SE2250877A1 - Method of detecting phase failure in a power converter - Google Patents

Abstract

A method of detecting a failure of a phase of a diode rectifier stage of a power converter comprising the rectifier stage including a capacitor, and an inverter stage, the capacitor forming a DC link between the rectifier stage and the inverter stage, wherein the inverter stage is connected to an electric motor, the method comprising: a) powering the rectifier stage, b) providing a rectifier stage load which discharges the capacitor while maintaining the motor in a standstill state, and c) determining whether any phase of the rectifier stage has failed based on a voltage measured across the capacitor while step b) is performed.

Description

METHOD OF DETECTING PHASE FAILURE IN A POWERCONVERTER TECHNICAL FIELD The present disclosure generally relates to power converters including a rectifier stage and an inverter stage.

BACKGROUND The loss of one phase at the input of a diode-rectifier input stage of a drive isa not uncommon failure. However, when at standstill or at low power outputthe drive may not detect it and continue to work normally. Once a high poweroutput is required from the drive, a potentially dangerous situation occurs:high input current may trip remaining input power, or the drive itself maytrip as DC-link voltage drops outside of its allowed range. When this occurs,control of the load is lost, which in particular in motion applications may result in damage of the load or its surroundings.

Fault detection methods for this fault exist and can be found in literature.Whereas some methods rely on measuring rectifier input voltages orcurrents, which is typically not possible in a normal drive due to lack ofsensors, methods monitoring the DC-link voltage or possibly also the motor-side current also exist. However, these methods work only if a significant loadcurrent is being drawn. Whereas this would work during operation, nothingwould prevent the drive from starting operation while a phase fault is present.

SUMMARY A general object of the present disclosure is to provide a method that solves or at least mitigates the problems of the prior art.

There is hence according to a first aspect of the present disclosure provided amethod of detecting a failure of a phase of a diode rectifier stage of a power converter comprising the rectifier stage including a capacitor, and an inverter 1O stage, the capacitor forming a DC link between the rectifier stage and theinverter stage, wherein the inverter stage is connected to an electric motor,the method comprising: a) powering the rectifier stage, b) controlling thepower converter to provide a rectifier stage load which discharges thecapacitor while maintaining the motor in a standstill or essentially standstillstate, and c) determining whether any phase of the rectifier stage has failed based on a voltage measured across the capacitor while step b) is performed.

By controlling the power converter to provide a rectifier stage load whichdischarges the capacitor while the motor maintains its standstill or essentiallystandstill state, a phase failure can be detected without the motor being inmotion or at least without being set in any significant motion. Thus, the riskof damaging the motor or its load due to a phase failure is eliminated or at least reduced.

The method may beneficially be performed before each start of the motor and/ or during periods of low-power operation.

If the rectifier stage comprises a 6-pulse diode rectifier, the ripple of the DC-link voltage contains a frequency component at 6 times the grid frequency, ormore generally the supply frequency, in case the rectifier stage is healthy. Inan open-phase case the ripple comprises a frequency component at 2 timesthe supply frequency. Therefore, a power pulse of about 1 period of the grid voltage is expected to be sufficient to detect a fault.

According to one embodiment step a) involves powering the rectifier stagewith one or more pulses each with a duration corresponding to 90% to 110% of a period of the voltage powering the rectifier stage.

Thus, the power consumption of the rectifier stage load is reduced comparedto feeding the rectifier stage with a voltage for a longer period. Moreover, itreduces the amount of motion that the motor could reach in case such a strategy is being employed.

The voltage applied in step a) may be a grid voltage. 1O According to one embodiment step b) involves switching in a braking resistoron the DC link.

According to one embodiment step b) involves controlling the inverter stagesuch that it injects a current that generates a pulsating positive and negative torque with a zero-torque average.

According to one embodiment step b) involves controlling the inverter stage such that it injects a non-torque producing current into the motor.

According to one embodiment the motor is an induction motor and the non- torque producing current is a magnetising or de-magnetising pulse.

According to one embodiment the motor is a reluctance motor or apermanent magnet motor and the non-torque producing current is in a direction orthogonal to a constant torque line.

According to one embodiment step c) involves analysing a spectrum of the voltage.

