Control method and control system for enhancing endurance to anomalous voltage for doubly-fed induction generator

Provided are a control method and system for enhancing an endurance capability to an abnormal voltage of a wind turbine generator system. The control method, includes; providing a doubly-fed wind turbine generator system connected to a power grid; detecting a voltage of the power grid, and determining whether the voltage of the power grid has a fault; when the voltage of the power grid has a fault, detecting a voltage of the DC buses, and determining whether the voltage of the DC buses exceeds a limit value; when the voltage of the DC buses exceeds the limit value, performing integrated system coordination control according to an abnormal operating condition mode; and when the voltage of the power grid returns to a normal range, performing integrated system coordination control according to a normal operating condition mode.

CROSS-REFERENCES TO RELATED APPLICATIONS

This is a National Stage Application, filed under 35 U.S.C. 371, of International Patent Application No. PCT/CN2017/073115, filed on Feb. 8, 2017, which claims priority to Chinese patent application No, 2016101142516 filed on Mar. 1, 2016, contents of both of which are incorporated herein by reference in their entireties.

TECHNICAL FIELD

The present disclosure belongs to the field of the operation control of wind power generation, and, in particular, relates to a control method and system for enhancing an endurance capability to an abnormal voltage of a doubly-fed wind turbine generator system.

BACKGROUND

Since a stator of doubly-fed wind turbine generator system is directly connected to an electric power grid, the doubly-fed wind turbine generator system is susceptible to the disturbance of a voltage of the electric power grid. At present, a wind power grid-connected standard only focuses on a fault ride-through capability of the wind turbine generator system, and is insufficient in requirements for small voltage dip of the electric power grid and endurance to a low voltage and a high voltage. Facts prove that these aspects have a great impact on a continuous operation of the doubly-fed wind turbine generator system as well.

In terms of the endurance to the high voltage and the low voltage, there is a general requirement that the doubly-fed wind turbine generator system should operate normally between −10% and 10%, and no specific requirement for an operation capability at a lower voltage or a higher voltage. For the doubly-fed wind turbine generator system, it can be seen from simple analysis that an insufficient output voltage of a converter under the high voltage causes that the power is difficult to fed into the electric power grid; the converter easily generates an overcurrent under the low voltage. In addition, a capacity of a machine/network converter is not balanced, which may further cause an increasing in a voltage of buses, triggering a protection action. Active load shedding in a variable speed-pitching manner is one method to alleviate low voltage overloading. In a case of small voltage dip in the electric power grid, control strategies are often used to suppress the overcurrent of the rotor. Such a method is effective when the rotor converter provides a sufficient voltage, and a control model is accurate enough, which is very difficult to implement in practice.

In terms of the endurance to the high voltage and the low voltage, there is no comparatively perfect technology compromising the two. For a case of the low voltage, although the active load shedding by variable speed-pitching may theoretically prevent the converter from overloading, since a voltage fluctuation is a frequent event, such a pneumatic load shedding scheme undoubtedly increases a load of the doubly-fed wind turbine generator system and affects its service life.

An excitation system of the doubly-fed wind turbine generator system based on hybrid energy storage of a supercapacitor and a battery exists, the system controls the characteristic of the power outputted from a stator side of a doubly-fed generator by controlling the excitation of the rotor of the doubly-fed generator. The scheme has shortcomings that the excitation control algorithm is very complicated, and is difficult to implement and high in cost. Moreover, during a failure of a blower, a rotor-side converter may be disconnected from the electric power grid for a short-time, the doubly-fed wind turbine generator system cannot be effectively controlled.

In addition, there is further an apparatus for achieving a low voltage ride-through capability of the wind turbine generator system by employing the supercapacitor. In this scheme, the supercapacitor is mainly used for low voltage ride-through during a failure of a wind power plant, and cannot complete power adjustment in a normal operation according to a scheduling command. In addition, the scheme needs to especially set up a low voltage ride-through control system in the wind power plant, which has many problems such as more implementation investment and large space occupation, and corresponding operation control strategies are not provided.

SUMMARY

In view of this, it is necessary to provide a control method and system for enhancing an endurance capability to an abnormal voltage of a wind turbine generator system, to solve problems of transient instability of DC buses and intermittent operation of the wind turbine generator system.

