Abstract
Accurately fitting rational functions to the frequency response of modal impedances is crucial for including frequency dependency in lumped parameter models of transmission lines. Vector fitting is widely used for fitting rational functions to the frequency response of modal impedances and then an R-L equivalent circuit is obtained from the rational functions. A single-step method based on the properties of Foster equivalent circuit is proposed to directly fit an R-L equivalent circuit to the frequency response of modal impedances. The positive peaks, negative peaks and the positive zero crossings of the slope change plot of the frequency response are found to provide a good approximation of zeros, poles and the flat region locations of the frequency response. An enhanced fitting algorithm is proposed based on these observations. A close enough fitting is achieved using the proposed method with less number of passive elements. Using the proposed model in EMTP-RV, 400, 765, 1200 kV transmission lines and a 11-bus 500 kV network are simulated for switching transients. The results are compared with the constant parameter cascaded (Formula presented.) -model and the universal line model in EMTP-RV. The switching transient results of the proposed model are found to be comparable to the Marti's model in EMTP-RV. © 2021 The Authors. IET Generation, Transmission & Distribution published by John Wiley & Sons Ltd on behalf of The Institution of Engineering and Technology
Highlights
High penetration of renewables through power converters has reduced the gap between the electro-mechanical dynamic transients and the electromagnetic transients to a negligible extent [1]
This paper presents a simplified algorithm to fit Foster R-L equivalent circuit to the modal impedances of a lumped parameter frequency dependent (LPFD) transmission line
Unlike the previous approaches used for fitting where an equivalent circuit is obtained from the transfer function of the frequency response, the proposed approach directly fits an equivalent circuit to the modal impedance frequency responses
Summary
High penetration of renewables through power converters has reduced the gap between the electro-mechanical dynamic transients and the electromagnetic transients to a negligible extent [1]. Even though [10] rightly represented frequency dependent line, it resulted in higher number of passive elements in fitted circuit which increased the order of state space matrix. A single step methodology is proposed in [15], which drastically reduces the number of passive elements required for fitting the frequency response of the characteristic impedance This method is shown to produce same accuracy as that of vector fitting used in Marti’s model of EMTP-RV. In [16], similar approach is used to fit an R-L equivalent circuit to the modal impedances of LPFD line proposed in [10] This approach depends on the local minimization of fitting error by splitting the entire frequency range into multiple windows. The proposed LPFD line simulation results are found to be comparable to the EMTP-RV’s Marti’s model
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