Parallel UPS Configurations
Running two or more UPS in parallel (and thus configuring a parallel UPS system) enables customers to either add redundancy or greater capacity. There are, therefore, two primary types of parallel UPS configuration: parallel-redundancy and parallel capacity UPS. In this article, we outline each and it’s place in today’s power protection scenarios, alongside presenting information on the four key challenges of parallel UPS systems.
In a parallel UPS system, the key challenges are:
• Controlling how the separate UPS ‘co-operate’ as one unified system.
• Synchronising the output of each UPS so it can flow into a shared output.
• Balancing the load equally amongst all UPS in the configuration.
• And if a problem occurs, identifying the UPS with the problem and decommissioning it.
A parallel-redundant UPS system is one in which two or more modules are installed on the same system in what is termed an N+X arrangement (N being the power capacity required by the connected loads and X being the number of modules in addition to that capacity). Parallel-redundancy allows for the failure of one single UPS module in the configuration without the need for the protected load to be transferred to mains power. In such an event, the other UPS modules (all of which have spare capacity) can take over the total load.
The advantages of a parallel-redundant UPS system are that if offers a far higher level of availability than parallel-capacity systems due to the extra capacity within each UPS module. This can be utilised if one breaks down or has to be taken out of service for maintenance. The system is also less likely to fail than other parallel configurations because there are fewer breakers and no step loads (modules are online the whole time). It is also expandable should the power requirement grow and the hardware arrangement is simple and cost-effective.
A parallel-capacity UPS system is created by connecting multiple UPS modules in parallel but without leaving room for redundancy. This means that each is operating at full capacity (this is also known as a total power system). This configuration will not increase overall system resilience but it does optimise system efficiency as each UPS is operating at full capacity. The system also provides availability over and above what it would be through mains power alone.
Parallel-capacity UPS systems normally have a by-pass to mains. If one module in the system breaks down, the rest (unlike a parallel-redundant configuration) will be unable to accommodate the additional load so it will be switched to by-pass and thus to mains, which is an unprotected power source.
Controlling Parallel UPS
Control of parallel UPS is normally managed using a master/slave arrangement whereby a master controller acts as the brains of the system and determines operating parameters: how UPS modules synchronise their outputs, how they share loads and where their control information comes from. It does, however, require a mess of control wiring between UPS modules.
A peer-to-peer control system is an alternative and it works in a similar way to peer-to-peer computing, with each UPS assessing its own operating parameters and determining how to interface with the others – and there is no need for control wiring.
Different applications (such as server rooms, laboratories and telephone exchanges) will have their own, unique set of requirements for resilience, redundancy, capacity, efficiency and operation.
To increase safety, security and resilience in parallel UPS installations, Castell (or ,strong>trapped key interlocking systems) are deployed.
Castell keys are a system of locks and keys that sequentially (and electronically) control equipment and machinery to ensure safe operation. In parallel UPS, for example, a Castell key operated bypass switch would offer safe and resilient operation of the bypass supply (to prevent the load from accidentally going into bypass mode). This may also help to increase availability in operations such as data centres where five-nines availability (99.999%) is desirable.
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