A DEVICE FOR CONTROLLING ELECTRIC POWER

Abstract: In a device for transmitting electric power between a direct voltage side (1) and an alternating side (2) of a converter (3) means (34-36) are adapted to detect a component possibly occurring in an electric quantity and having a frequency substantially corresponding to the frequency of the alternating voltage on the alternating voltage side of the converter on the direct voltage side of the converter and through the phase position of the component determined from which phase leg of the converter it comes and send this information to an arrangement (24) controlling the different power semiconductor members (19) of the converter and being designed to adapt the control so as to eliminate said electric component on the direct voltage side.

Description: A DEVICE FOR CONTROLLING ELECTRIC POWER FIELD OF THE INVENTION AND PRIOR ART The present invention relates to a device for transmitting electric power between a direct voltage side and an alternating voltage side of a converter having at least two phase legs having each two current valves connected in series and a phase connected to a point between the current valves, said current valves comprising a controllable power semiconductor member and a rectifying member connected in anti-parallel therewith, in which it comprises at least one sensor adapted to detect the phase current in the phases and an arrangement connected to the sensor for controlling the power semiconductor members depending upon information about the phase current received from said sensor. Such devices have a number of different fields of use, but for illuminating the invention the case of such a device in connection with transmitting electric power between a railway feed line and an electric multiple phase motor of a railway vehicle located on said alternating voltage side will be discussed hereinafter, although a use of a device of this type in all types of industrial driving systems is within the scope of the invention. The power semiconductor members are usually controlled by said arrangement according to a pulse width modulation pattern so as to carry out a conversion between direct voltage and alternating voltage and conversely, in which the frequency of this control may be in the order of 0.5 or a few kHz, whereas the frequency of the voltage obtained thereby on the alternating voltage side is considerably lower, for example 0-150 Hz. The optimum of a device of this type is that all commutations of the power semiconductor members are symmetrical, but the reality " may look different, and one of the power semiconductor members, which may for example be thyristors or IGBTs, may be turned on quicker, but be turned off slower, than the other power semiconductor members of the converter. This means then that the width of the different voltage pulses delivered on the phase in question will be larger for the different power semiconductor member, so that the phase voltage in the phase in question will contain a direct voltage component differing from half of the intermediate link voltage, which in its turn also means that the phase current will contain a direct current component. This direct current component in the phase current will conversely be modulated by the fundamental frequency voltage of the output voltage of the converter, i.e. the voltage having a frequency just as high as the frequency of the phase voltage, and give rise to an alternating current component on the direct voltage side having said fundamental frequency and also a voltage ripple with the fundamental frequency over the two poles of the direct voltage side. This gives rise to a number of problems discussed below, and it is accordingly desired to eliminate said direct cur- rent component in the phase current and thereby the fundamental frequency current and the voltage ripple on the direct voltage side. A problem is that the direct voltage side and a feeding network connected thereto, possibly through a further converter, are disturbed, which in the case of a railway is expressed by the fundamental frequency current, when the network is fed by a direct voltage and fundamental frequency current modulated by the fundamental frequency voltage of the network converter when the network is fed by an alternating voltage, in the line current of the railway feed line, which may disturb the signal system existing for determining the position of railway vehicles, should the frequency of said fundamental frequency voltage coincide with the carrier frequency of the signal system. Another disadvantage consists in that electrical equipment " driven by the multiple phase current, such as a motor of an engine, may earlier be saturated as a consequence of the direct voltage component disturbing, which means that it is not possible to take the power desired out of the motor. In the case of a device in a railway vehicle there has to be a possibility to vary the number of revolutions of said motor, so that also the frequency of the fundamental frequency component will vary. This means that the reactance of an inductor of a filter arranged on the direct voltage side for eliminating fundamental frequency components or multiples thereof has to be given a high value within a large frequency range. If the fundamental frequency component on the direct voltage had been neglecti- ble, this inductor could have been smaller than otherwise, since the filter then would not have to be efficient until higher frequencies. However, it is not possible to neglect the asymmetry in the converter, the vehicle has to be provided with a filter being much larger or heavier than otherwise needed, so that an engine may for example be forced to carry several hundreds of kilograms extra during the entire lifetime thereof. The regulating system used for controlling the power semiconductor members manages to control the power semiconductor members so that asymmetries may theoretically be eliminated at low frequencies, up to about 20 Hz, but if the frequency is increased further it does not have the time to compensate for asymmetries determined. A problem with using said sensors in a regulating system for compensating asymmetries resides also in the fact that they may have a certain remanence and always in- dicate a certain direct current, so that conclusions concerning the direct current component on the alternating voltage side may be completely false when using these sensors for that task, so there is even a risk for erroneously detecting the