Harmonic distortion of the main power supply is a phenomenon caused by switching, particularly of high speed power switches as can be found in Variable Frequency Drives. This high speed switching causes harmonics currents which are usu­ally the multiples of the supply fun­damental frequency, produced by 'non-linear' loads such as the AC to DC power conversion circuits in the Variable Frequency Drives.

For example, on a 50Hz supply, the 5th harmonic is 250 Hz, 7th har­monic is 350 Hz, etc. These are called 'integer harmonics' - i.e. exact multiples of the supply fre­quency. The average value of all the har­monics is the Total Harmonic Distortion or THD. With the increased use of large var­iable frequency drives the danger of the effect of high THD levels has increased too. Classification societies use a value for the THD of 5% or less for use on ships.

The main effects and dangers of high THD levels are:

The following basic types of vari­able frequency drive systems are available which are shown in the diagram on this page:

One-way rectifiers (Amber)

The 3-phase AC from the switch­board main bus-bars is rectified by 6 diodes into 6 currents DC which are brought together resulting in a pulsating DC. See diagram. This DC is the sum of the three phases, where the negative part of each si­nus is made positive. This forms a DC current with 6 pulses per origi­nal cycle, and no possibility of feed­ing back to the switchboard. This DC is transformed into 3-phase AC again through inverters with ad­justable voltage and frequency.

One-way rectifiers (Yellow) 

Between the bus-bar and the rec­tifiers, behind the main switches, transformers of the double stock type are installed. A double stock type transformer has two seconda­ry windings, one in star and one in delta, so producing 6 sinus curves each. The output of one transform­er is brought out of phase as much as 30 degrees. The voltage is not necessarily changed. The thus pro­duced 12 currents are rectified sim­ilarly to the situation above, and is rectified to a 12-pulse DC. This 12-pulse DC is changed into the desired current in inverters, in volt­age and frequency. This output is used in two consumers, running in phase. The distortion on the main bus-bars is considerable reduced.

One-way rectifiers (Blue).

The same as above, but the output of the second transformer is shifted another 15 degrees. The consumers, identical, are using 12 pulses each, but 15 degrees out of phase relative to each other. The distortion on the bus-bar is now 24 pulses, and has less effect again.

One-way rectifiers (Another blue).

Each inverter supplied by two dou­ble stock transformers, resulting in 24 pulses to each consumer, a fur­ther reduction of the distortion.

Active front-end converter (Green).

This means that the input is not just a rectifier which is controlled by the input voltage, but a control­lable device. Controllable devices can stop and pass voltage without the restrictions of a rectifier, so in­dependent of the input voltage. These devices, thyristors, transis­tors, IGBT's and whatever types are used, can lead power from the switchboard to the consumer and back from the consumer to the switchboard. Active also means that the convert­er takes power from the switch­board in a controlled way, thus minimising harmonics. Transform­ers are only required when the volt­ages differ substantially.

Harmonics created by converters, supplying consumers, are absorbed by the generators energising the switchboard. The impedance of the generators gives an indication of the capability to absorb harmonics. A low impedance will absorb more harmonics than a high impedance, but is also capable to create a high­er short-circuit current, requiring more expensive switchgear.