Microgrid converter protection method and system based on multi-mode cooperative control

Abstract

The invention discloses a microgrid converter protection method and system based on multi-mode cooperative control, and relates to the technical field of microgrid protection, and the method comprises the steps: monitoring each phase voltage current and neutral current of a four-leg converter side in real time, and entering an alarm mode, an emergency mode or a normal mode when corresponding conditions are satisfied; in an alarm mode, triggering a maximum sinusoidal current utilization unit SCU and a fast virtual impedance current limiter FVI to cooperatively limit current, and activating an overload phase balancer OPB to dynamically adjust a voltage reference value so as to maintain three-phase voltage balance; in the emergency mode, the SCU and the FVI are activated to perform selective current limiting on the fault phase, and the non-fault phase maintains normal voltage output; in the normal mode, only the OPB is activated. According to the method, cooperative operation of multiple protection functions is ensured, and finally, the uninterrupted high-quality power supply target of the micro-grid under various fault working conditions is achieved.

Description

Technical Field

[0001] This invention relates to the field of microgrid protection technology, and in particular to a microgrid converter protection method and system based on multi-modal collaborative control. Background Technology

[0002] With the large-scale application of renewable energy in microgrids, voltage source converters (VSCs) have become key equipment for realizing distributed energy grid connection. However, they still face significant challenges in dealing with short-circuit faults, overloads, and voltage imbalances. Especially in islanded operation of microgrids, neutral line overloads and asymmetrical faults can easily lead to converter overcurrent, often forcing traditional protection schemes to shut down, severely affecting the continuity of power supply.

[0003] In existing technologies, converter protection is mainly divided into two categories: hardware protection and algorithm protection. Hardware protection schemes include physical protection measures such as overcurrent fuses and redundant designs, while the mainstream algorithm protection schemes are mainly implemented through control strategies, such as the current reference saturation method. Regarding fault handling mechanisms, traditional protection schemes cannot achieve differentiated control between faulty and non-faulty phases, leading to a decline in the power supply quality of healthy phases; the neutral line protection function is missing, and when the system experiences severe imbalance, the neutral line current may exceed the equipment's tolerance capacity; existing protection schemes have insufficient dynamic response characteristics, and the delay in fault detection and action causes semiconductor devices to withstand excessive transient inrush currents. Furthermore, the lack of coordination mechanisms between different protection functions may lead to control conflicts in complex fault scenarios.

[0004] While the latest four-arm converter design improves neutral line control, it still assumes over-capacity neutral line design and exhibits significant voltage overshoot during fault recovery. These existing technologies all demonstrate clear limitations in handling complex fault conditions. Summary of the Invention

[0005] To address the aforementioned problems and technical requirements, the inventors propose a microgrid converter protection method and system based on multi-modal collaborative control. This method achieves stable voltage maintenance of the healthy phase during faults, establishes an active neutral current limiting mechanism, and optimizes the dynamic response characteristics of the protection system. An intelligent coordination strategy is designed to ensure the coordinated operation of multiple protection functions, ultimately achieving the goal of uninterrupted and high-quality power supply to the microgrid under various fault conditions. The technical solution of this invention is as follows:

[0006] Firstly, this application provides a microgrid converter protection method based on multi-modal collaborative control. This method integrates three functional modules: a maximum sinusoidal current utilization unit (SCU) for waveform-maintaining limiting of fault phase current; an overload phase balancer (OPB) for neutral current suppression and voltage symmetry maintenance; and a fast virtual impedance current limiter (FVI) for transient overcurrent suppression. These three modules achieve functional linkage through an intelligent coordinated state machine, forming the following hierarchical protection mechanism:

[0007] When an overload is detected, the alarm mode is entered. In this mode, the SCU and FVI are triggered to limit the current together, and the OPB is activated to dynamically adjust the voltage reference value to maintain the three-phase voltage balance.

[0008] When a short circuit is detected, it automatically switches to emergency mode. In this mode, the SCU and FVI are activated to selectively limit the current of the faulty phase, while the non-faulty phases maintain normal voltage output.

[0009] After the fault is cleared, the system smoothly returns to normal mode, in which OPB is reactivated.

