What’s the difference between centralised and decentralised UPS bypass modes? Our Technical Services Manager Jason Yates explains in our latest Tech Talk.
An uninterruptible power supply (UPS) switches to bypass mode when a fault, such as an inverter failing, means the unit can no longer support the critical load. Transferring to bypass provides an automated path around the UPS so the load runs off mains supply until the power protection system is back online.
There are two main approaches to UPS bypass – centralised and decentralised, also known as distributed. Both methods fulfil the same end goal of guaranteeing power continuity, with the primary difference the location of the static bypass within the overall UPS configuration.
As Jason explains in the video, in a centralised approach, there is a separate bypass cabinet that contains the static bypass switch and all other components. Whereas in a decentralised or distributed system, each individual UPS has its own distinct static bypass.
How Do I Choose Between A Centralised Or Decentralised UPS Bypass?
This Tech Talk video outlines the main factors that need to be considered before installing a UPS system. There are obvious cost implications, both in terms of the number of bypass switches and components, as well as switchgear. Resilience is another critical aspect to consider, whether that’s operational resilience or resilience during maintenance.
Other criteria covered by Jason include a site’s footprint and future planning, plus the issue of power balancing and how it is far more tricky to achieve in the decentralised method compared to a centralised system with a single UPS bypass line, one of the reasons the latter is often the preferred approach for larger installations.
Video Transcript
Hi and welcome to this week’s Tech Talk. Today we’re going to be looking at parallel UPS systems and in particular, we’re going to focus on both the decentralised and the centralised approach.
Now the terms decentralised and centralised effectively refers to the location of the static bypass within the overall UPS system. Now the primary reason to have a static bypass within the UPS system is to provide the UPS with an automated route of power around itself should an issue arise within the UPS and it can no longer adequately support the critical load.
In addition to this, the static bypass can also be utilised in order to clear issues on the outgoing side of the UPS, such as overloads or short circuits. Therefore, should an issue arise, the UPS system will simply transfer the load on to the static bypass, allow the issue to be resolved, and once it’s resolved can transfer the load back on to the output and the inverter completely seamlessly.
Now on the board here we have two systems. We’ve got the decentralised and we’ve got the centralised. The main difference between the two is the location of the static bypass. In the decentralised system, we have three UPSs, one, two, three. Now obviously you can use either more or less UPS systems and that applies to both the decentralised and the centralised.
Whereas in the decentralised system we have three UPS systems, and inside each of the UPS there is a dedicated static bypass per UPS. Therefore the bypasses are split across the UPS modules and hence they are decentralised.
However, if we look at the centralised system we again have the same three UPS systems, albeit this time we don’t have any static bypasses within the UPS modules. Therefore, because it is a centralised system all of the static bypass components are located in a centralised bypass cabinet and hence the term centralised.
Now both decentralised and centralised systems are perfectly viable UPS systems for every kind of power situation. Whereas some people do prefer the decentralised over the centralised, and in some cases consultants and customers have their own definitive preference as to which one they employ.
Now if you’ve got to make that decision about which version to go with, we can discuss just some of the key points which may help you make that final decision.
Which UPS Bypass Approach Costs More?
Firstly if we look at costs. In terms of costs, we can split this down firstly into the actual UPSs themselves. In the decentralised system, you’ve got three UPS systems, one, two, and three, each with their own static bypasses. Whereas in the centralised system you’ve again got the three UPS modules.
However, because you’ve got the additional fourth cabinet, which contains the static bypass and all the controlling architecture within that system, the centralised bypass solution is typically slightly more expensive than the decentralised approach and therefore just something to consider in terms of the overall equipment.
In addition to this, you must consider the switchgear requirements. In the decentralised system where we have simple single-phase single supply input to each of the UPS systems i.e. the breaker that supplies the UPS will feed both the rectifier and the bypass line. In this scenario, the switchgear would only require to be fitted with three input supply breakers and three outgoing load switches.
