An integrated active filter and energy storage converter and its control method

Abstract

This invention discloses an integrated active power filter and energy storage converter device. The device includes a shared power conversion main circuit, a unified core controller, and a mode management module. The AC side of the power conversion main circuit is coupled to the power grid via a connecting reactor, and the DC side is connected to an energy storage unit. The core controller receives operating mode commands output by the mode management module and controls the power conversion main circuit to dynamically switch between active power filter compensation mode, energy storage converter mode, and hybrid operating mode. An integrated control method for active power filter and energy storage converter is also disclosed. This invention deeply integrates the functions of an active power filter and an energy storage converter on a single hardware platform, achieving dynamic on-demand capacity allocation and solving the problems of high cost, poor coordination, and low equipment utilization in existing discrete solutions. Simultaneously, it can adaptively switch operating modes according to the power quality status of the power grid, prioritizing power quality while also considering energy dispatch requirements.

Description

Technical Field

[0001] This invention relates to an integrated active filter and energy storage converter and its control method, belonging to the field of power electronics technology. Background Technology

[0002] In modern power systems, especially in industrial power distribution applications with a large number of nonlinear loads (such as frequency converters and electric arc furnaces), and in power generation and user sides with fluctuating renewable energy sources (such as photovoltaics and wind power), the power system faces two increasingly prominent technical problems: First, the power quality deteriorates, mainly manifested as severe harmonic pollution, reactive power deficit, and three-phase imbalance; second, power fluctuations are severe, requiring the configuration of energy storage systems to smooth out peak shaving and valley filling, and improve the stability of the power grid and the power quality.

[0003] Active power filters (APFs), as the mainstream power electronic devices for mitigating grid harmonics, work by detecting harmonic components in the load current in real time and controlling the converter to inject a compensating current into the grid that is equal in magnitude but opposite in direction to the harmonic current, thereby canceling the harmonics and making the grid-side current approach a sine wave. Power storage converters (PCSs) are key interface devices connecting energy storage batteries and the grid. They control the charging and discharging process of the energy storage batteries, enabling bidirectional energy flow between the grid and the energy storage system, and performing functions such as load tracking, peak shaving and valley filling, and frequency regulation.

[0004] Currently, the conventional technical solution in industry to address the aforementioned harmonic mitigation and energy regulation issues is to deploy two separate devices, the APF and the PCS. However, this separate deployment of the APF and PCS has the following inherent drawbacks in practice: First, the equipment investment and operating costs are high. Because two complete converter main circuits and control systems are required, there is redundant configuration of hardware such as power switching devices, sensors, controllers, cabinets, and cooling systems, significantly increasing the initial investment. At the same time, two independent units require more installation space, increasing investment in switchgear, cables, and other supporting facilities, and doubling the workload for subsequent maintenance, resulting in high operation and maintenance costs.

[0005] Secondly, the system suffers from poor coordination, posing potential operational safety hazards. As independent devices, the APF and PCS are typically managed by different controllers, lacking information exchange and global coordination mechanisms. When a sudden change occurs in grid load harmonics, requiring the APF to operate at full capacity for compensation, if the PCS is also in a high-power charging or discharging state at the same time, the output currents of the two devices will overlap at the grid connection point. This could cause the upstream transformer or feeder current to exceed its rated capacity, leading to overload tripping or even equipment damage, posing a threat to the safe and stable operation of the power grid.

[0006] Third, low equipment utilization leads to resource waste. The capacities of APF and PCS are typically designed based on the worst-case operating conditions for their respective application scenarios; that is, the APF needs to consider the maximum harmonic current, and the PCS needs to consider the maximum charging and discharging power. However, in actual operation, both are not simultaneously operating at full load most of the time. For example, at night, when the load is light, the harmonic content is low, and the APF capacity is idle; while during the day, when there is ample sunlight, the photovoltaic output is high, which may require the PCS to charge at full power, but the harmonics may be lower at this time. This discrete approach cannot achieve dynamic capacity sharing, resulting in low average equipment utilization and wasted hardware resources.

[0007] Fourth, the system is highly complex and difficult to integrate. The grid connection design of the two independent systems, the relay protection configuration, and the monitoring and communication network are all quite complex, increasing the difficulty of system integration and engineering implementation. At the same time, the coordination and control logic between multiple devices is complex, the debugging cycle is long, and operation and maintenance are inconvenient.

