WO2010063882A1 - An isolated electrical system - Google Patents

An isolated electrical system Download PDF

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Publication number
WO2010063882A1
WO2010063882A1 PCT/FI2009/050936 FI2009050936W WO2010063882A1 WO 2010063882 A1 WO2010063882 A1 WO 2010063882A1 FI 2009050936 W FI2009050936 W FI 2009050936W WO 2010063882 A1 WO2010063882 A1 WO 2010063882A1
Authority
WO
WIPO (PCT)
Prior art keywords
excitation
electrical system
current
synchronous
generator
Prior art date
Application number
PCT/FI2009/050936
Other languages
French (fr)
Inventor
Juha Jaakko PYRHÖNEN
Janne Nerg
Jorma Haataja
Original Assignee
Lappeenrannan Teknillinen Yliopisto
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Lappeenrannan Teknillinen Yliopisto filed Critical Lappeenrannan Teknillinen Yliopisto
Publication of WO2010063882A1 publication Critical patent/WO2010063882A1/en

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Classifications

    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02K—DYNAMO-ELECTRIC MACHINES
    • H02K1/00—Details of the magnetic circuit
    • H02K1/06—Details of the magnetic circuit characterised by the shape, form or construction
    • H02K1/22—Rotating parts of the magnetic circuit
    • H02K1/223—Rotor cores with windings and permanent magnets
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02K—DYNAMO-ELECTRIC MACHINES
    • H02K21/00—Synchronous motors having permanent magnets; Synchronous generators having permanent magnets
    • H02K21/02—Details
    • H02K21/04—Windings on magnets for additional excitation ; Windings and magnets for additional excitation
    • H02K21/042—Windings on magnets for additional excitation ; Windings and magnets for additional excitation with permanent magnets and field winding both rotating
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02P—CONTROL OR REGULATION OF ELECTRIC MOTORS, ELECTRIC GENERATORS OR DYNAMO-ELECTRIC CONVERTERS; CONTROLLING TRANSFORMERS, REACTORS OR CHOKE COILS
    • H02P9/00—Arrangements for controlling electric generators for the purpose of obtaining a desired output
    • H02P9/14—Arrangements for controlling electric generators for the purpose of obtaining a desired output by variation of field
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02P—CONTROL OR REGULATION OF ELECTRIC MOTORS, ELECTRIC GENERATORS OR DYNAMO-ELECTRIC CONVERTERS; CONTROLLING TRANSFORMERS, REACTORS OR CHOKE COILS
    • H02P9/00—Arrangements for controlling electric generators for the purpose of obtaining a desired output
    • H02P9/14—Arrangements for controlling electric generators for the purpose of obtaining a desired output by variation of field
    • H02P9/26—Arrangements for controlling electric generators for the purpose of obtaining a desired output by variation of field using discharge tubes or semiconductor devices
    • H02P9/30—Arrangements for controlling electric generators for the purpose of obtaining a desired output by variation of field using discharge tubes or semiconductor devices using semiconductor devices
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02P—CONTROL OR REGULATION OF ELECTRIC MOTORS, ELECTRIC GENERATORS OR DYNAMO-ELECTRIC CONVERTERS; CONTROLLING TRANSFORMERS, REACTORS OR CHOKE COILS
    • H02P9/00—Arrangements for controlling electric generators for the purpose of obtaining a desired output
    • H02P9/14—Arrangements for controlling electric generators for the purpose of obtaining a desired output by variation of field
    • H02P9/26—Arrangements for controlling electric generators for the purpose of obtaining a desired output by variation of field using discharge tubes or semiconductor devices
    • H02P9/30—Arrangements for controlling electric generators for the purpose of obtaining a desired output by variation of field using discharge tubes or semiconductor devices using semiconductor devices
    • H02P9/305—Arrangements for controlling electric generators for the purpose of obtaining a desired output by variation of field using discharge tubes or semiconductor devices using semiconductor devices controlling voltage
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02K—DYNAMO-ELECTRIC MACHINES
    • H02K21/00—Synchronous motors having permanent magnets; Synchronous generators having permanent magnets
    • H02K21/02—Details
    • H02K21/04—Windings on magnets for additional excitation ; Windings and magnets for additional excitation
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02P—CONTROL OR REGULATION OF ELECTRIC MOTORS, ELECTRIC GENERATORS OR DYNAMO-ELECTRIC CONVERTERS; CONTROLLING TRANSFORMERS, REACTORS OR CHOKE COILS
    • H02P2101/00—Special adaptation of control arrangements for generators
    • H02P2101/10—Special adaptation of control arrangements for generators for water-driven turbines

