Fault ride-through method based on hybrid synchronous control voltage / current source mode switching

Abstract

The invention relates to a fault ride-through method based on hybrid synchronous control voltage / current source mode switching, aiming at fault ride-through (FRT) challenge in a novel power system, and breaking through the limitation of the existing solution from three key dimensions of overcurrent limitation, fault synchronization and reactive power support. On the basis of a hybrid synchronous control strategy, the damping characteristic of the system is remarkably improved, and the synchronous stability of the inverter during the fault period is effectively guaranteed; meanwhile, the intelligent control of smooth mode switching can be completed without complex switching logic or power grid information by utilizing simple detection of a saturation signal of a current limiter and combining with a mixed voltage / current source control architecture. Besides, the method has excellent short-circuit ratio (SCR) adaptive capacity, stable operation can be kept under different power grid strength conditions, and an innovative solution is provided for friendly improvement of new energy grid connection and transient performance optimization of a power system.

Description

Technical Field

[0001] This invention belongs to the field of fault ride-through technology, and in particular to a fault ride-through method based on hybrid synchronous control voltage / current source mode switching. Background Technology

[0002] Grid-connected inverters (GFM) exhibit current following and voltage forming characteristics, similar to traditional synchronous generators (SGs). Mainstream grid control methods include droop control, virtual synchronous generator (VSG) control, and hybrid synchronous control (HSC). Unlike synchronous generators, which can withstand 5-7 pu short-circuit currents, inverters are limited to 1.2-2 pu short-circuit currents due to semiconductor device limitations. Overcurrent limiting is crucial for GFM inverters. Maintaining grid synchronization under fault conditions and providing voltage support according to grid specifications are also fundamental requirements for GFM inverter applications.

[0003] Current research focuses on enhancing the fault ride-through (FRT) capability of GFM inverters, primarily addressing three key levels: 1) overcurrent limiting; 2) synchronization stability; and 3) active fault ride-through service. Overcurrent limiting is the primary requirement for ensuring inverter survival under fault conditions. Building upon current limiting, the second level requires maintaining synchronization stability, especially during faults. The third level, active fault ride-through service, must support frequency / voltage under fault conditions and enable flexible fault recovery from fault operation to normal operation. This level allows the inverter to inject fault current, contributing to grid recovery capabilities and complying with grid specifications.

[0004] The above-mentioned FRT requirements are divided according to three levels of control objectives. Figure 4 This is crucial for the large-scale deployment of GFM inverters. Although some methods have been proposed, a unified solution that can address all three aspects simultaneously still needs to be explored.

[0005] The differences compared to existing technologies are as follows:

[0006] I. Comparison with patent CN202411676429.2 "An inverter control method and system based on power value tuning";

[0007] Patent CN202411676429.2 provides an inverter control method and system based on power value tuning. This method has a clear control principle and is relatively simple. It switches power commands on top of traditional VSG control, providing sufficient reactive power output while offering inertial support to the grid system. Simultaneously, to address potential overcurrent issues during faults, a current limiting circuit is added to the control loop to maintain the inverter's safe and stable operation. However, this method only considers the occurrence of a fault and maintaining synchronous stability and support during the fault; it does not consider how the inverter switches back to grid-connected mode after fault recovery, potentially leading to the risk of being unable to exit the current limiting mode.

[0008] The fault ride-through method for voltage and current source mode switching based on hybrid synchronous control proposed in this invention can achieve smooth mode switching by simply integrating parameter changes and control delays, without the risk of failing to exit current saturation.

[0009] II. Comparison with patent CN202410787854.2 "A droop control inverter current limiting control method and system";

[0010] Patent CN202410787854.2 discloses a current-limiting control method and system for a droop-controlled inverter. It accurately derives the virtual impedance and active power reference value constraints when the fault current equals the current-limiting value through phasor analysis. Considering the most severe current-limiting scenario, it designs a virtual impedance-current difference proportionality coefficient and an active power reference value-current difference proportionality coefficient. During a fault, the adaptive current change proportionally increases the virtual impedance and decreases the active power reference value, meeting the current-limiting requirements under different grid voltage dips. By adjusting the virtual impedance and active power reference value, the fault current is indirectly limited, preserving the voltage source characteristics of the droop-controlled inverter, and significantly enhancing the system's fault ride-through capability. However, this method cannot provide reactive power support to the grid during a fault.