There is according to a second aspect of the present disclosure provided apower system comprising: a power converter including a diode rectifier stage,an inverter stage, and a capacitor forming a DC link between the rectifierstage and the inverter stage, a control system, and an electric motorconnected to the inverter stage, wherein the control system is configured to perform the method of the first aspect.

Generally, all terms used in the claims are to be interpreted according to theirordinary meaning in the technical field, unless explicitly defined otherwiseherein. All references to "a/ an /the element, apparatus, component, means,etc. are to be interpreted openly as referring to at least one instance of theelement, apparatus, component, means, etc., unless explicitly stated otherwise. 1O BRIEF DESCRIPTION OF THE DRAWINGS The specific embodiments of the inventive concept will now be described, by way of example, with reference to the accompanying drawings, in which:Fig. 1 schematically shows a circuit diagram of a power system; and Fig. 2 shows a method of detecting a failure of a phase of a diode rectifier stage of a power converter.

DETAILED DESCRIPTION The inventive concept will now be described more fully hereinafter withreference to the accompanying drawings, in which exemplifyingembodiments are shown. The inventive concept may, however, be embodiedin many different forms and should not be construed as limited to theembodiments set forth herein; rather, these embodiments are provided byway of example so that this disclosure will be thorough and complete, andwill fully convey the scope of the inventive concept to those skilled in the art.

Like numbers refer to like elements throughout the description.

Fig. 1 shows an example of a power system 1. The power system 1 comprises a POWCI' COIIVCITGI' The power converter 3 comprises a rectifier stage 5 and an inverter stage 7.The power converter 3 may alternatively also be referred to as an electric drive.

The exemplified rectifier stage 5 has an input side configured to be connected to a three-phase power source configured to generate voltages va-vc.

The rectifier stage 5 is a diode rectifier stage. The rectifier stage 5 comprises aplurality of diodes, such as six diodes D1-D6 arranged in a six-pulse rectifier configuration.

The rectifier stage 5 comprises a capacitor C. The rectifier stage 5 has an output side with two DC terminals. The capacitor C is connected across the 1O two DC terminals. The capacitor C forms a DC link between the rectifier stage and the inverter stage 7.

The power converter 3 may comprise a braking resistor R connected acrosstwo DC terminals, i.e., in parallel with the capacitor C, between the rectifier stage 5 and the inverter stage 7.

The power converter 3 may comprise a switch S connected in series with thebraking resistor R. The switch S may be a mechanical switch or an electronicswitch such as a transistor, e.g., an Insulated Gate Bipolar Transistor (IGBT), configured to be switched between an open state and a closed state.

The inverter stage 7 may have any configuration of controllable switches such as thyristors or transistors, e.g., IGBTs.

The power system 1 comprises an electric motor M connected to the inverter 7. The inverter 7 is configured to control the motor M.

The power system 1 comprises a control system 9 configured to control the inverter 7.The control system 9 is configured to control the switch S.

The control system 9 is configured to receive measurement of a voltage across the DC link, i.e., over the capacitor C.

Fig. 2 shows a flowchart of a method of detecting a failure of a phase of the rectifier stage 5.

In a step a) the rectifier stage 5 is powered. Thus, the rectifier stage 5 is fed with alternating voltages va-vc of the three electrical phases.

Step a) may involve powering the rectifier stage 3 with one or more pulses.Each pulse may for example have a duration corresponding to 90% to 110%of a period of the voltage, such as a grid voltage, powering the rectifier stage5. The duration could alternatively be longer, or the powering may involve feeding the rectifier stage 5 with an alternating voltage for many periods. 1O In a step b) the power converter 3 is controlled to provide a rectifier stageload which discharges the capacitor C while maintaining the motor M in a standstill or essentially standstill state.

According to one example, step b) involves switching in the braking resistor Ron the DC link. The braking resistor R is thus set from its open state to its theclosed state by the control system 9. The braking resistor R thus acts as arectifier stage load, discharging the capacitor C. A voltage ripple, which is notpresent when the rectifier stage 5 has no load and the motor M is at standstill, will thus appear over the capacitor C.

