Pre-Commissioning Tests and Functionality Assessment of a Bulk Power Transformer

(1)

Pre-Commissioning Tests and Functionality Assessment

of a Bulk Power Transformer

Peter Michael Enyong

Department of Electrical/Electronic Engineering Technology; Auchi Polytechnic, Auchi, Nigeria;

Abstract – A step-up bulk-power transformer had just been installed for the Unit 1 of Egbin Thermal Power Plant in the Lagos area, Nigeria. It was required by due practice to ascertain that it was in good functional condition and that it was installed to international standards. In this paper, the author presents details of the physical inspection and thirteen tests that were carried out. The observations from inspection and the results of the tests were analyzed with a view to ascertaining the condition and availability of the equipment for service delivery. The analysis indeed proved the power transformer to have satisfied international standards in virtually all aspects of the transformer tests considered and that it did not suffer defects arising from transportation hazards. To that extent the equipment was properly installed only that it yielded over 50% excess resistance than the factory stipulated (reference) resistance on the high voltage side which was, however, not satisfactory.

Keywords – Bulk Power Transfer, Transformer Tests.

I. INTRODUCTION

The step-up bulk-power transformer in question was a 270MVA, 16/330kV, 50Hz, YNd1 MITSUBISHI

two-winding transformer installed by MELCO (Nig.) LIMITED in 1985 for Unit 1 of the Egbin Thermal Power Plant, Lagos. Such a transformer is otherwise called generator transformer; the name which arises from the practice that it is always located as close to the associated generator as possible, in order to transfer bulk power from the generator to a given transmission line system [1]. After it was successfully installed it had to be thoroughly inspected and tested before putting it into service.

By way of organization of the paper, the next section shall deal with General Physical Inspection Exercise; to be followed by the third section entitled Actual Pre-Commissioning Test Exercises; whilst the fourth and final section takes care of Summary and Conclusion.

II. GENERAL PHYSICAL INSPECTION EXERCISE

Details of the various aspects of the apparatus inspected and the observations made are as given in Table I(a).

Table I(a): Record of observations from general physical inspection/check

S/No. ASPECTS OF INSPECTION/CHECK FINDINGS REMARKS

1 Bushing oil level Bushings filled up to mark Good

2 Transformer oil level Conservator oil level indicator pointer on 35o Good [2] 3 Fitting of accessories Accessories in their proper positions and due

directions

Good

4 Condition of fitted accessories No dents, no caving in, no misalignment Good 5 Tightness of fastenings Torque wrench tightness sound heard when

applied [see Table I(b)]

Good

6 Signs of transformer oil leakage No dripping from flanges and corks, no oil marks on tank walls, plinth or ground

Good

7 Positions of valves Valves in their correct operational positions Good 8 Condition of insulators No cracks, no chippings-off, no breakage Good 9 Condition of breather Silica-gel bright blue, oil-seal in place with

correct oil level

Good [3]

10 Earthing connections HV neutral point & equipment body all duly connected to earth.

Good

11 Equipment painting Cement Gray oil-paint applied Good [3]

12 Condition of transformer immediate surrounding

(2)

Spanner, Bolt and Nut [4] S/

No.

SPANNER SIZE

BOLT/NUT SIZE

TORQUE (Nm)

i M08 M05 5.7

ii M10 M06 7.9

iii M13 M08 17

iv M17 M10 28

v M19 M12 45

vi M22 M14 61

vii M24 M16 91

viii M30 M20 136

ix M32 M22 170

x M36 M24 125

III. ACTUAL PRE-COMMISSIONING TEST

EXERCISES

The actual pre-commissioning exercises involving various tests carried out on the generator transformer yielded results as detailed in the subsections that follow.

A. Core & Winding Insulation Resistance (IR) Test.

The test was carried out at an Oil Temperature of 28oC by applying 5kV(d.c.) for 1 minute in each case using a 5kV Megger Insulation Tester (Make – Yew, Type – 3213).

