A network construction energy storage system for off-grid operation of offshore wind power and a stable operation control method
By using a grid-type energy storage system and virtual impedance control, the problems of power generation fluctuations and fault overcurrents in the off-grid operation of offshore wind power have been solved, achieving stable and efficient power management.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- POWERCHINA HUADONG ENG CORP LTD
- Filing Date
- 2026-04-03
- Publication Date
- 2026-07-14
AI Technical Summary
In the off-grid operation of offshore wind power, the energy storage system is affected by wind speed and marine environment, resulting in large fluctuations in power generation. Lithium batteries have short lifespans and high costs. Traditional control strategies cannot effectively balance supply and demand, and are prone to overcurrent problems in case of failure.
A grid-type energy storage system is adopted, which connects multiple offshore wind turbines in parallel, combined with an H-bridge cascaded energy storage grid-connected converter and virtual impedance control to achieve stable operation.
It effectively mitigates wind power fluctuations, improves system stability and anti-interference capabilities during faults, reduces fault current, and enhances the energy conversion efficiency and expansion convenience of energy storage systems.
Smart Images
Figure CN122394021A_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of power electronics technology and grid-based control technology, specifically relating to a grid-based energy storage system and a stable operation control method for offshore wind power operating off-grid. Background Technology
[0002] The international community is accelerating its transition to a cleaner, low-carbon energy structure. Offshore wind power, with its abundant resources, high power generation efficiency, and lack of land occupation, has become a key area for renewable energy development. According to data from the International Energy Agency (IEA), global offshore wind power capacity has maintained rapid growth in recent years. However, traditional offshore wind power largely operates on a grid-connected model, relying on a robust power grid infrastructure to transmit electricity to onshore load centers. This can be difficult to meet actual needs in certain scenarios, leading to the development of off-grid operation models.
[0003] Off-grid operation of offshore wind power is mainly used on remote islands and marine engineering facilities, mostly in areas far from the mainland. These scenarios are often in remote locations, and extending the onshore power grid is extremely costly. If traditional power supply methods such as diesel generators are relied upon for a long time, not only will the energy supply be unstable and the operating costs high, but it will also generate a large amount of pollutant emissions. However, offshore wind power is significantly affected by natural factors such as wind speed, waves, and tides, and its output is highly intermittent and random. Off-grid systems cannot rely on onshore power grids for peak shaving and must rely entirely on energy storage systems to balance supply and demand. At the same time, in the high salt spray and high humidity marine environment, the current mainstream energy storage technology, lithium batteries, has obvious shortcomings: shortened lifespan and high cost for large-capacity energy storage. To solve this problem, a collaborative control technology for a grid-based energy storage system is proposed to support the stable operation of off-grid offshore wind power.
[0004] With continuous breakthroughs in offshore wind power technology leading to reduced development costs and increasing power generation efficiency, the issue of energy consumption is becoming increasingly prominent. To address this, power grids are planning high-voltage transmission lines and flexible DC transmission lines, but construction costs remain a concern. This patent proposes a new solution to the energy consumption problem for off-grid operation of offshore wind power by utilizing energy storage technology. Excess energy generated within a certain period is stored through a PCS (Power Conversion System), where improved grid-based control ensures stable off-grid operation of the energy storage system in the face of fluctuations in offshore wind power generation or system failures, reducing voltage and power fluctuations. If the PCS adopts grid-based control, it is equivalent to a voltage source externally, and the output current is highly dependent on the external system state. When the system experiences grounding or other faults, overcurrent problems arise. Currently, overcurrent is mainly limited by current limiters. Although this strategy can accurately control fault current, the converter changes from grid-based control to grid-following control, losing its ability to build upon grid inertia and frequency. Summary of the Invention
[0005] The first objective of this invention is to provide a grid-connected energy storage system for offshore wind power off-grid operation, addressing the aforementioned problems.
[0006] To achieve the above objectives, the present invention adopts the following technical solution:
[0007] A grid-connected energy storage system for offshore wind power off-grid operation comprises multiple offshore wind turbines connected in parallel. The offshore wind turbines are connected to an AC collection submarine cable via a DC / DC converter on the turbine side and an AC / DC converter on the grid side. The AC collection submarine cable is connected to the AC busbar on the turbine side via a transformer on the grid side. The AC busbar on the turbine side transmits power to the AC grid through a flexible DC transmission system on one hand, and is connected to an energy storage battery through an energy storage grid-connected converter on the other hand.
