High Voltage Engineering Practice: A Vehicle for

INTERNATIONAL CONFERENCE ON ELECTRIC POWER ENGINEERING (ICEPENG 2015) OCTOBER 14-16, 2015

High Voltage Engineering Practice: A Vehicle for Enhanced National Grid Performance L. C. EKECHUKWU PhD, MNSE

T. C. MADUEME PhD, MNSE, MNIEEE

NEWS ENGINEERING (NIG) LTD. ABUJA, NIGERIA. [email protected]

DEPT. OF ELECTRICAL ENGINEERING, UNIVERSITY OF NIGERIA, NSUKKA, NIGERIA. [email protected]

Abstract-The grid system is a configuration of generating stations, high voltage transmission, sub-transmission, and distribution lines, with all their associated auxiliary elements, insulations, communication networks and protective gears. The entire length is interrupted at various points known as transformer stations for the purpose of voltage transformations, in conformity to the designed grid configuration. The safety, reliability and efficiency of power delivery rest on the quality of the associated dielectrics alongthe lines and substations. This paper is aimed at highlighting the importance and roles of quality insulation and dielectrics in power system grids. Philosophy of some dielectric tests and measurements such as insulation resistance, polarisation index (PI), breakdown voltage (BDV), tangent (loss angle) delta(δ), are illustrated for the comprehension of import and applications of High Voltage Engineering (HVE) practice for a reliable grid performance. Capacitor characteristics and similarity in system dielectrics are examined. Types and significance of some vital high voltage tests are mentioned. Results of conducted tangent delta (δ) tests on a 132/11 kV, 63 MVA transformer bushings of the Ajaokuta integrated Steel Plant, Ajaokuta, Kogi State, Nigeria are presented and analysed with reference to the manufacturer’s limits. The test results substantiate high voltage engineering practice as an indispensable vehicle for an efficient grid performance. Key Words: High Voltage, Grid, Dielectrics, Capacitance, Leakage Current, Insulation resistance, Tests

I.

INTRODUCTION

While Thomas Alva Edison (1847 – 1931) patented low voltage discovery and supply through a very limited distance, with voltage drop constraints, Nikola Tesla (1856 – 1943)conceived the concept of alternating

transmission and transfer capability P in an AC system are the following: (1)

P = V2/Z

(2) Where S, the apparent power is in kVA, V the

line-to-line voltage is in Kilovolts (kV),I the line current in Amperes (A), and Z, the wave impedance of a transmission line is in ohms. This equation clearly shows that transmission of a fixed amount of Kilovolts Amp (kVA) over a transmission line, at a preferable higher voltage, is accompanied with a corresponding lower current, for a balanced power transfer. Table 1 shows the relationships between a typical power transfer capability of an alternating current (AC) system, operating voltage and transfer distance.

Table 1 Power Transfer Capability of An AC System And Its Relationship With Operating Voltage And Transfer Distance 220 330 500 750 1000 V, (kV) 256 250 Z, (Ohms) 400 303 278 121 360 900 2200 4000 P, (MW) 100 200 Around ˃ ˃ Transfer – – 1000 1000 1000 Distance, 300 600 (km) Source [2]

current in 1886, alongside using transformers for voltage

Lower currents are highly advantageous, as the watt

step-up, thereby reducing proportionally the line current

losses and voltage drops due to the lower conductor

with

[1].

resistance, are smaller. Consequently and economically,

Important relationships in connection with power

a smaller cross-sectional area of the transmission line,

consequent

reduced

voltage

drops(IR)

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INTERNATIONAL CONFERENCE ON ELECTRIC POWER ENGINEERING (ICEPENG 2015) OCTOBER 14-16, 2015

with lesser quantity of materials involved in its production, has substantial economic advantage. This

Conductor s

permits the use of thinner conductors. Transmission at higher voltages advantageously result in lower line voltage drop and heat losses (I2R),but renders the high voltage (HV) lines environmentally unsafe to living organisms, making the systems susceptible to ground

