Partial discharge
Encyclopedia
In electrical engineering
, partial discharge (PD) is a localised dielectric breakdown of a small portion of a solid or fluid electrical insulation
system under high voltage
stress, which does not bridge the space between two conductors. While a corona discharge
is usually revealed by a relatively steady glow or brush discharge
in air, partial discharges within solid insulation system are not visible.
PD can occur in a gaseous, liquid or solid insulating medium. It often starts within gas voids, such as voids in solid epoxy insulation or bubbles in transformer oil
. Protracted partial discharge can erode solid insulation and eventually lead to breakdown of insulation.
s. Since discharges are limited to only a portion of the insulation, the discharges only partially bridge the distance between electrodes. PD can also occur along the boundary between different insulating materials.
Partial discharges within an insulating material are usually initiated within gas-filled voids within the dielectric. Because the dielectric constant
of the void is considerably less than the surrounding dielectric, the electric field
across the void is significantly higher than across an equivalent distance of dielectric. If the voltage stress across the void is increased above the corona
inception voltage (CIV) for the gas within the void, then PD activity will start within the void.
PD can also occur along the surface of solid insulating materials if the surface tangential electric field is high enough to cause a breakdown along the insulator surface. This phenomenon commonly manifests itself on overhead line insulators, particularly on contaminated insulators during days of high humidity. Overhead line insulators use air as their insulation medium.
in series with the sample and then view the generated voltage on an oscilloscope
via a matched coaxial
cable.
When PD occurs, electromagnetic waves propagate away from the discharge site in all directions. Detection of the high-frequency pulses can identify the existence and location of partial discharge.
Whilst tan delta measurement allows detection of water trees, partial discharge measurement is suitable for detection and location of electrical trees.
Data collected during the procedure is compared to measurement values of the same cable gathered during the acceptance-test.
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 lead-covered paper-insulated cable. Partial discharge measurement is routinely carried out to asses the condition of the insulation system of rotating machines (motors and generators), transformer
s and gas-insulated switchgear
.
s. On an oscilloscope
, the discharges look like randomly occurring 'spikes' or pulses.
The usual way of quantifying partial discharge magnitude is in picocoulombs. 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.
, would change the voltage across the terminals by an amount equivalent to the PD event. This can be modeled by the equation:
The 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 picocoulombs.
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 no voltage area. Calibrators are usually disconnected during the discharge testing.
In wideband
detection, the impedance usually comprises a low Q
parallel-resonant RLC circuit
. This circuit tends to attenuate the exciting voltage (usually between 50 and 60 Hz
) and to amplify the voltage generated due to the discharges.
. The effects of PD within high voltage
cables and equipment can be very serious, ultimately leading to complete failure. The cumulative effect of partial discharges within solid dielectrics is the formation of numerous, branching partially conducting discharge channels, a process called treeing
. Repetitive discharge events cause irreversible mechanical and chemical deterioration of the insulating material. Damage is caused by the energy dissipated by high energy electrons or ions, ultraviolet light from the discharges, ozone attacking the void walls, and cracking as the chemical breakdown processes liberate gases at high pressure. The chemical transformation of the dielectric also tends to increase the electrical conductivity of the dielectric material surrounding the voids. This increases the electrical stress in the (thus far) unaffected gap region, accelerating the breakdown process. A number of inorganic dielectrics, including glass
, porcelain
, and mica
, are significantly more resistant to PD damage than organic and polymer
dielectrics.
In paper-insulated high-voltage cables, partial discharges begin as small pinholes penetrating the paper windings that are adjacent to the electrical conductor or outer sheath. As PD activity progresses, the repetitive discharges eventually cause permanent chemical changes within the affected paper layers and impregnating dielectric fluid. Over time, partially conducting carbonized trees are formed. This places greater stress on the remaining insulation, leading to further growth of the damaged region, resistive heating along the tree, and further charring (sometimes called tracking). This eventually culminates in the complete dielectric failure of the cable and, typically, an electrical explosion
.
PD generally dissipates energy in the form of heat. Sometimes, however, the energy is lost in the form of sound and light, such as the hissing and dim glowing from the overhead line insulators. Heat energy dissipation may cause thermal degradation of the insulation, although the level is generally low. For high voltage equipment, the integrity of the insulation can be confirmed by monitoring the PD activities that occur through the equipment's life. To ensure supply reliability and long-term operational sustainability, PD in high-voltage electrical equipment should be monitored closely with early warning signals for inspection and maintenance.