[0008] While adopting the above technical solutions, the present invention may also adopt or combine the following technical solutions:
[0009] As a preferred technical solution of the present invention: the energy storage grid-connected converter switches between grid-connected and off-grid operation states through a flexible DC side circuit breaker.
[0010] As a preferred technical solution of the present invention: under the grid-connected operation mode of the energy island, a constant power control strategy is adopted, and the active power and reactive power commands are given by the energy island centralized control system; under the off-grid operation mode, a droop control strategy with additional virtual impedance is adopted to control the AC bus voltage on the wind turbine side.
[0011] As a preferred technical solution of the present invention: the wind turbine-side DC / DC converter, in the grid-connected operation mode of the energy island, adopts a maximum power point tracking control strategy, which enables the wind turbine to track the maximum power point at different wind speeds by real-time detection of wind speed and tracking the maximum power generation curve.
[0012] As a preferred technical solution of the present invention: the energy storage grid-connected converter adopts an H-bridge cascaded structure, which is divided into 3 phases and 3 bridge arms. Each bridge arm consists of dozens to hundreds of power modules and a filter inductor connected in series. The sub-modules adopt a full-bridge topology structure, including power modules and battery modules.
[0013] The second objective of this invention is to provide a stable operation control method for a grid-connected energy storage system operating off-grid in offshore wind power.
[0014] Therefore, the above-mentioned objective of the present invention is achieved through the following technical solution:
[0015] A stable operation control method for an offshore wind power off-grid energy storage system, based on the aforementioned system, includes a power droop control stage. The power droop control comprises two parts: active power-frequency droop control and reactive power-voltage droop control. The active power-frequency control employs a virtual synchronous machine strategy, while the reactive power-voltage control employs a QV droop strategy, as shown in the following formula:
[0016]
[0017] In the formula, J is the virtual inertia constant; D is the damping coefficient; n is the reactive droop coefficient; P ref and Q ref For input active power and input reactive power reference values; P and Q are output active power and output reactive power; ω0 and ω are rated angular frequency and virtual angular frequency; U n This is the rated voltage at the grid connection point.
[0018] As a preferred embodiment of the present invention: the inner loop of the droop control loop adopts a decoupled control current loop, and at the beginning of the current loop of the controller, the mutually coupled components in the actual circuit are introduced. First, place it into the loop, and make sure the sign is exactly the opposite of the sign of the coupling term in the circuit model to cancel out the coupling terms and eliminate the coupling effect.
[0019] As a preferred embodiment of the present invention, the control method further includes reducing the fault current by adding a virtual impedance in the control stage.
[0020] This invention provides a grid-connected energy storage system and a stable operation control method for offshore wind power operating off-grid, which has the following beneficial effects:
[0021] 1) This invention combines an offshore wind power off-grid operation system with an energy storage system. Utilizing the margin of the energy storage system, it enables the on-grid / off-grid conversion of the offshore wind power system. The energy storage system smooths out power fluctuations caused by wind energy fluctuations, achieving off-grid operation. Simultaneously, the energy storage system employs a multi-level cascaded H-bridge converter technology (direct-connected energy storage system). Its modular cascaded topology, compared to low-voltage parallel energy storage systems that step up the voltage to the grid via transformers, provides advantages such as high energy conversion efficiency, convenient construction and expansion, no need for multi-level control command distribution, and fast response speed.
[0022] 2) This invention enables the grid-connected converter to simulate the output characteristics of a synchronous generator based on power droop control, possessing frequency regulation capabilities similar to a synchronous generator, and providing damping and inertia support for the system. Based on the phase-locked loop principle and active-frequency droop control, the angular frequency under different states is obtained; based on reactive-voltage droop control, a converter output voltage reference is obtained, enabling the converter to synchronously follow the reference voltage.
[0023] 3) This invention is based on grid-type control, which provides inertial and damping support to the system. At the same time, it adds virtual impedance to the traditional droop control, which effectively reduces the fault power and the transient inrush current at the moment of the fault, reduces the overall fluctuation of the power grid when a fault occurs, and improves the system's anti-interference capability. Attached Figure Description
[0024] Figure 1 This is a topology diagram of the grid-connected and off-grid energy storage structure of the offshore wind turbine power transmission system provided by the present invention.
