The presence of partial discharges (PDs) causes difficulty in sustaining the isolation of conductive materials, and these damages can worsen, leading to equipment loss, and in more severe scenarios, they can increase the risk of fires, electric shocks, equipment deterioration, or even explosions.
What are partial discharges?
Partial discharges are rapid and small breakdowns in the insulation of an insulating material or on its surface, which can be characterized as incomplete, intermittent, and rapid pulsating electrical signals, on the order of nanoseconds. It is important to note that partial discharges do not traverse the entire path between the conductors.
These discharges occur due to the potential difference provided by an active electric field, they last less than 1 ms and are generally accompanied by sound, light, heat, and chemical reactions. They can be initiated by vacuums, imperfections, and cracks in dielectric components of the equipment and can be present in solid or gaseous materials. As a consequence, they can cause insulation deterioration leading to further complications.
The presence of PDs causes difficulty in sustaining the isolation of conductive materials, and these damages can worsen, leading to equipment loss, and in more severe scenarios, they can increase the risk of fires, electric shocks, equipment deterioration, and even explosions.
How and why do partial discharges occur?
Partial discharges can occur in various components of the electrical system, such as transformers, cables, insulators, circuit breakers, surge arresters, switches, and generators.
These unwanted events can be triggered by a variety of factors, including:
Aging assets;
Adverse weather conditions;
Manufacturing defects;
Design errors;
Lack of insulation;
Mechanical failures;
Low-quality materials;
Overloading of the electrical system.
Characteristics of partial discharge formation:
Cavities, incrustations by foreign bodies, heterogeneous dielectrics, electric track formation, and erosion;
Surface problems due to corona effect or formation of bands with different electrical permeabilities (such as tips and other irregular effects);
Dividing joints between different insulating materials.
Effects of its formation:
All types of partial discharges manifest themselves through high-frequency signals, such as light, sound, and/or electric arcs. In addition, depending on the dielectric medium, whether it be gas, liquid, or solid, chemical reactions may occur.
These events are characterized by a variety of physical and chemical phenomena that occur at the site, such as:
Conducted and radiated electromagnetic pulses;
Brightness;
Acoustic noise;
Temperature increase;
Chemical reactions.
Classification of partial discharges:
1. Surface discharge
Also known as surface tracking or formation of dry bands and wet bands, it commonly occurs due to moisture and lack of maintenance, leading to insulation breakdown. Generally, this failure spreads until complete loss of the equipment.
This type of PD occurs when the electric field component tangential to the surface exceeds the critical value determined, in this case, the Partial Discharge deterioration process begins. This process can lead to insulation breakdown, known as tracking.
2. Corona discharge
Also known as external discharge, it occurs when the discharge encounters air directly and diffuses through the pointed or sharp edges of a conductor, emitting sound and radiofrequency. In the initial stages of voltage induction, they can be characterized by surface brightness and discharge currents due to gas formation on the material surface. In this chemical process, gaseous media are incorporated that can generate ozone causing more fissures in the insulation of materials composed of polymers. The corona effect also generates erosion effects in solid materials, common to aluminum electrodes or degradation of HDPE (high-density polyethylene) and silicone.
3. Void discharge
These are insulation faults that occur mainly in solids, such as cables, bushings, and junctions, leading to the formation of gases in cavities and/or cracks in empty spaces. It is also known as internal discharge. When they occur in liquid materials, they commonly occur in gas bubbles due to the heating of water vapor created in regions with high electric field intensity.
4. Arc discharge
It is a longer electrical discharge when the electrical breakdown of a gas occurs, producing plasma dissipating through the air or other non-conductive medium.
5. Mixed discharges
This phenomenon occurs when there are thermal expansions or mechanical stresses, forming internal delamination of the dielectric, or by the formation of tips on conductors with insufficient or worn coatings.
Diagnosing partial discharges:
To properly measure partial discharge, it is necessary to consider the type of equipment and its insulation. For example:
In gas-insulated substations, it is possible to measure using ultrasonic sensors;
In insulators, it is perceptible by light through anechoic chambers;
In generators or large motors through the electrical method by connecting capacitive sensors to the stator;
In transformers by gas chromatography, as oil degrades with the formation of activation gases or by leakage current analysis.
HVEX offers a wide range of partial discharge measurement and monitoring equipment. If you want to learn more about our solutions, please contact one of our consultants to find the best option for your case.
Diagnosis methodologies of partial discharges
1. Electrical Method
It is characterized by the measurement of small signals through capacitive couplers. Calibration is performed as a function of the apparent load, with a standardized calibration procedure, and the values of the measured PDs in pC can be compared between different circuits and test objects. This method can also be adopted to test the integrity of the insulation for most voltage equipment, as well as acceptance tests for equipment due to high precision and sensitivity.
Advantages:
The main advantages of the electrical method for measuring PDs are:
Calibration is performed as a function of the apparent load;
Due to the standardized calibration procedure, the PD values measured in pC can be compared between different circuits and test objects;
It is possible to identify the type of PD source;
It can be adopted to test the integrity of the insulation for most voltage equipment;
It can be used for equipment acceptance tests due to high precision and sensitivity.
Disadvantages:
It offers greater complexity in locating the PD source;
With the increase of the capacitance of the tested object, the sensitivity decreases;
More prone to electromagnetic interference.