Test Results:

(i) Core-to-Earth = 1005Megohms (ii) HV-to-Earth = 1254Megohms (iii) LV-to-Earth = 1030Megohms (iv) HV-to-LV = 1510Megohms

Analysis:

As in [5], a new transformer should give Core-to-Earth IR value greater than 1000Megohms; a service-aged transformer should give a value greater than

100Megohms; whereas, IR values of 10 to 100Megohms are indicative of deteriorating insulation between the core and ground. For the windings generally IR values of 600Megohms and above are acceptable for transformers rated 10MVA and above [6]. Hence, the transformer of this work did possess good IR property.

B. Core & Winding Polarization Index (PI) Test.

This test was effected under the same condition as in (a) above. The same apparatus and method was used, only that the 5kV(d.c.) was applied for 10 minutes in each case here.

Test Results:

(i) Core-to-Earth = 1628Megohms (ii) HV-to-Earth = 2090Megohms (iii) LV-to-Earth = 1700Megohms (iv) HV-to-LV = 2566Megohms

Analysis:

Polarization Index (PI) = {10 min IR value}/{1 min IR value}. Very good PI values are usually above 2.0 [7]. But, the limit of acceptable values is 1.6 as in [8]. The Egbin Station generator transformer gave PI values of not less than 1.6 which is good enough.

C. Turns Ratio Test Exercise.

This involved the use of Transformer Turns Ratio Test Set (Make – KEIHIN DENSOKKI, Type – TR 10).

Test Results:

Results of the exercise were as presented in Table II

Analysis:

The percentage deviation from the rated turn ratio, Trated, should be within ± 0.5% [9].

However, acceptable values of %deviation extend up to ±1.0% according to [10]. It is therefore clear from Table II that the transformer turns were produced to standard judging from the maximum %deviation from tap to tap.

Table II: Results of the Turns Ratio Test Exercise Tap

Pos.

Tap Voltage

Turns Ratio as Measured Rated Turns Ratio (Trate)

Max ± (%) Deviation from Trate (H1-H0)/

(x1-x2)

(H2-H0)/ (x2-x3)

(H3-H0)/ (x3-x1)

1 (346.50/√3)/16.0 12.523 12.535 12.535 12.504 + 0.248

2 (338.25/√3)/16.0 12.235 12.236 12.236 12.206 + 0.246

3 (330.00/√3)/16.0 11.961 11.955 11.957 11.908 + 0.445

4 (321.75/√3)/16.0 11.660 11.663 11.668 11.610 + 0.500

5 (313.50/√3)/16.0 11.381 11.383 11.383 11.312 + 0.628

D. Voltage Vector Relationship Test Exercise.

AVO Multi-meter Model 8MK.V was use in conducting this test.

Test Results:

Results of the exercise were as presented in Table III.

Table III: Results of the Voltage Vector Test Exercise Test

Winding

Voltage Applied

Terminals Linked

H.V. 420V

(a.c.)

H1-to-x1 Measured Voltage

Between H2 & x2

Between H2 & x3

Between H3 & x2

Between H3 & x3 405V

(a.c.)

435V (a.c.)

405V (a.c.)

(3)

Analysis:

The magnitude of measured voltages between the stipulated terminals in Table III should show the relationship given in [11] as

(H2-x3) > TEST VOLT > (H2-x2) = (H3-x2) = (H3-x3)

Consider the arbitrary Yd transformer winding configuration in Fig.1(a) where H1 and x1 terminals are short-circuited. The condition or equation stated above can only be realized when the equilateral triangle representing the LV winding rotates clockwise through 30o in order to assume the position in Fig.1(b).

Fig.1. (a): Arbitrary Yd Configuration

Fig.1. (b): Actual Yd1 Configuration

Consequently, the chief criterion is that the LV voltage (and current) vectors must lag those of the HV by 30o [12, 13]. As the readings in Table III actually satisfy the given equation, the transformer has proved itself truly a Yd1 power transformer.

E. Dissipation Factor Test Exercise.

The dissipation factor is also referred to as insulation power factor or tangent delta all of which point to material dielectric loss [14]. Schering Bridge (Make – HISSIN DENKI, Type – SH/M) was used; the Oil Temperature during test being 30oC.

