Detecting polyphase machine faults via current deviation

• US 6,172,509 B1
• Filed: 02/11/1999
• Issued: 01/09/2001
• Est. Priority Date: 02/11/1999
• Status: Expired due to Fees

• Current Assignee: Air Force Government of The United States of America As Represented By The Secretary
• Original Assignee: United States Of America As Represented By The Secretary Of The Air Force
• Inventors: Cash, Marcus A., Habetler, Thomas G.
• Primary Examiner(s): Metjahic, Safet
• Assistant Examiner(s): Deb, Anjan K.
• Application Number:
  US09/248,731
• Time in Patent Office: 698 Days
• Field of Search: 324/546, 324/772, 324/57, 324/58, 324/59, 324/64, 324/65, 702/182, 702/185, 318/434
• US Class Current: 324/546
• CPC Class Codes: G01R 31/346 Testing of armature or field windings

Abstract

An electrical winding fault detection system wherein turn-to-turn and other winding faults in a polyphase alternating current machine are identified through processing of measured machine winding current flow changes, i.e., through identification of changes in electrical balance conditions in the machine. This disclosed processing includes scaling or normalization of measured operating currents using current measurements made while the machine is known to be in fault-free normal operating condition and altering the normalization by a relationship between average current flow in the fault-free condition and average current flow in the monitored changing load condition. Machine operation from a balanced energization source is preferred for successful use of the system; it is therefore especially suited to multiple phased machines operated from electronically derived alternating current energy. The invention is disclosed primarily in terms of a motor-operated machine; however, application to generator (i.e., alternator), machines and transformers having balanced loads is also contemplated.

Claims

1. The disturbed electrical balance method of detecting occurrence and severity of winding turn faults in a polyphase alternating current rotating electrical machine, said method comprising the steps of:

determining instantaneous current values for each phase of said polyphase electrical machine;

calculating moving root-mean-square phase current values from said measured instantaneous phase current values;

storing winding turn fault-free healthy operation values of said calculated root-mean-square phase currents;

scaling ongoing calculated root-mean-square phase current values according to both said stored winding turn fault-free healthy operation values of said calculated root-mean-square phase currents and according to changes in a mathematical average of said calculated root-mean-square phase currents;

computing a scaled current difference-based measure of fault-caused current divergence of said scaled ongoing calculated root-mean-square current values;

evaluating a time-derivative of said computed scaled current difference-based measure of fault-caused current divergence; and

announcing presence of a suspected machine winding fault in response to magnitudes of said computed fault-caused current divergence and said evaluated time derivative values.

2. The method of detecting occurrence and severity of winding turn faults of claim 1 wherein said polyphase electrical machine is an electric motor and further including the step of energizing said electric motor from an electronic inverter balanced source of polyphase electrical energy.

3. The method of detecting occurrence and severity of winding turn faults of claim 1 wherein said polyphase electrical machine is an alternating current generator and further including the step of loading said generator with a balanced polyphase electrical load.

4. The method of detecting occurrence and severity of winding turn faults of claim 1 wherein said polyphase electrical machine is a three-phase machine inclusive of electrical windings connecting in one of a wye and a delta electrical configurations.

5. The method of detecting occurrence and severity of winding turn faults of claim 1 wherein said step of computing a scaled current difference-based measure of fault-caused current divergence includes the steps of:

determining a mathematical difference between each scaled ongoing calculated root-mean-square phase current value and a scaled representation of an average phase current value;

squaring each of said determined mathematical differences; and

generating a sum of said squared mathematical differences.

6. The method of detecting occurrence and severity of winding turn faults of claim 1 wherein said moving root-mean-square phase current values comprise one of:

an integral number of cycles of alternating current in time duration; and

one cycle of alternating current in time duration.

7. The method of detecting occurrence and severity of winding turn faults of claim 1 wherein said turn fault-free healthy operation current values comprise current values determined under a selected one of conditions of:

no machine load;

full machine load; and

intermediate machine load.

