Network construction type converter control method and system suitable for multiple power grid intensities

Abstract

The invention provides a network construction type converter control method suitable for multiple power grid intensities, and belongs to the field of new energy grid-connected stability. The method comprises the steps that the actual output power of an inverter is calculated according to inverter side capacitor voltage and grid-connected current; the actual output power of the inverter and the power reference value control an output internal potential amplitude reference value and an internal potential phase angle through the virtual synchronous generator; subtracting the voltage drop of the grid-connected current on the virtual impedance from the internal potential amplitude reference value to obtain a virtual internal potential; inputting the virtual internal potential as a reference voltage into a voltage and current double-loop controller, inputting a grid-connected current into a proportional differential controller, subtracting the output of the proportional differential controller from the output of the voltage and current double-loop controller, inputting an obtained result into an inverter through an SPWM modulator, and controlling the inverter; the invention also provides a control system of the network-forming converter. Grid-connected current proportional differential feedback is introduced, the inner loop control bandwidth is kept constant in the dynamic change process of the power grid strength, and power-voltage loop harmful coupling is eliminated.

Description

Technical Field

[0001] This invention relates to the field of new energy grid connection stability technology, and in particular to a grid-type converter control method and system applicable to multiple grid strengths. Background Technology

[0002] With the increasing proportion of new energy sources in the power grid, the drawbacks of currently widely used grid-forming converters, such as their inability to autonomously construct voltage and support grid frequencies, are becoming increasingly apparent. This is particularly problematic in weak grid conditions, where they are prone to system instability, posing a significant challenge to the stable operation of the power grid. Therefore, in recent years, grid-forming (GFM) converters with active support capabilities have received widespread attention. GFM converters synchronize their output power with the grid and simulate the external characteristics of a synchronous machine, ensuring the inertia and damping required by the grid. Furthermore, they possess advantages such as autonomously constructing output voltage without relying on the AC grid and providing rapid frequency response, effectively improving grid stability.

[0003] Grid-Forming Converters (GFMCs) employ vector voltage control and operate as AC voltage sources. Because of their identical control structure, GFMCs can function normally regardless of grid connection. GFMC control comprises an external power control loop and internal voltage control. The power loop uses a virtual synchronous generator (VSG) or virtual synchronous inverter control to simulate the inertia and droop characteristics of a synchronous generator. Regarding the inner loop control, in islanded mode, LC filters are typically used in GFMC applications to provide a pure voltage output with low switching harmonics. In grid-connected mode, due to the presence of the grid's equivalent impedance, the LC filter on the inverter output side will be converted to an LCL filter.

[0004] The inner loop controller of a GFMC is typically designed in islanded mode. Generally, due to grid impedance, the response speed of the inner loop in a grid-connected GFMC with dual-loop voltage control is much slower than in islanded mode. Furthermore, the voltage control bandwidth of a GFMC decreases with increasing grid strength, while the power loop bandwidth increases with grid strength. In strong grids with low grid impedance or in wind turbines with small capacity and low droop coefficients, the power loop bandwidth can be very high. This means that even if the inner loop is designed to be fast in islanded mode, the assumption of decoupled control bandwidth in grid-connected mode may not hold. A slow dynamic response inner loop can lead to coupling between the external power control loop and the internal voltage control loop, potentially resulting in unsatisfactory or even unstable power control dynamics. Under strong grid conditions, bandwidth coupling between the two loops can cause system instability.

[0005] Traditional methods indirectly affect bandwidth by adjusting the inertia or integral coefficients, but cannot actively maintain a constant voltage loop bandwidth, making it difficult to adapt to dynamic changes in grid strength. Therefore, there is an urgent need for a control method that fixes the voltage loop bandwidth, eliminating harmful power-voltage loop coupling by blocking the dynamic changes in voltage loop bandwidth with grid strength, and fundamentally improving the overall stability of grid-connected converters under both strong and weak grid conditions. In the existing technology, Chinese invention patent application CN119994949A, "Control Method and System for Grid-Connected Converters Based on Virtual Impedance Feedforward Controller," addresses the synchronous frequency resonance problem caused by power loop coupling by suppressing resonance through inductor current feedforward compensation. However, this method cannot eliminate harmful power-voltage loop coupling. Summary of the Invention

[0006] The technical problem to be solved by this invention is: how to solve the problem that the voltage inner loop control bandwidth of a grid-connected converter in grid-connected mode is easily coupled with the power outer loop control due to changes in grid strength, causing system oscillation.