Insulators

faults. While research works on conducting materials have been aimed at delivery of electricity at costeffective

and

minimal

ohmic

losses

of

high

Silver Copper Aluminium Iron Carbon(grani te) Water(sea) Water(fresh) Water(distilled) Porcelain Glass Air SF6

y, (ϭ) S/m 6.17x107 5.8x107 3.82x107 1.03x107 1.0x107

ty,ϵr -

4

80 6 5 1.0006 1

10-3 2.10-4 10-10 10-10 -

conductivities but zero permittivity as shown in Table

Source [1]

2,the emergence and applications of non-conducting

II. THE BASIC ENGINEERING CONCEPT AND

materials of very low conductivity and high permittivity

REQUIREMENTS

in power systems have been speedily undergoing several

ENGINEERING PRACTICE.

investigations, also, for higher quality classes of

A. High Voltage Classifications

insulations and dielectrics. This is expedient for the safe-

The IEC definition [2] of high voltage circuits are those

handling of high and extra-high system line voltages.

at applied potentials of more than 1000 V AC and at least

Emerged challenges in transmitting stepped-up voltages

1500V DC. In AC Transmissions high voltage is

along overhead power lines and high voltage system

considered at value range of 35kV – 220kV; extra high

elements, their effects and that of environment (e. g.

voltage (EHV) ≥ 330kV and ˂ 1000kV while ultra high

humidity, rains, lightning strokes, dusts and other

voltage (UHV) levels are 1000kV and above. HVDC

pollutions)on all associated dielectrics and insulations,

levels are below 600kV while UHVDC is above 600kV.

FOR

HIGH

VOLTAGE

necessitated dynamic research and development, leading to the discipline of High Voltage Engineering. Simply put, High voltage engineering practice embraces the study of dielectrics, their characteristics, tests and maintenance, applications to and performance in an integrated

grid.

The

practice

of

High

voltage

engineering, therefore, scopes through types and classes of insulations and their correct applications and behaviours for respective designed and rated generating, transmitting

and

appropriating

relevant

insulations for operations.

High

distributing tests

on

equipment: their

and

associated

safe, efficient and reliable grid voltage

testing

remains

an

indispensable culture in h.v. engineering practice for a reliable grid performance. Table 2 shows properties of

B. Design & Operational Characteristics of Dielectrics for Reliable Grid Performance High Voltage engineering studies basically lie in the fundamental design, concept and roles of the simple capacitor and capacitance phenomenon of a two opposite-facing charged conducting materials (plates or electrodes) separated by a non conducting material called the dielectric (Fig 2). In an operating circuit, the stressed dielectric, usually in the form of gas (e.g. air, Sulphur hexafluoride SF6) or solid (e.g. paper, wood, porcelain, mica, glass, etc) or liquid (e.g. wax, transformer, mineral or vegetable oil, etc), separates the two energised conducting plates facing each other. At a gradual increase of the applied voltage, the ‘sandwiched’ dielectric allows minute leaks of current, called leakage

some conductors and insulating materials.

currents, due to the material’s inherent impurities and Table 2 Properties of Insulating Materials Material

Some

Conductors

Conductivit

and

Permittivit

electrostatic character of capacitors: good dielectrics however disallows flow of high currents across it, till a

threshold voltage is attained across the plate, at which L. C. EKECHUKWU, T. C. MADUEME

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INTERNATIONAL CONFERENCE ON ELECTRIC POWER ENGINEERING (ICEPENG 2015) OCTOBER 14-16, 2015

the dielectric breaks down to conduct electricity. At this juncture the applied potential drives appreciable current through the dielectric. The voltage value at which the dielectric snaps into becoming a conductor is known as the breakdown voltage(BDV) of the particular dielectric and is a function of: (1) purity (quality) of the dielectric, (2)shape of the opposite-facing conducting plates, otherwise known as the electrodes, which is usually of disc, spherical and mushroom designs (Fig 1a) for HV test equipments. (3) the texture of the dielectric determined by its permittivity, (4) thickness of dielectric or distance D, between the plates (Fig 2a) and (5) environmental factors. Absolute perfect dielectric is considered to have zero leakage (capacitance) current, which is practically impossible due to manufacturing processes and material characteristics. Thus, under voltage stress, all dielectrics depending on their degree of purity have the tendency of passing through them, milli/micro/nano values of currents, called leakage capacitance, conduction and absorption currents, whose measured values depend on the prevailing voltage. These currents, under the prevailing voltage stress, give rise to the four respective resistances that total to what is known as the insulation resistance.

