This paper is devoted to an accurate investigation on the various dielectric and physicochemical properties of two kinds of transformer oils, which are submitted to accelerated thermal aging in presence of copper and presspahn. The first one is a non-treated new oil while the second one namely reclaimed oil, is extracted from transformer commissioned in 1989. The used oil is reclaimed in laboratory by adsorption process, in order to restore its quality by removing contaminants and restoring its insulating properties. The two kinds of oils are submitted to accelerated thermal aging under two temperatures 100°C and 120°C, in order to assess the stability of both oils. The investigated parameters are dissipation factor, acidity, hydrogen gas, viscosity, resistivity and index color, according to specifics standards. It’s shown that reclaimed oil degrades more quickly during thermal aging than new oil in presence of copper and presspahn. The results reveal instability of reclaimed used oil after accelerated thermal aging process.
Global warming has been a burning issue since the 1980s, and SF6 has been one of the most potent global-warming gases used by the power industry. SF6 is used in high-voltage circuit breakers. They are prone to leakage into the atmosphere, creating adverse effects on the environment. Replacing SF6 with supercritical CO2 as the arc quenching medium in circuit breakers can be a much better alternative for battling global warming. The prototype of a 72 kV, 20 kA (peak) supercritical CO2-based circuit breaker is being developed and will be tested using a synthetic test circuit. 20 kA fault current will be passed through the test breaker. The high current is generated using a capacitor bank and an inductor (resonant circuit). COMSOL simulation was used to obtain the required inductance and ensure that no flashover occurred between the turns of the inductor.
The residual life of a transformer is commonly associated with the estimated life of the insulation system, namely the solid insulation. While the temperature is the most relevant parameter for the calculation of the solid insulation life consumption, the influence of the water and oxygen content must be taken in consideration. The traditional paper life curves, commonly called "Arrhenius Curves" are traditionally focused on the hydrolytically catalyzed thermal degradation of the paper, assuming water content of less than 0.5% in the paper. Only recently, when the latest version of the IEC loading guide was published, charts presenting life expectancy curves for different levels of moisture and oxygen were introduced. The inclusion of such parameters into the calculation affects the largely, reducing the calculated lifespan of transformers to as much as two thirds of that predicted by the traditional curves. However, the challenge lays on how to estimate the actual water content in the paper in different regions of the windings, since there is currently no viable way to measure such values. This article presents an approach for continuously and iteratively estimate water content in different regions of the windings, based on the known initial conditions and the effective loading and ambient temperatures. The results from the model are then compared with the information obtained from online moisture and temperature sensors, and an artificial intelligence model is used to minimize the deviations. Thus, the proposed methodology allows assessing paper degradation rate more accurately. This information is of paramount relevance for asset management and for strategic loading decisions, allowing the inference of a failure risk associated both with the paper degradation and risk of bubbling, i.e., microbubbles formation.
HV insulation components work in a very harsh environment. These elements, depending on their application, are exposed to high voltage and high temperature (>100C), outdoor work, dielectric gases, and many other conditions. For many years such elements have been produced mainly from thermoset materials with filler, in particular epoxy resin. In general, the current material and its processing lead to a labor-intensive and complex manufacturing process. Additionally, epoxy material is very problematic to recycle thus, it would be desirable to find a material which is easier to process and more ecofriendly. Nowadays, we see more and better polymers like thermoplastics that seem to be good alternative. But their implementation is not an easy task and requires performing a lot of tests which are very challenging. The paper consists of selected HV insulation requirements and examples of tests that must be performed to find the best material candidates. Performed analyses and tests for different thermoplastic materials show its potential for implementation as alternative to epoxy resin. This way seems to be the right one as thermoplastic material offer a lot of benefits especially in terms of sustainability and circular economy.
This paper presents a high-frequency equivalent circuit model applicable for partial discharge (PD) location in an inductive test of a 6.6 kV-VT cast resin transformer (CRTr), in which a voltage is applied between the high-voltage terminals. Proposed equivalent circuit consists of the winding inductance, and the capacitance between adjacent inter-windings and that between the winding and ground of CRTr. The proposed circuit allows to simulate PD phenomena occurring in the windings. The estimation of the dominant frequency f r for PD detection can be achieved through the frequency characteristics of the impedance Z(f) measurement of CRTr and the calculation of its high-frequency equivalent circuit. This allows for the identification of PD and noise in the field, leading to improved defect detection sensitivity. Measurements of PD current waveforms were also made in a 6.6 kV VT with the inductive test circuit. Measured results were compared with waveforms calculated using the equivalent circuit. As a result, it was found that the polarity of the rising part of the current and dominant frequency components of simulated PD current waveforms were almost consistent with experimental ones.
