WO2007059985A1

WO2007059985A1 - Method for operating a power converter

Info

Publication number: WO2007059985A1
Application number: PCT/EP2006/011280
Authority: WO
Inventors: Waldemar Belwon; Johan Ulfstedt-Mattsson
Original assignee: Bombardier Transportation Gmbh
Priority date: 2005-11-24
Filing date: 2006-11-24
Publication date: 2007-05-31
Also published as: GB0523975D0; RU2396689C2; GB2432732A; EP1955432A1; RU2008125201A

Abstract

The invention relates to an arrangement for operating a converter, wherein the converter comprises a plurality of electronic valves (4) which are repeatedly switched on and off during operation of the converter in order to perform a current conversion of a direct current into an alternating current and/or vice versa. The arrangement comprises a plurality of valve control units (3), wherein each of the valve control units (3) is adapted to control the switching process of one of the electronic valves (4) . Furthermore, the arrangement comprises a converter control device which is connected to each of the plurality of valve control units (3) and which is adapted to control an operation of the plurality of valve control units (3), thereby operating the converter. At least one of the valve control units (3) comprises a voltage measurement device (14, 15, 16) which is adapted to measure a voltage across the valve (4) that is controlled by the valve control unit (3). The voltage measurement device (14, 15,16) is connected to the converter control device via a signal conversion unit (13) and via a signal line (7b).

Description

METHOD FOR OPERATING A POWER CONVERTER

The invention relates to a method and an arrangement of/for operating a converter, wherein the converter comprises a plurality of electronic valves which are repeatedly switched on and off during operation of the converter in order to perform a current conversion. The switching process of each of the electronic valves is controlled by a valve control unit which is directly connected to the electronic valve. In particular, the converter may be a DC (direct current) to AC (alternating current) converter which performs a conversion of a direct current into an alternating current and/or vice versa. More particularly, the invention relates to the field of converters for high-power applications such as providing electric energy to driving motors of railroad traction vehicles.

Converters of the type indicated before are well known in practice. For example, the converter may be a DC to AC converter, wherein the DC-side of the converter (first converter) is connected via a DC intermediate circuit to an AC to DC converter (second converter) which is connected to an AC power supply network. The AC-side of the first converter may be connected to a three-phase alternating current load, such as an asynchronous motor. A converter control device is provided which controls the operation of the electronic valves of the converter, thereby controlling the operation of the converter. There is a valve control unit for each of the valves and each of the valve control units is connected to the converter control device. For fast transfer of control signals, which are transferred from the converter control device to the valve control units, the control signals may be light pulse signals in an optical signal line, such as a glass fibre cable. Corresponding signal converters for conversion of analogue or digital signals into optical signals or vice versa are provided for the converter control device and for the valve control units. Such a converter usually requires information about the DC voltage on the DC-side of the converter. Formerly, resistive voltage devices were used to measure the DC voltage. In recent times, Hall Effect voltage measurement devices have been introduced. However, costs and effort for producing and mounting such measurement devices as well as for corresponding cabling are high. In addition, a Hall Effect measurement device requires power supply. Furthermore, redundant measurement devices may be necessary in order to guarantee the operation of the converter.

It is an object of the present invention to provide an arrangement as described before which can be produced and installed at reduced effort and costs. Preferably, the reliability and precision of the voltage measurement should be increased at the same time, compared to a single conventional voltage measurement device.

It is another object to provide a corresponding method of operating a converter.

According to a basic idea of the present invention, existing voltage measurement devices, which are integrated in the valve control units, are to be used to provide the converter control device with information about the DC voltage across the respective valve (i.e. the valve which is controlled by the valve control unit). The term "across" means the voltage between the two contacts or connections of the valve which are connected to each other when the valve is switched on and which are insulated against each other when the valve is switched off.

