ndt basics of partial discharge

1. 1
Module 5

2. What is Non-destructive Testing?
• It is a wide group of analysis techniques used in
science, industry & Engineering to evaluate the
properties of Insulating material, component or system
without causing damage.
• It is very difficult to test the quality of insulating
material after it forms part of equipment, suitable tests
must be done to ensure in the said range of operation.
• • Non-Destructive Testing (NDT) is ensure that the
material is not
• destroyed as in the case of high voltage testing.
•

3. NDT continued
• NDT helps to estimate the electrical properties
such as resistivity, dielectric constant & loss factor
, loss angle & dielectric loss over a wide
frequency range.
•
• • NDT is commonly used in , mechanical
engineering, petroleum engineering, electrical
engineering, civil engineering, systems
engineering, aeronautical engineering, medicine,
and art.
•

4. • Dielectric loss measure the quantity of a
• dielectric material’s inherent dissipation of
• electromagnetic energy converted into heat.
•
• • The dielectric loss can be expressed in terms
• of loss angle δ.
•
• • An efficient dielectric material supports a
varying charge with minimal dissipation of energy
in the form of heat.

5. • tan δ is called as loss angle or dissipation factor testing or
• loss tangent.
• • Helps to determine the quality of the cable insulation.
• • The cable and its insulation is compared to a parallel
plate capacitor.
• • In a perfect capacitor, the voltage and current are
phase
• shifted 90 degrees.
• • If there are impurities in the insulation, like water
trees, electrical trees, moisture and air pockets, etc. the
resistance of the insulation decreases.

6. • Resulting in an increase in resistive current
through the insulation.
• • It is no longer a perfect capacitor

7. • 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/DIELECTRIC LOSS
• • This “Loss Angle” is measured and analyzed.

8. • Fig represents an equivalent circuit of a cable,
The tangent of the angle δ is measured
•
•
•
• tan δ helps to find out the life expectancy of
dielectrics
on δ=0
tan δ= IR/ IC

13. How to measure dielectric loss andloss angle of a dielectric material?
.HIGH VOLTAGE SCHERING BRIDGE
2.TRANSFORMER RATIO ARM BRIDGE

14. The Schering Bridge is an electrical circuit used for measuring
the insulating properties of electrical cables and equipment.
• Accurate value of capacitance can be measured by using Schering bridge.
• It is used to measure insulation properties such as dielectric loss and loss angle.
• It has the advantage that the balance equation is independent of frequency.

16. We can be able to find the Unknown
capacitance
by using Schering bridge , ie
C1= C2R4/R3
R1=R3C4/C2
tan δ= wR1C1

17. Transformer Ratio Arm Bridge
For high frequency measurements, the arm with high resistance leads
to difficulties due to their residual inductance, capacitance and
skin effect.
If the length of arm is large, shielding difficulties

18. This bridge provide more accurate results for the small capacitance
measurements.
• Using at least two arms.
• Two types of Transformer Ratio Arm bridges
are available

19. a)Voltage Ratio Arm bridge-
-Suitable for high frequency low voltage application
b)Current Ratio Arm Bridge
-Used for high voltage low frequency
application

20. Transformer voltage ratio arm bridge

21. • In practical situation due to the presence of
magnetizing current and the load currents, the
voltage ratio slightly differ from the turns
ratio.
• Those errors are ratio errors & load error.

22. b.Transformer current ratio arm bridge
• In practical situation due to the presence of
magnetizing current and the load currents, the
voltage ratio slightly differ from the turns
ratio.
• • Those errors are ratio errors & load error.

24. • Current ratio
method
• •Full voltage is
applied across test
• capacitor
• • Standard capacitor
has to be built
• For high voltage.

25. Cf- Balancing capacitor
Cs- Standard capacitor
Ef – Proportional emf
At balance , there is no voltage
across the current
Comparator winding

28. • microvoid projectio
n
crack

29. Partial discharges
• Corona or gas discharge- due to non uniform
•
• field, sharp edge of electrodes etc.
•
•
• 2.Surface discharge- interfacing of different
•
• dielectric material results over stress
•
•
• 3.Cavity discharge- over stress due to cavities
•
•
• 4.Treeing channels

30. Partial Discharge
• Electrical discharge:
• Partial discharge:
• Discharge inception voltage
• Discharge extinction voltage
• Discharge magnitude
• Discharge energy
• Average current:
• Quadratic rate
• Discharge detector
• Sensitivity
• Resolution

31. Partial Discharge Measurement
Ca<<Cb<<Cc
Electrical insulation with imperfections or voids leading to partial discharges can
be represented by an electrical equivalent circuit

32. • Earlier the testing of insulators and other equipment was based
on the insulation resistance measurements, dissipation factor
measurements and breakdown tests.
• It was observed that the dissipation factor (tan delta) was
voltage dependent and hence became a criterion for the
monitoring of the high voltage insulation.
• weak points in an insulation like voids, cracks, and other
imperfections lead to internal or intermittent discharges in
the insulation.
• These imperfections being small were not revealed in
capacitance measurements but were revealed as power loss
components in contributing for an increase in the
dissipation factor. In modern terminology these are
designated as "partial discharges" which in course of time
reduce the strength of insulation leading to a total or
partial failure or breakdown of the insulation.