Definitions

  • the invention relates to an isolated electrical system comprising a synchronous generator having a rotor provided with at least two permanent magnet poles and with at least two electrically excited poles. Furthermore, the invention relates to an excitation device for excitation of a synchronous electrical machine. Furthermore, the invention relates to a method for energizing an isolated electrical system. Furthermore, the invention relates to a method for providing excitation of a synchron- ous electrical machine.
  • An isolated electrical system such as e.g. an electrical network of a ship, is typically energized with a synchronous generator that is arranged to produce alternating voltage.
  • a synchronous generator that is arranged to produce alternating voltage.
  • the increasing requirements relating to efficiency and optimal usage of energy direct interests towards permanent magnet synchronous generators because the efficiency of a permanent magnet synchronous generator in the power range typically used in isolated grids such as ships can be 1 ...2 percentage units better than that of a corresponding electrically excited synchronous generator.
  • a permanent magnet synchronous generator is well suitable for a rigid electrical network system in which the amplitude of voltage is substantially constant.
  • a permanent magnet synchronous generator is not so suitable for an isolated electrical system because, in many isolated electrical systems, the following requirements are imposed on a generator: (i) the amplitude of the stator voltage has to be able to be regulated within pre-determined voltage limits, e.g. ⁇ 10%, and (ii) the generator has to be able to produce a pre-determined short- circuit current for a pre-determined time after a beginning of a short-circuit situation, e.g. the short-circuit current may need to be at least three times the nominal current (3 * I N ) for at least e.g. two seconds after the beginning of the short-circuit situation.
  • Publication US2003011257 discloses a hybrid permanent magnet synchronous generator for an electrical system of a vehicle, e.g. a car.
  • the hybrid permanent magnet synchronous generator comprises a stator surrounding a rotor, a gap between the stator and the rotor, permanent magnets integrated into the rotor, and excitation coils integrated into the rotor.
  • the hybrid permanent magnet synchronous generator comprises at least two permanent magnet poles and at least two electrically excited poles.
  • the electrically excited poles are used for voltage regulation.
  • the publication US2003011257 presents a solution to the above-mentioned requirement related to the voltage regulation. No solution is, however, presented to the above-mentioned requirement related to the short- circuit current.
  • An isolated electrical system comprises:
  • a synchronous generator having a rotor provided with at least two permanent magnet poles and with at least two electrically excited poles, and - an excitation device for producing excitation current for the at least two electrically excited poles,
  • the excitation device is arranged to control the at least two electrically excited poles to contribute the excitation produced with the at least two permanent magnet poles as a response to a situation in which a short-circuit takes place in the isolated electrical system.
  • the synchronous generator is preferably dimensioned in such a manner that:
  • the at least two permanent magnet poles are capable of providing excitation corresponding to the operation of the synchronous generator when the excitation current is substantially zero and the stator current and stator voltage have their nominal values
  • the at least two permanent magnet poles are capable of providing excitation corresponding to the operation of the synchronous generator when the syn- chronous generator is short-circuited and the stator current is a first predetermined factor times a nominal value of the stator current, and
  • the at least two permanent magnet poles and the at least two electrically excited poles are together capable of providing excitation corresponding to the operation of the synchronous generator when the synchronous generator is short-circuited and the stator current is a second pre-determined factor times the nominal value of the stator current, the second pre-determined factor being greater than the first pre-determined factor.
  • the stator voltage and/or reactive power can be adjusted by controlling the excitation current.
  • Sufficient short-circuit current is achieved by controlling the at least two electrically excited poles to contribute the excitation produced with the at least two permanent magnet poles as a response to a situation in which short-circuit takes place in the isolated electrical system.
  • the first and second pre-determined factors can be chosen to have such values that the requirement related to the short-circuit current is fulfilled.
  • the first and second pre-determined factors can be, for example, two and three, respectively.
  • a new excitation device for excitation of a synchronous electrical machine.
  • the synchronous electrical machine can be a generator or a motor, or can operate either as a generator or as a motor.
  • An excitation device according to the invention comprises:
  • a regulating circuitry coupled to an output of the rectifier for providing regu- lated current for the excitation of the synchronous electrical machine
  • a new method for energizing an isolated electrical system comprising a synchronous generator having a rotor provided with at least two permanent magnet poles and with at least two electrically excited poles.
  • the method comprises controlling the at least two electrically excited poles to contribute the excita- tion produced with the at least two permanent magnet poles as a response to a situation in which a short-circuit takes place in the isolated electrical system.
  • a new method for providing excitation of a synchronous electrical machine comprises:
  • figure 1 a illustrates an isolated electrical system according to an embodiment of the invention
  • figure 1 b shows a cross-section view of a synchronous generator of an isolated electrical system according to an embodiment of the invention
  • figure 2 illustrates an excitation device according to an embodiment of the invention
  • FIG. 3 illustrates an excitation device according to an embodiment of the invention
  • figure 4 is a flow chart of a method according to an embodiment of the invention for energizing an isolated electrical system
  • figure 5 is a flow chart of a method according to an embodiment of the invention for providing excitation of a synchronous electrical machine.
  • Figure 1a illustrates an isolated electrical system 100 according to an embodiment of the invention.
  • the isolated electrical system comprises a synchronous generator 101.
  • Figure 1 b shows a cross-section view of the synchronous generator 101.
  • the synchronous generator has a rotor 106 provided with six permanent magnet poles 110-115 and with two electrically excited poles 108 and 109.