8. The method of detecting occurrence and severity of winding turn faults of claim 1 wherein said step of scaling ongoing calculated root-mean-square phase current values includes scaling according to a mathematical scaling factor of form:

${\mathrm{\alpha <br><br>}}_a=\frac{\left(I_a^h+I_b^h+I_c^h\right)}{I_a^h·<br><br>\left(I_a+I_b+I_c\right)},$

wherein I_a, I_b and I_c represent moving-root-mean-square machine phase currents and I^h_a, I^h_b and I^h_c represent healthy operation moving-root-mean-square machine phase currents.

9. The method of detecting occurrence and severity of winding turn faults of claim 1 wherein said step of scaling ongoing calculated root-mean-square phase current values includes:

storing a healthy machine, normal operation, winding turn fault-free, root-mean-square current value for each phase of said machine; and

determining an average value of said stored healthy machine phase current values.

10. The method of detecting occurrence and severity of winding turn faults of claim 1 further including performing selected of said determining, calculating, storing, scaling, computing, evaluating and announcing steps in repeating iteration in an ongoing cycle of machine monitoring.

11. The method of detecting occurrence and severity of winding turn faults of claim 1 further including performing said determining, calculating, scaling, computing, evaluating and announcing steps in a shortened and repeating iteration cycle following an initial iteration cycle inclusive of said storing step.

12. The method of detecting occurrence and severity of winding turn faults of claim 1 wherein said step of announcing presence of a suspected machine winding fault includes comparing said computed fault-caused current divergence and said evaluated time derivative values with a threshold value that is independent of machine currents.

13. The method of detecting occurrence and severity of winding turn faults of claim 1 wherein said step of determining instantaneous current values for each phase of said polyphase electrical machine includes computing a current value for one phase of a three-phase machine from measured current values in two other phases of said machine.

14. The winding fault detecting method of operating a phase current-monitored polyphase alternating current rotating machine comprising the steps of:

scaling monitored ongoing root-mean-square phase current values of said machine to both winding turn fault-free healthy operation values of said phase currents and to changes in a mathematical average of said monitored phase currents;

determining one of a statistical standard deviation and a statistical variance representations of summed differences between scaled phase current values and scaled mathematical average phase current values in said machine; and

indicating a winding fault detection event in response to one of said statistical representations exceeding a selected magnitude.

15. The winding fault detecting method of claim 14 wherein said machine is an electrical motor and wherein said monitored phase currents are ongoing root-mean-square current values determined over intervals of one alternating current cycle of said motor.

16. The winding fault detecting method of claim 14 further including the step of determining said ongoing root-mean-square current values from a plurality of measured instantaneous motor phase currents.

17. The winding fault detecting method of claim 14 wherein said step of scaling monitored ongoing root-mean-square phase current values of said machine includes:

determining a winding turn fault-free root-mean-square healthy operation value of each said phase current;

determining an average of said winding turn fault-free root-mean-square healthy operation current values;

determining a monitored interval root-mean-square value of each said phase current;

determining an average of said monitored interval root-mean-square phase current values; and

forming a mathematical quotient from said average of winding turn fault-free root-mean-square healthy operation current values and said average of monitored interval root-mean-square phase current values.

18. The winding fault detecting method of claim 14 further including the steps of:

evaluating a time-derivative of a selected one of said statistical standard deviation and said statistical variance representations of summed differences;

comparing said time derivative magnitude with a selected winding fault current rate of change-determined threshold value; and

indicating a winding fault detection event in response to said time derivative magnitude exceeding said threshold value.

19. The phase current based alternating current machine winding fault detection method comprising the steps of:

comparing time segregated values of corresponding phase currents in said machine to determine changes in machine electrical current balance;

said comparing step including compensating for load change-induced average phase current changes in said machine;

said comparing step additionally including determination of a statistical difference measurement between said time segregated values of corresponding phase currents; and

communicating a determination of suspected machine winding fault in response to detection of a selected degree of machine electrical current balance change.

