Partial Discharge (PD)

Partial discharge (PD) is a sign of deteriorating insulation, which might foretell an electrical failure of high-voltage equipment and result in highly expensive repairs. The existence and amplitude of Partial discharge (PD) are significant criteria to measure for the early diagnosis of deteriorating insulation quality and the evaluation of produced, installed, or repaired product quality, making the phenomena of enormous practical relevance to the electric power sector. Interpreting the Partial discharge (PD) data and figuring out how long an insulation system may last before experiencing a destructive and expensive breakdown is one of the most challenging challenges.

Iconic Engineering Limited possesses extensive experience in the domain of Partial Discharge (PD) testing and offers a variety of equipment incorporating diverse measurement technologies to cater to a wide range of Partial Discharge (PD) testing applications.

What is Partial discharge (PD)?

IEC 60270: High-voltage test techniques-Partial discharge measurements is the most significant standard that any expert would mention when discussing Partial discharge (PD). This standard is applicable to the measurement of Partial discharge (PD) in electrical systems or equipment when testing with either DC voltage or AC voltage up to 400 Hz.

According to IEC 60270 Partial discharge (PD):

is “A localized electrical discharge that only partially bridges the insulation between conductors and which can or cannot occur adjacent to a conductor”.

In general, Partial discharge (PD) develops in parts of the insulation or on the insulation's surface where there are larger concentrations of local electric field stress, such as a cavity in a solid dielectric, a protruding point, or low-density sections of a liquid dielectric. When the mean-free route is sufficiently short or the electric field is strong enough to induce impact ionization and produce an electron avalanche (also known as a "streamer"), the process of Partial discharge (PD) begins. Additionally, the discharge process has to be initiated by a free electron, which may be produced by cosmic rays or naturally occurring radiation. As a result, there is a statistical temporal lag associated with Partial discharge (PD) commencement.

Partial discharge (PD) test circuit for alternating voltages:

According to IEC 60270, Partial discharge (PD) measurements may be made using a variety of test circuits. Five acceptable test circuit examples are included in the standard. But they may all be condensed into a single schematic, as seen in Figure 1.

Figure 1: Partial discharge (PD) measurement circuit according to IEC60270

Theoretically, sensitivity is maximized when  > > . However, the majority of programmed do not work well in this circumstance. This is mostly caused by the extra load that the high-voltage source would have to provide. IEC 60270 advises adopting a  value of 1nF or greater as a result.

Each component in the test circuit has to have background noise that is suitably low. Signals that are detected during the Partial discharge (PD) measurement but do not come from the test item are referred to as this background noise. A high-voltage (HV) filter may sometimes be able to aid in attenuating undesirable signals that are being transmitted from the high-voltage (HV) source. The filter further enhances the sensitivity of the test circuit by preventing Partial discharge (PD) pulses from passing through the high-voltage (HV) transformer.

The test circuit shown in Figure 1 may also be utilized in a few other ways. To put the measuring impedance in series with the test object, for instance, the test object and coupling capacitor might be switched. The use of a bushing capacitance  in lieu of the coupling capacitor is another approved test circuit. As a result, the bushing tap is joined to the coupling device. The coupling element is often a quadrupole, which is a four-terminal network. It transforms the output voltage from the Partial discharge (PD) current pulse. Additionally, it often has certain filtering capabilities that separate the test voltage to enable synchronization as well as quick overvoltage prevention.

Partial discharge (PD) test circuit for direct voltage:

Figure 1 shows how closely the test circuit used for direct voltage testing resembles the test circuit used for alternating voltage. The coupling capacitor, however, cannot be utilized to determine the voltage. An extra DC voltage divider is necessary if a voltage reference is wanted.

In addition, the measured values and assessment standards vary from those used with alternating voltage. The voltage waveform shows no polarity reversal. As a result, the insulating relaxation properties will have a significant impact on the discharge pulse repetition rate. The blog is all about measuring alternating voltage. Clause 11 of IEC 60270 contains further information on Partial discharge (PD) measuring under direct voltage.

A Partial discharge (PD) produces a unipolar current pulse at its source that has a continuous and wide frequency range (from DC to several hundred megahertz). The path this pulse takes from its source to the sensor, however, introduces distortion. Because of this, it is impossible to directly quantify the localized charge at the discharge point.

A Partial discharge (PD) measurement may be used to obtain a variety of measurable values. Wherever practical, IEC 60270 advises using the apparent charge. The charge that, if quickly injected between the test object's terminals in a predetermined test circuit, would provide the same result on the measuring device as the actual Partial discharge (PD) current pulse is known as the apparent charge of a Partial discharge (PD) pulse.

