How to choose a medium voltage VFD:
Cooling type
This post looks into drive classification according to cooling. Same as for instance electric motors also VFDs can be divided in two main groups with regards to VFD cooling type:
- Air cooled drives
- Liquid cooled drives
Air cooling is known for its simplicity. Air cooled drives have one or more cooling fans, usually installed on the roof. The air inlet is often from the front side of the enclosure. The air is blown through the inside of the drive and cools down the main power hardware. Due to poor cooling capability of air (low relative thermal capacity) a large quantity of the medium has to pass through the drive. In case of multiple drives installed in the same room there could be a ‘storm’ inside – not very comfortable for personnel. It is recommended to have suitable dust filters in the inlet section and these need to be regularly replaced or cleaned. Typical interval is from few months up to 1-2 years depending on the particles contained. The ingress protection of the enclosure is typically IP21 or IP31. Air cooled drives create increased noise: sound pressure level is typically in the range 80 to 85 dB(A). Another disadvantage is the fact that all heat losses are evacuated directly into the electric room that must be re-cooled by HVAC. The investment cost (CapEx) as well as operational costs (OpEx) of HVAC are significant when considering the entire life cycle of the equipment. HVAC is also not very reliable making the whole system vulnerable. As a general rule the cooling fans shall have n+1 redundancy to contribute to high availability of the VFD. Same rules is recommended for HVAC as well.
Liquid cooling is much more efficient and is therefore the preferred method in higher power range. Deionized water is the most frequent liquid medium. 1 volumetric unit of water has the same thermal capacity as 4’000 volumetric units of air. Depending on ambient temperature range an anti-freeze (ethylene glycol or propylene glycol) might be added which causes derating in rang of few percent. This derating can be partly or fully compensated if the actual water temperature is lower than design water temperature. Advantage of liquid cooled drives is lower sound pressure level in the range 70 to 75 dB(A) and mainly the fact that majority of heat loss is evacuated into cooling system and does not load the electric room. The enclosure can be made with high ingress protection, e.g. IP54. Same as for air cooled systems a redundancy in cooling concept is advised. Commonly there are two cooling pumps, one running and one redundant. There is an automatic switchover from one to the other one after several hours of operation to ensure equal aging. When selecting liquid cooled VFD we recommend a high quality manufacturer with long history and experience with this type of drives. It will minimize risk of issues such as water leakage that you definitely don’t want to experience. The cooling system shall be designed in a way that maintenance including e.g. exchange of deionizer bottle can be performed while the VFD is operational. Is there no cooling water at site? No problem at all! You can still use liquid cooled drive attached to a water/air heat exchanger located nearby (next to the building, on the roof etc).
Following comparison illustrates why water is the cooling medium of choice in higher power range:
From above table it is clear that water is much more efficient coolant than air. Specific (mass) heat capacity of water is about 4.23 times higher than air. Moreover, the specific density of water is about 784 times higher than air. Combining both you get the impressive difference of factor more than 3’300. To evacuate the same amount of losses 3’300 times larger volume of air is needed compared to water. If you are familiar with large motors and generators, you probably know that high power generators often use water cooling for stator and hydrogen for rotor. Air cooling is simply not sufficient for the high power density used in the construction of these machines.
Selection of VFD cooling type is important decision step. The user can choose between air cooled and liquid cooled VFD. At low power air cooled drives are the ‘standard’ that is usually sufficient. At very high power liquid cooled drives are the only solution available. In between there is an overlap of both technologies. Do not compare just price of the equipment, but especially the life cycle cost including re-cooling, project specific ambient conditions etc.
A short summary on VFD cooling type is provided below:
References
[1] Medium voltage VFD portfolio, https://new.abb.com/drives/medium-voltage-ac-drives
[2] How to choose a medium voltage VFD – entire series, https://mb-drive-services.com/category/choose-mv-vfd/