A control method for enhancing an endurance capability to an abnormal voltage of a wind turbine generator system includes:

providing a doubly-fed wind turbine generator system connected to a power grid, where the doubly-fed wind turbine generator system includes a wind turbine, a gearbox, a generator, a converter and a supercapacitor energy storage apparatus, the supercapacitor energy storage apparatus includes a DC-DC converter and a supercapacitor, the converter includes a grid-side converter, a rotor-side converter and DC buses connected between the grid-side converter and the rotor-side converter, and the supercapacitor is electrically connected with the DC buses via the DC-DC converter;

detecting a voltage of the power grid, and determining whether the voltage of the power grid has a fault;

when determining that the voltage of the power grid has the fault, detecting a voltage of the DC buses, and determining whether the voltage of the DC bus exceeds a limit value;

when determining that the voltage of the DC bus exceeds the limit value, performing an integrated system coordination control on the supercapacitor energy storage apparatus and the converter according to an abnormal operating condition mode; and

when determining that the voltage of the power grid returns to a normal range, performing the integrated system coordination control on the supercapacitor energy storage apparatus and the converter according to a normal operating condition mode.

A control system for enhancing an endurance capability to an abnormal voltage of a wind turbine generator system includes:

a doubly-fed wind turbine generator system connected with a power grid, which includes a wind turbine, a gearbox, a generator, a converter and a supercapacitor energy storage apparatus, where the supercapacitor energy storage apparatus includes a DC-DC converter and a supercapacitor, the converter comprises a grid-side converter, a rotor-side converter and DC buses connected between the grid-side converter and the rotor-side converter, and the supercapacitor is electrically connected with the DC buses via the DC-DC converter; and

an integrated system coordination controller, which is configured to: when determining that a voltage of the DC buses exceeds a limit value, perform an integrated system coordination control on the supercapacitor energy storage apparatus and the converter according to an abnormal operating condition mode; and when that a voltage of the power grid returns to a normal range, perform the integrated system coordination control on the supercapacitor energy storage apparatus and the converter according to a normal operating condition mode.

The control method and system for the endurance capability to the abnormal voltage of the wind turbine generator system by employing a supercapacitor energy storage system, which are provided by the present disclosure, may be used for enhancing a fault handling capability of the doubly-fed wind turbine generator system, and enhancing the doubly-fed wind turbine generator system's endurance to small transient disturbance of the voltage of the power grid and continuous operation ability under the high/low voltage on the whole without changing an original control strategy of the wind turbine generator system. The control method and system is simple and reliable in control structure and algorithm as well as good in effect, and will not affect the operation and service life of other components of the wind turbine generator system.

DETAILED DESCRIPTION

In order to make objectives, technical solutions and advantages of the present disclosure be clear, the present disclosure will be further described in detail below with reference to accompanying drawings and embodiments. It is understood that the specific embodiments described herein are merely illustrative of the present disclosure and are not intended to limit the present disclosure.

An embodiment of the present disclosure provides a control method for enhancing an endurance capability to an abnormal voltage of a wind turbine generator system. The method is implemented by disposing a supercapacitor energy storage apparatus2in the doubly-fed wind turbine generator system. With reference toFIG. 1, the embodiment of the present disclosure further provides a doubly-fed wind turbine generator system with a supercapacitor energy storage system, which includes a generator5, a converter7and a supercapacitor energy storage apparatus2.

The doubly-fed wind turbine generator system may further include a wind turbine3and a gearbox4. The wind turbine3drives the generator5to operate through the gearbox4, so as to convert wind energy into electrical energy.

The doubly-fed wind turbine generator system may further include a transformer6, and the doubly-fed wind turbine generator system is connected to a power grid8via the transformer6.

The doubly-fed wind turbine generator system may further include a crowbar protection circuit1, which is connected to a rotor. When a voltage of the converter7exceeds a set threshold, the crowbar protection circuit1is turned on to bypass a current on the rotor. The transformer6(e.g., a box-type transformer substation) may be a boosting transformer. In one embodiment, the transformer6is a 0.69/35 kV boosting transformer with a rated capacity of 3 MVA.

The generator5may be a doubly-fed induction generator, including a stator and a rotor, where the stator is connected to the power grid8via the transformer6, and the rotor is connected to the power grid8via the converter7.

