A METHOD AND DEVICE FOR DEMAGNETIZING BRUSHLES SYNCHRONOUS MACHINES

Abstract: In a synchronous generator of brushless type where the field winding (2) of the rotor of the main device is fed via a co-rotating rectifier (8) from the rotor windings (4) in the rotor to a supply device on the same shaft, a damping resistance is put into the excitation current circuit, connected in parallel with a turn-off thyristor (10). With de-coupling of the excitation, the thyristor (10) is turned off by means of a turn-off pulse transmitted by means of a slip-ring arrangement (11, 12).

Description: A method and device for demagnetizing brushless synchronous machines The invention relates to a procedure according to the preamble of claim 1 and an arrangement according to the preamble of claim 5 respectively. A synchronous generator of the normal type can be seen as an assembly of a main device and a supply device. The main device has a three-phase-wound stator which supplies the output power, and a rotor with field windings which are supplied with an adjustable direct current. The supply device is a generator which is intended to produce current for the field windings of the main device's rotor. Presently the supply device is normally an alternating- current device. With non-brushless synchronous generators the alternating-current voltage from the supply device is rectified in a rectifier and the rectified current is fed to the main device's rotor via brushes. When semi-conductor rectifiers became available at a reasonable price in the 1960's it was considered suitable to have brushless synchronous generators, i.e. to have the field winding in the rotor supplied from the supply device's rotor with an intermediary co-rotating rectifier. This means that the main device's rotor can be supplied with direct current without using brushes in that a co- rotating rectifier rectifies its three-phase current and delivers it to the synchronous generator's rotor. The regulation of the synchronous generator occurs in brushless devices by the current coming from the supply device being regulated by acting on the current in its field winding which is of course stationary. In this way a brushless synchronous generator can in principle be regulated in a completely normal and known manner as far as concerns its normal power regulation of magnitude and phase. A particular regulation problem remains however which has up till now remained unsolved, namely the fast regulation of the main device's voltage down to zero. Clearly it is easy to regulate the current of the field winding down to zero by acting on the current of the field winding of the supply device so that the delivered armature e. .f. becomes zero. However, the magnetic energy then remains in the field winding of the synchronous generator and prevents a quick down-regulation process. If the load line of the synchronous device is then lightly loaded with an essentially capacitative load, even the voltage can rise momentarily which can be a great disadvantage. A serious voltage rise, particularly for the safety systems, can unnecessarily actuate error corrections. Brushless synchronous generators have up to now been used to a limited extent as hydro-electric generators, in particular for large ones, but only rarely as turbo¬ generators. The background to why they have received relatively limited application despite obvious advantages has to do with said well-known problems of regulating-down the stator voltage of the synchronous generator, which can be solved far more easily with brush-supplied synchronous generators since fixed regulation devices can then be installed. Known and applied solutions include coupling-in a damping resistance in series, polarity-reversal of the field-winding's current source for coupling-in reversed current and so-called vibration regulators. Recently, controlled thyristor current converters have been used which allow smooth regulation and inversion. If one disregards all practical considerations, there is nothing preventing similar devices of known type, and which are generally present in brush-fed synchronous generators, from being mounted on the rotating shaft so that even with brushless devices a regulation of, in principle, a known type can be obtained. However, in such a case, a continuous and certain supply of the control signals to the rotating control equipment has to be arranged, either via slip-rings or wirelessly. This is obviously complicated even at moderate rotational speeds. At higher rotational speeds, e.g. for steam turbine generators, the additional difficulty arises of mounting the control equipment rotating and exposed to high centrifugal forces. Since the requirements for freedom from breakdown in generators in the mains supply are particularly high, there has rightfully been hesitance concerning introducing such complicated and, in principle, breakdown-prone equipment and therefore, as far as known, has lead to stopping all similar constructions on paper or possibly which have led to prototypes. As with the rotor field winding supply of modern type brushless synchronous generators the problem is still present with excitation energy in the field winding of the main device, meaning in a typical case that it can take about 5 seconds to lower the voltage to half and that, with an unloaded line (capacitative loading), a voltage rise can also occur during the process. It is therefore and object