Test Results:

Results of the exercise were as presented in Table IV.

Analysis:

Dissipation factor, DF, is very sensitive to temperature; its value not changing with applied voltage. Thus, the average oil temperature at the time of testing must be recorded. However, the reference temperature commonly used is 20oC and correction factors need to be applied when testing at different temperatures [9]. Table V provides the applicable correction factors at various temperatures.

Table IV: Results of the Dissipation Factor Test Exercise S/

No.

WINDING CONNECTIONS DURING TEST

VALUES OBTAINED tan δ

(%)

Capacitance (uF)

1 HV-to-(LV&E) 0.70 0.041

2 LV-to-(HV&E) 0.48 0.059

3 HV-to-LV 0.70 0.036

(NB: HV=High voltage Winding; LV=Low voltage

Winding; E=Earth or ground).

Table V: DF Correction Factor for Various Temperatures [15]

TEST TEMP.

(oC)

CORR. FACTOR

(K2)

TEST TEMP.

(oC)

CORR. FACTOR

(K2)

10 0.80 45 1.75

15 0.90 50 1.95

20 1.00 55 2.18

25 1.12 60 2.42

30 1.25 65 2.70

35 1.40 70 3.00

40 1.55

The following equation, DF(20) = DF(θ)/K2, demonstrates the application of the correction factor; where DF(20) and DF(θ) are DF values at 20oC and Test Temperature in oC, respectively. Thus, for our test temperature of 30oC the DF values corresponding to the reference temperature of 20oC are: 0.384 for the LV-to-(HV&E) test and 0.56 for both (LV&E) and HV-to-LV tests. Obviously, the second DF value of 0.56 is not satisfactory.

F. Transformer Winding Resistance Test Exercise.

The ammeter-voltmeter method was adopted as in [16], being reflected in Fig. 2(a) for the HV winding resistance test exercise; whilst Fig. 2(b) summarizes that of the LV winding.

Fig.2. (a): Ammeter-Voltmeter Method of DC Resistance Measurement for the HV Winding

(4)

inspection, it can be seen that Vo/Io = 2R/3 in the case of the LV winding. Hence, R = 1.5(Vo/Io).

For each measurement the steady-state resistance value was obtained by leaving the set-up for 5 – 10 minutes before taking readings of Vo and Io. The transformer oil temperature was 28oC on the average. D.C. ammeter and voltmeter; rheostat; toggle switch; D.C. source were chief among the apparatus used.

Test Results:

Results of the exercise were as presented in Table VI.

Analysis:

The factory test results available (as reference data in this perspective) were

(1) Rr(HV) = 0.35272 ohm @ 10.2oC (oil temp.) for H1-H0 on Tap Position No.1.

(2) Rr(LV) = 0.002553 ohm @ 10.2oC (oil temp.) for x1-x2. For the purpose of comparison the corresponding

value of Rr(HV) for 28oC which may be designated as Rm(HV), shall be computed using a formula obtainable from [5] and given here as

Rm(HV) = Rr(HV){(Tm+234.5)/(Tr+234.5)} = 0.35272{(28+234.5)/(10.2+234.5) = 0.37838

where Tm is the test temperature, whilst Tr is the factory reference temperature. This is to be compared with the average resistance of all the three phases for each tap position which has been computed and presented in Table VII (short Table).

It is clear that the average measured winding resistance per tap position is over 1.5times the factory reference resistance. This realization is really not satisfactory. But, the actual standard criterion for judgment which matters most is that the measured resistance for a phase winding should not deviate more than ±2% from the average resistance of all the three phases on a given tap position,

for a good enough result [6]. However, the very maximum acceptable deviation is ±5% [5, 7]. In all these the transformer measured phase winding dc resistance values were quite unsatisfactory.

