TECHNICAL FIELD OF THE INVENTION

The present invention relates to a system for feeding electric power, which system comprises an input filter with an input con figured to be connected to a DC energy source, a DC intermediate link connected to an output of the input filter, a converter with an input connected to the DC intermediate link and an output config ured to be connected to a said first consumer of electric power, a unit configured to control the converter to obtain feeding of electric power requested by a said first consumer independently of varia tions of the voltage on the DC intermediate link, and an assembly configured to act stabilizing on the voltage on the DC intermediate link upon a variation of this voltage, as well as a method for feed ing electric power according to the preamble of the appended in dependent method claim. The first consumer of electric power may be any consumer of elec tric power, such as an electric machine and in the case of a said system on board a track-bound vehicle for the propulsion of the vehicle or in an auxiliary power system providing electric power to a distribution network to sockets, lighting, heating, and other ap- pliances on board the vehicle. The DC intermediate link may be connected to a DC energy source in the form of electric batteries or a DC-supply line, such as for feeding electric power to track- bound vehicles. The converter may be of any type, such as a DC/AC or DC/DC converter, controlled by a said unit to switch, such as by Pulse With Modulation (PWM), for converting the power to be fed to said first consumer.

A said input filter is arranged to smooth the current drawn from the DC energy source through the DC intermediate link. To allow good suppression of high frequency switching harmonics through cost and energy efficient filters these filters typically become poorly damped. When the converter, as here, shows a constant power load (CPL) behavior, which means that the power consumed by a said first consumer is approximately independent of the voltage on the DC intermediate link, a decrease/increase of the DC interme diate link voltage requires an increase/decrease of consumed cur- rent to keep the power consumption unaffected. In connection with a poorly damped input filter, such behavior is likely to result in system instability, leading to system shutdown due to under- or overvoltage trips. This is the reason for providing such a system with an assembly configured to act stabilizing on the voltage on the DC intermediate link upon a variation of this voltage. However, the objective of the stabilization is not only to stabilize the system but also to obtain a desired damping for avoiding occurrence of over and under voltage trips.

BACKGROUND ART

It may be attempted to solve the problem of obtaining said stabili- zation by adding passive damping components, such as additional or larger filters, but that would result in the drawback of adding complexity, cost, weight, space, and total power losses of the sys tem. This is the reason for attempting to obtain said stabilization in the form of so called active stabilization instead by controlling the converter to add a first power component to the power to be fed to a said first consumer to obtain stabilization of the DC link voltage upon occurrence of a variation of this voltage. Such a sta bilization action does not involve the drawbacks mentioned of add ing passive components in the form of filters. However, also this way of obtaining said stabilization has drawbacks, since internal high-priority power-control objectives of the power load may inter fere with the power modifications generated for stabilization . In such cases, neither system stability nor fulfillment of the internal control objectives may be assured. Internal control objectives in- elude protection functions like current-, torque-, and power limita tions, but also performance objectives requiring large control bandwidth. One example is traction control of electrical vehicles, where fast power control may be required to maximize adhesion during slippery road or track conditions. Limitations of active sta bilization may also be caused by physical limitations of power con trol, such as blocked converters. The application of the present invention on systems in track-bound vehicles is mainly discussed in this disclosure so as to illuminate the invention and the problems to be solved thereby without for that sake restricting the invention to that application.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a system of the type defined in the introduction being improved with respect to such systems already known by addressing the problems thereof discussed above.

This object is according to the invention obtained by providing such a system with the features listed in the characterizing part of appended patent claim 1 .

