cugre 2103 presentation a_portillo

AC Insulation Design of Power Transformers

FundamentalsEng. Álvaro PORTILLO LAURINO

Transformer ConsultantBrenda 5920, Montevideo, CP11400, Uruguay

Phone: (+598) 26007982 e-mail: [email protected] Power Transformers

Workshop - Challenges and SolutionsCUGRE - 29th October 2013 Salto - Uruguay

SummaryIn this work we will try to give an overview of

the insulation design process of high voltage "core-type" power transformers operated in AC networks for engineers involved in design

review tasks

CUGRE - 29th October 2013 Salto - Uruguay

International Power Transformers Workshop - Challenges and Solutions 2

review tasksAll presented values and formulas are only

orientations and can vary widely between manufacturersEmphasize the concepts

Introduction• The power transformers in electric networks are subjected:

- permanently to continuously operating voltages- sometimes to transient overvoltages caused by faults, switching operations or lightning strikes

• To probe his ability to work for many years in service, withpermanent and transient voltage conditions, the transformersare subjected to factory acceptance dielectric tests

• This tests trying to represent the different conditions that the• This tests trying to represent the different conditions that thepower system can impose to the transformer

• This tests are the result of more than 100 years of experienceand is generally accepted that if a transformer successfullypasses these tests they have a very high probability of workfor many decades in service without dielectric problems

• The challenge for the transformer designer is define aninsulation structure which comply with the dielectric tests



Introduction• The purpose of transformer insulation is to isolate parts or

electrodes at different potentials from one another but thedesign of an insulation structure is not only define thisdistances inside the transformer

• Previous to this is necessary to define completely: - the geometry and number of insulation barriers between windings and between windings and ground - the insulation material type best suited for each part of the transformer the transformer - the thickness of the conductor insulation - if is necessary or not the use of static end rings in the windings - the type of winding (interleaved or not)

• These definitions have a big influence in the voltagedistribution inside the transformer and in the electric fieldsthat appears during the dielectric tests



Introduction• Once the material types and geometry is completely defined

voltage distributions outside and inside the windingsaccording to the test voltages and to the correspondingwinding connection during test are calculated

• For AC voltages (50 to 200 Hz) the voltage distribution followslinearly the number of turns and can be calculated veryprecisely

• The calculation of impulse voltage distribution requires thesimulation of the transformer by means of an equivalentcircuit consisting of lumped R, L and C elements

• Then with this voltage distribution using simple analyticalformulae or numerical methods (like Finite Elements Method)is possible to calculate the electric field or electrical stress ineach point inside the transformer



Introduction

kV → kV/mm1.503e+007 : >1.582e+0071.424e+007 : 1.503e+0071.345e+007 : 1.424e+0071.266e+007 : 1.345e+0071.187e+007 : 1.266e+0071.107e+007 : 1.187e+0071.028e+007 : 1.107e+0079.492e+006 : 1.028e+0078.701e+006 : 9.492e+0067.910e+006 : 8.701e+006

Density Plot: |E|, V/m

1.503e+007 : >1.582e+0071.424e+007 : 1.503e+0071.345e+007 : 1.424e+0071.266e+007 : 1.345e+0071.187e+007 : 1.266e+0071.107e+007 : 1.187e+0071.028e+007 : 1.107e+0079.492e+006 : 1.028e+0078.701e+006 : 9.492e+0067.910e+006 : 8.701e+0067.119e+006 : 7.910e+0066.328e+006 : 7.119e+0065.537e+006 : 6.328e+0064.746e+006 : 5.537e+0063.955e+006 : 4.746e+0063.164e+006 : 3.955e+0062.373e+006 : 3.164e+0061.582e+006 : 2.373e+0067.910e+005 : 1.582e+006<0.000e+000 : 7.910e+005



Density Plot: |E|, V/m

7.910e+006 : 8.701e+0067.119e+006 : 7.910e+0066.328e+006 : 7.119e+0065.537e+006 : 6.328e+0064.746e+006 : 5.537e+0063.955e+006 : 4.746e+0063.164e+006 : 3.955e+0062.373e+006 : 3.164e+0061.582e+006 : 2.373e+0067.910e+005 : 1.582e+006<0.000e+000 : 7.910e+005

ITAIPÚ Autotransformer

470/470/157 MVA525/241.5/13.8 kV

Introduction• The calculated electrical stress (kV/mm) in each point P must

be less than the admissible dielectric strength (kV/mm) of theinsulating material used in this point P for this test condition

• If not, the insulation design is modified and verified again, and this procedure iteratively must leads to an optimised solution

• Normally the design is defined in a way that the test stressdoes not exceed the PD inception values of the insulatingmaterials

• Finally the success of the dielectric design depends on selecthigh quality insulating materials with narrow dimensiontolerances and shape stability and applying adequatestabilization, drying and impregnation processes to theinsulation materials



Transformer Insulations• The transformer insulations are usually classified in:

– external or major insulations– internal or minor insulations

• External or major insulations include principally insulations outer the windings:

– winding to winding (gaps between windings)(gaps between windings)

– phase to phase– windings to ground

(to core legs, to core yokes and to tank)– winding leads

(connections between windings, connections from windings to bushings, connections from windings to OLTC , etc.)