[0025] Figure 2 This is a topology diagram of an H-bridge cascaded battery energy storage system.
[0026] Figure 3 The equivalent circuit and droop control block diagram of the CHB energy storage grid-connected converter.
[0027] Figure 4 This is a block diagram of the closed-loop control of voltage and current for an energy storage grid-connected converter.
[0028] Figure 5 The control block diagram after introducing virtual impedance into the converter.
[0029] Figure 6 The equivalent circuit diagram for connecting the converter to the virtual impedance.
[0030] Figure 7 This is the equivalent circuit diagram of a grid-type energy storage system.
[0031] In the diagram: 1-Offshore wind turbine; 2-Wind turbine-side DC / DC converter; 3-Wind turbine grid-side AC / DC converter; 4-Wind turbine grid-side transformer; 5-AC grid-side transformer; 6-Wind turbine-side MMC converter station; 7-Onshore MMC converter station; 8-Grid-side transformer; 9-AC grid; 10-Energy storage grid-connected converter; 11-Energy storage battery; 12-Wind turbine-side AC bus. Detailed Implementation
[0032] The present invention will be described in further detail with reference to the accompanying drawings and specific embodiments.
[0033] like Figure 1As shown, an off-grid energy storage system for offshore wind power consists of multiple offshore wind turbines 1 connected in parallel. The offshore wind turbines 1 are connected to an AC collection submarine cable via a wind turbine-side DC / DC converter 2 and a wind turbine-side AC / DC converter 3. The AC collection submarine cable is connected to the wind turbine-side AC bus 12 via a wind turbine-side transformer 4. The wind turbine-side AC bus 12 transmits energy to the AC grid 9 through a flexible DC transmission system on one hand, and is connected to the energy storage battery 11 through an energy storage grid-connected converter 10 on the other hand. This realizes bidirectional energy flow from the wind turbine to the energy storage grid, from the onshore grid to the energy storage battery, and from the wind turbine to the onshore grid.
[0034] The flexible direct transmission system includes an AC grid-side transformer 5, a wind turbine-side MMC converter station 6, an onshore MMC converter station 7, and a grid-side transformer 8. The electricity generated by the offshore wind turbine 1 is transmitted to the onshore MMC converter station 7 via a DC submarine cable through the wind turbine-side MMC converter station 6. The onshore MMC converter station 7 is then stepped down by the grid-side transformer 8 and connected to the AC power grid 9.
[0035] The energy storage grid-connected converter switches between grid-connected and off-grid operation modes via a flexible DC-side circuit breaker. In grid-connected operation on the energy island, a constant power control strategy is employed, with active and reactive power commands provided by the energy island's centralized control system. In off-grid operation, a droop control strategy with added virtual impedance is used to control the AC bus voltage on the wind turbine side. The wind turbine-side DC / DC converter, in grid-connected operation on the energy island, employs a maximum power point tracking (MPPT) control strategy. By real-time wind speed monitoring and tracking the maximum power generation curve, the wind turbine can operate at its maximum power point under different wind speeds.
[0036] The energy storage grid-connected converter adopts an H-bridge cascaded structure. Cascaded H-bridge energy storage grid-connected converters can be directly connected to 10 kV and above power grids without the need for a step-up transformer, making it a preferred solution for realizing ultra-large capacity energy storage systems. The voltage and current stress of a single H-bridge unit is low, and the multi-cascaded level structure allows the power conversion system to maintain excellent harmonic performance at low switching frequencies. Its modular design gives it high operating efficiency and easy implementation of redundancy protection. Its topology is as follows: Figure 1As shown, the system is divided into three phases and three arms. Each arm consists of dozens to hundreds of power modules and a filter inductor connected in series. The sub-modules adopt a full-bridge topology, including power modules and battery modules. When selecting the number of power module sub-modules N, the range of values for the number of power modules should be determined based on the grid voltage level requirements, grid strength coefficient, and battery SOC coefficient. Then, the upper and lower limits of the number of modules should be determined based on the system's reliability requirements. A mathematical model of the converter should then be established, and calculations should be performed using models with different numbers of modules, and their stability analyzed. Finally, the number of power modules should be determined based on the converter efficiency and system reliability. The converter controls the switching of the sub-module's switching devices to achieve the transformation of the sub-module's output level. From the equivalent model of the converter, based on Kirchhoff's voltage law and after coordinate transformation, the following equation can be obtained:
[0037]
[0038]
[0039] After voltage-current decoupling control, the following can be obtained: Figure 4 The diagram shows a dual closed-loop control block diagram. Decoupling control ensures that the d-axis and q-axis components of the inverter's output voltage and current do not interfere with each other, reducing control errors and accelerating the system's dynamic response.