It is evident that in the operating power system network connections, short or long line transmission capacitances are introduced by phase-to-phase and phase-to-ground transmission lines. Air serves as their dielectric while the

Fig. 1 High voltage (a) testing vessel for liquid dielectric (b) simplified test circuit for liquid & solid dielectrics

(a) Oil Testing Vessel AVO Foster OTS60 With Mushroom-Type electrodes for Liquid DielectricBreakdown Voltage (BDV) Test. (Courtesy of Ajaokuta Steel Company Limited Ajaokuta) (b)Simplified circuit diagram

conducting lines and ground serve as the plates. In transformers and motors, the windings on one hand and the earthed machines’ bodies on the other hand are the electrodes,

while

conductors’

insulations,

the

impregnated papers, wedges, woods and oils serve as dielectrics. At high voltage (HV) injection test, insulation resistance test and/or any other non- destructive tests, the test equipment outputs are connected at any two convenient points considered as the electrodes for the tested material or specimen.

a)

Simple Capacitor

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dielectrics such as HV transformer bushings, instrument

transformers,

power

cable

insulations, transformer oils and other solid dielectrics like mica, glass, porcelain, etc. A relatively pure capacitor has current leading, by almost 90o, the applied voltage. Presence of

+

+

─

V

impurities like water provides path for resistive components of leakage current in dielectrics which is in phase with the operating voltage across the dielectric. The magnitude of the

─

resistive current tends to zero as the dielectric (b) Simplified circuit diagram Fig. 2

impurity tends to zero thus crediting the

Charged Parallel Arranged Plates

Showing Lines of Electrostatic Forces C. Significance of parameteric tests & measurements for the evaluation of quality status of dielectrics for the enhancement of grid performance

capacitive current maximum and 90O leading the applied voltage. Since no insulator is absolutely pure, a defining ratio-relationship between the magnitudes of the resistive and capacitive components of the resultant leakage currents is required for appraisal of degree of

Tremendous roles are played by insulators in the entire

quality of dielectrics. The resultant leakage

energised grid network, vis a vis:i) security of equipment

current IL lags the vertical y-axis capacitance

from damages at transients - like voltage surges. (ii)

current component, IC, by an angle delta (δ),

security of operating and straying personnel and

called the ‘loss angle’, whose magnitude

mammals from accidental contact with isolated and un-

depends on the purity of the dielectric material,

insulated equipment of the grid. Consequently and as a

while it leads the horizontal x-axis resistance

matter of priority in grids’ safety and reliable operation,

current component, IR, by angle theta (Ѳ),

vital high voltage tests are recommended on associated

known as the power factor angle, as seen in Fig

dielectrics of all power system elements of the grid. This should guarantee reliability and efficiency in grid

3. This appraising ratio-relationship

is

performance. The most vital tests amongst others are: (1)

known as tan δ or tan ‘loss angle’ of the

Tangent (Loss angle) Delta, δ and Power Factor (2)

dielectric. Thus, tan δ =

Insulation Resistance Measurements (3) Breakdown

as IR → 0. Tan δ measurements of grid element

Voltage Tests (4) Leakage Current Measurement (5) Polarisation Index Measurements.

dielectrics therefore remain a very important practice in high voltage engineering (HVE) practice, for assessing the purity and quality of

a) Dielectric Tests •

and is equal to zero

Tangent (Loss Angle) δ Tests, Power &

dielectrics for a continuity or fresh application to the grid. Tangent δ, Dissipation and Power

Dissipation Factor Tests. This is an HV non-destructive test of insulation materials. It is mathematically modelled from the capacitance theory of the capacitor, in determining the tangent δ (loss angle), power factor and dissipation factor measurement of

