Medium%20Voltage%20Induction%20Motor%20Protection%20and%20Diagnostics

1
Medium Voltage Induction Motor Protection and
Diagnostics
Yi Du Pinjia Zhang Prof. Thomas G.
Habetler School of Electrical and Computer
Engineering Georgia Institute of
Technology Atlanta, GA
2
Medium Voltage Facilities
3
Medium Voltage Supply
4
Medium Voltage Laboratory
5
(No Transcript)
6
(No Transcript)
7
(No Transcript)
8
(No Transcript)
9
(No Transcript)
10
(No Transcript)
11
(No Transcript)
12
(No Transcript)
13
(No Transcript)
14
(No Transcript)
15
Outline

• Introduction
• Heat transfer inside Motors
• Thermal Model-Based Approaches
• Parameter Model-Based Approaches
• Other Approaches

16
Medium voltage induction motors

• Mostly used in the petroleum, chemical, mining
  and other industries,
• Rated from 2300 V to 13200 V,
• They are rotor limited during starting, and
  stator limited under overload.

17
Overload Protection

• Malfunctions of these motors are very costly due
  to loss of productivity,
• The winding insulation failure is a typical
  malfunction, which is often caused by overload.

18
Conventional Overload Relays

• Conventional overload relays utilize simple
  thermal models and embedded temperature sensors.
• Simple thermal models can not estimate the rotor
  temperature.
• Disintegration of the connection, noise
  interference, and large time constant of the
  sensors often result in false alarm or trips.

19
Requirements

• Track the thermodynamic behavior of the motor's
  stator and rotor under steady and transient state
  conditions.
• It should also take into account the important
  differences in the thermal behavior due to the
  motor size and the type of construction and
  ventilation.

20
Possible Approaches

• Higher order thermal model-based approaches
• Model the thermal behavior of the motor. The
  thermal parameters are calculated from the motor
  dimensions and offline experiments. This
  approach is robust, but measurements need be made
  for each motor.
• Parameter-based approaches
• Estimate the temperature from the variation
  of the resistance of the stator and rotor. This
  method can respond to changes in the cooling
  conditions, and is accurate, but it is generally
  too sensitive.

21
Outline

• Introduction
• Heat Transfer inside Motors
• Thermal Model-Based Approaches
• Parameter Model-Based Approaches
• Other Approaches

22
Motor Losses

• The temperature rise inside a motor is caused by
  the losses accumulated in the motor.

23
Loss Segregations
Loss segregation for 15Hp motor
Wcopper Wcore Wfw Wstray
2 Pole 53 9 29 9
4 Pole 55 15 18 12
6 Pole 62 13 12 13
Compared with low power motors, high power motors
have larger percentage of core loss and stray
loss, and smaller percentage of copper loss.
Therefore, the thermal model only considering
copper loss is not suitable for large motors.
Loss segregation for 2002000Hp motors
Wcopper Wcore Wfw Wstray
2 Pole 29 15 36 20
4 Pole 35 18 24 23
6 Pole 37 23 18 22
24
Heat Transfer
The heat transfer inside a motor can be
classified into

• Conduction - transfer of heat due to the
  temperature difference.
• Shaft rotor iron rotor winding,
• Stator winding stator iron Frame,
• Convection - transfer of heat due to the fluid
  motion.
• Frame - external air, stator/rotor airgap, rotor
  endcap air, ...
• Radiation - transfer of heat by electromagnetic
  radiation.
• Radiation is ignored since the motor temperature
  is relatively low.

25
Thermal resistance and thermal capacitance

• Thermal behavior of the motor can be analyzed by
• Finite element methods (Time consuming)
• Lumped-parameter thermal network, composed of
  thermal resistors, thermal capacitors and heat
  sources.
• Some thermal resistances and thermal capacitances
  can be calculated directly from the motor
  dimensions.
• Other thermal resistances are complex and can
  only be measured online.
• Stator core to frame conduction resistance
• Endwinding cooling resistance
• Frame to ambient convection resistance

26
Thermal Network

• Given the difficulties to calculate certain
  thermal parameters, detailed thermal models can
  not guarantee good accuracy.
• Simplification of the thermal network is
  preferred for online monitoring.
• On the other hand, the thermal network should be
  complex enough to estimate the hot spot
  temperature.

27
Outline

• Introduction
• Heat Transfer inside Motors
• Thermal Model-Based Approaches
• Parameter Model-Based Approaches
• Other Approaches

28
Thermal Model-based Approaches

• Use thermal network to model the thermal behavior
  of the motor.
• The Motor is divided into homogenous components
  wherein each part has a uniform temperature and
  heat transfer coefficients.
• The heat flow paths are determined and thermal
  resistors are added between the nodes.
• Losses and thermal capacitors are allocated to
  each node.

