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Understanding Power Factor Testing Resultsrelative to

Modern Instrument Transformer Insulation Designs

Spring 2009, Doble Conference

Nick S. Powers ABB KuhlmanRolando Gomez Arteche

Introduction:Power factor testing of insulation systems is a well understood concept, but applying it toinstrument transformers of all types and sizes can be difficult as it is not necessarily intuitive.This paper will provide insight into the different transformer insulation structures, and how bestto apply power factor testing to truly measure the dielectric integrity of the transformers.Designs to be detailed are-

Single Stage Inductive Voltage TransformersMulti-Stage or Cascade Inductive Voltage TransformersCoupling Capacitor Voltage Transformers

Station Service Voltage Transformers (Test similar to VTs)Current TransformersSingle Phase Combination Current/Voltage Transformers (Test similar to VTs)

From experienced gained over the last several years, a review of some difficult testing issueswill be discussed and solutions for proper testing proposed.

Modern Instrument Transformer Insulation Designs:Although the primary purpose for instrument transformers is to accurately transform the voltageand current information from a high level primary down to a standardized low power output, themost critical criteria that defines long term survival is the major insulation system. This task ismade even more difficult as these devices not only must survive both the steady state operatingcondition, but also must function satisfactorily up to even higher level transient conditions. Oil-filled insulated instrument transformers have operated satisfactorily under these conditions formany decades, and their reliability is not only designed and built into the device at themanufacturing facilities, but also confirmed via testing at both the factory as well as in the fieldby the end users.

Instrument transformers are generally considered to be stable passive units, requiring little or nomaintenance throughout their lifetime, which can be in the range of 25 to 30 years. It is difficultto know, however, the actual condition of the installed transformer and if it fails, it can failunexpectedly and in some instances violently.


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As the confirmation of the insulation state is of such critical importance, users primarily usesome form of insulation power factor testing to provide benchmarks for initial acceptance(generally 0.5% power factor or less), and for continued acceptance of the condition of theinstalled base of instrument transformers.

Different test configurations have been used in order to measure the Power Factor for various

type of instrument transformers. Experiences, customs and usage have determined theapproaches taken by the users. These different approaches have been a reason for

disagreement with the customers due to inaccurate measured values. So toprovide necessary information to reach the common goal of insuring reliableperformance of the critical high voltage instrument transformers on the utilitygrid, we must first define fully the insulation system.

High voltage insulation systems:Inductive voltage transformer (VT)- These transformers are also calledmagnetic voltage transformers and can be classified into two basic forms. Thefirst configuration is called the single stage inductive voltage transformer wherethe unit consists of a single primary coil coupled through an steel core to

secondary windings (Figure 1). In well designed units, the primary andsecondary windings are separated with an insulation structure that isconservatively design to control the dielectric stress both between the windings,as well as over the insulators surface.

This is done using high quality kraft paper and intermediate shield layers, alongwith relatively large smooth line and ground shields or electrodes as shown in

Figures 2, 3, and 4.Topreventhighvoltagesfrom being

capacitivecoupledinto the

secondary windings from the high primary voltage, a reliable inductive design alsoincludes a ground shield placed between the two windings. The placement of thisshield, while helping to contain the high voltage dielectric stress to the majorinsulation, can also be problematic for interpreting power factor results due to itsproximity to the low end of the high voltage winding. This will be discussed later inthis paper.

The second design configuration is shown in Figure 5, and is a multi-stageinductive voltage transformer that is referred to as a cascade design inductive

voltage transformer. In this unit, two or more coils with individual insulationstructures as shown in the previous photos are coupled together through the steelcore. This allows reasonable sized insulation structures to be unit in tandem todistribute the stress line to ground through the voltage transformer. With thesemultiple insulation structures, power factor testing can be complicated and withsome tests, misleading.

Fig. 1

Fig. 5

Fig 2

Fig 3

Fig 4


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Coupling capacitor voltage transformer (CCVT) This design differs fromthe inductive design in that it uses a capacitive stack as a voltage dividerso as to introduce acontrolled voltage intothe base region of the

CCVT shown in Figure 6.In the base, which caneither be air insulated oroil-filled, the voltage isapplied to a reactor thatshifts the voltage signalback into phase with theprimary signal and thenthis medium voltage istransformed down to thefinal output voltage ratedfor the instrumentation equipment. A detail is shown in Figure 7.

