A grid-type converter overcurrent suppression system and method

Abstract

This invention relates to an overcurrent suppression system and method for grid-connected converters, belonging to the field of converter control. It adds a reactive power supplementary current loop to the reactive power control loop of the grid-connected converter control system to track reactive current during grid connection failures of renewable energy units. It also adds an active power supplementary current loop to the active power control loop of the grid-connected converter to track active current during grid connection failures of renewable energy units. This rapidly suppresses overcurrent in the grid-connected converter, quickly limiting the current to within the converter's current carrying capacity and ensuring the voltage source characteristics of the converter during fault periods.

Description

Technical Field

[0001] This invention relates to the field of converter control, and in particular to an overcurrent suppression system and method for a grid-type converter. Background Technology

[0002] In recent years, under the dual-carbon development background, the penetration rate of renewable energy sources such as wind power and photovoltaics connected to the grid through power electronic devices has been continuously increasing. This helps to change the classic grid operation mode dominated by synchronous motors. Compared with synchronous generators, which can withstand up to 7 times their rated current, converters can only withstand 20% to 50% of overcurrent. Therefore, how to suppress overcurrent during faults has become a key factor in the normal operation of new energy units.

[0003] Grid-type converters have attracted widespread attention due to their ability to actively control the frequency and voltage output of new energy units. Currently, there are three main strategies for suppressing overcurrent during short circuits. The first is to switch to current source mode during a fault, directly controlling the current. Under this control, the converter exhibits current source characteristics externally, but cannot effectively control the voltage, thus causing voltage stability problems during the fault period and the fault recovery phase.

[0004] The second method is a current-limiting strategy based on virtual impedance. This involves adjusting the reference voltage according to the difference between the actual current value and the set value, effectively adding impedance to the circuit. A major problem with this method is the accuracy of current control. The virtual impedance is calculated based on voltage dips and system loop parameters, and the accuracy of these parameters directly determines the accuracy of current control. Furthermore, these system loop parameters are difficult to obtain in actual operation. Another major problem with virtual impedance is that, because the inductor current cannot change instantaneously, the addition of virtual reactance may result in a decaying DC component in the short-circuit current, affecting the current control effect.

[0005] The third method is to change the power reference value of the outer power loop. Under this control method, the control target is still power, so accurate current control cannot be achieved. On the other hand, since the power reference value acts on the outer loop, the response speed is slow, and the larger DC component of the current in the early stage of the fault cannot be suppressed in time. Summary of the Invention

[0006] The purpose of this invention is to provide an overcurrent suppression system and method for grid-connected converters, so as to quickly suppress the overcurrent of grid-connected converters during grid connection failures of new energy units.

[0007] To achieve the above object, the present invention provides the following solutions:

[0008] A grid-type converter overcurrent suppression system, the system comprising: a grid-type converter control system, a reactive power supplementary current loop, and an active power supplementary current loop;

[0009] A reactive power supplementary current loop is set in the reactive power control loop of the grid-connected converter control system; the reactive power control loop is used to switch the internal PI-based reactive power outer loop to a reactive power supplementary current loop during the grid connection failure of the new energy unit; the reactive power supplementary current loop is used to output the d-axis voltage reference value during the fault period, with the q-axis current reference value and the actual q-axis current value during the fault period as inputs during the grid connection failure of the new energy unit.

[0010] An active power supplementary current loop is set in the active power control loop of the grid-connected converter. During a grid connection failure of a new energy unit, the active power supplementary current loop inputs the d-axis current reference value and the actual d-axis current value during the fault period at its input terminal, and connects to the VSG control loop inside the active power control loop at its output terminal. The VSG control loop switches the active power reference value to the active power reference value during the fault period, and outputs the equivalent voltage source angle during the fault period based on the active power reference value during the fault period and the output of the active power supplementary current loop.

[0011] During a fault, the d-axis voltage reference value, the equivalent voltage source angle during a fault, the three-phase current at the outlet of the grid converter, and the three-phase voltage across the filter capacitor are modulated by the abc / dq conversion module, the voltage and current inner loop control module, the dq / abc conversion module, and the pulse width modulation module in the grid converter control system to generate a modulation signal. The modulation signal is then applied to the grid converter to limit the current of the grid converter to within the converter current carrying capacity range.

[0012] Optionally, the system further includes: a low-pass filter structure and a first switch;

[0013] The output of the active power supplementary current loop is connected to the input of the low-pass filter structure, the output of the low-pass filter structure is connected to one end of the first switch, and the other end of the first switch is connected to the VSG control loop.

[0014] The first switch is used to connect during a grid connection failure of the new energy unit and disconnect during normal grid connection of the new energy unit; the low-pass filter structure is used to suppress the frequency fluctuations in the initial stage of the active power additional current loop connection and eliminate frequency static error.

[0015] Optionally, the passband gain of the low-pass filter structure is 0.3, and the cutoff angular frequency is 70.57.

[0016] Optionally, the system further includes: a toggle switch;

[0017] The first input terminal of the switching switch is input to the active power reference value during normal operation, the second input terminal of the switching switch is input to the active power reference value during a fault, and the output terminal of the switching switch is connected to the VSG control loop.