Table VI: Results of the HV Winding Resistance Test Exercise Tap Pos

Winding 1 2 3 4 5

H1-H0

Vo = 0.420 Io = 0.727 R = 0.578

Vo = 0.415 Io = 0.730 R = 0.568

Vo = 0.408 Io = 0.730 R = 0.559

Vo = 0.409 Io = 0.749 R = 0.546

Vo = 0.391 Io = 0.735 R = 0.531

H2-H0

Vo = 0.421 Io = 0.726 R = 0.580

Vo = 0.413 Io = 0.727 R = 0.568

Vo = 0.409 Io = 0.731 R = 0.560

Vo = 0.442 Io = 0.799 R = 0.553

Vo = 0.399 Io = 0.735 R = 0.543

H3-H0

Vo = 0.421 Io = 0.726 R = 0.580

Vo = 0.414 Io = 0.728 R = 0.569

Vo = 0.437 Io = 0.780 R = 0.560

Vo = 0.434 Io = 0.790 R = 0.549

Vo = 0.397 Io = 0.735 R = 0.540

Oil Temp. 26oC 26oC 28oC 28oC 30oC

(NB: Values of R include the d.c. resistance of the connecting leads. The LV d.c. winding resistance was, however, not measured during the exercise).

Table VII: Average Resistance of all the Three Phases for each Tap Position Tap Pos

Winding 1 2 3 4 5

HV 0.628Ω 0.621Ω 0.633Ω 0.630Ω 0.603Ω

G. Auxiliary-Circuit Insulation Resistance Test

Exercise.

Auxiliary circuits on a bulk power transformer are circuits associated with the equipment local protection, metering and control devices. They often involve intricate cabling works and meant for no more than 440V 3-phase or 240V single-phase supply as the case may be. Oil Temperature was 28oC and the duration of injection of d.c. was 1 minute in each case. A 500V Megger Insulation Tester (Make – Yew) was used.

Test Results:

Results of the exercise were as detailed in Table VIII.

Analysis:

(5)

Table VIII: Record of Insulation Resistance (IR) Readings of Auxiliary Circuits S/

No.

DESCRIPTION OF AUXILIARY CIRCUIT

READINGS

Phase-to-Earth Phase-to-Phase

1 Main Power Supply Circuit 100 MΩ 200 MΩ

2 Cooling Fan Circuits

i) Groups 1 & 2 fan circuits 10 MΩ each --

ii) Two-fan circuit 200 MΩ --

iii) One-fan circuit 950 MΩ --

3 Winding Temperature Thermometer

i) Alarm system circuit 100 MΩ --

ii) Trip system circuit 30 MΩ --

4 Oil Temperature Thermometer

5 Oil Level Indicators

i) Conservator oil level indicator circuit 100 MΩ --

ii) On-load tap-changer oil level indicator circuit 30 MΩ

6 Sudden Pressure Relay Circuit Pressure Relay not

connected --

7 Buchholz Relay

8 Pressure Relief Device Circuit 1000 MΩ --

H. Bushing Current Transformer (BCT) Insulation

Resistance Test Exercise:

The same test equipment and method as applicable to the auxiliary-circuit insulation resistance test exercise above were used here.

Test Results:

Results of the exercise were as presented in Table IX.

Analysis:

A table of insulation resistance values and the associated voltages is given in [17] which is tailored here to suit the purpose of this test as it is acceptable generally for all electrical equipment (see Table X); the applicable test temperatures of 25oC through 30oC ( or a rough average of 28oC) having been considered appropriate to that effect .

Table X: Acceptable Values of Insulation Resistance obtained during Tests [7]

S/ No.

EQUIPMENT RATED VOLTAGE

TEST VOLTAGE

(Vdc)

ACCEPTABL E READING

(MΩ)

1 <1000Vac 500Vdc >5

2 1000 to 2500Vac 1000Vdc >100 3 2501 to 5000Vac 2500Vdc >100

4 >5000Vac 5000Vdc >100

Since the test voltage in this case was 500Vdc it follows that the generator transformer gave an excellent value of insulation resistance.

I. Bushing Current Transformer Polarity Test

Exercise:

The induction-kick method with d. c. voltage was used as shown in the apparatus arrangement of Fig. 3(a); whilst Fig.3(b) shows details of the BCT winding per phase (the 1st in this case).