Station Service Voltage Transformer (SSVT)Figure 8 This design is a variation of the singlestage inductive transformer that uses the same

condenser style conservative design of kraft paper and shield layers todistributed the dielectric stress from line to ground across the insulatorsurface, as well as through the major insulation to ground. The SSVTcan have either circular disc wound coils connected together, or a layerwound design with kraft paper inner layers and special resin coatedpaper to improve short-circuit strength. Figure 9 shows a condenserbushing ready to be applied to an assembled core/coil, and Figure 10is an assembled core/coil inside the SSVT Tank.

The condenser style inner bushingextending from the dome to theprimary coil located in the baseregion has smooth line electrodes,as well as ground electrodes tofully control the voltage stress

within the major insulation of the transformer. The transformer

inner condenser bushing normally does not have a test tap to fullyisolate the insulation structure within the transformer housing, buta variation of this type tap has been optionally offered at the230kV level for the SSVT.

This design has a relatively large core and increased secondary copper cross section forsupplying power requirements for substation auxiliary power. From a dielectric perspective, thisdevice can be treated as an inductive voltage transformer and tested in a similar manner.

Fig. 8

Fig. 6

Fig. 7

Fig. 9Fig. 10


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Current Transformer (CT) There are two distinctly differentconfigurations of wound oil-filled current transformers available in themarket today. In a design where the primary conductor is fully insulatedwith the major insulation, and is routed down into the lower section of thetransformer where partially insulated steel cores and secondary windings

are applied over the primary is called a hairpin design. If the majorinsulation is applied directly to the secondary windings and steel cores ina toroidal fashion, with an downward inner condenser bushing, thisdesign is call a head or inverted eyebolt design. Both are tested in asimilar manner, so for simplicity, we will use the inverted eyebolt designshown in Figure 11 for illustration purposes in this paper.

In the inverted eyeboltdesign, the toroidalcores are located up inthe dome region of thecurrent transformer,

and the secondaryleads are taken fromthe dome down to the

low voltage box at the base of the CT via ahollow electrode that provides the form over

which the inner bushing major insulation isapplied. The core shield and the CT secondarywinding on toroidal core are shown in Figure 12.Figure 13 illustrates a fully taped insulated coilready for assembly.

The electrode in the center of the insulated coil is

either solidly grounded at the base, or is suppliedwith a tap that has a jumper to ground that can beisolated and used for power factor or dissipationfactor testing. This normally grounded tap tied tothe hollow electrode provides safe passage forthe secondary leads down the high voltageinsulated inner bushing. In CTs where thisdissipation tap is supplied, this helps isolate themajor insulation from the base and secondarywindings in the CT for ease in obtaining a reliablepower factor reading. Details on this test formatwill be discussed later.

Figure 5Fig. 11

Fig. 12

Fig. 13


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Combination Single Phase Metering Units (MU or CT/VT)- Where the users want to reduce thesize of the metering transformer footprint within the substation, manufacturers produce ametering transformer that consists of both a single stage inductive voltage transformer and acurrent transformers. The majority of the voltagetransformer is located in the base where the unit

consists of a single primary coil coupled throughan steel core to secondary windings (Figure 14).The high voltage electrode that supports theinner condenser bushing is taken up through theporcelain adjacent to the inner bushing electrodeof the current transformer.

An inverted eyebolt current transformer design islocated up in the dome, with its inner bushingelectrode running downward next to the innerbushing of the voltage transformer. To insurereliable long term performance free of partial

discharges, these electrodes must be alignedsuch that the dielectric field plots from each ofthe two insulation structures are equal so as notto stress one to the other and cause extremevoltage stress.

With the dielectric stresses properly controlled,the single phase metering units have givenreliable insulation performance comparable tothe separate CT and VT. With this nesting ofthe two insulation structures, substantial spacesavings are achieved, which greatly reduces the

real estate needed for installation of the meteringtransformers.

Fig. 14


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Proper Power Factor Testing of the Instrument Transformer Insulation:Inductive Voltage Transformers - Testing of inductive voltage transformers are tested inaccordance with the following test format. This generally gives reasonable results in evaluatingthe insulation conditions on these transformers.

However, there have been reported cases of test readings on the Test 1 (Overall) test wheresome of the inductive transformers show greater than 0.5% PF results. The results from thisoverall test reflect not only the loss current from the inner bushing insulation at reasonablelevels, but the high dielectric losses from the high capacitance pressboard insulation at the lowend of the HV winding to ground due to the proximity of the H0(H2) shield/bushing to insulationground shield.

