Advantages of liquid cooled drives
This article deals with liquid cooling for high power variable frequency drives (VFD). What is the principle of liquid cooling? What kind of liquid can be used for cooling? What are the advantages of liquid cooled drives? Continue reading this post on advantages of liquid cooled drives to find out more.
Considerations on VFD cooling type
Cooling method is an important factor when selecting a medium voltage VFD. Some considerations on cooling type have already been mentioned in [1]. This time we go a bit deeper and focus on the advantages of liquid cooled drives.
The considerations about VFD cooling type might be:
– Power density
– Re-cooling of the room
– Ambient conditions
– Sound level
– Auxiliary consumption
– Heat recovery
– System cost
– Reliability
The above list does not aspire to be complete, but provides couple of aspects to think about when making a decision on cooling method of VFD.
VFD rating and cooling type
The power range up to approx. 2 MVA is dominated by air cooled VFDs. Such cooling is relatively simple, cost effective and in given power range the heat loss is not a real burden. Liquid cooled drives at this lower power (from medium voltage perspective) are available, but less common. Then there is an overlap area where both air cooled and liquid cooled VFDs are being used and the selection depends on multiple factors, such as user preference, installation, ambient conditions, availability of site cooling water etc. As the power rating of the VFD further increases, the liquid cooling becomes the selection of choice.
Advantages of liquid cooled drives
In fact, water is an amazing liquid. Over 70% of Earth surface is covered by oceans and seas. Our body contains lot of water. And we cannot survive more than approx. 3 days without drinking water. In addition to all this, water is perfectly suited as a cooling medium.
Coming to the point: What are the key advantages of liquid cooled drives? We mention some of them based on our experience from medium voltage VFD business.
(i) Higher cooling efficiency and more effective re-cooling
Liquid cooling is extremely efficient. Due to high specific thermal capacity we need relatively small amount of water to evacuate considerable amount of heat out of the VFD.
The cooling system has its own consumption. You shall eventually clarify whether this consumption is included in the efficiency figures of the VFD or not. In any case, using liquid cooling system helps to reduce the auxiliary consumption.
Here is an example for comparison:
To remove 25 kW of heat loss from an electric room three cooling fans are used. Each fan is rated 2.8 kW. That means total consumption of 8.4 kW. The ratio between evacuated losses and consumed power is 8.4 kW/25 kW = 0.336 = 33.6%.
To remove 500 kW heat loss of a large VFD a 30 kW cooling water pump is used. Therefore, we get a ratio of 30 kW/500 kW = 0.06 = 6%.
The effectiveness of liquid cooling over air cooling is obvious. Liquid cooled drives dissipate most of the total losses, in the rule 90% to 98%, into the cooling liquid. Only small portion is radiated into ambient making the re-cooling of the electric room or e-house simple.
(ii) Protection against ambient
Majority of air cooled drives inherently do not allow very high IP rating of the cabinets unless there is some special air ducting system or so. And even with air duct the inlet air normally enters the VFD through openings in the enclosure. Filters are used to prevent dust and particles entering the VFD. However, in dusty environment a more frequent replacement of filters is needed.
Liquid cooled VFDs are available with high ingress protection as standard. The enclosure can be rated up to IP54 or higher as standard. Such high IP rating protects the VFD against dust, conductive particles etc. It makes it also much easier to get the VFD certified for instance for marine and offshore application as it is typically more rigid and more robust against vibration. Finally, high IP rating is a much better precondition for arc proof design.
(iii) Lower sound level
Liquid cooled VFDs have lower sound pressure level when comparing them with air-cooled VFDs [2]. That fact contributes positively to overall lower noise level within the installation and will be appreciated by the local crew on site (especially if there is other equipment inside the same room, such as switchgear, MCC etc). For typical values refer to the table in our previous post [2].
(iv) More comfort for operators and/or maintenance team
Besides lower noise level there is also much lower air flow inside the room making it more pleasant when staying inside. Unlike air cooled VFDs there is no need to blow several m³ of air volume per second.
(v) Heat recovery
Liquid cooling supports heat recovery from the cooling circuit to further boost the system efficiency. Such practice had already been used few years ago. One such example are VFDs running the feedwater pumps in power plants in Denmark and other Scandinavian countries. Every such thing is a step forward towards better sustainability.
Effectiveness of liquid cooling
Liquid cooling is the most convenient method to dissipate large amount of losses. Water as cooling medium is extremely efficient. The reason lies in its high specific thermal capacity. That means that relatively small mass of water can absorb large quantity of heat (losses).