According to one example, step b) involves controlling the inverter stage 7 bymeans of the control system 9 such that it injects a non-torque producingcurrent into the motor M. The inverter stage 7 and the motor M thus act as arectifier stage load, discharging the capacitor C. A voltage ripple will thus appear over the capacitor C.

In one example, the motor M is an induction motor and the non-torque producing current is a magnetising or de-magnetising pulse.

In one example, the motor M is a reluctance motor or a permanent magnetmotor and the non-torque producing current is in a direction orthogonal to a constant torque line.

In one example, step b) involves controlling the inverter stage 7 such that itinjects a current that generates a pulsating positive and negative torque witha zero-torque average. The torque thus remains zero and the motor M will maintain its standstill or essentially standstill state.

The use of the braking resistor R and the control of the inverter stage 7 toinject the non-torque producing current into the motor M or inject a currentthat generates a pulsating positive/ negative torque with a zero-torqueaverage may be employed in combination or alone. For example, if the power system 1 does not comprise a braking resistor, the injection of a non-torque 1O producing current into the motor M or injecting a current that generates a pulsating torque may be performed.

In a step c) it is determined whether any phase of the rectifier stage has failedbased on a voltage measured across the capacitor C while step b) isperformed. The voltage used in step c) is thus measured while step b) isperformed. The control system 9 is configured to receive the measurement of the voltage across the capacitor C.

Step c) may involve transforming the measured voltage to a frequencydomain and analysing the spectrum of the voltage. For example, a Fouriertransform may be used to transform the voltage to the frequency domain. Incase a frequency component is present at 6 times the frequency of the voltageapplied to the rectifier stage 5, when 6 diodes are used, it is determined thatall phases are healthy. In case a frequency component is present at 2 timesthe frequency of the voltage applied to the rectifier stage 5, it is determinedthat one of the phases is an open phase. In this case, the frequency component has a higher amplitude than if the rectifier stage 5 is healthy.

The inventive concept has mainly been described above with reference to afew examples. However, as is readily appreciated by a person skilled in theart, other embodiments than the ones disclosed above are equally possible within the scope of the inventive concept, as defined by the appended claims.

Claims (9)

1. A method of detecting a failure of a phase of a diode rectifier stage (5) ofa power converter (3) comprising the rectifier stage (5) including a capacitor(C), and an inverter stage (7), the capacitor (C) forming a DC link between therectifier stage (5) and the inverter stage (7), wherein the inverter stage (7) is connected to an electric motor (M), the method comprising:a) powering the rectifier stage (5), b) controlling the power converter (3) to provide a rectifier stage loadwhich discharges the capacitor (C) while maintaining the motor (M) in astandstill state, and c) determining whether any phase of the rectifier stage (5) has failedbased on a voltage measured across the capacitor (C) while step b) is performed.

2. The method as claimed in claim 1, wherein step a) involves powering therectifier stage (5) with one or more pulses each with a durationcorresponding to 90% to 110% of a period of the voltage powering the rectifier stage (5).

3. The method as claimed in claim 1 or 2, wherein step b) involves switching in a braking resistor (R) on the DC link.

4. The method as claimed in any of the preceding claims, wherein step b)involves controlling the inverter stage (7) such that it injects a current thatgenerates a pulsating positive and negative torque with a zero-torque average.

5. The method as claimed in any of claims 1-3, wherein step b) involvescontrolling the inverter stage (7) such that it injects a non-torque producing current into the motor (M). 1O

6. The method as claimed in claim 5, wherein the motor (M) is aninduction motor and the non-torque producing current is a magnetising or de-magnetising pulse.

7. The method as claimed in claim 5, wherein the motor (M) is areluctance motor or a permanent magnet motor and the non-torque producing current is in a direction orthogonal to a constant torque line.

8. The method as claimed in any of the preceding claims, wherein step c) involves analysing a spectrum of the voltage.

9. A power system (1) comprising: a power converter (3) including a diode rectifier stage (5), an inverterstage (7), and a capacitor (C) forming a DC link between the rectifier stage (5) and the inverter stage (7),a control system (9), andan electric motor (M) connected to the inverter stage (7), wherein the control system (9) is configured to perform the method as claimed in any of the preceding claims.