A METHOD FOR CONTROLLING A LINE CONVERTER ON BOARD A TRACK-BOUND VEHICLE

Abstract

In a method for controlling a line converter on board a track-bound vehicle semiconductor devices of current valves of the line converter are controlled to be turned on and off so as to prevent the current (I) through a secondary winding of a transformer to which midpoints of phase-legs of the converter are connected to pass zero and shift direction other when the voltage across the secondary winding shifts direction by a start of a new half period of an AC line voltage across the windings of the transformer.

Claims

1. A method for controlling a line converter (12) on board a track-bound vehicle (1), said converter having at least one bridge with two phase-legs (20, 21) connected in parallel between opposite poles (22, 23) of a DC intermediate link (60) and having each at least two current valves (24-27) connected in series, each said current valve comprising a semiconductor device (28-31) of turn-off type and a rectifying member (32-35) connected in anti-parallel therewith, a midpoint (36, 37) of each phase-leg dividing the phase-leg in two identical halves being connected to an opposite side of a secondary winding (4) of a transformer with respect to a said midpoint of the other phase-leg, said transformer having a primary winding (40) connected to an AC supply line (2) for track-bound vehicles, comprising controlling said semiconductor devices (28-31) to be turned on and off to prevent the current (I) through said secondary winding (4) of the transformer to pass zero and shift direction other than when the voltage across said primary winding (40) shifts direction by a start of a new half period of the AC line voltage across the windings of the transformer (3).

2. The method according to claim 1, comprising a first control scheme according to which said semiconductor devices (28-31) are controlled so that if a current through the secondary winding (4) of the transformer having a certain direction reaches zero, it will remain zero until starting to flow in the same certain direction again during one and the same half period of said AC line voltage and accordingly be discontinuous.

3. The method according to claim 2, wherein the control of said semiconductor devices (28-31) of the line converter (12) is carried out according to said first control scheme only when the electric power transferred from the AC supply line (2) to said vehicle (1) through the transformer (3) is below a predetermined level, such as 30% or 20% of the maximum electric power transferable to said vehicle.

4. The method according to claim 2, wherein the speed of said vehicle (1) is measured and said first control scheme is used for controlling the semiconductor devices (28-31) of the line converter (12) only when the speed of the vehicle is below a predetermined level, such as 20 km/h, 10 km/h or 5 km/h.

5. The method according to claim 2, wherein the time (T1) the semiconductor device (28-31) of a current valve (24-27) is kept conducting once turned on for said first control scheme is calculated by using the following formula when electric power is fed from the AC supply line by the line converter to the DC intermediate link $T_1=\sqrt{\frac{2.Math.T_p.Math.L.Math.\left(U_d-u\right)}{U_d.Math.u}}.Math.\sqrt{I_{\mathrm{ref}}}$ in which T1=time of conducting of semiconductor device Tp=period time of switching Ud=DC intermediate link voltage u=AC line voltage transformed to the secondary side, absolute value Iref=current reference absolute value L=inductance of transformer.

6. The method according to claim 2, wherein the time (T1) the semiconductor device (28-31) of a current valve (24-27) is kept conducting once turned on for said first control scheme is calculated by using the following formula when electric power is fed from the DC intermediate link by the line converter to the AC supply line $T_1=\sqrt{\frac{2.Math.T_p.Math.L.Math.u}{U_d.Math.\left(U_d-u\right)}}.Math.\sqrt{I_{\mathrm{ref}}}$ in which T1=time of conducting of semiconductor device Tp=period time of switching Ud=DC intermediate link voltage u=AC line voltage transformed to the secondary side, absolute value Iref=current reference absolute value L=inductance of transformer.

7. The method according to claim 2, comprising a second control scheme in the form of a normal Pulse Width Modulation including a continuous current flowing through said secondary winding (4) of the transformer (3), and shifting from said first control scheme to said second control scheme when the electric power transferred from the AC supply line (2) to said vehicle (1) exceeds a certain percentage of the maximum electric power transferable from said AC supply line to the vehicle, such as 20% or 30% thereof.

