Modeling, Control and Steady State Analysis of Back To Back VSC HVDC System
Abstract
This paper proposes a dynamic model of a VSC (voltage source converter) based Back to Back HVDC system and its control technique. From the system model, the corresponding relationship between the controlling and the controlled variables of the VSC is determined. The vector control technique is used to control the working of converters. The control structure consists of outer control loop and inner control loop. The outer loop controller controls the active and reactive power separately. The inner loop controller which is a fast working controller provides the reference d- axis and q- axis voltages which are required by the PWM for generating the triggering pulse. The validity of the model and the feasibility of the control method have been proved by the simulation results. In this paper the system performance is studied under steady state condition.
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Introduction
Due to advancement of high power switching devices VSC HVDC system outperform the CSC HVDC system. VSC HVDC transmission system, a newly developed technology, is based on VSC technology. The VSC HVDC is capable of supplying power to both active and passive electrical systems operating conditions it is capable of supplying or absorbing reactive power [6, 7 and 8]. Using VSC and PWM makes possible fast and better control of power flow. And also mitigates harmonics in AC current and AC voltage effectively and improves power factors of the connected AC systems. For construction of VSC fully controlled semiconductor switch is used (e.g. IGBTs). The switching of VSC HVDC is controlled by Pulse Width Modulation (PWM). In this paper, a brief analysis is conducted of the operation principles and control patterns of VSC HVDC.
The paper is organized as follows:
Section II provides an introduction to the topologies of VSC and the operating principle. A single line diagram of the VSC HVDC system is provided with detailed explanation of each element associated with the system.
Section III provides the operating principle of VSC HVDC.
Section IV presents the vector control method for VSC HVDC.
Section V presents the modeling of VSC HVDC system.
Section VI provides the simulation results of the proposed system. The steady-state condition is established and operational performance and stability of the system is studied.
Conclusion
The development of VSC in last decade generated more research interest in its power system applications. The vector control method of the VSC HVDC is investigated and the performance is tested in steady state conditions. The vector control strategy which control active power, reactive power and DC voltage, are presented and evaluated for the VSC HVDC transmission system which connects two AC grids. From the results, it is concluded that the system is responding in a faster way, a better quality of AC parameters can be obtained and the reactive and active power can be controlled effectively. Vector control technique implements the feed forward closed loop control technique so that the coupling between electrical quantities is removed.