  • Each electrical- Iy excited pole comprises a respective portion of excitation windings 150.
  • Each permanent magnet pole comprises a piece of permanent magnet material.
  • the piece of permanent magnet material of the permanent magnet pole 115 is denoted with the reference number 116.
  • a direction of magnetization of a piece of permanent magnet material is depicted with an arrow in figure 1 b.
  • the isolated electrical system 100 comprises an excitation device 102 that is arranged to produce excitation current for the electrically excited poles of the synchronous generator 101.
  • the excitation current flows in the excitation windings 150 of the electrically excited poles.
  • the excitation device 102 is arranged to control the electrically excited poles to contribute the excitation produced with the permanent magnet poles as a response to a situation in which short-circuit takes place in the isolated electrical system.
  • the electrically excited poles are arranged to strengthen the stator flux linkage in a short circuit situation in order to provide sufficient short-circuit current.
  • a control unit 128 is arranged to detect short-circuit situations and to control a regulating circuitry 122 of the excitation de- vice 102 so that the excitation produced with the electrically excited poles contributes the excitation produced with the permanent magnet poles when short-circuit has been detected.
  • FIG 1 a other parts of the isolated electrical system 100 are depicted with a cross-hatched area 103.
  • the isolated electrical system 100 can be, for example, an electrical network of a ship or some other vehicle, or an electrical network of an isolated research station or the like.
  • the synchronous generator is driven with a power engine 104 that can be, for example, a diesel- engine, a steam-turbine, a water-turbine, or a gas-turbine.
  • the isolated electrical system 100 can also comprise more than one synchronous generator.
  • the synchronous generator 101 is preferably dimensioned in such a manner that:
  • the stator voltage Us and/or reactive power Q (VAr) can be adjusted by controlling the excitation current.
  • a sufficient short-circuit current is achieved by controlling the electrically excited poles to contribute the excitation produced with the permanent magnet poles as a response to a situation in which short-circuit takes place in the isolated electrical system 100.
  • the first and second pre-determined factors C1 and C2 can be chosen to have such values that the requirement related to the short-circuit current is fulfilled.
  • the excitation device 102 comprises an electric double-layer capacitor (EDLC) 120 capable of providing excitation energy in the situation in which there is short-circuit in the isolated electrical system, i.e. when the supply voltage of the rectifier 121 is small or even zero.
  • EDLC electric double-layer capacitor
  • An electric double-layer capacitor is often called a "super capacitor”.
  • the excitation device 102 comprises a circuitry 123 for changing the polarity of the excitation current.
  • the electrically excited poles can be used not only for strengthening the stator flux linkage, i.e. for contributing the excitation provided with the permanent magnet poles, but also for weakening the stator flux linkage. This gives more freedom in the control of the stator voltage and/or of the reactive power.
  • the permanent magnet poles of the synchronous generator comprise damper wind- ings.
  • a bar of a damper winding of the permanent magnet pole 110 is denoted with the reference number 117.
  • the electrically excited poles of the synchronous generator comprise damper windings.
  • a bar of a damper winding of the electrically excited pole 108 is de- noted with the reference number 118.
  • An isolated electrical system comprises a brushless excitation device.
  • the brushless excitation device is not shown in figure 1 a but an exemplifying brushless excitation device will be presented later in this document with reference to figure 3.
  • the brushless excitation device com- prises:
  • the second rectifier comprises a circuitry for changing polarity of the excitation current.
  • the brushless excitation device comprises an electric double-layer capacitor at the output of the first rectifier.
  • the electric double-layer capacitor is capable of providing excitation energy in the situation in which there is short-circuit in the isolated electrical system.
  • the synchronous generator illustrated in figure 1 b has six permanent magnet poles and two electrically excited poles. It should be noted, however, that the number of permanent magnet poles and the number of electrically excited poles can be varied so that there is at least two permanent magnet poles and at least two electrically excited poles.
  • the synchronous generator illustrated in figure 1 b is a salient pole machine. It should be, however, noted that it is as well possible to have a cylindrical rotor in which excitation windings are located in slots that are on the surface of the rotor and permanent magnets can be either inside the rotor or on the surface of the rotor.
  • FIG. 2 illustrates an excitation device 202 according to an embodiment of the invention for excitation of a synchronous electrical machine.
  • the synchronous elec- trical machine can be a generator or a motor, or can operate either as a generator or as a motor.
  • the excitation device comprises a rectifier 221 arranged to take energy from an electrical network connected to stator windings of the synchronous electrical machine.
  • the excitation device comprises a regulating circuitry 222 coupled to an output of the rectifier for providing regulated current I 1 - for the excita- tion of the synchronous electrical machine.
  • the regulated current i.e.
  • the excitation current is conducted to the excitation windings 250 of the synchronous electrical machine via slip-rings 224 and 225 and slip-ring brushes 226 and 227.
  • the slip-rings are located on the shaft 207 of the synchronous electrical machine.
  • the excitation device comprises an electric double-layer capacitor 220 at the output of the rectifier 221 for providing energy to the regulating circuitry 222 in a situation in which there is short-circuit in the electrical network.
  • a control unit 228 is arranged to control the regulating circuitry 222 on the basis of measured stator voltage Us of the synchronous electrical machine and/or measured reactive power Q of the synchronous electrical machine.
  • the control unit 228 is preferably arranged to set the regulated current I 1 - to a pre-determined value as a response to a situation in which short-circuit is detected, e.g. the stator voltage Us is detected to be smaller than a pre-determined limit value, i.e. Us ⁇ Uslimit.
  • the control unit 228 can be a part of the excitation device. Alternatively, the control unit can be a separate device and the excitation device comprises an input interface for receiving a control signal 229 that is used for regulating the level of the regulated current I 1 -.
