A four-loop control system and low voltage ride through control method for a VSG
Patent Information
- Application Number
- CN202310765437.3
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- Filing Date
- 2023-06-27
- Publication Date
- 2026-11-03
- Estimated Expiration
- 2043-06-27
AI Technical Summary
现有的VSG低电压穿越控制方法在电网故障时无法满足无功电流注入的时限要求，基于模式切换的方法影响电网稳定性，而基于控制优化的方法响应速度较慢。
A four-loop control system is adopted, including a power loop, a power angle loop, a voltage loop, and a current loop. Combined with direct voltage control (DVC), the inverter's voltage source characteristics are preserved during grid faults. The capacitor voltage phase information is obtained through a phase-locked loop for feedforward control, enabling the inverter to respond quickly and switch smoothly.
During grid faults, the inverter achieves rapid response and smooth control mode switching, ensuring that a specified amount of power can be injected to maintain stable grid operation.
Smart Images
Figure CN116742707B_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of grid-connected inverter control technology, and relates to a four-loop control system and low voltage ride-through control method for VSG. Background Technology
[0002] Distributed generation technology based on renewable energy sources (such as solar, wind, and hydropower) is one of the important means for humanity to address the energy crisis and environmental pollution, and has received increasing attention in recent years. Among these technologies, grid-connected inverters, as the interface between renewable energy generation units and the power grid, play a crucial role in converting the electricity generated by renewable energy sources into AC form for transmission to the grid. As the capacity of distributed power sources connected to the grid increases, they should not merely serve as a simple energy source for the grid, but should also possess sufficient voltage amplitude and frequency support capabilities to maintain the stable operation of the grid.
[0003] Under this demand, the concept of virtual synchronous generator (VSG) was proposed by scholars. The basic idea of VSG is to enable the grid-connected inverter to simulate the characteristics of a synchronous generator through control, namely inertia, primary frequency regulation characteristics, and primary voltage regulation characteristics, so that the grid-connected inverter has a rotational inertia similar to that of a synchronous generator rotor to suppress grid frequency fluctuations.
[0004] There are two main control strategies for Low Voltage Ride-through (LVRT) technology: one is the mode-switching-based LVRT method, and the other is the control optimization-based LVRT method. The mode-switching-based LVRT method has a fast reactive current response during faults, but the VSG voltage source characteristics switch to current source characteristics, which is detrimental to the stability of high-voltage and high-efficiency power systems. The control optimization-based LVRT method retains the VSG electromechanical oscillation element, resulting in a slower power loop response. During grid faults, it cannot meet the time requirements for reactive current injection. Summary of the Invention
[0005] In view of this, the purpose of this invention is to provide a four-loop control system and low voltage ride-through control method for VSG. Based on the analysis of the changes in key inverter variables and power angle curves during grid faults, a direct voltage control LVRT method is proposed. This method retains the inverter voltage source characteristics during grid faults, directly controls the power angle and output voltage amplitude, and can complete the specified amount of power injection within the time specified by the grid connection standard.
[0006] To achieve the above objectives, the present invention provides the following technical solution:
[0007] Option 1: A four-loop control system for a VSG, comprising a power loop, a power angle loop, a voltage loop, and a current loop. The power loop supports the power system, outputting the commanded power angle and voltage amplitude. The power angle loop smoothly switches between VSG control modes, inputting the commanded power angle and outputting the commanded phase of the capacitor voltage for dq / abc coordinate transformation. The voltage and current loops form the inner loops, used to improve the transient performance of the control system.
[0008] Furthermore, in the power angle loop, the phase information of the capacitor voltage is obtained through the phase-locked loop as feedforward control and superimposed with the output of the power angle loop controller to improve the dynamic performance of the power angle loop controller during grid voltage fluctuations, so as to enable the control system to have a fast response capability.
[0009] Furthermore, the power loop includes an active loop and a reactive loop, wherein the active loop outputs the commanded power angle and the reactive loop outputs the voltage amplitude, and the voltage amplitude serves as the input to the voltage loop.
[0010] Option 2: Low-voltage ride-through method based on VSG four-loop control system. This method is based on direct voltage control (DVC) and adopts a VSG-DVC-VSG control mode switching sequence. That is, when the grid voltage is normal, the inverter uses VSG control mode, and when the grid voltage fails, the inverter uses DVC control mode to achieve low-voltage ride-through.
