Causes and effects of harmonics in electrical power systems

Power quality is an estimate of how stable the electrical system is, often this is described as “power quality health.” This is measured on three-phase electrical systems using instrumentation that considers several variables. Troubleshooting power quality issues will help your facility save money by optimizing energy use and protect equipment from future damage. The first step to evaluating power quality is to capture data from equipment, infrastructure, and the service panel. The primary effects of poor power quality effects include:

• Dips and swells – voltage lower or higher than expected
• Harmonics – frequency effects caused either by the power supply or by equipment operating within the system
• Unbalance – the effect of voltage or current variations on each of the electrical phases
• Flicker – effects caused by repetitive switching of electrical loads such as arc furnaces or other processes

What are harmonics in electricity?

Harmonics are currents or voltages with frequencies that are integer multiples of the fundamental power frequency, which in the U.S. is 60 Hertz. If the first fundamental frequency is 60 Hz, then the second is 120 Hz, and the third is 180 Hz. Here are a few examples of issues that might be related to harmonics.

• Flickering lights are a common symptom of a power quality issue. A potential source of flickering lights is machinery with rapid fluctuations in load current or voltage. These machines include large motors when they are starting up, machinery with cyclo-converters such as rolling mill drives and mine winders, as well as machines that use static frequency converters such as AC motors and electric arc furnaces.
• Overheated transformers and tripped breakers could be a sign of harmonic issues, which occur when non-linear loads that draw current in abrupt pulses, rather than in a smooth sinusoidal manner, cause harmonic currents to flow back into other parts of the power system.

Total Harmonic Distortion

The state of the harmonics in the system can be expressed in many ways and the first is the Total Harmonic Distortion or THD. The THD is the sum of all the harmonic effects; usually this is measured up to the 50th multiple of the fundamental frequency of the power system (60 Hz), at 3kHz or according to some guidance the 40th multiple (2.4kHz). This value of THD in terms of power quality health is most often applied to the voltage. Guidance states that the voltage harmonic effects should be less than 8 percent relative to the fundamental. Values above the stated 8 percent should be investigated further.

The first level of investigation would be to identify the percentage of each individual harmonic, 2nd, 3rd, 4th, 5th—up to 50th. This is indicated either live on a measurement instrument or on a chart from logged and downloaded data—this is visualized as a “harmonic spectrum.”

This sample harmonic spectrum shows a very typical scenario. The voltage THD is mid-range at about 3.5 percent on each phase. Note how the largest harmonics are on the 5th and 3rd, respectively, and soon after the 7th, the harmonics decline very quickly. These harmonics are generated by switched mode power supplies used by electronic equipment such as computers, monitors, televisions, and LED lighting. To a certain extent these are the harmonics that equipment manufacturers allow—inside their devices they have electronic filters that prevent the higher harmonics from being generated. The prevention, or mitigation, is done by the addition of simple networks of passive components such as resistors, capacitor, and inductors. By including this simple mitigation in the product, it enables the manufacturer to supply products that meet the required EMC standard.

If we consider current harmonics, we see a very different picture:

Here we have what seems to be a very surprising level of distortion—up to 40 percent. This is interesting, but not really that important. For one thing, the current is low in this case compared to how much current the circuit is concerned with. We describe these two values as IL (load current) and ISC (short circuit current). When ISC is sufficiently higher than IL, the THD for current is not important. The reason for this is that if there is a big difference in these currents, the voltage harmonics are not likely to be affected. This concept is baked into the IEEE 519 Standard (Recommended Practice and Requirements for Harmonic Control in Electric Power Systems).

Causes of electrical harmonics

In an industrial environment, the causes of harmonic distortion are most often the electrical equipment in an operation. Modern industrial plants contain many pieces of equipment that may contribute to the overall distortion—a few obvious examples include variable frequency drives and electrical motors driven by inverters. These drives take in the regular AC voltage and current to convert that into DC and then create a variable frequency output so that the motors can be controlled more precisely. When the current is drawn into the inverter it is not taken as a pure sinewave but takes current irregularly to charge the components that are on the front end of the inverter. This irregular current draw distorts the current and, consequently, the voltage. These inverters may be used to drive motors that are part of the industrial process such as pumping cooling or heating water, liquid materials, moving conveyors, or cooling fans. Other types of electronic controls will also be part of the process, and each one will create some distortion. When all this equipment is connected on the same network, the distortion will increase overall.

Equipment manufacturers have a responsibility to ensure that their equipment does not create levels of distortion that is not allowable, which is why equipment EMC standards are in place to prevent this. But in some cases where a user creates a unique combination of higher-than-expected distortion on an electrical network that is close to capacity, the distortion can become severe and affect other pieces of equipment. For example, old transformers were not designed with harmonics in mind necessarily, although it’s been some time since industrial power electronics were introduced and at their inception the majority of the loads in a facility were linear (where current and voltage are directly proportion—think of a simple resistive load). If harmonics are high, the distortion can cause older transformers to overheat and there are two problems with this. First, the heat being generated wastes energy and second, it is likely to damage the transformer, sometimes catastrophically.

How to reduce harmonics in power systems

There are two possible solutions for this.

1. Reduce the harmonics by installation of filters
2. Replace the transformer with a high K factor which can handle the distortion

Of these two solutions, both have merits and cost implications.

• Installation of filters can prove to be very effective economically and technically depending on the source of the harmonic distortion. To discover the specific source(s) requires harmonic surveys of the equipment connected to the system. A good place to start is with the largest electronic drives—consider which equipment draws the highest current, such as large drives or high-power UPS systems, to find out which ones have the highest THD present. Collect as much harmonic data as possible over a few days to see how the THD changes and identify the worst-case scenarios. This data can then be shared with a filter supplier who can advise on suitable solutions per load. It may only be one or two pieces of equipment that are causing the problem. The worst case is that you need a larger system, but, once again, a supplier can advise on a suitable solution.
• Replacing transformers is more difficult. It still requires harmonic surveys to discover K-factor—the heating effect due to harmonics. The K factor is derived from the harmonics using an IEEE recommended method, however, a suitable instrument calculates this for you. K-factor rated transformers are more expensive than standard transformers and downtime for installation of a new transformer can prove disruptive and create significant downtime. However, in some cases, this might be the only viable solution.

The recurring theme here is that measurements make the difference. Knowing your system health is important for maintaining your equipment to get the best value from it and to maintain reasonable energy usage. Power quality surveys should be considered as routine maintenance and by making semi-regular measurements you can discover any changes that may be occurring so you can find potential problems and fix them early. The period between surveys is up to the users’ judgement, but they need to consider their expectations of system reliability—the higher the expectations, the more regular the survey. This could be monthly, quarterly, every half-year, or, if you feel you’re really in control, annually. Regularly performing surveys should not be that much of a chore if you organized.

• Choose the measurement location wisely. Look for critical points on the network where equipment might cause problems and in which equipment might be more sensitive.
• Install at the same location every time.
• Listen for clues from equipment operators about their experiences with what’s going on at their level—they have some of the best information.
• Observe trends and compare apples to apples for simple correlation.
• Save historical data.
• Document adds, moves, and changes, and update electrical diagrams.
• Measure and log over a few days. One hour will only tell you what happened then—you need to consider more than one typical 24-hour period to see the rhythm of equipment operation.

By gathering this survey data, you can be in control of your electrical system, operating it in an efficient manor and maintaining the longevity of your electrical equipment.