Mitigating harmonics in electrical systems
Solutions at the nonlinear load
As an alternative to the systemic approach to harmonic mitigation, some components may be more economically viable for facilities where the potential for injection of excessive current harmonics is limited to a few specific loads.
A line reactor is the simplest solution for reducing harmonic current caused by nonlinear loads, typically converter-based devices such as VFDs. Inductors or isolation transformers, installed in series with and ahead of the load, can reduce the harmonic current content up to 50%, depending on the amount of impedance added to the line, to approach TDD levels of 30% to 40%. The most common values of ac line reactors are 3% and 5%. Typically, line reactors are less expensive than transformers.
In lieu of inserting line reactors in series with a VFD, a dc choke can be added to the drive’s dc bus, reducing approximately the same degree of harmonics as the ac reactor. The advantage of applying dc chokes is that they are typically physically smaller and are often mounted inside the VFD. Many VFDs can be ordered from the manufacturer with dc chokes already installed.
Passive filters are comprised of static, linear components such as inductors, capacitors, and resistors arranged in predetermined fashion to either attenuate the flow of harmonic currents through them or to shunt the harmonic component into the filter circuit. There are several types of passive filters, but the most effective type is the low-pass broadband filter, which offers great performance and versatility with lower risk of resonance with the line.
Figure 7 shows a typical tuned harmonic filter and a broadband filter circuit. In the tuned filter, the inductor (Lp) and capacitor (C) provide a low impedance path for a single (tuned) frequency. An inductor on the line side, (Ls) is required to detune the filter from the electrical system and other filters’ resonance points. This type of filter is very application specific. It can mitigate only a single frequency, and it injects leading reactive current (kVAR) at all times. But it is economical if you need to deal with only a dominant harmonic in the facility. It normally can reach a TDD target of 20%.
A broadband filter is designed to mitigate multiple orders of harmonic frequencies. Notice the similarity and the difference of the circuit from the tuned filter. Both inductors (L) could have impedances greater than 8%, which means there could be a 16% voltage drop across the filter. Its physical dimension is normally very large, and it generates significantly high heat losses, typically greater than 4%. A well-designed broadband filter can meet a TDD target of around 10%.
Low-pass harmonic filters have gained popularity due to their ability to attenuate multiple harmonic frequencies to achieve low levels of residual harmonic distortion. The typical low-pass filter configuration includes one or more series elements plus a set of tuned shunt elements. The series elements increase the input circuit’s effective impedance to reduce overall harmonics and detune the shunt circuit resonance. The shunt elements are tuned to attenuate most of the remaining circuit’s harmonics, primarily the 5th and 7th order harmonics. This type of filter is most commonly applied in series with and ahead of 6-pulse rectifier loads. Note that the harmonic distortion is reduced at the input stage of this filter. However, the load side will have significant current and voltage distortion, and thus it is recommended that only nonlinear loads be connected. Further, due to the series reactance, low-pass filters produce a voltage drop under loaded conditions, while voltage boosting will occur under no-load conditions, so some low pass harmonic filters may not be suitable for use with SCRs.
Engineers have many options available for mitigating harmonic current distortion. There is also the option of taking no action. However, this runs the risk of reduced equipment life, failure of sensitive microprocessor-controlled equipment, downtime, safety risks, and potentially even utility penalties. The best economical and technical solution is not the same for all cases, and a thorough cost/benefit consideration of the application is necessary to evaluate and select the optimized solution to a facility’s harmonics problems.
Whichever method is selected for a specific application, as a general rule, the greatest benefit is realized when harmonic mitigation solutions are placed close to the loads generating excessive harmonic currents (see “Rules of thumb”). With this topology, the electrical system can be more effectively used for real work, and the probability of creating resonance and harmonic related is significantly reduced.
Nicholas Rich is principal and senior electrical engineer at Interface Engineering. He has more than 25 years of experience in designing electrical power distribution, lighting, and communications systems.