With reference toFIG. 2together, two ends of the supercapacitor energy storage apparatus2are electrically connected with two DC buses78of the converter7, respectively. The converter7may further include a rotor-side converter72and a grid-side converter74electrically connected by the two DC buses78, as well as a capacitor76two ends of which are electrically connected to the two DC buses78respectively. The supercapacitor energy storage apparatus2and the capacitor76are connected in parallel.

The supercapacitor energy storage apparatus2includes a DC-DC converter24and a supercapacitor22. The supercapacitor22is mounted to the DC buses78of the converter7and electrically connected to the DC buses78of the converter7via the DC-DC converter24, particularly, connected to two DC buses78of the converter7via the DC-DC converter24. An equivalent circuit of the supercapacitor22inFIG. 2includes an equivalent capacitor Cscand an equivalent resistor Rreswhich are connected in series. A rated voltage of the supercapacitor22preferably ranges from 450 V to 960 V, and the power and the capacity of the supercapacitor22theoretically have no upper limits. In one embodiment, the supercapacitor has a voltage capacity Escof 4 F, a power of 350 kW, and a rated voltage of 800 V.

The DC-DC converter24is preferably a buck-boost bidirectional converter including a first fully-controlled switch S1, a second fully-controlled switch S2and a boosting inductor L. When electrical energy is transferred from the capacitor76of the DC buses78to the supercapacitor22, the supercapacitor22is charged, and the DC-DC converter24operates in a buck mode. When the electrical energy is transferred from the supercapacitor22to the capacitor76of the DC buses78, the supercapacitor22is discharged, and the DC-DC converter24operates in a boost mode. By adjusting a duty ratio of the switches in the DC-DC converter, specifically duty ratios of the fully-controlled switch S1and/or the fully-controlled switch S2, the voltage the supercapacitor22may be adjusted, and the duty ratio may be varied between 0 and 1. A voltage of the DC-DC converter24is higher than a voltage of the supercapacitor22. In one embodiment, in order to control the voltage of the DC buses78, the DC-DC converter24is switched between the buck mode and the boost mode continuously.

In one embodiment, a first end of the boosting inductor L is connected in series with the first fully-controlled switch S1, and is connected to a first DC bus78of the converter7through the first fully-controlled switch S1, and a second end of the boosting inductor L is electrically connected to a first electrode of the supercapacitor22. A second electrode of the supercapacitor22is electrically connected to a second DC bus78of the converter7. The second fully-controlled switch S2is connected between the first end of the boosting inductor L and the second electrode of the supercapacitor. The second fully-controlled switch S2and the supercapacitor22are both connected in parallel with the capacitor76of the DC buses78.

The first fully-controlled switch S1includes a first transistor and a first diode connected in parallel. A drain of the first transistor is connected to an anode of the first diode and to the first end of the inductor L. A source of the first transistor is connected to a cathode of the first diode and connected to one DC bus78of the converter7. The second fully-controlled switch S2includes a second transistor and a second diode connected in parallel. A source of the second transistor is connected to a cathode of the second diode and to the first end of the inductor L. A drain of the second transistor is connected to an anode of the second diode and electrically connected to the second electrode of the supercapacitor22.

The control method for enhancing the endurance capability to the abnormal voltage of the wind turbine generator system by employing the supercapacitor energy storage apparatus may ensure transient stability of the DC buses78and uninterrupted operation of the wind turbine generator system.

With reference toFIG. 3, a control method for enhancing an endurance capability to an abnormal voltage of a wind turbine generator system, which is provided by the embodiment of the present disclosure, further includes: integrated system coordination control on the supercapacitor energy storage apparatus2and the converter7, and a control process may include the following steps.

A voltage of the power grid is detected and it is determined whether the voltage of the power grid has a fault.

When it is determined that the voltage of the power grid has a fault, a voltage of DC buses78of the converter7is detected, and it is determined whether the voltage of the DC buses78exceeds a limit value or not.

When it is determined that the voltage of the DC buses78exceeds the limit value, a flag bit of the voltage of the power grid is set to “abnormal”, and integrated system coordination control is performed on the supercapacitor energy storage apparatus2and the converter7according to an abnormal operating condition mode.

When it is determined that the voltage of the power grid returns to normal (that is, in a normal voltage range of the power grid), the flag bit of the voltage of the power grid is set to “normal”, and integrated system coordination control is performed on the supercapacitor energy storage apparatus2and the converter7according to a normal operating condition mode.