  • An excitation device comprises a circuitry 223 for changing polarity, i.e. direction, of the regulated current I 1 -.
  • the control signal 230 is used for determining the polarity of the regulated current.
  • FIG. 3 illustrates a brushless excitation device 302 according to an embodiment of the invention for excitation of a synchronous electrical machine.
  • the excitation device comprises a rectifier 321 arranged to take energy from an electrical network connected to stator windings of the synchronous electrical machine.
  • the excitation device comprises a regulating circuitry 322 coupled to an output of the rec- tifier for providing regulated current I 1 - for the excitation of the synchronous electrical machine.
  • the excitation device comprises an electric double-layer capacitor 320 at the output of the rectifier 321 for providing energy to the regulating circuitry 322 in a situation in which there is a short-circuit in the electrical network.
  • the excitation device comprises an excitation generator 334 on a shaft of the synchron- ous electrical machine.
  • the regulating circuitry 322 is arranged to feed the regulated current I 1 - to a static field winding 331 of the excitation generator.
  • the excitation device comprises another rectifier 333 in a rotating part 340 of the excitation generator.
  • the rectifier 333 is arranged to couple rotating armature windings 332 of the excitation generator to excitation windings 350 of the synchronous electrical machine.
  • a control unit 328 is arranged to control the regulating circuitry 322 on the basis of measured stator voltage Us of the synchronous electrical machine and/or measured reactive power Q of the synchronous electrical machine.
  • the control unit 328 is preferably arranged to set the regulated current I 1 - to a pre-determined value as a response to a situation in which a short-circuit is detected, e.g. the stator voltage Us is detected to be smaller than a pre-determined limit value, i.e. Us ⁇ Uslimit.
  • the control unit 328 can be a part of the excitation device. Alternatively, the control unit can be a separate device and the excitation device comprises an input interface for receiving a control signal 329 that is used for regulating the level of the regulated current I 1 -.
  • the rectifier 333 comprises a circuitry 323 for changing polarity of output current of the rectifier 333, i.e. for changing polarity of excitation current l e of the synchronous electrical machine.
  • the control signal 330 is used for determining the polarity of the excitation current l e .
  • the control signal 330 can be transferred to the rotating part 340 of the excitation device, for example, using slip-rings, a capacitive coupling, an inductive coupling, a radio link, or an optical link.
  • the excitation device is not a brushless excitation device in the strict sense if slip-rings are being used for transferring the control signal 330.
  • the slip-rings and respective slip-ring brushes for the control signal 330 can be, however, dimen- sioned for a significantly smaller current compared with those that are used for conducting the excitation current of a synchronous electrical machine.
  • Figure 4 is a flow chart of a method according to an embodiment of the invention for energizing an isolated electrical system.
  • the isolated electrical system comprises a synchronous generator having a rotor provided with at least two perma- nent magnet poles and with at least two electrically excited poles.
  • the at least two electrically excited poles are controlled in the phase 401 to contribute the excitation produced with the at least two permanent magnet poles as a response to a situation in which a short-circuit takes place in the isolated electrical system.
  • the at least two permanent magnet poles are capable of providing excitation corresponding to the operation of the synchronous generator when the excitation current is substantially zero and the stator current and stator voltage have their nominal values
  • the at least two permanent magnet poles are capable of providing excitation corresponding to the operation of the synchronous generator when the synchronous generator is short-circuited and the stator current is a first predetermined factor times a nominal value of the stator current, and
  • the at least two permanent magnet poles and the at least two electrically excited poles are together capable of providing excitation corresponding to the operation of the synchronous generator when the synchronous generator is short-circuited and the stator current is a second pre-determined factor times the nominal value of the stator current.
  • the second pre-determined factor is greater than the first pre-determined factor.
  • the first pre-determined factor can be for example 2 and the second predetermined factor can be for example 3.
  • an electric double-layer capacitor is used for providing excitation energy in the situation in which there is short-circuit in the isolated electrical system.
  • the polarity of the excitation produced with the at least two electrically excited poles is altered during operation.
  • a brushless excitation device is used.
  • the brushless exci- tation device comprises:
  • the polarity of the output current of the second rectifier is altered for altering the polarity of the excitation produced with the at least two electrically excited poles.
  • an electric double-layer capacitor is used at the output of the first rectifier in order to provide excitation energy in the situation in which there is short-circuit in the isolated electrical system.
  • the isolated electrical system is an electrical network of a ship.
  • the permanent magnet poles of the synchronous generator comprise damper windings.
  • the electrically excited poles of the synchronous generator comprise damper windings.
  • FIG. 5 is a flow chart of a method according to an embodiment of the invention for providing excitation of a synchronous electrical machine.
  • the synchronous electrical machine can be a generator or a motor, or can operate either as a gene- rator or as a motor.
  • energy is taken with a rectifier from an electrical network connected to stator windings of the synchronous electrical machine.
  • energy is stored into an electric double-layer capacitor at the output of the rectifier.
  • the output current of the rectifier is regulated for providing regulated current for the excitation of the synchronous electrical machine.
  • the energy is stored into the electric double-layer capacitor in order to be able to provide the regulated current in a situation in which there is short-circuit in the electrical network.
  • the polarity of the regulated current is altered for altering the polarity of the excitation of the synchronous electrical machine.
  • a brushless excitation device In a method according to an embodiment of the invention for providing excitation of a synchronous electrical machine, a brushless excitation device is used.
  • the brushless excitation device comprises:
  • the polarity of the output current of the other rectifier is altered for altering the polarity of the excitation of the synchronous electrical machine.