Factor tests of a dielectric inform of its degree of purity and quality, for application or continuity of usage, in the grid. It is expedient to note that tan δ measurement should be carried out at the normal operating frequency of the grid,

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so as to give the actual leakage current through the tested

say HV bushing of a 132kV (or more)power transformer,

dielectric when in service. Since frequency determines

in a way that corresponds to the way the specimen is

and is inversely proportional to the magnitude of

usually connected and energised in normal circuit, till the

capacitive reactance of dielectric (

bridge balances. In the case of a current transformer, the

), very

high frequency will make the capacitive reactance very low, resulting in quite high leakage current measurement, and conversely, lower leakage current value than actual at testing with lower frequency than the grid’s normal value of 50Hz as is in Nigeria.

secondary winding and core, connected together, would form the lower terminal of Rx/Cx in Fig. 4b, while the primary winding would be connected to the high voltage terminal of the bridge. Although the usual model for an insulating material (dielectric) is a parallel RC circuit, it is possible to derive an equivalent series RC circuit (Rx and Cx in Fig. 4b).

I=Charging current Current

I

c

I

VAR=I.V.Sin θ

L

δ θ

V=Voltage

W=I.V.Cosθ Fig. 3 Power Factor, Loss angle and Dissipation Factor

(a)

IL = Leakage current IC = Capacitive current = Resistive current

•

I

R

Measurement of tanδ and capacitance with the Schering Bridge The non-destructive measurement of tan δ of dielectric specimen of equipment, such as high voltage bushings of transformers provides information on quality status of

(b)

the insulation. The capacitance and tan δ of insulators is measured by high voltage test equipment called the

Fig, 4 A typical h.v. bushing & basic schematic

Schering bridge, through simultaneous adjustment of

Schering circuit for tangent delta tests

equipments variables and application of test voltage of about 10kV,at the operating frequency, to the specimen,

•

Breakdown Voltage BDV (Destructive) Tests and Leakage Current Measurements

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This test, though seldom used, is considered destructive

lattice arrangements of molecules and insulation bonding

during the test period, for the fact that the dielectric is

while at rest, thus offering least resistance to any current

subjected to almost 200% higher than the normal

flow through the dielectric. At voltage application, with

operating designed values, at which duration the lattice

time, these molecules polarise to form high resistance to

bonding

temporary

further flow of current. The ratio of measured resistance

disorientation. Apart from effect of the presence of

at 600 seconds (10 minutes) R10 to measured resistance at

impurities in dielectrics, fluctuations in voltage stresses

60 seconds (1 minute) R1, i. e. R10/R1 is known as

due to transients and extreme environmental temperature

Polarisation Index (PI). How rapidly this happens tells us

changes and weather conditions also ‘restructure’ the

about the quality condition of the specimen. Capacitance

lattice arrangement of dielectrics, with time, and with a

current affects the first few seconds of PI test;

consequential effects on its quality. Consequently, high

Conduction current should be essentially zero if the

voltage injection tests are carried out on the dielectric at

windings are dry; leakage current is constant over time

very long frequency intervals, to ascertain its fitness for

while absorption current indicates the health of the

continuity on the grid. The test is specifically meant to

insulation. PI measures insulation resistance between the

expose any weak spot of a long serving insulation or

conductor and ground, making it mandatory for the test

further widens an already unidentified ruptured spots of

to be carried out on entirely insulated windings of motors

the insulation. The HV injection values are based on (i)

and/or any other machines, for the avoidance of skewed

how long the dielectric has been in service,(ii) grid’s

(false) result values due to winding parts exposed to

operational history or immediate reason for the test and

vicinity air dielectrics. Accepted dielectric PI ratios [3]

