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JEET, Vol. 11, No. 1, January 2016 
An Accurate Model of MultiType Overcurrent Protective Devices Using Eigensystem Realization Algorithm and Practice Applications
ChaoYuan Cheng 
Area 
A  Electric Power Engineering 
Abstract 
Accurate models of the characteristics of typical inversetime overcurrent (OC) protective
devices play an important role in the protective coordination schemes. This paper presents a novel
approach to determine the OC protective device parameters. The approach is based on the Eigensystem
Realization Algorithm which generates a state space model to fit the characteristics of OC protective
devices. Instead of the conventional characteristic curves, the dynamic state space model gives a more
exact fit of the OC protective device characteristics. This paper demonstrates the feasibility of
decomposing the characteristic curve into smooth components and oscillation components. 19
characteristic curves from 13 typical and 6 nontypical OC protective devices are chosen for curvefitting.
The numbers of fitting components required are determined by the maximum absolute values of
errors for the fitted equation. All fitted equations are replaced by a versatile equation for the
characteristics of OC protective devices which represents the characteristic model of a novel flexible
OC relay, which in turn may be applied to improve the OC coordination problems in the subtransmission
and distribution systems. 
Keyword 
Curve fitting,Eigensystem realization algorithm,Novel flexible overcurrent relay,Multifunction equation,Overcurrent protective device.

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