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Control Of Eletrric Generators (AREA)

Abstract

An isolated electrical system (100) is energized with a synchronous generator (101) having a rotor provided with permanent magnet poles and also with electrically excited poles. The system comprises an excitation device (102) for producing excitation current for the electrically excited poles. The stator voltage and/or reactive power can be adjusted by controlling the excitation current. A sufficient short-circuit current is achieved by controlling the electrically excited poles to contribute the excitation produced with the permanent magnet poles as a response to a situation in which short-circuit takes place in the isolated electrical system.

Description

An isolated electrical system
Field of the invention
The invention relates to an isolated electrical system comprising a synchronous generator having a rotor provided with at least two permanent magnet poles and with at least two electrically excited poles. Furthermore, the invention relates to an excitation device for excitation of a synchronous electrical machine. Furthermore, the invention relates to a method for energizing an isolated electrical system. Furthermore, the invention relates to a method for providing excitation of a synchron- ous electrical machine.
Background
An isolated electrical system, such as e.g. an electrical network of a ship, is typically energized with a synchronous generator that is arranged to produce alternating voltage. The increasing requirements relating to efficiency and optimal usage of energy direct interests towards permanent magnet synchronous generators because the efficiency of a permanent magnet synchronous generator in the power range typically used in isolated grids such as ships can be 1 ...2 percentage units better than that of a corresponding electrically excited synchronous generator. A permanent magnet synchronous generator is well suitable for a rigid electrical network system in which the amplitude of voltage is substantially constant. On the other hand, a permanent magnet synchronous generator is not so suitable for an isolated electrical system because, in many isolated electrical systems, the following requirements are imposed on a generator: (i) the amplitude of the stator voltage has to be able to be regulated within pre-determined voltage limits, e.g. ±10%, and (ii) the generator has to be able to produce a pre-determined short- circuit current for a pre-determined time after a beginning of a short-circuit situation, e.g. the short-circuit current may need to be at least three times the nominal current (3 * IN) for at least e.g. two seconds after the beginning of the short-circuit situation.
The above-mentioned requirements are quite easily fulfilled with an electrically excited synchronous generator since the excitation current is adjustable, and the armature reaction may be compensated by the excitation current, whereas a permanent magnet synchronous generator has to be quite strongly over-dimensioned in order to fulfill the above-mentioned requirements. This weakens the competitive- ness of a permanent magnet synchronous generator for use in an isolated electrical system.
Publication US2003011257 discloses a hybrid permanent magnet synchronous generator for an electrical system of a vehicle, e.g. a car. The hybrid permanent magnet synchronous generator comprises a stator surrounding a rotor, a gap between the stator and the rotor, permanent magnets integrated into the rotor, and excitation coils integrated into the rotor. Hence, the hybrid permanent magnet synchronous generator comprises at least two permanent magnet poles and at least two electrically excited poles. The electrically excited poles are used for voltage regulation. Hence, the publication US2003011257 presents a solution to the above-mentioned requirement related to the voltage regulation. No solution is, however, presented to the above-mentioned requirement related to the short- circuit current.
Summary
In accordance with a first aspect of the invention, there is provided a new isolated electrical system. An isolated electrical system according to the invention comprises:
- a synchronous generator having a rotor provided with at least two permanent magnet poles and with at least two electrically excited poles, and - an excitation device for producing excitation current for the at least two electrically excited poles,
wherein the excitation device is arranged to control the at least two electrically excited poles to contribute the excitation produced with the at least two permanent magnet poles as a response to a situation in which a short-circuit takes place in the isolated electrical system.
The synchronous generator is preferably dimensioned in such a manner that:
- the at least two permanent magnet poles are capable of providing excitation corresponding to the operation of the synchronous generator when the excitation current is substantially zero and the stator current and stator voltage have their nominal values,
- the at least two permanent magnet poles are capable of providing excitation corresponding to the operation of the synchronous generator when the syn- chronous generator is short-circuited and the stator current is a first predetermined factor times a nominal value of the stator current, and
- the at least two permanent magnet poles and the at least two electrically excited poles are together capable of providing excitation corresponding to the operation of the synchronous generator when the synchronous generator is short-circuited and the stator current is a second pre-determined factor times the nominal value of the stator current, the second pre-determined factor being greater than the first pre-determined factor.
With the above-described dimensioning, only small excitation current is needed during nominal operation. The stator voltage and/or reactive power can be adjusted by controlling the excitation current. Sufficient short-circuit current is achieved by controlling the at least two electrically excited poles to contribute the excitation produced with the at least two permanent magnet poles as a response to a situation in which short-circuit takes place in the isolated electrical system. The first and second pre-determined factors can be chosen to have such values that the requirement related to the short-circuit current is fulfilled. The first and second pre-determined factors can be, for example, two and three, respectively.
In accordance with a second aspect of the invention, there is provided a new excitation device for excitation of a synchronous electrical machine. The synchronous electrical machine can be a generator or a motor, or can operate either as a generator or as a motor. An excitation device according to the invention comprises:
- a rectifier for taking energy from an electrical network connected to stator windings of the synchronous electrical machine,
- a regulating circuitry coupled to an output of the rectifier for providing regu- lated current for the excitation of the synchronous electrical machine, and
- an electric double-layer capacitor at the output of the rectifier for providing energy to the regulating circuitry in a situation in which there is a short circuit in the electrical network.
In accordance with a third aspect of the invention, there is provided a new method for energizing an isolated electrical system, the isolated electrical system comprising a synchronous generator having a rotor provided with at least two permanent magnet poles and with at least two electrically excited poles. The method comprises controlling the at least two electrically excited poles to contribute the excita- tion produced with the at least two permanent magnet poles as a response to a situation in which a short-circuit takes place in the isolated electrical system.
In accordance with a fourth aspect of the invention, there is provided a new method for providing excitation of a synchronous electrical machine. The method comprises:
- taking, with a rectifier, energy from an electrical network connected to stator windings of the synchronous electrical machine,
- regulating output current of the rectifier for providing a regulated current for the excitation of the synchronous electrical machine, and - storing energy into an electric double-layer capacitor at the output of the rectifier for being able to provide the regulated current in a situation in which there is a short circuit in the electrical network.
A number of exemplifying embodiments of the invention are described in accompanied dependent claims.
Various exemplifying embodiments of the invention both as to constructions and to methods of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific exemplifying embodiments when read in connection with the accompanying drawings.
The verb "to comprise" is used in this document as an open limitation that does not exclude the existence of also unrecited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated.
Brief description of the figures
The exemplifying embodiments of the invention and their advantages are explained in greater detail below in the sense of examples and with reference to the accompanying drawings, in which:
figure 1 a illustrates an isolated electrical system according to an embodiment of the invention,
figure 1 b shows a cross-section view of a synchronous generator of an isolated electrical system according to an embodiment of the invention, figure 2 illustrates an excitation device according to an embodiment of the invention,
figure 3 illustrates an excitation device according to an embodiment of the invention,
