Fault ride-through control strategy for direct-drive wind turbine based on smooth switching under unbalanced conditions

Abstract

The present application relates to the imbalance working condition based on the smooth switching of direct drive fan fault ride through control strategy, including the following steps, the mathematical model of the grid side inverter under the imbalance working condition is established, and the power expression of the grid side inverter under the imbalance working condition is given based on instantaneous power theory.Based on the established mathematical model and the given power expression, the smooth switching control strategy is designed and realized, so that the double common frequency oscillation of the active power output by the grid side converter and the grid current imbalance degree are reduced at the same time.The present application can well inhibit the double common frequency oscillation of the active power output by the grid side converter of direct drive wind turbine under the imbalance working condition and greatly reduce the grid current imbalance degree, so as to stabilize the DC side voltage during asymmetric fault and meet the grid current requirement.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of power electronics, and particularly relates to a direct-drive wind turbine fault ride-through control strategy based on smooth switching under unbalanced conditions. BACKGROUND

[0002] The direct-drive permanent magnet synchronous wind turbine generator set adopting the double-converter topology structure of grid-side and machine-side has a faster regulating speed, can output stable active power, and has good reactive power regulating performance. Meanwhile, due to the isolation effect of the grid-side inverter and the DC side circuit, the direct-drive permanent magnet wind turbine generator set has certain superiority in the low voltage ride-through process compared with the commonly used doubly-fed wind turbine generator set. Therefore, the direct-drive permanent magnet wind turbine generator set is more and more widely applied in large-scale wind farms.

[0003] However, due to the special topology of the double-converter of the grid-side and the machine-side, when the grid voltage drops, the direct-drive permanent magnet synchronous wind turbine generator inevitably has the phenomenon of overcurrent of the grid-side inverter and is often accompanied by the rise of the DC side voltage. Therefore, when the voltage drop amplitude is large, if no measures are taken to suppress or improve the overcurrent and overvoltage, the inverter and the DC side capacitor will be damaged, which may cause the direct-drive wind turbine to be off-grid, expand the fault scale and cause great harm to the power grid. Therefore, with the expansion of the scale of wind power, China has formulated a series of technical requirements for wind farm grid-connected power generation, especially exact requirements for low voltage ride-through capability.

[0004] The traditional fault ride-through strategy is that the machine-side rectifier is responsible for controlling the active power output by the permanent magnet synchronous generator, the grid-side inverter is responsible for realizing the reactive power support to the grid and the DC side voltage control, and the DC side consumes the excess power through the energy dissipation resistor to suppress the DC side overvoltage. Considering that the grid voltage is unbalanced due to the unbalanced grid load, and most of the faults in the grid are asymmetric faults, and the traditional fault ride-through control often does not involve suppressing the active power and reactive power double-frequency fluctuations and the grid current imbalance caused by the negative sequence current under asymmetric faults, therefore, the traditional control strategy needs to be improved under asymmetric fault conditions.

[0005] In the asymmetric fault ride-through process, the grid-side inverter is switched to positive sequence and negative sequence current control, and the smooth switching between the optimized current given value and the control strategy is used to achieve the purpose of simultaneously suppressing the active power double-frequency and the grid current imbalance. At the same time, the machine-side rectifier is switched to the direct voltage control to stabilize the DC side voltage, and the positive sequence and negative sequence currents of the grid-side inverter have a larger regulating margin to adapt to various faults. SUMMARY

[0006] To solve the above problems, the application proposes a direct-drive wind turbine fault ride-through control strategy based on smooth switching under unbalanced conditions. The smooth switching control strategy of the grid-side inverter based on positive and negative sequence voltage orientation is as shown in Figure 1 The smooth switching control strategy of the machine-side rectifier based on stator voltage orientation is as shown in Figure 2

[0007] The application adopts a switching control strategy, that is, the traditional control strategy is adopted under non-fault (balanced) conditions, and the grid-side inverter is switched to control the positive and negative sequence currents when the grid voltage is unbalanced, so that the positive and negative sequence currents are optimally tracked without difference. At the same time, the machine-side rectifier is switched from the traditional active outer loop control to the direct voltage outer loop control at the time of fault, which stabilizes the DC side voltage while allowing the grid-side inverter positive and negative sequence currents to have a larger regulation margin to adapt to various faults and reduce the action time of the energy consumption resistor, and has better fault ride-through capability than the traditional control strategy.

