Long term dga trend evaluation of transmission power transformers

The Dissolved Gas Analysis (DGA) of insulating oils has, over the last few decades, become a widely used technique for detecting and diagnosing incipient electrical failures inside the windings of power transformers. Since the decomposition of oil indicates a threat to the operational safety of these expensive machines, electrical engineers focus their attention on the amount and nature of the gases evolved. Based upon the relationship that was established between the results of DGA and the potential cause of such deficiencies, appropriate preventive measures are taken to protect the transformer. However, the side effects of a hydrocarbon chain breakdown on the insulating properties of oil are generally not taken into consideration. When a hydrogen atom is knocked out from a hydrocarbon it means that the two electrons of a covalent bond were homolytically separated. While the hydrogen content of oil and other low molecular weight gases is carefully monitored, the decay products generated by the chemically reactive large fragments of decomposed molecules are ignored. Significant technological progress has been made in the laboratory analysis of oil, which offers the opportunity to reveal the link between gassing and the deterioration of liquid insulation quality under service conditions. This paper describes the unavoidable implications of secondary chemical reactions, caused by the gas evolvement of oil subjected to thermal and electrical stress.

Increasing pressure from both customers and regulators to maintain and enhance service reliability, while at the same time controlling costs has put power utilities on their toes. In view of this, asset management has become an increasingly important aspect of power business strategies such that electricity companies can utilize assets to the fullest while maintaining system reliability. This paper discusses the transformer fleet in the most obvious region of the Nigerian Electricity Industry. 15 years data (1990–2004) on transformer failure was collected and collated for the transformer population of the region. Statistical means was used to express the age distribution of transformer for 10year spread (2007–2017) and identified that about 58.82% of these transformers will exceed the average technical life. It thus, present the need for a proper replacement/refurbishment expenditure profile for transformers over the next 10 years; development of a user friendly condition assessment and prioritization tool for field personnel and management reporting, such that the subsequent failure rate of transformers can be reduced.

Surge analysis for lightning strike on overhead lines of wind farm

Some clients request the stability study of a protection system in designing power system to evaluate whether protection relays properly operate in case of an internal fault and if they do not function in the events of severe external fault. High fault currents could cause Current Transformers (CTs) to be saturated. A critical concern is the performance of fast protective relay (instantaneous or differential elements) during severe saturation of CTs. Time domain transient simulation software is widely used for the study of these types of works. In this paper, a relay stability study of 115 kV and 34.5 kV transmission and distribution systems was carried out and evaluated utilizing the PSCAD software, which provides several modeling methodologies for various electrical equipment such as transmission line, power transformer, current transformer and protective relays. For the purpose of this study, the modeling method of CT is the most important factor because it influences heavily on the protection relay operation. It is very important to model CT as close as the real characteristics of the CT. Two types of CT models provided by the PSCAD were discussed and compared with each other and an appropriate CT modeling methods were covered in this study. This study is mainly based on digital relays, which prevail recently in the relay market. Digital relay characteristics like anti-aliasing filter, digital filter and various setting function are embedded into PSCAD.

In the process of power supply to final consumers, an important amount of energy is lost in the transmission and distribution systems due to technological and non-technological losses. Technical losses represent an economic loss with environmental effects at national level, and its optimization should be performed from an overall national's perspective, regardless of the institutional organization of the sector and ownership of operating electricity utilities. The volume of energy losses in the power transmission and distribution systems is important, as it is generally up to 3 - 13%. The volume of power losses in Romanian power transmission and distribution systems is important, the level being of about double to the average in Europe. The current paper presents the research results of the authors regarding the real time monitoring and analysis systems for power losses, in electrical transmission and distribution systems.Within the research project, it was developed a dedicated system, successfully tested within a 5 years partnership with CNTEE “Transelectrica” - SA, on a transformer of 250/250/80MVA 400/121/20 kV from 400/110 kV Pelicanu Substation and respectively on the 400 kV overhead power line (118 km) Bucuresti Sud - Pelicanu.The tests results showed the importance of evaluation by real-time measurements, compared to the results of evaluations by calculations, of active power losses at power transformers and overhead power lines, respectively. Values several times higher were found for the values of active power losses measured online, in real time, than those determined by calculations.Online measurements have shown that there may be cases when large industrial consumers produce current and voltage harmonics well above the maximum allowable limits specified in the standards. The harmonics are transmitted practically undamped by the windings of the power transformers and on the transmission power lines (in the case of tests, even more than 100 km away from the place of their production).The online measurement of active power losses on the 400 kV overhead power line, 119 km, showed that corona losses are insignificant especially during periods of atmospheric humidity below 85%, and that active losses on power lines are practically resistive losses. The resistive losses on the measured high voltage overhead power lines are much higher than the calculated ones. The difference is determined by the fact that the calculation of active power losses on power lines includes the value of ohmic resistances of conductors evaluated based on line conductor resistivity or possibly based on ohmic resistance measured at a certain moment, and not the real value of the ohmic resistance of the line in real time (which changes over time due to the change in the state of the multitude of joints of the OHL conductor sections).

Energy conservation and energy efficiency are one of the most important areas of the Russian economy modernization. The reactive power flowing through them has a significant impact on power losses in 6–10 kV distribution networks. Asaresult, electricity losses in individual networks reach 40%. Consequently there are excess losses, which must be paid for from profit. Therefore energy saving and increasing energy efficiency in distribution networks are an urgent task, the need to solve which is due to various technical and economic reasons. This work is aimed at developing measures to optimize the transformers loading of an oil pumping station with the compensating devices introduction. The dependences of the relative change in the transmission capacity, power losses and voltage losses with a change in the reactive power factor have been obtained. A model has been developed in the Matlab Simulink program to assess the power supply system efficiency of an oil pumping station. The power supply system simulation model of the oil pumping station makes it possible to select the compensating devices capacity and installation locations in the projected and existing electrical network in order to minimize electricity losses, as well as to control reactive power depending on power transformers load factor. The results obtained can be used in the development of energy conservation programs in the power grid complex.

The oil-immersed power transformer is the core equipment for energy transmission and distribution power system. Assessing the condition of Oil-paper insulation system accurately is tremendously significant. Polarization and depolarization current measurement (PDC), which is based on the dielectric response, is currently widely used for condition assessment of transformer insulation as non-destructive testing methods. This paper based on the equivalent model of Oil-paper Insulation structure-X model, the characteristic parameters extracting for dielectric in time domain was worked out. Then an accelerated aging experiment at 130°C was done. Polarization and depolarization current were measured under different aging degrees. Characteristic parameters of them were also calculated, to investigate the influence of aging degree on polarization and depolarization current. Experimental results show that, polarization and depolarization current values and the corresponding characteristic parameters would changed regularly with aging degree. In guarantee under the condition of water content is consistent, the polarization and depolarization currents can be used to assess the aging state of transformer oil-paper insulation.