Nowadays, with the development of the power electronics industry and the achievement of high-level DC voltage, the issue of DC power transmission has been raised again. In cases such as high power transmission over long distances, power transmission using underground and submarine lines, connection of two networks with different frequencies or different phases, connection of wind farms to the power grid and control of power passing between different points in the electricity market, High voltage DC power transmission systems are a good solution. Power controllers on AC lines operate on the basis of reactive power control, but since the reactive power passing through the DC lines is zero, these controllers cannot be used on DC lines. The methods used to control the power passing through DC lines are generally divided into four categories, which include changing the line resistance using a variable resistor, controlling the voltage at the beginning or end of the line using a series variable voltage source, controlling the voltage at the beginning or end of the line using simple DC/DC converter and IPFC converter.Due to high losses the method of using variable resistance is not practical, and if a DC / DC converter is used, the total power passing through the line also passes through the DC / DC converter, which increases the losses as well as the high cost of the converter. In the series voltage injection method, the large number of switches as well as the need for a transformer increases the cost. In the method of using IPFC converter, not only the total power of the line does not pass through the converter, but also no transformer is needed, so among the existing methods, using IPFC converter is the best method in general. Although in DC networks the number of lines connected to one terminal can be more than two lines and therefore IPFC converters must be designed with several output terminals, so far only one IPFC converter with more than two output terminals has been designed.In this dissertation, these methods are introduced and the advantages and disadvantages of each are examined. The following is a way to improve the control system of one of the IPFC converters and introduce a multi-terminal IPFC converter based on this converter. By improving the control system, the amount of harmonic injection of the IPFC converter into the line current is reduced and its stability is also improved. A new IPFC converter with multi-terminal capability is also introduced. The multi-terminal IPFC converters introduced in this thesis have fewer switches, less injection harmonics and a better control system than the only IPFC converters ever presented. keywords: 1-DC power transmission, 2- Power electronics, 3- DC power controllers, 4- IPFC converter