[0040] At the moment a grid fault occurs, because the output voltage of the energy storage grid-connected converter cannot change instantaneously, a significant voltage difference will exist between the converter output and the grid. For example... Figure 7 As shown, the voltage difference between the converter output and the power grid can be expressed as U. s -U g This voltage difference will be applied to the output impedance Z. C and grid-side impedance Z g The fault current can be large, damaging components and causing system instability. Traditional control methods cannot solve this problem, so a fault current limiting strategy is needed. In this case, a virtual impedance is introduced into the network-type control described above to limit the fault current and enhance the system's anti-interference capability.
[0041] A stable operation control method for an offshore wind power off-grid energy storage system is disclosed. This method employs a grid-connected PCS system, consisting of DC / DC and DC / AC modules. The energy storage battery is located on the low-voltage side of the DC / DC converter and connected to the DC / AC converter's DC side via a DC / DC boost converter. The AC side is connected to the AC bus. This connection method results in a low DC-side voltage, requires fewer batteries to be connected in series, simplifies converter modulation strategy design, and increases battery capacity utilization. Based on the aforementioned system, the method includes a power droop control loop. This power droop control comprises active-frequency droop control and reactive-voltage droop control. It also includes reducing fault current by adding a virtual impedance to the control loop and limiting the converter's output current by adding a virtual impedance Z(s) to the control loop to reduce the converter's output voltage. The active-frequency control uses a virtual synchronous machine strategy, and the reactive-voltage control uses a QV droop strategy, as shown in the following formula:
[0042]
[0043] In the formula, J is the virtual inertia constant; D is the damping coefficient; n is the reactive droop coefficient; P ref and Q ref For input active power and input reactive power reference values; P and Q are output active power and output reactive power; ω0 and ω are rated angular frequency and virtual angular frequency; U n This is the rated voltage at the grid connection point. In a synchronous generator, constrained by its own physical parameters, its moment of inertia is usually fixed, and the generator inertia time constant calculated from it is also a constant value. However, in a converter, the control parameters have greater flexibility and can be adjusted as needed, without being limited by the inherent mode of the synchronous generator.
[0044] Active power and reactive power are calculated using the following formula:
[0045]
[0046]
[0047] In the formula u d u q i d i q The converter outputs three-phase voltage U pccx and three-phase current I pccx (x=a,b,c) The d-axis and q-axis components obtained after the dq transformation. P ’ Q ’ The output active power and output reactive power are obtained after filtering.
[0048] The inner loop of the droop control loop adopts a decoupled control current loop. At the beginning of the controller's current loop, the coupling elements in the actual circuit are introduced. By first placing the variable into the loop and ensuring its sign is exactly opposite to that of the coupling term in the circuit model, the coupling terms can be canceled out, eliminating the coupling effect. Decoupling control ensures that the d-axis and q-axis components of the inverter's output voltage and current do not interfere with each other, reducing control error.
[0049] Example
[0050] In this embodiment of the invention, the offshore wind turbine power transmission system is as follows: Figure 1 As shown, it includes an offshore wind turbine 1, a wind turbine-side DC / DC converter 2, a wind turbine-grid-side AC / DC converter 3, a wind turbine-grid-side transformer 4, an AC grid-side transformer 5, a wind turbine-side MMC converter station 6, an onshore MMC converter station 7, a grid-side transformer 8, an AC grid 9, an energy storage grid-connected converter 10, an energy storage battery 11, and a wind turbine-side AC bus 12.
