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Low resistance testing

 

Subject (1)

Effects of Low Resistance Measurement Instruments on Protection and Control Devices

Low resistance micro ohm measurement is a routine diagnostic test performed on Circuit Breakers (CB) and bus bars in medium and high voltage substations. This test is important to check the integrity of CB contacts and detection of high resistance joints and terminations for installed bus work. Although the test is very simple and easy to perform, it can prove problematic for other devices connected to the same circuit. 

The DC current used for measuring the low resistance can affect the protection and control circuitry in adverse ways. Typical problems encountered are Current Transformer (CT) magnetization, accidental tripping of differential relays and inconsistent and unreliable low resistance measurements. The paper will address the root cause of these encountered problems. It will recommend precautions to take prior to performing the test and discuss in detail the recommended practices to avoid any misoperation of connected protection and control devices. 

The recommended practices will be supported by both a real life case study and lab simulations to thoroughly explain the impact of quality of DC current used for the low resistance measurements, effect of in service tap CT ratio, and associated settings of connected relays. This paper will provide an insight into the factors that can go wrong during micro ohm measurements and how to prevent those using a proactive approach. 

Cable testing

 

Subjects (4)

Condition based replacement of medium voltage cables saves millions

 The method for diagnosis of water trees in XLPE cables based upon measurements of dielectric losses and capacitance as function of frequency has been verified. Based upon diagnostic measurements in 1996 a replacement strategy was decided upon for the Vattenfall 24 kV network of Botkyrka and the most severely degraded cables were replaced. This paper reports on the measurements made in 1996 and selective repeated measurements in 2002. The development of the ageing is analysed The failure statistics are reported and the effectiveness of the diagnostic method is proved. The costs savings using a replacement strategy based upon diagnostic measurements is presented.

By Roland Eriksson, SM, IEEE, Peter Werelius, Member IEEE, Lars Adeen, Peter Johansson and Henrik Flodqvist

Diagnostics of Water Tree Deteriorated XLPE Cables

IEEE Transactions on Dielectrics and Electrical Insulation

A high voltage dielectric spectroscopy system has been developed for diagnostics of water tree deteriorated extruded medium voltage cables. The technique is based on the measurement of non-linear dielectric response in the frequency domain. Today’s commercially available system is capable of resolving low loss and small variation of permittivity as a function of frequency and voltage. Experience from more than 200 field measurements was combined with laboratory investigations. Small samples were used in an accelerated ageing test to elucidate the correlation between water tree growth and dielectric response. Furthermore, field aged cables were investigated in the laboratory. It has been shown that the dielectric response of water tree deteriorated XLPE cables can be recognised and classified into different types of responses related to the ageing status and breakdown strength. The influence of termination and artefacts such as surface currents was investigated. The measurement method enables us to separate the response of the cable from the influence of accessories. Finally, two different field studies of the implementation of the diagnostic method are presented. The field studies show that the fault rate decreased significantly when replacement strategy was based on the diagnostic criteria formulated.

By Peter Werelius, Peter Thärning, Roland Eriksson, Björn Holmgren and Uno Gäfvert

Innovative Test and Diagnostic System for medium voltage cables – PD monitored withstand testing versus non destructive Partial Discharge (PD) diagnosis

For testing and partial discharge (PD) diagnostic purposes many different test voltage wave shapes and frequencies have been established over the past years. Their application is well proven and is guided by IEEE 400 norm. For
testing purposes the voltage need to produce enough stress to lead failures to breakdown. The very low frequency (VLF) waveforms turn out to be very effective and economical for that purpose. For PD diagnostic voltages are needed, with waveforms close to power frequency and in its application non destructive for the test object. VLF voltages could cause in case of long testing times during PD diagnosis unwanted breakdowns at weak spots, even if the applied voltage is not that high like it is used for withstand testing. Damped AC voltage (DAC), which is close to power frequency, is well proven to be very effective for partial discharge diagnosis and causes nearly no risk for breakdown due to the short excitation time even for critically aged cables. A new test and diagnostic system combining both, providing an effective test voltage for withstand testing and being non destructive for diagnostic measurements, is introduced recently. This paper describes the application and comparison of the new test and diagnostic system for partial discharge diagnosis by using DAC. Furthermore true VLF cosine rectangular withstand testing with accompanying PD monitoring is discussed. It is demonstrated that cosine rectangular VLF waveform delivers comparable results of PD parameter to judge the severity and to locate PD defects in MV cable systems.

