Doubly-fed fan high-voltage ride-through method based on energy storage and crowbar protection

Abstract

The invention aims to solve the problems of grid-side transient overcurrent and out-of-control DC bus voltage regulation and control of a grid-side converter when a double-fed fan has a high-voltage ride-through fault, and even causes large-amplitude oscillation of bus voltage and active power after the fault is ended. On the basis of the condition, a high-voltage ride-through control strategy combining grid-side energy storage and Crowbar protection is provided. The method provided by the invention has the beneficial effects that the transient overcurrent of the grid side is suppressed through the Crowbar device, the reactive regulation capability of the fan itself is exerted by improving the reactive instruction of the grid side so as to provide reactive support for the power grid, and the stability of the DC bus voltage is maintained and the power of the grid side is regulated through the energy storage device. According to the method, the fan can be helped to cope with power grid voltage sudden rise faults of different degrees, and the double-fed fan can be helped to complete high voltage ride through.

Description

Technical Field

[0001] The present invention relates to the field of renewable energy power generation and grid connection, and in particular to a high voltage fault ride-through of a doubly-fed induction generator (DFIG) based on hardware protection. Background Art

[0002] With the increase of global environmental awareness, the proportion of clean energy usage has increased significantly. As an economical and environmentally friendly energy source, wind energy continues to grow in share in the power system. However, with the large-scale access of wind power to the grid, many problems will also arise. Since wind energy is an intermittent energy source, the power quality is unstable, which can easily cause fluctuations in wind power. When wind power is connected to the grid, it poses a challenge to the overall stability of the power grid and is prone to power safety accidents. Doubly fed wind turbines are sensitive to fluctuations in the power grid. Wind turbines are easily affected by sudden increases in grid voltage and fail, and may even cause wind turbines to be disconnected from the grid and decoupled. Therefore, in the case of a sudden increase in grid voltage, it is very necessary to study the fault ride-through of doubly fed wind turbines.

[0003] The main causes of grid voltage surge faults are mainly divided into the following categories: (1) sudden current disturbance; (2) single-phase grounding fault causes single-phase reclosing, resulting in asymmetric grid voltage surge; (3) sudden removal of a large number of loads can also cause grid voltage surge faults; (4) during the recovery phase of low voltage faults, if the compensation capacitors cannot be withdrawn in time, it will cause a sudden surge in grid voltage, forming a low-high voltage chain fault.

[0004] At present, according to the degree of grid voltage surge, the high voltage ride-through protection strategies proposed at home and abroad can be summarized into the following two aspects: For faults with a small degree of grid voltage surge, the control strategy of improved DFIG is often adopted, such as improving the reactive power command value, virtual impedance control and demagnetization control on the grid side. However, when the grid voltage surge is large, it is impossible to achieve fault ride-through only by optimizing the control strategy of DFIG. It is necessary to invest in hardware protection circuits to ensure the safe operation of DFIG, such as Crowbar protection, Chopper protection and dynamic reactive power compensation technology. The HVRT control strategy without hardware circuit has the advantages of economic efficiency and cost saving, but the control strategy of this method is relatively complex. In the face of faults with large surge amplitude, its reliability is not high enough. The grid-side converter will also be overmodulated due to the excessive grid voltage, causing grid-side power oscillation. The control strategy using Crowbar protection can suppress the overcurrent on the rotor side, and the control strategy using Chopper protection can suppress the overvoltage of the DC bus. Both hardware protection strategies can deal with the problems caused by the voltage surge on the machine side, but their defects are that they need to be frequently switched in the circuit, which is easy to cause power oscillation of the system and also easy to cause power waste. Energy storage devices are mainly used to smooth power fluctuations and provide reactive power support in terms of voltage ride-through. However, in the event of a serious fault, they cannot help wind turbines cope with the impact of transient overcurrents, and they also ignore the reactive power regulation capabilities of the wind turbines themselves.

[0005] Therefore, it is necessary to study the high voltage ride-through problem of doubly-fed induction motors at this stage and propose a fault ride-through method for doubly-fed induction motors based on hardware protection. Summary of the invention

[0006] In view of the problems existing in the prior art, the present invention provides a doubly-fed induction motor fault ride-through method based on hardware protection. The method suppresses the overcurrent on the grid side and the overvoltage on the DC bus through three different methods of coordinated control, namely, grid-side energy storage protection, crowbar protection and adjustment of the grid-side reactive power command value, so as to achieve high voltage ride-through of the doubly-fed induction motor.

