If the stator and rotor overcurrents are not limited during the high voltage ride-through period, it will affect the normal operation of the wind turbine and reduce the service life of the DFIG. Therefore, during the high voltage ride-through period, corresponding control strategies should be adopted to reduce the rotor current and stator current of the DFIG.

[0105] In summary, there are two key problems during DFIG high voltage ride-through: (1) DC bus voltage out of control; (2) DFIG stator current overcurrent. Therefore, the following will propose corresponding control strategies for these two problems to improve the high voltage ride-through capability of DFIG.

[0106] ●High voltage ride through control strategy

[0107] A. Reactive power support control strategy

[0108] Since the grid code requires that the wind turbine should not be disconnected from the grid for a long time after a voltage surge, the high voltage ride-through period is a steady-state process. Therefore, and Can be ignored, GSC output voltage U gout It can be simplified into formula (16):

[0109]

[0110] In order to more intuitively show the GSC output voltage U goutThe q-axis component i of the GSC output current gq and the d-axis component i gd The relationship between , the data of 10MW wind turbine is substituted into equation (16), and the surface of GSC output voltage versus current is plotted. Figure 2 Schematic diagram of the curve of the GSC output voltage changing with the current in the present invention. Figure 2 As shown, the output current d-axis component i gd Determine, the output current g-axis component i gq ≤0, the output voltage U gout With |i gq | first decreases and then increases. Therefore, in order to ensure the stability of the DC bus voltage, the output current g-axis component i gd To control the output voltage U gout satisfy Output current g-axis component i gq The value range of is as shown in formula (17):

[0111]

[0112] The output current g-axis component i gq When ≤0, it means that the GSC absorbs reactive power from the grid and provides reactive support to the grid. This ensures the stability of the DC bus voltage while meeting the reactive power support requirements in the grid guidelines.

[0113] For traditional DFIG wind power generation systems, due to its single power flow, GSC cannot fully exert its reactive power support capacity during HVRT. In order to ensure the stability of the DC bus voltage, it is necessary to maintain a balance in active power transmission between GSC and RSC. Therefore, in the traditional control algorithm of GSC, the output current g-axis component i gd Depends on the output power of the RSC, but the output power of the RSC fluctuates with the wind speed, so the output current g-axis component i gd It also changes with wind speed and cannot be adjusted flexibly according to needs. The restrictions, |i gd |It is impossible to get the minimum value during the high voltage ride-through period, so |i gq |It is impossible to take the maximum value, and the reactive power support capacity of GSC cannot be fully utilized.

[0114] After the introduction of the energy storage system ESS, the flexibility of the DFIG power generation system is significantly increased, which can be divided into the following three situations: (1) When the ESS is not working, the operation is the same as that of the traditional DFIG; (2) When the RSC output power is large, the ESS is charged and absorbs part of the power generated by the RSC, and the rest of the power is transmitted to the grid through the GSC; (3) When the RSC output power is small, the ESS discharges to supplement the power shortage of the grid. Therefore, after adding the ESS, the output current g-axis component i gd It is not affected by wind speed and can be adjusted at will according to needs, which greatly increases the flexibility of GSC. Therefore, during HVRT, GSC can fully support the grid with reactive power, and the reference value of the g-axis component of the output current i gq_ref Take the maximum value or I in formula (17) GSCmax , and the reference value of the output current d-axis component i gq_ref As shown in formula (18):

[0115]

[0116] In summary, the reactive power support control strategy containing ESS can be obtained. Figure 3 FIG. 1 is a schematic diagram of the reactive power support control strategy including ESS in the present invention. Figure 3 As shown, in normal operation, by controlling the output current d-axis component i gd Ensure DC bus voltage U dc The DC-DC converter compares the active power reference value with the actual active power output of the DFIG to determine the output / absorption power of the ESS and smooth the power fluctuation of the DFIG. During the HVRT period, the DC bus voltage U dc To ensure the stability of the ESS control algorithm, a DC bus voltage outer loop is added to the ESS control algorithm. The reference value of the ESS power is calculated through the voltage outer loop. At this time, the GSC is fully supported by the reactive power of the grid. Therefore, the reference value of the g-axis component of the output current is i gq_ref Determined by equation (18), the reference value of the d-axis component of the output current is

[0117] B. Equivalent Dynamic Crowbar (EDCrowbar) Control Strategy

[0118] In the DFIG power generation system, since the DFIG stator is directly connected to the grid, the stator current i s It cannot be controlled directly. However, from equation (15), it can be seen that the DFIG stator current i s and rotor current i r By reducing the rotor current i r Or increase the rotor current d-axis component ird The stator current i can be reduced s , but the rotor current i r is a square term, the rotor current d-axis component i rd is a first-order term, raising i rd After i r will also increase accordingly, so the present invention chooses to reduce the rotor current i r The stator current i is reduced by s .

