3 big issues for Medium Voltage drives and how to address them

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Guillaume Fontes
Chief Scientific Officer at Power Design Technologies

Variable Speed Drives (VSDs) have been available for over 35 years and are used in a wide range of applications in industry and transport, such as wind turbines, mining, marine, and naval applications. A thorough exploration and benchmark of your medium voltage drive design options will help you reduce the safety & financial risks during your drive exploitation. Let’s have a look.

Around 60% of electrical energy used in industry is consumed by these VSDs[1]. Demand for these drives has been growing for a few years, especially at the megawatt level (i.e. at the medium voltage level), due to the increasing need for efficient motor control because of rising energy prices and environmental concerns.

In this context, a typical use case is the retrofit of an existing fixed speed induction motor originally powered straight from the grid. In this case, the introduction of a variable frequency drive is very interesting because:

  • it saves a motor replacement,
  • it increases motor efficiency,
  • it increases safety.

But, as it used to be a motor originally powered straight from the grid, attention must be paid about motor winding stress. Based on my experience with some Medium Voltage (MV) drive development companies and as a CSO for a power converter design software company, here are below three big issues for Medium Voltage drives and how I would go about addressing them:

1. Total Harmonic Distortion (THD)

In MV drives, power quality issues are mostly caused by the Pulse Width Modulation (PWM) waveform which injects a high frequency noise in the motor. The Total Harmonic Distortion (THD) of the current must be limited to preserve motor windings at all load levels and all speeds[2]. If not, your motor would be facing added losses and its integrity would be threatened, especially in the case of motor retrofitting.

2. Voltage Overshoots

Moreover, in VSD’s applications, the potential of future 15kV SiC power switches must be carefully watched as it implies very high dv/dt on the voltage waveform which may cause voltage overshoots that degrade the insulation of the motor winding. Once again, this issue is major for MV drives, especially in the case of motor retrofitting.In MV drives, power quality issues are mostly caused by the Pulse Width Modulation (PWM) waveform which injects a high frequency noise in the motor. The Total Harmonic Distortion (THD) of the current must be limited to preserve motor windings at all load levels and all speeds[2]. If not, your motor would be facing added losses and its integrity would be threatened, especially in the case of motor retrofitting.

3. Costs

Current solutions of MV drive use IGBT modules with high voltage ratings (up to 6500 V) which are the most expensive. The cost of this IGBT modules is not linear with the voltage rating, so a financial consideration would be to prefer to use modules with lower voltage ratings. Yet, this is not always possible as direct associations of modules in series present some limits and require some snubbers which create adding losses.

Current solutions: sine filter and 3-level inverters reduce the risks, at least partially…

In order to limit the financial & safety risks, MV drive specifications are frequently reused from past projects with little assessment of how they would meet different application needs[3]. Not to mention the time pressure that reduces the period you could allocate to the exploration of new operational needs.

In such cases, the use of 3-level inverters (such as NPC or T-type topologies) with a sine filter is then a classical solution. But it probably requires a bulky and costly output sine filter in order to limit the THD and voltage overshoots, and the IGBT modules used may have high voltage ratings and thus may be expensive.

…So here is how I would address these threats

The answer may not be a surprise for you. It’s about further increasing the number of levels (to 5 for example). It can change the game, first by reducing the current harmonics, secondly by reducing the spike amplitude which both lead to significant filter downsizing.

At last, it also opens options such as the use of cheaper low-voltage IGBT modules, which may decrease the whole cost of the power converter.

Yet, if multilevel topologies are undoubtedly interesting for these applications, many questions can be raised to consider such a degree of freedom:

  1. Which topology fits better to my application?
  2. How many voltage levels?
  3. Depending on the number of voltage levels, which power switch?
  4. What about the switching frequency?

Such questions must be raised with a benchmark process to quickly evaluate these options with at least these 3 key parameters: the multilevel topology, the power switch reference, and the switching frequency. Then all these solutions can be compared so that the designer of the drive inverter will be able to choose the best one according to different criteria: mass, efficiency, cost…

Bearing in mind these 3 big threats & benchmarking carefully your design options for these 3 key parameters of your inverters are a good starting point. However, as each application will be unique, the best thing you can do is that you make sure that you review all your design options of power converters (technology, voltage…).

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