What's the Harm in Harmonics?
A concise report on the effects of harmonic distortion on computers
- The adverse effects of harmonics on electrical power systems are well known. International
standards exist to help manage the injection of harmonic currents into the power system.
- A pure sine wave consists of a single fundamental frequency. Most waves are complex
and can be broken down into a series of harmonics. For instance, the 3rd harmonic of the
fundamental frequency 60 Hz is 180 Hz. The opposite end of the scale from a pure sine wave is
a square wave, which contains all the odd harmonics of a fundamental frequency starting with
the 3rd harmonic and extending to infinity.
- Harmonic distortion is a measure of the ratio between the fundamental and the harmonics that
are present in a wave-shape.
- Distortion is caused by non-linear loads drawing non-sinusoidal currents.
- Non-linear loads include such office equipment as computers (draw 3rd and 5th harmonics);
lighting (ballasts produce 3rd and 5th), transformers (3rd component), power converters
(variable speed drives, UPS, electronic ballasts produce 3rd, 5th, 7th, and 11th), and
commercial appliances such as photocopiers and printers.
- The harmonic currents in a distribution system have the following undesirable effects:
- Significant effects on the performance of computer power supplies.
- Heating of magnetic devices, which can cause premature insulation failure and
- Triple harmonics produce higher than expected neutral currents, potentially resulting in
insulation damage and breakdown due to temperature rise.
- High neutral currents cause additional voltage drops, reducing available voltage and
introducing even more distortion at the load. The resulting neutral-to-ground voltage also
causes parasitic ground currents.
- Additional losses resulting in higher energy costs.
- Failure of power factor correction capacitors or increased resonance that leads to an
even higher order of distortion.
- High frequency fields that can introduce buzz into telephone lines and corrupt data in
adjacent data lines.
- Stray magnetic fields can introduce annoying interference patterns in VDTs.
- Undesirable ground currents may cause upstream GFI (ground fault interrupter) to trip,
degrading power availability.
Computer power supplies are usually designed to operate over a range of AC input voltages.
They produce a DC voltage that is affected by the wave shape of the AC waveform. Harmonic
distortion has the effect of actually reducing the computer power supply’s operating voltage.
That variation is compounded by the normal variation of 10%. The end result is that the computer
supply may malfunction, hiccup, or fail to provide the required output logic voltages, resulting in
potential bit errors. A further negative side-effect of effectively lowering the computer’s operating
voltage range is the reduction of hold-up time, essential for the ability of the power supply to
ride through power sags (caused by other equipments’ startups). The result is more potential bit
errors. The simple fact is that the harmonic distortion prevalent in the modern office has a direct
pathway to your computer power supplies, and once there it can cause data loss and other
Series 9 and Ferro UPSs Address Harmonics
Power Consultants Warn:
- A site may not have an overload or unsafe condition now, but the owner needs to be on
guard against this harmonic threat.
Effects of Harmonics
- High levels of harmonics result in erratic or subtle malfunctions of equipment that can have
- Overheating could cause overvoltage damage and premature failure to computers and
- High harmonic levels can fool certain control circuits into erroneous operation. CPU clock
timing operations can be falsely triggered.
- Intermittent electrical noise is intense enough to corrupt digital signals and cause
- Distortion of voltage is severe enough to impair ride-through capabilities of computers and
- Harmonic voltages have virtually no effect on modern computers, but can cause
overheating in some cases.
- The UPS may operate in backup mode much more often than usual due to the poor quality of
the AC power source (source with severe line voltage distortion).
Independent Evaluation of Competitor Series 5 UPS
- The APC Smart-UPS® momentarily transfers to inverter for almost any voltage anomaly.
- UPS equipment worked properly with harmonic distortion levels below 5% THD. As distortion
levels increased, more interaction was noted. At 8.66% THD, the competitor UPS would not
operate from the test voltage until the sensitivity switch on the back of the unit was set to a more
What Does It Mean?
- Harmonics will cause sensitive equipment to crash and fail prematurely.
- Line-interactive technology may not protect the sensitive equipment from harmonics.
- Online and Ferro UPSs are recommended to protect sensitive equipment from harmonics.