fundamental-class-high-voltage.pdf

8/9/2019 fundamental-class-high-voltage.pdf

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Considerations in the Specification of

High Voltage Test Systems

By: Jeffrey A. BrittonAssistant Chief EngineerPhenix Technologies Inc.


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Fundamental Classification of

High Voltage Test Systems

- High Voltage AC Test Systems

- High Voltage DC Test Systems


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Types of High Voltage ACTest Systems

Note: For the context of this discussion, High Voltage isgenerally considered to be any AC or DC voltage inexcess of 20 kV.

- AC Dielectric Test Systems

- Resonant AC Test Systems


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Issues to Be Considered inthe Specification of a

Standard AC Dielectric TestSystem vs. An AC Resonant

Test System

- Load Type (Capacitive / Resistive):

- Standard AC Dielectric Test Systems maybe used to test Capacitive or Resistive testobjects, and may or may not includereactive compensation.

- Resonant AC Test Systems are exclusively

for high voltage testing of Capacitive testobjects.

- A standard AC Dielectric Test System mustbe used for loads which have time varyingcharacteristics, such as polluted insulatortests.


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- Power Requirements:

- Standard AC Dielectric Test Systems which

are not reactively compensated require aninput service equal to the output HV testingkVA of the test system, plus test systemlosses.

- AC Resonant Test Systems are typicallyemployed when test objects represent a

relatively large capacitance, such as longlengths of HV cable or large generator ortransformer windings.

- The required input power is reduced to onlythat of the ohmic (real power) losses withinthe system, typically 1/10th to 1/40thof therequired HV testing kVA.


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-Limitation of transients during specimen failure:

- Generally speaking, destructive transientsare reduced with AC Resonant TestSystems as compared to standard ACDielectric Test Systems, as the amount offollow through energy from the line islimited by the impedance of the regulatorand exciter transformer.

- Overvoltage transients are virtuallyeliminated in Series AC Resonant TestSystems, as the voltage is only presentwhen the resonant condition exists. Theresonant condition is immediatelydisrupted when a specimen failure occurs,and transient overvoltages are notproduced.


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- Waveshape requirements, distortion, harmoniccontent:

- Generally speaking, AC Resonant TestSystems produce a less distortedsinusoidal HV output than a standard ACDielectric Test System of similar ratings.

- In practice, total harmonic content is

typically less than 5% at full load fortraditional standard AC Dielectric TestSystems. For AC Resonant Test Systems,total harmonic content is typically muchless than 1% at full load.

- Waveform distortion produced by StandardAC Dielectric Test Systems in practicebecomes less prominent as the kVA ratingof the system increases.

- Higher output waveform distortion instandard AC Dielectric Test Systems alsooccurs as the step-up ratio of thetransformer increases.


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- Initial System Costs

- Generally speaking, the purchase of an AC

Resonant Test System represents a higherinitial cost than a standard AC Dielectric TestSystem with similar ratings. This is due to thehigh design and manufacturing costs for thevariable - gap HV reactor.

- As the amount of required HV testing power

increases, AC Resonant Test Systemsbecome increasingly competitive withstandard AC Dielectric Test Systems. In somecases it is possible to supply an AC ResonantTest System more economically that astandard AC Dielectric Test System. Eachcustomers testing requirements must beevaluated individually to determine anoptimum solution.


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Issues to Be Considered inthe Specification of a

Standard AC Dielectric TestSystem

- Physical Package: Tank Type vs. InsulatingCylinder Type

- In General, there are two basic constructiontechniques used in the design of oil insulatedhigh voltage test transformers used in ACDielectric Test Systems. These are: Tank Typeand Cylinder Type.


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Factors Influencing TheChoice of Physical

Packaging for HV TestTransformers

- Required Output Voltage

- Tank Type HV Test Transformer

- In this design, the oil insulated high voltagetransformer is contained within aconducting vessel, typically a welded steeltank.

- The transformer core is connected to the

same potential as the tank body. For asingle transformer (non-cascadedtransformer) design, the tank is normallygrounded to earth at the same physicallocation as the return side of the testspecimen.


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- The HV output is brought out via HVbushing. The fact that a bushing output is

required limits the voltage magnitude whichcan be developed in a single tank. Voltagesin excess of 350 kVrms start to becomeimpractical, as the physical dimensions ofthe output bushing become difficult to dealwith.

- Cascade connections of multiple Tank TypeTransformers in series are possible toobtain voltages higher than 350 kVrms, butare usually impractical due to the extremeexpense involved in building corona shieldsand insulating structures to support higherstages.


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- Cylinder Type HV Test Transformer

- In this design, the oil insulated high voltagetransformer is contained within aninsulating cylinder, typically fiberglass or,for small transformers rated equal to or lessthan 10 kVA and 200 kV, PVC.

- On relatively large transformers (in excess

of 10 kVA and 200 kV), the top and bottomof the cylinder are capped with conductingheader plates. These plates often servedirectly as the output terminations.

- The transformer core may be tied to thebottom header plate potential, or may betied to an intermediate high voltagepotential within the cylinder.

- The HV output is normally taken from thecorona rings which are bolted to the topheader plate, or from an output spinningbolted through the top plate of theinsulating cylinder on small transformers.

Since no bushing is involved, there are novoltage limitations imposed by the outputtermination.


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Solution:The capacitive reactance presented by the bushingat 60 Hz. is:

XfC x

Mc = = =1

2

1

2 60 400 106 63

12 ( )( )( )( )

.