[0018] The switching switch is used to connect the first input terminal when the new energy unit is running normally in grid connection, and to switch to the second input terminal when the grid connection of the new energy unit fails.

[0019] A method for suppressing overcurrent in a grid-connected converter, the method being applied to the aforementioned grid-connected converter overcurrent suppression system, the method comprising:

[0020] During the grid connection failure of a new energy unit, the reference value of the dq axis current during the fault period is determined based on the grid connection point voltage.

[0021] Input the reference value and actual value of the q-axis current during the fault into the reactive power supplementary current loop, and output the reference value of the d-axis voltage during the fault.

[0022] Based on the reference values ​​of d-axis voltage and d-axis current during the fault, determine the reference value of active power during the fault.

[0023] The active power reference value during the fault period is input into the VSG control loop in the active power control loop, and the d-axis current reference value and the actual d-axis current value during the fault period are input into the active power additional current loop in the active power control loop, and the equivalent voltage source angle during the fault period is output.

[0024] The reference value of d-axis voltage during the fault period, the angle of equivalent voltage source during the fault period, the three-phase current at the outlet of the grid converter, and the three-phase voltage across the filter capacitor are converted by abc / dq, the voltage and current inner loop control is applied, the dq / abc conversion is applied, and the pulse width modulation is applied to the grid converter to limit the current of the grid converter to the current carrying capacity of the converter.

[0025] Optionally, the formula for determining the q-axis current reference value during a fault is:

[0026]

[0027] In the formula, i qref This is the reference value for the q-axis current during the fault, u d K represents the voltage amplitude at the grid connection point. d I is the reactive current gain coefficient. N Rated current;

[0028] The formula for determining the d-axis current reference value during a fault is:

[0029]

[0030] In the formula, i dref i is the reference value for the d-axis current during the fault period. d0 I is the reference value of the d-axis current before the fault. max This is the overcurrent limit for the converter.

[0031] Optionally, the formula for calculating the d-axis voltage reference value during the fault period is as follows:

[0032]

[0033] u dref (s)=u drefpu (s)E0

[0034] Where u drefpu (s) represents the per-unit value of the voltage amplitude during the fault, k rp k ri These are the proportional-integral coefficients, i qrefpu 、i qpu These are the reference per-unit value and the actual per-unit value of the q-axis current, respectively, E 0pu The reference voltage per unit value; u dref (s) represents the reference value of the d-axis voltage during the fault period, and E0 represents the reference voltage.

[0035] Optionally, the formula for calculating the active power reference value during the fault is as follows:

[0036]

[0037] In the formula, P r ' ef i is the reference value for active power during the fault period. dref The reference value for the d-axis current during the fault is u. dref This is the reference value for the d-axis voltage during the fault period.

[0038] Optionally, the active power reference value during the fault period is input into the VSG control loop in the active power control loop, and the d-axis current reference value and the actual d-axis current value during the fault period are input into the active power supplementary current loop in the active power control loop, outputting the equivalent voltage source angle during the fault period, specifically including:

[0039] Input the reference value and actual value of the d-axis current during the fault period output by the voltage loop into the active additional current loop in the active control loop, and output the angular frequency deviation value.

[0040] The angular frequency deviation value is filtered using a low-pass filter structure to obtain the filtered angular frequency deviation value.

[0041] The active power reference value and the filtered angular frequency deviation value during the fault period are both input into the VSG control loop, and the equivalent voltage source angle during the fault period is output.

[0042] Optionally, the process for determining the filter parameters of the low-pass filter structure is as follows:

[0043] The transfer function of the low-pass filter structure is determined as follows: In the formula, G LF (s) is the transfer function, a1 is the passband gain, and ω LF The cutoff frequency is s, the complex frequency is s = jω, and ω is the angular frequency;

[0044] When the cutoff frequency of the active filter is 75Hz, the gain of the low-pass filter structure at the cutoff frequency is expressed as 20log. 10 G LF (s) = -3;

[0045] Based on the transfer function and the gain amplitude at the cutoff frequency, the relationship between the cutoff frequency and the passband gain is determined as follows:

[0046] The range of values ​​for the preset passband gain is determined, and the amplitude-frequency response curve is determined based on the relationship between the cutoff frequency and the passband gain.

[0047] Based on the selection criteria of large amplitude variation and small phase variation, the optimal passband gain is 0.3 and the optimal cutoff frequency is 70.57 on the amplitude-frequency response curve.

[0048] According to specific embodiments provided by the present invention, the present invention discloses the following technical effects:

[0049] This invention discloses an overcurrent suppression system and method for grid-connected converters. A reactive power supplementary current loop is added to the reactive power control loop of the grid-connected converter control system to track the reactive current during grid connection failures of renewable energy units. An active power supplementary current loop is also added to the active power control loop of the grid-connected converter to track the active power current during grid connection failures of renewable energy units. This rapidly suppresses overcurrent in the grid-connected converter, quickly limiting the current to within the converter's current carrying capacity and ensuring the voltage source characteristics of the converter during fault periods.