[0008] The technical scheme of the application is as follows:

[0009] First, the mathematical model of the grid-side inverter under unbalanced conditions is established, and the power expression of the grid-side inverter operating under unbalanced conditions is given based on the instantaneous power theory, thereby giving the optimized current command value.

[0010] The established mathematical model and the given power expression are used to design a fault ride-through control strategy based on smooth switching, thereby reducing the double public frequency oscillation of the active power output by the grid-side converter, the grid current imbalance degree and the action time of the crowbar resistor, so as to stabilize the DC voltage and suppress the grid current imbalance during fault ride-through, and reduce the heat dissipation pressure of the crowbar resistor.

[0011] The mathematical model of the grid-side inverter under unbalanced conditions is established based on the above-mentioned direct-drive wind turbine fault ride-through control strategy based on smooth switching under unbalanced conditions:

[0012]

[0013] Formula (1) is the voltage equation of the grid-side inverter of the direct-drive wind turbine under unbalanced conditions.

[0014] The power expression of the grid-side inverter under unbalanced conditions is given according to the instantaneous power theory based on the above-mentioned direct-drive wind turbine fault ride-through control strategy based on smooth switching under unbalanced conditions. The apparent power, active power and reactive power are given by the following formulas respectively:

[0015]

[0016]

[0017] ​The formula (2) is apparent power, the formula (3) is active power and reactive power, and it is not difficult to see from the formula (3) that the output active power of the grid-side inverter works in an unbalanced working condition and exists in double-frequency fluctuation, thereby causing DC voltage fluctuation.

[0018] In the above unbalanced working condition based on the smooth switching of the direct-drive wind turbine fault ride-through control strategy, the relationship between the active power components (P0, P1, P2) and the grid-side positive and negative sequence currents is observed, and in the case of positive and negative sequence grid voltage orientation, the optimized current instruction value for simultaneously suppressing the double-frequency oscillation of active power and the grid current imbalance is given.

[0019]

[0020] The formula (4) shows the optimized grid-side inverter positive and negative sequence current instruction value.

[0021] In the above unbalanced working condition based on the smooth switching of the direct-drive wind turbine fault ride-through control strategy, the positive and negative sequence voltage orientation control strategy containing a resonant controller (used to control the negative sequence current in the positive sequence rotating coordinate system, which is a double-frequency component) is designed according to the positive and negative sequence current given value, so that the grid-side inverter positive and negative sequence current can follow the instruction value without error.

[0022] In the above unbalanced working condition based on the smooth switching of the direct-drive wind turbine fault ride-through control strategy, the stator voltage orientation DC voltage control strategy for switching in the unbalanced working condition is designed, the grid-side inverter suppresses the output active double-frequency fluctuation and the grid-side negative sequence current, and controls the DC voltage at the same time, so as to realize the fault ride-through under asymmetric fault together with the grid-side converter and the DC side energy dissipation resistor.

[0023] In the above unbalanced working condition based on the smooth switching of the direct-drive wind turbine fault ride-through control strategy, considering that the roles of the DC voltage outer loops in the grid-side inverter and the machine-side rectifier in regulating the DC voltage are not exactly the same, which directly leads to the fact that when the DC voltage outer loop output current instruction value will appear mutation when the grid-side control DC voltage is switched to the machine-side control DC voltage in the normal working condition (and the same reason when the machine-side control DC voltage is switched to the grid-side control DC voltage after fault recovery). Therefore, the present application designs and realizes the smooth switching between different control strategies.

[0024] The advantage of the application is that the resonant controller is designed in the rotating coordinate system to make the current track the given value, while the switching control is adopted to make the machine side inverter undertake the task of stabilizing the DC voltage in the fault ride-through process, and the grid side inverter undertakes the task of regulating the positive sequence and negative sequence current in the fault ride-through process, so that the action time of the energy consumption resistor can be effectively reduced, the DC voltage can be stably maintained, and further improvement is made on the basis of the switching control to realize the smooth switching between different control strategies. Compared with the traditional control strategy, the multi-target collaborative control is realized, the loss is reduced, and the influence of the grid side DC voltage outer ring on the current control is avoided. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 The grid side inverter switching control strategy of the application.

[0026] Figure 2 The machine side rectifier switching control strategy of the application.

[0027] Figure 3 The simulation waveform under the traditional control strategy.

[0028] Figure 4 The simulation waveform under the improved control strategy.

[0029] Figure 5 The simulation waveform when the direct switching is adopted.