The kVA load imposed upon the HV TestTransformer at 350 kVAC is:

LOAD VX

VA kVAc

= =

2

18 476 18 5, .

This represents 18.5% of the HV Test Transformers1 Hour On / 1 Hour Off Duty output kVA, and 26.4%of the HV Test Transformers Continuous Dutyoutput.

If 100 kVA of HV Testing Power was required, theHV Test Transformer, Regulator and Input Servicewould have to be sized to cover the load presentedby the HV Bushing.

- In cases where a relatively high outputvoltage is required at a relatively low kVA, acylinder is often a better choice, as there are

no power losses to a HV bushing.

- In theory, if proper cooling is provided, thereis no upper kVA limit to either design.


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- Duty Cycle and Thermal Issues

- Tank Type HV Test Transformer

- Tank Type HV Test Transformers are bettersuited for continuous or long term dutyoperation if no external cooling system is tobe provided.

- The steel tank design exhibits better heatdissipation characteristics than a fiberglassor plastic cylinder, which tends to limit heattransfer from the oil to the outside.

- To increase kVA capacity or duty cycle in agiven physical size, additional coolingmeasures may be used to transfer heataway from the coils. Possible measuresinclude:

1) Use of radiators on the tank to increasethe effective surface area of the tank forcooling purposes. This is the mosteconomical and easily implemented

solution. No customer action is required.


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2) Use of radiators on the tank inconjunction with fans outside the tank.

This is also quite economical and easyto implement, and only requires thecustomer to supply AC power to thefans.

- Although more elaborate cooling systemsinvolving heat exchangers can be used if

more cooling is required, such measuresare very seldom required for tank type HVtest transformers.

- Specification of actual duty cycle isextremely important. Proper specification ofduty cycle will lead to an optimal solutionfor the customer in terms of cost, physicaldimensions, and performance.


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- Cylinder Type HV Test Transformer

- Cylinder Type HV Test Transformers are notgenerally suited for continuous or longterm duty unless external cooling isprovided. This is not normally a limitationhowever, as most hipot tests performedwith this equipment are of short duration,often lasting only a few minutes.

- Higher kVA capacities and increased dutycycles up to and including continuous dutyare available with the use of externalcooling systems. Possible measuresinclude:

1) Use of an oil-to-air heat exchangerexternal to the tank, to exhaust heat intofree air. This is somewhat costly, andthe customer must provide power to theoil pump(s), power to the fan(s) on theheat exchangers, and plumbing to carrythe oil from the tank to the heatexchanger. Such measures are normally

required only for systems designed tooperate for extended periods of time.


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2) Use of an oil-to-water heat exchangerexternal to the tank, to exhaust heat

either into a continuous cool watersupply or into a re-circulated watersupply cooled by a chiller if acontinuous cool water supply isunavailable. This is the most expensiveand difficult to implement solution, asthe customer must provide power to the

oil pumps, plumbing for the coolingwater, and either a continuous supply ofcooling water or power to operate achiller. This measure would normallyonly be required for a very large unitdesigned to operate at true continuousduty.



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Factors Influencing TheUse of Fixed Reactive

Compensation in ACDielectric Test Systems

- Type of Load

- Reactive Compensation is only an option with

stable capacitive test objects. With a fewexceptions, almost all test objects representcapacitive loads under hipot test.

- Notable exceptions are switchgear insulators,and fiberglass bucket truck booms, whoseresistive leakage current component is larger

than the capacitive charging current. Thesetest objects appear as very light resistiveloads under hipot test.

- If the system is designed to draw full loadcurrent at both no load, and at full capacitiveload, the only difference being the phase

angle of the input current with respect to theinput voltage, any pure resistive componentof current will overload the primary of the testsystem. Capacitance is therefore required onthe output.


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- AC Dielectric Test Systems with reactivecompensation should not be used to test

loads with time varying characteristics, suchas polluted insulators. The magnitude andphase angle of the load current varies withthese types of loads, and reactivecompensation is not beneficial.


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- Form of the Reactive Compensation

- Use of External Fixed Low Voltage Reactors

- Use of one or more fixed value low voltagereactors connected in parallel with theprimary of the HV Test Transformer is oneof the most versatile solutions for reactivecompensation in AC Dielectric Test

Systems.

- Stepped variable compensation is possiblethrough power off switching of differentreactance values. Using this method, near100% compensation can be achieved usingthe proper combination of compensatingreactors.

- Protection circuitry is required within thetest system to guard against undesiredresonance between the compensatingreactors and the capacitive test object.Failure to include this protection can resultin large circulating resonant currents within

the system, which can damage the powercomponents in the primary circuit.


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- This type of compensation is quite versatilein that resistive loads can also be tested at

powers up to the rated power of theregulator, by simply not pulling in any ofthe fixed reactors. In this mode the testsystem may be operated as a normal ACDielectric Test System without reactivecompensation.

- Switching of the compensating reactorsmust always be done with the power off,and may either be accomplished manually,through physical reconnection of reactors,or electrically through the use ofcontactors.

- With this arrangement with proper sizing ofthe compensation reactors, the regulatorand input service to the test system can besized as small as 10% of the required HVtesting power.


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- Use of a Fixed Airgap in the Core of the HVTest Transformer

- Placement of a fixed airgap in the magneticcore of the HV Test Transformer is a verycommon technique for achieving 50%reactive compensation.

