فایل ورد کامل تحلیل و ارزیابی کارایی پیوندهای انتقال برق VSC-HVDC دو سطحی و سه سطحی

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تعداد صفحات این فایل: ۱۸ صفحه

بخشی از مقاله انگلیسیعنوان انگلیسی:Analytical efficiency evaluation of two and three level VSC-HVDC transmission links~~en~~

Abstract

An analytical method to calculate the efficiency of VSC-HVDC links based on two-level and three-level VSCs is presented. The method uses the average and root mean square of the VSC converter current to estimate the conversion losses in the converters (conduction and switching losses). The remaining power losses (DC cable transmission losses due to I2R, the losses in the coupling transformer, and AC harmonic filter losses) are evaluated using conventional well-known methods. Results obtained analytically are confirmed by digital simulations.

۱ Introduction

High-voltage DC transmission based on voltage source converters (VSC-HVDC) presents a solution for many problems face nowadays by power networks, such as, network congestions, grid re-enforcements, wind farms connection, multi-terminal operation and asynchronous connections [1–۳].

Currently, there are two established approaches for the construction of a VSC-HVDC system. The first approach uses a standard two-level converter or a neutral-point clamped converter with forced commutated devices such IGBTs. This approach imposes a high insulation requirement on the interfacing transformer due to the high dv/dt that results from switching high voltage with relatively low switching frequencies. This arrangement also requires fairly large filters at the output to attenuate the switching frequency components from the output voltage at the point of common coupling.

The second approach uses a two-switch modular multilevel converter with medium voltage devices such as 4.5 kV IGBTs. This approach produces lower dv/dt (allowing the use of a transformer with standard insulation requirements) and significantly lower voltage harmonic distortion (which may eliminate the need for the AC filters). This approach requires however, a large number of switching devices and capacitors and a relatively complex modulation strategy compared to the first approach.

Both approaches result in fast dynamic performance, independent control of active and reactive power, no commutation failures during ac faults, better fault ride-through capability, and the ability to provide damping and frequency support through active or reactive power modulation [4–۷]. However, both approaches produce higher conversion losses compared to the conventional HVDC system. Estimation of the power losses during the design stage of the VSC-HVDC is essential, because it allows the designers to optimize the overall system performance through a compromise of several design indices. It also helps in the selection of heat sinking equipment and cooling system for the converters.

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