SIGNAL CONDITIONING FEATURE
CHOOSING THE RIGHT HARMONIC FILTER I
Here, John Mitchell, global business development manager of CP Automation, shares his top tips for companies that want to commission or replace harmonic filters
n 1976, it was discovered that the bacteria causing Legionnaires disease had always been present in water, but it was the precise temperature of the water in heating, ventilation and air conditioning systems that facilitated the bacteria’s maximum reproduction levels. This is just one example of the unintended consequences of technology. A similar and more recent story comes
from the world of industry and features the growing problem of harmonic currents and utility level voltage distortion, as a result an increasing number of non-linear loads in industrial and commercial environments.
WHY ARE HARMONICS A PROBLEM? Non-linear loads include common office equipment like computers, printers and battery chargers, as well as industrial equipment like fluorescent lighting, variable speed drives (VSDs) and transformers. A load is considered non- linear if its impedance changes with the applied voltage. This change means the current drawn by the non-linear load will not be sinusoidal even when it is connected to a sinusoidal voltage. Non- sinusoidal loads contain harmonic currents that interact with the impedance of the power distribution system to create voltage distortion and power quality problems. Voltage distortion can cause significant
damage to plant or building equipment, as well as the mains power supply. Common symptoms of high harmonics levels include motor vibration, voltage notching, electromagnetic interference and overheating, all of which are harmful for equipment and result in heat loss and increased maintenance and energy costs. Harmonic filters do what they say on the
tin: they remove harmonics and correct the phase of the fundamental currents, converting non-linear loads into linear ones. So far, so good; however, when it comes to commissioning the right harmonic filter for your specific application, things can get complicated.
ACTIVE VERSUS PASSIVE The first thing you should decide is whether you need a passive or an active harmonic filter. The traditional option is an electro-mechanical or semiconductor controlled passive filter, used to minimise power quality problems in the network. These filters operate mainly on a fixed basis and are tuned to a harmonic
INSTRUMENTATION | MARCH 2018
order close to the order to be eliminated. Often new equipment is specified to
meet a THID%, but the problem for many plants is they do not know how bad their site is already. It is almost like fixing a sticky plaster to a deep wound. Instead, companies should look at what is physically and commercially viable in the long term. When making a decision, you can also
consider a mixed solution. By fitting passive filters on many applications, you should be able to add a smaller active solution, which can save a lot of costs depending on the plant. One drawback of passive filters is that they are most efficient when the load is operating above 80 per cent. On the other hand, active harmonic
filters continuously monitor the network and inject exactly the right amount of compensation current when it is needed. The filter compensates the harmonic current or voltage drawn by each load. This allows current waveform to be restored instantaneously and lowers current consumption. For installations in which current load changes constantly, active harmonic filters work best. They can filter harmonics over a wide range of frequencies and adapt to any type of load. Regardless of what type of harmonic
filter you decide to use, make sure it has the relevant UL certifications for the environment in which it is going to run. If unsure, you should always refer to an expert.
John Mitchell, global business development manager of CP Automation
SPACE AND CURRENT RATING REQUIREMENTS In environments where space is an issue it is important to consider the best fit for the purpose in choosing a solution. Modularity can play a key role in your decision, as it refers to the degree to which a system’s components can be separated and recombined. Apart from space savings, modularity also means cost savings because it makes installation and maintenance more straightforward, reducing the need for many connectors and cables. With active harmonic filters, if one of the modules has a fault, a technician can shut down and repair or replace it, while the rest of the filter continues to run in reduction mode. This way, harmonic levels are kept under control even during maintenance. One final benefit of modular
The first thing you should decide is whether you need a passive or an active harmonic fitler
harmonic filters is that their capacity can be increased or decreased depending on how the plant’s systems change, which can save significant costs in the long run.
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