- If only a single level of compensation is

utilized, 50% compensation maximizes thebenefit over the entire load range of no loadto full capacitive load, by cutting the size ofthe regulator and input service in half.

- In this case, the amount of reactivecompensation is fixed.

- This type of test system may only be usedfor capacitive test objects.

- This type of system is designed to drawrated input current both at no load, and atfull capacitive load. The phase angle of theinput current changes from near ninety

degrees lagging the input voltage to nearninety degrees leading the input voltage asthe capacitive load is increased from zeroto rated load.


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- Use of a Fixed HV Reactor in Parallel With theTest Object

- This method of reactive compensation isnormally the most expensive and difficult toimplement, as it requires high voltagereactors, rated for the maximum testvoltage, to be placed in parallel with the testobject.

- Stepped variable compensation is possibleby placing different fixed reactors inparallel with the test object. This is similarto AC Resonant Test System operation,without the ability to continuously adjustthe level of compensation as in a ResonantTest System.

- The high expense of this technique coupledwith the relatively low level of versatilitynormally makes this method ofcompensation unattractive as a solution.


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- Available Input Power

- The amount of available input power to thetest system often determines the type ofreactive compensation required.

- If more HV testing power is required than isavailable from an existing input service, andthe test load is largely capacitive, reactive

compensation using one of the previouslymentioned techniques may be the solution.

- Depending on the technique used and thetype of objects being tested, it may bepossible reduce the required input service toas little as 10% of the required HV testingpower.

- Typically, using the most common techniqueof including an airgap in the core of the HVTest Transformer, the required input powerwill be reduced to approximately 50% of therequired HV testing power. This reduces thesize of the regulator, input circuit breaker,

and contactors as well.


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- Available Output Power

- When using fixed compensation, it isimportant to recognize that output currentcapability varies with output voltage. Thismeans that the available HV testing powervaries non-linearly with output voltage.

- It is important to understand the output

characteristics of a reactively compensatedAC Dielectric Test System, as customersoften incorrectly assume that maximum ratedoutput current is available at any outputvoltage.

- In practice, in an AC Dielectric Test Systemsupplied with 50% fixed reactivecompensation, maximum rated output currentis only available at a single load point: tunedto the proper capacitance at maximum ratedoutput voltage.

- The following illustrative example, whichshows the calculations, will help to explain

the actual operation of an AC Dielectric TestSystem designed with 50% reactivecompensation on the primary side of the HVTest Transformer.


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Example:

Consider an AC Dielectric Test System rated at100 kV output at 100 kVA, 50 Hz. The regulator israted 400 V, 50 Hz. input, and 0 - 400 V, 0 - 50 kVAoutput. A fixed primary reactor rated at 50 kVARat 400 V, 50 Hz. is connected in parallel to theprimary winding of the HV Test Transformer.Neglecting real and reactive power losses withinthe system, what are the maximum available HVtesting power and current at 100 kV? At 50 kV?

Solution:

The easiest way to understand the operation of areactively compensated AC Dielectric Test Set isto look at the system from a power perspective.

The HV testing power available at a given voltage

is the sum of the available regulator output powerand the available compensation available fromthe fixed reactor at that voltage.

The regulator provides variable voltage atconstant current. The output power of theregulator is therefore proportional to outputvoltage. In this case, rated output current for the

regulator is calculated as:

IRatedPower

RatedVoltage

VA

VAreg = = =

50 000

400125

,


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The output power of the regulator as a function ofregulator output voltage is expressed as:

OutputPower OutputVoltage VA= * [ ]125

The compensation available from the fixedreactor is expressed as:

CompOutputVoltage

XVA

r

=

2

[ ]

Xris the inductive reactance of the

compensating reactor, in ohms. This may becalculated from the data given as follows:

XRatedVoltage

RatedPower VAr = = =

2 2400

50 0003 2

( )

,.

V

Therefore the available compensation from thefixed reactor is expressed as:

CompOutputVoltage

VA=2

3 2.[ ]

The available HV testing power is then:

TestPower OutputPower Comp VA

TestPower OutputVoltageOutputVoltage

VA

= +

= +

( ) ( )[ ]

( * ) (.

)[ ]1253 2

2


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Neglecting system losses, at 100 kV output, theregulator output voltage will be rated voltage (400V). The available HV testing power is therefore:

TestPower VA= + =( )( ).

, [ ]400 125400

3 2100 000

2

This equates to an available current of 1 A at 100kV.

At 50 kV, 1/2 of rated output voltage, neglectingsystem losses, the regulator output voltage willbe 1/2 rated voltage (200 V). The available HVtesting power is therefore:

TestPower VA= + =( )( ).

, [ ]200 125200

3 237 500

2

This equates to an available current of 0.75 A at

50 kV.

This demonstrates the principle that rated currentis not available from a reactively compensatedAC Dielectric Test System at all voltages.

- For each reactively compensated AC Dielectric TestSystem, a plot may be made which describes output

current and output power as a function of outputvoltage. This plot will vary with the percentage ofreactive compensation.


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- Initial System Costs

- The effect of including reactive compensationon the initial cost of an AC Dielectric TestSystem varies, depending on the type ofcompensation used, and the size of the testsystem.

- Including 50% reactive compensation in the

form of an airgap in the core of the HV testtransformer is the most economical solution.This solution is often less expensive than anormal AC Dielectric Test System with nocompensation. The savings are realized in thereduction in the size of the regulator, primarycircuit breaker and contactor, and primarywinding of the HV test transformer.

