All the electrical equipments are designed to run as per the specification under normal system conditions within the tolerance degree of electromotive force, current and frequence. However they are subjected to unnatural state of affairs rather frequently. This will take to gradual impairment of the wellness of the equipment eventually taking to its failure. Conventional protection systems operate under system mistake conditions and protect the equipment from terrible harm. The faster the mistake is cleared the less will be the cost of fix of the faulted equipment.

However, the system will be capable to un-scheduled outage due to the faulted equipment and consequence in big down times impacting the dependability of power supply.

It is hence desirable to supervise the wellness of the equipment either continuously or at specified period of intervals specially designed techniques and instrumentality which provide a warning signal if there is any impairment in the wellness of the equipment such fore-warning can forestall major perturbations because proactive care and disciplinary steps can be initiated at the appropriate clip by which the cost of fix and system down clip that can be reduced with improved dependability.

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Following techniques available to measure the status of power transformer used to quantify their province of wellness.

Tan-delta and cap measuring.

Polarizing index and recovery electromotive force

measuring.

Dissolved gas analysis.

Frequency response analysis.

Acoustic sensing of partial discharges.

( A ) Tan-delta & A ; Cap measuring

Deterioration of any insularity system is detected by mensurating the Tan delta and Cap of the majority

insularity through the capacitance lights-out provided in the bushing or direct energisation of the equipment and mensurating the escape current. For an ideal capacitance the shunt opposition ‘R ‘ is infinite and correspondingly tan delta is zero. But due to dielectric losingss practical insularity system are represented by C and R is parallel as shown in figure, loss in R ( V2/R ) corresponds to the dielectric loss.

If the insularity deteriorates due to internal mistakes or thermic emphasis, so losingss additions and tan delta additions. Similar fluctuation can besides be observed if tan delta and C at a peculiar electromotive force are plotted as a map clip. Tan delta additions with electromotive force because of the gradual origin of internal discharges. The sudden alteration in the gradient of tan delta is called the sunburn delta tip up and the electromotive force where it occurs is an indicating of the quality of insularity. Similarly electrical capacity besides changes with electromotive force. This alteration is because of the shorting of internal nothingnesss in the insularity system as the electromotive force additions, broad fluctuation in these parametric quantities indicate impairment of the insularity.

Consequences obtained during testing:

Rating: 16.9/6.9KV,15/18MVA, 3-phase, 50 Hertz

Trial Consequences:

1 ] HV V/s LV GROUNDED

kilovolt

ma

% DDF

CAPACITANCE ( pF )

2.5

8.43

0.386

10732.00

5.0

16.864

0.385

10732.36

10.0

33.732

0.384

10732.89

2 ] HV V/s LV UNGROUNDED

kilovolt

ma

% DDF

CAPACITANCE ( pF )

2.5

6.109

0.205

7776.72

5.0

12.220

0.205

7777.17

10.0

24.439

0.205

7776.19

3 ] LV V/s HV GROUNDED

kilovolt

ma

% DDF

CAPACITANCE ( pF )

2.5

12.938

0.500

16471.18

5.0

25.856

0.509

16475.07

Tan-delta and electrical capacity measurings obtained during before and after temperature rise trial

Rating: 50MVA, 132 / 33KV power transformer

Transformer – 1

Before temperature rise:

Applied electromotive force ( KV )

Tan I?

Capacitance

HV v/s LV

Ungrounded

5

10

0.00201

0.00202

6385.3

6385.3

HV v/s LV

land

5

10

0.00853

0.00820

10163.5

10166.42

LV v/s HV

land

5

10

0.00388

0.00372

17513.65

17516.42

After temperature rise:

Tan I?

Capacitance

0.00206

0.00202

6385

6386

0.00576

0.00575

10167.4

10167.9

0.00228

0.00232

17526.4

17525.9

Transformer-2:

Before temperature rise:

Applied electromotive force ( KV )

Tan I?