The step of detecting the voltage of the power grid and determining whether the voltage of the power grid has a fault may continue throughout the control process. The fault of the voltage of the power grid may specifically be that the voltage of the power grid is higher or lower than the normal voltage range of the power grid, and the normal voltage range of the power grid may be set as required. The limit value of the voltage of the DC buses78may also be set as required, for example, the limit value is 1.05 p.u. (per unit), that is, exceeds a normal value by 5% of the normal value.

The integrated system coordination control may specifically include control of the grid-side converter and control of the supercapacitor energy storage system, which may be performed simultaneously.

The process of performing integrated system coordination control according to the abnormal operating condition mode includes: the grid-side converter is controlled according to the abnormal operating condition mode, and a boost control is performed on the supercapacitor energy storage apparatus2. The process of performing integrated system coordination control according to the normal operating condition mode includes: the grid-side converter is controlled according to the normal operating condition mode, and a buck control is performed on the supercapacitor energy storage apparatus2.

With reference toFIG. 4, the grid-side converter has two control modes of a “normal operating condition” and an “abnormal operating condition” according to the flag bit of the voltage of the power grid. When the voltage of the power grid is normal (that, when the grid voltage is within the normal voltage range of the power grid), the grid-side converter is in the normal operating condition mode, and the grid-side converter controls the voltage of DC buses78to be constant. At this time, it is in an “active priority control” mode, and a certain reactive/voltage assisted control may be provided. When the voltage of the power grid is abnormal, the voltage of the power grid is higher than an upper limit of the normal voltage range of the power grid or lower than a lower limit of the normal voltage range of the power grid, the grid-side converter is in the abnormal operating condition mode, the grid-side converter performs strategy switching, the grid-side converter does not control the voltage of the buses any more, and at this time, it is in a “reactive priority control” mode.

In the “reactive priority control” mode, when the voltage of the power grid is lower than the lower limit of the normal voltage range of the power grid, the grid-side converter performs over-excitation control so as to support the voltage of the power grid, and injects a certain active power under a constraint of an apparent capacity. When the voltage of the power grid is higher than the upper limit of the normal voltage range of the power grid, the grid-side converter performs under-excitation control, and injects a certain active power under the constraint of the apparent capacity. The under-excitation control not only helps the recovery of the voltage of the power grid, but also may ensure that the grid-side converter is out of control due to over-modulation, that is, ensure that the grid-side converter is still controllable at an abnormal voltage.

With reference toFIG. 5, a control strategy of a supercapacitor energy storage system includes boost control and buck control.

Since the grid-side converter does not control the voltage of the DC buses78in the “reactive priority control” mode any more, an active power input by the rotor-side converter will cause the voltage of the DC buses78to increase. The boost control is as follows. When the voltage of the DC buses78exceeds the limit value (that is, when a flag bit of the voltage of the power grid is set to “abnormal”), the supercapacitor22is used as an input terminal of the DC-DC converter24, the capacitor76of the DC buses78is used as an output terminal of the DC-DC converter24, and the voltage of the DC buses78is adjusted to be within a normal voltage range of the DC buses, that is, the voltage of the DC buses78is controlled.

The boost control includes: detecting the voltage of the DC buses78and controlling the voltage of the DC buses78to be within the normal voltage range of the DC buses78by controlling duty ratios of the first fully-controlled switch S1and/or the second fully-controlled switch S2. The boost control may include: controlling the supercapacitor22to perform charge/discharge control on the capacitor76of the DC buses78by controlling the duty ratio of the second fully-controlled switch S2. When the voltage of the DC buses78is greater than the upper limit of the normal voltage range of the DC buses, the DC buses78may be caused to charge the supercapacitor22by controlling the duty ratio d2of the second fully-controlled switch S2(for example, making d2>0.5 and d1=1−d2), electrical energy is transferred from the capacitor76of the DC buses78to the supercapacitor22, so that the voltage of the DC buses78is reduced. When the voltage of the DC buses78is lower than the lower limit of the normal voltage range of the DC buses, the DC buses78may be caused to discharge the supercapacitor22by controlling the duty ratio d2of the second fully-controlled switch S2(for example, making d2<0.5, and d1=1−d2), and the electrical energy is transferred from the supercapacitor22to the capacitor76of the DC buses, so that the voltage of the DC buses78is increased, and finally, the voltage of the DC buses78is controlled to be within the normal voltage range of the DC buses.

