act as a cooling medium act as a dielectric media isol isolat ate e win windi ding ng from from the at atmo mosp sphe here re quench the arc
Possible contamination in transformer oil • So Sollid pa part rtiicle cle like like du dust st an and d fifibe berr • Absorption of Air • Moisture content • • •
Absorption of Gases Sludge formation Acidity in oil
The Electrical effects of contamination in oil • • •
Low dielectric strength. Low resistivity. Increase in Tan delta value.
The vacuum dehydrated oil purification system (VDOPS) • This machine is used for transformer oil filtration. • Free, emulsified and dissolved water and gaseous contamination can be removed by this machine. A high performance VDOPS removes • • •
100% of free and emulsified water, air and gases. 90% of dissolved water, air and gases. Up to 5-10 ppm water content in oil
Overall schematic diagram of an oil purification machine Oil in take Inlet pump
Heater Pre filter
Vacuum pump
Mechanical booster
The amount of heat is decided by oil vendor Filter is around 10 micron
Vacuum chamber
The vacuum chamber contains PDM for effective vacuum expose
Fine filter
Filter is around 0.5 micron
outlet pump
Oil outlet
Oil Purification Machine
System Principle •
Boiling point of the water is dependent on the pressure. Normally, at atmospheric pressure ( 1atm) the boiling point of water is 100 C.
•
Boiling point of the oil is around 250 C at 1 atm.
•
When the surrounding pressure is low the boiling point is also reduced for .
•
When oil is exposed to vacuum even at low temperature( around 150 F), the water molecular and other dissolved gases reach its boiling point and are boiled off effectively since the boiling point of the oil is very high.
Operation of Oil Filtration Machine •Oil is in taken through inlet valve •A inlet pump circulate the oil through the heater where the oil temperature is raised up to 150 F. The system heat is adjustable •This oil is filtered through a pre filter ( 10 micron) and pumped to a vacuum chamber. This removes major portion of coarse dirt. •Then the oil is pumped through vacuum chamber. •Vacuum chamber contains permanent dispersion media (PDM), a cylindrical type e emen . e o ows roug e pores o e n o s ower n or er o ensure more exposure of vacuum to the oil particles. This process, boils off dissolved water and gases effectively. •The boiled off process is called Degassing. •Then the dehydrated oil is collected at the bottom of the vacuum chamber and pumped through the fine filter (0.5 micron). •Then the oils is taken out through the outlet valve NOTE: At the beginning the outlet valve is closed and the oil is circulated through a by pass valve. This is done to make the system stable
Finding boiling point •
The equation used to find out the boiling point is
T(p)= T0 /[1+T0*ln(P0 /P)/(L/K0)] T(p) - boiling point in Kelvin - o ng po n n e v n a
L K0
e pressure
- Latent heat of vaporization/ molecule - Boltzman’s constant = 1.4 x 10-23 J/K
Boiling point of water at 5 Torr Data: 1 atm = 760 Torr = 760 mmHg Boiling point of water is = 100 C or 373 K at 1 atm L/K0 = 4800 K for water Boiling point of water at 5 Torr : T(p) = 373/[1+373xln(760/5)/4800] = 268.3 K = (268.3 – 273) C = - 4.7 C At -4.7 C water reaches its boiling point. So at this vacuum, 150F (around 66C) temperature is enough to boil off all the water content in the oil.
Data:
Boiling point of oil (Ex: Nytro Libra X) at 5 Torr
The following table shows the boiling point of Nytro Libra X at different pressure from vendor data Pressure ( ATM)
Temperature ( Celsius)
0.00001
40
0.001
102
0.01
146
0.1
207
1.0
294
5.0
382
10
437
50
593
So the boiling point of Nytro Libra X at 5 torr is 102 < T < 146 Celsius And also the boiling point at 1 torr is 102 < T < 146 Celsius
Boiling Point and the Temperature in Oil Purification Machine •
When the pressure reduces the boiling point also reduces.
•
According to the vacuum created in the oil purification machine, the temperature of the oil purification machine also need to be set
•
If the temperature is higher than the oil boiling temperature then the oil also will be vaporized
•
In our oil purification test we keep the pressure less than 5 Torr and the temperature around 70 Celsius.
•
This temperature is well below than the oil boiling temperature at 1 - 5 Torr so that the oil will not be vaporized.
Transformer oil filtration Transformer oil filtration is done through three main steps 1. Vacuum the transformer 2. oil filling to transformer 3. oil circulation When shipping the transformers are filled partially with oil. So there is a need of oil fillin for new transformers. Before filling the oil the transformer need to be vacuum. After filling the oil to required level, the total amount of oil must be circulated 2 or 3 times through oil filtration machine
Vacuum the transformer
The figure shows the transformer vacuuming diagram
Vacuum the transformer •
Close the valves V3, V7, V10
• • •
Open the valves V4, V5, V8, V9 Start the vacuum pump The degree of vacuum and the vacuum holding time should satisfy the following table Vacuuming of transformer can be done by “oil purification machine’s”
•
RATED VOLTAGE ( k V)
BEFORE OIL FILLING Degree of Vacuum
Vacuum Hold Time
Below 80
< 5 Torr
> 2 Hrs
Below 140
< 2 Torr
> 7 Hrs
Below 200
<2 Torr
> 12 Hrs
Below 500
< 1 Torr
> 24 Hrs
Oil Filling to Transformers
Oil purification machine Main Tank of Transformer
Oil purification machine and external oil tank
The figure shows the transformer oil filling diagram
Oil Filling to Transformers cont… •The external oil tank is filled with required amount of oil before starting oil filling to transformer. •This external tank oil filling also done through the oil purification machine •After finishing the vacuuming of transformer the oil purification machine is connected to the transformer as shown in the figure above. •Before filling the transformer the oil is circulated through the oil purification machine for 30 min to warm up the system. •Open the valves V1, V2, V3 •Start the oil purification machine •When the dial oil level indicator shows a reading more than a little minimum position perform the following procedure 1. temporarily stop the oil purification machine 2. close Valve V9 and V3 3. Close valve V8
Oil Filling to Transformers cont… •Open the valve V10 •Inject the nitrogen gas into the conservator tank (Inside air bag) until the degree of vacuum become 0 psi •Remove the nitrogen tank and keep valve V10 opened •Check the oil temperature versus oil level temperature curve •If the oil level is lower open valve V3 and operate oil purification machine to adjust the oil level according to “ Oil level temperature curve”
Oil Level Indicator
Oil Circulation
Oil Circulation •Open valves V3 and V7 •Circulate the oil in tank at 60- 70 Celsius not less than 2 cycles •Close the valves V3 and V7 •Slightly open Breather pipe flange P1 •Connect the nitrogen tank at valve V10 and inject the nitrogen at the pressure of 0.2 bar until the oil spurt from P1 . •Remove nitrogen tank and make the degree of pressure of conservative tank 0 psi. •Connect the silica jell breather
STANDARDS USED
Rated voltage/ kV
Break down Voltage of oil IEC
ASTM
Water Content in Oil
Below 80
> 50 kV
> 35 kV
< 20 PPM
> 12 HRS
Below 140
> 50 kV
> 35 kV
< 20 PPM
> 24 HRS
Below 200
> 50 kV
> 35 kV
< 20 PPM
> 36 HRS
Below 500
> 50 kV
> 35 kV
< 20 PPM
> 48 HRS
Elapse time
1. After oil filtration, the oil must satisfy these values 2. Elapse time: after oil filtration the transformer must not be energized before elapse time.