The latest 2011 edition of the IEEE C57.100 standard introduced a new sealed tube accelerated aging test (STAAT) procedure and methodology for evaluating the thermal index (TI) of liquid-immersed transformer insulation systems. The application of this standard becomes increasingly important, as new insulating materials (both solid and liquid) have been developed, and their thermal performance shall be qualified. The experimental work presented in this article investigated the effects of different test parameters on the aging test results and data interpretation. The new findings on the dependence of the aging process on the ratio and type of materials in the system, as well as on the selected aging criteria provide a better understanding of the accelerated aging and interpretation of the data along with practical recommendations for the improvement of the test procedure. In parallel, for the first time, the TI of the Transformerboard, as a component of the so-called industry-proven insulation system, was investigated and defined based on the standard industry test. The results confirmed that the Transformerboard’s thermal class is, at least, 120 °C; this validates an empirically based application of this material as a part of the 120 °C class insulation system in both power and distribution transformers.
In 2017, we in Andritz were performing a thermal cycling test in accordance with IEEE 1310 for a customer acceptance test. After approximately 200 thermal cycles, a fire started in the test equipment and destroyed the testing facility. The smoke and heat associated with the fire caused extensive damage to our entire facility (both office and shop). As a result, a major insurance led investigation was undertaken, though the results were inconclusive as to the cause of the fire. Through these investigations and our own internal investigation, we learned a significant amount about what safety precautions should be taken when designing and performing the thermal cycling tests. This paper will show the importance of understanding the technical risks associated with this test (not described in the IEEE 1310 document) and provide recommendations to test labs that may make their own facilities safer.
The influence of a magnetic field on partial discharge due to voids such as butt gaps in composite insulation systems is reported. Experimental characterizations of an emulated butt gap in polypropylene laminated paper (PPLP) layers at room temperature in pressurized nitrogen gas and at 77 K in liquid nitrogen are described. A reduction in the partial discharge inception voltage was observed both at room temperature and 77 K when a 170 mT magnetic field was applied to the butt gap region. The differences in the phase resolved partial discharge patterns among the different cases studied are discussed. It was concluded that the presence of a magnetic field influences the PD dynamics at cryogenic temperatures.
In order to improve the understanding of the correlation between specific partial discharge (PD) sites measured using an electromagnetic probe (EMP) with a Phase Resolved Partial Discharge (PRPD) acquisition system, a laboratory experiment has been initiated to characterize its sensitivity under well-controlled conditions. Laboratory samples were made using cylindrical epoxy disks containing a single spherical cavity. A first set of samples were made with a cavity centered within the insulation volume of different sizes. Another sample was made with a single cavity in contact with the high voltage electrode. The peak amplitude pulse value obtained from the current meter (in milliamps) of the EMP was recorded for each configuration at a constant voltage. PD measurements combining the EMP connected to a PRPD acquisition system were made to display the 3D pattern of the localized PD activity of each disk. In parallel to the EMP measurements, PD measurements were also made with a 1 nF capacitive coupler to serve as a reference pattern. This paper describes the sample testing protocol and introduces preliminary results of the characterization of the EMP on epoxy disks with a single spherical cavity. Quantification of PRPD patterns and current values are also presented in relation to the size and location of the void within the epoxy disks.
It has been reported that low resistive-corona armor tape (CAT) used in the stator coils of a rotating machine is deteriorated by partial discharge within void or delamination in the main insulation of the stator coil, resulting in an unacceptable high resistivity of the material. Hence, the lifetime evaluation of the CAT is an important issue for the reliable operation of the machine. In this study, the determinant of the lifetime of the CAT is experimentally investigated. As a result, it was clarified that the lifetime of the CAT was determined by ozone concentration produced by partial discharges and the duration of exposure time to ozone. Furthermore, the lifetime of CAT can be well organized by the discharge energy of partial discharge, which enables the lifetime estimation of the CAT under the given operating conditions of the machine.
Sequence impedance of a transmission line is a critical parameter in power transmission and distribution (T&D) system design, which is widely utilized in protection coordination. The sequence impedance for underground power cables is required by the protection engineers during the protection design and after the installation (commissioning). The calculated sequence impedance may contain large errors up to 50%, due to the uncertainties in the earth impedance, cable layout, etc. Therefore, the field measurement of sequence impedance of underground power cables is commonly required by the end-users. The measurement methods and the practical challenges are discussed in this paper. The testing lead effect, which is the main uncertainty in measuring short power cables, is discussed with a case study. A method to determine the testing lead effect through measurements is proposed.
In this work, the effects of temperature over a range of 20 – 100 °C on paper-oil insulation under repetitive transient voltages have been analyzed. The ageing of the paper-oil insulation is measured based on the dielectric frequency response, partial discharge inception voltage, and intensity of discharges. The Luminol™ TRi oil, considering its oxidation stability and negative index for gassing has been selected as test oil to evaluate its applicability as an alternative liquid insulation for transformers facing continuous high-frequency repetitive transient voltages. The results help in understanding the ageing response of turn-to-turn insulation in wind-turbine step-up transformers subjected to repetitive transient voltages at different operating temperatures.
The traditional method of quality control testing of magnet wire, consists of a withstand (high potential) testing of a reel of wire per NEMA MW 1000, which is typically performed using a batch process. This paper explores an inline testing method using a spark testing system that has been historically utilized for wire and cable quality control testing. The two methods employ different detection styles. The pros and cons of the two methods will be reviewed.