figure 4 is a flow chart of a method according to an embodiment of the invention for energizing an isolated electrical system, and
figure 5 is a flow chart of a method according to an embodiment of the invention for providing excitation of a synchronous electrical machine.
Description of the embodiments
Figure 1a illustrates an isolated electrical system 100 according to an embodiment of the invention. The isolated electrical system comprises a synchronous generator 101. Figure 1 b shows a cross-section view of the synchronous generator 101. The synchronous generator has a rotor 106 provided with six permanent magnet poles 110-115 and with two electrically excited poles 108 and 109. Each electrical- Iy excited pole comprises a respective portion of excitation windings 150. Each permanent magnet pole comprises a piece of permanent magnet material. The piece of permanent magnet material of the permanent magnet pole 115 is denoted with the reference number 116. A direction of magnetization of a piece of permanent magnet material is depicted with an arrow in figure 1 b.
The isolated electrical system 100 comprises an excitation device 102 that is arranged to produce excitation current for the electrically excited poles of the synchronous generator 101. The excitation current flows in the excitation windings 150 of the electrically excited poles. The excitation device 102 is arranged to control the electrically excited poles to contribute the excitation produced with the permanent magnet poles as a response to a situation in which short-circuit takes place in the isolated electrical system. In other words, the electrically excited poles are arranged to strengthen the stator flux linkage in a short circuit situation in order to provide sufficient short-circuit current. A control unit 128 is arranged to detect short-circuit situations and to control a regulating circuitry 122 of the excitation de- vice 102 so that the excitation produced with the electrically excited poles contributes the excitation produced with the permanent magnet poles when short-circuit has been detected. In figure 1 a, other parts of the isolated electrical system 100 are depicted with a cross-hatched area 103. The isolated electrical system 100 can be, for example, an electrical network of a ship or some other vehicle, or an electrical network of an isolated research station or the like. The synchronous generator is driven with a power engine 104 that can be, for example, a diesel- engine, a steam-turbine, a water-turbine, or a gas-turbine. The isolated electrical system 100 can also comprise more than one synchronous generator.
The synchronous generator 101 is preferably dimensioned in such a manner that:
- the permanent magnet poles 110-115 are capable of providing excitation corresponding to nominal operation of the synchronous generator, i.e. the excitation current can be substantially zero when the stator current ls and the stator voltage Us have their nominal values; i.e. Is = IN, and Us = UN, - the permanent magnet poles 110-115 are capable of providing excitation corresponding to the operation of the synchronous generator when the stator windings of the synchronous generator is short-circuited Us a 0 and the stator current is a first pre-determined factor times a nominal value of the stator current, i.e. ls = C1 * IN, and
- the permanent magnet poles 110-115 and the electrically excited poles 108 and 109 are together capable of providing excitation corresponding to the operation of the synchronous generator when the synchronous generator is short-circuited Us a 0 and the stator current is a second pre-determined factor times the nominal value of the stator current, i.e. Is = C2 x IN. With the above-described dimensioning, only small excitation current is needed during nominal operation. The stator voltage Us and/or reactive power Q (VAr) can be adjusted by controlling the excitation current. A sufficient short-circuit current is achieved by controlling the electrically excited poles to contribute the excitation produced with the permanent magnet poles as a response to a situation in which short-circuit takes place in the isolated electrical system 100. The first and second pre-determined factors C1 and C2 can be chosen to have such values that the requirement related to the short-circuit current is fulfilled. The first and second predetermined factors C1 and C2 can be, for example, C1 = 2 and C2 = 3.
In an isolated electrical system according to an embodiment of the invention, the excitation device 102 comprises an electric double-layer capacitor (EDLC) 120 capable of providing excitation energy in the situation in which there is short-circuit in the isolated electrical system, i.e. when the supply voltage of the rectifier 121 is small or even zero. An electric double-layer capacitor is often called a "super capacitor". In an isolated electrical system according to an embodiment of the invention, the excitation device 102 comprises a circuitry 123 for changing the polarity of the excitation current. Hence, the electrically excited poles can be used not only for strengthening the stator flux linkage, i.e. for contributing the excitation provided with the permanent magnet poles, but also for weakening the stator flux linkage. This gives more freedom in the control of the stator voltage and/or of the reactive power.
In an isolated electrical system according to an embodiment of the invention, the permanent magnet poles of the synchronous generator comprise damper wind- ings. In figure 1 b, a bar of a damper winding of the permanent magnet pole 110 is denoted with the reference number 117.
In an isolated electrical system according to an embodiment of the invention, the electrically excited poles of the synchronous generator comprise damper windings. In figure 1 b, a bar of a damper winding of the electrically excited pole 108 is de- noted with the reference number 118.
An isolated electrical system according to an embodiment of the invention comprises a brushless excitation device. The brushless excitation device is not shown in figure 1 a but an exemplifying brushless excitation device will be presented later in this document with reference to figure 3. The brushless excitation device com- prises:
- an excitation generator on a shaft 107 of the synchronous generator,
- a first rectifier coupled to alternating voltage of the isolated electrical system,
- a regulating circuitry for coupling an output of the first rectifier to a static field winding of the excitation generator, and
- a second rectifier in a rotating part of the excitation generator for coupling rotating armature windings of the excitation generator to the excitation windings 150 of the electrically excited poles of the synchronous generator.
In an isolated electrical system according to an embodiment of the invention, the second rectifier comprises a circuitry for changing polarity of the excitation current.
In an isolated electrical system according to an embodiment of the invention, the brushless excitation device comprises an electric double-layer capacitor at the output of the first rectifier. The electric double-layer capacitor is capable of providing excitation energy in the situation in which there is short-circuit in the isolated electrical system.
The synchronous generator illustrated in figure 1 b has six permanent magnet poles and two electrically excited poles. It should be noted, however, that the number of permanent magnet poles and the number of electrically excited poles can be varied so that there is at least two permanent magnet poles and at least two electrically excited poles. The synchronous generator illustrated in figure 1 b is a salient pole machine. It should be, however, noted that it is as well possible to have a cylindrical rotor in which excitation windings are located in slots that are on the surface of the rotor and permanent magnets can be either inside the rotor or on the surface of the rotor.
Figure 2 illustrates an excitation device 202 according to an embodiment of the invention for excitation of a synchronous electrical machine. The synchronous elec- trical machine can be a generator or a motor, or can operate either as a generator or as a motor. The excitation device comprises a rectifier 221 arranged to take energy from an electrical network connected to stator windings of the synchronous electrical machine. The excitation device comprises a regulating circuitry 222 coupled to an output of the rectifier for providing regulated current I1- for the excita- tion of the synchronous electrical machine. The regulated current, i.e. the excitation current, is conducted to the excitation windings 250 of the synchronous electrical machine via slip-rings 224 and 225 and slip-ring brushes 226 and 227. The slip-rings are located on the shaft 207 of the synchronous electrical machine. The excitation device comprises an electric double-layer capacitor 220 at the output of the rectifier 221 for providing energy to the regulating circuitry 222 in a situation in which there is short-circuit in the electrical network.
A control unit 228 is arranged to control the regulating circuitry 222 on the basis of measured stator voltage Us of the synchronous electrical machine and/or measured reactive power Q of the synchronous electrical machine. The control unit 228 is preferably arranged to set the regulated current I1- to a pre-determined value as a response to a situation in which short-circuit is detected, e.g. the stator voltage Us is detected to be smaller than a pre-determined limit value, i.e. Us < Uslimit. The control unit 228 can be a part of the excitation device. Alternatively, the control unit can be a separate device and the excitation device comprises an input interface for receiving a control signal 229 that is used for regulating the level of the regulated current I1-. An excitation device according to an embodiment of the invention comprises a circuitry 223 for changing polarity, i.e. direction, of the regulated current I1-. In the exemplifying case shown in figure 2, the control signal 230 is used for determining the polarity of the regulated current.
