A SYSTEM FOR ELECTRICALLY COUPLING A THREE PHASE GENERATOR

Abstract: For the electrical coupling in of a three phase windmill generator (G) to the mains when the generator is driven at a speed close to the synchronous speed a single triac (Tr) is caused to initially build up the current in one or two phases in a gradual manner, whereafter the remaining two phases or one phase, respectively, are or is connected to the mains through simple contactor means (2), whereby the traditional fluctuation problems are counteracted in a simple and cheap manner.

Description: A system for electrically coupling a three phase generator. The present invention relates to the electrical coupling in of a three phase generator, e.g. an asynchronous windmill generator, for operatively connecting the generator with the A.C. mains or generally with another already operative generator. A basic manner of coupling an asynchronous generator to the A.C. mains is to simply actuate a three poled contactor between the generator and the mains when the generator is rotated at a speed close to the relevant synchronous speed. It is well known, however, that this coupling method shows certain disadvantages, particularly with respect to the creation of very high current peaks and voltage fluctuations which, though soon dying out, may well cause considerable problems not only electrically, but also mechanically, since they are associated with strong torque pulsations of the entire rotary system. Some types of solutions to these problems have been proposed in the prior art. One type relates to the more detailed design and control of the contactor equipment, as it has been found that the said problems can be at least counteracted by causing the contactor equipment to effect the necessary switching operations in a sequential manner as far as the three phases are concerned. However, this measure alone has not proved too successful, because, as well known, almost unpredictable problems are liable to arise whenever a three phasegenerator * is operated in an electrically unbalanced manner. It is a general attitude, therefore, that any kind of changes should be effected simultaneously OMPI in all three phases. It should be mentioned here that the "phases" of a generator may be constituted by specific individual phase windings as connected between the respective phase terminals and a common point, the so-called neutral point or zero point. This applies to the so-called star-coupled generators, which are often used without any external connection of the zero point. In the so-called delta- or D-coupled generators the three windings are each connected between two phase terminals so as to diagram- atically form a triangle without any virtual zero-point and without one winding specifically referring to one phase. However, these two basic winding systems are fully equivalent as seen from the exterior phase termin- als and it is common knowledge that they should both be operated or affected in an electrically balanced manner. This also applies to another known type of solution, which is operatively very advantageous with respect to a "soft" coupling in of the generator, and which is based on the use of controlled valves such as triacs or thyristors, which are operated so as to gradually allow the phase currents to increase to full current in each of the phases or rather in all three phases uniformly, each phase terminal being connected to the mains through such a controlled valve. Hereby the said electrical and mechanical pulsation problems are widely eliminated, but the controlled valves are rather expensive elements, and as they are used in triplicate the entire coupling system is indeed expensive. It is the purpose of the invention to provide a generator coupling system, which is both relatively inexpensive and sufficiently efficient in use. The invention or a primary aspect thereof is based on the recognition that the said pulsation problems •are widely eliminated if only two or even oneof the generator. phases are allowed to experience a gradual current increase, whereafter the respective remaining phase or phases will be connectable with the mains by simple abrupt contaction. The pulsation problems are hereby essentially reduced, without the unbalanced coupling procedure giving rise to other problems. The pulsation problems are particularly reduced when the third phase is connected to the mains upon the two. other phases already having been coupled in, because the generator will then already be magnetized to such an extent that the electromotive force in the third phase winding is already practically in phase with and similar to the A.C. voltage in the corresponding wire of the mains, whereby the connection of the third phase does not give rise to considerable pulsations or current peaks. It will be appreciated that already hereby at least one of the three conventional controlled valves may- be replaced by a much simpler and cheaper contactor. However, as far as the prevailing type of generator is " concerned, viz. the generator "without zero", the described advantage will be still more significant, because ' in such a generator, as mentioned, the first phase to be coupled in is arranged, diagrammatically, in series with another phase, whereby the desired gradual current increase in two phases will be achievable by means of but a single controlled valve, inasfar as the other end of the series connected are connectable, initially, through a very simple contactor, if not