Protecting Critical Power Paths using Modular and Mono-Block UPS Systems

Last year we worked with two university data centres who were both looking to upgrade existing transformer-based UPS. Their legacy UPS systems were approaching end-of-life and would require a costly overhaul to maintain operational effectiveness with mandatory fan, capacitor and battery replacement. Each UPS was around ten years old. Though the installed UPS represented the best engineering expertise at their time of installation, operational efficiency was dismal compared to systems today. With a 50% loading the UPS were around 70% efficient. Generated heat also added to the cooling requirements and air conditioning costs.

Modern transformerless UPS designs can achieve 95-97.5% operating efficiency in online mode and over a much wider load range (down to as low as 25%). These figures apply to both the latest modular and mono-block UPS systems and it is here that each university chose a different implementation path.

The majority of UPS installations are mono-blocks (probably over 85%). This is the traditional UPS design of a single cabinet housing the UPS electronics (IGBT-based rectifier and inverter) and with the battery in the same cabinet or supplied in a separate one or on a battery stand. The mono-block UPS has a built-in static switch to provide the system with a safe path for failure to mains if the UPS overloaded or develops a fault condition. The UPS may also include an internal maintenance bypass.

If N+X resilience is required, two or more mono-block UPS systems can be operated in parallel. Each UPS has a parallel card installed, connected via cable and their firmware controls share the load with one UPS adopting a master and the other a slave approach. If one UPS should fail or be taken out of circuit, the other UPS system is sized to power the full load. Each UPS may have a battery set or share a single battery set. Operating efficiencies can be raised using Eco-mode (up to 99%) which mimics a line interactive configuration, allowing a UPS system to go into sleep-mode or the battery charger to power down until the DC level reaches a set voltage (over a 1-2 week period).

University ‘A’ chose a 60kVA rated single mono-block UPS with 20% spare capacity built-into the sizing. The university did not require N+x resilience and felt that their data centre load profile would not change significantly over the lifetime of the new UPS installation. As well as a more compact and efficient design the installation also benefit from a 3-year warranty; standard for most UPS systems today.

Most UPS manufacturers now offer a modular UPS system alongside their mono-block ranges. Where as a typical mono-block range may include discrete sizes i.e. 10, 20, 30, 40, 50…to 200kVA UPS systems, a modular range is based on multiples of a standard module e.g. 25kW or 40kW. You may notice a difference in how the systems are rated. Modular UPS are almost always kW-rated at unity power factor. Mono-block systems are typically kVA-rated at 0.9 power factor or less. For a 100kVA mono-block UPS this means 90kW of power if 0.9 rated.

University ‘B’ chose the modular approach and standardized on a 40kW module design. Modular UPS systems are installed within a suitably sized frame that can take a multiple number of the modules. The approach is very similar to that of a data centre server rack cabinet and the frames are designed to site ‘in-row’ with a front door and touch-screen master control and overview panel.

In a modular UPS the frame houses an overall maintenance and static bypass arrangement and the necessary slot-in connections for the modules. Each module houses its own static switch. Typical frame sizes can be 120kW, 200kW, 320 or 480kW. University ‘B’ chose a 120kW frame with two 40kW modules to provide N+1 protection for their 40kW load. The frame size also allowed further modules to be added at a later date allowing expansion to 80kW N+1 or 120kW full capacity.

As the UPS upgrade at university ‘B’ was also part of a wider energy efficiency upgrade, the new UPS installation helped to achieve a 50% reduction in energy usage. A further advantage was that the same modular UPS approach was to be adopted in the second campus data centre. As UPS modules can be ‘hot-swapped’, it is relatively easy to take a module out of service in one frame and slot-in to another. This strategy gave the university greater flexibility to meet future capacity changes within either data centre.

Modular UPS do cost more than mono-block systems, with a higher upfront capital investment. However, the approach can provide greater flexibility to meet expansion and also benefit in terms of reduced service and maintenance times. If a module requires swap-out it can typically be done within 30 minutes of arrival on site and any module can be taken out of circuit without the load losing full on-line power protection. If a mono-block requires maintenance or swap-out and it is not in a parallel configuration, the load can be exposed to a raw mains power supply with the potential risk of downtime if a power cut occurs.

To learn more about our standard and modular range of UPS systems please call the Eco Power projects team on 0800 612 7388 for a free site survey and project consultation.