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Application Guidelines for Transformer Connection and Grounding for Distributed Generation: An Update
Grounding of electrical power systems is a very important consideration when designing new systems or upgrade or retrofit existing systems with new or additional equipment. This has become an intriguing and challenging task with the proliferation of distributed generation (DG) in the conventional power distribution systems (defined here typically for voltages 12.47kV and below).
Power system is grounded so that they are able to control the voltage with respect to remote earth at zero potential [1, 2]. By having such a reference, electrical engineers can design systems to provide a path for a flow of current that will allow the detection of unwanted connections between energized conductors and ground. Grounding an electrical power system also provide the benefits of increased personnel safety and protects equipment from the harmful effects of transient over-voltages.
Grounding as a broad topic can be divided into two major categories: (a) focuses on the design of grounding systems for protecting personnel following the National Electric Code (NEC), and (b) focuses on the protection of equipment and the study of the effects of grounding on the operation of the electric power system. This paper focuses on the latter and the importance of the transformer connections and grounding. Traditional distribution system is radially configured. Fig. 1 depicts a simple radially fed power distribution system with single or three phase laterals where power flows from the source down to the loads (in one direction). In case of a fault, current also flows from the source to the fault.
This mode of operation is well understood by industry and utilities alike for decades and has kept distribution systems simple and relatively easy to design. Because of this simplicity, the (grounded) wye - (grounded) wye or delta- (grounded) wye transformer configuration has become standard on most distribution systems. The radial distribution system has worked well in terms of reliability, selectivity for fault isolation, and its simplicity and allows operators to quickly switch components without requiring engineering analysis. However, the past decade has seen the increased usage of small generation installations (called Distributed or Dispersed Generation or DG) on customer loads on the existing distribution system. This has significantly changed the topology of the distribution system with many issues that arise from how the distribution transformer is connected and/or grounded.
|Application Guidelines for Transformer Connection and Grounding for Distributed Generation: An Update|