20. Winding turn fault-protected polyphase alternating current rotating electrical machine apparatus comprising the combination of:

a polyphase alternating current electrical machine connected with a balanced polyphase source of electrical energy;

means for determining instantaneous current values for each phase of said polyphase electrical machine;

means for calculating moving root-mean-square phase current values from said measured instantaneous phase current values;

means for storing winding turn fault-free healthy operation values of said calculated root-mean-square phase currents;

means for scaling ongoing calculated root-mean-square phase current values according to both said stored winding turn fault-free healthy operation values of said calculated root-mean-square phase currents and according to changes in a mathematical average of said calculated root-mean-square phase currents;

means for computing a scaled current difference-based measure of fault-caused current divergence in said scaled ongoing calculation of root-mean-square current values;

means for evaluating a time-derivative of said computed scaled current difference-based measure of fault-caused current divergence; and

means for announcing presence of a suspected machine winding fault in response to magnitudes of said computed fault-caused current divergence and said evaluated time derivative values.

21. Winding fault-monitored polyphase AC induction motor apparatus comprising the combination of:

a source of balanced polyphase alternating current electrical energy;

a polyphase induction motor having electrical windings energized by said balanced polyphase alternating current electrical energy source;

a plurality of instantaneous electrical current flow-measuring elements disposed intermediate said source of balanced polyphase alternating current electrical energy and individual phases of said motor electrical windings;

first electrical current signal processing apparatus connected with said instantaneous electrical current flow-measuring elements and generating moving one cycle root-mean-square phase current signal values from a plurality of said instantaneous electrical current flow-measurements;

second electrical current signal processing apparatus connected with said first electrical current signal processing apparatus moving one cycle root-mean-square current signal values and scaling ongoing generated root-mean-square phase current values according to both stored winding turn fault-free healthy operation values of said one cycle root-mean-square phase currents and according to changes in a mathematical average of said root-mean-square phase current signal values;

third electrical current signal processing apparatus connected with said second electrical current signal processing apparatus and generating standard deviation signal values from selected scaled current signal difference values generated therein; and

threshold comparison circuitry connected with said third electrical current signal processing apparatus standard deviation signal values and generating a winding fault alarm signal in response to standard deviation signal values in excess of a selected threshold signal value.

22. The winding fault-monitored polyphase AC indication motor apparatus wherein said first, second and third electrical current signal processing apparatus and said threshold comparison circuitry elements comprise one of:

a programmed digital computer,

an array of electrical circuit hardware, and

a combination of a programmed digital computer and an array of electronic circuit hardware.

23. The method of detecting polyphase alternating current electrical machine winding faults comprising the steps of:

measuring a sequence of winding fault-free healthy operation instantaneous phase current values in said polyphase alternating current electrical machine;

calculating moving root-mean-square phase current values from said measured sequence of instantaneous phase current values;

computing a first average value, an average of said moving root-mean-square phase current values;

storing a first set of said winding turn fault-free healthy operation calculated root-mean-square phase current values, said first set including a stored value for each phase of said machine;

determining a second average value, an average of said stored set of winding turn fault-free healthy operation calculated root-mean-square phase current values;

normalizing subsequent moving root-mean-square phase current values for said machine to a respective one of said first set of said winding turn fault-free healthy operation calculated root-mean-square phase current values to form to a set of per unit phase current values;

scaling each of said per unit phase current values by a ratio of said second average value, of said stored set of winding turn fault-free healthy operation calculated root-mean-square phase current values, to said first average value, of said moving root-mean-square phase current values, to form a plurality of normalized scaled per unit phase current values; and

indicating a winding fault detection event in response to a statistical evaluation of said plurality of normalized scaled per unit phase current values exceeding a selected threshold value.

24. The method of detecting polyphase alternating current electrical machine winding faults of claim 23 wherein said statistical evaluation of said plurality of normalized scaled per unit phase current values comprises application of a selected one of a standard deviation and a variance statistical evaluations.

25. The method of detecting polyphase alternating current electrical machine winding faults of claim 24 wherein said step of indicating a winding fault detection event is also responsive to time rate of change of said selected standard deviation and variance statistical evaluations.