Fig. 3(a): Apparatus setup for the Polarity Test Exercise

(6)

Test Results:

Results of the exercise were as presented in Table XI. Table XI: Results of the Polarity D. C. Test GALVANOMETER (G) DEFLECTION

B.C.T Secondary Transformer Phase

1x1‒1x5 2x1‒2x5 3x1‒3x5

RED PHASE (+) (+) (+)

YELLOW PHASE

(+) (+) (+)

BLUE PHASE (+) (+) (+)

NEUTRAL (+) (+)

No 3rd Neutral

B.C.T

Analysis:

The transformer as reflected in Fig.5 is of subtractive polarity [16]. With the switch, S, in the ON position, the galvanometer did give a (+) deflection. This was indicative of the fact that the B.C.T in question was of the right polarity marks [16].

J. Bushing Current Transformer Winding Resistance

Test Exercise.

The resistance bridge method was adopted. The average oil temperature was 30oC. In each case the apparatus setup was allowed to stabilize before reading was taken. A Wheatstone Bridge Set (Make ‒ YEW, Type ‒ 2755) was used.

Test Results:

Results of the exercise were as presented in Table XII.

Analysis:

The measured secondary winding resistance of a B.C.T may be considered good if it does not deviate more than ±2% from the average secondary winding resistance of all the three related phase B.C.T’s.

The same assessment yardstick applies to a neutral B.C.T. This follows the general standard in [6] for the winding resistance of 3-phase transformers which stipulates that the measured resistance for a phase winding should not deviate more than ±2% from the average resistance of all the three phases on a given tap position.

Table XII: Results of the B.C.T Secondary Winding Resistance Test

K. Bushing Current Transformer Magnetization Test

Exercise.

A variable voltage supply was connected across the terminals of the full length of a given B.C.T secondary winding and the current measured at different voltage values using a suitable ammeter. It is important to note that the ammeter was connected absolutely in series with the B.C.T winding to avoid reading the voltmeter branch current, which in some cases could be of the same order of the B.C.T magnetizing current [17]. As the magnetizing current was not going to be sinusoidal, an ammeter of the moving-iron type was used [18]. And it is often found that

current transformers with secondary ratings of 1Amp or less do have a knee-point voltage higher than the local mains supply [18]. Hence, the B.C.T secondary being

rated 1Amp, it was mandatory to use a step-up interposing transformer in order to obtain the necessary voltage for proper assessment of the magnetizing curve [18]. Moreover, the average oil temperature at the time of testing was 30oC. Among the apparatus used were: 0 – 260V variable auto-transformer; 220/6600V 1-phase step-up transformer; 6600/110V single-phase potential transformer; 0 – 1A moving-iron ammeter; 0 – 200V voltmeter.

Test Results:

Results of the exercise were as presented in Table XIII.

Analysis:

It can be observed from Table XIII that the magnetizing currents for the same input voltages are close to one another in respect of the No.1 and No.2 sets of B.C.Ts on B.C.T

SERIAL NUM-BER

ASPECT OF SECONDARY WINDING TESTED

B.C.T SECONDARY WINDING RESISTANCE (Ω) RED

PHASE

BLUE PHASE

THE NEUTRAL

1

1x1‒1x5 6.454 6.513 6.527 6.566

1x2‒1x5 5.314 5.375 5.386 5.417

1x3‒1x5 2.867 2.902 2.919 2.919

1x4‒1x5 1.863 1.892 1.913 1.900

2

2x1‒1x5 6.504 6.540 6.532 6.552

2x2‒1x5 5.366 5.411 5.392 5.402

2x3‒1x5 2.904 2.936 2.910 2.912

2x4‒1x5 1.888 1.917 1.904 1.897

3

3x1‒1x5 4.828 4.890 4.906 No 3rd Neutral

Bushing Current Transformer

(B.C.T)

3x2‒1x5 4.009 4.066 4.095

3x3‒1x5 2.201 2.252 2.287

3x4‒1x5 1.455 1.501 1.530

(7)

all the phases and neutral. Taking average current of both sets of B.C.Ts, values of current are obtained applicable to both sets as given in Table XIV for the purpose of generating magnetizing curves (i.e. 4 in number) common to both B.C.T sets. Also, as the currents for No.3 set of B.C.Ts are not too different from one another on all the

phases, an average value can be taken in each case (see Table XV) for the purpose of plotting one magnetizing curve for this set. Figure 4 shows the curves for the Nos.1 & 2 B.C.T sets; whilst Fig. 5 reflects the one curve for No.3 B.C.T set. MATLAB was used in accomplishing the plots.

