A control method for maintaining current limiting and stability of a virtual synchronous generator for asymmetric faults

Abstract

The application discloses a kind of control methods suitable for virtual synchronous generator to keep current limiting and stability of asymmetric fault, it is related to new energy grid connection technology field, the method first introduces double second-order generalized integrator (DSOGI) to carry out positive and negative sequence decomposition to voltage and current, constructs positive sequence current limiting channel and negative sequence suppression channel, realizes positive sequence current accurate limiting and negative sequence component effective inhibition;Second, the positive sequence equivalent network power model is established, the positive sequence active regulation coefficient is calculated and the active reference value is adaptively corrected, so that it matches the actual power transmission capacity under fault, so as to inhibit power angle deviation and oscillation;Finally, the smooth recovery strategy without mode switching is designed, and the active reference value recovery rate is dynamically adjusted according to the grid voltage recovery state.The application considers the current safety, three-phase balance, transient stability and disturbance-free recovery during fault ride-through without switching the grid-connected control, and improves the grid-connected reliability of grid-connected converter.

Description

Technical Field

[0001] This invention relates to the field of new energy grid connection technology, specifically a control method for maintaining current limiting and stability of a virtual synchronous generator suitable for asymmetrical faults. Background Technology

[0002] With the high proportion of new energy sources being integrated into the power system, grid-connected energy storage converters, due to their voltage source characteristics and virtual inertia support capabilities, have become key equipment for improving the stability of new power systems. Virtual Synchronous Generator (VSG) control technology simulates the mechanical and electromagnetic equations of a synchronous generator, enabling the power electronic converter to exhibit external characteristics similar to a synchronous machine.

[0003] However, in actual power grid operation, asymmetrical faults such as single-phase ground faults and two-phase short circuits occur frequently, leading to a significant coexistence of positive-sequence voltage drops and negative-sequence components at the grid connection point. Traditional VSG control strategies are mostly designed based on the assumption of a three-phase symmetrical ideal power grid. Under asymmetrical fault conditions, there is a strong coupling effect between the negative-sequence voltage component and the positive-sequence control loop, which can easily cause problems such as three-phase output current imbalance, output current limiting failure, and transient power angle instability, seriously threatening the grid connection safety and grid fault ride-through capability of the converter.

[0004] In the existing technology, there are three main types of independent improvement attempts for the control of VSG under asymmetric faults, but significant shortcomings still exist: 1. The current limiting control does not decouple the positive and negative sequence components, resulting in insufficient current limiting accuracy and three-phase current imbalance. Existing VSG current limiting schemes (such as virtual impedance current limiting and inner current loop saturation limiting) typically act directly on the total three-phase current or the current in the non-separated stationary coordinate system, without independently suppressing the negative sequence component. Under asymmetrical fault conditions, the negative sequence current component can still flow through the low-impedance path, leading to: The current limiting circuit can only clamp the total current amplitude and cannot accurately limit the positive sequence fundamental current; The three-phase output current is severely unbalanced, making it difficult to meet the power quality requirements of grid connection.

[0005] 2. The disconnect between current limiting and transient power angle stability control leads to power angle drift and oscillation. Most solutions only focus on current amplitude limits during faults, without simultaneously correcting the active power reference value. When the grid positive-sequence voltage drops, causing a decrease in power transmission capacity, the VSG still maintains the active power reference value before the fault, resulting in an imbalance between active power supply and demand. This drives the virtual power angle to continuously accelerate its shift, and induces severe power angle oscillations at the moment the fault is cleared, even leading to loss of synchronization.

[0006] 3. The fault recovery strategy is crude, relying on network construction / following mode switching, which poses a risk of secondary impact. Some solutions switch the VSG to grid-based current source control during a fault to achieve current limiting, and then switch back to grid-based control after the fault is cleared. This type of mode switching strategy: It disrupts the voltage source support characteristics of the VSG and weakens the inertia and damping contribution during the fault. The lack of coordination between the recovery process and the grid voltage recovery status and current limiting status can easily lead to excessively fast recovery, causing secondary overcurrent or secondary power angle oscillation. Summary of the Invention

[0007] The purpose of this invention is to provide a control method for maintaining current limiting and stability of a virtual synchronous generator suitable for asymmetrical faults, so as to solve the problems mentioned in the background art.