[0119] In DFIG power generation systems, Crowbar circuit is one of the widely used methods to limit DFIG rotor overcurrent. Figure 4 It is the rotor side equivalent circuit diagram of the DFIG power generation system using Crowbar circuit during normal operation and HVRT period. Figure 4 Medium r , R r Represent the rotor inductance and rotor resistance respectively, R Cro Represents the current limiting resistor of the Crowbar circuit. Figure 4 As shown in the figure, when the DFIG is operating normally, switch S1 is disconnected, S2 is closed, RSC is operating normally, and the Crowbar circuit is disconnected; when a PCC overvoltage fault occurs, switch S1 is closed, S2 is disconnected, RSC is disconnected, and the Crowbar circuit is connected to the rotor, which effectively reduces the DFIG stator and rotor currents. However, when the Crowbar is put into operation, the RSC is disconnected from the DFIG rotor, resulting in an uncontrolled rotor, increasing the risk of rotor out of control. In addition, the rotor is disconnected from the grid, which will cause a loss of power on the rotor side.

[0120] Depend on Figure 4 It can be seen that during the high voltage ride-through period, the Crowbar circuit is actually the impedance connected in parallel to the DFIG rotor. According to formula (12), the rotor side equivalent circuit diagram without the Crowbar circuit can be drawn. Figure 5 This is the equivalent circuit diagram of the rotor side without the Crowbar circuit. Figure 5 The equivalent impedance Z of RSC RSC It can be expressed by formula (19):

[0121]

[0122] Therefore, if the RSC can be equivalent to a Crowbar circuit during high voltage ride-through, the power generation system can suppress the overcurrent phenomenon of the stator during high voltage ride-through without adding a Crowbar circuit, and can ensure that the RSC is always connected to the rotor, reducing the risk of DFIG out of control.

[0123] Before the high voltage ride-through fault occurs, the DFIG is in rated working state. According to the reactive power support strategy described in IV-A, during the high voltage ride-through, the slip rate s of the DFIG is less than 0, the rotor and stator generate active power, and the stator absorbs reactive power, that is, P s <0, P r <0, Q s > 0. Therefore, knowing the DFIG operating parameters, the steps for drawing the vector diagram are as follows:

[0124] 1) The positions of the stator voltage vector and current vector can be determined by the operating status of the DFIG.

[0125] 2) The position of the stator flux vector can be obtained through the stator voltage equation (1).

[0126] 3) The position of the rotor current vector can be obtained through the stator flux equation (2).

[0127] 4) The position of the rotor flux vector can be obtained through the rotor flux equation (2).

[0128] 5) The position of the rotor voltage vector can be obtained through the rotor voltage equation of formula (1).

[0129] 6) Obtain the stator and rotor voltage, flux, and current vector diagrams.

[0130] Figure 6 is the stator and rotor voltage, flux, and current vector diagram. Then through Figure 6 The rotor side equivalent circuit vector diagram can be obtained. Figure 7 It is the equivalent circuit vector diagram of the rotor side. Figure 7 Medium θ Z represents the rotor impedance angle, θ RSC Represents the RSC equivalent impedance angle. It changes only with time, so at a fixed moment, when When changing, Will follow changes, so it can be controlled by Make Get the minimum value, then Minimum. is the radius, The starting point is the center of the circle. The value range of Draw out the range of values After that, you can get the corresponding Figure 8 is the rotor voltage The value range and different rotor voltage Lower rotor impedance voltage The change of Figure 8 It can be seen from (b) that when and When the directions are the same, The smallest, so at this time Also takes the minimum value. Fig. 9 is the rotor impedance voltage Vector illustration of the smallest hour.

[0131] Obviously, the RSC equivalent impedance Z RSC The impedance angle θ RSC It should satisfy formula (20):

[0132] θ Z +θ RSC =180° (20)

[0133] Therefore, in the equivalent Crowbar control strategy, after satisfying the condition of equation (20), the RSC equivalent impedance Z RSC It can be expressed as Z RSC =kZ r , where k is the impedance factor of the equivalent Crowbar. Substituting it into equation (12) yields the new rotor voltage equation (21):

[0134]

[0135] For the actual operation of DFIG r >>R r , so R r can be ignored, then equation (21) can be simplified to obtain equation (22)

[0136]

[0137] Therefore, the rotor current equation is (23):

[0138]

[0139] The rotor voltage can be expressed as equation (24):

[0140]

[0141] because and The direction is the same, which means that the RSC equivalent impedance is positive, that is, k ≥ 0. According to the space vector modulation theory, the rotor voltage should satisfy Substituting it into formula (24), we can get the value range of k as formula (25):

[0142]

[0143] In the range of equation (25), the rotor current i r and rotor voltage u r The value of k decreases and increases with the increase of k, so in order to ensure the normal operation of DFIG and have the ability to suppress the stator and rotor currents, the value of k is particularly important. When Crowbar is used to protect DFIG, the rotor and stator currents of DFIG will be much smaller than the currents during normal operation. Therefore, the stator and rotor currents will generate a large impact current during recovery. In fact, as long as the stator current of DFIG during HVRT is the same as that during normal operation, it is possible to effectively protect DFIG while reducing the value of k, thereby avoiding rotor voltage. Too high to satisfy In order to meet the above conditions, the equivalent Crowbar circuit dynamic impedance k can be calculated by the DFIG stator current during normal operation. dyn .