- Including fixed reactive compensation in theform of a single compensating reactorconnected in parallel with the HV testtransformer will typically cost about the sameas a normal AC Dielectric Test System with noreactive compensation. In this case, there is

no cost savings in the HV test transformeritself, and any savings in regulator costs areusually absorbed by the provision of theadditional low voltage reactor.


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- Including stepped variable reactivecompensation in the form of several lowvoltage reactors of various compensation

values typically adds considerable versatilityto the test system, and also adds cost.Although there is some savings in thereduction in the size of the regulator, this isnormally more than absorbed by the cost ofthe additional low voltage reactors, thecontactors to switch them in and out, and the

associated control and protection systemcosts.

- Reactive compensation in the form of HVcompensating reactors connected in parallelwith the test object typically addsconsiderable cost to the system. Generallyspeaking, the benefit derived by this solutiondoes not justify the cost.

- In all cases where reactive compensation isused, there should be some savings inreduction of the required input service to thetest system. This is an indirect savings, but asavings just the same.


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- System Operating Costs

- Maintenance costs for compensated and un-compensated AC Dielectric Test Systems arevery similar, as very little maintenance isrequired for these systems.

- The regulator is the only main componentwhich requires periodic maintenance.

Decreasing the size of the regulator by usingreactive compensation may provide a smalldecrease in maintenance costs for reactivelycompensated systems.

- For very large AC Dielectric Test Systems,there may be a considerable long termsavings in power cost for customers whosepower billing includes a power factor penalty.


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Factors Influencing the Typeof Regulator Used in an AC

Dielectric Test System

- Solid State Regulators

- Not traditionally used in high power highvoltage AC test systems

- Solid State Regulators typically generateundesireable harmonic content, anddistortion of the output voltage from that of atrue sinewave.

- Introduction of high frequency noise can alsobe a problem, which is not acceptable forconducting HV withstand testing and makingpartial discharge measurements.

- The ability of solid state components towithstand the violent transients which can be

produced during HV test specimen failuresdoes not match that of variable transformers.


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- Variable Transformer Type Regulators

- Traditionally used in high power high voltageAC test systems.

- Some of the reasons why variabletransformers continue to find favor in highvoltage AC testing are:

1) Variable Transformers represent a welldefined technology, with many years ofproven performance in this application.

2) Variable Transformers exhibit highreliability with a relatively low amount ofrequired maintenance.

3) The simplicity and relatively low cost ofvariable transformer design andproduction makes them attractive for usein custom test systems.

4) Modular designs allow for regulation ofvoltage at practically unlimited power

levels.


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5) Variable transformers exhibit a hightolerance for short term overloads, makingthem ideal for use in AC Dielectric Test

Systems where a high power requirementexists for a short time.

6) Variable transformers exhibit a hightolerance to the transients which areproduced during test specimen failures(flashovers).

7) Variable transformers do not contribute toharmonic content, nor produce waveformdistortion. In fact, certain type of variabletransformers, especially two windingvariable transformers (non-autotransformers) provide a line filteringaction which blocks the passage of highfrequency disturbances existing on thepower line.

- Several different types of variable transformerdesigns are in common use in AC DielectricTest System regulators. The type of variabletransformer used if most often determined by

application. Designs commonly in use includeToroidal Variable Autotransformers, ColumnType Variable Transformers, and Thoma TypeVariable Transformers.


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- Characteristics and Applications of ToriodalType Variable Autotransformers

- Main Characteristics

- A Toroidal Type Variable Autotransformeris constructed by winding a single layerof magnet wire onto a toroidal core. Asliding or rolling brush assembly movesradially over a surface of the winding

which has been ground to expose theconductor.

- A "quasi stepless" variable sinusoidalvoltage is available at the brush, withnegligible distortion.

- The series impedance will typically rangebetween 30% and 1% as the brushposition varies from 10% to 100%.

- Voltage resolution is typically around 0.7Volts / turn.

- Efficiency is normally between 92% and

98%.

- Input voltage ranging between 120 V and300 V. Units may be connected in seriesto provide regulation up to 600V.


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- Theoretically unlimited kVA rating whenstacked and ganged.

- In practice, use of Toroidal Type VariableAutotransformers becomes mechanicallycumbersome above 200 kVA due tocomplex motor and chain drivemechanisms required to synchronize anddrive the brush holder assemblies.

- A limited amount of voltage step-up maybe achieved, approximately 15% - 20%,but this is normally at a somewhat de-rated output current.

- Overload withstand capability is good,but not as good as other types of variabletransformers, particularly when slidingcontacts are used, as do most U.S.manufacturers.


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- Applications in AC Dielectric Test Systems

- Input voltages up to 600 V

- In practice, powers up to approx. 200kVA, at 50% duty cycle. (1 hour on / 1hour off)

- Single or three phase applications


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- Characteristics and Applications of ColumnType Variable Transformers


- A Column Type Variable Transformer mayeither be constructed as a Two WindingVariable Transformer, with a primary anda secondary, or as a VariableAutotransformer.

- Generally speaking, the regulatingwinding of a Column Type VariableTransformer is constructed by winding asingle layer of flat magnet wire on edgeonto an insulating tube. A rolling brushassembly moves axially over a surface ofthe winding which has been ground toexpose the conductor.

