WO2019157626A1

WO2019157626A1 - Demagnetization protection of a permanent magnet generator

Info

Publication number: WO2019157626A1
Application number: PCT/CN2018/076638
Authority: WO
Inventors: Heng DENG; Hao Hua; Yong Liu; Kurt Andersen
Original assignee: Envision Energy (Jiangsu) Co., Ltd.
Priority date: 2018-02-13
Filing date: 2018-02-13
Publication date: 2019-08-22
Also published as: CN111869065A; CN111869065B

Abstract

Disclosed is a wind turbine generator (WTG) with a permanent magnet generator system. The WTG has at least one permanent magnet arranged to magnetically interact with a magnetic flux from at least one first winding configured to produce a first magnetic flux and at least one second winding arranged relative to the at least one first winding and to produce a second magnetic flux countering the first magnetic flux. The at least one first winding and the at least one second winding may individually be controlled by a controller.

Description

Demagnetization protection of a permanent magnet generator

Field of the Invention

The present invention relates to a wind turbine generator (WTG) with a permanent magnet generator system. The WTG has at least one permanent magnet arranged to magnetically in-teract with a magnetic flux from at least one first winding configured to produce a first mag-netic flux and at least one second winding arranged relative to the at least one first winding and to produce a second magnetic flux countering the first magnetic flux. The at least one first winding and the at least one second winding may individually be controlled by a control-ler.

Background of the Invention

The permanent magnet (PM) machines benefit from high efficiency and high power density due to the employment of high energy-product, PM machines and thus the elimination of excitation coils. Nevertheless, the PM machines confront the risk of irreversible demagnetiza-tion on PMs if the extremely high armature currents occur and the corresponding demagnetiz-ing electromagnetic field is generated, which happens with short-circuit fault. In order to ad-dress the problem of irreversible demagnetization, high-grade PMs with a higher demagneti-zation limit are required, which significantly increase the machine cost. Therefore, it would be helpful to find an approach to reduce the resulting demagnetizing electromagnetic field on the PMs.

Dual-winding (more-winding) machines are attracting increasing attention in high power ap-plications since they benefit from the improved fault-tolerant capability and reduce the capac-ity requirement on single converter. In the dual-winding PM machines, there are two individ-ual armature winding sets and they can operate independently. If the short-circuit fault hap-pens to one winding set (fault winding) and thus its armature currents rise dramatically, with which a strong demagnetizing electromagnetic field occurs and threats the PMs, the other winding set (healthy winding) can be controlled with a specific strategy to generate a magnet-izing electromagnetic field, which counters against the demagnetizing field of the fault wind-ing and thus protects the PMs from the irreversible demagnetization. It should be emphasized that this PM protection approach can be realized by directly controlling the healthy winding during fault operations and there is no cost increase.

Description of the Invention

An objective is achieved by a wind turbine generator (WTG) with a permanent magnet gen-erator system. The WTG has at least one permanent magnet arranged to magnetically interact with a magnetic flux from at least one first winding configured to produce a first magnetic flux and at least one second winding arranged relative to the least one first winding and to produce a second magnetic flux countering the first magnetic flux. The at least one first wind-ing and at least one second winding may individually be controlled by a controller.

In an aspect, such as normal operation, the second winding will generate a second magnetic flux together with the first magnetic flux. That is the second winding flux would positively flow together with first winding flux, i.e. in the same direction during normal operation. The second magnetic flux only counters against the first one during a non-normal operational event including as an example a short-circuit

Thereby, the system may reduce the risk of demagnetizing the permanent magnet (PM) . Hence, permanent damage to a wind turbine generator with a permanent magnet generator is reduced, and expensive repair or change of permanent magnets is reduced.

Permanent magnet generators (PMG) enjoy the benefits of high efficiency and high power density, and permanent magnet (PM) machines are of increasing importance in the applica-tions ranging from wind power generation, electric vehicle, and industrial traction.