[0050] The present invention still incorporates a low-pass filter structure in the active power control loop. The low-pass filter structure can suppress the frequency fluctuations during the initial connection of the active power additional current loop and eliminate static frequency errors. Attached Figure Description

[0051] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0052] Figure 1 This is the topology of the grid converter control system provided in the embodiments of the present invention;

[0053] Figure 2This is a control block diagram of the reactive power control loop provided in an embodiment of the present invention;

[0054] Figure 3 This is a control block diagram of the active power control loop provided in an embodiment of the present invention;

[0055] Figure 4 This is a control block diagram of the VSG control loop provided in an embodiment of the present invention;

[0056] Figure 5 This is a control block diagram of the active power supplementary current loop provided in an embodiment of the present invention;

[0057] Figure 6 This is a schematic diagram of a low-pass filter structure provided in an embodiment of the present invention;

[0058] Figure 7 This is a schematic diagram of the amplitude-frequency response curve provided in an embodiment of the present invention; Figure 7 (a) in the figure is a graph showing the relationship between amplitude and frequency. Figure 7 (b) in the figure is a graph showing the relationship between phase and frequency;

[0059] Figure 8 A schematic diagram of the abc / dq transformation provided in an embodiment of the present invention;

[0060] Figure 9 This is a block diagram of voltage and current dual closed-loop control provided in an embodiment of the present invention;

[0061] Figure 10 This is a control block diagram for generating modulation signals provided in an embodiment of the present invention;

[0062] Figure 11 A flowchart of an overcurrent suppression method for a grid-type converter provided in an embodiment of the present invention;

[0063] Figure 12 This is a comparison chart of the transmission power of the original control and the additional current control; Figure 12 (a) in the figure is a comparison of active power output. Figure 12 (b) in the figure is a comparison of reactive power output;

[0064] Figure 13 This is a comparison diagram of the output three-phase currents of the original control and the additional current control; Figure 13 (a) in the diagram is the output three-phase current diagram under the original control. Figure 13 (b) in the diagram is the output three-phase current diagram of the additional current control;

[0065] Figure 14 This is a comparison diagram of the dq-axis components of the output current between the original control and the additional current control. Figure 14 (a) in the figure is a comparison diagram of the d-axis components of the output current. Figure 14(b) in the figure is a comparison diagram of the q-axis components of the output current;

[0066] Figure 15 A comparison chart of the voltage amplitude at the PCC point between the original control and the additional current control;

[0067] Figure 16 A comparison chart of the transmission power under the control method of changing the outer loop power reference value and the additional current control method; Figure 16 (a) in the figure is a comparison chart of active power transmission. Figure 16 (b) in the figure is a comparison chart of reactive power transmission;

[0068] Figure 17 A comparison chart of the PCC point voltage amplitude under the control method of changing the outer loop power reference value and the additional current control method;

[0069] Figure 18 Output current diagram for changing the outer loop power reference value control method;

[0070] Figure 19 Output current diagram for additional current control method;

[0071] Figure 20 Voltage diagram of PCC point for current limiting control;

[0072] Figure 21 To control the PCC point voltage with additional current;

[0073] Figure 22 A comparison chart of the PCC point voltage amplitude under current limiting control and additional current control. Detailed Implementation

[0074] 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 creative efforts are within the scope of protection of the present invention.

[0075] The purpose of this invention is to provide an overcurrent suppression system and method for grid-connected converters, so as to quickly suppress the overcurrent of grid-connected converters during grid connection failures of new energy units.

[0076] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0077] When a fault occurs during the grid connection of a new energy unit, the limited current carrying capacity of the converter can cause device damage, leading to the unit disconnecting from the grid. To solve the above problem, this invention provides a grid-connected converter overcurrent suppression system, including: a grid-connected converter control system, a reactive power supplementary current loop, and an active power supplementary current loop.

[0078] A reactive power supplementary current loop is installed within the reactive power control loop of the grid-connected converter control system. This loop is used to switch the internal PI-based reactive power outer loop to the reactive power supplementary current loop during grid connection failures of renewable energy units. The reactive power supplementary current loop uses the q-axis current reference value and the actual q-axis current value during the fault period as inputs, and outputs the d-axis voltage reference value during the fault period.

[0079] An active power supplementary current loop is installed in the active power control loop of the grid-connected converter. During grid connection faults in renewable energy units, the active power supplementary current loop receives the d-axis current reference value and the actual d-axis current value during the fault period at its input terminal, and connects to the VSG control loop inside the active power control loop at its output terminal. The VSG control loop switches the active power reference value to the active power reference value during the fault period and outputs the equivalent voltage source angle during the fault period based on the active power reference value during the fault period and the output of the active power supplementary current loop.

[0080] During a fault, the d-axis voltage reference value, the equivalent voltage source angle during a fault, the three-phase current at the outlet of the grid converter, and the three-phase voltage across the filter capacitor are modulated by the abc / dq conversion module, the voltage and current inner loop control module, the dq / abc conversion module, and the pulse width modulation module in the grid converter control system. The modulated signal is then applied to the grid converter to limit the current of the grid converter to within the converter's current carrying capacity.