Capacitance

HV v/s LV

Ungrounded

5

10

0.00121

0.00122

6266.3

6266.3

HV v/s LV

land

5

10

0.00143

0.00155

10145.5

10046.42

LV v/s HV

land

5

10

0.00141

0.00149

16598.65

16600.42

After temperature rise:

Tan I?

Capacitance

0.00125

0.00126

6266.5

6266.3

0.00130

0.00128

10142.4

10046.9

0.00134

0.00139

16599.4

16576.9

Then two parametric quantities moderately signify the overall measure of complete insularity system comprising of oil, imperativeness boards, paper, ceramic stuff etc. Hence, the measuring of sunburn delta & A ; C for measuring the majority insularity system quality is frequently used for transformers, generators, motors and bushings.

Decision:

Before and after temperature rise trial shows between LV to HV and HV to LV is demoing fluctuation in tanI? and electrical capacity due to thermal ageing consequence in the cellulose stuff ( craft paper ) .

Interpretation of trial consequences:

Correct reading of trial consequences require equipment cognition and features of the insularity used. Changes in the in the electrical capacity of an insularity indicates presence of wet bed, short circuit or unfastened circuits in the insularity system. Dissipation factor measurings may bespeak the undermentioned defects in the insularity of electrical equipments.

1 ) Cellulosic impairment due to clip and temperature.

2 ) Ingress of H2O, taint by C sedimentations, bad oil, soil and other chemicals.

3 ) Severe escape over surfaces and through clefts.

4 ) Nothingnesss.

An addition in dissipation factor in an oil filled bushing may bespeak that oil is contaminated whereas addition in both electrical capacity and tan delta indicates that the immersion of wet. For a capacitor type bushing which has shorted bed, the electrical capacity value may increase where as the dissipation factor may stay changeless.

( B ) . Polarizing index and recovery electromotive force measuring:

Insulation impairment take topographic point because of wet incursion and deposition of insularity impairment takes topographic point because of wet incursion and deposition of carry oning drosss. Enough attention should be taken to guarantee that the insularity system of the equipment is protected from these two agents through hermetically sealing the equipment or by frequent drying and glade through physical agencies. Both Polarizing Index ( P.I ) Recovery Voltage ( R.V ) given an estimation of the wet content absorbed by the insularity and supply a good appraisal of its quality.

( 1 ) . Polarizing Index = IR value after 10min

IR value after 1min

( 2 ) . Absorption Index = IR value after 60sec

IR value after 15sec

The insularity sample is charged with a electromotive force Un by shuting switch t1 and insularity being a loss capacitance gets charged to Us. Then t1 is opened and the sample is short circuited through t2. The current I fluxing through t2 is plotted as a map of clip. The hold of the current I show the quality of insularity.

The ratio of I2/I1 as shown in the figure above is defined as polarized index. Insulation opposition and polarising index AVO makes meggar 5KV applied electromotive force.

Transformer-1: –

Rating: KVA: 7250, KV: 5500 / 3160

H.V TO L.V

H.V TO EARTH

L.V TO EARTH

IR after 10min / 1min

14500 / 5750

5290 / 3900

10600 / 4910

Polarizing index

2.51

1.35

2.16

Transformer-2: –

Rating: KVA 15000 / 18000 KV 16.5 / 6.9

H.V TO L.V

H.V TO EARTH

L.V TO EARTH

IR after 10min / 1min

18300 / 5020

7130 / 5210

12800 / 6220

Polarizing index

3.645

1.368

2.05

The higher the ratio the drier the insularity is. In the instance of transformer a PI value non less than 1.3 is desirable. Higher Pi values are obtained by subjecting the transformer to drying operations. Too much of drying of insularity becomes brittle. Measurement of PI is done sporadically for all equipment with composite insularity to determine the wellness of the equipment.

