Report 3

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Report 3: Standalone Three-leg Converter

Three level inverters are more commonly used where medium or high voltages are handled such as high voltage direct current transmission, standalone power systems (SAPS) and heavy-duty industrial equipment [1]. The advantages they hold over the two level inverter in these applications is that the switching of the device will have less power loss, the ac-side voltages will contain less harmonic components and they do not require expensive filters at low switching frequencies [1]. As such, the three level converter provides a more efficient and reliable converter for the overall SAPS than a two level converter. The standalone three level or three leg converter can be implemented in a few different configurations [2]. The three common types of multilevel converters are the diode clamped multilevel inverter as seen in [2, Fig. 1], the capacitor clamped or flying capacitors multilevel inverter as seen in [2, Fig. 2] and the cascaded inverter with separate DC sources as seen in [2, Fig. 3].

In a diode clamped converter diodes create more AC voltage levels by utilizing diodes to create more paths for current, this allows the AC terminal to temporarily connected to the DC levels [3]. A common variant of the diode clamped converter is the neutral point converter (NPC), where the clamping diodes link the AC terminal to the neutral leg of the DC link [3]. So long as the DC link capacitors are well balanced there will never be a switching state where a switch will need to block more than half of the DC link voltage, this allows the NPC to handle double the power of a two level converter.

In a flying capacitor converter the capacitors lack a direct connection to a common DC link, which is where the term flying capacitors originate from [3]. These capacitors help control direct voltages that will provide additional voltage levels [3]. These converters may contain different current directions through the flying capacitors between two states, this allows the converter to balance the capacitor voltage by choosing the desired state whenever zero voltage occurs [3]. The direction of the AC-side current is chosen when the capacitor voltage drops below a certain level, and since the other side of the AC-side current contains an interchanged state it is always possible to correct deviations in the flying capacitor voltage during the next switching cycle [3]. Just like with the NPC the switches will never find themselves in a state where they will be required to block more than half of the DC link voltage which gives the flying capacitor converter the same power ratings [3].

Cascaded multilevel converters utilize submodules, the series-connection of converter elements, to create more voltage levels while handling higher voltages [3]. All the submodules are derived from half-bridge and full-bridge submodules, in a half-bridge submodule a two-level phase leg in parallel with a DC capacitor shall maintain a direct voltage [3]. Here two possible switching states, bypass and insertion, are possible [3]. These possible combinations make the output voltage equal Vo or zero depending on our current switching state [3]. In a full-bridge submodule a second phase leg is connected in parallel to the same capacitor [3]. Now the possible switching states is four for the entire circuit, this allows for the output voltage to gain a third value, -Vo [3]. Half-bridge submodules are good in DC applications because it only outputs a single output voltage Vo when the switches allow signals to pass, full-bridge submodules work best with AC applications because the output voltage is bipolar allowing the module to deliver alternating voltages or a combination of AC and DC [3].

 

 

 

References

[1] K. Sharifabadi, L. Harnefors, H. P. Nee, S. Norrga, and R. Teodorescu, Design, control, and application of modular multilevel converters for HVDC transmission systems. Chichester, West Sussex, United Kingdom: IEEE Press, Wiley, 2016.

[2] S. Gudadhe, S. Rodge, and S. Timande, “Comparative Study Between Two And Three Level Converter For Electrical Application,” International Journal of Advances in Engineering & Technology, vol. 9, no. 2, pp. 210–217, Apr. 2016.

[3] B. Wu, High-power converters and AC drives. Hoboken, NJ: Wiley-IEEE Press, 2006.