Power electronic converter and a method for controlling it

A power electronic converter comprises controllable switches (104-109) and a control system (110) for operating the controllable switches. The control system is configured to control the controllable switches so as to produce at least one test voltage pulse. The power electronic converter comprises a voltage sensor (111) for detecting a reflected voltage that is caused by a reflection of the test voltage pulse in an electric system connected to the power electronic converter. The control system is configured to control the operation of the power electronic converter at least partly in accordance with information based on the detection of the reflected voltage. The information can be used for example for determining forbidden voltage pulse lengths which would cause reflection-based over-voltages and/or for detecting faults such as short circuits and earth faults.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Stage application of International Patent Application No. PCT/EP2016/054076, filed on Feb. 26, 2016, which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

The disclosure relates to a method for controlling a power electronic converter. Furthermore, the disclosure relates to a power electronic converter and to an electric drive. Furthermore, the disclosure relates to a computer program for controlling a power electronic converter.

BACKGROUND

In many cases, there can be a need for a cable between a power electronic converter and an electric machine so that the length of the cable can be as much as e.g. 300 meters or even more. The power electronic converter can be for example a frequency converter and the electric machine can be for example an alternating current motor and/or generator or a transformer. The reason for the need for the long cable can be for example a lack of room for the power electronic converter in the vicinity of the electric machine and/or ambient conditions unsuitable for the power electronic converter in the vicinity of the electric machine.

An inherent inconvenience related to a long cable between a power electronic converter and an electric machine is that the control of the electric machine is more challenging than in a case where a power electronic converter in the vicinity of an electric machine. The control is more challenging because finiteness of the signal propagation speed on the cable and reflections may cause situations where instantaneous voltages and currents at the power electronic converter differ from the corresponding instantaneous voltages and currents at the electric machine. A further inconvenience related to a long cable is that voltage reflections may cause over-voltages at the electric machine. In a disadvantageous case, the reflection-based over-voltages may damage the insulations of the electric machine. A traditional approach for reducing the reflection-based over-voltages is to set a lower limit to the temporal widths of voltage pulses produced by the power electronic converter. An inconvenience related to this approach is that the lower limit has to be sufficiently long, i.e. to include a sufficient safety margin, in order to achieve an acceptable operation in different situations. Therefore, in some cases, the application of the lower limit may unnecessarily degrade the quality of the control.

SUMMARY

In accordance with the present invention, there is provided a new power electronic converter that can be for example a frequency converter. A power electronic converter according to the invention comprises:electric terminals for connecting to an external electric system,controllable switches connected to the electric terminals,a control system for operating the controllable switches so as to control voltages of the electric terminals, anda voltage sensor connected to the electric terminals.

The control system is configured to:control one or more of the controllable switches so as to produce at least one test voltage pulse at the electric terminals,receive, from the voltage sensor, a sensor signal indicative of a reflected voltage detected from the electric terminals, the reflected voltage arriving from the external electric system and being caused by a reflection of the test voltage pulse, andcontrol the operation of the power electronic converter at least partly in accordance with information based on the sensor signal.

In a power electronic converter according to an exemplifying and non-limiting embodiment of the invention, the information based on the sensor signal is used for determining allowed voltage pulse widths in order to avoid too high reflection-based over-voltages. In a power electronic converter according to an exemplifying and non-limiting embodiment of the invention, the above-mentioned information is used for detecting faults such as e.g. short circuits and earth faults. In a power electronic converter according to an exemplifying and non-limiting embodiment of the invention, the above-mentioned information is used for computing an estimate for the distance from the power electronic converter to a detected fault.

An electric drive according to the invention comprises:an electric machine,a power electronic converter according to the invention, anda cable between the electric machine and the power electronic converter.

The power electronic converter can be for example a frequency converter and the electric machine can be for example an alternating current motor and/or generator or a transformer.

