Mitigating the threat of harmonics to maintain data centre reliability
Data centres contain sensitive equipment designed to store vast amounts of information, which is always accessible to its users. It is crucial that nothing compromises the reliability of data centre systems but, because of the large amounts of energy they consume, several potential faults can occur which threaten their stability.
Harmonics are one of the key issues affecting data centre reliability and can occur with the presence of equipment which uses non-linear energy loads that do not draw an electronic current from the utility with a smooth, continuous wave.
Variable-frequency drives (VFDs) are one of the primary consumers of non-linear energy loads in data centres and are used to substantially reduce the large quantities of energy consumed by heating, ventilation, air-conditioning and refrigeration (HVACR) systems. HCAVR systems play a vital role in regulating the facility’s temperature, an essential function considering the amount of heat generated, but they are a major cause of harmful harmonics because of the use of VFDs.
Other examples of equipment that has the potential to create harmonics in the system include uninterruptible power supplies (UPS) and battery storage systems, as well as LED lighting, photocopiers, and electronically commutated motors.
The wave form distortions which create harmonic currents generate power losses that add heat to the power chain, increasing the risk of downtime by causing equipment to malfunction. The more power there is in the network, the bigger the harmonic currents will be.
According to ABB, a company which specialises in energy saving drives and motors, the day-to-day costs of harmonic-induced system inefficiencies are often hidden and overlooked. For example, a transformer or motor that runs hotter means it is using energy in an inefficient manner, since energy is being used to create the heat instead of powering other loads in the building. This leads to the facility’s HVACR system to operate at a higher capacity, thus consuming more power to remove the waste heat.
Frank Taaning Grundholm, vice president of global HVACR sales at ABB Motion, says that mitigation is essential in minimising the risk posed by harmonics. “From a system perspective there is of course the opportunity to simply over-dimension the cables, fuses, and switchgear,” he says. “But if there is no mitigation at all, the need for over-dimensioning can be up to 30 per cent.
“In addition to the harmonics-related higher capital, operating costs might be further increased by the possibility of penalty charges from electrical utilities for harmonics content in the network.”
There are various methods of mitigation a data centre can employ, depending on the specifications for each centre. By calculating the total harmonic distortion of the voltage (THDV), which has the greatest impact on the quality of power the end-user receives, a data centre manager can compare different mitigation solutions.
Generally, ABB advise that on projects where drives make less than 30 per cent of the building transformer’s capacity, it is acceptable to use all six-pulse drives, whereas on projects with a higher percentage of drive loading, a combination of six-pulse drives on smaller devices, and active front end drives on larger devices, is optimal.
Another solution is to utilise an ultra-low harmonic (ULH) drive designed specifically for HVACR applications to ensure low harmonic content, even at partial loads at the source. “Adding ULH to the chillers, large pump and fan sets can eliminate the need for external harmonic mitigation solutions for the whole system,” Grundholm adds.
Addressing harmonics at an early stage with such mitigation techniques allows for other parts of the electrical infrastructure to cost less, with further long-term savings achieved through higher efficiencies and longer-lasting equipment.
Other data centre reliability concerns
Harmonics are not the only consideration for a data centre manager when examining ways to increase the facility’s reliability. The power factor (PF) – a measure of how effectively equipment uses electricity – of the entire network can be affected by various types of equipment used in a data centre. Data servers, for example, are capacitive loads whereas the cooling equipment has large inductive loads. Both loads are types of reactive power that impact PF, causing energy losses and destabilising the operation of power system equipment such as UPSs and generators.
“For effective electricity usage and power system reliability, data centre PF should be close or equal to unity (one),” Grundholm continues. “ULH drives can compensate reactive power in the network and deliver unity power factor without the need for external solutions.”
Likewise, radio frequency interference (RFI) can also affect the power quality. Data centres use a high amount of variable speed equipment to save energy, generating both radiated and cable borne electromagnetic noise which is transmitted to neighbouring buildings. This radiation might also affect IT equipment, and data centre security and performance.
The British Standards Institution maintains that inadequate electromagnetic compatibility (EMC) can result in errors and the incorrect operation of electronic devices, including degraded functionality and, potentially, complete failure.
“Negligence in working to the correct EMC standards can cause severe issues,” Grundholm concludes. “Not only will there be a need to install external filters at additional cost later on, managing high frequency noise after the installation is completed can be an almost impossible task.
“This is because high frequencies might radiate rather than be contained solely within the cables. If, as an example, mains and motor cables are installed in the same cable tray, the radio frequency filters are essentially without function and the only solution is to move one of the two cables to a separate tray at substantial cost.”
By being aware of potential issues affecting the reliability of a data centre facility, a manager can enact mitigation measures which will not only keep the facility functioning in a dependable way but protect expensive equipment from degrading and save the long-term cost of replacing appliances.