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Qualitrol-Iris
Power
DIAGNOSTIC NEWS
January 2013
Why Partial Discharge Suppression Coatings are Needed in Stator
Windings
By: Greg Stone Inside this issue: Why Partial Discharge Sup‐
pression Coatings are Needed in Stator Windings 1,3 Blake Lloyd New President of IEEE‐IAS 1 MCPDTracII—New Continuous PD Monitor for Distribution Class Switchgear 2 2013 Training Courses 2 Flux Probes 3 QCMC 2013 and IRMC 2013 4 Qualitrol‐Iris Power Customer Service Department 4 Stress relief coatings are important insulation system components in stator windings operating at 6 kV or above. These coatings are present to prevent partial discharges (PD) occurring on the surface of the stator bars or coils. They prevent PD from occurring in any air gap that might be present between the coil/bar surface and the stator core, or in the endwinding near the end of the stator core. PD gradually degrades the groundwall insulation, and can create significant amounts of ozone. The coating in the slot is usu‐
ally a black paint or tape and is often referred to as a partly conductive or semiconductive coating or shield. The stress control coating on the end‐
winding area just outside of the slot is usually grey or red in color (although it is sometimes not visible since it is overcoated with an insulating varnish). This coating is also called the silicon carbide coating. SLOT SEMICONDUCTIVE COATING The reason PD may occur between the coil and the core is similar to the reason PD can occur in air pockets within the groundwall. Since coils and bars are fabricated outside of the stator core, they must be thinner in the narrow dimen‐
sion than the width of the core's steel slots, oth‐
erwise, the coils/bars cannot be inserted into the slot. Thus an air gap between the coil/bar surface and the core is inevitable. The global VPI process may fill in much of the gap with epoxy or polyes‐
ter between the coil surface and the core, and thus in theory does not require stress relief coat‐
ings. However, because of thermal cycling con‐
siderations, most machine manufacturers will still use a semiconductive coating with global VPI stators.
Continued on page 3... Blake Lloyd New President of IEEE Industry Application Society
UPCOMING EVENTS 2013
EPRI—TGUG Users Group Jan 21‐22 Savannah, GA Energy Generation Conference Jan 29‐31, Bismarck, ND NETA Power Test Feb 18‐21 2013 New Orleans, LA Westem Turbine User Meeting Mar 10‐13 San Diego, CA CEATI Hydropower Workshop Mar 12‐13 Las Vegas, NV 501 F&G User Group Meeting Mar 18‐21 Charlotte, NC Blake Lloyd, a co‐founder of Iris Power‐Qualitrol, and its current Director of Development, took over the role of President of the IEEE Industry Application Society (IAS) beginning January 1, 2013. IEEE is the world’s largest organization of technical professionals with more than 400,000 mem‐
bers of which more than half are from outside the USA. IEEE puts on more than 1,350 conferences annually (4 per day on average!) and publishes more than 100 leading journals on all aspects of electrical engineering. The IAS is one of the main societies in the IEEE and has approximately 10,000 members with over 170 local Chapters around the world. IAS sponsors over 20 technical conferences per year, has 2 highly rated technical publica‐
tions, and supports relevant Standards work of concern to petrochemical, pulp and paper, mining and cement industries. IAS is one of the least academic societies in the IEEE with over 80% of members from industry. The IAS is or‐
ganized into 4 departments including Manufacturing Systems, Process Indus‐
tries, Power Systems, and Industrial Power Conversion. Blake will lead the IAS for a 2‐year term as President. In this role he is responsible for the management of the society business and interfacing to other IEEE entities. This is the highest level in IEEE that any Qualitrol asso‐
ciate (or indeed Danaher associate) has ever attained, and yields Outgoing President, Bruno Lequesne exceptionally high visibility for Qualitrol amongst the world’s electri‐
(right), passes the gavel to Blake Lloyd