However, the irreversible demagnetization is a critical limit to PMs which may occur with extremely high armature currents (e.g. in short-circuit operation) . The irreversible demagneti-zation of PMs would damage the machines permanently and it must be avoided in PM ma-chines.

In a dual-winding PM machine as suggested, the windings can be controlled independently.

During normal operations, a similar or identical control strategy can be applied to the two windings, namely the first winding and the second winding, and the windings then contribute to the power generation simultaneously.

In a case of fault or non-normal operation, say if the short-circuit fault happens to one wind-ing, e.g. the first winding (which may be termed a fault winding) , and the resulting demagnet-izing electromagnetic field threats the permanent magnets, the other winding, i.e. the second winding (which may be termed a healthy winding) can be controlled with a specific strategy to generate a magnetizing electromagnetic field to protect the PMs.

And the risk of irreversible demagnetization of permanent magnets can be greatly reduced.

In general and as an example, if a short-circuit fault happens to one winding, e.g. the first winding or the fault winding, an extremely high armature current occurs in the fault winding and thus the strong demagnetizing electromagnetic field is generated. The magnetic field may alter and adversely affect the magnetization of the permanent magnet.

Consequently, the permanent magnets are confronted with a high risk of irreversible demag-netization.

In example, there may be two independent windings in the machine and two corresponding converters.

If a short-circuit fault happens in the first winding (fault winding) and the demagnetizing cur-rents and electromagnetic field occur, then the second winding (the healthy winding) can be utilized by actively injecting the magnetizing armature currents with the reversed polarity to those in the fault winding.

Thus, the electromagnetic field generated by the healthy winding will cancel out the demag-netizing field of the fault winding, which contributes to the low resultant electromagnetic field, and thus greatly reduces the risk of irreversible demagnetization of PMs.

In an aspect, the controller and PMG system are configured to detect a fault in a first winding and to generate a control that will activate a second winding to generate a second magnetic flux to counteract the first magnetic flux resulting from the fault in the first winding.

The detection of a fault may be performed using current, voltage, or short circuit detecting means. There may be means for detecting a B-field. The controller may be programmed or wired to introduce or inject a current into a second winding to generate a second magnetic flux, which in this case may be considered a corrective flux, that will counteract or cancel the effect of the first magnetic flux, which in this case may be considered an error flux.

The controller may be programmed, and the magnetic flux is a result of calculations.

The controller may be provided with a table with tabulated flux values and/or tabulated cur-rent values.

In an aspect, the at least one first winding is operated by at least one first converter and wherein the at least one second winding is operated by at least one second converter.

Each converter is adapted to provide the currents to windings as required.

In an aspect, the controller and system are configured to detect a short circuit type of fault in the at least one first winding. As an example, the controller and system are configured to de-tect a high current in the at least one first winding.

In an aspect, a first axis of a first winding is substantially coaxial with a second axis of a sec-ond winding. As an example, the first and second windings may be arranged in a cavity in a stator. The cavity may be so formed that the windings fit into the cavities and are aligned co-axially.

In an aspect, the at least permanent magnet is arranged on a rotor, and the first and second windings are arranged in a stator.

An objective of the invention is achieved by a method of protecting a wind turbine generator (WTG) with a permanent magnet generator comprising at least on permanent magnet, at least one first winding and at least one second winding. The method of protecting a WTG may comprise the following acts.

There may be an act of detecting a fault in the first winding. There may be an act of estimat-ing the resulting first magnetic flux on the permanent magnet.

There may be an act of applying a second magnetic flux from the second winding counteract-ing the resulting first magnetic flux on the permanent magnet.

In an aspect, the act of detecting a fault comprises an act of detecting a short circuit in the first winding. In an aspect, the act of applying a second magnetic flux may be performed by applying a second current into the second winding.

In an aspect, the act of generating a second magnetic flux in the second winding is performed by injecting a second current from a second converter.