permanently connected with the relevant wire of the mains. Thus, the desired result will hereby be achiev¬ able with the use of only one controlled valve in stead of three, whereby the total coupling system is produce- able at remarkably reduced costs.. In addition to the said cost reduction the coupling system of the invention will even be less sensitive »to OMPI . electrical noise, because of the reduced number of potentially -noise sensitive controlled valves. The invention may be seen " as a further development of the invention as disclosed in the International Patent Application No. PCT/DK82/00037 (WO 82/03952) as published after the priority date of the present application. In that application is disclosed the use of a contactor for first connecting two of the phase terminals of the generator with the corresponding phase wires of the mains, either through some kind of- an impedance, which is active though one or more steps for limiting the current build-up and which is later on short circuited so as to be inoperative in normal operaton of the generator, or/and at such a specif c moment of the transient change of the mains voltage that the phase connection will already hereby cause a minimum of following fluctuation problems. In that application has been cited some prior art relating to three phase "motors" "with zero", and it will be appreciated that the present invention .relates to "generators'*, primarily even -"without zero". The coupling in of a motor from stillstand is quite different from the coupling purpose * of the invention. It should be mentioned that the present invention may well be combined with the said earlier invention, e.g. by arranging for the first two phases to be connected through a controlled valve, which is caused to gradually build up the current to an inter¬ mediate value, e.g. corresponding to the valve being a controlled one-way rectifier, whereafter the controlled valve is short-circuited through a contactor, which is preferably actuated at a moment, where the transient voltage over the contactor terminals is at maximum or close to maximum. in the following the invention is described in -more detail with reference to the drawing, in which: Fig. 1 is a diagram of a coupling system according to the invention. Figs. 2έ . and 3--are diagrams ■-f modified coupling systems, and Figs. 4 and 5 are diagrams of further modified . coupling systems. In Fig. 1 is shown the three phases R, S and T of the mains 2 as connectable with a generator G, which is typically an asynchronous windmill generator. In the T-phase connection is inserted a triac Tr, which may be substituted by' two antiparallel thyristors, and a contactor 1. This contactor 1 is double and is arranged so as to be operable to additionally connect the S-phase with the generator. A further double contactor 2 is provided for switching on the R-phase and at the same time, via contacts 3,short-circuit the triac Tr. When the speed of the windmill at least roughly corresponds to the synchronous speed of the generator G the contactor 1 is actuated so as to establish a direct switching on of the S-phase and at the same time connect the triac Tr in the T-phase, whereby the triac will now be connected in the circuit from the T-phase through the two respective and series connected phases, of the generator to the S-phase. The triac control unit, designated 4, initially holds the triac closed, but is then actuated so as to gradually open the triac, whereby the current is allowed to rise gradually during a following time interval, which may be of the magnitude 10-100 msec or even longer. When the two generator phases have thus been coupled in, the connection of the third phase, R, is rather unqualified, because the generator will now be magnetically preadapted for its connection with the third phase. It is sufficient, therefore, to simply . actuate the contactor 2 some time after the completion OMPI of the current rise through the triac. This lapse of time, of course, should not be unnecessarily * long, and in a preferred system the contactor 2 is actuated by a timer 400 msecs. after the actuation of the contactor 1. The contactor 2 will even cause the triac Tr to be short-circuited through the contacts 3, whereafter the triac may simply be switched off. It is deemed unnecessary at this place to describe in more detail the design and the operation of the triac and its associated control unit, because the entire triac unit may be similar to those already used in the prior art for the individual and * simultaneous coupling- of all three phases. The contactor 1 is not principally required at all, but it will normally be desirable and even prescribed that a full galvanic separation between the generator and the mains can be established. In Fig. 2 the contactor 1 of Fig. 1 has been omitted, and contactor 2 has been divided into two separate contactors 2R and 2T. The triac has been replaced by a single controlled thyristor Ty. As a first step the control unit 4 is actuated so as to cause the thyristor * 10 * to.