WO2001017084A1

WO2001017084A1 - Intelligent synchronous machine with rotating sensor and processing unit

Info

Publication number: WO2001017084A1
Application number: PCT/SE2000/001604
Authority: WO
Inventors: Lars Gertmar; Arne Nysveen; Jan Ove Gjerde; Per-Anders LÖF
Original assignee: Abb Ab
Priority date: 1999-08-27
Filing date: 2000-08-23
Publication date: 2001-03-08
Also published as: SE516401C2; AU6884300A; SE9903026D0; SE9903026L

Abstract

The invention relates in general terms to a synchronous machine (1), a control method therefore, an electric power plant comprising such a machine and a method for control of the electric power plant. The synchronous machine (1) comprises a power electronic converter device (8), a winding provided rotor (3) with a co-rotating processing unit (10), and a likewise corotating communication unit (11) for wireless information transfer to a stationary processing unit (12). To the rotor (3) is also at least one corotating sensor (15-17) for measuring mechanical and/or electrical quantities associated with the rotor (3). The processing units (10, 12) evaluate locally measured quantities and controls the power electronic converter device (8) based on these evaluations.

Description

Intelligent synchronous machine with rotating sensor and processing unit.

TECHNICAL FIELD

The present invention relates generally to design, control, operation and protection of power converters and in particular to power converters comprising synchronous machines with rotor windings. The invention also concerns electric power plants comprising controllable synchronous machines, and control of such plants.

BACKGROUND

Electrical machines can in general be used for both generator operation and motor operation. During generator operation, a mechanical power is converted into an electric power. During motor operation, the conversion takes place in the opposite direction, i.e. from an electrical power to a mechanical one. Common for these two aspects is that a conversion, where electrical powers are included, takes place. An electrical power converter thus comprises electrical machines in both these aspects.

Electrical power converters are today to a large extent composed by synchronous machines, at least for electrical power 'converters that are intended for connection to general electric power networks. It is very important that the operation of synchronous machines takes place in concordance with the electric power network they are connected to and discrepancies normally result in losses or inefficient utilisation. One therefore often wishes to control the operational modes of the synchronous machines in a fast and efficient manner. Such a control can e.g. comprise control of the power 'conversion, relation between reactive and active power, torque, frequency and rotational speed, voltages, currents, overload protections etc. Generally, a synchronous machine thus constitutes a converter for active power and at the same time a simple and easily available setting tool for reactive power. As means for achieving a control of these quantities, a power electronic converter is normally used, which controls the current that passes through the rotor windings. By power electronic converters, here generally mean means that influence amplitude, phase and/ or frequency of an electrical current, which inter alia comprises ac-to-dc-converters, dc-to-ac-converters, frequency converters, phase shifters and different types of current amplifying means as well as combinations thereof.

The magnetisation of synchronous machines with rotor windings, i.e. current feed to the rotor windings, can be carried out in two ways. It can either be made brushless, by use of a magnetisation machine, or by slip rings and brushes. In the latter method, the current that will be provided to the rotor windings is transferred between the stationary and the rotating part of a synchronous machine by an arrangement of brushes and slip rings. Such a solution has, however, all the problems that are connected with movable contacting tools, such as sparking, interferences, material wear etc. Many larger synchronous machines are therefore today using so called brushless magnetisation. Brushless magnetisation takes place by means of a co- rotating exciter machine and a power electronic converter. Power electronic converters rotating with the shaft, in the form of ac-to-dc-converters, are present since about 25 years as brushless exciters for feeding the field winding in the rotor. The power electronic converter being co-rotating with the shaft is substantially/ most often designed with diodes, and the field current is controlled via the stator current of the exciter, so that the electronics of the power electronic converter on-board in the rotor is simple to design in traditional analogous technique.

One example of an embodiment of a brushless exciter is apparent from the ABB brochure "Brushless exciter", SEGEN/HM 8-001. From the brochure, it is evident that the exciter is an alternating current machine, whose stator is provided with sailent poles and whose rotor has a three-phase alternating current winding for exciting a power electronic converter. The direct voltage of the power electronic converter is then connected to the field winding of the synchronous machine. The voltage of the synchronous machine can be regulated by influencing the magnetisation of the exciter via its field winding.

A disadvantage with a brushless exciter of the above type, with a power electronic converter that is based on diodes, is the slow dynamics of the system. When using a so-called PSS ("Power System Stabilizer"), it is necessary that the magnetisation system has a sufficiently short response time. This can be achieved by designing the ac-to-dc-converter by thyristors. Such a system is described in the American patent US 3,671,850, where one uses radio communication to determine the control angle for the thyristors in the rotating thyristor rectifier. Here, analogous technique is used, which makes the system difficult to dynamically adapt, at changes of parameters included in the regulation, such as the resistance of the rotor.

The use of slip rings and brushes, both the ac-to-dc-converter and the control electronics are typically stationary, whereby the readymade current is transferred to the rotor.

A special embodiment of magnetisation at a with constant rotational speed rotating electrical machine is described in the patent application PCT/EP 98/007744, for "Power flow control" in a transmission line. The stator windings of the electrical machine are here connected in series with the conductors of the transmission line without connected neutral point. The rotor of the electrical machine is provided with two or three 90 or 120 degrees, respectively, electrically phase-shifted direct-current rotor windings for control of amplitude and phase of the voltage of the electrical machine. The excitation of the rotor windings takes place via a co-rotating magnetisation exciter and power electronic converter (ac-to-dc-converter) for each one of the rotor windings. In the same patent application, a more developed "Power flow control" is furthermore described, with a, to the same shaft as the first electrical machine connected, second electrical machine, which however is connected in shunt to the transmission line but also has co-rotating power electronic converters. The control and regulation is, however, sparingly discussed.

Since installations of large synchronous machines constitute large investments, it is of particular importance to ensure that synchronous machines do not operate in such a way that one risks damages on the machine and unscheduled operational interruptions. Therefore, safety devices are connected to most synchronous machines, which should supervise the condition of the synchronous machines and in an appropriate manner interrupt or limit such operation that may damage the machine or be disadvantageous for the operation of the electric power network. The safety devices may consist of pure protections, turning off the operation at noticed faults. They may also consist of limitation devices, which in a suitable manner changes or limits the operation to permitted conditions.

In the following description, it is assumed that the armature windings of the machine are placed in the stator and that field current (magnetisation current) thereby is in the rotor. This is, however, without importance for the basic idea of the invention.

