Composite short-circuit current control strategy for offshore wind power short-circuit fault

Abstract

The invention provides a composite short-circuit current control strategy for an offshore wind power short-circuit fault, and the strategy comprises the steps: forbidding an integral term of outer ring PI control, and coordinating the distribution of reactive power and active power to dynamically adjust an inner ring voltage reference value. And when a serious short-circuit fault occurs in the flexible DC power transmission system, freezing the integrator controlled by PI, retaining the current value of the integral term, and continuing to use the proportional term to maintain the response. In order to maintain voltage stability and reduce voltage difference of driving short-circuit current, on the basis of known active power and reactive power feasible regions, reactive power and active power distribution should adopt a reactive power priority strategy to increase voltage recovery capability, accelerate fault recovery and guarantee system safety. According to the invention, the short-circuit current during the short-circuit fault of the offshore wind power flexible direct-current power transmission system is reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of short-circuit current suppression in flexible direct current systems, and in particular relates to a composite short-circuit current control strategy for offshore wind power short-circuit faults. Background Art

[0002] With the continuous development of high-voltage direct current (HVDC) transmission technology, the Flexible Direct Current (VSC-HVDC) system, with its advantages such as the ability to independently regulate active and reactive power, and its lack of commutation failure and reactive power compensation issues, has made it play an increasingly important role in power grids. When a Flexible Direct Current (VSC) transmission project transmits low power, the short-circuit current it contributes to an AC system short-circuit fault is negligible compared to the short-circuit current provided by the AC system itself. Therefore, it is generally believed that Flexible Direct Current (VSC) transmission systems do not provide short-circuit current. However, for AC systems with large short-circuit capacity, the AC short-circuit current contributed by the Flexible Direct Current (VSC-HVDC) converter may cause the overall system short-circuit current to approach or exceed the rated breaking current of the circuit breaker, making the short-circuit fault difficult to clear.

[0003] Therefore, short-circuit current suppression strategies for flexible DC systems are gaining increasing attention. To prevent short-circuit current from flowing excessively, some literature has proposed using dual-vector control to convert AC quantities to DC quantities between two coordinate systems, followed by tracking and regulation using a proportional-integral controller. This control method offers excellent control effectiveness, but drawbacks include a complex control structure and errors in the positive- and negative-sequence decomposition. Other literature addresses the threat posed by negative-sequence current to the safe and continuous operation of MMC-HVDC systems during asymmetric AC faults. By adding positive- and negative-sequence damping to the positive- and negative-sequence inner-loop control systems in synchronously rotating coordinate systems, the negative-sequence current reference value is set to zero, accelerating the tracking of the inner-loop control output to the reference value, thereby achieving negative-sequence current suppression. Furthermore, some literature considers that increasing an equivalent impedance from the fault point to the MMC virtual potential can prevent current overshoot during AC faults, but this comes at the cost of increased construction costs. Therefore, a control strategy using a virtual impedance to prevent current overshoot has been proposed. Ultimately, this approach incorporates the virtual voltage generated by the virtual impedance into the outer-loop control, thus saving the cost of adding an equivalent impedance. Therefore, exploring the ways to suppress the short-circuit fault current of offshore wind power entering the large receiving power grid through flexible direct current, and optimizing and forming a composite control strategy for flexible direct current short-circuit current that is suitable for the large receiving power grid, are of great significance for reducing the short-circuit current of the flexible direct current system, enhancing the voltage recovery capability, and accelerating the recovery of short-circuit faults. Summary of the Invention

[0004] The present invention aims to provide a composite short-circuit current control strategy for offshore wind power short-circuit faults. This strategy, focusing on the MMC of offshore wind power flexible direct current transmission systems, reduces the short-circuit current during short-circuit faults in offshore wind power flexible direct current transmission systems. By prioritizing reactive power in power allocation, the system's fault recovery capability is accelerated. The technical solutions employed are as follows:

[0005] A composite short-circuit current control strategy for offshore wind power short-circuit faults includes the following steps:

[0006] Real-time detection of the voltage V at the flexible direct current common connection point PCC a and current I ac , when V a < voltage threshold or I ac > current threshold, the converter MMC sends a command to the outer loop power controller to disable the PI control integral link;

[0007] At the same time, according to the feasible domain of the power reference value, the inner loop voltage reference value u output by the waveform modulation module is dref 、u qref Make adjustments.

