ASTM D1868-20 Standard Test Method for Detection and Measurement of Partial Discharge (Corona) Pulses in Evaluation of Insulation Systems

The standard "ASTM D1868-20 Standard Test Method for Detection and Measurement of Partial Discharge (Corona) Pulses in Evaluation of Insulation Systems" developed by the American Society for Testing and Materials (ASTM) describes a test method for detecting and measuring partial discharge (corona) pulses at the terminals of an insulation system under an applied test voltage and for determining the partial discharge (corona) onset and extinction voltages as the test voltage is increased and decreased. This test method is useful for determining integral quantities such as average current, quadratic ratio, and power, as well as quantities such as apparent charge and pulse repetition rate. This test method is useful for test voltages ranging in frequency from zero (direct voltage) to about 2000 Hz.

This test method is directly applicable to a simple insulation system that can be represented as a two-terminal capacitor. This test method is also applicable to insulation systems (with distributed parameters) such as high-voltage cables. In such systems, attention should be paid to the phenomena of attenuation and reflection.

For the purposes of this standard, the following ranges and definitions apply:

Ionization refers to the process by which electrons are lost from or transferred to neutral molecules or atoms, creating positively or negatively charged particles.
Partial discharge (corona) refers to an electrical discharge that only partially bridges the insulation between conductors. This electrical discharge, governed by the transient gas ionization process, may take the form of a spark characterized by a narrow discharge channel or a diffuse glow with an enlarged or significantly widened discharge channel. Partial discharges occur in gas-filled cavities enclosed within insulation systems and are initiated when the voltage across the gaps changes by a value equal to their breakdown voltage.
Corona is a partial discharge in gases adjacent to a conductor. The term has also been used to refer to partial discharges in general.
Continuous partial discharges (continuous corona) are discharges that recur at fairly regular intervals, for example approximately every cycle of an alternating voltage or at least once per minute for an applied direct voltage.
The partial discharge (corona) onset voltage is the lowest voltage at which sustained partial discharges above a specified magnitude (which may define the allowable background noise limit) occur as the applied voltage is gradually increased. Many test and sample parameters can affect this value and in some cases repeatability may be difficult to achieve.
The partial discharge (corona) extinction voltage is the highest voltage at which partial discharges above a specified magnitude no longer occur as the applied voltage is gradually reduced above the initial voltage. Many test and sample parameters can affect this value and repeatability may be difficult to achieve in some cases.
Partial discharge pulse voltage is the terminal pulse voltage resulting from partial discharge represented as a voltage source suddenly applied in series with the capacitance of the insulation system under test and detected at the terminals of the system under open circuit conditions. The form, rise time and magnitude of the voltage of the partial discharge pulse depend on the geometry of the cavity, its size, the nature of its boundaries, the type of gas and the pressure in it, and the parameters of the conduction medium between the discharge area and the partial discharge pulse detector. Partial discharge pulses of the spark type discharge have considerably shorter rise times than those of the glow type.
The amount of partial discharge (terminal corona charge) is the magnitude of the individual discharge in an insulation system and is expressed in terms of the charge transfer measured at the system terminals. The measured charge is usually not equal to the charge transferred at the discharge point and is related to the discharge energy. For a small sample, which can be treated as a simple lumped capacitor, it is equal to the product of the partial discharge pulse voltage and the capacitance of the insulation system.
The partial discharge (corona) level is the magnitude of the largest current discharge that occurs during observation of continuous discharges.
The average discharge (corona) current is the value obtained by dividing the sum of the absolute magnitudes of the individual discharges in a given time interval by that time interval.
The squared ratio is the sum of the squares of the individual discharge magnitudes in a given time interval divided by that time interval.
Partial discharge (corona) energy is the energy drawn from the test voltage source as a result of individual discharge.
Partial discharge (corona) power loss is obtained by dividing the sum of the energies drawn from the test voltage source by the individual discharges occurring in a certain time period, by that time period.
Partial discharge apparent power loss is the time period sum of all corona pulse amplitudes multiplied by the thermistor test voltage.
The partial discharge (corona) pulse rate is the average number of discharge pulses occurring per second or other specified time interval. The number of pulses may be limited to pulses above a predetermined threshold magnitude or to pulses between specified lower and upper magnitude limits.
A partial discharge pulse is a voltage or current pulse that occurs at a specific location in the circuit as a result of a partial discharge.

During testing, the insulation system of a specimen is energized in a test circuit from a high voltage source. A partial discharge (corona) in the specimen causes a sudden charge transfer and a voltage pulse across the specimen terminals. A measuring device connected to the terminals is calibrated and responds to the voltage pulse in terms of the charge transferred across the terminals.

The presence of partial discharges (corona) at operating voltage in an insulation system has the potential to cause a significant reduction in the life of the insulation material. Some materials are more susceptible to this type of discharge damage than others.

The presence of partial discharges (corona) in an apparently solid insulation is a possible indication of the presence of internal voids. Partial discharge tests are 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.

Our organization, which has been trying to support businesses from every sector with its testing, measurement, analysis and evaluation studies carried out in a wide range for years, has a strong staff that closely follows the developments in the world in the field of science and technology and constantly improves itself. In this context, testing services in accordance with the "ASTM D1868-20 Standard test method for the detection and measurement of partial discharge (corona) impacts in the evaluation of insulation systems" standard are also provided to businesses.