PD can be prevented through careful design and material selection. In critical high voltage equipment, the integrity of the insulation is confirmed using PD detection equipment during the manufacturing stage as well as periodically through the equipment's useful life. PD prevention and detection are essential to ensure reliable, long-term operation of high voltage equipment used by electric power utilities
.
Electrical engineering
Electrical engineering is a field of engineering that generally deals with the study and application of electricity, electronics and electromagnetism. The field first became an identifiable occupation in the late nineteenth century after commercialization of the electric telegraph and electrical...
, partial discharge (PD) is a localised dielectric breakdown of a small portion of a solid or fluid electrical insulation
Electrical insulation
thumb|250px|[[Coaxial Cable]] with dielectric insulator supporting a central coreThis article refers to electrical insulation. For insulation of heat, see Thermal insulation...
system under high voltage
High voltage
The term high voltage characterizes electrical circuits in which the voltage used is the cause of particular safety concerns and insulation requirements...
stress, which does not bridge the space between two conductors. While a corona discharge
Corona discharge
In electricity, a corona discharge is an electrical discharge brought on by the ionization of a fluid surrounding a conductor that is electrically energized...
is usually revealed by a relatively steady glow or brush discharge
Brush discharge
A brush discharge is a type of corona discharge that takes place between two electrodes embedded in a nonconducting medium and is characterized by nonsparking, faintly luciferous furcations composed of ionized particles....
in air, partial discharges within solid insulation system are not visible.
PD can occur in a gaseous, liquid or solid insulating medium. It often starts within gas voids, such as voids in solid epoxy insulation or bubbles in transformer oil
Transformer oil
Transformer oil or insulating oil is usually a highly-refined mineral oil that is stable at high temperatures and has excellent electrical insulating properties. It is used in oil-filled transformers, some types of high voltage capacitors, fluorescent lamp ballasts, and some types of high voltage...
. Protracted partial discharge can erode solid insulation and eventually lead to breakdown of insulation.
Discharge mechanism
PD usually begins within voids, cracks, or inclusions within a solid dielectric, at conductor-dielectric interfaces within solid or liquid dielectrics, or in bubbles within liquid dielectricDielectric
A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing dielectric...
s. Since discharges are limited to only a portion of the insulation, the discharges only partially bridge the distance between electrodes. PD can also occur along the boundary between different insulating materials.
Partial discharges within an insulating material are usually initiated within gas-filled voids within the dielectric. Because the dielectric constant
Dielectric constant
The relative permittivity of a material under given conditions reflects the extent to which it concentrates electrostatic lines of flux. In technical terms, it is the ratio of the amount of electrical energy stored in a material by an applied voltage, relative to that stored in a vacuum...
of the void is considerably less than the surrounding dielectric, the electric field
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
across the void is significantly higher than across an equivalent distance of dielectric. If the voltage stress across the void is increased above the corona
Corona discharge
In electricity, a corona discharge is an electrical discharge brought on by the ionization of a fluid surrounding a conductor that is electrically energized...
inception voltage (CIV) for the gas within the void, then PD activity will start within the void.
PD can also occur along the surface of solid insulating materials if the surface tangential electric field is high enough to cause a breakdown along the insulator surface. This phenomenon commonly manifests itself on overhead line insulators, particularly on contaminated insulators during days of high humidity. Overhead line insulators use air as their insulation medium.
Partial discharge equivalent circuit
The equivalent circuit of a dielectric incorporating a cavity can be modeled as a capacitive voltage divider in parallel with another capacitor. The upper capacitor of the divider represents the parallel combination of the capacitances in series with the void and the lower capacitor represents the capacitance of the void. The parallel capacitor represents the remaining unvoided capacitance of the sample.Partial discharge currents
When partial discharge is initiated, high frequency transient current pulses will appear and persist for nano-seconds to a micro-second, then disappear and reappear repeatedly. PD currents are difficult to measure because of their small magnitude and short duration. The event may be detected as a very small change in the current drawn by the sample under test. One method of measuring these currents is to put a small current-measuring resistorResistor
A linear resistor is a linear, passive two-terminal electrical component that implements electrical resistance as a circuit element.The current through a resistor is in direct proportion to the voltage across the resistor's terminals. Thus, the ratio of the voltage applied across a resistor's...