[0008] To address the aforementioned issues, some studies have attempted to combine APF with energy storage systems, such as by connecting energy storage batteries in parallel to the DC side of the APF. However, this approach typically still uses the original control architecture of the APF, treating energy storage only as an energy buffer on the DC side. It fails to achieve deep integration and unified scheduling from the hardware platform to the control strategy, and cannot fundamentally solve the problems of capacity sharing and collaborative control.

[0009] Therefore, there is an urgent need in this field for an integrated solution that can deeply integrate APF and PCS functions to overcome the problems of high cost, poor coordination, low utilization and system complexity of separate deployment in the existing technology. Summary of the Invention

[0010] The purpose of this invention is to address the shortcomings of separate deployment of APF and PCS in the prior art by providing an integrated active power filter and energy storage converter device, and also to provide an integrated control method for active power filter and energy storage converter, so that the device capacity can be dynamically allocated between active power and reactive / harmonic power to achieve coordinated and optimized operation, thereby improving equipment utilization and system reliability.

[0011] To solve the above-mentioned technical problems, the present invention adopts the following technical solution: an active filtering and energy storage converter integrated device, including a shared power conversion main circuit, a unified core controller and a mode management module; The AC side of the power conversion main circuit is coupled to the power grid through a connecting reactor, and the DC side is provided with an interface for connecting the energy storage unit and a DC side support capacitor. The power conversion main circuit adopts a single converter topology as a shared hardware platform to realize the functions of active power filter and energy storage converter. The core controller is electrically connected to the power conversion main circuit, and the mode management module is signal-connected to the core controller. The core controller receives operating mode commands output by the mode management module and controls the power conversion main circuit to dynamically switch between the following three operating modes: Active filter compensation mode injects compensation current into the power grid to offset load harmonics and / or reactive current; Energy storage converter mode controls the bidirectional transmission of active power between the power grid and the energy storage unit; In hybrid operating mode, active filtering compensation and energy storage converter functions are executed simultaneously.

[0012] The aforementioned device, wherein the core controller includes: The detection module is used to sample grid voltage, load current and converter output current in real time; A harmonic and reactive power detection module, connected to the detection module, is used to separate harmonic components and reactive power components from the load current; The instruction generation unit is connected to the harmonic and reactive power detection module and the mode management module, and is used to generate the target output current instruction of the converter according to the operating mode instruction. The PWM drive module has its input terminal connected to the instruction generation unit and its output terminal connected to the power switching device of the power conversion main circuit, and is used to generate PWM drive signals.

[0013] In the aforementioned device, the instruction generation unit generates a harmonic compensation current instruction as the target output current instruction based on the harmonic and reactive power detection results in active filter compensation mode. In energy storage converter mode, the fundamental active current command is generated based on the active power command as the target output current command. In the hybrid operating mode, the harmonic compensation current command and the fundamental active current command are superimposed to generate a composite target output current command.

[0014] In the aforementioned device, the mode management module generates or selects an operating mode instruction based on at least one input signal, including scheduling instructions from the upper-level energy management system, real-time monitored power grid power quality parameters, and user-manually set instructions; and when the power grid power quality parameters exceed a preset threshold, it prioritizes triggering or maintaining an active filter compensation mode or a hybrid operating mode to ensure power quality.

[0015] The aforementioned device, wherein the power quality parameter includes total harmonic distortion (THD), and the mode management module is used to select the energy storage converter mode when the THD is lower than a first threshold. Select the hybrid working mode when THD is higher than or equal to the first threshold and lower than the second threshold; When THD is higher than or equal to the second threshold, select the active filter compensation mode.

[0016] In the aforementioned device, the topology of the power conversion main circuit is one of a two-level voltage-source converter, a three-level neutral-point clamping converter, a T-type three-level converter, or a modular multilevel converter; the energy storage unit is a lithium battery energy storage group, and its terminal voltage provides DC-side voltage support for the power conversion main circuit.

[0017] In the aforementioned device, the capacitance value of the DC-side supporting capacitor simultaneously meets the requirements of the active filtering function for supporting rapid voltage fluctuations and the requirements of the energy storage and converter function for DC-side voltage stability. The inductance value of the connecting reactor is determined based on the total current ripple requirement when the power conversion main circuit simultaneously handles the maximum expected harmonic current and the rated active current under its rated capacity.

