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Compared to conventional ac cables, the current rating of HVDC cable circuits is more complicated because the permissible current is not only limited by the operating temperature, but also by the electrical stress within the insulation. Therefore the IEC60287 current rating standard is considered insufficient for some HVDC applications because it fails to include the stress limitation. This paper presents an analytical methodology to calculate an electrical stress-limited rating for HVDC cables, which considers the maximum dielectric stress as the current limiting factor. This paper represents the first part of a project which has comprehensively reviewed cable rating calculations for mass impregnated HVDC cable systems.

Disconnecting switch operations in gas insulated equipment cause transient voltages with risetimes as steep as 5 to 20 nanoseconds and magnitudes as high as 2.5 pu. There is very little information on the effect of these transients on oil-paper insulated equipment. There have been reports, however of transformer and bushing failures caused by these transients. The electrical breakdown characteristics of oil-paper insulation under steep front impulse were studied in this project, which was co-sponsored by the Canadian Electrical Association and BC Hydro. V/sub 50/ (50% breakdown probability voltage) breakdown data was obtained with steep front (10 ns/2500 /spl mu/s), lightning and switching impulse waveforms. Insulation breakdown voltage vs breakdown time (V-t) data and multiple impulse breakdown data were obtained with the steep front impulse waveform. The V/sub 50/ results showed that the breakdown strengths were lower for steep front impulses than for lightning impulses. The multiple impulse breakdown results showed that oil-paper insulation breakdown strength can be lower than 100 kV/mm. These results are alarming, since they suggest that oil-paper insulated equipment subjected to steep front transients will fail at voltages below the lightning impulse design level (BIL). The volt-time data had a discontinuity. The breakdown process at risetimes below about 50 ns was different from the breakdown process at risetimes above 50 ns. >

Dielectric testing techniques, in both time and frequency domains, are currently widely used by power utilities for assessment of the condition of transformer oil-paper insulation systems. However, it has been reported that results of these tests are highly influenced by the operating temperature during measurements. The distribution, migration and equilibrium of moisture between oil and paper in a complicated insulation system is highly temperature dependent. It requires adequate experience and proper understanding to interpret the dielectric response results in the presence of temperature variations and thermal instability. Proper analysis of the dielectric test result is only possible with an understanding of the physical behavior of the insulation system in response to temperature. A circuit model, which describes the dielectric behavior of the transformers main insulation system, has been investigated in this paper. The values of the parameters of the model have been identified from the dielectric tests. A correlation has been observed between the operating temperature and the equivalent model parameters that can be used as additional information for better interpretation of the dielectric test results. This paper thus reports a detailed study on the effects of temperature on dielectric measurements of a transformer under controlled laboratory conditions. Some results of practical on-site testing are also presented to demonstrate the possibility of errors that may be introduced in dielectric test results analysis unless temperature effects are taken into consideration.

Knowledge about moisture content in oil impregnated paper insulation is essential when estimating remaining lifetime of power transformers. Direct evaluation of moisture content is rarely possible due to inaccessibility of the internal insulation system in transformers. Therefore, various indirect estimation techniques are utilized. Frequency domain spectroscopy (FDS) measurements of transformer insulation belong to this group. To perform high quality interpretation of results of FDS measurements a good knowledge on dielectric responses in oil impregnated pressboard and paper is required, especially as it refers to their variation with water content and temperature. The aim of this paper is to provide an open access to the frequency domain spectra of oil impregnated paper and pressboard samples, which can then be used in modeling of the results of diagnostic measurements in power transformers.

An extensive revision of the IEEE C37.011 "Application Guide for TRV for AC High-Voltage circuit Breakers" has been in process for the last four years. The main aim was to introduce the two- and the four- parameter description of the transient recovery voltage (TRV) as used in IEC Switchgear standards that have been introduced in the revisions of Standards C37.04 and C37.06. These two standards are being concurrently revised, but IEEE C37.011 provides the necessary explanations on TRV characteristic in advance. Non-specialist engineer users of the guide will better understand these standards

Test methods for evaluating flashover voltage of ceramic and nonceramic insulators under ice, snow, and cold-fog conditions are recommended. The paper describes the procedure to be followed at various stages of an evaluation test including preparation of the test object, insulator precontamination, ice, snow, and cold-fog deposits as well as voltage applications and flashover voltage evaluation. Some relevant parameters for ice accretion, flashover voltage evaluation, artificial pollution, and power source requirements are taken from IEC 60129, IEC 60507, and IEEE Std.-4-1995 on standard techniques for high-voltage testing.