Preferably, the measurement results of a plurality of the valve control units are transferred to the converter control device. Consequently, there is redundant information and/or information for different switching states of the converter (a switching state of the converter is defined by the respective switching states of the valves). For example, the converter comprises three branches, wherein each of the branches comprises a series connection of two of the valves, wherein each of the branches is connected to the DC voltage and wherein a contact of each of the branches, in between the two valves, is connected to an AC-side of the converter. In this case, each of the valve control units can measure the DC voltage if the corresponding controlled valve is in its OFF state and if the other valve of the same branch is in the ON state. Thus, it is preferred to measure the DC voltage when the corresponding valve is in its OFF state, in particular shortly before the valve is switched on. For example, the measurement may be triggered depending on the phase and frequency of a periodic switching cycle of the valve, wherein it is taking into account that the lengths of the OFF state periods varies. Alternatively, the measurement might be performed continuously or quasi-continuously (i.e. repeatedly after short intervals) and non- plausible values of the voltage measurement are excluded from transfer to the converter control device or excluded from being used by the converter control device. For example, the converter control device might process the measurement values in digital form so that a plausibility check may easily be performed by software.

In practice, the valves may be IGBTs (Insulated Gate Bipolar Transistors). However, any other valve which can be controlled by a valve control unit may be used as well in connection with the invention.

The invention comprises the following advantages:

- Since the precision of voltage measurement performed by the valve control units is high and since, according to the preferred embodiment, measurement values of different valve control units are used by the converter control device, the process of switching on and off the valves can be performed with higher confidence that the measurement result is correct. Thus the timing of the switching process can be optimised, thereby reducing switching losses and increasing safety.

- Additional measurement devices, such as a Hall effect measurement device, can be omitted. Corresponding cabling and installation effort can be omitted as well. The power supply system of the converter control arrangement can be simpler and smaller dimensioned. The weight can also be reduced.

- If the measurement results of different valve control units are used, the reliability of the measurement is high.

Compared to conventional arrangements, minor amendments to the hardware might be necessary. In particular, the measurement signal is to be transferred from the valve control unit to the converter control device. However, since there are usually signal lines for transferring signals from the valve control units to the converter control device, the additional effort is low. The corresponding insulation requirements regarding the electrical insulation between the measurement device in the valve control unit on one hand and the signal line on the other hand are fulfilled by existing solutions. Thus, the additional costs for using the voltage measurement in the valve control unit compared to a separate, additional measurement device are low.

Furthermore, a method of operating a converter is proposed, wherein the converter comprises a plurality of electronic valves which are repeatedly switched on and off during operation of the converter in order to perform a current conversion. For example, the conversion may be a conversion of a direct current into an alternating current and/or vice versa. The switching process of each of the electronic valves is controlled by a valve control unit which is directly connected to the electronic valve. An operation of the plurality of valve control units is controlled by a converter control device. The method includes the following: performing a voltage measurement by at least one of the plurality of valve control units, wherein a voltage across the valve is measured in the voltage measurement while the valve is off; and transferring at least one measured value which has been obtained by the voltage measurement from the valve control unit to the converter control device

Preferably, the converter control device processes the at least one measured value in a process of controlling the operation of the plurality of valve control units. According to the best mode of the invention, measured values which have been obtained by and transferred from different valve control units (in particular from all of the valve control units of the converter) to the converter control device are processed by the converter control device. The measurement value(s) may be used for calculating the timing of switching on and off the valves of the converter. For example, a model which describes the operation of the load that is connected to the converter may be used for the calculation. The voltage across the valve in the OFF state is an important input variable to the model, besides other input quantities, such as measured currents in the connections between the converter and the load.

In particular for the purpose of a continuous operation, the converter control device may repeatedly receive measured values from the at least one valve control unit and may use at least some of the received values for controlling the operation of the plurality of valve control units.

For example, the converter control device can handle the values measured by different valve control units in the following manner:

When the operation of the converter control device has been started it issues commands, for example to a first and a second valve which are connected in series to each other in order to provide the current of the same phase, so that these two valves are never in their ON state at the same time. This is the usual way of controlling a PWM (Pulse Width Modulation) power converter. This means that only one valve is in the OFF state during most of the time, except for short periods when both valves are OFF. These periods are provided for safety reasons, since there will be short circuit if both valves are ON. These short periods of time are called "dead time".

As a result, the converter control device can be provided with measured values nearly continuously (or repeatedly according to a sampling rate of the measurement and/or of the signal transfer from the valve control unit to the converter control device) during the whole operation. Except for the dead times, one of the valves of each branch will provide the converter control device with measured values about the voltage across the branch. Optionally, the dead times may be extended by short phases during which the respective other valve of the same branch is switched on, so that the measured values are taken and/or are transferred and/or sampled by the converter control device only outside of these extended dead times.