[0081] The topology of a grid converter control system is as follows: Figure 1 As shown, the framework of the grid-type converter control system is mainly divided into two parts: calculating the equivalent voltage source angle θ, frequency ω, and amplitude u. dref The external power loop control circuit, along with the internal voltage and current control circuit containing all further control actions, ultimately generates the modulation signal required for PWM (Pulse Width Modulation). The main circuit of the grid-type converter control circuit includes a DC power supply, an inverter circuit, an LCL filter, and a grid equivalent circuit. The U in the DC power supply... dc Indicates DC voltage, i dc C represents direct current. dc This represents the capacitor. The inverter circuit is a grid-connected converter. PCC represents the grid connection point, Z in the grid equivalent circuit. g V represents the equivalent reactance of an AC power grid. gThis represents the equivalent voltage of an AC power grid. g This represents the current after LCL filtering.

[0082] See Figure 1 The working principle of each module in the grid-type converter control system is as follows:

[0083] Power calculation: Input quantities include the three-phase voltage u across the filter capacitor. abc and the loop current i after LCL filtering gabc The output includes active power P. e and active power Q e .

[0084] Active power control loop: Inputs include active power P e Reference angular velocity ω0 and active power reference value P ref The output is the equivalent voltage source angle θ. Without an additional active current loop, the active control loop uses VSG (Voltage Sag Generator) control. By simulating the synchronous generator governor and rotor motion equations, it achieves operating characteristics similar to a synchronous generator, providing the system with virtual inertia and synchronous power capabilities. Its topology is as follows: Figure 4 As shown.

[0085]

[0086] Where J represents virtual inertia, and D... p This represents the damping coefficient. ω0 is the reference angular velocity, and ω is the actual angular velocity T generated by the VSG. em The output torque can be expressed as:

[0087]

[0088] T* is the reference torque generated by the virtual speed governor, which can be expressed as:

[0089]

[0090] Reference active power P * Generated by the speed controller, it can be calculated using the following formula:

[0091] P * =P ref -k p (ω-ω0) (4)

[0092] In the formula k p This represents the P-ω coefficient.

[0093] From equations (1)-(4), we can obtain:

[0094]

[0095] In the formula, T0 = P ref / ω0,D=D p +k p / ω0, D represents the equivalent damping. The equivalent voltage source angle θ can be calculated according to formula (5).

[0096] abc / dq transformation: Based on the equivalent voltage source angle, the three-phase current i at the outlet of the grid-type converter... abc and the three-phase voltage u across the filter capacitor abc Perform abc / dq transformations respectively to obtain the dq-axis current components i at the outlet of the grid-type converter. dq and the dq-axis voltage component u across the filter capacitor dq .

[0097] Reactive power control loop: Inputs include reactive power Q e Reference voltage E0 and reactive power reference value Q ref The output is the d-axis voltage reference value u. dref .

[0098] Voltage loop: Employs voltage-oriented control, setting the q-axis voltage reference value to 0, i.e., u qref =0. Input quantities include the d-axis voltage reference value u. dref and the dq-axis voltage component u across the filter capacitor dq The output includes the d-axis current reference value i. dref and q-axis current reference value i dref .

[0099] Current loop: Input quantities include d-axis current reference value i dref q-axis current reference value i dref The dq-axis current component at the outlet of the grid-type converter is i dq The output includes the d-axis voltage value v. id and q-axis voltage value v iq .

[0100] dq / abc transformation: Based on the equivalent voltage source angle θ, the voltage value v along the d-axis... id and q-axis voltage value v iq Perform dq / abc conversion to output three-phase voltage.

[0101] PWM: The input is a three-phase voltage, which generates a modulation signal. The modulation signal is used as a drive signal to act on the grid-type converter.

[0102] The overcurrent suppression system proposed in this invention is mainly achieved through additional current loops (active additional current loop and reactive additional current loop). The additional current loop is divided into two current components along the dq axis, which correspond to the active loop and the reactive loop, respectively.

[0103] (1) Improvement of reactive power control loop

[0104] During normal operation, the voltage loop control target is the reactive power setpoint, and the voltage reference value is generated through a PI-based reactive power outer loop. In case of a fault, an additional current loop is used to calculate the voltage reference value, with its input being the q-axis current reference value and its actual value. During a fault, switch S... q Disconnect, S PIq Closed loop, reference voltage is given by additional current loop, control block diagram as follows: Figure 2 As shown. Switch S q The current path is the reactive power outer loop based on PI, S PIq The corresponding path is the reactive power additional current loop.

[0105] The per-unit expression for voltage amplitude during a fault is:

[0106]

[0107] where k rp k ri i represents the proportional-integral coefficient of the additional loop PI controller, i qrefpu 、i qpu These are the reference per-unit value and the actual per-unit value of the q-axis current, E. 0pu This is the reference voltage per unit value.

[0108] The voltage amplitude is:

[0109] u dref (s)=u drefpu (s)E0 (7)

[0110] E0 is the reference voltage.