Recently developed monitoring-systems show the need as also the possibilities for online- and offline-onsite-diagnostics. The limit in sensitivity for electrical pd-measurements according to IEC 60270 which is reached using filters for sinusoidal disturbances and compensation for corona impulses is still not satisfactory. The acoustic partial discharge measurement (pd-measurement) is also a well known and reliable method often used by transformer manufacturers in testing. In laboratory setups a gain in sensitivity of the acoustic in comparison to the electrical pd measurements could be established. This is reached by adapting the sensors to the propagation path, a modern but moderately priced acquisition hardware and the introduction of new signal processing algorithms. The developed sophisticated measuring systems show remarkable results in online acoustic pd-measurements onsite.

Reliability and power quality are two of the most discussed topics in the utility industry today. Many groups have performed power quality studies with varying results. One large industrial customer (Champion International) in northern New York State, which experienced numerous process interruptions in its paper mill due to power system disturbances, is the focus of this study. Monitoring methods, results, inadequacies of standards, and unique mitigation techniques used to conquer their problems are discussed in this paper.

Many utilities still have paper-insulated pipe type cables in service. In the near future they may have to replace some of these systems due to local increases in electric power demand or aged cable insulation. In many cases the steel pipes are well protected against corrosion and thus suitable for further use. For retrofitting in such installations, XLPE-insulated cables offer several advantages. Felten & Guilleaume Energietechnik AG has developed a new three-core XLPE-insulated 64/110 kV cable with reduced insulation thickness (10 mm), that can be pulled into a pipe. In co-operation with VEW ENERGIE AG, the cable performance is being investigated in a field test, that started in October 1996. This long-term loading cycle test will last for three years. The test voltage, applied between conductor and earth, amounts to 127 kV. The temperature is measured with the help of an optical-fiber measurement system. This device is computer controlled and works as a remote monitoring system.

This paper briefly describes the special publication, "Fault and Disturbance Data Requirements for Automated Computer Analysis", prepared by working group III of the Relaying Practices and Consumer Interface Protection Subcommittee of the IEEE Power System Relaying Committee. The publication describes the analysis objectives and the data necessary to facilitate automated analysis.

A review is given of the performance of the +or-250 kV HVDC mass-impregnated, solid-type submarine cables installed between Denmark and Norway. The cables, which have been partly overloaded, have successfully met all requirements. Three mechanical failures have occurred, two due to heavy beam trawls and the third during stormy weather. On two occasions the cables were repaired in stormy weather. They have been hit by a great number of trawlboards and fisherboats' anchors without any significant damage. Tests and investigations of the insulation have demonstrated that this type of cable can be used for far higher voltages than assumed before. Migration of the impregnant is of less significance than previously believed. The cable can tolerate very high mechanical strains, such as those arising during layering at very great sea depths. Bending tests were made on cable pieces at tensions up to 72 metric tons; the results of the subsequent electrical tests were excellent, and no visible traces of damage of the insulation were found. The design stresses are considered, and suggestions are made for the design of cable for very deep waters. >

Nowadays, there is an increasing interest in using transformer monitoring systems. Some monitoring systems use models to predict the value of certain variables. One of the key variables to be monitored is moisture content in oil, as moisture has a high influence on transformer exploitation. In this paper, the theoretical foundation of a model for monitoring moisture in transformer oil is reported. The moisture model estimates moisture content in transformer oil under certain working conditions. Comparing the value of estimated moisture with the measured one, the model is able to detect failures producing an abnormal water content in the transformer. The model considers the equilibrium relations between paper and oil, the transients that take place until the steady-state equilibrium is attained, and the water increase caused by the insulating paper ageing. In addition, the model can alert to the possibility of oil saturation and the consequent free water formation in the transformer. In this paper, the theoretical foundation of the moisture model is described; in a companion paper, its experimental verification is reported.

For Part I see ibid., p.1417-22, 2005. In the companion paper, a model to detect failures leading to an increase of the water content in transformer oil was proposed. In this paper, the experimental verification of the model is presented. The model was tested in a 1500-kVA test transformer constructed as a reduced scale model of a 60-MVA 220-kV transformer. In order to load the transformer to have the capability of varying load and power factor over a wide range, the opposition method described in IEC 60076-2 Standard was used. Sensors to measure temperature and moisture in oil were installed in the test transformer. The model was adjusted and validated using a set of data corresponding to different operating conditions. The transformer was submitted to an accelerated aging process studying the model response in such a case. In order to verify the model's ability to detect failures, a water ingress in the transformer was simulated.

