Power quality, harmonics and ride-through

A data centre needs not just enough power but power of the right quality: the right voltage, at the right frequency, with a clean waveform and without interruptions. Lesson 1 noted that UPS systems protect the IT load from most disturbances and that data centres can disturb the grid in return. This lesson goes into detail. It classifies power quality disturbances, explains the voltage tolerance curves equipment is designed to, quantifies harmonics and their effects on transformers, neutrals and generators, examines voltage unbalance and power factor, and shows why the loads that are not on UPS, above all the cooling plant, decide whether a brief grid disturbance becomes a facility outage.

Kinds of disturbance

Power quality disturbances are classified (for example in IEEE 1159) by duration and magnitude:

• Transients: very short spikes or oscillations, microseconds to a few milliseconds, from lightning or switching capacitors and loads.
• Sags (dips) and swells: short reductions or increases in RMS voltage, half a cycle to a minute, mostly caused by faults elsewhere on the grid. Sags are by far the most common disturbance affecting industrial customers.
• Interruptions: loss of voltage, from momentary (a utility recloser operating, under a few seconds) to sustained.
• Long-duration variations: overvoltage or undervoltage lasting minutes or more.
• Waveform distortion: harmonics, interharmonics and notching from power electronics.
• Voltage unbalance and frequency variations.

A fault on a transmission line tens of kilometres away can cause a sag to 50% or 60% of normal voltage for 100 ms at a data centre, until the protective relays on that line clear it. Such events happen many times a year at most sites.

Voltage tolerance curves

The ITI (CBEMA) curve, published by the Information Technology Industry Council, describes the voltage disturbances that IT equipment should tolerate. Its main points, for 120 V equipment, are:

• Complete loss of voltage for up to 20 ms (about one cycle) with no interruption of function.
• Voltage as low as 70% for up to 0.5 s, and 80% for up to 10 s.
• Steady-state voltage within ±10% of nominal.
• Swells up to 120% for 0.5 s and 110% thereafter, with higher short transients.

Modern server power supplies, with their internal hold-up capacitors (ELE-202, Lesson 10), usually do better than the curve. But the UPS makes the question moot for the IT load: a double-conversion UPS supplies the load from its batteries throughout any sag. The equipment that is exposed is everything not on UPS.

Making mechanical loads ride through

Approaches include drives with ride-through settings that keep running at reduced speed or restart automatically ("flying restart"), feeding critical pumps, fans and controls from UPS (mechanical UPS), chilled-water or ice thermal storage that carries the cooling load for 10 to 20 minutes, and chillers with fast-restart features. The design should specify what disturbance the whole facility, not just the IT load, must ride through.

Harmonics

Electronic loads draw non-sinusoidal current. A non-sinusoidal periodic waveform can be expressed as a sum of sinusoids at the fundamental frequency (60 Hz) and its integer multiples, the harmonics (180 Hz is the 3rd, 300 Hz the 5th, and so on). The total harmonic distortion of a current is

and the RMS current, which is what heats conductors and transformers, is \(I_{rms} = I_{1}\sqrt{1 + THD_{I}^{2}}\).

Single-phase switch-mode power supplies without power factor correction draw strongly distorted current dominated by the 3rd harmonic. Three-phase rectifiers in UPS systems and drives produce mainly the 5th, 7th, 11th and 13th harmonics. Modern server supplies with active power factor correction draw nearly sinusoidal current at full load (current THD below 10%), but distortion rises at light load.

Effects of harmonics

• Transformer heating: eddy-current losses rise roughly with the square of harmonic order. Transformers serving distorted loads are derated or specified with a K-factor rating, where \(K = \sum (I_{h}/I_{rms})^{2}h^{2}\). For the panel above, \(K \approx 2.2\), so a K-4 transformer would be suitable.
• Neutral overloading from triplen harmonics (Lesson 6).
• Voltage distortion: harmonic currents flowing through the source impedance distort the voltage seen by every other load, and can upset generators, drives and sensitive controls.
• Resonance: capacitors for power factor correction or filtering can resonate with the system inductance at a harmonic frequency, amplifying it dangerously.
• Generator problems: generators have higher impedance than the utility, so the same harmonic currents cause more voltage distortion when the site runs on generators.

Limits and mitigation

IEEE 519 sets limits for harmonics at the point of common coupling (PCC) with the utility. For systems up to 1 kV, voltage THD should not exceed 8%, and for 1 kV to 69 kV, 5%; current distortion limits depend on the ratio of the available short-circuit current to the load current, with stricter limits for large customers on weak systems. Mitigation options include active front-end UPS rectifiers and drives that draw near-sinusoidal current, passive tuned filters, active harmonic filters that inject cancelling currents, multi-pulse rectifiers with phase-shifting transformers, and choosing IT equipment with good power factor correction.

In Canada, utilities set harmonic and power quality requirements in their connection standards, generally based on IEEE 519 or the IEC 61000 series; Hydro-Québec, Hydro One and others publish their own technical requirements for large customers.

Voltage unbalance

In a three-phase system the three line voltages should be equal. Unbalance, caused by unequal single-phase loading or utility conditions, is expressed (in the NEMA definition) as the maximum deviation from the average line voltage divided by the average. Small voltage unbalance causes large current unbalance in motors and extra heating: as a rule of thumb, the percentage increase in motor temperature rise is about twice the square of the percentage voltage unbalance, and motors must be derated above 1% unbalance.

Frequency, generators and grid ride-through settings

On the utility, frequency is held very close to 60 Hz by the whole interconnected grid. On generators it is not: when a large load is added or removed, the engine slows or speeds up until its governor responds, and the frequency can swing by several percent for a few seconds. Voltage also dips and recovers as the voltage regulator responds. Double-conversion UPS systems isolate the IT load from these swings, but they have input tolerances of their own: if frequency or voltage moves outside the UPS input window, the UPS transfers to battery, and if its bypass is not synchronized it cannot transfer to bypass safely. Generator and UPS settings must therefore be coordinated so that normal generator transients stay inside the UPS input window.

The same settings matter to the grid. When every UPS on a campus is set to transfer to battery at the slightest voltage dip, a single transmission fault can disconnect hundreds of megawatts of load at once (Lesson 2), and the sudden loss of load raises grid frequency and voltage. Grid operators increasingly ask large loads to specify their voltage and frequency ride-through behaviour, to avoid unnecessary transfers, and to reconnect gradually rather than all at once after a disturbance. Setting a UPS to ride through brief sags on its rectifier, where its design allows, and staging the return to utility power across a campus are simple, low-cost contributions to grid stability.

Power factor of IT loads

Lesson 1 discussed power factor and utility billing. Within the data centre, the IT load presents a special case: server power supplies with active power factor correction have a power factor near unity at full load, but their input filter capacitors draw a small leading (capacitive) current. At light load, when a facility is new and mostly empty, the IT load can present a leading power factor of 0.9 or lower. Most generators are rated for lagging power factor and can be pushed into unstable operation by significant leading load, because the leading current magnetizes the generator and its voltage regulator loses control. UPS systems with active front ends also present near-unity power factor to the generator. Designers check the generator's capability curve against the expected load power factor at all stages of build-out.

Measuring power quality

Power quality is measured with permanently installed meters at key points (service entrance, generator buses, UPS inputs and outputs) and with portable analysers for investigations. Class A instruments to IEC 61000-4-30 give comparable, traceable results. The measurements, recorded continuously with waveform capture of every event, let engineers correlate equipment trips with grid disturbances, verify harmonic compliance and understand how the facility behaves on generator. They feed the electrical power monitoring system (Lesson 10).