From Safe-Swappability to true System Scalability

In this article we provide some insights on the quick and easy, interruption-free replacement of UPS-modules in order to achieve true system scalability.

1. Quick and Easy, Interruption-Free replacement of modules

The most evident requirement to fulfil this first function, the “quick and easy way”, is the weight of the module. Ideally, a module should weight so little that it could be quickly and easily handled by one single person. According to most international working standards, a module should not exceed 30kg in order to be carried be one single person. Today, modules of smaller rating (20kW) may weight approximately 1 kg per kW or close to 20kg. Larger modules of 30 to 40kW may weight up to 1.5kg per kW or 60kg, but must be carried by two persons, as per working standards.

Another requirement to fulfil this first function is a secure and fast disconnection of the module from the bus bar, as well as a quick shut down. This is possible first by a full front access to the UPS and the modules, a quick access to the locking elements (screws, etc) and a high quality, full plugging system of each module. Such plug system should include input from raw mains, inverter output, battery in and out connections and control and communication.

Even though most vendor of modular UPS claim that their modules are “hot swappable”, several architectural choices have lead to first execute a few preparatory steps before the modules can be extracted from the frame. Such steps are typically interventions from the side and the rear, including cabling de-assembly, etc. In case of critical applications, this type of “hot swapability” isn’t good enough.

Modular systems may be required to be “safe-swappable”. This means that the modules can be retrieved from the system without decreasing the maximum safety and availability of operation. In other words, the load must stay on inverter when modules are exchanged or added. The UPS is not switched to static or manual bypass.

Finally, at the end of such “quick and easy” operation, the new module must reset automatically and switch on again to the load safely.

There is a ”quick and easy” way to know if a modular UPS system fulfils its promises: a demonstration of a system in operation at site or in the factory under working conditions.

2. System Scalability – “Pay as you grow” – and System Flexibility

Modular UPS vendor have taken in chord the “pay as you grow” claim which suits very well this kind of UPS architecture.

It is important however not to focus on the claim but to be aware of the differences which make the scalability a secure and cost efficient operation.

All features listed above basically prevail. It is particularly critical when adding UPS modules to the system in order to continuously protect the increasing load that this operation is done without switching to bypass. Also, it is preferable that minimal setting must be made before switching the modules to the loads, or if necessary, that these can be done safely. Typical settings are mains voltage and battery configuration.

Finally, when the new modules is switched to the load, it is important that a controlling mechanism insure a correct load sharing between the modules, since load sharing is essential for the quality of the load supply. Cross currents between the modules must be avoided. To cope with such issue, some vendors use a particular advantage of the Decentralized Architecture, the Master and Slave principle. One module is designated as the Master and all other slaves. The new added module will be necessarily a slave. The Master’s control circuit measures the difference between the actually measured and the correct value continuously and send to all slave modules a unique parameter so that all can similarly regulate the current. This principle applies during normal operation as well.

True scalability and overall flexibility are then given when system up-grading is done without jeopardizing the protection of the load, when any module of the UPS type can be added to the system without exhaustive settings and when load sharing is regulated optimally when switching the new module.

Fig. 1: System Scalability (Traditional UPS System). In this example, the UPS system is never more than about 35% loaded. A traditional UPS has a maximum efficiency of about 93% when fully loaded dropping to about 90% at 50% loading and much less at 35% loading.

Fig. 2: System Scalability (Modular UPS System). Due to the scalability of modular UPS systems it is possible to add power as the Data Centre requirements grow (without increasing footprint).

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