Inductive (magnetic) voltagetransformers for 115kV and higherapplication are only supplied as line toground connected transformers, and assuch have an isolated H0 bushing toallow for separating the primarywinding from base/ground for testingthe insulation. The insulation that mustbe verified to meet dielectricrequirements is that found in the innerbushing controlling the stresses lineground across the surface of theinsulator and also the turn to turn andlayer to layer insulation inside theprimary coil. This can be viewed as thecapacitances shown in Figure 15.

As can be seen, the C1 capacitance ofthe inner bushing insulation is impacted

H0

Ca =Insulator CapacitanceC1 =Major Insulation CapacitanceC1=Capacitance between Ground

Shield & Coil Shield

Ca

C1C1

Fig 15


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by the C1 capacitance which has the most impact on the power factor results on this unit.

Power factor, of course, is also impacted by environmental issues and how they affect Cainsulator capacitance. Insulator cleanliness, humidity conditions and insulator moisture iscritical power factor measurements. Even altitude can have an impact on the Ca power factor.This issue has been particularly serious when dealing with cascade transformers, however this

problem can also be noticed on single stage inductive voltage transformers. The goal of thisinformation is to provide a specific testing configuration for newer design inductive designs thatwill truly reflect the major insulation and not be impacted by the non-energized low endinsulation. In general, there are 4 configurations for testing the line to ground connectedinductive voltage transformer on site shown in Figure 16.

Inductive VT Measurement Configurations in the field:

Configuration 1Represents the best practical configuration that is actually used in the factory,which requires isolating the equipment base from ground.Configuration 2Matches the Doble overall test/Test 1, but the reading will be impacted by theH0 (non-energized) insulation to ground shield.Configuration 3Configuration measures mainly losses of the H0- base is isolated during test.Configuration 4 This format is the H1 Cross Check method and will give best practical methodfor testing the single stage inductive VTs.

Inductive VT Equivalent Circuits:Test configuration equivalent circuits for the above are as follows:

Configuration 1: (Recommended withactual designs and per factory test).

Measures the major insulation

Sensible to the physical state ofthe porcelain and its parasite-capacitances.

Test set

5kVCONFIGURATION 3

Test set

5kV

UST

Floating tankTest set

5kV

Test set

5kV

CONFIGURATION 1 CONFIGURATION 2

CONFIGURATION 4

Fig. 16

Guard

H0

GSTUST

GSTOverall


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5kV

Gnd H0

Gnd X1, Y1

LV Lead To meter

Fig. 17

The transformer must be completely insulated from ground in order to measureConfiguration 2:

Measures the major insulation and the pressboard cylinder located at the end of the HV coil. (Thispressboard cylinder has a very high capacitance value and high dielectric losses. This format is notrepresentative of the major insulation condition).

Sensible to the physical state of the porcelain and its parasite-capacitance.

Inductive VT Equivalent Circuits: (Contd)

Configuration 3:

Measures the dielectric losses only of theHV coil on the pasteboard cylinder (Thispressboard cylinder has a very highcapacitance value and high dielectriclosses. And its not representative of theMajor insulation condition.)

The transformer must be completelyinsulated from ground in order to measure

Configuration 4:

Measures the major insulation Its insensitive to the physical state of the porcelain and its parasite-capacitance.

The transformer does not have to be completely isolated from the ground to take the measurements

This feature is not presently available on most inductive VTs. It will require modification to the designof the transformer

Proper Inductive VT Physical Test Connections:

For testing power factorresults in the field, Figure

17 is per the Configuration1 given above and willprovide the closest resultsto the factory values. Thisrequires that the base beisolated from the ground,the H0 bushing and thesecondary terminals aregrounded, and with the LVLead attached to the base,and the H1 is energized.


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Proper Inductive VT Physical Test Connections: (contd)The closest Doble test that will yield results comparable to factory test results, when the overalltest GST Test 1 is indicating high power factor results in the field, would be per Figure 18 below,the H1 Cross Check test. By guarding out the H0 terminal, and connecting the LV Lead to thegrounded base and energizing the H1 terminal, this test removes the effect of the high

capacitance pressboard cylinder between the H0 and ground shield from the reading.

Station Service Voltage Transformer and Combined CT/VT:

As station service voltage transformers and current/voltage combined units are structuredelectrically similar to the single stage VT, they can be tested per the instruction noted above andas shown in Figures 19 and20, respectively. Shown areexamples of the units beingtested using the Doble GSTH1 cross check test whichgives a reasonableapproximation for insulationpower factor in each of thesetwo designs.