[0010] Its further technical solution is that the methods for switching into the three modes include:

[0011] When the amplitude of any phase current or the amplitude of the neutral line current exceeds the maximum allowable value, the alarm mode is activated.

[0012] When any phase voltage is detected to drop below the dynamic hysteresis lower limit threshold and any phase current amplitude exceeds the maximum allowable value, the emergency mode is activated.

[0013] When all phase voltages recover to above the dynamic hysteresis upper limit threshold and no phase current amplitude exceeds the maximum allowable value, the system enters normal mode.

[0014] The further technical solution is that the SCU performs the following operations:

[0015] The original current reference value is obtained from each phase voltage loop, and the current amplitude of the original current reference value of each phase is extracted by amplitude calculation.

[0016] When the current amplitude of any phase is detected to exceed the maximum allowable value, a proportional coefficient is generated according to the dynamic adjustment formula, and the original current reference value is corrected.

[0017] The corrected current reference value is injected into the corresponding current loop to achieve fault phase current limiting protection.

[0018] Its further technical solution is that OPB performs the following operations:

[0019] The original current reference value and neutral line current measurement value are obtained from each phase voltage loop, and the current amplitude of each phase original current reference value and neutral line current measurement value is extracted by amplitude calculation;

[0020] The normalized overload index is calculated based on the current amplitude of each phase and the neutral line current amplitude.

[0021] The parameters are input into the PI controller to generate dynamic adjustment coefficients, and the reference values ​​of each phase voltage are uniformly corrected.

[0022] The corrected phase voltage reference values ​​are injected into the corresponding voltage loops to achieve neutral line current limiting and voltage symmetry maintenance.

[0023] The further technical solution is that the FVI performs the following operations:

[0024] The original current reference value is obtained from each phase voltage loop, and the current amplitude of the original current reference value of each phase is extracted by amplitude calculation.

[0025] When the current amplitude of any phase exceeds the set threshold, the dynamic virtual impedance parameter is calculated;

[0026] The dynamic virtual impedance parameters are processed by a high-pass filter to extract transient components, which then generate a compensation voltage.

[0027] Injecting the compensation voltage into the modulation wave generation unit achieves transient overcurrent suppression protection.

[0028] Secondly, this application also provides a microgrid converter protection system based on multi-modal cooperative control, comprising:

[0029] The circuit section adopts a four-arm converter and an LCL filter structure located on the AC side, and selects one set of arms for independent control of the neutral line;

[0030] The control section employs a voltage loop and current loop control structure, and embeds a maximum sinusoidal current utilization unit (SCU), a fast virtual impedance current limiter (FVI), and an overload phase balancer (OPB) that exchange information via a data bus.

[0031] The SCU is deployed between the voltage loop output and the current loop input of each phase to dynamically adjust the current reference value to achieve current limiting protection.

[0032] The FVI is connected in parallel to each phase voltage control loop, and its output is simultaneously applied to the modulation wave generation unit to generate dynamic virtual impedance parameters to achieve transient overcurrent suppression protection.

[0033] The OPB connects the neutral current detection channel and the voltage loops of each phase, and is used to dynamically adjust the voltage reference value to achieve three-phase voltage balance and neutral current limiting protection.

[0034] A three-level state machine connects and controls the SCU, FVI, and OPB to execute the microgrid converter protection method based on multi-modal collaborative control as described in the first aspect, thereby achieving multi-module hierarchical collaborative protection.

[0035] The beneficial technical effects of this invention are:

[0036] In the microgrid converter protection method and system based on multimodal collaborative control provided by this invention, the coordinated operation of three core modules—SCU, OPB, and FVI—achieves full-process protection from fault detection to restoration to normal operation. The system adopts a three-level state machine control architecture, intelligently switching between normal, alarm, and emergency modes based on real-time monitoring data of the converter's phase current and voltage and neutral line current, and using dynamic hysteresis upper and lower thresholds ζ. off ζ on Ensuring switching stability. This method overcomes the limitations of traditional protection technologies in terms of operating speed (<10ms), current limiting accuracy (±1%), and voltage imbalance (<2%), making it particularly suitable for microgrid applications with high power quality requirements. Experimental verification shows that this scheme can effectively suppress short-circuit current transient overshoot (reducing it from 1.72pu to 1.25pu) and significantly improve voltage imbalance from 10.5% to within 2%. Attached Figure Description