[0007] The technical solution adopted by the present invention is: A method for fault ride-through of a doubly-fed induction motor based on hardware protection comprises the following steps: S1. Use Crowbar device to suppress overcurrent on the grid side; When the grid voltage suddenly rises, the RSC converter and GSC converter in the doubly-fed induction motor will have overcurrent problems, and the current impact in the grid-side converter is even greater; the control strategy of grid-side energy storage can absorb the excess power generated on the grid side during the fault, but it cannot effectively deal with the problem of transient overcurrent on the grid side; S1.1 Determination of rotor dynamic mathematical model The voltage equation and flux equation of the doubly-fed induction motor in the synchronous speed coordinate system are: The flux equation of the doubly-fed induction motor is: From the voltage equation and flux equation of the doubly-fed induction motor, the rotor dynamic mathematical model can be obtained: S1.2 Analysis of the grid-side crowbar protection circuit being activated to suppress overcurrent on the generator side In the dynamic mathematical model of the rotor, the transient resistance expression of the rotor can be obtained: From the transient resistance expression of the rotor, it can be seen that by changing the value of the stator resistance, the transient resistance of the rotor can be changed accordingly, thereby achieving the purpose of suppressing the transient current of the rotor. When the grid voltage rises suddenly, the grid-side Crowbar protection circuit is activated, which can effectively suppress the overcurrent on the stator side and protect the grid-side converter; S1.3 Selection of the resistance value of the crowbar on the grid side The application of the crowbar is to suppress the overcurrent on the grid side. By selecting the appropriate crowbar resistance value, the equivalent resistance on the stator side can be changed. When the stator current is suppressed, the current injected into the GSC by the grid will decrease accordingly, and the overcurrent on the grid side can also be suppressed. The selection of the crowbar resistance value on the grid side can be obtained from the voltage equation of the doubly fed induction motor in the synchronous speed coordinate system: S2. Use energy storage devices to smooth the active power on the grid side; When the grid voltage suddenly rises, as the grid voltage rises, power backflow occurs on the grid side of the wind turbine, which will cause the active power on the grid side to be too large, thus causing power mismatch between the two converters. Therefore, it is proposed to add an energy storage device on the grid side to absorb the unbalanced power on the grid side during the fault period; S2.1 Power analysis on the grid side during steady-state operation When the grid voltage is normal, the wind turbine runs stably. At this time, the GSC converter stably transmits power to the grid. The active power on the grid side is equal to the active power on the converter side: S2.2 Power analysis on the grid side when a sudden surge fault occurs When the grid voltage suddenly rises, as the grid voltage rises, power backflow occurs on the grid side of the wind turbine, which will cause the active power on the grid side to be too large, thus causing power mismatch between the two converters. Therefore, it is proposed to add an energy storage device on the grid side to absorb the unbalanced power on the grid side during the fault: The energy storage device can absorb the excess power generated on the grid side during the fault period, thereby achieving stable power transmission between the RSC and the GSC and achieving the purpose of stabilizing the DC bus voltage; S2.3 Parameter selection of energy storage system when a sudden surge fault occurs in the power grid The energy storage system is installed on the grid side of the Crowbar protection circuit. Considering the setting of the rotor side energy storage parameters, the grid side energy storage battery voltage should be greater than twice the stator side phase voltage peak. Since the output voltage of the GSC is limited by the DC bus side voltage, the maximum output voltage of the GSC can be determined: From the above formula, we can further get the expression of the grid-side battery voltage: Since the grid side adopts the reactive power command value improvement strategy, it can reduce the generation of active power on the grid side during the fault period by providing a certain amount of reactive power support to the grid, which can further reduce the capacity of the grid side energy storage battery and save economic costs; S3. Use the grid-side reactive power command value to provide reactive power support to the grid When a sudden rise fault occurs, the GSC converter loses control due to transient overvoltage on the grid side. In order to achieve controllable operation of the GSC converter, reactive current compensation is required. By increasing the reactive current, the voltage division effect of the grid-side inductance can be enhanced, which can improve the imbalance of the GSC voltage vector and facilitate the recovery of the grid voltage. S3.1 Power analysis on the grid side during steady-state operation When the grid is operating in a steady state, there is no exchange of reactive power between the grid-side converter and the grid; S3.2 Power analysis on the grid side when a sudden surge fault occurs When the grid voltage suddenly rises, the wind turbine's own reactive power support capability can be brought into play, and the inductive reactive current can be injected into the grid through the grid-side converter. The improvement of the reactive power command value on the grid side can be adjusted according to the sudden rise of the grid voltage, but due to the limitation of the converter capacity, the reactive power regulation capability on the grid side is limited; 1) When the grid voltage surge is shallow, that is, when the grid voltage surge is 1.1pu to 1.2pu; In order to help wind turbines achieve high voltage ride-through during faults, wind turbines are required to have a certain reactive power support capability. The dynamic inductive reactive current required to be provided by wind turbines in my country is: 2) When the grid voltage surge is deep, that is, when the grid voltage surge is 1.2pu to 1.3pu; For DFIG, its rated active power is generated by both the stator side and the rotor side, and there is a slip relationship between the active power generated by the rotor side and the active power generated by the stator side: The active power generated by the rotor side is injected into the grid through the grid-side converter. Therefore, the active power generated by the stator, the active power generated by the rotor and the active power generated by the grid side have the following relationship: The powerless capability of the grid-side converter is limited by the capacity of the converter: Therefore, the reactive power limit of the grid-side converter is: The reactive current limit of the grid-side converter can be further obtained: S4. Grid-side collaborative control strategy design

