Abstract

High Voltage Direct Current (HVDC) transmission systems are used all over the world in frequency-conversion schemes, non-synchronous AC interconnections, submarine DC-links and in many other projects where they enhance AC system stability. HVDC system has many advantages such as fast power flow control, stable and economical operation for long-distance transmission. But the large-scale usage of HVDC transmission is limited by several factors and one of the most important factors is the high cost of conversion equipment. A proposed converter model for the HVDC rectifier system is presented for a possible reduction of the high cost of conversion equipment. This thesis presents an investigation into the viability of a hybrid diode-thyristor HVDC rectifier. In a standard HVDC system, a 12-pulse thyristor-bridge is used. In this thesis, the CIGRE benchmark based HVDC system operating with a weak AC system is presented as the standard model. In the proposed hybrid circuit, the lower 6-pulse thyristor-bridge (which is connected with a Y-Y transformer), is replaced by a cheaper diode-bridge. This reduces the overall capital cost of the 12-pulse rectifier, but has implications regarding the operational behaviour of the HVDC terminal and system. The operational behaviour of this hybrid diode-thyristor rectifier under static and dynamic conditions and the comparison with the standard model are verified with the well-known simulation package called EMTP-RV. Due to unbalanced harmonic cancellation between diode-bridge and thyristor-bridge, the proposed HVDC system generates more characteristic harmonics than the standard model on both AC and DC side. So, an extra filtering unit is needed which will increase the cost of the proposed HVDC model. In the proposed model, only one controller block is needed for the thyristor-bridge as the diode-bridge does not need the controller block. So the cost will be reduced in the proposed model due to the requirement of a single controller block. The controller for optimizing the PI parameters of the proposed HVDC model is designed by using ITAE (Integral of Time multiplied by the Absolute value of the Error) criterion, which is one of the synthetic indexes for evaluating the control system's performance. DC fault is the most severe fault for the converter valves at the rectifier end and it represents one serious operational difficulty with the introduction of the proposed model. It is very difficult to recover from a DC fault like the standard model because of the uncontrolled diode-bridge. So an AC breaker (which is costly equipment) is added to the system to recover from the fault