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It is good practice to have n+1 redundancy even at the standby generation level, whereas it is critical for UPSs to go with 2 (n+1) redundant designs. (If n is the minimum requirement to support the critical load, for example, 2 x 500 kVA UPSs, then n+1 redundancy would mean 3 x 500kVA UPSs and 2 (n+1) would mean 2 systems of 3 x 500 kVA each. It is important to utilize an external Centralized Static Bypass (CSB) per each parallel redundant UPS system. CSB provides a high degree of reliability when compared to simple modular parallel UPS systems because a CSB static switch is rated for the system load, while the modular UPS system depends on a static switch that is rated only for the individual module rating, i.e. typically only 20 percent of the load. (Figures 1 and 2)

Blade Servers

Since the growth of blade servers, it is good to have generator sets that have excellent compatibility with leading power factor imposed by blade servers. This will provide added reliability in the event of the entire UPS system going into bypass mode during a mains loss situation. Since most IT loads generate harmonics, it is good practice to limit propagation by using Cancellation Transformers. This helps to reduce the size of the neutral conductor, saving copper cost and eliminating fire risk and nuisance tripping of circuit breakers. UPSs need to be provided with suitable harmonic filters to ensure that the current distortion (THDI) level is held at five percent or lower, regardless of loading on the UPS system.

In order to limit the damage caused due to a faulty source, the Static Transfer Switches (STS) need to be used at the Power Distribution Unit (PDU) level so that any fault is limited to that part of the circuit, and system integrity is unaffected. These STS units are fast-acting (4 millisecond switching time); hence they can switch the critical load from one source to the other without jeopardizing functionality of the servers.

Built-in Design

Each UPS system needs to have built-in redundancy and reliability at the design stage. Once the design is checked for any dormant or hidden single points of failure, it is good practice to carry out factory witness tests for each individual unit, from UPS systems to switch gear to standby generator sets. Even during factory witness testing, it is good practice to simulate short circuits on the load to help measure the fault tolerance level of the UPS system and its components. Also, it is important to evaluate 100 percent stepload performance of the UPS system and standby generators. It is recommended that critical component monitoring of systems such as UPS battery banks are deployed, as this type of monitoring will help the facilities management team take proactive steps to avoid an internal disaster in a data recovery center.

There is no substitute for planned and regular maintenance including thermal imaging of critical components such as UPSs, batteries during discharge, PDUs and switch gear.

Shri Karve is Director of Business Development for MGE UPS Systems, a world leader in providing high quality power solutions that increase power availability and system uptime. For more information visit www.mgeups.com