The limits for the level a synchronous machine can be utilised is often set by considerations regarding temperatures, e.g. the temperatures of the stator and field windings. The currents in the windings, permitted by the safety and limiting devices, are in general estimated by means of simple theoretical models. For the field winding, a limit for the current is typically set by the magnetisation equipment. This may have a limit at e.g. nominal field current. Larger machines are often equipped with a field current limiter that besides a momentary limiter may comprise a time delay that allows for a certain over-current during a shorter period. This limitation is, however, static and does not take the actual thermal condition of the synchronous machine into account. The stator winding at alternating current machines is normally protected by an over-current protection. At overload of the machine, such as that the current exceeds a limit that is given by nominal power, the machine is disconnected. Synchronous machines may be equipped with a stator current limiter and /or an under- magnetisation limiter. These limiters may either automatically control the field current or give an alarm to the machine operator, which manually can control the field current and in such a way control the reactive power so that the operational point is kept within the permitted working area in a well-known "P-Q-circle diagram", which for anyone skilled in the art often is called capability diagram.

Alternating current machines with a power over 5 MVA are today equipped with resistive temperature meters (e.g. PtlOO elements) that are placed in or in close vicinity to the stator winding. These give a good information about the working temperature of the winding. These are connected to a protection that disconnects the machine when the temperature reaches over a certain limit. These limits are typically determined from the temperature class or from measurements during the commissioning of the machine.

A problem with machine protections and machine limiters according to prior art is that they in many cases are based on coarse static models about loss generation and conduction, and temperature rise. The actual conditions, such as variation in the temperature of the ambience, are generally considered very little. In order to achieve a protection also for rather extreme conditions, large safety margins have to be used. In less extreme cases, this leads to that the protections are tripped unnecessarily early in a process. Furthermore, if a load drop or fault in the electric power network causes an unfavourable distribution of reactive and active power, this may easily lead to a temperature and/ or current increase in certain parts of an alternating current machine. If the protection is set to a far too low level, such an electrical network situation may lead to that the protection of the machines is activated and that the machine thereby is turned off. This may in its turn lead to an aggravated state for the electric power network. Also other types of potential faults and breakdown risks are interesting to supervise. The condition of the insulation, such as detection of earth fault leak currents and mechanical machine states, such as mechanical stress, vibrations, cooling temperatures and flows constitute possible sources for discovering operational changes that could be able to lead to different types of breakdowns. Supervision of stationary parts can easily be provided and is well known within prior art. Supervision of rotating components is more difficult and has up to now been limited to a few special cases.

S.T. Chow and CH. Chew has in "Radio telemetry system for vibration measurement of rotating machinery", Conf. Proc. TENCON'84, Int. Conf. on Consumer & Industrial Electronics & Applications, New York, NY, USA, 1984, pp. 3-5, described a device with strain gauges on the rotor blades in a turbine, where the signals are transferred to a measurement and analysis instrument for evaluation. "Unique sensor applications for hydroelectric generator rotor-mounted sensor scanning technology", by J.S. Edmonds and T.L. Churchill, IEEE Techn. Appl. Conf. Northcon/96, Conference record, IEEE, New York, NY, USA, 1996, pp. 73-77, is also an example of transferring of measurement values from rotating parts. The same is true for

"The measurement of strain on a laminated disc flywheel" by A. Owens and P. Williams, Int. Power Generation, Vol. 6, No. 6, 1983, pp. 24-27 and "Rotor monitoring and protection for large generators" by R.H. Regan and K. Wakeley, Proc. Electrical Machines and Drives, 1995, pp. 203-207.

Common for such systems is that a special solution for transferring of measurement data is taking place from a rotating sensor to a central measurement and evaluation system. A continuous supervision is thus possible to occur, but with help of a considerable amount of transferred data. In the cases where the results are used for controlling the device, such a control takes place according to conventional principles. Since synchronous machines involve large and costly installations, the devices are often dimensioned in order to withstand normal operational conditions with large margins, in particular concerning the electrical insulation. Furthermore, margins are normally present in several steps, which results in that the total margin becomes unjustified large. This may e.g. occur by that a manufacturer of an insulation material states a certain nominal temperature, at which the material has a certain life time. Such a temperature is often given with a certain margin. The material may then be classified in a material class, whereby the temperature limit of the class normally is used for all materials included in the class. Furthermore, a machine manufacturer uses the material in his construction, whereby he adds his own constructional margins. Finally, the vendor of a machine may specify certain permitted operational conditions, but when an operator works out his own operator instructions, further safety margins are added in order to compensate for minor mistakes in the operation. This gives in many cases a total margin that is so large that only a too small part of the possible performance of the material is utilised, which today in many cases is not economically optimum.

The problems with synchronous machines according to prior art is that they to a large extent lacks an, in the situation of today, reliable and dynamic control concerning e.g. magnetisation of the rotor. Furthermore, there are in the synchronous machines of today inherent constructional margins that are not efficiently utilised. Furthermore, the protection devices for synchronous machines according to prior art are unsatisfactory regarding flexibility and supervision possibilities.

SUMMARY

A general object of the present invention is to provide a synchronous machine with intelligence in order to be able, in an improved manner, to reach an optimum control of the operational conditions of the machine. An object with the present invention is also to utilise measurements of, to the rotating parts of an electrical machine, associated electrical and mechanical quantities as basis for the control. In the present description it is intended that the term mechanical quantities also comprises thermal quantities. Another object is to, by continuous supervision of critical quantities, utilise in the synchronous machine existing constructional margins for an efficient operation and a flexible protection of the machine. A subordinated object with the present invention is to provide an electrical machine, where processing and/ or measurement of to rotating parts of the electrical machine associated quantities takes place locally, in direct connection to the actual part of the electrical machine. A further object with the present invention is to provide an electric power plant with increased possibilities to planned and/or co-ordinated power changes.

These objects are fulfilled by devices and methods, which presents the, in the enclosed device and methods claims, stated characteristics. In general words, a synchronous machine is provided, which comprises a power electronic converter device, a winding provided rotor with a co-rotating processing unit, and a similarly co-rotating communication unit for wireless information transfer to a stationary unit. Also at least one co-rotating sensor exists at the rotor for measuring of mechanical and/ or electrical quantities associated with the rotor.

SHORT DESCRIPTION OF THE DRAWINGS

The invention and further objectives and advantages that are achieved thereby are best understood by reference to the below description and the enclosed drawings, in which:

Fig. 1 shows a block diagram of an embodiment according to the present invention, with brushless magnetisation; Fig. 2 shows a block diagram of another embodiment according to the present invention, with magnetisation via slip rings and brushes;

Fig. 3 shows a block diagram of a third embodiment according to the present invention, with double rotor windings; Fig. 4 is a combined capability and phasor diagram for a synchronous machine in generator operation;

Fig. 5a shows a block diagram of an electric power plant with two synchronous machines according to the present invention;

Fig. 5b shows a block diagram for communicating electric power plants;

Fig. 6 shows a flow diagram for a control method for a synchronous machine according to the present invention; and

Fig. 7 shows a flow diagram for a control method for an electric power plant according to the present invention.

