high voltage testing of transformer

7/25/2019 High Voltage Testing of Transformer

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High Voltage Testing Of Transformer

Introduction

A transformeris a static device that transfers electrical energyfrom one circuitto another

through inductively coupledconductorsthe transformer's coils. A varying currentin the

first or primarywinding creates a varyingmagnetic fluxin the transformer's core and thus a

varying magnetic fieldthrough the secondarywinding. This varying magnetic field inducesa

varying electromotive force (EMFor !voltage! in the secondary winding. This effect is

called mutual induction.

"f a loadis connected to the secondary# an electric current will flow in the secondary winding

and electrical energy will $e transferred from the primary circuit through the transformer to

the load. "n an ideal transformer# the induced voltage in the secondary winding (Vs is in

proportion to the primary voltage (Vp# and is given $y the ratio of the num$er of turns in the

secondary (Ns to the num$er of turns in the primary (Np as follows%

&y appropriate selection of the ratio of turns# a transformer thus allows an alternating current

(Avoltage to $e !stepped up! $y maing Nsgreater than Np# or !stepped down! $y maing

Nsless than Np.

"n the vast ma)ority of transformers# the windings are coils wound around aferromagnetic

core# air*coretransformers $eing a nota$le exception.

Transformers range in si+e from a thum$nail*si+ed coupling transformer hidden inside a stage

microphoneto huge units weighing hundreds of tons used to interconnect portions ofpower

grids. All operate with the same $asic principles# although the range of designs is wide. ,hile

new technologies have eliminated the need for transformers in some electronic circuits#

transformers are still found in nearly all electronic devices designed forhousehold (!mains!

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http://en.wikipedia.org/wiki/Electrical_energyhttp://en.wikipedia.org/wiki/Electrical_energyhttp://en.wikipedia.org/wiki/Electrical_networkhttp://en.wikipedia.org/wiki/Inductive_couplinghttp://en.wikipedia.org/wiki/Inductive_couplinghttp://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Magnetic_fluxhttp://en.wikipedia.org/wiki/Magnetic_fluxhttp://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/Electromagnetic_inductionhttp://en.wikipedia.org/wiki/Electromotive_forcehttp://en.wikipedia.org/wiki/Volthttp://en.wikipedia.org/wiki/Mutual_inductionhttp://en.wikipedia.org/wiki/Mutual_inductionhttp://en.wikipedia.org/wiki/Electrical_loadhttp://en.wikipedia.org/wiki/Alternating_currenthttp://en.wikipedia.org/wiki/Alternating_currenthttp://en.wikipedia.org/wiki/Magnetic_corehttp://en.wikipedia.org/wiki/Magnetic_corehttp://en.wikipedia.org/wiki/Magnetic_corehttp://en.wikipedia.org/wiki/Transformer#Coreshttp://en.wikipedia.org/wiki/Microphonehttp://en.wikipedia.org/wiki/Microphonehttp://en.wikipedia.org/wiki/Power_gridhttp://en.wikipedia.org/wiki/Power_gridhttp://en.wikipedia.org/wiki/Mains_electricityhttp://en.wikipedia.org/wiki/Mains_electricityhttp://en.wikipedia.org/wiki/Electrical_networkhttp://en.wikipedia.org/wiki/Inductive_couplinghttp://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Magnetic_fluxhttp://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/Electromagnetic_inductionhttp://en.wikipedia.org/wiki/Electromotive_forcehttp://en.wikipedia.org/wiki/Volthttp://en.wikipedia.org/wiki/Mutual_inductionhttp://en.wikipedia.org/wiki/Electrical_loadhttp://en.wikipedia.org/wiki/Alternating_currenthttp://en.wikipedia.org/wiki/Alternating_currenthttp://en.wikipedia.org/wiki/Magnetic_corehttp://en.wikipedia.org/wiki/Magnetic_corehttp://en.wikipedia.org/wiki/Transformer#Coreshttp://en.wikipedia.org/wiki/Microphonehttp://en.wikipedia.org/wiki/Power_gridhttp://en.wikipedia.org/wiki/Power_gridhttp://en.wikipedia.org/wiki/Mains_electricityhttp://en.wikipedia.org/wiki/Electrical_energy


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voltage.Transformers are essential for high*voltage electric power transmission# which

maes long*distance transmission economically practical.

High Voltage Transformer

-igh voltage transformers convert votages from one level or phase configuration to another#

usually from higher to lower. They can include features for electrical isolation# power

distri$ution# and control and instrumentation applications. -igh voltage transformers usually

depend on the principle of magnetic induction $etween coils to convert voltage andor current

levels.