2. Acoustic Method
Partial discharge detection by the acoustic method aims to record the acoustic signal generated during its occurrence. Such signals result from a pressure wave generated by the release of energy from partial discharges, and their detection should be performed using suitable acoustic sensors. In general, the frequency range over which the sensors must operate is between 20-250 kHz.
In this methodology, the adequacy of the sensors must be considered, considering the propagation of waves in the test object to ensure high precision and sensitivity in the records.
The sizing of acoustic sensors for partial discharge detection should be done considering the medium in which these should capture the wave. Microphones are commonly used in air and other gases, while piezoelectric material hydrophones are used in liquid media.
Advantages:
The main advantages of the acoustic method for measuring PDs:
Non-invasive, relatively cheap, and easy to use;
Greater immunity to electromagnetic interference;
Efficient location of PD sources.
Disadvantages:
Calibration of the acoustic signal in relation to the partial discharge load is not possible;
Not as convenient for continuous monitoring of PDs;
Not very useful for estimating the risk of defects in transformers, due to the high attenuation of the acoustic signal;
Slow data processing.
3. Ultra-High Frequency (UHF) Method
This methodology is based on detecting the electromagnetic emissions from the discharge locations in the insulation produced by partial discharge pulses.
The advantage of the UHF method is that the signal-to-noise ratio obtained, under "in loco" conditions, is higher when compared to PD measurement described in IEC 60270, a fact that allows greater sensitivity and precision during processing. This occurs because, at high frequencies, electromagnetic waves are more attenuated as they propagate through the medium.
Thus, the electromagnetic noise, responsible for polluting the measured signals, generated farther from the UHF sensor when compared to the PD source, undergoes greater attenuation than the electromagnetic emissions from the discharge itself, resulting in higher quality measurement due to higher signal-to-noise ratio.
Advantages:
The main advantages of the UHF method for measuring partial discharges are:
Greater immunity to noise, which means that the UHF method can be used for PD measurement in noisy environments;
Good precision for locating defects compared to other methods;
Pulse shapes and associated frequencies are preserved by UHF sensors. This aspect is important because different types of defects can generate PD signals of different shapes and frequencies, which in many cases helps in the identification and location of discharges;
High success in practical measurement and monitoring experiments in gas-insulated substations (GIS).
Disadvantages:
In the UHF method, measurements cannot be calibrated in terms of apparent discharge load;
The UHF method is less flexible, since UHF sensors and related accessories are generally specific to a type of equipment;
High percentage cost compared to other methods.
4. Optical Method
The aim of the optical methodology is to detect ultraviolet or infrared radiation emitted by partial discharges using receivers that depend on the surrounding medium (air, sulfur hexafluoride (SF6), and oil) for measurement.
Advantages:
Among the main advantages of the Optical method for measuring partial discharges are:
All measurements are galvanically isolated, so there are no influences on the measurements, which are immune to unwanted signals.
Disadvantages:
Accessibility of the sensor inside the equipment;
Possibility of calibration loss of optical sensors;
Opacity of the medium, especially in insulating oils and signal loss.
5. Chemical Method
The chemical method for PD detection is one of the most traditional and consists of chromatographic analysis of gases dissolved in the transformer's insulating oil, which originate from the chemical reactions that occur due to the degradation of insulating materials of the equipment itself.
The methodology is based on the fact that the quantity and types of gases dissolved in the oil contain information about faults in the insulation system.
According to the IEEE Std C67-113 standard, each type of defect generates one or more types of gases in the insulation.
Thus, chromatographic analysis is very useful for its ability to assess the various types of gases and enable the identification and diagnosis of various defects, from the initial stage to the most advanced. Among the gases commonly generated by internal defects, according to Gutnik, the following stand out:
Combustible gases: Hydrogen (H2), Methane (CH4), Ethane (C2H6), Ethylene (C2H4), and Acetylene (C2H2);
Non-combustible gases: Oxygen (O2), Nitrogen (N2), Carbon Monoxide (CO), and Carbon Dioxide (CO2).
Advantages:
Advantages of the chemical method, according to BRUNINI:
Efficiency in identifying PDs through chromatographic analysis of the gases present in the oil;
A methodology that is well-established with several consolidated researches in this area;
Allows easy identification of the evolution of gas formation over time.
Disadvantages:
Impossibility of measuring the apparent load of PDs;
Does not allow the location of PD pulses;
Online measurement equipment is expensive and mostly require carrier gas or moving parts.
Those sensitized by fixed electrodes do not have sufficient resolution in parts per million to detect occurrences at the beginning of PD formation.
Protection against partial discharges: essential measures for high-quality electrical equipment!
In summary, measurement is feasible in all cases, but each equipment requires a specific solution. If the analyzed equipment presents partial discharges above normative values, its withstand capability is likely to be low, affecting its reliability and leading to a continuous degradation process.
To ensure protection against partial discharges in electrical equipment, it is essential to implement preventive measures. This includes conducting tests according to manufacturing standards, using protective devices, and adopting preventive maintenance with high-quality materials and components.
Furthermore, it is essential to carry out periodic tests and diagnostics on the equipment to ensure its integrity and proper functioning.
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