[0008] Preferably, the inner loop voltage reference value u dref 、u qref The adjustment specifically includes the following steps:

[0009] Step A: According to the reactive power reference value at the time of the fault, the maximum allowable reactive power Q is found in combination with the feasible region of the power reference value. refmax , according to Q refmax Get the corresponding maximum inner loop voltage reference value u of the q axis qrefmax , and finally find the reduction coefficient k added to the q axis qreduce :

[0010]

[0011] k qreduce =u qrefmax / u qref

[0012] Where, X is the system reactance; δ is the power angle; V grid -Grid voltage;

[0013] Q ref -Reactive power reference value; V1-q axis inner loop voltage reference value;

[0014] Step B: Find Q based on the feasible region of the power reference value refmax The corresponding active power P ref Then calculate the corresponding d-axis inner ring voltage reference value u according to the formuladref′ , and finally find the reduction coefficient k added to the d axis dreduce :

[0015]

[0016] k dreduce =u dref′ / u dref

[0017] Among them, P ref - Active power reference value; V2-d axis inner loop voltage reference value.

[0018] Preferably, the feasible range of the power reference value is calculated as follows:

[0019] The upper bound of the power feasible region is:

[0020]

[0021] P ref - Active power of converter station under fault, Q ref - Reactive power of converter station under fault, A, B, and C are all system constants;

[0022] At maximum operating DC voltage U dc As the boundary, the lower bound of the power feasible region is obtained:

[0023]

[0024] Among them, P ref0 - Initial active power of the converter station before the fault, T - fault duration, C eq -DC capacitance, K U Maximum allowable DC voltage coefficient.

[0025] Preferably, the inner loop voltage reference value U output by the waveform modulation module new_dref 、U new_dref for:

[0026] U new_dref =U dref *k dreduce

[0027] U new_dref =U qref *k qreduce

[0028] Among them, U new_dref -Adjusted d-axis inner ring voltage reference value;

[0029] U new_qref - Adjusted Q-axis inner loop voltage reference value.

[0030] This paper takes the MMC of offshore wind power flexible direct current transmission system as the research object, and proposes a multi-time scale coordinated composite short-circuit current control strategy that adjusts the inner loop voltage reference value by disabling the outer loop PI control integral term and coordinating the distribution of reactive power and active power.

[0031] Compared with the prior art, the advantages of the present invention are:

[0032] When a serious short-circuit fault occurs in the flexible DC transmission system, the control strategy detects the short-circuit current and issues an instruction to disable the integral term of the outer loop PI control.

[0033] In order to maintain voltage stability, the distribution of reactive power and active power should adopt a reactive power priority strategy to speed up voltage recovery, reduce the voltage difference that drives short-circuit current, speed up fault recovery, and ensure system safety.

[0034] Because the function of reactive power is to raise the voltage at the fault point, reactive power priority will accelerate the rise of voltage and reduce the voltage difference.

[0035] In summary, the present invention can respond to faults quickly. After disabling the integral term of the outer loop PI control, the delay link of the outer loop control is reduced, so that the system can respond to short-circuit faults more quickly and quickly change the output characteristics of the converter, thereby effectively controlling the rising trend of the short-circuit current in a short time, so that the fault current can be reduced to a safe range more quickly, creating favorable conditions for the rapid clearing of the fault, helping to reduce the interference and impact of the fault on the overall operation of the system, and improving the power supply reliability and continuity of the system.

[0036] Adjusting the inner loop voltage reference value can precisely control the converter output voltage, thereby precisely regulating the short-circuit current. The voltage reference value output by the inner loop voltage is the output voltage of the MMC.

[0037] Compared with some other more general short-circuit current control strategies, this composite strategy can flexibly set the inner-loop voltage reference value according to the actual situation of the short-circuit fault, achieve refined control of the short-circuit current, keep it within the expected reasonable range, and better meet the requirements of safe and stable operation of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 This is the structural diagram of the offshore wind power flexible direct current transmission system;

[0039] Figure 2 This is the topology diagram when MMC is working normally;

[0040] Figure 3 This is the control block diagram of the composite short-circuit current control strategy for offshore wind power short-circuit faults;

[0041] Figure 4 Schematic diagram of the power feasible region;

[0042] Figure 5 Flowchart of method for disabling outer loop PI integrator;

[0043] Figure 6 Coordinated control flow chart for multiple time scales. DETAILED DESCRIPTION

[0044] The following, with reference to a schematic diagram, provides a more detailed description of the composite short-circuit current control strategy for offshore wind turbine short-circuit faults. This schematic diagram illustrates a preferred embodiment of the present invention. It should be understood that those skilled in the art may modify the present invention described herein while still achieving its beneficial effects. Therefore, the following description should be understood as a general guideline for those skilled in the art and not as a limitation of the present invention.