-igh voltage transformers can $e configured as either a single*phase primary configuration or

a three*phase configuration. The si+e and cost of a transformer increases when you move

down the listing of primary windings. /ingle*phase primary configurations include single#

dual# 0uad (121# 3*lead# and ladder. A 3*4ead primary re0uires more copper than a 5uad

(121 primary. A 4adder is the least economical primary configuration. Three*phase

transformers are connected in delta or wye configurations. A wye*delta transformer has its

primary winding connected in a wye and its secondary winding connected in a delta. A delta*

wye transformer has its primary winding connected in delta and its secondary winding

connected in a wye. Three phase configuration choices include delta * delta# delta * wye (6#

wye (6 7 wye (6# wye (6 7 delta# wye (6 7 single*phase# delta 7 single phase# and

international. 8rimary fre0uencies of incoming voltage signal to primaries availa$le for

power transformers include 39 -+# :9 -+# and ;99 -+. 39 -+ is common for European

power. :9 -+ is common in ated 8ower of the transformer is the sum of the ?A (?olts x Amps for all of the

secondary windings. =utput choices include A or @. For Alternating urrent waveform

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output# voltage the values are typically given in >M/ values. onsult manufacturer for

waveform options. For direct current secondary voltage output# consult manufacturer for type

of rectification.

-igh voltage transformers can $e constructed as either a toroidal or laminated transformer.

Toroidal transformers typically have copper wire wrapped around a cylindrical core so the

magnetic flux# which occurs within the coil# doesn't lea out# the coil efficiency is good# and

the magnetic flux has little influence on other components. 4aminated transformers contain

laminated*steel cores they are also called E*" transformers. These steel laminations are

insulated with a nonconducting material# such as varnish# and then formed into a core that

reduce electrical losses. 8ower transformers can $e one of many types. These include

autotransformer# control transformer# current transformer# distri$ution transformer# general*purpose transformer# instrument transformer# isolation transformer# potential (voltage

transformer# power transformer# step*up transformer# and step*down transformer. Mountings

availa$le for high voltage transformers include chassis mount# dish or dis mount# enclosure

or free standing# h frame# and 8& mount

Testing Of Transformer

As regards complex electrical e0uipment such as high voltage power transformers# internal

insulation is su$)ect to defects due to several reasons associated to $ad material# design#

manufacturing processesor resulting from shipment.

=n*site electrical tests are for the test voltage to simulate on the transformer under testing the

e0uivalent stresses which may $e esta$lished during service condition.

&asically# electrical tests on power transformers are grouped in type and routine tests. The

goal of a routine test is to chec correct manufacture of -? insulation while the goal of a

type test is to confirm correct design of -? insulation.

"n addition# the application of on*site tests may $e a$le to $e separated in%

commissioning tests% as part of the on*site e0uipment commissioning procedure in order to

demonstrate that shipment and erection have not caused any new defects to -? insulation

on*site repair or refur$ishment% as part of the repair or refur$ishment procedure in order to

demonstrate that repair or refur$ishment have $een successfully completed and -? insulation

is free of dangerous defect and

diagnosis% as part of a diagnostic procedure in order to provide reference values to further

tests or to confirm results o$tained from other types of test.

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Bp to date# on*site high voltage withstand tests including partial discharge monitoring and

measurements are the most significant tests in order to 0uantify -? insulation 0uality. The

use of a separate -? source is more informative than measurement at normal operation

voltage# as it allows investigation of the -? insulation performance with voltage.

Alternating voltages are most important for on*site tests . =ther voltage shapes for simulation

of overvoltages have $een used however# they are strongly dependent on availa$ility of on*

site testing systems.

The application of -? on*site tests has $een a good practice in /outh America. /ince CDD1#

on*site -? tests have $een performed in more than CC9 power transformers ranging from

9M?A to 339M?A# CC3? to :3? (A and :99? (@. 4arge electric power utilities

and industrial plants are the main customers to this technology.

HV ON-SITE TEST SETUP

To perform -? on*site tests# a complete set of mo$ile testing e0uipment is made availa$le at field#

including%

varia$le fre0uency :9*1;9-+ motor*generator group. There are three motor*generator groups

availa$le% 99?A# G39?A and 1M?A. The proper group is selected according to transformer

power and voltage

step*up and regulating transformers

reactive power compensating capacitors and reactors

no*load and load measuring system and

partial discharges measuring and monitoring system as per "E:99:* and "E:919.

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HV ON-SITE TESTS APPLIED O! DIA"NOSIS

"n many cases# -? on*site tests have $een used for diagnostic purposes on large power transformers.

