The trouble with harmonics

Controls, Drives & Automation: Tell us about your professional background and your role at Quantum controls.

Jorgo Kristo: My first exposure to Electrical Engineering was at school when I was completing my NVQ Level 3. From the first year I really liked the field of Electrical Engineering and it sparked to me a passion and desire to learn even more. The desire for more advance knowledge has led me to pursue a BSc in Electrical Engineering; Power Quality in particular has caught my attention, as it is a field that is not so widely known but is considered one of the most important fields in electrical engineering due to the high increase of non-linear loads.

Power Quality has many aspects of which a critical one is harmonics. Indeed, working with VSDs, which are classified as non-linear loads or harmonic producing loads, it was obvious that they affect power quality.

Following a postgraduate degree in Electrical Power Engineering, I joined Quantum Controls.

CDA: What are harmonics and how are they caused?

JK: Electrical power systems operate at a certain voltage level set by the regulatory authorities, and constant frequency level regardless of the voltage level. The frequency at which the electricity is generated and maintained is 50Hz in the UK. The voltage waveform generated by generators can be considered as pure sinusoidal waveform. However, due to the increasing number of the non-linear loads (the current they draw from the network is not proportional to the applied voltage and Ohms’ law cannot describe the relation between the voltage and the current) connected to the electrical network, the waveform now is distorted containing harmonics.

Harmonics are voltage and current periodic waveforms, whose frequency is an integer multiple of the fundamental/system frequency, 50Hz as mentioned above. Harmonic is defined as the presence of distortion in voltage and current waveforms. A harmonic distortion is the steady state deviation of current or voltage waveform from its ideal sinusoidal waveshape of the electrical system frequency.

Harmonics are generated by non-linear loads, such as VSDs, power converters, switch mode power supplies, PCs, LEDs, to name a few; their design and operational characteristics generate harmonics. Since they are using semiconductor devices (electronic switches) which can change state (ON/OFF) very rapidly, when they are operating at very high frequencies during commutation between the phases and because they draw partially power from the network only during a fraction of the power frequency period in way of pulses, this generates harmonic currents, which lead to harmonic voltage – which is the voltage drop of the harmonic current with the system impedance.

CDA: What are the problems caused by harmonics?

JK: Harmonic distortion is an undesirable event in the electrical power system. Harmonics are claimed to cause several issues in the electrical power systems and their components:

- Increase the RMS value of the line current flowing into the cables. The cables not only carry the fundamental current but also the current harmonics which will require increase in the size of the cable and increase the losses.

- Thermal effects on the transformers. The RMS value of the current will be higher including harmonics. Harmonic losses in terms of heat dissipation on the windings; this extra heat can have adverse effects in reducing the lifespan of the transformer insulation that eventually will lead to transformer failure.

- Neutral conductor overloading when supplying single-phase non-linear loads.

- Thermal effects on rotating machines such as motors that can cause premature failure.

- Torsional pairs including equipment fatigue, increased vibration, bearing wear out, etc.

- Undesired operation of fuses and circuit breakers.

- Undesired effects on capacitor banks used for power factor correction purposes.

- Poor power factor; extra care should be given when performing power factor correction when harmonics are present.

- Excessive power losses and reduced energy efficiency due to skin effect and due to RMS current being higher including harmonics.

- Metering equipment not being able to measure accurately due to harmonic disturbances.

- Under operation or even non-operation of other electronic equipment that are sensitive to voltage disturbances.

CDA: Are harmonics a growing issue and, if so, why?

JK: There has been a big increase of non-linear loads connected to the electrical power network. The use and operation of these loads causes issues in the electrical power systems they are fed from. The common theme is that they all produce harmonics.

The challenge for the electrical regulators and system designers is to design the new future systems and/or to incorporate/adapt the existing one into an environment with high levels of harmonics. The sources of harmonics in the future are expected to be diverse and numerous. The problem grows with the increased use of sensitive electronics and automated processes in the industrial sector, extensive use of VSDs for process controls and optimisation, PCs,  and digital communications. In the utility sphere, incorporating distributed energy resources using renewable energy generation equipment and energy storage devices, where power converters are used to interface and condition power between generation and consumers. Power quality, including harmonics, is one of the critical aspects that must be maintained and regulated to avoid any disturbances and unwanted events.

CDA: How can harmonics be compensated for or reduced?

JK: There are several techniques and elimination methods to tackle harmonics. In general context, we can refer to harmonic filters, such as passive and active harmonic filters and the combination of the two which is called hybrid filter. Also, harmonic cancellation can be achieved via the use of multi-pulse converter configurations in some cases. Series line reactors can be used in series with the harmonic-producing load. There are some other methods that are harder to implement; for example, active harmonic filters can mitigate harmonic currents, improve power factor, and improve current unbalancing.

CDA: Can you tell us about a specific case where Quantum Controls has successfully solved problems caused by harmonics?

JK: Quantum was appointed to monitor a site for harmonic issues when a VSD rated at 315kW was causing issues in neighbouring equipment. We analysed the data and found that the THD(V) was above the recommended limits. The power factor was low and the current distortion very high. For this case we suggested to fit a series passive filter. Since the installation of the filter, the harmonic levels have improved and the issues resolved.

CDA: Are there any other issues to do with harmonics you would like to bring to the attention of the readers?

We would recommend monitoring the supply to their sites so they can have a clear picture of the health and operational performance of their electrical systems. To ensure they are within the recommended harmonic levels and standards and that their systems is operating efficiently. It can deteriorate the equipment, increase the losses, reduce the performance, and shorten the lifespan of any electrical components.

There is the illusion that poor power factor is purely due to inductive reactive power and that a capacitive reactive compensation (capacitor banks) will solve the problem, reduce the losses, and improve the power factor to higher values. But the fact is that by installing capacitor banks without considering the harmonics, not only the power factor is not getting improved, but also there is a chance that a resonance condition could occur leading to greater issues and eventually fail the capacitor banks and/or blow the fuses.

If you are like most Engineers I speak to, you are frustrated at the lack of knowledge on the topic of Harmonics, misinformation provided by ‘technical sales people’ and quite possibly the support you're getting to fix the issue, then you should consider a low cost but comprehensive harmonic survey from Quantum Controls which will:

1. Undertake a comprehensive site survey, which will not in any way interfere with production

2. Identify if the 11 key problems caused by Harmonics are in line with current regulations on your own sites Power System

3.  Issue a detailed, yet easy to understand report, that will show you your exact current harmonic footprint and detail any recommendations that may be needed to rectify any of the 11 possible problems and bring your site up to full legal compliance.

Understanding Harmonics does not have to be a struggle. Book your site survey via the website below.

Jorgo Kristo is senior electrical engineer-service team leader at Quantum Controls

https://www.quantum-controls.co.uk/services/harmonic-survey/