Kee Electric – Franklin, Brentwood

Generally, when buying an unemotional item, like an Uninterruptible Power Supply (UPS), we look for the lowest price of a simplest unit that meets requirements. However, generous rewards await those that look around, and consider, smart and cost effective solutions before buying a UPS.

UPS systems protect computers when utility power is available and continue to supply conditioned power from batteries at power outage. According to the specific application, basic requirements should be defined, such as the type of the UPS, its Capacity (Size) in VA (Volt-Ampere) or kVA (kilo-VA) units, and the battery backup time.

After you know the basic requirements you can start shopping for the best price of a suitable UPS. You can however make better. By surveying the market, looking around, checking what is available at what prices, and considering the creative recommended guiding points, you can make dramatic cost saving, generally  reaping additional performance benefits.

Here are seven points to consider for making a smart cost effective decision when buying a Power UPS:

1. As a rule:  For better protection, Line Interactive type should be preferred over Standby type and On Line type over Line Interactive type. Therefore, unless other considerations prevail, choose the better system if its price is only slightly higher.

2. Compare specified UPS price to the price of next size system. Consider buying bigger unit, if the added price isn’t significant. Bigger UPS enables to increase future loads, it has bigger battery with increased backup time, and extends battery life time.

3. Consider buying a bigger system if you need to increase battery capacity, beyond the standard battery coming with the UPS. You might pay for a bigger UPS, already equipped with the battery size you need, considerably less than the cost of the smaller system with extended battery.

4. Check the cost effect of splitting the load or buying systems which can operate in parallel. In some cases the cost of two systems, each rated half total capacity, may be lower than a single UPS system.

5. Consider using several stand alone single phase UPS systems, instead of a three phase UPS, provided loads and installation allow. Generally, the cost of two or three single phase units is less than one three phase system. Multiple systems may in certain cases increase power reliability by adding redundancy.

6. Consider the cost of using three single phase UPS systems, each fed from different phase, instead of a single three phase UPS systems, provided loads and installation allow. This configuration allows continuing partial operation, in case of a single UPS or input phase failure.

7. In redundant configuration, two or more units work in parallel, while at least one is a spare unit. Thus full power is supplied even if a single UPS fails. Check possible combinations to find the lowest cost. For instance, for a redundant system supplying 20kVA load, compare the cost of two parallel 20kVA systems, to a single three parallel systems 10kVA each. (Common mistake assumes that the last solution is always cheaper, due to lower total power).

How much can be gained?

Here are some examples:

Based on average prices of  low power UPS systems, the price of an Online UPS is almost twice compared to Line Interactive type. This ratio decreases when power grows. You will probably have to add only 10% to buy a 5kVA Online system.

You will need probably to add only 20% to increase an Online UPS from 1kVA system to 1.5kVA or  from 20kVA three-phase UPS to a 30 kVA unit.

Three 5kVA single phase UPS systems, each per phase, might cost about 30% less, compared to a single three-phase 15kVA UPS.

Three 30kVA units in parallel redundant configuration for a 60kVA load, can save as much as  25% (compared to two 60kVA units.

Implementing the suggested cost effective points, before buying an Uninterruptible Power Supply, may lead to a better solution with dramatically lower cost. Local availability and price of the specific system, as well as additional technical restraints on using the suggested solutions, should all be checked before a final decision is made.

Parallel uninterruptible power supplies can be installed with a single or dual ac input. It is imperative, however, that the input supply is correctly rated and cabling sized appropriately. The overall rating for a parallel system must be adequate to supply the chosen method of parallel operation, be it a parallel-capacity or parallel-redundant system.

In the case of a parallel-capacity configuration, the ac supply must be rated to the maximum input current of each uninterruptible power supply module and multiplied by the total number of modules used.

In the case of a parallel-redundant system, there is a bit more choice; the overall supply rating can be either the same as that used for a parallel-capacity design, which is the preferred method, or equal to the total current required by each of the individual UPS modules, plus 10 percent for redundant modules to cover charging and system losses.

Cable Sizing

Sizing cables is not necessarily complicated but it needs to be done correctly so as to avoid costly and disruptive downtime once the installation has been commissioned. Input cables must be sized in accordance with the rating of the supply of each UPS they are connected to. Output cables must be rated to either the supply or the maximum output power available from each UPS module (whichever of these is the greater). Input and output cables should be equal in length to ensure an even power distribution, should the UPS need to transfer loads to bypass.

Input Supply Isolators and Circuit Breakers

The use of isolators on the input of each of the uninterruptible power supply modules provides a means of isolating any one for service, removal or decommissioning. Circuit breakers are preferred as they can also provide further protection, specifically from over-currents (discrimination).

On the output of an uninterruptible power supply, isolators are preferred instead of circuit breakers as they prevent automatic disconnection of any one UPS module, which could otherwise disrupt operation of the parallel system during use. System resilience is increased by interlocking output isolators. This can also prohibit unauthorised operation. Future capacity expansion of the system is assured, relatively easily and inexpensively, through the installation of a spare switchgear set.

Dual Input Connection

It is common practice when connecting critical applications to increase resilience of an uninterruptible power supply by attaching it to a dual input ac power source rather than relying on a single one. This allows the bypass and rectifier to be supplied from separately derived sources, which could be different sub-stations, to mitigate a single-point-of-failure.

With transformer-based UPS this is made easier as they are usually supplied with a dual input option as standard. The rectifier and bypass supplies of this type of uninterruptible power supply are independent of each other and can be powered from two separate ac sources.

In the case of transformerless UPS systems, the rectifier and bypass supplies require a common neutral connection to be made within the UPS itself, which makes it a little trickier to utilise with a dual input supply without it being installed as a factory fit option.

Parallel uninterruptible power supply connection need not be any more complicated than any other type of power protection system as long as a few simple steps are followed and the system is correctly sized and rated.