Visuth Lorch

Transformer Failure Due to Circuit-Breaker-Induced Switching Transients

Switching transients associated with circuit breakers have been observed for many years. Recently this phenomenon has been attributed to a significant number of transformer failures involving primary circuit breaker switching. These transformer failures had common contributing factors such as 1) primary vacuum or SF-6 breaker, 2) short cable or bus connection to transformer, and 3) application involving dry-type or cast coil transformers and some liquid filled. This paper will review these recent transformer failures due to primary circuit breaker switching transients to show the severity of damage caused by the voltage surge and discuss the common contributing factors. Next, switching transient simulations in the electromagnetic transients program (EMTP) will give case studies which illustrate how breaker characteristics of current chopping and re-strike combine with critical circuit characteristics to cause transformer failure. Design and installation considerations will be addressed, especially the challenges of retrofitting a snubber to an existing facility with limited space. Finally, several techniques and equipment that have proven to successfully mitigate the breaker switching transients will be presented including surge arresters, surge capacitors, snubbers and these in combination.

Transformer failure due to circuit breaker induced switching transients

Switching transients associated with circuit breakers have been observed for many years. Recently, this phenomenon has been attributed to a significant number of transformer failures involving primary circuit-breaker switching. These transformer failures had common contributing factors such as the following: 1) primary vacuum or SF-6 breaker; 2) short cable or bus connection to transformer; and 3) application involving dry-type or cast-coil transformers and some liquid-filled ones. This paper will review these recent transformer failures due to primary circuit-breaker switching transients to show the severity of damage caused by the voltage surge and discuss the common contributing factors. Next, switching transient simulations in the electromagnetic transients program will give case studies which illustrate how breaker characteristics of current chopping and restrike combine with critical circuit characteristics to cause transformer failure. Design and installation considerations will be addressed, particularly the challenges of retrofitting a snubber to an existing facility with limited space. Finally, several techniques and equipment that have proven to successfully mitigate the breaker switching transients will be presented, including surge arresters, surge capacitors, snubbers, and these in combination.

Medium voltage switching transient induced potential transformer failures; prediction, measurement and practical solutions

During commissioning of a large data center, while switching medium-voltage circuit breakers without any appreciable load, several potential transformers failed catastrophically. A detailed investigation, including a computer simulation, was performed. Ferroresonance produced by switching transients associated with opening and closing the vacuum breakers was determined to be the cause. The analysis also determined that the close-coupled power transformers were also in jeopardy. Field inspections involving grounding improvements coupled with solution simulations were made. High-speed switching transient measurements were performed to verify the analysis and the surge protective device solution (arresters and snubbers). This paper walks the reader through problem recognition, simulation, field measurements, and solution implementation. Special focus will be made on the field measurement verification.

Vacuum Circuit Breaker Transients During Switching of an LMF Transformer

Switching transients associated with circuit breakers have been observed for many years. With the widespread application of vacuum breakers for transformer switching, recently, this phenomenon has been attributed to a significant number of transformer failures. Vacuum circuit breaker switching of electric arc furnace and ladle melt furnace (LMF) transformers raises concern because of their inductive currents. High-frequency transients and overvoltages result when the vacuum breaker exhibits virtual current chop and multiple re-ignitions. This paper will present a detailed case study of vacuum breaker switching of a new LMF transformer involving current chopping and restrike simulations using the electromagnetic transients program. A technique that involves a combination of surge arresters and snubbers will be applied to the LMF to show that the switching transients can be successfully mitigated. Additionally, some practical aspects of the physical design and installation of the snubber will be discussed.

Power Quality Investigation of Back-to-Back Harmonic Filters for a High-Voltage Anode Foil Manufacturing Facility

The problem, as described by this high-voltage anode foil manufacturing facility, has included loss of the harmonic filter banks during utility power disturbances. During such disturbances, one or more formation machine(s) trip and rectifier(s) fault, both of which are critical to the continuous operation of the process. One recent disturbance resulted in significant failure of components in both filter banks. A power quality investigation commenced to determine the cause of the filter bank failure. This paper describes the results of the analytical methods employed. Field harmonic measurements determined the presence of voltage and current distortion and established an appropriate model for harmonic analysis of various likely system conditions. Harmonic analysis explained the tuning of the existing filter banks and changes to tuning that occurred since installation several years ago, as well as determined the harmonic loading on each stage of the filters. Transient simulations identified existing problems and areas of concern, especially in regard to transient overvoltages that resulted when switching either of the filter banks. Recommendations included alternatives to avoid possible failures in the future, most significantly, the re-design of one the harmonic filters and the application of surge protection.