- Either a primary winding (for TwoWinding Variable Transformer designs) ora compensation winding speciallydesigned to reduce series impedance (forVariable Autotransformer designs) wound

onto a smaller insulating tube, which fitsinside the regulating winding tube.


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- These tubes are placed concentricallyonto the core steel, which is typically

stacked from cut lamination steel. Thisallows for conventional single phase orthree phase core construction.

- A "quasi stepless" variable sinusoidalvoltage is available at the brush, withnegligible distortion.

- For Variable Autotransformer designs,the series impedance typically rangesbetween 10% and 1% as the brushposition varies from 10% to 100%.

- For Two Winding Variable Transformerdesigns, the series impedance is typicallynearly constant as the brush positionvaries from 10% to 100%, and is usuallybetween 6% and 12%.

- Voltage resolution is typically less than0.7 Volts / turn.

- Efficiency is normally between 95% and98%.

- Input voltage ranging between 120 V and600 V.


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- Unlimited kVA rating when ganged.

- A limited amount of voltage step-up maybe achieved, approximately 15% - 20%,but this is normally at a somewhat de-rated output current.

- Line separation (isolation) is possible, inthe Two Winding Variable Transformer

design, which is necessary for someapplications.

- Excellent overload withstand capability,superior to Toroidal VariableAutotransformers.

- More suited to long term testing andheavy industrial use than the ToroidalVariable Autotransformer.

- Generally more expensive than ToroidalVariable Autotransformers in applicationsbelow 200 kVA.


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- Input voltages up to 600 V

- Unlimited kVA rating

- Single and three phase applications


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- Characteristics and Applications of ThomaType Variable Transformers


- A Thoma Type Variable Transformer isconstructed strictly as a Two WindingVariable Transformer, with a primary andsecondary.

- The regulating (secondary) winding isconstructed by winding a single layer ofnon-insulated wire or bar onto a rotatingcylinder. A moving brush assembly,which is precisely gear timed to followthe secondary winding, serves as thecontinuous tapping point to provide acontinuously variable, pure sinusoidalwaveform without distortion.

- A the primary winding is situated insidethe secondary winding, both of which areinstalled on the center leg of single phaseshell type core constructed of cutlamination steel.

- These unit are usually oil insulated, andare contained in welded steel tanks.


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- As the moving brushes do not traverseturns, but track the winding continuously,the Thoma Type Variable Transformer

does not generate electromagneticinterference (EMI).

- The series impedance is constant as thebrush position varies from 10% to 100%,and is usually between 4% and 10%.

- Voltage resolution is infinite.

- Efficiency typically around 98%.

- Input voltages up to approximately 20 kV

- Single units up to approximately 2000kVA are available. kVA rating is unlimitedwhen regulators are ganged in parallel.

- Output voltage is usually designed to be0 to 1000 V, but this can vary somewhat.

- Line separation (isolation) is provided.

- Excellent overload withstand capability

- Suited for long term or continuousoperation at full load, with no movementof the brushes.


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- Input voltages up to 20 kV

- In practice, kVA ratings range fromapproximately 200 kVA upwards.Maximum kVA is unlimited.

- Single and three phase applications


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Notes Regarding SystemImpedance in AC Dielectric

Test Systems

- General HV Testing

- In typical HV Testing applications, a testtransformer impedance of approximately8% to 12% based on the transformer's ratedkVA is acceptable.

- The relatively high impedance of a standardAC Dielectric Test System transformerhelps to limit the energy released duringspecimen failure. This reduces undesirabledamage which may be inflicted on thespecimen itself, as well as on the system.

- In a typical AC Dielectric Test System, avariable autotransformer is normally usedas the regulator. As noted earlier, theimpedance of the variable autotransformervaries with brush position, and may range

from 1% to as high as 30%, depending onthe type of variable transformer used. Thisis acceptable for most HV testingapplications.


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- Special HV Testing

- In certain types of high voltage testing, such

as testing of polluted or contaminated HVinsulators, the test object represents anunstable resistive / capacitive load, whosecharacteristics are time dependent.

- In this special type of testing a very strongtest voltage supply is required. Low and

constant impedance is needed to maintain astable test voltage output under conditions ofvery high and erratic partial discharge.

- Special designs are required for the HVtransformer and regulator to achieve a lowconstant impedance. These types of systemsare more expensive than normal AC DielectricTest Systems due to the special design, andthe increased physical and electrical strengthbuilt into such systems.

- The best way to insure optimal function of agiven test system is to determine the type oftesting to be performed, and include this in

the specification given to the equipmentsupplier in the specification stage of a test

system purchase.


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Issues to Be Considered inthe Specification of an AC

Resonant Test System

- Physical Package: Tank Type vs. InsulatingCylinder Type

- In General, there are two basic constructiontechniques used in the design of oil insulatedhigh voltage tuneable reactors used in ACResonant Test Systems. These are: GroundedTank Type systems and Modular CascadeCylinder Type systems.


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Factors Influencing TheChoice of Physical

Packaging for AC ResonantTest Systems

- Grounded Tank Type HV Reactor

- In this design the oil insulated high voltagereactor is contained within a groundedconducting vessel, typically a welded steeltank.

- The reactor core and drive frame areconnected to ground potential within the tank

body.

- Grounded Tank Type AC Resonant TestSystems may be operated in either series orparallel resonant mode.


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- Partial discharge measurement is normallymade via an external coupling capacitor,

which may also serve as a preload for seriesmode resonant operation, in the absence of atest object within the specified tuning rangeof the HV reactor.