Figure 3 illustrates a brushless excitation device 302 according to an embodiment of the invention for excitation of a synchronous electrical machine. The excitation device comprises a rectifier 321 arranged to take energy from an electrical network connected to stator windings of the synchronous electrical machine. The excitation device comprises a regulating circuitry 322 coupled to an output of the rec- tifier for providing regulated current I1- for the excitation of the synchronous electrical machine. The excitation device comprises an electric double-layer capacitor 320 at the output of the rectifier 321 for providing energy to the regulating circuitry 322 in a situation in which there is a short-circuit in the electrical network. The excitation device comprises an excitation generator 334 on a shaft of the synchron- ous electrical machine. The regulating circuitry 322 is arranged to feed the regulated current I1- to a static field winding 331 of the excitation generator. The excitation device comprises another rectifier 333 in a rotating part 340 of the excitation generator. The rectifier 333 is arranged to couple rotating armature windings 332 of the excitation generator to excitation windings 350 of the synchronous electrical machine.
A control unit 328 is arranged to control the regulating circuitry 322 on the basis of measured stator voltage Us of the synchronous electrical machine and/or measured reactive power Q of the synchronous electrical machine. The control unit 328 is preferably arranged to set the regulated current I1- to a pre-determined value as a response to a situation in which a short-circuit is detected, e.g. the stator voltage Us is detected to be smaller than a pre-determined limit value, i.e. Us < Uslimit. The control unit 328 can be a part of the excitation device. Alternatively, the control unit can be a separate device and the excitation device comprises an input interface for receiving a control signal 329 that is used for regulating the level of the regulated current I1-.
In an excitation device according to an embodiment of the invention, the rectifier 333 comprises a circuitry 323 for changing polarity of output current of the rectifier 333, i.e. for changing polarity of excitation current le of the synchronous electrical machine. In the exemplifying case shown in figure 3, the control signal 330 is used for determining the polarity of the excitation current le. The control signal 330 can be transferred to the rotating part 340 of the excitation device, for example, using slip-rings, a capacitive coupling, an inductive coupling, a radio link, or an optical link. The excitation device is not a brushless excitation device in the strict sense if slip-rings are being used for transferring the control signal 330. The slip-rings and respective slip-ring brushes for the control signal 330 can be, however, dimen- sioned for a significantly smaller current compared with those that are used for conducting the excitation current of a synchronous electrical machine.
Figure 4 is a flow chart of a method according to an embodiment of the invention for energizing an isolated electrical system. The isolated electrical system comprises a synchronous generator having a rotor provided with at least two perma- nent magnet poles and with at least two electrically excited poles. In the method, the at least two electrically excited poles are controlled in the phase 401 to contribute the excitation produced with the at least two permanent magnet poles as a response to a situation in which a short-circuit takes place in the isolated electrical system.
In a method according to an embodiment of the invention for energizing the isolated electrical system:
- the at least two permanent magnet poles are capable of providing excitation corresponding to the operation of the synchronous generator when the excitation current is substantially zero and the stator current and stator voltage have their nominal values,
- the at least two permanent magnet poles are capable of providing excitation corresponding to the operation of the synchronous generator when the synchronous generator is short-circuited and the stator current is a first predetermined factor times a nominal value of the stator current, and
- the at least two permanent magnet poles and the at least two electrically excited poles are together capable of providing excitation corresponding to the operation of the synchronous generator when the synchronous generator is short-circuited and the stator current is a second pre-determined factor times the nominal value of the stator current.
The second pre-determined factor is greater than the first pre-determined factor. The first pre-determined factor can be for example 2 and the second predetermined factor can be for example 3. In a method according to an embodiment of the invention for energizing the isolated electrical system, an electric double-layer capacitor is used for providing excitation energy in the situation in which there is short-circuit in the isolated electrical system.
In a method according to an embodiment of the invention for energizing the isolated electrical system, the polarity of the excitation produced with the at least two electrically excited poles is altered during operation.
In a method according to an embodiment of the invention for energizing the isolated electrical system, a brushless excitation device is used. The brushless exci- tation device comprises:
- an excitation generator on a shaft of the synchronous generator,
- a first rectifier coupled to alternating voltage of the isolated electrical system,
- a regulating circuitry for coupling an output of the first rectifier to a static field winding of the excitation generator, and
- a second rectifier in a rotating part of the excitation generator for coupling rotating armature windings of the excitation generator to excitation windings of the electrically excited poles of the synchronous generator.
In a method according to an embodiment of the invention for energizing the iso- lated electrical system, the polarity of the output current of the second rectifier is altered for altering the polarity of the excitation produced with the at least two electrically excited poles.
In a method according to an embodiment of the invention for energizing the isolated electrical system, an electric double-layer capacitor is used at the output of the first rectifier in order to provide excitation energy in the situation in which there is short-circuit in the isolated electrical system.
In a method according to an embodiment of the invention for energizing the isolated electrical system, the isolated electrical system is an electrical network of a ship. In a method according to an embodiment of the invention for energizing the isolated electrical system, the permanent magnet poles of the synchronous generator comprise damper windings.
In a method according to an embodiment of the invention for energizing the iso- lated electrical system, the electrically excited poles of the synchronous generator comprise damper windings.
Figure 5 is a flow chart of a method according to an embodiment of the invention for providing excitation of a synchronous electrical machine. The synchronous electrical machine can be a generator or a motor, or can operate either as a gene- rator or as a motor. In the phase 501 , energy is taken with a rectifier from an electrical network connected to stator windings of the synchronous electrical machine. In the phase 502, energy is stored into an electric double-layer capacitor at the output of the rectifier. In the phase 503, the output current of the rectifier is regulated for providing regulated current for the excitation of the synchronous electrical machine. The energy is stored into the electric double-layer capacitor in order to be able to provide the regulated current in a situation in which there is short-circuit in the electrical network.
In a method according to an embodiment of the invention for providing excitation of a synchronous electrical machine, the polarity of the regulated current is altered for altering the polarity of the excitation of the synchronous electrical machine.
In a method according to an embodiment of the invention for providing excitation of a synchronous electrical machine, a brushless excitation device is used. The brushless excitation device comprises:
- an excitation generator on a shaft of the synchronous electrical machine, the regulated current being fed to a static field winding of the excitation generator, and
- another rectifier in a rotating part of the excitation generator for coupling rotating armature windings of the excitation generator to excitation windings of the synchronous electrical machine. In a method according to an embodiment of the invention for providing excitation of a synchronous electrical machine, the polarity of the output current of the other rectifier is altered for altering the polarity of the excitation of the synchronous electrical machine. The specific examples provided in the description given above should not be construed as limiting. Therefore, the invention is not limited merely to the exemplifying embodiments described above.

