ASTM D1868-20

Standard Test Method for Detection and Measurement of Partial Discharge (Corona) Pulses in Evaluation of Insulation Systems

SIGNIFICANCE AND USE
5.1 The presence of partial discharges (corona) at operating voltage in an insulation system has the potential to result in a significant reduction in the life of the insulating material. Some materials are more susceptible to such discharge damage than others. This characteristic can be investigated using Test Method D2275.  
5.2 The presence of partial discharges (corona) in an apparently solid insulation is a potential indication of the existence of internal cavities. Partial discharge tests have been useful in the design and inspection of molded, laminated, and composite insulation, as well as specimens in the form of cables, capacitors, transformers, bushings, stator bars, and rotating machines (1-9), (13), (12). See also AEIC CS5-87, ICEA T-24-380, IEEE 48, IEEE C57 113-1991, IEEE C57 124-1991, and IEEE 1434-2005.  
5.3 Partial discharge (corona) inception and extinction voltages are used in the determination of the limiting voltage at which an insulation system will operate free of such discharges. The extinction voltage is often substantially lower than the inception voltage. Where the operating voltage is below the inception voltage but above the extinction voltage, it is possible that a transient over-voltage will initiate discharges which then continue until the voltage is lowered below the extinction voltage. Inception and extinction voltages depend upon many factors, including temperature and the rate at which the voltage is changed. After a time at a voltage, it is possible that discharges will start and stop in a nonuniform and unpredictable fashion, especially for discharges within cavities in certain materials, in particular if the discharge degradation products formed are conductive (1), (5).  
5.4 The magnitude (pulse height) of a partial discharge is an indication of the amount of energy that it dissipates in the insulation system. Partial discharge magnitude and pulse rate are useful in estimating the rate, or change of rate, at which deterio...
SCOPE
1.1 This test method covers the detection and measurement of partial discharge (corona) pulses at the terminals of an insulation system under an applied test voltage, including the determination of partial discharge (corona) inception and extinction voltages as the test voltage is raised and lowered. This test method is also useful in determining quantities such as apparent charge and pulse repetition rate together with such integrated quantities as average current, quadratic rate, and power. This test method is useful for test voltages ranging in frequency from zero (direct voltage) to approximately 2000 Hz.  
1.2 This test method is directly applicable to a simple insulation system that can be represented as a two-terminal capacitor (1), (2).2  
1.3 This test method is also applicable to (distributed parameter) insulation systems such as high-voltage cable. Consideration must be given to attenuation and reflection phenomena in this type of system. Further information on distributed parameter systems of cables, transformers, and rotating machines will be found in Refs (1-9). (See AEIC CS5-87, IEEE C57 113-1991, IEEE C57 124-1991, and IEEE 1434-2005.)  
1.4 This test method can be applied to multi-terminal insulation systems, but at some loss in accuracy, especially where the insulation of inductive windings is involved.  
1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use. Specific precaution statements are given in Sections 8 and 14.  
1.6 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendation...

General Information

• Status: Published
• Publication Date: 29-Feb-2020
• ICS: 29.080.30 - Insulation systems
• Technical Committee: D09 - Electrical and Electronic Insulating Materials
• Drafting Committee: D09.12 - Electrical Tests

Relations

• Replaces: ASTM D1868-13 - Standard Test Method for Detection and Measurement of Partial Discharge (Corona) Pulses in Evaluation of Insulation Systems
• Effective Date: 01-Mar-2020
• Refers: ASTM D149-20 - Standard Test Method for Dielectric Breakdown Voltage and Dielectric Strength of Solid Electrical Insulating Materials at Commercial Power Frequencies
• Effective Date: 01-Jan-2020
• Refers: ASTM D618-08 - Standard Practice for Conditioning Plastics for Testing
• Effective Date: 01-Nov-2008
• Refers: ASTM D2275-01(2008)e1 - Standard Test Method for Voltage Endurance of Solid Electrical Insulating Materials Subjected to Partial Discharges (Corona) on the Surface
• Effective Date: 01-May-2008
• Refers: ASTM D3382-07 - Standard Test Methods for Measurement of Energy and Integrated Charge Transfer Due to Partial Discharges (Corona) Using Bridge Techniques
• Effective Date: 01-May-2007
• Refers: ASTM D618-05 - Standard Practice for Conditioning Plastics for Testing
• Effective Date: 01-Nov-2005
• Refers: ASTM D149-97a(2004) - Standard Test Method for Dielectric Breakdown Voltage and Dielectric Strength of Solid Electrical Insulating Materials at Commercial Power Frequencies
• Effective Date: 01-Mar-2004
• Refers: ASTM D2275-95 - Standard Test Method for Voltage Endurance of Solid Electrical Insulating Materials Subjected to Partial Discharges (Corona) on the Surface
• Effective Date: 10-Mar-2001
• Refers: ASTM D2275-01 - Standard Test Method for Voltage Endurance of Solid Electrical Insulating Materials Subjected to Partial Discharges (Corona) on the Surface
• Effective Date: 10-Mar-2001
• Refers: ASTM D3382-95 - Standard Test Methods for Measurement of Energy and Integrated Charge Transfer Due to Partial Discharges (Corona) Using Bridge Techniques
• Effective Date: 01-Jan-2001
• Refers: ASTM D3382-95(2001)e1 - Standard Test Methods for Measurement of Energy and Integrated Charge Transfer Due to Partial Discharges (Corona) Using Bridge Techniques
• Effective Date: 01-Jan-2001
• Refers: ASTM D618-00 - Standard Practice for Conditioning Plastics for Testing
• Effective Date: 10-Nov-2000
• Refers: ASTM D149-97a - Standard Test Method for Dielectric Breakdown Voltage and Dielectric Strength of Solid Electrical Insulating Materials at Commercial Power Frequencies
• Effective Date: 01-Jan-1997
• Referred By: ASTM D2275-22 - Standard Test Method for Voltage Endurance of Solid Electrical Insulating Materials Subjected to Partial Discharges (Corona) on the Surface
• Effective Date: 01-Mar-2020
• Referred By: ASTM D3032-21a - Standard Test Methods for Hookup Wire Insulation
• Effective Date: 01-Mar-2020

Overview

ASTM D1868-20: Standard Test Method for Detection and Measurement of Partial Discharge (Corona) Pulses in Evaluation of Insulation Systems provides guidance for detecting and quantifying partial discharges, commonly referred to as corona, within electrical insulation systems. This standard test method is essential for evaluating the integrity and performance of insulating materials and systems in high-voltage electrical equipment. It helps identify defects and imperfections, such as internal cavities, which may compromise reliability or reduce the service life of the insulation.

Partial discharge testing according to ASTM D1868-20 is applicable to a wide range of insulation systems, including cables, capacitors, transformers, bushings, stator bars, and rotating machines. By offering procedures to determine key parameters such as partial discharge inception and extinction voltages, this standard plays a critical role in quality assurance and system design.

Key Topics

• Detection of Partial Discharge (Corona) Pulses: The method covers the identification and measurement of corona pulses at the terminals of insulation systems subjected to an applied test voltage.
• Measurement of Critical Voltages: Procedures are provided to determine the inception voltage (the lowest voltage at which continuous discharges occur) and extinction voltage (the highest voltage at which such discharges cease).
• Quantitative Analysis: The method enables assessment of apparent charge, pulse repetition rate, average current, quadratic rate, and power dissipation associated with partial discharges.
• Applicability to Various Systems: The test can be performed on simple two-terminal systems and complex, distributed parameter systems like high-voltage cables, with specific guidance on adjusting for attenuation and reflection phenomena.
• Significance of Findings: The presence of partial discharge often signals the existence of internal defects or cavities within an insulation system. Regular testing helps in identifying vulnerable materials and assessing the risk of premature insulation failure.

