WO2018227486A1

WO2018227486A1 - System adapted for operating generator

Info

Publication number: WO2018227486A1
Application number: PCT/CN2017/088478
Authority: WO
Inventors: Silvio Semmer
Original assignee: Envision Energy (Jiangsu) Co., Ltd.
Priority date: 2017-06-15
Filing date: 2017-06-15
Publication date: 2018-12-20
Also published as: CN110870166A; DK181215B1; CN110870166B; DK201970706A1

Abstract

A system (1, 2) adapted for operating generator (4) performs a re-magnetisation of permanent magnet poles (6) in a generator operating in a nacelle (102) of a wind turbine (100). The system (1, 2) is adapted for re-magnetising one or more permanent magnets (8). Hereby, it is possible to achieve a re-magnetisation of permanent magnet poles (6) in generators (4). In normal operation, re-magnetisation should not be necessary, but in situations where a short cut of the electric connections of the generator (4) has been performed one or more of the permanent magnets (8) can have lost their magnetisation.

Description

System adapted for operating generator

Field of the Invention

The present invention relates to a system adapted for operating generator， which sys-tem comprises a control system and an inverter， which generator comprises poles formed by permanent magnets placed at a back iron in a rotor or stator， which rotor or stator comprises at least a first winding system consisting of at least one system of coils， which rotor in normal operation rotates in relation to the stator， where change of magnetic field in the coils generates electric current in the coils， which electric current is sent through wires to a control system， which control system comprises at least one converter.

Object of the Invention

It is the object of the patent application to perform a magnetisation and or re-magnetisation of permanent magnet poles in a generator operating in a nacelle of a wind turbine.

Description of the Invention

The object of the invention can be achieved by a system as disclosed in the preamble of the claim 1 and modified by a system adapted for magnetisation and or re-magnetising one or more permanent magnets， the system is adapted to generate an AC or DC current in one or more of the coils for preheating one or more magnets， the sys-tem is further adapted to generate DC impulse current in one or more coils for a short period for generating a strong magnetic field in one or more magnets.

Hereby， it is possible to achieve a magnetisation and or re-magnetisation of permanent magnet poles in generators. In many generator applications it is possible simply to replace a magnetic pole and then let the generator start up as usual. However， espe-cially in wind turbines placed at open sea， it is very important that re-magnetisation is possible of the magnets because service and repair is very difficult. In bad weather situations， for example in the North Sea， it can take weeks or maybe even months be-fore wind and waves allow access to a wind turbine. Even if service personnel have access to the wind turbine， it is very critical to perform a repair of a generator where magnet poles have to be replaced simply because there is so little open space around a generator in a nacelle of a wind turbine. In normal operation， re-magnetisation should not be necessary， but in situations where a short circuit of the electric connections of the generator has been performed one or more of the magnets can have lost partially or completely their magnetisation.

In some situations， all magnets are losing their magnetism and in that situation the generator will produce nearly no power. By this patent application it is possible， may-be after stopping the generator， to perform at first a preheating of the magnets depend-ing on type of magnets； the temperature can be different but probably the preheating of the magnets has a limit at approximately 140℃.

Many magnets need preheating before re-magnetisation because they can be re-magnetised with a lower magnetic field if they are preheated. Preheating could be per-formed in different ways， but maybe the fastest and most effective preheating is per-formed by adding an AC current to the coils that generate a magnetic field. The oscu-lating magnetic field in the magnets will automatic generate heat in the magnets.

It is preferred that the temperature of the magnets is under control during the heating process in order to prevent an overheating. When the heating is ended， it is possible that the coils are instead connected to a DC source where a DC current is flowing and in that way generates a magnet field passing through the magnets.

The current in the coils has to have sufficient current in order to perform a total mag-netisation of the magnets. In order to get full magnetisation it is very important that the magnetic field generated by the coils is passing through the magnets and further into the back iron placed behind the magnet， and the magnetic field is then probably passing through neighbour magnets which are to be magnetised in the opposite direc-tion. In that way the magnetic field can be sent back to the neighbour coil which also generates a magnetic field in the opposite direction. During the magnetisation process， it is very important to limit the current in the coils into the maximum current that the coils can accept. The magnetisation process can be very short， not more than 2 sec-onds， and in that way the heating in the magnetic coils can be accepted even if the current passing through the coils is much higher than normal. As soon as re-magnetisation has been performed in one or more of the magnetic coils， it is possible to stop the process and let the generator start normal operation. The generator can start after the magnets have been cooled down to around 60 degC. At this temperature， the magnets have regained full magnetic performance and the generator will be able to deliver rated torque again. Therefore， as soon as a failure of a magnetic field is detect-ed，probably by controlling the wave form of the generated electric current， it is possi-ble to indicate one or more magnets which have been without a magnetic field.