[0045] like Figures 1 to 6 ,A composite short-circuit current control strategy for offshore wind power short-circuit faults includes the following steps: when the flexible DC system fails, the outer loop PI integrator is disabled, and reactive power is allocated first.

[0046] Real-time detection of the voltage V at the flexible direct current common connection point PCC a and current I ac , when V a < voltage threshold or I ac When the current is greater than the threshold, the flexible DC system fails.

[0047] 1. Disable the outer loop PI integral term:

[0048] like Figure 5 As shown, when the MMC is working normally, the outer loop power controller first outputs the reference current of the d-axis and q-axis respectively according to the different control quantities. and

[0049] The inner loop current controller outputs the reference value U of the AC side voltage of the converter station according to the AC side current value and the reference current value. dref and U qref .

[0050] Ultimately, the trigger pulse will control the working status of the upper and lower bridge arm sub-modules according to the voltage reference value, achieving the purpose of controlling the amplitude and phase of the AC side voltage V, thereby ensuring that the AC side current can quickly track the reference current and eventually be consistent with the reference current, realizing the control of electrical quantities such as power or voltage of the converter station.

[0051] When a fault occurs in the flexible DC system, the voltage at the flexible DC common connection point (PCC) will decrease, and the actual active power and reactive power provided by the flexible DC system will decrease. However, the active power and reactive power command indicators will not change. Therefore, after the active current and reactive current pass through the PI control link, the active current and reactive current command indicators of the outer loop will continue to increase to the limit value of the outer loop control, resulting in excessive short-circuit current at the PCC point, affecting the stability of the power grid and equipment safety.

[0052] Therefore, this embodiment considers starting from the PI control link to limit the short-circuit current.

[0053] First, the voltage and current signals on the AC side of the system are monitored in real time. When the voltage at the PCC point is detected to be lower than the set threshold or the current exceeds the set value, it is determined that a short circuit fault has occurred in the system.

[0054] When a short-circuit fault is detected in the system, the MMC immediately issues a command to disable the outer-loop PI control integrator. This command is quickly transmitted to the outer-loop PI controller, i.e., the outer-loop controller, through the control system's communication network or hardware circuit.

[0055] After receiving the disable command, the outer loop PI controller stops the integrator adjustment function in less than 2 milliseconds, retains the current value of the integral term, and continues to use the proportional term to maintain the response, avoiding excessive current reference values ​​due to integral accumulation, reducing short-circuit current, improving system dynamic performance, and enhancing system stability.

[0056] like Figure 3 As shown, the converter MMC sends a command to disable the outer loop PI control integrator to the outer loop power controller; the outer loop power controller outputs the d-axis reference current q-axis reference current

[0057] like Figure 5 As shown, when only the proportional term kp is used, the input of the integrator module includes the reference value and the actual input value.

[0058] The reference value refers to the reactive power and active power originally set by the system.

[0059] The actual input value refers to the reactive power and active power at the PCC fault point.

[0060] The outer loop power controller adopts fixed active power control and fixed reactive power control, and performs PI adjustment with the actual input PCC point reactive power and active power, and then obtains a d-axis current reference value and a q-axis current reference value to input into the inner loop current controller.

[0061] 2. Prioritize reactive power allocation:

[0062] According to the instantaneous power theory, it can be known that the feasible range of active power and reactive power of the converter station under the maximum allowable AC current limit is related to the AC bus voltage of the receiving converter station.

[0063] Taking the maximum allowable AC current as the boundary, the upper bound of the power feasible region can be obtained:

[0064]

[0065] P ref is the active power of the converter station under fault conditions, Q ref is the reactive power of the converter station under fault conditions, and A, B, and C are constants determined by the system's current capacity, transformer equivalent reactance, and AC bus equivalent reactance.