(iii) the provided norms for the test. The idea is that by a

ranges between 2 and 5 as values ˂ 2 suggest dirty or

gradual increase, the applied voltage attains the rupturing

moist winding and values ˃ 5 suggest very dry, brittle

or momentary ‘destruction’ value of the dielectric to

insulation. High voltage pressed-coil insulation (VPI)

conduct. The recorded value becomes the BDV or

systems are exception with PI values between 1 and 2,

‘worth’ value of the material against future transient

considered as perfect good windings. Newer epoxy

over-voltages. Fig 1b is a basic schematic diagram of a

insulation systems have fast reaction time with an almost

specimen subjected to a high voltage breakdown test.

instant polarisation of molecules, such that the PI is

•

structure

is

subjected

to

Insulation Resistance and Polarisation Index

perfectly, almost unity. It should be noted that an energised winding by Megger, Hipot or in service

Measurements. This measurement is a non destructive test usually

operation already has the insulations polarised and will

carried out on power system specimens such as power

certainly offer wrong PI if measured immediately after

cables, circuit breakers, transformer bushings and

shut down. Because it takes time for the molecules to

windings, generators and motor windings, bus bars,

assume their ‘at rest’ positions, it is always advised that

reactors, etc. Insulation resistance values are affected,

the windings must be de-energised long enough to return

most times, by variables such as [3], (1) type of

to rest before commencing PI measurements. Else the

insulation (2) age of the material (3) surface area and (4)

dielectric will take lesser time to polarise, rendering false

environmental factors like moisture and contaminations

PI values.

(dusts).

The

ratio

of

the

applied

voltage

and

any/combination of capacitance, conduction, leakage and

III. METHODOLOGY

absorption generated currents totals to the insulation

Tan (loss angle) δ Test of Bushings of 132/11 kV,

resistance value of the tested specimen. Dielectrics are

63MVA Transformer by Schering Bridge method[4] was

characterised by random orientation and haphazard

carried out at higher oil temperature. Thus the

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transformer is heated by the no-load magnetising current

capacitance CD-P and tan δ taken, using the Schering

with the automatic cooling system demobilised to

bridge instrument. The entire bushing assembly is held in

prevent automatic cut-in of cooling fans. Thus, Oil Temp

position by the flange, firmly bolted to the earthed

o

at High Side, Th side = 44 C; Ambient Air Temp Tamb =

transformer body. Connections are made between D and

34.5oCAverage Oil Temp, Tave =

F. Capacitance CP-F, between the last metallic band

o

C.

(electrode) P and earthed flange F, is taken during

The vertical bushing input bar, D shown in Fig. 4a has

measurement.

several sheets of oil-impregnated paper wound around it.

Cx

is

obtained

; where

from

the

is the value of chosen

Each layer is separated and held in position by a

formula

cylindrical metallic sheet. The alternating arranged

standard capacitor for the test and is equal to 49.91pf;

cylindrical papers and metal sheets are bound in

is a constant resistor equal to 3183 ῼ;

diminishing area sizes down till the last metallic band,

obtainedfrom the bridge measuring device P5023 of

accessed through the test tap point P. Connections are

former soviet make. At zero balance of galvanometer, G,

made between D and P, and measurement of series

all indicated resistance values are summed up to give

is

A. Manufacturer’s Recommended Test Values Table 3 shows the manufacturer’s specifications of bushing oil when tested at 34 degrees centigrade. Table 3. Temperature of Bushing Oil at Tests = 34OC Tested Phase of Bushing/Plan t No. A (R)

Bushing

Test

Calculated

Measured

Terminal

Voltage

Capacitance,

tgδ of HV Bushing, in

limits of HV Bushing, in

tested

(kV)

(pf)

%, & Angular Degrees

%, & Angular Degrees

D–P

35

C1=342

(calculated)

0

0.56; (0.0097 )

tgδ

Remarks

˂1.2; (0.020)

satisfactory satisfactory

T322

P–F

5

C3=420

1.0; (0.01 )

˂3; (0.050)

B (Y)

D–P

35

C1=325

0.47; (0.0080)

˂1.2; ((0.020))

satisfactory

D88453

P–F

5

C3=460

0.9; 0.0010)

˂3; (0.050)

satisfactory

C(B)

D–P

35

C1=348

0.41; (0.0070)