By an arrangement configured to sense at least one parameter associated with the system indicating whether the control unit is able to obtain said stabilization by adding a said first power com- ponent, it may be detected if any of the above situations where the modification of the control of the converter for obtaining said sta bilization has drawbacks occur and another stabilization procedure would be preferred. In such a case a said second consumer of electric power connected in parallel with the converter to the out- put of the input filter may be controlled to consume a second power component to one of a) assist the control of the converter to obtain said stabilization and b) alone take care of this stabilization when the assembly is unable to obtain this alone by adding a said first power component at the control of the converter. The shortcom- ings of the stabilization by using the control of the converter men tioned above may then be addressed by instead using the control of said second consumer to consume a said second power com ponent. However, it is pointed out that stabilization by controlling the converter will be used alone when this is possible, since such a control will result in a minimum or no additional power losses, but as soon as this converter control is not able to obtain said stabilization alone, i.e. in a favourable way, the stabilization aimed at may be obtained by using the option of controlling said second consumer to consume a second power component. According to an embodiment of the invention said second power consumer comprises a series connection of a resistance and a semiconductor switch connected to the DC intermediate link be tween the input filter and the converter, and said control member of the system is configured to control said second consumer to consume a said second power component by controlling the semi conductor switch so as to consume electric power by a current then flowing through the resistance, which is preferably realized by a resistor. Thus, a said second power component for obtaining said stabilization of the DC intermediate link voltage may then be con- sumed by heat generated in the resistance by the current flowing therethrough . Such a second consumer of electric power is in the case of a said system on board a track-bound vehicle already there and has not to be added, since it is there for acting as a so called brake chopper. Such a chopper enables controlled deenergizing of the energy stored in the DC intermediate link, such as in said input filter, allow protection at large DC intermediate link voltages and can be used to consume electric power in case of braking of the vehicle without any possibility to feed back power generated in said first consumer in the form of an electric machine to the DC- supply line. The latter possible use is the reason for calling such a consumer a brake chopper.

According to another embodiment of the invention said arrange ment is configured to sense the electric current flowing through said converter, and said control member of the system is config ured to control said second consumer to consume a said second power component to obtain said stabilization when the current sensed exceeds a predetermined value making a stabilization by adding a said first power component resulting in a possible ex ceeding of a minimal margin remaining to a maximum current al lowed for the converter. This means that no additional current mar- gins, and by that no overdimensioning of the system is required for obtaining the stabilization aimed at when the current is close to the maximum current allowed for the converter, since it will then be shifted to obtain said stabilization through the control of the second consumer instead.

According to another embodiment of the invention said arrange ment is configured to sense whether said converter is blocked or not, and said control member of the system is configured to control said second consumer to consume a second power component for obtaining said stabilization when the converter is blocked. Thus, damping of the system with blocked converter may be improved for obtaining said stabilization by controlling said second con sumer. According to another embodiment of the invention the system is configured to feed electric power to a first consumer in the form of an electric machine.

According to another embodiment of the invention said arrange- ment is configured to sense the torque generated by the electric machine, and said control member of the system is configured to control said second consumer to consume a said second power component to obtain said stabilization when the torque sensed ex ceeds a predetermined value making a stabilization by adding a said first component to resulting in a possible exceeding of a min imal margin remaining to a maximum torque allowed for the elec tric machine.

According to another embodiment of the invention such a system on board a track-bound vehicle is configured to sense the rotation speed of the electric machine, and said control member of the sys tem is configured to control said second consumer to consume a said second power component for obtaining said stabilization when the speed sensed is below a predetermined value. This means that limited power modification possibilities of the motor (electric machine) at low speeds of the electric machine can be compensated by using said second power consumer, such as a brake chopper, to improve damping of the system.

According to another embodiment of the invention the system is configured to feed electric power to a first consumer on board a track-bound vehicle, such as for propulsion of the vehicle or in an auxiliary power system of the vehicle.

According to another embodiment of the invention the system is configured to feed electric power to an electric machine on board a track-bound vehicle for propulsion of the vehicle, said arrange ment is configured to sense a parameter indicating the slippery degree of tracks upon which a said vehicle is moving , and said control member of the system is configured to control said second consumer to consume a second power component to obtain said stabilization when slippery tracks demanding a slip slide control of the vehicle are sensed. This means that the stabilization control, which is carried out by the use of said second consumer, such as a brake chopper, will not interfere with the torque regulation (slip slide control) necessary for obtaining efficient influence of the electric machine upon the wheels of the vehicle in slippery condi tions.

According to another embodiment of the invention said second consumer is a so called brake chopper controllable to consume electric power upon braking of the vehicle without possibility to feed electric power back to said DC energy source. It is preferred to use such a brake chopper as said second consumer, since it is in any case already there in a track-bound vehicle not adding any extra costs. The invention also relates to a method for feeding electric power according to the appended independent method claim. The fea tures and advantages of such a method and the embodiments thereof defined in dependent claims appear clearly from the above discussion of the system according to the invention.

