modular systems
BALANCING ACT
Wendy Torell of Schneider Electric discusses how N + 1 UPS designs can help find a balance between redundancy , cost and speed of deployment .
The primary purpose of any uninterruptible power supply ( UPS ) is to ensure back up power to the equipment in a data centre in the event that the electrical or mains supply becomes compromised . But how do you ensure the UPS itself doesn ’ t lead to downtime ?
The answer lies in the utilisation of redundant systems . For highly critical loads , the deployment of a dual-path architecture , in which two separate power paths , each supported by a separate UPS , is commonplace . While this offers the best protection in that there is a fully redundant UPS to take over should the other one fail or require maintenance , such a solution also represents the most expensive option .
There are more cost effective redundancy options in which an acceptable level of resilience can be provided using N + 1 systems . A number of N + 1 configurations are possible , with a highly modular option offering a good trade-off between system availability and cost .
N + 1 options Virtualisation is now the norm in many data centres , in particular those providing colocation services . In these scenarios , applications are not hosted on dedicated individual servers with their own integral storage arrays ; rather they are allocated virtualised resources according to need from racks of servers and storage which host multiple applications and services , frequently on behalf of a number of clients .
In such scenarios , fault tolerance is provided by software that automatically reassigns an application to a properly functioning server resource in the event that a malfunction occurs in the IT equipment . In such situations , instead of having a 2N arrangement - or full redundancy for each piece of critical infrastructure such as a UPS - an N + 1 arrangement can be more cost effective and still provide adequate back up . In an N + 1 layout , one extra unit can provide resilience to ‘ N ’ required units .
There are several different configurations through which N + 1 redundancy can be achieved . In an Isolated Redundant configuration there is a main or ‘ primary ’ UPS module that normally feeds the load , but this primary module has a separate input for the static bypass circuit . The secondary UPS module feeds the static bypass of the main UPS module or modules and is completely unloaded in normal operating mode .
A Parallel Redundant configuration consists of several UPS modules of the same size and capacity connected in parallel on to a common output bus . This approach is more common than Isolated Redundant .
In both of the above N + 1 configurations , they are comprised of separate UPS units , one of which provides redundancy for all the others . An alternative approach is the Internally Modular Redundant configuration in which the ‘+ 1 ’ redundant UPS is built inside the frame of the system , thereby sharing backplane , control system and battery plant within the UPS .
Compared with the Parallel Redundant configuration , the Internally Modular configuration offers some advantages in terms of capital cost and reduced deployment times .
Considering the trade-offs between such systems , each rated at 1,000kW ,
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