AU2020455056A1

AU2020455056A1 - Low-voltage ride-through control method and system for wind turbine generator

Info

Publication number: AU2020455056A1
Application number: AU2020455056A
Authority: AU
Inventors: Minghao Han; Yu Li; Zhen Li; Ruiqi WANG; Zhenbin ZHANG
Original assignee: Shandong University
Current assignee: Shandong University
Priority date: 2020-06-22
Filing date: 2020-08-04
Publication date: 2022-05-12
Anticipated expiration: 2040-08-04
Also published as: WO2021258508A1; CN111682587A; AU2020455056B2; CN111682587B

Abstract

Disclosed is a low-voltage ride-through control method for a wind turbine generator. The method comprises the following steps: receiving real-time voltage monitoring data of a power grid, and active power currently emitted by a wind turbine generator; and according to the voltage data, determining whether a voltage drop occurs in the power grid, involving: if a voltage drop occurs, further determining whether the voltage drop is within a set range, and if the voltage drop exceeds the set range, cutting off the wind turbine generator; if the voltage drop is within the set range, calculating reactive power that can be currently provided, and determining whether a reactive power demand can be met while the current active power is maintained; and if so, continuing to run until a fault is recovered or the wind turbine generator is cut off when a specified time is reached, otherwise, gradually modifying active and reactive given values. In the technical solution, by means of a control policy, the reactive output capability of a system is improved when a voltage drop occurs in a power grid, such that low-voltage ride-through is realized, without the need to additionally provide a hardware protection circuit.

Description

LOW VOLTAGE RIDE THROUGH CONTROL METHOD AND SYSTEM FOR WIND-DRIVEN GENERATOR TECHNICAL FIELD

The present invention relates to the field of low voltage ride through technologies for wind power generation systems, and in particular, to a low voltage ride through control method and system for a wind-driven generator.

BACKGROUND

The description in this section merely provides background information related to the present disclosure and does not necessarily constitute the prior art. The wind power system is sensitive to the change in the voltage of the power grid. When a voltage drop occurs in the power grid, the imbalance of mechanical and electrical power in this transient process affects the stable operation of the wind generation set. An overcurrent and additional torque generated by the generator may damage devices. In addition, the short term failure of the power grid may cause the disconnection of the wind generation set from the power grid. The disconnection of the wind generation set changes the power distribution and affects the stability of the entire system. Therefore, as the proportion of wind power generation systems in renewable energy gradually increases, phenomena such as wind curtailment and energy limitation occur. In order to ensure the stability of the power grid connected to wind power, industrial standards for low voltage ride through of the wind power are stipulated. That is to say, the wind generation set maintains fault ride-through within the voltage time range specified by the curve shown in FIG. 5. After the voltage changes back to normal, the active power output of the wind generation set is rapidly recovered. The wind generation set should have the capability of reactive current injection. The response time for injecting the reactive current is not greater than 75 ins. The injection duration is not less than

550 ins. The effective injecting value is calculated by using I, >1.5x (0.9-UT,)In . In the

equation, IT is the effective injection value of the reactive current, is the rated current of

the wind generation set, and UTP is the per-unit value of the voltage at the test point. A conventional wind-driven generator realizes low voltage ride through by methods such as arranging an additional hardware protection circuit such as a Crowbar protection circuit or using a direct-current support capacitor. The conventional method of arranging an additional hardware protection circuit can effectively adjust the active power balance of the power system, so as to protect the excitation converter and the rotor winding when the power grid fails. However, this may cause the wind farm to lose controllability within a short period. A switching operation of the Crowbar circuit may cause transient impact in the system. In this case, the induction motor will absorb a large amount of reactive power from the system, which is difficult to meet the low voltage ride through standards stipulated. Since the wind power generation system is usually based on maximum power point tracking control, the protection strategies using the additionally arranged hardware often ignore the reactive power adjustment capability of the converter, failing to make full use of the idle capacity of the converter. Therefore, in practical applications, in order to ensure the low voltage ride through to meet the standards, a reactive compensator is additionally provided along with the protection circuit to provide reactive power, so as to assist the recovering of the voltage of the power grid. Such a low voltage ride through mechanism requires the additional arrangement of a large number of hardware devices and ignores the reactive output capability of the wind power system, which inevitably leads to an increase in system costs and reduction of work efficiency and makes it difficult to achieve an economic low voltage ride through effect. In addition, because the wind power system is a typical multi-time scale dynamic system, the additional arrangement of hardware devices also increases the difficulty of design and control. Moreover, the Crowbar circuit used consumes a large amount of active power during the voltage drop, resulting in energy loss.