The process of this application typically starts $ased on previous events such as%

detected event of in oil dissolved gas generation increase given up partial discharge as a possi$le

diagnosis using dissolved gas analysis methods or

detected mechanical event such as overacceleration during a shipment operation.

"n several cases# -? induced voltage with partial discharge electrical and acoustic monitoring has

$een successfully used to detect and locate partial discharge in large power transformers.

As an example# figure ; shows a ;*year old 99M?A# 339CGC.G? on*load regulating transformer

under on*site testing at a su$station yard

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@uring the test# partial discharge activities were measured (up to 399p at C9HBn and located in

the -? winding exit areas. Figure 3 shows the results of 8@ location through the application of

acoustic sensors.

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The transformer has $een visually inspected internally and partial discharges evidences have $een

located in the area indicated $y the previous test induced test. After that# the transformer has $een

shipped $ac to factory and disassem$led for complete repair.

#

Im$ulse Voltage Test Of Transformers

During the Lightning Impulse (LI) test of transformer windings with a low impedance

it is difficult to ensure a minimum time to half-value of 40 s in accordance with IEC

00!-" and IEC 000-#$ %his is caused &' the oscillating discharge

determined &' the impulse voltage test generator capacitance and the transformer

impedance$ In most cases using special adapted circuits can solve the pro&lem$

1. Impulse voltage test generator with capacitive load

or the LI testing of &asic arrangements &ut also of different electrical components a

purel' capacitive load can &e assumed$ %he impulse voltage shape generated &' an

impulse voltage test generator &ased on the *+, multiplier circuit can &e

descri&ed &' two eponential functions with different time constants$ .hereas the LI

front time %# according to IEC 000-# /# is essentiall' determined &' the

resistance of the front resistor +s located in the impulse voltage test generator and

the load capacitance Ct1 see fig$ #1 the time to half-value %2 is determined &'

the impulse capacitance of the impulse capacitor Ci and the resistance of the tail

resistor +p &eing part of the impulse voltage test generator$ *ccording to IEC

0000-# there are the following time parameters and tolerances for the standard LI

#$2305 ront time %# 6 #$2 s 7 "0 8 %ime to half-value %2 6 0 s 7 20 8

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Impulse voltage test generator withinductive load

In most of the cases power transformers cannot &e assumed as a purel' capacitive

load for the LI testing$ 9suall' the LI test voltage is applied to one winding terminal ofthe transformer to &e tested1 whereas all other terminals are connected with the

earth$ :ere&'1 not onl' the input capacitance of the transformer winding acts as the

load for the impulse voltage test generator &ut also its impedance to all other short-

circuited windings$

%he principal circuit (fig$ #) must &e etended &' the transformer inductance Lt that is

connected in parallel to the test capacitance Ct

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%here&' the inductance Lt of the load &ecomes smaller with decreasing impedance

voltage vimp81 with decreasing rated phase-to-phase voltage ;


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Projection of an impulse voltage test generator for the LI test of

power transformers

%he main technical data of the transformers to &e tested1 li>e the circuitr' and the

arrangement of the windings1 their rated voltage1 rated power1 impedance voltage

and not at least the rated fre=uenc' determine essentiall' the total charging voltage

and the stage energ' of an impulse voltage test generator for the LI test$ %he total

charging voltage of the impulse voltage test generator should lie for LI testing "0 8

? 0 8 a&ove the highest re=uired LI test voltage$ In man' cases the value of "0 8

is sufficient for routine tests$ If development tests are to &e carried out1 a total

charging voltage1 which lies 0 8 a&ove the highest rated LI test voltage1 is

recommended$ If the eception @earthing via termination resistorsA is not considered1the re=uired impulse capacitance Ci re= can &e calculated for each winding voltage

level acc$ to e=uation ()$ %a>ing into consideration the different circuitr' options of

the impulse voltage test generator (parallel connection of stages1 partial operation)

and the a&ove aspects regarding the total charging voltage the stage charging

energ' can &e calculated in principle for each possi&le test case$ Bormall' a stage

energ' of ? #0 > per #00- >;-stage and a stage energ' of #0 ? 20 > per

200->;-stage will &e sufficient$ .hereas the lower values appl' to transformers with

lower power1 the higher values appl' to transformers with higher power (fig$ 4)$

ften1 impulse voltage test generators for power transformer testing have an energ'

of # > per 200->;-stage1

Extension of the loading range of impulse voltage test generators

ften it is re=uired to test transformer with such a high power1 for which the eisting

impulse voltage test generator has not &een originall' meant$ In such cases it is

necessar' to utilise all reserves of the eisting impulse voltage test generator$

Increasing the effective impulse capacitance

%he following generall' >nown measures can &e ta>en5

a) +unning the impulse voltage test generator in partial operation1 i$e$ with the

minimum num&er of stages1 &eing necessar' to reach the re=uired test voltage level$