˂1.2; ((0.020))

satisfactory

79540

P–F

5

C3=410

0

Manufacturers

0

0

˂3; (0.05 )

1.0; (0.01 )

satisfactory

D= Bushing input bar (1st electrode); P=Extreme metal sheet (2nd electrode), F=Earthed Flange; R3 = R1+R2; CX=C1 or C3; C1=Measured capacitance across D-P terminals (1st & 2nd electrodes); C3=Measured capacitance across P-F terminals (2nd & 3rd electrodes)

IV. TEST RESULTS The results of the tests carried out on a 63 MVA, 132/11kV power transformer bushing are shown in table 4 and table 5 respectively. Table 4 Tests Results on operating bushings of 132/11kv, 63 MVA power transformer of Ajaokuta Steel Company(Manufactured In Former Soviet Union); Temperature of Bushing Oil is 39OC. Tested Phase Bushing/Pl ant No.

Insulation

Tested

Test

Calculated

Measured tgδ of HV

Manufacturer’s

Resistance

Specimen

limits of HV Bushing,

Voltag

Capacitance

Bushing,

Across

e,

,

Angular Degrees

P-E(Mῼ)

(kV)

(pf)

A (R) T322

D–P 3000

B (Y) D88453

3000

C(B) 79540

3000

10

C1=357.2

in

%,

&

in

tgδ

Remarks

%, & Angular

Degrees 0

0.745; (0.013 ) 0

P–F

5

C3=524.5

2.34; (0.04 )

D–P

10

C1=336.6

0.549; (0.00950) 0

˂1.2; (0.020)

satisfactory

˂3; (0.050)

satisfactory

˂1.2; ((0.020))

satisfactory satisfactory

P–F

5

C3=559.3

1.245; (0.021 )

˂3; (0.050)

D–P

10

C1=363.4

2.4; (0.0420)

˂1.2; ((0.020))

non-satisfactory

P–F

5

C3=558.6

6.8; (0.1190)

˂3; (0.050)

non-satisfactory

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INTERNATIONAL CONFERENCE ON ELECTRIC POWER ENGINEERING (ICEPENG 2015) OCTOBER 14-16, 2015

D= Bushing input bar (1st electrode); P=Extreme metal sheet (2nd electrode), F=Earthed Flange; R3 R1+R2; CX=C1 or C3; C1=Measured capacitance across D-P terminals (1st & 2nd electrodes); C3=Measured capacitance across P-F terminals (2nd & 3rd electrodes)

Table 5. Repeated test (5 days later) on ht bushings of 132/11kV, 63MVA power transformer after replacement of phase c bushing of previous poor tan δ readings. Temperature of Bushing Oil ts 37OC. Tested Phase Insulation Bushing/Plant Resistance No. Across

Tested

Test

Calculated

Measured tgt δ

Manufac

Specimen

Voltage,

Capacitance,

&

turer’s

(kV)

(pf)

Angular Degrees

Remarks

tgt δ limits

P-E

&Angular

before

Degrees

HV injection (Mῼ) A (R)

3000

T322 B(Y)

P–F 3000

D88453 C(B)

D–P

3000

10 5

C1=355.4 C3=529

0.525; (0.0090) 0

1.50; (0.026 ) 0

˂1.2; (0.020) 0

˂3; (0.05 )

satisfactory satisfactory

0

D–P

10

C1=336.64

0.525; (0.009 )

˂1.2; ((0.02 ))

satisfactory

P–F

5

C3=570.63

1.0; (0.0170)

˂3; (0.050)

satisfactory

D-P

10

C1=359.88

0.48; (0.0080)-Replaced ˂1.2; ((0.020))

79537

satisfactory

bushing P-F

5

C3=507.58

0.98; (0.0170)-Replaced ˂3; (0.050)

satisfactory

bushing

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D= Bushing input bar (1st electrode); P=Extreme metal sheet (2nd electrode), F=Earthed Flange; R3 R1+R2; CX=C1 or C3; C1=Measured capacitance across D-P terminals (1st& 2ndelectrodes); C3=Measured capacitance across P-F terminals (2nd & 3rd electrodes)