[0008] Adding Crowbar and energy storage system on the grid side of DFIG can collaboratively realize high voltage ride through of DFIG. The collaborative control strategy of grid side energy storage system and Crowbar protection (referred to as grid side energy storage control strategy) is as follows: 1) When the grid-side current exceeds the set value, the grid-side crowbar is put into operation, the grid-side converter is temporarily bypassed, and the energy storage device is not put into operation; 2) When the grid-side current is within the set value, the crowbar on the grid side is removed, the grid-side converter resumes normal operation, and the energy storage device is put into operation to absorb excess power on the grid side; 3) After the fault is resolved, the energy storage device feeds the stored energy back to the wind turbine’s own electrical equipment through the auxiliary converter.

[0009] The control structure includes: a grid-side crowbar switching control link, a grid-side reactive power instruction control link, a grid-side energy storage device control link, and a converter vector modulation link.

[0010] It can be seen from the scheme that the beneficial effects of the present invention are: protecting the grid-side converter by analyzing the suppression of the stator-side current by the grid-side crowbar, analyzing the limit of the reactive support capability provided by the grid-side converter to provide dynamic reactive support for the power grid, and using the grid-side energy storage device to achieve power stability at the grid-side end and maintain the stability of the DC bus voltage. The high voltage ride-through method proposed in the present invention can help the wind turbine to achieve fault ride-through under different degrees of sudden rise, and can also play the role of the wind turbine itself to provide a certain reactive support, effectively enhancing the fault ride-through capability of the doubly-fed wind turbine; BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The drawings described herein are used to provide a further understanding of the present application and constitute a part of the present application. The illustrative implementation examples and descriptions of the present application are used to explain the present application and do not constitute an improper limitation on the present application. In the drawings: Figure 1 This is the topological structure diagram of the grid-side energy storage protection system; Figure 2 This is the control structure diagram of the grid side crowbar switching; Figure 3 It is the structure diagram of the grid-side reactive power command control; Figure 4 This is the grid-side energy storage control structure diagram; Figure 5 It is the network side collaborative control strategy diagram; Figure 6 It is a comparison diagram of the grid-side current under the GSC control strategy and the control strategy proposed by the present invention (1.2pu sudden increase amplitude); Figure 7 It is a comparison diagram of DC bus voltage under the GSC control strategy and the control strategy proposed by the present invention (1.2pu sudden rise amplitude); Figure 8 It is a comparison diagram of grid-side current under the GSC control strategy and the control strategy proposed by the present invention (1.3pu sudden increase amplitude); Fig. 9 It is a comparison diagram of DC bus voltage under the GSC control strategy and the control strategy proposed by the present invention (1.3pu sudden rise amplitude); Fig.10 This is a comparison diagram of the grid-side current under chopper protection and the strategy proposed by the present invention (1.3pu surge amplitude); Fig.11 This is a comparison diagram of the DC bus voltage under chopper protection and the strategy proposed by the present invention (1.3pu surge amplitude); Fig.12 This is a comparison diagram of the grid-side current under the stator-side series resistance protection and the strategy proposed by the present invention (1.3pu sudden rise amplitude); Fig.13This is a comparison diagram of the DC bus voltage under the stator side series resistance protection and the strategy proposed by the present invention (1.3 pu sudden increase amplitude).