[0144] Assume that the stator current i in normal operation is s =I sn , will I sn , Equation (23) is substituted into the current equation (15) and the equation (26) about k is obtained after sorting:

[0145]

[0146] in

[0147] Solving equation (26) to obtain K is equation (27):

[0148]

[0149] The expression of k obtained by formula (27) is formula (28):

[0150]

[0151] In formula (25), k ≥ 0 is required, because Therefore, it is impossible to obtain this value. Combining equations (25) and (28), we can get the value of k as equation (29):

[0152]

[0153] From formula (29), we can see that when k dyn When the value range requirement of formula (25) is met, let k = k dyn The stator current of DFIG can be stabilized at the normal operating level during HVRT, and a stronger stator current suppression capability can be obtained by increasing k according to actual needs; when k dynWhen the value range requirement of formula (25) cannot be met, it means that the impedance factor k that can make the stator current of the DFIG lower than the normal operating level during the HVRT period cannot be found, that is, the stator current suppression ability of the equivalent Crowbar strategy is insufficient. At this time, other strategies should be adopted in coordination.

[0154] According to the HVRT control strategies proposed in parts A and B above, the overall control method of the DFIG-ESS wind storage power generation system during the high voltage ride-through period can be obtained. Fig.10 This is the overall control block diagram of the DFIG-ESS wind storage power generation system during the high voltage ride-through period of the present invention. Fig.10 , the specific steps of the high voltage ride through control method of the equivalent dynamic Crowbar doubly fed wind storage system of the present invention can be obtained.

[0155] Fig.11 Flow chart of the high voltage ride through control method of the equivalent dynamic Crowbar wind storage system with no power support of the present invention. Fig.11 As shown, the specific steps of the high voltage ride-through control method of the equivalent dynamic Crowbar wind storage system with no power support of the present invention include:

[0156] S1101: Deployment of energy storage systems:

[0157] The energy storage system ESS is connected in parallel to both ends of the DC bus of the wind power generation system DFIG through a bidirectional Buck-Boost DC-DC converter.

[0158] S1102: Monitor the grid connection point voltage U of the wind power generation system DFIG pcc , and determine the grid voltage U pcc Is it less than the preset voltage threshold? If yes, go to step S1103, otherwise go to step S1104.

[0159] S1103: Conventional vector control:

[0160] It is determined that the wind power generation system is in normal operation and the following methods are used for control:

[0161] The grid-side converter GSC is based on the d-axis component reference value i of the GSC output current gd_ref and the q-axis component reference value i gq_ref The GSC output voltage is vector controlled, where the d-axis component reference value i of the GSC output current is gd_ref Determine using the following method:

[0162] Get the current DC bus voltage U dc , according to the preset DC bus voltage reference value U dc_ref Calculate the DC bus voltage difference ΔUdc :

[0163] ΔU dc =U dc_ref -U dc (30)

[0164] Then the DC bus voltage difference ΔU dc Input PI controller to get the d-axis component reference value i gd_ref .

[0165] q-axis component reference value i gq_ref Determine using the following method:

[0166] Get the current grid connection point PCC voltage U pcc , according to the preset grid connection point PCC voltage reference value U pcc_ref Calculate the difference ΔU pcc :

[0167] ΔU pcc =U pcc_ref -U pcc (31)

[0168] Then the difference ΔU pcc Input PI controller to get q-axis component reference value i gq_ref .

[0169] The energy storage system ESS is based on the energy storage system current reference value I ESS_ref For the energy storage system current I ESS Control is performed, where the energy storage system current reference value I ESS_ref Determine using the following method:

[0170] Get the current output active power P out , according to the preset reactive power reference value P out_ref Calculate the output energy storage system power reference value P ESS_ref :

[0171] P ESS_ref =P out_ref -P out (32)

[0172] Then the energy storage system current reference value I is calculated using the following formula ESS_ref ;

[0173]

[0174] Among them, U ESS Indicates the current voltage of the energy storage system.

[0175] The rotor-side converter RSC is based on the d-axis component reference value i of the rotor currentrd_ref and the q-axis component reference value i rq_ref The rotor voltage is vector controlled, where the d-axis component of the rotor current reference value i rd_ref Determine using the following method:

[0176] Get the current active power P s , according to the preset active power reference value P s_ref Calculate the difference ΔP s :

[0177] ΔP s =P s_ref -P s (34)

[0178] Then the difference ΔP s Input PI controller to get the d-axis component reference value i rd_ref .

[0179] q-axis component reference value i rq_ref Determine using the following method:

[0180] Get the current reactive power Q s , according to the preset reactive power reference value Q s_ref Calculate the difference ΔQ s :

[0181] ΔQ s =Q s_ref -Q s (35)

[0182] Then the difference ΔQ s Input PI controller to get q-axis component reference value i rq_ref .