- All Phenix Technologies HV reactors arespecified to provide exceptionally clean high

voltage output, with a standard specificationof less than 10pC partial discharge at ratedoutput voltage, with no external high voltagefiltering required. As an option, all HV reactormay be supplied with a specification of lessthan 2pC partial discharge at rated outputvoltage, with no external high voltage filteringrequired.

- High voltage filtering techniques and shieldedtest rooms may be employed for testingwhere even less than 2pC must be measured.


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- Modular Cascade Cylinder Type HV Reactor

- In this design, the oil insulated high voltage

reactor is mounted within a conductingcylinder, typically steel, which is suspendedvertically between two insulating cylinders,typically fiberglass. The top and bottom of themodule are capped with conducting headerplates, typically steel, which often servedirectly as the output terminations.

- The conducting cylinder, reactor core anddrive frame are connected to an intermediatepotential (1/2 of the rated output voltage ofthe module) within the conducting cylinder.

- Phenix Technologies Modular CascadeCylinder Type HV Reactors may only beoperated in series resonant mode.

- The HV output is normally taken from thecorona rings or special output electrode (forvoltages in excess of approximately 800 kV)which are bolted to the top plate. Since nobushing is involved, there are no voltage

limitations imposed by the outputtermination.


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- Modular Cascade Cylinder Type HV Reactorsare normally designed to be stacked with theHV reactor windings connected electrically in

series. This allows for theoretically unlimitedmaximum test voltages. Units with outputvoltages well in excess of 2 MV are currentlyin operation.

- Voltage measurement is usuallyaccomplished via an external voltage divider

capacitor, which may also serve as a thepreload capacitor for series mode resonantoperation. With the appropriate couplingcoupling circuitry, this same capacitor mayalso be used for partial dischargemeasurement.

- Unless a preload capacitance is connectedwhich is within the specified tuning range ofthe HV reactor, full voltage cannot bedeveloped with no load capacitance present.

- Modular Cascade Cylinder Type HV Reactorsmay be supplied with paralleling bars, whichmay be installed to make various series /

parallel connections among the modules.This provides the same effect as providingoutput taps, which allows for tuning over agreater capacitance range.


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- A minimum 20 to 1 tuning range is typical foreach output tap supplied.

- All Phenix Technologies HV reactors arespecified to provide exceptionally clean highvoltage output, with a standard specificationof less than 10pC partial discharge at ratedoutput voltage, with no external high voltagefiltering required. As an option, all HV reactormay be supplied with a specification of less

than 2pC partial discharge at rated outputvoltage, with no external high voltage filteringrequired.

- High voltage filtering techniques and shieldedtest rooms may be employed for testingwhere even less than 2pC must be measured.


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1) Use of radiators on the tank to increasethe effective surface area of the tank for

cooling purposes. This is the mosteconomical and easily implementedsolution. No customer action is required.

2) Use of radiators on the tank inconjunction with fans outside the tank.This is also quite economical and easy

to implement, and only requires thecustomer to supply AC power to thefans.

- Although more elaborate cooling systemsinvolving heat exchangers can be used ifmore cooling is required, such measuresare very seldom required for tank type HVreactors.



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- Modular Cascade Cylinder Type HV Reactors

- Modular Cylinder Type HV Reactors are not

generally suited for continuous or longterm duty unless special provisions aremade to provide additional cooling.

- Cooling for long term or continuous dutyoperation may be provided through simpleaddition of radiators to the center

conducting cylinder of each reactor moduleto increase the effective surface area forcooling.

- For even higher kVA capacities andincreased duty cycles up to and includingcontinuous duty, the use of externalcooling systems may be required. Possiblemeasures include:

1) Use of an oil-to-air heat exchangerexternal to the tank, to exhaust heat intofree air. This is somewhat costly, andthe customer must provide power to theoil pump(s), power to the fan(s) on the

heat exchangers, and plumbing to carrythe oil from the tank to the heatexchanger. Such measures are normallyrequired only for systems designed tooperate for extended periods of time.


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2) Use of an oil-to-water heat exchangerexternal to the modules, to exhaust heat

either into a continuous cool watersupply or into a re-circulated watersupply cooled by a chiller if acontinuous cool water supply isunavailable. This is the most expensiveand difficult to implement solution, asthe customer must provide power to the

oil pumps, plumbing for the coolingwater, and either a continuous supply ofcooling water or power to operate achiller. This measure would normallyonly be required for a very large unitdesigned to operate at true continuousduty.



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Determination of the QualityFactor "Q" In The

Specification of an ACResonant Test System

- Determination of the System "Q" is one of themost critical aspects of properly specifying aresonant test system, as the system "Q" directly

impacts the size of the test system inputservice, isolation transformer (if required), testregulator, low voltage power line filters andexciter transformer.

- From theory, the Quality Factor "Q" of aresonant circuit may be mathematically

expressed as the ratio of output reactive powerto input real power. Or:

QkVAR

kWout

in

=

- In a properly tuned resonant circuit, the reactive

powers absorbed by the variable HV reactor andthe test capacitance are equal and 180 degreesout of phase in time. This energy is transferredback and forth between the test object and theHV reactor at power frequency.


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- The only power which must be supplied by anexternal source to maintain this resonantcondition is the ohmic (real power) losses

produced by power which is dissipated withinthe test circuit.