Table XIII: Results of the B.C.T Magnetization Tests B.C.T FULL

SECONDRAY WINDING

RED PHASE

BLUE PHASE

THE NEUTRAL

V mA V mA V mA V mA

1x1‒1x5 For No.1 B.C.Ts

0 0 0 0 0 0 0 0

400 11 400 11 400 11 400 5

600 13 600 13 600 14 600 10

700 15 700 15 700 15 700 12

800 16.5 800 17 800 17 800 15

900 21.5 900 24 900 21 900 22

1000 47 1000 55 1000 52 1000 36

1100 288 1100 290 1100 290 1100 230

2x1‒2x5 For No.2 B.C.Ts

0 0 0 0 0 0 0 0

400 11 400 11 400 11 400 6

600 13 600 13 600 14 600 8

700 15 700 15 700 15 700 10

800 17 800 17 800 19 800 13

900 26 900 24 900 30 900 18

1000 65 1000 53 1000 71 1000 33

1100 450 1100 300 1100 410 1100 230

3x1‒3x5 For No.3 B.C.Ts

0 0 0 0 0 0 No 3rd Neutral

Bushing Current Transformer

(B.C.T)

200 10 200 11 200 10

300 15 300 14 300 13

400 23 400 23 400 19

500 65 500 65 500 52

600 657 600 690 600 680

Table XIV: Average Magnetizing Currents relative to Nos.1&2 BCT Sets B.C.T FULL

SECONDRAY WINDING

RED (R) PHASE

YELLOW (Y) PHASE

BLUE (B) PHASE

THE NEUTRAL

V mA V mA V mA V mA

(x1‒x5) For both No.1 and No.2 B.C.Ts

0 0 0 0 0 0 0 0

400 11 400 11 400 11 400 5.5

600 13 600 13 600 14 600 9

700 15 700 15 700 15 700 11

800 16.75 800 17 800 18 800 14

900 23.75 900 24 900 25.5 900 20

1000 56 1000 54 1000 61.5 1000 34.5

(8)

Red Phase

Fig.4(b) Magnetizing Curve For B.C.Ts Nos.1&2 on the Yellow Phase

Fig.4(c) Magnetizing Curve For B.C.Ts Nos.1&2 on the Blue Phase

Fig.4(d) Magnetizing Curve For B.C.Ts Nos.1&2 on the Neutral

Table XV: Average Magnetizing Currents relative to No.3 BCT Set

No.3 B.C.Ts

FULL SECONDARY WINDINGS (x1‒x5)

V 0 200 300 400 500 600

mA 0 10.5 14 21 58.5 675.7

Fig.5. Magnetizing Curve for No.3 B.C.Ts on all the Phases

It is to be realized that the performance requirements of a B.C.T, like any other current transformer, are often specified in terms of the knee-point voltage [19]. The knee-point voltage of the excitation or magnetization characteristics or curves is the point at which 10% further increase in the voltage applied to the current transformer secondary will produce a 50% increase in the exciting or magnetizing current flow [17]—[19]. Thus, beyond the knee-point the current transformer becomes saturated.

(9)

L. Buchholtz Relay Function Test Exercise.

The ALARM CONTACTS were tested by supplying nitrogen gas through the gas sampling valve into the top chamber and the TRIP CONTACTS were tested by releasing the pressure (of about 0.15Kg/cm2) from the top of the transformer, thus causing a pressure surge. A hand pump was used.

Test Results:

Alarm contacts made at the right level of oil fall in the upper chamber with 450cm3 of gas. Trip contacts made in the lower chamber following the pressure surge created.

