Editing Partial discharge (section)

==Discharge detection and measuring systems==
With the partial discharge measurement, the dielectric condition of high voltage equipment can be evaluated, and [[electrical treeing]] in the insulation can be detected and located. Partial discharge measurement can localize the damaged part of an insulated system.

Data collected during partial discharge testing is compared to measurement values of the same cable gathered during the acceptance-test or to factory quality control standards. This allows simple and quick classification of the dielectric condition (new, strongly aged, faulty) of the device under test and appropriate maintenance and repair measures may be planned and organized in advance.

Partial discharge measurement is applicable  to cables and accessories with various insulation materials, such as [[polyethylene]] or paper-insulated lead-covered (PILC) cable. Partial discharge measurement is routinely carried out to assess the condition of the insulation system of rotating machines (motors and generators), [[transformer]]s, and gas-insulated [[switchgear]].

=== Partial discharge measurement system ===

A partial discharge measurement system basically consists of:

* a cable or other object being tested
* a coupling capacitor of low inductance design
* a high-voltage supply with low background noise
* high-voltage connections
* a high voltage filter to reduce background noise from the power supply
* a partial discharge detector
* PC software for analysis

A partial discharge detection system for in-service, energized electric power equipment:
* a cable, transformer, or any MV/HV power equipment
* Ultra High Frequency Sensor (UHF) Detection Bandwidth 300&nbsp;MHz-1.5&nbsp;GHz
* High Frequency Current Transformer (HFCT) Bandwidth 500&nbsp;kHz-50&nbsp;MHz
* [[Ultrasonic transducer|Ultrasonic microphone]] with center frequency 40&nbsp;kHz
* Acoustic Contact Sensor with detection bandwidth 20&nbsp;kHz - 300&nbsp;kHz
* TEV sensor or coupling capacitor  3&nbsp;MHz-100&nbsp;MHz
* Phase-resolved analysis system to compare pulse timing to AC frequency

=== The principle of partial discharge measurement ===
A number of discharge detection schemes and partial discharge measurement methods have been invented since the importance of PD was realized in the late 20th century. Partial discharge currents tend to be of short duration and have rise times in the [[nanosecond]] realm. On an [[oscilloscope]], the discharges appear as evenly spaced burst events that occur at the peak of the sinewave. Random events are arcing or sparking.
The usual way of quantifying partial discharge magnitude is in pico[[coulomb]]s. The intensity of partial discharge is displayed versus time.

An automatic analysis of the reflectograms collected during the partial discharge measurement – using a method referred to as [[time domain reflectometry]] (TDR) – allows the location of insulation irregularities. They are displayed in a partial discharge mapping format.

A phase-related depiction of the partial discharges provides additional information, useful for the evaluation of the device under test.

===Calibration setup===
The actual charge change that occurs due to a PD event is not directly measurable, therefore, ''apparent charge'' is used instead. The apparent charge (q) of a PD event is the charge that, if injected between the terminals of the [[device under test]], would change the voltage across the terminals by an amount equivalent to the PD event. This can be modeled by the equation:

:<math>q= C_b \Delta(V_c)</math>

Apparent charge is not equal to the actual amount of changing charge at the PD site, but can be directly measured and calibrated.  'Apparent charge' is usually expressed in pico[[coulomb]]s.

This is measured by calibrating the voltage of the spikes against the voltages obtained from a calibration unit discharged into the measuring instrument. The calibration unit is quite simple in operation and merely comprises a square wave generator in series with a capacitor connected across the sample. Usually these are triggered optically to enable calibration without entering a dangerous high voltage area. Calibrators are usually disconnected during the discharge testing.