Phase to Phase Insulation



Winding to Ground Insulation



HV Winding Lead Insulation



Connections from Windings to OLTC



Transformer Insulations

• Internal or minor insulations include principally insulations inside the windings:

– conductor to conductor – turn to turn– turn to turn– section to section (axially along windings) – layer to layer



Turn to Turn Insulation



Section to Section Insulation



Transformer Insulations• Other essential elements in order to achieve a good dielectric

design:

– Angle caps – Angle rings– Static end rings– Static end rings– Internal surge arresters



Angle Caps



Angle Rings



Static End Rings



Internal Surge Arresters



Power System OvervoltagesThe transformers during operation are subject continuously to operating voltages and occasionally to overvoltagesThe overvoltages occurring in the power systems can bedivided into:– lightning overvoltages

aperiodic voltage waves with duration of one to tens of microsecondsmicroseconds

– switching overvoltagesdamped oscillatory voltage waves with duration up to thousands of microseconds

– temporary overvoltagesvoltage waves at or close to the power frequency lasting for few minutes



Dielectric TestsTests intended to verify the insulation withstand to operational

voltage and to transient overvoltages• The Applied Voltage Test at industrial frequency (50 or 60 Hz)

With the applied voltage test the withstand strength of theexternal insulations (winding to winding and windings toearth) to service and temporary overvoltages is verifiedIn this test there is not turn-to-turn voltage

• The Induced Voltage Test (short and long duration) at• The Induced Voltage Test (short and long duration) atindustrial frequency (between 100 to 200 Hz)With the induced voltage test we verified principally theinternal insulations (turn to turn, section to section) and alsoexternal insulations (phase to phase, winding to winding andwindings to earth) to service and temporary overvoltagesThe long duration induced voltage test with PD measurementis intended to verify that the transformer will be free ofharmful partial discharges under normal operatingconditions



Dielectric Tests• The Switching Impulse Test is intended to verify the capability

of insulation to withstand slow rise time (greater than 100 µs)transient voltages typically associated with switchingoperations in serviceWith this test the internal and external insulations are verifiedto switching transientsThe fundamental test wave frequency is in the order of 2.5 kHzThe voltage impulse shall have a time to peak of at least 100 μs,a time above 90 % of the specified amplitude of at least 200 μs,a time above 90 % of the specified amplitude of at least 200 μs,and a time to zero of a minimum of 1000 μs.



This impulse wave shape is purposely differentfrom the standard waveshape of 250/2500 μsrecommended in IEC 60060-1, since IEC60060-1 is intended for equipment without asaturable magnetic circuit.

The time to peak is chosen to be long enoughto give an essentially linear voltagedistribution along the windings

Dielectric Tests• The Lightning Impulse Test (full wave and chopped wave) is

intended to verify the insulation withstand to fast rise time (around 1 µs) transients overvoltages occurring in the power system as a result of lightning strikesWith this test the internal and external insulations are verified to lightning transientsThe fundamental test wave frequency is in the order of 250 kHzThe chopped wave test voltage impulse has a higher peak value and contains higher frequency components than the full wave impulseand contains higher frequency components than the full wave impulse



Dielectric Tests - Connections



Voltage Distribution in WindingsThe distribution of voltage to ground along the coils (Fig.a), for the differenttests, is illustrated in the following figures:



Fig.b shows the distribution of voltage in the applied voltage test (not turn-to-turn voltage)Fig.c shows the distribution of voltage in the induced voltage test (voltageinductively distributed, proportional to the number of turns, through allwindings)Fig.d shows the distribution of voltage in the atmospheric impulse test(oscillating voltages that produces non-uniform stresses in winding undertest).