In accordance with the present invention, there is provided also a new method for controlling a power electronic converter. A method according to the invention comprises:controlling one or more controllable switches of a power electronic converter so as to produce at least one test voltage pulse at electric terminals of the power electronic converter,receiving a sensor signal indicative of a reflected voltage detected from the electric terminals of the power electronic converter, the reflected voltage arriving from an electric system connected to the electric terminals of the power electronic converter and being caused by a reflection of the test voltage pulse, andcontrolling the operation of the power electronic converter at least partly in accordance with information based on the sensor signal.

In accordance with the invention, there is provided also a new computer program for controlling a power electronic converter. A computer program according to the invention comprises computer executable instructions for controlling a programmable control system of a power electronic converter to:control one or more controllable switches of the power electronic converter so as to produce at least one test voltage pulse at electric terminals of the power electronic converter,receive a sensor signal indicative of a reflected voltage detected from the electric terminals of the power electronic converter, the reflected voltage arriving from an external electric system connected to the electric terminals of the power electronic converter and being caused by a reflection of the test voltage pulse, andcontrol the operation of the power electronic converter at least partly in accordance with information based on the sensor signal.

In accordance with the invention, there is provided also a new computer program product. The computer program product comprises a non-volatile computer readable medium, e.g. a compact disc “CD”, encoded with a computer program according to the invention.

A number of exemplifying and non-limiting embodiments of the invention are described in accompanied dependent claims.

Various exemplifying and non-limiting embodiments of the invention both as to constructions and to methods of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific exemplifying embodiments when read in connection with the accompanying drawings.

DETAILED DESCRIPTION

The specific examples provided in the description below should not be construed as limiting the scope and/or the applicability of the accompanied claims. Lists and groups of examples provided in the description are not exhaustive unless otherwise explicitly stated.

FIG. 1ashows a schematic illustration of an electric drive that comprises an electric machine116, a cable115, and a power electronic converter100according to an exemplifying and non-limiting embodiment of the invention. One end of the cable115is connected to the armature winding of the electric machine116and the other end of the cable115is connected to the power electronic converter100. In the exemplifying case shown inFIG. 1a, the power electronic converter100is a frequency converter and the electric machine116is a three-phase alternating current machine that can be e.g. an induction machine, an electrically excited synchronous machine, or a permanent magnet synchronous machine.

The power electronic converter100comprises electric terminals101,102, and103for connecting to an external electric system. In this exemplifying case, the cable115and the electric machine116represent the electric system which is external from the viewpoint of the power electronic converter100. The power electronic converter100comprises controllable switches104,105,106,107,108, and109connected to the electric terminals101-103as illustrated inFIG. 1a. The controllable switches can be for example insulated gate bipolar transistors “IGBT”, gate turn off thyristors “GTO”, or some other suitable controllable switch components. In the exemplifying case shown inFIG. 1a, the power electronic converter100comprises a capacitive intermediate circuit112and a converter unit113for transferring electric energy between an alternating voltage power grid114and the capacitive intermediate circuit112. As illustrated inFIG. 1a, the capacitive intermediate circuit112is connected to the controllable switches104-109via a positive rail117and a negative rail118.

The power electronic converter100comprises a control system110for operating the controllable switches104-109so as to control voltages of the electric terminals101-103. InFIG. 1a, the three phases of the power electronic converter100and of the electric machine116are denoted as U, V, and W, the voltages between the electric terminals101-103of the power electronic converter are denoted as VC UV, VC VW, and VC UW, and the voltages between the electric terminals of the electric machine116are denoted as VM UV, VM VW, and VM UW. The power electronic converter100comprises a voltage sensor111connected to the electric terminals101-103. The voltage sensor111can be configured to detect the phase voltages of the electric terminals101-103with respect to the ground, or the voltage sensor can be configured to detect the phase-to-phase voltages, i.e. the main voltages, VC UV, VC VW, and VC UWbetween the electric terminals101-103.

