Download D N IAGNOSTIC EWS

Iris Power LP
APRIL 2009
I N T HI S IS S U E:
Upgrading MotorTrac to
PDTrac
Spotlight on Contract
Administration
Upcoming Events
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UPCOMING EVENTS
IRMC 2009
May 11-14, 2009
New Orleans, LA
IEEE IAS/PCA
Cement
May 31-June 4,2009
Palm Desert, CA
IEEE Pulp and
Paper
June 21 - 26, 2009
Birmingham, AL
Waterpower 2009
July 27-30, 2009
Spokane, WA
IEEE 2008 PCIC
Finepoint Circuit
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DIAGNOSTIC NEWS
POWER
Sept. 14-16, 2009
Ahahaim, CA
Oct. 5-9, 2009
Atlanta, Georgia
Your Source For Monitoring the Reliability of Electrical Equipment
EFFECT OF INVERTER DRIVES ON MOTOR STATOR
INSULATION RATED >2000 V
By Greg Stone
In past issues of the Diagnostic News we have discussed how modern inverter fed drives (IFDs) can lead to partial discharge (PD) and stator winding failure in random wound
motor stators rated as low as 440 V. In this article I will concentration the effect of the
voltage source type of pulse width modulated (PWM) IFDs on stators rated about 2400
V and above. These stators are usually if the “form wound” type. Unlike the situation in
low voltage motors, PD is not the only concern.
IFDs of the PWM voltage source type are now being produced up to 13.8 kV. Presently,
such drives can produce up to about 2000 pulses per second that may be 1000 V or so
in magnitude. At the higher voltages, these drives tend to have many levels, where the
switching devices are cascaded on one another to reach the desired terminal voltage.
The resulting waveform is characterized by the step voltages from transistor switching
at the “clock” frequency of the drive, as well as by the peak voltage of the applied voltage and the “fundamental” frequency which determines the running speed of the motor.
The IGBT and ICBT switching devices created short risetime voltage pulses that generate frequencies up to a few megahertz. In addition, and as with low voltage drives, the
transmission line effects between the power cable and the motor can increase the magnitude of the voltage beyond the step created by switching a DC voltage. The repetitive,
short risetime voltage surges (or jumps) result in a more complicated impact on the insulation system, than occurs in random wound stators.
Turn Insulation PD
The repetitive, short risetime voltage impulses may cause a high percentage of the voltage step at the motor terminals to appear between the turns in the first coil. If there are
any small voids adjacent to the turn insulation, which is often inevitable even with the
best global VPI processing, then partial discharges may result. The electron and ion
bombardment of the insulation by the PD will gradually erode the insulation. Modern
mica paper turn insulation is one of the most PD-resistant materials ever developed –
but even this material will gradually puncture if the PD magnitude is high enough and
enough PD pulses occur over time. If less resistant materials such as Dacron and glass
are used as the turn insulation, failure may follow relatively rapidly (which is why converter duty motors should have mica paper turn insulation). Once puncture of the turn
insulation occurs, a large current will flow around the affected turn, which will rapidly melt
the copper and groundwall insulation in the vicinity of the original puncture. A ground
fault will soon occur. Thus the design of the turn insulation must be evaluated to determine how it resists this PD failure process.
Iris Power ~ 3110 American Dr. ~ Mississauga ~ Ontario ~ Canada ~ L4V 1T2
Phone: 905-677-4824 ~ Fax: 905-677-8498 ~ E-mail: [email protected] ~ www.irispower.com
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Effect of Inverter Drives on Motor Stator Insulation Rated >2000 V Continued...
How significant this failure process is depends on the risetime
of the voltage steps (the shorter, the more likely the process),
the magnitude of the steps (which in turn depends on the DC
bus voltage) and the switching frequency of the converter (the
higher the frequency, the greater the number of PD pulses will
occur per second, and thus the faster the aging rate).
Groundwall Insulation Heating
The PWM converter voltage waveform can lead to increased
groundwall insulation heating, which can increase the winding
temperature and thus accelerate the normal thermal aging
processes described above. The epoxy mica insulation material has a property called dielectric loss or dissipation factor.
Ideally the insulation acts as a pure capacitance, and when a
voltage is applied, there is no power loss and thus no selfheating of the insulation. However, real materials such as
epoxy mica contain polar molecule groups that tend to move
when a changing electric field is placed across them. This
vibrational motion of the molecules causes heating of the
material. For epoxy mica, about 0.5% of the 50 or 60 Hz
capacitive current in the insulation goes to molecular losses
that cause heating. This is termed the dissipation factor
(which is essentially the same as the power factor). In a conventional motor, this “dielectric loss” is very minor compared to
the copper I2R and core losses.
