13-04-2013, 04:54 PM
Improved Coordination of Backup Protection for Transformers in IEC61850-based Substation System
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ABSTRACT
A distance relay scheme is commonly used for backup
protection. This scheme, called a step distance protection, is
comprised of 3 steps for time differential and regions. According
to conventional step distance protection scheme, Zone 2 can
exceed the ordinary coverage excessively in case of a transformer
protection relay. In this case, there can be overlapped protection
area from the backup protection relays, and malfunctions can be
occurred when any fault occurs in the overlapped protection area.
Distance relays and overcurrent relays are used for backup
protection generally, and both relays have this problem, the
malfunction from the overlapped protection area. The method of
modifying Zone 2 may not be obvious to handle this problem
because this method, modifying the common rule, can cause
another coordination problem. To overcome this problem clearly,
this paper describes an improved backup protection coordination
scheme using an IEC61850-based distance relay for transformer
backup protection.
INTRODUCTION
differential relay scheme is generally the primary method
of a transformer protection, and an overcurrent relay or a
distance relay scheme is used as a backup. As a general
rule in Korea, a distance relay scheme is the method used for
backup protection of transformers. And same as above in case
of a transmission line protection. A phase distance relay and
ground distance relay use almost the same algorithm and
parameter settings. The only difference is in the compensation
portion depending on the fault types.
In these days, overcurrent relays seem not to take charge of
major role for a trip, but it is still used for fault detection
widely, and under the specific system it still used for a trip.
Table I shows protection relays for backup protections with
time delay [1].
PROPOSED COORDINATION METHOD
This paper proposes an improved coordination method to
cope with the problems described above, without requiring
any modification of the zone or time delay.
IEC61850- based substation system.
The stated scope of IEC 61850 is communications within a
substation to coordinate the various items of control and
protection equipment. The main features of IEC 61850 are as
follows:
• Interoperability between the intelligent electric devices
(IEDs) from various manufacturers to create an integrated
system.
• High-speed data transfer among IEDs using a peer-topeer
communication model instead of the master-slave
communication model used in other protocols.
• Data definition based on an advanced object-oriented
model containing complete data specifications instead of a
single-oriented model with each data type defined by a
numeric address.
SIMULATIONS
System Configuration
A system including transformers was designed as shown in
Figure 7 to test the proposed coordination method. There was
a distance relay for transformer backup protection located on
the C-Bus, and a distance relay and time-overcurrent relay for
transmission line protection located on the B-Bus. An
overlapping protection area exists in this system because of
the transformer’s impedance.
Table II shows the relevant data for this system. These data
are typical of those used by the Korea Electric Power
Corporation. The fault resistance, source impedance, and
saturation characteristics of the transformer were not
considered in this simulation.
Thev.A and Thev.B are the Thevenin equivalent source
models.
CONCLUSION
An improved method of coordinating the backup protection
for transformers in an IEC61850-based substation system was
proposed. The main concepts were demonstrated using
simulations with data from the Korean power system. A
RTDS was used for hardware simulation to generate the input
signals. Cisco’s MMS-EASE Lite was used to create the
IEC61850 network.
This paper has shown that the proposed method eliminates
the coordination error from the distance relay when a fault
occurs in the overlapping protection area behind the
transformer. The proposed method is thus capable of dealing
with coordination problems in the substation system.