Vacuum Circuit Breaker Transients during Switching of an LMF Transformer

Switching transients associated with circuit breakers have been observed for many years. With the wide-spread application of vacuum breakers for transformer switching, recently this phenomenon has been attributed to a significant number of transformer failures. Vacuum circuit breaker switching of electric arc furnace and ladle melt furnace transformers raises concern because of their inductive currents. High frequency transients and overvoltages result when the vacuum breaker exhibits virtual current chop and multiple re-ignitions. This paper will present a detailed case study of vacuum breaker switching of a new ladle melt furnace transformer involving current chopping and re-strike simulations using the electromagnetic transients program. A technique that involves a combination of surge arresters and snubbers will be applied to the ladle melt furnace to show the switching transients can be successfully mitigated. Additionally, some practical aspects of the physical design and installation of the snubber will be discussed.

Voltage Distortion on an Electrical Distribution System

This article described a harmonic analysis study that determined that the proposed midfrequency welder would produce unacceptable voltage distortion on the utility electrical distribution system, especially an adjacent manufacturing facility with extensive electronic controls as well as on the house load associated with the midfrequency welder. The voltage distortion was characterized by significant notching and false zero crossings, both of which could affect electronic control circuits, while the latter could affect timing circuits such as the welder. The article then showed that the installation of line chokes in combination with a fifth harmonic single-tuned filter improves voltage distortion to within acceptable limits. Finally, the article explained that additional enhancements including the use of phase-shifting transformers to achieve a 12pulse or 24-¿ulse effective system may entirely eliminate the need for installing a filter. This knowledge of the harmonics nature of the midfrequency welder load and effective harmonic mitigation methods provided valuable data to the welder manufacturer so that changes could be made in the design stage to achieve the utility criteria.

Harmonic Filter Analysis and Redesign for a Modern Steel Facility with Two Melt Furnaces Using Dedicated Capacitor Banks

This modern steel making facility relies on reactive compensation at the substation level in the form of harmonic filtering to support voltage for casting operations, and at the melt furnace level in the form of capacitor banks to support LMF operations. A history of harmonic filter failures at the substation, and intermittent capacitor unit failures at the melt furnace banks, culminated in a recent catastrophic failure of the filter reactor. This paper describes the analytical methods used to redesign the harmonic filter to achieve the desired level of compensation yet avoid future failure of the filter components. Field measurements showed the harmonic filter provided a low impedance path for harmonic currents produced during arcing of either or both melt furnace transformer resulting in overload of the filter reactor. Preliminary harmonic analysis enabled temporary detuning of the harmonic filter in the field to avoid this damaging resonance condition permitting steel making operations to continue while redesigning the filter. Detailed harmonic analysis and filter design considered a wide range of normal and emergency system conditions, especially the effects of extended line outages by the utility for reconductoring. Recommendations included specifications for the redesigned harmonic filter as well as changes to improve performance of the melt furnace capacitor banks

Transformer failure due to circuit breaker induced switching transients appplicable to the cement industry

Switching transients associated with circuit breakers have been observed for many years. Recently this phenomenon has been attributed to a significant number of transformer failures involving primary circuit breaker switching. These transformer failures had common contributing factors such as 1) primary vacuum or SF-6 breaker, 2) short cable or bus connection to transformer, and 3) application involving dry-type or cast coil transformers and some liquid filled. This paper will review these recent transformer failures due to primary circuit breaker switching transients to show the severity of damage caused by the voltage surge and discuss the common contributing factors. Next, switching transient simulations in the electromagnetic transients program (EMTP) will give case studies which illustrate how breaker characteristics of current chopping and re-strike combine with critical circuit characteristics to cause transformer failure. Design and installation considerations will be addressed, especially the challenges of retrofitting a snubber to an existing facility with limited space. For the cement industry, situations where circuit breaker induced switching transients are likely to damage transformers will be discussed. Finally, several techniques and equipment proven to successfully mitigate the breaker switching transients will be presented including surge arresters, surge capacitors, snubbers and these in combination.

Evaluation of the proposed retirement of a condensing turbine generator on the paper mill electrical distribution system and utility ties

The problem, as described by this paper mill, is the desire to retire an inefficient condensing steam turbine generator for economic reasons and purchase additional power from the utility. The condensing unit, along with a modern extraction steam turbine generator and two utility ties, presently supply power to this three-bus medium voltage system via sync bus and reactor ties. A power system study commenced to determine the impact of the proposed change on the existing paper mill electrical distribution system as well as the existing utility ties. This paper describes the results of the analytical methods employed: power quality measurements, load flow analysis, short circuit analysis and transient stability simulations. Recommendations included alternatives to accommodate retirement of the condensing steam turbine generator, most significantly, the installation of a new power transformer as a third utility tie and the application of reactive compensation as filters at the three medium voltage busses.