presence of such a direct current component also in the absence thereof and that a direct current component in the phase current is created by controlling the power semiconductor member in question " through the arrangement so that the entire situation is made worse in this way. SUMMARY OF THE INVENTION The object'of the present invention is to provide a device of the type defined in the introduction, through which the problems discussed above are solved to a large extent. This object is according to the invention obtained by providing such a device with means adapted also to comprise means arranged to detect a fundamental frequency component, i.e. a component having a frequency substantially corresponding to the frequency of the alternating voltage on the alternating volt- age side of the converter, possibly occurring for an electric quantity on the direct voltage side of the converter, and by means of the phase position of said component determine from which phase leg it comes and send this information to said control arrangement, which is arranged to adapt the control of the power semiconductor member in question so as to eliminate said electric fundamental frequency component on the direct voltage side. By detecting said component with the so called fundamental fre- quency on the direct voltage side of the converter and determine which phase leg the component comes from, and then adapt the control of the power semiconductor member in question by the control arrangement thereto in this way, it is possible to eliminate the direct current component of the phase current and thereby the fundamental frequence component of the current and the voltage on the direct voltage side by simple means. The electric quantity in question may be either the current or the voltage on the direct voltage side. By detecting a possible asymmetry in the commutations of the power semiconductor members by measuring the results of the asymmetry in this way, the existence thereof may be determined in a completely reliable " way, and by determining which phase leg the asymmetry comes from and of which type and magnitude it is the control arrangement may control the power semiconductor member for eliminating the asymmetry, in which it all the time may be checked that the control just taking place is correct through said detection. An asymmetry fault of this type is for the rest substantially depending upon the fundamental frequency on the alternating voltage side, such as for example the number of revolutions of an electric motor of an engine, so it will be sufficient to arrive to the optimum compensating adjustment at one occasion, and this may then be maintained independently of changes of the frequency of the alternating voltage. Accordingly, the disadvantages connected to the presence of a fundamental frequency component on said direct voltage side may in this way be eliminated, so that said signal system is not disturbed any longer, the motor of a railway vehicle may deliver the power desired and a filter on the direct voltage side has not to be able to eliminate any fundamental frequency component and does therefore not have to be efficient until higher frequencies, so that the inductor thereof may be made considerably smaller and thereby lighter. According to a preferred embodiment of the invention said means comprise a band pass filter with a variable frequency band adjustable according to the frequency of the alternating voltage on the alternating voltage side of the converter. Said component may hereby be distinguished and reliably detected independently of possible changes of the frequency of the alter- nating voltage side. The control arrangement has al! the time information about the frequency in question, since this controls the power semiconductor members for obtaining this frequency, and this arrangement may accordingly easily deliver this information to the band pass filter so as to set the frequence band thereof. According to another preferred embodiment of the invention said means is adapted to detect an alternating current component possibly occurring in the current on the direct voltage side of the converter. This constitutes an advantageous possibility being easy to realize to determine said component on the direct voltage side and thereby asymmetry of the commutations of the power semiconductor members. According to another preferred embodiment of the invention said means is adapted to detect an alternating current component possibly occurring in the voltage between the two poles of the direct voltage side. This constitutes also a possibility easy to realize to determine said fundamental frequency component on the direct voltage side, and an advantage of this solution is that a device of this type is normally already provided with a sensor for measuring the voltage between the two poles of the direct voltage side, so that no new sensor has to be arranged for measuring a possible alternating voltage component of this voltage, but this information may be taken from the information de- livered by the sensor already there. According to another preferred embodiment of the invention said means comprises members for integrating the alternating component of said electric quantity over a determined period of time so as to determine a value upon which the control of said power semiconductor member is based . According to another preferred embodiment of the invention the direct voltage side of the converter is adapted to normally feed electric power to the alternating voltage side for driving electric equipment arranged there, which advantageously is an electric motor, for example in a railway vehicle. The converter is then advantageously adapted to provide the electrical equipment with an alternating voltage having a frequency varying according to variations over the time of the need of the equipment, i.e. the frequency of the alternating voltage driving the equipment re- " quired by the equipment. This is necessary for being able to vary the number of revolutions of an electric motor of a railway vehicle. According to another preferred embodiment of the invention the converter is a part of an alternating current converter with said direct voltage side as an intermediate link and a second converter on the opposite side of the intermediate link with respect to the first converter mentioned above, said intermediate link having at least a first capacitor arranged between the two poles for defining the direct voltage therebetween, and in a further