[0011] The fault ride-through method based on voltage and current source mode switching of hybrid synchronous control proposed in this invention provides sufficient reactive power to support the grid voltage during the fault period according to the grid connection criteria. Summary of the Invention

[0012] To address the aforementioned technical challenges, this invention proposes a fault ride-through method based on hybrid synchronous control voltage / current source mode switching. Addressing the challenges of fault ride-through (FRT) in novel power systems, this invention overcomes the limitations of existing solutions from three key dimensions: overcurrent limiting, fault synchronization, and reactive power support. Based on a hybrid synchronous control strategy, this invention significantly improves system damping characteristics, effectively ensuring the synchronous stability of the inverter during faults. Simultaneously, by utilizing the simplified detection of the current limiter's saturation signal, combined with a hybrid voltage / current source control architecture, it achieves intelligent control that enables smooth mode switching without complex switching logic or grid information. Furthermore, this invention possesses excellent short-circuit ratio (SCR) adaptability, maintaining stable operation under varying grid strength conditions, providing an innovative solution for improving the grid-friendliness of renewable energy integration and optimizing the transient performance of power systems.

[0013] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0014] The fault ride-through method based on hybrid synchronous control voltage / current source mode switching includes the following steps:

[0015] Step 1) Measure the output current and voltage of the inverter and calculate the corresponding output power P and Q;

[0016] Step 2) Determine the internal potential phase angle by measuring the inverter's output voltage and calculating the inverter's output power;

[0017] Step 3) Calculate the current command i output by the controlled voltage control module by measuring the output voltage of the inverter. dqref1 ;

[0018] Step 4) Calculate the current setpoint i output by the controlled current control module by measuring the output voltage and current of the inverter. dqref2 ;

[0019] Step 5) by i dqref1 and i dqref2 The calculated current given value i dqref ;

[0020] Step 6) Current setpoint i dqref After processing by the current limiter, a limited current reference i is generated. dqlim ;

[0021] Step 7) The confined current reference i obtained from the previous step dqlim The inverter is controlled by a pulse width modulation (PWM) unit using a modulated signal.

[0022] As a further improvement to the present invention, the calculation of output power P and Q in step 1) is as follows:

[0023]

[0024]

[0025] In the formula, This refers to the active power output of the inverter. This refers to the reactive power output of the inverter. Let d be the d-axis component of the inverter's output voltage in the dq coordinate system. Let q be the output voltage of the inverter in the dq coordinate system. Let d be the d-axis component of the inverter's output current in the dq coordinate system. Let q be the q-axis component of the inverter's output current in the dq coordinate system.

[0026] As a further improvement of the present invention, the internal potential phase angle in step 2) is the internal potential phase angle based on hybrid synchronous control;

[0027] The internal potential phase angle generation calculation based on hybrid synchronous control is as follows:

[0028]

[0029] Where θ represents the reference angle generated by hybrid synchronous control, used for the dq / abc variation in inverter control, ɷ n and ɷ v P represents the reference angular frequency and the output angular frequency, respectively. ref P and V are the inverter's active power reference value and output active power, respectively. pcc_q This represents the q-axis component of the output voltage at the PCC, where m is the droop active power factor and k is the input voltage. pcc_q coefficient.

[0030] As a further improvement of the present invention, step 3) of the controlled voltage source control module uses PI voltage control to obtain the current given i output by the controlled voltage control module. dqref1 :

[0031]

[0032] Among them, i dref1 and i qref1 The current reference value i output by the controlled voltage control module dref The components on the d-axis and q-axis, k vp and k vi These are the proportional and integral parameters of the controlled voltage source loop, E ref Indicates voltage amplitude reference, v pcc_d V represents the d-axis component of the output voltage at point PCC. pcc_q The q-axis component of the output voltage at point PCC is represented, where s is the Laplace operator, and idref1 and i qref2 The current reference value is calculated by the controlled voltage source control module.

