To get the best solution, you need a clear understanding of the sources and mitigation technologies currently available
No harmonic mitigation solution is ideal for every application. So, once you or your client decides to reduce harmonic distortion on a facility electrical power system, you must consider several factors in selecting the ideal mitigation method. Only when you've considered these factors can you determine the best economical and technical solution.
Let's examine a step-by-step approach to effective harmonic evaluation and mitigation techniques currently available.
Step 1: Define and quantify the problem
First, you must determine the magnitude of harmonic distortion at different points in the electrical system, and identify whether or not any equipment failures or malfunctions are the result of these harmonics. You can measure the magnitude of harmonics for existing systems by using owned or leased harmonic measurement equipment, or calculating it for proposed systems or additions to systems. Take measurements at the secondary side of supply transformers and generators, as well as at the input conductors for large power electronics loads and at the input to sensitive equipment.
The waveforms and harmonic spectra for current will appear different for measurements taken at the power source, which feeds multiple loads, than for individual loads. For an individual linear load, such as a motor, the current waveform should appear to be a sine wave, whereas it will appear distorted for an individual nonlinear load (Fig. 1). The distortion of individual nonlinear loads will vary somewhat, depending on several factors such as source impedance, load current, and system voltage distortion. The waveform measured at the common power source will result from the combination of all of the individual loads.
The mixture of linear and nonlinear loads operating from a common power source has a significant effect on the distortion present on the electrical system. In cases where mostly linear loads are connected, pure sinusoidal current can overwhelm the system with fundamental current, causing the harmonic current to have less of an effect on the overall system. A system with predominantly nonlinear loads will exhibit a more distorted waveform at the power source.
Step 2: Determine the power quality objectives
Once you've identified the major sources of harmonics and quantified the magnitude of harmonic distortion at various points on the system, you must determine the acceptable level of harmonic distortion. This may simply be a desire to have all electrical equipment functioning properly with a high probability for meeting normal life expectations. Or, the objective may be a specific target for operational and energy efficiency. There may be an engineering specification that dictates compliance with a power quality standard or special industry requirements. In many cases, the objective is to meet the harmonic current and voltage distortion limits established by IEEE Std-519, “IEEE Standard Practices and Requirements for Harmonic Control in Electrical Power Systems,” as shown in the Table.
By considering the problem and the objectives, you can help determine the optimum points at which to apply mitigation equipment within a facility, such as at individual sources of harmonics, at a group of harmonic generating loads, at a group of mixed (linear and nonlinear) loads, or at a central point in the facility electrical distribution network. As shown in Fig. 2 (click here to see Fig. 2), whether you apply the mitigation equipment near the load or at a central point upstream, such as at the service entrance, the end result for the electric utility can be the same. However, the major difference is the level of power quality achieved within the facility itself.
Generally speaking, you realize the benefit derived from the application of harmonic mitigation equipment at all points that are upstream of the point where this equipment is connected to the electrical system. This suggests that to receive the greatest internal benefit, you should consider applying harmonic mitigating equipment as close to the source of harmonics as possible. This approach will typically achieve the lowest consumption of reactive and harmonic energy, the least distortion of system voltage, and the best overall power quality for both the facility and the electric utility combined. Any reduction of harmonics within the facility will be transferred to the electric utility system.
Step 3: Consider alternative mitigation methods
There are several basic techniques you can employ to reduce electrical system harmonic distortion. Within each of these, there are several different alternative methods to consider. The basic methods for mitigating harmonics include:
Increasing effective source impedance,
Diverting harmonics to an alternate path,
Employing a hybrid method combining both items above, and
Increasing effective source impedance
You can predict the harmonic current distortion for a 6-pulse rectifier (common in variable-frequency drives) based upon the effective source impedance, which is essentially the voltage drop across the power source at the operating load current. You can increase the effective source impedance by adding either a line reactor or an isolation transformer ahead of the nonlinear load.