Applications

The ASTM D1868-20 partial discharge test method delivers substantial practical value across multiple sectors:

• Design and Development: Manufacturers of electrical insulation systems can evaluate prototypes for susceptibility to partial discharge and optimize materials and construction methods accordingly.
• Quality Control: Routine use during production ensures consistency and reliability in cables, capacitors, transformers, and other high-voltage components.
• Commissioning and Maintenance: Utilities and industrial users apply this method to assess new installations or diagnose existing equipment, helping prevent unexpected failures.
• Research and Material Selection: Engineers and researchers can compare the partial discharge characteristics of different insulating materials, supporting informed decisions in system design.
• Safety and Compliance: By identifying the inception and extinction voltages, operators ensure that insulation systems operate well below the thresholds where damaging partial discharge may occur.

Related Standards

For comprehensive assessment and alignment with industry best practices, ASTM D1868-20 should be considered alongside the following related standards:

• ASTM D149 - Test Method for Dielectric Breakdown Voltage and Dielectric Strength of Solid Electrical Insulating Materials
• ASTM D2275 - Test Method for Voltage Endurance of Solid Electrical Insulating Materials Subjected to Partial Discharges
• ASTM D3382 - Test Methods for Measurement of Energy and Integrated Charge Transfer Due to Partial Discharges
• AEIC CS5-87 - Specifications for Thermoplastic and Crosslinked Polyethylene Insulated Shielded Power Cables
• ICEA T-24-380 - Guide for Partial Discharge Procedure
• IEEE 48 - Standard Test Procedures and Requirements for High Voltage Alternating Current Cable Terminations
• IEEE C57 113-1991 - Guide for Partial Discharge Measurement in Liquid-Filled Transformers and Shunt Reactors
• IEEE C57 124-1991 - Recommended Practice for Detection of Partial Discharge and Apparent Charge Measurement in Dry-Type Transformers
• IEEE 1434-2005 - Guide to the Measurement of Partial Discharges in Rotating Machinery

