There's a Disturbance in the Force

Posted by  Mark Maher 25-Aug-2017 12:08:04

mark-maher
Mark is our Technical Manager based at the Drives Core Competence Centre in Dudley

Harmonic DistortionAs the volume of electronic equipment in use continues to grow across the commercial and industrial sector, the issue of power quality is becoming ever more pressing.

The term ‘harmonic distortion’ is confusing to many, yet its principles can be made simple when you start with the basics. Commence ‘Harmonics 101’: The electricity that is supplied via the mains has a continuously changing voltage. The voltage has a sinusoidal waveform, oscillating 50 times per second (AKA 50Hz Alternating Current).

When you consider a simple electrical device, such as a kettle, whilst in use its heating element draws electrical current, in the same smooth (sinusoidal) waveform of the mains voltage. This is known as a ‘linear load’, because the current drawn is exactly proportional to the voltage that is applied. There is no distortion from a linear load.

Electronic devices do not draw current in the same way. Instead, they often draw current only at the peaks of the voltage waveform. Instead of a smooth (sinusoidal) current draw, it is taken in a discontinuous way – in sudden lumps during each sine wave. The sudden surge of current will tend to pull down the voltage at that instant – with the effect of distorting the sinusoidal voltage waveform.

The term ‘Harmonic Distortion’ is the name given to the rather technical (and complicated mathematical) description of a distorted sine wave. It refers to the principle that any periodic wave, of any shape, is in fact, made up of several sinusoidal waves, of differing frequencies. Very often the frequencies are exact multiples of each other – these are called Harmonics. The metric of ‘Total Harmonic Distortion’ highlights what proportion of a wave is not purely sinusoidal i.e. – how bad it is.

Voltage – this is what is presented through the national grid. We need it to be a clean and accurate 50Hz sine wave as that is what most equipment is designed to operate from – and any deviations are most likely going to cause trouble.

Current – this is what equipment draws from the national grid. The more current that is drawn ‘discontinuously’, the harder it is to maintain the clean 50Hz voltage required.  As a consequence, limits are now imposed to restrict how much ‘distorted current’ that can be drawn.

So, it is now clear that there are two types of (Total) Harmonic Distortion – THDi for the amount of distorted current that a device will draw, and THDv for how distorted the voltage waveform is. It is important to note here that THDv is caused by THDi.

It is common (but not desirable) for THDi to be quite high, but THDv should always be low.  The correlation between the amount of THDi, and how it effects THDv requires a lot of detailed knowledge about your electrical infrastructure, something most people cannot answer. It’s therefore very tempting to invest in equipment with the lowest levels of Harmonic Distortion.

Balancing Energy Consumption, Costs and Harmonic Distortion

When you consider the sheer volume of mains powered electronic equipment within a business, it is easy to see the potential scale of the problem. One of the greatest culprits of harmonic distortion is in fact the variable speed drive. If left unchecked, the distorted voltage can increase stresses in all electrical equipment. The very nature of Harmonic Distortion effectively means that our 50Hz rated equipment, is also being supplied at much higher frequencies. This causes several problems, most significantly excess heat in motors, transformers, and power factor correction equipment to name a few. This in turn results in equipment failure and overarching damage to the power system, which is an added cost burden to business and potentially even a fire risk.

Amidst businesses the problem is a major concern and so to for the distribution and transmission networks who are most frequently left at the mercy of distortions within the power network – making it incredibly hard in terms of supply and demand issues. One key problem is that the harmonics on one power line may differ minute-to-minute, hour-by-hour as the equipment load changes. 

As a means of targeting this growing issue, international standards have been set by the IEEE and more specifically the Energy Networks Association (ENA) in the UK. As a result these factors are considered as part of the design process of an electrical installation, ensuring there is minimal disruption to everyday applications.

Unfortunately in industrial applications however, much of the equipment responsible for harmonic distortion is the same equipment that businesses have invested to help support greater energy efficiencies across its operations – namely variable speed drives.

When you consider that the effectiveness of equipment is dependent on the quality of power entering into the system, any disturbance in the electrical power supply by any electrical device, from a mobile phone charger or lighting systems right through to a VSD, will affect the process. As the latter are often high powered, they can cause quite significant voltage fluctuations possibly pulling around 800 Amps. With so 

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many VSDs now in use throughout the marketplace it is in fact causing a big issue for the energy supply companies – causing restrictions that mean manufacturers must comply with a 30 per cent restriction on drive equipment.

Furthermore electrical regulations say that VSDs have to be designed to operate with minimal harmonic distortion. As a general rule under the IEEE-519 standards, drive equipment must represent more than 30 per cent of the building total load before harmonic mitigation measures are put into place.

For many companies the solution has often been to try to filter out the distortion after it happens, using passive filters that burn off the distortion or active filters to counter them, but this often results in higher energy consumption. In fact depending on the level of mitigation required, the use of harmonic filters can increase the total cost of the equipment dramatically.

A Cost Effective Solution

One of the latest innovations is to limit distortion at source, rather than minimise the effects of it once it happens - helping to make the process more efficient and reducing the overarching cost of the operation.  

Most drive equipment is reliant on a DC-Bus within the drive which comprises a bank of capacitors which store rectified (DC) energy for use by the switching power electronics, and electric motor. The way the capacitors are charged up within the inverter, is directly responsible for the harmonic distortion caused by a VSD.

To help resolve this issue, a new solution has been brought to market, that has changed the DC-bus technology in its latest variable speed drive,  which means that it doesn’t generate as much distortion in the first place –  no expensive, and efficiency sapping filters are required. This has also addressed a common reliability issue, as the electrolytic capacitors which are usually used in the DC bus, have a limited lifespan.

The new solution is simple to use yet feature packed, for the efficient control of fan and pumping applications. The new low harmonic DC bus technology also brings the additional benefit of a higher power factor, and has even improved efficiency. Available in an IP66, or IP55 enclosure, it’s also easy to install in plant rooms – or where there is limited space for a conventional electrical panel.

In terms of harmonic distortion, the manufacturers and the users of electrical equipment each have a role to play in ensuring standards are met which in turn will help make their own processes more efficient. For some time VSDs have been seen as the saviour of the manufacturing world in terms of efficiency yet, without careful consideration of harmonics and ultimately distortion, businesses are ultimately flogging a dead horse. 

For more information on ERIKS please visit: www.eriks.co.uk or contact your usual ERIKS service centre.

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Topics: News, Efficiency, TCO, Power Transmission

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