Radiocommunications Agency  EMC Awareness

Power harmonic reduction

What this technique is used for

Power harmonic reduction is used to prevent the switch-mode power converters used in power supplies, variable speed motor drives, uninterruptible power supplies, and other high-power power conversion applications from interfering with telephones and radio reception.

How this technique is used

Installation engineers can only consider filtering the a.c. power supply input to the power converter.

How power harmonic problems arise

Harmonics of the a.c. power supply frequency are caused by the non-linear load presented by the semiconductor switching in the off-line AC-DC converter circuits. Because of their high rates of dV/dt and dI/dt, their harmonics can extend to at least the 40th harmonic (2kHz for a 50Hz supply). These harmonics all fall in the audio frequency band, where they can cause interference with audio circuits such as those used in telephones.

Differential audio frequency emissions pass easily through mains distribution transformers into the medium-voltage (MV) and high-voltage (HV) distribution networks. These couple to other equipment by…

• Direct conduction via the a.c. power distribution network

• Earth potential differences caused by the power supply network’s neutral conductor being earthed at more than one point, and/or by common-mode return currents flowing in the soil

• Near-field magnetic induction from the power distribution cables

• Common-mode coupling from the power distribution cables.

The non-linear loads only generate differential-mode noise. But MV and HV power distribution networks are very large, so even at such low frequencies the imbalance in the power cables causes a significant amount of differential-mode to common-mode conversion (longitudinal conversion loss), resulting in significant levels of common-mode voltages.

Telephone networks are also distributed, and have significant levels of common-mode to differential mode conversion due to imbalance. This makes it very easy for audio frequency noise in the MV and HV power distribution to interfere with telephones.

Key issues in employing this technique

Passive filtering: notch versus low-pass

At the high powers associated with such converters, filtering requires large and costly components. Filtering lower frequencies also requires significantly larger filter components. Filtering harmonic noise from the power input can mean filtering down to its 5th harmonic (250Hz for a 50Hz mains powered system, 83.33Hz for a 16.67Hz traction power system).

Ordinary low-pass EMC filters with corner frequencies low enough to give attenuation at such low frequencies would be very large, heavy and costly. So resonant filter designs are mostly used instead, with notch characteristics. Notch filters are designed to resonate at a particular harmonic in such a way as to attenuate it strongly. Several of them may be required, each tuned to a different frequency.

Notch filters are employed until the frequency gets high enough for the size and cost of low-pass filters to be reasonable.

‘Active’ filtering

A technique known as ‘active filtering’ has been developed in recent years for reducing harmonic disturbances in the a.c. mains power supply and reducing the demands on the passive EMC filters (in some cases eliminating the need for notch filters altogether).

Despite its name this is not a filtering technique at all, it is a high-power signal processing technique that uses fast-switching semiconductors. Most ‘active mains harmonic filters’ are supplied as separate units for solving harmonic problems in existing installations, but there seems no reason why the technology cannot be integrated with an “active rectifier’ circuit. Low-pass filtering may still be needed

Improving the balance of the conductors

If the power and/or telephone cables could have improved balance, and/or were only earthed at one point, this interference would be reduced. But because of the huge investment already made in installing these cables all over the country over the last 60 years, this is not regarded as a practical solution.