By integrating ASTM D1868-20 into routine insulation system evaluation, organizations can enhance equipment reliability, extend operational life, and meet critical safety and performance standards in the electrical and power industries.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: D1868 − 20
Standard Test Method for
Detection and Measurement of Partial Discharge (Corona)
Pulses in Evaluation of Insulation Systems
This standard is issued under the fixed designation D1868; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the U.S. Department of Defense.
1. Scope* 1.6 This international standard was developed in accor-
dance with internationally recognized principles on standard-
1.1 This test method covers the detection and measurement
ization established in the Decision on Principles for the
of partial discharge (corona) pulses at the terminals of an
Development of International Standards, Guides and Recom-
insulation system under an applied test voltage, including the
mendations issued by the World Trade Organization Technical
determination of partial discharge (corona) inception and
Barriers to Trade (TBT) Committee.
extinction voltages as the test voltage is raised and lowered.
This test method is also useful in determining quantities such
2. Referenced Documents
as apparent charge and pulse repetition rate together with such
2.1 ASTM Standards:
integrated quantities as average current, quadratic rate, and
D149Test Method for Dielectric Breakdown Voltage and
power. This test method is useful for test voltages ranging in
DielectricStrengthofSolidElectricalInsulatingMaterials
frequency from zero (direct voltage) to approximately
at Commercial Power Frequencies
2000Hz.
D618Practice for Conditioning Plastics for Testing
1.2 This test method is directly applicable to a simple
D2275Test Method for Voltage Endurance of Solid Electri-
insulation system that can be represented as a two-terminal
cal Insulating Materials Subjected to Partial Discharges
capacitor (1), (2).
(Corona) on the Surface
1.3 This test method is also applicable to (distributed
D3382Test Methods for Measurement of Energy and Inte-
parameter) insulation systems such as high-voltage cable.
grated Charge Transfer Due to Partial Discharges (Co-
Consideration must be given to attenuation and reflection
rona) Using Bridge Techniques
phenomena in this type of system. Further information on
2.2 IEEE Standards
distributed parameter systems of cables, transformers, and
IEEE 48Standard Test Procedures and Requirements for
rotating machines will be found in Refs (1-9). (See AEIC
High Voltage Alternating Current Cable Terminations
CS5-87, IEEE C57 113-1991, IEEE C57 124-1991, and IEEE
IEEE 1434-2005Guide to the Measurement of Partial Dis-
1434-2005.)
charges in Rotating Machinery
IEEE C57 113-1991Guide for PD Measurement in Liquid-
1.4 This test method can be applied to multi-terminal
Filled Power Transformers and Shunt Reactors
insulation systems, but at some loss in accuracy, especially
IEEE C57 124-1991Recommended Practice for the Detec-
where the insulation of inductive windings is involved.
tion of PD and the Measurement of Apparent Charge in
1.5 This standard does not purport to address all of the
Dry-Type Transformers
safety concerns, if any, associated with its use. It is the
2.3 Other Documents:
responsibility of the user of this standard to establish appro-
AEIC CS5-87Specifications for Thermoplastic and Cross-
priate safety, health, and environmental practices and deter-
linked Polyethlene Insulated Shielded Power Cables
mine the applicability of regulatory limitations prior to use.
th 5
Rated 5 through 35 kV (9 Edition) October 1987
Specific precaution statements are given in Sections 8 and 14.
1 3
This test method is under the jurisdiction of ASTM Committee D09 on For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Electrical and Electronic Insulating Materials and is the direct responsibility of contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
Subcommittee D09.12 on Electrical Tests. Standards volume information, refer to the standard’s Document Summary page on
Current edition approved March 1, 2020. Published March 2020. Originally the ASTM website.
approved in 1961. Last previous edition approved in 2013 as D1868–13. DOI: Available from Institute of Electrical and Electronics Engineers, Inc. (IEEE),
10.1520/D1868-20. 445 Hoes Ln., Piscataway, NJ 08854-4141, http://www.ieee.org.
2 5
Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof Available from Association of Edison Illuminating Companies (AEIC), P.O.
this test method. Box 2641, Birmingham, AL 35291-0992, http://www.aeic.org.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D1868 − 20
ICEA T-24-380Guide for Partial Discharge Procedure detected at the terminals of the system under open-circuit
conditions. The shape, rise time, and magnitude of the voltage
3. Terminology
V of the partial discharge pulse are dependent upon the
t
geometry of the cavity, its size, nature of its boundaries, the
3.1 Definitions:
typeofgasandthepressurewithinaswellastheparametersof
3.1.1 The following terms are presented in a developing
the transmission medium between the discharge site and the
sequence; it is best that they be read in their entirety.
partialdischargepulsedetector.Thepartialdischargepulsesof
3.1.2 ionization—the process by which electrons are lost
the spark-type discharge will have substantially shorter rise
from or transferred to neutral molecules or atoms to form
times than those of the glow-type (10).
positively or negatively charged particles.
3.1.9 partial discharge quantity (terminal corona charge)
3.1.3 partial discharge (corona)—an electrical discharge
(Q)—themagnitudeofanindividualdischargeinaninsulation
that only partially bridges the insulation between conductors. t
system expressed in terms of the charge transfer measured at
This electrical discharge, which is governed by the transient
the system terminals. The measured charge is in general not
gaseous ionization process, can assume the form of either a
equal to the charge transferred at the discharge site, and does
sparkcharacterizedbyanarrowdischargechanneloradiffused
have a relation to the discharge energy. For a small specimen
glow having an expanded or substantially broadened discharge
that can be treated as a simple lumped capacitor, it is equal to
channel. The partial discharges occur in gas filled cavities
the product of the capacitance of the insulation system and the
occluded within insulating systems and are initiated whenever
partial discharge pulse voltage, that is:
thevoltageacrossthecavitieschangesbyavalueequaltotheir
breakdown voltage (5).
Q 5 C V (1)
t t t
3.1.4 corona—visible partial discharges in gases adjacent to
where:
a conductor. This term has also been used to refer to partial
Q = partial discharge quantity, C,
t
discharges in general.
C = capacitance of the specimen insulation system, F, and
t
3.1.5 continuous partial discharges (continuous corona)—
V = peak value of the partial discharge pulse voltage ap-
t
dischargesthatrecuratratherregularintervals;forexampleon
pearing across C,V.
t
approximately every cycle of an alternating voltage or at least
3.1.10 partial discharge (corona) level—the magnitude of
once per minute for an applied direct voltage.
the greatest recurrent discharge during an observation of
3.1.6 partial discharge (corona) inception voltage (PDIV
continuous discharges.
[CIV])—the lowest voltage at which continuous partial dis-
3.1.11 average discharge (corona) current (I)—the sum of
t
charges above some stated magnitude (which may define the
the absolute magnitudes of the individual discharges during a
limit of permissible background noise) occur as the applied
certain time interval divided by that time interval. When the
voltage is gradually increased (Note 1). Where the applied
discharges are measured in coulombs and the time interval in
voltage is alternating, the PDIV is expressed as 1/√2 of the
seconds, the calculated current will be in amperes.
peak voltage. Many test and specimen parameters can affect
t
Σ Q 1Q 12222Q
thisvalue,andinsomecasesreproducibilitymaybedifficultto
t 1 2 n
I 5 (2)
t
achieve. t 2 t
1 0
NOTE1—ManyfactorsmayinfluencethevalueofthePDIVandPDEV where:
includingtherateatwhichthevoltageisincreasedordecreasedaswellas
I = average current, A,
t
theprevioushistoryofthevoltageappliedtothespecimen.Inmanycases
t = starting time, s,
it may be difficult to obtain the same value with subsequent tests.
t = completion time, s, and
Moreover, the “continuous” character of the partial discharges is
Q,Q,Q = partial discharge quantity in a corona pulse 1
sometimes quite difficult to define, and an arbitrary judgment in this
1 2 n
respect may lead to different values of the PDIV or PDEV. through n, C.
3.1.7 partial discharge (corona) extinction voltage (PDEV
3.1.12 quadratic rate—the sum of the squares of the indi-
[CEV])—the highest voltage at which partial discharges above
vidual discharge magnitudes during a certain time interval
some stated magnitude no longer occur as the applied voltage
dividedbythattimeinterval.Thequadraticrateisexpressedas
is gradually decreased from above the inception voltage (see
(coulombs) per second.
Note 1).Where the applied voltage is alternating, the PDEVis
3.1.13 partial discharge (corona) energy (W)— the energy
expressedas1/√2ofthepeakvoltage.Manytestandspecimen
