WO2021001005A1

WO2021001005A1 - System for operating a generator

Info

Publication number: WO2021001005A1
Application number: PCT/DK2020/050200
Authority: WO
Inventors: Peter BUNDSGAARD RASMUSSEN
Original assignee: Belt Generators Ivs
Priority date: 2019-07-01
Filing date: 2020-07-01
Publication date: 2021-01-07

Abstract

The present invention relates to a system and a method for connecting at least one generator primary from a wind turbine to a frequency converter for generating AC power for a supply network, which generator comprises a plurality of independent coils for generating independent phases, which are sent to a plurality of rectifiers. A first object of the pending invention is to achieve a highly reliable system for con- necting a generator to an inverter and further to a supply network. A second object is to let a generator with a failure in one coil/section/phase continue operating with a reduced power production. The scope of the invention is that the output of the rectifi- ers can be combined in one or more DC bridges, which DC bridges are connected to a common frequency converter, which inverter is generating AC current and AC voltage for a supply net-work. Hereby can be achieved that independent coils as independent sections of a generator can by isolated cabling be connected directly to rectifier means and further from the rectifier means the power is transmitted as DC to a common DC bridge.

Description

System for operating a generator

Field of the Invention

The present invention relates to a system for connecting at least one generator primary from a wind turbine to a frequency converter for generating AC current and AC volt age for a supply network, which generator comprises a rotor and a stator, which gen erator comprises a plurality of independent coils for generating independent phases, which independent phases are sent to a plurality of rectifiers.

The present invention further relates to a method for operating a generator primary in a wind turbine, as disclosed in the claims, where the generator comprises a rotor rotat ed by wind energy from a wind turbine, which rotor generates a rotating magnetic field in a plurality of independent stator coils organized as sections for generating AC current and AC voltages, which AC current and AC voltages are rectified into DC current and voltage for supply of an inverter generating AC current and AC voltage for a supply network, where independent phases are transmitted through independent cables to a plurality of rectifiers.

In order to avoid heating of the permanent magnets the magnet rotor yoke is made from laminated iron in combination with the multiphase and/or sectioned stator and divided segmented magnets in axial and/or tangential directions.

Background of the Invention

EP 1416604 A2 concerns an electric circuit for a power plant, preferably a wind tur bine, including a driven generator with rotor and stator, where the stator of the genera tor is divided into separate sections that each is connected to an independent rectifier circuit. Each rectifier circuit is connected to an independent direct current circuit, which in turn is connected to the input of at least one independent inverter. The out puts of the independent inverters are connected to a supply network. Permanent mag nets are heated up, when operating with sectioned stator, where one or more sections fail and having a solid steel rotor yoke carrying the magnets. This will clearly lead to a system, which is sensitive to errors in the generator. If an error occurs, the system may have to shut down or operate at reduced power.

Object of the Invention

A first object of the pending invention is to achieve a highly reliable system for con necting a generator to an inverter and further to a supply network.

A second object is to let a generator with a failure in one coil/secti on/phase continue operating with a reduced power production with rated power on the remaining coils/sections/phases.

A combination of a segmented stator and laminated back iron in the rotor enables in dividual independent sections to be switched off without heating the magnets.

Description of the Invention

The scope of the invention can be achieved by a system as disclosed in the opening paragraph and further modified in that the output of the rectifiers may be combined in a DC bridge, which DC bridge is connected to a common frequency converter, which converter is generating AC current and AC voltage for a supply network.

A wind turbine system for connecting at least one generator to a frequency converter for generating AC current and AC voltage for a supply network, which generator comprises a rotor and a stator, which rotor comprises a laminated yoke, which stator is divided into independent sections, which independent sections comprises a plurality of independent coils for generating independent phases, which independent phases out of at least three phases or more are sent to a plurality of rectifiers, wherein the output of the rectifiers from each independent coils is combined in one or more common DC bridge with other independent coils, which DC bridge is connected to a one or more common frequency converter, which frequency converter is generating AC current and AC voltage for a supply network.