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&) witching a certain num&er of generator stages respectivel' in parallel and

connect this parallel stages in series to reach the re=uired test voltage$

Increasing the parallel resistorsIf the time to half-value remains onl' a few &elow the permitted lower limit %2 min 6

40 s1 it is possi&le to reach a value of %2 40 s &' increasing the tail resistors +p$

9suall' the tail resistors meant for switching impulse voltage can &e applied$ *

further increase of the resistance of the tail resistors +p a&ove the resistance value

for the I generation does not have an' result$

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ig$ 5 Impulse ;oltage %est 'stem I< #032000 F (#0 >1 2000 >;) with impulse

voltage divider and chopping multiple spar> gap1 with a stage energ' of # > &eing used for theLI test of power transformers up to 24 >;

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5.3. Decreasing the damping of the test

circuit

*s alread' mentioned in chapter 21 if the circuit damping is to high1 a time to half-value of %2 40

s is not reached even with a sufficient of the impulse voltage test generator (Ci Ci re=)1 see fig$ "$

%he front and tail resistors in the impulse voltage test generator are mostl' responsi&le for that

damping$ %he damping caused &' the tail resistors + p can &e considera&l' eliminated &' their

increase1 as alread' recommended in chapter $2$ $ or a further reduction of the damping the

resistance of the front resistor +s has to &e reduced$ %his would cause a shorter front time %# of the LI$

%o >eep the front time %# unchanged1 the capacitance of the load has to &e increased corresponding

to the reduction of the resistance of the front resistor +s$ %his is easil' realised &' connecting

an additional capacitor in parallel to the transformer winding to &e tested$ 9nfortunatel'1 the effect of

this method is limited1 &ecause a reduction of the resistance of the front resistor + s will lead to

oscillations on the front of the LI voltage soon1 which ma' eceed the permitted limit for the overshootof 8 3#3$

pplication of the !"laninger#circuit!

%he disadvantage of oscillations on the voltage front after a reduction of the front resistor + s is

completel' avoided with a circuit invented &' FL*BIBFE+ $ :ere&' the front resistor responsi&le

for the voltage front remains unchanged &ut it is &ridged &' an additional inductance formed &' an air-

coil

%he Flaninger-coil must have an inductance value ca$ #00 ? 200 :1 to &e ineffective for the fastimpulse front and to &ridge the front resistor +s during the much longer impulse tail$ o he front of the

LI impulse remains unchanged and the tail is etended$ Conse=uentl' an additional resistor + t has to

&e switched in parallel to the load inductance Lt1 to form a true voltage divider consisting of + s33Lg and

+t33Lt

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ig$ !5 LI test of power transformers &' using the Flaninger-circuit1 adGustment of the voltage

shape at the voltage crest &' means of an additional resistor + t (optimal adGustment +t 6 "00

hm for this eample)

ig$ H5 LI test of power transformers &' using the Flaninger-circuit1 adGustment of the time to

half-value %2 and the amplitude of opposite polarit' d &' means of the tail resistor + p (optimal

adGustment +p 6 0 hm for this eample1 %2 40 s1 d J 0 8) .ith a Flaninger-circuit the front time

%#1 the time to half-value %2 and the amplitude of opposite polarit' d of the LI test voltage can &e set

almost independentl'1 i$e$ %# with the tail resistor +s1 %2 and d with the resistors +p und +t (fig$ !

and H)$ * variation of the Flaninger-coil inductance is as a rule not necessar'$ %he Flaningercircuit

ena&les for LI testing the most effectiveadaptation of the impulse voltage test generatorand the

transformer to &e tested$ *n eistingimpulse voltage test generator can &e utilisedoptimall'$

$. %onclusion

%he testing of power transformers with LI test voltage acc$ to the IEC standards presupposes

special >nowledge of the interaction &etween the impulse voltage test generator and the inductive

load$ or eample1 there eists a close connection &etween the main data of the transformer to

&e tested and the re=uired impulse capacitance of the impulse voltage test generator$ %here are

also re=uirements related to the damping characteristic of the test circuit to utilise an eisting

impulse voltage test generator optimall'$ ome &asic aspects and circuitries were descri&ed

in this paper$

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Oxidation of oil: Oxidation usually results in the formation of acids and

sludge in the transformer liquid. It is mainly due to exposure to air and

high operating temperatures.