SUMMARY

In order to overcome the above disadvantages of the prior art, the present invention provides a low voltage ride through control method for a wind-driven generator. By means of the control strategy, the reactive output capability of the system is improved when the voltage drop occurs in the power grid. In this way, the low voltage ride through is achieved without requiring the arrangement of an additional hardware protection circuit. To achieve the foregoing objective, one or more embodiments of the present invention provide the following technical solutions: A low voltage ride through control method for a wind-driven generator includes the following steps: receiving real-time monitored data of a voltage of a power grid and an active power currently outputted by the wind-driven generator; determining, according to the voltage data, whether a voltage drop occurs in the power grid; determining, if the voltage drop occurs, whether the voltage drop is within a set range; and if the voltage drop exceeds the set range, removing the wind-driven generator; or if the voltage drop is within the set range, calculating a reactive power that is currently available, and determining whether a reactive power requirement is satisfied while maintaining the current active power, and if so, performing continuous operation until a fault is removed or a specified time is reached and then removing the wind-driven generator, or otherwise, gradually correcting set values of the active power and the reactive power. One or more embodiments provide a low voltage ride through control system for a wind driven generator. The system includes: a voltage real-time monitoring module, configured to receive real-time monitored data of a voltage of a power grid and an active power currently outputted by a wind-driven generator; a voltage drop determination module, configured to determine, according to the voltage data, whether a voltage drop occurs in the power grid; and a voltage drop control module, configured to: if the voltage drop occurs, determine whether the voltage drop is within a set range; and if the voltage drop exceeds the set range, remove the wind-driven generator; or if the voltage drop is within the set range, calculate the reactive power that is currently available, and determine whether the reactive power requirement is satisfied while maintaining the current active power, and if so, perform continuous operation until a fault is removed or a specified time is reached and then remove the wind-driven generator, or otherwise, gradually correct set values of the active power and the reactive power. One or more embodiments provide an electronic device. The electronic device includes a memory, a processor, and a computer program stored in the memory and executable on the processor. When the processor executes the program, the low voltage ride through control method for a wind-driven generator is performed. One or more embodiments provide a computer-readable storage medium, storing a computer program. When the program is executed by a processor, the low voltage ride through control method for a wind-driven generator is performed. One or more embodiments provide a converter, storing a computer program, where when the program is executed by a processor, the low voltage ride through control method for a wind-driven generator is performed. The foregoing one or more technical solutions have the following beneficial effects:

By only changing the control strategy without arranging any additional hardware protection circuit, the present invention improves the reactive output capability of the wind

power generation set during the low voltage ride through, effectively overcomes the transient impact and energy loss caused by the arrangement of the additional hardware circuit, reduces

the system costs and improves the electrical energy conversion efficiency of the system.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings constituting a part of the present invention are used to

provide a further understanding of the present invention. The exemplary examples of the present invention and the related descriptions are used to explain the present invention, and are not intended to constitute an improper limitation on the present invention. FIG. 1 is a relation curve between a rotor power coefficient and a pitch angle and a tip speed ratio.

FIG. 2 is a relation curve between a rotational speed and a power of a wind turbine. FIG. 3 shows an equivalent circuit of a wind power integration converter.

FIG. 4 shows an adjusting interval of a reactive power. FIG. 5 shows a low voltage ride through standard. FIG. 6 is a flowchart of a low voltage ride through control strategy according to an embodiment of the present invention.