29
First - order Thermal Model

• Used in conventional relays for its simplicity,
• Do not consider the rotor winding temperature,
• The stator winding temperature is given by,

30
First - order Thermal Model

• Thermal resistance Rth and thermal capacitance
  Cth can be directly calculated from the trip
  class t6x and the service factor SF.
• Assume the loss Ploss equals the stator copper
  loss

• Cth is calculated using the trip class and the
  winding insulation class.

31
First - order Thermal Model

• Temperature rise is a complex combination of
  distributed thermal capacitances and resistances,
  single time constant is not enough.
• Therefore, large margin is needed for safety and
  the motor is over protected.
• The rotor temperature can not be monitored.

32
Second - order Thermal Model

• Stator and rotor are modeled separately,
• Model B eliminate the node while maintaining the
  same function.
• Parameters are calculated from offline
  experiments

Model A
Model B
33
Second - order Thermal Model

• Model C simplifies the rotor side, and less
  parameters are needed.
• Second-order thermal model is a good tradeoff
  between accuracy and complexity.

Model C
34
Higher - order Thermal Model

• Model the hot spot, such as end windings,
  seperately.
• The thermal model becomes complex and it is
  difficult to identify the parameters.

35
Outline

• Introduction
• Heat Transfer inside Motors
• Thermal Model-Based Approaches
• Parameter Model-Based Approaches
• Other Approaches

36
Parameter-based Approaches
Estimate the temperature from the variation of
the stator winding resistance and the rotor bar
resistance.
k1 is 234.5 for 100 IACS conductivity copper
It is an online method and can respond to changes
in the cooling conditions.
37
Rotor Resistance

• Rotor resistance can be calculated in the
  synchronous reference frame with the d-axis
  aligned with the stator current. Under the steady
  state, the rotor resistance, which is independent
  of the stator resistance, is given by

• Rotor resistance can also be calculated in the
  stationary reference frame and rotor reference
  frame.
• By these methods, the rotor resistance is
  independent of the stator resistance and is less
  sensitive to the parameter variations.

38
Stator Resistance

• Stator resistance is generally calculated based
  on rotor resistance.
• In the synchronous reference frame with the
  d-axis aligned with the stator current, the
  stator resistance is given by,

• Rotor speed can be calculated from the stator
  current harmonics.

39
Outline

• Introduction
• Heat transfer inside Motors
• Thermal Model-Based Approaches
• Parameter Model-Based Approaches
• Other Approaches

40
Neural Network - based Approaches

• Neural networks have been proposed to estimate
  the stator resistance and rotor resistance.
• The advantages are that they do not require the
  motor parameters and can be easily implemented.

• The drawbacks are they are still sensitive to the
  parameter changes since the network is trained
  using the data based on certain parameters.

41
Hybrid Approaches

• Combine thermal model based approaches with
  parameter based approaches,
• Rotor temperature is estimated by parameter
  based approaches since it is less sensitive to
  the parameter variations,
• Stator temperature is monitored by thermal model
  based approaches.

42
Signal Injection-based Approaches

• The stator resistance is estimated from the dc
  components of the voltage and current.
• Relatively accurate since it is not affected by
  the inductance of the motor.
• It is intrusive and introduces torque
  oscillation.

43
Overview of Fault Diagnostics for MV Motors
Induction Motor Fault Categories
Distribution of MV Induction Motor Failures
44
OUTLINE

• Overview of Fault Diagnostics for MV Motors
• Bearing Failure and its Diagnostic
• Stator Winding Inter-turn Fault and its
  Diagnostic
• Rotor Fault and its Diagnostic
• Broken Rotor Bar End-Ring Faults and their
  Diagnostic
• Rotor Eccentricity and its Diagnostic
• Conclusions

45
Overview of Fault Diagnostics for MV Motors
Induction Motor Fault Categories
Distribution of MV Induction Motor Failures
46
Analysis of Fault Diagnostics for MV Motors

• Main differences between MV motors and small
  low-voltage motors
• High Insulation Requirement for Stator Winding
  stator winding inter-turn fault
• Large Output Torque rotor and bearing- related
  mechanical faults
• High Thermal Stress stator insulation failure
  and rotor-related faults

47
Bearing Failure Monitoring

• Bearing failure is the most common fault for MV
  motors.
• Reasons for Bearing Failure
• Electrical Stress
• Stator, rotor or input voltage unbalance causes
  unbalanced magnetic flux, which induces shaft
  current, and potential voltage between bearing
  and ground.
• Mechanical Stress
• Friction and rotor eccentricity can cause
  mechanical failure of bearings.
• Thermal Stress
• Overheat causes the failure of lubricant, which
  lead to friction.