8. The method according to claim 7, wherein the time of conducting (T1) of the semiconductor device (28-31) of each current valve (24-27) for said second control scheme is calculated by using the following formula when electric power is fed from the AC supply line by the line converter to the DC intermediate link: $T_1=\frac{L}{U_d}.Math.\left(I_{\mathrm{ref}}-I\right)-\frac{T_p}{U_d}.Math.u+T_p$ in which T1=time of conducting of semiconductor device Tp=period time of switching Ud=DC intermediate link voltage Iref=current reference absolute value I=current absolute value L=inductance of transformer u=AC line voltage transformed to the secondary side, absolute value.

9. The method according to claim 7, wherein the time of conducting (T1) of the semiconductor device (28-31) of each current valve (24-27) for said second control scheme is calculated by using the following formula when electric power is fed from the DC intermediate link by the line converter to the AC supply line: $T_1=\frac{L}{U_d}.Math.\left(I_{\mathrm{ref}}-I\right)+\frac{T_p}{U_d}.Math.u$ in which T1=time of conducting of semiconductor device Tp=period time of switching Ud=DC intermediate link voltage Iref=current reference absolute value I=current absolute value L=inductance of transformer u=AC line voltage transformed to the secondary side, absolute value.

10. The method according to claim 8, wherein (T1), is calculated according to the first and second control scheme in parallel, the two values of (T1) so obtained are compared and the control scheme associated with the formula resulting in the lowest value of (T1) is selected for the control of the semiconductor device (28-31) of each current valve (24-27).

11. The method according to claim 2, wherein during said first control scheme only a semiconductor device (28-31) of one current valve (24-27) of each phase-leg is turned on at a time.

12. The method according to claim 2, wherein when electric power is fed from the AC supply line through said line converter to said DC intermediate link, said first control scheme is carried out by keeping all but one of the semiconductor devices (28-31) turned off and during a first half period of the AC line voltage turning on and off, only either in a first (20) of said phase-legs the semiconductor device (28) of the current valve (24) connected to a first (22) of said poles of said DC intermediate link (60) or in a second (21) of said phase-legs the semiconductor device (31) of the current valve (27) connected to a second (23) of said poles and correspondingly during the other half period of the AC line voltage, turning on and off only either in said first (20) phase-leg the semiconductor device (29) connected to said second pole (23) or in said second (21) phase-leg the semiconductor device (30) connected to said first pole (22).

13. The method according to claim 2, wherein when electric power is fed from the AC supply line through said line converter to said DC intermediate link, said first control scheme is carried out by either turning on the semiconductor devices (28, 30) of the two current valves (24, 26) connected to a first (22) of said poles of the DC intermediate link (60) to be conducting during the same pulse period or turning on the semiconductor devices (29, 31) of the two current valves (25, 27) connected to a second pole (23) of said DC intermediate link (60) to be conducting during the same pulse period.

14. The method according to claim 13, wherein the semiconductor devices of the current valves connected to said first pole (22) and said second pole (23) of the DC intermediate link are alternately turned on.

15. The method according to claim 2, wherein when electric power is fed from said DC intermediate link through said line converter to the AC supply line, the first control scheme is carried out by, during a first half period of the AC line voltage, in a first (20) of said phase-legs keeping a semiconductor device (28) connected to a first (22) of said poles of said DC intermediate link (60) continuously turned on and in a second (21) of said phase-legs turning on and off the semiconductor device (31) connected to a second of said poles (23) of the DC intermediate link (60) to conduct in pulses and, correspondingly during the next half period of the AC supply line voltage, in said second phase-leg (21) keeping the semiconductor device (30) of the current valve (26) connected to said first pole (22) continuously turned on and in said first phase-leg (20) turning on and off the semiconductor device (29) connected to said second pole (23) to conduct in pulses.

16. A computer program comprising computer program code for bringing a computer to implement a method according to claim 1 when the computer program code is executed in the computer.