In order to realize control decoupling of the voltage of the DC buses78from the supercapacitor energy storage system and the DC unloading circuit (that is, the crowbar protection circuit1), the limit value of the voltage of the DC buses78may be set to be slightly smaller than a triggering action value of the DC unloading circuit, so as to prevent the DC buses78and the DC unloading circuit from operating simultaneously, causing the voltage of the DC buses78to be instable. In addition to this, it is necessary to consider the operating voltage range of the supercapacitor22during the boost control, a gain factor is dynamically adjusted, so that an injection/absorption power of the supercapacitor22may be adjusted to prevent the voltage of the supercapacitor22from exceeding the operating range. For example, the gain factor is set to “0” or “1”. When the voltage of the supercapacitor22exceeds the upper/lower limit, the gain factor is set energy storage system does not control the voltage of the DC buses78of the converter7any more, otherwise, the gain factor is set to “1”, which indicates that the supercapacitor energy storage system controls the voltage of the DC buses78.

The supercapacitor22has an allowable operating voltage range (for example, 450 V to 960 V). To prevent the supercapacitor22from overvoltage or undervoltage, control, that is, buck control, of the voltage of the supercapacitor22may be achieved by the DC-DC converter24. Particularly, the voltage of the DC buses78is monitored while the boost control is performed. When the voltage of the DC buses78returns to the normal voltage range of the DC buses (that is, when the flag bit of the voltage of the power grid is set to “normal”), buck control is performed, the supercapacitor22serves as an output terminal of the DC-DC converter24, the capacitor76of the DC buses serves as an input terminal of the DC-DC converter24, and the voltage of the supercapacitor22is adjusted to be within the normal voltage range of the supercapacitor, that is, the voltage of the supercapacitor22is controlled.

The buck control includes: monitoring the voltage of the supercapacitor22and controlling the voltage of the supercapacitor22to be within the normal voltage range of the supercapacitor by controlling the duty ratios of the first fully-controlled switch S1and/or the second fully-controlled switch S2. The buck control may include: causing the capacitor79of the DC buses to control the charge/discharge of the supercapacitor22by controlling the duty ratio of the first fully-controlled switch S1. When the voltage of the supercapacitor22is greater than an upper limit of the normal voltage range of the supercapacitor22, the supercapacitor22may be discharged by controlling the duty ratio d1of the first fully-controlled switch S1(for example, making d1>0.5 and d2=1−d1), electrical energy is transferred from the supercapacitor22to the capacitor76of the DC buses78, such that the voltage of the supercapacitor22is reduced. When the voltage of the supercapacitor22is lower than a lower limit of the normal voltage range of the supercapacitor, the supercapacitor22may be charged by controlling the duty ratio d1of the first fully-controlled switch S1(for example, making d1<0.5, and d2=1−d1), and the electrical energy is transferred from the capacitor76of the DC buses78to the supercapacitor22, such that the voltage of the supercapacitor22is increased, and finally, the voltage of the supercapacitor22is adjusted.

With reference toFIG. 6, an embodiment of the present disclosure further provides a control system for enhancing an endurance capability to an abnormal voltage of a wind turbine generator system. The control system includes the doubly-fed wind turbine generator system and an integrated system coordination controller9. The integrated system coordination controller9determines whether a voltage of a power grid has a fault. When it is determined that the voltage of the power grid has a fault, the integrated system coordination controller9determines whether a voltage of DC buses78exceeds a limit value or not. When the voltage of the DC buses78exceeds the limit value, the integrated system coordination controller9performs integrated system coordination control on a supercapacitor energy storage apparatus2and a converter7according to an abnormal operating condition mode. When the voltage of the power grid returns to normal, the integrated system coordination controller9performs integrated system coordination control on the supercapacitor energy storage apparatus2and the converter7according to a normal operating condition mode.