in series with the sample and then view the generated voltage on an oscilloscope
Oscilloscope
An oscilloscope is a type of electronic test instrument that allows observation of constantly varying signal voltages, usually as a two-dimensional graph of one or more electrical potential differences using the vertical or 'Y' axis, plotted as a function of time,...
via a matched coaxial
Coaxial
In geometry, coaxial means that two or more forms share a common axis; it is the three-dimensional linear analogue of concentric.Coaxial cable, as a common example, has a wire conductor in the centre a circumferential outer conductor and an insulating medium called the dielectric separating...
cable.
When PD occurs, electromagnetic waves propagate away from the discharge site in all directions. Detection of the high-frequency pulses can identify the existence and location of partial discharge.
Discharge detection and measuring systems
With the partial discharge measurement, the dielectric condition of high voltage equipment can be evaluated, and treeing in the insulation can be detected.Whilst tan delta measurement allows detection of water trees, partial discharge measurement is suitable for detection and location of electrical trees.
Data collected during the procedure is compared to measurement values of the same cable gathered during the acceptance-test.
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
Polyethylene
Polyethylene or polythene is the most widely used plastic, with an annual production of approximately 80 million metric tons...
or lead-covered paper-insulated cable. Partial discharge measurement is routinely carried out to asses the condition of the insulation system of rotating machines (motors and generators), transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
s and gas-insulated switchgear
Switchgear
The term switchgear, used in association with the electric power system, or grid, refers to the combination of electrical disconnects, fuses and/or circuit breakers used to isolate electrical equipment. Switchgear is used both to de-energize equipment to allow work to be done and to clear faults...
.
Partial discharge measurement system
A partial discharge measurement system basically consists of:- a test object
- 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
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 realised early in the last century. Partial discharge currents tend to be of short duration and have rise times on the order of nanosecondNanosecond
A nanosecond is one billionth of a second . One nanosecond is to one second as one second is to 31.7 years.The word nanosecond is formed by the prefix nano and the unit second. Its symbol is ns....
s. On an oscilloscope
Oscilloscope
An oscilloscope is a type of electronic test instrument that allows observation of constantly varying signal voltages, usually as a two-dimensional graph of one or more electrical potential differences using the vertical or 'Y' axis, plotted as a function of time,...
, the discharges look like randomly occurring 'spikes' or pulses.
The usual way of quantifying partial discharge magnitude is in picocoulombs. 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. 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 testDevice under test
Device under test , also known as unit under test , is a term commonly used to refer to a manufactured product undergoing testing.-In semiconductor testing:...
, would change the voltage across the terminals by an amount equivalent to the PD event. This can be modeled by the equation:
The 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 picocoulombs.
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 no voltage area. Calibrators are usually disconnected during the discharge testing.
Laboratory methods
- Wideband PD detection circuits
In wideband
Wideband
In communications, wideband is a relative term used to describe a wide range of frequencies in a spectrum. A system is typically described as wideband if the message bandwidth significantly exceeds the channel's coherence bandwidth....
detection, the impedance usually comprises a low Q
Q factor
In physics and engineering the quality factor or Q factor is a dimensionless parameter that describes how under-damped an oscillator or resonator is, or equivalently, characterizes a resonator's bandwidth relative to its center frequency....
parallel-resonant RLC circuit
RLC circuit
An RLC circuit is an electrical circuit consisting of a resistor, an inductor, and a capacitor, connected in series or in parallel. The RLC part of the name is due to those letters being the usual electrical symbols for resistance, inductance and capacitance respectively...
. This circuit tends to attenuate the exciting voltage (usually between 50 and 60 Hz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
) and to amplify the voltage generated due to the discharges.
- Tuned (narrow band) detection circuits
- Differential discharge bridge methods
- Acoustic and Ultrasonic methods
Field testing methods
The field testing of plant and equipment in service by the above methods, although it has been done, is not very convenient. Field testing for PD activity emphasizes basic indication of discharge activity over absolute accuracy of measurement. For example, transient earth voltage detectors sense the electromagnetic field at gaps in the metal enclosures of switchgear; by use of two or more sensors and measurement of the differences in reception time of an impulse, location of the impulses may be inferred.Effects of partial discharge in insulation systems
Once begun, PD causes progressive deterioration of insulating materials, ultimately leading to electrical breakdownElectrical breakdown
The term electrical breakdown or electric breakdown has several similar but distinctly different meanings. For example, the term can apply to the failure of an electric circuit....