[0111] (2) Improvement of active power control

[0112] Due to the presence of inertia and damping elements, VSG has good anti-interference capabilities and can suppress peak values ​​during frequency fluctuations, but it will affect the active power tracking speed. Therefore, to improve the active power response speed during faults, an additional PI loop (active power supplementary current loop) is added. Switching between fault and normal operation control is achieved by switching SPI and SP, while simultaneously modifying the active power reference value during faults. The control block diagram is as follows: Figure 3 As shown.

[0113] 1. Change in active power reference value

[0114] In case of a fault, switch S p (Toggle switch) and S PI (First switch) is turned on, and the active power reference value switches to P. r ' ef , P r 'ef The d-axis current reference value i during the fault can be obtained. dref And the voltage amplitude u generated by the reactive power additional current loop dref The calculated expression is as follows:

[0115]

[0116] In the formula, i dref u is the reference value for the d-axis current. dref This is the reference value for the d-axis voltage during the fault period.

[0117] 2. Additional current loop design

[0118] The active power supplementary current loop structure during the fault is as follows: Figure 5 As shown, its output Δω PI1 Acting on the damping term D p This provides negative feedback to the damping term, reducing ΔT and thus accelerating the response speed of the active power loop. When the system is stable, the additional PI loop input is 0, i.e., ΔT = 0, and the active power stabilizes at the active power reference value P given by the speed regulation link. * The angular frequency is stable at ω0.

[0119] The addition of a PI loop can cause significant frequency fluctuations. To address this, a low-pass filter (LF) is added after the PI loop. This suppresses large frequency fluctuations during the initial switching phase and eliminates static frequency errors. The low-pass filter structure is as follows: Figure 6 As shown.

[0120] The transfer function of the filter stage in the above diagram is:

[0121]

[0122] Given a source filter cutoff frequency of 75Hz, the gain of the filter at the cutoff frequency can be expressed as:

[0123] 20log 10 |G LF (s)|=-3 (10)

[0124] Substituting equation (12) into equation (13), we get:

[0125]

[0126] Taking a1 = 0.1 to 1.7, the corresponding ω is calculated. LF The amplitude-frequency response curve is as follows Figure 7 As shown. Based on the amplitude-frequency response curve analysis, the filter parameters a1 = 0.3 and ω can be selected. LF =70.57.

[0127] (3) abc / dq transformation

[0128] The active power loop output θ acts on the subsequent electrical quantities to transform from the abc three-phase stationary coordinate system to the dq coordinate system.

[0129] Figure 8 in,i abc u abc These represent the three-phase current at the converter outlet and the three-phase voltage across the filter capacitor, respectively. gabc i is the loop current after LCL filtering; dq , udq are the dq-axis current components at the converter outlet and the dq-axis voltage components across the filter capacitor, respectively. gdq The loop current dq-axis component after LCL filtering is θ, where θ is the equivalent voltage source angle during the fault.

[0130] (4) Voltage and current inner loop control

[0131] See Figure 9 This invention employs voltage-oriented control, setting the q-axis voltage reference value to 0, i.e., u qref =0, then the reference value of the d-axis voltage during the fault period is the voltage amplitude u. dref During a fault, the reactive power supplementary current loop is used, and the inner loop adopts voltage and current dual inner loop control, based on the inductor current i. dq Grid-side voltage u dq PI control, dq-axis cross-coupling compensation and voltage u dq Current i gdq Low-pass compensation is used to achieve static error-free tracking of voltage and current reference values. f L f These represent the filter capacitor and inductor, respectively, and k vp k vi These represent the proportional and integral coefficients of the voltage loop PI, respectively.

[0132] (5) Modulation signal generation

[0133] See Figure 10 Based on the equivalent voltage source angle θ during the fault, the voltage and current inner loop output v id v iq After coordinate transformation, v is converted to a three-phase stationary coordinate system. abc , v abc The modulation signal generated by PWM is ultimately applied to the grid-type converter.

[0134] This invention proposes an additional PI control grid-connected controller with a low-pass filter structure, which controls the dq axis current components separately. By analyzing the amplitude-frequency characteristics of the low-pass filter structure, suitable filter parameters are obtained. The proposed strategy, while ensuring that the system frequency meets the grid connection requirements, rapidly limits the converter current to the allowable range and ensures the voltage source characteristics of the converter during faults.

[0135] This invention also provides an overcurrent suppression method for a grid-type converter, which is applied to the aforementioned overcurrent suppression system for a grid-type converter, such as... Figure 11 As shown, the method includes the following steps:

[0136] Step S1: During the grid connection failure of the new energy unit, determine the reference value of the dq axis current during the fault period based on the grid connection point voltage.

[0137] This scheme employs voltage-oriented control, setting the q-axis voltage reference value to 0, i.e., u qref =0, corresponding u q The actual value is also controlled to be 0, therefore u q =0, then the d-axis voltage is the voltage amplitude, and the active power and reactive power at point PCC can be expressed as:

[0138]

[0139] Therefore, i can be used. q Characterizing reactive power, i d The active power loop is characterized by its primary action on active current, while the reactive power loop acts on reactive current.