By Daniel Götz, Frank Petzold,  Hein Putter Seba KMT Baunach, Germany,  Sacha Markalous, Marco Stephan Seba KMT, Radeberg, Germany and Henning Oetjen, Megger, Valley Forge, PA, USA

Investigations on a Combined Resonance/VLF HV Test System – Partial Discharge (PD) characteristics at VLF and DAC voltages

The necessity of HV cable testing is a given fact. So far two testing methods, 24 hour soak or resonance testing are commonly used. This paper describes a comparison of test  power demand dependent from voltage shape and test capacitance as well as the power consumption of test systems for different test techniques. Furthermore an optimised technology for combining advantages of resonance and VLF principle is described and case studies of field applications of this technology are discussed. Moreover this paper presents the results of investigations regarding the noise sources created by the HV equipment and the methods for successful reduction and suppression to levels below 10 pC, which is a relevant value for on-site PD measurements. Case studies from field application show a good comparability of PD measurement results obtained with DAC and the optimised resonance/VLF technology. The new test equipment demonstrates that withstand testing on test objects with capacitances up to 25 µF are very successful and monnitored PD testing as well as diagnostic PD testing are conclusive.

By F. Petzold, H.T. Putter, D. Götz, H. Schlapp, S. Markalous SebaKMT GmbH, Baunach/Radeburg, Germany

Insulation testing

 

Subjects (2)

Mordern ART (Accuracy in Resistance Testing)

EuroTechCon 2014 - Warwick, UK

Temperature has a large influence on the value of insulation resistance being measured at any instance. As is often the case the object temperature is greater than the ambient. This will give errors in values being obtained at that time (compared to any specific 20ºC condition for example). This paper will demonstrate the advantages of applying a variable low frequency AC waveform and, using temperature-to-frequency conversion techniques developed, and also the ability to correct any resistance value automatically to a to a known temperature corrected result.

By Alan Purton

The characteristics of insulation resistance

The Institution of Electrical Engineering | Vol. 52, No. 224 | pp 51 - 83

During recent years a great deal of valuable research work has been done to increase our knowledge of the properties of insulating materials, yet notwithstanding the progress so made the natural laws governing insulation resistance are but little understood. So little, that if at the outset of this paper a plausible statement were made to the effect that the insulation resistances of an electrical system depended mainly upon the dielectric properties ofthe insulating materials, it might easily pass unchallenged. Possibly some objectors might be found among those who have to maintain the insulation of electrical plant; for no one who has had much experience of the behaviour of insulation in practice, could fail to be struck by the disparity between insulating materials under test in the laboratory and the same materials under the ordinary conditions of use.

By S. Evershed, Member

Relay testing

 

Subjects (2)

Approximating a power swing and out-of-step condition for field testing

PAC World 2015 | University of Strathclyde, Glasgow, UK | June 2015

Testing a power swing or out-of-step scenario on modern protective relays can be a tricky task.  Past methods of testing power swing and out of step conditions have often involved imprecise methods of applying voltages and currents to simulate impedances seen by the relay.  By manually ramping the impedance trajectory, or playing several vector states where a specific impedance was applied, it was possible to initiate a power swing block or out-of-step trip in a protective device.  However, with more advanced algorithms implemented into modern protective relays, previous methods of testing might not work.

One method that has proven to work is applying a simulation of the power system, complete with all of the necessary sources and impedances of the elements under study.  The output of the simulation can then be stored in a format suitable for field testing such as COMTRADE.  This format can be played to the protective relays and the response measured.  Although this method is effective, it can be daunting to personnel who may be required to test these schemes, but who may not have a background in power system protection or simulation.  It is for this reason that a simplified method of testing power swing and out of step conditions without the use of complex simulations is desired.