[0183] S1104: High voltage ride through control:

[0184] It is determined that the wind power generation system is in the HVRT operation state. The grid-side converter GSC cooperates with the energy storage system ESS to provide reactive power support to suppress the DC bus overvoltage during the HVRT period. The specific method is as follows:

[0185] The grid-side converter GSC is based on the d-axis component reference value i of the output current gd_ref and the q-axis component reference value i gq_ref The GSC output voltage is vector controlled, where the d-axis component of the output current is referenced by q-axis component reference value i gq_ref Determine using the following method:

[0186]

[0187] The energy storage system ESS is based on the energy storage system current reference value I ESS_ref Stable DC bus voltage U dc , where the energy storage system current reference value I ESS_ref Determine using the following method:

[0188] Get the current DC bus voltage U dc , according to the preset DC bus voltage reference value U dc_ref Calculate the DC bus voltage difference ΔU dc :

[0189] ΔU dc =U dc_ref -U dc (37)

[0190] Then the DC bus voltage difference ΔU dc Input PI controller to get the output energy storage system power reference value P ESS_ref .

[0191] Then the energy storage system current reference value I is calculated using the following formula ESS_ref ;

[0192]

[0193] Among them, U ESS Indicates the current voltage of the energy storage system;

[0194] The rotor-side converter RSC simulates the Crowbar circuit to perform equivalent dynamic Crowbar circuit control to suppress the stator and rotor currents of the DFIG during the HVRT period. The specific method is as follows:

[0195] The rotor-side converter RSC is based on the d-axis component reference value i of the rotor current rd_ref and the q-axis component reference value i rq_ref , using vector control method to control the rotor current i r Control is performed, where the d-axis component of the rotor current reference value i rd_ref and the q-axis component reference value i rq_ref Determine using the following method:

[0196] The impedance factor k of the equivalent Crowbar is calculated according to the following formula:

[0197]

[0198] Among them, k dyn represents the equivalent Crowbar circuit dynamic impedance calculated in advance by the DFIG stator current during normal operation, Represents the port voltage when the rotor is open circuit in the rotor coordinate system.

[0199] Then the rotor current in the rotor coordinate system is calculated using the following formula

[0200]

[0201] Among them, L s , L r and L m They represent stator inductance, rotor inductance and field inductance respectively. represents the stator flux in the rotor coordinate system, represents the leakage inductance coefficient;

[0202] The rotor current The d-axis component and q-axis component of the d-axis component are respectively used as the d-axis component reference value i rd_ref and the q-axis component reference value i rq_ref .

[0203] In this embodiment, the specific method for the grid-side converter GSC to perform vector control is:

[0204] Get the d-axis component i of the current GSC output current gd and the q-axis component i gq , and then use the following formula to calculate the d-axis component difference Δi gd and the q-axis component difference Δi gq :

[0205] Δi gd =i gd_ref -i gd (41)

[0206] Δi gq =i gq_ref -i gq (42)

[0207] The d-axis component difference Δi gd and the q-axis component difference Δi gq Input the corresponding PI controller to obtain the d-axis output voltage control value Δu gd And q-axis output voltage control quantity Δu gq .

[0208] The d-axis component reference value u of the GSC output voltage is calculated using the following formulas: gd_ref and the q-axis component reference value u gq_ref :

[0209] u gd_ref =Δu gd +u gd -ω g L f i gd+R f i gd (43)

[0210] u gq_ref =Δu gq +u gq -ω g L f i gq +R f i gq (44)

[0211] Among them, u gd 、u gq They represent the d-axis component and q-axis component of the GSC output voltage, ω g Indicates the grid voltage frequency, L f , R f are the inductance and resistance of the filter between GSC and the grid, i gd 、i gq They represent the d-axis component and q-axis component of the GSC output current respectively.

[0212] The d-axis component reference value u of the GSC output voltage gd_ref and the q-axis component reference value u gq_ref According to the vector angle θ of the current power grid g The reference value of the GSC output voltage is converted into a three-phase coordinate system, and then input into a PWM (Pulse-Width Modulation) controller to generate a control signal of the grid-side converter GSC to control the GSC output voltage.

[0213] In this embodiment, the specific method for the rotor side converter RSC to perform vector control is:

[0214] Get the d-axis component i of the current rotor current rd and the q-axis component i rq , and then use the following formula to calculate the d-axis component difference Δi rd and the q-axis component difference Δi rq :

[0215] Δi rd =i rd_ref -i rd (45)

[0216] Δi rq =i rq_ref -i rq (46)

[0217] The d-axis component difference Δi rd and the q-axis component difference Δi rqInput the corresponding PI controller to obtain the d-axis rotor voltage control value Δu rd and q-axis rotor voltage control quantity Δu rq .