Un & lt ; =36KV 36 & lt ; =Un & lt ; & lt ; 70 70 Un & lt ; =170 170CUn

Restricting values of the H2O 40 35 30 20

Content for all ( ppm )

Restricting values of tan I? 1.5 0.8 0.3 0.2

For all at 90 deg C & A ; 50Hz

Like PI, RV ( Recovery Voltage ) besides gives a step of the wet content in the insularity system. Insulation specimen is charged with electromotive force Ue by shuting switch t1 for a period technetium. Then exchange t1 is opened and exchange t2 is closed short circuiting the charged specimen. Switch t2 is closed for a continuance t2 and so is opened. The unfastened circuit electromotive force V is plotted as a map of clip. The electromotive force across the specimen recovers, goes to a maximal value Um at thulium and decays thenceforth as shown in fig above. The polar compounds in the insularity whose clip changeless lies in between Te and td are responsible for the visual aspect of recovery electromotive force across the specimen.

Time T where Um/Uc peaks gives an indicant of the wet content of the insularity. Recovery electromotive force method ( besides called as return electromotive force ) applied to dielectric stuff is based on the polarisation and relaxation procedure taking topographic point inside the insularity when the specimen is electrically simulated. The scope of technetium used in this measuring scope from 1 milliseconds to 1000 seconds.

The insularity system with higher wet content has a comparatively fast polarisation response. Consequently the insularity will be about wholly polarized and depolarized at along bear downing ( technetium ) and short circuiting ( td ) times. Almost no residuary energy is available after the short circuit stage. Hence low recovery electromotive force valves will be step for adequate bear downing times as shown in fig.5 merely at shorter bear downing clip will the relationship between the polarisation and depolarisation be such that maximal Um value can look.

Hence, for higher wet content, the planetary upper limit of the polarisation moves to lower clip changeless parts. This method is really frequently is applied to measure the insularity quality of power transformer. The RV method indicates the wet contained in the composite insularity.

In transformer, wet migrates from paper to oil as the temperature increases. Therefore, under light burden conditions, the oil can demo low H2O content. Hence, measuring of ppm degree of wet in the oil entirely will non give the right position of insularity. Hence Rv measuring is preferred since it indicates the planetary quality of the composite insularity.

C ) Dissolved gas analysis:

Mineral oil is used in many transformers both as insularity to the unrecorded parts and as a coolant for heat dissipation. Contamination of this all due to moisture impregnation greatly affects its insularity belongingss. This wet taint can be due to leakage through the enclosure of the moist air from outside or from the paper insularity used for the unrecorded parts.

Particularly in transformers the paper insularity is hydroscopic and when the temperature of the insularity is high wet will migrate to the environing all. Detection of this wet in all is really of import since it affects its BDV.

During operation, due to over burden, thermic cycling and electrical emphasis under unnatural electromotive force conditions the insularity deteriorates consequences in internal discharges. These are micro discharges normally referred to as partial discharges because these do non amount to a sum flashover which represents a mistake. The consequence of micro discharges is that they bit by bit weaken the insularity eventually taking to a major mistake.

Detection of these discharges at the beginning will take to preventative care and will cut down the cost of harm due to ruinous failure of the equipment.

Dissolved gas analysis ( DGA ) is the most popular technique used to analysing the quality of internal insularity in transformer. Depending on the strength of internal discharges and the hot musca volitanss developed due to overloading or any other mistakes such as short circuits in magnetic nucleus etc.

The dissociation of oil leads to development of different gases which get absorbed in the oil. These gases are analyzed utilizing gas chamotograph and based on the concentrating of these dissolved gases the internal insularity jobs can be identified.

Table gives the allowable concentration in ppm of the major gas dissolve vitamin D in the oil. These bounds addition with the age of the transformers.

