Impact of Distributed Resources on Distribution Relay Protection
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Introduction
The use of distributed resources has increased substantially since 1998 because of the potential to provide increased reliability and lower cost of power delivery to customers. The addition of a distributed resource (DR) to a power system, particularly to the distribution system, introduces system conditions not otherwise encountered. These conditions can have a serious impact on the operation and integrity of the electric power system as well as cause damaging conditions to equipment. The main objective of this paper is to study some of these issues, particularly with respect to system protection. These issues include:


DR Interface Transformer Connections
The selection of the interconnection transformer connection has a major impact on how the dispersed generator will interact with the utility system. [3] There is no universally accepted “best” connection. Figure 1 shows five commonly used connections. Each of these connections has advantages and disadvantages to the utility with both circuit design and protection coordination affected. Each connection should be addressed by the utility as they establish their interconnect requirements.


High Side Delta or Ungrounded Wye
Consider the first three connections: Delta (HV)/Delta (LV), Delta (HV)/Wye-Gnd (LV) and Wye-Ungnd (HV)/Delta (LV) where (HV) indicates the primary winding and (LV) indicates the secondary winding. The major concern with these connections is in the area of circuit design. An advantage of this connection is that there is no source of zero sequence current to impact the utility ground relay coordination. Referring to figure 1, for ground faults at F1 and F2, all of the fault current will come from the utility.