In a typical electrical system, the supply transformer is the primary source of impedance. Transformer nameplate impedance is based upon full load operating conditions. It defines the full load voltage drop associated with the inductive reactance of the transformer, which is a function of XL (ohms) and the current. When a load draws current that is lower than the full load capacity of the transformer (or reactor), the effective impedance is lower than the stated nameplate value. This is because the ohms are the same, but the current is lower. Another way to look at it is that the ohms of the transformer are a smaller percentage of the ohms of the load. An easy way to evaluate the effective source impedance, relative to an individual load, is by using the following equation:
In many cases, the transformer power capacity is much greater than the capacity of individual loads, so the effective impedance relative to individual loads may be much lower than shown on the transformer nameplate. The percent of harmonic current distortion (THDI) will typically be higher for small loads than for large loads due to their lower effective impedance.
Example: A 50kVA load operating at full load when supplied from a 500kVA transformer (rated 5% impedance) will have an effective impedance of 0.50% (5%×50÷500) and total harmonic current distortion of about 100% THDI. A 250kVA load, supplied from the same transformer, sees an effective impedance of 2.5% (5% × 250÷500) and thus is expected to cause approximately 50% THDI.
Therefore, you can reduce harmonic current distortion by increasing the effective source impedance relative to individual loads. You most commonly accomplish this by simply adding a series-connected line reactor at the input to various loads that use 6-pulse rectifiers, such as VFDs. Typical amounts of line reactor impedance range from 2% to 5% impedance. Keep in mind that the percent impedance also refers to the voltage drop at full load operating conditions.
If you add a 4% impedance line reactor to a system, then the total effective impedance will be the sum of the impedances of the line reactor, supply transformer, utility power source impedance, and the conductors between the transformer and the load. Therefore, at full load conditions, the total percent impedance, using a 4% impedance line reactor, will typically be about 4% to 5%, and the drive input voltage will be 95% to 96% of the system line voltage. Because motors in the United States are typically rated at about 95.8% of system voltage (460V vs. 480V), this results in an ideal situation in terms of motor performance and will result in total harmonic current distortion at the individual (6-pulse) load of about 35% to 38% THDI. However, if the drive was operating at 50% load, the effective percent impedance would be one-half of the effective full load impedance — and the percentage of harmonic current distortion would be higher than at full load.
Example: A 100-hp adjustable-speed drive is supplied from a 1,000kVA transformer, with 5.0% impedance (effective input impedance is 5.00×100÷1000=0.50%). Utility and conductor impedances provide an additional 0.25% impedance. A line reactor of 4% impedance is added, resulting in total full load effective impedance of 4.75%. At full load, the predicted harmonic current distortion will be about 36% THDI. Now let's consider what happens at about 2/3 load. The effective input impedance is now 4.75%×2÷3=3.16% impedance. Although the magnitude of fundamental and harmonic rms currents will both be reduced, the actual percentage of total harmonic current distortion will be higher (about 45% THDI).
Diverting a specific harmonic frequency away from power source
This technique combines reactors and capacitors to form a tank circuit that offers a lower impedance path to harmonic frequencies than the power source. In this sense, the filter is “tuned” to provide a low-impedance path for the unwanted harmonic frequency. Here, the tuned filters are connected in parallel with the power source to divert harmonics into the filter and thus prevent some portion of the harmonic currents from flowing into the power source. The full magnitude of harmonic current will flow between the nonlinear load and the filter, but lower harmonic current will flow into the power source.
This type of filter is suitable for use with single or multiple loads, including a combination of both linear and nonlinear loads. The design of these filters is based on the magnitude of harmonics to be filtered, the harmonic frequency desired to be removed from the power system, and the impedance of the power source. Special design consideration must be taken to assure that connection of the harmonic filter will not cause a resonance condition on the power system, which can result in amplification of certain harmonic frequencies.
You can install tuned filters at individual loads (Fig. 3 on page 34) or at a central location for bulk facility filtering (Fig. 4 on page 34). One note of caution: Full harmonic current will continue to flow between the nonlinear load and the tuned harmonic filter. As such, the benefit provided by this technique is only realized by that part of the power system upstream of the filter connection point. If you apply a bulk filter on a main switchboard or transformer, the harmonics will continue to flow on the facility (internal) power system between the connection point of the filter and the downstream loads.
