Purpose: Some previous researchers work on the HVDC Monopolar systems using Matlab/Simulink environment to carry out their analyses. We continue this discussion of Monopolar HVDC systems. Methodology: The analyses that are carried out include double phase-to-ground fault, single phase-to-ground fault and three phase-to-ground fault at the AC side of the rectifier and the DC line to ground fault.  Findings: It is discovered, among others, that for three phase-to-ground fault, the voltages at the AC side of the rectifier, the DC line voltage and the voltage at the inverter side of the load are completely zero but that the current in the rectifier will increase from its standard value though will be less harmful and the current of the inverter side will decrease from its original value. It is further discovered that the fastest transient will be the case when fault is applied to the DC transmission system and that transmission network is of critical importance in high voltage transmission lines and engineers can also use this result to identify different faults in the transmission lines and the care of power quality is highly important because of electrical energy demand. Unique Contribution to Theory, Practice and Policy: The current in the rectifier decreases from its standard value but will be less harmful. The causes of decrease in DC transmission line depend on severity of the fault such as single phase to ground, double line to ground and three phase-to ground at the rectifier side.

Existing studies show that several performance issues will arise in the HVDC link during the three phase-to-ground fault at the side of the inverter and that the DC voltage will oscillate around zero and will not affect the rectifier of the AC system though the inverter of the AC system, and the AC voltages will become zero and the AC currents will show high amplitude as well as minor disturbances. It has also been argued that when the fault is applied on a single-phase to ground fault at the inverter side on the AC side, the voltage will decrease. In this paper, we focus on single line-to-ground fault, double line-to-ground fault, and three phase-to-ground fault at the inverter of the AC system and their behavior on the DC link as well as on the AC system of the rectifier with detailed simulations. A high voltage direct current (HVDC) Monopolar system is modeled using a Matlab/Simulink software package for the research. The results show that during the three phase-to-ground fault at the AC system of the inverter, the DC voltage will increase with a bogus waveform and the currents of the AC system at the rectifier will collapse to zero.At the double phase-to-ground fault level, the DC voltage will experience an increase in waveform while the currents of the AC system of the rectifier will experience different disturbances. At the single phase-to-ground fault level, the DC voltage will remain stable and the rectifier side of the AC system will also experience a stable state for both currents and voltages.

It has been argued that when a single-line-to-ground fault is applied at the inverter on the alternating current side, the voltage will drop. Likewise, when a fault is applied on a LLG fault at the inverter, the currents of the alter-nating current system of the rectifier will experience various disturbances. In this study, we focus on a converter station on the rectifier side of a HVDC system. Three fault cases, namely, D4D5 open-circuit fault, D1D2 open-circuit fault, and D3D4D6D2 open-circuit fault and their behavior on the alternating cur-rent system of the rectifier, AC system of the inverter as well as the direct current link with detailed simu-lations were carried out. An HVDC mono-polar system was modeled using the MATLAB/Simulink software environment. It was discovered that the D4D5 and D1D2 open-circuit faults and the AC system of the rectifier side would both have the same increase in AC voltages, whereas the behavior of the AC currents of both faults would remain normal with no effect. It was also shown that during the D3D4D6D2 open-circuit fault, the alternating current volt-ages and currents at the inverter side would experience zeros in a short time and then rise with a false sinusoidal waveform