What do I need to Know About ‘Low Harmonic Drives’?

The Truth You NEED to Know About ‘Low Harmonic Drives’

What are Harmonics and Harmonic Distortion?

In electrical networks, harmonics are a bit like an unwanted guest at a party. If you connect a device to an AC electrical supply and it draws non-sinusoidal current, it will create some form of harmonics. Typically these are devices where the connected AC supply is initially converted into an intermediate dc voltage. This can include equipment such as computers (or anything with a switch-mode power supply), LED lighting and variable speed drives. The general term for a device that draws non-sinusoidal current is a ‘non-linear load’. Harmonic distortion is characterised by its repetitive and continuous deformation of the original sinusoidal wave form.

Why Do I Need to Manage Harmonics?

In the UK, engineers are compelled to manage harmonics on their sites due to stringent legislation and regulatory frameworks aimed at safeguarding the stability and reliability of the electrical grid. Harmonics not only lead to inefficient power consumption and increased energy costs but also have the potential to disrupt the operation of sensitive equipment and, more critically, compromise the overall quality of power supplied by the grid. In response to these concerns, UK engineers are mandated to comply with regulations like the Electricity Safety, Quality, and Continuity Regulations (ESQCR) and the Grid Code, which establish strict limits on harmonic emissions to ensure the integrity of the national grid. Management of harmonics is therefore an essential engineering practice and a legal requirement for network operators in the UK to uphold the reliability of the electrical infrastructure and protect the interests of both consumers and businesses. Huge fines can be given to consumers who don’t adhere to the limits on harmonic emissions imposed on them by their supplier and in the worst case scenarios companies can have power consumption limitations imposed on them by the utility provider or even their electrical supply withdrawn. It’s no wonder that harmonics can cause engineers some anxiety. In order for a network operator to be satisfied that a consumer is meeting the required limits for harmonics the consumer is required to meet the limits set out by the Energy Networks Association Engineering Recommendations G5/5 (note that it is also possible for the operator to reference the limits set out by an older document, G5/4-1 instead, even though this was superseded in June 2020). One significant change between G5/4-1 and G5/5 is the increased range over which limits are applied. G5/4-1 was only concerned with harmonic orders up to the 50th whereas this is extended up to the 100th order in G5/5. It’s important to be aware though, just because you have non-linear loads such as variable speed drives connected in your installation it doesn’t necessarily follow that the magnitude of the resulting harmonics is a real cause for concern. It’s important to evaluate the system carefully to determine if any mitigating action is actually needed – we will discuss this further later.

What is a ‘Low Harmonic Drive’?

To explain what a ‘low harmonic drive’ is, first we need to consider how a typical / standard variable speed drive is constructed, if this is something you already know then please bear with us.
• A standard variable speed drive (VSD) will typically contain six diodes which convert AC into DC. These diodes result in six pulses of current being drawn from the supply (hence why these drives are also known as ‘six pulse drives’). Anything which does this, will naturally cause harmonic distortion due to the pulsing nature of the current drawn from the supply
• A ‘low harmonic drive’ typically comprises two power sections that are constructed from Insulated Gate Bipolar Transistors (IGBTs). One to provide current to the motor and one to convert the AC supply current into dc current (this supply section is known as an ‘Active Front End / AFE’ and we generally refer to a low harmonic drive with an AFE as an ‘AFE drive’). This switches at much higher frequency than just six times per mains cycle as occurs in the standard drive and the resulting harmonic distortion is very low as a result

The Issues with ‘Low Harmonic Drives’

Many of our customers have been visited by salespeople, explaining that they need to replace all of their VSDs with these new ‘Low Harmonic Drives’. As a company we find this very concerning, as do Danfoss who are one of our Drive Partners. Please note, Danfoss offer ‘Low Harmonic AFE Drives as well as standard 6-pulse drives and offer a balanced view in what the best solution is to a customer’s application. We spoke at length to Country Manager UK & Ireland Alan Baird and Applications Manager, Stuart Bratton at Danfoss to get you the FACTS on these drives. Like Danfoss, our mission is to give you accurate information, so you can make an informed decision for yourself and your company.

  • AFE drives only mitigate lower order harmonics and are ineffective for anything above the 50th order
  • AFE drives only mitigate their own harmonics, they don’t do anything to reduce the harmonic current or voltage distortion created by other elements of the electrical system, or indeed any background distortion that is present on the supply already
  • AFE drives are less efficient than a standard VSD and so will cost you more to run over time (as well as being more expensive initially)
  •  AFE drives can cause issues when on the same network as a standard 6 pulse VSD
  •  AFE drives are the best choice for an application where the drive needs to regenerate braking power into the supply
  • The additional heat created by an AFE drive (due to its lower efficiency) also requires additional cooling which adds further operational costs
  • Harmonics should be measured or modelled before you decide if you need to mitigate them; you may find you don’t need to do anything to mitigate them
  • There is never a ‘one size fits all’ solution, so if you do have harmonics which need to be mitigated then you should explore all of the options available to you, other options include passive and active harmonic filters for example
  • Active Harmonic Filters are not the same as Active Front Ends – Active harmonic filters are highly efficient and are installed in PARALLEL with a standard drive (not in SERIES) which makes the overall solution more efficient than an AFE drive

We’re also getting reports of salespeople telling customers that if they weren’t to utilise ‘low harmonic drives’ they would need to install larger circuit breakers and cables if they used a standard VFD instead. This is not necessarily true. Whilst it is true that harmonic distortion will increase the overall RMS current, in the vast majority of cases it would be very unlikely that the cable or breakers would increase in size compared to a low harmonic drive installation.

Your Decision

Before looking at solutions to harmonics on an existing application you should do a harmonic survey. For new installations you should speak to your drive supplier and ask them to model the system to determine if any mitigation is likely to be needed. If harmonic mitigation is needed, then you should investigate the different solutions and pick the most viable one. Ultimately this is your decision, but we’ve made a summary to assist you below.
• All AC drives, including AFE drives, have side effects; they produce harmonics
• Too high levels of harmonic distortion reduce reliability, affect production quality, and increase operating costs
• Do not try to mitigate your harmonics without considering if this is required
• You should evaluate your full system before looking at harmonic mitigation, for accuracy this means completing a harmonic survey or accurately modelling the system
• There is no single solution / product that fits every scenario
• All means of harmonic mitigation will have a cost – if operating costs are important some solutions may be more attractive than others

If you need more help with harmonics, or you’re not sure you’ve been given the correct information then email us and we’ll be glad to assist you. Email us at [email protected]

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