چكيده به لاتين
Modular multilevel converters are highly interested by researchers due to their outstanding characteristics compared to traditional converters. The advantages of modular multilevel converters are the lack of high voltage tolerance switches, low harmonic content, simple structure, modularity and the ability to high voltage levels. One of the problems of modular multilevel converters is balancing the voltage of capacitors to reduce losses and circulation current. In this thesis, we have tried to provide two low cost and easy methods for balancing capacitor voltage in modular multilevel converter. In the first method, capacitors are balanced using the current direction without sorting the voltage of the capacitors.
Balancing in first method can be performed by performing low calculations with low switching frequency. First, large interval is divided into smaller intervals. The intervals are checked in order and the required capacitor of the capacitor modulation system enters the circuit. If the arm current is positive, the investigation of the intervals starts from the lower intervals to the upper intervals, and if the arm current is negative, the investigation of the intervals starts from the upper intervals and is carried out towards the lower intervals, in the second method, balancing the voltage of the capacitors with low calculations without the need for the sensor is done well. In this method, no current and voltage sensors are required and there is no need to sort voltage and has low calculations.
Also, switching losses in this method is low. First, all capacitors must experience all the places where the rest of the capacitors are located. When a capacitor enters orbit in a special range, the rest of the capacitors enter the circuit in a certain order. By operation, the increase and decrease in voltage of all capacitors will be the same. This same voltage change is rooted in the fact that the capacitors are located in the same circuit. Both of these methods have the ability to implement with any modulation methods. Both balancing methods can be implemented with any modulation system. In the following, a method is presented to reduce the number of voltage sensors in modular multilevel converter. Finally, a simple and easy way to balance the voltage of capacitors is provided. In this method, the voltage of capacitors is compared only to their nominal value. With the help of arm current, appropriate capacitors enter the orbit.
Simulation results show a 70 % reduction in switching losses and a 90% reduction in the volume of calculations compared to the traditional method. The results of both methods are displayed by simulation in SIMULINK environment of MATLAB software.