[0018] An integrated control method for active filtering and energy storage converter, applied to the aforementioned integrated device, includes the following steps: S1. Real-time acquisition of grid-side voltage and current signals and DC-side voltage signals; S2. Determine the target operating mode of the device according to the current operating mode command; calculate the instantaneous target output current command of the power conversion main circuit based on the target operating mode and the collected signals. S3. Drive the power conversion main circuit through the current closed-loop control algorithm so that its output current tracks the instantaneous target output current command.

[0019] The aforementioned method is used to calculate the instantaneous target output current command: In active filter compensation mode, the instantaneous target output current command is the harmonic compensation current command; In energy storage converter mode, the instantaneous target output current command is the fundamental active current command; In hybrid operating mode, the instantaneous target output current command is calculated using the following formula: i_ref = i_p + i_q + i_h Where i_ref is the instantaneous target output current command, i_p is the fundamental active current component, i_q is the reactive compensation current component, and i_h is the harmonic compensation current component.

[0020] The aforementioned method employs a proportional resonant control or quasi-proportional resonant control algorithm for its current closed-loop control.

[0021] Compared with the prior art, the present invention has at least the following beneficial effects: The device of this invention is highly integrated, significantly reducing costs. It achieves two functions through a single converter main circuit and a single control system, saving on hardware costs such as power switching devices, sensors, chassis, and heat sinks, and greatly reducing initial investment and floor space.

[0022] This invention enables dynamic capacity allocation, improves equipment utilization, and breaks the fixed limits of traditional discrete equipment capacity. The total apparent power capacity of the device can be dynamically allocated to active power and reactive / harmonic power according to real-time demand. For example, when harmonics are low, energy scheduling can be fully implemented; when harmonics increase, more capacity is automatically allocated for compensation, realizing "on-demand allocation" of capacity, which greatly improves equipment utilization and economy.

[0023] This invention employs collaborative intelligent control to enhance system reliability. It achieves global optimization through a unified control platform and ensures the priority of governance effects through an adaptive switching mechanism based on power quality status. When harmonics exceed the standard, it automatically limits charging and discharging power to ensure that the system does not exceed the limit, thereby improving the intelligence and safety of operation.

[0024] This invention offers rapid response and flexible, scalable functionality. Its software-defined functional model enables the system to quickly adapt to changes in grid demand. This integrated architecture also facilitates the expansion with other advanced functions, such as voltage sag compensation and active grid support, enhancing the equipment's added value and adaptability. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the overall system structure of the present invention; Figure 2 This is a schematic diagram of the internal logic function modules of the core computing control unit of this invention; Figure 3 : This is a schematic diagram of the mode switching logic flow of the present invention.

[0026] Figure description: 100-Power conversion main circuit, 200-Core controller, 201-Detection module, 202-Harmonic and reactive power detection module, 203-Instruction generation unit, 204-PWM drive module, 300-Mode management module. The present invention will be further described below with reference to the accompanying drawings and specific embodiments. Detailed Implementation

[0027] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments. It should be understood that these embodiments are for illustrative purposes only and do not constitute any limitation on the scope of protection of the present invention. All equivalent substitutions or improvements made based on the technical solutions of the present invention are within the scope of protection of the present invention.

[0028] Example 1: As Figure 1 As shown, this embodiment provides an integrated active filtering and energy storage converter device, including a shared power conversion main circuit 100, a unified core controller 200, a mode management module 300, and an energy storage unit.

[0029] The power conversion main circuit 100 adopts a three-phase T-type three-level voltage source converter topology, serving as a shared hardware platform for realizing the active power filter (APF) and energy storage converter (PCS) functions. This topology consists of 12 power switches S1-S12 and two DC-side support capacitors Cdc1 and Cdc2. The AC side of the power conversion main circuit 100 is connected to the grid / load common connection point bus via a three-phase connecting reactor L (filter inductor) and a grid-connected contactor KM. The positive and negative terminals of the DC side of the power conversion main circuit 100 are directly connected to the DC output terminal of the energy storage unit. The energy storage unit uses a lithium battery energy storage pack with a nominal voltage of 400V. This voltage serves as the DC-side voltage support for the power conversion main circuit 100 and provides the required high-frequency current path for the APF function.

