High Voltage Testing of Transformer
-
Upload
srinivas-cnu -
Category
Documents
-
view
237 -
download
1
Transcript of High Voltage Testing of Transformer
-
7/25/2019 High Voltage Testing of Transformer
1/18
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!
SeminarsTopics.com
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 -
7/25/2019 High Voltage Testing of Transformer
2/18
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
SeminarsTopics.com
http://en.wikipedia.org/wiki/Mains_electricityhttp://en.wikipedia.org/wiki/Mains_electricityhttp://en.wikipedia.org/wiki/Electric_power_transmissionhttp://en.wikipedia.org/wiki/Mains_electricityhttp://en.wikipedia.org/wiki/Electric_power_transmission -
7/25/2019 High Voltage Testing of Transformer
3/18
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.
SeminarsTopics.com
-
7/25/2019 High Voltage Testing of Transformer
4/18
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.
SeminarsTopics.com
-
7/25/2019 High Voltage Testing of Transformer
5/18
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
SeminarsTopics.com
-
7/25/2019 High Voltage Testing of Transformer
6/18
@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.
SeminarsTopics.com
-
7/25/2019 High Voltage Testing of Transformer
7/18
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 #$2 s 7 "0 8 %ime to half-value %2 6 0 s 7 20 8
SeminarsTopics.com
-
7/25/2019 High Voltage Testing of Transformer
8/18
Impulse voltage test generator withinductive load
In most of the cases power transformers cannot &e assumed as a purel' capacitive
load for the LI testing$ 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
SeminarsTopics.com
-
7/25/2019 High Voltage Testing of Transformer
9/18
%here&' the inductance Lt of the load &ecomes smaller with decreasing impedance
voltage vimp81 with decreasing rated phase-to-phase voltage ;
-
7/25/2019 High Voltage Testing of Transformer
10/18
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$
SeminarsTopics.com
-
7/25/2019 High Voltage Testing of Transformer
11/18
&) 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$
SeminarsTopics.com
-
7/25/2019 High Voltage Testing of Transformer
12/18
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 >;
SeminarsTopics.com
-
7/25/2019 High Voltage Testing of Transformer
13/18
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
SeminarsTopics.com
-
7/25/2019 High Voltage Testing of Transformer
14/18
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$
SeminarsTopics.com
-
7/25/2019 High Voltage Testing of Transformer
15/18
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.
SeminarsTopics.com
-
7/25/2019 High Voltage Testing of Transformer
16/18
,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$
SeminarsTopics.com
-
7/25/2019 High Voltage Testing of Transformer
17/18
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.
SeminarsTopics.com
-
7/25/2019 High Voltage Testing of Transformer
18/18
SeminarsTopics.com