drawnfromthetestvoltagesourceastheresultofanindividual
parameters can affect this value, and in some cases reproduc-
discharge.ItistheproductofthemagnitudeQofthatdischarge
ibility may be difficult to achieve.
and the instantaneous value V of the voltage across the test
3.1.8 partial discharge pulse voltage (V)—the terminal
t
specimen at the inception of the discharge (11). Thus the
pulse voltage resulting from a partial discharge represented as
discharge energy of the ith pulse is:
a voltage source suddenly applied in series with the capaci-
W 5 Q V (3)
i i i
tance of the insulation system under test, and that would be
where:
W = the discharge energy, W·s(=J),
i
Available from The Insulated Cable Engineers Association, Inc. (ICEA), P.O.
Q = the partial discharge magnitude (see 3.1.9), and
I
Box 2694, Alpharetta, GA 30023, http://www.icea.net.
D1868 − 20
measuring instrument coupled to the terminals to respond to
V = theinstantaneousvalueoftheappliedtestvoltageatthe
i
the voltage pulse in terms of the charge transferred at the
time of the discharge, V.
terminals.
3.1.14 partial discharge (corona) power loss (P)—the sum-
mation of the energies drawn from the test voltage source by
5. Significance and Use
individual discharges occurring over a period of time, divided
5.1 The presence of partial discharges (corona) at operating
by that time period.
voltage in an insulation system has the potential to result in a
i5m significantreductioninthelifeoftheinsulatingmaterial.Some
P 5 Σ Q V (4)
i51 i i
T
materials are more susceptible to such discharge damage than
others. This characteristic can be investigated using Test
where:
Method D2275.
P = the discharge power, W,
T = the time period, s,
5.2 The presence of partial discharges (corona) in an appar-
m = the number of the final pulse during T, and
ently solid insulation is a potential indication of the existence
QV = the discharge energy of the ith pulse (see 3.1.13).
i I
of internal cavities. Partial discharge tests have been useful in
thedesignandinspectionofmolded,laminated,andcomposite
When partial discharge pulse-height analysis is performed
insulation, as well as specimens in the form of cables,
overaone-secondinterval,thenthepowerdissapated,P,canbe
capacitors, transformers, bushings, stator bars, and rotating
determined from:
machines (1-9), (13), (12). See also AEIC CS5-87, ICEA
t
P 5Σ n Q V (5)
j51 j j j
T-24-380,IEEE48,IEEEC57113-1991,IEEEC57124-1991,
where: and IEEE 1434-2005.
P = pulse discharge power loss, W,
5.3 Partial discharge (corona) inception and extinction volt-
n = recurrence rate of the jth discharge pulse in pulses/
j
ages are used in the determination of the limiting voltage at
second,
which an insulation system will operate free of such dis-
Q = the corresponding value of the partial discharge quan-
j
charges. The extinction voltage is often substantially lower
tity in coulombs for the particular pulse, and
than the inception voltage. Where the operating voltage is
V = instantaneous value of the applied voltage in volts at
j
belowtheinceptionvoltagebutabovetheextinctionvoltage,it
which the jth discharge pulse takes place (6).
is possible that a transient over-voltage will initiate discharges
If the assumption (12) is made that V∆C . C∆V (where
which then continue until the voltage is lowered below the
j j t j
∆C isincrementalcapacitancerisein C duetothedrop∆V in
extinction voltage. Inception and extinction voltages depend
t j
j
V as a result of the jth discharge), then the above summation
uponmanyfactors,includingtemperatureandtherateatwhich
j
must be multiplied by 1/2. However, this assumption is not
the voltage is changed.After a time at a voltage, it is possible
usually borne out in practice. that discharges will start and stop in a nonuniform and
unpredictable fashion, especially for discharges within cavities
3.1.15 partial discharge apparent power loss (P )—the
a
in certain materials, in particular if the discharge degradation
summationoveraperiodoftimeofallcoronapulseamplitudes
products formed are conductive (1), (5).
multiplied by the rms test voltage.
5.4 Themagnitude(pulseheight)ofapartialdischargeisan
P 5 I V (6)
a t t
indication of the amount of energy that it dissipates in the
where: insulation system. Partial discharge magnitude and pulse rate
are useful in estimating the rate, or change of rate, at which
P = apparent power loss in time interval (t −t ), W,
a 1 0
deterioration is produced.
I = average corona current, A, and
t
V = applied rms test voltage, V.
s
5.5 In general, the occurrence of partial discharges is not
directly related to the basic properties of a solid insulating
3.1.16 partial discharge (corona) pulse rate (n)—the aver-
material, but usually results from overstressing of gaseous
age number of discharge pulses that occur per second or in
occlusions or similar imperfections or discontinuities in an
some other specified time interval. The pulse count may be
insulating system. It is possible that partial discharges will
restricted to pulses above a preset threshold magnitude, or to
originate at locations such as on the leads or terminals without
those between stated lower and upper magnitude limits.
resulting in any hazard within the main part of the insulation
3.1.17 partial discharge pulse—a voltage or current pulse
system.
that occurs at some designated location in a circuit as a result
of a partial discharge.
6. Interference
6.1 It is possible that radiated or conducted electrical
4. Summary of Test Method
disturbances from sources other than the test specimen will
4.1 A specimen insulation system is energized in a test
interfere with the measurement of partial discharges. The
circuitbyahigh-voltagesource.Apartialdischarge(corona)in
magnitude of disturbances reaching the measuring instrument
the specimen will cause a sudden charge transfer and a
mustbekeptsmallrelativetothemostsensitivemeasurements
resulting voltage pulse at the specimen terminals. Calibrate a to be made.
D1868 − 20
6.2 The following techniques are useful to reduce interfer- to realize the desired circuit sensitivity (see circuit sensitivity
ence from radiation: (1) shielding the test circuit, or (2) expressionsinX1.1).Avalueof100pFisoftensatisfactoryfor
conducting the test in a shielded room. The following tech- low-capacitancespecimens.Forhighercapacitancespecimens,
niqueisusefultoreduceinterferencebyconduction:theuseof avalueof2500pFisfoundtobeadequate.Ingeneral,ahigher
a low-pass filter in the voltage supply circuit. capacitance value will improve circuit sensitivity but will
requireincreasedchargingcurrentfromthetestvoltagesupply.
6.3 It is possible that corona on the connecting leads
The coupling capacitor shall not introduce into the circuit
betweenthetestvoltagesourceandthespecimenwillinterfere
pulses of sufficient magnitude to interfere with the most
withthemeasurement.Suchinterferenceislikelytobeavoided
sensitive measurement.
iftheleadsaresmooth-surfacedandofsufficientdiameter,with
spherical terminals. 7.3 Measuring Impedance:
7.3.1 The measuring impedance shall have a value at test
6.4 It is often possible to identify interference from the
frequencythatislowincomparisonwithothercircuitelements
display of an oscilloscope coupled to the measuring circuit,
to prevent the appearance of an excessive portion of the test
withitshorizontaldeflectionrelatingtotheinstantaneousvalue
voltage at the input terminals of the measuring device. The
ofthetestvoltage.Forexample,pulsesthatappearonlyduring
measuring impedance is usually inductive or resistive.
the negative half-cycle of the alternating test voltage are often
7.3.2 Aninductiveimpedance,shuntedbystraycapacitance
the result of corona originating on the connecting lead or
(consisting of connection cables and component mountings),
terminal rather than the result of partial discharges within the
produces an oscillatory response to a partial discharge pulse.
specimen.
Thepersistenceoftheoscillationsfacilitatespulseobservation,
6.5 Itisusuallypossibletocontrolinterferencebytheuseof
but reduces resolution between pulses. Resolution can be
time window circuits that suppress the measuring device input
changed by the use of a shunting resistor to damp the
during the portion of the test voltage wave when partial
oscillation or by modifying the frequency response of the
dischargesdonotoccur.Usingthistechniquetakecaretoavoid
discharge measuring system.
the loss of wanted signals. Other more sophisticated interfer-
7.3.3 A resistive impedance, shunted by stray capacitance,
ence suppression techniques of signal processing can be used
produces an exponentially decaying step response to a dis-
(1).
charge pulse. A resistive impedance is used to provide maxi-
6.6 Some interference sources are characterized by well mum pulse resolution and where the impedance must be
defined three dimensional distributions of discharge pulse adjusted, as in a bridge circuit.
magnitude, its phase relationship to the applied voltage and its
7.4 Measuring Devices:
recurrence rate or, alternatively, their pulses exhibit certain
7.4.1 A cathode-ray oscilloscope is used to display and
pulse shape attributes. Neural networks can be taught to
measure the discharge pulses that appear across the measuring
recognize these specific features of the interference generated
impedance. The pulses are amplified by the vertical deflection