[0140] During a fault, the active and reactive current components are set according to the low-voltage ride-through requirements for wind power grid connection. According to wind power grid connection regulations, the grid voltage drop is 0.2–0.9U. N At that time, the reactive current input from the wind turbine into the system shall not be less than 1.5 (0.9-U). s )I N Therefore, the reference value for the dq-axis current during low-voltage ride-through is given by the following formula:

[0141]

[0142]

[0143] In the formula, i d0 K is the reference value of the d-axis current before the fault. d The reactive current gain coefficient is generally not less than 1.5. Since this invention employs d-axis voltage orientation, u... d I represents the voltage amplitude at the grid connection point. N For the rated current, I max This is the overcurrent limit for the converter.

[0144] The above-obtained reference values ​​for the dq-axis current components are used as inputs to the active power supplementary current loop and the reactive power supplementary current loop, i.e., the control target.

[0145] Step S2: Input the reference value of q-axis current and the actual value of q-axis current during the fault into the reactive power supplementary current loop, and output the reference value of d-axis voltage during the fault.

[0146] Step S3: Determine the active power reference value during the fault period based on the d-axis voltage reference value and the d-axis current reference value during the fault period.

[0147] Step S4: Input the active power reference value during the fault into the VSG control loop in the active power control loop, and input the d-axis current reference value and the actual d-axis current value during the fault into the active power supplementary current loop in the active power control loop, and output the equivalent voltage source angle during the fault.

[0148] Combination Figure 3 The specific output process of the equivalent voltage source angle during the fault period is as follows: The fault period d-axis current reference value and actual d-axis current value output from the voltage loop are input into the active power supplementary current loop in the active power control loop, and the angular frequency deviation value is output. The angular frequency deviation value is filtered using a low-pass filter structure to obtain the filtered angular frequency deviation value. Both the fault period active power reference value and the filtered angular frequency deviation value are input into the VSG control loop to output the fault period equivalent voltage source angle.

[0149] Step S5: During the fault, the reference value of the d-axis voltage, the angle of the equivalent voltage source during the fault, the three-phase current at the outlet of the grid converter, and the three-phase voltage across the filter capacitor are converted by abc / dq, the voltage and current inner loop control is performed, the dq / abc conversion is performed, and the pulse width modulation is performed to generate a modulation signal. The modulation signal is then applied to the grid converter to limit the current of the grid converter to within the current carrying capacity of the converter.

[0150] Combination Figure 1 The specific implementation process of step S5 is as follows:

[0151] Based on the equivalent voltage source angle during the fault, the abc / dq transformation is performed on the three-phase current at the outlet of the grid converter and the three-phase voltage across the filter capacitor to obtain the dq-axis current component at the outlet of the grid converter and the dq-axis voltage component across the filter capacitor.

[0152] Voltage-oriented control is adopted to control the q-axis voltage reference value to 0, and input the dq-axis voltage component across the filter capacitor and the d-axis voltage reference value during the fault period into the voltage loop, and output the dq-axis current reference value.

[0153] Input the dq-axis current reference value and the dq-axis current component at the output of the grid-type converter into the current loop, and output the dq-axis voltage.

[0154] The dq-axis voltage is transformed by dq / abc to obtain the three-phase voltage;

[0155] The three-phase voltage is input into the PWM to generate a modulation signal, which is then applied to the grid-type converter.

[0156] The simulations of this invention were conducted in the Matlab / Simulink environment to test and verify the effectiveness of the proposed control strategy. The system parameters are shown in Table 1. To simulate power grid faults, direct control... Figure 1 The equivalent voltage vg of the power grid in this invention is set to the occurrence of a three-phase short-circuit fault in the system at t=0.3s and the fault clearing at t=0.7s. The simulation duration is 1.2s and the overcurrent limit of the converter is set to 1.5pu.

[0157] Table 1 Simulation System Parameters

[0158] project Numerical <![CDATA[Frequency reference f0 (Hz)]]> 50 <![CDATA[Voltage amplitude reference E0 (V)]]> 690 <![CDATA[DC bus voltage V dc (V)]]> 1200 <![CDATA[Converter-side inductor L cf (mH)]]> 1.5 <![CDATA[Filter capacitor C f (uF)]]> 30 <![CDATA[Grid-side inductor L g (H)]]> 1 <![CDATA[Line resistance R L (Ω)]]> 0.3 <![CDATA[Line inductance L L (mH)]]> 0.4 <![CDATA[Power reference S n (kVA)]]> 30

[0159] (1) Comparison of the effects of the original control and the additional current control

[0160] Taking a severe fault where the grid voltage drops to 0.2 pu as an example, under the original control strategy, the active power reference value remains unchanged, and the active power reference value is given by the droop element of the analog speed governor. Figure 12 As can be seen from the active power output curve, due to the presence of VSG inertia and damping elements, the response speed is slow, so the active power output cannot be stabilized quickly. However, the additional current control, by changing the active power reference value during the fault period and supplementing it with a suitable damping coefficient, greatly accelerates the adjustment speed of active power, achieving stability within 0.1s.

[0161] Depend on Figure 12 As can be seen from the reactive power output curve, the additional current control, due to its good dynamic response, can quickly provide stable reactive power to the system to maintain the stability of the grid connection point voltage.

