Keep It Simple Stupid

Keep the VFD design simple stupid

How to apply the principle in medium voltage drives? There are several areas such as hardware design, protection concept or software architecture. KISS aims to eliminate the root cause of the problem rather than solving the consequences.

Hardware design

VFD topology utilizing medium voltage semiconductors with high power density and low parts count is one example. Instead of using large quantity of components and engineering complex redundancy concepts the same or even better reliability figures can be achieved with low parts count and no semiconductor redundancy.

• 5 MW, 4.16 kV neutral point clamped (NPC) inverter:
  • 12 active switches
  • 18 diodes
• 5 MW, 4.16 kV multi-cell inverter:
  • 48 active switches
  • 48 diodes

And it does not involve power semiconductors only. Similar situation is for example with cooling pipes in case of a liquid cooled VFD. Inverter with high quantity of semiconductors also requires large quantity of plastic pipes distributing the water to the individual heat sinks. Thus increasing the probability of leakages.

Absence of fuses

Use of fuses in medium voltage VFDs is tricky.  The fuse needs to have very quick response to provide required protection of semiconductor devices. Fuses also naturally age making the maintenance somewhat cumbersome. And again, there is the topic of quantity. In the example of a 5 MW, 4.16 kV multi-cell inverter a total amount of 36 fuses is needed (some manufacturers reduce the quantity to 24 pcs using two fuses per cell). A fuseless design will be more in line with KISS principles. It reduces the parts count, simplifies the maintenance job throughout the lifetime of equipment and boosts the availability.

Sensorless speed control

Elimination of a speed encoder can also be seen as a step towards KISS. Performance of an encoder is affected by multiple factors such as physical distance between inverter and motor, type of signal, interfences with other equipment (EMC topic), installation aspects (e.g. unbalance), environmental factors (dust, vibration) etc.

In sensorless variant the speed is calculated in VFD software based on electrical signals that are already available. Of course, there are high performance applications that require speed and/or position encoder [3].

Filterless design

If possible, a filterless VFD design is preferred. This applies mainly for the grid side filter. Since the grid parameters are project-specific a grid filter is usually an engineered solution. The purpose of a filter is to absorb specific harmonics and eventually compensate the reactive power. However, it is rather challenging to make a filter design that is robust and stable during grid faults and other unusual operation states.

On the machine side there is practically always some kind of filter (as minimum a dv/dt filter reducing the voltage slope and ensuring compatibility within the inverter-cable-motor system [4]). However, machine side has somewhat different aspects. On one hand there is the variable output frequency. On the other hand it is a clearly defined system with well known machine parameters making it easier to manage.