This paper will talk about a non-traditional method of that utilizes the superposition of two waveforms of dissimilar frequencies to achieve a power swing and out-of-step condition.  The rate of change of impedance can be controlled as well as the minimum and maximum impedances, the number of pole slips, as well as the starting phase angle relationships.  These parameters can be manipulated via basic formulas suitable for beginning field personnel.

By Jason Buneo and Dhanabal Mani Megger, Ltd

Lessons Learned from a 400kV Busbar Misoperation Utilizing the IEC 61850 Standard

PAC World Americas 2014 | Raleigh, North Carolina | Sept, 2014

The implementation of IEC 61850 standard for substation design and commissioning is fast-phased method of defining grid protection schemes throughout the world.  The protection logic that involves dc control circuits are executed internally in the Intelligent Electronic Devices (IEDs) and effectively communicated between the IEDs via Generic Object Oriented Substation Events (GOOSE). Any error in the mapping of GOOSE signals will result in undesired operation of the protection schemes.

The main buses in power substations are designed to carry load currents through the individual feeders as well as high amplitude currents during bus fault conditions. Any delay in fault isolation or improper relay operation could result in severe damage to the substation buses, and the equipment connected to them. Therefore, proper design and testing of the bus-bar protection scheme is required to ensure safe and reliable operation of the substation. The complex protection schemes, such as bus-bar and breaker failure protection are relatively easier to design using the modern IEC 61850 standard. However, the implementation of these schemes in the real world poses certain unique challenges.

This paper discusses the investigation of the tripping of a 400 kV substation due to improper operation of a bus-bar protection scheme. This incident happened when a Zone 2 fault occurred on one of the 400 kV line feeders, immediately triggering a breaker-failure condition. Under a normal trip scenario, the zone 2 timer will time out and the line IED will issue a trip signal to the line breaker to isolate the fault. The line IED will also then issue a Breaker-Failure Initiate (BFI) signal to bus-bar IEDs through GOOSE messages. The breaker-failure condition is only declared when the line breaker fails to trip within a specified breaker-failure time.  However, in this case, the breaker-failure condition was initiated before the Zone 2 timer expired instead of after.

An investigation was carried out to determine the reason for declaring a breaker-failure condition even before zone 2 tripping of the line IED. Further analysis of the IEC 61850 network and GOOSE configurations led to the conclusion that the BFI signal was mapped incorrectly. The bus-bar IEDs were configured to receive a BFI signal through GOOSE messaging for a fault pick-up signal instead of a fault trip signal by protection IEDs.  This minor error caused the entire substation to be out of service. This paper discusses the methods of testing so that would help prevent this situation.

By Dhanabal Mani, Vijay Shanmugasundaram, IEEE Member and Jason Buneo, IEEE Member

Transformer testing

 

Subjects (12)

Analysis of HV bushing insulation by dielectric frequency response

2016 Electrical Insulation Conference (EIC), Montréal, Qc, Canada

Discover how various different test methods can allow you to better assess the insulation systems of EHV bushings. This paper aims to expand your knowledge regarding the practicality of various test methods by looking at the field testing of oil-impregnated paper (OIP) EHV bushings with methods such as capacitance and power factor/dissipation factor (tanδ) measurement and more advanced testing methods, including dielectric frequency response (DFR) and dissolved gas analysis (DGA).

By I. Güner, Hydro Quebec, D. Robalino, Megger USA, and P. Werelius, Megger Sweden 

HV and EHV bushing condition assessment - field experience

2016 CIGRE-IEC Colloquium, Montréal, QC, Canada 

Garner in-depth knowledge of how dielectric frequency response (DFR), and dissolved gas analysis (DGA) can identify the risk of EHV bushing failure from analysing various field testing case studies. The paper also looks at correlation of these two advanced diagnostics techniques as well as the practicality of other testing methods, such as capacitance and power factor/dissipation factor measurements.