[0030] Figure 6 The simulation waveform when the smooth switching is adopted. DETAILED DESCRIPTION

[0031] The traditional fault ride-through control strategy of the direct drive wind turbine usually does not involve the suppression of the double fundamental frequency oscillation of active power and the suppression of the grid-connected negative sequence current under unbalanced conditions, or only involves one of them and does not consider the improved control of the machine side rectifier. However, under unbalanced conditions, if the active double frequency fluctuation is not suppressed, the DC voltage will produce double frequency fluctuation, which will endanger the safety of the DC side capacitor; at the same time, if the grid-connected negative sequence current is not suppressed, the grid-connected current requirement under the grid voltage imbalance cannot be met.

[0032] The application gives the grid side positive and negative sequence current command value under the condition that the relationship between the known power components (P0, P1, P2) and the grid side positive and negative sequence currents is known. And under the condition of positive and negative sequence voltage orientation, smooth switching is adopted at the same time, so that the grid side converter switches to the single current loop based on the PI-R (proportional integral resonant) controller under the fault condition to achieve the purpose of multi-target collaborative control, and the machine side control switches to the DC voltage control based on the stator voltage orientation, which cooperates with the grid side to adapt to different fault conditions and reduces the action time of the energy consumption resistor.

[0033] The application mainly comprises the following steps: establishing a mathematical model of the grid-side inverter under unbalanced working conditions, and giving a power expression of the grid-side inverter under unbalanced working conditions based on the instantaneous power theory. Based on the established mathematical model and the given power expression, a smooth switching control strategy is designed and implemented to simultaneously reduce the double grid frequency oscillation of the active power output by the grid-side converter and the grid current unbalance degree. When the direct-drive wind turbine works under unbalanced working conditions, the control strategy of the grid-side inverter is as follows: the DC voltage U dc is controlled by the traditional control strategy, and the positive and negative sequence dq current command values of the grid-side are tracked by a single current loop; the control strategy of the machine-side converter is as follows: the active power outer loop of the traditional control strategy is switched to the DC voltage outer loop.

[0034] In order to make the above-mentioned purposes, features and advantages of the application more obvious and easy to understand, the application will be further described in detail below with reference to the drawings.

[0035] Step 1: a mathematical model of the grid-side inverter under unbalanced grid conditions is established, and a power expression of the grid-side inverter is established according to the instantaneous power theory, thereby giving an optimized current command value:

[0036] Step 1: the mathematical model of the grid-side inverter under unbalanced grid conditions is

[0037]

[0038] Formula (1) is the voltage equation of the positive sequence component of the grid-side inverter under unbalanced working conditions in the positive sequence synchronous rotating coordinate system and the voltage equation of the negative sequence component in the negative sequence synchronous rotating coordinate system.

[0039] Step 2: based on the instantaneous power theory, an expression of the output power of the grid-side inverter working under unbalanced working conditions is given.

[0040]

[0041]

[0042] Formula (2) is the complex power output by the grid-side inverter, and formula (3) is the active power output by the grid-side inverter, wherein P1cos(2ω g t) and P2cos(2ω g t) are both oscillation powers of 2 times the grid frequency.

[0043] Step 3: the relationship between the active power components (P0, P1, P2) and the positive and negative sequence currents of the grid-side inverter is observed, and based on the basic condition of the positive and negative sequence grid voltage orientation, the positive and negative sequence current command values are given.

[0044]

[0045]

[0046] Formula (5) is the relationship between active power components (P0, P1, P2) and grid-side positive and negative sequence currents It can be seen from formula (5) that direct equation solving cannot make active power components (P0, P1, P2) and grid-side negative sequence currents meet the requirements at the same time. In the traditional control strategy, the mode of controlling P1 and P2 to be 0 (at this time, the negative sequence current is not 0) or controlling the negative sequence current to be 0 (at this time, P1 and P2 are not effectively controlled) is usually adopted. The present application gives the optimized current command value shown in formula (4) to achieve the purpose of simultaneously suppressing the double public frequency oscillation of active power and the grid-side negative sequence current.