Analysis:

Buchholz alarm and trip systems observed to be functional judging from the test results.

M. Transformer Oil Dielectric Strength Test

Exercise.

For this exercise, five samples of the oil were taken with oil-sampling bottles that were rinsed many times with the same transformer oil and a small amount of the oil was first made to flow out of the tap and run to waste before samples were then taken. The electrode gap was set to 2.5mm. The SANMI Oil Tester, No.JIS C2101, Range 0 – 60kVdc was used. The test voltage (dc) was applied gradually each time within 1 minute.

Test Results:

Results of the exercise were as presented in Table XVI. Table XVI: Results of the Oil Dielectric Strength (DS)

Tests

(NB: B.D.V – Breakdown Voltage; D.S. – Dielectric Strength)

Analysis:

The breakdown voltage (B.D.V) should not be less than 30kV for transformers rated 287.5kV and above, and 25kV for those rated below 287.5kV [5]. If the B.D.V falls below these values, the oil should be reclaimed. It follows that a transformer oil D.S should not be lower than 10kV/mm at an electrode gap of 2.5mm or 6.25kV/mm at a gap of 4mm. Here, only the B.D.V test was carried out. Although a significant test, this was grossly inadequate for the transformer oil property evaluation, because moisture in combination with oxygen and heat will have cellulose insulation destroyed in a transformer long before the B.D.V or D.S of the oil gives a clue that anything was going wrong [20]. However, with average B.D.V of 56.8kV and D.S of 22.72kV/mm the oil proved

sufficiently capable of withstanding the expected stresses of the transformer rated voltages.

N. Dial Oil & Winding Temperature Thermometer

Test Exercise

The indicator bulb of the dial thermometer was suspended in an oil bath together with an accurate mercury thermometer; care being taken to avoid their touching the side or bottom of the container.

Hotplate was used to heat the oil while stirring and the readings of the two thermometers were taken as the temperature increased. Oil bath test kit (Qualitrol), hotplate, mercury thermometer were thus chief amongst the apparatus used.

Test Results:

Results of the exercise were as presented in Table XVII.

Analysis:

The error temperature for any of the dial thermometers should be within ±5oC [5]. Thus, both dial thermometers were in good working condition.

IV. SUMMARY & CONCLUSION

A. Summary

From the physical inspection report as detailed in Table I(a), the transformer was satisfactory in terms of external facility and surrounding conditions. The thirteen test exercises covered in this paper include: (1) core & winding dielectric property test (2) turns ratio test (3)

voltage vector relationship test (4) dissipation factor test

(5) transformer winding resistance test (6) auxiliary-circuit insulation resistance test (7) bushing current transformer insulation resistance test (8) bushing current transformer polarity test (9) bushing current transformer resistance test (10) bushing current transformer magnetization test (11) buchholz relay functional test (12)

transformer oil dielectric strength test and (13) dial temperature thermometer test.

B. Conclusion

From the test results as provided in this work and the analysis as presented, the generator transformer yielded acceptable pre-commissioning test results, excepting (of course) the HV winding resistance test results which were over 1.5times the factory reference resistance on each tap. The apparatus was, however, commissioned; but not without the strong recommendation for an effective cooling measure to avoid over-heating due to substantial additional copper losses (of over 50%). Effective use of the transformer cooling fans was thus the near solution.

Table XVII: Results of the Dial Temperature Thermometers

(NB: O.T.T – Oil Temp. Thermometer; W.T.T – Winding emp. Thermometer)

TEST TEMP. (oC) 120 100 90 80 70 60 50 40

O.T.T READING (oC) 119 100 89.5 79.5 70 60.5 50.5 40.5 ERROR (oC) - 1 0 - 0.5 - 0.5 0 +0.5 +0.5 +0.5 W.T.T READING (oC) 120.5 100.5 90.5 80.5 70.5 60 50 40

ERROR (oC) +0.5 +0.5 +0.5 +0.5 +0.5 0 0 0

SAMPLES 1 2 3 4 5

B.D.V (kV) 55 55 58 58 58 AVERAGE B.D.V(kV) 56.8kV

(10)

REFERENCES

[1] Asea Brown Boveri (1988): Switchgear Manual; Mannheim, Germany; 8th_{Edition, ABB Publication; p.430.}

[2] Enyong P. M. (1994): Report on the Installation of a 16MVA

Power Transformer; National Electric Power Authority, Sapale

Power Station; An unpublished Technical Report; p.13. [3] Brown Boveri (1978): Three-Phase Transformers 50 to

2500kVA; Publication No. DTR 51085 E; p. 8-10.

