If you don’t know what electrical harmonics are, then you’re in good company. Outside of electrical engineering circles, they’re often widely misunderstood, and their impact can be easily underestimated. What starts off as a flickering light can easily snowball into a facility-wide shutdown if the problem is not identified and remedied quickly.
What are harmonics?
For an electrical grid operating in ideal conditions, the current is in theory a pure sine wave, containing no harmonics. In practice, no electrical grid is ever a pure sine wave, and so some degree of harmonic distortion will always be present. In small amounts harmonics are completely harmless, and almost unnoticeable. However, if left to accumulate unchecked, they can lead to malfunctioning equipment and reliability issues. For data centres, where reliability is of utmost importance, this can add up to a serious headache.
The prime culprit of harmonics is non-linear loads, which are widespread in modern systems due to the use of AC to DC converters such as switch mode power supplies and diode-based rectifiers. This includes computers, printers, televisions, servers, uninterruptible power supplies, EC fan solutions, integrated pump and drive solutions, and variable speed drives. These loads draw current in pulses rather than in a smooth wave, and the high-frequency, non-sinusoidal currents that this generates subsequently create harmonics within the power system.
Excessive harmonics result in nuisance trips and malfunctioning equipment, as well as reducing system efficiency and increasing losses. Equipment running inefficiently or erratically will also generate more heat, which can lead to premature equipment failure, and create more work for cooling systems.
Harmonics are present in nearly every electrical system, and in small amounts they’re unlikely to cause any major issues. The problem comes when more non-linear loads are added to the network. As data centres race to increase capacity and pack more power density into smaller spaces, the risk of harmonics grows with each new device connected to the network. This makes data centres, which are essentially packed from floor-to-ceiling with harmonic-generating loads, uniquely exposed.
Mitigation methods
Plenty of big data centre operators have underestimated harmonics and ended up running into this problem, resulting in downtime and costly retrospective fixes. It’s an increasingly common problem as facilities attempt to expand their capacity without taking appropriate measures to ensure the electrical infrastructure is able to support it. One way of addressing high harmonic content is to oversize equipment like transformers and back-up generators. However, the oversizing required is often fairly substantial, making this a relatively high cost solution.
A facility using 6-pulse drives with no in-built harmonic mitigation will typically have to oversize its generator by around 50 percent. As well as sending costs skyrocketing, using oversized equipment reduces system efficiency. Furthermore, it not only potentially takes up precious space that could otherwise be used for more computing equipment, but also generates more heat.
Harmonic filters are another option, and are typically used on EC fan systems or drive systems with no in-built mitigation. These can be installed in the first instance, or retrofitted at a later date, which makes them a popular solution, as they can be installed as and when they are needed. However, filters are by no means a perfect solution; they create additional power requirements, and are also an additional point of failure. If your filter goes down then so too does your mitigation, which can potentially leave your whole facility exposed.
Why mitigate when you can prevent at source?
Implementing mitigations as and when you need them is one solution, however there are trade-offs to this approach. For a start, there’s a risk that the effects of harmonics won’t manifest themselves until it’s too late, resulting in costly downtime. There is also the cost of implementing harmonic filters, which take up precious space in already densely-packed facilities. An alternative approach is to prevent harmonics from being generated in the first place.
Variable speed drives are typically used throughout data centres, particularly in cooling systems. While drives are technically non-linear loads, they are also the only such commonly-used devices with their own built-in harmonic mitigation.
Ultra-Low Harmonic drives use active front end (AFE) technology to essentially cut off the harmonics at source. AFE drives use IGBTs instead of diodes to convert AC to DC, and have an active supply unit and integrated line filter. In practice, they can help to reduce harmonic content produced by the drive by up to 90 percent compared to conventional drives.
To put that into context: a conventional rectifier with no harmonic mitigation can have a total harmonic current distortion (THDi) of around 95-120%. A rectifier with a DC choke has 35-45% THDi. In contrast, an AFE drive can prevent harmonics from being generated in the first place, resulting in a THDi of just 3% – a reduction of around 90% compared to no mitigation at all.
Electrical equipment works most efficiently and reliably when it is supplied by clean, distortion-free power. Selecting the right type of drive can ensure that harmonic content is reduced, without having to invest in separate filters, or oversize equipment. Prevention, as they say, is always better than cure.