[0007] This invention solves the above-mentioned technical problems through the following technical solution: a grid-type converter control method applicable to multiple grid strengths, the method comprising:

[0008] The actual output power of the inverter is calculated based on the inverter-side capacitor voltage and grid-connected current.

[0009] The actual output power of the inverter and the power reference value are controlled by a virtual synchronous generator to determine the output internal potential amplitude reference value and the internal potential phase angle.

[0010] The virtual internal potential is obtained by subtracting the voltage drop of the grid current across the virtual impedance from the reference value of the internal potential amplitude.

[0011] The virtual internal potential is used as the reference voltage input to the voltage-current dual-loop controller, and the grid-connected current is input to the proportional-derivative controller. The output of the voltage-current dual-loop controller is subtracted from the output of the proportional-derivative controller, and the result is input to the inverter through the SPWM modulator to control the inverter.

[0012] This invention introduces proportional-derivative feedback of grid-connected current to maintain a constant inner loop control bandwidth during dynamic changes in grid strength. This makes the inner loop controller unaffected by changes in grid-side inductance, blocks the dynamic changes in voltage loop bandwidth with grid strength, eliminates harmful coupling between power and voltage loops, and ensures that the inner loop controller can maintain a constant bandwidth under any grid strength. This fundamentally improves the overall stability of grid-connected converters under both strong and weak grid conditions.

[0013] Preferably, the voltage and current dual-loop controller includes a voltage loop controller using quasi-proportional resonant control and a current loop controller using proportional control. The proportional coefficient of the proportional-derivative controller is equal to the negative of the proportional coefficient of the current loop, and the derivative coefficient of the proportional-derivative controller is equal to the negative of the filter inductance on the output side of the inverter.

[0014] The proportional gain of the proportional-derivative controller is equal to the negative of the proportional gain of the current loop, eliminating the pole at s=0 in the open-loop transfer function from the inverter output voltage to the inverter side current. This allows the grid-connected converter to have similar high-frequency characteristics in both islanded and grid-connected modes. The derivative of the proportional-derivative controller is equal to the negative of the filter inductance on the inverter output side, ensuring that the grid-connected converter has identical low-frequency characteristics in both islanded and grid-connected modes. This eliminates the influence of grid-side impedance on the inner loop control, giving the grid-connected converter the same inner loop control bandwidth in grid-connected and islanded modes. This further improves the inner loop voltage tracking performance of the grid-connected converter in both islanded and grid-connected modes, enhancing power control stability, especially when the power control bandwidth is high in strong grid-connected modes.

[0015] In addition, the proportional-derivative controller has simple parameter design. After determining the system parameters and current loop control parameters, the parameters required for the grid-connected current state feedback function can be obtained without additional parameter tuning process.

[0016] Preferably, the voltage loop controller G v (s) is: Current loop controller G c (s) is: G c (s)=K cp Among them, K vp K cp These are the proportional gains of the voltage loop and current loop, respectively, where ω0 is the rated grid frequency, and K... vr For the resonant gain, ζ r s is the damping coefficient of the quasi-proportional resonant controller, and s is a complex frequency domain variable.

[0017] Preferably, the proportional-derivative controller G ad (s) is: G ad (s)=-K cp -sL1; where K cp L1 is the proportional gain of the current loop, L2 is the output filter inductor of the inverter, and s is a complex frequency domain variable.

[0018] Preferably, the closed-loop transfer function G of the voltage-current dual-loop controller DCGCF (s) is:

[0019]

[0020] Among them, G v(s), G c (s) represent the voltage loop controller and the current loop controller, respectively, G d (s) is the delay function, L1, C f These are the inverter output side filter inductor and filter capacitor, respectively. g Let be the equivalent inductance of the power grid, and s be a variable in the complex frequency domain.

[0021] Preferably, the grid-connected converter has exactly the same low-frequency characteristics in both islanded and grid-connected modes.

[0022] Preferably, the actual output power of the inverter is:

[0023]

[0024] Among them, E m E represents the output voltage amplitude of the inverter. g ∠0° represents the grid voltage, δ represents the phase angle of the VSG output voltage, and the relationship between the phase angle and the inverter output angular frequency is: δ=∫(ω-ω0)dt, where ω0 is the rated grid frequency, X g This refers to the line reactance.