[0011] At the moment of failure, the inverter switches from VSG control mode to DVC control mode, keeping the output voltage amplitude and power angle of VSG unchanged from the pre-fault values; and by controlling the input of the power angle loop and voltage loop, the inverter provides power support to the grid.
[0012] At the moment of fault recovery, the inverter switches from DVC control mode to VSG control mode, and makes the input value of DVC control mode equal to the output value of VSG control mode before the grid fault to achieve a smooth switch of control mode.
[0013] Furthermore, when the inverter operates in low voltage ride-through mode using DVC control mode, the current that the inverter needs to inject into the grid and the power corresponding to the required current are calculated according to the following formula. Then, the phase difference between the required capacitor voltage and the grid voltage, i.e., the power angle δ, is calculated based on the power. fc And the required capacitor voltage amplitude U fc δ fc U fc As input to the power angle loop and voltage loop:
[0014]
[0015]
[0016]
[0017]
[0018]
[0019] In the formula, i f,q I represents the reactive current that the inverter needs to output when the grid voltage drops. N U represents the rated current of the inverter. g Indicates the grid voltage, i f,d P represents the active current that the inverter needs to output when the grid voltage drops. fr Q fr L represents the active power and reactive power injected into the grid by the inverter when the grid voltage drops. g and R g L represents the line inductance and line resistance, respectively. v and R v These represent the virtual inductance and virtual resistance, respectively, and ω represents the frequency of the inverter output voltage.
[0020] Furthermore, the output value of the VSG control mode includes the voltage amplitude E. nr and work angle δ nr The input values for DVC control mode include the power angle loop command input δ. r and the instruction input E of the voltage loop r .
[0021] The beneficial effects of this invention are as follows: This invention achieves low voltage ride-through during grid faults by switching the control mode of the grid-connected inverter. It can retain the voltage source characteristics of the inverter during grid faults, directly control the power angle and output voltage amplitude, and complete the power injection of a specified amount within the time specified by the grid connection standard. This ensures that distributed power sources can inject sufficient power into the grid during grid faults and maintain the stable and continuous operation of the grid.
[0022] Other advantages, objectives, and features of the invention will be set forth in part in the description which follows, and in part will be apparent to those skilled in the art from the following examination, or may be learned from practice of the invention. The objectives and other advantages of the invention can be realized and obtained through the following description. Attached Figure Description
[0023] To make the objectives, technical solutions, and advantages of the present invention clearer, the preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings, wherein:
[0024] Figure 1This is a schematic diagram of the VSG four-ring control system structure;
[0025] Figure 2 The timing diagram for low voltage ride-through control;
[0026] Figure 3 This is the ideal power angle curve during a power grid fault.
[0027] Figure 4 This is a schematic diagram of low voltage ride-through control based on DVC control. Detailed Implementation
[0028] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the illustrations provided in the following embodiments are only schematic representations of the basic concept of the present invention. Unless otherwise specified, the following embodiments and features can be combined with each other.
[0029] Please see Figures 1-4 This describes a four-loop control system for VSG and a low-voltage ride-through control method. For example... Figure 1 The diagram shows the model structure of a four-loop control system, established in the dq rotating coordinate system of the inverter. This invention adds a power angle loop to the standard three-loop control system to enable smooth switching between VSG and DVC modes under grid fault and normal operating conditions. Under normal operating conditions, a well-designed power angle loop has virtually no impact on VSG performance. Under fault conditions, the addition of the power angle loop facilitates DVC control and control mode switching. In the control system, the power loop supports the power system; its active power loop output is the VSG command power angle, and its reactive power loop output is the voltage amplitude. The power angle loop implements power angle control. Its input is the commanded power angle, connecting to the power loop. The power angle feedback value is the difference between the capacitor voltage phase and the grid voltage phase. The phase information is obtained through a phase-locked loop (PLL). The output of the power angle loop serves as the commanded phase of the capacitor voltage, providing the phase required for dq / abc coordinate system transformation. The capacitor voltage phase information obtained through the PLL is superimposed on the output of the power angle loop controller as feedforward control, improving the dynamic performance of the controller during grid voltage fluctuations and enabling the control system to have a fast response capability. H(s) is the transfer function of the power angle controller, which is the PI controller. The voltage loop and current loop form the inner loops of the VSG control system, with the voltage loop on the outside and the current loop on the inside. The output of the voltage loop controller is the input of the current loop controller.