17. A computer program product comprising a data storing medium readable by a computer, in which the computer program code of a computer program according to claim 16 is stored on the data storing medium.

18. An electronic control unit (12) of a track-bound vehicle comprising an execution means (50), a memory (51) connected to the execution means and a data storing medium (53) connected to the execution means (50), in which the computer program code of a computer program according to claim 16 is stored on said data storing medium (53).

19. A track-bound vehicle comprising an electronic control unit (12) according to claim 18.

20. The method according to claim 3, wherein the speed of said vehicle (1) is measured and said first control scheme is used for controlling the semiconductor devices (28-31) of the line converter (12) only when the speed of the vehicle is below a predetermined level, such as 20 km/h, 10 km/h or 5 km/h.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0059] With reference to the appended drawings, below follows a specific description of embodiments of the invention cited as examples.

[0060] In the drawings:

[0061] FIG. 1 is a very schematic view illustrating an electric system in which a line converter which may be controlled through a method according to the present invention is included,

[0062] FIG. 2 is a schematic view of a said line converter,

[0063] FIGS. 3a and 3b are graphs of the current versus time and current harmonics resulting therefrom versus frequency during a time period of the fundamental voltage when controlling a line converter as shown in FIG. 2 through a prior art Pulse Width Modulation scheme and at a comparatively high fundamental current value,

[0064] FIGS. 4a and 4b are graphs corresponding to those of FIGS. 3a and 3b resulting when controlling the line converter through a method according to an embodiment of the present invention but at a comparatively low fundamental current value,

[0065] FIGS. 5a and 5b illustrate the control of the current valves of the line converter in FIG. 2 when controlling the line converter according to a said first control scheme and when electric power is fed from the AC supply line by the line converter to the DC intermediate link, i. e. in normal motor operation, for a positive half period of voltage and current, and FIG. 5c illustrates the discontinuous current resulting from this control,

[0066] FIGS. 6a, 6b and 6c correspond to FIGS. 5a, 5b and 5c but for a negative half period of the current,

[0067] FIGS. 7a-7c and 8a-8c correspond to the FIGS. 5a-5c and 6a-6c, respectively, when electric power is fed from the DC intermediate link by the line converter to the AC supply line, i. e. for generator operation,

[0068] FIG. 9 is a graph of the current through the secondary winding of the transformer of the line converter in FIG. 2 during a time period of the voltage of the AC supply line when carrying out a control according to a simplified embodiment of said first control scheme, and

[0069] FIG. 10 is a schematic view illustrating an electronic control unit for implementing a method according to the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0070] We assume that the line converter shown in FIG. 2 is controlled through the control unit 16 by applying a first control scheme comprising an algorithm for calculating the time T.sub.1 the semiconductor device of a current valve is kept conducting once turned on, and that T.sub.1 is calculated according to 1) when electric power is fed from the AC supply line by the line converter to the DC intermediate link and according to 2) when electric power is fed from the DC intermediate link by the line converter to the AC supply line:

[00005]$\begin{array}{cc}{T}_1=\sqrt{\frac{2.Math.T_p.Math.L.Math.\left(U_d-u\right)}{U_d.Math.u}}.Math.\sqrt{I_{\mathrm{ref}}}& 1)\\ T_1=\sqrt{\frac{2.Math.T_p.Math.L.Math.u}{U_d.Math.\left(U_d-u\right)}}.Math.\sqrt{I_{\mathrm{ref}}}& 2)\end{array}$ [0071] in which [0072] T.sub.1=time of conducting of semiconductor device [0073] T.sub.p=period time of switching [0074] U.sub.d=DC intermediate link voltage [0075] u=AC line voltage absolute value [0076] I.sub.ref=current reference absolute value [0077] L=inductance of transformer

[0078] The current will then in the case of a low load during one half period be discontinuous and positive and during the next half period discontinuous and negative as shown in FIG. 4a. The current harmonics shown in FIG. 4b will then result, and it is seen that the disturbing harmonics around 1800 Hz are substantially weaker than in the case of a Pulse Width Modulation control as shown in FIG. 3b. The lower the current the higher the degree of reduction of said harmonics when using the first control scheme according to the present invention.