DETAILED DESCRIPTION

A basic embodiment of the present invention is schematically illustrated in Fig. 1. A synchronous machine, generally denoted by 1, comprises a stator 2 and a rotor 3. The stator 2 is provided with alternating current windings 4, through which, during operation, an alternating current flows, which is provided by supply terminals 5. The rotor 3 is arranged around a rotating shaft 6, and comprises rotor windings 7 or field windings. These rotor windings 7 are during operation provided with magnetisation current that is controlled by a power electronic converter 8.

According to the present invention, the synchronous machine 1 is provided with a, with the rotating shaft 6, co-rotating processing unit 10. The co- rotating processing unit 10 is connected to a, with the rotating shaft 6, co- rotating communication means 11. This communication means 11 is arranged to send and receive data by a wireless information transfer. The co- rotating processing unit 10 is furthermore connected to the power electronic converter 8, for control of its operation. A stationary processing unit 12 is connected to a stationary communication means 13. The stationary communication means 13 is arranged to send and receive data through the wireless information transfer with the co-rotating communication means 11. The stationary processing unit 12 is preferably also connected to additional external processing units via at least one communication device 14. A number of, with the rotating shaft 6, co-rotating sensors 15, 16 and 17 sense mechanical and/ or electrical quantities, which are associated with the rotor 3, and the sensors 15, 16 and 17 are connected to the co-rotating processing unit 10 for transmission of measurement data. In the present embodiment, the sensors 15 and 16 measure the temperature of the rotor winding 7 and the sensor 17 measures the actual magnetisation current of the rotor. In a similar way, a number of stationary sensors 18, 19 and 20, which sense mechanical and/ or electrical quantities associated with stationary parts of the synchronous machine 1, are connected to the stationary processing unit 12 for transmission of measurement data. In the present embodiment, the sensors 18 and 19 measure the temperature of the stator winding 4 and the sensor 20 measures the actual current of the stator. The co-rotating processing unit 10 treats the measurement data that is transferred from the co-rotating sensors, 15, 16 and 17, and data received form the stationary processing unit 12, if any. Based on these treated data, the co-rotating processing unit 10 controls the function of the power electronic converter 8 in an appropriate way.

This basic concept makes it possible to control the synchronous machine 1 in a dynamic and efficient way. With the rotor 3 associated quantities are measured locally, which normally gives a higher reliability than remote measurements. These reliable measurement data are sent to the co-rotating processing unit 10 via permanent connections, which present very large bandwidths and constitute thus no obstacle for the communication. The co- rotating processing unit 10 treats the incoming measurement data locally, and can, based on this, give control signals to the power electronic converter how a continued operation should be. The aspects of the magnetisation of the rotor that are determined by quantities achieved from the rotor are preferably controlled by signals that are treated totally within the rotating part of the synchronous machine. The invention thus gives a double function. On one hand, it gives a setting tool or speed means for control of desired operation of the machine unit (as it is defined below), determined among other things from external considerations or requests. On the other hand, it provides a supervision means for the machine due to the possibility for measuring of local quantities.

If measurement data from the co-rotating sensors 15, 16 and 17 causes any measures that concerns non-rotating parts of the synchronous machine 1, the co-rotating processing unit 10 extracts these data and transfer them to the stationary processing unit 12 via the communication means 11, 13. The stationary processing unit 12 takes thereafter care of these data for further processing.

In a corresponding way, if data that has its origin from the stationary sensors 18, 19 and 20, is of importance for the control of the power electronic converter 8, the stationary processing unit 12 extracts appropriate information, and sends it via the communication means 11, 13 to the co- rotating processing unit 10. This is also valid for data and instructions that are obtained from other places and are intended for the co-rotating processing unit 10. The co-rotating processing unit 10 then controls the power electronic converter 8 by means of received data.

The control of the synchronous machine can, as is indicated above, also comprise "external" data, which may be constituted by technical, economical, administrative as well as other control information. The actual data exchange with information about the condition of the synchronous machine as well as of the electric power network can be performed in close relation to the synchronous machine, for instance via different supervision units, which are further described below.

By placing a processing unit co-rotating with the rotor, one achieves a possibility of local treatment of locally achieved data. Thereby, the amount of data that has to be transferred between stationary and movable parts is minimised. The co-rotating positioning of the processor unit also provides a possibility for autonomous operation of the rotor, during possible breakdowns in the communication with stationary parts. A fault on the communication devices may thereby not mean an immediate turning-off of the whole machine, but the machine may continuously be run, even though more carefully. The co-rotating processing unit can thereby control the operation of the rotor according to predetermined guidelines independent of the stationary parts.

Anyone skilled in the art realises that one also may use a redundancy in the communication, i.e. provide alternative communication paths, if the regular communication is knocked out. Such a communication may e.g. be constituted by brushes and slip rings or parallel similar communication paths, which are activated when needed.

In the embodiment, shown in fig. 1, a so-called inverted synchronous machine 26 is used as a magnetisation machine. The stator windings 4 of the main machine are connected to an electric power network via the supply connections 5 and a power transformer 23. A smaller power transformer 22 converts the alternating voltage in the supply connections 5 in order to supply an AC-to-DC converter 21 with suitable alternating current. The AC- to-DC converter 21 is controlled by the stationary processing unit 12, so that the stator 24 of the magnetisation machine 26 is supplied with an appropriate direct current. An alternating current is then induced in the rotor windings 25 of the magnetisation machine 26, which current constitutes the exciting current to the power electronic converter 8. The power electronic converter 8 is in this case preferably a thyristor rectifier.

In an alternative embodiment, an asynchronous machine with multiphase windings can be used as magnetisation machine. This has the advantage that it can supply power also to a rotor at standstill.

Fig. 2 shows another embodiment of the present invention. This is in large similar to the embodiment shown in fig. 1 and similar parts are denoted by the same reference numbers and are not further discussed. Only the existent differences are described here below. In this embodiment, an arrangement with slip rings and brushes 31 is used to transfer the magnetisation current to the rotor 3. The information concerning the desired control of the magnetisation current is in this case transferred from the co-rotating processing unit 10 to the stationary processing unit 12. A power electronic converter 30 is then controlled by the stationary processing unit 12 according to the instructions achieved from the co-rotating processing unit 10. The power electronic converter 30 is in this case positioned stationary, which means that the need for communication between the processing units 10, 12 in general increases.

Fig. 3 shows a further embodiment of the present invention. Also this embodiment has large similarities with Fig. 1, and similar parts are denoted by the same reference numbers and are not further discussed. Only the existent differences are described here below. The rotor 3 is provided with two field windings 38, 39. These field windings are arranged with displaced magnetic axes. The windings 38 and 39 are supplied with current from a double power electronic converter 40 via the connection lines 41 and 42, respectively. As in Fig. 1, the embodiment in Fig. 3 is provided with a synchronous machine 26 as magnetisation machine. The device with double field windings has certain advantages concerning controllability, stability and so on. Such a type of machine is advantageously used together with an electrical machine according to the earlier mentioned patent application PCT/ EP98/ 007744.