Gass

Less than old ages

Age of Trf 4-10 old ages

More than 10 old ages

1

Hydrogen ( H2 )

100-150

200-300

200-300

2

Methane ( CH4 )

50-70

100-150

200-300

3

Acetylene ( C2H4 )

20-30

30-50

100-150

4

Ethylene ( C2H4 )

100-150

150-200

200-400

5

Carbon-dioxide ( CO2 )

3000-3500

4000-5000

10000-12000

6

Ethane ( C2H6 )

30-50

100-150

800-1000

7

Carbon

monoxide ( CO )

200-300

400-500

600-1000

D.G.A consequences:

Rating: 8MVA, 33 / 700V

Before commencing:

Transformer 1

Transformer 2

Transformer 3

ppm

ppm

ppm

Hydrogen ( H2 )

255

64

92

Oxygen ( 02 )

12273

13148

12588

Nitrogen ( N2 )

21834

24773

22044

Methane ( CH4 )

NO

NO

NO

Ethylene ( C2H4 )

NO

NO

NO

Ethane ( C2H6 )

NO

NO

NO

Acetylene ( C2H2 )

NO

NO

NO

Propylene + propane ( C3H6 + C3H8 )

NO

NO

NO

Carbon dioxide ( CO2 )

329

369

260

Carbon monoxide ( CO )

33

59

42

Similarly it is possible to supervise evolved gases specifically hydrogen which gets collected in the curator. Hydrogen sensor is placed in the pipe linking the armored combat vehicle to the curator conventional Bucholz relay is besides located in the same pipe.

After one twelvemonth:

Transformer 1

Transformer 2

Transformer 3

ppm

ppm

ppm

Hydrogen ( CH2 )

680

33

84

Oxygen ( O2 )

18175

24073

13243

Nitrogen ( N2 )

60870

80135

46618

Methane ( CH4 )

933

Nothing

Nothing

Ethylene ( C2H4 )

140

Nothing

Nothing

Ethane ( C2H6 )

1984

Nothing

Nothing

Acetylene ( C2H2 )

235

Nothing

Nothing

Propylene+propane

( C3H6+C3H8

1163

Nothing

Nothing

Crobon dioxide ( ( CO2 )

954

815

290

Carbon monoxide ( CO )

203

273

171

Note:

Transformer 1 indicates electrical discharges.

Transformers 2nd and 3rd are found normal.

Hydrogen gas evolved for all types of mistakes in the transformer. It has low solubility in all and most of the generated gas goes out and in the procedure actuates the Hydrogen sensor. The on line Hydrogen detection is more effectual and faster than the bucholz relay.

Deterioration of cellulose stuff with clip is due to deploy merization of its molecular construction. This debasement is accelerated by chemical, mechanical, thermic and electrical emphasis that take topographic point inside the transformer. Degree of polymerisation is step on sample paper insularity and their value is less than 150 the paper is non fit for farther utilizations as insularity. So an alternate method to measure the quality of cellulose insulating used in transformer is the estimation the content of furanic compound dissolved in the oil.

Using higher public presentation burden chromatograph ( HPLC ) the furanic content can be estimated from the oil sample and there is a direct relationship between DP and the degree of furanic compound ( 2 Furfural ) . Any value less than 10 ppm of 2 Furfural can be considered as non acceptable for the transformer to go on to work.

C ) Frequency response analysis:

Rating: 16.9/6.9KV,15/18MVA, 3-phase, 50 Hertz

When there are more than one twist in the equipment their will be matching between the twists giving rise to common induction and electrical capacity. Any mistake in the equipment will ensue in short circuit between bends to land or in distortion of twists due to electro magnetic forces ensuing from the flow of short circuit currents. Such alterations in the twists will ensue in the twists fluctuation of the induction Ls and M electrical capacity Cs and Cg.

When a ladder web stand foring a twist is excited by an impulse, each node of the twists a.b.c exhibition oscillatory electromotive forces matching to the natural frequence of the web. There will be a figure of such frequences. Each spiral of the twist resonates at its natural frequence 1/a?s ( LC ) . A twist of transformer has a big figure of natural frequences. If such a twist is exited by a variable frequence changeless electromotive force beginning. Then as the beginning frequence is changed, the current I through the twist will travel through a figure of maximal and minimal values as shown in the fig11. The extremum and troughs correspond to the natural frequence nowadays in the twists.