Voltage Distribution in Windings• In the atmospheric impulse test the voltage distribution

depends of the capacitances and inductances (self andmutual) of the windingsThe initial voltage distribution inside the windings iscapacitive and at the end of the transient this voltagedistribution is inductiveDuring the transient the voltage in each point of the windingis oscillatory with frequencies equal to the naturalfrequencies of the transformer and with a dampingis oscillatory with frequencies equal to the naturalfrequencies of the transformer and with a dampingdepending of the transformer losses

• In the case of switching impulse test the voltage distributionis almost linear, similar to that experience during an inducedvoltage withstand test, and when specifying switching impulsetest is not performed short-duration induced voltage test(Table 1 of IEC 60076-3:2013)



External Insulation DesignIn a paper-oil insulation system, stressed with AC voltage: • the maximum admissible field stress of pressboard is higher

than 20 kVrms/mm• for an oil gap of around 5 mm values less than 12 kVrms/mm are

admissible• This difference is augmented by the fact that the permittivities

of the two materials differ by a factor 2, resulting in fieldvalues twice as high in oil that in the adjacent board

• Furthermore, the relative strength of oil for an increasing gap• Furthermore, the relative strength of oil for an increasing gapwidth decreases:

Therefore, in a paper-oil insulation system the solid material isused only to subdivide oil gaps and to insulate electrodes. Thedesign of such systems concentrates in general on the electricstrength of the oil gaps and of solid-liquid interfaces



External Insulation Design – Weidmann Curves

These curves express the maximum admissible design value as a value of uniform electric field of low probability of partial discharge inception for 1 min AC test voltage (less than 1%)



External Insulation Design

• Partial discharges should be excluded even during dielectric tests of insulations structures

• This design concept is extremely important

• Localization of partial discharges during transformer testing is unsafe and should be avoided to the greatest possible extent



External Insulation Design• Another type of breakdown that can occur in insulation

structures consisting of solids and fluids is creep breakdown• This occurs along a solid surface that is in contact with a

liquid or gas • These potential breakdown surfaces are nearly unavoidable in

insulation design.insulation design.• In the end insulation area the pressboard must be used in

such a manner that creep stress are practically precluded • To achieve this the pressboard-oil boundary

surfaces must run, as far as possible, parallel to the equipotential surfaces.



External Insulation DesignFor the solid-liquid interfaces the maximum admissible creep tangential stress EC-AC along clean pressboard surfaces in degassed oil, in terms of the creep distance dC (in mm) along the surface, can be calculated using the following formula:

EC-AC in kVrms/mm, 1% probability of PD inception



External Insulation Design• Several parameters influence the breakdown behaviour of

transformer oil and in consequence to this the oil-designcurves

• The most evident parameter is the duration of voltageapplication on an insulation configuration

• Breakdown test has shown that oil-paper insulation exhibits• Breakdown test has shown that oil-paper insulation exhibitsan exponential decrease of strength when the duration of thevoltage application is increased

• This volt-time breakdown characteristic can be represented with a equation of the type:



External Insulation Design - DIL• Design curves must reflect this dependence; therefore these

curves are defined for a reference duration of 1 minute, AC,power frequency

• A multiplication factor is introduced to adapt the designcurves for different time duration, e.g. lightning impulse (BIL),switching impulse (SIL), 1 hour induced voltage, etc.switching impulse (SIL), 1 hour induced voltage, etc.

• This factor is called Design Insulation Level (DIL) and itincreases (reduces) the respective design curve value if thevoltage application time is shorter (longer) than 1 minute:



External Insulation Design - DIL



External Insulation DesignThe evaluation of extenal paper-oil insulation systems consist ofcalculation of stress and the subsequent comparison of stressvalues with admissible design valuesThe calculation of stress is divide into three parts:

• Calculation of Voltage distributions within windings according tothe specific test voltages and to the corresponding windingsconnections during the tests

• These voltages are converted to the equivalent voltage at 1• These voltages are converted to the equivalent voltage at 1minute power frequency voltage

• The maximum of these equivalents in each insulation clearance willdefine the insulation design in this insulation clearance

• Finally, usually using the Finite Elements Method (FEM) oranalytical formulas for simple geometries, the electric field withinthe insulation clearances is determined for the maximumequivalent voltage.CUGRE - 29th October 2013 Salto - Uruguay


External Insulation DesignWinding to Winding Insulation

W

inding

Wind

ing

End collars

Key spacers

Angle Rings

Radial Spacers LV

HV



LV W

inding

HV W

inding

Pressboard barriers

Pressboard barriers

Sticks

External Insulation DesignFrom the point of view of the electric field calculation thisconfiguration is very simple. Disregarding curvature effects, theelectric field is uniform along all the height of the windings andcan be calculated using the elementary formulas of a planecapacitor:

Where U is the applied voltage between the windings, dOil is thetotal oil width, dPsb is the total pressboard width and (ε = 2.2)and (ε = 4.4) are the permittivities of oil and pressboardrespectively.