[0025] Preferably, the reference value of the internal potential amplitude E and the internal potential phase angle δ are respectively:

[0026]

[0027] E = E m +k q (Q ref -Q e )

[0028] δ=∫(ω-ω0)dt

[0029] Among them, P e Q e These represent the active power and reactive power output by the VSG, respectively. ref Q ref These are the reference values ​​for active power and reactive power, respectively; ω0 is the rated grid frequency; ω is the actual angular frequency of the inverter output; J and D are the VSG moment of inertia and damping coefficient, respectively; and E... m k is the output voltage amplitude of the inverter. q This is the reactive power droop coefficient.

[0030] Preferably, the calculation process of the output of the voltage and current dual-loop controller includes: subtracting the virtual internal potential from the inverter-side output voltage to obtain a first difference value; subtracting the output value of the first difference value after adjustment by the voltage loop controller from the inverter-side output current to obtain a second difference value; and obtaining the output of the voltage and current dual-loop controller after adjustment by the current loop controller.

[0031] This invention also provides a grid-type converter control system suitable for multiple grid strengths, the system comprising:

[0032] The power calculation module is used to calculate the actual output power of the inverter based on the inverter-side capacitor voltage and grid-connected current.

[0033] The outer loop control module is used to control the output internal potential amplitude reference value and internal potential phase angle of the inverter by comparing the actual output power of the inverter with the power reference value through a virtual synchronous generator.

[0034] The virtual impedance control loop is used to subtract the voltage drop of the grid current across the virtual impedance from the reference value of the internal potential amplitude to obtain the virtual internal potential.

[0035] The voltage and current dual-loop control module is used to input the virtual internal potential as the reference voltage into the voltage and current dual-loop controller, and input the grid-connected current into the proportional-derivative controller. The output of the voltage and current dual-loop controller is subtracted from the output of the proportional-derivative controller, and the result is input into the inverter through the SPWM modulator to control the inverter. Attached Figure Description

[0036] Figure 1 A control block diagram of a grid-type converter control method applicable to multiple grid strengths provided in Embodiment 1 of the present invention;

[0037] Figure 2 This is a block diagram of a traditional voltage and current inner loop control.

[0038] Figure 3 The open-loop transfer function G of the voltage loop in islanded mode using traditional dual-loop control. LC (s) and the open-loop transfer function G of the voltage loop in grid-connected mode LCL Bode plot of (s);

[0039] Figure 4 The inner loop control block diagram with grid-connected current negative proportional differential feedback in the grid-connected current control method for grid-type converters applicable to multiple grid strengths provided in Embodiment 1 of the present invention;

[0040] Figure 5 The voltage loop Bode plot with grid-connected current negative proportional differential feedback in the grid-connected current control method for grid-connected converters applicable to multiple grid strengths provided in Embodiment 1 of the present invention;

[0041] Figure 6(a) shows the adoption of Figure 2 Bode plot of the closed-loop transfer function of a traditional voltage-current inner-loop control system;

[0042] Figure 6(b) shows the adoption of Figure 4 Bode plot of the closed-loop transfer function of an inner-loop control system with negative proportional-derivative feedback of grid-connected current. Detailed Implementation

[0043] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below in conjunction with specific embodiments and with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0044] Example 1

[0045] Figure 1 The diagram shows the main circuit and control block diagram of the grid-type converter of this invention. The main circuit adopts a T-type three-level inverter circuit, the outer power loop is controlled by a virtual synchronous generator (VSG), and the inner loop adopts voltage and current dual closed-loop control based on grid current feedback. Figure 2 This is a block diagram of the traditional voltage and current dual-loop control for a grid-type converter in the αβ coordinate system. Figure 2 In the middle, voltage loop controller G v (s) Quasi-proportional resonance (QPR) control is adopted, and the current loop controller G is used. c (s) Proportional control is adopted, and voltage loop controller G is used. v The expression for (s) is:

[0046]

[0047] Current loop controller G c The expression for (s) is:

[0048] G c (s)=K cp

[0049] Among them, K vp K cp These are the proportional gains of the voltage loop and current loop, respectively, where ω0 is the rated grid frequency, and K... vr For the resonant gain, ζ r s is the damping coefficient of the quasi-proportional resonant controller, and s is a complex frequency domain variable.

