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How to choose the right UPS system for the job

Schneider Electric UPS

Marc Garner, VP Secure Power Division at Schneider Electric, details the considerations you should make when selecting a reliable UPS system. 

Uptime and resiliency remain key topics of discussion within digital transformation, but the choice of which backup power solution to deploy is often a secondary consideration. This seems counterintuitive when trying to safeguard business and mission-critical applications.

Despite the growing popularity of Lithium-ion (Li-ion) battery technology technologies, many data centres and electrical critical infrastructure installations continue to rely on uninterruptible power supply systems powered by valve-regulated lead-acid (VRLA) batteries. 

The reason many businesses option for VRLA batteries is due to the misconceptions regarding their price and performance. However, that’s not exactly the whole story regarding the total cost of ownership. 

Schneider Electric’s Battery Technology for Data Centres white paper details studies which found some interesting comparisons regarding the use of VRLA vs Li-ion. The studies found that over a 10-year period, Li-ion delivered a total cost ownership between 10% and 40% lower than an equivalent UPS using VRLA. 

The different safety considerations when choosing a UPS

Another key consideration for electrical and IT professionals is safety, which is crucial when designing, specifying and deploying UPS systems. According to the 2017 Centrica Resilience Report, 11% of businesses say that their employees have been put in dangerous or life threatening situations as the result of energy-related failures.

Short circuits in a UPS can cause battery damage resulting, at best, in the loss of a UPS or short-term power outage, and at worst significant damage to an entire data centre, loss of revenue or of life. Careful consideration, therefore, must be paid at the design and specification stages to ensure that the primary task of providing continuous power in the event of an outage is not undermined by a fault that could have been prevented with proper planning.

Today various techniques and products are available to safeguard against short circuits, including DC circuit breakers, DC fuses and DC switches. Fuses and thermal-magnetic circuit breakers operate based on the heating produced by overloads or fault currents flowing through them, yet consequently, the ambient temperature within a facility can have an effect on the trip characteristics of both types.

Fuses, for example, are nonetheless the lowest cost protection devices but, unlike switches and circuit breakers, are not resettable and must be replaced. Circuit breakers with electronic trip units, however, are not affected by ambient temperature levels and neither are DC disconnect switches. In many installations, DC protection devices may include one or more kinds of components depending on the particular requirements of the system.

Nonetheless, when configuring a UPS system for a customer installation, there is a standard set of procedures to follow.

Select and size the battery

Battery capacity, in Ah or Wh, is the principal but not the only consideration when deploying a UPS. Other parameters to determine include detailed load profiles, specified backup time, UPS characteristics such as inverter or battery charger efficiency, charging and discharging performance, and redundancy requirements. Here it is essential you have specified the right size UPS for the assets you’re trying to protect from downtime.

Calculate discharging current and selecting conductor size

UPS battery charge and discharge cycles must be analysed to ensure they remain in a safe and reliable condition. Depending on the type of battery technology selected, its lifecycle and replacement schedules will differ greatly, which is another crucial consideration for electrical professionals when specifying a battery type.

In fact, the impedance of the battery system affects arc flash calculations, which are essential for ensuring safety. This includes conductors, inter-cell connectors between individual battery units, inter-tier and inter-aisle connectors for rack-mounted systems, and connectors between a UPS and its battery banks. All of which contribute to the overall impedance.

In order to minimise the voltage drop across conductors, or identify the right level of protection for the system, the minimum conductor cross section can be calculated. This will ensure that the UPS will provide optimum performance as and when required, but particular attention should also be paid to its lifecycle, anticipating the need for regular or proactive maintenance.

Calculating short circuit currents

As well as charging and discharging cycles, short circuit scenarios must also be analysed so that adequate safety features can be built in at the design and specification stage. It is important to consider that some features, including battery system voltage and internal resistance varies with the charge of the battery and the condition of its health. Further, as a battery becomes close to the end of discharging or as it gets older, its voltage decreases and its resistance increases, thereby affecting the value of short circuit current.

Finally, if the current becomes too low it may not be enough to open the protective circuit breaker or fast fuse, leading to the risk of overheating. Power protection devices must therefore be chosen to take account of the varying short circuit currents during different modes of operation.

Site and system protection devices

The optimum way to safeguard against UPS battery malfunction is to use IoT-enabled devices that provide data-driven insights into the health and status of a UPS, or its battery system. Indeed as conditions evolve over time and temperature fluctuations are experienced, the protection systems must be able to respond in a timely manner to prevent a malfunction or increase risk of downtime.

Today intelligent UPS solutions can monitor the health of each individual battery unit, ensuring normal operation during charging and discharging modes, while using both smart alarms and analytics to alert electrical professionals to system issues, or abnormal operating conditions.

However, given the trade off between price and performance between different types of UPS systems, many power supplies may also have a more rudimentary system-level protection. Here a simple checklist for assembling custom battery and UPS systems, rather than using a vendor-manufactured solution, should comprise the following:

  1. Follow all of the steps outlined above closely, beginning with battery selection.
  2. Analyse and validate the safety of the UPS under normal and abnormal operations.
  3. Perform strict quality control of all critical devices including UPS, batteries and protective components.
  4. Ensure adequate spare parts are kept in an accessible inventory.
  5. Establish and follow a proactive maintenance or digital servicing plan.

Whether deploying a pre-specified UPS, or a customised power protection solution, it is crucial to ensure adequate safeguards are in place to prevent UPS battery malfunction. For successful digital transformation projects, the need for resilient and uninterruptible power is paramount, but it’s essential that IT and electrical professionals also follow a safe specification process from the start.

Tools to guide the specification process.

To help guide electrical specifiers, contractors and resellers in the UPS selection process, Schneider Electric has announced a series of updates to its EcoStruxure Specifier Tool; adding single and three-phase uninterruptible power supply (UPS) technologies that enable partners to quickly specify projects and complete tenders.

Available free via the Schneider Electric partner portal, the tool enables consultants and engineers to specify UPS requirements from 1-1500kVA rating, with advanced and innovative features such as efficiency, modularity, battery type, architecture and redundancy.

The new update also provides partners with access to pre-tested electrical reference designs, enabling them to identify ways to increase project revenue, such as through the integration of electric vehicle (EV) charging stations in smart buildings and data centres, and by bidding on larger-scale electrical projects.

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