Harmonics - fighting the enemy from within

The push for ‘greener’ electricity through more sophisticated electrical technologies has, in many cases, increased the negative consequences of harmonics on our electrical infrastructure. Electronic ballasts for lighting, switchmode power supplies for PCs and variable speed drives for motor controls are becoming ubiquitous in our homes and workplace. But while they’re improving our energy efficiency, they’re also making the electricity supply dirty.

According to Wikipedia, “Harmonics are electric voltages and currents that appear on the electric power system as a result of certain kinds of electric loads. Harmonic frequencies in the power grid are a frequent cause of power quality problems.” These are problems introduced onto the electrical system from within, not from the outside. The online encyclopaedia goes on to warn that harmonic frequencies in the power grid are a frequent cause of power quality problems. The good news is that they can be controlled, since they can be dealt with internally.

When it comes to supply protection, all too often the electrical industry immediately thinks about keeping out the electrical disturbances travelling along the grid and making their way into buildings and onto loads - things like voltage surges from lightning strikes or other external anomalies. But it seems that we have actually been polluting the electricity supply from within our own premises through the increased deployment of technologies that are intended to improve energy efficiency. So, while many of these things are succeeding in helping to reduce our energy consumption, they’re also causing major problems that are often not linked to the harmonics introduced onto the network by them.

In Australia’s conventional electricity supply, voltage varies sinusoidally at 50 Hz. When a linear electrical load is connected to the system, it draws a sinusoidal current at the same frequency as the voltage. When a non-linear load is connected to it, the load draws current that is not necessarily sinusoidal. This current waveform can become quite complex, depending on the type of load and its interaction with other components of the system. And that digression from a pure sine wave can have significantly negative effects on electrical equipment and operations within businesses and homes.

Wesley Stephenson, NHP Product Manager for Power Quality, states that this trend is a cause for concern and should be addressed sooner rather than later if we are to avert major operational problems for our clients and to truly optimise their energy efficiencies: “The community has had a singular focus on energy efficiency, but little thought has been given to the consequential issues, like the impact many of these initiatives have on the supply.

“In offices, energy-efficient fluorescent lighting uses electronic ballasts, PCs using switchmode power supplies are everywhere and heating, ventilation and air conditioning (HVAC) systems use variable speed drives (VSDs) for more precise climate control. And the situation is becoming the same in the modern home. As good as they are for energy-efficient operation, they all put out considerable harmonics which can be counterproductive if not corrected.”

The effects of harmonics

“Harmonics pollute the supply, but in most cases you cannot easily identify what problems are caused by them,” continues Stephenson. “Some of the more common manifestations of harmful harmonics include lighting flicker, shortened equipment life spans, erratic equipment behaviour, nuisance tripping of electronic circuit protection devices, errors in digital monitoring devices such as power meters and reduced power factor.”

One of the major effects that harmonics has on power systems is increasing the current in the system, especially third-order or triplen harmonics, which cause a sharp increase in the zero-sequence current, therefore increasing the current in the neutral conductor significantly.

Harmonics can also have adverse effects on electric motors. Electric motors experience hysteresis loss caused by eddy currents set up in the iron core of the motor, which are proportional to the frequency of the current. Since the harmonics are at higher frequencies, they produce more core loss in a motor than the power frequency would. This results in increased heating of the motor core, which (if excessive) can shorten the life of the motor.

Why are harmonics important?

Stephenson cautions that we shouldn’t be complacent about harmonics: “Don’t dismiss harmonics as just ‘noise’. Rather, treat it as something that needs to be eradicated.

“We receive numerous requests from customers for quotes on ‘harmonics mitigation systems’ that have been specified in tenders. But when customers receive our quotes, they are concerned by the significant cost associated with harmonic mitigation equipment. Unfortunately, they fail to see the value that either active or passive harmonic mitigation products can provide to their clients. Some of them elect to omit the harmonics mitigation system altogether from their quote, even though it is specified, believing it to be an unnecessary element in the electrical system.

“Contractors need to understand the benefits of reducing harmonics for the end user and make sure they communicate it to their clients, who may not understand the damaging effects of ‘dirty power’ caused by harmonics. If the application is mission critical, such as in a medical facility or a data centre, the up-front cost of a comprehensive harmonics mitigation system can seem high, but it is insignificant in comparison to the consequences of downtime caused by harmonics. In non-mission-critical applications, the cost of the mitigation system can be kept down by providing it at the areas likely to be affected by harmonics. If costs need to be contained, address the main polluters then reassess. This action may suffice in reducing harmonic content to adequate levels.”

Testing for harmonics

Testing for harmonics is a complex process and requires not only the right test equipment, but also an understanding of the nature of harmonics. According to Westek Electronics Managing Director John Thompson: “Power quality testing requires instrumentation that is capable of measuring harmonics, inter-harmonics, flicker as well as spikes and voltage sags and swells. The need for power quality testing isn’t confined to supply authorities and companies. It’s equally applicable to internal reticulation for commercial and industrial installations. Although specific instruments are available for flicker testing, the combination of this test with that of harmonics is far more cost effective, since flicker is affected by changes in harmonic spectra, inter-harmonics and impedance changes.