[0050] The computational delay and PWM loading delay are simulated as a delay function G. d (s), the expression is:

[0051]

[0052] Among them, T s During the switching cycle, in LC (islanded) mode, the voltage U from the inverter output voltage...i Inverter-side current I L Open-loop transfer function T iLC (s) is:

[0053]

[0054] Open-loop transfer function G of voltage loop in islanded mode LC (s) is:

[0055]

[0056] When a grid-connected converter is connected to the grid, the filter changes from LC type to LCL type due to the presence of grid-side inductance. At this time, the inverter output voltage U... i Inverter-side current I L Open-loop transfer function T iLCL (s) is:

[0057]

[0058] Open-loop transfer function G of voltage loop in grid-connected mode LCL (s) is:

[0059]

[0060] Based on the open-loop transfer function G of the voltage loop in islanded mode LC (s) and the open-loop transfer function G of the voltage loop in grid-connected mode LCL (s) It can be seen that the open-loop transfer functions of LC and LCL modes are different. Specifically, the open-loop transfer function of LCL mode has one more zero located at the origin than that of LC mode. The fundamental reason for this is that the open-loop transfer function T iLCL (s) compared to the open-loop transfer function T iLC (s) The addition of an extra pole located at the origin will make G LCL The number of types (TN) of (s) decreases by 1, resulting in a significant change in the low-frequency characteristics of the system.

[0061] The system parameters are shown in Table 1, and the control parameters are shown in Table 2. Substitute these parameters into the open-loop transfer function G. LCL (s), G LC Bode plot of (s) is as follows Figure 3 As shown, it can be seen that the open-loop transfer function G of the voltage loop in grid-connected mode is... LCL (s) and the open-loop transfer function G of the voltage loop in islanded mode LC (s) have similar high-frequency characteristics, but in the low-frequency range, due to the difference between the two modes TN, the open-loop transfer function G of the voltage loop in the grid-connected mode... LCL(s) The rate of change of the amplitude-frequency curve is greater than that of the open-loop transfer function G of the voltage loop in islanded mode. LC (s) is large, and the open-loop transfer function G of the voltage loop in grid-connected mode is large. LCL The initial phase of (s) also leads the open-loop transfer function G of the voltage loop in islanded mode. LC (s)90 degrees, so that the open-loop transfer function G of the voltage loop in grid-connected mode is... LCL (s) Compared to the open-loop transfer function G of the voltage loop in islanded mode LC The low-frequency loop gain of (s) is poor, and the response speed of the voltage and current inner loop in grid-connected mode is much slower than that in islanded mode.

[0062] Table 1 System Parameters

[0063] parameter Value parameter Value <![CDATA[U dc / V]]> 800 <![CDATA[U g / V]]> 380 <![CDATA[L1 / mH]]> 0.2 <![CDATA[C f / μF]]> 60 <![CDATA[L g / mH]]> 0.6 <![CDATA[f0 / Hz]]> 50 <![CDATA[S n / AND]]> 125000 <![CDATA[T s / μs]]> 62

[0064] Table 2 Control Parameters

[0065] parameter Value parameter Value J 2 D 150 <![CDATA[D q ]]> 200 <![CDATA[K q ]]> 1 / 100 <![CDATA[K vp ]]> 3.5 <![CDATA[K vr ]]> 160 <![CDATA[ζ r ]]> 1 / 100 <![CDATA[K cp ]]> 0.2 <![CDATA[K adP ]]> -0.2 <![CDATA[K adD ]]> <![CDATA[-0.6×10 -3 ]]>

[0066] Because the response speed of the voltage and current inner loop of a grid-connected converter is much slower in grid-connected mode than in islanded mode, when the grid impedance is low (strong grid), the control bandwidth of the voltage and current inner loop will decrease, causing the bandwidth of the power loop and voltage loop to become close, resulting in coupled oscillation.

[0067] To address the issue of system oscillations caused by the voltage inner loop control bandwidth being affected by grid strength variations and easily coupling with the power outer loop control in grid-connected mode, this embodiment provides a control method for grid-connected converters applicable to multiple grid strengths, including the following steps:

[0068] Step 1: Based on the inverter-side capacitor voltage u abc and grid-connected current i abc The actual output power of the inverter is calculated. The three-phase current i output from the inverter side is collected using sensors. Labc Inverter-side capacitor voltage u abc and grid-connected current i abc Calculate the actual output power of the inverter, including the active power P. e and reactive power Q e The calculation method is as follows:

[0069]

[0070] Among them, E m E represents the output voltage amplitude of the inverter. g∠0° represents the grid voltage, δ represents the phase angle of the VSG output voltage, and the relationship between the phase angle and the inverter output angular frequency is: δ=∫(ω-ω0)dt, where ω0 is the rated grid frequency, X g This refers to the line reactance.