development of this embodiment the intermediate link has a filter formed by a second capacitor and at least an inductor and adapted to remove oscillations of the double multiple of the fre- quency of the alternating voltage on said opposite alternating voltage side occurring on the direct voltage side. By arranging said means of the device according to the invention said filter may in this way be designed to remove oscillations having a frequency of the double multiple of the frequency of the alternating voltage of the opposite alternating voltage side. According to another preferred embodiment of the invention said means are adapted to carry out said detection within the circuit formed on said direct voltage side of a capacitor branch con- necting the poles of the intermediate link and the first converter, since the detection is then not disturbed and will thereby be reliable to an optimum . Further advantages as well as advantageous features of the in- vention will appear from the following description and the other dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS With reference to the appended drawings, below follows a de- scription of preferred embodiments of the invention cited as ex- " amples. In the drawings: Fig 1 is a simplified circuit diagram illustrating a device according to a first preferred embodiment of the invention, Fig 2 illustrates schematically the construction of a con- ventional signal system for detecting the presence of a train on a certain section of the railway track, which is shown for explaining how the invention solves a problem of such a system, and Fig 3 is a view corresponding to Fig 1 of a device according to a second preferred embodiment of the invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE I NVENTION A device for transmitting electric power between a direct voltage side 1 and an alternating voltage side 2 of a converter 3 is schematically illustrated in Fig 1 . More exactly, this device is intended to be a part of an alternating current converter for transmitting electric power from a one phase railway feed network for railway vehicles to a motor 4 of a railway vehicle driven by a three phase alternating voltage for driving the vehicle, in which except for a change of the number of phases of the alter- nating voltage also the frequency thereof will be changed. The frequency of the railway feed line is normally 16 2/3 Hz (in some countries 50 Hz), while the frequency of the alternating voltage delivered to the motor 4 preferably is variable and typically within the range of 0-150 Hz. Such a converter has in a conventional way except for the first converter 3 a direct voltage intermediate link 5 with two poles 6, 7 and a capacitor 8 fof defining the direct voltage between the two poles. The intermediate link has also an inductor 9, which together with the capacitor 8 forms a LC-filter, which will be described further below. A second converter not shown is arranged on the opposite side of the intermediate link to the first converter 3 between the intermediate link and the one phase line. This constitutes conventional technique, and the construction of the converter 3 and the part of the control thereof also constituting conventional technique will now be explained . The converter 3 has three phase legs 10-12 with two current valves 13-18 connected in series each, each of which consist of a controllable power semiconductor member 19, such as an IGBT (Insulated Gate Bipolar Transistor) or a GTO (Gate Turn- off Thyristor) and a rectifying member connected in anti parallel therewith in the form of a rectifying diode 20. A phase conductor 21 -23 of each phase of the alternating voltage side of the alternating current converter is connected to a mid point of each phase leg between the two current valves of the phase leg. By controlling the power semiconductor members 19 in conventional way through a control arrangement 24 schematically indicated to be turned on and off according to a pulse width modulation pattern with a high frequency, in the order of 0.5 or a few kHz, an alternating voltage having a lower frequency, for exam- pie 50 Hz, determined by the arrangement 24 may be obtained on the alternating voltage side. The device has also sensors 25-27 connected to the different phase of the alternating voltage side for measuring the phase current thereof. These sensors are normally Hall-element-sensors, and they send information about the phase current to the control arrangement 24, which uses this information for controlling the power semiconductor members 19. The description so far concerns conventional technique, and the characterizing for the present invention will now be described. As already mentioned in the introductory portion of the description of this disclosure asymmetries may occur on the alternating voltage side and these may then propagate to the direct voltage side, should a power semiconductor member not react exactly as the other upon control signals received thereby, for example be turned on more rapidly, but be turned off slower than the other control power semiconductor members. The pulses delivered by the current valve in question will in such a case be slightly longer than those delivered by the second current valve in the same phase leg , so that the overall voltage of the phase in question over the time will not be half the intermediate link voltage but will differ from half the intermediate link voltage with a polarity of the pole of the defective current valve. Thus, the phase in question gets a direct voltage component differing from half the intermediate link voltage, which is comparatively small, maybe in the order of 10 mV, but since the motor 4 preferably has a low reactance so as to keep the losses therein low this voltage may give rise to a substantial direct current component of the phase current of this phase, for example in the order of several amperes. This direct current component generates on the other through the regulation of the converter on the direct voltage side an alternating voltage component having the same frequency as the alternating voltage of the alternating voltage side, i.e. with the fundamental frequency. This results in an alternating current part on the direct voltage side as well as an alternating voltage component, a so called ripple voltage, of the voltage between the two poles 6, 7 of the direct voltage side. These components have to be removed and the disadvantages associated therewith have been thoroughly discussed further