DETAILED DESCRIPTION

It should be noted that, the following detailed descriptions are all exemplary, and are intended to provide further descriptions of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those usually understood by a person of ordinary skill in the art to which the present invention belongs. It should be noted that the terms used herein are merely used for describing specific

implementations, and are not intended to limit exemplary implementations of the present invention. As used herein, the singular form is also intended to include the plural form unless the context clearly dictates otherwise. In addition, it should further be understood that, terms "include" and/or "comprise" used in this specification indicate that there are features, steps,

operations, devices, components, and/or combinations thereof. The embodiments in the present invention and features in the embodiments may be mutually combined in case that no conflict occurs.

Maximum power point tracking is the basic principle for the control of wind-driven generators, and is also an important means of increasing the utilization rate of wind energy. Maximum power point tracking control is for the purpose of enabling the wind-driven generator to output a maximum power at different wind speeds. When the wind-driven generator normally operates, a mechanical power P outputted by the wind-driven generator is

P=- C,(p,A)pAv 3 , where C, is a rotor power coefficient, p is an air density, A is an 2 area of a stream surface, and v is the wind speed. The rotor power coefficient C, is a

function of a pitch angle 8 and a tip speed ratio A . The relationship between C, and 8 and

A is shown by a performance curve of a wind turbine, as shown in FIG. 1. Obviously, when the pitch angle is constant, a A exists at a same wind speed to cause C, to reach the

maximum. In this way, a relation curve among different wind speeds, rotational speeds, and powers is obtained, as shown in FIG. 2. The reactive support of the wind-driven generator needs to be implemented by using a converter. The converter between the wind-driven generator and the power grid is mainly configured to transmit the active power between a wind power generation set and the power grid. However, in a case that the active power is lower than an apparent power of the converter, the converter may alternatively provide reactive support for an alternating current power grid by controlling a power factor of the wind power integrated into the power. Therefore, the nature of the reactive output capability of a wind power generation system is power exchange among three phases. Due to the limitation of the apparent power of the wind power generation system itself, an increase in the active power may reduce the output of the reactive power. An equivalent circuit of a wind power integration converter is shown in FIG. 3. U is an output voltage of the converter, EN is a voltage of a grid-connection point, x is

equivalent inductance, and a is a phase angle difference between the output voltage of the converter and the grid-connection point. According to the equivalent circuit, expressions of

the active power and the reactive power are derived as: P= ENUsina and x

E NU E2N Q= cosa N An adjusting interval of the active power and the reactive power is x x obtained, as shown in FIG. 4. The adjusting interval is an area formed by A, B, C, and D.

P,,a, represents the active power obtained when the maximum power tracking is realized. A shaded area is an area in which Q is a negative value, which indicates that the reactive power can be generated in the area. Except for the above derivation range, an influence of the capacity of the converter on the reactive output needs to be considered. Embodiment I The present embodiment discloses a low voltage ride through control method for a wind driven generator. A core point is performing optimization based on a maximum power point tracking strategy, to improve the power quality of the system during normal operation. In this way, the reactive output capability of the system is improved during a voltage drop of the power grid, thereby achieving low voltage ride through. Specifically, the method includes the following steps. Step 1: A controller receives real-time monitored data E of a voltage of a power grid and an active power Ppre currently outputted by the wind-driven generator. Step 2: Determine whether the power grid normally operates, if so, perform step 3, and if not, determine whether a drop is within a set range, if the drop exceeds the set range, remove the wind-driven generator, if the drop is within the set range, calculate a current maximum apparent power Smax and an available reactive power Q, and then perform step 4. In this embodiment, if the voltage E > 0.9 EN, it indicates that the drop does not occur, and the power grid normally operates. If E < 0.9 EN, it is further determined whether the voltage drop is within a certain range. If 0.2 EN < E < 0.9 EN, the current maximum apparent power Smax and the available reactive power Q are calculated. In order to ensure the low voltage ride through to remove the wind-driven generator without a protection action, an output current of a converter is required to be within 1.1 times a rated current of the converter. The maximum apparent power may be calculated by using Smax = 3 EN*1. IN. EN is a voltage value (shown in FIG. 3) of a grid-connection point, and IN is the rated current Qs2max7 -P 2 of the converter. Step 3: A voltage optimization control parameter is obtained based on a maximum power point tracking control strategy. Step 4: A current active power is maintained, and a required reactive power is generated. That is to say, it is determined, according to an instantaneous maximum apparent power limitation, whether a requirement for the switched reactive power is satisfied without changing the active power. If so, continuous operation is performed until a fault is removed or a specified time is reached, and then the wind-driven generator is removed. Otherwise, set values of the active power and the reactive power are gradually corrected according to a tailored control strategy. Specifically, if the maximum reactive power calculated in step 2 that can be provided by the converter is greater than the required reactive power Q",,, the

