tan delta

Hi guest

Many problems can cause the high of Tan Delta, to analysis of transformer, other test should be carried out also, such as :

1. Tip-up

2. Core Excitation

3. Capacitance

4. DGA

5. Water content by PDC or RVM test

6. DFR or FDS

7. Bushing tap or Hot Collar

8. SFRA

9. TTR

10. IR / PI

11. Cross winding DC Resistance

12. Etc

The complete test can help you to identify what is the Root Cuase the Problems, so you can easy to make a correction.

What test that you have ?

What is the power factor value and method ? Did you test on the bushing its self ( ex, hot collar ?)

Tip Up test.

The Tip-Up Test can be performed by varying the applied test voltage from Zero Volts to the maximum in several equal steps, Calculate the percent power factor at each voltage .

If the Power Factor does not vary as the test voltage is changed, moisture is the probable cause of the high Power Factor, if the Power Factor increases as the test voltage increases, the the ionization is occurring causing carbonization of the oil and transformer winding

EXCITATION CURRENT

The purpose of this test is to detect short-circuited turns, poor electrical connections, core de-laminations, core lamination shorts, tap changer problems, and other possible core and winding problems.

On a three-phase, wye/delta or delta/wye transformer test, the excitation current pattern will be two phases higher than the remaining phase. Compare the two higher currents only. If the excitation current is less than 50 milliampere (mA), the difference between the two higher currents should be less than 10%. If the excitation current is more than 50 mA, the difference should be less than 5%. In general, if there is an internal problem, these differences will be greater. When this happens, other tests should also show abnormalities, and an internal inspection should be considered. The results, as with all others, should be compared with factory and prior field tests.

CAPACITANCE

This test measures and records the capacitance (including bushings) between the high and low voltage windings, between the high voltage winding and the tank (ground), and between the low voltage winding and the tank (ground). Changes in these values as the transformer ages and events occur, such as nearby lightning strikes or through faults, indicate winding deformation and structural problems such as displaced wedging and winding support.

DGA

Use the following method to analysis:

1. Use 9 (nine) key gases

2. Roger Ratio

3. Donernberg Ratio

4. Duval Triangle

Water content by PDC or RVM test

PDC can measure automatically the water content in the cellulose and in the oil,

Water in a transformer is measured either as water content or as water saturation. The typical range for water content in the cellulose insulation of a transformer is between 0.5 % under new conditions and about 5 % under aged and wet conditions. For water saturation, would expect a typical range between 0.5 % for a new transformer and up to 30 % or sometimes even 50 % for an extremely wet, aged transformer. Water solubility will be low for new oil but it will increase ever faster with the aging of the oil.

If high power factor in your XFMER is cause water content, you can dry out by the following method (chose the best method as the transformer conditios).

A Number of HEATING methods are used any method should take into account the relatively large quantities of heat required overheating during drying must be avoid, normally 90⁰ C is the max of temperature for heating.

The drying methods as follow to be divided by 3 conditions

1. Factory and Repair Shop Methods

a. Oven Drying

If the corrective works is being done in a service shop, the core and winding can be removed from the tank and dried as a unit in an oven, larger unit are dried in their tank and or may be placed in an insulated temporary enclosure.

b. Dry Air

Keeping the insulated transformer in a confined enclosure and blowing by the dry air, this method need long time to achieved the temperature.

2. Field Methods with Power Off

a. Heating by the Circulating current in the winding (short circuit)

A source of power to heat the transformer is connected on one winding, with the other winding short circuited. Current up to full rated current can be used continuously to heat forced oil cooled transformers. The temperature should be monitored so that heating is controlled , temperature should high enough to drive out the moisture but not so high as to damaged the insulation, keep the temperature not over than 90⁰C. when temperature reached app 900C reduced the transformer power supply.

b. Heating by Circulating Forced Hot Air

With the oil removed the transformer tank is blanketed in order to minimize heat loss and keep it at uniform temperature to prevent condensation in the tank interior, suitable duct and blowers or fan are assembled to blow clean dry air (max 90⁰C) through the core and winding either at the top or bottom.

c. Heating by Circulating Hot Oil

This method requires an external heating system, including a suitable oil filter with a vacuum type drier or filter press plus heaters to provide round 90⁰C hot oil to be circulated until a satisfactory dry out.

d. Heating by the oil immersed Resistor

This methods consists of drying out the complete insulation system in the units tank by installed the specially construction resistor unit. It is lowered between the winding at the bottom of the tank. Resistor units are spaced symmetrically as possible around the inside of the tank in order to distribute of the heat. The resistor can be connected to single phase ac or dc low voltage supply. The temperature must not more than 90⁰C where is most normally required in the oil and on the outside layer insulation.

If the problem is cause by the internal fault in the winding , such as short winding, bad contacts, core fault, etc, power factor can't to be improve without any repair or replacement even the oil is new.

Good Luck.

Rgds

Siswanto

sis_cahya@yahoo.com