[0162] Figure 13 and Figure 14 The output three-phase current waveforms and dq-axis components under the original control and the proposed control are compared. The comparison shows that the original control has a slow response speed and cannot suppress the overcurrent in the early stage of the fault in time. When the grid voltage drops to 0.2 pu, the converter is subjected to a continuous overcurrent of more than 10 pu, which will damage the grid-connected power electronic devices and lead to the unit disconnection. In contrast, the proposed control responds quickly after the system fault and stabilizes the output current at the set value within 0.1s.

[0163] Figure 15A comparison of the voltage amplitude at the PCC point under the original control and the proposed control shows that the reactive power reference value of the outer loop under the original control is no longer suitable for operation during the fault period, and cannot generate a suitable reference voltage. Furthermore, due to the untimely response of the outer loop, the voltage cannot be stabilized quickly. Observation shows that within 0.1 seconds after the fault, the additional current control quickly generates a suitable reference voltage. After that, the converter operates as a voltage source, which can provide a higher and more stable voltage support for the PCC point compared to the original control.

[0164] (2) Comparison of the effects of changing the outer loop power reference value and additional current control

[0165] Taking a grid voltage drop of 0.5 pu as an example, under the control method of changing the outer loop power reference value, the reactive power reference is 0.5 pu and the active power reference is 0. The current suppression measures described above, which change the outer loop power reference value, are compared with the additional current loop control proposed in this invention. The changes in various electrical quantities during the fault process are as follows: Figure 16-19 As shown.

[0166] The change in the outer loop power reference value failed to fully utilize the converter capacity, and the overly conservative setting of the active power reference value resulted in a very low d-axis current component, ultimately causing the fault current to fall below the rated current. The strategy proposed in this invention, however, can act rapidly after a fault and stabilize the output current at the set value of 1.5 pu within 0.1 seconds.

[0167] The results above show that the additional current control strategy proposed in this invention is superior in terms of dynamic response, and can quickly suppress overcurrent and stabilize it at the set value, providing more stable voltage support for the system.

[0168] (3) Comparison of the effects of current limiting and additional current control

[0169] Taking a grid voltage drop to 0.2 pu as an example, the above current limiting control is compared with the additional current loop control proposed in this invention. The changes of various electrical quantities during the fault process are as follows: Figure 20-22 As shown.

[0170] Current limiting control operates as a current source during fault periods. Compared to the proposed additional current control, it eliminates the need for an outer and inner voltage loop. The lack of a voltage loop prevents effective control of the PCC point voltage. Figure 20 The voltage waveform under current limiting control is severely distorted. However, under the control proposed in this invention, the voltage waveform is very stable. Figure 22 It can be seen that both control strategies can provide some voltage support during the fault. Under current limiting control, the voltage cannot be controlled in time, so the voltage is always on a downward trend. However, under additional current control, the converter operates as a voltage source, which can provide more stable voltage support for the PCC point.

[0171] In summary, when the converter operates as a voltage source under additional current control, it can suppress overcurrent and provide more stable voltage support, which is more conducive to the stable operation of the unit during faults and the voltage recovery after the fault is cleared.

[0172] To address the problems of existing current suppression strategies, this invention proposes a fault current suppression strategy based on an additional current loop, which continues to operate as a voltage source during faults. The key to this strategy lies in obtaining a suitable reference voltage during faults, which can quickly suppress overcurrent while providing a certain voltage support.

[0173] The various embodiments in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the various embodiments can be referenced to each other.

[0174] Specific examples have been used to illustrate the principles and implementation methods of this invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of this invention. Furthermore, those skilled in the art will recognize that, based on the ideas of this invention, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of this invention.

Claims

1. An overcurrent suppression system for a grid-type converter, characterized in that, The system includes: a grid-type converter control system, a reactive power supplementary current loop, and an active power supplementary current loop; A reactive power supplementary current loop is set in the reactive power control loop of the grid-connected converter control system; the reactive power control loop is used to switch the internal PI-based reactive power outer loop to a reactive power supplementary current loop during the grid connection failure of the new energy unit; the reactive power supplementary current loop is used to output the d-axis voltage reference value during the fault period, with the q-axis current reference value and the actual q-axis current value during the fault period as inputs during the grid connection failure of the new energy unit. An active power supplementary current loop is set in the active power control loop of the grid-connected converter. During a grid connection failure of a new energy unit, the active power supplementary current loop inputs the d-axis current reference value and the actual d-axis current value during the fault period at its input terminal, and connects to the VSG control loop inside the active power control loop at its output terminal. The VSG control loop switches the active power reference value to the active power reference value during the fault period, and outputs the equivalent voltage source angle during the fault period based on the active power reference value during the fault period and the output of the active power supplementary current loop. During a fault, the d-axis voltage reference value, the equivalent voltage source angle during a fault, the three-phase current at the outlet of the grid converter, and the three-phase voltage across the filter capacitor are modulated by the abc / dq conversion module, the voltage and current inner loop control module, the dq / abc conversion module, and the pulse width modulation module in the grid converter control system to generate a modulation signal. The modulation signal is then applied to the grid converter to limit the current of the grid converter to within the converter current carrying capacity range.

