The total energy consumed by an industry is greatly determined by the overall efficiency of the electrical system. Power factor compensation and harmonic filtering systems are commonly used in industries to maximize energy efficiency. A previous article provides a broad introduction to power factor and methods that are commonly used for power factor compensation. In this article, we are going to primarily focus on harmonics in power systems and some of the solutions that are commonly used to mitigate them.
Causes of harmonics in power systems
In our article on power factor correction, we saw that inductive loads can be broadly categorized into linear and non-linear loads. In the case of linear loads, the current drawn by the equipment is proportional to voltage. This means that, just like the voltage, the current drawn by the load has a sinusoidal waveform. On the other hand, in the case of non-linear loads, the current drawn by the load is non-sinusoidal.
Some of the most common non-linear loads in today’s industries include variable speed drives, switch-mode power supply (SMPS) systems, uninterruptible power supply (UPS) systems, welding machines, static converters, induction furnaces, and battery chargers.
Figure 1 and Figure 2 show an example of a non-linear load and current waveforms for a linear and a non-linear load, respectively.
The interaction between the impedance of a power distribution system and the harmonic components contained in a non-linear current distorts the waveform of the supply voltage. Such voltage distortions can have considerable effects on the performance of other loads as well as the power distribution equipment.
Let us now consider a source of AC voltage at a fundamental frequency, f. Harmonics are current or voltage components that oscillate at frequencies that are multiples of the first harmonic (fundamental frequency). For example, the third, the fifth, and the seventh harmonics are equal to 3f, 5f, and 7f, respectively. If the AC supply voltage is at 60 Hz, the third, the fifth, and the seventh harmonics will be 180 Hz, 300 Hz, and 420 Hz, respectively. Figure 3 shows the fundamental, the third harmonic and the distorted waveform.
Presence of harmonic frequencies causes some components of the electrical distribution system to resonate with response to the magnetic fields produced by the higher frequency harmonics. This mechanical resonance causes some electrical equipment such as transformers to vibrate and emit buzzing sounds corresponding to the harmonic frequencies present in the power system. In most electrical distribution systems, it is common to find 3rd to 25th harmonic frequencies.
Since the distorted waveforms are periodic waves, it is possible to reconstruct the harmonic components contained in a waveform. Such reconstructions are done by employing Fourier analysis. Figure 4 and Figure 5 show a complex waveform and the equivalent harmonic components, respectively. How synthesis of nonlinear waveforms is done using Fourier analysis is beyond the scope of this article.
The magnitude of voltage distortion due to non-linear currents is mainly determined by the impedance of the system and levels of fault current and harmonic current. The total distortion caused by non-linear loads connected to the electrical distribution system is commonly given in terms of total harmonic distortion (THD). This measure is mathematically given by
Effects of harmonic distortions
Harmonics can significantly affect the overall performance of an electrical distribution network. To start with, high frequency harmonics are a major cause of overheating in cables, transformers, and other components of an electrical distribution system. In extreme cases, the overheating caused by harmonics can cause fires. In addition, studies have shown that harmonic resonance can cause circulating currents and high voltages in electrical distribution networks.
Harmonics can considerably impact the performance of electrical equipment. Excessive voltage distortions can also cause electrical equipment to malfunction or fail. Moreover, many studies have linked harmonics and reduction in the service life of electrical equipment.
Other effects of harmonic distortion include generator failures, metering issues, and false tripping of circuit breakers. Harmonic distortion can also cause a significant reduction in the power factor of an electrical installation. If this happens, the electricity bill can shoot up since installations with poor power factor are penalized by utilities.
We have seen that harmonics can considerably affect the performance of electrical systems and increase operational costs. For industrial facilities, it is important to perform a harmonic distortion analysis when constructing or expanding an installation. There are various methods that are used in today’s industries to reduce harmonic distortion. For this article, we will focus our attention on two types of harmonic filters: passive harmonic filters and active harmonic filters.
One of the simplest ways of reducing harmonics is by adding an inductor on the line side of the device generating harmonics. Such an inductor is commonly referred to as a line reactor. Figure 6 above shows a three-phase line reactor connected on the line side of a frequency drive.
Line reactors are low cost devices but they are less effective in minimizing harmonics. A line reactor can be combined with an LC-filter in shunt arrangement to form a harmonic filter that is commonly referred to as a harmonic trap filter. This filter is connected on the line side of the device that is generating harmonics and is capable of absorbing several harmonics depending on how it is tuned. Figure 7 above shows a harmonic trap filter connected on the line side of a frequency drive.
A line reactor can be connected in parallel with a capacitor to create a harmonic filter that is commonly referred to as a broadband filter. Unlike a harmonic trap filter, this filter does not require tuning and provides more protection to the device that is generating harmonics. A broadband filter is capable of filtering harmonics of all frequencies. Apart from filtering harmonics, this filter also prevents system resonance and improves the power factor. Compared to DC chokes or line reactors, harmonic trap and broadband filters are more effective and more expensive to implement. Figure 8 above shows a broadband filter connected in series with a frequency drive.
Active harmonic filters utilize active components, such as IGBTs, to cancel harmonics in an electrical distribution system. An active harmonic filter can be connected either in series or parallel with the device that is generating the harmonics. Figure 9 shows an active filter connected in series with a frequency drive while Figure 10 shows an active filter connected in parallel with a frequency drive.
The following approaches are also commonly used to minimize harmonics in electrical distribution systems and installations:
· Pulse converters
· DC chokes
· Separating neutral wires for each phase.
· Using an over-sized neutral wire.
The current drawn by a non-linear load is non-sinusoidal and can cause distortion of the supply voltage waveform. The number of non-linear loads connected to power distribution systems is growing every day and so is the harmonics problem. Harmonic currents cause heating losses in power systems and reduce the overall energy efficiency of an electrical distribution system. There are different methods of mitigating harmonics and industrial consumers are required to employ them