The influence of protection system failures and preventive maintenance on protection systems in distribution systems

Abstract

Current transformer (CT) saturation may cause a variety of protective relays to malfunction. The conventional method to deal with the problem is overdimensioning of the core so that CTs can carry up to 20 times the rated current without exceeding 10 percent ratio correction. However, this not only reduces the sensitivity of relays, but also increases the CT core size.Hitherto there is no satisfactory method available that would fully cope with the problem of CT saturation; protective relay engineers have thus to take account of this drawback when laying down the specifications in terms of relay performance.This paper presents a technique of estimating the secondary current corresponding to the CT ratio and it can be incorporated within the digital protective relay algorithm.The equivalent circuit of a CT is shown in Figure 1. In Figure 1, at any instant the relationship between ip(t) and is(t) is:As is(t) is the measured secondary current, if ie(t) can be deduced, then the secondary current corresponding to the CT ratio -zp(t) can be estimated.If the burden of a CT is a resistive inductive burden of the type(& = Rb + jwLb), at any time the relationship between the core flux q(t) and the secondary current is(t) is given by:As all the secondary circuit parameters are known, the core flux can be calculated. The initial core flux cp(to) can also be calculated using a full cycle window of the secondary current in the steady state.The magnetization curve and the hysteresis curve are shown together in Figure 2. Beyond the positive and negative saturation points, the magnetizing current is the same as the exciting current. Between the points, however, the exciting current has two different values for every one value of the flux. The difference between them is equal to or less than half the width of the major loop. As the latter is small, the difference between them is even smaller so that exciting current can be assumed to,be the same as the magnetizing current. Thus, -zp(t) can be calculated by simply adding the magnetizing current to the measured secondary current.This paper proposes a novel compensating algorithm for secondary currents using the magnetization curve, and which accurately estimates the secondary current corresponding to the CT ratio even when the CT is saturated. It is clearly shown that the error is maintained at a low level even when there is severe CT saturation such as under large offset primary currents and in the presence of remanent flux. It has a number of significant attributes such as improvements in the sensitivity of relays to low level internal faults, maximizing the stability of relays for An electric power system comprises generation, transmission and distribution. Although distribution systems have received less attention than generating systems and composite generating and transmission systems, analysis of the customer failure statistics shows that distribution systems are responsible for as much as 90 percent of the unavailability of supply to a customer. Such statistics reinforce ...

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