Claims

What is claimed is:
1. An isolated electrical system (100) comprising:
- a synchronous generator (101 ) having a rotor (106) provided with at least two permanent magnet poles (110-115) and with at least two electrically excited poles (108, 109), and
- an excitation device (102, 202, 302) for producing excitation current for the at least two electrically excited poles, characterized in that the excitation device is arranged to control the at least two electrically excited poles to contribute the excitation produced with the at least two permanent magnet poles as a response to a situation in which a short-circuit takes place in the isolated electrical system.
2. An isolated electrical system according to claim 1 , wherein the excitation device comprises an electric double-layer capacitor (120, 220, 320) capable of providing excitation energy in the situation in which there is a short-circuit in the iso- lated electrical system.
3. An isolated electrical system according to claim 1 or 2, wherein the excitation device comprises a circuitry (123, 223, 323) for changing polarity of the excitation current.
4. An isolated electrical system according to claim 1 , wherein the excitation de- vice (302) comprises:
- an excitation generator (334) on a shaft of the synchronous generator,
- a first rectifier (321 ) coupled to alternating voltage of the isolated electrical system,
- a regulating circuitry (322) for coupling an output of the first rectifier to a static field winding (331 ) of the excitation generator, and
- a second rectifier (333) in a rotating part of the excitation generator for coupling rotating armature windings (332) of the excitation generator to excitation windings (350) of the electrically excited poles of the synchronous generator.
5. An isolated electrical system according to claim 4, wherein the second rectifier comprises a circuitry (323) for changing polarity of the excitation current.
6. An isolated electrical system according to claim 4 or 5, wherein the excitation device comprises an electric double-layer capacitor (320) at the output of the first rectifier (321 ), the electric double-layer capacitor being capable of providing excitation energy in the situation in which there is a short-circuit in the isolated electrical system.
7. An isolated electrical system according to claim 1 , wherein the isolated electrical system is an electrical network of a ship.
8. An isolated electrical system according to claim 1 , wherein the permanent magnet poles of the synchronous generator comprise damper windings (117).
9. An isolated electrical system according to claim 1 or 8, wherein the electrically excited poles of the synchronous generator comprise damper windings (118).
10. An isolated electrical system according to claim 1 , wherein:
- the at least two permanent magnet poles are capable of providing excitation corresponding to the operation of the synchronous generator when the excitation current is substantially zero and stator current and stator voltage have their nominal values,
- the at least two permanent magnet poles are capable of providing excitation corresponding to the operation of the synchronous generator when the synchronous generator is short-circuited and the stator current is a first predetermined factor times a nominal value of the stator current, and
- the at least two permanent magnet poles and the at least two electrically excited poles are together capable of providing excitation corresponding to the operation of the synchronous generator when the synchronous generator is short-circuited and the stator current is a second pre-determined factor times the nominal value of the stator current, the second pre-determined factor being greater than the first pre-determined factor.
11. An excitation (202, 302) device for excitation of a synchronous electrical ma- chine, the excitation device comprising a rectifier (221 , 321 ) for taking energy from an electrical network connected to stator windings of the synchronous electrical machine and a regulating circuitry (222, 322) coupled to an output of the rectifier for providing regulated current for the excitation of the synchronous electrical machine, characterized in that the excitation device comprises an electric double- layer capacitor (220, 320) at the output of the rectifier for providing energy to the regulating circuitry in a situation in which there is a short-circuit in the electrical network.
12. An excitation device according to claim 11 , wherein the excitation device comprises a circuitry (223) for changing polarity of the regulated current.
13. An excitation device according to claim 11 , wherein the excitation device further comprises: - an excitation generator (334) on a shaft of the synchronous electrical machine, the regulating circuitry (322) being arranged to feed the regulated current to a static field winding (331 ) of the excitation generator, and
- another rectifier (333) in a rotating part of the excitation generator for coupling rotating armature windings (332) of the excitation generator to excitation windings (350) of the synchronous electrical machine.
14. An excitation device according to claim 11 , wherein the other rectifier comprises a circuitry (323) for changing polarity of output current of the other rectifier.
15. A method for energizing an isolated electrical system, the isolated electrical system comprising a synchronous generator having a rotor provided with at least two permanent magnet poles and with at least two electrically excited poles, characterized in that the method comprises controlling (401 ) the at least two electrically excited poles to contribute the excitation produced with the at least two permanent magnet poles as a response to a situation in which a short-circuit takes place in the isolated electrical system.
16. A method according to claim 15, wherein:
- the at least two permanent magnet poles are capable of providing excitation corresponding to the operation of the synchronous generator when the excitation current is substantially zero and stator current and stator voltage have their nominal values, - the at least two permanent magnet poles are capable of providing excitation corresponding to the operation of the synchronous generator when the syn- chronous generator is short-circuited and the stator current is a first predetermined factor times a nominal value of the stator current, and
- the at least two permanent magnet poles and the at least two electrically excited poles are together capable of providing excitation corresponding to the operation of the synchronous generator when the synchronous generator is short-circuited and the stator current is a second pre-determined factor times the nominal value of the stator current, the second pre-determined factor being greater than the first pre-determined factor.
17. A method for providing excitation of a synchronous electrical machine, the method comprising taking (501 ), with a rectifier, energy from an electrical network connected to stator windings of the synchronous electrical machine, and regulating
(503) output current of the rectifier for providing regulated current for the excitation of the synchronous electrical machine, characterized in that the method further comprises storing (502) energy into an electric double-layer capacitor at the output of the rectifier for being able to provide the regulated current in a situation in which there is a short-circuit in the electrical network.
PCT/FI2009/050936 2008-12-01 2009-11-19 An isolated electrical system WO2010063882A1 (en)

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Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5502368A (en) * 1994-06-06 1996-03-26 Ecoair Corp. Hybrid alternator with voltage regulator
WO2003038989A1 (en) * 2001-10-09 2003-05-08 Abb Oy Method and apparatus for magnetizing a synchronous generator
US20060119206A1 (en) * 2002-08-14 2006-06-08 Akemakou Antoine D Double-excitation rotating electrical machine for adjustable defluxing

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5502368A (en) * 1994-06-06 1996-03-26 Ecoair Corp. Hybrid alternator with voltage regulator
WO2003038989A1 (en) * 2001-10-09 2003-05-08 Abb Oy Method and apparatus for magnetizing a synchronous generator
US20060119206A1 (en) * 2002-08-14 2006-06-08 Akemakou Antoine D Double-excitation rotating electrical machine for adjustable defluxing

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