One of the most frustrating electrical problems to solve is when equipment behaves erratically or repeatedly fails for no apparent reason. These issues can be the hardest to troubleshoot, especially on a large site with a lot of electrical equipment. Often when the equipment is tested in a separate location it works perfectly, but when placed on site it just malfunctions.

The root cause of many such problems is power quality. Sometimes it will be as simple as a voltage dip or swell that causes it. However if the problem is persistent and repetitive then it could be caused by harmonics. The easiest way to find out the cause is to use a power quality analyzer.

What are harmonics?

sinusiodal waveform

Photo courtesy of Creativity103(CC Attribution)

AC power has a voltage and current that alternates at a fixed rate (frequency) of 50 or 60 cycles per second (Hz). The voltage and current should go up and down in a pattern called a sine wave, which has a well-defined, very specific curve. However sometimes the voltage and current deviate from this perfect curve, giving a distorted waveform. Whenever the AC curve is being distorted into a different looking curve, there will be harmonics present. Measuring harmonics (usually done with a power quality analyzer) is a process whereby the curve is analysed down into many smaller “perfect sine wave” curves with higher frequencies, which add up to make the overall distorted curve. Each of these smaller perfect sine wave curves has a different frequency (cycles per second). Normally with a repeating distortion pattern, the frequencies of these components will be odd-numbered multiples of the base frequency (which is 50 or 60 Hz). These are called harmonics. So the base frequency (50 or 60 Hz) is the first harmonic, and the 3rd harmonic would be 3 times the base frequency, making it 150Hz for a 50Hz system. The 5th harmonic would then be 250Hz etc.

How are harmonics caused?

The main reasons for harmonics to appear in power systems are from large numbers of technology devices such as computers, that use “switch mode” power supplies. Harmonics can also be caused by variable speed drives, electronic ballasts (for fluorescent / sodium / mercury lights), UPS units for computers, and many other devices. Harmonics appear due to a distorted current waveform being drawn by the equipment. This happens because modern devices with electronic power supplies tend to draw their current in short “spikes”, rather than smooth sine waves. These spikes of current being drawn by lots of devices can add up across the power system to make the voltage also become distorted in the site. In severe cases the voltage distortion can be so bad that the mains utility cannot correct it, and hence the voltage waveform becomes distorted for all neighbouring properties using the same branch circuit. This may result in the electricity utility approaching the customer to demand that they correct the problem.

What problems do harmonics cause?

Harmonics can cause excess heat in the power distribution network

Photo courtesy of vaxomatic(CC Attribution)

Harmonics in the current can cause harmonics in the mains voltage. Since the same supply voltage is ultimately provided to all equipment on the site, all equipment would then receive power with a distorted voltage waveform. This can make the equipment generate more waste heat, and can make computer based equipment malfunction. The harmonics can also be induced into communications wiring, causing noisy phone lines or lost data. Finally irregular harmonics can also cause lights and screens to flicker. With 3 phase equipment, harmonics can also cause a large amount of current to flow in the neutral wire, whereas without the harmonics the neutral current would normally be almost zero. This means that the building wiring needs to include larger wires or busbars for the neutral. Excessive harmonics can also cause problems for power utilities, whereby extra current flows in the neutral wire and also inside their transformers. Extra currents cause extra heat to be generated, and this can ultimately cause transformers to fail.

Using a Power Quality Analyzer to measure harmonics

LDW-6095K 3 Phase Power Analyzer
To find out how bad a site’s harmonics are, you need a Power Quality Analyser that calculates and displays the harmonics and THD. As the harmonics can occur on both the voltage and current differently, it is important that the meter can measure both independently. The meter will normally show a figure for THD, which is Total Harmonic Distortion. This gives you a quick idea of how distorted the waveform is overall, allowing you to compare different circuits of the site. To diagnose a harmonics problem you will need to use the power analyser to measure harmonics over a period of time, on each of the branch circuits. This will allow you to identify which circuits are causing the most harmonic problems. Once you know which branch circuits are to blame, you can then physically check what equipment is run on those circuits, and if possible do some tests by switching off the suspect equipment and measuring harmonics again.

Correcting the harmonics problem

To correct the harmonics issue, you will need to decide on the best course of action based on the particular site, the symptoms, and the equipment that is causing it. Here are a few things that are commonly done to deal with problems from harmonics:
  • Upgrade equipment to low harmonic models (eg. electronic ballasts, computer power supplies)
  • Install harmonic filters on the circuits in question. This could be a generic power filter, or a filter specifically tuned to match the non linear load.
  • Wire a separate branch circuit to the equipment causing the problem, using oversized cables to minimise voltage drops, and keeping cables clear of any communication wiring
  • Install a harmonic cancelling transformer, or phase shifting transformer
  • In three phase systems, install oversized neutral conductors to cope with the extra current from harmonics