[0218] The d-axis component reference value u of the rotor voltage is calculated using the following formulas: rd_ref and the q-axis component reference value u rq_ref :

[0219] u rd_ref =Δu rd -ω r σL f i rq +u Zd +e rd (47)

[0220] u rq_ref =Δu rq +ω r σL f i rd +u Zq +e rq (48)

[0221] Among them, ω r represents the rotor voltage frequency, i rq 、i rd They represent the d-axis component and q-axis component of the rotor current, u Zd 、u Zq They represent the d-axis component and q-axis component of the rotor impedance voltage, respectively, and e rd 、e rq They respectively represent the d-axis component and q-axis component of the port voltage when the rotor is open circuit.

[0222] The d-axis component reference value u of the rotor voltage rd_ref and the q-axis component reference value u rq_ref According to the current rotor voltage vector angle θ r It is converted into a rotor voltage reference value in a three-phase coordinate system, and then input into a PWM controller to generate a control signal of the rotor-side converter RSC to control the rotor voltage.

[0223] In order to better illustrate the technical effect of the present invention, a specific example is used to verify the present invention through simulation experiments. In this embodiment, a wind turbine with a rated power of 9MW consisting of 6 1.5MW DFIGs is used, and a DFIG-ESS wind energy storage system with a capacity of 1MW and a voltage of 800V is built in MATLAB / Simulink to verify the above control strategy.

[0224] Example 1 Grid-side control strategy

[0225] The comparative experiment of the grid-side control strategy in this embodiment includes two existing grid-side control methods as comparative methods, namely:

[0226] (1) Adding a control strategy for a static VAR compensator (STATCOM), for details, see “Wessels, Christian and Friedrich Wilhelm Fuchs. “High voltage ride through with FACTS for DFIGbased wind turbines.” 2009 13th European Conference on Power Electronics and Applications (2009): 1-10.”, where the converter capacity of the STATCOM is the same as that of the GSC;

[0227] (2) High voltage ride-through control strategy using current domain method. For details, see “Liu, Peilin, Guofang Zhu, Lei Ding, Xuesong Gao, Chunjiang Jia, Chong Ng and VVTerzija. “High-voltage ride-through strategy for wind turbine with fully-rated converter based on current operating range.” International Journal of Electrical Power & Energy Systems (2022): n.pag.”;

[0228] According to the previous theoretical analysis, DFIG needs to absorb reactive power and reduce the grid connection point voltage under the premise of stabilizing the DC bus voltage during HVRT. Therefore, the result curve of the comparative experiment includes the DC PCC voltage, bus voltage, reactive power and GSC d-axis current i gd curve.

[0229] Since the control strategy proposed in the present invention can reduce the PCC voltage to 1.2pu in an HVRT fault when the PCC voltage rises to 1.3pu, the current China Power Grid Guidelines require that when the PCC voltage is between 1.1 and 1.2pu, the DFIG should ensure that it operates without being disconnected from the grid for 10s. Therefore, the fault duration was set to 10s in the experiment. At 8s, the PCC voltage rose by 0.3pu, triggering the high voltage ride-through control strategy of the DFIG. At 18s, the PCC voltage returned to normal.

[0230] Fig.12 : is a comparison chart of the network side HVRT strategy experimental results of the present invention and the comparative method in this embodiment. Fig.12 It can be seen that the three high voltage ride-through strategies can suppress the PCC voltage. Among them, the current domain control strategy does not add any hardware equipment and has a lower cost than the other two strategies. Fig.12 It can be seen that this method can effectively suppress the fluctuation of DC bus voltage, but it is obviously inferior to the other two methods of adding hardware circuits in reducing PCC voltage and reactive power support. Fig.12 The current domain and the strategy of the present invention gd Curve, i in the current domain method during HVRT gd =-0.8pu, the maximum value of the GSC output current used in the experiment is 1.3pu, so the current domain method is insufficient to give full play to the reactive power support of GSC, and the high voltage ride-through capability of DFIG is limited. g , DC bus voltage U dc , power generation system parameters and formula (17) to calculate i gd Theoretical target value i gd_ref =-1.27pu, with GSC control algorithm tracking i gd_ref , while ensuring the stability of the DC bus, the reactive power support capability of GSC is fully utilized.

[0231] In addition, according to Fig.12Comparing the results of the other two strategies, it can be seen that since the converter capacity used by STATCOM is the same as that of GSC, the HVRT strategy proposed in this invention shows similar PCC voltage suppression capability and reactive power absorption capability as STATCOM, which further proves that the method proposed in this invention can maximize the reactive power support capability of GSC. Compared with the HVRT control strategy of this article, the use of STATCOM adds a converter and transformer, and the hardware cost is much higher than the control strategy proposed in this article. However, during a fault, STATCOM only needs to control reactive power. Therefore, the use of STATCOM responds more quickly when a fault occurs. In addition, since reactive power support is completed by the STATCOM device, the oscillation of the DC bus voltage will be smaller than that of the strategy proposed in this article. However, the China Power Grid Guidelines require that when the PCC voltage is between 1.2 and 1.3 pu, the DFIG should remain connected to the grid for 500 ms. When the control strategy of the present invention is adopted, the PCC can be reduced from 1.3 pu to 1.2 pu within 500 ms. Therefore, the response speed of the control strategy of the present invention does not affect the normal execution of the control strategy. In addition, in the strategy proposed by the present invention, the oscillation range of the DC bus voltage after the GSC absorbs reactive power is only ±0.04 pu, which has no obvious effect on the operating stability of the power generation system.