The system provides a plurality of independent coils generating independent phases. The independent phases may be out of at least three phases or more, where each of the independent phases is led through each of a predefined rectifier. The output from each of the rectifiers is combined in one or more common DC bridges, preferably active DC bridges. The DC bridge or bridges are connected to frequency converter or con verters, where the DC bridge or bridges are proving the frequency converter or con- verier with a substantially constant current and voltage output. The frequency convert er or converters are capable of generating a substantially stable AC current and AC voltage for a supply network. This will ensure a reliable and durable system in opera tion. Hereby can be achieved that independent coils as independent sections of a generator can by isolated cabling be connected directly to rectifier means and further from the rectifier means the power is transmitted as DC to a common DC bridge to which there is a further connection to the inverter circuit which is from the DC generating basic current and voltage for a supply network. Because the connection directly towards the DC bridge is performed total independently of each other results in that any failure in rectifiers or in coils in the generator has a minimal effect in the total power produc tion. This means that a wind turbine for example placed at open sea will be able to operate with less power production but still have the effect of producing up to full load on remaining independent sections in operation until it is possible to perform service at the wind turbine.

The generator has a rotor equipped with permanent magnets which permanent mag nets are located on the rotor and the rotor is divided into independent sections. The rotor is equipped with surface mounted or buried permanent magnets mounted on lam- inated iron as a yoke. The yoke of the rotor is made from laminated iron to limit heat losses. It is combined with a stator divided into sections connected to individual recti fiers with a common DC bus. The combinations enable rated power on the sections also if one or more sections are switched off due to a failure. The laminated iron yoke in the rotor secures that the magnets do not heat up during unbalanced operation with the stator sections due to varying magnetic field in the border region between the sec tions. Using solid steel as rotor back iron will heat up the permanent magnets.

In a preferred embodiment for the system can the cabling from the independent coils/sections/phases be electric isolated from each other until the connection to the rectifiers representing parallel groups of coils in each stator phase. The advantage is to avoid interconnection between the sections by bus bars.

The stator is organized in independent phases more than 3 for individual connection to individual rectifiers. Alternatively the stator is organized in independent phases equal to 3 for individual connection to individual rectifiers. The wiring from the independ ent coils as independent phases are independent and electric isolated from each other until the connection to the rectifiers. The independent coils having an identical phase are serial connected in order to achieve the voltage for the DC bridge. Each independ ent section might consist of more parallel groups of serial connected coils.

In a further preferred embodiment, the system comprises a control system for indicat ing the frequency and the phase of the supply network, which control system gener ates a control signal for the frequency converter.

The system may comprise a control system for indicating the frequency and the phase of the supply network, which control system generates a control signal for the fre quency converter. Hereby can be achieved that at least at starting production of power, the inverter starts producing power which is synchronised to the power that is already at the public grid.

A bridge coupling of rectifiers may be provided as an active bridge coupling. The bridge coupling may be connected to an AC source, which for example could be a wind turbine generator only supplying with a plurality of independent phases. The active bridge may for example be an IGBT bridge, where the rectifiers are all con nected with switching transistors. The active bridge may be a controllable. The switching transistors are able to control the rectifiers in a short cut in that the switch ing transistors are capable of immediately closing the rectifiers. The rectifiers may then first start conducting after next positive voltage.