[0079] FIGS. 5a-5c relate to the use of said first control scheme for motor operation and the case of a positive half period. FIGS. 5a and 5b show the control orders to the semiconductors of a first and a second phase leg, respectively. A control order=0, i. e. with a value on the time axis, means that none of the semiconductors is turned on. A positive control order, i. e. with a value above the time axis, means that the semiconductor connected to the positive pole of the intermediate link is turned on and the other semiconductor is off. A negative control order, i. e. with a value below the time axis, means that the semiconductor connected to the negative pole of the intermediate link is turned on and the other semiconductor is off. It is seen that only the lower semiconductor device 29 of the first phase-leg 20 and the upper semiconductor device 30 of the second phase-leg 21 are turned on, which results in a prevention of the current I (FIG. 5c) to be negative.

[0080] FIGS. 6a-6c show the control during the negative half period for motor operation, in which only the upper semiconductor device 28 of the first phase-leg 20 and the lower semiconductor device 31 of the second phase-leg 21 are turned on.

[0081] FIGS. 8a and 8b illustrate how the first control scheme is carried out for generator operation of the line converter according to FIG. 2 during a negative half period of the voltage. The lower semiconductor device 29 of the first phase-leg 20 is kept continuously turned on and the upper semiconductor device 30 of the second phase-leg 21 is alternately turned on and turned off. This prevents the current I (see FIG. 8c) from turning negative, so it will be discontinuous in the case of a low load current.

[0082] FIGS. 7a-7c are used corresponding to FIGS. 8a-8c for generator operation during a positive half period of the voltage, in which the lower semiconductor device 31 of the second phase-leg 21 is continuously turned-on and the upper semiconductor device 28 of the first phase-leg 20 is alternately turned on and turned off, which results in a prevention of the current I through the secondary winding of the transformer to be positive, so that it will be discontinuous in the case of a low load current.

[0083] FIG. 9 illustrates a development of the current through the secondary winding of the transformer through which the line converter according to FIG. 2 is connected when controlling the line converter according to a simplified embodiment of said first control scheme at low current operation, in which alternately the two upper semiconductor devices 28, 30 of the two phase-legs and the two lower semiconductor devices 29, 31 of the two phase-legs are turned on and off. This control scheme may only be carried out when the current is controlled in phase with the voltage, i.e. during motor operation. It will then not be necessary to know when the voltage passes zero and a new half period begins for obtaining a discontinuous current for low current operation of the line converter as illustrated for the current I in FIG. 9.

[0084] Computer program code for implementing a method according to the invention is with advantage included in a computer program which can be read into the internal memory of a computer, e.g. the internal memory of an electronic control unit of a track-bound vehicle. Such a computer program is with advantage provided via a computer program product comprising a data storage medium which can be read by a computer and which has the computer program stored on it. FIG. 10 illustrates very schematically an electronic control unit 16 comprising an execution means 50, e.g. a central processor unit (CPU) for execution of computer software. The execution means 50 communicates with the memory 51, e.g. of the RAM type, via a data buss 52. The control unit 16 comprises also a non-transitory data storage medium 53, e.g. in the form of a flash memory or a memory of the ROM, PROM, EPROM or EEPROM type. The execution means 50 communicates with the data storage medium 53 via the data buss 52. The computer program comprises computer program code for implementing a method according to the invention, e.g. in accordance with the embodiments disclosed above is stored on the data storage medium 53.

[0085] 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.

[0086] The method according to the invention may be applied to a line converter having more than one bridge, such as for example six bridges connected to one and the same transformer. Each current valve of the converter may have a plurality of semiconductor devices connected in series and controlled simultaneously as one single semiconductor device. The rectifying members shown in the line converter above may also stand for a plurality of rectifying members connected in series.