- The real power losses must be supplied by theinput service to the test system via the isolationtransformer (if required), regulator and excitertransformer. The required output power as well

as the system "Q" must therefore be determinedin advance to properly size the input powercomponents.

- Losses in the resonant circuit are a result of HVreactor losses, and test load losses. Thesecomponents are described in more detail asfollows:

- Reactor Losses: This represents the realpower dissipated in the HV reactor itself at agiven output voltage and current. This powerloss results from resistive losses in the HVreactor windings, magnetic losses in the HVreactor core steel, and any other stray losses

in the tank or frame structure.


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- Test Load Losses: This represents the realpower dissipated in the test object at a givenvoltage and current, which includes losses in

insulation of the test specimen due toresistive leakage current, losses in anytermination equipment used to make a coronafree connection to the test object, and anyother stray losses external to the HV reactor.

- The required Test Specimen Reactive Power

may be calculated as:

Test Power =V

XkVAR

output

c

2

[ ]

Where Xcrepresents the capacitive reactance of

the load, calculated as:

XfC

cload

= 12

[ ]

Where frepresents the power frequency [Hz]

And Cloadrepresents the capacitance of the test

object [F].


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Mode of Operation:Series Resonant Mode vs.

Parallel Resonant Mode

- The decision on mode of operation, eitherseries resonant or parallel resonant, is madeaccording to the test object and the

measurements to be carried out.

- In most cases, tests can be performed in eithermode of operation, however certain type oftesting are easier to perform in one mode thananother.

- Since with Phenix Technologies resonant testequipment parallel mode operation is onlypossible with Grounded Tank Type ResonantTest Systems, the question of mode ofoperation should be addressed in thespecification stage of an equipment purchase.


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- Series Resonant Operation

- Series resonant operation is better suited forsensitive partial discharge measurements.

- Line noise from the power line is bettersuppressed in series mode operation. Theseries resonant circuit represents a low passfilter with a pole located at the resonant

frequency, which in this case is the powerfrequency of 50 or 60 Hz.

- The energy released during specimen failureis considerably lower in series resonant modeoperation than in parallel mode operation.

- Precise tuning is more difficult in seriesresonant operation than in parallel resonantoperation, and voltage control is less stable.The series resonant operating point isextremely sensitive to small tuning changes.

- Operation at full output voltage is notpossible in series resonant operation without

a capacitive preload present at the output ofthe HV reactor.


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- Parallel Resonant Operation

- Parallel resonant operation provides a more

stable output voltage for unstable testspecimens such as large generator windings,or other test specimens with unstable ohmiccomponent due to partial discharge or coronalosses.

- In parallel resonant operation, the rate of rise

of the test voltage is stable with the rate ofrise of the regulator output.

- Tuning changes appear as currentfluctuations at the regulator and excitertransformer, rather than voltage fluctuationsat the output of the HV reactor.

- Parallel resonant operation permits operationat full voltage with no load present at theoutput of the HV reactor. This feature can bequite useful when type testing very shortlengths of cable which require very littlepower.


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Issues to Be Considered inthe Specification of a HVDC

Dielectric Test System

- Voltage Required

- The maximum DC voltage required must bespecified. This depends on the objects to be

tested, and the test procedures of thecustomer. These procedures may beregulated by national or internationalstandards for the objects under test.

- Some HVDC Test Systems are fixed in theiroutput voltage capability, whereas others are

offered as modular systems whose maximumvoltage output is expandable via addition ofmore modules.

- At some point in the expansion of a modularHVDC Test System, increases in maximumsystem voltage will come at the expense of

maximum output current, as the regulator andtransformer(s) of lower stages must supplypower to all subsequent stages.


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- Maximum Current Required

- Generally speaking, the maximum currentrequired of any HVDC Test System isdetermined by two parameters:

1) The sum of the resistive leakage currentand any partial discharge current of thetest object at maximum test voltage

2) The amount of time allowed to reach thedesired test voltage

- From theory, the test specimen current whenperforming HVDC tests is comprised of fourcomponents. These are:

1) The capacitive charging current due to theapplication of direct voltage to the testspecimen:

This current represents electrical chargebeing transferred onto the test object. Thecharge transferred to the test object

represents potential energy which isstored in the electric field of the testobject's capacitance. It is this electric fieldwhich stresses the insulating materialwhich is under test.


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2) The dielectric absorption current due toslow charge displacements within the

insulation of the test object:

This component of current may persist forperiods of a few seconds up to periods ofseveral hours. It is this component ofcurrent which is of interest when doingpolarization index tests on winding

insulation in rotating electrical machines.

3) The continuous leakage current of the testobject insulation at maximum test voltage,which is the steady state direct currentwhich has to be supplied to maintain aconstant direct voltage after components1) and 2) have decayed to zero:

Neglecting partial discharge losses, thiscurrent is the minimum current whichmust be supplied to the test object toreach and maintain the desired maximumtest voltage. Theoretically, as long as atleast this much current is available from

the HVDC Test System, the test object caneventually be charged to maximumvoltage.


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4) Partial discharge currents due to partialbreakdowns in the test object insulation,or corona losses in air due to inadequate

conductor sizes or inadequate voltageclearances to other objects.

- Generally, all of the above components aresmall compared to component 1), thecapacitive charging current. The amount ofcurrent required therefore normally reduces

to a question of how much time can beallowed to charge a given test object to thedesired test voltage.