In operation there will of course be the possibility of multiplying this circuit, so that not only three phases are to be converted into DC, but a plurality of phases. There may be provided more than one common DC bridge handling a plurality of phases in one system. The more than one DC bridges may be connected to one or more common frequency converter. The frequency converter is generating a common AC current and common AC voltage for a supply network. The system may easily be controlled in order to ensure a high performance and avoid the system shutting down due to errors. Especially this invention makes it possible because the system as disclosed is able to operate even if there are failures in one or more circuits, for example if one coil in a generator is not operating, the generator will be able to continue operating with a lim ited power generation. In a further preferred embodiment for the system can the system operate with a volt age of about 750 V or higher measured between independent phases. Hereby can be achieved that the generator is operating with a relative median voltage and with a rela tive high current. Also for medium voltage these features are relevant. The relative high current that is necessary because the voltage is kept relative median level is not giving any problems because each of the phases or the coils are not coupled in parallel but each coil or sections are maybe serial connected coils are sent through their own cables. Therefore, a high number of parallel cables are used to transmit the power from the generator to the rectifiers and from the rectifiers directly to a bridge. Because the power is divided in parallel cables also heating of cables is reduced simply be- cause also the power dissipation is divided into the number of parallel cables.

In a further preferred embodiment for the system can the independent coils having identical phase be serial connected in order to achieve the voltage for the DC bridge. Hereby can be achieved that generators operating with a lower voltage can be used simply because one or more of the coils having the same phase can be serial connect ed. Therefore, to achieve a voltage of approx. 1,200 V or higher at the bridge two or more coils can be serial connected and thereby also connected to the same cable to wards a common rectifier. Hereby is achieved that even the same electronic circuit can be used for different kind of generators.

The scope of the invention can be further achieved by a method for operating a gener ator, primary in a wind turbine , where the generator comprises a rotor rotated by wind energy from a wind turbine, which rotor generates a rotating magnetic field in a plu rality of independent stator coils for generating AC current and AC voltages, which AC current and AC voltages are rectified into DC current and voltage for supply of an inverter generating AC current and AC voltage for a supply network where independ ent phases are transmitted through independent cables to a plurality of rectifiers, wherein the rectified current and voltage are combined into common DC bridges, where the DC bridges are connected to one or more common frequency converters, for generating AC current, and AC voltage for one or more supply network.

Because of a rectified current and voltage, which are combined into a common DC bridge, and where the DC bridge is connected to a common frequency converter for generating AC current and AC voltage for a supply network, it possible for the entire system to operate even if there are failures in one or more circuits. For example if one coil in a generator is not operating, the generator will be able to continue operating with a limited power generation A DC bridge can be configured using standard diodes; however, these rectifier cir cuits’ causes larger energy losses in the DC bridge than active bridges. The heat ac cumulation in the DC bridge may over time cause damage to the bridge, especially if the bridges are used in high voltage installations handling high current. This disad vantage is prevented by using a DC bridge which is an active bridge. The active bridge comprises for example switching transistors or MOSFETs, which is more pow er efficient.

Hereby can be achieved that by this method a generator, preferably in a wind turbine, can be connected to a power inverter/converter in a highly effective way. Because each sector or each coil from a generator is independent connected to rectifier means and from that rectifier the power is connected to a common DC bridge from where a common inverter generates AC power for a supply network. Because there are parallel cables from the generator then the total power is limited in the cables. Also the semi conductors are limited in their demands to current simply because there is a high number of parallel rectifier means.

In that way it is able to produce a relative high reliable system because if one of the rectifiers or one of the coils has a malfunction and they are not working, the rest of the generator and also the rest of the rectifiers will still continue operation. For a wind turbine a defect section will of course reduce the total power production but it is high ly effective that the power production can continue until repair is possible. Especially for wind turbines placed at open sea it is highly effective that the wind turbine can continue to operate even if there are smaller failures in the wind turbines simply be- cause repair can be rather difficult because of high wind and high waves at the open sea and therefore a delay of weeks or months is possible.

Description of the Drawing

Fig. 1 shows a schematic view of a system for a generator.

Fig. 2 shows a segment of a stator and rotor.

Fig. 3 shows an inner rotor.

Fig. 4 shows surface mounted permanent magnets shown as a rotor segment with three poles.