[0071] Step 2: The actual output power of the inverter and the power reference value are controlled by a virtual synchronous generator to output the internal electromotive force amplitude reference value E and the internal electromotive force phase angle δ. Through outer-loop power control, the grid-connected converter is kept synchronized with the grid, simulating the droop characteristics of a synchronous generator. The outer-loop power control includes a reactive voltage control loop and an active frequency control loop, controlling the reactive power Q... e and the set reactive power reference value Q ref Input reactive voltage control loop, reactive power Q e With reactive power reference value Q ref By performing differential calculations and obtaining the reference value E of the internal potential amplitude through droop control, the active power P is then... e and active power reference value P ref Input the active frequency control loop and calculate the internal potential phase angle δ. The internal potential amplitude reference value E and the internal potential phase angle δ are respectively:

[0072]

[0073] E = E m +k q (Q ref -Q e )

[0074] δ=∫(ω-ω0)dt

[0075] Among them, P e Q e These represent the active power and reactive power output by the VSG, respectively. ref Q ref These are the reference values ​​for active power and reactive power, respectively; ω0 is the rated grid frequency; ω is the actual angular frequency of the inverter output; J and D are the VSG moment of inertia and damping coefficient, respectively; and E... m k is the output voltage amplitude of the inverter. q This is the reactive power droop coefficient.

[0076] Step 3: Subtract the grid-connected current i from the reference value of internal potential amplitude E. abc The voltage drop across the virtual impedance yields the virtual internal potential e. By inputting the reference value of the internal potential amplitude and the grid-connected current into the virtual impedance control loop, the internal impedance characteristics (mainly reactance) of the synchronous generator are simulated, thereby enhancing system stability, improving power distribution, and limiting fault current.

[0077] Step 4: Using the virtual internal potential e as the reference voltage, input voltage and current dual-loop controller, and grid-connected current i abc Input proportional-derivative controller G ad (s) The output of the voltage and current dual-loop controller is subtracted from the output of the proportional-derivative controller, and the result is input to the inverter through the SPWM modulator to control the inverter.

[0078] The virtual internal potential e serves as the reference voltage input voltage and current dual-loop controller. The virtual internal potential e is related to the inverter-side output voltage u. abc The difference is calculated to obtain the first difference value, which is then processed by the voltage loop controller G. v (s) The adjusted output value and the inverter-side output current i Labc The difference is calculated to obtain a second difference value, which is then processed by the current loop controller G. c (s) After adjustment, the output of the voltage and current dual-loop controller is obtained.

[0079] The voltage and current dual-loop controller includes a voltage loop controller G employing quasi-proportional resonant control. v (s) and the current loop controller G using proportional control c (s), proportional-derivative controller G ad The proportional coefficient of (s) is equal to the negative of the proportional coefficient of the current loop, and the proportional-derivative controller G ad The differential coefficient of (s) is equal to the negative of the filter inductance on the output side of the inverter.

[0080] Voltage loop controller G v (s) is:

[0081]

[0082] Current loop controller G c (s) is:

[0083] G c (s)=K cp

[0084] Among them, K vp K cp These are the proportional gains of the voltage loop and current loop, respectively, where ω0 is the rated grid frequency, and K... vr For the resonant gain, ζ r s is the damping coefficient of the quasi-proportional resonant controller, and s is a complex frequency domain variable.

[0085] Ignoring control delay, proportional-derivative controller G ad (s) is:

[0086] G ad (s)=-K cp -sL1

[0087] Among them, K cp L1 is the proportional gain of the current loop, L2 is the output filter inductor of the inverter, and s is a complex frequency domain variable.

[0088] See Figure 4 The open-loop transfer function G of the voltage loop of a grid-connected converter in grid-connected mode LCL_PD (s) is:

[0089]

[0090] The closed-loop transfer function G of the dual-loop controller with grid current feedback (DCGCF) in the grid-type converter of this invention. DCGCF (s) is:

[0091]

[0092] Among them, G v (s), G c (s) represent the voltage loop controller and the current loop controller, respectively, G d (s) is the delay function, L1, C f These are the inverter output side filter inductor and filter capacitor, respectively. g Let be the equivalent inductance of the power grid, and s be a variable in the complex frequency domain.