Q'1SAE se=2S generated required reactive power Q' may be simply calculated as EN according to power regulations. S is the capacity of the converter, and AE is a difference of the voltage for the grid-connection point before and after failure. When the integration point voltage is detected to be recovered, the maximum power point tracking control strategy is used for performing operation. If the voltage is not recovered for a long time or is lower than 20% of the rated value after the voltage drop, a wind-driven generator is removed by means of quick response.

The tailored control strategy is expressed as FP(t)=e-a t " +(1-e-a t . a is a

) Q(t) = S& -_ P(tY)

variable in an interval of [0, 1], and a value of the variable is adjusted in real time according to an amplitude and a change in the change rate of the voltage drop. Pr is an output of the

active power when a voltage change is detected. Generally, the set value of the reactive power of a wind power generation system during the grid-connected operation that uses the maximum power point tracking control strategy is set to zero. That is to say, the wind power generation system itself does not involve the adjustment of the reactive power, and needs to mate with a thermal power generating unit to perform dispatching of the reactive power. Therefore, when the voltage drop occurs due to a failure, a hardware protection circuit needs to be mated to achieve an actual state of the low voltage ride through. Therefore, if the reactive power is to be adjusted according to the control strategy, an original maximum power point tracking mode needs to be changed to form an optimized control strategy that may generate the reactive power by sacrificing a part of the active power without requiring the maximum power generation efficiency. When the voltage drop is detected, according to the low voltage ride through standards and by considering the possible noise influence, when the voltage of the power grid is reduced below 90% of the rated voltage, the original maximum power point tracking operation mode is used, and a set value of the original reactive power is no longer used. The P= P required power is generated as pre with a set value of the current active power Q= Qset maintained unchanged. In this way, it can be ensured that the strong influence on the control of the active power is not generated. Then, it is determined, according to an instantaneous maximum apparent power limitation, whether a requirement for the switched reactive power is satisfied without changing the active power. If the generated reactive power may achieve the low voltage ride through standard, continuous operation is performed until the fault is removed or the specified time is reached, and then the wind-driven generator is removed. Otherwise, the set values of the active power and the reactive power are gradually corrected

P(t)=e'at P,+(1-e-at )Pt according to the tailored control strategyof , so as to avoid Q(t)= S -P(t2

causing the strong influence on the system, thereby achieving the requirements for the active and reactive changes during the low voltage ride through process. In the equation, a is a variable in an interval of [0, 1], and a value of the variable is adjusted in real time according to an amplitude and a change in the change rate of the voltage drop. P, is an output of the

active power when the voltage change is detected. When a controller detects that the integration point voltage is recovered, the maximum power point tracking control strategy is used for performing operation. If the voltage is not recovered for a long time or is lower than % of the rated value after the voltage drop, a wind-driven generator is removed by means of quick response. The above specific process is shown in FIG. 6. For the wind-driven generator during the normal operation, such a power adjustment strategy may also be used to improve the reactive adjustment capability of the wind power.