discharge pulse patterns and distinguish them from the actual
amplifier, which must respond to the important frequency
dischargepulsepatternsemanatingfromthecavitieswithinthe
componentsofthepulses.Bandwidthintherangefrom25kHz
insulating systems (14).
to several hundred thousand Hz have been found satisfactory.
The height of the vertical deflection caused by a partial
7. Apparatus
discharge can be related to the discharge magnitude. In a
7.1 Test Voltage Supply:
common arrangement, the vertical deflections are superim-
7.1.1 The voltage supply must be capable of energizing the
posed on an elliptical oval trace synchronized with the test
testcircuit,includingthespecimen,overarangeofvoltagesto
voltage. Thus the point on the voltage wave at which each
the maximum desired test value. The requirements in Test
dischargeoccurscanbevisualized.Equiptheoscilloscopewith
Method D149 are recommended. The frequency of the supply
a beam-brightening or pulse-stretching circuit to facilitate the
voltage shall preferably be the frequency that will be used in
observation of peak deflections.
service of the specimen.
7.4.2 The partial discharge level can be measured by a
7.1.2 The voltage supply must not introduce into the mea-
peak-readingvoltmeterconnectedacrossthemeasuringimped-
suring circuit pulses of sufficient magnitude to interfere with
ance. The meter must be able to respond accurately over a
the most sensitive measurement. The internal impedance in-
range of pulse rates between 1 and at least 1000 per second.
cluding the shunt capacitance of the voltage supply must not
7.4.3 The average discharge current can be measured by an
significantly reduce the sensitivity of the measurement. To
instrument that responds to the average value of the rectified
assist in meeting these two requirements, insert a supply
discharge pulses that occur in a certain time interval. Take
impedance, preferably in the form of a low-pass filter, in the
precautions to avoid oscillations in the measuring circuit that
output circuit of the voltage supply.
could reduce accuracy. Alternatively, determine the average
7.1.3 The voltage supply shall include a voltmeter that
discharge current from a pulse height distribution curve (see
responds to the peak voltage applied to the test circuit and
7.4.7).
permits measurements of the voltage directly across the insu-
7.4.4 The quadratic rate can be measured by an instrument
lation system.
that responds to the mean square value of the discharge pulses
7.2 Coupling Capacitor (C )—The capacitance value of that occur in a certain time interval. A suitable rectifier or a
cc
thiscapacitorisselectedinrelationtoothercircuitcomponents thermal detector is suitable provide the squaring function.
D1868 − 20
7.4.5 The apparent discharge power loss can be determined cient magnitude to interfere with the most sensitive
from the area under a pulse height distribution curve (see measurement, and it must have a voltage rating equal to the
7.4.7).Auseful approximation of the discharge power loss for highest test voltage.
analternatingtestvoltageisobtainablefromaninstrumentthat
7.6 Calibration Pulse Generator—Thepulsegeneratormust
responds to the product of the average discharge current and
have an output impedance of not more than 100 Ω capable of
the RMS value of the test voltage.
withstanding the charging current at test voltage of the cali-
bration capacitor. When connected to the circuit the generator
NOTE 2—Method A of Test Methods D3382 is suitable to determine
power loss due to continuous partial discharges under alternating voltage.
must produce pulses having a rise time of 0.1 µs or less and a
Alternatively, Method B of Test Methods D3382 is suitable to obtain
decaytimetohalfcrestofgreaterthan1ms.Provisionmustbe
dischargeenergypercycleinjoulesandthenmultiplyingbythefrequency
made for determining the crest value of the output pulse. A
to obtain the loss in watts.
square-wave generator that meets these requirements is a
7.4.6 A pulse counter responsive to either positive or
satisfactory pulse source. The maximum repetition rate of the
negative pulses is suitable to measure the partial discharge
pulse generator shall be 12.5 kHz.
(corona) pulse rate, n. A rectifier bridge is suitable to obtain
total pulse counts of the combined positive and negative pulse
8. Hazards
responses. Suitable pulse-shaping circuits applied across the
8.1 The portion of the test area that includes the test
measuring impedance must be employed to ensure that each
specimen and other components of the test circuit to be
discharge pulse is recorded as a single event.When oscillatory
energized at high voltage shall be blocked from easy access
responses are involved, a demodulation circuit with proper
and marked by warning signs. Doors or gates to this area shall
active filtering constitutes an effective means for obtaining
be provided with switches interlocked with the test voltage
smooth pulses having a single polarity (13). When high pulse
supply system.
resolution is desired and the pulses are already unidirectional
butcontainsuperimposedhigh-frequencyoscillations,multivi- 8.2 Provision shall be made for the remote (automatic)
brator circuits are suitable to carry out pulse shaping (15).The
grounding of the high-voltage circuit. The high-voltage circuit
insertionofanamplifierwithasuitablebandwidthbetweenthe shall not be approached unless a ground is applied. This
measuring impedance and the pulse-shaping circuit will pro-
precautionisespeciallyimportantwhendirect-voltagetestsare
videadequatesensitivity.Acontrolledgatingsystemissuitable employed, since the circuit has the potential to remain charged
to prevent the counting of pulses below selected magnitude
at high voltage after the energizing source is interrupted.
levels.Forexample,inhighambientnoiseenvironmentsitwill
8.3 Surge voltage protectors must be applied to those parts
be potentially useful to avoid counting pulses below a preset
of a measuring circuit that must be accessible during the
level.
measurement, such as the measuring device and calibration
7.4.7 Single- or multichannel pulse-height analyzers can be
pulse generator, to minimize danger from accidental high
utilized to obtain curves of the partial discharge pulse rate, n,
voltage which could result from the failure of the test
as a function of the discharge magnitude, Q. The area under
specimen, high-voltage calibration capacitor, or other circuit
such pulse height distribution curves is given by
element. The instrumentation must be enclosed in a grounded
`
case.
A~n, Q! 5 * n~Q!dQ (7)
8.4 Itisrecommendedthatthesafetypracticesasoutlinedin
and is dimensionally equal to the corona current and is
paragraph 2.0 to 2.8 of Ref (16) be followed.
proportional to the partial discharge power loss.As in the case
of pulse counters (7.4.6), the use of proper pulse shaping and
9. Sampling
amplification circuitry is of the utmost importance to ensure a
9.1 Because this test method is used on a variety of
meaningful pulse-height analysis.
insulating materials and systems, no one sampling procedure
Computerized techniques are frequently followed to obtain
can be specified. Material specification methods will often
partial discharge pulse phase-resolved data (14). A digital
provide guidance. For some insulation systems, each produc-
partial discharge detector is utilized for the acquisition of all
tion unit is tested individually as a quality control measure.
the quasi-integrated pulses; it then sorts and quantifies the
discreet pulses by their magnitude, Q, the corresponding phase
10. Test Specimens
angle or discharge epoch, φ, with respect to the applied
sinusoidal voltage at which they occur and their respective 10.1 It is acceptable for specimens to be entire insulation
recurrence rate, n. An analysis software is then employed to systems with normally included electrodes. It is acceptable to
plot the bivariate distribution of (Q~φ~n). From the resulting make discharge measurements between any two insulated sets
three dimensi
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: D1868 − 13 D1868 − 20
Standard Test Method for
Detection and Measurement of Partial Discharge (Corona)
Pulses in Evaluation of Insulation Systems
This standard is issued under the fixed designation D1868; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the U.S. Department of Defense.
1. Scope*
1.1 This test method covers the detection and measurement of partial discharge (corona) pulses at the terminals of an insulation
system under an applied test voltage, including the determination of partial discharge (corona) inception and extinction voltages
as the test voltage is raised and lowered. TheThis test method is also useful in determining quantities such as apparent charge and
pulse repetition rate together with such integrated quantities as average current, quadratic rate, and power. TheThis test method
is useful for test voltages ranging in frequency from zero (direct voltage) to approximately 2000 Hz.2000 Hz.
1.2 TheThis test method is directly applicable to a simple insulation system that can be represented as a two-terminal capacitor
(1),(2)).
1.3 TheThis test method is also applicable to (distributed parameter) insulation systems such as high-voltage cable.
Consideration must be given to attenuation and reflection phenomena in this type of system. Further information on distributed