Fig. 5 shows a bridge coupling of rectifiers, which are connected to an AC source. Detailed Description of the Invention

Fig. 1 shows a schematic view of a system 2 for a generator 4 for a wind turbine. The generator 4 comprises a rotor 16 indicated by an arrow and a stator 18. The stator 18 comprises independent coils 20.1-12. These independent coils 20.1-12 generate inde pendent phases 22.1-12. These independent phases 22.1-12 are in cabling transmitted to a plurality of rectifiers 24.1-12. The output of rectifiers 24.1-12 are sent to a com mon DC bridge 26. The DC bridge 26 is supplying DC to at least one frequency con verter 8. This frequency converter is delivering three-phase AC which is connected to a supply network 14. In operation the independent coils 20.1-12 will generate independent DC power 22.1- 12 towards the plurality of rectifiers 24.1-12, which is supplying DC at the DC bridge 26. Because there is 12 independent phases connected by single rectifiers to the DC bridge, the DC bridge will have a DC voltage nearly without any ripple. The higher the number of independent coils 20-20n, the lower level will the ripple have at the DC bridge 26.

The major effect of the pending patent application is that if a failure occurs in one of the independent coils 20.1-12 or in one of the rectifiers 24.1-12 the generator 4 will be able to operate but with a small reduction in the power production that is generated and can be supplied with a supply network 14. This is very important because wind turbines are operating in harsh environments where for example repair is very diffi cult. For example at wind turbines placed at open sea in the North Sea it can take weeks before the weather allows service at a wind turbine. It is therefore extremely important if a wind turbine is able to continue production of power even if there is a minor failure and therefore a reduction in the maximum power that can be supplied.

It is possible to operate with more frequency converters 8 in order to avoid stop of power production. Further it is possible that also the plurality of rectifiers 24.1-12 are a complicated circuit which maybe is failure safe in that the component as such com prises extra rectifier components so even if one component is having a failure, a paral lel component can take over and therefore normal operation continues. Fig. 2 indicates a segment of a stator and rotor. Fig. 2 indicates an outer rotor having rotor structure independent sections 38 and a rotor laminated yoke 40 carrying mag nets 34. Further is indicated fastening dove tails 42 for fastening the coils 20-20n. Fur ther is indicated a stator laminated yoke 42 and a stator structure segment 44. As indi cated here the number of coils are very high and some of these coils generate the same phase of power which can be combined in a serial connection or in some situations in a parallel combination and in that way generate independent phases 22-22n.

Fig. 3 indicates an inner rotor, which inner rotor indicates an air gap against the stator 46. Further are indicated permanent magnets 34 and also a rotor laminated pole shoe 48.

In operation the magnets will generate a magnetic field that is conducted in the lami nated pole shoe towards not shown coils in a stator. Fig. 4 shows surface mounted permanent magnets shown as a rotor segment with three poles. Here is indicated an air gap between the stator and rotor 46. Further are indicat ed permanent magnets 34 and below is indicated rotor laminated yoke 40 and below is further indicated rotor structure independent sections. In this rotor the magnets are directly generating the magnetic field towards the stator coils. Hereby can be achieved that the permanent magnets are fastened to the rotor laminat ed yoke and the use of rotor laminated pole shoe is hereby avoided.

Fig. 5 discloses a bridge coupling of rectifiers 124, which are connected to an AC source 122, which for example could be a wind turbine generator only supplying with three independent phases. The bridge at fig. 5 is an IGBT bridge because the rectifiers 124 are all connected with switching transistors 132, which are connected parallel to the rectifiers 124. The switching transistors 132 are able to control the rectifiers in a short cut in that the switching transistors 132 will immediately close the rectifiers 124 and they will first start conducting after next positive voltage. Further at fig. 5 a kind of battery 126 is indicated which also could be a DC power link connected to an in verter.

