18 PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 90 NR 8/2014 Paweł DAWIDOWSKI 1 , Kacper SOWA 2 , Mariusz STOSUR 1 , Marcin SZEWCZYK 1 , Przemysław BALCEREK 1 ABB Corporate Research Center - Poland (1), AGH University of Science and Technology (2) Reduction of THD by switching frequency optimization in three-level NPC inverter Abstract. This paper aims to describe the effect of switching frequency selection for three-level NPC inverter and its impact on the quality of output waveforms for different cable lengths connecting the inverter with motor. Authors propose an optimal switching frequency selection as an option to overall limitation of THD. Traditional and the most popular techniques to limit the THD have been reviewed in order to compare with the proposed method. Simulation verification shows that harmonic spectrum generated by the inverter with the switching frequency selected at the minimum equivalent impedance of the network cable resulting in decrease of total level of harmonics contents. Streszczenie. W artykule została przedstawiona analiza wyboru częstotliwości przełączania trójpoziomowego falownika napięcia, oraz jej wpływu na jakość przebiegów wyjściowych dla różnych długości kabli. Autorzy proponują wybór częstotliwości przełączania, jako jedną z opcji ograniczenia współczynnika THD. W artykule przedstawiono tradycyjne i najbardziej popularne techniki ograniczenia współczynnika THD w celu ich porównania z proponowaną metodą. Wyniki symulacyjne wykazały, że ilość harmonicznych generowanych przez falownik jest mniejsza, jeśli falownik pracuje przy częstotliwości, dla której impedancja zastępcza kabla jest najmniejsza. (Redukcja współczynnika THD przez optymalizację częstotliwości przełączania trójpoziomowego falownika NPC) Keywords: NPC inverter, cable connection, voltage reflections, LC filter. Słowa kluczowe: trójpoziomowy falownik napięcia, połączenie kablowe, odbicia fali elektromagnetycznej, filtr LC. doi:10.12915/pe.2014.08.04 Introduction Application of multilevel inverters leads to many advantages like e.g. limited steepness of the voltage applied (dv/dt), which substantially reduces the total harmonic distortions of the output waveforms. Advanced control schemes such as direct torque control (DTC) allows process optimization and increase of an efficiency of an electrical energy conversion. Also, classical V/f based control for such converters is useful and suitable for many applications [1, 2]. Nevertheless such multilevel inverters generate also some crucial problems, which essentially reduce motor lifetime. They might cause a premature failure of winding insulation, a bearing failure, higher electromagnetic interference levels (EMI) and wave reflections in the case of long cable connection between a drive and motor. It is thus worth to investigate the long cable connections cases as they starts to play significant role, especially in the oil and gas industry. In such cases the high reliability is expected from the drive system as it operates in harsh environment, and the replacement of a faulty component might be extremely costly or impossible. The traveling wave phenomena occurs in the case of a surge impedances mismatch [3, 4]. Additionally the PWM based control results in generation of higher harmonic content. These phenomenon are responsible for insulation degradation, partial discharges and additional heat losses in a power cable [5, 6]. In this paper the interaction between the drive switching frequency and a power cable length is investigated. The goal is to find the influence of those factors on the total harmonic distortion THD level at the output of the inverter filter. A three-level neutral point clamped inverter (3L-NPC) topology case study is presented with different cable lengths. Fig.1. Three-level NPC inverter feeding the induction motor drive via the cable connection The considered system consists of the drive unit, an optimal LC filter selected according to [9], a long cable connection and the induction motor as presented in Fig. 1. 3L-NPC inverters are widely used in many high power MV applications. A comprehensive circuit analysis of such converters can be found in [7, 8]. To limit overall harmonic content and dv/dt the output of the inverter is connected to the LC filter. This allows to mitigate overall effects caused by the converter dv/dt. The filter design is a complex task as the several technical requirements must be met. The filter resonance frequency must be higher than the fundamental frequency and lower than the lowest bandwidth of the switching frequency. One need to select the filter topology, inductances and capacitances of the filter to limit filter while the overall goal is to minimize the filter weight taking into account the physical limitation of each component [9]. In the case where a long cable between drive and motor is present, also the design of the cable has a significant influence on the harmonic levels in voltage and current waveforms as demonstrated in [12]. Special performance is required for cables working under extreme environmental conditions. These conditions determine the complicated, multilayered structure, consisting of many different parts: conductor, insulation, shield and heavy armor which results in a number of nonlinear effects. Another possibility to reduce weight and size of a filter components is to increase the switching frequency of the drive. Although this increases the value of the switching losses in semiconductors, it is still cost effective and commonly practiced [10, 11]. Since the lower switching frequency became a requirement for high power application, in order to reduce the losses an alternative methods should be investigated. The power losses should be kept at the possible low level in order to improve efficiency. The use of semiconductors operated on lower voltage reduce the switching losses and therefore the switching frequencies can be increased. Lower voltage of switches is a natural consequence of the inverter topology used, specifically on the number of voltage levels generated by the inverter. The main remark is that the size of the filter can be effectively reduced by increasing the switching frequency due to the