Claims

1. A high voltage fault ride-through method for a doubly-fed wind turbine based on energy storage and crowbar protection, characterized in that: The paper analyzes the problem that when a sudden surge fault occurs in the power grid, the transient overvoltage on the grid side exceeds the modulation range of the grid-side converter, causing transient overcurrent on the grid side and overvoltage on the DC bus side, and causing bus voltage oscillation in the later stage of the fault. The grid-side crowbar method is used to suppress transient overcurrent shocks on the grid side. The maximum reactive power support capacity of the grid-side converter is analyzed, and the unbalanced power on the grid side is smoothed according to the coordination of the grid-side reactive power command value and the grid-side energy storage device to maintain the stability of the DC bus voltage. Through the coordinated control strategy of the grid-side crowbar, the grid-side reactive power command value and the grid-side energy storage device, the high-voltage fault ride-through of the doubly-fed wind turbine is finally achieved: The improved control method for high-voltage ride-through of a doubly-fed induction motor includes the following steps: S1. Use a crowbar protection device to adjust the reactive power command value on the grid side and use an energy storage device to enhance the high voltage ride-through capability of the wind turbine; S1.1 Cutting crowbar protection device The doubly-fed wind turbine determines whether to activate the crowbar device based on the collected wind speed, grid-side current and DC bus voltage. When the grid-side overcurrent or DC bus-side overvoltage occurs, the crowbar protection device is activated to suppress the grid-side current within a safe range. After the fault ends, the crowbar protection device is closed in a timely manner. S1.2 Selection of grid-side reactive power command value The selection of the grid-side reactive power command value can provide certain reactive power support to the grid; when the grid voltage surge is shallow, that is, when the grid voltage surge is 1.1pu to 1.2pu, reactive power support is provided in accordance with my country's wind turbine fault ride-through standard; when the grid voltage surge is deep, that is, when the grid voltage surge is 1.2pu to 1.3pu, the required reactive power is provided according to the maximum reactive power support capability of the GSC itself; S1.3 Switching on and off of energy storage devices The doubly-fed wind turbine determines whether to put the energy storage device into operation based on the collected DC bus voltage and overcurrent on the grid side. When overcurrent occurs on the grid side and the DC bus voltage fluctuates greatly, the energy storage protection device is put into operation to smooth out the unbalanced power on the grid side and maintain the stability of the DC bus voltage. After the fault ends, the energy storage protection device is removed in a timely manner. S2. Use the coordinated control strategy of grid-side crowbar protection, grid-side reactive power command value and grid-side energy storage device to help the double-fed wind turbine complete high voltage ride-through; When the DFIG operates normally, the grid-side Crowbar and energy storage device do not work; when a sudden rise fault occurs in the DFIG, it will cause grid-side overcurrent. As the grid voltage increases, it exceeds the modulation range of the GSC converter, causing bus voltage fluctuations. Therefore, at the initial stage of the fault, it is necessary to detect whether the grid-side current exceeds the maximum current value of the GSC converter. If the grid-side current is within the limit, the energy storage is put into operation, and the reactive power command value on the grid side is adjusted to regulate the unbalanced power on the grid side. If the grid-side current exceeds the limit, the Crowbar is first put into operation to limit the grid-side current to the maximum tolerance range of the converter, and then the energy storage device is put into operation and the reactive power command value on the grid side is adjusted. During the fault period, the DC bus voltage is detected. If the bus voltage exceeds the allowable range, the Crowbar is put into operation again until the bus voltage does not exceed the set threshold. When the bus voltage is within the allowable range, the energy storage device is controlled to be cut out in time; The control structure includes: a grid-side crowbar switching control link, a grid-side reactive power instruction control link, a grid-side energy storage device control link, and a converter vector modulation link.

2. A method for high voltage fault ride-through of a doubly-fed induction motor based on hardware protection according to claim 1, characterized in that: Through the high-voltage fault ride-through method of the doubly-fed induction motor, it is possible to suppress the transient overcurrent impact on the grid side, smooth the power fluctuation on the grid side and maintain the stability of the DC bus voltage, and at the same time give full play to the reactive power support capability of the wind turbine itself, provide dynamic reactive power support to the power grid, and effectively enhance the fault ride-through capability of the doubly-fed wind turbine.

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