It is important to note that assessing the condition of equipment also allows the possibility to predict failure. This thesis focuses on establishing condition assessment techniques for major power equipment such as power transformers.
DISCUSSION
CONCLUSIONS
LIST OF TABLES
LIST OF ILLUSTRATIONS
LIST OF SYMBOLS! ABBREVIATIONS
INTRODUCTION
- Importance of High Voltage Power Equipment in Substations
- Life Assessment and Life Extension
- Economic Considerations in Condition Assessment
- Aims of this Study
- Structure of Thesis
Therefore, similar plants installed in different areas of the system will ultimately exhibit very different conditions. The purpose of this thesis is to present condition monitoring techniques (including condition diagnosis) to assess the condition of the large high voltage plant equipment installed in transmission system substations and to provide recommendations for life extension.
FAILURE MODES AND DIAGNOSTIC METHODS FOR TRANSFORMERS AND CIRCUIT BREAKERS
- Power Transformers
- Deterioration Factors and Failure Mechanisms
- Instrument Transformers .1 Introduction
- Diagnostic Methods
- Circuit Breakers .1 Background
- Stresses In-Service
- Deterioration Factors and Failure Mechanisms
- Assessment of Diagnostic Methods
The moisture content in the oil can be an indication of the moisture content in the solid insulation. Depending on the location and severity of the problem, adjustments can be made on site.
CONDITION MONITORING, CONDITION DIAGNOSTICS AND CONDITION ASSESSMENT APPLIED TO
TRANSFORMERS AND CIRCUIT BREAKERS
Condition Diagnostics
Once the moisture content of the oil for a given temperature is determined, the corresponding moisture content for the paper can be estimated by on-line or off-line methods.
Paper Insulation Condition
The application of the curve can be understood by looking at some examples taken from reference [50]. CIGRE also reported that radio interference voltmeters can be used and are of the narrowband type.
Mechanical Condition
- Voltage Transformers (Off-Line)
Voltage impulse application with a view to observing the transfer function of the transformer can be considered as a significant condition measurement tool. The response of the transformer before re-c1amping is compared with the results of a reference transformer. EPRI also reported that all manufacturers of current transformers experienced some failures, but most of the failures involved four manufacturers.
The presence of contaminants, moisture and oxygen can cause insulation oil to degrade. Researchers Gabriel and Bradley also stated that the testing of voltage transformers may be due to the history of use. Any increase in leakage current may indicate that the insulation capacitance has changed.
Circuit Breakers
- Instrument Transformers
- Circuit Breakers
Each time the circuit breaker operates, operation data can be stored and compared with previous operations. Various researchers [31-34] have reported that the circuit breaker parameters that can be monitored are classified into the following categories. In addition, parameters can be classified as suitable for offline monitoring or online monitoring, or both.
To preserve the life of a power transformer, the load management of a unit can be extremely important. There are several techniques that can be used to set the warp winding offset in transformers. Sokolov et al [83] reported that instantaneous failure can be caused by thermal runoff or electrical origin.
JI YES
Conclusions
Condition monitoring using on-line and/or off-line techniques is a key component of condition assessment. Condition monitoring may be necessary to detect imminent breakdowns in electrical equipment and enable maintenance to be prioritized Le. Because of the emphasis on reduced maintenance costs and life extension, it is desirable to have control systems that will provide information to determine when maintenance should be performed and to implement measures or practices to extend life. Equipment can be replaced if it does not meet system reliability requirements or when the cost of life extension measures exceeds the cost of replacement.
The purpose of maintenance and condition monitoring is to maintain the reliability level of existing equipment and determine the necessary measures that can be taken to extend its service life. It is clear that through regular maintenance (tap changer only) and condition monitoring, the transformer in Croydon was identified as critical and measures were taken to extend its lifespan. Based on the above discussion, the best approach would be to use condition monitoring techniques to identify impending failures, enable maintenance prioritization, and assist utilities in the decision to refurbish (life extension) or replace equipment.
LIFE EXTENSION
- Power Transformers
- Internal Insulating System and Components
- Voltage Transformers
- Circuit Breakers
- Analysis of Various Factors
- Conclusions
Particles can negatively impact dielectric strength, especially when combined with moisture in the oil. Extreme care must be taken to avoid air contamination when conducting tests to determine the oxygen content of the oil. Drain and refill the oil, but this may not solve the problem. The oil impregnated into the paper can make up a significant percentage of the total oil.
In addition to the cost of the new circuit breaker itself, the following possible costs summarized below can be considered. The life of a power transformer can be significantly extended by controlling the internal system properties, i.e. on-line drying systems can be extremely effective when installed on new transformers so that the moisture level in the oil and paper insulation system is maintained.