The peak return topographic point when the beginning frequence coincides with any of the series resonant frequences of the twists 1/a?s ( CgLs ) and a trough return topographic point where the beginning frequence coincides with the parallel resonance frequences of the weaving 1/a?s ( CsKs ) when the twist is healthy and its mechanical structural is non atltered these resonant frequences which are fundamentally map of L & A ; C will stay invariant and if the above experiment as shown in fig is repeated indistinguishable consequences will be obtained. In fig the current I is a map of electromotive force applied.

In order to do the consequence independent of outside variables but wholly dependent on weaving parametric quantities merely, the secret plan is made between electric resistance ( Z=V/I ) and the frequence. In this instance wherever current I is maximal Z will be minimal and when I is minimal Z will be maximal.

However the nature of secret plan will be remain same and the fluctuation of Z indicates the assorted natural frequences of the weaving under consideration. Equally long as the twist remains same the fluctuation of Z with I will be indistinguishable. Hence this secret plan is considered as an electrical signature of the twist.

If due to any mistake, the weaving gets displaced or see a few short circuits either between spirals or between spirals to land so due to fluctuations are L and C natural frequences get alterations and so there will be deformation in the fluctuation ‘Z ‘ . It can be said the electrical signature of the twist alterations.

Probable mistake sensing by FRA:

Frequency Ranges

Portable Mistakes

5 Hz to 2KHz

Shorted bends, unfastened circuit, residuary magnetic attraction or nucleus motion

50 Hz to 20 KHz

Bulk motion of weaving comparative to each other

500 Hz to 2 MHz

Distortion with in a twist

25 Hz to10 MHz

Problems with weaving leads and/ or prove lead arrangement

The chief advantage of this method of status monitoring is that FRA trial can be conducted at site where equipment is situated

Tocopherol ) Measurement of partial discharges by acoustic sensing:

Measurement of partial discharges is one of the of import diagnostic techniques for measuring the status of power equipments. The conventional electrical method for measuring of partial discharge is good established and widely accepted. However, the electrical partial discharge method has certain restrictions which have resulted in development of jumping techniques.

Acoustic Emission ( AE ) sensing technique is one among them. Development of high frequence detectors high velocity instrumentality and necessary package has led to the success of the AE techniques internal partial discharge create acoustic moving ridges in the supersonic scope from 50 to 150 KHz.this is because of sudden prostration of nothingness due to internal discharges doing force per unit area moving ridges.

This accoustic signals travel through the insularity medium which includes both solid ( paper ) and liquid ( oil ) stages and make the metallic enclosure ( armored combat vehicle ) acoustic detectors holding frequence set from 50 to 150 KHz mounted on the outer surface of the armored combat vehicle. These detectors pick up the signal which is generated inside the equipment due to internal discharges.

It is possible to travel the accoustic detector over the armored combat vehicle to different topographic points and picked up the ensuing signal, through triangulation method it is possible to turn up this beginning of this signal inside the equipment ( transformer ) by mounting these detectors at three different topographic points on the armored combat vehicle ( on the side of the armored combat vehicle ) and at the same time entering the picked up signals.

The acoustic sensor is a portable it can be taken to site ; the normal hearable noise of the environment and that produced by the equipment will non impact the detector as these detectors picked up signals in scope 50 to 150 KHz which is above hearable scope. The electromagnetic moving ridges generated in the substation due to corona and curving besides will non impact the detectors because the electromagnetic noise is above 500 KHz.

