When designing a power continuity plan for a data centre or any other type of critical operation, power quality should feature high on the tick list. Poor power quality can be as dangerous to critical loads as a partial or complete mains power supply failure and lead to intermittent data corruption and hardware failure explains Robin Koffler, general manager, Riello UPS
The actual quality of a mains power supply is measured in terms of its waveform, voltage and frequency and the presence or not of a variety of power problems including blackouts and momentary interruptions, sags, brownouts, surges, spikes and transients, electrical noise, frequency variations, and harmonics.
Of the eight most common types of power problem, harmonics is perhaps the least understood and planned for. Harmonics are voltage or current waveforms the frequencies of which are multiples of the fundamental. In Europe the fundamental frequency is 50Hz (50 cycles per second) and the multiples are ordered into a specific sequence. For example, the 2nd harmonic is 100Hz (2x50Hz), 3rd harmonic 150Hz (3x50Hz) and so on.
Often overlooked as a potential threat, harmonic pollution is a growing problem in the UK because its presence can be disruptive not only to a polluted site but also other customers of the utility provider. Most consumers are unaware that they are responsible, under the terms of their electricity supply contract, for the total harmonic values generated at the Point of Common Coupling (PCC) and building incomer when new installations are made. Acceptable levels are published within Engineering Recommendation G5/5-1, from The Energy Networks Association (www.energynetworks.org). The critical point for new installations is that where the specified harmonic levels cannot be met, consumers must secure approval from their utility provider before connection or face penalties.
Within a new data centre, for example, the most prevalent loads will be high-end servers and associated data processing hardware, which are powered by Switch Mode Power Supplies (SMPS). It is the power supplies themselves which form the load on the electrical supply, and being non-linear draw their own power in regular modulated pulses of current, rather than as a continuous linear current. This action can lead to the generation of high levels of harmonics, especially where a large number are supplied from a three-phase mains power supply. Of the harmonics generated, it is the set known as Triple-Ns or Triplens whose harmonic orders are multiples of three and include 3rd, 9th and 15th. Other harmonic orders, not in phase with one another, simply cancel each other out and though still considered to degrade power quality, their impact is less severe.
When harmonics are present in a mains power supply they can lead to voltage distortion, overheating of building wiring circuits, neutral conductors, supply transformers and switchgear, and nuisance tripping of breakers. Harmonics can also cause disruption to equipment on the same supply and lead to random failures. Within a data centre environment their presence can therefore prove disastrous.
It is the 3rd order harmonics which potentially are the most serious within a three-phase mains supply (Fig. 1.11) due to the summing effects within its neutral conductor. As these harmonics are multiples of three, they are all in phase with one another and therefore their magnitudes are added together. Their effect is to greatly increase the current flowing within the neutral and this can lead to potential overload and affect associated switchgear.
Whilst, it is accepted practice to balance loads across the phases of a three phase mains power supply, even this will not counter-act the impact of Triplens. This is because Triplens can generate neutral currents up to 1.73 (?3) times the average currents present. This additional loading (and heat generation) can degrade upstream neutral conductors and/or wiring insulation leading to potential breakdown and a fire hazard if unmanaged.
Whilst SMPS may be the most common source of harmonics, others include: rectifiers, variable speed drives, discharge lamps, fluorescent lighting, mercury and sodium lamps. When designing a power continuity plan, it is therefore vital that all site loads are assessed for their impact and effect on overall power quality.
Mitigating Harmonic Pollution
Data centres are now one of the most concentrated users of SMPS in the racks of servers they deploy over relatively small footprints. Whilst most users apply a concerted effort to manage the resultant air conditioning demands, few realise the potential harm that can be done due to harmonic pollution.
Within such an environment it is common to select a centralised approach to power continuity and install an Uninterruptible Power Supply (UPS) and standby generator. Whilst the UPS will power critical loads, the generator will provide power to essential services such as air conditioning and security systems, as well as provide backup to the UPS should its battery set be discharged.
The UPS can therefore be considered to fit ‘in-line' between the loads and the mains power supply. In addition to providing power protection to the loads, it should also protect the mains power supply itself from any harmonics generated by the loads themselves.
However, it is again not commonly known that UPS themselves, by way of their design, also generate harmonic pollution. For any UPS this is typically stated as Total Harmonic Distortion (THDi). Care has to be taken when comparing different THDi values as these can differ when contrasting the two different types of on-line UPS (transformer-based and transformerless) and also with regard to the percentage of load applied for each measurement.
Within a UPS it is the rectifier that connects to the mains power supply and converts the mains alternating current (ac) into the levels of direct current (dc) required to power the inverter and charge the battery.
For transformer-based UPS, rectifiers are typically six or twelve-pulse, dependent upon the thyristor number and configuration. A six-pulse rectifier at full load will typically generate a THDi of around 29% and a 12-pulse around 8%. To reduce these values further a passive harmonic filter can be installed alongside the UPS. The obvious disadvantages of this approach being increased capital cost, wiring, installation, loss of efficiency and increased footprint. Harmonic filters can be added post-installation but further installation costs and downtime need to be planned for.
Transformerless UPS have a different type of front-end whose configuration is usually that of a rectifier-booster. THDi levels of less than 7% can be achieved and reduced to less than 4% when an active harmonic filter is installed. For some designs, the harmonic filter may be positioned inside the UPS cabinet reducing impact on overall footprint but still resulting in higher capital and operating costs.
Harmonics is just one research and development area for UPS manufacturers with the goal of producing zero-impact uninterruptible power supplies. The latest developments offer THDi levels of less than 3% using IGBT-based (Insulated Gate Bipolar Transistors) rectifiers and it is forecast that this approach will be adopted as a standard for UPS up to 200kVA or more over the next one to two years. Achieving a zero-impact also covers the areas of operating efficiency and input power factor. Such designs can now offer efficiencies of 96-98% and input power factors close to unity. Their cumulative effective offers high reliability systems that can achieve 35% energy savings and quicker capital payback than traditional UPS.
Harmonics is therefore a ‘hot-topic' when it comes to power continuity but one most people avoid. The subject can appear complex but requires thought at the planning stage of any new installation if the systems are to deliver the benefits intended and satisfy the requirements of the utility providers who in the end has the ultimate power.
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