Because the tuned filter offers low impedance at the tuned frequency, it's possible to attract harmonics from any source connected to the power system. For this reason, when you apply tuned filters on individual loads, you must also apply a series-connected reactor at the input to the filter. This will prevent the tuned network from attracting harmonics from other loads and becoming overloaded. This reactor essentially detunes the filter (increases its impedance) with respect to loads connected upstream.
When you apply a tuned filter as a bulk solution to serve multiple loads, this filter is usually designed with the ability to switch one or more tuned circuits into and out of the system as required. This allows you to adjust the filter capacity based on the actual load conditions. It also helps prevent overcompensation by controlling the amount of capacitance to be added to the system.
Employing a hybrid filter to minimize and divert harmonics
One of the most effective harmonic filters for use with single or multiple nonlinear loads is the low pass harmonic filter, as shown in Fig. 5 on page 36. This hybrid filter combines the tuned filter with additional line reactors and can be designed to achieve residual harmonic current distortion levels less than 5% THDI. When all three stages are used, it can prevent the attraction of harmonics from other loads, and can also prevent harmonic resonance relative to both the input and output circuitry.
Three stages of filtering are combined in this type of filter to limit harmonic distortion (for VFDs) to the range of 3% to 5%. Harmonic distortion may be higher for DC drives and similar power conversion schemes using phase-controlled SCRs. Series impedance (input stage plus output stage) reduces the magnitude of harmonic current produced by the 6-pulse rectifier of the VFD load. The shunt circuit is tuned to remove most of the 5th harmonic, which is normally the most predominant in 3-phase, 6-pulse rectifier applications.
The input and output stage reactors play another important role. Because the tuned section must be tuned close to the 5th harmonic frequency, it would normally offer low impedance to 5th harmonic currents from virtually any other nonlinear load on the system. This would attract harmonics from other loads and could overload this section of the filter. In addition to increasing the effective source impedance, the input and output stage reactors also detune the shunt filter stage, relative to the input and output sides of the total filter. The filter is thus detuned relative to other loads connected upstream as well as from the perspective of the connected load. This prevents high peak current and high di/dt rates in the input rectifiers of the connected load. In fact, the current waveform at the input to the VFD, looks exactly like the input current waveform of a VFD with input line reactor. When all three stages are used, a well-designed low pass filter has the capability to achieve harmonic distortion levels of as low as 5% THDI when operating between 0% to 100% load.
Harmonic cancellation using 12- and 18-pulse converters
There are two basic methods that accomplish cancellation of harmonic currents: passive technology using phase shifted transformer windings and active filters using IGBT transistor technology.
Twelve- and 18-pulse rectification methods have been used for many years, especially in motor drive applications. A 12-pulse converter uses two 6-pulse bridge rectifiers that are supplied from two different power sources. These power sources are phase shifted by 30 electrical degrees, resulting in cancellation of the 5th and 7th harmonics. Similarly, the 18-pulse converter uses three sets of 6-pulse bridge rectifiers that are supplied from three different power sources, each of which are phase shifted by 20 electrical degrees. This arrangement results in cancellation of the 5th, 7th, 11th, and 13th harmonics.
The advantage of these types of power converters is that fewer harmonics are produced than by the standard 6-pulse rectification method. You can determine the harmonics produced by any given rectification method by using this equation.
where h = harmonic number, k = a constant (1,2,3…) and p = number of pulses
Therefore, a 12-pulse converter, which has 12 diodes and creates 12 pulses of DC bus voltage per cycle of AC input voltage, will produce harmonic numbers 11, 13, 23, 25, 35, 37, etc. An 18-pulse converter will produce only harmonic numbers 17, 19, 35, 37, etc. These compare to the standard 6-pulse converter, which produces 5, 7, 11, 13, 17, 19, 23, 25, etc., harmonics. Using an 18-pulse converter will typically reduce harmonics to approximately 5% THDI at full load conditions. A 12-pulse converter typically achieves about 10% to 12% THDI at full load.