The control system110is configured to control one or more of the controllable switches104-109so as to produce at least one test voltage pulse at the electric terminals101-103. The control system110is configured receive, from the voltage sensor111, a sensor signal indicative of a reflected voltage detected from the electric terminals101-103. The reflected voltage arrives from the cable115and is caused by a reflection of the above-mentioned test voltage pulse. The control system110is configured to control the operation of the power electronic converter100at least partly in accordance with information based on the sensor signal indicative of the reflected voltage.

A test voltage pulse can be produced for example between the phases U and V so that the controllable switches104-107are first in the non-conductive state, then the controllable switches104and107are switched to the conductive state for a time period corresponding to the temporal length of the test voltage pulse, and thereafter at least one of the controllable switches104and107is switched back to the non-conductive state. As at least one of the controllable switches104-107is in the non-conductive state after the test voltage pulse has been produced, the voltage sensor111is capable of measuring the reflected voltage from between the electric terminals101and102because at least one of the electric terminals101and102is in a high-impedance “Hi-Z” state and thus the intermediate circuit112does not determine the voltage between the electric terminals101and102. The temporal length of the test voltage pulse can be for example from 0.5 microseconds to few, e.g. 3, microseconds. In many cases, the temporal length of the test voltage pulse is advantageously from 0.5 to 1.5 microseconds.

The information based on the reflected voltage can be utilized in many ways in the control of the power electronic converter100. Some exemplifying and non-limiting ways to utilize the above-mentioned information are presented below.

In a power electronic converter according to an exemplifying and non-limiting embodiment of the invention, the control system110is configured to measure a time from the production of the test voltage pulse to the detection of the reflected voltage and to control the operation of the power electronic converter at least partly in accordance with the measured time. For example, the control system110can be configured to determine, on the basis of the measured time, allowed pulse widths for the voltages of the electric terminals101-103in order to avoid reflection-based over-voltages at the electric machine116. The allowed pulse widths PW, i.e. allowed temporal lengths of voltage pulses, can be for example according to the following:
2N+1.5<PW/T<2N+2.5 orPW/T≥4.5,  (1)
where T is the time measured from the production of the test voltage pulse to the detection of the reflected voltage and N is zero or a positive integer. The measured time T represents the round-trip time which is 2L/v, where L is the physical length of the cable115and v is the propagation speed of the test voltage pulse and of the reflected voltage.

The principles utilized in the above-mentioned exemplifying and non-limiting embodiment of the invention are explained below with reference toFIG. 1b.FIG. 1bshows a schematic illustration of the electric machine116, the cable115, and the electric terminals101-103. The graph120shows the waveform of the voltage VM UVbetween the phases U and V of the electric machine116in an exemplifying case where the voltage VC UVbetween the electric terminals101and102has an upward directed voltage step from 0 to UDCat the time instant 0. Furthermore, the graph120shows the waveform of voltage VM UV, Idealwhich corresponds to a hypothetic case where the cable115is a lossless ideal transmission line and the electric machine116constitutes an ideal open end where the reflection coefficient r is +1. As can be seen from the waveforms of the voltages VM UVand VM UV, Idealshown in the graph120, the propagation time from the electric terminals101-103to the electric machine116is T/2 and thus the round-trip time is T. A graph121shows the waveforms of the voltages VM UV, Idealand VM UVin an exemplifying case where the voltage VC UVhas a first upward directed voltage step from 0 to UDCat the time instant 0, a downward directed voltage step from UDCto 0 at the time instant T, and a second upward directed voltage step from 0 to UDCat the time instant 2T. As can be seen from the waveform of VM UV, Idealshown in the graph121, the voltage oscillations occurring at the electric machine116and caused by the first and second upward directed voltage steps are constructively superimposed. The voltage oscillation occurring at the electric machine116and caused by the above-mentioned downward directed voltage step has such a temporal phase that it does not cancel the above-mentioned voltage oscillations caused by the above-mentioned upward directed voltage steps. Thus, reflection-based over-voltages may occur at the electric machine116. A graph122shows the waveforms of the voltages VM UV, Idealand VM UVin an exemplifying case where the voltage VC UVhas a first upward directed voltage step from 0 to UDCat the time instant 0, a downward directed voltage step from UDCto 0 at the time instant 2T, and a second upward directed voltage step from 0 to UDCat the time instant 3T. In this exemplifying case, the voltage oscillation occurring at the electric machine116and caused by the first upward directed voltage step is cancelled by the voltage oscillation caused by the downward directed voltage step. Thus, the peak values of the voltage VM UVat the electric machine116are smaller than those in the case illustrated by the graph121.

