Method for detecting a short circuit and control unit

Abstract

A method for detecting a short circuit in a winding of an electric motor being driven by a drive, the method includes measuring respective phase currents of the windings, transforming the phase currents to negative sequence components, and comparing the negative sequence components with respective baseline values. A corresponding control unit is further disclosed.

Claims

1. A method for detecting a short circuit in a winding of an electric motor being driven by a drive, the method comprising the following steps: measuring respective phase currents of the windings, transforming the phase currents to negative sequence components, comparing the negative sequence components with respective baseline values, and detecting a short circuit if the negative sequence components deviate more than a respective absolute or relative threshold from the respective baseline value.

2. The method according to claim 1, wherein the transforming the phase currents to negative sequence components is per-for-med using a rotating reference frame, rotating at an output frequency of the drive.

3. The method according to claim 1, wherein the baseline values are given dependent upon the output frequency.

4. The method according to claim 1, wherein a negative sequence component corresponds to a current component or har-monics rotating in opposite direction to a main rotating magnetic field inside an air gap of the motor.

5. The method according to claim 1, wherein the method further comprises the following step: measuring a dc link voltage.

6. The method according to claim 5, wherein the dc link voltage is measured at rectified output voltages and/or at connection points between respective diodes.

7. The method according to claim 5, wherein the method further comprises the following steps: transforming the dc link voltage to a reference frame rotating with a reference frame rate, and obtaining a resonance signature by taking a square root magnitude of the dc link voltage in the reference frame.

8. The method according to claim 7, wherein the dc link voltage in the reference frame is passed through a low pass filter before taking a square root magnitude.

9. The method according to claim 7, wherein the reference frame rotates with an output frequency of the drive, multiplied by a value given by 2/n, wherein n is an integer value.

10. The method according to claim 9, wherein n is in integer value of a maximum output frequency divided by a grid frequency.

11. The method according to claim 7, wherein the detecting a short circuit is paused while a grid unbalance is detected by detecting presence of harmonics in the resonance signature.

12. The method according to claim 7, wherein a short circuit is also detected if the drive operates in a resonance band and no grid unbalance is detected based on the resonance signature.

13. The method according to claim 12, wherein each resonance band is defined around a power grid frequency multiplied by an integer.

14. A control unit for an electric motor, the control unit being configured to perform the method according to claim 1.

15. The method according to claim 2, wherein the baseline values are given dependent upon the output frequency.

16. The method according to claim 2, wherein a negative sequence component corresponds to a current component or har-monics rotating in opposite direction to a main rotating magnetic field inside an air gap of the motor.

17. The method according to claim 3, wherein a negative sequence component corresponds to a current component or har-monics rotating in opposite direction to a main rotating magnetic field inside an air gap of the motor.

18. The method according to claim 2, wherein the method further comprises the following step: measuring a dc link voltage.

19. The method according to claim 3, wherein the method further comprises the following step: measuring a dc link voltage.

20. The method according to claim 4, wherein the method further comprises the following step: measuring a dc link voltage.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0080] The invention will now be further described with respect to the accompanying drawings.

[0081] FIG. 1 shows an electric motor and a control unit,

[0082] FIG. 2 shows a frequency spectrum,

[0083] FIG. 3 shows another frequency spectrum,

[0084] FIG. 4 shows a flow diagram,

[0085] FIG. 5 shows first signal processing entities,

[0086] FIG. 6 shows second signal processing entities,

[0087] FIG. 7 shows third signal processing entities, and

[0088] FIGS. 8a and 8b show a Vdc2f calculation and a corresponding flag response.

DETAILED DESCRIPTION

[0089] FIG. 1 shows schematically a control unit 10 and a connected electric motor 20. The motor 20 has three phases u, v, w that are connected to the control unit 10. The phases are connected to each other at a central point. The control unit 10 is typically configured to provide electric power to the electric motor 20, and also to perform certain control and surveil-lance functions.

[0090] When a motor is connected to the control unit 10, or at any other point in time when it is required, the control unit 10 may perform a method as described herein.

[0091] FIG. 2 shows an ASM stator winding inter-turn fault and generated harmonics spectrum. It can be seen that negative frequency components are present, which are, in the present case, represented by the upwards pointing arrow at the left end of the horizontal axis. This can be used in order to determine a short circuit fault.

[0092] FIG. 3 shows negative sequence and third harmonics components of stator winding current as transformed to 0 Hz and 4 f. Also there, negative sequence components are present. Also components at 0 Hz are present.