[0008] To achieve the above objectives, the present invention provides the following technical solution: A control method for maintaining current limiting and stability in a virtual synchronous generator suitable for asymmetrical faults, characterized by comprising the following steps: Step 1: Collect the three-phase output voltage at the grid connection point of the grid-connected energy storage converter. and three-phase output current The measured values ​​are used to obtain the voltage and current in the stationary coordinate system through Clarke transformation. Quantity: Step 2: Convert the voltage in the stationary coordinate system Current The component input is a dual second-order generalized integrator (DSOGI) positive-sequence decomposition module, which separates the voltage and current into positive-sequence components to obtain the positive-sequence voltage component. negative sequence voltage component Positive and negative sequence components of current The positive and negative sequence components of voltage are represented as follows: The positive and negative sequence components of the current are represented as follows: This represents the orthogonal signal operator generated by DSOGI, based on the internal potential amplitude output by the virtual synchronous generator. The voltage in the stationary coordinate system is obtained by transformation, and then input into the positive and negative sequence decomposition module of the dual second-order generalized integrator DSOGI to separate the voltage into positive and negative sequences, thereby obtaining the positive-sequence virtual internal potential. ; Step 3: Subtract the positive-sequence virtual internal potential component from the positive-sequence voltage component at the grid connection point to obtain the voltage across the positive-sequence virtual impedance. Then, input the voltage across the positive-sequence virtual impedance into the virtual impedance circuit to obtain the positive-sequence current reference component before current limiting. in, For virtual resistance, The virtual inductance is represented by a virtual resistance. and virtual inductance Composition, its plural form is ,in The virtual impedance amplitude is The virtual impedance angle is ,in For virtual sensory resistance, The system angular frequency; Step 4: Establish the positive sequence current reference amplitude based on the positive sequence current component: in, These are the positive sequence current reference components before current limiting, and when the positive sequence current reference amplitude... Less than or equal to the preset current limit value When, the positive sequence current reference value remains unchanged; when Greater than the preset current limit value When using a vector amplitude current limiting method to limit the positive sequence current reference quantity, the expression is as follows: in, These are the positive sequence current reference components after current limiting. This current limiting method limits the amplitude of the positive sequence current reference vector while keeping the phase angle of the positive sequence current reference vector unchanged, so that the output current of the positive sequence control channel does not exceed the preset current limiting range. In the negative sequence suppression channel, the negative sequence current reference value is set to zero, that is: A negative sequence compensation control quantity is generated through a negative sequence current control loop to suppress the negative sequence component in the output current. Step 5: Calculate the positive sequence current limiting factor Determined based on the positive sequence current reference vector magnitude and the preset current limit value, its expression is: Step 6: Calculate the positive-sequence equivalent active power and positive sequence active power regulation coefficient During a fault, the active power transmission capacity under the positive-sequence equivalent network Represented as: In the formula, This represents the amplitude of the positive-sequence component of the grid voltage during the fault period. The amplitude of the virtual synchronous generator potential. This is the virtual impedance amplitude. This is the virtual impedance angle. For the grid-side equivalent inductive reactance, where the positive-sequence virtual power angle is... Defined as: In the formula, The phase angle of the internal potential of the virtual synchronous generator. The phase angle of the positive sequence component of the grid voltage. The positive sequence active power regulation coefficient The ratio of transmittable active power during a fault in a positive-sequence equivalent network to the reference value of active power before the fault is expressed as: In the formula, The active power reference value before the fault is used, based on the positive sequence active power adjustment coefficient. The corrected active power reference value during the fault period is: Step 7: After the power grid fault is cleared, adjust the positive-sequence active power regulation coefficient at the instant the fault is cleared. The system recovers the active power reference value smoothly under the conditions of positive sequence voltage recovery and current limiting in the power grid. When the system is in a state of minor fault, a ramp recovery method is used: where, This is the time to clear the fault. For the active power recovery slope, when Increase to the reference value of active power before the fault season: And maintain normal operation. When the system is in a moderate or severe fault state and a first-order inertial recovery method is used, the active power reference value satisfies: in, The active power recovery time constant is adjusted by... The size of the value ensures a smooth recovery of the active power reference value after the fault is cleared, preventing the output current from re-entering the current-limiting state due to an excessively fast recovery process. Meanwhile, the positive sequence active power adjustment coefficient After the fault is cleared, the system gradually recovers to level 1. The recovery process is represented as follows: in, This represents the positive-sequence active power regulation coefficient at the time of fault clearance. To adjust the coefficient and restore the slope, when When restoring to 1, let: Then exit the fault recovery state.

