Analytical and Numerical Power Loss Analysis in Modular Multilevel Converter Marcin Zygmanowski 1 , Bogusław Grzesik 1 1 Power Electronic, Electrical Drive and Robotics Department Silesian University of Technology Gliwice, Poland 1 marcin.zygmanowski@polsl.pl, 1 bogusław.grzesik@polsl.pl Marek Fulczyk 2 , Radosław Nalepa 3 ABB 2 ABB Oy, Distribution Automation, Vaasa, Finland 3 ABB Corporate Research Center, Cracow, Poland 2 marek.fulczyk@fi.abb.com, 3 radoslaw.nalepa@pl.abb.com Abstract—This paper presents two approaches to semiconductor power loss analysis in Modular Multilevel Converters (MMC) controlled without feedback of the switching cell capacitor voltage. The first one is a novel analytical approach - basing on an averaged model of the MMC. The second one is basing on detailed model in Matlab-Simulink - used as a reference for the analytical one. Analyses are performed for five MMC switching cells scenario. The analytical method has been proven as sufficient and fast solution for the MMC power losses estimation. Keywords—modular multilevel converter; power loss estimation; open-loop control I. INTRODUCTION Nowadays multilevel converters [1] are becoming a more and more popular solution in medium voltage drives [2]. Among these converters the modular multilevel converter (MMC) seems to be the newest one. It is also believed that this converter will be standardized at high and ultra-high voltage applications, particularly for HVDC systems [1], [3]. The most important advantages of the MMC include the lack of one high or medium voltage dc-link capacitor bank and full modularity. These features make the MMC suitable for medium voltage applications, e.g. medium voltage electric drives [2]. In high power applications high efficiency of the converter is particularly important, so power loss analysis should be addressed at an early stage of converter design. In the MMC the power losses calculations are particularly complex due to a large number of switches operating under pulsating dc capacitor voltages and non-sinusoidal currents [4]. The specific operation of the modular multilevel converter causes that typical well-known estimated power loss equations valid for two-level converters are not applicable for the MMC. Although some papers investigate the power loss issue in MMC [5], [6] the authors of this paper will discuss this topic in more detail - taking into account all phenomena and components influencing power losses. These will include parasitic components of non-ideal MMC and operation of he PWM modulator with a submodule selector. In the paper the authors will present two different approaches to power loss analysis. The first one is based on the averaged model [7] based on a number of assumptions, e.g. sinusoidal ac currents i V , constant dc-link voltage v dc , ideal MMC components (switches, capacitors, etc.) and sinusoidal modulating signals. The analytical loss model of the MMC will include conduction losses and switching losses. The second model in the paper is the simulation model developed in Matlab-Simulink. The investigated converters will have five submodules in each converter arm (Fig. 1), and will operate under the open loop control [8]. II. ANALYSIS OF MMC CONDUCTION LOSSES A. MMC averaged model Due to the complexity of MMC topology, some assumptions have to be made before the analytical formulae of MMC power losses are established. In this analysis only one submodule (SM in Fig. 1.b) is taken into consideration, assuming that other submodules dissipate the same amount of losses. The conduction losses are derived from averaged arm currents and modulating signals taken from the lossless MMC model. Conduction losses in converter transistors and diodes can be calculated from instantaneous powers given as (1).                     2 Tcon T T T0 T TT 2 Dcon D D D0 D DD p t v ti t V i t ri t p t v ti t V i t ri t        In (1) the on-state characteristics of transistor and diode are approximated with piecewise linear functions as v T = V T0 +r T i T and v D = V D0 +r D i D . In (1) it is assumed that during the off-state of the switch its current is zero. For calculating the averaged values of conduction losses the transistor and diode currents have to be analytically expressed. For this reason the switch currents, which depend on the modulating signals and submodule output current have to be then averaged over the switching period. The transistor T 1 and diode D 2 conduct current only when the arm current flows in the opposite direction (in Fig. 1.b, i arm < 0), the switches T 2 and D 1 conduct when i arm > 0. Conduction of particular switches depends on the switching states s 1 s 2 which can be either s 1 s 2 = 01 or s 1 s 2 = 10, which has been presented in Table 1. The dead time is not taken into consideration in this analysis. Switching states result from the application of the PWM method, in which the triangular carrier signals s N1 and s N2 , are compared to modulating signals s M1 and s M2 given as (2) – Fig. 2. This paper has been prepared under a project financed by ABB Corporate Research Center in Cracow, Poland.