Harmonics and the hidden cost of poor Power Quality

In an ideal world, the AC voltage delivered by your utility would be a pure sine wave of 50Hz or 60Hz frequency, if you were to view it on an oscilloscope it would look like the waveform in Figure 1 below.  

We do not live in an ideal world so in reality; this ideal wave gets distorted as it makes its way from the point of generation across the power transmission network where it arrives at your premises. Measuring this distortion is the essence of power quality. In addition to the distortion the waveform suffers as it transits the power grid, distortion can also result from equipment within your building. 

There are many facets to power quality, but in this blog, I am going to focus on harmonics. What they are, how they commonly originate, the problems they can cause and how ResourceKraft can help you detect, analyse and mitigate them… 

What are Harmonics anyway…. 

Harmonics are high-frequency distortions of the nominal voltage and current AC waveforms (Figure 2). Of primary concern are the voltage harmonics as voltage is common to all users of the electrical supply, current harmonics which are drawn by and restricted to a local installation (user) are what creates these voltage harmonics. Because of the interconnected nature of the electrical grid, these harmonics propagate to and can potentially affect other connected installations. The only way to limit voltage harmonics is to minimize current harmonics.

There are three very distinct types of harmonics. Positive harmonics that rotate with the same sequence as the fundamental such as the 7^th harmonic. Negative harmonics that rotate with the opposite sequence to that of the fundamental such as the 5^th harmonic and Triplen harmonics which do not rotate at all because they are in phase with each other and so are referred to as ‘zero sequence’. 

Examples of this are the 3^rd and 9^th harmonics as shown below in Table 1. 

The ratio of harmonic content to the fundamental waveform (the ideal “pure” sine waveform referred to in Figure 1) is defined as Total Harmonic Distortion (THD). THD is expressed as a percentage of the fundamental, the higher that percentage the more distorted the waveform. 

Total Demand Distortion (TDD) is a moving average of THD but uses a much longer averaging time, up to 30 mins and is based on a systems rated current. 

This is measured at the point of common coupling between the grid and the installation with limits defined in the IEEE 519-1992 Standard. 

Both distortion values are monitored, reported & accessible remotely using ResourceKraft Resolve Power Quality Monitoring. 

Where do Harmonics come from…. 

Harmonics are often generated by the operation of certain types of equipment commonly found in most commercial or industrial businesses. Examples include variable speed drives, switched-mode power supplies, equipment using inverters or rectifiers and in general anything that chops or splits the fundamental frequency sine wave. Even fluorescent lighting, battery chargers, and computers can contribute to harmonics on an electrical system. 

Problems caused by Harmonics…. 

Depending on the nature and extent of the harmonics many issues can arise from them. For instance, the presence of negative sequence harmonics in an electric motor can oppose the natural rotation of the fundamental frequency (and thus opposes the motor rotation) meaning the motor now needs to work harder to achieve the same torque. The additional energy needed to overcome the opposing force is converted into heat in the motor, which may eventually cause winding damage through overheating and potentially early motor failure. 

Electronic controls and automated processes are often dependent on the zero-crossing for precise timing. Harmonics can significantly alter or cause multiples of this zero-crossing leading to erratic functioning of this equipment. 

Localized current harmonics have the potential to generate system-wide voltage harmonics and these fluctuating voltages can be radiated from the powerlines acting as an antenna. The resulting emitted radio waves can interfere with telecommunications.  

Older watt-hour meters may return incorrect readings due to the additional torque placed on the induction disc by harmonics, these devices are calibrated for accurate operation on the fundamental frequency only. 

Harmonics currents whilst of no benefit on an electrical system still need to be conducted through it. The energy in these harmonics must be accounted for when designing an electrical system or it may lead to overheating of the conductors.

See below as an example of where the design for just carrying the fundamental frequency current can lead to undersized conductors. 

How can Resourcekraft Resolve help…. 

Resolve Power Quality Monitoring supports remote monitoring and configuration. This capability allows monitoring of power quality without the need to repeatedly visit the site. It can monitor both AC and DC power systems. Taking 512 samples per cycle it has the power to analyze and measure harmonics in real-time and records emissions from 2kHz to 150kHz.  

We recently worked with an electrical engineering contractor to help them identify the root cause of equipment reliability issues they were encountering at a customer’s industrial site. Some interesting Resolve data captures from this site are shown below. We first identified issues in relation to flicker and harmonics on this site (Tuesday, July 9^th) and following remedial works by the contractor we proved the effectiveness of the solutions they implemented to mitigate the identified power quality issues (Tuesday, August 13^th). 

As can be seen from the above, the resolve power quality system allows you to examine in minute detail the quality of the power on your premises. The cloud-based system allows archiving and historic analysis so power quality can be correlated with production or equipment issues. The “always-on” monitoring with alerts enables you to take immediate action when a problem is discovered.