In operation there will of course be the possibility of multiplying this circuit, which is disclosed at fig. 5 so that not only three phases are to be converted into DC, but the circuit could be multiplied several times. For example as disclosed in fig. 1 we are operating with twelve phases. There is no doubt that four different circuits as indicated at fig. 5 could be in operation, but also in generators for much larger wind turbines with a higher number of phases. Especially this invention makes it possible because the system as disclosed is able to operate even if there are failures in one or more cir cuits, for example if one coil in a generator is not operating, the generator will be able to continue operating with a limited power generation.

Reference signs:

System (2)

Generator (4)

Frequency converter (8)

AC current (10)

AC voltage (12)

Supply network (14)

Rotor (16)

Stator (18) Independent coils (20-20n) Independent phases (22-22n) Plurality of rectifiers (24-24n) DC bridges (26)

Control system (30)

Control signal (32)

Permanent magnets (34) Magnet segments (36)

Rotor structure segments (38) Rotor laminated yoke (40)

Fastening dove tail (42)

Stator laminated yoke (42) Stator structure segment (44) Air gap against stator (46) Rotor laminated pole shoe (48)

AC source (122)

Diodes from IGBT (124)

DC power (126)

Control switches (132)

Claims

1. System (2) for connecting at least one generator (4) primary from a wind turbine to a frequency converter (8) for generating AC current (10) and AC voltage (12) for a supply network (14), which generator (4) comprises a rotor (16) and a stator (18) which rotor (16) comprises a laminated yoke (40), which stator (18) is divided into independent sections, which independent sections comprises a plurality of independ ent coils (20-20n) for generating independent phases (22-22n), which independent phases out of at least three phases or more (22-22n) are sent to a plurality of rectifiers (24-24n), characterized in that the output of the rectifiers (24-24n) from each inde- pendent coils (20-20n) is combined in one or more common DC bridges (26) with other independent coils (20-20n), which DC bridges (26) are connected to a one or more common frequency converter (8), which frequency converter (8) is generating AC current (10) and AC voltage for a supply network (14). 2. System according to claim 1, characterized in that the generator (4) has a rotor

(16) equipped with permanent magnets (34), which permanent magnets (34) are locat ed on the rotor (16) and the rotor (16) is divided into independent sections (36).

3. System according to claim 1 or 2, characterized in that the yoke (40) is made from laminated iron to limit heat losses.

4. System according to claims 1-3, characterized in that the stator (18) is organized in independent phases (22-22n) more than 3 or equal to 3 for individual connection to individual rectifiers (24-24n).

5. System according to claim 4, characterized in that the wiring from the independ ent coils (20 -20n) as independent phases (22-22n) are independent and electric isolat ed from each other until the connection to the rectifiers (24-24n). 6. System according to one of the claims 1-5, characterized in that the system com prises a control system (30) for indicating the frequency and the phase of the supply network (14), which control system (30) generates a control signal (32) for the fre quency converter (8).

7. System according to claims 1-6, characterized in that the system operates with a voltage of about 750 V or higher measured between independent phases (22-22n).

8. System according to claims 1-7, characterized in that the voltage at the DC bridge (26) is about 1200 V or higher.

9. System according to claims 1-8, characterized in that independent coils (20-20n) having identical phase are serial connected in order to achieve the voltage for the DC bridge (26).

10. Method for operating a generator (4) primary in a wind turbine , as disclosed in the claims 1-9, where the generator (4) comprises a rotor (16) rotated by wind energy from a wind turbine , which rotor (16) generates a rotating magnetic field in a plurality of independent stator coils (20-20n) for generating AC current (10) and AC voltages (12), which AC current (10) and AC voltages (12) are rectified into DC current and voltage for supply of an inverter (8) generating AC current (10) and AC voltage for a supply network (14), where independent phases (22-22n) are transmitted through in dependent cables to a plurality of rectifiers (24-24n), characterized in that rectified current and voltage are combined into common DC bridges (26), where the DC bridg- es (26) is connected to one or more common frequency converters (8) for generating AC current (10) and AC voltage for one or more a supply network (14).