LIST OF SYMBOLS! ABBREVIATIONS
CHAPTER 2: FAILURE MODES AND DIAGNOSTIC METHODS FOR TRANSFORMERS AND CIRCUIT BREAKERS
2.2 Instrument Transformers .1 Introduction
2.2.6 Diagnostic Methods
Allan et al [21] stated that condition monitoring systems should be based on diagnostic methods that do not interfere with operation, leaving off-line tests to periodic and special maintenance or situations. Information obtained from these condition monitoring systems will then be used to determine the maintenance schedule and replacement (end of safe, useful life) date. The on-line concept is to obtain every possible service minute from instrument transformers by retiring them to avoid in-service failures. Table 2.2 highlights diagnostic methods most widely used for instrument transformers,together with references.
ONI APPLICATION
PROBLEMS DIAGNOSTIC METHODS OFF TO: REFERENCES
LINE CTs VTs CVTs
Mechanical 1. Pressure inside enclosure ON X X X [26]
2. Pressure valve ON X X X [26]
3. Bellow position ON X X X [26]
4. Oil level indicator ON X X - [26]
5. Inspection for oil leakage ON X X X [26]
6. Water content in the oil ON X X - [52]
Thermal 7. Inspection of contacts ON X - - [26]
8. Gas-in-oil chromatography ON X X - [13,17]
9. Thermal scanning ON X - X [26,27,29]
Chemical 10.Oil testing (neutralization value,corrosive sulphur,
oxidation,viscosity ,etc.) ON X X - [30]
Dielectric 11.Gas-in-oilchromatography ON X X - [13,27]
12.H2detection ON X X - [19,26,79]
13.Oil dielectric strength ON X X - [12,27,30]
14. PD measurement ON X X X [19,21,26-28]
15.Zero sequence checking OFF - - X [27]
16. Tan (delta) measurement ON X - X [18,21,22,27,80]
Table 2.2: Diagnostic methods used In Instrument transformers (paper-oil).
In order to facilitate the monitoring of the instrument transformer condition, the installation of a low cost monitoring system should be economically justified in the case of high voltage systems. CIGRE reported [19] that these types of devices used depend on the sealing and oil compensation systems adopted and that they may include pressure sensors, pressure valves, bellow position indicators and gas detection systems.
Visual checks are generally performed to identify traces of oil originating from hair- line cracks on the porcelain and for connection discolouration indicating overheating of poor or loose contact. Depending on the circumstances the following diagnostic techniques are usually applied:
2.2.6.1 Tan (delta) or Loss Angle Measurement
The tan (delta) or loss angle measurement is a measure of the power loss in the insulating material and therefore a general indication of the quality of the dielectric.
Some types of current transformers are provided with a connection point at the lowest capacitive grid in order to measure the loss angle. Therefore,it is possible to measure the loss angle of the main insulation between this terminal and the bottom flange of the current transformer.
In the case of voltage transformers, it is possible to measure the loss angle of the insulation between the shielding grid and the secondary winding. However,this is not
15
common practice. The value of the loss angle depends on a number of factors such as voltage, temperature and design features. IEC 358 [22] recommends that measurements be carried out at higher temperatures (about 90 °C) at which the presence of contaminant may be detected as a function of the voltage.
2.2.6.2 Analysis of Gases Dissolved in Oil
Most types of instrument transformers are provided with an oil valve from which it is possible to take samples of oil. International standard IEC 567 [23] states that oil sampling must be done with caution if performed while the unit is in service. This is due to the fact that air bubbles created during sampling may cause failure. For gas analysis and their interpretation,the same methods as with power transformers are in use [24,25].
Precautions may be taken during oil sampling, because the volume of oil is small and may require refilling.The presence of a large amount of hydrogen is an indication of partial discharge activity in the oil between the paper layers. According to EPRI research [17] the formation of x-wax in these areas is typical.
Various researchers [19,26,79] have reported that voltage transformers have been provided with hydrogen detection probes. These researchers stated that the hydrogen concentration in oil is checked after diffusion from the oil through a membrane by a thermal conductivity detector.
2.2.6.3 In-Service Partial Discharge Detection
Various researchers [27,28] reported that some utilities employ this in-service partial discharge detection to detect discharge activity in instrument transformers by means of acoustic sensors fixed with magnets on the outer tank surface. Gabriel et al [27]
stated that this method is reliable for detecting the presence of large partial discharges (more than 1000 pC).High intensity partial discharges may result in rapid violent failure of equipment.
2.2.6.4 Thermal Scanning
Thermal scanning may be used systematically in substations. Moja [29] stated thermal scanning reveals overheating at the level of the connecting contacts and that it might be possible to detect the oil level within the sealed apparatus.
2.2.6.5 Other Oil Checks
International specification IEC 422 [30] states that an inspection of the internal insulation should be performed every 3 to 5 years. In the case where there are'leak- free' cable terminations, ageing may occur and result in the formation of moisture from the paper or of other substances that may have adverse chemical effects on the insulatinq oil.
!his, i.n t~rn, influences the breakdown strength or the thermal stability of the insulation I~ an unfavourable manner.As all these phenomena can result in physical and chemical changes of the insulating oil, certain basic measurements are
important. These are breakdown strength, water content, neutralisation value, dielectric interfacial tension,dielectric loss angle,etc.
Presently, the condition of instrument transformers may be monitored systematically.
The application of computer-based techniques with advanced procedures for data acquisition and processing favours on-line methods. However, the extensive application of these techniques depends on the availability of simple and reliable diagnostic and surveillance sensors. According to EPRI research [20] internal pressure, gas detection and bellow position sensors can now be mounted on instrument transformers for extra and ultra high voltage systems.
2.3 Circuit Breakers