[0030] The timing relationship of the VSG low voltage ride-through control method based on DVC is as follows: Figure 2 As shown, the implementation logic is as follows:
[0031] Under normal operating conditions, the inverter adopts the VSG control strategy, and the controlled variables are the feedback values of the active power P and reactive power Q output by the inverter. The VSG operates at the power angle curve I.
[0032] When a power grid fault occurs, the capacitor voltage U at the time of the fault is directly controlled. fc and work angle δ fc This refers to Direct Voltage Control (DVC), which enables features such as... Figure 3 The trajectory of the power angle is shown. At the moment of the fault, the power angle δ is maintained. fc Keep the capacitor voltage U constant and change it. fc The switching of the power angle relationship from curve I to curve IV is achieved, limiting the instantaneous current; during a fault, the power angle δ is changed. fc This allows it to slide along curve IV, or the capacitor voltage U can be changed. fc This enables the switching of the power angle curve, allowing for flexible control of the inverter's active and reactive power outputs.
[0033] After the fault is cleared, the inverter control strategy is switched from DVC to VSG, ensuring that the inverter output power remains consistent before and after the fault.
[0034] To achieve inverter power angle during grid faults according to Figure 3 The simplest way to achieve the trajectory shown is to directly control the inverter port voltage, i.e., the DVC control strategy. However, considering that the inverter must inject a specified amount of power into the grid during a fault, to achieve low-voltage ride-through with controllable transient current and steady-state power, the relationship between the inverter output power and the port voltage amplitude and power angle must first be established. According to the national standard GB / T 34120—2017, when a low-voltage fault occurs in the grid, the energy storage inverter can inject reactive current into the grid and satisfy the following conditions:
[0035]
[0036] In the formula, i f,q I represents the required reactive current output when the voltage drops. N This indicates the inverter's rated current.
[0037] When a grid fault occurs, the energy storage inverter injects reactive current into the grid, and the active current and reactive current do not exceed the rated current, i.e., the following conditions are met:
[0038]
[0039] In the formula, i f,dThis indicates the required active current output when the voltage drops.
[0040] VSG is based on grid voltage orientation, and has e q =u c,q =0, where u c,q The q-axis component of the capacitor voltage gives the required active power P during low-voltage ride-through. fr and reactive power Q fr They are respectively:
[0041]
[0042] Based on the steady-state expression for the output power of a virtual synchronous generator, if the power that the inverter needs to inject into the grid during a grid fault is known, then the power angle δ of the capacitor under this operating condition is... fc Port voltage U fc It can be represented as:
[0043]
[0044]
[0045] Based on the above formula, the relationship between the inverter output current, power angle, and port voltage when the grid voltage drops is established. Therefore, the requirements for the reactive current and active current injected into the grid by the inverter can be converted into requirements for the voltage amplitude and power angle of the inverter capacitor port.
[0046] Low-voltage ride-through based on DVC involves two mode switches: at the moment of a fault, VSG control switches to DVC control, and after the fault is cleared, DVC control switches back to VSG control. Therefore, ensuring a smooth transition between these two modes is essential for achieving low-voltage ride-through. For the former, since the DVC control strategy directly controls the inverter output voltage amplitude and power angle, reasonable control commands and system bandwidth can avoid transient impacts during switching. For the latter, there are uncertainties between the voltage amplitude and phase output by the VSG control strategy and the actual grid voltage during switching.
[0047] DVC control is essentially embedded as an inner loop within VSG control. By ensuring that the DVC control strategy commands and VSG output variables correspond equally during mode switching, a smooth transition from DVC to VSG can be achieved. The VSG low-voltage ride-through control scheme designed for this purpose is as follows: Figure 4 As shown, the specific steps are as follows:
[0048] Under normal operating conditions, signal switches S1, S2, and S3 are switched to position 1, and the inverter adopts the VSG control strategy, that is, the voltage amplitude E output by the VSG is... nr As the command input e of the voltage and current inner loop d The power angle δ of the VSG outputnr δ serves as the command input for the power angle control loop. r That is, e d =E r =E nr e q =0, δ r =δ nr .
[0049] When a grid voltage fault occurs, since the DVC loop is the inner loop of the VSG control loop, signal switches S1, S2, and S3 can be directly switched to position 2. The inverter operates in low-voltage ride-through mode using the DVC strategy, with δ r =δ fr E r =E fr P = P0, Q = Q0, where δ fr E fr These are the power angle command and voltage command in DVC mode, respectively. P and Q represent the feedback values of the inverter output power, and P0 and Q0 represent the command input of VSG. Then, the VSG output voltage amplitude and power angle can remain unchanged from the values before the fault. The power that the inverter needs to inject into the grid is calculated according to equations (1) to (3), and U is obtained through equation (4). fc δ fc As the command input for the voltage and current inner loop and the power angle loop, i.e., E fr =U fc δ fr =δ fc .