“High sampling speed is necessary in order to capture dynamic effects such as inrush current, changes in harmonic spectra because of load variations, inter-harmonics through leakage of VSD inverter signals via the DC link to the input converter stages and sub-harmonics caused by rotor slot deformation and rotor vibration.”

Solving harmonics

“There are a variety of solutions available to reduce the effects of harmonics, mostly by filtering the distortions,” Stephenson outlines. “These include line reactors, passive and active harmonic filters, plus active front-end variable speed drives for motor starting applications.

Passive harmonic filters are a widely used and simple solution, but over time may lose effectiveness as their components age. They may also become overwhelmed by harmonic sources throughout the network, so some consideration should be given to selecting the most suitable solution for the application.

“Harmonic mitigation is the objective of good electrical design,” continues Stephenson. “What is deployed must be appropriate to the environment. For example, in process plants where continuous operation is paramount, or in high-risk facilities such as mine sites and water treatment plants, consultants will typically specify that harmonics must be under a certain limit, typically voltage harmonics less than 8%. In some cases, this limit can be much lower.

“The main concern is voltage distortion because of its potentially damaging effects to loads. But current distortion is what should be mitigated because this is what actually causes voltage distortion. The way to mitigate harmonic currents is to monitor the harmonic currents being produced and insert a current waveform which will modify the harmonic current waveform back to a sinusoidal signal. An active filter provides the best remedy in most circumstances because it analyses what the disturbance actually looks like and injects the mitigation current waveform into the system to cancel out the disturbance, resulting in a pure current sine wave. This in turn yields a pure voltage sine wave and negates the harmonic disturbances and their potentially negative effects.

“To address harmonics on an existing site, a good start is to understand what you are dealing with before taking any remedial action. Arrange to have a full power quality analysis conducted at the site to identify the problems - dirty power, exposure to lightning risks, power factor correction requirements, flicker and voltage sags, to name just a few.

“Armed with that information, consider the solution options and as many of them as practicable. That will give you choices to apply the most cost-effective solution for your specific situation. In most cases, active filtering is the best and easiest solution, but it is also the most expensive. Depending on the site and the nature of the harmonic disturbances, it may be the most cost-effective, long-term solution. For example, if the disturbances are loads distributed throughout a large facility, a single active filter located at the main board may ultimately cost less to provide and install than, say, many different types of passive filters located all over the facility.

“In greenfield sites, factoring in harmonic mitigation at the design stage is an important step. Identify the harmonic producers from the client’s inventory of equipment and provide the most suitable filters for them. So in a water treatment plant for example, where VSDs are extensively used, connect appropriately sized passive filters locally to them. VSDs greater than 15 kW are major sources of disturbances, so look to mitigate harmonics at these locations. Again, if there are many sources of disturbances, perhaps factor a centrally located active filter to cover the entire electrical system.

“The same strategy can be applied to a commercial facility, like an office or retail centre that deploys a number of modern HVAC units, all equipped with VSDs. As with the scenario above, target the main harmonic producers, typically the large VSD loads.

“In an environment with switching power supply loads, again such as an office or a retail centre, the most effective mitigation option is to utilise a central active filter, or distributed active filters if the load is large enough.

“When it comes to addressing sites with a large number of fluorescent lights with electronic ballasts, this scenario needs to be assessed on a case-by-case basis. Typically, an installation with a harmonics problem caused by fluorescent lights with electronic ballasts will have a huge number of them. If there is a filtering option for the light fitting, then the number of installations can be prohibitive. In these cases, an active filter is the easiest - and most likely the lowest cost - solution.

“In a mission-critical site, such as a data centre or medical facility, there will typically be different areas where harmonic mitigation can be applied. HVAC systems lend themselves to passive filtering options, whereas the switching harmonic sources within the site are dealt with most easily with an active filter.”

Thompson adds some additional guidance on dealing with the effects of harmonics on electric motors: “One of the frequently ignored harmonic effects is the straining of transformer capacity. High levels of harmonic current cause high heat loads in transformers because of the increased eddy current losses in the windings over and above the level due to the fundamental (50 Hz) current as well as increased iron losses. These additional losses are proportional to the squares of the frequency and harmonic current. The ends of transformer windings are particularly susceptible to overheating because of a concentration of leakage magnetic fields. In practice, the presence of high levels of harmonic current requires the de-rating of the transformer. There are, however, problems associated with de-rating transformers. These can include primary overcurrent protection levels that, when set lower to take account of the de-rating factor, may cause tripping of protection gear during inrush current conditions and also higher than necessary core losses through the use of oversize cores. The use of ‘K-factor’ transformers, on the other hand, takes into account the harmonic levels and is therefore a much better solution. This is particularly so when the growth of an electrical installation is taken into account. A de-rated transformer is more likely to become overloaded as more and more harmonic loads are connected onto the circuit.”