cal engineers. Qualitrol‐Iris Power—3110 American Dr.—Mississauga—Ontario—Canada—L4V 1T2 Telephone: 905‐677‐4824; Fax: 905‐677‐8498; Email: [email protected]; www.irispower.com—www.qualitrolcorp.com Page 2
MCPDTracII – A New Continuous PD Monitor for
Distribution Class Switchgear
2013 Training Courses EL CID—ACE Course March 26‐28 and September 24‐26 Toronto, Canada Hydrogenerator Maintenance Course September 10‐12 Denver, CO, USA Partial Discharge Course September 17‐19 Charlotte, NC, USA For more information, contact: [email protected] The MCPDTracII is a new product from Qualitrol
‐Iris Power that is specifically designed to measure the partial discharge (PD) in motor control centers or other air insulated switchgear (AIS) rated up to 35 kV. PD in such switchgear leads to tracking and eventual failure. The MCPDTracII can monitor multi‐
ple sets of 80 pF EMC couplers that detect partial discharge. The sets of PD sensors are connected at intervals in a long isolated phase bus (IPB) or within AIS cubicles. As the product name suggests, this continuous monitor is an extension of the PDTracII and built on the TracII platform introduced last year. This second generation of continuous monitors provides supe‐
rior features to the original Trac product line, such as multiple selection of test ranges, configuration by memory stick and comprehensive data archive with phase‐resolved PD plots. For partial discharge monitoring of switchgear, couplers are connected to the main bus‐bars every two or three cubicles or panels apart. The array of couplers will allow the location of partial discharge to be determined using the time‐of‐flight technique. If the MCPDTracII detects increasing or significant discharge activity, a TGA‐B portable instrument can be used to determine the circuit where the dis‐
charge occurs, by varying the delay time parameter entered in the PDLitePro software and watching the change in pulse classification when the delay time is gradually shortened. For day‐to‐day monitoring, the MCPDTracII is applied to measure the installed coupler sets one by one. The multi‐
plexor fitted in the MCPDTracII unit can accept up to 48 couplers, or 16 sets (Figure 1). It will step through each of the cou‐
pler sets and take measurements. Compared to the Fig. 1 Multiplexor in MCPDTracII previous PDTrac or BusTrac monitors, which were designed to read only three and six couplers, re‐
spectively, one MCPDTracII can monitor 16 times the number of couplers that are connected to a PDTrac. This means savings to the customer but still complete coverage of all installed couplers. The first MCPDTracII unit was installed to moni‐
tor a line of 15 kV switchgear in Houston, Texas (Figure 2). The switchgear assembly consists of two switchgear sections that are connected by a nor‐
mally‐open bus‐tie circuit breaker. PD sensors are Fig. 2 MCPDTracII mounted in a 15 kV switchgear connected at the ends of the bus‐bars. They were installed by the switchgear manufacturer and Iris Field Service checked the “delay time” of coupler pairs after erection of the switchgear at site. Ini‐
tially two PDTrac units were supplied to cover the two bus‐bars and one set of couplers was wired to the input of each PDTrac. Extrapolating the same idea, an MCPDTracII monitor can handle more than one switchgear line‐
up or switchboards if the MCPDTracII can be placed somewhere such that the coaxial cables from the PD sensors do not exceed 100 feet or 30 m. A new version of PDTracPro software is used for instru‐
ment configuration and data download (Figure 3). The appearance of the user interface is the same, except that when an MCPDTracII unit is attached, an Fig. 3 PDTracPro User Interface additional step of mapping the coupler set to the appropriate input channel of the multiplexor is necessary. Over the last few years in North America, Iris couplers have been installed in a number of refiner‐
ies and utility distribution systems. These custom‐
ers consider their switchgear to be critical equip‐
ment in the electrical power distribution system and partial discharge monitoring is key to keeping track of the health of the switchgear.  For more information about the exciting new fea‐