After a maximum apparent power is calculated, the required power is generated asFr '"e Q =Qse,

with a set value of the current active power maintained unchanged. In a case that the generating capacity can satisfy user demands, reactive support is performed on the power grid. In this way, the power quality of the system can be improved, use efficiency of the wind power generation is improved, dependence on other power generation modes is reduced, and the requirements for reactive power compensation hardware are reduced. In the present embodiment, based on the maximum power point tracking strategy and the power characteristics of the converter, the idle capacity of the converter is fully utilized, so as to improve the power quality of the system during the normal operation. The voltage change is detected when the voltage drop occurs in the power grid, and the set values of the active power and the reactive power are adjusted, so as to ensure that the reactive output of the system can satisfy the requirement for the low voltage ride through without the need to arrange an additional hardware protection circuit. Therefore, the transient impact and the energy loss caused by the addition of the hardware circuit are effectively overcome. In addition, system costs are reduced, and the electrical energy conversion efficiency of the system is enhanced. The present invention is based on the maximum power point tracking control strategy commonly used by a wind power generation set. Since the wind-driven generator has a non linear characteristic, the maximum power point outputted by the wind-driven generator varies with the magnitude of the wind power. In order to ensure the maximum output of the active power, the output of the active power is controlled by adjusting a torque component of a rotor converter of the wind-driven generator, so as to ensure that the wind-driven generator operates with an optimal power curve. When the generated energy is sufficient to support loads, a certain reactive power is outputted to the power grid by adjusting the output power, thereby improving the power quality of the system. In an electrical power system of an inductive circuit, the occurrence of the voltage drop indicates insufficiency of the reactive power. Therefore, a capacitive reactive power should be provided to reduce the degree of voltage distortion caused by inductive reactive power disturbance due to the voltage drop, thereby avoiding a further voltage drop. In this case, the original control strategy needs to be adjusted according to the low voltage ride through standard, so that the reactive power can be outputted as much as possible to support the voltage by using the idle capacity of the converter while ensuring the maximum power point tracking. When a major fault occurs, the output of the active power can be flexibility reduced, to meet a reactive current output standard of low voltage ride through, thereby converting more reactive power. In this way, the voltage of the power grid is easily recovered. Then when the voltage of the power grid is recovered, the active output is gradually increased to the rated power, thereby improving the low voltage ride through capability. Embodiment II The present embodiment is intended to provide a low voltage ride through control system for a wind-driven generator. The system includes: a voltage real-time monitoring module, configured to receive real-time monitored data of a voltage of a power grid and an active power currently outputted by the wind-driven generator; a voltage drop determination module, configured to determine, according to the voltage data, whether a voltage drop occurs in the power grid; and a voltage drop control module, configured to: if the voltage drop occurs, determine whether the voltage drop is within a set range; and if the voltage drop exceeds the set range, remove the wind-driven generator; or if the voltage drop is within the set range, calculate a reactive power that is currently available, and determine whether a reactive power requirement is satisfied while maintaining the current active power, and if so, perform continuous operation until a fault is removed or a specified time is reached and then remove the wind-driven generator, or otherwise, gradually correct set values of the active power and the reactive power. Embodiment III The present embodiment is intended to provide an electronic device. An electronic device includes a memory, a processor, and a computer program stored in the memory and executable on the processor. When the processor executes the program, the low voltage ride through control method for a wind-driven generator described in the embodiment is performed. Embodiment IV The present embodiment is intended to provide a computer-readable storage medium. The computer-readable storage medium stores a computer program. When the program is executed by a processor, the low voltage ride through control method for a wind-driven generator described in the embodiment is performed. Embodiment V The present embodiment is intended to provide a converter. The converter is configured to store a computer program. When the program is executed by a processor, the low voltage ride through control method for a wind-driven generator described in the embodiment is performed. The steps involved in the foregoing Embodiment 2, to Embodiment 5 correspond to method Embodiment 1. For a specific implementation, refer to related descriptions of Embodiment 1. The term "computer-readable storage medium" should be understood as a single medium or a plurality of media including one or more instruction sets, and should also be understood as including any medium. The any medium can store, encode, or carry an instruction set used for being executed by a processor, and cause the processor to perform any method in the present invention. A person skilled in the art should understand that the modules or steps in the present invention may be implemented by using a general-purpose computer apparatus. Optionally, they may be implemented by using program code executable by a computing apparatus, so that they may be stored in a storage apparatus and executed by the computing apparatus. Alternatively, the modules or steps are respectively manufactured into various integrated circuit modules, or a plurality of modules or steps are manufactured into a single integrated circuit module. The present invention is not limited to any specific combination of hardware and software. The foregoing descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. A person skilled in the art may make various alterations and variations to the present invention. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention shall fall within the protection scope of the present invention. The specific implementations of the present invention are described above with reference to the accompanying drawings, but are not intended to limit the protection scope of the present invention. A person skilled in the art should understand that various modifications or deformations may be made without creative efforts based on the technical solutions of the present invention, and such modifications or deformations shall fall within the protection scope of the present invention.

