Simple Thermal Modeling of Oil-Immersed Distribution Transformers in PEA. Thailand.

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

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

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

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. Provincial Electricity Authority, Thailand 4

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. Provincial Electricity Authority, Thailand 5

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. Provincial Electricity Authority, Thailand 6

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

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. Provincial Electricity Authority, Thailand 8

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

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

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

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

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

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

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

16. EQUATION FITTING sum of deviations squared (SDS.) FOR 19 EXAMPLES , SDS = 69.59 C2 average percent absolutedeviation (APD.) FOR 19 EXAMPLES , APD = 10.19% Provincial Electricity Authority, Thailand 16

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 • Temperature rise at 80% of rated load is 35.63 C. Provincial Electricity Authority, Thailand 17

18. Question Provincial Electricity Authority, Thailand 18