The invention also relates to a driving arrangement for a track- bound vehicle as well as a track-bound vehicle according to the appended claims for such an arrangement and vehicle.

Further advantages as well as advantageous features of the in vention will appear from the description following below.

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the appended drawings, below follows a specific description of an embodiment of the invention cited as an example. In the drawings:

Fig. 1 illustrates very schematically a system for feeding electric power to an electric machine on a track- bound vehicle according to an embodiment of the invention,

Fig. 2 illustrates the principal upon which the present in vention is based, Fig. 3 illustrates the voltage stabilizing function of a said assembly in a system according to an embodi ment of the invention by changing the proportions of said first and second power components, and Fig. 4a-d are graphs showing simulation results of a stabi lization action carried out upon a variation of the DC intermediate link voltage in a system accord ing to the invention.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

The general construction of a system for feeding electric power to a first consumer of electric power in the form of an electric machine 1 on board a track-bound vehicle 2 for propulsion of the vehicle is schematically illustrated in Fig. 1 . The system has an input filter 6 with an input configured to be connected to a DC energy source in the form of a DC supply line 5. . The input filter is in the form of an LC filter 6, with an inductor 7 having an inductance 8 and a re sistance 9 and a capacitor 10. A DC intermediate link 3 is con nected to an output 14 of the input filter. A converter 12, here a voltage source inverter, has an input 13 connected to the DC in termediate link 3 and an output 15 through which the converter delivers AC electric power to the electric machine 1 . A control unit

16 is configured to control the converter by using Pulse Width Mod ulation to feed the AC electric power to the electric machine. The switching of the power semiconductor devices, such as IGBTs, of the converter will cause harmonics on the input side of the con verter suppressed by said filter 6. The control unit is configured to obtain feeding of electric power requested by the electric machine independently of variations of the voltage on the DC intermediate link. A second consumer of electric power in the form of a resistor

17 is connected in series with a semiconductor switch 18, such as an IGBT, in parallel with the capacitor 10 of the LC filter 6 to the output 14 of the filter. This constitutes a so-called brake chopper 19, through which power generated when braking the vehicle by the electric machine 1 then functioning as a generator can be con sumed when there is no possibility to feed the power generated back to the DC-supply line 5. A control member 20 of the system is configured to control the semiconductor switch 18 and by that said second consumer to consume electric power through current flowing through and heating the resistor 17. This control member 20 may be integrated in the control unit 16 as indicated in Fig. 1 or be formed by a separate unit.

The system has an assembly configured to act stabilizing on the voltage of the DC intermediate link upon a variation of this voltage. If, for instance, said voltage suddenly decreases that would in the case of controlling the converter to feed a constant power to the electric machine result in a sudden increase of the current to keep the power unaffected, which would lower the voltage further, and accordingly result in an instability of the DC intermediate link volt age. However, this behavior may be counteracted by then making the control unit 16 to control the converter 12 to add a first power component to the power to be fed to the electric machine, so that there will be no decrease of the current to the electric machine but a slightly short time increase of the power fed thereto. This means that efficient stabilization of the DC link voltage upon occurrence of a variation of this voltage is obtained without causing any addi tional power losses. However, in some situations it is not possible to efficiently carry out such a stabilization procedure. In such a case the control member 20 may instead control the brake chopper 19 to consume a second power component for obtaining said sta bilization of the DC intermediate link voltage upon occurrence of a variation of this voltage. The system comprises an arrangement configured to sense at least one parameter associated with the system indicating whether the control unit is able to obtain said stabilization by adding a said first component to the control of the converter. The control unit may then decide how the two possible stabilization procedures shall be combined. This is schematically illustrated in Fig. 2 showing 30 as the total power modification for stabilization in the form of the power component to be added to the power consumed by the electric machine for obtaining stabili zation. This may be done by a first power component 31 to be consumed by the electric machine and/or a second power compo nent 32 consumed by the brake chopper (second consumer). The upper and lower limits of a limitation block 33 are dynamically var ied with the operating point of the system to realize required sta bilization performance with minimal use of the brake chopper to minimize power losses. A limitation block 34 in the form of a block for“lower limitation to zero” is inserted, since the power reference to the brake chopper would be positive if negative deviations can not be realized by the brake chopper. However, the brake chopper control may be provided with an offset enabling a dynamic reduc tion of the power consumption of the chopper and thus obtaining of such negative deviations.