of the invention to achieve a practical, low cost solution to the problem underlying the invention being, upon lowering the voltage of a synchronous generator of brushless type with the main device's field supply by means of a shaft-mounted rectifier, from a co- rotating armature to a supply device, to increase the speed of elimination of the excitation energy. This and other objects are achieved in accordance with the invention by a procedure and arrangement as defined in the characterizing portion of claim 1 and claim 5 respectively. According to the invention it is intended that turning-off the thyristor should occur by means of a current pulse which is preferably supplied via brushes even if wireless transmission is possible. Even in such cases the generator must still be regarded as brushless since no brushes are present which transmit anything other than signal currents. Prior art brushless synchronous generators are also provided with brushes for signal transmission for monitoring, e.g. for faulty earth protection. The brushes of the invention can thus be suitably arranged on common brush rockers together with other signal-transmitting brushes, for contact with a slip-ring array. It may also be noted that brushes for the excitation current in non- brushless synchronous generators can transmit in the order of 100-1 000 A at 100-1 000 V depending on the size of the device. Control currents for thyristors of the turn-off type are under similar excitation currents in the order of 1-5 A at voltages of a few volts. The subject of the invention is thus justifiably referred to as brushless. In accordance with the invention the lowering-time can be drastically shortened by the effective time constant in the field circuit being reduced by a factor typically equal to 5. In this way the risk of voltage rise is reduced which, as with e.g. a 400 kV mains, can have serious consequences even with moderate amounts, e.g. 10%. The invention will now be described with reference to a non-limiting embodiment depicted in the drawings. Fig. 1 shows a schematic circuit diagram of a brushless synchronous generator equipped according to the principle of the invention. Fig. 2 shows a generator rotor on the same shaft as a rotor of a supply device. * What Fig. 1 shows is a schematic representation of a 5 synchronous generator with rotor windings on the same main device and on the supply device mounted on the same shaft (not shown in Fig.l). The main device has a three-phase power winding 1 on its stator and a field winding 2 for direct current on its rotor. The supply device 3 has a 10 three-phase winding 4 on its rotor and a field winding 5 with connections 6 and 7 on its stator. That which is shown in Fig. 1 between the generator's field winding 2 and the supply device's three-phase winding 4 is thus mounted on a rotatable shaft. 15 A six-pulse rectifier 8 conventionally constructed with semi-conductor diodes is connected to the three-phase winding 4 and supplies a direct current to the field winding 2 during normal operation via a conducting 20 thyristor which can be turned off, preferably a GTO thyristor. A damping resistance 9 is connected in parallel with the thyristor 10. For control of the thyristor via its control electrode, which thyristor is thus mounted in position on the rotatable system in the synchronous 25 generator, there is a slip-ring arrangement 11 with a slip- ring on the shaft and brush on a brush rocker. Another similar slip-ring arrangement allows the connection with the thyristor's 10 anode and can at the same time possibly perform other functions (not described here) e.g. such as 30 signal or protective earth for the metering system. * During the process when the device is started for phasing- in, the thyristor 10 should be activated by a turn-on pulse, e.g. via the connections 13 and 14 and the slip-ring 35 arrangements 11 and 12. The thyristor 10 will then be conductive for as long as the field current is fed to the field winding 2. It is also possible to arrange it such that the thyristor 10 is activated by the voltage across the resistance 9, which arises when the excitation voltage starts to be supplied, being made to affect the thyristor (not shown) . Turning-off the thyristor 10 requires that, upon demagnetization, this effect is compensated by a turn- off signal being delivered during the whole process via slip-ring arrangements. The skilled man will appreciate that this can be arranged in many different ways. The arrangement according to the invention does not result in any notable safety problems. If the thyristor 10 becomes unusable, this occurs mainly always in a form that the thyristor is short-circuited and thus uncontrollable, meaning that the method according to the invention can no longer be applied. This does not lead to any catastrophic result. The normal control of the device during operation occurs in a manner well-known to the skilled man via the stationary field winding 5 of the supply device 3, said winding being supplied with a variable direct current from a source 15. If however a fast reduction of the excitation current in the field winding 2 is required it is insufficient, in accordance with the invention, to reduce the voltage from the supply device 3 since the current in the field winding 2 with its high impedance continues its circuit path through the rectifier 8. With capacitative load this can also imply a temporary voltage