[0009] Compared with the prior art, the beneficial effects of the present invention are: (1) Positive and negative sequence decoupling current limiting significantly improves current limiting accuracy and current quality. By introducing a dual second-order generalized integrator (DSOGI) positive-sequence decomposition module into the VSG control loop, voltage and current are decoupled into positive-sequence control channels and negative-sequence suppression channels, respectively. The current limiting circuit only acts on the positive sequence current reference vector and uses a vector amplitude scaling method to accurately limit the positive sequence current amplitude while keeping the phase angle unchanged. The negative sequence current reference value is forced to zero and is independently suppressed by the negative sequence control loop, effectively eliminating the pollution of the three-phase current by the negative sequence component, ensuring the balance of the three-phase output current during the fault period, and meeting the grid connection specifications.

[0010] (2) Active power reference adaptive matching positive sequence transmission capability enhances transient power angle stability. A positive-sequence equivalent network power transmission model considering virtual impedance and grid equivalent inductive reactance is constructed. The positive-sequence equivalent transmissible active power is calculated, and a positive-sequence active power adjustment coefficient is generated accordingly to dynamically correct the active power reference value during fault periods. This ensures that active power commands are always matched with the grid's transmission capacity after a power drop, eliminating the imbalance between active power supply and demand at the source. It effectively suppresses the acceleration shift of the virtual power angle and the power angle oscillation after fault clearing, and significantly improves the transient stability of VSG under asymmetric faults.

[0011] (3) The smooth recovery strategy avoids secondary impacts and maintains the network structure characteristics throughout the process. Design a smooth recovery strategy for fault clearing, and adaptively select either ramp recovery or first-order inertial recovery mode based on the degree of positive sequence voltage recovery of the power grid, current limiting status, and kz changes: The recovery process does not require switching from grid-based control to grid-following control, and the voltage source characteristics and inertia support capability of the VSG are preserved throughout the process; By adjusting the recovery slope and time constant, secondary overcurrent and power angle oscillation caused by excessively fast recovery are avoided, thus achieving a seamless switch from fault conditions to normal operation.

[0012] (4) Multi-objective collaborative optimization, with overall performance superior to a single improvement scheme. This invention organically integrates four control objectives—current limiting control, negative sequence suppression, power angle stabilization, and smooth recovery—into the same VSG control framework, solving the problems of conflicting or incomplete objectives in existing technologies. It achieves controllable current, power balance, power angle stabilization, and disturbance-free recovery throughout the fault ride-through process, making it particularly suitable for grid-connected energy storage converters in scenarios with a high proportion of new energy access. Attached Figure Description

[0013] Figure 1 This is a schematic diagram of the method of the present invention.

[0014] Figure 2 This is a flowchart of the fault clearing and smooth recovery strategy in an embodiment of the present invention. Detailed Implementation

[0015] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0016] In this embodiment of the invention, a control method for maintaining current limiting and stability of a virtual synchronous generator suitable for asymmetrical faults is as follows: Figure 1 This paper addresses the application scenario of grid-connected energy storage converters operating continuously in the grid under asymmetrical grid voltage dip faults. Its key feature is the proposal of a control method that, based on the virtual synchronous generator current-limiting stability control method, incorporates a dual second-order generalized integrator positive-sequence decomposition (DSOGI) control module. This module decomposes the voltage and current components during the fault into positive-sequence and negative-sequence components, constructing separate positive-sequence control and negative-sequence suppression channels. This allows the current-limiting control to apply to the positive-sequence current reference while suppressing the negative-sequence current component, thereby improving the three-phase current imbalance and inaccurate output current limiting caused by negative-sequence component coupling under asymmetrical faults in the original current-limiting stability control method. Furthermore, a positive-sequence active power reference adaptive adjustment module is added to calculate the positive-sequence active power adjustment coefficient. And using the positive sequence active power adjustment coefficient The active power reference value during the fault period is corrected to match the actual power transmission capacity under the positive-sequence equivalent network, thereby improving the transient stability angle offset and power angle oscillation problems during asymmetric faults and fault clearing processes; then, a fault clearing smooth recovery strategy is added, such as... Figure 2 Based on the positive sequence voltage recovery state, current limiting state, and positive sequence active power regulation coefficient of the power grid The system automatically adjusts the recovery process of the active power reference value according to the changes in the power supply, so that the converter can simultaneously achieve asymmetrical fault current limitation, negative sequence current suppression and transient power angle stability improvement without switching to grid-based control.