By I. Güner, Hydro Quebec, D. Robalino, Megger USA, and P. Werelius, Megger Sweden 

 

Integrated test van for maintenance and diagnosis of power transformers

2016 International Conference on Condition Monitoring and Diagnosis – Xi’an – China

Consider the integrated transformer test van as a viable means to reduce testing times and increase safety. It is equipped to run several routine electric test techniques, such as insulation testing and winding resistance, as well as more advanced diagnostic methods, such as DFR and SFRA, and it is all controlled by a central computer hub. This paper looks at the abilities and advantages of these complex vehicles in detail and how you can use them across a plethora of applications. 

By D. Denissov and A. Aleev, Megger

DFR techniques for insulation assessment of bushings and transformers

Learn more about Dielectric Frequency Response (DFR) and how it’s data can be used in combination with the mathematical modelling of oil-paper insulation to assess moisture concentration. This paper gives you a brief background to DFR and looks at how insulation modelling can be used to improve our understanding of insulation properties and how this can be used for the insulation assessment of bushings and instrument transformers.

By Matz Ohlen and Peter Wereliu, Megger Sweden and Denis Denisov and Lutz Hulka, Megger GmbH

A Comprehensive Approach to CT Field Diagnostics

Current transformers (CT) serve as a critical and essential interface in the world of electrical power. The role of a CT, at first glance, may appear to be a simple one but, CTs function as one of the most important components of the electrical system infrastructure. The performance of these devices is reflected in the correct operation of metering devices, relays and circuit breakers. CTs differ from other transformers in that their robust construction and rugged design cause them to be uniquely applicable to long-term operation with minimum maintenance. In a high voltage current transformer (HV CT) the insulation system is limited to off-line testing, and oil sampling during operation is forbidden. Therefore a comprehensive, simple approach to evaluating the condition of low voltage, medium voltage and high voltage CTs is paramount for testing service companies and utility operators.

By Diego Robalino, Megger

Transformer Testing Techniques & Standard

Transformer manufacturers and field operators have always benefitted when new technologies are applied during design, manufacturing, commissioning, and operational processes that improve the quality and reliability of electrical apparatuses. The new computational tools and the continuous research by individuals in the academic, public, or private sector have created better materials capable of withstanding demanding service conditions, saving space, and minimizing energy losses. As manufacturing technology advances at this rapid pace, testing methodologies must evolve to keep pace. Advances in power electronics and computer technology lead to more accurate, reliable, portable, and user-friendly instruments in the field.

By Diego Robalino, Megger

Individual temperature compensation

TRANSFORMERS MAGAZINE | Volume 2, Issue 3 | pp 42-47

Dielectric testing techniques in the time and frequency domains are increasingly being used by transformer manufacturers, power utilities and researchers for transformer oil-paper insulation systems condition assessment. Since 1997, when the first portable device designed to carry out dielectric response tests in the frequency domain in the field was put on the market, the technology has evolved and new features have been incorporated. One of these features is becoming a "must have" tool for power transformer dielectric condition assessment: individual temperature compensation.

By Diego Robalino, Megger 

Frequency to temperature domain of transformer liquid insulation

IEEE Conference on Electrical Insulation and Dielectric Phenomena (CEIDP) | 2014

The dielectric response of the overall transformer insulation is an invaluable source of information for those involved in the manufacture, operation and maintenance of power and distribution transformers. One of the greatest advances in the use of the transformer insulation dielectric response is without a doubt the possibility of performing advanced diagnostics of the oil-paper insulation to determine the moisture concentration of the solid insulation and the quality of the liquid insulation. Moreover, based on the knowledge acquired, the dielectric response in the frequency domain allows correct definition of the thermal behavior of dielectric parameters such as power factor or dissipation factor. Dissipation factor or power factor are dependent on frequency and temperature. It is clear for field users that the thermal behavior of the dielectric parameters is related to the condition of the overall insulation system and not just to the construction or nameplate of the transformer. Therefore, the application of the dielectric response in the frequency domain is a more accurate method to determine the real thermal behavior of the power factor. In the context of this document, authors investigate different liquid insulation materials with different aging/quality conditions, their dielectric response in the frequency domain and the conversion of this response into the temperature domain.