The sampling rate of the voltage measurement across the valve performed by valve control units is high compared to transient behaviour of the voltage to be measured (i.e. how fast the DC-voltage can change over time). Therefore, a good mean value can be obtained from the values which are taken over a single switching cycle (e.g. switch on first valve - switch off first valve - switch on second valve - switch off second valve).

In systems which comprise three phases (and therefore three branches) the phase signals comprise a phase shift of 120 degrees. This means that the converter control device can generate a mean value of the voltage across the valves based on values obtained from all six valves and/or a by comparison of mean values based on voltage measurements from each phase. For example, the converter control device may be adapted to exclude the mean value from the branch, wherein this excluded mean value differs and/or fluctuates most of all three branches.

The mean value obtained or another measured value may be used in a manner by the converter control device for controlling the arrangement. Ways of using and processing the information about the voltage are known in the art.

The at least one valve control unit may produce an analogue signal which represents the measured value and the analogue signal may be converted into a digital signal and/or into a pulse signal (for example: a light pulse signal) which is transferred to the converter control device. An existing type of measurement device in the valve control unit comprises a resistive voltage divider and, in addition, a capacitive voltage divider (divider arrangement). This type of measurement device can be used as well for the measurement values which are to be transferred to the converter control device. The analogue signal produced by the divider arrangement may be a voltage signal which corresponds to the voltage to the measured. This voltage signal may be compared with a reference signal. One way to produce the reference signal and to measure the collector emitter voltage is to linearly increase the reference voltage and to measure the time needed for the reference voltage until it is equal to the voltage to be measured. A clock of the valve control unit can be used for this purpose.

Furthermore, the invention includes an arrangement for operating a converter, wherein the converter comprises a plurality of electronic valves which are repeatedly switched on and off during operation of the converter in order to perform a current conversion. For example, the conversion may be a conversion of a direct current into an alternating current and/or vice versa. The arrangement comprises the following:

- a plurality of valve control units, wherein each of the valve control units is adapted to control the switching process of one of the electronic valves;

- a converter control device which is connected to each of the plurality of valve control units and which is adapted to control an operation of the plurality of valve control units, thereby operating the converter; wherein at least one of the valve control units comprises a voltage measurement device which is adapted to measure a voltage across the valve that is controlled by the valve control unit and wherein the voltage measurement device is connected to the converter control device.

According to the best mode of the arrangement, each of the valve control units comprises one exemplar of the voltage measurement device.

The at least one valve control unit may comprise a signal conversion unit which is adapted to convert an analogue signal of the measurement device into a digital signal and/or into a pulse signal to be transferred to the converter control device.

Preferably, the converter control device comprises a processing unit which is connected to the at least one valve control unit and which is adapted to process measured values received from the at least one valve control unit in a process of controlling the operation of the plurality of valve control units.

Advantages and further embodiments of the arrangement are described above with reference to the method according to the present invention.

An example of the present invention (which corresponds to the best mode of the invention) will be described in the following with reference to the accompanying drawing. The figures of the drawing show:

Fig. 1 schematically an arrangement 1 comprising a DC to AC converter and a converter control device, Fig. 2 schematically one of the electronic valves shown in Fig. 1 and details of the corresponding valve control unit and Fig. 3 details of an arrangement for measuring the voltage.

The DC to AC converter shown in Fig. 1 comprises six electronic valves 4a to 4f, which are IGBT's in the example. Each of the valves 4a to 4f is combined with a valve control unit 3a to 3f which is connected to the gate of the respective valve 4a to 4f. The six valves 4a to 4f are arranged in three branches, wherein each of the branches comprises two of the valves 4a, 4b; 4c, 4d; 4e, 4f, which are connected in series to each other, so that each of the branches connects the electric lines 2a, 2b of a DC intermediate circuit. A contact (indicated by a small circle) in between the two valves of each branch is connected to one or three AC lines 6a, 6b, 6c.