In order to avoid the adverse effect illustrated by the graph121, a voltage pulse between the electric terminals101and102and starting at the time instant 0 should not end on areas marked with the label “Forbidden” in the graph120. Instead, the voltage pulse should end on the areas marked with the label “Allowed.” The area where the time is greater than 4.5T is marked as “Allowed” because the oscillation of the voltage VM UVis assumed to be attenuated so much that the adverse effect illustrated by the graph121does not take place in a harmful extent. The areas marked as “Allowed” in the graph121correspond to allowed pulse widths which fulfill the above-presented equation (1). Therefore, the minimum pulse width does not need to be e.g. about ≈5T based on the attenuation of oscillations but it is also possible to use shorter pulse widths from 1.5T to 2.5T. Thus, there is no need to degrade the quality of the control of the electric machine116in the same way as in cases where the reflection-based over-voltages are avoided by using a minimum pulse width which has a safety margin sufficient for different situations.

In a power electronic converter according to an exemplifying and non-limiting embodiment of the invention, the control system110is configured to use the sensor signal indicative of the reflected voltage for detecting faults such as for example short circuits, earth faults, and conductor breaks. For example, the control system110can be configured to set a data interface119of the power electronic converter100to indicate a fault on the cable115and/or the electric machine116in response to a situation in which a first arriving edge of the reflected voltage has an opposite polarity with respect to the polarity of the test voltage pulse. This approach is suitable for detecting short circuits and earth faults whose impedance is smaller than the characteristic impedance of the cable115. Thus, the reflection coefficient r at the fault is negative and thereby the first arriving edge of the reflected voltage has an opposite polarity with respect to the test voltage pulse. The control system110can be further configured to compute an estimate for the distance from the power electronic converter100to the fault on the basis of a pre-determined parameter indicative of the propagation speed v of the test voltage pulse and the time t measured from production of the test voltage pulse to detection of the reflected voltage. The estimate of the distance is vt/2.

In a power electronic converter according to an exemplifying and non-limiting embodiment of the invention, the control system110is configured to control the controllable switches104-109so as to produce test voltage pulses between different ones of the electric terminals101-103and to detect differences between reflected voltages detected from between the different ones of the electric terminals. For example, a first test voltage pulse can be produced between the phases U and V, i.e. between the electric terminals101and102, a second test voltage pulse can be produced between the phases V and W, and a third test voltage pulse can be produced between the phases U and W. Differences between waveforms of the reflected voltages corresponding to the test voltage pulses, i.e. asymmetry, are indicative of a fault in the cable115and/or in the electric machine116. The control system is configured to set the data interface119to indicate a fault for example in response to a situation in which differences in amplitudes of the reflected voltages and/or differences in oscillating frequencies of the reflected voltages exceed pre-determined threshold values. For a further example, the control system110can be configured to set the data interface119to indicate a fault on a phase U, V, or W in response to a situation in which a time measured from the production of the test voltage pulse at the phase under consideration to the detection of the reflected voltage from the phase under consideration is shorter than a corresponding time measured for at least one of the other phases. The situation of the kind described above indicates that there is a fault in the cable115. The fault can be an earth fault, a short circuit between two phases, or a break in one or two phase conductors. The control system110can be configured to compute an estimate for the distance from the power electronic converter110to the fault on the basis of the time measured for the phase under consideration and a pre-determined parameter indicative of the propagation speed of the test voltage pulses.

