GB2204958A - Measuring field current in synchronous generators - Google Patents

Description

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Process and device for measuring field current

22 0 4 95 c, 0 The subject-matter of this invention consists of a process and a device for measuring the field current of a synchronous generator with a brushless excitation system.

It is well-known that a so-called brushless excitation system consists of (figure 1) Z - an alternating current generator of the synchronous type, with a fixed inductor and a rotating rotor-; - a rotating converter mounted on the same shaft; a voltage regulator; an auxiliary 'exciter alternator (in the classical version).

As can be seen in the diagram of figure -1, the use of an exciter coaxial to the main machine makes it possible to convert part of the mechanical power available on the shaft of the unit directly into excitation power.

The power generated by the exciter alternator (alternating current) is transferred to the rotating rectifier bridge and from this, once it has been trans formed into a direct supply, transferred directly to the field of the main machine, by means of suitable connections, without requiring the use of slip rings and brushes, and therefore with no points of interruption.

This known classical solution offers many adva ntages the first of these is the extremely limited maintenance due to the absence of brushes.

The absence of rings, howevey, prevents access to the field of the main machine and in particular prevents reading of the field current, significant in itself and due to the possibility it would provide to obtain the average temperature of the winding by reading it in conjunction with the voltage.

The rotor current of the main machine is thus reconstructed, based on the numerical results of the standard calculation codes used by designers, starting from the field current of the au,.,.iliary exciter alternator.

As an alternative, again according to known techniques, the rotor current of the main machine may be read directly by means of a shunt and a telemetering system; these devices are however complex and expensive, and for a wide range of machines.

invention is to overcome these disillustrated in detail below, it ofadvantages:

are not practicable The purpose of this advantages and, as fers the following - simplicity; - inexpensiveness 1 - 3 high reliability; good degree of precision as a measuring element. The solution offered by this invention consists of collecting by means of a measuring coil on the fixed part the flux generated by the current in the connections between the rectifier bridge and the field on the main machine at the axial portion running along the shaft, practically on the surface or in any case in the accessible part of same.

Actually, direct current flows through the connections, but on the fixed part the movement of the shaft creates a flux variable in time, having an intensity proportional to the direct current.

This also makes it possible to take into account, by updating the coefficient of proportionality, any variations in the speed of the unit (always envisaged in the context of specifications, as frequency variations, generally in the region of 5 % more or less).

The essential characteristics of the invention are summarized and schematically illustrated in the claims; the aims, advantages and characteristics thereof are also given in the following description concerning forms of embodiment chosen by way of example only, with particular reference tP the attached drawings in which - figure 1 is a simplified diagram of a synchronous - 4 j generator with a brushless excitation system of the known type, already described in broad outlines in the introductory part of the description; figure 2 shows schematically in the form of a transverse cross-section, the position of the measuring coil according to the invention in rela tion to the rotating shaft and the relevant bars; figure 3A is a front view of a particular form of application of the invention; figure 3B shows a side view of the same appl i cation; figure 4 is a schematic illustration of the cross section of the measuring coil.

Figure I has already been illustrated and therefore there follows simply an indication, in the proper order, of the various parts of the known type of generator to which the figure refers the auxiliary exciter alternator.

2 indicates the voltage regulator (AVR).

3 indicates the rectifier bridge.

4 indicates the field winding of the exciter alternator.

indicates the inducer winding of the exciter alternator.

6 indicates the rotating rectifier bridge.

synchronous 1 indicates -1 7 indicates the field winding of the synchronous generator (main machine).

8 indicates the rotor winding of the synchronous generator (main machine).

With reference to figure 2, saute shows, as stated above, the position of the measuring coil BM in relation to the rotating shaft A, rotating at a frequency (f).

In this figure, R.1 indicates the radius of the shaft, corTip,rJLseU typically between -100 and 300 rrtrri.

R2 indicates the distance of the outer edge of the bar 5 frofft the centre of the shaft A, RE generally being greater than R1 by about 5 - 15 mm.

R3 indicates the distance of the internal edge of the measuring coil BM from the centre of the shaft. In general R3 is greater than R2 by about 20 - 40 mut.

(Sp) and (Am) indicate respectively the thickness of the bar and its width.

In general Sp = 5 - 10 mut while Am = 30 - 60 mm.

