The overview of protection schemes for distribution
systems containing micro-grid
CHEN Jian, HE Zheng-you, Member, IEEE, JIANG Wei
School of Electrical Engineering
Southwest Jiaotong University
Chengdu, Sichuan, China, 610031
chenjian0301@163.com, hezy@home.swjtu.edu.cn
Abstract—The traditional protection scheme of distribution
networks is affected due to the distributed generators connecting
to the power system. In order to ensure the protection works
correctly, various improved and new schemes have been
presented. This paper reviews the development on protection
scheme for distribution system containing micro-grid.
Meanwhile, the principles and characteristics of these schemes
are summarized.
Keywords-distributed generation; micro-grid; distribution
network; protection scheme
I. INTRODUCTION
Distributed Generation (DG) is a small power generator
which is designed for satisfying the demand of specific
customers and supporting economy operation of original
distribution networks. It is always installed close to the
customers. Typical DG mainly includes the fuel cell, wind
power, small hydro-electric power station, solar cell array and
biological electricity generation etc. The application of DGs
can reduce the cost of transmission, increase power quality,
and easily meet the short-term proliferating power
requirement. However, it brings some adverse influences to
the distribution network when accessing to it. In order to make
the best of DGs’ advantages and weaken its negative impacts,
the conception of micro-grid is proposed. The CERTS
(Consortium for Electric Reliability Technology Solution of
the USA) defines “micro-grid” as a power supply system
composed of DGs and loads. It can provides the electricity and
heat. It operates as a single controlled power unit with the
inverter as its main part, and meets the requests of power
quality and power supply security.
Fig.1 shows the basic architecture of distribution networks
containing micro-grid. The distribution networks become
multi-supplied networks after micro-grids are connected. The
topology structure, load flow distribution, and the magnitude
and direction of the fault current become completely different
from what they were. The original relays, which were
designed based on traditional radial network, could not
response correctly due to the impacts of micro-grids. So
innovative protections for distributed system containing
micro-grids should be researched. In this paper, the protection
schemes for both micro-grids connected and islanded modes
are summarized.
Fig.1 The basic architecture of distribution networks
containing micro-grid
II. G
RID-CONNECTED PROTECTION
Micro-grid can operate in both parallel and islanded modes
with different short-circuit currents in different modes. When
it operates in islanded mode, the fault current is limited to
twice rated-current that is not large enough for the traditional
over-current protection to trip. Therefore, the new protection
scheme should respond to its internal and external faults
selectively, rapidly, sensitively and reliably.
Early in the development of distributed generation
technologies, the IEEE Std 1547-2003 [3] dictates that all DGs
should be disconnected from the main grid immediately if a
fault occurs to ensure existing protection relay works
correctly. However, that guide has two defects. Firstly, it is
possible that the DGs are still connecting before the protection
trips because of their time-coordination problem, which may
result in protection failed [4-5]. Secondly, not all DGs affect
the existing protection and automatic reclosing. Considering
making full use of the power generation capacity and
emergency backup capabilities of DGs, improving power
supply reliability and power quality [9], it is not appropriate to
disconnect all DGs blindly when fault occurs. Therefore,
advanced protection scheme should be able to diagnose and
978-1-4244-6255-1/11/$26.00 ©2011 IEEE