The excellent specific thermal capacity of water can be demonstrated on a simple example from everyday life. Let’s assume that you want to make a tea using an electric kettle. That goes very fast you might say. And yes, it just takes few minutes. However, look at the numbers closer. The electric kettle has rather high input power, often 1 kW or more. It is a very powerful electric heater.
My electric kettle has exact 1000 W stated on the nameplate. If I want to make a tea I would use 0.5 l of water. Let’s assume that the initial water temperature is approx. 20°C. To bring the water to boiling state the electric kettle shall increase its temperature by 80°C or by 80 K (Kelvin), respectively.
Using simple equation from high school we calculate the energy to be delivered:
With my 1 kW electric kettle it will take little less than 170 s, i.e. close to 3 minutes. We were putting 1 kW of power into this small volume of 0.5 l for almost 3 minutes to get a temperature rise of 80 K. That is a lot of energy.
Vice versa, you can also imagine hot long it takes to cool the water down. I typically have to wait 25-30 minutes before I can drink my tea. Again, it is the high specific thermal capacity that causes rather slow cooling down of water.
Here a short video of water heating seen through a thermal imaging camera. Exact this camera (FLIR E6) is available to rent for attractive conditions.
As you can see, it took approx. 1 minute to heat up the water from 80°C to 100°C.
Redundancy of liquid cooling
Liquid cooling circuit features similar options like air cooling when it comes the redundancy. A common feature is to have redundant cooling pumps with an automatic switch-over. In normal operation the pumps make the switch-after defined number of running hours to ensure similar wear of both of them. In fact, most common redundancy is 2 x 100%, but in some cases 3 x 50% might be used as well.
The heat exchanger might be be redundant per default, but redundancy can be realized on request. Sensors for temperature, pressure or conductivity usually also have redundant execution.
Deionizer bottle can also be realized as redundant although it is not necessary as it can be replaced while the VFD is in operation. The operator is notified well in advance that the bottle needs to be replaced.
Reliability and availability of liquid cooled drives
Many people have the opinion that liquid cooled drives might have more reliability issues. Is such concern justified?
Well, the reliability of a liquid cooling system is depends on the system design and design features, quality processes, site conditions or service and maintenance activities. Liquid cooled systems can be designed and manufactured with excellent reliability.
Field experience in large turbogenerators
Take a medium/large turbogenerator in a conventional power plant. Such a machine typically has a water cooled stator and hydrogen cooled rotor. In fact, some companies use water to cool the rotor as well. Obviously, these turbogenerators are still, despite of the rapid growth of renewables, the backbone of power generation. Almost every larger thermal power plant has such machines. The power generation infrastructure is extremely critical with high requirements on reliability. And yet, liquid cooling does the perfect job there.
Field experience in hydro power plants
Recently we have written about renaissance of hydro power plants in Europe and worldwide [3]. The hydrogenerators are almost exclusively water cooled [4]. In fact, the water is used as cooling medium for all system components in such plant: transformer oil to water heat exchanger (OFWF cooling), converter (in case of variable speed) is water cooled and already mentioned motor-generator uses water cooling. The design lifetime of hydro power plants often exceeds 40 years. If water cooling would not be reliable, hardly anybody would use it.
Above examples demonstrate that liquid cooling works in a very reliable fashion when done properly. If there are issues with liquid cooling, they can usually be traced back to improper design or poor quality rather than the cooling method itself.
Field experience in wind converters
There is a large installed base of frequency converters in wind turbine applications. Especially the offshore turbines often use medium voltage liquid cooled converters. Imagine how difficult it is to access such installation offshore. That is the best proof that liquid cooling can work in very reliable way.
What do below applications have in common?
Large turbogenerators ♦ Wind generators and wind converters ♦ Hydro generators and hydro frequency converters
They all rely on efficient and reliable liquid cooling.
Conclusion
There are many advantages of liquid cooled drives. Most significant of them are better cooling efficiency, minimized demand for re-cooling of electric room, reduced acoustic sound level or option for heat recovery.
References
[1] How to choose a medium voltage VFD: Cooling type, https://mb-drive-services.com/choose-mv-vfd-cooling/
[2] Acoustic noise level, https://mb-drive-services.com/acoustic-noise-level/
[3] Pumped storage hydro power plants: Introduction, https://mb-drive-services.com/pumped-storage-hydro-power-plants/
[4] Pumped storage hydro power plant Dlouhe Strane, https://mb-drive-services.com/pspp_dlouhe_strane/
[5] Medium voltage AC drives, https://new.abb.com/drives/medium-voltage-ac-drives