In a preferred embodiment for the invention， the generator can comprise at least a sec-ond winding system consisting of at least one second system of coils. In generators where more than one winding is operating with the same permanent magnet， it is pos-sible that these windings are operating together in order to re-magnetise the permanent magnet. In situations where for example two coils are operating they can generate a combined magnetic field which field will pass through the permanent magnet. In that way a much higher magnetic field is to be achieved.

In a further preferred embodiment for the invention， re-magnetisation can be per-formed by a first and second winding system operating in parallel at least in relation to magnets that have to be re-magnetised. Hereby， it can be achieved that different wind-ing systems can be operating in parallel for hereby generating a combined magnetic field.

In a further preferred embodiment for the invention， re-magnetisation can be per-formed by a first and second coils operating in serial at least in relation to magnets that have to be re-magnetised. Hereby， it is achieved that the same current can be used twice for generating the magnetic field. In winding systems where more coils are used in relation to each of the magnets， it is possible to connect all the coils in serial in or-der to achieve the highest possible magnetic field that is possible with the current that is flowing through the coils.

In a further preferred embodiment for the invention， the DC current in the coils can be increased for a short period for generating a magnetising field in one or more magnets， which magnetic field is above the coercivity of the magnets. Hereby， it is achieved that the permanent magnet is re-magnetised into the coercivity of the magnet.

In a further preferred embodiment for the invention， the DC current in the coils can be increased for a short period for generating a magnetising field in one or more magnets， which magnetic field is above 1.5 the coercivity of the magnets. By generating a mag-netic field which is higher than 1.5 of the coercivity of the magnet， a re-magnetisation of the permanent magnets will take place immediately. Therefore， the current in the coils only have to be flowing for a relative short period， not more than 2 seconds. Be-cause of the fact that the coils at the same time are heated up to a temperature where the coercivity of the magnet is lower than at normal operation， then at the increased temperature it is possible to reach more than 1.5 of the coercivity of the magnets by a current in the coils which is not much higher than the current that the coils are con-ducting when they are in normal operation.

In a further preferred embodiment for the invention， all magnets in the generator can be heated by an AC current in the coils. Hereby， all magnets in a generator can be preheated at the same time. This is rather important because if a really bad failure has occurred in a generator， all magnets can be full or partly de-magnetised. Therefore， it is very important that all magnets can be re-magnetised in the same operation. As an alternative， the magnets could be preheated and re-magnetised one by one. Hereby， it can be achieved that the power for preheating and for re-magnetisation is reduced， which can be achieved by a smaller converter connected to a grid.

In a further preferred embodiment for the invention， the DC current in the coils can be increased for a short period for generating a magnetising field in one or more magnets， which magnetic field is above 1.5 the coercivity of the magnets. Hereby， the current in all the coils can be connected to a DC source and hereby generating the magnetic field that is passing through all the magnets in the generator and thereby performing the re-magnetisation.

In a further preferred embodiment for the invention， the patent application can con-cern a method for re-magnetisation of permanent magnets in generators as previous disclosed， where the system performs a test sequence for detecting failure in one or more of the magnets， and the system starts a re-magnetisation sequence by stopping the generator， letting first and second winding system operating in parallel and letting first and second coils operating in serial， performing a preheating of the magnets that have to be re-magnetised by connecting the serial connected coil to a AC source， per-forming a re-magnetisation of the magnets by connecting the serial connected coil to a DC source， for a short period for re-magnetising the magnets.

Hereby， it can be achieved that as soon as a de-magnetisation of one or more perma-nent magnets have been indicated， maybe after a failure in operation of the generator， it is possible immediately to let the system start the process as disclosed in this meth-od.

By preheating all the permanent magnets by an AC connection to the coils， it is possi-ble then by connecting the coils afterwards to a DC source to achieve a magnetic field passing through the magnets so they are re-magnetised and they are magnetised up to more or less the same level as when the generator was started the first time. Also in situations where the magnets are being weak， maybe after long time operation， it is possible， maybe in a situation with a wind turbine in a non-operation mode because of no wind， to perform a re-magnetisation of all the magnets that are forming the mag-netic poles.

This invention relates to a model structure of stator and magnet pole of a permanent magnet direct drive generator. It is a part of a generator， and it includes at least one magnet pole and at least one winding wire which covers at least one magnet pole.

The magnet pole is fixed to a structure， such as a rotor yoke. The winding wire is also fixed to a structure， such as a stator lamination. There is a uniform air gap between the magnet pole and the winding wire， the magnet pole can make a relative motion against the winding wire. The magnet pole is made from magnet material and the winding wire is made from metal electrical conductive material， such as copper， aluminum， etc.