above, but the influence thereof on the signal system of the railway will now briefly be described with reference to Fig 2. The signal system utilizes a so-called track line for detecting the presence of a train on a certain section 28 of a railway track 29. " A transmitter 30 emits for this sake a carrier frequency current in one rail to a receiver 31 in the other end of the section and the current returns then in the other rail, such as indicated through the arrows 32. The arrows 33 indicate the very traction current, which may be in the order of 1000 A, while the signal current may be in the order of 1 A. If a train is present within the track section 28 the wheel axles thereof will short- circuit the rails and no current will reach the receiver 31 , and it is thereby indicated that the track section is occupied. The signal current has typically a carrier frequency of some tenths Hz or slightly higher, and if now the frequency of the fundamental frequency component on the direct voltage side coincides with this frequency the signal system may be disturbed and false information of presence or absence of a train may be delivered. For reliably eliminating a direct current component possibly existing in any phase current and thereby indirectly a fundamental frequency component on the direct voltage side a member 34 has been arranged in the device according to Fig 1 for measur- ing the current of the connection of the intermediate link to the first converter 3 so as to detect an alternating current component with fundamental frequency possibly existing there. A band pass filter 35 with a variable frequency band is for this sake connected to a member 34 and adjustable according to the fre- quency of the alternating voltage on the alternating voltage side of the converter, in which it all the time gets information about this so called fundamental frequency from the arrangement 24. Furthermore, a member 36 for integrating the alternating current component with fundamental frequency over a determined pe- riod of time is connected to the band pass filter 35 so as to determine an effective value of the alternating current component, which is then sent to the control arrangement 24 for constituting the basis of the control of the power semiconductor members. The phase position of the alternating current component is then also determined in the member 36, and information thereabout is also sent to the control arrangement 24, so that this will be aware of exactly which phase leg has generated the alternating current component on the direct voltage side. The control arrangement will then modify the control of exactly that phase leg thereby so that the alternating current component is eliminated, i.e. such modification takes place until no alternating current component may be determined by the member 34 any longer. It may then in the practice be proceeded in such a way that a certain value is added to or subtracted from the signal coming from the centre of the phase in question for obtaining a balance of the phase leg in question. The very control takes place more in detail in the following way: 1. The measuring signal for the intermediate link current re- ceived from the member 34 is filtrated by the band pass filter 35, the mid frequency of which is controlled by the fundamental frequency prevailing. 2. The signal filtrated phase locks through the integrator 36 an oscillator, in which an integrator thereof controls the phase position/frequency and another integrator controls the amplitude. The integrators are utilized since the fundamental frequency current in the intermediate link shall be compensated away. 3. The presumed current vector according to point 2 has through the phase locking the same phase position as the real direct current component in the measuring signal of the load currents. 4. The presumed current vector is subtracted from the load cur- rent vector measured. 5. The amplitude of the presumed current vector is adjusted until the fundamental frequency component of the intermediate current measured is minimized or eliminated. Since the asymmetry fault is independent of the frequency it will " in this way be sufficient to remove the asymmetry once, and the compensating will then take place automatically also when the frequency on the alternating voltage side is changed by changing the number of revolutions of the motor 4. A device according to a second preferred embodiment of the invention is illustrated in Fig 3, which differs from the one shown in Fig 1 only by the fact that one member 34' for measuring the direct voltage between the two poles 6, 7 of the direct voltage side, i.e. across the capacitor 8 defining this voltage for determining an alternating voltage component having said fundamental frequency of this voltage, is arranged instead of the current measuring member 34 of the device according to Fig 1 . The function of this embodiment is for the rest the same as for the one according to Fig 1 , and an adaption of the control by the control arrangement 24 of the power semiconductor members accordingly takes place here, so that the fundamental frequency component of the voltage on the direct voltage side is eliminated. The invention is of course not in any way restricted to the preferred embodiments described above, but many possibilities to modification thereof will be apparent to a man skilled in the art without departing from the basic idea of the invention as defined in the claims. As already mentioned, the invention is not restricted to the railway area, but it finds a use in nearly any industrial driving systems whatsoever, especially where it is required that the feeding network must not be disturbed. It is neither necessary that the voltage of the alternating voltage side has three phases, but it could theoretically be of the multiple phase type with a different number of phases. Although it is shown in the figures that said sensors for measuring the phase currents are arranged at each of the phases, this is not necessary, but it is possible to manage with one sensor less than the number of phases, i.e. in the present case with two sensors, since conclusions may be drawn from the measur- ing results thereof with respect to the phase current of the phase having no sensor, and the patent claim definition "at least one sensor adapted to detect the phase current of the phases" is intended to include such an embodiment as well, where the phase current of one of the phases is not directly, but only indi- rectly detected.

