Analysis of proposed controlling topology of the HVDC link between Bangladesh and India

The HVDC (high voltage DC) transmission becomes a popular means of power transfer in complex electrical power system network. The use of semiconductor switching device makes the operation of the HVDC system stable and reliable. The determination of proper control topology of the converter and inverter plays the most significant role for uninterrupted operation of the system. The selection of conduction angle, delay angle, advance angle, extinction angle and overlap angle of the switching devices needs to be properly coordinated with the switching mode of converter and inverter. The effect of harmonics causes distortion in system parameters which hampers the maximum power transfer through the DC link. In this paper, a new controlling topology of the monopolar HVDC link is proposed. The ZCS (zero current switching) is used for switching of converter and inverter. The main feature of this controlling topology is that both the converter and inverter operate at constant current mode and there exists a current margin between the two currents. The PI (proportional integral) controller continuously checks and monitors the system parameters with the reference values and thereby corrects the operation of the switching devices according to the system requirements. The selection of large current margin and minimum overlap angle (1⁰-3⁰) helps in preventing crossover between operational characteristics of converter and inverter. In conventional controlling topologies, multiple crossovers hamper the stable operation due to improper selection of current margin and thereby harmonics imposes perturbation in the HVDC system. In this proposed controlling topology, the voltage dependant current limit (VDCL) ensures large current margin so that the converter and inverter constant current modes do not interact with each other and thereby harmonics cannot be superimposed on system parameters like voltage or current. The selection of delay angle within the range of 7<- up>0 to 15⁰ ensures better system operation by minimizing perturbation during fault. A MATLAB Simulink model based on the monopolar HVDC system between Bangladesh and India is used for analysis of the proposed controlling topology. From the simulation results it can be inferred that the voltage, current and power waveforms of the HVDC system are stable. When the HVDC link becomes unbalance then the system voltage decreases to about 5-10% of the rated value of 1 p.u. The deviation of the active and reactive power is approximately 8% and 21% respectively from the reference value. The effect of harmonics and third harmonic distortion (THD) are relatively negligible (about 1.4% and 2% respectively of the rated values). The controlling mechanism restores the system stability within few seconds after fault.