[0030] The core controller 200 uses a digital signal processor (DSP) and a field-programmable gate array (FPGA) as its core computing and control units, and its internal logic functional modules are divided as follows: Figure 2 As shown, it specifically includes: Detection module 201: Real-time sampling of the three-phase voltage (Ua, Ub, Uc), load three-phase current (iLa, iLb, iLc), and converter output three-phase current (ica, icb, icc) of the power grid through Hall voltage and Hall current sensors, while simultaneously acquiring the DC side voltage Udc.

[0031] Harmonic and reactive power detection module 202: Its input is connected to the detection module 201, and it is used to separate the harmonic current component i_h and the reactive current component i_q from the load current in real time. In this embodiment, the ip-iq detection algorithm based on instantaneous reactive power theory is used.

[0032] Command generation unit 203: Its input terminals are connected to the harmonic and reactive power detection module 202 and the mode management module 300, respectively. This unit generates the instantaneous target output current command i_ref of the power conversion main circuit 100 based on the operating mode command issued by the mode management module 300, combined with the received harmonic current component i_h, reactive current component i_q, and the externally given active power command Pref.

[0033] PWM drive module 204: Its input is connected to the instruction generation unit 203, and its output is connected to the drive circuits of each power switch in the power conversion main circuit 100 via optical fiber. This module uses a current closed-loop control algorithm (a quasi-proportional resonant controller is used in this embodiment) to make the actual output current of the converter accurately track the target output current instruction i_ref, and generates 12 PWM drive signals (PWM1-PWM12) through carrier phase-shift sinusoidal pulse width modulation (SPWM).

[0034] The mode management module 300 is signal-connected to the core controller 200. Its input terminal receives scheduling instructions from the upper-level energy management system (EMS) and power quality parameters (such as total harmonic distortion rate THD) calculated by the detection module 201. The mode management module 300 generates operating mode instructions according to a preset strategy and sends them to the instruction generation unit 203.

[0035] The working process of the device in this embodiment is as follows: After the system is powered on and initialized, the mode management module 300 determines the current operating mode based on the received upper-level instructions and the real-time monitored THD value. Assuming that the THD is 5% at this time and no charging or discharging instructions are issued from the upper level, the mode management module 300 selects the "active filter compensation mode".

[0036] In the "active filter compensation mode", the instruction generation unit 203 receives the harmonic current component i_h and the reactive current component i_q from the harmonic and reactive current detection module 202, and generates the target output current instruction i_ref = -(i_h + i_q), that is, injects a compensation current into the grid that is equal in magnitude and opposite in direction to the load harmonic and reactive current.

[0037] The PWM drive module 204 compares i_ref with the actual output current, adjusts the error through a quasi-PR controller, and generates a modulation wave signal. After comparing the modulation wave with the triangular carrier wave, PWM1-PWM12 drive signals are generated to control the on / off state of the power switching transistors S1-S12, so that the converter output current accurately tracks i_ref, thereby achieving harmonic mitigation and reactive power compensation.

[0038] Example 2: This example, based on Example 1, focuses on illustrating the mode switching logic of the device under different operating modes. For example... Figure 3 As shown: This embodiment also provides an adaptive mode switching strategy based on power quality status, which is executed in the mode management module 300: The mode management module 300 continuously monitors the total harmonic distortion (THD) at the point of common coupling of the power grid and has two preset thresholds: the first threshold THD1 = 3% and the second threshold THD2 = 8%.

[0039] When THD < 3%, it indicates that the power grid has good power quality. If the upper-level EMS has charging / discharging needs at this time, the mode management module 300 selects "energy storage converter mode", and the device focuses on performing energy dispatching tasks such as peak shaving and valley filling.

[0040] When 3% ≤ THD < 8%, it indicates that the power grid has a certain degree of harmonic pollution but has not yet seriously exceeded the standard. The mode management module 300 automatically selects the "hybrid operating mode". While executing the preset charging and discharging plan, the device uses the remaining capacity to compensate for harmonics and reactive power.

[0041] When THD ≥ 8%, it indicates that the grid harmonic pollution has seriously exceeded the standard, and power quality has become the primary threat. The mode management module 300 forces the selection of "active filter compensation mode" and sends a power limit or shutdown request to the upper-level EMS to suspend or limit the PCS charging and discharging function, so as to use the full capacity of the device for harmonic and reactive power compensation and restore the grid power quality as quickly as possible.