The integrated system coordination controller9includes a converter controller91and an energy storage apparatus controller92. The energy storage apparatus controller92and the supercapacitor energy storage apparatus2together form a supercapacitor energy storage system. When the voltage of the DC buses78exceeds the limit value, the energy storage apparatus controller92controls the duty ratios of switches in the DC-DC converter24and performs the boost control. When it is determined that the voltage of the power grid returns to within the normal range, the energy storage apparatus controller92controls the duty ratios of the switches in the DC-DC converter24and performs the buck control.

The energy storage apparatus controller92performs the boost control, particularly, controls the voltage of the DC bus78to be within the normal voltage range of the DC bus.

In one embodiment, when the voltage of the DC buses78is greater than an upper limit of the normal voltage range of the DC buses, the energy storage apparatus controller92may control a duty ratio d2of the second fully-controlled switch S2(for example, make d2>0.5 and d1=1−d2) to enable the DC buses78to charge the supercapacitor22, electrical energy is transferred from the capacitor76of the DC buses78to the supercapacitor22, so that the voltage of the DC buses78is reduced. When the voltage of the DC buses78is lower than a lower limit of the normal voltage range of the DC buses, the energy storage apparatus controller92may control a duty ratio d2of the second fully-controlled switch S2(for example, make d2<0.5, and d1=1−d2) to enable the DC buses78to discharge the supercapacitor22, and the electrical energy is transferred from the supercapacitor22to the capacitor76of the DC buses, so that the voltage of the DC buses78is increased.

The energy storage apparatus controller92performs the buck control, particularly, controls the voltage of the supercapacitor22to be within a normal operating voltage range of the supercapacitor.

In one embodiment, when the voltage of the supercapacitor22is greater than an upper limit of the normal voltage range of the supercapacitor, the energy storage apparatus controller92may control the duty ratio d1of the first fully-controlled switch S1(for example, make d1>0.5 and d2=1−d1) to discharge the supercapacitor22, electrical energy is transferred from the supercapacitor22to the capacitor76of the DC buses78, so that the voltage of the supercapacitor22is reduced. When the voltage of the supercapacitor22is lower than a lower limit of the normal voltage range of the supercapacitor, the energy storage apparatus controller92may control the duty ratio d1of the first fully-controlled switch S1(for example, make d1<0.5, and d2=1−d1) to charge the supercapacitor22, and the electrical energy is transferred from the capacitor76of the DC buses to the supercapacitor22, so that the voltage of the supercapacitor22is increased, and finally, the voltage of the supercapacitor22is adjusted.

In addition, the control system for enhancing the endurance capability to the abnormal voltage of the wind turbine generator system may further include a DC bus voltage detection module93and a power grid voltage detection module94. In one embodiment, the control system may further include a supercapacitor voltage detection module95. The DC bus voltage detection module93, the power grid voltage detection module94, and the supercapacitor voltage detection module95detect the voltage of the DC buses, the voltage of the power grid, and the voltage of the supercapacitor22, respectively, and transmit detection results to the integrated system coordination controller9.

By means of the above control method for enhancing the endurance capability to the abnormal voltage of the wind turbine generator system, the supercapacitor energy storage system may realize the following controls.

Its operating voltage is maintained when the voltage of the power grid is normal.

When the voltage of the power grid is abnormal, since the grid-side converter is switched to the “reactive priority control” mode, in order to balance a generator/network power, the supercapacitor controls the voltage of the DC buses to ensure continuous operation of the wind turbine generator system. Moreover, the supercapacitor may also suppress a transient process of the voltage of the DC buses due to voltage dip of the power grid.

The present disclosure provides a control method for the endurance capability to the abnormal voltage of the wind turbine generator system by employing a supercapacitor energy storage system, which may be used for enhancing a fault handling capability of the doubly-fed wind turbine generator system. The control method solves continuous operation problems of the doubly-fed wind turbine generator system in the cases of small temporary disturbance of the grid voltage, and under a high/low voltage without drastically changing the original control strategy and control structure of the wind turbine generator system. The algorithm of the control method is simple and reliable as well as good in effect, and the operation and service life of other components of the wind turbine generator system are not affected.

The above embodiments are only used to illustrate the technical solutions of the present disclosure and are not intended to limit them. Although, the present disclosure has been described in detail with reference to the above embodiments, those skilled in the art may still make modifications or equivalent substitutions on the specific embodiments of the present disclosure, and any modifications or equivalent substitutions made without departing from the spirit and scope of the disclosure may be within a protective scope of claims to be approved by the present disclosure.