Drives&Controls & BACK TO BASICS n SPONSORED BY When it comes to MV voltage source inverter (VSI) drive topologies, there are two main options: neutral point clamped (NPC) and cascaded H-bridge (CHB). Alberto Ricci, ABB’s UK product manager for MV drives, compares and contrasts them. There are several di erent MV drive topologies. In many cases, users aren’t aware of how this choice a ects their potential total cost of ownership for operating the drive. There are two main options: neutral point clamped (NPC) and cascaded H-bridge (CHB) topologies. ABB manufactures both. Let’s start by looking at their construction. CHB drives consist of multiple series-connected LV (low-voltage) power modules, or “cells”. LV IGBTs are connected in series to produce an MV level output. By contrast, the NPC topology uses MV semiconductor devices (IGCTs or IGBTs). These are arranged in three “legs”, with the centre connected to a neutral point (0V DC) on the DC bus – hence the “neutral point clamped” name. Now that we have the rough basics of the design, we can compare the key di erences between the two: n Complexity CHB drives have a more complex construction than NPC and contain more components. For a typical 3.3kV inverter, a CHB drive has 36 IGBTs, while an NPC version has 12 IGCTs. n Maintenance There are di erences in service intervals and costs associated with each topology. CHB drives require more frequent and expensive maintenance, with power cells and other components needing to be replaced two or three times in a typical 20-year lifetime. Maintaining NPC drives according to manufacturer schedules is important, but their main components, such as power switching devices or capacitors, do not need to be replaced as part of planned maintenance. n Transformer The multi-winding input transformer cannot be separated – it is always integrated with the drive. Where space is limited, NPCs can o er an advantage by separating the input transformer from the converter cabinet. Also, if the transformer can be located outdoors, its heat losses will be taken care of by the great British weather, instead of needing to pay for additional cooling. n Totex CHB drives generally have a lower Capex cost, but their Opex costs during their lifetimes, may be higher. Each application will have di erent requirements and factors to consider, and each topology option has merits in di erent circumstances. The NPC topology usually provides excellent performance, including higher reliability and availability due to its relatively simple design. This makes it easier and more cost-e ective to maintain. Although the upfront cost of a CHB drive is initially attractive and makes for a strong commercial proposition – especially at lower power ratings – consideration should also be given to the total maintenance requirements and the reduced exibility of the transformer location. When making the evaluation, remember that Totex = Capex + Opex, including all of the above elements. Unit 3B, Lythalls Lane Industrial Estate, Lythalls Lane, Coventry CV6 6FL UNITED KINGDOM T. +44 2476 30 7722 | E. sales@andantex.co.uk www.andantex.co.uk MV drives: which topology is best?
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