2. The system according to claim 1, characterized in that, The system also includes: a low-pass filter structure and a first switch; The output of the active power supplementary current loop is connected to the input of the low-pass filter structure, the output of the low-pass filter structure is connected to one end of the first switch, and the other end of the first switch is connected to the VSG control loop. The first switch is used to connect during a grid connection failure of the new energy unit and disconnect during normal grid connection of the new energy unit; the low-pass filter structure is used to suppress the frequency fluctuations in the initial stage of the active power additional current loop connection and eliminate frequency static error.

3. The system according to claim 2, characterized in that, The low-pass filter structure has a passband gain of 0.3 and a cutoff angular frequency of 70.

57.

4. The system according to claim 1, characterized in that, The system also includes: a switching switch; The first input terminal of the switching switch is input to the active power reference value during normal operation, the second input terminal of the switching switch is input to the active power reference value during a fault, and the output terminal of the switching switch is connected to the VSG control loop. The switching switch is used to connect the first input terminal when the new energy unit is running normally in grid connection, and to switch to the second input terminal when the grid connection of the new energy unit fails.

5. A method for suppressing overcurrent in a grid-type converter, characterized in that, The method is applied to the overcurrent suppression system of the grid-type converter according to any one of claims 1-4, and the method includes: During the grid connection failure of a new energy unit, the reference value of the dq axis current during the fault period is determined based on the grid connection point voltage. Input the reference value and actual value of the q-axis current during the fault into the reactive power supplementary current loop, and output the reference value of the d-axis voltage during the fault. Based on the reference values ​​of d-axis voltage and d-axis current during the fault, determine the reference value of active power during the fault. The active power reference value during the fault period is input into the VSG control loop in the active power control loop, and the d-axis current reference value and the actual d-axis current value during the fault period are input into the active power additional current loop in the active power control loop, and the equivalent voltage source angle during the fault period is output. The reference value of d-axis voltage during the fault period, the angle of equivalent voltage source during the fault period, the three-phase current at the outlet of the grid converter, and the three-phase voltage across the filter capacitor are converted by abc / dq, the voltage and current inner loop control is applied, the dq / abc conversion is applied, and the pulse width modulation is applied to the grid converter to limit the current of the grid converter to the current carrying capacity of the converter.

6. The method according to claim 5, characterized in that, The formula for determining the q-axis current reference value during a fault is: In the formula, i qref This is the reference value for the q-axis current during the fault, u d K represents the voltage amplitude at the grid connection point. d I is the reactive current gain coefficient. N Rated current; The formula for determining the d-axis current reference value during a fault is: In the formula, i dref i is the reference value for the d-axis current during the fault period. d0 I is the reference value of the d-axis current before the fault. max This is the overcurrent limit for the converter.

7. The method according to claim 5, characterized in that, The formula for calculating the d-axis voltage reference value during the fault period is as follows: u dref (s)=u drefpu (s)E0 Where u drefpu (s) represents the per-unit value of the voltage amplitude during the fault, k rp k ri These are the proportional-integral coefficients, i qrefpu 、i qpu These are the reference per-unit value and the actual per-unit value of the q-axis current, respectively, E 0pu The reference voltage per unit value; u dref (s) represents the reference value of the d-axis voltage during the fault period, and E0 represents the reference voltage.

8. The method according to claim 5, characterized in that, The formula for calculating the active power reference value during the fault period is as follows: In the formula, P′ ref i is the reference value for active power during the fault period. dref The reference value for the d-axis current during the fault is u. dref This is the reference value for the d-axis voltage during the fault period.

9. The method according to claim 5, characterized in that, The active power reference value during the fault period is input into the VSG control loop in the active power control loop, and the d-axis current reference value and the actual d-axis current value during the fault period are input into the active power supplementary current loop in the active power control loop. The equivalent voltage source angle during the fault period is output, specifically including: Input the reference value and actual value of the d-axis current during the fault period output by the voltage loop into the active additional current loop in the active control loop, and output the angular frequency deviation value. The angular frequency deviation value is filtered using a low-pass filter structure to obtain the filtered angular frequency deviation value. The active power reference value and the filtered angular frequency deviation value during the fault period are both input into the VSG control loop, and the equivalent voltage source angle during the fault period is output.

10. The method according to claim 9, characterized in that, The process for determining the filter parameters of the low-pass filter structure is as follows: The transfer function of the low-pass filter structure is determined as follows: In the formula, G LF (s) is the transfer function, a1 is the passband gain, and ω LF The cutoff frequency is s, the complex frequency is s = jω, and ω is the angular frequency; When the cutoff frequency of the active filter is 75Hz, the gain of the low-pass filter structure at the cutoff frequency is expressed as 20log. 10 |G LF (s)|=-3; Based on the transfer function and the gain amplitude at the cutoff frequency, the relationship between the cutoff frequency and the passband gain is determined as follows: The range of values ​​for the preset passband gain is determined, and the amplitude-frequency response curve is determined based on the relationship between the cutoff frequency and the passband gain. Based on the selection criteria of large amplitude variation and small phase variation, the optimal passband gain is 0.3 and the optimal cutoff frequency is 70.57 on the amplitude-frequency response curve.

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