In the case of either 12- or 18-pulse converters, if the 6-pulse bridge rectifiers are connected in parallel, as shown in Fig. 6 on page 38, then the magnitude of harmonic current distortion may increase appreciably as the load is reduced and with unbalanced line voltages. If the bridge rectifiers are series connected, as shown in Fig. 7, then low harmonics can be achieved throughout a wider range of load conditions. Series connection will require lower voltage (12-pulse uses ½ line voltage for each of two 30° phase-shifted windings to supply each rectifier bridge; 18-pulse uses 1/3 line voltage for each of three 20° phase-shifted windings to supply each rectifier bridge). The parallel connection of bridge rectifiers uses full voltage with reduced bridge current, while series connection uses reduced voltage and full current through the bridge rectifiers.
You can achieve “pseudo” 12-pulse systems by supplying similar loads from two separate transformers or windings that are 30° displaced. Here, you use one delta winding to feed one set of 6-pulse loads and one wye winding to supply another similar set of 6-pulse loads. Cancellation of harmonics can occur to the extent that these loads are balanced and to the extent that the six independent transformer phase voltages are balanced. “Pseudo” 18-pulse conversion can be achieved in a similar manner, but it becomes increasingly difficult to balance nine phase voltages and maintain similar current through all three sets of 6-pulse bridge rectifiers and transformer windings to achieve desired results.
Harmonic cancellation — active filters
This technique uses an insulated gate bipolar transistor (IBGT) based device that, in many ways, is similar to a VFD. In some cases, the active filter is incorporated directly into the VFD front end.
The basic operation of this type of filter involves measurement and analysis of the input current waveform, with the injection of the inverse harmonic current waveform. You typically select the active filter capacity based upon harmonic cancellation current requirements, which is accomplished by determining the magnitude of harmonic current desired to be removed from the system. When properly selected, an active filter will typically reduce harmonics to residual levels of about 5% THDI. You connect a standalone active filter in parallel with the power system. It is suitable for a mixture of both nonlinear and linear loads.
Making sense of mitigation
System harmonic distortion levels depend on a number of factors, such as impedance, mixture of loads (linear and nonlinear), background voltage distortion, operating conditions (percent load), and converter type. You can consider several alternative methods to mitigate harmonics. Your decision will be based on the objectives for facility power quality and the budget available. Each technique has its own set of merits and limits, and each offers a respective range of harmonic mitigating performance.
Understanding the sources and magnitude of harmonics, distribution of linear and nonlinear loads, any power quality objectives, and various mitigation solutions available will help you arrive at the best economical and technical solution.
Houdek is president of Allied Industrial Marketing, Milwaukee.
Use line reactors with about 4% impedance to lessen harmonic current distortion right at the input to each VFD. This will reduce individual load harmonics to about 35% to 38% (lower if the drive also has a DC bus choke), protect the VFD rectifiers from transient overvoltage, reduce the harmonic distortion at the upstream transformer or generator, and reduce the cost of any additional bulk harmonic filtering equipment under consideration.
Apply tuned harmonic filters as close as possible to the source of harmonic current. This will improve the power quality of the facility power system as well as points upstream. Both the facility and the utility will benefit. Harmonic distortion may be reduced to levels of 5% to 30% THDI, depending on filter design and point of connection.
Use a low pass harmonic filter whenever you need an economical solution for meeting very low limits for harmonic current distortion. The effectiveness of the low pass filter is enhanced when the phase-to-phase inductances of all filter magnetic components are balanced and maintained within a tolerance of about 3% or better. The low pass harmonic filter is suitable for use in supplying one or more nonlinear loads, such as VFDs. Do not connect linear loads, such as motors, to the output of a low pass filter due to the high harmonic content in this stage.
Use 12- or 18-pulse converters having series-connected bridge rectifiers to achieve the lowest harmonic distortion levels over the widest range of operating conditions.
Reduce the size and cost of the active filter by applying 4% impedance line reactors to each individual VFD on the system so the cancellation current requirement for the active filter is reduced.