1、英文原文CONSIDERATIONS IN UNIT SUBSTATION DESIGN TO OPTIMIZERELIABILITY AND ELECTRICAL WORKPLACE SAFETYByDavid B. Durocher Senior Member, IEEE Industry Manager Eaton CorporationABSTRACTMany legacy low and medium-voltage unit substations installed today are based upon older designs that took advantage of
2、 reduced first cost “opportunities” allowed by existing instal ation codes and standards.Fast-forward to how these substation designs fair in safety and reliability today, particularly in industrial process applications found in cement, pulp and paper, petroleum & chemical and others, some of theexe
3、rcised “opportunities” applied in the past begin to look more like liabilities than assets. Legacyengineering decisions once thought to be prudent take on new meanings today, particularly when these decisions are viewed through the lens of emerging new workplace safety standards. The critical issue
4、ofaddressing destructive arc-flash hazards associated with persons working on or around energizedelectrical equipment must now be considered. Because traditional substation designs often appeared to involve some compromise regarding both safety and reliability, a design team of a major process indus
5、try user took a fresh look at unit substation design. The design review took place in conjunction with construction of a Greenfield plant built in the spring of2009 in the USA. This paper will review the design limitations of traditional unit substation configurations, offer an overview of the alter
6、natives considered by the Greenfield site project team, and discuss technical and safety validation of the design that was ultimately selected and installed. Economic comparisons to traditional designs, changes in the owner operating and safety procedures for plant personnel as a result of the engin
7、eering design changes, and overall design acceptance by operations will also be reviewed in this paper. Index Terms Process Industries, Power Distribution, Unit Substations, Design for Safety, Electrical Workplace Safety. INTRODUCTION Low and medium voltage unit substations are applied universal y a
8、cross most every industry. At the tree-top level, unit substations are used simply to transform medium-voltage, typically 15 to 25kV, to a lower distribution voltage, typically 0.48 to 4.16kV, for application in supporting a host of various motor andprocess equipment loads. Fig. 1 shows a typical lo
9、w-voltage unit substation. In this case, the primary assembly at the left is a medium-voltage fused load break switch. For this example, we will assume theprimary voltage is 13.8kV. For assemblies in North American industry, this assembly is typically designed to metal-enclosed switchgear standard A
10、NSI/IEEE Standard C37.20.3 1. This assembly includes a load-break isolation switch with ratings of 600 or 1200 amperes and a medium-voltage current-limiting fuse, appropriately sized to protect the transformer. The primary switchgear is close-coupled to a substation transformer, either dry-type or l
11、iquid filled. The substation transformer is designed to ANSI/IEEE Standard C.57.12 with wall-mounted primary and secondary bushings. There are many different substation transformer design alternatives to choose from, beyond the scope of this paper. Good information on the alternatives can be found i
12、n other technical papers, including 3. In this case, the transformer rating is shown at 2000kVA. With a secondary distribution voltage at 480Y/277 volts, the low-voltage bushings are shown close-coupled to metal enclosed low-voltage switchgear. In Fig. 1, the low-voltage switchgear consists of a 320
13、0 ampere secondary main bus and secondary metering, with no secondary main circuit breaker, connected to four 1200 ampere feeder circuit breakers. There are again variations on low-voltage switchgear designs. For process industry applications, most frequently these assemblies are manufactured to UL1
14、558 Standards 4.DESIGN CONSIDERATIONSIn anticipation of the upcoming project, the design team for the Greenfield site took on the task of investigating existing unit substation configurations carefully to identify where there may be some inherent hidden flaws in the design. It is important to note t
15、hat prevailing codes and standards regarding installation of this equipment had an impact on the unit substation design. In the US, the prevailinginstallation document that applies is the National Electrical Code (NEC) 5. Lets investigate two areas of this code that impact the design and instal atio
16、n of the unit substation presented here. NEC Article 240.21(C)2 Overcurrent Protection Article 240.21(C) of the NEC addresses required overcurrent protection, specifical y related to transformer secondary conductors. The article states that “a set of conductors feeding a single load shall bepermitted to be connected to a transformer secondary, w