[0093] Proportional-derivative controller G ad The proportional gain (s) is equal to the negative of the current loop proportional gain, which can eliminate the inverter output voltage U i Inverter-side current I L Open-loop transfer function T iLCL (s) The pole located at s=0, through zero-pole cancellation, enables the grid-type converter to have similar high-frequency characteristics in islanded and grid-connected modes; the proportional-derivative controller G ad The differential coefficient of (s) is equal to the negative of the filter inductance on the inverter output side, ensuring that the grid-connected converter exhibits identical low-frequency characteristics in both islanded and grid-connected modes. Substituting the system parameters from Table 1 and the control parameters from Table 2 into the open-loop transfer function G of the voltage loop in grid-connected mode... LCL_PD (s), thus obtaining the voltage loop Bode plot with proportional-derivative feedback of the grid current, as shown. Figure 5As shown, when both modes have similar high-frequency characteristics, regardless of the grid-side inductance, the LC mode and LCL mode have exactly the same low-frequency characteristics, eliminating the influence of grid-side impedance on the inner loop control. This allows the grid-connected converter to have the same inner loop control bandwidth in grid-connected mode as in islanded mode, further improving the inner loop voltage tracking performance of the grid-connected converter in both islanded and grid-connected modes, and enhancing power control stability, especially when the power control bandwidth is high in the grid-connected mode of a strong power grid.

[0094] Furthermore, by introducing proportional differential feedback of grid-connected current, the inner loop control bandwidth remains constant during dynamic changes in grid strength. This ensures that the inner loop controller is unaffected by changes in grid-side inductance, blocks dynamic changes in voltage loop bandwidth with grid strength, eliminates harmful coupling between power and voltage loops, and allows the inner loop controller to maintain constant bandwidth under any grid strength, fundamentally improving the overall stability of grid-connected converters under both strong and weak grid conditions.

[0095] The proportional-derivative controller (PDC) has simple parameter design. Once the system parameters and current loop control parameters are determined, the parameters required for the grid-connected current state feedback function can be obtained without additional parameter tuning. This invention has a certain degree of universality; regardless of whether the current loop is proportional control, proportional-resonant control, or proportional-integral control, this method can ensure that both islanded and grid-connected modes exhibit the same low-frequency characteristics.

[0096] Example 2

[0097] This embodiment provides a grid-type converter control system suitable for multiple grid strengths, including:

[0098] The power calculation module is used to calculate the actual output power of the inverter based on the inverter-side capacitor voltage and grid-connected current; the actual output power of the inverter is:

[0099]

[0100] Among them, E m E represents the output voltage amplitude of the inverter. g ∠0° represents the grid voltage, δ represents the phase angle of the VSG output voltage, and the relationship between the phase angle and the inverter output angular frequency is: δ=∫(ω-ω0)dt, where ω0 is the rated grid frequency, X g This refers to the line reactance.

[0101] The outer loop control module is used to control the output of the internal electromotive force (EMF) amplitude reference value and internal EMF phase angle via a virtual synchronous generator, based on the actual output power and power reference value of the inverter. The internal EMF amplitude reference value E and the internal EMF phase angle δ are respectively:

[0102]

[0103] E = E m +k q (Q ref -Q e )

[0104] δ=∫(ω-ω0)dt

[0105] Among them, P e Q e These represent the active power and reactive power output by the VSG, respectively. ref Q ref These are the reference values ​​for active power and reactive power, respectively; ω0 is the rated grid frequency; ω is the actual angular frequency of the inverter output; J and D are the VSG moment of inertia and damping coefficient, respectively; and E... m k is the output voltage amplitude of the inverter. q This is the reactive power droop coefficient.

[0106] The Virtual Impedance control loop is used to subtract the voltage drop of the grid current across the virtual impedance from the reference value of the internal potential amplitude to obtain the virtual internal potential.

[0107] The voltage and current dual-loop control module is used to input the virtual internal potential as the reference voltage into the voltage and current dual-loop controller, and input the grid-connected current into the proportional-derivative controller. The output of the voltage and current dual-loop controller is subtracted from the output of the proportional-derivative controller, and the result is input into the inverter through the SPWM modulator to control the inverter.