[0093] FIG. 4 shows a flow diagram of a typical process, which will be described in the following.

[0094] The block 1 initializes the proposed condition monitoring function.

[0095] Block 2 sets the threshold limit for detecting stator winding inter-turn fault.

[0096] Block 3 of the flow chart shown in FIG. 4 samples three phase winding currents Isu, Isv, Isw, of the motor, dc link voltage, duty ratios d.sub.u, d.sub.v, d.sub.w of voltage of phase u, phase v, and phase w, and an output frequency of the drive.

[0097] Block 4 transforms these winding currents and voltages to sequence components of voltage and current and calculates negative sequence components of current and signal which correspond to unbalance or weak grid condition Vdc2f.

[0098] Block 5 determines if a drive output frequency lies within the resonance band. As shown in the flowchart of FIG. 4, this block decides, whether the output frequency WsRef of the drive falls within the resonance band (frequency of grid close to output frequency of the drive, Wgrid=WsRef). If the output frequency of the drive lies outside the resonance band than the drive knows there is no influence of grid on the stator winding monitoring signal as shown in Block 7, the grid resonance flag is set to zero.

[0099] Block 6 activates when the drive output frequency falls within the resonance band or a pre-defined frequency band. Now the drive monitors a grid resonance indicator signal Vdc2f and if it is less than the preset threshold value again the drive determines that grid condition is not influencing the stator winding monitoring signal. If this condition is true, it is then used in block 8 to indicate that drive should not generate stator winding warning and alarm because false positives can be generated due to unbalance grid condition.

[0100] Block 9 gets activated in case of a positive logic decision of block 6 and the drive now determines that the grid is healthy or the operating point of the drive is outside of a frequency range where the grid can influence the stator winding monitoring signal and in this case false warning and alarm cannot be generated by the drive.

[0101] Block 10 determines if a drive output frequency lies in the resonance band and if the resonance flag is set i.e. resonance has occurred, then stator winding signal monitoring time counters are reset to zero as shown in Block 12 in order to avoid false positive warnings and alarms.

[0102] Block 11 counts time duration for warning and alarm signal when there is no resonance condition and the stator winding monitoring signal is higher than the threshold limit of warning or alarms.

[0103] In the process in block 11, a status flag is generated for warning level-1, warning level-2, or alarm in case of stator winding faults occurring, fault signatures crossing a threshold, or magnitude and time duration of these faults exceeding pre-programmed countervalue. Block diagrams of methods used to extract negative and positive sequence components of current and voltage are shown in FIGS. 5 to 7. The process utilizes simple low-pass filters to filter out high frequency components from transformed negative and positive sequence x and y components of currents and negative sequence components of voltages. Especially, the process shown in these figures and the following description can be used in order to extract sequence components, as mentioned elsewhere herein.

[0104] FIG. 8a shows the block diagram of the method how to calculate Vdc2f from the dc link voltage Vdc and by using the output frequency of the drive for reference frame transformation. The corresponding FIG. 8b shows that the resonance flag is generated when the drive is running within the resonance band and the gird is unbalance i.e. running at 750 rpm (it is an 8 pole motor) corresponds to a 50 Hz output frequency of motor. The diagram of FIG. 8b was made with a 3% unbalance in the grid, the speed was changed from 740 rpm to 760 rpm and back to 740 rpm in repetitive steps with 50% load.

[0105] Processing starts with an Alpha-Beta to x-y transformation of currents I or voltages V for the respective phases u, v, w. Regarding the currents, negative sequence components Isxneg, Isyneg (FIG. 5), and positive sequence components Isxpos, Isypos (FIG. 6) are calculated for the axes x, y. Regarding voltages, negative sequence components Vsxxneg, Vsyneg are calculated for the axes x, y. The respective values are low-pass (LP) filtered and are subject to a calculation of the square root of the sum of the squares of the respective x and y components. The results are negative sequence components Ineg of the current, positive sequence componence Ipos of the current, and negative sequence components Vneg of the voltage.

[0106] The proposed algorithm works during normal grid conditions as well as in case of weak grid (unbalance mains voltage condition). The algorithm may perform detection of stator winding inter-turn short circuit faults for ASM, PMSM and SynRM. The algorithm is able to discriminate healthy and faulty stator winding with both unbalance and balance grid condition.

[0107] The proposed algorithm was verified through experimentation and observation.

[0108] While the present disclosure has been illustrated and described with respect to a particular embodiment thereof, it should be appreciated by those of ordinary skill in the art that various modifications to this disclosure may be made without departing from the spirit and scope of the present disclosure.