A converter control device for controlling the operation of the valve control units 3a to 3f is connected to the valve control units 3a to 3f via signal lines 7a to 7f. During operation of the converter, the converter control device 5 outputs pulse signals to the valve control units 3a to 3f, in particular light pulse signals. For example, the respective valve control unit 3 is triggered to switch on the assigned valve 4, if the beginning of a light pulse is received by the valve control unit 3. If the light pulse terminates, the valve control unit 3 is triggered to switch off the assigned valve 4.

According to the present invention, each of the signal lines 7a to 7f is also used for feedback of signals from the valve control units 3 to the converter control device 5, including signals which represent measurement values of the measured voltage across the respective valve. Each of the signal lines 7a to 7f comprises a plurality of signal connections, for example a plurality of glass fibre cables.

The converter control device 5 comprises a processing unit 8 which is connected to the valve control units 3a to 3f via the signal lines 7a to 7f. For simplicity, only the connection of signal line 7b to the processing unit 8 is shown in Fig. 1 and Fig. 2. The processing unit 8 processes the measured voltage values and calculates the timing of switching on and off the valves 4.

Fig. 2 shows a preferred embodiment of the valve control units 3a to 3f, for example valve control unit 3b. For clarity reasons, Fig. 2 only shows components of the valve control unit 3b which are used for voltage measurement. Other components, such as components for driving a current to or from the gate of IGBT 4b, are omitted.

The valve control unit 3b comprises a resistive voltage divider, which consists of a series connection of two resisters 15, 16. The series connection is connected at one end to a point in the branch of IGBTs 4a, 4b, namely in between the IGBTs 4a, 4b. The other end of the series connection is connected to the side of the branch which is connected to DC line 2a.

There is a capacitive voltage divider connected in parallel to the resistive voltage divider. The capacity voltage divider comprises a first capacitor 18 and a second capacitor 19. The first capacitor 18 is connected in parallel to the resistor 15. The second capacitor 19 is connected in parallel to the resistor 16. The resistive voltage divider and the capacitive voltage divider form a divider arrangement.

The divider arrangement divides the voltage across IGBT 4b, i.e. the voltage across the collector C on one side and the emitter E on the other side, so that a significantly smaller voltage than the voltage across the IGBT 4b can be measured. For example, the ratio of the voltage divider may be in the range of 500:1 to 4000:1 , e.g. 2000:1.

In the example shown in Fig. 2, the divided voltage across resistor 16 and across capacitor 19, i.e. between contacts A₁ B, is fed to a comparator 14. This comparator compares the voltage signal in line 12 (which connects contact A with one input of comparator 14) with a reference signal in line 17 which is connected to a second input of comparator 14. The reference signal may be, for example, a voltage signal in the range of 0 to 10 volt, in particular 0 to 5 volt, depending on the voltage to be measured. A respective comparison signal is available at the output of comparator 14 and is transferred to a signal converter, which may be part of a digital processing unit 13 of the valve control unit 3b. An output of the signal converter is connected to signal line 7b.

Figure 3 shows details of a preferred embodiment for measuring the voltage. Same reference numerals refer to the same elements or devices as described in connection with figure 2. The comparator 14 is connected to a first electrode (drain) of a transistor 21 , for example a MOSFET, via a resistor 23. The connection from the comparator 14 to the resistor 23 is made by line 17. The drain electrode of transistor 21 is as well connected to a constant current source 24. The input of the current source 24 is connected to a power source (e.g. the plus pole of the power source). A second electrode (source) of the transistor 21 is connected to contact B (which might be connected to the negative pole of the power source, or to ground potential). Furthermore, the control electrode (gate) of the transistor 21 is connected with the digital processing unit 13. In addition, line 17 is connected to contact B via a capacitor 22.

The voltage measurement is performed when the transistor 4b is in its OFF state. For example, the measurement can repeatedly be performed during the OFF state. According to a preferred embodiment, the digital processing unit 13 detects or determines that the transistor 4b is in the OFF state and a counter device 20 of the digital processing unit 13 outputs a signal to the control electrode of the transistor 21 so that the transistor 21 is switched off. As a result, the constant current source 24 starts charging the capacitor 22 via the resistor 23 instead of passing through drain-source of transistor 21. At the same time, the counter device 20 starts counting the periodic clocks signals of a clock (not shown in the figure).

