Tan Delta Testing

 
Why is Insulation Tested ?

 

All transformers, high voltage switch-gear, motors and electrical equipment accessories have a high voltage lifespan. From the first day of use the equipment is subject to thermal and mechanical stresses, foreign particle ingress and variations in temperature and humidity. All of these influences raise the working temperature of the equipment when switched on. This heating accelerates chemical reactions in the electrical insulation, which result in a degradation of the dielectric characteristics. This process has an avalanche character

i.e. the changing electrical characteristics of the insulation increase the loss factor and produce heating which further degrades the insulation.
 
If the loss factor of the insulation is periodically monitored and recorded, it is possible to predict and / or avoid catastrophic failure of the electrical equipment.

What is Dissipation Factor  tan δ?

 

Significance and Use
 
Dissipation Factor (or Power Factor)—This is a measure of the dielectric losses in an electrical insulating liquid when used in an alternating electric field and of the energy dissipated as heat.
 
A low dissipation factor or power factor indicates low ac dielectric losses. Dissipation factor or power factor may be useful as a means of quality control, and as an indication of changes in quality resulting from contamination and deterioration in service or as a result of handling.

Power factor ?
 
The power factor of an AC electrical power system is defined as the ratio of the real power flowing to the load to the apparent power in the circuit, and is a dimensionless number in the closed interval of −1 to 1 .
 
 
PF= (real power/apparent power)

 
Significance of Capacitance and Dissipation Factor
 A large percentage of electrical apparatus failures are due to a deteriorated condition of the insulation
 
Ø Changes in the Normal capacitance of insulation indicate such abnormal conditions as the presence of a moisture layer, short circuits, or open circuits in the capacitance network. Dissipation factor measurements indicate the following conditions in the insulation of a wide range of electrical apparatus:
 
 
 Ø Chemical deterioration due to time and temperature, including certain eases of acute deterioration caused by local overheating.
•    Contamination by water, carbon deposits, bad oil, dirt and other chemicals.
 
•    Severe leakage through cracks and over surfaces.
 
•    Ionization.

 Working Principle C & Tan delta Test set
 
There are three basic kinds of capacitance, tan d and power factor test instruments in use. Although the high accuracy Schering Bridge must be balanced manually and the balance observed on a null indicator, it has been widely sold and used for decades. The capacitance and dissipation factor can be calculated by reading the position of the balance elements.
 
 
The automatically balanced C & Tan delta measuring instrument performs measurement by the differential transformer method. The automatic balancing makes operation very easy. The double vector-meter method is essentially an improvement of the differential transformer method .
 
How Does it Work ?
 
In a perfect capacitor, the voltage and current are phase shifted 90 degrees and the current through the insulation is capacitive. If there are impurities in the insulation, like those mentioned above, the resistance of the insulation decreases, resulting in an increase in resistive current through the insulation. It is no longer a perfect capacitor. The current and voltage will no longer be shifted 90 degrees. It will be something less than 90 degrees. The extent to which the phase shift is less than 90 degrees is indicative of the level of insulation contamination, hence quality/reliability. This “Loss Angle” is measured and analyzed.

APPLICATIONS OF TAN DELTA TEST SET
 
•   Power cables & Accessories
•   Shunt reactors
 
•   Capacitors
 
•   Power transformers
 
•   Liquid & solid insulations
 
•   Instrument Transformers
 
•   Bushings
 
•   Switch-gears

MEASUREMENT & ANALYZING FEATURES
 
 TEST MODE – UST
TEST MODE – GST

Analysis of Results
 
•        High tan – delta ( between 0.007 and 0.01 ) ingress of moisture is                   bushing insulation .
  
•        Very high tan delta ( beyond 0.01 ) high Humidity , Strong ageing                of insulation .
•        Low tan delta , weak potential connection .
•        High capacitance , this indicate partial breakdown