An object of the invention is achieved by a method of reducing demagnetization of a perma-nent magnet in a permanent magnet generator in a wind turbine generator (WTG) . The meth-od of reducing demagnetization comprises the following acts.

There may be an act of determining an error magnetic flux on the permanent magnet.

There may be an act of applying a corrective magnetic flux counter to the error magnetic flux on the permanent magnet.

In an aspect, the act of determining the error magnetic flux includes an act of determining a first error current in a first winding. In an aspect, the act of applying a corrective magnetic flux includes an act of injecting a second corrective current in a second winding.

An objective of the invention may be achieved by a method of operating a wind turbine gen-erator (WTG) having a permanent magnet generator. The method of operating the WTG may comprise applying the acts outlined herein.

An objective of the invention may be achieved by a device for operating a wind turbine gen-erator, WTG, having a tower supporting a nacelle supporting a rotor with blades and a drivetrain operatively connected to a permanent magnet generator (PMG) system.

The PMG may comprise at least one permanent magnet arranged to magnetically interact with a magnetic flux from at least one first winding configured to produce a first magnetic flux and at least one second winding arranged relative to the least one first winding. The sec-ond winding is configured to produce a second magnetic flux countering the first magnetic flux. At least one first winding and at least one second winding are individually controlled by a controller.

The second winding may also be configured to produce a second magnetic flux, say during normal operation, that will add or contribute to the first magnetic flux.

The device comprises sensors and means adapted to execute the actions outlined.

An objective may also be achieved by a computer program product comprising instructions to cause the device to execute the actions outlined.

The systems and actions outlined are not limited to the examples. In example, the system and method cannot only be applied to the dual-winding machines (with first winding and second winding) , but also to the more-winding machines (with first winding, second winding, third winding ...) . In general there may be multiple windings or multiple sets of windings.

Also the systems, arrangements, methods and actions do not only suit a generator in a wind turbine, but also find use in other generator or motor arrangements. In particular in high pow-er level and/or high fault-tolerance applications such as ship propulsion, aircrafts, electric vehicle drive, etc.

Description of the Drawing

The invention is described by example only and with reference to the drawings, whereon:

Fig. 1 illustrates a wind turbine generator (WTG) with a permanent magnet generator;

Fig. 2 illustrates a permanent magnet generator (PMG) ;

Fig. 3 illustrates a section of a permanent magnet (PM) with first and second windings;

Fig. 4 illustrates schematics of a dual converter system adapted to a dual winding PM;

Fig. 5 illustrates a circuitry of a converter system to a PMG;

Fig. 6 illustrates three phase currents, currents and torque during normal operation of a PMG in a WTG;

Fig. 7 illustrates demagnetization of a PM due to a short circuit fault in a first winding;

Fig. 8 illustrates mitigated demagnetization of a PM in a system with PM protection; and

Fig. 9 illustrates a PM without protection where the demagnetization occurs and a PM with protection that is safe.

Detailed Description of the Invention

Item	No
Wind turbine generator, WTG	1
Tower	2
Nacelle	3
Rotor	4
Blade	5
Drivetrain	6
Generator	10
Permanent magnet generator, PMG	12
Permanent magnet generator system, PMG system	14
Permanet magnet	20
Permanent magnetic flux	22
Rotor	24
Shaft	25
Stator	26
Cavity	28
Winding	30
First Winding	31
Second Winding	32
First (winding) axis	34
Second (winding) axis	35
Coil	36
Magnetic flux	40

First magnetic flux	41
Second magnetic flux	42
First current	43
Second current	44
Error magnetic flux	46
Corrective magnetic flux	47
Fault	50
Healthy	51
Short circuit fault	52
Error Current	54
Corrective current	55
Converter	60
First conyerter	61
Second converter	62
Demagnetization	70
With PM protection	72
Without PM protection	73
Controller	100

Fig. 1 illustrates a wind turbine generator (WTG) 1 having a tower 2 supporting a nacelle 3 supporting a rotor 4 with a blade 5. The rotor 5 is directly coupled to a drivetrain 6 connected to a shaft 25 (not shown) driving a generator 10 (not shown) .

