Temperature Rise TestsCentre for Power Transformer Monitoring, Diagnostics

and Life Management (transformerLIFE)Monash University, Australia

Oleg RoizmanIntellPower, Australia

Spring 2009 IEEE Transformers Committee MeetingMiami FL, 21 April 2009

Valery DavydovMonash University

Special Test Transformer Nameplate Data

Year of Manufacture 2006

Rated Power HV/MV for ONAN,kVA

468/468

Rated Voltage HV/MV/LV, kV 22/4.5/0.415

Rated Current HV/MV Amp 12.3/60.0

Cooling Types ONAN, ONAF, OFAF

Number of Phases 3

Vector Symbol YNyn0yn0

Mass Untanking, kg 2420

Mass Each Cooler (excluding oil), kg 115

Mass Total (including oil), kg 6850

Insulating Oil Each Cooler, l 43

Insulating Oil Total, litre 2650

Oil Circulation, l/min 1200

Temperature Sensors and DAQ System

• 16 Fiber Optic sensors• 24 Thermocouples, including magnetic and thermal

ribbon types• 9 RTDs, including those of moisture/temperature

transmitters• More than 60 channels of information stored at 1 min

interval

Location of Thermocouples and RTDsin Test Transformer

Top Oil Temperatures

Top Core Yoke Temperatures

Top Rings Temp

Top Radiator

Bottom Radiator

Location of Fibre Optic Probes and Core Thermocouples

Fibre Optic Probe Installation in MV Winding, Phase B

Fibre Optic Probe Installation in HV Winding

Effect of Measuring Instrumentation

• In the following 4 slides, comparisons are made for the two windings of Phase B (OFAF)– MV (layer type)– HV (disc type)

• The comparisons are made for results obtained during the Temperature Rise Tests conducted at Monash using two different instrumentation sets for winding resistance measurement

Comparison of Tavr and FO for MV OFAFPhase B, 100A

30

35

40

45

50

55

60

65

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

t, min

T, d

eg C

FO12 MV top FO13 MV middle FO14 MV bottom θwm(i) as measured θw(i) as calculated

Avr wnd. T

Top wnd. temp

Mid wnd. temp

Bot wnd. T

1st set of winding resistance measurement instrumentation was used

Comparison of Tavr and FO for HV OFAFPhase B, 100A

35

37

39

41

43

45

47

49

51

0 2 4 6 8 10 12 14 16

t, min

T, d

eg C

FO7 HV top FO9 HV middle FO11 HV bottomMeasured average winding temperature Calculated average winding temperature

Tawr

1st set of winding resistance measurement instrumentation was used

OFAF (100A) HV Phase B

60

65

70

75

80

85

90

0:00:00 0:02:53 0:05:46 0:08:38 0:11:31 0:14:24 0:17:17 0:20:10 0:23:02

Twarm Twarm fitted IEC 20mins Twarm fitted IEC 15mins Twarm fitted IEC 5minsTwarm fitted exp 5mins FO7 FO1 FO9

2nd set of winding resistance measurement instrumentation was used

OFAF(100 A) MV Phase B

60

70

80

90

100

110

120

0:00:00 0:02:53 0:05:46 0:08:38 0:11:31 0:14:24 0:17:17 0:20:10 0:23:02

Time since shut down (mins)

T, d

eg C

Twarm Twarm fitted IEC 20mins Twarm fitted IEC 15mins Twarm fitted IEC 5minsTwarm fitted exp 5mins FO13 FO12 FO14

It could be seen that depending on duration of test variations in Twnd (R) is ~10 ºC

2nd set of winding resistance measurement instrumentation was used

Blocked Coolers: 60A

70

72

74

76

78

80

82

84

86

88

90

7:12 8:24 9:36 10:48 12:00 13:12 14:24 15:36 16:48 18:00 19:12

T, d

egC

FO6 FO15 FO16

Blocked Coolers 60 A

Phase A Phase C Phase B

~4 hours!