[0042] This dynamic priority switching mechanism fully demonstrates the advantages of the collaborative intelligent control of this invention, ensuring the safety and reliability of system operation.

[0043] Example 3: This example focuses on the parameter design considerations for the DC-side support capacitor and connecting reactor to better support the deep integration of APF and PCS functions.

[0044] The selection of capacitance values ​​for DC-side support capacitors Cdc1 and Cdc2 must consider the requirements of both functions: On the one hand, from the perspective of APF functionality, the DC-side capacitor needs to provide a low-impedance path for high-frequency harmonic currents to suppress DC-side voltage ripple caused by harmonic current injection. The larger the capacitance value, the smaller its impedance to high-frequency components, and the better the effect of suppressing voltage fluctuations.

[0045] On the other hand, from the perspective of PCS function, the DC-side capacitor needs to maintain the stability of the DC voltage. Especially when the active power changes suddenly, the energy stored in the capacitor can buffer the instantaneous power imbalance, prevent the DC voltage from dropping or rising sharply, and provide a stable feedback for the voltage outer loop control.

[0046] By establishing a system simulation model and conducting experimental verification, this embodiment determines that the range of the total DC capacitance value Cdc (the equivalent capacitance after Cdc1 and Cdc2 are connected in series) must meet two conditions: at the maximum compensation harmonic current frequency, the impedance of the capacitor is lower than the set value (e.g., 0.1Ω); at the same time, when the PCS has a maximum power step (e.g., from -100kW to +100kW), the DC voltage fluctuation range is controlled within ±5% of the rated voltage.

[0047] The final values ​​were Cdc1 = Cdc2 = 4700μF, with an equivalent total capacitance of 2350μF.

[0048] When selecting the inductance value for the connected reactor L, both current ripple suppression requirements and dynamic response speed must be considered. In mixed operating mode, the current flowing through the reactor includes the fundamental active current and harmonic compensation current with abundant high-frequency components. A larger inductance value results in better suppression of switching frequency ripple, but it limits the rate of change of current, affecting the APF's compensation effect on higher harmonics and the PCS's fast response capability.

[0049] In this embodiment, based on the rated capacity (100kVA), DC side voltage (800V), switching frequency (10kHz), and the highest harmonic to be compensated (e.g., 50th, 2500Hz) of the power conversion main circuit 100, preliminary calculations were performed using the formula L = Udc / (8 * fsw * ΔImax). After simulation optimization, the inductance value of the connecting reactor L was finally selected as 0.3mH. This value can meet the compensation requirements for the main harmonic frequency band while ensuring that the total current ripple is less than 15% of the rated current.

[0050] In summary, this invention deeply integrates the functions of APF and PCS through a shared power conversion main circuit and a unified core controller. Furthermore, it achieves flexible switching and coordinated control of three operating modes through a mode management module, effectively solving the problems of high cost, poor coordination, and low utilization rate inherent in the separate deployment of existing technologies. The above description is merely a preferred embodiment of this invention and is not intended to limit the invention. For those skilled in the art, various improvements and modifications can be made without departing from the spirit and principles of this invention, and these improvements and modifications should also be considered within the scope of protection of this invention. This invention discloses an integrated active power filter and energy storage converter device. The device includes a shared power conversion main circuit, a unified core controller, and a mode management module. The AC side of the power conversion main circuit is coupled to the power grid via a connecting reactor, and the DC side is connected to an energy storage unit. The core controller receives operating mode commands output by the mode management module and controls the power conversion main circuit to dynamically switch between active power filter compensation mode, energy storage converter mode, and hybrid operating mode. An integrated control method for active power filter and energy storage converter is also disclosed. This invention deeply integrates the functions of an active power filter and an energy storage converter on a single hardware platform, achieving dynamic on-demand capacity allocation and solving the problems of high cost, poor coordination, and low equipment utilization in existing discrete solutions. Simultaneously, it can adaptively switch operating modes according to the power quality status of the power grid, prioritizing power quality while also considering energy dispatch requirements.