By D.M.Robalino,and R.E. Alvarez, Megger

On-site testing of special transformers

Proceedings of the 10th IET International Conference on AC and DC Power Transmission

The increased usage of the phase shifting transformers in various fields within power systems has introduced new challenges in performing on-site maintenance testing. These special transformers were used as converter transformers, rectifier transformers, harmonic mitigating transformers, zigzag transformers and quadrature boosters with unconventional vector groups, having an additional phase shift such as +/-7.50, +/-150 combined with normal ‘Delta and Star’ configurations. The challenges now faced by the increased use of these transformers are testing them for accuracy on the voltage ratio and phase shift at site during commissioning and maintenance. Different test methods were performed / studied and were compared. A prototype of a three-phase converter transformer with variable ratio taps and vector groups was constructed and tested. The prototype transformer is constructed to have variable parameters for test trials on different vector groups and ratios. The three phase converter transformer prototype will have options to regulate the output voltage on varying input voltage using the tap changer and also can vary the phase shift by varying the vector group combinations. On-site testing of ratio and phase shift will be done on different types of traction rectifier transformers. The approach for factory testing and onsite testing of the special transformers will be different. The effect on the phase shift values when the ratio taps are changed will be the focus and the results compared. The study on the importance of phase shift on these special transformers is presented. The test results of different test methods using modern portable test equipment and conventional classical methods were compared and presented. The results of the ratio testing and the phase shift measurement variation during vector group variation and tap change were presented.

By Simanand Gandhi Jeyaraj, Megger Limited, Robert Milne, UK Power Networks and Grant Mitchell, Transmag Transformers

Accurate temperature correction

Annual Report Conference on Electrical Insulation and Dielectric Phenomena (CEIDP), 2011

Typically, dissipation factor (DF) or power factor (PF) test is carried out in the field following well known procedures. It is not necessary to emphasize the importance of dielectric test for power system operators. Accurate recording of insulation temperature values during the test is critical but not always feasible in the field. DF measured values are later normalized to a 20°C base for future comparison and trending. Nevertheless, as stated in several international publications, accuracy of temperature correction is still under investigation because temperature correction factors (TCF) from reference tables do not consider the percentage moisture concentration of the insulation system. The existing Temperature Correction Tables correspond to a variety of insulation materials and construction of different high voltage electrical equipment and components. Therefore, the application of state-of-the-art technologies to determine “specific” temperature correction factors for DF, is essential to provide reliable interpretation of results and proper equipment condition assessment. Frequency Domain Spectroscopy (FDS) in conjunction with DF Analysis are a powerful tool to determine the percentage moisture concentration in solid insulation capable to estimate Individual Temperature Compensation (ITC) of DF measured data of power transformers. Throughout this document, field experience is summarized when a three-winding transformer is removed from service and put to a series of testing procedures including DF, FDS and tip-up test on the high voltage bushings. Results of the analysis, experimental data and conclusions made based on the obtained results are presented herein.

By D.M. Robalino

Bushing insulation based on dielectric response

Modern technology and developments in signal acquisition and analysis techniques have provided new tools for transformer diagnostics. Of particular interest are dielectric response measurements where insulation properties of oil-paper systems can be investigated. Dielectric Frequency Response, DFR (also known as Frequency Domain Spectroscopy, FDS), was introduced more than 20 years and has been thoroughly evaluated in a number of research projects and field tests with good results. DFR data in combination with mathematical modeling of the oil-paper insulation is proven as an excellent tool for moisture assessment. Since the modeling theory contains influence of temperature, DFR and modeling can be used to calculate the temperature dependence of the insulation system.
This paper gives a background to DFR, insulation modeling and how these tools can be utilized to improve understanding of insulation properties and in particular how this can be used for bushing diagnostics.

By Matz Ohlen and Peter Werelius, Megger

Demagnetisation of GOES

Grain-oriented electrical steels (GOES) are the basic component of transformer cores, and their magnetisation processes are very complex, depending on the amplitude and frequency of excitation. However, their demagnetisation frequency is of less significance because good quality procedure can result in the same demagnetised state. This document discusses these procedures in greater detail.