Halting harmful harmonics: Page 2 of 4

April 10, 2018 // By John Mitchell
Halting harmful harmonics
The rise of non-linear loads in industrial environments over the last two decades has resulted in the growing problem of harmonic currents and utility-level voltage distortion. Facing a lack of awareness, the industry has struggled to implement effective mitigation techniques. 

Active vs passive

Passive and active solutions can be installed in both series and parallel (shunt) configurations. Series solutions operate in line with the load, meaning that units must be sized for the full current load. Shunt units can be sized only for the harmonic disturbance. There is a clear decision to be made between series-passive, shunt-passive, series-active and shunt-active solutions.

 

Series-passive

The most straight forward series-passive solution can be achieved using a line reactor. This is a three-phase choke placed in front of the rectifier. A line reactor provides a low cost way to reduce current harmonics, whilst adding a level of protection to the rectifier. However it's not perfect, it's not suitable for large drives and on its own, will be unable to meet IEEE519 standards.

The next option is to use a series harmonic filter. It provides more effective compensation than a line choke, significantly reducing total harmonic distortion (THD). Although a series harmonic filter works well as a "catch-all" it is grid sensitive and may lead to interaction. It's also bulky and not particularly suited to dynamic applications, working best on pumps and fans on a reasonably well balanced supply.

Of course, truly balanced supplies are few and far between. Any unbalance on the supply can cause damage as a result of overloading and overheating. Series harmonic filters lack upgradeability, monitoring and redundancy, meaning that if the filter fails, the drive fails.

The last series-passive solution is multi-pulse. Multi-pulse is a multi-winding transformer with phase shift in the windings. Because every secondary winding has its own rectifier, an 18-pulse configuration can target and effectively cancel out the 18th, 19th, 35th and 37th harmonics.

The downside of using multi-pulse is that it's very sensitive to voltage unbalance. On an 18-pulse drive under 50% load, when the unbalance is increased from 0% to 3% the current THD increases from 10% to 35%. At less than 100% load, the current THD doubles from 8% to 16%. When using multi-pulse, consideration needs to be given to planning the drive system and deployment as units are often large, heavy and difficult to retrofit.

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