JI YES
CHAPTER 5: ELECTRICAL BREAKDOWN AND PARTIAL DISCHARGES IN LIQUIDS
5.2 Literature Review
5.2.1 Detection Methods
The current methods utilized for the detection of partial discharges fall into three general categories,which may be summarized as follows:
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5.2.1.1 Electrical Detection
Partial discharges cause high frequency, low amplitude perturbations in the applied voltage and current waveforms. Electrical discharge detection techniques are intended to detect these perturbations. At least two standard circuits exist (as well as several variations) which enable detection of either Radio Interference Voltage (RIV) or PD level (pico-coulombs) associated with the phenomenon [87,88]. Unfortunately these techniques are not very effective,typically as a result of excessive interference signals (electromagnetic interference) or excessive test object capacitance. A further limitation of this technique is that it is generally not possible to indicate the location of a detected partial discharge source.
5.2.1.2 Chemical Detection
In a transformer the insulation is primarily oil impregnated cellulose and it is usually submerged in a dielectric liquid. In the majority of cases the liquid is mineral oil.
Consequently, partial discharges cause the formation of degradation products from both the effected insulation and oil. These products are principally gaseous and are absorbed into the surrounding volume of oil. Above this oil is usually a blanket of nitrogen and those gases absorbed in the oil that would eventually appear in the head space as they reach an equilibrium situation with the liquid.
Detection of partial discharges then involves the analysis of oil for specific absorbed gases.The presence of hydrogen and/or acetylene gas is an indication of PD activity and arcing respectively [14]. This technique is free of the noise associated with electrical detection methods. This is an accepted and recognised technique in the field.
Problems associated with chemical methods are due to the fact that a PD source evolves a small volume of gas as compared to the large volume of oil present in the system. Consequently there can be a long time delay between the initiation of a PD source and the evolution of enough gas for it to be detectable. For this reason the information tends to be historic.Oil analysis would indicate that a problem exists,but does not allow for an evaluation of the instantaneous condition of a transformer. As the gas and oil samples are taken from large reservoirs, there is also little information regarding the physical location of the source.
5.2.1.3 Acoustic Emission Sensing
Partial discharges are pulse like phenomenon,which cause mechanical stress waves to exist in the material involved. These stress waves propagate through the surrounding transformer oil and can be detected by sensors attached to the tank wall.
Much work has been done in the detection of these high frequency signals [89-92]
but transformers are mechanically noisy, there has been considerable difficulty in differentiating between partial discharge produced signals and others.
One of the advantages of the acoustic method is its ability to locate a source provided its presence can first be detected.The success of this method is based on the sensitivity of the acoustic sensor [91].This method is less sensitive for sources inside the winding structure [92].
5.2.2 Measurement and Interpretation
The technique of measuring and analysing partial discharges occurring in insulation structures or assemblies can be used to detect weaknesses before they lead to catastrophic failure. In a transformer, partial discharges cause transient changes of voltage to earth at every available winding terminal.
Commercially available detectors are used to detect radio interference voltage, or radio influence voltage [87], or measure the intensity of partial discharges expressed in pico-coulombs [88]. Partial discharge magnitude expressed in pico-coulombs is now established in IEC and various other documents [93-96]. There exists well- documented literature [87,95,96] on partial discharge patterns, their evaluation of results and determination of origin, etc,to assist practitioners in the interpretation of results.
5.2.3 Localisation Techniques
The possibility of partial discharge location determination is a major features of acoustic discharge detection. Location can be based on either measurement of the time of signal arrival at a sensor [89,91,95,97-101]or on the measurement of a signal level [88], provided that the discharge source is at a fixed position. In practical situations, localisation is based on time-of-flight measurements requiringtwo or more simultaneous measurements in order to facilitate triangulation to determine the source location. With the use of two or three sensors (all-acoustic system) an accuracy margin of ±10 em is possible [95].
