Thomas J. Dionise

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.

Assessing the Performance of a Static Var Compensator for an Electric Arc Furnace

The advantages of a static var compensator (SVC) for electric arc furnace (EAF) and ladle melt furnace (LMF) applications are well known. The SVC minimizes the impact of the EAF and LMF on the utility and improves the efficiency of both furnaces. In this application, it was desirable to quantify the performance of the SVC. This paper describes power quality measurements that were taken on the electrical distribution system to evaluate the performance of the SVC. The purpose of the power quality measurements was to monitor the voltage regulation, harmonics, flicker, and other power quality quantities at the 138-kV utility point of common coupling (PCC), as well as the 34.5-kV system serving the SVC, EAF, and LMF. The measurements and subsequent analysis established a baseline for the SVC performance and identified areas of concern. This paper describes the analysis of the measurements and evaluation of the SVC performance.

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.

Harmonic analysis and multi-stage filter design for a large bleach production facility

Modern sodium chlorate or sodium hypochlorite-making processes (commonly known as “bleach”) takes salt and water through an electrolytic process. The electrolytic process requires a significant amount of DC power. Bleach production plants, equipped with rectifiers, can generate potentially damaging harmonic currents if not mitigated or controlled. This paper presents a combination of harmonic mitigation methods applied at a large bleach production facility. The harmonic mitigation methods consisted of transformer phase shifting and multi-stage harmonic filter banks applied at 34.5 kV to satisfy IEEE Std 519–1992 harmonic distortion limits [1] at the point of common coupling. The authors faced several challenges with the harmonic filter design that included various operating conditions within the facility as well as numerous utility substation loading and capacitor combinations.

Comprehensive Analysis to Specify a Static Var Compensator for an Electric Arc Furnace Upgrade

This steel maker is upgrading the Electric Arc Furnace (EAF) Melt Shop to increase production and improve metallurgical quality of their product. The increased electrical load and harmonic generation for the new furnace operation necessitated the installation of a static var compensator (SVC) to meet utility interconnect requirements. This paper describes a sequence of comprehensive analysis performed to specify the SVC including Load Flow Analysis, Flicker Analysis and Harmonic Analysis Studies. The analysis presented in this paper sized and evaluated the proposed SVC reactive compensation to mitigate flicker, harmonics and power factor for the new EAF and LMF installations to comply with the utility interconnect requirements.

A Systematic Approach for Medium-Voltage Power Factor Correction Design

The value proposition for the installation of power factor (PF) correction (PFC) to avoid costly utility penalties is easily understood. The attractive payback of such PFC opportunities often pushes the project ahead without engineering considerations. In the case of medium-voltage PFC, lack of upfront engineering can lead to unexpected consequences in the performance of the PFC equipment ranging from nuisance trips and blown fuses to catastrophic failure in the extreme. In this paper, the authors propose a systematic approach to the design of PFC for medium-voltage systems. Three actual case studies were selected to illustrate this systematic approach and show that it can be applied regardless of the size, type, and complexity of the facility. Such a methodology is composed of load flow and harmonic measurements, harmonic analysis, utility billing demand and PF analysis, and reactive compensation specification in the form of either capacitors or harmonic filters, if needed. Diverse and competing design constraints, including daily and week load cycles, nearby utility capacitors, switched or stepped requirements, specifics of utility tariffs, loads with high harmonic content, and harmonic resonance, are identified, analyzed, and then incorporated into the solution to ensure successful performance of the PFC equipment.

Assessing the performance of a Static VAR Compensator for an electric arc furnace

The advantages of a static var compensator (SVC) for electric arc furnace (EAF) and ladle melt furnace (LMF) applications are well known. The SVC minimizes the impact of the EAF and LMF on the utility and improves the efficiency of both furnaces. In this application, it was desirable to quantify the performance of the SVC. This paper describes power quality measurements that were taken on the electrical distribution system to evaluate the performance of the SVC. The purpose of the power quality measurements was to monitor the voltage regulation, harmonics, flicker, and other power quality quantities at the 138-kV utility point of common coupling (PCC), as well as the 34.5-kV system serving the SVC, EAF, and LMF. The measurements and subsequent analysis established a baseline for the SVC performance and identified areas of concern. This paper describes the analysis of the measurements and evaluation of the SVC performance.