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Effects of Modern Variable Speed
Drives on Motor Winding Insulation
Blake Lloyd, FIEEE
Qualitrol - Iris Power
[email protected]
About Iris Power
• Founded in 1990 by 4 individuals from Ontario Hydro's Research
Division, one of the largest electrical utilities in North America.
• Core Business is CBM Products & Services for machines > 3.3 kV.
• Size: ~ 2Employees
• Head Office: Toronto
• Satellite Offices:
–
–
–
–
–
Houston, Texas
Mumbai, India
Beijing, China
Watford, UK
Sao Paulo & Rio de Janeiro, Brazil
• + Agents located worldwide
My home, Toronto
Outline
• Characteristics of voltage source PWM
drives
• Impact on random wound (LV) motor
stators
• Impact on form wound (MV) stators
• New standards to qualify insulation systems
for converter fed motors
• Offline PD detection under surges
• Online PD detection with PWM drives
Why VSDs
Process control requirements
Improved energy conservation below full load
Reduced need for gears
Less wear and tear during motor starting
High Power (>750 HP) AC Drive Sales
Billions of $
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$6
$5
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04
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Year
07
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WHAT ARE VSDs or CONVERTER
FED DRIVES (CFDs)
• Converters consist of a rectifier and inverter to
create an output voltage of any desired AC
frequency
• Speed of the motor is directly proportional to the
frequency, thus by controlling the AC frequency,
can control the speed
• The rectifier changes 50/60 Hz AC voltage to DC
• The inverter changes DC to AC of a selected
frequency
• Most common type of converter is the voltage
source, pulse width modulated (PWM) converter
WHAT ARE VSDs or CONVERTER FED
DRIVES (CFDs)
• Most common switching device in a converter is
the insulated gate bipolar junction transistor
(IGBT)
• IGBT has very fast turn on time (< 100 ns), which
reduces the heat generated by the transistor, and
thus reduces drive size and cooling needs
• IGBT voltage source PWM drives now operating
up to 13.8 kV, using multiple stages
3 Level Converter Output
(<690 V)
5 level Converter Output
(>2000 V)
V
U pk/pk
t
U' pk
Oscilloscope Traces Of Surge
(top @drive, rest at the motor)
0
Voltage, ac
-1000
-2000
0
0.1
0.2
0.3
0.4
Time, microseconds
0.5
Problems VSDs may Cause
• Overheating at lower speeds due to inadequate
cooling
• Overheating due to supply harmonics
• Circulating currents in bearings due to rotor
voltage build-up
• Stator insulation problems due to repetitive
voltage surges from voltage source PWM drives
This presentation concentrates on the last issue
Random Wound Stator Winding
End Turn Phase
Paper.
Slot Liners – Ground
Wall Insulation
Wedges - Ground
Wall Insulation
Slot
Separator
Turn Insulation
Insulation Problems in Random
Wound Stators
• Fast risetime of switching transient as well as
possible voltage doubling caused by transmission
line impedance mismatches results in high coil
inter-turn voltages
• For long cable lengths (>30m), surge voltage may
be up to two times DC bus voltage
• If regenerative braking (traction motors),
overvoltage may be even higher
Insulation Problems in Random
Wound Stators
• Short risetime voltage surges have frequencies up
to 3 MHz (for 100 ns risetime)
• High frequency component is blocked by
inductance of the coils, and instead passes through
the capacitance to ground
• Result is that as much as 70% of the terminal
voltage is dropped across first coil
• Leads to very high turn to turn stress
Magnitude (%)
Voltage Drop Across Phase End
Coil as a Function of Surge
Risetime
80.0
70.0
60.0
50.0
40.0
30.0
20.0
10.0
0.0
0
0
10
0
0
0
20 30 40
0
0
0
0
50 60 70 80
tr (ns)
0
0
90 100
Insulation Problems in Random
Wound Stators
• If air pockets between turns, or between turn to
ground or phase to phase, may produce partial
discharge (PD)
• PD is the electrical breakdown of the air in the
void (occurs at 3 kV/mm in dry air at 100 kPa)
• Discharges are sparks that can erode the organic
insulation components in the stator insulation
systems (magnet wire film, slot liner, phase
paper).