increase for the synchronous device. In order to reduce these effects and to make the reduction of the excitation current quicker, the resistance 9 is coupled-in in accordance with the invention, due to a turn- off pulse being supplied via the connections 13 and 14. Through a damping which is too fast, over-voltages can in certain cases occur across the field winding resulting in breakdown. In order to overcome this and still achieve an optimal damping it can be useful in certain cases to mount a plurality of resistors which are coupled-in through turning-off of each one's thyristor (not shown in the figures). The reduction process can then be adjusted so that the damping at the start is limited to that which can be tolerated by the field winding, so that it can be increased gradually during the subsequent process. It is suitable to construct the damping resistance so that the impedance or the damping resistance becomes as purely resistive as possible, normally by means of known two-wire (bifilar) winding. GTO thyristors ("Gate Turn Off") are commercially available components and suitable data is available from the manufacturers, such that it is known for the skilled man how the actual turning-on and turning-off of the thyristor should be effected. The control circuit which should be connected to the terminals 13 and 14 thus requires no further explanation. Fig.2 shows schematically the rotating parts in a synchronous generator with a shaft 20 on which is located a rotor 21 for the actual generator and a rotor 22 for the synchronous generator's supply device. On the shaft 20 there are also slip-rings 23, intended to monitor as well as to turn-off the thyristor 10 shown in Fig.l. The windings 4 according to Fig.l are placed in the rotor 22 whilst the windings 2 are placed in the rotor 21. The resistance 9 and the thyristor 10 can suitably be placed either on the rotor 21 of the main device, e.g. such as at position 24, or on the rotor 22 of the supply device at 24' . In the latter case these components can be suitably mounted together with the rectifier means 8 onto a dynamically well-balanced and centrifugal force- withstanding unit.

Claims: Claims 1. Procedure for demagnetizing a brushless synchronous generator, the field winding (2) of which is arranged in its rotor (22) which is rotatable on a shaft, said field winding being fed by direct current from a rectifier (8) rotating therewith and coupled to a rotor (22) of a supply device (3) co-rotating with the generator, the field winding (5) of said supply device being supplied from a variable current source (15) , characterized in that a damping resistance (9) is connected in series with the field winding (2) of the synchronous generator, said damping resistance being connected in parallel with a controlled turn-off thyristor (10) which during normal operation is conductive and which is provided with a control electrode to which, for demagnetization, a turn-off pulse is fed for turning off the thyristor (10) so that the current flowing in the field winding is forced to go through the damping resistance. 2. Procedure according to claim 1, characterized in that the thyristor's (10) control electrode is coupled to a slip ring (11) mounted on said shaft, said slip ring being in contact with a brush. 3. Procedure according to claim 1, characterized in that the damping resistance is dimensioned so that the current circuit of the field winding (2) upon coupling-in of the damping resistance (9) obtains a considerably reduced time constant, preferably by a factor of about 5. 4. Procedure according to claim 1, characterized in that the damping resistance and the thyristor are mounted on the rotor of the supply device. 5. Procedure according to claim 1, characterized in that the damping resistance and the thyristor are mounted on the rotor of the synchronous device. 6. Brushless synchronous generator comprising a supply device mounted on its shaft (20), whereby the field winding (2) of the synchronous generator is arranged on its rotor (21) which is rotatable on the shaft whilst the rotor (22) of the supply device has windings (4) coupled via a co- rotating rectifier (8) to the synchronous generator's field winding (2) , characterized in that a damping resistance (9) is connected in series between the rectifier (8) and the field winding (2) of the synchronous generator, said damping resistance being connected in parallel with a turn- off thyristor (10), the control electrode of which is coupled to a slip-ring (11) mounted on the shaft, against which slip-ring runs a brush connectable to a current source arranged for turning-on and turning-off the thyristor. 7. Brushless synchronous generator according to claim 6, characterized in that the damping resistance is arranged upon coupling-in to reduce the effective time-constant for the current circuit including the field winding (2), preferably by a factor of about 5. 8. Brushless synchronous generator according to claim 6, characterized in that said damping resistance and the thyristor are mounted to the rotor (22) of the supply device. 9. Brushless synchronous generator according to claim 6 , characterized in that said damping resistance and the thyristor are mounted to the rotor (21) of the synchronous device.

Inventors: IVARSSON CLAES, IVARSSON, CLAES
Applicants: ASEA BROWN BOVERI, ASEA BROWN BOVERI AB
Application year: 1993
Main classification: H02P9
IPC codes: H02P9