parameter systems of cables, transformers, and rotating machines will be found in Refs.Refs (11-9), (2),(3), (4),(5),(6),(7),(8), and
(9). (See AEIC CS5-87, IEEE C57 113-1991, IEEE C57 124-1991, and IEEE 1434-2005.)
1.4 TheThis test method can be applied to multi-terminal insulation systems, but at some loss in accuracy, especially where the
insulation of inductive windings is involved.
1.5 This standard does not purport to address all of the safety problems,concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appropriate safety safety, health, and healthenvironmental practices and
determine the applicability of regulatory limitations prior to use. Specific precaution statements are given in Sections 8 and 14.
1.6 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
D149 Test Method for Dielectric Breakdown Voltage and Dielectric Strength of Solid Electrical Insulating Materials at
Commercial Power Frequencies
D618 Practice for Conditioning Plastics for Testing
D2275 Test Method for Voltage Endurance of Solid Electrical Insulating Materials Subjected to Partial Discharges (Corona) on
the Surface
D3382 Test Methods for Measurement of Energy and Integrated Charge Transfer Due to Partial Discharges (Corona) Using
Bridge Techniques
This test method is under the jurisdiction of ASTM Committee D09 on Electrical and Electronic Insulating Materials and is the direct responsibility of Subcommittee
D09.12 on Electrical Tests.
Current edition approved Nov. 1, 2013March 1, 2020. Published December 2013March 2020. Originally approved in 1961. Last previous edition approved in 20072013
as D1868 – 07.D1868 – 13. DOI: 10.1520/D1868-13.10.1520/D1868-20.
The boldface numbers in parentheses refer to the list of references at the end of this test method.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D1868 − 20
2.2 Other Documents:IEEE Standards
AEIC CS5-87 Specifications for Thermoplastic and Crosslinked Polyethlene Insulated Shielded Power Cables Rated 5 through
th 4
35 kV (9 Edition) October 1987
ICEA T-24-380 Guide for Partial Discharge Procedure
IEEE 48 Standard Test Procedures and Requirements for High Voltage Alternating Current Cable Terminations
IEEE 1434-2005 Guide to the Measurement of Partial Discharges in Rotating Machinery
IEEE C57 113-1991 Guide for PD Measurement in Liquid-Filled Power Transformers and Shunt Reactors
IEEE C57 124-1991 Recommended Practice for the Detection of PD and the Measurement of Apparent Charge in Dry-Type
Transformers
2.3 Other Documents:
AEIC CS5-87 Specifications for Thermoplastic and Crosslinked Polyethlene Insulated Shielded Power Cables Rated 5 through
th 5
35 kV (9 Edition) October 1987
ICEA T-24-380 Guide for Partial Discharge Procedure
3. Terminology
3.1 Definitions:
3.1.1 The following terms are presented in a developing sequence; it is best that they be read in their entirety:entirety.
3.1.2 ionization—the process by which electrons are lost from or transferred to neutral molecules or atoms to form positively
or negatively charged particles.
3.1.3 partial discharge (corona)—an electrical discharge that only partially bridges the insulation between conductors. This
electrical discharge, which is governed by the transient gaseous ionization process, can assume the form of either a spark
characterized by a narrow discharge channel or a diffused glow having an expanded or substantially broadened discharge channel.
The partial discharges occur in gas filled cavities occluded within insulating systems and are initiated whenever the voltage across
the cavities changes by a value equal to their breakdown voltage (5).
3.1.4 corona—visible partial discharges in gases adjacent to a conductor. This term has also been used to refer to partial
discharges in general.
3.1.5 continuous partial discharges (continuous corona)—discharges that recur at rather regular intervals; for example on
approximately every cycle of an alternating voltage or at least once per minute for an applied direct voltage.
3.1.6 partial discharge (corona) inception voltage (PDIV [CIV])—the lowest voltage at which continuous partial discharges
above some stated magnitude (which may define the limit of permissible background noise) occur as the applied voltage is
gradually increased (Note 1). Where the applied voltage is alternating, the PDIV is expressed as 1/√2 of the peak voltage. Many
test and specimen parameters can affect this value, and in some cases reproducibility may be difficult to achieve.
NOTE 1—Many factors may influence the value of the PDIV and PDEV including the rate at which the voltage is increased or decreased as well as
the previous history of the voltage applied to the specimen. In many cases it may be difficult to obtain the same value with subsequent tests.
Moreover, the “continuous” character of the partial discharges is sometimes quite difficult to define, and an arbitrary judgment in this respect may lead
to different values of the PDIV or PDEV.
3.1.7 partial discharge (corona) extinction voltage (PDEV [CEV])—the highest voltage at which partial discharges above some
stated magnitude no longer occur as the applied voltage is gradually decreased from above the inception voltage (see Note 1).
Where the applied voltage is alternating, the PDEV is expressed as 1/=21/√2 of the peak voltage. Many test and specimen
parameters can affect this value, and in some cases reproducibility may be difficult to achieve.
3.1.8 partial discharge pulse voltage (V )—the terminal pulse voltage resulting from a partial discharge represented as a voltage
t
source suddenly applied in series with the capacitance of the insulation system under test, and that would be detected at the
terminals of the system under open-circuit conditions. The shape, rise time, and magnitude of the voltage V of the partial discharge
t
pulse are dependent upon the geometry of the cavity, its size, nature of its boundaries, the type of gas and the pressure within as
well as the parameters of the transmission medium between the discharge site and the partial discharge pulse detector. The partial
discharge pulses of the spark-type discharge will have substantially shorter rise times than those of the glow-type (10).
3.1.9 partial discharge quantity (terminal corona charge) (Q )—the magnitude of an individual discharge in an insulation
t
system expressed in terms of the charge transfer measured at the system terminals. The measured charge is in general not equal
Available from the publication department of the Association of Edison Illuminating Companies, 600 N. 18th St., PO Companies (AEIC), P.O. Box 2641, Birmingham,
AL 35291-0992. 35291-0992, http://www.aeic.org.
Available from theThe Insulated Cable Engineers Association, Inc., PO Box 440, South Yarmouth, MA 02664.Inc. (ICEA), P.O. Box 2694, Alpharetta, GA 30023,
http://www.icea.net.
Available from Institute of Electrical and Electronics Engineers, Inc. (IEEE), 445 Hoes Ln., P.O. Box 1331, Piscataway, NJ 08854-1331, http://www.ieee.org.
Available from Institute of Electrical and Electronics Engineers, Inc. (IEEE), 445 Hoes Ln., Piscataway, NJ 08854-4141, http://www.ieee.org.
D1868 − 20
to the charge transferred at the discharge site, and does have a relation to the discharge energy. For a small specimen that can be
treated as a simple lumped capacitor, it is equal to the product of the capacitance of the insulation system and the partial discharge
pulse voltage, that is:
Q 5 C V (1)
t t t
where:
Q = partial discharge quantity, C,
t
C = capacitance of the specimen insulation system, F, and
t
V = peak value of the partial discharge pulse voltage appearing across C , V.
t t
where:
Q = partial discharge quantity, C,
t
C = capacitance of the specimen insulation system, F, and
t
V = peak value of the partial discharge pulse voltage appearing across C , V.
t t
3.1.10 partial discharge (corona) level—the magnitude of the greatest recurrent discharge during an observation of continuous
discharges.
3.1.11 average discharge (corona) current (I ) —the sum of the absolute magnitudes of the individual discharges during a
t
certain time interval divided by that time interval. When the discharges are measured in coulombs and the time interval in seconds,
the calculated current will be in amperes.
t
Q 1Q 1222222Q
1 2 n
(
t
I 5 (2)
t
t 2 t
1 0
t
Σ Q 1Q 12222Q
t 1 2 n
I 5 (2)
t
t 2 t
1 0
where:
I = average current, A,
t
t = starting time, s,
t = completion time, s, and
Q , Q , Q = partial discharge quantity in a corona pulse 1 through n, C.
1 2 n
where:
I = average current, A,
t
t = starting time, s,
t = completion time, s, and
Q , Q , Q = partial discharge quantity in a corona pulse 1 through n, C.
1 2 n
3.1.12 quadratic rate—the sum of the squares of the individual discharge magnitudes during a certain time interval divided by
that time interval. The quadratic rate is expressed as (coulombs) per second.
3.1.13 partial discharge (corona) energy (W)— the energy drawn from the test voltage source as the result of an individual
discharge. It is the product of the magnitude Q of that discharge and the instantaneous value V of the voltage across the test
specimen at the inception of the discharge (11). Thus the discharge energy of the ith pulse is:
W 5 Q V (3)
i i i
where:
W = the discharge energy, W·s( = J),
i
Q = the partial discharge magnitude (see 3.1.9), and
I
V = the instantaneous value of the applied test voltage at the time of the discharge, V.