Picocoulombs (pC) are used to measure apparent charge. The value is determined for a single theoretical Partial discharge (PD) event by integrating the current across time. In the test circuit, the data acquisition unit measures the voltage drop across a measuring impedance and transforms it into a current.

However, Partial discharge (PD) seldom happens in isolated single instances when there is alternating voltage. The typical output of a defect that causes Partial discharge (PD) is a pulse train with stochastically distributed charge values. As a result, clause 4.3.3 of IEC 60270 proposes utilizing what is known as the biggest regularly recurring Partial discharge (PD) magnitude and a weighting procedure. The Partial discharge (PD) instrument typically shows the derived data automatically.

As was previously noted, Partial discharge (PD) pulses have a steady and wide frequency content and may thus be monitored at many frequencies. The impact of various frequency ranges is beyond the purview of this article. To measure Partial discharge (PD) with wide-band Partial discharge (PD) equipment, IEC 60270 includes the following requirements:

30 kHz ≤  ≤ 100 kHz

 ≤ 1 MHz

100 kHz ≤ ∆f ≤ 900 kHz

Were,

         Lower Limit Frequencies

         Upper Limit Frequencies

∆f        Total Bandwidth

Test procedure:

There is no particular test technique specified in IEC 60270. They are often offered by the testing guidelines or the relevant technical committee for a particular test item.

The test voltage levels and frequency, the order and length of the voltage applications, and the pass/fail criteria are all defined by a typical test method. These requirements may involve figuring out the Partial discharge (PD) inception and extinction voltages.

The inception voltage is the lowest voltage at which Partial discharge (PD) initially appears after the voltage has been raised from a level at which it does not. The applied voltage at which Partial discharge (PD) stop occurring when the applied voltage is reduced from a higher value at which Partial discharge (PD) is detected is known as the extinction voltage.

Typically, the applied voltage is increased to a level that, for a predetermined amount of time, surpasses the partial discharge test voltage. We refer to this as pre-stressing. After that, the applied voltage is lowered to the designated Partial discharge (PD) test voltage and kept there for a certain period of time. Then, at predetermined intervals or at the conclusion of this period, the Partial discharge (PD) amount is measured.

Figure 2 provides an illustration of a typical test sequence with pre-stressing.

Figure 2: Typical test sequence with pre-stressing

The Different Types of PD Measurement Technologies:

Depending on the necessary application, Iconic Engineering Limited provides a broad range of various Partial discharge (PD) measuring systems. Electrical conventional detection techniques and non-conventional detection methods make up the majority of the technologies.

Conventional Partial Discharge (PD) Detection: All of the information in this blog article relates to the IEC 60270 conventional or electrical Partial discharge (PD) detection technique. These techniques are for "offline" Partial discharge (PD) testing when the excitation voltage is supplied by an external source.

For Partial discharge (PD) measurements of high voltage (HV) equipment, such as transformers, generators, switchgear, and cable systems employing applied voltage in the lab or on-site testing conditions, we Iconic Engineering Limited can provide you with precise and dependable solutions.

Iconic Engineering Limited has a Precision Acoustic Imager that is designed to detect gas and vacuum leaks as well as Partial discharge (PD) and corona discharge. The innovative technology of the acoustic imaging camera makes it a cost-effective, secure, useful, and cozy tool for teams who examine and repair industrial high-voltage and power distribution equipment.

Iconic Engineering Limited has a Portable Partial discharge (PD) Diagnostics System, which enables precise localization of Partial discharge (PD) sources throughout the length of a cable utilizing the "Time of Flight" PD measuring approach, while doing Partial discharge (PD) testing of medium voltage cables using Very Low Frequency (VLF, 0.1 Hz) voltages.

Non-Conventional Partial Discharge (PD) Detection:

In addition to the offline method used in IEC 60270, there are alternative ways to identify the existence of Partial discharge (PD). These novel technologies, which perform "online" tests while being employed for on-site Partial discharge (PD) testing, depend on sensors. Due to the measurement not requiring a galvanic connection to the high voltage (HV) conductors, the voltage provided to the DUT does not need to be turned off for testing.

Iconic Engineering Limited has the Online Partial Discharge Spot Tester which is a mobile Partial discharge (PD) detector that measures Partial discharge (PD) signals in switchgear and other grounded surfaces using capacitive TEV ("transient earth voltage") sensors. Additionally, the equipment can monitor Partial discharge (PD) signals in cables that are inductively linked from the cable sheath or ground thanks to the employment of sensitive High-Frequency Current Transformer (HFCT) sensors.