Harmonic Analysis
What Is A Harmonic Analysis?
Harmonic Analysis is the procedure of measuring and studying the characteristics of an electrical power system to identify, predict and mitigate issues associated with high levels of harmonics.
It involves installing specialist power analysers, for a minimum of a week and a maximum sample interval of 10 minutes, to monitor the load and collect vital information on the day-to-day running of an industrial or commercial building.
In addition to harmonic analysis, the advanced analysis equipment PureSine employ also records other important power quality indicators.
The information collected is studied and presented in a technical report detailing findings that include but are not limited to:
- The total harmonic distortion (THD) levels.
- The individual harmonic levels and whether they exceed IEC or EREC G5/5 design limits (depending on the application).
- Concerns about the general power quality levels.
- Details of any short term issues identified as a by-product of the analysis (transients, voltage dips etc.)
- Most importantly, recommendations for improvements.
What are Harmonics?
When we think of ac voltages and currents, we normally visualise perfect sine waves with a frequency of 50 Hz. Unfortunately, given the rapid increase in IT and process control equipment, this is now rarely the case.
Figure 1 shows the three-phase voltage and current waveforms captured on a major shopping centre incoming LV supply. It is quite apparent that the current waveform is far removed from the ideal sinewave, being noticeably distorted.
A waveform such as that shown in Figure 1 is quite typical for large commercial / industrial sites and it is the presence of harmonics that results in such distortion.
Figure 1 – Voltage and current waveforms captured at major shopping centre
Harmonics are frequencies that are superimposed upon the 50 Hz signal that are an integer multiple of 50 Hz; i.e. the 3rd harmonic is 150 Hz, the 5th harmonic is 250 Hz and so on.
These harmonics are not just a mathematical representation of a distorted waveform, they are real and their effects are predictable and measurable. Figure 2 shows a breakdown of the load current harmonics, which shows that the 5th and 7th harmonics are predominant, which is very typical for this type of load.
Figure 2 – Harmonic profile of shopping centre load current
These harmonic currents are produced by the load and flow back to the supply, passing through the impedance of the cables, transformer etc. as they do so. As these harmonic currents flow through the system, they produce harmonic voltage drops which in turn distort the supply voltage. The voltage waveform in Figure 1 exhibits such distortion and the voltage harmonic distortion profile shown in Figure 3 shows a harmonic profile very similar to that of the current. This clearly illustrates the load current passing through the system impedance and deforming the voltage.
Figure 3 – Harmonic profile of shopping centre voltage
What Causes Harmonics?
Harmonics are produced by loads that draw a current that is not directly proportional to the voltage, often referred to as non-linear load. The type of loads that draw such current are generally loads that incorporate some form of rectification and include the following: –
- UPSs
- Inverter drives
- IT equipment (the power supplies therein)
- DC drives
What Issues Can Harmonics Cause?
Harmonics can cause a multitude of different issues, the most common of which are detailed below: –
- Damage to power factor correction due to harmonic absorption of capacitors or even harmonic magnification due to harmonic resonance
- Overheating of ac motors and even mechanical damage due to pulsating torques on motor shafts
- Interference induced into comms / IT cables if not correctly segregated from power cables carrying distorted current, resulting in noise on telephony systems and possible data issues
- Premature failure of light fittings
- Premature failure of dc power supplies
Are There Harmonic Limits?
Yes – in terms of actual limits, the 61000 series of IEC standards specify harmonic current injection limits such as 61000-3-4. Generally, however, it is a better idea to try and comply with the Engineering Recommendation G5/5 to ensure a suitable environment for other equipment to operate in.
How We Can Help
With over 30 years of experience in the field of harmonics, including doctoral research in power system harmonic resonance, PureSine can offer a comprehensive harmonic diagnostic, assessment and solution package.
In addition to harmonic analysis carried out to the requirements of 61000-4-13 (covering both the 61000 series and G5/5 analysis requirements), PureSine can carry out full G5/5 assessments, covering the connection of new load. This assessment includes capturing harmonic data for the pre-existing load, gathering data on the additional load and modelling of the harmonic levels resulting from the connection of the new load. Harmonic analysis can be carried out post load installation to ensure the accuracy of the assessment and to verify compliance with G5/5.
Should harmonic filtration prove necessary, PureSine can specify and implement active harmonic filtration schemes to minimise load current harmonics. Again, post installation harmonic analysis can be carried out to verify the efficacy of the harmonic filters proposed.
If a harmonic resonance is diagnosed, it’s likely to be a result of interaction between system harmonics and the capacitors within a ‘standard’ or ‘plain’ power factor correction unit. In this case, PureSine can design and install a detuned power factor correction system that will eliminate any harmonic base level magnification or even reduce base harmonic levels.
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