i
where:
W = the discharge energy, W·s( = J),
i
Q = the partial discharge magnitude, (see 3.1.9), and
I
V = the instantaneous value of the applied test voltage at the time of the discharge, V.
i
3.1.14 partial discharge (corona) power loss (P)—the summation of the energies drawn from the test voltage source by
individual discharges occurring over a period of time, divided by that time period.
i5m
P 5 Q V (4)
( i i
T
i51
i5m
P 5 Σ Q V (4)
i51 i i
T
D1868 − 20
where:
P = the discharge power, W,
T = the time period, s,
m = the number of the final pulse during T, and
Q V = the discharge energy of the ith pulse (see 3.1.13).
i I
where:
P = the discharge power, W,
T = the time period, s,
m = the number of the final pulse during T, and
Q V = the discharge energy of the ith pulse (see 3.1.13).
i i
When partial discharge pulse-height analysis is performed over a one-second interval, then the power dissapated, P, can be
determined from:
i
P 5 n Q V (5)
j j j
(
j51
t
P 5 Σ n Q V (5)
j51 j j j
where:
P = pulse discharge power loss, W,
n = recurrence rate of the jth discharge pulse in pulses/second,
j
Q = the corresponding value of the partial discharge quantity in coulombs for the particular pulse, and
j
V = instantaneous value of the applied voltage in volts at which the jth discharge pulse takes place (6).
j
where:
P = pulse discharge power loss, W,
n = recurrence rate of the jth discharge pulse in pulses/second.
j
Q = the corresponding value of the partial discharge quantity in coulombs for the particular pulse.
j
V = instantaneous value of the applied voltage in volts at which the jth discharge pulse takes place (6).
j
If the assumption (12) is made that V ΔC . C ΔV (where ΔC is incremental capacitance rise in C due to the drop ΔV in V
j j t j j t j j
as a result of the jth discharge), then the above summation must be multiplied by ⁄2 . 1/2. However, this assumption is not usually
borne out in practice.
3.1.15 partial discharge apparent power loss (P )—the summation over a period of time of all corona pulse amplitudes
a
multiplied by the rms test voltage.
P 5 I V (6)
a t s
P 5 I V (6)
a t t
where:
P = apparent power loss in time interval (t − t ), W,
a 1 0
I = average corona current, A, and
t
V = applied rms test voltage, V.
s
where:
P = apparent power loss in time interval (t − t ), W,
a 1 0
I = average corona current, A, and
t
V = applied rms test voltage, V.
s
3.1.16 partial discharge (corona) pulse rate (n)—the average number of discharge pulses that occur per second or in some other
specified time interval. The pulse count may be restricted to pulses above a preset threshold magnitude, or to those between stated
lower and upper magnitude limits.
3.1.17 partial discharge pulse—a voltage or current pulse that occurs at some designated location in a circuit as a result of a
partial discharge.
4. Summary of Test Method
4.1 A specimen insulation system is energized in a test circuit by a high-voltage source. A partial discharge (corona) in the
specimen will cause a sudden charge transfer and a resulting voltage pulse at the specimen terminals. Calibrate a measuring
instrument coupled to the terminals to respond to the voltage pulse in terms of the charge transferred at the terminals.
D1868 − 20
5. Significance and Use
5.1 The presence of partial discharges (corona) at operating voltage in an insulation system has the potential to result in a
significant reduction in the life of the insulating material. Some materials are more susceptible to such discharge damage than
others. This characteristic can be investigated using Test Method D2275.
5.2 The presence of partial discharges (corona) in an apparently solid insulation is a potential indication of the existence of
internal cavities. Partial discharge tests have been useful in the design and inspection of molded, laminated, and composite
insulation, as well as specimens in the form of cables, capacitors, transformers, bushings, stator bars, and rotating machines
(11-9),(2),(3), (4),(5),(6),(7),(8),(9),(13), and (12). (SeeSee also AEIC CS5-87, ICEA T-24-380, IEEE 48, IEEE C57 113-1991,
IEEE C57 124-1991, and IEEE 1434-2005.)1434-2005.
5.3 Partial discharge (corona) inception and extinction voltages are used in the determination of the limiting voltage at which
an insulation system will operate free of such discharges. The extinction voltage is often substantially lower than the inception
voltage. Where the operating voltage is below the inception voltage but above the extinction voltage, it is possible that a transient
over-voltage will initiate discharges which then continue until the voltage is lowered below the extinction voltage. Inception and
extinction voltages depend upon many factors, including temperature and the rate at which the voltage is changed. After a time
at a voltage, it is possible that discharges will start and stop in a nonuniform and unpredictable fashion, especially for discharges
within cavities in certain materials, in particular if the discharge degradation products formed are conductive (1),(5).
5.4 The magnitude (pulse height) of a partial discharge is an indication of the amount of energy that it dissipates in the insulation
system. Partial discharge magnitude and pulse rate are useful in estimating the rate, or change of rate, at which deterioration is
produced.
5.5 In general, the occurrence of partial discharges is not directly related to the basic properties of a solid insulating material,
but usually results from overstressing of gaseous occlusions or similar imperfections or discontinuities in an insulating system. It
is possible that partial discharges will originate at locations such as on the leads or terminals without resulting in any hazard within
the main part of the insulation system.
6. Interference
6.1 It is possible that radiated or conducted electrical disturbances from sources other than the test specimen will interfere with
the measurement of partial discharges. The magnitude of disturbances reaching the measuring instrument must be kept small
relative to the most sensitive measurements to be made.
6.2 The following techniques are useful to reduce interference from radiation: (a1) shielding the test circuit, or (b2) conducting
the test in a shielded room. The following technique is useful to reduce interference by conduction: the use of a low-pass filter in
the voltage supply circuit.
6.3 It is possible that corona on the connecting leads between the test voltage source and the specimen will interfere with the
measurement. Such interference is likely to be avoided if the leads are smooth-surfaced and of sufficient diameter, with spherical
terminals.
6.4 It is often possible to identify interference from the display of an oscilloscope coupled to the measuring circuit, with its
horizontal deflection relating to the instantaneous value of the test voltage. For example, pulses that appear only during the negative
half-cycle of the alternating test voltage are often the result of corona originating on the connecting lead or terminal rather than
the result of partial discharges within the specimen.
6.5 It is usually possible to control interference by the use of time window circuits that suppress the measuring device input
during the portion of the test voltage wave when partial discharges do not occur. Using this technique take care to avoid the loss
of wanted signals. Other more sophisticated interference suppression techniques of signal processing can be used (1)). .
6.6 Some interference sources are characterized by well defined three dimensional distributions of discharge pulse magnitude,
its phase relationship to the applied voltage and its recurrence rate or, alternatively, their pulses exhibit certain pulse shape
attributes. Neural networks can be taught to recognize these specific features of the interference generated discharge pulse patterns
and distinguish them from the actual discharge pulse patterns emanating from the cavities within the insulating systems (14).
7. Apparatus
7.1 Test Voltage Supply:
7.1.1 The voltage supply must be capable of energizing the test circuit, including the specimen, over a range of voltages to the
maximum desired test value. The requirements in Test Method D149 are recommended. The frequency of the supply voltage shall
preferably be the frequency that will be used in service of the specimen.
7.1.2 The voltage supply must not introduce into the measuring circuit pulses of sufficient magnitude to interfere with the most
sensitive measurement. The internal impedance including the shunt capacitance of the voltage supply must not significantly reduce
the sensitivity of the measurement. To assist in meeting these two requirements, insert a supply impedance, preferably in the form
of a low-pass filter, in the output circuit of the voltage supply.
D1868 − 20
7.1.3 The voltage supply shall include a voltmeter that responds to the peak voltage applied to the test circuit and permits
measurements of the voltage directly across the insulation system.
7.2 Coupling Capacitor (C )—The capacitance value of this capacitor is selected in relation to other circuit components to
cc
realize the desired circuit sensitivity (see circuit sensitivity expressions in X1.1). A value of 100 pF is often satisfactory for
low-capacitance specimens. For higher capacitance specimens, a value of 2500 pF is found to be adequate. In general, a higher
