Modular multilevel converters（MMCs） have been increasingly used in power systems. Their main circuit parameter design determines the initial cost and performance of the systems using them. We have studied the parameter design of several important components of the main circuit of MMC

such as link transformers

arm reactors

sub-module capacitors

and IGBT modules. Firstly

based on the fundamental equivalent circuit of MMC

a method is proposed for the determining the rated no-load voltage of link transformer at valve side

which further confirms that the range of（1.00

1

.05）/2 times of the voltage should be selected at DC side. Secondly

by introducing the concept of equivalent capacity dischar

ging time constant

it is found that the fluctuation ratio of sub-module capacitor voltage varies with the time constant in a relationship that rarely changes for different projects

therefore

the time constant is universally applicable and the general parameters of sub-module capacitors can be obtained accordingly. Analyzing several practical projects

we have obtained a recommended value of the equivalent capacity discharging time constant

which is 40 ms. Thirdly

the voltage and current load of sub-module power electronic devices under four extreme operating conditions are investigated using analytical formulae of the MMC. It is discovered that the current load changes greatly according to the operating condition. Finally

by introducing the concepts of phase unit series resonance angle frequency and circulating current resonance angle frequency

we have established a principle for determining the parameters of arm reactor

and through analyzing some corresponding parameters of existing projects

the recommended value of phase unit series resonance angle frequency is given as the rated angle frequency of connected power system.