[0066] Taking the maximum operating DC voltage as the boundary, the lower bound of the feasible region of the power reference value can be obtained:

[0067]

[0068] P ref0 is the initial active power of the converter station before the fault, which is a constant value;

[0069] T is the fault duration;

[0070] C eq is the DC capacitance, which is a constant value;

[0071] K U is the maximum allowable DC voltage coefficient, which is a constant value.

[0072] Based on the obtained feasible region with reference values, the voltage reference values ​​of the d-axis and q-axis outputs of the inner loop voltage control are adjusted.

[0073] Since the q-axis voltage reference value is adjusted first during fault recovery, reactive current is quickly injected to support the AC voltage. Specifically, it includes:

[0074] Step A: According to the reactive power reference value at the time of the fault, the maximum allowable reactive power Q is found in combination with the feasible region of the reactive power reference value. refmax , according to Q refmax Get the corresponding maximum inner loop voltage reference value u of the q axis qrefmax , and finally find the reduction coefficient k added to the q axis qreduce :

[0075]

[0076] k qreduce =u qrefmax / u qref

[0077] Among them, X-system reactance, is a constant value; δ-power angle, is a constant value; V grid -Grid voltage, which is the measured value; Q ref -Reactive power reference value; V1-q axis inner loop voltage reference value.

[0078] like Figure 4 As shown, according to the reactive power Q at the time of the fault (falling within the area enclosed by the lower boundary and the upper boundary), a vertical line is drawn. One of the intersection points of the vertical line and the upper boundary is (Q refmax , P ref ).

[0079] Step B: Find Q based on the feasible region of active power reference value refmax The corresponding active power reference value P ref Then calculate the corresponding d-axis inner ring voltage reference value u according to the formula dref′ , and finally find the reduction coefficient k added to the d axis dreduce :

[0080]

[0081] k dreduce =u dref′ / u dref

[0082] Among them, P ref - Active power reference value; V2-d axis inner loop voltage reference value.

[0083] The inner loop current controller of the converter MMC outputs the inner loop voltage reference value u dref 、U qref The modulation module to the converter MMC generates a voltage waveform.

[0084] The inner loop voltage reference value U output by the waveform modulation module new_dref 、U new_dref for:

[0085] U new_dref =U dref *k dreduce

[0086] U new_qref =U qref *k qreduce

[0087] Among them, U new_dref -Adjusted d-axis inner ring voltage reference value;

[0088] U new_qref - Adjusted Q-axis inner loop voltage reference value.

[0089] The flexible DC system is mainly divided into three modules: main circuit module, control system module and waveform modulation module.

[0090] like Figure 3 As shown, the control system module is divided into an outer loop power controller and an inner loop current controller. The active power reference value P is set according to the required reactive power Q and active power P. ref and reactive reference value Q ref , and input them into the outer loop power controller, which then outputs the reference current to the inner loop current controller according to the active and reactive reference values;

[0091] The inner loop current controller can obtain the voltage reference value U according to the input current reference value dref 、U qref And the modulation coefficient (reduction coefficient), and then get the new voltage reference value U that meets the reactive priority new_dref 、U new_qref , and then set the new voltage reference value U new_dref 、U new_qref Input to the waveform modulation module;

[0092] Then the waveform modulation module obtains the voltage reference value U new_dref 、U new_qref To modulate and output the voltage waveform to the main circuit module, so that the active and reactive power of the main circuit module can meet the requirements.

[0093] That is, u is calculated according to the formula in steps A to B. qrefmax 、u dref′ It is not possible to directly change the inner loop output voltage reference value, so the reduction coefficient is set to make the original inner loop voltage reference value become u qrefmax 、u dref′ .

[0094] Although reactive power does not perform work in the power system, it supports AC system voltage recovery and plays a key role in maintaining system voltage stability. By increasing the output of reactive power, the AC system voltage level can be effectively raised and short-circuit currents driven by voltage differences can be reduced.

[0095] Active power can effectively reduce the fluctuation of DC voltage and ensure the stable operation of the system.

[0096] When adjusting the inner loop voltage reference value, the system needs to prioritize the allocation of reactive power to support the voltage recovery at the PCC point of the AC system.

[0097] Increasing reactive power can effectively raise the voltage level of the AC system and reduce short-circuit currents driven by voltage differences. The purpose of reactive power in a flexible DC system is to provide reactive power to raise the voltage.

[0098] like Figure 6 As shown, in a flexible DC system, the composite short-circuit current control strategy proposed in the present invention can be divided into two response time periods.