Fig. 2 illustrates a generator 10 with a shaft 12, which generator 10 is a permanent magnet generator (PMG) 12 type of generator and the PMG 12 may be seen as a permanent generator system (PMG system) 14 or part of such. That the generator 10 itself may need interfacing according to a particular implementation. Depending on the actual situation, the permanent generator, PMG 12, may be a unit as depicted in figure 2. When having installed that perma-nent generator system, the PMG system 14 may be the PMG 12 or the PMG 12 with required connectors, communications, etc.

Fig 3 and 4 illustrate parts of a permanent magnet generator (PMG) system 14 with a an illus-trative arrangement of windings shown in fig. 4 and a controller 100 and converter 60 ar-rangement arranged with a permanent magnet generator (PMG) 12.

Fig 3 and 4 illustrate a section of a permanent magnet 20 arranged to magnetically interact with a magnetic flux 40 from at least one first winding 31 configured to produce a first mag-netic flux 41 and at least one second winding 32 arranged relative to the at least one first winding 31 and to produce a second magnetic flux 42 countering the first magnetic flux 41 and wherein at least one first winding 31 and at least one second winding (32) individually are controlled by a controller 100.

The second winding 32 is also arranged relative to a first winding 31 to produce a second magnetic flux 42 supporting or contributing the first magnetic flux 41.

The first and

second windings

31, 32 are arranged in a cavity 28 in a stator 26 of the perma-nent magnet generator (PMG) 12.

The controller 100 and PMG system 14 are configured to detect a fault 50 (not shown) in a first winding 31 and to generate a control that will activate a second winding 32 to generate a second magnetic flux 42 to counteract the first magnetic flux 41 resulting from the fault in the first winding 31.

Fig. 3 illustrates exemplary aspects of the arrangements in the PMG.

The first winding 31 arranged relatively to a second winding 32 so that a first axis 34 of the first winding 31 is substantially coaxial with a second axis 35 of the second winding 32.

In more details, fig. 3 shows a section of a cross section of a dual three-phase 360-stator-slot/120-rotor-pole of an outer-rotor machine. The two sets of windings (the first winding 31 and second winding 32) are employed in the machine to constitute the dual-winding topolo-gy, and each winding 31, 32 includes three phases (A, B, C) .

The overlapping distributed

windings

31, 32 are located on the stator 26 whilst the permanent magnets 20 are surface-mounted on the rotor 24 yoke. In this embodiment of a PMG 12, the two separate sets of windings 30 (the first winding 31 and the second winding 32) each in-cludes three phases (A, B, C) . In each stator slot or cavity 28, a coil 36 belonging to two dif-

ferent winding sets

31, 32 are arranged with a first axis 34 aligned with a second axis 35.

Fig. 5 illustrates an exemplary implementation of a circuitry with a first converter 61 and a second converter 62 connected to respective first winding 31 and second winding 32 in a permanent magnet generator.

Fig. 6 illustrates normal operation of a PMG 12 as exemplified. Fig. 6A shows the currents in windings A, B, C. Fig. 6B shows the currents Id and Iq, where the d-axis and q-axis current in the synchronous rotating frame, which is equivalent to the Ia, Ib, Ic in the three-phase sta-tionary frame. . Fig. 6C shows the torque.

Fig 7 illustrates aspects of a fault 50 resulting in demagnetization 70 in a permanent magnet 20 without PM protection 73.

Fig 7A illustrates a fault 50 in a first winding 31 and the resulting first magnetic flux 41, which is an error magnetic flux 46 resulting from the error current 54. In this situation with-out PM protection 73, the second winding 32 carries a second current 44 resulting in a second magnetic flux 42 that is normal.

Fig. 7A shows the d-axis and q-axis current of fault and healthy windings after short-circuit, without PM protection strategy.