[0051] Offshore wind turbine 1 is connected to an AC collection submarine cable via a turbine-side DC / DC converter 2 and a grid-side AC / DC converter 3. The AC collection submarine cable is then connected to the turbine-side AC bus 12 via a transformer. The AC bus transmits power to the AC grid via a flexible DC transmission system and is also connected to a battery via an energy storage grid-connected converter. In this embodiment, the energy storage grid-connected converter uses the following method: Figure 2 The diagram illustrates a two-stage isolated H-bridge cascaded energy storage system topology. In off-grid operation of offshore wind turbines, the energy storage converter employs a virtual impedance-improved droop control strategy, with active and reactive power commands provided by the centralized control system. This invention, based on power droop control, enables the grid converter to possess frequency regulation capabilities similar to a synchronous generator, providing damping and inertia support for the system. According to active-frequency droop control, the angular frequency under different conditions is obtained; according to reactive-voltage droop control, when the wind turbine system voltage drops, the converter output quickly follows the fluctuation, ensuring that the converter output voltage remains synchronized with the wind turbine system voltage. The control strategy of this invention is power droop control, which, by simulating the operation of a synchronous generator, enables the grid converter to possess frequency regulation capabilities similar to a synchronous generator, providing damping and inertia support for the system. Based on active power-frequency droop control, the angular frequency of different states is obtained; based on reactive power-voltage droop control, when the voltage of the wind turbine system drops, the converter output can quickly follow the fluctuation to ensure synchronization with the voltage of the wind turbine system. When the wind turbine system fails, the converter maintains system stability through a process similar to the primary frequency regulation of a generator.
[0052] Depend on Figure 3 The equivalent model of the converter shown can be obtained by applying Kirchhoff's voltage law and performing coordinate transformation as follows:
[0053]
[0054]
[0055] At the beginning of the controller's current loop, introduce the mutually coupled components from the actual circuit. First, the loop is placed in the circuit, and its sign is exactly opposite to that of the coupling term in the circuit model. This cancels out the coupling terms and eliminates their coupling effects. After voltage-current decoupling control, the following can be obtained: Figure 4 The diagram shows a dual closed-loop control block diagram. Decoupling control ensures that the d-axis and q-axis components of the inverter's output voltage and current do not interfere with each other, reducing control errors and accelerating the system's dynamic response.
[0056] If a grid-side fault occurs in the wind turbine system, the output voltage of the energy storage grid-connected converter cannot change instantaneously, resulting in a significant voltage difference between the converter output and the grid. This voltage difference applies to both the output impedance and the grid-side impedance, leading to a large fault current that can easily damage power devices. Therefore, a fault current limiting strategy is introduced, which reduces the fault current by adding a virtual impedance to the control loop. By adding a virtual impedance Z(s) to the control loop, the converter's output voltage is reduced, thus limiting the converter's output current. Without adding a virtual impedance, the output current is:
[0057]
[0058] In the formula i gabc For the converter grid-connected current, U refabc U is the reference value for the three-phase output voltage of the converter. gabc Let G(s) be the grid-side voltage, G(s) be the converter control transfer function, and Z0(s) and Z2(s) be the voltage across the grid. g (s) represent the converter output impedance and the grid connection impedance, respectively.
[0059] By adding a virtual impedance in the control loop, it is equivalent to adding impedance between the converter and the wind turbine system grid, thereby reducing the fault current. The output current is then calculated using the following formula:
[0060]
[0061] Based on the above analysis, the reactive power control block diagram of the energy storage grid-connected converter system after introducing virtual impedance can be obtained as follows: Figure 5 As shown.
[0062] This control method effectively suppresses noise and distortion harmonics in the inverter's output current. By introducing a virtual inductance into the control system, the inverter's output impedance becomes inductive and its magnitude is controllable, significantly improving its robustness in parallel operation. Adjusting the magnitude and type of the virtual impedance can change the inverter's output impedance angle, allowing the system to remain stable over a wider impedance range, which is beneficial for maintaining the stability of weak power grid systems. Figure 1 As shown, when multiple inverters operate in parallel, even slight differences in output voltage amplitude and frequency can generate circulating current, increasing losses and affecting system efficiency. Figure 6 As shown, the virtual impedance is equivalent to connecting an "adjustable resistor" in series at the output of each inverter, which can increase the impedance value of the circulating current loop, significantly reduce the amplitude of the circulating current, and improve system stability.
