International Review of Electrical Engineering (I.R.E.E.), Vol. 5, N. 5

September-October 2010

Manuscript received and revised September 2010, accepted October 2010 Copyright © 2010 Praise Worthy Prize S.r.l. - All rights reserved

2350

A New Approach for Protection and Control of Distribution Systems with DG

Fatih M. Nuroğlu, Ayşen Basa Arsoy Abstract – The paper aims to propose a Central Coordination Relay (CCR) to provide a solution to islanding protection, which became one of the major issues as distributed generating resources penetrate into electrical distribution systems. The operating procedures of the relay are identified after defining islanded areas and fault areas. The relay is modeled and its response in a real based small-scale system is tested using DigSilent PowerFactory. It is shown that the CCR can maximize the DG capacity usage and improve service continuity in any fault conditions. Copyright © 2010 Praise Worthy Prize S.r.l. - All rights reserved. Keywords: Distributed Generation, Intentional Islanding, Central Coordination Relay

I. Introduction Utilization of small sized generators within MV/LV

distribution systems has increased rapidly for more than two decades. Distributed generation (DG) including renewable resources, micro-turbine or fuel cell, has been alternative/supplementary to the generation from large synchronous generators connected to transmission system.

Distribution systems are normally designed so that the sole source is the transmission grid. Presence of the distributed resources in distribution grid, ‘multisourced grid’, may bring positive or negative effects with the severity depending on the factors of the location, type and size of the source, and grid configuration. At least, one of these factors have an impact on protection, control, reliability, and quality of the system [1-5]. The impact of distributed generation on the system is evaluated in [6] by several indices. Utility perspective of DG interconnection to distribution systems is given in [7].

As DG penetrates into distribution systems, protection issues need to be reevaluated. Challenges and issues on protection are discussed and some technical approaches are developed in Refs.[8-13].

Current standards [14] state that a DG should not continue to energize the local system as soon as the loss of grid or fault is detected to prevent voltage and frequency unbalances or fault feeding. Islanding techniques detecting loss of grid are explored in [15-19]. Without proper islanding, the resources are not utilized more efficiently and the benefits of DG are not maximized. Intentional islanding letting DGs in service could be a powerful means for improving service continuity of distribution systems.

An adaptive protection scheme is proposed in [20] for distribution systems with high penetration of DG by dividing the system into zones having reasonable load-DG balance. Ref. [21] carried out a similar study with an adaptive configuration depending on local generation/state during the loss of main.

In [22], it is stated that intentional islanding is preferred by utilities due to improved system reliability and service continuity. A synchronous generator and double-fed generator with governor and exciters control can be operated in islanding mode. Another result given in [22] is that total generation must be greater than the total load, and a synchronization check relay is needed for restoration of the system.

From this point of view, a new approach is introduced in this paper. A design of a Central Coordination Relay (CCR) sited on a substation is developed for control and protection coordination of the feeders. The developed relay is not only set for current and time, but also status of circuit breakers of the islanded areas and DG’s availability.

This paper first reviews the protection issues on distribution systems with DG focusing on islanding. Then, a procedure dividing distributed generating systems into new operating zones is presented. Later, the principle of CCR design and operation steps are explored followed by application of CCR into a real based small scale system using DigSilent PowerFactory.

II. Protection Issues Conventional power distribution systems are designed

so that the system is radial in structure with uniflow of power due to single source from transmission network. Protection of this type of systems is relatively easy, consisting of fuses, re-closers and some cases relays with