[0099] First, in the fast time scale of millimeter level, when the system fails, the integral link of the outer loop PI control is disabled to quickly cut off the cumulative effect of the integral and limit the continuous rise of the short-circuit current.

[0100] Then, during the slow time scale period of the control strategy, the short-circuit current amplitude is limited by the reference value of the dynamic inner loop voltage to maintain the dynamic stability of the system.

[0101] 4.1 Rapid flow limiting period

[0102] The PCC point voltage is monitored in real time within milliseconds at the initial stage of a fault. Within 2ms after the fault is detected, the integral function of the outer loop PI controller is frozen, and the update of the power or voltage reference value is stopped to avoid outer loop regulation delays or malfunctions, giving priority to ensuring reactive current to support the AC voltage.

[0103] 4.2 Dynamic recovery period

[0104] After the current is initially suppressed, if the PCC voltage is detected to be restored to V ac >0.7pu, enter the recovery process;

[0105] According to the fixed slope (k ramp =0.1pu / ms) gradually restores the outer loop PI control reference value, and the outer loop readjusts the DC voltage to generate the active current reference value.

[0106]

[0107] P max - Maximum power variation;

[0108] 4.3 Steady-state transition period

[0109] After the fault is cleared, the system transitions to normal operation and the integral coefficient k of the outer loop PI controller is i Gradually recover to the rated value according to the exponential curve.

[0110] During the restoration process, reactive power priority restrictions are gradually lifted and the i d with i q , and release the limit value adjustment of the inner loop voltage.

[0111] The above description is merely a preferred embodiment of the present invention and does not limit the present invention in any way. Any person skilled in the art who, without departing from the scope of the present invention, makes any equivalent substitution, modification, or other changes to the technical solution and technical content disclosed in the present invention shall be deemed to be within the scope of the present invention and still fall within the scope of protection of the present invention.

Claims

1. A composite short-circuit current control strategy for offshore wind power short-circuit faults, characterized in that: The following steps are involved: Real-time detection of the voltage and current at the flexible direct current common connection point (PCC). When the voltage is less than the voltage threshold or the current is greater than the current threshold, the converter MMC sends a command to the outer loop power controller to disable the PI control integral link. At the same time, according to the feasible domain of the power reference value, the inner loop voltage reference value u output by the waveform modulation module is dref 、u qref Make adjustments.

2. The composite short-circuit current control strategy for offshore wind power short-circuit fault according to claim 1 is characterized in that , inner loop voltage reference value u dref 、u qref The adjustment specifically includes the following steps: Step A: According to the reactive power reference value at the time of the fault, combined with the feasible region of the power reference value, find the maximum allowable reactive power Q refmax , according to Q refmax Get the corresponding maximum inner loop voltage reference value u of the q axis qrefmax , and finally find the reduction coefficient k added to the q axis qreduce : k qreduce =in qrefmax / in qref Where, X is the system reactance; δ is the power angle; V grid -Grid voltage; Q ref -Reactive power reference value; V1-q axis inner loop voltage reference value; Step B: Find Q based on the feasible region of the power reference value refmax The corresponding active power P ref Then calculate the corresponding d-axis inner ring voltage reference value u according to the formula dref′ , and finally find the reduction coefficient k added to the d axis dreduce : k dreduce =in dref′ / in dref Among them, P ref - Active power reference value; V2-d axis inner loop voltage reference value.

3. The composite short-circuit current control strategy for offshore wind power short-circuit fault according to claim 1 is characterized in that: The feasible region of the power reference value is calculated as follows: The upper bound of the power feasible region is: P ref - Active power of converter station under fault, Q ref - Reactive power of converter station under fault, A, B, and C are all system constants; At maximum operating DC voltage U dc As the boundary, the lower bound of the power feasible region is obtained: Among them, P ref0 - Initial active power of the converter station before the fault, T - fault duration, C eq -DC capacitance, K U - Maximum permissible DC voltage coefficient.

4. The composite short-circuit current control strategy for offshore wind power short-circuit fault according to claim 1, characterized in that: The inner loop voltage reference value U output by the waveform modulation module new_dref 、U new_dref for: IN new_dref =U dref *k dreduce IN new_qref =U qref *k qreduce Among them, U new_dref -Adjusted d-axis inner ring voltage reference value; U new_qref - Adjusted Q-axis inner loop voltage reference value.

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