Due to the constant current produced by the constant current source 24, the reference voltage across capacitor 22 rises linearly with time. Consequently, the time which is measured by counting the clock signals is proportional to the reference voltage. When the reference voltage reaches the voltage to be measured, the comparator 14 outputs a signal to the counter device 20 and the counting process is stopped. The counted time is a measure for the collector emitter voltage. For calibration, a calibration signal (voltage) may be provided to contact A (figure 2).

Optionally, a digital filter might be implemented in digital processing unit 13, so that the results of repeated measurements can be filtered and so that a corresponding filtered value is output to the converter control device via line 7b.

Since each of the six valve control units 3a to 3f transfers measurement values to the converter control device 5, multiple redundant measurement values can be used by the converter control device 5 for controlling the operation of the converter. Thus, the reliability of the measurement is high and the costs for obtaining and transferring the measurement values to the converter control device 5 are low.

Claims

1. A method of operating a converter (3, 4), wherein the converter (3, 4) comprises a plurality of electronic valves (4) which are repeatedly switched on and off during operation of the converter (3, 4) in order to perform a current conversion, in particular a conversion of a direct current into an alternating current and/or vice versa, wherein the switching process of each of the electronic valves (4) is controlled by a valve control unit (3) which is directly connected to the electronic valve (4), wherein an operation of the plurality of valve control units (3) is controlled by a converter control device (5) and wherein the method includes the following: performing a voltage measurement by at least one of the plurality of valve control units (3), wherein a voltage across the valve (4) is measured in the voltage measurement while the valve (4) is off; and transferring at least one measured value which has been obtained by the voltage measurement from the valve control unit (3) to the converter control device (5)

2. The method of the preceding claim, wherein the converter control device (5) processes the at least one measured value in a process of controlling the operation of the plurality of valve control units (3).

3. The method of the preceding claim, wherein the converter control device (5) repeatedly receives measured values from the at least one valve control unit (3) and uses at least some of the received values for controlling the operation of the plurality of valve control units (3).

4. The method of one of the two preceding claims, wherein the measured values which have been transferred from different valve control units (3) to the converter control device (5) are processed by the converter control device (5) in the process of controlling the operation of the plurality of valve control units (3).

5. The method of the preceding claim, wherein at least some of the measured values are taken in the same switching state of the converter (3, 4) or in the same switching state of a plurality of the valves (4) of the converter (3, 4).

6. The method of one of the preceding claims, wherein the voltage measurement is performed by each of the plurality of valve control units (3) and at least one measured value is transferred from each of the valve control units (3) to the converter control device (5).

7. The method of one of the preceding claims, wherein the at least one valve control unit (3) produces an analogue signal which represents the measured value and wherein the analogue signal is converted into a digital signal and/or into a pulse signal which is transferred to the converter control device (5).

8. The method of the preceding claim, wherein the pulse signal is a light pulse signal.

9. An arrangement (1 ) for operating a converter, wherein the converter (3, 4) comprises a plurality of electronic valves (4) which are repeatedly switched on and off during operation of the converter (3, 4) in order to perform a current conversion, in particular a conversion of a direct current into an alternating current and/or vice versa, wherein the arrangement (1 ) comprises the following: a plurality of valve control units (3), wherein each of the valve control units (3) is adapted to control the switching process of one of the electronic valves (4); a converter control device (5) which is connected to each of the plurality of valve control units (3) and which is adapted to control an operation of the plurality of valve control units (3), thereby operating the converter (3, 4); wherein at least one of the valve control units (3) comprises a voltage measurement device (14, 15, 16) which is adapted to measure a voltage across the valve (4) that is controlled by the valve control unit (3) and wherein the voltage measurement device (14, 15, 16) is connected to the converter control device (5).

10. The arrangement of the preceding claim, wherein each of the valve control units (3) comprises the voltage measurement device (14, 15, 16).

11. The arrangement of one of the preceding claims, wherein the at least one valve control unit (3) comprises a signal conversion unit (13) which is adapted to convert an analogue signal of the measurement device (14, 15, 16) into a digital signal and/or pulse signal to be transferred to the converter control device (5).

12. The arrangement of one of the preceding claims, wherein the converter control device (5) comprises a processing unit (8) which is connected to the at least one valve control unit (3) and which is adapted to process measured values received from the at least one valve control unit (3) in a process of controlling the operation of the plurality of valve control units (3).