The result is illustrated in fig 7B, where demagnetization 70 results in a permanent magnet 20 having an altered or destroyed permanent magnet flux 22.

Fig. 7C shows tabulated, e.g. pre-stored in a lookup table, current patterns, which would be injected into the healthy winding after detection of a short circuit.

Thus, different current patterns can have different effects on PM protection. The look-up ta-ble can be saved in a controller and the best PM protection may be achieved by selecting the appropriate current patterns for a healthy winding.

Fig. 8A and 8B illustrate aspects of a fault 50 resulting in no, less or an acceptable change in permanent magnetic flux 22 in a permanent magnet 20 with PM protection 72.

Fig 8A illustrates a fault in a first winding 31 and the resulting first magnetic flux 41, which is an error magnetic flux 46. The system is with PM protection 72 and a corrective current 55 is applied as a second current 44 in the second winding 32 (healthy winding) . The result is illustrated in fig. 8B, where, compared to the demagnetization 70 shown in fig. 7B, the per-manent magnetic flux 22 in the permanent magnet 20 is basically unchanged or changed to an acceptable level.

In detail, fig. 8A shows the d-axis and q-axis current of fault and healthy windings after a short-circuit in a system with PM protection strategy.

Fig. 9 compares the flux density distributions along the PM magnetization direction among the worst time during short-circuit fault 52, where the conventional method without protec-tion 73 and the proposed method or use of system with protection 72 using a second winding 32 protection strategy as presented.

As an example, in the illustrated dual-winding PGM 12, the minimum flux densities along the magnetization direction inside the PM 20 are shown in fig. 9, in which the short-circuit fault 52 happens to the first winding 31 at a specific time. T

In detail, fig. 9A shows the variation of the minimum flux density against time and fig. 9B, I shows the flux density distributions at the worst time. Fig. 9B, I is identical to fig. 7B while fig. 9B, II is identical to fig. 8B.

Results with two control strategies are presented and compared. One strategy is a convention-al or normal control without PM protection 73. Another strategy is a control with PM protec- tion 72, which control actively injects magnetizing currents into the second winding 32 (i.e. the healthy winding) .

It can be seen that the lowest flux density after short-circuit fault 52 is 0.14T with the conven-tional control strategy without PM protection 73. This is improved to 0.29T with PM protec-tion 72 as proposed and used as a control strategy.

Therefore, the risk of a demagnetization 70 of PMs 20 has been reduced. Moreover, the flux 40 density distributions inside the PMs 20 at the worst time with the two control strategies are compared in fig. 9B. It is observed that the PM 20 with the proposed control method or with PM protection 72 has a significantly permanent magnet 20 higher flux 22 density than that with the conventional control method (i.e. without PM protection 73) .

In fig. 9B, the first plot (with the label of ’Risky area’ ) shows the PM without protection and the demagnetization occurs; while the second plot shows the PM with protection that is safe.

In summary, fig. 7A illustrates the currents after short-circuit and fig. 7B shows the flux den-sity distributions in a permanent magnet without PM protection, i.e. the conventional method.

Fig. 8A shows the currents after a short circuit and fig. 8B shows the flux density distribu-tions in a permanent magnet with PM protection, i.e. the proposed method.

The effects or results of the two methods are compared in fig. 9 where fig. 9A shows the var-iation of flux density against time and where the results from fig. 7B and fig. 8B are present-ed in fig. 9B (I and II) for direct comparison.