Pressure-relief diaphragm broken:This is due to an internal fault causing

excessive internal pressures or the transformer liquid level being too high

or excessive internal pressure due to loading of transformer.

Discoloration of transformer liquid: Discoloration is mainly caused by

carbonization of the liquid due to switching core failure or

contaminations.

Leakage of transformer liquid: !ea"age can occur through screw #oints

around gas"ets welds casting pressure$relief device and so on. The

main causes are improper assembly of mechanical parts improper % lters

poor #oints improper % nishing of surfaces defects in the material used or

insu& cient tightness of mechanical parts.

oisture condensation:The main causes for moisture condensation are

improper ventilation in open$type transformers and a crac"ed diaphragm

or lea"ing gas"ets in sealed$type transformer.

!as-sealed transformer troubles: In gas$sealed transformers additional

problems can be the loss of gas oxygen content above '( or gas

regulator malfunctions. These problems are caused by gas lea"s above

the oil lea"y valve seats insu& cient gas space and)or insu& cient *

ushing of gas space with nitrogen.

Transformer s"itching equipment troubles: +any transformers are

equipped with tap chargers and other switching equipment. The problems

associated with these transformers may be excessive wearing of contacts

mechanism overtravel moisture condensation in mechanism liquid and

others.

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,xcessive contact wear is due to loss of contact pressure from wea"ened

springs or a contact$ma"ing voltmeter set at too narrow a bandwidth or

insu& cient time delay. +echanism overtravel usually is due to defectiveor improper ad#ustment of controller contacts. +oisture condensation is

due to improper ventilation and carbonization is due to excessive

operation and lac" of % ltering. Other

problems such as control fuse blowing and mechanism motor stalling are

due to short circuits in the control circuit mechanical binding or low$

voltage conditions in the control circuitry

AC Hi-Pot Test

The - hi$pot test is applied to evaluate the condition of transformer

windings.

This test is recommended for all voltages especially those above /0.' "1.

2or routine maintenance testing of transformers the test voltage should

not exceed 3'( of factory test voltage. 4owever the hi$pot test for

routine maintenance is generally not employed on transformers becauseof the possibility of damage to the winding insulation. This test is

commonly used for acceptance testing or after repair testing of

transformers. The - 41 test value should not exceed 5'( of the factory

test value. 6hen - hi$pot testing is to be used for routine maintenance

the transformer can

be tested at rated voltage for / min instead of testing at 3'( of factory

test voltage. The - hi$pot test values for voltage systems up to 37 "1 areshown in Table '.7. Testing procedures and test connections are similar to

the D hi$pot tests

TTR Test

The TT8 test applies voltage to one winding of a transformer and detects

the

voltage being generated on another winding on the same core. In the caseof a low voltage hand$cran" powered TT8 9 1 - is applied to the low$

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voltage winding of the transformer under test and a reference transformer

in the TT8 set. The 41 windings of the transformer under test and the TT8

reference transformer are connected through a null detecting instrument.

-fter polarity has been established at 9 1 when the null reading is zerothe dial readings indicate the ratio of the transformer under test.

In the case of an electronic TT8 test set a voltage :typically 9; 1 -< is

applied to the 41 winding of the transformer under test. The voltage

generated on the low$voltage winding is measured and the voltage ratio

between high and low windings is calculated. 1oltage ratio is

proportionally equal to turns ratio. The hand$cran" powered TT8 the

handheld electronic TT8 and the three$phase electronic TT8 are through

c respectively.

The TT8 test provides the following information=

It determines the turns ratio and polarity o f single$ and three$phase

transformers one phase at a time.

It con% rms nameplate ratio polarity and vectors.

It determines the ratio and polarity :but not voltage rating< oftransformers without mar"ings.

Tests include all no$load tap positions on a transformer. Tests include all

load taps on load tap changer :!T< transformers if connected for voltage

ratio control. On !T transformers connected for phase angle control ratio

and polarity are performed in neutral positions only. If tested on load taps

readings may be ta"en for reference for future comparison but will

deviate from nameplate ratings. !T taps may be con% rmed byapplication of low three$phase voltage and reading volts and the phase

angle for each.

Identify trouble in transformer windings such as open$circuit and

short$circuits of turn$to$turn sensitivity. The standard deviation as

de% ned by ->?I)I,,, '5.@A.;;$A;;3 ?ection 7.@ states that results

should be within ;.'( of nameplate mar"ings with rated voltage

applied to one winding. The TT8 with accuracy of ;.@( is acceptedas a referee.

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high voltage testing of transformer

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