ELECTRICAL BREAKDOWN AND PARTIAL DISCHARGES IN LIQUIDS
- Introduction
- Literature Review
- Detection Methods
- Synthetic Oils
- Breakdown Process
- Mechanism of Hydrogen Generation
- Partial Discharge Sources
- Electrical Measurement of Partial Discharges
- Acoustic Detection of Partial Discharges
- Signal Absorption and Reflection
In power transformers, the mediums involved are liquid (oil) and solid (steel wall of tank). The speed of the longitudinal wave in oil is about 1400 m/s [99]. Discharges are initiated in the water globules due to the lower permittivity of the water compared to the insulating liquid. Sound energy, which is proportional to the square of the pressure, increases by 26% for every decibel increase [101].
Propagation of the acoustic PD signal through the oil directly to the sensor located on the tank wall. The transmission time for the shortest (ts) path corresponds to the point where 8=a =vol Vm-. It should be applied evenly to the transducer's clean mounting surface before placement.
PARTIAL DISCHARGES - EXPERIMENTAL ASPECTS
- Introduction
- Test Set-Up
- Test Procedure
- Part 1: Measurement Instrumentation and Equipment Testing
- Results and Observations
- Oil Sample (Verification of Gas Sensor Accuracy)
- Sensitivity of Electrical and Acoustic Detection Systems
- Detection and Verification of Location (Fixed PD Source)
- Combination of Two Real Time Diagnostic Techniques The hydrogen gas sensor readings taken on
- Discussion
- Conclusions
- Recommendation for Future Work
Two piezoelectric acoustic sensors 81 and 82 (Appendix A) were placed in locations L1 or L2 on the transformer tank. The positioning of the sensors is illustrated in figure 6.2. 14. The test voltage was further reduced until no PD activity could be observed on the corona detector screen. For an rp= 20 urn (tip radius of a stainless steel needle), the electric field at the tip of the discharge is calculated using equation 6.1 for the various PDIVs achieved at the predetermined test gap lengths. The calculated values are summarized in Table 6.3.
However, the difference in mean values can be attributed to the limitations of the measuring equipment, ie. 080. The sensors were originally placed at location 1 in order to reach the audio signals in the immediate vicinity of the discharge site and at the same time maintain a safe distance to the high-voltage test supply (through-phase terminal B). The current waveforms of the electrical and acoustic signals obtained at the two locations on the surface of the tank are shown in Figure 6.7(a). In Figures 6.8(a) and (b), the signal is attenuated and the arrival times are slightly longer.
ECONOMIC CONSIDERATIONS IN CONDITION ASSESSMENT
- Statutory Requirements
- Operating Cost Reduction
The impact of a circuit breaker failure depends on the extent and status of the busbars in the substation as well as the availability of protection. Service experience has shown that the reliability of the circuit breaker depends on its duty cycle. Avon substation has two feeds from Mersey and two from Impala under normal operating conditions. Most of the real power requirements are supplied by the Mersey substation.
This fault will initiate a bus trip, which disconnects the portion of the busbar to which the breaker is connected. The bus strip operation will result in the isolation of the zone to which the breaker is connected. The impact of an RT fault is dependent on the layout and status of the busbars in the substation as well as the availability of the protection. Due to the construction of these units, there is a fifty percent (P=0.5) chance that the fault will be reflected on the P1 (primary) side as opposed to the P2 (secondary) side of the transformer.
Scenario 2
- Break-Even Cost of Unsupplied Energy
The results for the installation of the new transformers and the old transformer currently in service are presented in Table 7.7. See Appendix I for NPV calculation. The cost of maintaining the existing SFs breakers is not taken into account as they remain unchanged. The planned maintenance cycle of the spare oil breakers is different from the installed oil supply breakers. If the single unit is replaced, the cost of maintaining the new transformers (R 10 000 per year) is much less compared to the existing unit (R 740 000). The corresponding savings IS.
The results for the installation of the new transformer and the existing transformer currently in use are shown in Table 7.12. See Appendix J for the NCV calculation. This research focused on power transformers. The findings of the study were as follows: To investigate these issues, a small-scale experimental setup representative of power transformers in the field was used. The test object was a small distribution transformer equipped with a point-to-sphere electrode geometry in the tank.
BASSETTO A,MAK J;Analysis of the degree of polymerization of paper samples from service-aged transformers, annual international conference of double customers, Doble Engineering Company, 1990. BASSETTO A, MAK J; Determination of the degree of polymerization of paper microspecimens from transformer windings, Annual International Conference of Doble Customers, Doble Engineering Company, 1991. GHASEMI P, HALEM S, TAPPER M; Partial Discharge Detection Using Acoustic Emission Techniques, Annual International Conference on Doble Customers, Doble EngineeringCo, 1998.
SOKOLOV V, MAJAKOV V; On-Site Measurement of Partial Discharge on Power Transformers, Doble Annual International Subscribers' Conference. GLODJO A; Field Experience with Online Multigas Monitors, Doble Annual International Customer Conference, Doble Engineering Co, 1998. PETROU JP, KOPACZYNSKI DJ, LACHMAN MF; Expert Systems Enable Continuous In-Service Diagnostics of Installed High Voltage Plant, Doble Clients Annual International Conference, USA, 1996.
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