With particular reference to figures 3A and 3B, in this working drawing the measuring coil (BM), the bars (S), the brushless shaft (A) and the rotating rectifier bridge 6 already indicated in the preceding schematic figures can be recognized.

The alternator rotor 102 and the connections 101 be- 6 - tween the bars (S) and the rotating rectifier bridge 6 are shown.

Figure 4 has been included with the sole purpose of giving an indication of the size and characteristics of the rrteasuriTg coil BM.

The height H of the coil may be comprised between 8 and 13 rrim, and its outside diameter D2 may have the same values.

As far as concerns the winding, its inside diameter D.1 is comprised between 5 and.10 mm; again in this case the height of t h e winding varies between t bl e same 1 i rri i t s.

This makes it possible to understand how the positioning of the coil entails no constructional problems, considering its small dimensions.

In any case the coil must be of a size and have a number of turns such as to guarantee:

(a) A good reading in any significant load condition. To this end the product of NA (number of revolutions by section) must vary between 250 and 2000 depending on the field current, the number of turns of the alternator, the number of connections and the diameter of the shaft.

(b) Minimization of the disturbances. main parameter to be considered is In this case the represented by 1 the uniformity of the [Tiagnetic field inside the coil, so that the height of the coil must be as small as possible.

(c) Minimization of dimensions. The purpose of this is to make the same coil applicable to all the special shapes which may be encountered in differently structured machines.

(d) The coil must be easy to fit into place. In this respect the application system must be able to ensure perfect positioning in the radial direction of the axis of the coil and the correct distance of the coil from the connections; maximum repeatability of positioning in the event of dismantling or replacement must also be guaranteed. Acceptable values for the distance between the coil and the connection are comprised between 20 and 40 rrim.

The voltage signal collected at the two ends of the coil may be processed to obtain two different indications:

- the absolute value (intensity) of the current; __ the harmonic content of the waveform of the voltage read.

The first item of information is obtained in a manner well-known in itself, by means of a peak voltage measuring device which is the parameter most closely 1 - 8 related to the field current.

The second item of information is significant diagnostic point of view in the case of bridges with a single parallel diode for each Direct calibration of the device is not normally required for rotating diode exciter alternators of a standardized series: in those cases in which calibration is necessary, this can be done by means of a suitable calculation code, prepared specifically for this purpose taking into account the size of the shaft and of the measuring coil and the nature of the surrounding materials, or else by direct measurement of the current using removable test rings.

The degree of precision obtainable without particularly sophisticated solutions is in the region of 5 %.

However even a precision of 5 % is encouraging, although it is well to remember that in these cases since the field winding of the machine is not accessible, the possibility of monitoring the progress of the current variations continuously is even more important than a very high degree of precision.

The above in order to ascertain on a continuous basis any abnormal conditions and avoid more serious problems by relating field current variations to load variations in order to distinguish between normal and abnormal f rom a rectif ier branch.

11 0 CY variations. 1 Although for descriptive reasons based on the foregoing description way of example only, many changes treade in the embodiment of the Invention. These changes- and variations shall sidered to be based on the following th is invention i S and illustrations by and variations rriav be however be conclaims.

Claims (6)

1. A process for the determination of the field current of a synchronous generator equipped with a brushless excitation system, characterized by the fact that it envisages the measurement of the magnetic flux generated by the current in the connections between the rectifier bridge and field of the main machine. on the fixed part, at the height of the axial section which runs along the shaft.

2. Process according to the preceding claim, characterized by the fact that this measurement is done by means of a coil.

3. Device capable of implementing the process according to the foregoing claims, characterized by the fact that it envisages a measuring coil inserted in a corresponding support prepared on the fixed part of the synchronous generator. in correspondence with the axial section running along the shaft. in the vicinity of the rectifier bridge-main alternator coupling.

4. Device according to the preceding claim characterized by the fact that this measuring coil has a diameter and axial dimensions comprised between 5 and 15MM.

5. A process for measuring field current substantially 1 as hereinbefore described with reference to the accompanying drawings.

6. A device for measuring'field current substantially as hereinbefore described with reference to the accompanying drawings i Published 1988 at The Patent Office, State House, 66.171 High Holborn, London W01R 4TP. Further copies may be obtained from The Patent Office, Sales Branch, St Mary Cray, Orpm9ton, Kent BR5 3RD. Printed by Multiplex techniques ltd, St Mary Cray, Kent. Con. 1187.