External Insulation DesignTo simplify we suppose that all cylinders have the same width and allthe oil ducts have the same width (this is not usual in practical cases):

In this case, for AC test (1 min, 50 Hz), with degassed oil andinsulated electrodes, applying equation for oil and equation forpressboard the design conditions will be:pressboard the design conditions will be:



External Insulation DesignLike an example, consider the design of a 245 kVrms main gapThe voltages to be applied in the tests are:

Um 245/ SI 850/ LI 1050/ LIC 1155/ AC 460 kVAccording to DIL factor approach the main gap will be designed for:






Both Winding to Winding Insulations (LV-HV gaps) are equivalents from the point of view of the dielectric design


Phase to phase insulation Winding to core return legs insulation



Winding to core return legs insulation

Winding to tank insulationDisregarding curvature effects these

insulations are designed in the same way of winding to winding insulation

External Insulation DesignNon-uniform electric fields

• End winding insulation• Winding to core yokes insulations• Winding leads insulations



External Insulation Design• For non-uniform electrical fields a very conservative approach

would be to limit local maximum stresses to values given bythe oil design curves for the full length gapIn this case the larger part of the gap is not stressed at thelimit of its dielectric strengthThis is not satisfactory as it leads to excessive dimensions andhigh costshigh costs

• On the other hand it would be risky to compare the averageelectric field stress with the design curvesIn highly non-uniform fields average values can be lowcompared with the maximum value in the gapThese highly stressed gap parts intervals might beoverstressed



External Insulation Design• The method for determining the electric strength in the case

of non-uniform fields was developed and verified byexperiment [Weidmann]

• Suppose a oil gap d with highly non-uniform electric fieldprofileBeginning in the high field region, average stresses arecalculated for gap intervals which are sucessively increasingfrom up to the total length:from up to the total length:

These average stresses are compared with the oil designcurves values for a gap interval zThe dielectric strength must be higher than the averagestress for all the gap intervals z from z = 0 to z = d



External Insulation DesignDesign curves for non-uniform electric fields



This method is widely used to define maximum permissiblevoltages in insulating structures with highly non-uniformedelectrical fields and breakdown tests showed that thesevoltages are equivalent to a breakdown probability of 2%.

External Insulation Design - Example

Case 1 Case 2 Case3



External Insulation DesignExample



q( r ) Minimum Oil Gap 1 Oil Gap 2 Oil Gap 3Case 1 1.19 @ r =125 mm −−−−− −−−−−

Case 2 1.18 @ r =110.5 mm 2.30 @ r =177.5 mm 3.39 @ r =245 mmCase 3 1.62 @ r =54 mm 1.64 @ r =73.5 mm 1.64 @ r =245 mm

Internal Insulation Design• For the design of internal insulations the same rules explained

in external insulations can be applied with the exception ofDIL factor

• DIL must not be applied for internal design of the windingsinsulation

• These insulations shall be designed for each type of stress• These insulations shall be designed for each type of stress(service, AC tests, impulse tests, etc.)

• The internal insulation design is strongly dependent of thetype of winding and of the measures taken by the designer toimprove the lightning impulse voltage distribution by meansof winding series capacitance increase (interleaved diskwindings and intershield disk windings)



Internal Insulation DesignTurn to Turn Insulation - Kraft Paper - Admissible Field Stress



n = 0.22 for impulse testsn = 0.33 for AC tests

Internal Insulation DesignSection to Section Insulation

A oil duct is generally provided between sections of the disc-type and helical-type windings to ensure the requireddissipation of heatdissipation of heatDue to the creepage distance formed by the pressboard spacersplaced between the sections, the admissible voltage stress inthese ducts is much lower than that tolerable for an oil gap ofequal thicknessSome manufacturers use curves defining the admissible sectionto section strength in function of the thickness of conductorinsulation, parametric in the distance between the sections.



Internal Insulation DesignSection to Section Insulation

A more direct approach is to use directly an equation for theadmissible creepage stress in uniform fields for AC (50 Hz, 1min) and lightning impulse voltages:

Other values that are verified are the oil electric field at theinternal and external edges of the conductors of each sectionThe limits for these stresses are around 11 kV/mm for AC (50Hz, 1 min) and 29 kV/mm for lightning impulse voltages.All these values and formulas are only orientations and can varywidely between manufacturers



Thank you very much for your attentionfor your attention



cugre 2103 presentation a_portillo

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