The rotor 3 is in the embodiment in Fig. 3 provided with a number of additional sensors 43-46, which are connected to the co-rotating processing unit 10. Two sensors 43, 44 are arranged to the connection lines 41 and 42, respectively, in such a way that they can sense both the current and the voltage in the respective connection line 41 and 42, respectively. The sensors

43 and 44 are also arranged in order to sense disturbances on the current and voltage signals, respectively, whereby they also can be used as detectors for partial discharges, PD. This is described more in detail below. A vibration sensor 45 is arranged at the rotor body for detection of vibrations of the rotor, and a torque sensor 46 is also arranged at the rotor shaft 6, for measuring the shaft torque. Also these functions are described more in detail below.

It is obvious for anyone skilled in the art that such additional sensors 43-46 are not specifically usable for the synchronous machine shown in Fig. 3, but additional sensors may be present together with any type of synchronous machine.

The angle between the voltage vector and the so-called q axis, determined by the rotating magnetic flux, is referred to as the load angle. This angle constitutes a mechanical quantity, which can be measured via a, with the rotor co-rotating, sensor by for instance determining when the voltage vector reaches its maximum in relation to the rotating magnetic flux vector. The load angle constitutes an important parameter in order to determine the stability properties of the generator towards the external electric power network, which is described further below.

An electrical quantity that can be measured from the stationary stator side is the phase angle, which is the angle between voltage and current on the stator side.

The recent development of e.g. insulating material for large machines, concerning thermal ageing, production methods, etc. has enabled an increased utilisation of the capability of the machines. This is discussed more in detail below. These advantages and possibilities may, however, not be utilised so easily during operational conditions, unless reliable operational information is available. Protection and control of stationary components, such as stator windings and bearings are today rather common. This makes it possible to utilise the capability of the machines concerning the stationary parts. Supervision of the operational conditions of the rotor is, however, today not as common, but nevertheless very important for an economical utilisation of the machines. For a good utilisation and a relevant protection of electrical synchronous machines, it is among other things of interest to measure the true rotor winding temperature. This quantity contains lots of information about the operational conditions of the rotor. Furthermore, it may also be of interest to, preferably simultaneously, measure the true rotor currents, rotor voltages, vibrations of the rotor, partial discharges in the insulation of the rotor, the insulation resistance of the rotor, shaft torque and load angle. If damping windings are used, corresponding quantities are also of interest to know.

Furthermore, the condition of the insulation is important for the control and planning of the operation of the machine. The condition of the insulation can be monitored by measuring, either continuously or intermittently, different quantities. The methods will vary depending on the type of winding and voltage. By measuring the insulation resistance between winding and ground (iron parts of the machine), one may detect faults in the insulation, such as for instance mechanical wear, contamination or collection of humidity. For the rotor winding, this can be performed by measuring current and voltage

(maybe by using a measurement bridge for increased accuracy) and for a stator winding, one may perform the same by measuring the leak current. Partial discharges (PD) are present in cavities in the insulation, but may also be discharges in air between the insulation and iron parts of the machine. In the stator, the winding insulation is normally made with a semiconductive outer coating and measurement of PD therefore gives a good indication of the winding insulation itself. The discharge level is normally rather high in traditional insulation (typically larger than 10 000 pC) and it is normally of interest to measure changes in the discharge level in order to detect the degradation of the insulation. For traditional design of the field winding in the rotor, the voltage is rather low and therefore there is no need for any semiconductive coating between the insulation and the iron parts. Measurement of PD will therefore for a traditional design not have the same value. However, a change of the design of the rotor winding, such as use of two separate windings, as mentioned above and/ or increased voltage level transients for increased dynamics in the regulation may bring measurements of values of partial discharges up again in order to supervise the condition of the insulation. Measurement of the insulation resistance and/ or PD can also be used for the regulation of the operation of the machine itself. One will then e.g. during a strong temporary overload of the machine get mechanical tensions between the winding and the iron in the machine since the temperature difference is large. This may in the worst case damage the insulation and continuous measurement of PD can then be used in order to limit the set-point value at temporary overload of the machine. Overload of the machine is discussed further below.

Additional quantities that can be of interest to measure can e.g. be the mechanical vibrations in the rotor. A changed appearance of the vibration spectrum may indicate incipient mechanical problems, and may also bring about changed operation control, in order to avoid inconvenient breakdowns.

The shaft torque is also information that is of interest for controlling the synchronous machine. Such quantities can be measured in a number of conventional ways, e.g. by strain gauges mounted at the shaft. By help of these sensors, torques, both static and oscillating, may easily be detected.

Information about the shaft torque can for instance be used in order to protect large turbo machines against subsynchronous resonance, SSR. At the interaction between the electric power network and a turbo generator, SSR may lead to torsion interaction that results in heating of the shaft, which in turn may lead to breakdowns. This type of destructive torsion interaction may arise if there are "unfortunate" relations between the eigen- frequencies for the long shaft in a large turbo aggregate (including generator and turbines) and the electric power network. The risk for that it may arise subsynchronous (0 - 50 alt. 60 Hz) resonances is largest for large turbo generators in the vicinity of serially compensated power lines. In the following, it will be described how and why one requests to control a synchronous machine. In figure 4, a capability diagram is shown, which corresponds to a synchronous machine in stationary operation. The synchronous machine is assumed to, in order to simplify the reasoning, have a round rotor. P denotes the active power and Q the reactive power to /from the machine. In figure 4, one has assumed that the pole voltage has its rated value Us, IA denotes the stator current and jXs denotes the synchronous reactance. An inner magnetomotoric force EA, which is controlled by the field current IF, is formed. The current in the windings has given rated values IFN,

ISN, which should not be exceeded during stationary conditions. Thus, permitted stationary operational conditions are limited to an area within the stator current limitation 57 and the field current limitation 56, where both IF≤IFN and IA≤IAN. This means that the area 53 is not stationary permitted since IA is limiting and the area 54 is not stationary permitted since IF is limiting. The rated powers PN and QN correspond to the state when both the stator and rotor currents assume their rated values.

At use of a synchronous machine in undermagnetised operation, further limitations ensue. One limitation is constituted by the load angle that for a synchronous machine in general increases with decreasing magnetisation at constant converted active power. This means that if the operational point is brought sufficiently far out to the left in figure 4, the load angle will eventually reach the stationary (theoretically) maximum value of 90 degrees, when the machine becomes unstable. This constitutes a further limitation of the useful area in the PQ-diagram. Thus, there is a stability limitation 58 for undermagnetised operation, which may follow the path that is sketched in figure 4. This results in that the area 59 is not available for operation.