[0037] Figure 1 This is a schematic diagram of a microgrid converter protection system architecture based on multi-modal cooperative control provided in this application;

[0038] Figure 2 This is a flowchart of a microgrid converter protection method based on multi-modal cooperative control provided in this application;

[0039] Figure 3 This is the logic block diagram of the maximum sinusoidal current utilization unit (SCU) provided in this application;

[0040] Figure 4 This is the logic block diagram of the overload phase balancer OPB provided in this application;

[0041] Figure 5 This is the logic block diagram of the fast virtual impedance current limiter (FVI) provided in this application. Detailed Implementation

[0042] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings.

[0043] One embodiment of this application provides a microgrid converter protection system based on multi-modal cooperative control, such as... Figure 1As shown, the system includes a circuit section and a control section. The circuit section employs a four-arm converter, with four IGBT arms forming the main power circuit, and a fourth arm specifically designated for independent neutral line control. A bus capacitor is configured on the DC side, and an improved LCL filter structure is used on the AC side. In one possible implementation, the improved LCL filter structure includes a converter-side inductor L... inv,k and L inv,ne Grid-side inductance L g,k and L g,ne and filter capacitor C f,k and C f,ne With damping resistor R f,k and R f,ne Serial combination.

[0044] The control section adopts a voltage loop and current loop control structure. In this embodiment, both the voltage loop and current loop are designed based on the PR controller and interact with the protection function module through a data bus. In addition, three protection function modules are embedded in the control section: an overload phase balancer (OPB) is connected to the neutral line current detection channel and works with a dedicated bridge arm to adjust the zero-sequence component; a maximum sinusoidal current utilization unit (SCU) is connected to the output node of each phase voltage loop and generates dynamically adjusted current reference values ​​through real-time calculation; and a fast virtual impedance current limiter (FVI) is connected in parallel to each phase voltage control loop, and its output dynamic virtual impedance parameters are fed back to the main controller when a mode switching command is issued by the state machine, specifically only effective in alarm or emergency modes. Each module interacts with other modules via an optimized data bus. The OPB shares adjustment commands with each phase voltage loop to dynamically adjust the voltage reference value to achieve three-phase voltage balance and neutral line current limiting protection. The SCU is deployed between the output of each phase voltage loop and the input of the current loop to dynamically adjust the current reference value to achieve current limiting protection. The output of the FVI also acts on the modulation wave generation unit to generate dynamic virtual impedance parameters to achieve transient overcurrent suppression protection.

[0045] The control section also includes a three-level state machine, which, together with other parts, forms a hierarchical and collaborative protection hardware platform. This state machine is used to execute a microgrid converter protection method based on multi-modal collaborative control, realizing hierarchical and collaborative protection of the three protection functional modules. The details are described below.

[0046] like Figure 2 As shown, one embodiment of this application also provides a microgrid converter protection method based on multi-modal cooperative control. The method process is implemented through a three-level state machine, and the specific operation mechanism is as follows:

[0047] The system monitors the phase voltage, current, and neutral current on the four-arm converter side in real time, and enters alarm mode, emergency mode, or normal mode when corresponding conditions are met. The system is typically in normal mode during initial operation, at which point it continuously monitors the phase voltage and current. k (k = a, b, c) and neutral current i ne When the amplitude i of any phase current is detected k,max Or neutral current amplitude i ne,max Exceeding the maximum allowed value I max When this occurs, it indicates that the converter current is overloaded, triggering alarm condition f. alert =1, the system enters alarm mode.

[0048]

[0049] f i,k =i k,max >I max

[0050] In alarm mode, the three modules SCU, FVI and OPB work together: SCU limits the amplitude of current in each phase, FVI suppresses transient overshoot, and OPB maintains three-phase voltage balance by adjusting the voltage reference value.

[0051] When any phase voltage v is detected k Falling to the lower limit threshold of the dynamic hysteresis loop The following exists, and any phase current amplitude i k,max Exceeding the maximum allowed value I max When a continuous overcurrent occurs, it indicates a short-circuit fault in the converter, triggering emergency condition f. cont =1, the system switches to emergency mode.

