1. Provincial Electricity Authority, Thailand 1 Simple Thermal Modeling of Oil- Immersed Distribution Transformers in PEA. Thailand. Power Quality Control Section Provincial Electricity Authority

2. Provincial Electricity Authority, Thailand 2 CONTENTS  OBJECTIVE  INTRODUCTION  SOPE OF WORK  THEORY  EXPERIMENTAL WORKS  CONCLUSION

3. Provincial Electricity Authority, Thailand 3 OBJECTIVE  To predict load current by measure top surface of transformer and ambient temperature.

4. Provincial Electricity Authority, Thailand 4 INTRODUCTION  Transformers are the largest part of investments of distribution system  It is advisable to know the insulation condition, operating life, and loading capacitance of a transformer to ensure its economical operation and secured service.

5. Provincial Electricity Authority, Thailand 5 INTRODUCTION  Rule of Thumb for PEA. “ Load current of transformers should not be greater than 80% of rated current.”  Measure load current of transformer. To ensure its operation.

6. Provincial Electricity Authority, Thailand 6 INTRODUCTION  Now PEA. use IR. Camera to do preventive maintenance for distribution system.  We found many electrical connection problems. Easy for us to identify problems and solve them.

7. Provincial Electricity Authority, Thailand 7 INTRODUCTION  For distribution transformer, there is no temperature base for each loading current to consider the operating temperature.

8. Provincial Electricity Authority, Thailand 8 SCOPE OF WORK  Consider 250 kVA (22kV/400-230V) Transformers in 22 kV distribution system, Chiangmai, Thailand.  Neglect heat that generate from harmonic current.  Assume load variation by time of transformers is very small, raw data may be for steady state.  Assume all transformers are healthy. No arc, corona, partial discharge inside transformer.

9. Provincial Electricity Authority, Thailand 9 THEORY  Losses in transformers  Heat transfer  Equation fitting  Thermal- Electrical analogy

10. Provincial Electricity Authority, Thailand 10 LOSSES IN TRANSFORMER  Core loss  Copper loss %Load Ploss Ploss = Pcore + Pcu

11. Provincial Electricity Authority, Thailand 11 1 st LAW of THERMODYNAMIC %Load Ploss %Load Heat Ploss TR. T surface T amb Q Electrical losses = HEAT that transfer to surrounding Steady state, no accumulated energy

12. Provincial Electricity Authority, Thailand 12 EQUATION FITTING %Load Heat %Load ΔTΔT Q = UA. ΔT T oil Q Q T amb T surface ΔT = Q / UA ΔT = a(%L) 2 +b(%L)+c in consideration range UA is constant

13. Provincial Electricity Authority, Thailand 13 THERMAL-ELECTRICAL ANALOGY Q = UA. ΔT T oil Q Q T ambT surface 1/UA Q = ΔT/(1/UA) Q Q T surface T amb 1/UA I I V1 V2 R R I = ΔV/R

14. Provincial Electricity Authority, Thailand 14 RESULT  Load Current  Ambient Temperature  TR. Surface max. Temperature 19 examples

15. Provincial Electricity Authority, Thailand 15 EQUATION FITTING dTmax = 0.0039(%Load) 2 + 0.0323(%Load) + 8.0857 EQUATI ON:

16. Provincial Electricity Authority, Thailand 16 EQUATION FITTING sum of deviations squared (SDS.) SDS = 69.59 C 2 FOR 19 EXAMPLES, SDS = 69.59 C 2 average percent absolute deviation (APD.) APD = 10.19% FOR 19 EXAMPLES, APD = 10.19%

17. Provincial Electricity Authority, Thailand 17 CONCLUSION  The top surface temperature of transformer is function of loading capacitance and ambient temperature. dTmax = 0.0039(%Load) 2 + 0.0323(%Load) + 8.0857 35.63 C.  Temperature rise at 80% of rated load is 35.63 C.

18. Provincial Electricity Authority, Thailand 18 Question