December 16, 2015

Don’t let harmonics get you down

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~ Top tips for choosing the right harmonic filter ~

How to choose the right harmonic filter
In 1976, it was discovered that the bacteria causing Legionnaires disease, an atypical strain of pneumonia, 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. Here, John Mitchell, global business development manager of supply, repair and field service specialist, CP Automation, shares his top tips for companies that want to commission or replace harmonic filters.

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 tricky.

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 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’s 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%.

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's going to run. If unsure, you should always refer to an expert.

Space and current rating requirements
In environments where space is an issue, such as commercial buildings in urban spaces, data centres, the mining industry or a plant floor, 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 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.

No heat excess
Another memorable thing about 1976 was the particularly hot summer that year. Harmonic currents can induce additional heating in generators, which can have a negative effect on the entire system. In the long run, harmonics can also lead to heating of busbars, cables and equipment, cause thermal damage to induction motors and generators and thermal tripping of safety devices, like fuses or sensors in breakers.

In this already sizzling environment, any additional source of heat needs to be thought through. Luckily, some active filters, like the Comsys ADF series, produce up to a third less heat than others due to their switching techniques of the insulated-gate bipolar transistors (IGBTs). Equally, the REVCON RHF Series of passive filters offer up to 99.5% efficiency.

When comparing a filter, be it active or passive, you should look closely at the total cost of ownership for the end user. After all heat loss is a direct cost in energy bills. This is particularly important on the shop floor, in marine and offshore applications or any other environments where space and heat is particularly precious.

Intuitive software
To make users’ lives easier, active harmonic filters come with intuitive software and remote data access. This means authorised users can access real time information and see live harmonics data by using a PC, laptop, tablet or smartphone with internet access. Most modern harmonic filters have web based tools that require no special software installation or licences.

Remote data access means more flexibility and greater peace of mind for engineers and facilities managers, who no longer have to be on site to resolve an issue. If process variables go beyond set parameters and alarms go off, the device immediately notifies the person or team responsible and they can access live information. This should help them make informed decisions, regardless of where they are in the world.

Holistic approach
Before commissioning a harmonic filter for your application, it’s important to assess the entire system, calculate the harmonics and size the right solution for your specific set up. It is not enough to look at one troublesome application individually; instead, you need to look at the plant or entire operation as a whole. Often what looks like the problem can actually be an effect rather than a cause.

Companies should identify and understand all the components installed on site when it comes to both linear and non-linear loads. They should also be aware of the transformer size and the rated short-circuit breaking current. Only after understanding the system in its entirety, can a company make an informed decision on what type of harmonic filter it needs, as well as what capacity and additional features the filter should have.

CP Automation recommends performing a survey of the plant and capturing as much information as possible over several days. After this initial analysis, we can recommend the most appropriate product and install it without significant disruptions.

After the harmonic filter has been live for a several days, another survey should be performed to check if all problems have been resolved. This ensures the product is appropriate and it gives companies real peace of mind.

The increasing levels of harmonic currents in industrial and commercial applications are certainly an unintended consequence of rapid technology uptake. Luckily, like the Legionnaires disease bacteria problem, the solution is simple, sustainable and inexpensive. Moreover, if you’re unsure of what harmonic filter your system needs, help is never too far away.

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