Locating the position of a discharge is also possible when used in conjunction with electrical sensing systems. Here an electrically detected pulse is recorded on one channel of the measurement equipment and the outputs of several acoustic sensors recorded on other channels. This technique is often used at a factory or laboratory due to the problems inherent in electrical detection methods. It has been reported that a precision better than ±5 em may be obtained with the use of a combined electrical and acoustic system [91]. By employing advanced signal processing techniques to this system,the accuracy can be improved by 60% [99].Provided that it may be possible to differentiate the partial discharge signals from other noises, the time delays associated with sensor locations may be used to calculate the source location.
5.2.4 Acoustic Wave Propagation
Acoustic signals produced by a partial discharge can be divided into two waves i.e.
longitudinal and shear waves. In power transformers, the mediums in concern are liquid (oil) and solid (steel wall of tank).The speed of the longitudinal wave in oil is approximately 1400 m/s [99]. Corrections to speed of sound for temperature and moisture content are not generally made to increase accuracy because the uncertainties due to material propagation are usually much larger [101]. However, in the steel wall of the transformer tank, both longitudinal and shear waves propagate with velocities of 5200 m/s and 3200 m/s respectively [101].
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5.2.5 Discharge Inducing Defects - Model Geometries
In order to analyse discharge activity patterns and other important characteristics, researchers have performed experimental investigations based on a range of electrode or model geometries [95]. These geometries represent various types of discharge defects, which can be present in oil filled transformers. Discharge sources have also been activated under oil by the use of simple gapped electrode arrangements such as a point-to-hemisphere [94],sphere-to-sphere [60] and point- to-plane [102]. The point is usually a stainless steel needle with a tip radius ranging from 20 to 50 microns and a sphere diameter ranging from 12.5to 25mm.
5.2.6 On-Line Fault Gas Monitoring
Analyzing fault gases to diagnose problems in power transformers is universally accepted and has been used for many years. Dissolved Gas Analysis remains a key technique in establishing fault mechanisms. The introduction of in-situ on-line analysis brings new possibilities. Real-time gassing, including trends, can be associated with specific events, even on a daily basis, and yield information closely related to the unit's problems. On-line gas analysis offers the ability to make an assessment of the transformers operatingcondition close to real time.
Several on-line gas detection devices have been evaluated to establish their performance [103]. Based on this evaluation, one monitoring system (Hydran 201 i) has proven to be reliable in the field.This system monitors only one key fault gas (Hydrogen) [104] - necessary for detecting discharges in transformers. The manufacturer of Hydran specifies the analytical performance in terms of accuracy to be ±10% of reading ±25 ppm of dissolved hydrogen gas.
5.3 Breakdown in Insulating Liquids
There are two main types of insulating liquids that are commonly used in high voltage applications. These are petroleum and synthetic based insulating oils [105]. Other oils used are synthetic hydrocarbons and halogenated hydrocarbons. For very high temperature applications silicone oils and fluorinated hydrocarbons are generally used.
5.3.1 Petroleum Oils
Petroleum insulating oils are mainly used in transformers (oil-immersed), circuit breakers,capacitors,bushings and power cables.In transformers the oil impregnates the paper insulation and provides insulation between the live and grounded parts.
The oil is also used for cooling by convection.
The oils are obtained by fractional distillation of crude oil. The oils are divided into two main classes based on their chemical compositions.
•
Paraffin base (or methane series) characterised by the formulae CnH 2n+2[106]- Due to oxygen in the air the paraffin-based hydrocarbons are liable to deteriorate by oxidation [106].
• Naphthene base, characterised by the chemical formulae CnH2n [106]
- The naphthene hydrocarbons are more stable since they have a higher oxidation resistance [106].
Before application, the oil has to be purified, dried and free of acid (acid increases the oil conductivity). Highly purified oils are of lighter colour while oxidised oils are darker. Oxidised oil forms sludge that settles on the windings and hampers effective cooling and results in overheating and ageing of the solid insulation.