[0232] In summary, the control strategy of the present invention considers the reactive power support strategy of ESS during HVRT, which can effectively suppress the DC bus voltage oscillation while providing reactive power support to the grid and reducing the PCC voltage. Compared with the current domain control strategy, it has a significant improvement, and the more expensive method of adding STATCOM has certain advantages in some performance, but compared with the strategy proposed in the present invention, the overall effect is similar and the gap is not large.

[0233] Example 2 Rotor side control strategy

[0234] In this embodiment, the rotor side control strategy comparison experiment uses two existing control strategies as comparison methods:

[0235] (1) Add Crowbar circuit

[0236] (2) The demagnetization control strategy proposed in the literature “Yang, Jin, John Edward Fletcher and John O'Reilly. “A Series-Dynamic-Resistor-Based Converter Protection Scheme for Doubly-Fed Induction Generator During Various Fault Conditions.” IEEE Transactions on Energy Conversion 25 (2009): 422-432.”

[0237] During the HVRT process, the stator current will increase compared to normal operation. The method proposed in the present invention reduces the stator current by reducing the rotor current. Therefore, the comparative experimental result curve includes PCC voltage, stator current, and rotor current.

[0238] Fig.13 : is a comparison chart of the experimental results of the wind turbine side HVRT strategy of the present invention and the comparative method in this embodiment. Since the equivalent impedance in the equivalent Crowbar adaptive control strategy proposed by the present invention will change according to the change of the grid connection point voltage, Fig.13 The PCC voltage increases by 1.3 pu at 8 s, decreases to 1.2 pu at 13 s, and recovers at 18 s. In fact, during high voltage ride-through, as long as the stator current can be lower than the current during normal operation, the DFIG can be protected. Therefore, in the experiment, the equivalent dynamic Crowbar strategy adopts two ideas: tracking the normal stator current and maximally suppressing the stator current. Two different sets of equivalent Crowbar impedance values ​​are selected. The former uses dynamic impedance and the latter uses fixed impedance.

[0239] contrast Fig.13 The stator current and rotor current curves in the figure show that the Crowbar circuit has the strongest ability to suppress DFIG stator overcurrent. After the fault occurs, the RSC is disconnected and the Crowbar circuit is connected. The rotor current i r =0, but after the fault is eliminated, RSC is reconnected, and both the stator and rotor will generate large inrush currents, affecting the DFIG. The equivalent Crowbar control strategy with fixed impedance has a stronger ability to suppress stator current than the demagnetization control strategy. At this time, the impedance factor k=0.19. The inrush current of the DFIG stator and rotor after the fault is eliminated is also larger than that of the demagnetization control strategy; and the dynamic impedance equivalent Crowbar control adopted in this paper only aims to maintain the stator current the same as normal operation during high voltage ride-through, so compared with other control strategies, i during HVRT smaximum, but the inrush current is minimum after the fault is cleared.

[0240] Through the analysis of the above comparative experiments, it can be seen that the equivalent dynamic Crowbar circuit proposed in the present invention can effectively suppress the increase of stator current during HVRT. Compared with directly adding a Crowbar circuit, the impact current of this strategy when the fault is eliminated is smaller. In addition, by selecting different impedance factors, the equivalent dynamic Crowbar circuit can produce different stator current suppression capabilities, which is more flexible than the demagnetization algorithm and can meet more usage scenarios. Among them, the use of dynamic impedance factors to track the normal operating stator current can play a protective role during HVRT, while achieving the minimum impact current after the fault is eliminated.

[0241] Embodiment 3 overall control strategy.

[0242] The experiment in this embodiment combines the control strategies mentioned above to achieve overall control of DFIG during HVRT and comprehensively improve the high voltage ride-through capability of DFIG. Therefore, the comparative experiment in this embodiment selects two other overall strategies for improving the high voltage ride-through capability of DFIG:

[0243] (1) The addition of a stator-side superconducting fault current limiter (SSFCL)-Chopper device in the literature “Zou, Zhi-Ce and Jun Yao. “Efficient HVRT Scheme of DFIG Using a SSFCL-Chopper Considering Grid Voltage Recovery.” IEEE Transactions on Applied Superconductivity 31(2021):1-3.”

[0244] (2) The DFIG full-process high voltage ride-through strategy based on dynamic adjustment of rotor reactive current proposed in the literature "Zhang Xu, Wang Yi, Liu Bowen, et al. DFIG full-process high voltage ride-through strategy based on dynamic adjustment of rotor reactive current [J]. Automation of Electric Power Systems, 2022, 46(22): 142-150."