[0033] As a further improvement of the present invention, step 4) of the controlled current control module adopts decoupled PQ control to obtain the current reference i output by the controlled current control module. dqref2 :

[0034]

[0035] Among them, i dref2 and i qref2 The current reference value i calculated by the controlled current source control module dref Components on the d-axis and q-axis, P * lim and Q * lim These represent the reference values ​​for active and reactive power, respectively, v pcc_d V represents the d-axis component of the output voltage at point PCC. pcc_q This represents the q-axis component of the output voltage at point PCC.

[0036] To meet power grid specifications, P * lim and Q * lim The design is as follows:

[0037]

[0038]

[0039] Among them, S rated V represents the rated apparent output power, and λ represents the voltage sag ratio when reactive power is injected into the grid, as typically defined in grid specifications. g P is the grid voltage. ref This is the active power reference value of the inverter, E is the internal potential voltage, and ɷ n and ɷ v These represent the reference angular frequency and the output angular frequency, respectively, and m is the active droop coefficient.

[0040] As a further improvement of the present invention, the current given value i in step 5) dqref ;

[0041]

[0042] Where D represents the ratio between the controlled voltage source and the controlled current source loop;

[0043] When a fault occurs, the current limiter triggers current limiting and generates a mode switching signal, adjusting D from 0.5 to 1, thereby switching the inverter from hybrid voltage / current source mode to current source mode. After the fault is cleared, when P returns to P... ref Afterwards, the switching is delayed for a period of time to avoid transient oscillations. The inverter activation mode is switched, and D changes from 1 to 0.5.

[0044] As a further improvement of the present invention, the confined current reference i in step 6) dqlim ;

[0045] .

[0046] Among them, I max The maximum current amplitude set for the inverter, i dref and i qref The given current values ​​i are respectively dqref Components on the d-axis and q-axis.

[0047] Beneficial effects:

[0048] This invention proposes a fault ride-through method based on hybrid synchronous control for voltage / current source mode switching. This method significantly enhances system damping, effectively solves the synchronous stability problem of inverters during faults, and achieves intelligent and smooth mode switching without complex switching logic or additional grid information. It robustly suppresses overcurrent and provides effective reactive power support to maintain voltage. At the same time, its excellent short-circuit ratio (SCR) adaptability ensures stable operation under different grid strength conditions, significantly improves the friendliness of new energy grid integration, and optimizes the transient performance of the power system. Attached Figure Description

[0049] Figure 1 This is a flowchart of the present invention;

[0050] Figure 2 This is a schematic diagram of the control scheme of the present invention;

[0051] Figure 3 This is a diagram of mode switching signals;

[0052] Figure 4 The hierarchical structure required for the FRT of the GFM inverter;

[0053] Figure 5 Diagram of the experimental platform;

[0054] Figure 6 The voltage and current waveforms are shown during the fault occurrence phase.

[0055] Figure 7 The output power and frequency response diagrams during the fault period are shown.

[0056] Figure 8 The voltage and current waveforms are shown after the fault has been cleared.

[0057] Figure 9 The output power and frequency response diagrams are shown after the fault is cleared. Detailed Implementation

[0058] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments:

[0059] This invention proposes a fault ride-through method based on voltage / current source mode switching using hybrid synchronous control, as shown in the flowchart below. Figure 1 As shown. Figure 1 In the diagram, the purple box represents the synchronization control module, which generates the inverter phase angle through hybrid synchronization control; the yellow box represents the controlled voltage source control module, which generates current setpoint 1; the green box represents the controlled current source control module, which generates current setpoint 2; and then, through weighted mixing, the final current setpoint is generated; the white box represents the mode switching control module.

[0060] First, construct the hybrid synchronous control loop for the grid-connected (GFM) inverter; second, design the hybrid voltage / current source control architecture; third, refine the mode switching loop and current limiter. The specific steps are as follows:

[0061] 1. Based on hybrid synchronous control, generate the phase angle for inverter dq / abc conversion:

[0062] The synchronization loop used to generate the phase angle θ of the abc / dq transformation is based on v pcc_q The hybrid synchronous control formula is as follows:

[0063]

[0064] Where θ represents the reference angle generated by hybrid synchronous control, used for the dq / abc variation in inverter control. ref and ɷ represent the angular frequency reference value and the output angular frequency, respectively. P ref P and V represent the inverter's active power reference value and output active power, respectively. pcc_q This represents the q-axis component of the output voltage at the PCC. m is the droop active power factor, and k is the input voltage. pcc_q coefficient.