[0052]

[0053]

[0054] In emergency mode, OPB is disabled, and only SCU and FVI are activated to selectively limit the current of the faulty phase, while the non-faulty phases maintain normal voltage output.

[0055] After the fault is cleared, when all phase voltages recover to the upper limit threshold of the dynamic hysteresis loop... The above and none of the phase current amplitudes exceed the maximum allowable value I max When (i.e., no overcurrent occurs), the recovery condition f is triggered. normal =1, the system returns to normal mode.

[0056]

[0057]

[0058] In normal mode, only the OPB is activated to maintain three-phase voltage balance. In this embodiment, ζ is set. off >ζ on This is to avoid mode oscillations caused by critical voltage fluctuations.

[0059] like Figure 3 As shown, the SCU achieves precise current limiting protection by dynamically adjusting the current reference value. Specifically, it performs the following operations: First, it obtains the original current reference value from each phase voltage loop. The current amplitude of each phase's original current reference value is extracted through real-time amplitude calculation. When the current amplitude of any phase is detected Exceeding the maximum allowed value I max At that time, the proportional coefficient SCU is generated according to the dynamic adjustment formula. k and the original current reference value The correction is then made. The dynamic adjustment formula is defined as follows:

[0060]

[0061] The proportionality coefficient SCU calculated using the above formula is... k Compared with the original current reference value The product is multiplied to output a current reference value that maintains a sinusoidal characteristic and is limited. The corrected current reference value is injected into the corresponding current loop through a dedicated interface node in the control architecture, forming a closed-loop regulation path.

[0062] The SCU design employs dynamic scaling instead of traditional hard limiting, achieving precise current limiting while maintaining current waveform quality and the closed-loop regulation function of the voltage control loop. The independent processing mechanism for each phase enables selective current limiting for asymmetrical faults, effectively solving the waveform distortion and system instability problems caused by traditional solutions.

[0063] like Figure 4 As shown, the OPB achieves neutral current limiting and three-phase voltage balance through dynamic voltage regulation, specifically performing the following operations: First, it obtains the original current reference value from each phase voltage loop. and neutral line current measurement value i ne The system extracts the current amplitude of each phase's original current reference value and the measured neutral current value through rapid amplitude calculation. Then, based on the current amplitude of each phase... and neutral current amplitude Calculate the normalized overload index Specifically, the amplitude of each phase current and neutral current amplitude After the comparator selects the maximum value (i.e., identifies the most severe overload phase / neutral line), the normalized overload index is calculated.

[0064]

[0065] Will The input PI controller generates a dynamic adjustment coefficient α(t) and adjusts the reference values ​​of each phase voltage. A unified correction was performed, and the corrected reference values ​​for each phase voltage were obtained. Inject the corresponding voltage loop. The dynamic adjustment coefficient is expressed as:

[0066]

[0067] Then, based on α(t), the reference values ​​of each phase voltage are... The expression for unified correction is:

[0068]

[0069] in, To adjust the error, the error passes through a low-pass filter (LPF) before entering the PI controller, k p k i These are the parameters for the PI controller. Through the above PI control, the three-phase voltage can be reduced proportionally to the overload ratio. Therefore, the neutral line current decreases with the phase voltage. When e(t)→0, the neutral current converges to I. max This enables neutral line current limiting and three-phase voltage balancing functions.

[0070] In one possible implementation, the PI controller also integrates an anti-saturation mechanism, via k lim Parameters limit the integral output of the PI controller to prevent integral saturation during regulation. Furthermore, in the OPB module, the emergency mode variable cmv is used as a system status flag; when the state machine issues a mode switching instruction f... cont =1, cmv is set to 1 to disable the OPB function and ensure stable voltage of non-faulty phases. During normal operation, it is set to 0 to keep OPB active.