Further limitations may be present for different types of synchronous machines, which contribute with further restrictions in the capability diagram, e.g. as a result of heating of coil ends, field current limitations for minimum current and so on. In the following discussions, we will, however, neglect such effects, in order to be able to keep the description simple and easily understandable. The principle of the invention is, however, valid also for other limiting factors.

Possible control of the operational conditions of the machine is thus limited to an area 55. A stationary operational state is given by the powers Pi and Qi, which are present within the area 55.

In electrical power networks of today, the available reactive power Q is often of great importance. Since there are plants that may constitute sources as well as sinks for reactive power Q, a balance has to be achieved. It is therefore sometimes of interest to change e.g. the reactive power Q that the generator "delivers" or "consumes" (source or sink). According to IEC 34-1, sect. 9, a synchronous generator is rated with a rated apparent power SN and its rated power factor cos φN. From these data, active and reactive rated power can be calculated (PN and QN, respectively). Corresponding reasoning can be made for synchronous motors. If the machine works at rated power PN and QN, one rapidly realises that an increase of the reactive power Q can not be performed without exceeding the rated currents IFN and IAN. It is relatively common that the generator works close to PN, while the reactive power can be varied with regard to the need in the electric power network. The possibilities to deliver or consume reactive power depend on where in the capability diagram the generator worked before the change. These possibilities are determined according to prior art by static constructional margins and calculation models.

The temperature in the windings of the rotor in a synchronous motor is a limiting parameter for the ability of the synchronous machine to produce electrical torque as well as its ability to produce reactive power via the winding of the stator to the electric power network. Limits for temperatures and currents are often set in order to protect the machine against damaging overloading. Existent synchronous machines have, however, often a large thermal margin because of that the designer and operator built in safety margins at the design of the machine or because of that new experience values have been achieved since the machine was set into operation. Creation of standards, customers demands, uncertainty in dimensioning and inherent tolerances in insulation margins, gives for large electrical generators and motors of today a considerable thermal capacity that for the moment can be utilised to increase the power output of the machine.

By being able to measure important quantities in a reliable way, it is, however, possible to achieve a safe way to supervise the synchronous machine so that no damages arise. This means that one deliberately during e.g. shorter periods may exceed nominal values regarding currents, voltages and temperatures, without jeopardising the operative operation of the synchronous machine. By introducing a supervision, the limitations of the operational conditions may thus be treated in a much more flexible manner. The fixed limitations according to prior art may thereby according to the present invention be treated in a more flexible manner. This means that certain of the limitations that have been shown in figure 4 no longer have to be absolute limitations, but deliberate and supervised exceeding of these limitations may in certain cases be permitted. One possibility to expand the area for possible operational points is to deliberately allow an accelerated ageing of the insulation material, i.e. a shortening of the life, by letting the machine work at a higher temperature than what is nominally stated. Such an operation may e.g. be performed against a higher economical benefit by this operation. One may thus "sell off a part of the life of the machine" if that is desired.

In a synchronous machine, one may easily control the currents through the rotor windings both concerning phase and amplitude. Such a control of e.g. a power electronic converter means may thus be used to change the operational point in the capability diagram in Fig. 4. Under controlled conditions, the operational point may then e.g. be moved outside the area 55, which gives possibilities to a flexible utilisation. In similar ways, the operational point may also be changed when measurements indicate that there is a probable fault in the plant, which should give rise to a careful utilisation. The operational point may then be moved to a very conservative operational state with additional safety margins, until the reasons for the fault indications are investigated. If e.g. the rotor vibrations increase in an unjustified manner, the shaft torque that arises may e.g. be changed by the choice of rotor current so that the mechanical load on the rotor decreases.

According to the present invention, the synchronous machine is provided by a number of sensors, preferably at both stationary and rotating parts. These sensors may be of the above-discussed types, or for measuring of other quantities that may be though to influence the desired operation of the synchronous machine. The sensors are connected to the processing units 10, 12, where a local evaluation of the measurement values is performed. The data transfer may here be performed by fixed connections, which easily can give desired bandwidth. In e.g. the cases where measurements from a stationary sensor indicates that the rotor current should be altered, a transfer of information has to be performed between the stationary parts of the synchronous machine to its rotating parts. In the same way, a transfer of information in the opposite direction may also be necessary. The amount of transferred data is, however, reduced by the local treatment in the stationary

12 and co-rotating 10 processing unit, respectively. An example of how one may reduce the amounts of data is e.g. that instead of transferring stationary values, one only transfers measured values at certified changes. Since the operational conditions to an overwhelming part are stationary for a large synchronous machine, the amount of transferred data is considerably reduced. The higher processing capacity and intelligence that is locally available, the less becomes the demand of information transfer between the processing units.

By placing processor capacity locally, close to the measurement points, the control of the power electronic converter can also be performed very fast and flexible. The dynamics in the control of the power electronic converter in e.g. Fig. 1 does not any more need to totally rely on control from the stationary parts, but may be performed directly from the local co-rotating processing unit 10, when concerning the information that likewise is obtained from the co-rotating parts.

According to the invention, the transmission is performed wireless between the rotating and stationary parts. A solution with transmission via slip rings and brushes should in principle be possible, but since data transfer in this way often is associated with high noise levels, mechanical wear and uncertainty in reliability, this is not to recommend. Wireless communication of different types can be used.

Transmission with inductive coupling can be used. Frequency modulation and time multiplexing are preferably used in such cases. Inductive coupling does, however, normally demand that one additional mechanical unit is connected to the rotor shaft, with corresponding stationary parts. This increases in an undesirable way the building length for such synchronous machines.

Transmission by different types of light signals, such as IR or visible light is also possible. Infrared communication according to IrDA (Infrared Data

Association) is rapidly developed and standardised systems are now available. Solutions are available e.g. by Counterpoint Systems Foundry, Inc.

In a preferred embodiment of the present invention, radio communication is utilised for the wireless information transmission. The radio technique is well developed for such applications as a result of the mobility demands concerning computers and telephone equipment. Power electronic converters of conventional type give normally rise to electromagnetic disturbances of different kinds. These disturbances are, however, normally mainly below a frequency of 100 MHz, why carrier frequencies over this value should be chosen for the communication. The disturbances of the power electronic converter do thereby not influence the communication in any significant degree. There are licence-free frequency band, such as the so called ISM- band (Industrial-Scientific-Medical), that is utilised for short range communication and that functions in electromagnetically disturbed surroundings. The frequency band at 2.45 GHz is e.g. of great interest.