-graduallyopenforfull current. Because the thyristor is a one-way rectifier, it will be open for the A.C. current in one direction only, i.e. it will be effectively open half the time only, whereby the current is correspondingly limited. However, already by the gradual build-up of the first half of the current the related fluctuation problems will be widely counteracted, and it may well be acceptable to thereafter just uncritically actuate the contactor 2T for short- circuiting the thyristor, which may thereafter be switched off. As in the said prior application it will be highly advantageous, however, to effect the said short-circuiting at a moment of time whereby the transient A.C. voltage between the contacts to be OMFI short-circuited is at or close to its maximum. The contactor 2R is caused to connect the generator with phase R shortly after the actuation of the contactor 2T. If desired, still in accordance with the said earlier application, it would be possible to arrange for the final coupling of phase T, Fig. 2, through two steps, because a first step, upon full opening of the thyristor Ty, could be to short-circuit the thyristor through an impedance and then cause this impedance itself to be short-circuited. In this manner, all according to the detailed circumstances, it may be preferable to make use of a few uncritical connector steps through associated impedances rather than arranging for the connection to be specifically controlled in response to a detection of the transient voltage passing through a maximum. In many windmills the main shaft is connected with two generators for operation at high and low speed respectively. Thus, if the speed changes, it will be required to switch off one generator and couple in the other. In the prior art such systems, therefore, have often included six expensive triacs for effecting the necessary couplings. In Fig. 3 of the drawing is shown such a system, including two generators G- j and G2 in which but a single triac is used. The system of Fig. 3 corresponds almost fully to Fig. 1 with the exeption that the triac Tr is connected with the two generators in parallel, through individual three phase contactors 5 and 6, whereby the same triac is usable for the initial connection of each generator separately or selectively according to the relevant actuations of the contactors 5 of 6, while the final phase connection is still effected by means of the contactor 2, which even serves to short-circuit the * triac. -£UREA OMPI In Fig. 3 it is shown by way of example that the two generators G. and G2 are "star-coupled" and "D- coupled", respectively. In the examples as here described the generators are "without zero", but it is clearly within the scope of the invention to use the coupling system for generators as zero coupled to the mains. A first possibility will be to simply disconnect the zero during the coupling in period by means of a separate contactor, whereby the coupling will correspond fully to the already described examples. Another possibility as illustrated in Fig. 4 is to let the zero connection z remain operative and to initially connect but a single phase T to the mains through the controlled valve Tr and thus through the zero connection, while a contactor 7 is thereafter actuated to connect both of the other phases to the mains at one time or one after the other. Optionally this contactor may also short-circuit the valve Tr. Yet another possibility as illustrated in Fig. 5 is to arrange the controlled valve Tr in the z-wire. With one generator winding already connected to the mains (T) , preferably through a separation contactor 8, this winding will be gradually energized when the valve Tr is opened. Shortly after that,the contactor 7 is actuated to connect the two other windings to - he respective main phases, whereafter the current in the z-wire will normally disappear. In these examples the valve Tr may be replaced with by a variable or shortable impedance or such impedances, as according to the said prior application. Also, similar elements may be placed even in the second phase connection and be operated for full connection of this phase or phase terminal before the third phase is finally connected to the mains. OMΠ

Claims: CLAIMS ; 1. A system for electrically coupling a three phase generator such as an asynchronous windmill generator to the A.C. mains in response to the generator rotating at a speed close to the relevant synchronous speed, said system * including controlled electrical valve means for effecting, upo actuation thereof, a gradual build-up of the current in order to counteract fluctuation problems as * connected with a sudden build-up of the current, characterized .in that the controlled valve means are provided in only one or two of the phase wires, while in the third wire a contactor is arranged, which is controllable so as to effect contaction in two or one phases, respectively, upon the magnetizing current having been initially and gradually build-up in the first phase or phases through the said controlled valve means. 2. A system according to claim 1, characterized in that the controlled valve is.-arranged in series with two series connected generator phases the third phase . of the generator is connectable with the corresponding phase of the mains through a simple contactor. 3. A system according to claim 2, characterized in that the said contactor is a double contactor serving additionally to short-circuit the said controlled valve. 4. A system according to claim 1, characterized in that the controlled valve or a corresponding element or elements for effecting a gradual current build-up, such as shortable impedances, is or are mounted in the "zero" wire connection between the mains and a virtual neutral point of the generator phase windings. OMPI WIPO

Inventors: DROENEN OLAV, DROENEN, OLAV
Applicants: DROENEN OLAV, DROENEN, OLAV
Application year: 1983
Main classification: H02J3
IPC codes: H02J3