Mitigating harmonics in electrical systems
Although devices using power electronics can produce distortion in electrical distribution systems, it’s up to the engineer to apply effective solutions to mitigate them.
- Understand current and voltage harmonics in electrical systems, and their negative effects on the facility electrical system.
- Know how electronic power equipment such as VFDs creates harmonics.
- Understand characteristic and noncharacteristic harmonics.
- Understand IEEE 519 guidelines for the reduction of electrical harmonics.
- Learn design techniques for mitigating harmonics with recommended applications.
Harmonics and detrimental effects
In North America, alternating current (ac) electrical power is generated and distributed in the form of a sinusoidal voltage waveform with a fundamental frequency of 60 cycles/sec, or 60 Hz. In the context of electrical power distribution, harmonics are voltage and current waveforms superimposed on the fundamental, with frequencies that are multiples of the fundamental. These higher frequencies distort the intended ideal sinusoid into a periodic, but very different shaped waveform.
Many modern power electronic devices have harmonic correction integrated into the equipment, such as 12- and 18-pulse VFDs and active front-end VFDs. However, many nonlinear electronic loads, such as 6-pulse VFDs, are still in operation. These nonlinear loads generate significant magnitudes of fifth-order and seventh-order harmonics in the input current, resulting in a distorted current waveform (see Figure 1).
The characteristics of the harmonic currents produced by a rectifier depend on the number of pulses, and are determined by the following equation:
h = kp ±1
- h is the harmonic number, an integral multiple of the fundamental
- k is any positive integer
- p is the pulse number of the rectifier
Thus, the waveform of a typical 6-pulse VFD rectifier includes harmonics of the 5th, 7th, 11th, 13th, etc., orders, with amplitude decreasing in inverse proportion to the order number, as a rule of thumb. In a 3-phase circuit, harmonics divisible by 3 are canceled in each phase. And because the conversion equipment’s current pulses are symmetrical in each half wave, the even order harmonics are canceled. While of concern, harmonic currents drawn by nonlinear loads result in true systemic problems when the voltage drop they cause over electrical sources and conductors results in harmonics in the voltage delivered to potentially all of the building electrical system loads—even those not related to the nonlinear loads. These resulting harmonics in the building voltage can have several detrimental effects on connected electrical equipment, such as conductors, transformers, motors, and other VFDs.
Conductors: Conductors can overheat and experience energy losses due to the skin effect, where higher frequency currents are forced to travel through a smaller cross-sectional area of the conductor, bunched toward the surface of the conductor.
Transformers: Transformers can experience increased eddy current and hysteresis losses due to higher frequency currents circulating in the transformer core.
Motors: Motors can experience higher iron and eddy current losses. Mechanical oscillations induced by current harmonics into the motor shaft can cause premature failure and increased audible noise during operation.
Other VFDs and electronic power supplies: Distortion to the increasing voltage waveform in other VFDs and electronic (switch mode) power supplies can cause failure of commutation circuits in dc drives and ac drives with silicon controlled rectifiers (SCRs).
Establishing mitigation criteria
The critical question is: When do harmonics in electrical systems become a significant enough problem that they must be mitigated? Operational problems from electrical harmonics tend to manifest themselves when two conditions are met:
- Generally, facilities with the fraction of nonlinear loads to total electrical capacity that exceeds 15%.
- A finite power source at the service or within the facility power distribution system with relatively high source impedance, resulting in greater voltage distortion resulting from the harmonic current flow.
IEEE 519-1992, Recommended Practices and Requirements for Harmonic Control in Power Systems, was written in part by the IEEE Power Engineering Society to help define the limits on what harmonics will appear in the voltage the utility supplies to its customers, and the limits on current harmonics that facility loads inject into the utility. Following this standard for power systems of 69 kV and below, the harmonic voltage distortion at the facility’s electrical service connection point, or point of common coupling (PCC), is limited to 5.0% total harmonic distortion with each individual harmonic limited to 3%.
In this standard, the highest constraint is for facilities with the ratio of maximum short-circuit current (ISC) to maximum demand load current (IL) of less than 20, with the following limits placed on the individual harmonic order: (Ref. Table 10.3, IEEE Std. 519)
- For odd harmonics below the 11th order: 4.0%
- For odd harmonics of the 11th to the 17th order: 2.0%
- For odd harmonics of the 17th to the 23rd order: 1.5%
- For odd harmonics of the 23rd to the 35th order: 0.6%
- For odd harmonics of higher order: 0.3%
- For even harmonics, the limit is 25% of the next higher odd harmonic.