[0017] Step 1: Collect the three-phase output voltage at the grid connection point of the grid-connected energy storage converter. and three-phase output current The measured values ​​are used to obtain the voltage and current in the stationary coordinate system through Clarke transformation. Quantity: Step 2: Convert the voltage in the stationary coordinate system Current The component input is a dual second-order generalized integrator (DSOGI) positive-sequence decomposition module, which separates the voltage and current into positive-sequence components to obtain the positive-sequence voltage component. negative sequence voltage component Positive and negative sequence components of current The positive and negative sequence components of the voltage are represented as follows: The positive and negative sequence components of the current are represented as follows: This represents the orthogonal signal operator generated by DSOGI. It is based on the internal potential amplitude output by the virtual synchronous generator. The voltage in the stationary coordinate system is obtained by transformation. Then, it is input into the positive and negative sequence decomposition module of the dual second-order generalized integrator DSOGI to separate the voltage into positive and negative sequences, thereby obtaining the positive-sequence virtual internal potential. .

[0018] Step 3: Subtract the positive-sequence virtual internal potential component from the positive-sequence voltage component at the grid connection point to obtain the voltage across the positive-sequence virtual impedance. Then, input the voltage across the positive-sequence virtual impedance into the virtual impedance circuit to obtain the positive-sequence current reference component before current limiting. in, For virtual resistance, This is a virtual inductance. The virtual impedance is composed of a virtual resistance. and virtual inductance Composition, its plural form is ,in The virtual impedance amplitude is The virtual impedance angle is ,in For virtual sensory resistance, This is the system angular frequency.

[0019] Step 4: Establish the positive sequence current reference amplitude based on the positive sequence current component: in, These are the positive sequence current reference components before current limiting. When the positive sequence current reference amplitude... Less than or equal to the preset current limit value When, the positive sequence current reference value remains unchanged; when Greater than the preset current limit value When using a vector amplitude current limiting method to limit the positive sequence current reference quantity, the expression is as follows: in, These are the positive sequence current reference components after current limiting. This current limiting method limits the amplitude of the positive sequence current reference vector while keeping the phase angle unchanged, ensuring that the output current of the positive sequence control channel does not exceed the preset current limiting range. In the negative sequence suppression channel, the negative sequence current reference value is set to zero, i.e.: A negative sequence compensation control quantity is generated through a negative sequence current control loop to suppress the negative sequence component in the output current.

[0020] Step 5: Calculate the positive sequence current limiting factor Determined based on the positive sequence current reference vector magnitude and the preset current limit value, its expression is: Step 6: Calculate the positive-sequence equivalent active power and positive sequence active power regulation coefficient During a fault, the active power transmission capacity under the positive-sequence equivalent network... Represented as: In the formula, This represents the amplitude of the positive-sequence component of the grid voltage during the fault period. The amplitude of the virtual synchronous generator potential. This is the virtual impedance amplitude. This is the virtual impedance angle. For the grid-side equivalent inductive reactance, where the positive-sequence virtual power angle is... Defined as: In the formula, The phase angle of the internal potential of the virtual synchronous generator. This represents the phase angle of the positive sequence component of the grid voltage.

[0021] The positive sequence active power regulation coefficient The ratio of transmittable active power during a fault in a positive-sequence equivalent network to the reference value of active power before the fault is expressed as: In the formula, This is the reference value of active power before the fault. Based on the aforementioned positive-sequence active power adjustment coefficient... The corrected active power reference value during the fault period is: Step 7: After the power grid fault is cleared, adjust the positive-sequence active power regulation coefficient at the instant the fault is cleared. The system performs smooth recovery control of active power reference values ​​during grid positive sequence voltage recovery and current limiting states. Figure 2 As shown.

[0022] When the system is in a state of minor fault, a ramp recovery method is used: where, This is the time to clear the fault. The active power recovery slope. When Increase to the reference value of active power before the fault season: And maintain normal operation.

[0023] When the system is in a moderate or severe fault state and a first-order inertial recovery method is used, the active power reference value satisfies: in, This is the active power recovery time constant. It can be adjusted... The size of the value ensures a smooth recovery of the active power reference value after the fault is cleared, preventing the output current from re-entering the current-limiting state due to an excessively fast recovery process.

[0024] Meanwhile, the positive sequence active power adjustment coefficient After the fault is cleared, the system gradually recovers to 1. The recovery process is represented as follows: in, This represents the positive-sequence active power regulation coefficient at the time of fault clearance. To adjust the coefficient and restore the slope. When When restoring to 1, let: Then exit the fault recovery state.

