A high voltage direct current (HVDC) transmission system is provided. The high voltage direct current (HVDC) transmission system includes a rectifier converting alternating current (AC) power into DC power; an inverter converting the DC power into the AC power; DC transmission lines W1 and W2 transmitting the DC power obtained from the rectifier through conversion to the inverter; a first active power measurement unit measuring first active power input to the rectifier; a second active power measurement unit measuring second active power output from the inverter; and a first control unit controlling the operations of the rectifier and the inverter based on the first active power measured and the second active power measured, wherein the first control unit senses oscillation generated in the HVDC transmission system and generates a control signal for damping the sensed oscillation to control one or more of the rectifier and the inverter.

A power transmission network includes an AC electrical system, an AC transmission link from the AC electrical system to AC-DC converter(s), and a control system. AC-DC converter(s) include an AC connecting point connected to the AC transmission link and a DC connecting point for connection to a DC transmission link. The control system operates each AC-DC converter in an AC voltage control mode as an AC slack bus to control a magnitude and/or a frequency of an AC voltage of the AC transmission link at a steady-state value and facilitating a power transfer between its connecting points to accommodate power generated or supplied to the AC electrical system. The control system further operates each AC-DC converter in a protection mode to protect each AC-DC converter from an overvoltage and/or an overcurrent.

A resonant power converter comprising electrical safety components comprising a combination of a diodes and a zener diodes coupled between DC conductors and an auxiliary switching circuit, the diodes being adapted to hinder the current from flowing from the auxiliary switching circuit to the negative DC conductor, and the zener diodes being adapted to allow current to flow from the negative DC conductor to the auxiliary switching circuit when the potential difference between the negative DC conductor and the phase conductor is above a threshold voltage. The Zener diodes being selected such that the threshold voltage of the Zener diodes is below the maximum blocking voltage of the transistors.

A resonant power converter comprising electrical safety components comprising a combination of a diodes and a zener diodes coupled between DC conductors and an auxiliary switching circuit, the diodes being adapted to hinder the current from flowing from the auxiliary switching circuit to the negative DC conductor, and the zener diodes being adapted to allow current to flow from the negative DC conductor to the auxiliary switching circuit when the potential difference between the negative DC conductor and the phase conductor is above a threshold voltage. The Zener diodes being selected such that the threshold voltage of the Zener diodes is below the maximum blocking voltage of the transistors.

A dual power switching system selects a first power source or a second power source to supply power to a load coupled to an inductive device. The dual power switching system includes a first thyristor, a second thyristor, a third thyristor, a fourth thyristor, and a controller. The controller calculates a total magnetic flux according to a current magnetic flux of the inductive device and an expected magnetic flux of the second power source. When the power-supplying source is switched, the controller turns off the first thyristor and the second thyristor, and determines whether a commutable time is reached according to a power parameter of the first static transfer switch and the second static transfer switch. When the commutable time is reached, the controller correspondingly turns on at least one of the third thyristor and the fourth thyristor according to the total magnetic flux being less than a threshold.