How to choose a medium voltage VFD: System integration

How to choose a medium voltage VFD:
System integration

As part of our series on how to choose a medium voltage VFD [1] we have another essential aspect to mention: System integration capability. What does it mean? How does the system integration look like? And most importantly: why does it even matter?

Importance of system integration capabilities

Remember that end user actually does not buy a variable frequency drive as such – he buys a variable speed drive system (power drive system in IEC terminology). If, for instance, the VFD does not fit the motor, it is a severe issue. Similarly, if the VFD-motor pair does not fit the driven machinery or process requirements the whole system is not useful. At this point it is probably clear that system integration is a key part of a successful and especially economical operation of variable speed equipment.

Therefore, it is desirable to have someone with system integration capabilities as a guide. Such system integrator might be a separate specialized company (for large and complex projects) or the drive system integration might be done by the VFD manufacturer (typically smaller projects or projects with low/medium complexity). The latter scenario requires a VFD manufacturer that does not just supply you with components, but walks with you the extra mile. Prerequisites are the right level of system know-how, experience, organizational setup and (not underestimate) a mindset.

What are the qualities of a system integration unit? For me it is a combination of know-how and engineering best practice along with customer-oriented approach.

System integration support

When do you need support from system integrator or system engineering team? Well, this is a very good question. Unfortunately, it is not so easy to answer. The point is that you may be totally unaware of a particular challenge or issue until you face it. In fact, you may be a very good planner. However, unless you have already executed lot of variable speed drive projects, a surprise can still pop up.

Now let’s look a bit closer. What are the fields of system integration? There is actually plenty of them. Starting with the “big ones” we can mention:

A. Integration into the grid

This part is obviously about power quality, such as harmonics and reactive power. The system designer can propose best VFD type based on grid data (short circuit power, resonances if known etc). You may need someone who is capable to perform a network study. Obviously, the goal is to minimize the impact of the variable speed drive system on power quality in terms of harmonic distortion and reactive power loading. The mitigation can be done on VFD level (e.g. “low harmonic” drive) or on a system level (virtual increase of pulse number, installation of filter etc). Experienced system designer can elaborate the best solution from technical and commercial point of view.

Keep in mind that it is not just the VFD impacting the grid but also the grid impacting the drive system. Grid faults, voltage unbalance, transient phenomena or existing harmonics are just few things to mention. They may affect the performance of your power drive system. In worst case these issues may lead to tripping of VFD or even hardware damages.

B. Electro-mechanical integration

Next section is related to the machine side. Matching the inverter with the motor (harmonics, voltage waveform) is just part of it. In the bigger picture it is then the drive system interacting with the driven load. Here comes the topic of torque ripple and torsional vibration. Skilled system integration partner will guide you from the beginning of the journey. He can provide inputs for the torsional analysis and dimensioning of mechanical components. Moreover, he would take care of optimal parameterization of VFD control, diagnostic on site and finally support in troubleshooting in case of any errors.

This is a classical example of issues that may not be thought about in the planning stage but pop up later in the project. Hence, it is important to have a speaking partner. In this case it is beneficial to have access to a system engineer from the VFD manufacturer’s company as he shall be specialist in the electro-mechanical interaction, but shall also know the specific VFD in detail.

C. Integration into plant protection scheme

Protection is a crucial topic. We have already discussed transformer protection and explained why a protection scheme used for power transformers may not work for drive transformer (at least not without adaptations). Key topic is the sensitivity and potential “white spots”. Also, the fault cases may look very different compared to a power transformer.

VFD shall protect itself and the components downstream. You may need to check how fast, how good and how robust such protection is and whether it really covers all operation scenarios. The upstream circuit breaker may be essential part of the protection scheme of the VFD. In such case, you need to ensure that the breaker fulfills the requirements of the VFD manufacturer. Type of the breaker, such as vacuum circuit breaker or SF6 type, can also influence other aspects like switching overvoltages.

Motor protection is mostly covered by the VFD. However, special considerations are needed in case of a soft starter or a drive system with synchronous bypass option. The motor needs its own protection (overcurrent, surge protection etc) when operating direct on-line. At the same time, standard motor protection elements may not work when supplied from a VFD because of non-sinusoidal waveform and variable frequency. Another challenge may be proper system protection for soft starters where the motor rating significantly exceeds VFD rating (soft starting at light load).

