Cell bypass - What you should know
Today we will discuss the cell bypass in cell-based medium voltage variable frequency drives (MV VFD). What is the purpose of such bypass? How is it technically implemented? What happens when the bypass is activated? And finally, how does cell bypass impact the availability of the VFD? If you are interested in some of these answers then make yourself comfortable and keep reading: cell bypass – things you should know.
Cell bypass as a common feature
Cell bypass is a popular feature in so called multi-cell VFD topologies. The drive consists of multiple cells per phase that are series connected to reach the desired nominal output voltage. Power cell, the building block of this type of converter, typically has rated voltage of 600 – 750 V depending on design or product variant.
Technical realization
The feature is usually realized as a mechanical bypass contact that can be closed very fast after detection of a failure.
An alternative solution is an additional semiconductor that bypasses the cell and is triggered after a cell failure is detected.
The power cell is also bypassed when there is a communication problem (e.g. broken fiber link connection).
Purpose of cell bypass
Due to higher quantity of low voltage components (compared to other medium voltage VFDs with different topologies) the availability of such a drive would generally not be sufficient. Therefore, a cell bypass was introduced to reach comparable availability as the competing technology on the market. Purpose of cell bypass is to bridge a faulty cell and keep the VFD in operation. Depending on the configuration the VFD is either N+1 (full performance can be preserved after loss of one cell per phase) or N-1 (operation with reduced performance after loss of cell). Cell bypass is generally believed to be something essential for high availability of cell based medium voltage drives.
So, the conventional wisdom may conclude:
cell bypass = high availability of your drive system
Cell bypass – things you should know
Now what if I told you that the thing with increased availability may be exact opposite? What if the conventional wisdom does not ultimately lead to right conclusion?
The fact is that the benefit of a cell bypass is largely overestimated. Having cell bypass is little bit like a very generic insurance. You keep paying regular fees and believe that you are protected. But just when something really happens you figure out whether the insurance covers your case or not. Similar situation is here. We certainly don’t say that cell bypass is useless. However, we want to make you aware that it is not a magic stick that solves any problem that may occur.
Reconfiguration after a cell failure
Cell bypass can isolate the power cell while leaving the other (series connected) power cell active. In this way, the VFD can stay in operation. However, how smooth is this transition? How long does it take?
According to the manufacturer, the required period to bypass a failed cell is about 250 ms. That is not as fast as you would maybe think. Following a cell bypass the other cells remain active. The neutral point shifts in order to produce a balanced output voltage to the motor. With one cell bypassed the VFD usually still produces sufficient voltage for the process to continue. That is at least the case for quadratic torque type of loads (T_load ∼ ω²). For constant torque applications it is more critical. Since the motor torque is being reduced with the square of the machine voltage, even a small voltage reduction may cause substantial reduction of motor torque.
Cell bypass may need maintenance
Most common variant of cell bypass is the mechanical contact. It probably does not surprise that such contact also needs a maintenance. Now, the periodicity of maintenance can present a drawback for the availability of your plant. Manufacturers typically recommend maintenance (or “test”) once per year. This is a potential conflict with operation philosophy of some plants that may require 3 to 5 years of uninterrupted service.
Transients when cell bypass is activated
Uninterrupted operation of a VFD after a cell failure might be a challenge…
This section is available for our premium subscribers. Interested? Purchase our premium subscription.
Fault tolerant design?
Multicell based VFDs with cell bypass option are often considered to be fault tolerant. Is it really so? This section is available for our premium subscribers. Interested? Purchase our premium subscription.
Plant availability
For certain processes it is not easy to simply restart and resume the operation. It may easily take several hours (or even days) until everything is ramped up again. We know it very well from grid disturbances that are very short, but if the VFD trips it takes a while to resume operation. Logically, same thing applies to cell bypass. No matter how fast it reacts if the system trips during or right after activation of the bypass there will be an impact on the operation.
Regarding other types of VFD bypass see HERE.
Summary on cell bypass
“Cell bypass allows the VFD to operate in the unlikely event of cell fault” stands in a marketing material of one major manufacturer. While the cell bypass sounds like a good option do not overestimate its effect.
First of all, we would have to question the wording “unlikely event”. Based on field experience, a failure of power cell is not that totally unlikely and in fact happens now and then.
Second, some manufacturers state that “the performance is uninterrupted”. That is an optimistic statement that may not be true all the time.
If your aim is to minimize the downtime then cell bypass might be an adequate solution. However, if you target an uninterrupted operation after a fault of power cell you shall be much more careful. I would personally not rely on it. In case your driven load is sensitive to transients (e.g. turbomachinery such as compressors) there is non-negligible likelihood of tripping following the transient caused by activation of cell bypass. That scenario was observed on real multicell VFDs with cell bypass option out in the field.
Another potential drawback is the maintenance of the bypass contact. While the marketing material may state up to 5 years of continuous running you shall check it more carefully. The maintenance manual may state something else (inspection every year). Check also the warranty conditions with regards to regular maintenance and the corresponding intervals.
Note also that cell bypass only helps in case of cell issue. There might be other failures, for example on the phase-shifting input transformer which is in this case an integral part of the VFD. Since we also rent VFD transformers, we have seen and heard about couple of such failures. This is when the plant availability really starts to suffer. It typically takes couple of weeks to get the drive up and running again. Instead of cell bypass one would wish a complete bypass of the drive to operate the motor at least with fix speed until the issue is resolved. That might actually be an option to sincerely consider providing that grid and motor nominal voltage is matching.
References
[1] Reliability and availability series, Link to article series
[2] Bypass in VFD applications – What does it actually mean? https://mb-drive-services.com/bypass-in-vfd-applications-what-does-it-actually-mean/
Your access to extensive drives know-how.