•

C

–

phase

bushing

dielectric

was

recommended for replacement; was effected and tested for tangent delta, with test values of0.48; (0.0080) and

V. ANALYSIS OF TEST RESULTS

0.98; (0.0170), as shown in Table 5,

The results of tangent delta tests informing the

measured across D – P and P – F

status of HV bushings of the three phases of a

electrode

typical 132/11kV, 63 MVA transmission power

These test values are almost as

transformer have been presented. As the power

qualitative

transformer is a vital power equipment that

factory and pre-operational values as

enhances efficient grid performance the following

shown in table 3.

findings and

measures to

forestall possible

•

transformer breakdown and system collapse are

The

manufacturing

manufacturer’s

Most of the obtained loss angle values are

and

demonstrating high level of purity

commissioning (1982) value for the

expected of bushing dielectrics for

tangent delta test of C-phase HT

grid

bushing, measured across terminal

transmission transformers.

in Fig. 4a, was 0.41; (0.0070) and 1.0; (0.010) respectively as shown in Table 3,was

quite

satisfactory

conforming

to

and

manufacturer’s

standard. By

the

as seen in Tables 3, 4, 5, thus

factory’s

electrodes D – P and P – F as shown

•

as

respectively.

within 1st and 2nd decimal zero ranges

hereby highlighted: •

terminals

June

•

network elements,

Deterioration

few

years

of

grid

as

dielectrics

is

unnoticeably gradual and could be misleading observations qualities operational

1988,

of

such

ascertained

if

judged only.

and

by

visual

Hence

their

suitability

for

continuity

must

be

through periodic

non

commissioning and operation of the

destructive and at seldom, high

transmission transformer, the tan δ

voltage injection tests.

value of C-phase bushing was tested and found deteriorated, while A & B

VI. CONCLUSION

phases tested qualitatively satisfactory

This paper has highlighted the importance, types of

as shown in Table 4. The tangent

dielectrics in high voltage networks and the tests

delta readings across electrodes D – P

that are performed to ensure reliability of the power

and P- F of C – phase bushing, was

grid. In this work it has also been ascertained that

found to be 2.4; (0.0420) and 6.8;

system equipment should be guided by high

(0.1190)

signifying

voltage practices, including subjection to high

highdifferentialsby1.2;

voltage tests of their dielectrics, so as to promptly

3.8 (0.069O) above

zero down delays in the identification, speed and

manufacturer’s limits of 1.2; ((0.02)

quantity, of deteriorating insulations, in the interest

and 3; (0.050) respectively.

of safety of equipment, personnel, reliability and

respectively,

substantial ((0.0220)) and

integrity of the integrated network as a whole. Test

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INTERNATIONAL CONFERENCE ON ELECTRIC POWER ENGINEERING (ICEPENG 2015) OCTOBER 14-16, 2015

results on a 63 MVA power transformer bushing oil has been presented and analyzed. It is certain that high voltage engineering practice will remain a sine qua non, in decades to come, a vehicle for the enhancement of national grid performance, as more research findings and innovations for further improvement of dielectric qualities are made public.

REFERENCES. 1

Holtzhausen J. P, Vosloo W. L. ‘High Voltage Engineering Practice and Theory’ Electrical Engineering Portals, https://app.box.com/s/x7froiv7q2lzpocbgh35, 2015

2

Jian L. I. ‘Introduction to Fundamentals of High Voltage Engineering’, Department of High

Voltage & Insulation Engineering,

Chongqing

University.

www.cee.cqu.edu.cn/myweb/upfile/200903091 45343432.pdf, March 9, 2009

3

Yung

Chuck,

EASA

Technical

Support

Specialist. ‘Use Polarisation Index Test to Determine

Condition/Health

of

Motor

Insulation’, www.easa.com. September 2000

4

Ajaokuta Steel Company limited Ajaokuta, Kogi State, Nigeria ‘High Voltage Tests & Maintenance Records’ 1988

159 L. C. EKECHUKWU, T. C. MADUEME