CHAPTER LIQUID FOR TRANSFORMER (OIL).pptx

TRANSFORMER LIQUID TYPES

1. CONDUCTION & BREAKDOWN IN LIQUID DIELECTRIC

2. TOPIC OUTLINE Introduction Classification of Liquid Dielectric Characteristics Of Liquid Dielectrics Conduction And Breakdown In Pure Liquid Conduction And Break Down In Commercial Liquids 2

3. 3.1 INTRODUCTION Liquid dielectrics are used mainly in HV cables and capacitor for filling up of transformers, circuit breakers etc. Act as heat transfer agents in transformer The most important factor that affects the electrical strength of an insulating oil, is the presence of water in the form of fine droplets in the oil. Dielectric strength of oil reduces more sharply if it contains impurities in addition to water. 3

4. ELECTRICAL EQUIPMENT USING LIQUID DIELECTRIC Power transformer 4 Power capacitor Switchgear

5. 3.2 CLASSIFICATION OF LIQUID DIELECTRIC  Transformer Oil (mineral oil)  Silicone Oil  Synthetic hydrocarbons  Chlorinated Hydrocarbon  Ester 5

6. 3.2.1 TRANSFORMER OIL  The cheapest and the most commonly used in power apparatus  Almost colorless liquid consisting of a mixture of hydrocarbons  Gradually ageing process, when the liquid in a transformer is subjected to prolonged heating at high temperature of about 95 °C  With time the oil becomes darker due to formation of acid and resins, or sludge in the liquid. 6

7. 3.2.2 SILICONE OILS As alternative to polychlorinated biphenyl (PCB) but they are expensive At a temperature 150 °C they exhibit high long-term thermal stability Resistant to chemicals and oxidation, even at higher temperature They can be used at higher temperatures than transformer oil 7

8. 3.2.3 SYNTHETIC HYDROCARBONS Polyolefins are the dielectrics of choice for application in power cables Over 55% of synthetics hydrocarbons produced worldwide today are Polyolefins. The most commonly use olefins are polybutylene and alkylaromatic hydrocarbons. The general characteristics are very similar to mineral oil. 8

9. 3.2.4 CHLORINATED HYDROCARBONS Produce Askarels and Polychlorinated biphenyl (PCB) Excellent electrical properties but in recent years their has been banned throughout the world, because they pose serious health hazards 9

10. 3.2. 5 ESTE R S currently two types of synthetic esters are being used are Organic ester and phosphate ester. Organic ester have: high boiling point, high fire point, good viscosity- temperature relationship, used extensively in capacitors. Phosphate ester have: high boiling point, low flammability, used in transformers (in hazardous areas) 10

11. 3.3 CHARACTERISTICS OF LIQUID DIELECTRICS Electrical Properties Heat Transfer Characteristic Chemical Stability 11

12. 3.3.1 ELECTRICAL PROPERTIES i. The electrical properties are essential in determining the dielectric performance of liquid dielectric are:  Capacitance per unit volume or relative permittivity  Resistivity  Loss tangent ( tan d) or power factor  Ability to withstand high dielectric stresses Permitivities of most the petroleum oils vary from 2.0 to 2.6 and Silicone oils from 2.0 t0 73 Resistivities used high voltage application more than 10 16 ohm-meter Power factor , determine power loss and is an important parameter in cable and capacitor. In case transformer, the dielectric loss in the oil is negligible when compared to copper and iron losses. Pure and transformer oil, power factor have very low power factor varying 10 -4 at 20°C and 10 -3 at 90°C . 12

13. ii. Dielectric Strength is the most important parameter. Its depends on the atomic and molecular properties of the liquid itself. In practical the dielectric strength depends on The material of the electrodes Temperature Type of applied voltage Gas content in the liquid. 13

14. 3.3.2 HEAT TRANSFER CHARACTERISTICS The main factors that control the heat transfer are thermal conductivity ( K ) and Viscosity ( v) Heat transferred mainly by convection. Under natural atm cooling condition convection (N) is given by 14 K = thermal conductivity ; A = Coefficient of expansion C = specific heat per unit volume, v = kinematics viscosity ;n = 0.25 - 0.33 N  f [ K 3 AC / v ] n

15. 3.3.3 CHEMICAL STABILITY  Insulating liquid are subjected to thermal and electrical stresses in the presence of materials like oxygen, water, fibers and etc.  These will cause degradation of the liquid which can result in corrosion, impairment of heat transfer, increased dielectric losses, discharge and arcing. 15

16. 3.4 CONDUCTION AND BREAKDOWN IN PURE LIQUIDS Fig 3.3 shows the characteristic of conduction current-electric field in a hydrocarbon liquid. The curve has three distinct regions. EET413 HIGH VOLTAGE ENGINEERING 16

17. C ONT… . At very low fields, the current is due to the dissociation (penceraian) of ions. With intermediate fields, the current reaches a saturation value, and at high fields the current generated because of the electron emission from the cathode gets multiplied in the liquid medium by a Townsend mechanisms. The current multiplication also occurs from the electrons generated at the interfaces of liquid and impurities. The breakdown voltage depends on the field, gap separation, cathode work-function and the temperature of the cathode. In addition, the liquid viscosity, liquid temperature, the density and the molecular structure of liquid also influence the breakdown strength of liquid. EET413 HIGH VOLTAGE ENGINEERING 17

18. C ONT … . The breakdown strength is more if the dissolved (terlarut) gases are electronegative in character (like oxygen). Similarly the increase in the partial pressure of oxygen evolved and the liquid hydrostatic pressure will increase the breakdown strength in n-hexane as shown in Fig 3.4 and 3.5 EET413 HIGH VOLTAGE ENGINEERING 18

19. 3.5CONDUCTION AND BREAKDOWN IN COMMERCIAL LIQUIDS Commercial insulating liquids are not chemically pure and have impurities like gas bubbles, suspended particles etc. These impurities reduce the breakdown strength. When breakdown occurs in these liquids, additional gases and gas bubbles are evolved and solid decomposition products are formed. EET413 HIGH VOLTAGE ENGINEERING 19

20. C ON T The breakdown mechanism depends on the nature and condition of the electrodes, the physical properties of the liquid and the impurities and gases present in the liquid. In general the breakdown mechanisms are classified as follows a ) Suspended Particle Mechanism Cavitation and Bubble Mechanism Thermal Mechanism EET413 HIGH VOLTAGE ENGINEERING 20

21. FORMULA

22. EXAMPLE In an experiment for determining the breakdown strength of transformer oil, the following observations were made. Determine the power law dependence between the gap spacing and the applied voltage of the oil. 22 Gap spacing (mm) 4 6 8 10 Voltage at breakdown (kV) 88 135 165 212

23. SOLUTION The relatio n s h i p be t w e e n the v olta g e and g a p is no r m a l ly given as V  Kd n Our objective is to find out values of K and n. Substituting values of two observations , we have 23

24. SOLUTION 24