The fault detections of the kind described above can be carried out for example during commissioning of the electric drive and/or during a break in the normal operation of the electric drive.

The control system110of the power electronic converter100may comprise one or more processor circuits, each of which can be a programmable processor circuit provided with appropriate software, a dedicated hardware processor such as for example an application specific integrated circuit “ASIC”, or a configurable hardware processor such as for example a field programmable gate array “FPGA”. Furthermore, the control system110may comprise memory which can be e.g. random access memory “RAM”. Furthermore, the control system110may comprise driver circuits for supplying control signals to the controllable switches104-109.

The power electronic converter100described above with reference toFIGS. 1aand 1bcan be deemed to be a power electronic converted that comprises:electric terminals for connecting to an external electric system,controllable switches connected to the electric terminals,means for operating the controllable switches so as to control voltages of the electric terminals,means connected to the electric terminals and for detecting one or more of the voltages of the electric terminals,means for controlling one or more of the controllable switches so as to produce at least one test voltage pulse at the electric terminals, andmeans for controlling the operation of the power electronic converter at least partly in accordance with information based on a sensor signal indicative of a reflected voltage detected from the electric terminals, the reflected voltage arriving from the external electric system and being caused by a reflection of the test voltage pulse.

A power electronic converter according to an exemplifying and non-limiting embodiment of the invention comprises means for measuring a time from the production of the test voltage pulse to the detection of the reflected voltage and for controlling the operation of the power electronic converter at least partly in accordance with the measured time.

A power electronic converter according to an exemplifying and non-limiting embodiment of the invention comprises means for determining, on the basis of the measured time, allowed pulse widths for voltages of the electric terminals of the power electronic converter. The allowed pulse widths can be for example according to the above-presented equation (1).

A power electronic converter according to an exemplifying and non-limiting embodiment of the invention comprises means for setting a data interface of the power electronic converter to indicate a fault on the external electric system in response to a situation in which a first arriving edge of the reflected voltage has an opposite polarity with respect to the polarity of the test voltage pulse.

A power electronic converter according to an exemplifying and non-limiting embodiment of the invention comprises means for computing an estimate of the distance from the power electronic converter to the fault on the basis of a pre-determined parameter indicative of the propagation speed of the test voltage pulse and a time measured from the production of the test voltage pulse to the detection of the reflected voltage.

A power electronic converter according to an exemplifying and non-limiting embodiment of the invention comprises means for controlling the controllable switches so as to produce test voltage pulses at different ones of the electric terminals and for detecting differences between reflected voltages detected from the different ones of the electric terminals.

A power electronic converter according to an exemplifying and non-limiting embodiment of the invention comprises means for setting the data interface of the power electronic converter to indicate a fault on a part of the external electric system connected to a first one of the electric terminals in response to a situation in which a time measured from the production of the test voltage pulse at the first one of the electric terminals to the detection of the reflected voltage from the first one of the electric terminals is shorter than the corresponding time measured for at least one of the other ones of the electric terminals.

A power electronic converter according to an exemplifying and non-limiting embodiment of the invention comprises means for computing an estimate of the distance from the power electronic converter to the fault on the basis of the time measured for the first one of the electric terminals and a pre-determined parameter indicative of the propagation speed of the test voltage pulses.

FIG. 2ashows a flowchart of a method according to an exemplifying and non-limiting embodiment of the invention for controlling a power electronic converter. The method comprises the following actions:action201: controlling one or more controllable switches of the power electronic converter so as to produce at least one test voltage pulse at electric terminals of the power electronic converter,action202: receiving a sensor signal indicative of a reflected voltage detected from the electric terminals of the power electronic converter, the reflected voltage arriving from an electric system connected to the electric terminals of the power electronic converter and being caused by a reflection of the test voltage pulse, andaction203: controlling the operation of the power electronic converter at least partly in accordance with information based on the sensor signal.