[0050] In the instant of fault recovery, δ r =δ nr e d =E nr Because of the large inner loop bandwidth, the inverter output power quickly adjusts to the pre-fault state. After one power frequency cycle of fault recovery, signal switches S1, S2, and S3 are switched to position 1. Since the power angle loop is the inner loop of the active power loop and the voltage loop is the inner loop of the reactive power loop, the output of the active power loop is equal to the input of the power angle loop, and the output of the reactive power loop is equal to the input of the voltage loop. Therefore, u i,abc No significant changes will occur, thus avoiding the impact caused by control mode switching. At this point, the rotor equation and excitation equation are balanced, and the VSG can be adjusted to its normal operating state.
[0051] Finally, it should be noted that 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 preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.
Claims
1. A four-loop control system for VSG, characterized in that: The system includes a power loop, a power angle loop, a voltage loop, and a current loop. The power loop supports the power system and outputs the commanded power angle and voltage amplitude. The power angle loop enables smooth switching of the inverter control mode during low-voltage ride-through, with the input being the commanded power angle and the output being the commanded phase of the capacitor voltage, used for dq / abc coordinate transformation. The voltage and current loops, as inner loops, improve the transient performance of the control system. When the inverter operates in low voltage ride-through mode using direct voltage control (DVC) mode, the current that the inverter needs to inject into the grid and the corresponding power are calculated according to the following formula. Then, the required capacitor voltage amplitude is calculated based on the power. The phase difference between the capacitor voltage and the grid voltage, i.e., the power angle. , and These serve as command inputs for the power angle loop and voltage loop, respectively. In the formula, This represents the reactive current that the inverter needs to output when the grid voltage drops. Indicates the inverter's rated current. Indicates the grid voltage. This indicates the active current that the inverter needs to output when the grid voltage drops. This represents the active power injected into the grid by the inverter when the grid voltage drops. This represents the reactive power injected into the grid by the inverter when the grid voltage drops. and These represent the line inductance and line resistance, respectively. and These represent virtual inductance and virtual resistance, respectively. This indicates the frequency of the inverter's output voltage.
2. The four-ring control system according to claim 1, characterized in that: In the aforementioned power angle loop, capacitor voltage phase information is obtained through a phase-locked loop as feedforward control and superimposed with the output of the power angle loop controller to improve the dynamic performance of the power angle loop controller during grid voltage fluctuations, thereby enabling the control system to have a fast response capability.
3. The four-ring control system according to claim 1, characterized in that: The power loop includes an active loop and a reactive loop. The active loop outputs a command power angle, and the reactive loop outputs a voltage amplitude. The voltage amplitude serves as the input to the voltage loop.
4. A low-voltage ride-through method for a VSG four-loop control system, applied to the four-loop control system according to any one of claims 1 to 3, characterized in that: This method is based on direct voltage control and uses a VSG-DVC-VSG control mode switching sequence. That is, when the grid voltage is normal, the inverter uses VSG control mode, and when the grid voltage fails, the inverter uses DVC control mode to achieve low voltage ride-through. At the moment of the fault, the inverter switches from VSG control mode to DVC control mode, keeping the output voltage amplitude and power angle of VSG unchanged from the value before the fault. And by controlling the inputs of the power angle loop and voltage loop, the inverter provides power support to the grid; At the moment of fault recovery, the inverter switches from DVC control mode to VSG control mode, and makes the input value of DVC control mode equal to the output value of VSG control mode before the grid fault to achieve a smooth switch of control mode.
5. The low-voltage ride-through method according to claim 4, characterized in that: The way to keep the output voltage amplitude and power angle of the VSG unchanged before the fault is to change the feedback input of the VSG from the actual power injected into the grid by the inverter to the control input of the VSG; wherein, under normal grid operating conditions, the control input is equal to the actual power injected into the grid by the inverter.
6. The low-voltage ride-through method according to claim 4, characterized in that: The output value of the VSG control mode includes voltage amplitude. E nr Harmony and angle The input values for the DVC control mode include the power angle loop command input. and voltage loop command input E r .