[4] Enyong P. M. (2004): Extension of NEPA and WRPC 33kV Power Supplies to Nigerian Gas Company Ltd. Complex in

Ekpan – Warri; An unpublished Detailed Technical Report; Vol.

II p.18.

[5] Hydroelectric Research & Technical Services Group (2000):

Facilities Instructions,Standards & Technique – Transformer

Maintenance; Denver, Colorado; Publication D-8450, FIST

3-30; p.75-78.

[6] Oyetola J. B. (2012): Transformer Testing & Measuring

Instruments (using PHCN as a Case Study): International Journal

of Engineering Research & Technology (IJERT); Vol. 1, Issue 9; p.4-7; 207-216.

[7] Hartford Steam Boiler Inspection & Insurance Company (2010):

Standard for an Electrical Preventive Maintenance (EPM)

Program; HSB Publication No.420 (rev 02/10); p.1-13.

[8] NEI Peebles Ltd. (1976): Operating and Maintenance

Instruction on Boiler Feed Pump Drives, East Pilton, Edinburgh

EH52XT, Peebles Electrical Machines Publishing.

[9] Steyn D. J. (2006): Power Transformer Field Testing; South Africa; Transmission Energize Publication; pp. 18-22.

[10] Pansini A. J. (1976): Basic Electrical Power Transformers; New Jersey; Hayden Book Co. Inc.; p. 129, 130.

[11] Enyong P. M. (1990): Statement of Experience and Project: An unpublished Technical Report submitted to Nigerian Society of Engineers, Warri Branch, for Corporate Membership Registration Purposes; p.19-26.

[12] Marrano S. J. & Dilouie C. (1998): Electrical System Design &

Specification Handbook for Industrial Facilities; India; The

Fairmont Press, Inc; p.146-148.

[13] Say M. G. (1976): Alternating Current Machines, 4th_Ed.,

London, Pitman Publishing Limited; p.166-169.

[14] Pabla A. S. (2000): Electric Power Distribution; New Delhi; Tata McGraw-Hill Publishing Company Ltd.; p.499.

[15] Aibeche A., Grouni S., Akroum H. & Boualegue K. (2013):

Experimental Research of Diagnostic Measurement Tests for

Power Transformers: Proceedings of International Conference

on Control, Engineering & Information Technology; Vol. 4, pp. 29-36.

[16] Enyong P. M. & Obaitan E. B. (2013): Electric Power System &

Machines Laboratory Manual; Benin-City; Lead Printing Press.

[17] Davis T.(1996): Protection of Industrial Power System; 2nd_Ed.;

Oxford; Newnes-Butterworth-Heinemann Publishing; p. 207-216.

[18] General Electric Company (1987): Protective Relays –

Application Guide; 3rd_{Ed; England; GCE Measurements}

Publishing; p.70-72; 397,398.

[19] General Electric Company (1980): Commissioning, Operating and Maintenance Instructions for High Speed Biased Differential Relays – Types DTH 13 & DTH 32; St. Leonards Works, England; GEC Measurements Publication R-57:7A; p.6, 7.

[20] Kelly J. J., Meyers S. D., Parish R. H. (1981): Transformer

Maintenance Guide; S. D. Meyers Co. Publishing.

AUTHOR'S PROFILE

ENYONG Peter Michael

is a Chief Lecturer in the Department of Electrical/Electronic Engineering of Auchi Polytechnic, Auchi, Nigeria. He hails from Uruan LGA via Uyo Capital City of Akwa-Ibom State, Nigeria; born on 15th_{January, 1955; had his B.Eng.}