Wave signifier shows AE signal and frequence spectrum that measured by utilizing VS30-V

Applied electromotive force ( KV )

Amplitude of Electrical PD ( soap ) , personal computer

Magnitude of acoustic PD ( soap ) , dubnium

10

& lt ; 500

& lt ; 35

12

550

48

14

800

50

16

1200

82

18

1500

85

20

2000

89

Acoustic PD and electrical PD at the same time measured as a map of applied electromotive force:

At the clip of commissing of the equipment ex. power transformer, at specified location on the armored combat vehicle the acoustic signals are recorded and kept as mention signatures. Periodically these signals are measured at the same rotary motions and any major fluctuation ( increasing amplitude ) in the signal picked up will bespeak some unnatural operation. Based on the farther probe be taken to rectify the job.

The chief advantage of this method is that equipment is kept energized while carry oning the trial.hence this is on line conditional monitoring.

LATEST CONDITION MONITERING TECHNIQUES:

Latest techniques in status monitoring and nosologies which can be powerful tool in future for observing transformer jobs can be summarized as follows:

Frequency Response Analysis ( FRA ) to look into for system resonance status and dynamic motions and sensing of weaving mechanical deformation.

PD measuring and acoustic localisation of mistakes.

Furfuraldehyde ( FFA ) analysis of oil ( HPLC chromatography ) to observe ripening of cellulosic stuff without taking paper samples.

On-line Tan Delta Monitoring of H.V. bushings: Signals from transducers connected to Voltage pat of bushings are collected and transmitted to user for treating the information by package and converted to dielectric loss angle and escape current values.

Polarization spectrum or Recovery electromotive force measuring ( RVM ) gives indicant of wet in insularity and possible paper ripening and oil status

On-line Gas Proctors:

On line H proctors ( e.g. HYDRANS ) provide earliest possible sensing of gas build up and alarm the user to the demand for elaborate research lab analysis

On line wet content measuring ( e.g. DOMINO, AQUAOIL etc. ) provides continues monitoring of H2O content in oil, bespeaking the position of solid insularity. The stored informations can be down loaded for analysis utilizing package. The package gives tendency analyses, generate graphs and studies.

On -line temperature monitoring -Direct measuring through fiber ocular detectors for uninterrupted monitoring of hot topographic point temperature to command lading pattern and thermic ripening of the transformer…

CONDITION MONITORING Parameters:

Dissolved Gas Analysis ( DGA ) provides an early warning of assorted inchoate mistakes in transformer twist and nucleus.

Oil parametric quantities proving: Low BDV indicates wet or atom in the oil, Moisture or sourness indicate oil status, merger phenol and methyl phenol indicate the happening of solid ripening in the paper barrier dielectric or packing stuff.

C and Tan delta measuring of bushing and twist assesses the status of insularity ( dry or wet ) of bushing and twist.

Weaving opposition measuring detects job in broken sub-conductors, weaving contact articulations and OLTC connexions.

IR measuring indicates the presence of taint ( soil, wet, etc. ) and stress debasement of insularity.

Turn ratio trial indicates job in weaving and verifies incorrect tap modifier connexions

Excitation/ Magnetization current trial locates mistakes in the magnetic nucleus construction such as shorted laminates or nucleus bolt insularity dislocation or shorted bends due to insularity failures, which have resulted in carry oning waies between weaving bends.

Decision:

The benefits of status monitoring can be summarized as below:

Reduced care cost.

Consequences provide a quality control characteristic.

Limits the chance of destructive failures, taking to betterments in operator safety and quality of supply.

For measuring possibility and badness of any failure and eventful fix activities.

Provide information on the works runing life, enabling concern determinations to be made either on works renovation or replacing.

Using status appraisal techniques, we have been able to observe:

Overheating of music director / lack in thermic design of transformer

Dislocation of stress shield at the underside of bushings

Loose magnetic wall shunts

Insulation failure between nucleus bolts and spiral support constructions

OLTC jobs like loose nut and bolts and connexions.

Lost/floating possible connexions to screening rings

Partial discharges between phonograph record or music directors

Overheating of tank portion, bolt etc.

OLTC jobs like loose nut and bolts and connexions

Failure of nucleus bolt lamination /shorting of nucleus lamination burrs

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