===Laboratory methods===
* Wideband PD detection circuits
*:In [[wideband]] [[detection]], the  impedance usually comprises a low [[Q factor|Q]] parallel-resonant [[RLC circuit]]. This circuit tends to attenuate the exciting voltage (usually between 50 and 60 [[hertz|Hz]]) and amplify the voltage generated due to the discharges.
* Tuned (narrow band) detection circuits
* Differential discharge bridge methods
* Acoustic and Ultrasonic methods

===Field testing methods===
Field measurements preclude the use of a [[Faraday cage]] and the energising supply can also be a compromise from the ideal. Field measurements are therefore prone to noise and may be consequently less sensitive.<ref>D. F. Warne ''Advances in high voltage engineering'',  [[Institution of Engineering and Technology|Institution of Electrical Engineers]], 2004 {{ISBN|0-85296-158-8}},  page 166</ref><ref name="IEEE">{{cite book|chapter=Testing Distribution Switchgear for Partial Discharge in the Laboratory and the Field|publisher=[[IEEE]]|date=2008-06-12|doi=10.1109/ELINSL.2008.4570430|isbn=978-1-4244-2091-9|title=Conference Record of the 2008 IEEE International Symposium on Electrical Insulation|last1=Davies|first1=N.|last2=Jones|first2=D.|pages=716–719}}</ref>

Factory quality PD tests in the field require equipment that may not be readily available, therefore other methods have been developed for field measurement which, while not as sensitive or accurate as standardized measurements, are substantially more convenient. By necessity field measurements have to be quick, safe and simple if they are to be widely applied by owners and operators of MV and HV assets.

'''Transient Earth Voltages (TEVs)''' are induced voltage spikes on the surface of the surrounding metalwork. TEVs were first discovered in 1974 by Dr John Reeves<ref>Davies, N., Tang, J.C.Y., Shiel, P., (2007), Benefits and Experiences of Non-Intrusive Partial Discharge Measurements on MV Switchgear, CIRED 2007, Paper 0475.</ref> of [http://www.eatechnology.com/ EA Technology]. TEVs occur because the partial discharge creates current spikes in the conductor and hence also in the earthed metal surrounding the conductor. Dr John Reeves established that TEV signals are directly proportional to the condition of the insulation for all switchgear of the same type measured at the same point. TEV signals are measured in mV and typically displayed in dBmV. TEV pulses are full of high frequency components and hence the earthed metalwork presents a considerable impedance to ground. Therefore, voltage spikes are generated. These will stay on the inner surface of surrounding metalwork (to a depth of approximately 0.5&nbsp;[[μm]] in [[mild steel]] at 100&nbsp;MHz) and loop around to the outer surface wherever there is an electrical discontinuity in the metalwork. There is a secondary effect whereby electromagnetic waves generated by the partial discharge also generate TEVs on the surrounding metalwork – the surrounding metalwork acting like an antenna. TEVs are a very convenient phenomenon for measuring and detecting partial discharges as they can be detected without making an electrical connection or removing any panels. While this method may be useful to detect some issues in switchgear and surface tracking on internal components, the sensitivity is not likely to be sufficient to detect issues within solid dielectric cable systems.

'''Ultrasonic''' measurement relies on fact that the partial discharge will emit sound waves. The frequency for emissions is "white" noise in nature and therefore produces ultrasonic structure waves through the solid or liquid filled electrical component. Using a structure borne ultrasonic sensor on the exterior of the item under examination, internal partial discharge can be detected and located when the sensor is placed closest to the source.

'''HFCT Method''' This method is ideal for detecting and determining the severity of the PD by burst interval measurement.  The closer the bursts get to "zero voltage crossing" the more severe and critical the PD fault is.  Location of the fault area is accomplished using AE described above.

'''Electro Magnetic Field''' detection picks up the radio waves generated by the partial discharge. As noted before the radio waves can generate TEVs on the surrounding metalwork. More sensitive measurement, particularly at higher voltages, can be achieved using in built UHF antennas or external antenna mounted on insulating spacers in the surrounding metalwork.

'''Directional Coupler''' detection picks up the signals emanating from a partial discharge. This method is ideal for joints and accessories, with the sensors being located on the semicon layers at the joint or accessory.<ref>Craatz P., Plath R., Heinrich R., Kalkner W.: Sensitive On-Site PD Measurement and Location using Directional Coupler Sensors in 110kV Prefabricated Joints, 11th ISH99, London, paper 5.317 P5</ref>