• Eventually PD causes stator failure
PARTIAL DISCHARGES (PD)
• Occur if there is an air space, the electric
stress exceeds about 3 kV/mm peak (in air
at 100 kPA), and there is an ‘initiatory’
electron
• Air space between adjacent magnet wires
Reducing Risk of Random Wound
Insulation Failure
By reducing Impact of surges:
• Bring motor close to drive (<15 m) to avoid voltage
doubling), but this makes voltage surge risetime shorter
(and thus more dangerous to the turn insulation)
• Increase the length of the power cable to the motor to
many 100s of meters, which will lengthen the surge
risetime (and decrease risk of turn failure), but still leaves
risk of ground and phase insulation failure
• Use surge arrestors and/or dv/dt filters to clip surge
magnitude and/or lengthen surge risetime, but they are
expensive
Reducing Risk of Random Wound
Insulation Failure
By making stator more resistant to surges:
• Use PD resistant magnet wire (some
improvement, but normally not sufficient)
• Eliminate PD up to the highest surge voltage
motor is likely to see by:
– better impregnation (trickle, VPI, etc) to eliminate
voids
– Larger conductors (reduces electric stress)
– Thicker turn, ground and/or phase insulation (again to
reduce electric stress)
Insulation Issues with Form
Wound Stators (2-13.8 kV)
• Motors now operating up to 65 MW, 13.2
kV driven by voltage source PWM
Converters
• Usually form wound windings
• Slot conductive coatings for 3.3 kV and
above (since higher peak voltages)
• Silicon carbide stress control at 6 kV and
above
MV Insulation System
Anatomy
Glass
Protectiv
e tape
Stress
Control
tape
Form-Wound Stator Coil Insulation Components
a phase insulation / overhang insulation
1 phase to phase
b mainwall insulation
2 phase to ground
c turn insulation
3 turn to turn
d corona protection shield
e stress grading region
Type II Insulation Stress
Control System
Stress Control
“Traditional” Insulation Aging
Mechanisms
• Improper cooling
• Contamination
• Poor thermal
conductivity
• Repeated starting
Thermal
• Insufficient inter-coil
clearance
• Groundwall
delamination
• Contamination
Electrical
Possible Effects of MV
Converters on Insulation
Four possible aging processes:
1. Turn insulation deteriorating due to PD
2. Groundwall insulation heating
3. Groundwall insulation PD
4. PD suppression coating degradation
Turn Insulation
• Voltage surge risetime will be longer than
for low voltage motors due to multiple
stages (typically >1 ms)
• Usually mica paper turn insulation which is
very resistant to PD
• Thus turn insulation failure much less likely
than for random wound stators
Mainwall Thermal Aging
• Insulation dielectric heating in addition to
temperature rise due to copper loss and core loss
• Increased dielectric loss due to the switching
frequency (currently around 2 kHz) and the
harmonics from fast risetime (100’s of kHz)
• loss tends to increase with frequency
• In addition, for epoxy mica above 120 C, the loss
increases even more since DF increases with T
• Higher operating temperature may lead to faster
thermal aging of groundwall or thermal runaway
Mainwall PD
• With power frequency, peak AC voltage is
1.4 times rms line to ground voltage
• With converters, the ratio of peak AC
voltage to rms voltage may be much higher
• Ratio depends on the number of stages in
the converter, and transmission line effects
• May lead to greater PD activity – and thus
faster electrical aging and ozone attack
Effect on Stress Control Coatings
• High switching frequency creates very high
capacitive currents through the ground insulation
(X = 1/(2pfC))
• capacitive currents flow through resistance of
stress control coatings
• May raise surface temperatures by 50 C
• coatings and adjacent groundwall degrade due to
oxidation
• May result in extreme surface PD activity and
ozone
Effect on Stress Control Coatings
• High frequency capacitive currents will also
change the electric field distribution at the
end of the stator cores
• Highest electric stress at slot end, rather
than end of silicon carbide coating
• Stress control deterioration seems to be the
biggest problem with MV motors right now
Frequency Effect on Stress Control
System
Conducting armour
Stator
Core
Semiconducting grading
(resistance - R)
Groundwall
(capacitance - C)
Copper
Xc = 1
2pfC
Thermal Maps of EW Heating Due
to 60 Hz AC and PWM Inverter
(Espino-Cortes, EI Magazine, Jan. 2007)
Avoiding Premature Failure
Some or all of the following could be used:
• Use next highest voltage class (e.g. 4 kV PWM
motor would use a 6.9 kV insulation system)
• Class H insulation system, for higher temperature
• Class H semicon coatings (since higher capacitive
currents)
• Three part stress control system (normal material
in the slot, highly conductive coating just outside
of the slot, and normal silicone carbide layer, but
interleaved connection
Standards to Qualify Insulation
Systems for Converter Duty Motors
IEC developing two standards:
– IEC 60034-18-41 for Type I insulation systems
(normally random wound stators), published 2007
– IEC 60034 – 18-42 for Type II motors (normally form
wound stators), published 2008
– Type I stators are designed to be PD free over its entire
life
– Type II insulation systems are expected to withstand
PD over its entire life
Type I (Random Wound) Stator
IEC Standard
• Manufacturer must prove the motor will not
have PD for the expected life of the motor
• Sets PD inception voltage limits, based on
severity of the surge environment
• In a severe environment, 460 V rated motor
must have a PDIV >2800V
Type I Tests
• Measure PDIV voltage on actual stators,
using short risetime voltage surges
• To ensure PD free over expected “life”,
PDIV under short risetime surges must
exceed specified limits after full thermal
aging test time (IEEE 117 or IEC 60034-1821)
Type II (Form Wound) Stator
IEC Standard
•
•
1.