[0108] The calculation process of the output of the voltage and current dual-loop controller includes: subtracting the virtual internal potential from the inverter-side output voltage to obtain the first difference value; subtracting the output value of the first difference value after adjustment by the voltage loop controller from the inverter-side output current to obtain the second difference value; and obtaining the output of the voltage and current dual-loop controller after adjustment by the current loop controller.

[0109] The voltage and current dual-loop controller includes a voltage loop controller using quasi-proportional resonant control and a current loop controller using proportional control. The proportional coefficient of the proportional-derivative controller is equal to the negative of the proportional coefficient of the current loop, and the derivative coefficient of the proportional-derivative controller is equal to the negative of the filter inductance on the inverter output side. Grid-connected converters exhibit identical low-frequency characteristics in both islanded and grid-connected modes.

[0110] Voltage loop controller G v (s) is: Current loop controller G c (s) is: G c (s)=K cp Among them, K vp K cpThese are the proportional gains of the voltage loop and current loop, respectively, where ω0 is the rated grid frequency, and K... vr For the resonant gain, ζ r s is the damping coefficient of the quasi-proportional resonant controller, and s is a complex frequency domain variable.

[0111] Proportional-derivative controller G ad (s) is: G ad (s)=-K cp -sL1; where K cp L1 is the proportional gain of the current loop, L2 is the output filter inductor of the inverter, and s is a complex frequency domain variable.

[0112] Closed-loop transfer function G of a dual-loop controller with grid current feedback DCGCF (s) is:

[0113]

[0114] Among them, G v (s), G c (s) represent the voltage loop controller and the current loop controller, respectively, G d (s) is the delay function, L1, C f These are the inverter output side filter inductor and filter capacitor, respectively. g Let be the equivalent inductance of the power grid, and s be a variable in the complex frequency domain.

[0115] Simulation Experiment

[0116] Figure 2 The closed-loop transfer function G of a traditional dual-loop controller CDC (s) is:

[0117]

[0118] Figure 3 Closed-loop transfer function G of a dual-loop controller with grid current feedback DCGCF (s) is:

[0119]

[0120] The system and control parameters are shown in Table 3:

[0121] Table 3 System and Control Parameters

[0122] parameter numerical values parameter numerical values <![CDATA[Input voltage U dc > 800V <![CDATA[Inverter-side inductor L1]]> 200uH <![CDATA[Grid voltage U g > 380V <![CDATA[Filter capacitor C f > 60uF Active power P 125KW reactive power Q 0KVar <![CDATA[Dead time T d > 1.5us <![CDATA[Current loop proportionality coefficient K cp > 00.12 <![CDATA[Voltage loop proportionality coefficient K vp > 8 <![CDATA[Voltage loop resonance coefficient K vr > 160

[0123] Based on a converter with a rated power of 125KW, the grid-side inductance Lg is set to 0.1mH, 1.7mH, and 2.7mH, corresponding to the strong grid, transition zone, and weak grid, respectively. The data in Table 3 are then substituted into the closed-loop transfer function G of the traditional dual-loop controller. CDC(s) and the closed-loop transfer function G of the dual-loop controller with grid current feedback DCGCF (s) The Bode plots of the system closed-loop transfer function are shown in Figures 6(a) and 6(b). According to the principle of automatic control, when the amplitude gain curve drops to -3dB, the corresponding frequency is the bandwidth frequency. As can be seen from the above Bode plots, the frequency at -3dB of the traditional voltage and current dual-loop control is significantly different, while the frequency of the amplitude-frequency curve at -3dB is the same for the dual-loop control with grid current feedback, regardless of whether it is a strong grid, a weak grid, or a transition zone. This means that the bandwidth of the inner loop control remains unchanged regardless of the strength of the grid, verifying the effectiveness of the control method in Example 1.

[0124] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A control method for grid-type converters applicable to multiple grid strengths, characterized in that: The methods include: The actual output power of the inverter is calculated based on the inverter-side capacitor voltage and grid-connected current. The actual output power of the inverter and the power reference value are controlled by a virtual synchronous generator to determine the output internal potential amplitude reference value and the internal potential phase angle. The virtual internal potential is obtained by subtracting the voltage drop of the grid current across the virtual impedance from the reference value of the internal potential amplitude. The virtual internal potential is used as the reference voltage input to the voltage-current dual-loop controller, and the grid-connected current is input to the proportional-derivative controller. The output of the voltage-current dual-loop controller is subtracted from the output of the proportional-derivative controller, and the result is input to the inverter through the SPWM modulator to control the inverter.