[0063] In summary, the technical solution of this invention, based on an offshore wind power grid-connected and off-grid operation system, achieves off-grid operation of offshore wind power by combining an energy storage system with a cascaded H-bridge structure. Based on grid construction control, reactive power-droop control ensures that the output voltage of the energy storage grid-connected converter remains synchronized with the voltage on the wind turbine system side, reducing the impact of phase angle on the energy storage converter during wind turbine system faults. By combining virtual impedance control, the resonant peak value is effectively suppressed and the peak fault current value is reduced when a fault occurs, improving the operational stability and robustness of the energy storage converter during faults.
[0064] The solution of this invention is to propose a design scheme for the transmission and consumption system of offshore wind energy, as well as a control scheme for the off-grid operation of offshore wind energy. The system is designed as follows: When offshore wind energy is operating normally, it is collected through the AC bus on the wind turbine side, passes through an energy storage system, and is then transmitted via a DC transmission line to the grid through a grid-connected inverter. When a fault occurs in the transmission system, the wind energy is controlled to flow to the energy storage system, thus realizing the off-grid operation of the offshore wind power station system, improving energy utilization while enhancing system stability. When offshore wind energy fluctuates, the energy storage system releases or stores excess energy as needed, thus acting as a buffer.
[0065] The above specific embodiments are used to explain and illustrate the present invention, and are only preferred embodiments of the present invention, not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made to the present invention within the spirit and scope of the claims shall fall within the protection scope of the present invention.
Claims
1. A grid-connected energy storage system for offshore wind power off-grid operation, characterized in that: Multiple offshore wind turbines are connected in parallel. The offshore wind turbines are connected to the AC collection submarine cable via the wind turbine-side DC / DC converter and the wind turbine-grid-side AC / DC converter. The AC collection submarine cable is connected to the wind turbine-side AC bus via the wind turbine-grid-side transformer. The wind turbine-side AC bus is connected to the AC grid through a flexible DC transmission system on one hand, and to the energy storage battery on the other hand through an energy storage grid-connected converter.
2. The grid-connected energy storage system for offshore wind power off-grid operation according to claim 1, characterized in that: The energy storage grid-connected converter switches between grid-connected and off-grid operation states via a flexible DC-side circuit breaker.
3. The grid-connected energy storage system for offshore wind power off-grid operation according to claim 1, characterized in that: In grid-connected operation mode on the energy island, a constant power control strategy is adopted, and the active and reactive power commands are given by the energy island centralized control system. In off-grid operation mode, a droop control strategy with additional virtual impedance is adopted to control the AC bus voltage on the wind turbine side.
4. The grid-connected energy storage system for offshore wind power off-grid operation according to claim 1, characterized in that: The wind turbine-side DC / DC converter, in the grid-connected operation mode of the energy island, adopts a maximum power point tracking control strategy. By detecting wind speed in real time and tracking the maximum power generation curve, the wind turbine can track the maximum power point at different wind speeds.
5. The grid-connected energy storage system for offshore wind power off-grid operation according to claim 1, characterized in that: The energy storage grid-connected converter adopts an H-bridge cascaded structure, which is divided into 3 phases and 3 bridge arms. Each bridge arm consists of dozens to hundreds of power modules and a filter inductor connected in series. The sub-modules adopt a full-bridge topology structure, including power modules and battery modules.
6. A method for stable operation control of a grid-connected energy storage system for offshore wind power operating off-grid, characterized in that: The method is based on the system described in any one of claims 1-5, and includes a power droop control stage. The power droop control comprises two parts: active-frequency droop control and reactive-voltage droop control. The active-frequency control employs a virtual synchronous machine strategy, and the reactive-voltage control employs a QV droop strategy, as shown in the following formula: In the formula, J is the virtual inertia constant; D is the damping coefficient; n is the reactive droop coefficient; P ref and Q ref P and Q are the input active power reference value and input reactive power reference value; P and Q are the output active power and output reactive power, respectively. ω0 and ω are the nominal angular frequency and the virtual angular frequency, respectively; U n This is the rated voltage at the grid connection point.
7. The method according to claim 6, characterized in that: The inner loop of the droop control loop adopts a decoupled control current loop. At the beginning of the controller's current loop, the coupling elements in the actual circuit are introduced. First, place it into the loop, and the sign is exactly the opposite of the sign of the coupling term in the circuit model.
8. The method according to claim 6, characterized in that: The control method also includes adding virtual impedance to the control stage.