David D. Shipp

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.

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.

Mitigating Arc-Flash Exposure

As more and more industries address arc-flash electrical safety concerns, they are discovering the high risk associated with what used to be normal maintenance tasks. In many cases, the excessively high arc-flash incident energies make it necessary that all maintenance must be done with equipment deenergized, which is not always acceptable to the process industries. This article addresses the multiple ways the authors have devised to significantly lower the arc-flash incident energy exposure by new system design and products, retrofits, retrofills, equipment modifications, and alternate protection settings. In most cases, National Fire Protection Association (NFPA) 70E-2009 hazard risk category (HRC) 2 or lower can be obtained. Several real-world examples are discussed.

Arc-Flash Incident Energy Reduction Using Zone Selective Interlocking

As a result of new requirements in the National Fire Protection Association (NFPA) 70E standard, many facilities are performing arc-flash hazard analyses to better understand how to protect personnel from the possibility of being injured in an arc-flash incident. In many cases in the petrochemical industry, it is unsafe or not practical to shut down electrical equipment to do work. This paper explores practicable options that a facility has to reduce incident energy levels in existing low-voltage systems where the arc-flash hazard analysis results in unacceptably high levels. It also explores a case study in the Gulf of Mexico, where an offshore oil production platform was able to cost effectively reduce incident energy levels from approximately 170 to under 15 by implementing a zone selective interlocking system into their low-voltage switchgear. The selection process logic involving equipment, personnel considerations, and commissioning are all addressed.

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.

Arc-flash incident energy reduction using zone selective interlocking

As a result of new requirements in NFPA 70E, many facilities are performing arc-flash hazard analyses to better understand how to protect personnel from the possibility of being injured in an arc-flash incident. In many cases in the petrochemical industry, it is unsafe or not practical to shut down electrical equipment to do work. This paper explores practicable options that a facility has to reduce incident energy levels in existing low-voltage systems where the arc-flash hazard analysis results in unacceptably high levels. It will also explore a case study, in the Gulf of Mexico, where an offshore oil production platform was able to cost effectively reduce incident energy levels from approximately 170 calories/cm2 to under 15 calories/cm2 by implementing a zone selective interlocking system into their low-voltage switchgear. The selection process logic involving equipment, personnel considerations and commissioning will all be addressed.

Design, Development and Testing of a Voltage Ride-Thru Solution for Variable Speed Drives in Oil Field Applications

ChevronTexaco initiated a plan in 1996 to develop the Boscan Oil Field just West of Maracaibo, Venezuela, that included the expansion and reconditioning of the existing 24 kV distribution system in order to assure a reliable service for the new and existing wells. About 75% of the southern field production uses electrical submersible pumps (ESP) while the remainder uses Beam Pumps and Progressive Cavity Pumps (PCP). All of the ESPs and PCPs, and some of the Beam Pumps are driven by variable speed drives (VSDs). About 255 km of overhead lines results in high exposure to lightning events resulting in subsequent sags and interruptions that cause well-site VSDs to trip, severely impacting production goals. A strategy was developed to combine VSD ride-thru with a recloser system so that most disturbances would have no effect on production. This paper describes the successful development, building and testing of a VSD ride-thru prototype for the PCP drives. The strategy of providing ride-thru for the PCP drives in conjunction with a recloser system is explained. Results of a technical literature survey and evaluation of possible solutions for Boscan Field are evaluated. Target specifications for a prototype ride-thru unit are given along with development plans and testing requirements. Based on this analysis, a prototype ride-thru module for the PCP drives was built using power electronic components and ultra-capacitors. This paper describes the design, development and testing of the ride-thru modules for the VSDs and describes the applicability of this solution to the overall industry.