Claims

CLAIMS What is claimed is:

1. A low voltage ride through control method for a wind-driven generator, the method comprising the following steps: receiving real-time monitored data of a voltage of a power grid and an active power currently outputted by the wind-driven generator; determining, according to the voltage data, whether a voltage drop occurs in the power grid; determining, if the voltage drop occurs, whether the voltage drop is within a set range; and if the voltage drop exceeds the set range, removing the wind-driven generator; or if the voltage drop is within the set range, calculating a reactive power that is currently available, and determining whether a reactive power requirement is satisfied while maintaining the current active power, and if so, performing continuous operation until a fault is removed or a specified time is reached and then removing the wind-driven generator, or otherwise, gradually correcting set values of the active power and the reactive power.

2. The low voltage ride through control method for a wind-driven generator according to claim 1, wherein if no voltage drop occurs, optimization control is performed on an output voltage based on a maximum power point tracking control strategy.

3. The low voltage ride through control method for a wind-driven generator according to claim 1, wherein during determining whether the voltage drop is within the set range, a current maximum apparent power is further calculated, and it is determined, according to an instantaneous maximum apparent power limitation, whether the reactive power requirement is satisfied while maintaining the current active power.

4. The low voltage ride through control method for a wind-driven generator according to claim 3, wherein if the reactive power requirement is not satisfied while maintaining the current active power, the set values of the active power and the reactive power are gradually corrected according to a tailored control strategy.

5. The low voltage ride through control method for a wind-driven generator according to claim 4, wherein the tailored control strategy is:

P(t) = e-at + (1- e-at'

Q(t) = 'S - P(t2 wherein a is a variable in an interval of [0, 1], a value of the variable is adjusted in real time according to an amplitude and a rate of change of the voltage drop, Smax is the current maximum apparent power, and P, is an output of the active power when a voltage change is detected.

6. The low voltage ride through control method for a wind-driven generator according to claim 2, wherein the real-time monitored data of the voltage of the power grid is continuously received, and when the voltage is recovered, the maximum power point tracking control strategy is used to perform optimization control on the output voltage.

7. A low voltage ride through control system for a wind-driven generator, the system comprising: a voltage real-time monitoring module, configured to receive real-time monitored data of a voltage of a power grid and an active power currently outputted by the wind-driven generator; a voltage drop determination module, configured to determine, according to the voltage data, whether a voltage drop occurs in the power grid; and a voltage drop control module, configured to: if the voltage drop occurs, determine whether the voltage drop is within a set range; and if the voltage drop exceeds the set range, remove the wind-driven generator; or if the voltage drop is within the set range, calculate the reactive power that is currently available, and determine whether a reactive power requirement is satisfied while maintaining the current active power, and if so, perform continuous operation until a fault is removed or a specified time is reached and then remove the wind-driven generator, or otherwise, gradually correct set values of the active power and the reactive power.

8. An electronic device, comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein when the processor executes the program, the low voltage ride through control method for a wind-driven generator according to any of claims I to 6 is performed.

9. A computer-readable storage medium, storing a computer program, wherein when the program is executed by a processor, the low voltage ride through control method for a wind driven generator according to any of claims 1 to 6 is performed.

10. A converter, storing a computer program, wherein when the program is executed by a processor, the low voltage ride through control method for a wind-driven generator according to any of claims I to 6 is performed.