. The effects of PD within high voltage
High voltage
The term high voltage characterizes electrical circuits in which the voltage used is the cause of particular safety concerns and insulation requirements...
cables and equipment can be very serious, ultimately leading to complete failure. The cumulative effect of partial discharges within solid dielectrics is the formation of numerous, branching partially conducting discharge channels, a process called treeing
Electrical treeing
Within the field of electrical engineering, treeing is an electrical pre-breakdown phenomenon. It is a damaging process due to partial discharges and progresses through the stressed dielectric insulation, whose path resembles the form of a tree....
. Repetitive discharge events cause irreversible mechanical and chemical deterioration of the insulating material. Damage is caused by the energy dissipated by high energy electrons or ions, ultraviolet light from the discharges, ozone attacking the void walls, and cracking as the chemical breakdown processes liberate gases at high pressure. The chemical transformation of the dielectric also tends to increase the electrical conductivity of the dielectric material surrounding the voids. This increases the electrical stress in the (thus far) unaffected gap region, accelerating the breakdown process. A number of inorganic dielectrics, including glass
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
, porcelain
Porcelain
Porcelain is a ceramic material made by heating raw materials, generally including clay in the form of kaolin, in a kiln to temperatures between and...
, and mica
Mica
The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition...
, are significantly more resistant to PD damage than organic and polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
dielectrics.
In paper-insulated high-voltage cables, partial discharges begin as small pinholes penetrating the paper windings that are adjacent to the electrical conductor or outer sheath. As PD activity progresses, the repetitive discharges eventually cause permanent chemical changes within the affected paper layers and impregnating dielectric fluid. Over time, partially conducting carbonized trees are formed. This places greater stress on the remaining insulation, leading to further growth of the damaged region, resistive heating along the tree, and further charring (sometimes called tracking). This eventually culminates in the complete dielectric failure of the cable and, typically, an electrical explosion
Explosion
An explosion is a rapid increase in volume and release of energy in an extreme manner, usually with the generation of high temperatures and the release of gases. An explosion creates a shock wave. If the shock wave is a supersonic detonation, then the source of the blast is called a "high explosive"...
.
PD generally dissipates energy in the form of heat. Sometimes, however, the energy is lost in the form of sound and light, such as the hissing and dim glowing from the overhead line insulators. Heat energy dissipation may cause thermal degradation of the insulation, although the level is generally low. For high voltage equipment, the integrity of the insulation can be confirmed by monitoring the PD activities that occur through the equipment's life. To ensure supply reliability and long-term operational sustainability, PD in high-voltage electrical equipment should be monitored closely with early warning signals for inspection and maintenance.
PD can be prevented through careful design and material selection. In critical high voltage equipment, the integrity of the insulation is confirmed using PD detection equipment during the manufacturing stage as well as periodically through the equipment's useful life. PD prevention and detection are essential to ensure reliable, long-term operation of high voltage equipment used by electric power utilities
Electrical power industry
The electric power industry provides the production and delivery of electric energy, often known as power, or electricity, in sufficient quantities to areas that need electricity through a grid connection. The grid distributes electrical energy to customers...
.
International Standards
- IECInternational Electrotechnical CommissionThe International Electrotechnical Commission is a non-profit, non-governmental international standards organization that prepares and publishes International Standards for all electrical, electronic and related technologies – collectively known as "electrotechnology"...
60270:2000/BS EN 60270:2001 "High-Voltage Test Techniques - Partial Discharge Measurements" - IEEE 400-2001 "IEEE Guide for Field Testing and Evaluation of the Insulation of Shielded Power Cable Systems"
- IEEE 1434-2000 "IEEE Trial-Use Guide to the Measurement of Partial Discharges in Rotating Machinery"
External links
- Comprehensive on-line resource on partial discharge
- About Partial Discharge
- Partial discharge measurement on power transformers
- What is Partial Discharge (PD)
- Advanced cable diagnostics by partial discharge measurement
- What is Partial Discharge in High Voltage Cables, Switchgear, Transformers and Motors?
- An introduction to Partial Discharge
- What is Partial Discharge?