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As a recognised and long established designer and manufacturer of state-of-the-art Uninterruptible Power Systems (UPS) with world-wide coverage, Chloride closely follow developments in technology and product application. Of particular interest at this time is the changing nature of the computer loads that UPS products have to support. Whilst we have always been aware of the non-linear effects of computer loads we recognise that certain modern day IT products such as blade servers can draw power from the supply with a leading displacement power factor (DPF).
This fundamental change in the nature of the loads that UPS are called upon to support has occurred largely unnoticed by the users and infrastructure managers. Indeed it comes as a surprise to many to discover that the loads that were originally considered lagging in nature have slowly changed as the IT community have progressively upgraded their systems with new blade servers Chloride’s objective is to ensure that our products are compatible with all types of load and that our designs are fully optimised for the actual loads encountered in the IT environment.
For many years the industry has measured, and specified, IT equipment loads with power factors in the range 0.6 to 0.9 lagging. Until recently the front end of computer power supplies consisted of little more than a full wave bridge rectifier followed by a large electrolytic capacitor. The resultant rectified DC voltage was applied to a switched mode converter which produced the low voltage supplies required by the computer. This low cost approach, driven by commercial pressures, had advantages of cost and in size and weight compared to more sophisticated designs but exhibited a very poor power factor and high harmonic feed-back into the mains supply.
These harmonics, principally 3rd, 5th and 7th, had to be supplied by the UPS and provided a significant design challenge to UPS manufacturers to provide these harmonic currents and the associated very high peak currents, whilst still maintaining the supply voltage wave-shape close to the ideal sinusoid. In three phase supplies the triplen harmonics (3rd, 9th, etc) combined in the neutral conductor necessitating an increase in cable size if overloading was to be avoided. Additionally when the UPS was not in circuit, or for those computer installations that did not include a UPS, these harmonic currents were returned to the public mains supply causing power loss, heating to the supply transformers and cables and voltage distortion to other consumers.
As a consequence, to meet these requirements, Chloride, and the majority of the UPS industry, designed products capable of providing high peak currents and standardised on 0.8 power factor (pf) lag as a means of determining nominal KVA/KW ratings. This rating convention has been adopted almost universally across the industry and so, for example, we have 100kVA/80KW as a standard frame rating.
The optimisation of the UPS for lagging PF loads was partially brought about by the necessity to provide output capacitors to filter the PWM waveform from the inverter switching bridges into an accurate sine wave. A disadvantage of these capacitors is that they affect the ability of the UPS to support leading PF loads resulting in a significant de-rating with that type of load. However this was of no consequence as IT loads were consistently lagging and leading power factor loads were actively discouraged due to their affects on diesel generator control systems.
Diagram 1 shows the de-rating required for leading power factor loads on earlier generations of UPS modules. This shows that the system can support only 77% of its nominal kW rating when supporting a load with a DPF of 0.9 lead.
Under pressure from EMC standards (EN/IEC 61000-3-2) to reduce input harmonic currents, the computer power supply manufacturers have re-designed their switched mode power supplies (SMPS). Their objective being to dramatically reduce input harmonics and to improve the operating input power factor. The result is that, at 100% load, the input current is virtually a pure 50 Hz sine wave, with little harmonics, and a power factor very close to unity.
However the situation at reduced load is not quite so rosy. Power supplies within IT equipment are rarely fully loaded: they are sized to support the maximum possible server configuration with capacity to spare for reliability, and who fills every slot in the rack?
Also the provision of power supply redundancy within the server by the fitting of two or more hot swap power supplies immediately reduces the normal operating load on each of the power supplies to significantly less than 50% of the nominal rating. At these low loads the input power factor of the power supply is no longer unity it becomes leading. The net result is that UPS may now expect to see aggregate loads normally operating with power factors in the range 0.9 lead to unity. More extreme leading power factors may occur at very low loads.
With the increasing deployment of blade servers, with so-called unity power factor input supplies and other low harmonic input current equipment it has become necessary to reconfigure the output filter stages of UPS systems to ensure compatibility with these loads and to optimise the utilisation of the UPS. Diagram 2 illustrates the different performance characteristics provided by this optimisation. In this article we look to explain how this change and future proofing has been relatively easy for us to achieve with our modern inverter technologies.
Fortunately inverter design in the last few years has evolved to embrace the use of higher switching frequencies. The semi-conductor manufacturers have developed ever improving IGBT power switching devices allowing faster switching in the conversion stage from DC to AC for a given power loss and thus heat output. If the power bridges are more efficient it allows the designer to adopt a higher pulse width modulation (PWM) switching frequency. By properly controlling the PWM the harmonic content of the AC waveform is reduced allowing the use of smaller AC filters and the need for large capacitor banks for harmonic filtration.
Thus the UPS can be optimised to handle these new leading power factor loads, without impacting on the ability to support a load exhibiting a more normal power factor. This is achieved by redesigning the UPS inverter switching bridges, output transformer (if the UPS is of a design that incorporates an inverter isolation transformer) and output harmonic filter to provide a symmetrical circular diagram as shown here.
By correct implementation of control algorithms, essential to ensure operating stability, loads of any power factor, from fully lagging, through unity to fully leading can be supported by a properly designed and controlled UPS
Chloride’s latest 70-NET, 80-NET and 90-NET ranges are all fully capable of supporting leading power factor loads. They do not require any de-rating and can thus be confidently connected to any load within the nominal kVA and kW ratings of the UPS regardless of power factor. In this way the loads of today, as well as those of yesteryear and the future, can be supported by Chloride UPS giving confidence to the infrastructure manager.