[0245] Fig.14 3 is a comparison chart of the overall HVRT strategy experimental results of the present invention and the comparative method in this embodiment. Fig.14 Includes PCC voltage curve, bus voltage, reactive power, GSC d-axis current i gd , DFIG stator current and rotor current curves.

[0246] pass Fig.14It can be seen that the high voltage crossing strategy using the SSFCL-Chopper device stabilizes the DC bus voltage during HVRT through the Chopper circuit on the grid side. The SSFCL device is connected between the grid connection point and the generator to limit the current between the stator and the rotor, so that the DFIG maintains the same current during HVRT as during normal operation. Compared with the control strategy of the present invention, this method has a faster response speed, but because SSFCL requires the use of superconducting materials, this method is also costly and difficult to implement. In addition, since this method does not provide reactive power support, the PCC voltage is not effectively suppressed during HVRT.

[0247] The DFIG full-process high voltage ride-through strategy based on dynamic adjustment of rotor reactive current does not add hardware devices and can improve the reactive power support capability of DFIG at a low cost. However, during the HVRT period, due to the insufficient reactive power support capability of this method, the PCC voltage only dropped from 1.3pu to 1.27pu. In terms of the control of DC bus voltage and stator current, this method is effective, but compared with the control algorithm of the present invention, its flexibility needs to be improved.

[0248] Based on the above experimental results, it can be seen that the present invention, in combination with the energy storage system, effectively suppresses the DC bus voltage oscillation and stator current during the HVRT period by controlling the RSC and the GSC.

[0249] Although the above describes the illustrative specific embodiments of the present invention to facilitate those skilled in the art to understand the present invention, it should be clear that the present invention is not limited to the scope of the specific embodiments. For those of ordinary skill in the art, as long as various changes are within the spirit and scope of the present invention as defined and determined by the attached claims, these changes are obvious, and all inventions and creations using the concept of the present invention are protected.

Claims

1. A high voltage ride-through control method for an equivalent dynamic Crowbar wind storage system with no power support, characterized in that: The following steps are involved: S1: The energy storage system ESS is connected in parallel to both ends of the DC bus of the wind power generation system DFIG through a bidirectional Buck-Boost DC-DC converter; S2: Monitor the grid connection point voltage U of the wind power generation system DFIG pcc , and determine the grid voltage U pcc Is it less than the preset voltage threshold? If yes, go to step S3, otherwise go to step S4; S3: Determine that the wind power generation system is in normal operation and use the following method to control it: The grid-side converter GSC is based on the d-axis component reference value i of the GSC output current gd_ref and the q-axis component reference value i gq_ref The GSC output voltage is vector controlled, where the d-axis component reference value i of the GSC output current is gd_ref Determine using the following method: Get the current DC bus voltage U dc , according to the preset DC bus voltage reference value U dc_ref Calculate the DC bus voltage difference ΔU dc : ΔU dc =U dc_ref -U dc Then the DC bus voltage difference ΔU dc Input PI controller to get the d-axis component reference value i gd_ref ; q-axis component reference value i gq_ref Determine using the following method: Get the current grid connection point PCC voltage U pcc , according to the preset grid connection point PCC voltage reference value U pcc_ref Calculate the difference ΔU pcc : ΔU pcc =U pcc_ref -U pcc Then the difference ΔU pcc Input PI controller to get q-axis component reference value i gq_ref ; The energy storage system ESS is based on the energy storage system current reference value I ESS_ref For the energy storage system current I ESS Control is performed, where the energy storage system current reference value I ESS_ref Determine using the following method: Get the current output active power P out , according to the preset reactive power reference value P out_ref Calculate the output energy storage system power reference value P ESS_ref : P ESS_ref =P out_ref -P out Then the energy storage system current reference value I is calculated using the following formula ESS_ref ; Among them, U ESS Indicates the current voltage of the energy storage system; The rotor-side converter RSC is based on the d-axis component reference value i of the rotor current rd_ref and the q-axis component reference value i rq_ref The rotor voltage is vector controlled, where the d-axis component of the rotor current reference value i rd_ref Determine using the following method: Get the current active power P s , according to the preset active power reference value P s_ref Calculate the difference ΔP s : ΔP s =P s_ref -P s Then the difference ΔP s Input PI controller to get the d-axis component reference value i rd_ref ; q-axis component reference value i rq_ref Determine using the following method: Get the current reactive power Q s , according to the preset reactive power reference value Q s_ref Calculate the difference ΔQ s : ΔQ s =Q s_ref -Q s Then the difference ΔQ s Input PI controller to get q-axis component reference value i rq_ref ; S4: The wind power generation system is determined to be in the HVRT operation state. The grid-side converter GSC cooperates with the energy storage system ESS to provide reactive power support to suppress the DC bus overvoltage during the HVRT period. The specific method is as follows: The grid-side converter GSC is based on the d-axis component reference value i of the output current gd_ref and the q-axis component reference value i gq_ref The GSC output voltage is vector controlled, where the d-axis component of the output current is referenced by q-axis component reference value i gq_ref Determine using the following method: The energy storage system ESS is based on the energy storage system current reference value I ESS_ref Stable DC bus voltage U dc , where the energy storage system current reference value I ESS_ref Determine using the following method: Get the current DC bus voltage U dc , according to the preset DC bus voltage reference value U dc_ref Calculate the DC bus voltage difference ΔU dc : ΔU dc =U dc_ref -U dc Then the DC bus voltage difference ΔU dc Input PI controller to get the output energy storage system power reference value P ESS_ref ; Then the energy storage system current reference value I is calculated using the following formula ESS_ref ; Among them, U ESS Indicates the current voltage of the energy storage system; The rotor-side converter RSC simulates the Crowbar circuit to perform equivalent dynamic Crowbar circuit control to suppress the stator and rotor currents of the DFIG during the HVRT period. The specific method is as follows: The rotor-side converter RSC is based on the d-axis component reference value i of the rotor current rd_ref and the q-axis component reference value i rq_ref , using vector control method to control the rotor current i r Control is performed, where the d-axis component of the rotor current reference value i rd_ref and the q-axis component reference value i rq_ref Determine using the following method: The impedance factor k of the equivalent Crowbar is calculated according to the following formula: Among them, k dyn represents the equivalent Crowbar circuit dynamic impedance calculated in advance by the DFIG stator current during normal operation, It represents the terminal voltage when the rotor is open circuit in the rotor coordinate system; Then the rotor current in the rotor coordinate system is calculated using the following formula Among them, L s , L r and L m They represent stator inductance, rotor inductance and field inductance respectively. represents the stator flux in the rotor coordinate system, represents the leakage inductance coefficient; The rotor current The d-axis component and q-axis component of the d-axis component are respectively used as the d-axis component reference value i rd_ref and the q-axis component reference value i rq_ref .