Guard H0

5kV

LV Lead Tometer

Temporarily removelead from

tank and guard H0

Fig. 19

Guard H0

Gnd Base, X1, Y1

5kV

LV Lead Tometer

Fig. 18

Highcapacitance

pressboard

cylinder


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A field test for PF Test that will best test insulation equivalent to the factory test for the SSVTand the CT/VT combination unit is the standard Doble GST H1 Cross check test that givesaccurate results for power factor and removes the effect of the H0 insulation issues. Insummary, here is the test setup that corresponds to Figures 19 and 20.

Removes the effect of H0 insulation Guard H0 Ground base and X1, Y1, Z1 Energize Primary H1@ 5kV

Cascade Inductive VT Physical TestConnections:Cascade VTs offer a more challenging, if notimpossible task to obtain representativepower factor results. In these designs withmultiple insulation structures, there exists avery complex equivalent circuit that is verydependant upon the environmentalconditions, and the parasitic capacitanceeffect upon the external insulators. Figure21 shows the capacitance that must beconsidered when evaluating the readings on

these multi-stage design VTs.

The techniques given for the single stageinductive VT will not necessarily work for theCascade design due to the intermediate core

coupling the coil sections together. These tests will tend to energize the core and result inexcitation current flow, which adds greatly to the normal capacitance currents and giveunacceptable power factor readings. Two variations of these tests are shown in Figure 22.

Fig. 21

5kV

Guard H0

Ground Base, X1, Y1, Z1

LV Terminals

LV Lead To meter

H0 affected by high PFressboard c linder

Fig. 20


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LV Lead To meter

5kV

Gnd H0

Gnd X1, Y1

Fig. 22

Test with Isolated Base will Energize the Core! Test with Grounded Base will Energize the Core!

Both the isolated base technique that is used on single stage VTs at the factory for obtainingrepresentative results for the majorinsulation power factor testing, aswell as the GST Doble H1 Cross

Check test both excite the coreand will result in energized currentflow. The main concern with theCascade Inductive transformer isthat many of the power factorconnections will excite the core,and in those cases the measuredlosses will include excitationcurrent, with no possibility ofguarding it. This is because theend lead of the upper winding isinternally attached to the

intermediate tank, as well as thestart lead of the lower winding. Theonly practical method in testing theCascade design is that shown inFigure 23, but this will be impactedby the insulation between thesecondary winding and its ownground shield!

Guard H0

Gnd Base, X1, Y1

LV Lead To meter

5kV

Jumper and

energize H1 & H0

Gnd Base X1,Y1

5kV

LV Lead To

Fig. 23


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H0 bushing is isolated from ground, and connected to H1, the base is grounded and as well asX1 and Y1 LV terminals are grounded, and the H1-H0 are energized at 5kV. Power factorvalues obtained by this method will be quite high, as well as the capacitance value because theinsulation between the secondary winding and ground consists of a relatively short thicknesspressboard cylinder. Typical pressboard power factor values will be around 1% or 1.5%. Soresults can be confusing. Contact the manufacturer to discuss any discrepancy.

In addition, Cascade VTs have very low internal capacitance values that force the measuredvalues to be completely dependant on the environment conditions and the parasite-developingcapacitance, yielding completely inaccurate results. These conditions can prevent the customerfrom testing the true insulating capacitance of the high tension winding.

Coupling capacitor voltage transformer Physical Test Connections:Testing on the coupling capacitor voltage transformers can be performed using typicalinformation from prior Doble test instructions. A complete test definition was provided in aDoble paper given in 2004 International Conference of Doble Clients*. In summary, the testinformation is as follows and is illustrated in Figure 24.

H1

C1

C2

Cn Total

Voltage

Gnd Sw

AF Switch

Fig. 24

CCVT Power Factor and Capacitance Test Preparation

External link for high frequency carrier injection removedIdentify C1-2+ C1-1 + C2 values on stack nameplatesPotential grounding switch position


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Coupling capacitor voltage transformers can be tested per the following table defining the tests

required, and the connections to be made based upon the number of capacitor sections on theCCVT being tested. This table and the illustration in Figure 24 supplies complete details forperforming power factor tests that will represent the insulation of the transformer.

*Table obtained from a prior Doble report presented on CCVT testing.

