CT Saturation Calculations: Are They Applicable in the Modern World?—Part I: The Question

Previously, ANSI/IEEE relay current transformer (CT) sizing criteria were based on traditional symmetrical calculations that are usually discussed by technical articles and manufacturers' guidelines. In 1996, IEEE Standard C37.110-1996 introduced (1+X/R) offset multiplying, current asymmetry, and current distortion factors, officially changing the CT sizing guideline. A critical concern is the performance of fast protective schemes (instantaneous or differential elements) during severe saturation of low-ratio CTs. Will the instantaneous element operate before the upstream breaker relay trips? Will the differential element misoperate for out-of-zone faults? The use of electromagnetic and analog relay technology does not assure selectivity. Modern microprocessor relays introduce additional uncertainty into the design/verification process with different sampling techniques and proprietary sensing/recognition/trip algorithms. This paper discusses the application of standard CT accuracy classes with modern ANSI/IEEE CT calculation methodology. This paper is the first of a two-part series; Part II provides analytical waveform analysis discussions to illustrate the concepts conveyed in Part I     

IEC medium-voltage circuit-breaker interrupting ratings-unstatedshort-circuit considerations

International Electrotechnical Commission medium-voltage circuit-breaker interrupting ratings are reviewed using typical petrochemical examples. This paper investigates the complexity of high-magnitude X/R ratios, and short-circuit current zero-axis crossings beyond the interrupting time of the standard circuit-breaker when used for generator applications. Guidelines are provided, enabling the engineer to correctly define the circuit-breakers and switchgear for these applications     

Secondary selective system residual bus transfer - a modern application approach

A traditional secondary selective residual bus automatic transfer scheme is reviewed, and the basic logic necessary for automatic transfer operation is discussed. Although many transfer scheme documents are available, the authors believe the scheme complexity should challenge application engineers to better understand the "why's" rather than the "how to's" of the application. A modern programmable logic controller approach is proposed as a contemporary solution to a complex control application.     

Is My UPS Distribution System Coordinated?

This paper reviews typical uninterruptible power supply (UPS) distribution systems for the petroleum and chemical industries, focusing on the selectivity of interrupting devices during short-circuit conditions. When selectivity is not achieved, alternative solutions are proposed. Distribution-system-selectivity comparisons are performed for standard circuit-breaker panelboards, fuse panelboards, and custom panelboards utilizing hydraulic magnetic circuit breakers. When remote panels are implemented, application considerations should include oversizing feeder cables or using step-up/step-down transformers. To determine the "ride through" response of operating plant individual UPS loads, the voltage-depression time during UPS distribution-system-fault clearing is compared with the load "ride through" capability. General guidelines are provided for improved UPS distribution-system performance     

Smart industrial substations

The smart industrial substations enhances plant electrical personnel safety by physically removing the operator from the arc flash zone. The smart industrial substations shifts electrical technician functions from time-consuming tasks in front of substations equipments to time-saving tasks away from the substations. The smart industrial substation is a "best fit" integration of traditional and modern substation devices with local/remote human-machine interfaces (HMIs), pagers, and cell phones.     

Is selectivity achieved in critical low-voltage UPS and standbygenerator power circuits?

This paper reviews typical critical low-voltage uninterruptible power supply (UPS) systems and standby generator power circuits used in the petrochemical industry to determine if selectivity is achieved between protective devices during short-circuit conditions. Alternate system configurations are proposed when complete selectivity is not achieved. Generator excitation systems and UPS support during fault conditions is discussed