tures of the MCPDTracII, please contact the Iris sales representative in your area. Qualitrol‐Iris Power—3110 American Dr.—Mississauga—Ontario—Canada—L4V 1T2 Telephone: 905‐677‐4824; Fax: 905‐677‐8498; Email: [email protected]; www.irispower.com—www.qualitrolcorp.com Page 3
Why Partial Discharge Suppression Coatings are Needed in Stator
Windings Continued from page 1 FLUX PROBES
Detecting shorted turns in synchronous rotor wind‐
ings is best done using mag‐
netic flux monitoring. Many machines are already equipped with the necessary flux probe installed in the air gap. Where there is no probe, Iris offers two options for a probe. If the air gap (the distance between the rotor and the stator) is less than about 50 mm, the best probe to install is the TFProbe, which is easily TFProbe
glued to a stator tooth. In many cases the TFProbe can be retrofitted when the rotor is within the motor or generator. If the air gap is greater than about 50 mm, then we recommend installation of the FFProbe. The FFProbe is "foldable", in that during rotor installation and re‐
moval, the probe will lie flat, reducing the risk of damage. The FFProbe is installed on a stator wedge, and regretta‐
bly, can only be installed when the rotor is not within the machine. FFProbe folded (top) and
in normal position
Figure 1 shows the gap that can occur in the slot, adjacent to the coil sur‐
face, since the coil is undersized. An electrical equivalent circuit is also shown. An equivalent circuit is shown in Figure 1. A surprisingly large percentage of the applied volt‐
age will appear across the air gap. If the electric stress (E = V/d) in the air gap exceeds 3 kV/mm, PD will occur, at least in an air‐cooled machine. This PD will eventually erode a hole through the groundwall, causing failure. Discharges on the coil/bar surface are sometimes referred to as slot discharge, since they can be seen in the slot. Un‐
der practical conditions, most stators rated 6 kV or more will experience this PD on the coil/bar surface. To prevent PD on the coil or bar surfaces, manufacturers have long been coating the coil/
bar in the slot area with a partly conductive coat‐
ing. The coating is usually a graphite loaded paint or tape. This coating is likely to be in contact with the grounded stator core at many places along the length of the slot. With a sufficiently low resis‐
tance (say a few kilohm), this coating is essentially at ground potential because of the contact with the core. Thus the voltage across any air gap is zero. PD cannot occur in the gap, because the electric stress will never exceed 3 kV/mm. The result is that the semiconductive coating prevents surface discharges in the slot. Note that the coat‐
ing cannot be highly conductive, since this will short out the stator core laminations, leading to vibration sparking. Semiconductive coatings on coils in the slot are not normally needed for stators rated 6 kV or less. Clearly this is because it is unlikely that the critical threshold of 3 kV/mm electric breakdown strength of air will occur at this low operating voltage, even if a substantial gap occurs between the coil and the core. The use of a thin groundwall in 4 kV stators is causing some motor manufactur‐
ers to use semiconductive material since it is pos‐
sible for the 3 kV/mm threshold to occur if the groundwall is thin. Similarly motors rated 3.3 kV and above connected to a PWM voltage source converter will also normally have the semicon‐
ducting coating, due to the high voltage spikes that can occur with such variable speed drives. SILICON CARBIDE COATING The low resistance semiconductive slot coat‐
ing usually extends only a few centimeters be‐
yond each end of the slot. The thin edge of the coating creates a very non‐uniform electric field at the end of the slot coating since the electric stress depends strongly on the inverse of the radius, just as the sharp edge of a knife concen‐
trates the mechanical force in a small area. This field would exceed 3 kV/mm, and PD would occur at the end of the coating. Such PD would eventu‐
ally destroy the insulation in the vicinity, leading to failure. Thus, just as for high voltage cables, the end of the semiconductive slot coating must be 'terminated'. The 'termination' method em‐
