Power quality
In a building full of computers, the neutral can carry more current than each phase. The third harmonics of the three phases do not cancel: they add.
01When power is “good quality”
The ideal grid delivers a perfect sinusoidal voltage, of constant amplitude and frequency and balanced three-phase. Power quality measures how far the real one departs from that ideal, and how those deviations affect equipment. The responsibility is shared: the utility answers for the voltage it delivers, and users, for the disturbances their loads inject. Regulators set quality limits for the technical product —the voltage and its disturbances— and for the technical service —the number and duration of outages—.
02The disturbances
| Disturbance | What it is | Typical causes | Effects |
|---|---|---|---|
| Voltage sag | A drop of 10 to 90% lasting milliseconds to seconds | Short circuits on the grid, starting of large motors | Restart of computers and drives, production stoppages |
| Transient overvoltage | Spikes of microseconds to milliseconds | Lightning, capacitor switching | Breakdown of insulation and semiconductors |
| Interruption | Voltage almost zero | Faults and protection operation | Loss of service |
| Harmonics | Components with a frequency that is a multiple of 50 Hz | Rectifiers, switching power supplies, drives, LED lighting | Heating, neutral overload, resonances |
| Flicker | Slow amplitude fluctuations | Arc furnaces, welders | Visible flickering of lighting |
| Imbalance | Phases of different amplitude or phase shift | Poorly distributed single-phase loads | Heating of three-phase motors |
The reference standards are IEC 61000-4-30, which defines how to measure each parameter, and EN 50160, which sets the characteristics of the voltage of the public grid. For the harmonics injected by users, IEEE 519 is also used.
03Harmonics
A nonlinear load draws a non-sinusoidal current even if the voltage is sinusoidal. The most common case is the rectifier with a filter capacitor, at the input of almost every electronic power supply: the capacitor charges only near the peak of the voltage, and the current arrives in narrow pulses. That current is decomposed into a Fourier series with odd harmonics, and its content is summarized in the total harmonic distortion:
The harmonics that are multiples of three (3rd, 9th, 15th…) of the three phases are in phase with each other, so in the neutral of a three-phase system they do not cancel: they add. An office building full of computers can have more current in the neutral than in the phases.
04The power factor with harmonics
With sinusoidal voltage, only the fundamental of the current produces active power. Harmonics increase the RMS current and the apparent power without contributing useful power. The true power factor has two factors:
The solution is in the power supply: active power factor correction (PFC), a boost converter that forces the input current to follow the shape of the voltage. The harmonic emission limits of IEC 61000-3-2 make it necessary in practice in equipment above 75 W. In existing installations, tuned passive filters or active filters that inject the opposite harmonic current are used.
A current with a fundamental in phase with the voltage and 3rd, 5th and 7th harmonics, with the signs of a rectifier with a capacitor (pulses at the voltage peak). At the top, one period of the voltage (gray) and the current; below, its spectrum. The same load is assumed on the three phases of a system with neutral.
05How it is measured
Power quality analyzers record for days the voltage and current of each phase and calculate with standardized methods the RMS values, the harmonics up to the 50th, sags, flicker and imbalance. Class A instruments, according to IEC 61000-4-30, give results that are comparable between instruments and can be used for complaints. All of that depends on measuring true RMS values, as seen in non-sinusoidal signals: an average-responding clamp greatly underestimates the current of a switching power supply.
06In the laboratory
With a power analyzer, measure the input current of a charger without PFC and of a computer power supply with PFC: waveform, THD, power factor and crest factor.
Feed three identical nonlinear loads, one per phase of a three-phase system with neutral, and measure the phase and neutral currents.
Install a recorder in a laboratory’s panel and analyze the sags and voltage harmonics recorded.
07Common mistakes
- Correcting a distorting load with capacitors. It does not improve the power factor and can cause resonance.
- Sizing the neutral thinner than the phases. With electronic loads it can carry more current.
- Measuring the current with an average-responding instrument. With narrow pulses it reads far less.
- Blaming the grid for what the installation itself injects. Voltage harmonics often come from the loads of the same building.
08Self-assessment
What THD does a current with 30% 3rd and 40% 5th have?
\( \sqrt{0.09 + 0.16} = 0.5 \): 50%.
What power factor does that load have if its fundamental is in phase with the voltage?
\( 1/\sqrt{1.25} \approx 0.89 \).
Three identical loads each have 5 A of 3rd harmonic. How much flows in the neutral?
15 A at 150 Hz.
Why does a three-phase drive without a neutral not load the neutral with third harmonics?
Because without a connection to the neutral the zero-sequence currents, such as the triplens, have nowhere to flow: a six-pulse three-phase rectifier does not produce a 3rd harmonic.
09Further reading
- Roger C. Dugan, Mark F. McGranaghan, Surya Santoso and H. Wayne Beaty. Electrical Power Systems Quality. 3rd ed., McGraw-Hill, 2012. The reference on disturbances, their measurement and their mitigation.
- Math H. J. Bollen. Understanding Power Quality Problems: Voltage Sags and Interruptions. IEEE Press, 2000. Voltage sags and their effects on equipment, in depth.
- Francisco C. De La Rosa. Harmonics and Power Systems. CRC Press, 2006. Harmonics in power systems, their sources, effects and filters.