As the PWM switching frequency increases, we find that the
power consumed increases linearly and so does the winding
temperature. Consequently PWM converters will cause the
stator winding insulation temperature to be higher than for a
conventional motor, all other things being equal, and therefore
thermally age faster. The current generation of medium voltage drives does not seem to produce significant increases in
temperature in the groundwall due to dielectric heating – but
this could change as the switching frequency and the magnitude of the voltage steps increase.
Groundwall Insulation PD
Most conventional stator windings rated 3.3 kV and above
may have PD within the groundwall, if voids of sufficient size
are present. The voids may be from poor manufacturing, or
occur over time as a result of delamination due to thermal
aging. Similarly, PD may occur as a result of the voltage
surges from converter drives. But the PD may be larger and
more frequent with an IFD, because the peak voltages are
usually higher than the peak voltage from a sinusoidal supply.
The peak-to-peak voltage can be higher than from a 50/60 Hz
supply due to the transmission line effects that may cause the
jump voltage changes that occur with converters to possibly
double. The peak to peak voltage in a drive system will be
worse if fewer stages are used in the converter (thus a greater
jump voltage occurs), the voltage risetime is shorter, and/or the
power cable between the converter and the motor is longer.
The rate of aging is primarily determined by the fundamental frequency of the converter.
PD Suppression Coating Degradation
The partly conductive coatings that normally cover the
coil insulation in the stator slot and the silicon carbide
material at the slot exits are intended to suppress the
probability of PD occurring on the coil surface in the slot
and just outside of it. Several studies have shown that
under PWM voltage, these coatings will operate at higher temperatures and thus increase the rate of thermal
aging, if they are not properly designed. Since PWM voltage waveforms contain voltages at high frequencies
(from the risetime of the voltage steps and the PWM
switching rate), higher capacitive currents flow through
the groundwall and then through the PD suppression
coatings. These higher currents create higher I2R losses in the coatings than would occur under 50/60 Hz operation - increasing the operating temperature of the coatings. The result is that an inadequately designed stress
relief system may create a temperature rise as much as
50 C above that which would occur with a 50/60 Hz sinusoidal voltage. This local heating of the coatings will
accelerate the thermal aging of both the coatings, and
the adjacent groundwall insulation. When the coatings
degrade, surface PD may be extensive, creating lots of
ozone gas. In the voltage source PWM medium voltage
converter fed motors in operation to date – this is the
problem that has created the greatest concern about the
service life of the stator insulation.
New IEC Standard
The IEC has expended considerable effort in the past 7
years to develop new technical specifications for the
insulation systems of converter driven motors. IEC has
just published IEC TS 60034-18-42 for form wound stators. This technical specification requires that motor
designers “qualify” the stator insulation system design as
being fit for PWM voltage source drives by undertaking 3
separate voltage endurance tests.
One voltage
endurance test to evaluate the PD resistance of the turn
insulation. The second test is to determine the voltage
endurance of the groundwall insulation. The last voltage
endurance test is to ensure that the stress relief coating
can survive the repetitive voltage surges from the IFD.
The voltage endurance tests for the turn and groundwall
insulation can be performed with 50 or 60 Hz voltage, if a
suitable adjustment for the IFD frequency is made.
However, the stress relief coatings must be tested with
voltage surges to properly replicate the failure process.
Endusers, as well as motor insulation systems designers
should become familiar with the new qualification test
procedures.
Iris Power ~ 3110 American Dr. ~ Mississauga ~ Ontario ~ Canada ~ L4V 1T2
Phone: 905-677-4824 ~ Fax: 905-677-8498 ~ E-mail: [email protected] ~ www.irispower.com
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UPGRADING MOTORTRAC TO PDTRAC
This note describes why and how to upgrade the MotorTrac
monitor, a single-ended continuous on-line PD monitoring
system, to PDTrac monitor.
HOW TO UPGRADE TO PDTRAC
INTRODUCTION
Iris Power offers several types of PD installations depending of the machine type. The single-ended BUS installation
with three 80 pF Epoxy Mica Capacitors (EMCs) in the terminal box is used mainly with medium voltage motors and
small generators with power cable with a length of 30
meters or longer.
The first continuous on-line PD monitoring system for single
ended installations that Iris Power offered was the
MotorTrac monitor. After several enhancements as
response to customer needs and availability of new technologies, the MotorTrac evolved to the current PDTrac monitor.
WHY PDTRAC?
The fundamental incremental feature of the PDTrac monitor
is the networking capability for remote configuration, testing,
and displaying. The following is a summary of the enhancements incorporated in the PDTrac over the MotorTrac monitor (for more details see the PDTrac Manual):
Networking option with,
RS485 communication
Ethernet communication
Remote configuration (alarm settings, sensitivity
ranges, etc.)