The article "Bluetooth - the universal radio interface for ad hoc, wireless connectivity" in Ericsson-Review, Vol. 75, No. 3, 1998, pp. 110-1 17, by J. Haartsen, describes commercially recently available technique for digital communication. Bluetooth is a radio interface in the frequency band of 2.45 GHz, which allows terminals to be connected and communicate wireless via a wireless LAN (Local Area Network) with short range. In Bluetooth, each unit can communicate simultaneously with several other units. Bluetooth uses a spectrum spreading technique with frequency jumps in order to divide the frequency band into several jump channels. During one connection, the transmitters jump from one channel to another in a pseudo- random manner. Wireless communication with up to 721 kbit/s can in this way be guaranteed in the present invention between rotating and stationary parts of the synchronous machine.

A preferred embodiment of the present invention utilises digital communication. It has during the later decades been a clear trend to miniaturise electronics for signal processing. Telecommunication can be said to be almost totally digital today in its essential parts. Digital communication has today very well developed methods for data compression, filtering, fault correction etc., which is not available in the same way at analogous communication. Signal processing in control and regulation circuits has also been miniaturised and it is easy to implement internal digital signal processing in e.g. power electronic converters, both ac-to-dc-converters and dc-to-ac-converters. To make such circuits with analogue technique does not bring about any additional value, since the digital resolution both concerning amplitude and time is sufficiently high. Digital communication is also to prefer as a result of other aspects such as the possibilities to set and trim the parameters of the regulation circuits remotely. An original parameter setting can thus be exchanged during operation to a modified parameter setting based on earlier operational data.

The stationary processing unit can be placed in such a position that it can work at normal conditions and consists preferably of a conventional microprocessor, which are well-known to anyone skilled in the art. This is therefore not further described. The co-rotating processing unit is on the contrary exposed for more abnormal conditions, e.g. centrifugal forces, vibrations and raised temperatures. These surrounding conditions thus have to be considered at the choice of processor and mechanical design of this and fitting thereof. Such processors are today available, mainly for military use, where the object among other things is to provide reliable shock protections.

Transmission of both reactive and active power contributes to power losses in electric power plants and electric power networks. Electric power plant is in this application referring to a construction comprising a group of synchronous machines, which are located within a limited area and are operated in a co-ordinated manner and which preferably belong to the same operator. An electric power plant according to the above definition, may typically consist of a number of synchronous machines, transformers, lines, cables, bus bars, disconnecting switches and circuit breakers and accompanying measuring and setting tools. The power conversion of the electric power plant can be influenced by the machine as well as other parts in the plant, such as phase compensating equipment, to which shunt reactors and shunt and serial capacitors can be counted. Other components in the power conversion process are constituted by for instance HVDC (High Voltage Direct Current) and FACTS (Flexible AC Transmission Systems) components and power electronic power converters for industry and distribution applications. The above mentioned components may furthermore be used in order to control the stationary and transient behaviour of the electric power network in normal operation as well as in fault cases. An electric power network is in this application referring to a network of connected electric power plants, which typically are spread over a wider geographical area.

Furthermore, shortage of reactive power, or an unfavourable distribution of it in an electric power plant or an electric power network, can lead to so- called voltage collapses. This is particularly important in connection with that it appears larger faults in the power network, e.g. when larger generators or motors suddenly falls out. When this occurs, the properties of the electric power network changes. Different regulators in the electric power network or the electric power plant will try to maintain the frequency and voltage within the predetermined limits. This can today among other ways be done by changing the active and reactive production at generators and by changing the transformation ratio through tap changer regulation in transformers. It has, however, at certain occasions been show that this has not been sufficient. If the need of reactive power is larger than what the electric power network can provide for, the transmission capability in the electric power network may break down very fast by that the maximum point on the so called PV-curve is passed or by that there are further disconnections of remaining production units as a result of overload. The limitation in the amount of transmitted power depends in many electric power networks on shortage of reactive power, above all at critical positions and not on that thermal limits for transmission lines are exceeded.

It is thus of great importance, in order for an operator of an electric power network or electric power plant to be able to control the voltage of the electric power network or electric power plant, to maintain an appropriate distribution of sources and sinks for reactive power, and to be able to change this distribution during shorter periods. Reactive power can not, unlike for active power, be transported any longer distances. This means that reactive power constitutes a "local" resource, which must be available in the area where it is needed, for instance after a disturbance. The reactive power is closely related to the voltage in the electric power network, while the active power in similar way is strongly connected to the frequency. A way to control the reactive power is to introduce phase compensating elements in the electric power network or the electric power plant. For large and complex networks, in particular if the load changes its nature from one time period to another, such solutions demand extensive analyses and simulations in order to determine location and electrical power rating for these phase compensators in an optimum way. Furthermore, procedures, for how these should be controlled for different operational conditions in the network, are required. Another measure for controlling the amount of reactive power may be to change the operational conditions for alternating current machines, e.g. synchronous machines, in order to get these to change the reactive power with respect to the present situation in the electric power network or electric power plant. Contributing causes to that a voltage collapse arises may be a high active as well as reactive load level (stressed electric power network), insufficient reactive resources (at least locally) together with different types of disturbances in the electric power network. If a disturbance for instance leads to a reduction of available reactive resources, this can result in a voltage collapse. It is important that the magnetisation or winding temperatures at the synchronous machine, at stationary conditions, does not exceed its nominal values. An optimum utilisation of available resources may thus constitute an important factor in order to guarantee the operation of electric power networks. Phase compensating equipment thus constitutes, together with the possibilities for an improved utilisation of the reactive resources in synchronous machines, important resources in order to control power flows in the electric power networks.

The present invention gives, however, possibilities for utilising different existing margins in machines in a more efficient and flexible way in order e.g. to be able to increase the reactive power without decreasing the total utilisation of the machine. Synchronous machines according to the present invention can thus be utilised by an electric power network or electric power plant in order to balance the relation between active and reactive power, in particular at critical situations.

The concept of utilisation of constructional margins may thus also be brought out on an electric power plant level or electric power network level.

Fig. 5a and 5b show schematically an electric power plant, in which a number of synchronous machines 62 are included. Together with control equipment for the machine, the synchronous machine constitutes a machine unit 71. Each machine unit 71 constitutes a part of an electric power plant (according to earlier definition), which via power lines or cables 61 is connected to an electric power network 60. The machines are equipped with local processing units 64, 68 and communication devices 69, 70 according to the present invention and data concerning the properties, operational conditions and status, both instantly and historically, of the machine can be available in a stationary processing unit 64 and /or a co-rotating processing unit 68 at each machine. A group of machine units 71 is generally controlled by a production management unit 66. According to the present invention, a number of communication devices 65 are established between the machine units 71, i.e. the stationary 64 and/or co-rotating processing units 68 of the machines and the production management unit 66. If for instance a fault occurs in the network, or if the electric power need is abnormal, the production management unit 66 can transfer information about this to the different processing units 64, 68, possibly related to a wish or demand of temporary being able to operate the machines in a specific manner. The respective processing units 64, 68 may in their turn inform the production management unit 66 about its present operational state and if any tendencies to network instability have been detected.