Claims

A wind turbine generator (WTG) (1) with a permanent magnet generator (PMG) system (14) comprising:

- at least one permanent magnet (20) arranged to magnetically interact with a magnetic flux (40) from

- at least one first winding (31) configured to produce a first magnetic flux (41) and

- at least one second winding (32) arranged relative to the at least one first winding (31) and to produce a second magnetic flux (42) countering the first magnetic flux (41) and wherein at least one first winding (31) and at least one second winding (32) are indi-vidually controlled by a controller (100) .
The wind turbine generator (1) according to claim 1, wherein the controller (100) and PMG system (14) are configured to detect a fault (50) in a first winding (31) and to generate a control that will activate a second winding (32) to generate a second magnetic flux (42) to counteract the first magnetic flux (41) resulting from the fault in the first winding (31) .
The wind turbine generator (1) according claim 1 or 2, wherein the at least one first wind-ing (31) is operated by at least one first converter (61) and wherein the at least one second winding (31) is operated by at least one second converter (62) .
The wind turbine generator (1) according to any one or more of claims 1 to 3, wherein the controller (100) and PGM system (14) are configured to detect a short circuit type (52) of fault (50) in the at least one first winding (31) .
The wind turbine generator (1) according to any one or more of claims 1 to 4, wherein the controller (100) and PGM system (14) are configured to detect an error current (54) in the at least one first winding (31) .
The wind turbine generator (1) according to any one or more of claims 1 to 5, wherein a first axis (34) of a first winding (31) is substantially coaxial with a second axis (35) of a sec-ond winding (32) .
The wind turbine generator (1) according to any one or more of claims 1 to 6, wherein the first and second windings (31, 32) are arranged in a cavity (28) in a stator (26) of the perma-nent magnet generator (12) .
The wind turbine generator (1) according to any one or more of claims 1 to 7, wherein the at least one permanent magnet (20) is arranged on a rotor (24) and the first and second wind-ings (31, 32) are arranged in a stator (26) .
A method of protecting a wind turbine generator (WTG) with a permanent magnet genera-tot system (14) comprising at least on permanent magnet (20) , at least one first winding (31) , and at least one second winding (32) , the method comprising acts of:

- detecting a fault (50) in the first winding (31) and estimating the resulting first mag-netic flux (41) on the permanent magnet (20) ;

- applying a second magnetic flux (42) from the second winding (32) counteracting the resulting first magnetic flux (41) on the permanent magnet (20) .
The method of claim 9, wherein the act of detecting a fault (50) comprises an act of de-tecting a short circuit (52) in the first winding (31) , and the act of applying a second magnetic flux (42) is performed by applying a second current (44) into the second winding (32) .
The method of claim 10, wherein the act of generating a second magnetic flux (42) in the second winding is performed by injecting a second current (44) from a second converter (62) .
A method of reducing demagnetization of a permanent magnet (20) in a permanent mag-net generator (12) in a wind turbine generator (WTG) (1) , the method comprising acts of:

- determining an error magnetic flux (46) on the permanent magnet (20) ;

- applying a corrective magnetic flux (47) counter to the error magnetic flux (46) on the permanent magnet (20) .
The method of 12, wherein the act of determining the error magnetic flux (46) includes an act of determining a first error current (54) in a first winding (31) , and the act of applying a corrective magnetic flux (47) includes an act of injecting a second corrective current in a sec-ond winding (32) .
A method of operating a wind turbine generator (WTG) (1) having a permanent magnet generator (20) , the method comprising acts from any one or more of claim 10 to 13.
A device for operating a wind turbine generator, WTG, (1) having a tower (2) supporting a nacelle (3) supporting a rotor (4) with blades (5) and a drivetrain (6) operatively connected to a permanent magnet generator system (14) comprising:

- at least one permanent magnet (20) arranged to magnetically interact with a magnetic flux (40) from

- at least one first winding (31) configured to produce a first magnetic flux (41) and

- at least one second winding (32) arranged relative to the at least one first winding (31) and to produce a second magnetic flux (42) countering the first magnetic flux (41) , and wherein at least one first winding (31) and at least one second winding (32) indi-vidually are controlled by a controller (10) ; the device comprising sensors and means adapted to execute the actions of the method ofclaim 1 to 14.
A computer program product comprising instructions to cause the device of claim 15 to execute the actions of claim 1 to 14.