Topic on its own is the handling of overload conditions for drive system components.

Other areas of drive system integration / system engineering

Besides above areas there are many other topics where system engineering capabilities are extremely helpful and can save lot of time and money. Some of the topics may even sound very trivial until you are in the middle of the project and come across that subject. Again, just few examples for illustration:

Cabling

What can be so special about cabling? Well, there are several things that can potentially go wrong. It starts with a selection of proper cable type and cable dimensioning. Next point is cable laying. What is the right insulation level of VFD cables exposed to non-sinusoidal voltages? What shall be the cross section of cable shield? And how where to actually ground that shield? There might also be some restrictions on maximum cable length (refer to our article on “long motor cables“). And what if the motor frequency goes far beyond 50 or 60 Hz like in high-speed applications?

Cabling has a significant impact on overall electromagnetic compatibility so it is highly recommended to use the best practice and advice from VFD manufacturer in order to avoid EMC issues.

Cooling and re-cooling

Another simple topic you may say. However, there are cases where a good engineering guideline is required. Let’s say that you have air-cooled VFD inside a building and want to install an air duct. It might make sense, especially when having multiple air-cooled VFDs inside the room. But how to do it? How to size the ducting fans? What else to consider? Will the VFD need some modifications to support such concept?

Generally you will need to know the heat load of the VFD to ensure proper re-cooling of the electric room where VFD is installed. Your system integrator may have few tips how to do it in the most efficient way.

Motor retrofit

There are cases where user wants to convert existing fix-speed motor into variable-speed operation. That is perfectly possible and had been done numerous times. Nonetheless, several considerations are necessary, be it the selection of a suitable VFD or necessary modifications on the motor. VFD manufacturer with system engineering capabilities most likely has a detailed and well-thought guideline to ensure a smooth and successful process of transforming a DOL motor into variable-speed machine. It will be a tremendous help to realize your project in time and without painful issues.

Replacements

Recently, several projects appeared that aim to replace a gas turbine or steam turbine with an all-electric drive system. In many cases there are interesting benefits when switching to an electric drive system. However, brownfield installations always need a special care. In this case a partner with system integration skills is highly recommended.

Details make the difference

The above list is not exhaustive. In fact, we could add many more items, both small and big. it is often about the details that make a difference. Details in design that can avoid few system trips and save plenty of money. Details that extend the equipment lifetime. And last but not least: details that increase the safety.

It does not have to cost you a fortune to get access to system integration expertise. You just need to select a partner that has such capability.

The probability that you come across a topic that would deserve support from system specialist is high. At the same time, the probability that you problem is entirely new and has not been dealt with before is very low. By that I want to emphasize that there are many solutions and best engineering practices.

Do you want to learn more? Purchase our Premium Subscription and ramp up your drives knowledge.

System integration: Final words

System integration capabilities play important role during planning and execution of drive projects. it is recommended to consider it when selecting a VFD manufacturer. Don’t just get tempted by attractive price but also make sure that the manufacturer can support you with system topics if necessary. There are small details that finally make a big difference. Keep in mind that something unexpected may come regardless how well you plan the things in advance. Therefore, it is vital to have a skilled and knowledgeable partner. We simply don’t know what we don’t know.

Getting access to system integration knowledge does not have to be too expensive. Renowned VFD manufacturers have lot of knowledge and guidelines to share with you. Many engineering guidelines have been created in the past years and you can take advantage of it. You may pay a bit more for the product but you get all this knowledge and experience included.

References

[1] How to choose a medium voltage VFD – full series, Link to article series

[2] Network harmonics – articles series, Link to article series

[3] Torsional – articles series, Link to article series

[4] Protection of VFD transformers, https://mb-drive-services.com/protection_of_vfd_transformers/

[5] Long motor cables, https://mb-drive-services.com/long-motor-cables/

For other resources to this article see directly hyperlinks in text.

How to choose a medium voltage VFD: System integration

Published by admin on May 8, 2021May 8, 2021