Usage of the coil in this invention：

1) at normal work situation， the wire makes a relative motion and cuts magnetic lines of flux， inducted current；

2) when the magnet is partially demagnetised due to short circuit etc.， the wire can build an electric circuit and impose required current to heat the magnet to a set high temperature；

3) when the magnet is partially demagnetised due to short circuit etc.， the wire can impose a required current， create a magnetic field to re-magnetise the magnet to nearly saturation.

Usage of the magnet pole in this invention：

1) provide magnetic field at a normal working situation；

2) when the magnet is partially demagnetised due to short circuit etc.， re-magnetised to nearly saturation by wire.

The aim of this invention is to make an assembly structure with magnet pole and coil which can be used to re-magnetise the magnet when it is de-magnetised by fault con-dition like short circuit.

Even the de-magnetisation on the magnet is a very small probability due to tempera-ture increasing and short circuit， but the result of the de-magnetisation can cause a decrease of the performance of the generator. To deal with this situation， the magnets have to be designed with higher coercivity to ensure that the generator can keep a whole life performance， which directly causes higher magnet cost.

This invention provides a magnetising function within the generator， so that lower coercivity magnet can be used， which means lower material cost. If the magnet got a partially demagnetisation， this invention can set up an heating electric circuit， the magnet’s temperature increased to a certain point， and the coil provides a magnetic field to re-magnetising the magnet pole. The magnetisation is returned to initial work-ing situation.

The re-magnetising field should be above 1.5 times of the coercivity. When magnet’s temperature increases， its coercivity decreases accordingly， the needed magnetising field also becomes lower. Comparing to room temperature， it is easy to magnetise a magnet at a higher temperature to achieve an equal effect. This invention’s heating function can help the magnet to increase to required temperature， therefore use a lower field to magnetise the magnet.

This invention is suitable for all permanent magnet synchronous machines， especially for permanent magnet synchronous generators like direct drive generators and also geared generators of wind turbines.

Description of the Drawing

Fig. 1 shows a wind turbine.

Fig. 2 shows a section of a generator stator and rotor.

Fig. 3 indicates the electric connection of a generator for two system usage.

Fig. 4 shows the electric connection to the different windings to the grid.

Fig. 5 indicates a coverture of the magnetism of a magnet that has been demagnet-ised and after that re-magnetised.

Fig. 6 discloses a system for operating the generator.

Detailed Description of the Invention

Fig. 1 shows a wind turbine 100 with a nacelle 102 and with three blades all connect-ed to a hoop 105. The nacelle 102 is carried at a tower 106. The hoop 105 is connected mechanically to a generator in order to let the rotor of the generator rotate in relation to a stator and in that way generate electric power.

Fig. 2 shows a sectional view of a generator 4 indicating an outer rotor 12. At this outer rotor 12 is placed magnetic poles 6 formed as permanent magnets 8. The outer rotor cooperates with an inner stator 14 which stator 14 comprises a first winding sys- tem 16 formed by coils 18. Between the outer rotor 12 and the stator 14 there will be formed an air gap. The magnets 8 are fixed to a not-shown back iron in order to gen-erate a magnetic circuit for the magnetic field which is generated by the permanent magnets 8.

Fig. 3 shows a first winding system 16 and a second winding system 32. The first winding system 16 comprises coils 18 and the second winding system 32 comprises coils 34. Both of the winding

system

16 and 32 are connected to a converter 26. In normal operation， the coils 18 will by change in the magnetic field through the coils generate an electric current 28 which is supplied to the converter 26. In normal opera-tion of the converter， the generated power will probably be converted into grid power which can be transmitted to the grid. Further， at fig. 3 it is indicated that system 1 and system 2 in the middle example is parallel connected. At the same time， the two

coils

34 and 18 are serial connected. The two coils are connected in a way where they will generate a common magnetic field which is to be used for re-magnetisation of the magnets. In the situation where there is a parallel coupling of the two winding system and serial coupling of the coils， a preheating of the magnets is performed， simply by letting the converter 26 generate an AC current into the two parallel coupled systems. The permanent magnet can in that way be preheated to a temperature at which the magnetic material can be re-magnetised most effectively. Further， at the third example indicated at fig. 3 it is shown that twice current from the converter is used just for one second. In that way， an extremely high magnetic field is generated through the mag-nets and a re-magnetisation is performed. Hereafter， the converter is performing a change so that the first and second winding

system

16 and 32 can be connected as in normal operation.

Fig. 4 shows the first winding system 16 and the second winding system 32. Further， coils 18 and 34 are indicated. It is clear from fig. 4 that the system is operating as two independent three-phase systems where three-phases and a common line are connect-ed to the converter. Probably， a converter has separate input for the two different sys-tems.