Claims

1. An integrated active filtering and energy storage converter, characterized in that, It includes a shared power conversion main circuit, a unified core controller, and a mode management module; The AC side of the power conversion main circuit is coupled to the power grid through a connecting reactor, and the DC side is provided with an interface for connecting the energy storage unit and a DC side support capacitor. The core controller is electrically connected to the power conversion main circuit, and the mode management module is signal-connected to the core controller. The core controller receives operating mode commands from the mode management module and controls the power conversion main circuit to dynamically switch between the following three operating modes: Active filter compensation mode injects compensation current into the power grid to offset load harmonics and / or reactive current; Energy storage converter mode controls the bidirectional transmission of active power between the power grid and the energy storage unit; In hybrid operating mode, active filtering compensation and energy storage converter functions are executed simultaneously.

2. The apparatus according to claim 1, characterized in that, The core controller includes: The detection module is used to sample grid voltage, load current and converter output current in real time; A harmonic and reactive power detection module, connected to the detection module, is used to separate harmonic components and reactive power components from the load current; The instruction generation unit is connected to the harmonic and reactive power detection module and the mode management module, and is used to generate the target output current instruction of the converter according to the operating mode instruction. The PWM drive module has its input terminal connected to the instruction generation unit and its output terminal connected to the power switching device of the power conversion main circuit, and is used to generate PWM drive signals.

3. The apparatus according to claim 2, characterized in that, In active filter compensation mode, the instruction generation unit generates a harmonic compensation current instruction as the target output current instruction based on the harmonic and reactive power detection results. In energy storage converter mode, the fundamental active current command is generated based on the active power command as the target output current command. In the hybrid operating mode, the harmonic compensation current command and the fundamental active current command are superimposed to generate a composite target output current command.

4. The apparatus according to claim 1, characterized in that, The mode management module generates or selects an operating mode instruction based on at least one input signal, including scheduling instructions from the upper-level energy management system, real-time monitored power grid power quality parameters, and user-manually set instructions; and when the power grid power quality parameters exceed a preset threshold, it prioritizes triggering or maintaining the active filter compensation mode or the hybrid operating mode.

5. The apparatus according to claim 4, characterized in that, The power quality parameters include total harmonic distortion (THD), and the mode management module is used to select the energy storage converter mode when the THD is lower than a first threshold. Select the hybrid working mode when THD is higher than or equal to the first threshold and lower than the second threshold; When THD is higher than or equal to the second threshold, select the active filter compensation mode.

6. The apparatus according to claim 1, characterized in that, The topology of the power conversion main circuit is one of a two-level voltage-source converter, a three-level neutral-point clamping converter, a T-type three-level converter, or a modular multilevel converter; the energy storage unit is a lithium battery energy storage group, whose terminal voltage provides DC-side voltage support for the power conversion main circuit.

7. The apparatus according to claim 1, characterized in that, The capacitance value of the DC-side supporting capacitor simultaneously meets the requirements of the active filtering function for supporting rapid voltage fluctuations and the requirements of the energy storage and converter function for DC-side voltage stability. The inductance value of the connecting reactor is determined based on the total current ripple requirement when the power conversion main circuit simultaneously handles the maximum expected harmonic current and the rated active current under its rated capacity.

8. An integrated control method for active filtering and energy storage converter, applied to the integrated device according to any one of claims 1 to 7, characterized in that, Includes the following steps: S1. Real-time acquisition of grid-side voltage and current signals and DC-side voltage signals; S2. Determine the target operating mode of the device according to the current operating mode command; calculate the instantaneous target output current command of the power conversion main circuit based on the target operating mode and the collected signals. S3. Drive the power conversion main circuit through a current closed-loop control algorithm so that its output current tracks the instantaneous target output current command.

9. The method according to claim 8, characterized in that, When calculating the instantaneous target output current command: In active filter compensation mode, the instantaneous target output current command is the harmonic compensation current command; In energy storage converter mode, the instantaneous target output current command is the fundamental active current command; In hybrid operating mode, the instantaneous target output current command is calculated using the following formula: i_ref = i_p + i_q + i_h Where i_ref is the instantaneous target output current command, i_p is the fundamental active current component, i_q is the reactive compensation current component, and i_h is the harmonic compensation current component.

10. The method according to claim 8 or 9, characterized in that, The current closed-loop control algorithm adopts proportional resonance control or quasi-proportional resonance control.

Images