- The total demand distortion (TDD) is 5.0%.
There are various harmonic mitigation methods available to address harmonics in the distribution system. They are all valid solutions depending on circumstances, each with their own benefits and detriments. The primary solutions are harmonic mitigating transformers; active harmonic filters; and line reactors, dc bus chokes, and passive filters.
Harmonic mitigating transformers
In a standard delta-wye transformer, zero-sequence currents flow through the secondary wye winding and are coupled into the primary delta winding where they are trapped (see Figure 2). These zero-sequence currents can cause excessive heating and voltage distortion. Harmonic mitigating transformers can be implemented in pairs to mitigate 5th, 7th, and higher-order harmonic currents by taking advantage of the transformer phase shifts relative to each other, to cancel a significant amount of the harmonic current at these higher frequencies.
One type of harmonic mitigating transformer uses a zig-zag configuration. The zig-zag transformer is configured by winding half of the secondary turns of one phase of the transformer on one leg of the 3-phase transformer, with the other half of the secondary turns on an adjacent phase (see Figure 3).
Note that harmonic mitigating transformers are not a panacea for the elimination of harmonics in an electrical system. Mitigation of 5th, 7th, and higher order harmonic currents requires the installation of multiple transformers with a 30-deg relative phase shift between the two, connected to a common bus in an electrical distribution system. Also, when mitigating these higher level harmonic currents by this means, balance of loads between the transformers is required. As shown in Figure 4, one transformer is a delta-zigzag configuration harmonic mitigating transformer with a 0-deg phase shift, and the second transformer is a delta-wye with a 30-deg phase shift.
Voltage distortion is normally greatest at the point where the equipment is connected to the distribution system. Therefore, to attain maximum benefit, harmonic mitigating transformers should be installed as close as practical to the load that they feed.
Installation of a non-phase-shift harmonic mitigating transformer provides an effective treatment of triplen (3rd, 9th, 15th, and so on) harmonic currents that are generated by loads connected to the transformer. Triplen harmonic currents are treated in the secondary windings of the transformer due to the transformer’s low zero-sequence impedance.
When a standard or K-rated delta-wye transformer is installed in an electrical distribution system, the addition of a non-phase-shift harmonic mitigating transformer offers an economical solution for treating higher order harmonic currents. The 30-deg phase-shift created between the standard or K-rated delta-wye transformer and harmonic mitigating transformer provides treatment of 5th, 7th, 17th, and 19th order harmonic currents to the extent of the balance of the load between the two transformers. In this configuration, the harmonic currents are canceled in the common electrical bus that feeds the transformers. Close coordination between the construction and location of the two transformers must be executed, as the impedance values of the transformers should be identical to receive the maximum mitigation of these higher-order harmonic currents.
Active harmonic filter (AHF)
The concept of an active filter is to produce harmonic components of the fundamental current waveform that are out of phase with—and thus cancel the harmonic components generated from—the nonlinear loads. Figure 5 conceptually illustrates how the harmonic current generated by the AHF is injected into the system to cancel harmonics from a VFD load. The AHF is installed as a parallel device and is scalable, making it a highly effective device that cancels multiple order harmonics in the distribution system. This method addresses harmonics from a systemic point of view and can save significant cost/space in many applications, with performance levels that can meet a TDD 5% target.
The active harmonic filter uses a current transducer to actively monitor the load current in real time to react to changes in load. Some AHFs are designed to also inherently synchronize the line current with the voltage to approach unity displacement power factor. The system typically performs fast Fourier transforms to calculate the amount of harmonics present for each harmonic order in the load current to determine the amplitude of the first 30 to 50 orders. The system logic processor filters out the fundamental frequency, and then directs the power converter to inject the phase-inverse of only the harmonic currents back into the circuit for cancellation of the harmonic content.
The benefits of AHFs include:
- Dynamic adjustment for virtual real-time correction of the nonlinear current
- Synchronization of the current and voltage waveforms
- Adjustment using a feedback loop to prevent leading power factor.
AHF equipment is available for implementation at the PCC of the facility to the utility, for connection to a distribution bus within 3-phase power distribution systems inside facilities, and within distribution and control equipment, such as motor control centers (see Figure 6).
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.