FIG. 2billustrates the above-mentioned action203in a method according to an exemplifying and non-limiting embodiment of the invention. In this exemplifying and non-limiting embodiment of the invention, the action203comprises the following sub-actions:sub-action204: measuring a time from the production of the test voltage pulse to the detection of the reflected voltage, andsub-action205: determining, on the basis of the measured time, allowed pulse widths for voltages of the electric terminals of the power electronic converter, the allowed pulse widths can be e.g. according to the above-presented equation (1).

FIG. 2cillustrates the above-mentioned action203in a method according to an exemplifying and non-limiting embodiment of the invention. In this exemplifying and non-limiting embodiment of the invention, the action203comprises the following sub-actions:sub-action206: setting a data interface of the power electronic converter to indicate a fault on the external electric system in response to a situation in which a first arriving edge of the reflected voltage has an opposite polarity with respect to the polarity of the test voltage pulse, andsub-action207: computing an estimate for the distance from the power electronic converter to the fault on the basis of a pre-determined parameter indicative of the propagation speed of the test voltage pulse and a time measured from the production of the test voltage pulse to the detection of the reflected voltage.

In a method according to an exemplifying and non-limiting embodiment of the invention, the action201shown inFIG. 2acomprises controlling the controllable switches of the power electronic converter so as to produce test voltage pulses at different ones of the electric terminals of the power electronic converter and the action203shown inFIG. 2acomprises detecting differences between reflected voltages detected from the different ones of the electric terminals of the power electronic converter.FIG. 2dillustrates exemplifying sub-actions of the action203:sub-action208: setting a data interface of the power electronic converter to indicate a fault on a part of the external electric system connected to a first one of the electric terminals of the power electronic converter in response to a situation in which a time measured from the production of the test voltage pulse at the first one of the electric terminals of the power electronic converter to the detection of the reflected voltage from the first one of the electric terminals of the power electronic converter is shorter than the corresponding time measured for at least one of the other electric terminals of the power electronic converter, andsub-action209: computing an estimate for the distance from the power electronic converter to the fault on the basis of the time measured for the first one of the electric terminals of the power electronic converter and a pre-determined parameter indicative of the propagation speed of the test voltage pulses.

A computer program according to an exemplifying and non-limiting embodiment of the invention comprises computer executable instructions for controlling a programmable control system of a power electronic converter to carry out actions related to a method according to any of the above-described exemplifying embodiments of the invention.

A computer program according to an exemplifying and non-limiting embodiment of the invention comprises software modules for controlling a power electronic converter. The software modules comprise computer executable instructions for controlling a programmable control system of the power electronic converter to:control one or more controllable switches of the power electronic converter so as to produce at least one test voltage pulse at electric terminals of the power electronic converter,receive a sensor signal indicative of a reflected voltage detected from the electric terminals of the power electronic converter, the reflected voltage arriving from an external electric system connected to the electric terminals of the power electronic converter and being caused by a reflection of the test voltage pulse, andcontrol operation of the power electronic converter at least partly in accordance with information based on the sensor signal.

The above-mentioned software modules can be e.g. subroutines or functions implemented with a suitable programming language and with a compiler suitable for the programming language and for the programmable control system under consideration. It is worth noting that also a source code corresponding to a suitable programming language represents the computer executable software modules because the source code contains the information needed for controlling the programmable control system to carry out the above-presented actions and compiling changes only the format of the information. Furthermore, it is also possible that the programmable control system is provided with an interpreter so that a source code implemented with a suitable programming language does not need to be compiled prior to running.

A computer program product according to an exemplifying and non-limiting embodiment of the invention comprises a computer readable medium, e.g. a compact disc “CD”, encoded with a computer program according to an embodiment of invention for controlling a power electronic converter.

A signal according to an exemplifying and non-limiting embodiment of the invention is encoded to carry information defining a computer program according to an embodiment of invention for controlling a power electronic converter.

The specific examples provided in the description given above should not be construed as limiting the scope and/or the applicability of the accompanied claims. Lists and groups of examples provided in the description given above are not exhaustive unless otherwise explicitly stated.