[0071] The OPB design addresses the shortcomings of traditional solutions in handling neutral line overload by directly incorporating the neutral line current into the control loop. Secondly, it employs a global voltage regulation strategy to maintain the symmetry of the three-phase voltage while limiting the overload current. Finally, an optimized low-pass filter (LPF) with a cutoff frequency of 100 rad / s ensures a smooth transition during regulation, preventing secondary impacts on the system. Experimental verification shows that in cases of severe current imbalance leading to neutral line overcurrent, when the neutral line current is approximately 1.15 pu and the OPB is not activated, the SCU and FVI functions cannot alleviate the problem. After the OPB is activated, the new steady-state voltage operating point is approximately 0.85 pu. Due to the decrease in phase voltage, the neutral line current decreases accordingly and is precisely limited to 1.0 pu. Therefore, the OPB module designed in this application can precisely limit the neutral line overload current from 1.15 pu to 1.0 pu, while reducing the voltage imbalance from 10.5% to below 2%, with no overshoot during dynamic regulation and a response time of less than 20 ms.

[0072] like Figure 5 As shown, FVI achieves transient overcurrent suppression through dynamic virtual impedance injection, specifically by performing the following operations: First, obtaining the original current reference value from each phase voltage loop. The current amplitude of the original current reference value for each phase is extracted through amplitude calculation. When the current amplitude of any phase is detected Exceeding the set threshold ζ m I max When calculating the dynamic virtual impedance parameters

[0073]

[0074] Where, ζ m This is the action threshold coefficient (typically 1.2 pu), used to set the activation critical point of the virtual impedance: only when... At this point, the FVI generates a virtual impedance. This design avoids interference with normal current fluctuations while providing a buffer for controllable overshoot in the early stages of a fault. FVI The gain coefficient of the virtual impedance is used to actively raise the equivalent output impedance during transient processes by linearly converting the current overshoot into virtual impedance.

[0075] After the transient components of the dynamic virtual impedance parameters are extracted by a high-pass filter (HPF), a compensation voltage is generated. Ultimately A modulation wave generation unit is injected to form a closed-loop current limiting control. The FVI module is activated in both alarm and emergency modes, but its transient suppression strength varies depending on the fault type (overload / short circuit).

[0076] This application verifies the functionality of the SCU and FVI. In the first case, under a short-circuit fault without FVI, using only the SCU, the transient current overshoot reaches 1.72 pu during the fault initiation period. In the second case, with FVI but without a high-pass filter and SCU, the current overshoot only reaches about 1.25 pu. In the third case, with both FVI and SCU activated simultaneously, this scheme exhibits precise current suppression and mitigation of the current overshoot to 1.25 pu. Therefore, it is proven that the SCU and FVI modules designed in this application can effectively suppress the transient overshoot of the short-circuit current, i.e., reduce it from 1.72 pu to 1.25 pu.

[0077] The above descriptions are merely preferred embodiments of this application, and the present invention is not limited to the above embodiments. It is understood that other improvements and variations directly derived or conceived by those skilled in the art without departing from the spirit and concept of the present invention should be considered to be included within the protection scope of the present invention.

Claims

1. A microgrid converter protection method based on multi-modal cooperative control, characterized in that, The method includes: Real-time monitoring of phase voltage, current and neutral line current on the four-arm converter side, and entry into alarm mode, emergency mode or normal mode when the corresponding conditions are met; In the alarm mode, the maximum sinusoidal current utilizer SCU and the fast virtual impedance current limiter FVI are triggered to limit the current together, while the overload phase balancer OPB is activated to dynamically adjust the voltage reference value to maintain the three-phase voltage balance. In the emergency mode, the SCU and FVI are activated to selectively limit the current of the faulty phase, while the non-faulty phases maintain normal voltage output. In the normal mode, only the OPB is activated to maintain three-phase voltage balance.

2. The microgrid converter protection method based on multi-modal cooperative control according to claim 1, characterized in that, The real-time monitoring of phase voltage, current, and neutral current on the four-arm converter side, and the entry into alarm mode, emergency mode, or normal mode when corresponding conditions are met, includes: When the amplitude of any phase current or the amplitude of the neutral line current exceeds the maximum allowable value, the alarm mode is activated. When it is detected that the voltage of any phase drops below the lower limit of the dynamic hysteresis threshold and the current amplitude of any phase exceeds the maximum allowable value, the emergency mode is entered. When all phase voltages recover to above the dynamic hysteresis upper limit threshold and no phase current amplitude exceeds the maximum allowable value, the system enters the normal mode.