[0047] Second step: based on the established mathematical model and the given power expression, a control strategy for switching under unbalanced conditions (the grid-side switching control strategy is shown in formula (4), and the machine-side switching control strategy is shown in formula (5)) is designed to simultaneously reduce the double public frequency oscillation of active power and the grid-side negative sequence current of the grid-side converter output, so that the DC voltage is stabilized during fault ride-through, and the energy consumption resistance action time is reduced to reduce energy loss while meeting the grid current requirements: Figure 1 Figure 2 Step 1: the single current loop control strategy of the grid-side inverter including a PI-R controller is designed from the positive and negative sequence current given values in formula (4), and the positive and negative sequence current of the grid-side inverter is realized without error tracking to the command value.

[0048] Step 2: the DC voltage outer loop control strategy of the machine-side rectifier is designed to control the DC voltage while the grid-side inverter suppresses the output of the double public frequency oscillation of active power, so as to realize fault ride-through under asymmetric fault in cooperation with the grid-side converter control.

[0049] Step 3: considering that the roles of the DC voltage outer loops in the grid-side inverter and the machine-side rectifier in regulating the DC voltage are not exactly the same, which directly leads to the fact that switching from the grid-side control of the DC voltage in normal conditions to the machine-side control of the DC voltage in fault conditions will cause the current command value output by the DC voltage outer loop to suddenly change (the same reason applies when switching to the normal condition control after fault recovery), thereby affecting the current inner loop and further deteriorating the control effect and affecting the stability of the DC voltage. Therefore, the present application clears the integrator output in the DC voltage outer loop PI controller of the grid-side inverter during the fault, and clears the integrator output in the DC voltage outer loop PI controller of the machine-side rectifier in normal conditions, and designs and realizes the smooth switching between different control strategies.

[0050] In specific implementation, as shown in formula (6), the DC voltage outer loop PI controller of the grid-side inverter is cleared during the fault, and as shown in formula (7), the DC voltage outer loop PI controller of the machine-side rectifier is cleared in normal conditions.

[0051] Figure 1 ​​As shown, based on the grid-side inverter model of the positive sequence two-phase rotation (dq) coordinate system and the optimized current given value, a PI-R controller is designed to realize the grid-side switching control strategy, which controls the positive and negative sequence currents to track the given value under unbalanced conditions; as Figure 2 As shown, the machine-side switching control strategy is designed based on the DC capacitor model and the machine-side rectifier model; meanwhile, Ctrl is the control signal of the double-input selector, when Ctrl is 1, the signal entering the output port A of the selector, and when Ctrl is 0, the signal entering the output port B of the selector, the control signal can be represented by the fault signal, that is, when a fault occurs, Ctrl is 1, and when there is no fault, that is, under normal conditions, Ctrl is 0, the DC voltage outer loop PI controller of the grid-side converter and the machine-side converter also clears the integrator output through the control signal, when the control signal is 1, the grid-side integrator output is cleared, and when the control signal is 0, the machine-side integrator output is cleared.

[0052] As shown, Figure 3 As shown, during 2.5 seconds to 3.5 seconds, a two-phase ground fault occurs in the power grid, when the traditional fault ride-through control strategy is adopted, the DC voltage rises to 1.10pu, and there is a large two-frequency oscillation (the difference between the peak and the trough is 0.1pu), and there is also a large negative sequence current (the amplitude is 0.1pu); as Figure 4 As shown, during 2.5 seconds to 3.5 seconds, a two-phase ground fault occurs in the power grid, when the improved fault ride-through control strategy is adopted, the DC voltage is stabilized around 1.0pu, and there is a significant improvement in suppressing the DC voltage two-frequency oscillation and the grid-connected negative sequence current compared with the traditional control strategy; as Figure 5 As shown, when the direct switching is adopted, the d-axis current command value of the machine-side rectifier will have a large mutation when switching from normal conditions to fault conditions, thereby affecting the stability of the DC capacitor voltage, and when the fault is recovered to normal conditions, the d-axis current command value of the grid-side inverter will also have a mutation, which will have a bad influence on the stability of the DC voltage; as Figure 6 As shown, after the smooth switching is adopted, the d-axis current command values of the machine-side and the grid-side converters do not have mutations, which is conducive to maintaining the stability of the DC voltage.

[0053] It should be understood that parts not elaborated in the specification are all prior art.

[0054] Although the specific embodiments of the present application are described above with reference to the drawings, it should be understood by those skilled in the art that these are only illustrative, and various modifications or changes can be made to these embodiments without departing from the principles and essence of the present application. The scope of the present application is only limited by the appended claims.

