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