ployed today uses silicon carbide. Silicon carbide is a special material that has an interesting property: as the electric stress in‐
creases in this material, its resistance decreases. In the past, silicon carbide was used in high volt‐
age surge arrestors to divert to ground high volt‐
age surges from lightning (i.e. have a low resis‐
tance state) while being fully insulating during normal operating voltage of a transmission line. When applied to stator coils and bars, the silicon carbide has a very low resistance in the high stress region at the end of the slot semiconduc‐
tive coating, and gradually increases its resistance further along the endwinding from the core. This varying resistance makes the electric field at the end of the semiconductive coating more uniform. Usually, the stress is reduced to below the critical 3 kV/mm (in air) that would initiate PD. Silicon carbide is usually mixed into a paint base, or incorporated into a tape that is applied to the coil/bar surface. The length of the silicon car‐
bide surface coating depends on the voltage rat‐
ing, but 5 to 10 cm is usual. Qualitrol‐Iris Power—3110 American Dr.—Mississauga—Ontario—Canada—L4V 1T2 Telephone: 905‐677‐4824; Fax: 905‐677‐8498; Email: [email protected]; www.irispower.com—www.qualitrolcorp.com Qualitrol‐Iris Power Customer Service Department QCMC 2013
With the success of the Qualitrol Condition Monitoring Conference in Dubai, UAE last November, we are pleased to confirm that there will be an‐
other QCMC in the Middle East in 2013. The QCMC is a technical conference offering both in‐
depth half‐day courses on con‐
dition monitoring methods, as well as presentations on state‐
of‐the‐art aspects of electrical equipment design, failure and repair. As with the North American based IRMC, this information is provided in a non‐commercial environment by world‐class experts offering a range of points‐of‐view. Visit the Qualitrol‐Iris Power website for more details as they become available. We continue to experience very encouraging growth, in terms of Sales and opportunities for our business globally. This is largely due to customers who believe in the benefits of our products and services. Iris has always recognized the impor‐
tance and value of supporting our customers and we remain committed to this approach with a focus on providing the best support that will assist our customers to achieve their goals. In our efforts for continuous improvement, our customers are encouraged to provide feedback to us so that we can build and establish robust proc‐
esses to meet the deliverables, as defined by cus‐
tomer requirements. Dr. Greg Stone was recently a keynote speaker to the IEEE International Conference on the Properties and Applications of Dielec‐
tric Materials, held in Bangalore, India. Our team is dedicated to providing Commer‐
cial and Technical support. The group is as follows, starting from the left in the picture: Siraj Shaikh—Senior Technical Support Specialist Rajiv Sharma—Manager, Global Customer Services (Generation) Lisa Zarudny—Sales Analyst James Thompson—Senior Technical Support Spe‐
cialist Byron Mazariegos—Product Specialist Madge Wozniczka—Sales Analyst Chintan Pandya—Inside Sales Representative Pablo Rojas—Senior Product Specialist Farhan Mithani—Sales Analyst IRMC 2013 June 17‐20 InterContinental New Orleans 444 St. Charles Ave. New Orleans, LA Qualitrol‐Iris Power will be hosting the 16th Annual Iris Rotating Machine Conference (IRMC) June 17‐20, 2013 in New Orleans, LA.
This annual conference is attended by industry professionals from around the world and covers topics such as: 
motor or generator failures 
new diagnostic test methods 
winding repair methods 
detection of rotor winding problems 
new developments in rotating machine insulation 
the effect of inverter fed drive surges on motor insulation 
case studies on motor and generator predictive maintenance Visit the Qualitrol‐Iris Power website for more details as they become available. Abstracts for papers are due by February 15. For more information, contact: [email protected] Qualitrol‐Iris Power—3110 American Dr.—Mississauga—Ontario—Canada—L4V 1T2 Telephone: 905‐677‐4824; Fax: 905‐677‐8498; Email: [email protected]; www.irispower.com—www.qualitrolcorp.com