In the event of multiple stack designs (C1 = C1-1 + C1-2 + .)

10kVClosedCarrier Pt/AFMidpointGST-GndC1-1

10kVClosedCarrier Pt/AFMidpointUSTC1-2 stack/Top

Test

kV

VoltageGnd

Switch

GroundEnergizeTest

ModeMeasure

10kVOpenCarrier Pt /AF*H1(HVTerm)GST-GndCn

*Carrier Link Opened.

2kVClosed---- *Carrier Pt/AFGST-GndC2

10kVClosedCarrier Pt /AF*H1 (HV Term)GST-GndC1

H1

C1

C2

Cn Total

Voltage

Gnd Sw

AF Switch

Fig. 24


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Current Transformer Physical Test Connections:In general, current transformer insulation testing is simpler to perform and the major insulation isrelatively accessible for power factor tests. As the major insulation

is supplied between the primary and theground shield, and the secondarywinding is not involved in the testing of

the major insulation, connections areeasier to perform. In some cases, thecurrent transformers are equipped withdissipation factor or power factor tapsas shown in Figure 25, and some donot have a shield ground external to thetransformer. Both designs will bereviewed and test format provided.

When viewing the CT insulation relativeto capacitances, noted in Figure 26 theC1 is the major capacitance and can be

checked using the Overall Test No. 1.Keep in mind that this test is alsosensitive to insulator capacitance Ca.

Any high reading on PF on the test, itmay be necessary to perform a hotcollar test on the insulation.

In current transformers not equippedwith an isolated dissipation factor or testtap, the typical connections can bemade as shown in Figure 27 for properPF results.

With this test, a jumper is placed acrossH1 to H2, the secondary winding andbase are connected to ground, and theLV lead is connected to the base of theCT. With the H1/H2 connectionenergized at 5kV, good PF results can

be obtained. In the case of this overalltest showing high on power factor, thena hot collar test can be performed toguard out any impact of the insulatorcapacitance.

Fig. 25

C1

Ca Fig. 26

5kV

LV Lead To meter

Gnd Base & X1LV Terminal

Fig. 27


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Current transformers with a test tap will allow isolating the major insulation ground electrode soas to only check the C1 capacitance of the design.

5kV

LV LeadTometer

Ground X1LV Terminal

Temporarily RemoveGround Connection

Fig. 28

Figure 28 shows the connections required forthe CT with test tap. The test tap can be isolated

from the metal base, the base and the LVterminal grounded, with the H1/H2 terminals

jumpered together, 5kV can be applied on theprimary of the current transformer, with the LVlead connected to the tap. This is an overallTest 1 configuration.

With the test tap design, to fully isolate theinsulator capacitance Ca from the C1 majorinsulation value, a UST can be performed on theTest tap, with the LV terminal grounded, theprimary H1/H2 conductor energized at 5kV, and

the LV Lead to the meter on the tap.This is a preferred method of obtaining truepower factor of the major insulation.

The tap insulation (C2) can also be measured byenergizing the tap with 2kV maximum, groundthe LV terminal, and guard the H1/H2 primary.

Upon completion of all tests, insure that the testtap is firmly connected back to the base for safeuse.


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Common Problems to Watch Out For:Some testing issues that we have seen repeatedly in application of instrument transformers are:

3. Effect of Radiated High Voltage EMF on Excitation Readings

Pickup of Energy Possible Highest reading under H0/H2 energized direction H1 terminal picks-up radiated energy

1. VT Overall PF & Importance of cross-check tests

If Overall PF Test (Test 1) is High, verify condition with Cross Check tests

Always check overall test against H1 Cross-Check Believe in the H1 cross-check value

2. Cleanliness of insulators and relative humidity

Epoxy surface on H0 (Moisture/Contamination) Wipe Porcelain Surfaces Perform Hot-Collar test as needed to check Insulator Never use alcohol to wipe insulators Condensation


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Transformer excitation tested with energized 138kV line nearby. 138kV line was 30 abovegrade and 13 away from VT-

Test results with EMF impacting the H1 terminal

Test results without EMF impacting the same transformer

Dissimilar excitation readingsdue to radiated field effect

Excitation readings equalaway from any radiated field


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UnderstandingPower Factor Testing Results

relative toModern Instrument Transformer

Insulation Designs

Presented by

Nick Powers - ABB Kuhlman&

Rolando Gomez-Arteche

DOBLE CONFERENCE March 2009

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