The system comprises an arrangement configured to sense at least one parameter associated with the system indicating whether the control unit 16 is able to obtain said stabilization by adding a said first component, and this arrangement is through an arrow indicated by 40 in Fig. 3 configured to sense the electric current flowing through the converter 12. The control member 20 is con- figured to control the second consumer, here the brake chopper 19, to consume a said second power component to obtain said stabilization when the current sensed exceeds a predetermined value making a stabilization by adding a said first component re sulting in a possible exceeding of a minimal margin remaining to a maximum current allowed for the converter. This means that the system has not to be overdimensioned to be able to obtain said stabilization by adding said first power components when control ling the converter which would require additional current margins for proper operation.

It is through an arrow 41 in Fig . 3 indicated that the arrangement is further configured to sense whether said converter 12 is blocked or not, and the control member 20 of the system is configured to control said second consumer, here the brake chopper 19, to con- sume a second power component for obtaining said stabilization when the converter is blocked, which results in an improved damp ing of the system.

It is by an arrow 42 in Fig . 3 indicated that the arrangement is further configured to sense the speed of the vehicle, and the con trol member 20 of the system is configured to control said second consumer, here the brake chopper 19, to consume a said second power component for obtaining said stabilization when the speed sensed is below a predetermined value. As mentioned before, lim ited power modification possibilities of the motor (electric machine 1 ) at low speeds can by this be compensated by using said second consumer to improve system damping.

It is in Fig. 3 an arrow 43 indicated how the arrangement is further configured to sense a parameter indicating the slippery degree of tracks upon which the vehicle is moving , and the control member 20 of the system is configured to control said second consumer, here the brake chopper 19, to consume a second power compo nent to obtain said stabilization when slippery tracks demanding a slip slide control of the vehicle are sensed. This means that the stabilization action is decoupled from the control of the power to the motor, so that this may be directed solely to meet the require ments of the slip slide control. 45 indicates the actual control to be carried out for obtaining said stabilization considering 40-43.

Fig. 4a-d are graphs illustrating how a desired power modification for obtaining a said stabilization may be obtained by combining the control of the converter (power motor control) and the control of the brake chopper (second consumer). Fig. 4a illustrates the DC intermediate link voltage U d versus time, and how this voltage is increased and decreased as a result of stepwisely changes of the DC supply voltage of the system. Fig. 4b is a graph of the power modification Pstab versus time requested for obtaining said stabi lization . Fig. 4c is a graph of the motor torque T of the electric machine 1 versus time showing how the desired power modifica tion is realized at the voltage decreases and at the first voltage increase by modifying the motor torque reference, whereas it is realized through the brake chopper at the second voltage increase as illustrated by Fig. 4d showing a graph of the actual brake chop per power P2, i.e. said second power component, versus time. The invention is of course in no way restricted to the embodiments described above, since many possibilities for modifications thereof are likely to be obvious to one skilled in the art without having to deviate from the scope of invention defined in the appended claims.

“Consumer of electric power” is to be interpreted as a member normally consuming electric power, but it is possible that such a member may also in some situations generate electric power, such as in the case of an electric machine on board a track-bound ve hicle when braking the vehicle. Said first and second power components may be positive or neg ative, which is also shown for the first power components added to the electric machine power control as shown in Fig. 4b upon a decrease of the DC intermediate link voltage. The system may of course feed electric power to a plurality of first consumers, such as electric machines in a railway vehicle.

A said arrangement may be configured to sense any number of parameters, also only one.

The input filter may have any other appearance than the LC filter shown in Fig. 1 .

“Sensing the speed” of the electric machine or vehicle shall not be interpreted as restricted to measuring the speed but also cover estimation thereof.