Definitions of standards IEC 60 and IEEE Std 4 are insufficient for the evaluation of lightning impulsc parameters of waveforms with oscillations andor overshoot. This has brought about different interpretations in the estimation of these parameters, especially when using digital recorders. Large discrepancies havc resulted from the different interpretations.The working group 33.03 of CIGRE has collected various opinions about this problem. These are presented, analysed and discussed in this paper. Different ways of determining mean curves of impulses with oscillations and/or overshoot are proposed. In addition, different alternatives for the definitions of overshoot amplitude and duration are suggested for impulses with overshoot.The problems and alternative solutions are discussed in the light of analyzing three impulses of the IEC Test Data Generator. This illustrates clearly the need to change the above mentioned standards. Simple, easily applicable, reproducible and technically justified definitions are required in order to take advantage of the accuracy and facilities of digital recorders in impulse measurements. This paper offers the basis for further discussion on the subject before WG 33 03 of CIGRE makes its final proposal. Further studies with practical test impulses are still needed.
Distributed generation units (DGUs) bring some problems to the existing protection system, such as those associated with protection blinding and sympathetic tripping. It is known that fault current limiters (FCLs) help minimize the negative impact of DGUs on the protection system. In this paper, a control-based FCL is proposed, i. e., the FCL is integrated into the DGU control law. To this end, a predictive control strategy with fault current limitation is suggested. In this way, a DGU is controlled, not only for power exchange with the power grid but also to limit its fault current contribution. The proposal is posed as a constrained optimization problem allowing taking into account the current limit explicitly in the design process as a closed-loop solution. A linear approximation is proposed to cope with the inherent nonlinear constraints. The proposal does not require incorporating extra equipment or mechanisms in the control loop, making the design process simple. To evaluate the proposed control-based FCL, both protection blinding and sympathetic tripping scenarios are considered. The control confines the DGU currents within the constraints quickly, avoiding large transient peaks. Therefore, the impact on the protection system is reduced without the necessity that the DGU goes out of service.
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