Outer raceway
Inner raceway
Ball
Cage
48
Bearing Failure Monitoring

• Classification of Bearing Failure
• Single Point Defects
• Outer raceway
• Inner raceway
• Ball
• Cage
• Generalized Roughness

• Existing Methods
• Standard vibration sensor method
• Chemical analysis method
• Temperature monitoring
• Acoustic emission method
• Sound pressure method
• Current signature spectra method

49
Current signature spectra methods

• 1.Single point defects
• Wavelet method
• Neural network clustering method
• Adaptive time-frequency method
• Park vector trajectory method
• Other methods
• 2.Generalized roughness
• Mean spectrum deviation method
• Fundamentally monitor the E-M torque harmonics
  corresponding to the mechanical vibration
  frequencies

50
Bearing Failure Monitoring

• is the power supply frequency is the
  vibration frequency
• is the corresponding stator current signature
  frequency.
• Challenges for MV motors
• For single point defects
• Poor Signal/Noise Ratio
• Due the large output torque, the torque
  vibration caused by bearing failure is more
  difficult to observe. So the low signal/noise
  ratio is a potential problem for current-based
  bearing diagnosis of large MV motors.
• For generalized roughness
• Separate measurement noise and bearing
  failure-related vibration noise

51
Stator Winding Inter-turn Faults

• Reasons for Stator Inter-turn Fault
• Electrical Stress
• High voltage causes winding insulation failure
• Thermal Stress
• Motor life is reduced by 50 for every 10C above
  limit
• Mechanical Stress
• Friction between stator and rotor caused by rotor
  eccentricity
• Other Stress

52
Stator Winding Inter-turn Faults

• Existing Methods
• Negative Sequence Current
• Negative Sequence Impedance
• E-M Torque Harmonics
• Current Spectrum
• Current Park Vector Trajectory
• Artificial Intelligent Methods
• Fundamentally Monitor the unbalance of stator
  winding

Stator Inter-turn Fault
53
Stator Winding Inter-turn Fault and its Diagnostic

• Challenges
• How to consider voltage unbalance in power supply
• How to consider original stator winding unbalance
• How to set threshold for negative-sequence
  impedance

54
Rotor-related Failures

• Rotor-related faults can be classified into
• Broken Rotor Bar
• Broken Rotor End-Ring
• Rotor Eccentricity (shaft misalignment)
• Reasons for rotor-related faults
• Mechanical stress including rotor eccentricity,
  and stator-rotor friction
• Thermal stress overheat in rotor can cause
  rotor deterioration
• Electrical stress frequency starting and
  overload operations can cause thermal stress due
  to large current unbalanced flux can induce
  unbalanced magnetic pull.

55
Broken Rotor Bar and End-Ring Faults

• Broken rotor bar fault can cause unbalanced
  magnetic flux, and thus torque oscillation and
  stator current harmonics.

• Due to large output torque, and large rotor
  current, broken rotor bar fault is more common on
  large MV motors than small motors
• The effects of broken rotor end-ring are the same
  as broken rotor bar, in the sense that the rotor
  flux is asymmetric, and induces harmonics in the
  stator current.

56
Broken Rotor Bar and End-Ring Faults

• Existing methods
• Signature current analysis
• EM torque harmonics monitoring
• Slot harmonic methods
• Starting current analysis
• Pattern recognition-based methods
• Artificial intelligence-based methods
• Other methods
• Fundamentally monitor the signature harmonics
  and slot harmonics in stator current

Broken Rotor Bar-related current harmonics
57
Rotor Eccentricity

• Rotor eccentricity is a possible reason for many
  kinds of motor faults, such as stator insulation
  failure, broken rotor bar and end-ring, and even
  shaft crack.

Rotor Shaft Crack

• Rotor eccentricity is mainly caused by shaft
  misalignment, when the geometric center of the
  rotor does not coincide with the center of the
  stator.
• The current harmonics related to rotor
  eccentricity are

58
Rotor-related Faults

• For MV motors, due to the high thermal stress on
  rotor, and the large output torque, especially
  the starting acceleration torque, rotor-related
  faults are quite common.
• The fundamental methods for rotor-related faults
  are current signature analysis, as the signature
  frequencies related to broken rotor bar or
  eccentricity are well-known.
• Challenges for MV motors
• Separating signature harmonics from load
  oscillation
• The signature harmonics in stator current are
  caused by the unbalanced rotor flux, but the same
  harmonics can also be caused by the load
  oscillation.
• Diagnostics for drive-connected motors
• the low-frequency harmonics can be cancelled or
  reduced by the controller.

59
Conclusions

• Motor faults diagnostics
• Stator Inter-turn Fault
• monitor the unbalance of stator winding
• Bearing Fault
• monitor the current harmonics caused by
  bearing-related torque vibration
• Rotor Fault
• Broken Rotor Bar/End-ring
• Rotor Eccentricity
• monitor the current signature harmonics caused
  by unbalanced rotor flux

60
Conclusion of Motor Faults and their Diagnostics
for MV Motors

• Challenges for fault diagnostics of MV motors
• Compensate for the effect of power supply and
  original motor unbalance
• Cancel the effect of load oscillation on
  diagnostics
• Reliable diagnosis even with low SNR
• Fault diagnostics for drive-connected systems
• Remote condition monitoring

61

• QUESTIONS?