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Iris Power LP APRIL 2009 I N T HI S IS S U E: Upgrading MotorTrac to PDTrac Spotlight on Contract Administration Upcoming Events Pg. 3 Pg. 4 Pg. 4 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 Breaker Conference 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 1 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 2 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 3 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 4