Serial fiber optic communication option
MotorTrac Monitor
2 D data display (recorded every 9 days)
Noise data recording, with superior algorithms for noise
discrimination
There are two options to upgrade
Input modules for ambient temperature, ambient humidity, and either voltage, power, current or stator temperature Uninstalling the existing MotorTrac monitor and installing
a new junction box (it does require shutdown). Installation
(the MotorTrac monitor has only one input for temperature)
according to the PDTrac Installation Guide.
Up to 8 analogue outputs for PD activity option
Using existing MotorTrac enclosure as coupler termina Friendly control and displaying software
tion box/junction box (it does not require shutdown).
PD data display in both PDTracPro and PDView
software (compared to MS Excel in MotorTrac).
Mount the PDTrac monitor close enough, within 50
cm (20 in), to the MotorTrac enclosure for the pro
Note: PDView is also used to view data from
vided 65 cm (30 in) coaxial jumper cables.
portable instruments.
Multi-language support (PDTracPro )
Open the front cover of the MotorTrac monitor and
Windows Vista compatibility
disconnect the internal BNC plugs from BNC jacks
(A, B and C) on the Termination Board (Figure 1).
Separate coupler termination box/junction box for tests
with portable instrument and safe and secure removal of
continuous monitor.
Route the provided jumper cables in a 2 cm (¾ in)
conduit to connect the PDTrac monitor to the BNC
Hazardous location certification option (ATEX or UL)
jacks (A, B and C) on the Termination Board of the
existing MotorTrac enclosure.
Nuclear containment option
OPC interface for data output option
Iris Power ~ 3110 American Dr. ~ Mississauga ~ Ontario ~ Canada ~ L4V 1T2
Phone: 905-677-4824 ~ Fax: 905-677-8498 ~ E-mail: [email protected] ~ www.irispower.com
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PD MONITORING ON MOTORS SUPPLIED FROM VFD
SPOTLIGHT ON CONTRACT ADMINISTRATION
In recent years rapid advances in the field of power electronics have lead to the development of solid state, power switching devices that produce continuous fast rise time voltage
surges that influence PD measurements. Such devices are
used in the modern variable frequency converter (VFC) drives
that control the speed of driven equipment by varying the
motor power supply characteristics. Iris already makes a line
of PD equipment for VFC drive motors rated <1000V (PD
Alert). The following considerations refer to the PD monitoring
for motors with form windings having voltage ratings of 2.3 kV
and above.
The Contract Administration Group, headed by Kim Zarb, is
responsible for preparing and issuing quotes, verifying the accuracy of purchase orders, sheduling site visits and issuing reports.
Madge, Reena and Olga are sales analysts, while Shelly and
Michelle work closely with Field Services.
From left to right: Madge Wozniczka, Olga Aviles, Reena Chadha,
Shelly Zikovic, Kim Zarb, Michelle Mathias
LCI Drive With Two-Winding Synchronous Motor
There is no standard solution for the application of periodic PD
monitoring to motors supplied from VFC’s and in the case of
voltage source pulse width modulated converter types, with
fast switching devices, this will be impractical with the present
commercial IRIS, PD measuring instruments.
It is much more likely that PD monitoring can be applied to
motors fed from current source converters, load commutated
converters and cyclo-converters. However, for these types of
drives the only practical way to determine whether PD monitoring can be applied to the motor is to first install EMC’s and
then take measurements when the motor is powered from its
associated VFC. In all of these cases it is likely that extra filtering would be needed ether in the TGA-B instrument or EMC
terminal box based on how the measurements would be done
(client TGA or Iris service).
Periodic PD monitoring can be applied to VFC supplied
motors if the drive has a 50/60Hz bypass providing, under normal operation, the voltage surges seen at the BNC termination
surge suppressors do not damage them. This needs to be
investigated.
Until more experience is gained PDTrac and BusTrac continuous PD monitoring instruments should not be applied to
motors supplied from VFC’s.
For more details please request the technical note “PD
Monitoring on Motors Supplied form VFD” from your Sales
Representative.
UPCOMING
EVENTS
IRIS ROTATING MACHINE
CONFERENCE 2009 (IRMC)
Join us for the 2009 IRMC in
New Orleans, Louisiana
May 11-14, 2008
Venue:
InterContinental New Orleans
444 St. Charles Ave.
New Orleans, LA 70130
COURSES
Iris will be running the following courses in
2009:
ACE Course
September 15-17, 2009, Toronto, ON
Hydrogenerator Maintenance Course
April 28 - 30, 2009, The Coeur d'Alene Resort
Partial Discharge Course
December 1-3, 2009, Long Beach, California
For more information, or to register, please contact
Michelle Mathias at [email protected].
Iris Power ~ 3110 American Dr. ~ Mississauga ~ Ontario ~ Canada ~ L4V 1T2
Phone: 905-677-4824 ~ Fax: 905-677-8498 ~ E-mail: [email protected] ~ www.irispower.com
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