Fig 5 shows a coordinate system 200 with curves 202-210. The curve 202 is the de-magnetising curve； it shows how the magnet is demagnetised， impose a minus mag- netic field， such as -320kA/m indicated as 203 at the curve 202， then the magnet gets partly demagnetised. After demagnetisation， this magnet cannot work as normal， it needs to be magnetised， the

curve

204， 206 is one of the magnetising curves when a plus magnetic field is imposed， such as +330kA/m＝Hcj. When this magnet is restored to a relative higher situation， it can work， but not enough. After demagnetisation， this magnet cannot work as normal， it needs to be magnetised， the dotted curve 210 is one of the magnetising curves when a plus magnetic field is imposed， such as +495kA/m＝1.5Hcj. When this magnet is restored to a similar saturated situation， it can work， almost enough. So in this invention， it is specified that 1.5 times or more of Hcj is the magnetising field to saturate the magnetisation.

At fig. 6 is indicated a converter 26 which has input from the first winding system 16 and the second winding system 32. In that way， the converter 26 receives two three-phase input of power which is independent on each other in normal operation. A con-verter performs a combination of the power input and change the frequency of the power so that the output 42 has a grid frequency and probably also has synchronisa-tion to the grid. Further， a control system 36 is connected to the converter. This con-trol system has a plurality of input lines， only one line is indicated 38 which is con-nected to a thermal detector 40 which is placed at one or more of the magnets in order to control the magnet temperature.

Coercivity is to be defined as property of a material determined by the value of the coercive force when the material has been magnetised to saturation.

A short period is defined to be a period of not more than 2 seconds.

Reference list

System (2)

generator (4)

poles (6)

permanent magnets (8)

back iron (10)

rotor (12)

stator (14)

first winding system (16)

coils (18)

electric current (20)

wires (22)

control system (24)

converter (26)

AC current (28)

DC current (30)

second winding system (32)

second system of coils (34)

wind turbine 100

a nacelle 102

blades 104

hoop 105

tower 106

coordinate system 200 .

de-magnetising curve 202

a minus magnetic field， such as -320kA/m indicated as 203

magnetising curves 204， 206

dotted curve

208， 210

Claims

System (2) adapted for operating generator (4) ， which system comprises a control system and an inverter， which generator (4) comprises poles (6) formed by permanent magnets (8) placed at a back iron (10) in a rotor (12) or stator (14) ， which rotor (12) or stator (14) comprises at least a first winding system (16) consisting of at least one sys-tem of coils (18) ， which rotor (12) in normal operation rotates in relation to the stator (14) ， where change of magnetic field in the coils (18) generates electric current (20) in the coils (18) ， which electric current (20) is sent through wires (22) to a control system (24) ， which control system (24) comprises at least one converter (26) ， characterised in that the system (2) is adapted for re-magnetising one or more permanent magnets (8) in the following steps of operation：

a. the system (2) is adapted to generate an AC current (28) in one or more of the coils (18) for preheating one or more magnets (8) ，

b. the system (2) is adapted to generate DC current (30) in one or more coils (18) for a short period for generating a strong magnetic field in one or more magnets (8) .
System (2) according to claim 1， characterised in that the generator (4) comprises at least a second winding system (32) consisting of at least one second system of coils (34) .
System (2) according to claim 2， characterised in that by re-magnetisation first (16) and second winding system (34) are operating in parallel at least in relation to magnets (8) that have to be re-magnetised.
System (2) according to claim 3， characterised in that by re-magnetisation first (18) and second coils (34) are operating in serial at least in relation to magnets (8) that have to be re-magnetised.
System according to claim 4， characterised in that the DC current in the coils (18， 34) is increased for a short period for generating a magnetising field in one or more magnets (8) ， which magnetic field is above the coercivity of the magnets (8) .
System according to claim 5， characterised in that the DC current in the coils (18， 34) is increased for a short period for generating a magnetising field in one or more magnets (8) ， which magnetic field is above 1.5 the coercivity of the magnets (8) .
System according to claim 6， characterised in that all magnets (8) in the generator (4) are preheated by an AC current in the coils (18， 34) .
System according to claim 7， characterised in that the DC current in the coils (18， 34) is increased for a short period for generating a magnetising field in one or more magnets (8) ， which magnetic field is above 1.5 the coercivity of the magnets (8) .
Method for re-magnetisation of permanent magnets (8) in generators (4) as dis-closed in one of the previous claims， characterised in that：

a. the system (2) performs a test sequence for detecting failure in one or more of the magnets (8) ，

b. the system (2) starts a re-magnetisation sequence

c. stopping the motor or generator (4)

d. letting first (16) and second winding system (34) operating in parallel and letting first (18) and second coils (34) operating in serial，

e. performing a preheating of the magnets that has to be re-magnetised by con-necting the serial connected coil (18， 34) to a AC source，

f. performing a re-magnetisation of the magnets by connecting the serial con-nected coil (18， 34) to a DC source， for a short period for re-magnetising the magnets (8) .