Such a configuration becomes particularly powerful if the production management unit 66 has access to the present operational state for the different machines, i.e. e.g. to temperature measurement values or other interesting quantities. The control of the synchronous machines 62 via the production management unit 66 may be based on internal measurement signals as well as local, regional or central input signals. Examples of internal measurement quantities are temperature measurement values from stator as well as rotor. The frequency constitutes an example of an interesting locally measurable input signal to a production management unit 66, while examples of regional and global input signals can be constituted by different indicators, for instance based on eigenvalue calculations, which states the risk for voltage instability. If the production management unit 66 has access to both the present operational state for the different machines and stored information about how large constructional margins that are to be utilised in each of the machines, this can be utilised in order to continuously update the operational plans for how one should be able to handle different types of faults or operational situations. The updating of operational plans can be built on mathematical relations for instance in the form of optimisation tools such as optimum load distribution, OPF, and stability indicators e.g. based on eigenvalue calculations as well as on measured, calculated or extrapolated data for the response of the machines with respect to interference of protections or limiters. If the production management unit 66 knows that a certain machine has a large margin to utilise, this can be requested at a fault situation, maybe in order to be able to rescue other machines with less margin in the network.

A database with data about the different machines is thus to prefer. The database comprises among other thing the data that has been measured with the above described sensors. Such information can at a later stage be used for analysis in order to increase the knowledge about the behaviour of the plant at normal operation as well as at disturbances. The information can also be used to analyse the efficiency of the plant and to be able to plan future operational ways. The database thus comprises both historical information concerning the earlier operation of the machines, but also information that is needed to achieve an adequate control of the machines.

The database may e.g. comprise the response of the machine to earlier occurred disturbances. The information stored in the database can as described above be based on measured, calculated as well as extrapolated data and mathematical calculations for instance in the form of optimising tools and eigenvalue calculations. Measurement data and other information that is stored in the database can with advantage be time stamped, for instance by using time specification via GPS satellites in order to synchronise the timing. The database can be available directly in the production management unit 66 and/or the local processing units 64, 68. The production management unit 66 therefore preferably comprises a memory means for storage of the database.

The network communication devices 65 can operate according to different communication methods according to prior art. Such methods can be based on fixed connections in the form of e.g. metallic wires or fibre optics, or on wireless transmission, such as radio or radio link. When connecting several production management units, a network of communication devices can be formed. Such a network contributes to alternative communication paths, which can be utilised in the cases where one or some of the communication devices by some reason are not available. By suitable combinations of redundant communication paths and storage of information in databases, the production management unit 66 can perform calculations and updates of operational plans, for instance by extrapolating earlier measurement data or results from eigenvalue calculations to estimate the condition in the machine, even if one or some of the communication devices and/ or sensors at the occasion do not work. The possibility to build up an operationally safe network is of particular large importance for instance at strained operational situations and during the operation recovery after a larger operational disturbance.

The concept with utilisation of constructional margins can, as earlier have been pointed out, be brought out to electric power network level. Means for control and supervision of the electric power network, a network supervision unit 72, is responsible for the operation of the electric power network in large, and can in corresponding way as been described above communicate with electric power plants, i.e. the production management units 66, or individual electrical machines 62, in order to be able to receive information about possible margins. These margins can then be used for an optimisation of larger areas or the total operation of the electric power network.

The information that is transferred from an electric power plant or individual electrical machine to a network supervision unit 72 on electric power network level, should preferably be of a more concise type than lower in the hierarchy. An electric power network operator has above all an interest in to know available power resources at different places in the network and the possible costs that are associated with utilisation of these. The detailed conditions concerning the temperature margins of the individual machines are of less interest, at least for a hierarchically superior electric power network operator. It is also of interest for an operator of an electric power plant to be able to limit the information exchange, since a part of the available information can be used as competitive means against competing operators.

The main task for the network supervision unit 72 is to guarantee the possibilities for a reliable and economically optimum operation of the electric power network. This may comprise communication of information exchange and set points as well as control signals to both production management units 66, individual electrical machines 62 and electric power networks 60. During normal operational conditions, the control of production management units 66 can be based on economical control signals, while it can be necessary to send direct control commands at serious operational disturbances, with the aim to save the integrity and continuous operation of the electric power network. The information exchange between the network supervision unit 72 and the other units should be performed in the form of messages, which besides address and message may comprise safety keys (coding) in order to limit the right to study the information. By utilising different authorisation criteria for different units (utilisers) in the communication system, a satisfactory secrecy against unauthorised distribution of information can be achieved. The possibility to determine which information each utiliser has access to reduces the risk for unauthorised distribution of sensitive information and enables that for instance co-operation partners can have access to more information than other totally external parties.

A preferred embodiment of a production management unit 66 in an electric power plant thus comprises at least one means for external communication. This means is arranged on one hand to receive and interpret messages from external units, such as e.g. an operator of an electrical power network and on the other hand to send out selected information to the external units.

A plausible communication sequence will now be described. An external unit sends a request if the electric power plant can increase its production of active power by 5 %, and to what price this can be done. The production management unit 66 of the electric power plant answers with an offer to a certain price. If the external unit can accept the offer, an order about increased power output is sent. The production management unit 66 of the electric power plant responds by carrying through the promised increase and sending back a confirmation of the order.

Fig. 6 shows a flow diagram for a basic control method for a synchronous machine according to the invention. The process begins in step 100. In step 102, a number of rotor quantities are measured, which are collected into a co-rotating processing unit in step 104. The co-rotating processing unit evaluates the measurement values in step 106. Rotor information that is of importance for stationary parts, and outer information that is important for the co-rotating processing unit are transferred between stationary and rotating parts in step 108. Based on the evaluated measurement results, the rotor current is then controlled in step 110 and the process is ended in step 112.

Fig. 7 shows a flow diagram for a basic control method for an electric power plant with at least one synchronous machine according to the invention. The process starts in step 100. In step 102, a number of rotor quantities are measured in the synchronous machine, which are collected into a co- rotating processing unit in step 104. The co-rotating processing unit evaluates the measurement values in step 106. Rotor information that is of importance for stationary parts of the synchronous machine and/ or the control of the electric power plant, and external information that is of importance for the co-rotating processing unit are transferred between stationary and rotating parts in step 108. Based on the evaluated measurement results, the operation and power conversion of the electric power plant is then controlled in step 111 and the process ends in step 112.

Anyone skilled in the art realises that different modifications and alterations can be made to the present invention without departing from the frame of the invention, which is defined by the enclosed patent claims.

