Partial Discharge Definition
Acoustic emission (AE) oscillation rate:
The number of acoustic emission oscillations that exceed the counter threshold level in a time interval, often 1s, or a number of cycles, depending on the instrument being used.
Partial discharge (PD):
An electrical discharge that only partially bridges the insulation between conductors.
Partial discharge (PD) acoustic sensor or sensing transducer:
A resonant, piezo-electric transducer that detects the mechanical stress waves that propagate from the partial discharge source through the internal construction materials and oil to the transformer tank wall. Note that the sensor is sensitive to stress waves in its frequency range that may not be from a partial discharge source.
Corona:
Visible partial discharges (PDs) in gases adjacent to a conductor.
Noise:
Unwanted disturbances superimposed on a useful signal that tend to obscure the signal’s information content.
Off-line testing:
Tests that are made with the rotating machine at standstill with the necessary test voltage applied to the winding, or portion thereof, from a separate test supply.
On-line testing:
Tests that are made with the rotating machine functioning to transmit and modify force, motion, or electricity.
Partial discharge extinction voltage (PDEV):
The highest voltage at which partial discharges (PDs) above some stated magnitude (which may define the limit of permissible background noise) no longer occur as the applied voltage is gradually decreased from above the inception voltage. PDEV, is expressed as 1/2 of the peak of the alternating voltage. Many factors may influence the value of PDEV including the rate at which the voltage is decreased as well as the previous history of the voltage applied to the winding or component thereof. In most cases, PDEV is less than PDIV.
Partial discharge inception voltage (PDIV):
The lowest voltage at which continuous partial discharges (PDs) above some stated magnitude (which may define the limit of permissible background noise) occur as the applied voltage is increased. PDIV is expressed as 1/2 of the peak of the alternating voltage. Many factors may influence the value of PDIV, including the rate at which the voltage is increased as well as the previous history of the voltage applied to the winding or component thereof.
Partial discharge (PD) power loss:
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.
Partial discharge (PD) quantity:
The magnitude of an individual discharge in an insulation system expressed in by of the apparent charge transfer (Q) measured at the terminals of the test object expressed in coulombs. In the case of complete windings, such measurements are limited to the frequency range of 10 kHz to 1 MHz, and the results obtained are a function of the bandwidth of the particular detection system.
Pseudoglow discharge:
A type of partial discharge (PD) characterized by current pulses of relatively small amplitude and, generally, a long rise time. As a result of the upper frequency limitation in their Fourier frequency spectrum, pseudoglow discharges are not readily detected by conventional PD detectors. Pseudoglow discharges are also characterized by a diffused glow that cannot be visually distinguished from that resulting from a true-glow discharge.
Pulse discharge:
A type of partial discharge (PD) phenomenon characterized by a spark-type breakdown. The resultant detected pulse discharge has a short rise time, and its frequency spectrum may extend as far as ≥100 MHz. Such a pulse discharge may be readily detected at the terminals of the winding or component under by means of conventional pulse detectors,that are generally designed for PD measurements within the frequency band from 10 kHz to several megahertz.
Pulse height analysis:
The measurement of the number of pulses occurring within a series of magnitude windows over a defined phase period, not exceeding one half-cycle, of the alternating voltage applied to the object under test. Sampling periods of one or more seconds may be involved.
Pulse phase analysis:
A type of pulse height analysis in which there is more than one phase window for each half-cycle of the alternating voltage applied to the object under test.
Pulseless-glow discharge:
A type of partial discharge (PD) phenomenon characterized by a diffused glow. The overall voltage waveform across a gap space undergoing pulseless-glow partial discharge does not indicate the presence of any abrupt collapse in voltage, except for the two at the beginning of each half-cycle. Although partial discharge energy is expended over the pulseless region, a conventional partial discharge detector will give no indication of this, as it will only respond to the two initiating breakdowns in each half-cycle.
Quadratic rate (D):
The sum of the squares of the individual discharge magnitudes during a certain time interval divided by that time interval.
Quadratic rate (D):
The sum of the squares of the individual partial discharge magnitudes during a certain time interval divided by that time interval.
Quasi-peak:
A quantity, measured with an RC weighting circuit having specified time constant(s), which is a fraction of the peak value of pulses of constant amplitude, the fraction increasing toward unity as the pulse repetition rate is increased.
Radio influence voltage (RIV):
The radio noise voltage appearing on conductors of electric equipment or circuits, as measured using a radio noise meter as a two-terminal voltmeter in accordance with specified methods.
Random partial discharges (PDs):
Partial discharges that recur infrequently with a repetition rate of less than one per second.
Resistance temperature detector (RTD):
A temperature detector that is usually a three-terminal resistor, either 10 Ω copper at 25 °C or 100 Ω platinum at 0 °C, whose sensing element is about 50 cm in length, which is encapsulated within an insulating substrate of suitable thermal rating. RTDs are usually installed between top and bottom bars (coil legs) in a given slot. The measuring leads should be positioned such that they do not interfere with the stress control coating.
Signal attenuation:
A decrease in signal magnitude from one point to another.
Signal reflection:
The result of a mismatch in the characteristic (surge) impedances of the signal transmission path in which a portion of the incident signal is reflected in the direction opposite to the incident signal.
Signal transmission:
The process by which the signal travels in a medium in the same direction as the incident signal.
Slot discharges:
Partial discharges that occur between the outer surface of a coil or bar and the grounded core steel.
Spectrum analysis:
Measurement of a signal over a range of frequencies using a partial discharge detector having a defined bandwidth resolution.
Stress control coating:
The paint or tape on the outside of the groundwall insulation that extends several
centimeters beyond the conductive (semiconductive) slot coating in high-voltage stator bars and coils. The stress control coating often contains silicon carbide particles that tend to linearize the electric field distribution along the coil or bar end turn. The stress control coating overlaps the conductive (semiconductive) coating to provide electrical contact between them.
Total integrated charge:
A quantity derived from pulse height.
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