Claims

1. A direct-drive wind turbine fault ride-through control strategy based on smooth switching under unbalanced conditions, characterized by: The following steps are involved: First, a mathematical model of the grid-side inverter is established under unbalanced conditions. Based on the instantaneous power theory, a power expression for the grid-side inverter operating under unbalanced conditions is given, thereby providing the optimized current command value. Based on the established mathematical model and the given power expression, a fault ride-through control strategy based on smooth switching is designed. This strategy reduces the double-frequency oscillation of the active power output of the grid-side converter, the grid current imbalance, and the crowbar resistor operation time. This stabilizes the DC voltage and suppresses the grid current imbalance during the fault ride-through process, while also reducing the heat dissipation pressure of the crowbar resistor. Observe the active power components ( 、 、 ) and the grid-side positive and negative sequence current ( 、 、 、 ) and, in the case of positive and negative sequence grid voltage orientation, gives the optimized current command value that simultaneously suppresses the double common frequency oscillation of active power and the grid current imbalance; (4) Formula (4) shows the positive and negative sequence current command values ​​of the grid-side inverter after optimization; During a fault, the integrator output in the grid-side inverter DC voltage outer loop PI controller is cleared to zero, and under normal operating conditions, the integrator output in the generator-side rectifier DC voltage outer loop PI controller is cleared to zero.

2. The direct-drive wind turbine fault ride-through control strategy based on smooth switching under unbalanced working conditions according to claim 1 is characterized by: Establish a mathematical model of the grid-side inverter under unbalanced grid conditions: (1) Formula (1) is the voltage equation of the grid-side inverter of the direct-drive wind turbine under unbalanced working conditions.

3. The direct-drive wind turbine fault ride-through control strategy based on smooth switching under unbalanced working conditions according to claim 1 is characterized by: Based on the instantaneous power theory, the power expression of the grid-side inverter under unbalanced working conditions is given; its apparent power, active power and reactive power are given by the following formulas respectively: (2) (3) Formula (2) is the apparent power, and formula (3) is the active and reactive power. It is not difficult to see from formula (3) that when the grid-side inverter works under unbalanced conditions, the output active power has double frequency fluctuations, which will cause DC voltage fluctuations.

4. The direct-drive wind turbine fault ride-through control strategy based on smooth switching under unbalanced working conditions according to claim 1 is characterized by: A positive- and negative-sequence voltage-oriented control strategy containing a resonant controller is designed based on the given positive- and negative-sequence current values, so that the positive- and negative-sequence currents of the grid-side inverter can follow the command value without error. The resonant controller is used to control the negative-sequence current, which appears as a double power frequency component in the positive-sequence rotating coordinate system.

5. The direct-drive wind turbine fault ride-through control strategy based on smooth switching under unbalanced working conditions according to claim 1 is characterized by: The stator voltage-oriented DC voltage control strategy used for switching under unbalanced conditions on the generator side can control the DC voltage while suppressing output active power double frequency fluctuations and grid-connected negative sequence current on the grid-side inverter. This strategy coordinates the grid-side converter and DC-side energy-dissipating resistors to achieve fault ride-through under asymmetric faults. This strategy includes: Grid-side switching control: Provides grid-side positive and negative sequence current command values. Under the conditions of positive and negative sequence voltage orientation, smooth switching is simultaneously implemented to switch the grid-side converter to a single current loop based on a proportional-integral resonant (PI-R) controller for multi-objective coordinated control under fault conditions. Generator-side switching control: Generator-side control switches to stator voltage-oriented DC voltage control, cooperating with the grid side to adapt to different fault conditions and reduce the action time of energy-consuming resistors.

6. The direct-drive wind turbine fault ride-through control strategy based on smooth switching under unbalanced working conditions according to claim 1 is characterized by: Taking into account that the roles of the DC voltage outer loop in the grid-side inverter and the machine-side rectifier in regulating the DC voltage are not exactly the same, this directly leads to the switching from the grid-side direct voltage control under normal operating conditions to the machine-side direct voltage control under fault conditions, which will cause the current command value output by the DC voltage outer loop to suddenly change. The same is true when switching to normal operating condition control after fault recovery. Smooth switching between different control strategies is designed and implemented. Specifically: based on the grid-side inverter model of the positive-sequence two-phase rotating (dq) coordinate system and the optimized current set value, the grid-side switching control strategy is implemented based on the PI-R controller, and the positive and negative sequence currents are simultaneously controlled under unbalanced conditions to track the set value; the machine-side switching control strategy is designed based on the DC capacitor model and the machine-side rectifier model.

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