[0065] 2. Design a hybrid voltage / current source control architecture:

[0066] Hybrid voltage / current source loop is used to generate current reference i dqref It consists of two parts: generating i dqref1 Controlled voltage source control and generation of i dqref2 Controlled current source control.

[0067] The controlled voltage source control uses conventional PI voltage control:

[0068]

[0069] Where, k vp and k vi These are the proportional and integral parameters of the controlled voltage source loop, respectively. E ref Indicates the voltage amplitude reference. pcc_d This represents the d-axis component of the output voltage at point PCC.

[0070] Controlled current source control uses decoupled PQ control to obtain the current reference value i. dqref2 :

[0071]

[0072] Among them, P * lim and Q * lim These represent the reference values ​​for active and reactive power, respectively. To meet power grid specifications, and because P... * lim and Q * lim The design is as follows:

[0073]

[0074]

[0075] Among them, S rated V represents the rated apparent output power, and λ represents the voltage sag ratio when reactive power is injected into the grid, as typically defined in grid specifications. g This is the grid voltage.

[0076] 3. Mode switching control:

[0077] Current reference i dqref is i dqref1 and i dqref2 The weighted average is shown below:

[0078]

[0079] Where D represents the ratio between the controlled voltage source and the controlled current source loop.

[0080] Changes in D Figure 3 As shown. When a fault occurs, the current limiter triggers current limiting and generates a mode switching signal, adjusting D from 0.5 to 1, thereby switching the inverter from hybrid voltage / current source mode to current source mode. After the fault is cleared, when P returns to P...ref Afterwards, the switching is delayed for a period of time to avoid transient oscillations. The inverter activation mode is switched, and D changes from 1 to 0.5.

[0081] 4. Circular limiter:

[0082] Current given value i dqref After processing by the current limiter, a limited current reference i is generated. dqlim :

[0083]

[0084] The inverter is controlled by a pulse width modulation (PWM) unit using a modulated signal.

[0085] A schematic diagram of a fault ride-through scheme based on voltage / current source mode switching using hybrid synchronous control is shown below. Figure 2 As shown.

[0086] 5. Experimental verification

[0087] Experiments were conducted using an RTU-BOX and an RTI-INV8020IR inverter to verify the feasibility of the proposed method. The experimental platform configuration is as follows: Figure 5 As shown. The inverter's switching frequency is 10kHz. A three-phase grid voltage drop to 0.25 pu is selected as the fault condition.

[0088] Figure 6 and Figure 7 Experimental results under the proposed control scheme are presented. The proposed control scheme effectively limits the output current within a threshold range under fault conditions. During a fault, the inverter's active power drops to near zero, while the reactive power reaches 206 Var, meeting the core requirements of the power grid. Furthermore, after a small fluctuation, the inverter's frequency remains consistent with the grid frequency, and the inverter maintains synchronization with the grid system.

[0089] Figure 8 and Figure 9 The experimental waveforms of the proposed control scheme after fault clearance are shown. After fault clearance, the inverter's output power recovers rapidly. Subsequently, the inverter activates mode switching, recovers quickly, and the mode switching is smooth with minimal oscillation.

[0090] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any other way. Any modifications or equivalent changes made based on the technical essence of the present invention shall still fall within the scope of protection claimed by the present invention.

Claims

1. A fault ride-through method based on hybrid synchronous control voltage / current source mode switching, characterized in that, Includes the following steps: Step 1) Measure the output current and voltage of the inverter and calculate the corresponding output power P and Q; Step 2) Determine the internal potential phase angle by measuring the inverter's output voltage and calculating the inverter's output power; Step 3) Calculate the current command i output by the controlled voltage control module by measuring the output voltage of the inverter. dqref1 ; Step 4) Calculate the current setpoint i output by the controlled current control module by measuring the output voltage and current of the inverter. dqref2 ; Step 5) by i dqref1 and i dqref2 The calculated current given value i dqref ; Step 6) Current setpoint i dqref After processing by the current limiter, a limited current reference i is generated. dqlim ; Step 7) The confined current reference i obtained from the previous step dqlim The inverter is controlled by a pulse width modulation (PWM) unit using a modulated signal.