High-voltage direct current transmission plays a central role due to the changing demands on the transmission grid resulting from the energy revolution. Especially with DC corona discharges, a measurable ion current is generated on the ground below the overhead line and can have an impact on the environment. This increases the importance of the qualification and reliable design of equipment exposed to DC voltage. Particularly in the case of bipolar transmission systems, grid operators are faced with new challenges because of the resulting corona losses. Due to the highly non-linear conditions in the simulation of bipolar corona discharges, robust and efficient numerical methods for the calculation of these phenomena are necessary because there is no analytical solution. This publication presents an efficient and generally applicable method for any overhead line configuration for the evaluation of the onset field strength. The main idea is to calculate the general integral conditions of electric discharges numerically using test particles along the electric field lines. For a meaningful analysis of the results, the results for the unipolar case are validated with the empirical Peek equation and it can be shown that the deviations are about 5%.
At Manitoba Hydro's newest hydraulic generating station, isolated phase bus (IPB) was installed to connect the generators to the step-up transformers. Partial discharge (PD) tests were performed on each section of IPB as part of the commissioning process. The tests exposed various issues related to both the design and assembly of the IPB on multiple units, including poor post insulator connections, contamination of bus components, and problems with seal bushings. Localization of all PD sources was accomplished using portable ultrasonic and ultraviolet detectors. Following each failed test, efforts were made to eliminate the source(s) of PD, and iterations of testing/troubleshooting were performed until a passing test result was obtained. This paper provides technical details about the PD tests performed, the test results, the issues found, and the remedial actions taken. The PD test proved to be a valuable commissioning tool that may have helped prevent future IPB in-service failures, and it also ensured that PD pulses in the IPB would not interfere with future online generator winding PD measurements.
Insulators' failures caused by any type of defect may result in serious technical and economic losses. In particular, contamination flashovers due to excessive pollution and humidity are one of the main problems encountered in power systems. Moreover, other types of defects like surface crack and internal void are other common examples of outdoor insulator defects that may lead to insulator failure. In this regard, detecting the defect using a non-intrusive method is important in terms of energy continuity and monitoring efficiency. Both vision (regular, IR and UV camera) and emission (RF, ultrasonic, and acoustic) based sensors have been deployed in the field to detect these defects. It has been reported that each sensor has certain pros and cons in terms of its ability to detect the different defects. Therefore, it is essential to investigate and compare the sensing performance of non-intrusive sensors for different defects. In this study, an ultrasonic sensor (20 - 100 kHz), an RF antenna (0.53 - 3 GHz) and an IR camera were used to detect dry band arcing, corona and surface discharge. All three sensors along with the standard partial discharge measurement system were used simultaneously for each measurement. The results show that sensing performance of the three sensors varies depending on the defect type.
As next generation aircraft are pushing to become all-electric, increasingly high operating voltages are being used to meet the power demands required on-board. There are however caveats to increasing the voltage. Increasing the voltage means that electrical aging of the insulation can occur more rapidly, and this is only exacerbated under low pressures experienced at high altitude flight. This increases the risk of achieving the inception voltage of partial discharges (PD) which accelerates the rate of electrical aging.This work explores the discharge behavior of a high voltage insulation system under a low-pressure environment using an ultra-high frequency (UHF) radio sensing system to capture the electromagnetic (EM) waves of the first sign of PD. In this example we present a case where a defect of a known size is created inside the insulation layer of a high voltage cable and show that it is possible to detect the inception of early PD from the EM waves. This allows the realization of the impact of the aerospace environment on the detection and the impact of PD on high voltage insulation systems, which will be crucial for condition monitoring and future lifetime prediction of in-service aircraft.
Offline and online testing revealed that the generator tightening mechanisms had loose parts and there was an increase in the level of online partial discharge. The discharge was concentrated around 0° and 180° or around 90° and 270°, and the vibration-sparking was assumed to be a disturbance in the stator winding of the generator. Several findings were obtained from field observations, including the presence of black grease at the core end, greasing at the binding on the circuit ring, and end winding. It was also found that the partial discharge inception voltage (PDIV) of the stator winding occurred at 1 kV, which was an unusual PD inception voltage for a 20 kV rated voltage of a turbogenerator. The purpose of this case study was to reduce the vibration-sparking symptoms on a 300 MW turbogenerator by reinforcing the stator end winding. The following methods were applied to reduce the vibration-sparking symptoms: identifying problems, reinforcing the end winding, measuring direct bump tests along with reinforcement, comparing PDIV before and after reinforcement, and comparing online partial discharge tests before and after reinforcement. The reinforcement on the end winding shifted the resonant frequency of the stator end winding away from the power frequency of 95110 Hz. It also reduced vibration in the slot bar to some degree, which decreased the online PD magnitude compared to previous levels. PD inception voltage of stator winding increased from 1 kV to 6 kV, which is normal for a 20 kV turbogenerator. Reinforcement of end winding has been proven as a method to reduce the vibration-sparking symptom, decrease online PD magnitude, and increase partial discharge inception voltage of stator winding.