33. • If the sites of partial discharges can be located
inside an equipment, like in a power cable or a
transformer, it gives valuable information to
the insulation engineer about the regions of
greater stress and imperfections in the
fabrication.
• Based on this information, the designs can be
considerably improved.

34. • Electrical discharge: The movement of electrical charges
through an insulating (dielectric) medium, initiated by electron
avalanches.
• Partial discharge: An electrical discharge that only partially
bridges the dielectric or insulating medium between two
conductors. Examples are: internal discharges,surface
discharges and corona discharges. Internal discharges are
discharges in cavities or voids which lie inside the volume of
the dielectric or at the edges of conducting inclusions in a solid
or liquid insulating media.

35. • Surface discharges are discharges from the
conductor into a gas or a liquid medium and
form on the surface of the solid insulation not
covered by the conductor.
• Corona is a discharge in a gas or a liquid
insulation around the conductors that are
away or remote from the solid insulation.

36. • Discharge inception voltage is the lowest voltage at which
discharges of specified magnitude will recur when an
increasing a.c. voltage is applied.
• Discharge extinction voltage is the lowest voltage at which
discharges of specified magnitude will recur when an applied
a.c. voltage, which is more than the inception
voltage, is reduced.
• Discharge magnitude is the quantity of charge, as measured at
the terminals of a sample due to a single discharge.
• Discharge energy is the energy dissipated by a single
discharge.

37. • Average current is the average value of the
discharge current during a cycle due to a
single or multiple discharges. Ia, the average
current over an interval T can be expressed as

40. • Self-restoring insulation
• Self-restoring insulation is the insulation which
completely recovers its insulating properties after a
• disruptive discharge caused by the application of a test
voltage.
• Non-self-restoring insulation
• Non-self-restoring insulation is insulation which loses
its insulating properties, or does not recover them
• completely, after a disruptive discharge caused by the
application of a test voltage.

41. Partial Discharge Measurement using strain
detector
•Transformer –
free from internal
discharges
•Filter – to filter
pulses from
transformer
Dis Advantage : external signal not fully rejected

42. • HV Transformer is free from discharge
• • The resonant filter is used to prevent any pulses
starting from the capacitance of the windings and bushings
of the transformer
• • Cx is the test object- 100 pF to 0.1pF
• • Cc is the coupling capacitor
• • Zm is the detection impedance
• • The signal is developed across detection impedance Zm
• • Then the signal is passed through band pass filter
(10kHz frequency) ,amplifier and displayed Unit (on the
CRO & multi channel analyzer unit).

43. PARTIAL DISCHARGE MEASUREMENT
CIRCUIT

46. Narrow band detection scheme
•Zm is a parallel L-C circuit tuned to 500Hz.
•The bandpass filter has a bandwidth of about ± 10
kHz.
Wide band detection scheme
•Zm is an R-C network connected to a double tuned
transformer.
•The bandwidth is about 250 kHz with centre
frequency between 50 to 200 kHz.
•A wide band amplifier is used, and the signal is
displayed on the CRO

47. Balanced Detection Method
•Bridges tuned & balanced
at 50Hz
•Filter: used to block 50Hz
components
• signals in the range
5-50kHz are allowed to
pass through the filter
•Any external interference
are balanced out

48. •Dummy sample is also
used with standard
condenser
•Capacitance & tan
delta of dummy sample
are made
approximately equal
Disadv : if two discharge occurs in both samples ,they cancel out
Advantage: better rejection of external noise
use of wide frequency band with better resolution of
individual pulses

49. Calibration of Discharge Detectors
•Signal voltage developed
across Zm depends on Cx & Cc
•Calibrated by injecting a pulse
having a charge of known
magnitude into the detector
•Magnitude of charge injected is
qk=CkVk
•The output voltage of the pulse
generator can be varied from
10μV-100v in steps
•Ck vk
•CkVk * ((Cx+Cc)/Cc)

50. Discharge Detection in Power Cables
If Cc is neglected in comparison
with the detection impedance Zi,
the voltage signal across the
detection impedance becomes,
Voltage wave is given by,