2. The grid-type converter control method applicable to multiple grid strengths according to claim 1, characterized in that: The voltage and current dual-loop controller includes a voltage loop controller using quasi-proportional resonant control and a current loop controller using proportional control. The proportional coefficient of the proportional-derivative controller is equal to the opposite of the proportional coefficient of the current loop, and the derivative coefficient of the proportional-derivative controller is equal to the opposite of the filter inductance on the output side of the inverter.

3. The grid-type converter control method applicable to multiple grid strengths according to claim 1, characterized in that: Voltage loop controller G v (s) is: Current loop controller G c (s) is: G c (s)=K cp Among them, K vp K cp These are the proportional gains of the voltage loop and current loop, respectively, where ω0 is the rated grid frequency, and K... vr For the resonant gain, ζ r s is the damping coefficient of the quasi-proportional resonant controller, and s is a complex frequency domain variable.

4. The grid-type converter control method applicable to multiple grid strengths according to claim 1, characterized in that: Proportional-derivative controller G ad (s) is: G ad (s)=-K cp -sL1; where K cp L1 is the proportional gain of the current loop, L2 is the output filter inductor of the inverter, and s is a complex frequency domain variable.

5. The grid-type converter control method applicable to multiple grid strengths according to claim 1, characterized in that: Closed-loop transfer function G of voltage-current dual-loop controller DCGCF (s) is: Among them, G v (s), G c (s) represent the voltage loop controller and the current loop controller, respectively, G d (s) is the delay function, L1, C f These are the inverter output side filter inductor and filter capacitor, respectively. g Let be the equivalent inductance of the power grid, and s be a variable in the complex frequency domain.

6. The grid-type converter control method applicable to multiple grid strengths according to claim 1, characterized in that: Grid-type converters have exactly the same low-frequency characteristics in both islanded and grid-connected modes.

7. The grid-type converter control method applicable to multiple grid strengths according to claim 1, characterized in that: The actual output power of the inverter is: Among them, E m E represents the output voltage amplitude of the inverter. g ∠0° represents the grid voltage, δ represents the phase angle of the VSG output voltage, and the relationship between the phase angle and the inverter output angular frequency is: δ=∫(ω-ω0)dt, where ω0 is the rated grid frequency, X g This refers to the line reactance.

8. The grid-type converter control method applicable to multiple grid strengths according to claim 1, characterized in that: The reference value of the internal potential amplitude E and the internal potential phase angle δ are respectively: E=E m +k q (Q ref -Q e ) δ=∫(ω-ω0)dt Among them, P e Q e These represent the active power and reactive power output by the VSG, respectively. ref Q ref These are the reference values ​​for active power and reactive power, respectively; ω0 is the rated grid frequency; ω is the actual angular frequency of the inverter output; J and D are the VSG moment of inertia and damping coefficient, respectively; and E... m k is the output voltage amplitude of the inverter. q This is the reactive power droop coefficient.

9. The grid-type converter control method applicable to multiple grid strengths according to claim 1, characterized in that: The calculation process of the output of the voltage and current dual-loop controller includes: subtracting the virtual internal potential from the inverter-side output voltage to obtain the first difference value; subtracting the output value of the first difference value after adjustment by the voltage loop controller from the inverter-side output current to obtain the second difference value; and obtaining the output of the voltage and current dual-loop controller after adjustment by the current loop controller.

10. A grid-type converter control system suitable for multiple grid strengths, characterized in that: The system includes: The power calculation module is used to calculate the actual output power of the inverter based on the inverter-side capacitor voltage and grid-connected current. The outer loop control module is used to control the output internal potential amplitude reference value and internal potential phase angle of the inverter by comparing the actual output power of the inverter with the power reference value through a virtual synchronous generator. The virtual impedance control loop is used to subtract the voltage drop of the grid current across the virtual impedance from the reference value of the internal potential amplitude to obtain the virtual internal potential. The voltage and current dual-loop control module is used to input the virtual internal potential as the reference voltage into the voltage and current dual-loop controller, and input the grid-connected current into the proportional-derivative controller. The output of the voltage and current dual-loop controller is subtracted from the output of the proportional-derivative controller, and the result is input into the inverter through the SPWM modulator to control the inverter.

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