With the centralised system you again you still have the same number of input breakers and output switches for the UPS modules. However, now you have a separate bypass cabinet which would also require an additional supply circuit breaker and an additional output load switch, and therefore that can increase the cost of the electrical switchgear around the system.
In addition to that, the bypass module would have to be rated to the maximum power connected to the output of the system, so the supply breakers to the bypass module could be significantly larger.
However, there’s something you must consider between these two systems in that the centralised system here is effectively dual input because you have the rectifier modules being fed from one source and you have the bypass module here being fed from another source or another breaker and therefore it’s effectively dual input.
Now if you wanted to apply that to the decentralised solution you would have to increase the number of breakers feeding each UPS to two, one for the rectifier and one for the bypass line. Therefore, the switchgear would have to contain six input circuit breakers, that’s two per UPS system. Therefore you must consider the installation method in terms of the additional costs of the actual switchgear, which is typically overlooked.
In addition to the cost, you’ve got to consider the initial investment and the future planning. Because at the end of the day, if you have a decentralised system you can simply add modules in the future and therefore increase the capacity of the overall system.
Whereas with a centralised system you must consider the rating of the bypass module, because if the bypass module at day one is not rated to deal with the load that will be applied at day two then potentially you could end up having to replace that module with a larger module to suit the expansion in load.
In addition to that, if you plan from day one with a much larger module and you will ever need then it could be a very expensive purchase and you could end up with a static bypass module which is far oversized for the final installation.
So those are some of the major costs that you need to consider.
UPS Bypass Resilience
The next thing to consider is the resilience of the system. Resilience can be broken down into two areas. Firstly, operational resilience. In general, both systems are just as reliable and resilient as each other. However, some people do consider having the single bypass module to be a potential single point of failure. which quite rightly it is.
Some people will say “well okay, in order to get over that I can just add in a second bypass module” and hence have two bypass modules to remove that single point of failure. However, by doing so you are simply mimicking the decentralised approach so it would be a very, very cost prohibitive method of installation.
In addition to the operation resilience, we’ve got to consider the maintenance resilience. With the decentralised system, if we need to maintain the UPS, we simply power down the UPS, maintain the UPS, and power the UPS system back on. However, whilst that UPS system is being maintained the remaining operational module can still support the critical load and provide static bypass availability at all times.
Whereas with the centralised system, again you can maintain the UPS modules with no problems whatsoever without dropping resilience. However when you need to maintain the static bypass module you’re effectively removing the bypass availability to all of the UPS system, so during maintenance, the resilience level is reduced.
How Does The Choice Of UPS Bypass Affect Power Balancing?
Now one of the biggest reasons why people install a centralised system is what’s referred to as power balancing whilst on the static bypass. Now what I mean by this is effectively in a decentralised system if we go to bypass we effectively bypass each of the three UPS systems.
Depending on the length of the cables that are fitted between the incomer and the load through the UPS system will dictate the current flowing through each of the UPS systems. Now if that’s not carefully installed from day one, potentially during bypass operation you could overload some conductors and therefore cause inadvertent nuisance tripping of the supply circuit breakers. So, that’s something you need to consider.
During extreme circumstances in order to aid the balancing of that current, you could have to fit line reactors in order to facilitate the balancing. However, line reactors would add a significant cost increase to the overall system.
Whereas with a centralised system because you’ve only got that single bypass line you’ve no longer got any power balancing issues. Hence for many for many large installations, the centralised approach with the single bypass line is the preferred choice.
Finally, we’ve got to consider floor space. Now floor space is going to be very heavily dictated by the method of installation because you’ve got to consider the switchgear floor space and then you’ve got to consider the module floor space.
Obviously, if floor space is at a premium you have to very carefully design the overall system that’s most suitable for your applications, but it’s something that’s imperative that you do consider.
Well, I hope you found this tech talk useful and I hope you learnt a bit of a little more around the decentralised and the centralised systems.
However, if you do have a question for us here at Riello UPS, then drop us an email to techtalk@riello-ups.co.uk. Thank you.