3. The microgrid converter protection method based on multi-modal cooperative control according to claim 1, characterized in that, The SCU performs the following operations: The original current reference value is obtained from each phase voltage loop, and the current amplitude of the original current reference value of each phase is extracted by amplitude calculation. When the current amplitude of any phase is detected Exceeding the maximum allowed value I max At that time, the proportional coefficient SCU is generated according to the dynamic adjustment formula. k The original current reference value is then corrected, where k = a, b, c; The corrected current reference value is injected into the corresponding current loop to achieve fault phase current limiting protection.

4. The microgrid converter protection method based on multi-modal cooperative control according to claim 3, characterized in that, The dynamic adjustment formula is:

5. The microgrid converter protection method based on multi-modal cooperative control according to claim 1, characterized in that, The OPB performs the following operations: The original current reference value and neutral line current measurement value are obtained from each phase voltage loop, and the current amplitude of each phase original current reference value and neutral line current measurement value is extracted by amplitude calculation; The normalized overload index is calculated based on the current amplitude of each phase and the neutral line current amplitude. The aforementioned indicators are input into the PI controller to generate dynamic adjustment coefficients, and the reference values ​​of each phase voltage are uniformly corrected. The corrected phase voltage reference values ​​are injected into the corresponding voltage loops to achieve neutral line current limiting and voltage symmetry maintenance.

6. The microgrid converter protection method based on multi-mode collaborative control according to claim 5, wherein the calculation of the normalized overload index based on the phase current amplitude and the neutral line current amplitude includes: The amplitude of each phase current and neutral current amplitude After the comparator selects the maximum value, the normalized overload index is calculated. Where I max This represents the maximum allowable value for the current.

7. The microgrid converter protection method based on multi-modal cooperative control according to claim 5, wherein the index is input into the PI controller to generate dynamic adjustment coefficients, and the reference values ​​of each phase voltage are uniformly corrected, including: The dynamic adjustment coefficient is expressed as: α(t)=1+k p ·e(t)+k i ∫e(t)dt Based on the dynamic adjustment coefficient, the reference values ​​of each phase voltage are... The expression to be uniformly corrected is: in, To adjust for error, k p k i These are the parameters for the PI controller.

8. The microgrid converter protection method based on multi-modal cooperative control according to claim 1, characterized in that, The FVI performs the following operations: The original current reference value is obtained from each phase voltage loop, and the current amplitude of the original current reference value of each phase is extracted by amplitude calculation. When the current amplitude of any phase is detected Exceeding the set threshold ζ m I max Calculate the dynamic virtual impedance parameters; The dynamic virtual impedance parameters are processed by a high-pass filter to extract transient components, which then generate a compensation voltage. The compensation voltage is injected into the modulation wave generation unit to achieve transient overcurrent suppression protection; Where, ζ m I is the action threshold coefficient. max This represents the maximum allowable value for the current.

9. The microgrid converter protection method based on multi-modal cooperative control according to claim 8, characterized in that, The calculation expression for the dynamic virtual impedance parameter is as follows: Where k FVI This is the gain coefficient for the virtual impedance.

10. A microgrid converter protection system based on multi-modal cooperative control, characterized in that, include: The circuit section adopts a four-arm converter and an LCL filter structure located on the AC side, and selects one set of arms for independent control of the neutral line; The control section employs a voltage loop and current loop control structure, and embeds a maximum sinusoidal current utilization unit (SCU), a fast virtual impedance current limiter (FVI), and an overload phase balancer (OPB) that exchange information via a data bus. The SCU is deployed between the voltage loop output and the current loop input of each phase, and is used to dynamically adjust the current reference value to achieve current limiting protection. The FVI is connected in parallel to each phase voltage control loop, and its output is simultaneously applied to the modulation wave generation unit to generate dynamic virtual impedance parameters to achieve transient overcurrent suppression protection. The OPB connects the neutral current detection channel and the voltage loop of each phase, and is used to dynamically adjust the voltage reference value to achieve three-phase voltage balance and neutral current limiting protection. A three-level state machine connects and controls the SCU, the FVI, and the OPB to execute the microgrid converter protection method based on multi-modal collaborative control as described in any one of claims 1 to 9, thereby realizing multi-module hierarchical collaborative protection.

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