2. The fault ride-through method based on hybrid synchronous control voltage / current source mode switching according to claim 1, characterized in that, The output power P and Q in step 1) are calculated as follows: ； ； In the formula, This refers to the active power output of the inverter. This refers to the reactive power output of the inverter. Let d be the d-axis component of the inverter's output voltage in the dq coordinate system. Let q be the output voltage of the inverter in the dq coordinate system. Let d be the d-axis component of the inverter's output current in the dq coordinate system. Let q be the q-axis component of the inverter's output current in the dq coordinate system.

3. The fault ride-through method based on hybrid synchronous control voltage / current source mode switching according to claim 1, characterized in that, In step 2), the internal potential phase angle is the internal potential phase angle based on hybrid synchronous control; The internal potential phase angle generation calculation based on hybrid synchronous control is as follows: ； Where θ represents the reference angle generated by hybrid synchronous control, used for the dq / abc variation in inverter control, ɷ n and ɷ v P represents the reference angular frequency and the output angular frequency, respectively. ref P and V are the inverter's active power reference value and output active power, respectively. pcc_q This represents the q-axis component of the output voltage at the PCC, where m is the droop active power factor and k is the input voltage. pcc_q coefficient.

4. The fault ride-through method based on hybrid synchronous control voltage / current source mode switching according to claim 1, characterized in that, In step 3), the controlled voltage source control module uses PI voltage control to obtain the current given i output by the controlled voltage control module. dqref1 : ； Among them, i dref1 and i qref1 The current reference value i output by the controlled voltage control module dref The components on the d-axis and q-axis, k vp and k vi These are the proportional and integral parameters of the controlled voltage source loop, E ref Indicates voltage amplitude reference, v pcc_d V represents the d-axis component of the output voltage at point PCC. pcc_q The q-axis component of the output voltage at point PCC is represented, where s is the Laplace operator, and i dref1 and i qref2 The current reference value is calculated by the controlled voltage source control module.

5. The fault ride-through method based on hybrid synchronous control voltage / current source mode switching according to claim 1, characterized in that, In step 4), the controlled current control module uses decoupled PQ control to obtain the current command i output by the controlled current control module. dqref2 : ； Among them, i dref2 and i qref2 The current reference value i calculated by the controlled current source control module dref Components on the d-axis and q-axis, P * lim and Q * lim These represent the reference values ​​for active and reactive power, respectively, v pcc_d V represents the d-axis component of the output voltage at point PCC. pcc_q This represents the q-axis component of the output voltage at point PCC. To meet power grid specifications, P... * lim and Q * lim The design is as follows: ； ； Among them, S rated V represents the rated apparent output power, and λ represents the voltage sag ratio when reactive power is injected into the grid, as typically defined in grid specifications. g P is the grid voltage. ref This is the active power reference value of the inverter, E is the internal potential voltage, and ɷ n and ɷ v These represent the reference angular frequency and the output angular frequency, respectively, and m is the active droop coefficient.

6. The fault ride-through method based on hybrid synchronous control voltage / current source mode switching according to claim 1, characterized in that, In step 5), the current given value i dqref ; ； Where D represents the ratio between the controlled voltage source and the controlled current source loop; When a fault occurs, the current limiter triggers current limiting and generates a mode switching signal, adjusting D from 0.5 to 1, thereby switching the inverter from hybrid voltage / current source mode to current source mode. After the fault is cleared, when P returns to P... ref Afterwards, the switching is delayed for a period of time to avoid transient oscillations. The inverter activation mode is switched, and D changes from 1 to 0.

5.

7. The fault ride-through method based on hybrid synchronous control voltage / current source mode switching according to claim 1, characterized in that, In step 6), the restricted current reference i dqlim ; ； Among them, I max The maximum current amplitude set for the inverter, i dref and i qref The given current values ​​i are respectively dqref Components on the d-axis and q-axis.

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