- The amount of time required to charge anobject may be approximately calculated,given the available charging current, thecapacitance of the test object, and the desiredtest voltage to be attained. This is done asfollows:

The total electrical charge qstored within agiven capacitance at a given voltage Visexpressed as:

q CV= coulombs][

Where: C= the test object capacitance [F]V= the desired test voltage [V]


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1 ampere of current is defined as the flow of 1coulomb of electrical charge past somereference point in a time interval of 1 second.

Hence the total amount of charge imparted toa test object with capacitance Cin a time tmay be expressed as:

q It= [coulombs]

Where: I = the direct charging current appliedto the test object [A] or [coulombs /second]t = the time for which the chargingcurrent Iwas applied [seconds]

The result, therefore, is:

CV It = [coulombs]

Solving this for time:

tCV

I= [seconds]


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This relationship will provide an approximateamount of time required to charge a given test

object C to a given voltage V, at a constantcharge rate of I, with the followingassumptions:

1) The charging current is maintainedconstant at I amperes, by an operator orautomatic control system. In practice, the

actual charging current may fall off a bit asthe rated voltage of the HVDC Test Systemis approached, due to RC time constantlimitations.

2) All other current components are smallcompared to the capacitive chargingcurrent of the test object. This is normallya good assumption.


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- The following illustrative example shows howthis equation may be applied to determine theamount of time required to charge a given

capacitance to a given voltage at a givencurrent.

Example:

Consider a reel of coaxial HV cable. Thecapacitance of the cable reel measured from the

center conductor to the ground shield is 5 uF, and itis desired to charge the cable to 160 kVDC towithstand test the insulation. Assuming a 200 kVHVDC Test Supply is available, and is rated for 5 mAoutput current. Neglecting dielectric absorbtion,leakage and partial discharge losses, approximatelyhow much time is required to charge the test objectto the desired voltage?

Solution:

Using the relationship tCV

I= onds][sec the

approximate time is calculated:

tx

= =

( )( , )

.

5 10 160 000

0 005 160

6F V

A seconds


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- Duty Cycle Required

- Normally, continuous current capability is not

required, as the charging current is onlypresent for a few minutes or seconds.

- The time interval for which a HVDC TestSystem to rated to supply a given currentessentially determines the maximumcapacitance which can be steadily charged to

rated test system voltage at that current. Thisdoes not mean that larger capacitancescannot be tested, but simply that they willhave to be charged at a lower current for alonger time interval.

- When purchasing a standard HVDC TestSystem, calculations can be performedsimilar to those given in the previous exampleto determine if the system will meet thedemands of particular test application.

- When purchasing a custom HVDC test systemit is important to provide the equipmentsupplier with as much information as

possible regarding the intended application ofthe system, to insure optimum performance.


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- Polarity Required

- DC hipot tests may generally be performedusing either polarity.

- Some HVDC Test Systems offer reversiblepolarity output. Others offer only a fixedpolarity output, which is determined by theinternal connection of the rectifier.

- The customer must determine based on theirspecific test requirements the requiredpolarity, and this must be included in thespecification for the specific test system.


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- Measurements to Be Made

- If a HVDC test is simply a withstand voltagetest, voltage may be the only criticalmeasurement. Voltage measurement byresistive voltage divider is the normal methodused for voltage measurement in most HVDCtesting applications.

- If leakage current through the insulation of atest object must be measured, special guard /ground metering circuitry may be employedto differentiate leakage current returning fromthe grounded side of the specimen fromoutput current measured at the return of thepower supply.

- If percentage ripple is important, rippledetection circuitry can be included to monitorthe ripple voltage and either illuminate anindicator if the percentage ripple increasesabove a preset limit, or display the percentripple directly.


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- Percentage Ripple Required

- Generally, unless a very high component ofleakage current is present in a test object, thecapacitance of the test object providesenough filtering effect to essentially eliminateripple once a steady direct test voltage isreached.

- Depending on the type of HVDC rectifiercircuit provided in the test system, the ripplemay be quite high during the charging of atest object. Once the object reaches full testvoltage the test object's own capacitanceholds the voltage constant, dropping only theamount permitted by the natural RC dischargerate between successive voltage peakscoming from the test supply.

- For HVDC testing of largely capacitive testobjects, even a single phase half waverectifier is usually adequate to maintain asufficiently low ripple at constant testvoltages.


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- For test objects with a larger resistive leakagecomponent, additional measures may be

taken to reduce the ripple voltage, such as theuse of full wave single phase rectifier circuits,high voltage filtering capacitors, or 3, 6 or 12pulse three phase rectifier circuits.

- Higher supply frequencies, supplied by solidstate power supplies, are sometimes used

when low ripple is required in the presence ofa large resistive leakage component. Thehigher frequency allows the test object's owncapacitance to provide better filteringperformance, as well as a reduction in thesize of additional filtering capacitors whichmay be used.


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- Type of Load

- In general, the type of load will determine themajor characteristics of the required HVDCtest system.

- Large capacitive loads with a small resistiveleakage component will require no filtering forripple reduction, but may require a high

output current capacity for quick chargetimes.

- Loads with large resistive leakage mayrequire special rectifier circuits and additionalfiltering to provide suitably low ripple output.

- Loads with a large resistive leakagecomponent may need a higher duty cycle,since the leakage current remains constant aslong as the voltage is applied. This is differentthan a large capacitive load with a smallleakage component, where the chargingcurrent decreases with time.

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fundamental-class-high-voltage.pdf