All electrical utilities deal with the task of determining the residual life of their in-service power transformers. Given the difficulties experienced with the use of first and second generations of markers (carbon oxides and 2-furfuraldehyde), several organizations are now considering the use of methanol for this purpose. Hydro-Quebec, which discovered this approach, uses this molecule on a regular basis to evaluate the state of the cellulose insulation of in-service power transformers and applies it to Electricite de France's nuclear power plant transformers. In this paper, some examples of the application of methanol in the field are presented against the information received from the early marker generations.

All 3-core cables require fillers to fill the space between insulated cores and the belt insulation or a sheath. The equations given in IEC 287 and in the Neher-McGrath paper (1957) for the internal thermal resistance of 3-core cables were developed for paper insulated cables. For such cables, it was assumed that the insulation and filler materials have the same thermal resistivities. In reality, in 3-core cables a variety of materials are used as fillers. The majority of these will have a higher thermal resistivity than the insulation. In the previous paper, a new formula was developed to compute the value of the internal thermal resistance of belted cables taking into account the thermal resistivity of the filler. In this paper, screened cables are considered and a new formula for the computation of the internal thermal resistance of such cables is presented. The effect of filler resistivity on cable ampacity is also discussed.

This paper describes the results of a long-term reliability test of a newly developed 500 kV DC oil-filled cable and accessories that transmit up to 2800 MW. This is the largest capacity ever used for submarine cables. The test line was 500 m and consist of submarine cables, land cables and several kinds of accessories. An optical fiber sensor was first installed in the oil duct of the cable to measure the conductor temperature directly under application of DC voltage. From 1994 to 1996, the authors carried out an accelerated electrical and thermal aging test equivalent to 40 years, a load cycle test, a polarity reversal test and a transient overloading current test. Satisfactory breakdown strength was confirmed in the residual test which followed the long-term test. These results have been instrumental to the decision to construct 500 kV DC commercial submarine cable lines.

A reduced wall 138kV HPOF pipe-type cable having a 700 kcmil (355 mm2) compact round copper conductor and 348 mils (8.8 mm) of insulation and two types of joints have been developed for re-conductoring existing 69kV HPOF feeders utilizing 5 inch (127 mm) outside diameter pipes on the Florida Power & Light Company (FP& L) system. Laboratory test results on the cable and two joints, one employing hard paper and the other high density crepe paper tapes, are reported. The commercial cable installation is described. Test results are also given on an improved design reduced wall 138kV HPOF pipe-type cable having a 2000 kcmil (1015 mm2) compact segmental copper conductor and 379 mils (9.6 mm) of insulation and a high density crepe paper joint intended for re-conductoring a 69kV HPOF pipe-type feeder using the existing 6-5/8 inch (168 mm) outside diameter pipes. The cable and joint satisfactorily passed all requirements of AEIC CS2-82 (4th Edition) [4].

The Neher/McGrath paper and the IEC Standard 60287-2-1 contain the formulae for the calculation of the external thermal resistance of cables installed in backfills or duct banks. These models are based on several assumptions, which often are not valid for practical installations. This paper examines the applicability of the Neher/McGrath model and proposes a new solution for the calculation of the external thermal resistance for such installations. Numerical examples illustrate the difference between the two methods.

This paper proposes a new modulated model predictive control method for the control of modular multilevel converters (MMCs) in voltage source converter-high voltage dc systems. The proposed method retains the advantages of the conventional finite control set-model predictive control methods by programing the nonlinear properties of the MMC into the design calculations while minimizing the ac line and circulating current ripples and steady-state error by generating modulated switching signals with a fixed switching frequency. In this paper, first, the predictive modeling of the MMC is provided. Next, the proposed control method is described. Then, the application of the proposed method to an MMC system is detailed. Finally, experimental results from an MMC system connected to a three-phase grid are provided to validate the theoretical outcomes.

When designing utility substations, it is necessary to consider all directional components of the electromagnetic reaction forces during short-circuit faults to properly account for their effects on substation equipment. IEEE 605 assumes straight long parallel conductors in its two-dimensional (2-D) force calculations while ignoring any bends and 3-D placement of conductors. CIGRE's Brochure 105 provides practical information on the mechanical effects of short-circuit currents in HV and EHV open air, conventional substations. Although these computational methods are easy to implement, they have several limitations and are sometimes too conservative. Previously published papers address such conservatism using dynamic analysis approaches that provide more accurate results when compared to experimental data. This paper provides two extensions to IEEE 605 and CIGRE 105 for calculating fault current reaction forces. A closed-form equation defining the simplified 2-D short-circuit force time history during a fault based on CIGRE 105 and a computationally inexpensive numerical 3-D force calculation technique are presented.