[0025] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

[0026] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A control method for maintaining current limiting and stability in a virtual synchronous generator suitable for asymmetrical faults, characterized in that, Includes the following steps: Step 1: Collect the three-phase output voltage at the grid connection point of the grid-connected energy storage converter. and three-phase output current The measured values ​​are used to obtain the voltage and current in the stationary coordinate system through Clarke transformation. Quantity: Step 2: Convert the voltage in the stationary coordinate system Current The component input is a dual second-order generalized integrator (DSOGI) positive-sequence decomposition module, which separates the voltage and current into positive-sequence components to obtain the positive-sequence voltage component. negative sequence voltage component Positive and negative sequence components of current The positive and negative sequence components of voltage are represented as follows: The positive and negative sequence components of the current are represented as follows: This represents the orthogonal signal operator generated by DSOGI, based on the internal potential amplitude output by the virtual synchronous generator. The voltage in the stationary coordinate system is obtained by transformation, and then input into the positive and negative sequence decomposition module of the dual second-order generalized integrator DSOGI to separate the positive and negative sequence of the voltage and obtain the positive sequence virtual internal potential. ; Step 3: Subtract the positive-sequence virtual internal potential component from the positive-sequence voltage component at the grid connection point to obtain the voltage across the positive-sequence virtual impedance. Then, input the voltage across the positive-sequence virtual impedance into the virtual impedance circuit to obtain the positive-sequence current reference component before current limiting. in, For virtual resistance, The virtual inductance is represented by a virtual resistance. and virtual inductance Composition, its complex form is ,in The virtual impedance amplitude is The virtual impedance angle is ,in For virtual sensory resistance, The system angular frequency; Step 4: Establish the positive sequence current reference amplitude based on the positive sequence current component: in, These are the positive sequence current reference components before current limiting, and when the positive sequence current reference amplitude... Less than or equal to the preset current limit value When, the positive sequence current reference value remains unchanged; when Greater than the preset current limit value When using a vector amplitude current limiting method to limit the positive sequence current reference quantity, the expression is as follows: in, These are the positive sequence current reference components after current limiting. This current limiting method limits the amplitude of the positive sequence current reference vector while keeping the phase angle of the positive sequence current reference vector unchanged, so that the output current of the positive sequence control channel does not exceed the preset current limiting range. In the negative sequence suppression channel, the negative sequence current reference value is set to zero, that is: A negative sequence compensation control quantity is generated through a negative sequence current control loop to suppress the negative sequence component in the output current. Step 5: Calculate the positive sequence current limiting factor Determined based on the positive sequence current reference vector magnitude and the preset current limit value, its expression is: Step 6: Calculate the positive-sequence equivalent active power and positive sequence active power regulation coefficient During a fault, the active power transmission capacity under the positive-sequence equivalent network Represented as: In the formula, This represents the amplitude of the positive-sequence component of the grid voltage during the fault period. The amplitude of the virtual synchronous generator potential. This is the virtual impedance amplitude. This is the virtual impedance angle. For the grid-side equivalent inductive reactance, where the positive-sequence virtual power angle is... Defined as: In the formula, The phase angle of the internal potential of the virtual synchronous generator. The phase angle of the positive sequence component of the grid voltage. The positive sequence active power regulation coefficient The ratio of transmittable active power during a fault in a positive-sequence equivalent network to the reference value of active power before the fault is expressed as: In the formula, The active power reference value before the fault is used, based on the positive sequence active power adjustment coefficient. The corrected active power reference value during the fault period is: Step 7: After the power grid fault is cleared, adjust the positive-sequence active power regulation coefficient at the instant the fault is cleared. The system recovers the active power reference value smoothly under the conditions of positive sequence voltage recovery and current limiting in the power grid. When the system is in a state of minor fault, a ramp recovery method is used: where, This is the time to clear the fault. For the active power recovery slope, when Increase to the reference value of active power before the fault season: And maintain normal operation. When the system is in a moderate or severe fault state and a first-order inertial recovery method is used, the active power reference value satisfies: in, The active power recovery time constant is adjusted by... The size of the value ensures a smooth recovery of the active power reference value after the fault is cleared, preventing the output current from re-entering the current-limiting state due to an excessively fast recovery process. Meanwhile, the positive sequence active power adjustment coefficient After the fault is cleared, the system gradually recovers to level 1. The recovery process is represented as follows: in, This represents the positive-sequence active power regulation coefficient at the time of fault clearance. To adjust the coefficient and restore the slope, when When restoring to 1, let: Then exit the fault recovery state.

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