2.
3.
Motors expected to endure PD
Qualify insulation system by three separate tests:
Voltage endurance test on turn insulation models
Voltage endurance test on ground insulation models
Voltage endurance test on stress relief coating models
(must use surge voltages)
•
Test 1 and 2 can be done with either 50/60 Hz or surge
voltages
A voltage endurance test applies a higher than normal
voltage and measures time to insulation failure
•
Off-Line PD detection with
Voltage Surges
• PD can occur due to short risetime,
relatively high magnitude voltage pulses
from converter fed drives (IFDs)
• Detection of PD in presence of invertors
voltage surges is a challenge – since the
risetimes are similar
• Desire by OEM’s to test in factory to
determine PDIV for motors
Surge PD Detector
use with Digital Oscilloscope Display
PD from Motor Stator Energized
by Surge Tester
IFD PD Measurements Offline
• Many of the major motor manufacturers are now
designing/processing insulation systems to be PD
free
• These OEMs use surge PD measurements to
evaluate different designs and processes
• At least one major OEM now does factory IFD PD
test on a high percentage of production motors to
prove motor suitable for use with IFDs
3.50
Maximum
3.00
Minimum
PDIV (kV)
2.50
2.00
1.50
1.6 kV min
1.00
0.50
0.00
Mfr A
Mfr B
Mfr C
Mfr D
Mfr E
Mfr F
On-Line PD Measurement Challenges in
Medium Voltage Motors Fed by VS
PWM Inverters
• Off-line and on-line electrical PD
measurement well-established for 50/60 Hz
• PD testing during short risetime voltage surges
from VS-PWM inverters more challenging
since surge has similar frequency content as
PD, but is >1000 times larger
• Need to measure PD in VHF (30-300 MHz)
frequency range to separate PD from surge
voltages
PD Sensors
• Capacitive couplers in VHF range with
additional filtering – 80 pF couplers with 40
MHz lower cutoff
• Further suppress drive switching voltage
surges with additional high pass filters (need to
be tuned for each application)
• Advantage of using 80 pF sensors is that one
can use the PD interpretation database
established for 50/60 Hz machines
AC Voltage Phase Reference
• Also require a fundamental frequency
voltage to synchronize PD with AC voltage
• Fundamental frequency can vary from 10
Hz to 120 Hz
• Due to switching noise on AC voltage,
found it necessary to use a capacitive
voltage divider mounted at the drive.
Portable TGA-B
Continuous PDTracII
On-Line PD Case Study
• 45 MW, 7.2 kV natural gas compressor motors
• System installed in 2007
• Used one 80 pF capacitive coupler per phase
PD Sensors on Motor Terminals
Measured PD and Noise from Operating
Variable Speed Motor at 100 Hz
PRPD Plot – 45 MW, 7.2 kV
Motor
12.5 MW, 3 kV Motor PD
3 .1 6 to 1 0 p p s
1 0 to 3 1 .6 p p s
3 1 .6 to 1 0 0 p p s
1 0 0 to 3 1 6 p p s
3 1 6 to 1 0 0 0 p p s
> 1000 pps
Su b s e t 8
M a g n itu d e
1 to 3 .1 6 p p s
750
P u ls e
[m V ]
B ip o l a r M a c h in e P D
-2 5 0
-2 5 0
-5 0 0
-5 0 0
-7 5 0
-7 5 0
750
500
500
250
250
0
0
-225
-180
-135
-90
-45
0
P h a s e A n g l e [d e g ]
45
90
Conclusions
1. Short risetime voltage surges from voltage
source PWM drives have caused premature
deterioration and failure due to ozone in the
stress relief coatings in service
2. Offline testing using a surge tester and special
electronics can be used to qualify motor
designs.
2. Practical measurements of on-line PD have
been demonstrated on dozens of motors fed
with VS-PWM inverters