Claims

1. A rotating electrical synchronous machine comprising a rotor (3) with windings (7; 38,39); a stator (2); at least one, with the rotor (3), rotating sensor (15-17,43-46); and at least one power electronic converter (8; 30; 40) for control of the current in the rotor windings (7; 38, 39), and characterised by a, with the rotor (3) rotating, processing unit (10; 68), connected to the co-rotating sensor (15-17, 43-46), for evaluation of measurement data received from the co-rotating sensor (15-17, 43-46) and for control of the power electronic converter (8; 30; 40); a stationary processing unit (12; 64); and a communication means (11, 13; 69, 70) for wireless information transferring between the co-rotating processing unit (10; 68) and the stationary processing unit (12; 64).

2. The rotating electrical synchronous machine according to claim 1, characterised in that the information transfer of the communication means

(11, 13; 69, 70) is made by radio signals.

3. The rotating electrical synchronous machine according to claim 2, characterised in that the radio signals have a carrier frequency exceeding 100 MHz.

4. The rotating electrical synchronous machine according to claim 1, 2 or 3, characterised in that the wireless information transfer of the communication means (11, 13; 69, 70) is digital.

5. The rotating electrical synchronous machine according to any of the claims 1 to 4, characterised in that the co-rotating sensor (15- 17, 43-46) measures one of the quantities: rotor winding temperature, current in the rotor winding, voltage in the rotor winding, partial discharges in the insulation of the rotor, the insulation resistance of the rotor the vibrations of the rotor, shaft torque, and load angle.

6. The rotating electrical synchronous machine according to any of the claims 1 to 5, characterised in that the power electronic converter is a, with the rotor (3) rotating, power electronic converter (8; 40), connected to and controlled by the co-rotating processing unit (10; 68).

7. The rotating electrical synchronous machine according to any of the claims 1 to 5, characterised in that the power electronic converter is a stationary power electronic converter (30), connected to and controlled by the stationary processing unit (12; 64).

8. The rotating electrical synchronous machine according to any of the claims 1 to 7, characterised by at least one stationary sensor (18-20), connected to the stationary processing unit (12; 64), whereby the stationary processing unit (12; 64) is arranged for evaluating data received from the stationary sensor (18-20).

9. The rotating electrical synchronous machine according to claim 8, characterised in that the stationary sensor (18-20) measures one of the quantities: stator winding temperature, current in the stator winding, voltage in the stator winding, partial discharges in the insulation of the stator, the insulation resistance of the stator, and phase angle.

10. An electric power plant comprising a number of rotating alternating current machines (1, 62), a production management unit (66), and characterised in that at least one of the rotating alternating current machines (1, 62) is a synchronous machine according to any of the claims 1 to 9.

11. The electric power plant according to claim 10, characterised by communication devices (14; 65) between the production management unit (66) and the stationary (12; 64) and/or the co-rotating (10; 68) processing unit of the synchronous machine.

12. The electric power plant according to claim 10 or 11, characterised in that the communication devices (14; 65) comprises physical lines in the form of metallic wires or optical fibres.

13. The electric power plant according to claim 10 or 11, characterised in that the communication devices (14; 65) comprises means for transmission via radio or radio link.

14. The electric power plant according to any of the claims 10 to 13, characterised in that the production management unit (66) comprises means for transmission of messages to and from external units.

15. A control method for rotating electrical synchronous machines, comprising the steps of: measuring quantities associated with the rotor (3); and controlling the rotor current of the synchronous machine (1; 62); and characterised by the steps of: collecting the measured rotor quantities in a, with the rotor (3) rotating, processing unit (10; 68); evaluating, in the co-rotating processing unit (10; 68), the measured rotor quantities; transferring of information wireless between the co-rotating processing unit (10; 68) and a stationary processing unit (12; 64); whereby the control of the rotor current of the synchronous machine

(1; 62) is based on the evaluation of the measured rotor quantities.

16. The control method according to claim 15, characterised in that the control of the rotor current of the synchronous machine (1; 62) is made by the co-rotating processing unit (10; 68).

17. The control method according to claim 15, characterised in that the control of the rotor current of the synchronous machine ( 1 ; 62) is made by the stationary processing unit (12; 64).

18. The control method according to any of the claims 15 to 17, characterised by the further steps of: measuring quantities associated with the stator (2); collecting the measured stator quantities in the stationary processing unit (12; 64); and evaluating, in the stationary processing unit (12; 64), the measured stator quantities.

19. The control method according to claim 18, characterised in that the control of the rotor current of the synchronous machine (1; 62) is based on the evaluation of the measured stator quantities.

20. The control method according to any of the claims 15 to 19, characterised in that the control of the rotor current of the synchronous machine (1; 62) is based on external instructions and/ or on pre-stored instructions and control parameters.

21. The control method according to claim 20, characterised by the further step of: modifying the control parameters for the control of the rotor current based on earlier operational data.

22. The control method according to any of the claims 15 to 21, characterised in that the transfer is made by radio signals.

23. The control method according to claim 22, characterised in that the radio signals have a carrier frequency exceeding 100 MHz.

24. The control method according to any of the claim 15 to 23, characterised in that the transfer is digital.

25. A control method for an electric power plant, comprising at least one synchronous machine (1; 62), comprising the steps of: measuring quantities associated with the rotor (3) of the synchronous machine, and controlling the power conversion of the electric power plant; and characterised by the steps of: collecting the measured rotor quantities in a, with the rotor rotating, processing unit (10; 68); evaluating, in the co-rotating processing unit (10; 68), the measured rotor quantities; transferring of information wireless between the co-rotating processing unit (10; 68) and a stationary processing unit (12; 64); whereby the control of the power conversion of the electric power plant is based on the evaluation of the measured rotor quantities.

26. Control method according to claim 25, characterised by the further steps of: measuring quantities associated with the stator (2); collecting the measured stator quantities in the stationary processing unit (12; 64); and evaluating, in the stationary processing unit (12; 64), the measured stator quantities.

27. The control method according to claim 26, characterised in that the control of the power conversion of the electric power plant is based on the evaluation of the measured stator quantities.

28. The control method according to claim 25, 26 or 27, characterised by the further step of: communicating data concerning the operation of the synchronous machine from the stationary (12; 64) and/or the co-rotating (10; 68) processing unit to a production management unit (66) belonging to the electric power plant.

29. The control method according to any of the claims 25 to 28, characterised by the further step of: communicating instructions and/or requests concerning the operation of the synchronous machine (1; 62) from the production management unit

(66) to the stationary (12; 64) and/or the co-rotating (10; 68) processing unit.

30. The control method according to any of the claims 25 to 29, characterised in that the communication between the production management unit (66) and the stationary (12; 64) and/or the co-rotating (10;

68) processing unit is made via metallic wires or optical fibres.

31. The control method according to any of the claims 25 to 29, characterised in that the communication between the production management unit (66) and the stationary (12; 64) and/or co-rotating (10; 68) processing unit is made via radio or radio link.

32. The control method according to claim 30 or 31, characterised by the step of: transmitting messages between the production management unit (66) and external units.