Blocked Coolers: 60 A

Blocked Cooling

68

70

72

74

76

78

80

82

84

0:00 2:24 4:48 7:12 9:36 12:00 14:24 16:48 19:12 21:36

T, d

egC

Twarm A Twarm fitted IEC (A) Twarm C Twarm fitted IEC © Twarm B Twarm fitted IEC (B)

Ph A

Ph C

Ph B

76.59+time*0.1618-4.331)exp(-time/*5.521 = Tw

2nd set of winding resistance measurement instrumentation was used

Blocked Coolers HV Phase B 60A

45

50

55

60

65

70

75

80

85

90

0:00 2:24 4:48 7:12 9:36 12:00 14:24 16:48 19:12 21:36

Time since shutdown (mins)

T, d

eg C

Twarm FO7 FO9 FO11 (TV4+TV5)/2

hottest measured temp mid measured temp

bottom measured tempWinding temp by resistance

ONAN(60A) HV phase B

45

50

55

60

65

70

0:00:00 0:02:53 0:05:46 0:08:38 0:11:31 0:14:24 0:17:17

Time since shutdown (mins)

T, d

eg C

Twarm Twarm fitted IEC 15mins Twarm fitted IEC 5mins Twarm fitted exp 5mins FO9 FO3 FO11

2nd set of winding resistance measurement instrumentation was used

ONAN(60A) MV phase B

40

45

50

55

60

65

70

75

0:00:00 0:02:53 0:05:46 0:08:38 0:11:31 0:14:24 0:17:17

Time since shutdown (mins)

T, d

eg C

Twarm Twarm fitted IEC 15mins Twarm fitted IEC 5mins Twarm fitted exp 5mins FO12 FO13 FO14

2nd set of winding resistance measurement instrumentation was used

Comparison of FO Data for Phases A, B & C

• In the following slide a comparison is made for Phases A, B and C for the HV (disc type) winding for the OFAF cooling mode

• The comparison is made for the measurements obtained during the Temperature Rise Tests conducted at Monash for the FO probes installed in the 2nd top disc of Phases A, B & C

• The differences in the FO measurement results were observed due to the following reasons:– Phase A is the most remote phase from the oil inlet pipe; the velocity

of oil flow through the winding ducts of Phase A is the lowest– Phase C is the closest phase to the oil inlet pipe; the velocity of oil flow

through the winding ducts of Phase C is the highest– Phase B is in the middle between the inlet pipe and Phase A

FO Sensors Data for Phases A, B and COFAF (100A)

55

60

65

70

75

80

85

90

95

8:24 9:36 10:48 12:00 13:12 14:24 15:36 16:48 18:00

Time, h:mm

Tem

pera

ture

, deg

C

FO6 FO15 FO16

Phase A

Phase B

Phase C

Cooling curves

Cooling Curves for A, B and C Phases, OFAF (100A), HV

60

65

70

75

80

85

90

0:00 2:24 4:48 7:12 9:36 12:00 14:24 16:48 19:12 21:36

Time since shutdown, min

Win

ding

tem

pera

utre

by

R, d

eg C

Phase B Phase C Phase A

2nd set of winding resistance measurement instrumentation was used

Effect of first valid time point for MV in OFAF

60

65

70

75

80

85

90

0:00:00 0:02:53 0:05:46 0:08:38 0:11:31 0:14:24 0:17:17 0:20:10 0:23:02

Time since shutdown (mins)

T, d

eg C

Twarm Twarm fitted IEC 20mins (from 1m45s) Twarm fitted IEC 20mins (from 4m)

1:45 min

4:00 min

Factors Affecting Winding Temperature Rise

• Winding resistance measuring equipment• Ambient temperature determination• Inadequate calculation of the average oil

temperature (leads to wrong g factor)• Accuracy of Load Loss measurement• Assumed total loss as sum of NL + LL• Effect of the Core temperature dynamics• Cold resistance measurement errors• Not reaching steady state before shutdown

Factors Affecting Winding Temperature Rise (cont’d)

• Connection circuit (two windings at a time)• Time interval – first and last data point resistance

measurement • Fitting curve method• Ambient oil temperature consideration

Conclusions

• Depending on winding time constant taking first resistance measurement at 4 min may be too long wait and could lead to significant error in determination of winding temperature at shutdown

• 10 min cooling curve period could be well justified for small and medium distribution transformers, but does not seem to be adequate for large power transformers, where 20 min should be considered as more appropriate

• 15 sec acquisition rate was found to be easily achievable with the modern acquisition systems and is recommended, especially for a winding with a short time constant

Conclusions (Cont’d)

• Considerations should be given to the following recommendations when FO sensors are used:

a. Number and locations of FO temperature probes should be determined on the basis of analysis of heat and mass transfer with assistance of numerical methods such as FEM and CFD;

b. Hot-spot temperature should be continuously measured by FO sensors installed in each winding of each phase and verified by calculations in accordance with the latest relevant standards and/or more detailed in-house thermo-hydraulic models;

c. Average winding temperature rise by resistance should be measured only in the winding of the phase with the highest hot-spot temperaturefound in b) unless the difference between that temperature and the average of all phases exceeds agreed value (e.g. 3 ºC).