Conexiones De Transformadores

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Distributed Generation Interconnection Transformer and Grounding Selection
R. F. Arritt, Member, IEEE, and R. C. Dugan, Fellow, IEEE

Fig. 1 Neutral Shift During Single Line to Grounded Fault (SLG) on Phase C (a) Pre-Fault Condition (b) Not Effectively Grounded System (c) Effectively Grounded System

II. DIRECT DG CONNECTION While it is possible to obtain DG rated for the utility mediumvoltage (MV) distribution system, there can be considerable risks in connecting such equipment directly to utility distribution system. This is particularly risky for installations fed by overhead lines. The insulation level of the machines may not coordinate with the utility system. Interconnecting DG through a transformer can eliminate many of the difficulties that may occur withdirectly-connected DG installations. Therefore, direct connection is often discouraged.

R. F. Arritt is with EPRI, Knoxville, TN 37932 USA (email: rarritt@epri.com) R. C. Dugan is with EPRI, Knoxville, TN 37932 USA (email: r.dugan@ieee.com

©2008 IEEE.

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ntroducing Distributed Generation (DG) into a distribution system designed for radial power flow creates many issues including but not limited to: harmonic concerns, system overvoltages, fault coordination, increased fault currents, insulation coordination, and islanding concerns. The addition of DG may also alter thedistribution of fault current affecting the existing overcurrent protection coordination. The added DG may expose the existing utility system to temporary system overvoltages that are damaging to the lineto-neutral-connected electrical equipment. For example, Fig. 1 illustrates typical phase voltages when a single line-toground (SLG) fault is applied at Phase C. Fig. 1(a) illustrates the pre-faultcondition, (b) illustrates a system that is not effectively grounded, and (c) illustrates a system that is effectively grounded. As can be seen in Fig. 1 (b), there are excessive voltages on the unfaulted phases that will cause damage to surge arresters and other electrical equipment. This condition may occur when a DG becomes islanded on the faulted section. This issue as well as other conditionsyielding overvoltages on the distribution system may be alleviated by choosing an appropriate interconnection transformer that maintains an effectively grounded system at all times. However, this may

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I. INTRODUCTION

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Index Terms—distributed generation; grounding; neutral reactor; isolation transformer

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0.1Abstract-- The interconnection of Distributed Generation (DG) into the power distribution system creates many challenges for utilities. Many of these challenges are related to the type of interconnection transformer (or lack thereof) and consequently the grounding used for the DG interconnection. There remains confusion about the various transformer types for DG interfacing. This is a critical pointbecause the selection of the correct interface with the local utility will simplify feasibility studies; therefore, decreasing the overall cost of the DG interconnect investigation to benefit all. This paper discusses the advantages and disadvantages of different interconnecting transformers. Neutral reactor sizing for grounding transformer connections is discussed.

change the flow of fault currenteffecting the existing ground fault protection. The selection of DG interconnection transformer will also have an effect on the insulation coordination and the flow of third harmonics.

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III. INTERCONNECTION TRANSFORMER CONFIGURATIONS Several of the concerns with the interconnection of DG can be addressed, or at least minimized, through means of an appropriately-configured...