2. The equivalent dynamic Crowbar wind-storage system high voltage ride-through control method according to claim 1 is characterized in that: The voltage threshold is 1.1 times the PCC standard voltage.

3. The equivalent dynamic Crowbar wind-storage system high voltage ride-through control method according to claim 1 is characterized in that: The specific method for the machine-side converter RSC to perform vector control in step S3 is: Get the d-axis component i of the current GSC output current gd and the q-axis component i gq , and then use the following formula to calculate the d-axis component difference Δi gd and the q-axis component difference Δi gq : Δi gd =i gd_ref -i gd Δi gq =i gq_ref -i gq The d-axis component difference Δi gd and the q-axis component difference Δi gq Input the corresponding PI controller to obtain the d-axis output voltage control value Δu gd and q-axis output voltage control quantity Δu gq ; The d-axis component reference value u of the GSC output voltage is calculated using the following formulas: gd_ref and the q-axis component reference value u gq_ref : at gd_ref =Δu gd +u gd -ω g L f and gd +R f and gd at gq_ref =Δu gq +u gq -ω g L f and gq +R f and gq Among them, u gd 、u gq They represent the d-axis component and q-axis component of the GSC output voltage, ω g Indicates the grid voltage frequency, L f , R f are the inductance and resistance of the filter between GSC and the grid, i gd 、i gq They represent the d-axis component and q-axis component of the GSC output current respectively; The d-axis component reference value u of the GSC output voltage gd_ref and the q-axis component reference value u gq_ref According to the vector angle θ of the current power grid g It is converted into a GSC output voltage reference value in a three-phase coordinate system, and then input into a PWM controller to generate a control signal for the grid-side converter GSC to control the GSC output voltage.

4. The equivalent dynamic Crowbar wind-storage system high voltage ride-through control method according to claim 1 is characterized in that: The specific method for performing vector control of the rotor side converter RSC in step S3 is: Get the d-axis component i of the current rotor current rd and the q-axis component i rq , and then use the following formula to calculate the d-axis component difference Δi rd and the q-axis component difference Δi rq : Δi rd =i rd_ref -i rd Δi rq =i rq_ref -i rq The d-axis component difference Δi rd and the q-axis component difference Δi rq Input the corresponding PI controller to obtain the d-axis rotor voltage control value Δu rd and q-axis rotor voltage control quantity Δu rq ; The d-axis component reference value u of the rotor voltage is calculated using the following formulas: rd_ref and the q-axis component reference value u rq_ref : you rd_ref =Δu rd -oh r σL f I rq +u Zd +e rd you rq_ref =Δu rq +oh r σL f I rd +u Zq +e rq Among them, ω r represents the rotor voltage frequency, i rq 、i rd They represent the d-axis component and q-axis component of the rotor current, u Zd 、u Zq They represent the d-axis component and q-axis component of the rotor impedance voltage, respectively, and e rd 、e rq They represent the d-axis component and q-axis component of the port voltage when the rotor is open circuit; The d-axis component reference value u of the rotor voltage rd_ref and the q-axis component reference value u rq_ref According to the current rotor voltage vector angle θ r It is converted into a rotor voltage reference value in a three-phase coordinate system, and then input into a PWM controller to generate a control signal of the rotor-side converter RSC to control the rotor voltage.

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