capacitance value will improve circuit sensitivity but will require increased charging current from the test voltage supply. The
coupling capacitor shall not introduce into the circuit pulses of sufficient magnitude to interfere with the most sensitive
measurement.
7.3 Measuring Impedance:
7.3.1 The measuring impedance shall have a value at test frequency that is low in comparison with other circuit elements to
prevent the appearance of an excessive portion of the test voltage at the input terminals of the measuring device. The measuring
impedance is usually inductive or resistive.
7.3.2 An inductive impedance, shunted by stray capacitance (consisting of connection cables and component mountings),
produces an oscillatory response to a partial discharge pulse. The persistence of the oscillations facilitates pulse observation, but
reduces resolution between pulses. Resolution can be changed by the use of a shunting resistor to damp the oscillation or by
modifying the frequency response of the discharge measuring system.
7.3.3 A resistive impedance, shunted by stray capacitance, produces an exponentially decaying step response to a discharge
pulse. A resistive impedance is used to provide maximum pulse resolution and where the impedance must be adjusted, as in a
bridge circuit.
7.4 Measuring Devices:
7.4.1 A cathode-ray oscilloscope is used to display and measure the discharge pulses that appear across the measuring
impedance. The pulses are amplified by the vertical deflection amplifier, which must respond to the important frequency
components of the pulses. Bandwidth in the range from 25 kHz to several hundred thousand Hz have been found satisfactory. The
height of the vertical deflection caused by a partial discharge can be related to the discharge magnitude. In a common arrangement,
the vertical deflections are superimposed on an elliptical oval trace synchronized with the test voltage. Thus the point on the voltage
wave at which each discharge occurs can be visualized. Equip the oscilloscope with a beam-brightening or pulse-stretching circuit
to facilitate the observation of peak deflections.
7.4.2 The partial discharge level can be measured by a peak-reading voltmeter connected across the measuring impedance. The
meter must be able to respond accurately over a range of pulse rates between 1 and at least 1000 per second.
7.4.3 The average discharge current can be measured by an instrument that responds to the average value of the rectified
discharge pulses that occur in a certain time interval. Take precautions to avoid oscillations in the measuring circuit that could
reduce accuracy. Alternatively, determine the average discharge current from a pulse height distribution curve (see 7.4.7).
7.4.4 The quadratic rate can be measured by an instrument that responds to the mean square value of the discharge pulses that
occur in a certain time interval. A suitable rectifier or a thermal detector is suitable provide the squaring function.
7.4.5 The apparent discharge power loss can be determined from the area under a pulse height distribution curve (see 7.4.7).
A useful approximation of the discharge power loss for an alternating test voltage is obtainable from an instrument that responds
to the product of the average discharge current and the RMS value of the test voltage.
NOTE 2—Method A of Test Methods D3382 is suitable to determine power loss due to continuous partial discharges under alternating voltage.
Alternatively, Method B of Test Methods D3382 is suitable to obtain discharge energy per cycle in joules and then multiplying by the frequency to obtain
the loss in watts.
7.4.6 A pulse counter responsive to either positive or negative pulses is suitable to measure the partial discharge (corona) pulse
rate, n. A rectifier bridge is suitable to obtain total pulse counts of the combined positive and negative pulse responses. Suitable
pulse-shaping circuits applied across the measuring impedance must be employed to ensure that each discharge pulse is recorded
as a single event. When oscillatory responses are involved, a demodulation circuit with proper active filtering constitutes an
effective means for obtaining smooth pulses having a single polarity (13). When high pulse resolution is desired and the pulses
are already unidirectional but contain superimposed high-frequency oscillations, multivibrator circuits are suitable to carry out
pulse shaping (15). The insertion of an amplifier with a suitable bandwidth between the measuring impedance and the
pulse-shaping circuit will provide adequate sensitivity. A controlled gating system is suitable to prevent the counting of pulses
below selected magnitude levels. For example, in high ambient noise environments it will be potentially useful to avoid counting
pulses below a preset level.
7.4.7 Single- or multichannel pulse-height analyzers can be utilized to obtain curves of the partial discharge pulse rate, n, as a
function of the discharge magnitude, Q. The area under such pulse height distribution curves is given by
`
A~n, Q! 5 n~Q!dQ (7)
*
and is dimensionally equal to the corona current and is proportional to the partial discharge power loss. As in the case of pulse
counters (7.4.6), the use of proper pulse shaping and amplification circuitry is of the utmost importance to ensure a meaningful
pulse-height analysis.
D1868 − 20
Computerized techniques are frequently followed to obtain partial discharge pulse phase-resolved data (14). A digital partial
discharge detector is utilized for the acquisition of all the quasi-integrated pulses; it then sorts and quantifies the discreet pulses
by their magnitude, Q, the corresponding phase angle or discharge epoch, φ, with respect to the applied sinusoidal voltage at which
they occur and their respective recurrence rate, n. An analysis software is then employed to plot the bivariate distribution of
(Q~φ~n). From the resulting three dimensional plot, the univariate distributions of (Q~φ) and (n~φ) can be obtained; alternatively,
they can be measured directly by means of hardware.
7.5 Calibration Capacitor (C )—It is important that the calibration capacitor not exceed 200 pF. If its position in the test circuit
c
subjects it to the test voltage, the capacitor shall not introduce into the measuring circuit discharge pulses of sufficient magnitude
to interfere with the most sensitive measurement, and it must have a voltage rating equal to the highest test voltage.
7.6 Calibration Pulse Generator—The pulse generator must have an output impedance of not more than 100 Ω capable of
withstanding the charging current at test voltage of the calibration capacitor. When connected to the circuit the generator must
produce pulses having a rise time of 0.1 μs or less and a decay time to half crest of greater than 1 ms. Provision must be made
for determining the crest value of the output pulse. A square-wave generator that meets these requirements is a satisfactory pulse
source. The maximum repetition rate of the pulse generator shall be 12.5 kHz.
8. Hazards
8.1 The portion of the test area that includes the test specimen and other components of the test circuit to be energized at high
voltage shall be blocked from easy access and marked by warning signs. Doors or gates to this area shall be provided with switches
interlocked with the test voltage supply system.
8.2 Provision shall be made for the remote (automatic) grounding of the high-voltage circuit. The high-voltage circuit shall not
be approached unless a ground is applied. This precaution is especially important when direct-voltage tests are employed, since
the circuit may has the potential to remain charged at high voltage after the energizing source is interrupted.
8.3 Surge voltage protectors must be applied to those parts of a measuring circuit that must be accessible during the
measurement, such as the measuring device and calibration pulse generator, to minimize danger from accidental high voltage
which could result from the failure of the test specimen, high-voltage calibration capacitor, or other circuit element. The
instrumentation must be enclosed in a grounded case.
8.4 It is recommended that the safety practices as outlined in paragraph 2.0 to 2.8 of Ref.Ref (16) be followed.
9. Sampling
9.1 Because this test method is used on a variety of insulating materials and systems, no one sampling procedure can be
specified. Material specification methods will often provide guidance. For some insulation systems, each production unit is tested
individually as a quality control measure.
10. Test Specimens
10.1 It is acceptable for specimens to be entire insulation systems with normally included electrodes. It is acceptable to make
discharge measurements between any two insulated sets of conductors or metallic parts. It is acceptable for other conductors or
metallic parts to be or not to be grounded, but their disposition shall be noted in the report.
10.2 Where the test specimen does not have its own electrode system, temporary electrodes must be applied. There shall be no
cavities between an electrode and the surface of the specimen insulation, since discharges in such cavities cannot be distinguished
from partial discharge within the specimen. A film of insulating paste or grease spread on the surface of the insulation before
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