Claims: Claims 1 . A device for transmitting electric power between a direct voltage side (1 ) and an alternating voltage side (2) of a converter (3) having at least two phase legs (10-12) having each two current valves (13-18) connected in series and a phase (21 -23) connected to a point between the current valves, said current valves comprising a controllable power semiconductor member (19) and a rectifying member (20) connected in anti-parallel therewith, in which it comprises at least one sensor (25-27) adapted to detect the phase current in the phases and an arrangement (24) connected to the sensor for controlling the power semiconductor members depending upon information about the phase current received from said sensor, character- ized in that it also comprises means (34, 34', 35, 36) arranged to detect a fundamental frequency component, i.e. a component having a frequency substantially corresponding to the frequency of the alternating voltage on the alternating voltage side of the converter, possibly occurring for an electric quantity on the di- rect voltage side of the converter, and by means of the phase position of said component determine from which phase leg it comes and send this information to said control arrangement, which is arranged to adapt the control of the power semiconductor member in question so as to eliminate said electric fun- damental frequency component on the direct voltage side. 2. A device according to claim 1 , characterized in that said means comprises a band pass filter (5) with a variable frequency band adjustable according to the frequency of the alternating voltage on the alternating voltage side of the converter. 3. A device according to claim 1 or 2, characterized in that said means (34-36) is adapted to detect an alternating voltage component possibly occurring in the current on the direct voltage side of the converter. 4. A device according to claim 3, characterized in that said means (34-36) are adapted to detect said alternating voltage component on the direct voltage side close to the converter (3). 5. A device according to claim 1 or 2, characterized in that said means (34', 35, 36) are adapted to detect an alternating voltage component possibly occurring in the voltage between the two poles (6, 7) of the direct voltage side. 6. A device according to claim 5, characterized in that said means are^ adapted to detect an alternating voltage component with said frequency possibly occurring by measuring the voltage over one or several capacitors (8) between the two poles of the direct voltage side for defining the direct voltage. 7. A device according to any of claims 1 -6, characterized in that said means comprise members (36) for integrating the alternating component of said electric quantity over a determined period of time so as to determine a value upon which the control of said power semiconductor members (19) is based. 8. A device according to any of claims 1 -7, characterized in that the direct voltage side (1 ) of the converter is adapted to normally feed electric power to the alternating voltage side for driving an electrical equipment (4) present there. 9. A device according to claim 8, characterized in that the converter (3) is adapted to provide the electrical equipment with an alternating voltage having a frequency varying according to variations of the need of the equipment over the time. 10. A device according to claim 8 or 9, characterized in that said equipment is an electric motor (4). 1 1 . A device according to any of claims 1 -10, characterized in that the converter (3) is a part of an alternating current con- verter with said direct voltage side as an intermediate link (5) and a second converter on the opposite side of the intermediate link with respect to the first converter mentioned above, said intermediate link having at least a first capacitor (8) arranged between the two poles for defining the direct voltage therebetween. 12. A device according to claims 4 and 1 1 , characterized in that said means are adapted to carry out said detection within the circuit formed on said direct voltage side of a capacitor branch connecting the poles of the intermediate link (5) and the first converter (3). 13. A device according to claim 1 1 or 12, characterized in that the intermediate link (5) has a filter formed by a second capacitor (8) and at least an inductor (9) and adapted to remove oscillations of the double multiple of the frequency of the alternating voltage on said opposite alternating voltage side occurring on the direct voltage side. \ 14. A device according to claim 1 1 , characterized in that the second converter is intended to be connected to a railway feed line conducting a one phase alternating voltage, and that the device is arranged on a vehicle driving on a railway for convert- ing the one phase voltage to a multiple phase voltage for transmitting power between the railway feed line and an electric motor (4) driving the vehicle. 15. A device according to any of the preceding claims, charac- terized in that the converter (3) is adapted to generate a three phase alternating voltage on said alternating voltage side.

Inventors: ALAKUELA MATS, BAENGTSSON HANS, ALAKUELA, MATS, BAENGTSSON, HANS
Applicants: ABB DAIMLER BENZ TRANSP, ALAKUELA MATS, BAENGTSSON HANS, ABB DAIMLER-BENZ TRANSPORTATION (TECHNOLOGY) GMBH, ALAKUELA, MATS, BAENGTSSON, HANS
Application year: 1999
Main classification: H02M1
IPC codes: H02M1, H02M7