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Power System Harmonic Fundamental Considerations: Tips and Tools for Reducing Harmonic Distortion in Electronic Drive Applications October 2011/AT313 by Larry Ray, P.E. Louis Hapeshis, P.E. Make the most of your energy Revision #1 10/11 SM Power system harmonic fundamental considerations Summary Abstract ....................................................................................................... p 3 Introduction ................................................................................................. p 4 Harmonic Distortion Basics .......................................................................... p 5 Get connected to power Voltage and Current Distortion ..................................................................... p 8 Harmonics and Power Factor – Displacement and Total ............................... p 11 Harmonic Mitigation – Two Passive Techniques ........................................... p 13 Common Harmonic Current Signatures ....................................................... p 15 Harmonic Distortion Simulation Methods...................................................... p 17 References .................................................................................................. p 22 AT313 | 2 Power system harmonic fundamental considerations Abstract This paper provides an overview of harmonic considerations for designing industrial and commercial electric power distribution systems. These power systems must serve a combination of loads, many of which produce non-sinusoidal current when energized from a sinusoidal AC voltage source. While conventional power distribution systems accommodate a significant amount of non-sinusoidal current, the design engineer can utilize existing IEEE guidelines and basic software tools to avoid some special circuit and load configurations that exacerbate harmonic distortion problems. Get connected to power AT313 | 3 Power system harmonic fundamental considerations Introduction Power system harmonic distortion has existed since the early 1900s, as long as AC power itself has been available. The earliest harmonic distortion issues were associated with third harmonic currents produced by saturated iron in machines and transformers, so-called ferromagnetic loads. Later, arcing loads, like lighting and electric arc furnaces, were shown to produce harmonic distortion as well. The final type, electronic loads, burst onto the power scene in the 1970’s and 80’s, and has represented the fastest growing category ever since. Get connected to power A better understanding of power system harmonic phenomena can be achieved with the consideration of some fundamental concepts, especially, the nature of non-linear loads, and the interaction of harmonic currents and voltages within the power system. AT313 | 4 Power system harmonic fundamental considerations Harmonic Distortion Basics What’s Flowing on the Wire? By definition, harmonic (or non-linear) loads are those devices that naturally produce a non-sinusoidal current when energized by a sinusoidal voltage source. Each “waveform” on the right, for example, represents the variation in instantaneous current over time for two different loads each energized from a sinusoidal voltage source (not shown on the graph). Get connected to power Graph 1 For each load, instantaneous current at some point in time (at the start of the graph, for example) is zero. Its magnitude quickly increases to a maximum value, then decreases until it returns to zero. At this point, the current direction appears to reverse – and the maximum-to-zero-magnitude trend repeats in the negative direction. This pattern is repeated continuously, as long as the device is energized, creating a set of largelyidentical waveforms that adhere to a common time period. Both current waveforms were produced by turning on some type of load device. In the case of the current on the left, this device was probably an electric motor or resistance heater. The current on the right could have been produced by an electronic variable-speed drive, for example. The devices could be single- or threephase, but only one phase current waveform is shown for illustration. The other phases would be similar. How to Describe What’s Flowing on the Wire? Fourier Series While the visual difference in the above waveforms is evident, graphical appearance alone is seldom sufficient for the power engineer required to analyze the effects of non-sinusoidal loads on the power system. The degree of non-linearity must be objectively established, and the method of quantifying the harmonic distortion must also facilitate future analysis and mitigation. AT313 | 5 Power system harmonic fundamental considerations Graph 2 One method of describing the non-sinusoidal waveform is called its Fourier Series. Jean Fourier was a French mathematician of the early 19th century who discovered a special characteristic of periodic waveforms. Periodic waveforms are those waveforms comprised of identical values that repeat in the same time interval, like those shown above. Fourier discovered that periodic waveforms can be represented by a series of sinusoids summed together. The frequency of these sinusoids is an integer multiple of the frequency represented by the fundamental periodic waveform. The waveform on the left above, for example, is described entirely by one sinusoid, the fundamental, since it contains no harmonic distortion. The distorted (non-linear) waveform, however, deserves further scrutiny. This waveform meets the continuous, periodic requirement established by Fourier. It can be described, therefore, by a series of sinusoids. This example waveform is represented by only three harmonic components, but some real-world waveforms (square wave, for example) require hundreds of sinusoidal components to fully describe them. The magnitude of these sinusoids decreases with increasing frequency, often allowing the power engineer to ignore the effect of components above about the 50th harmonic. The concept that a distorted waveform (even a square wave!) can be represented by a series of sinusoids is difficult for many engineers. But it is absolutely essential for understanding the harmonic analysis and mitigation to follow. It’s important for the power engineer to keep in mind a few facts: The equivalent harmonic components are just a representation – the instantaneous current as described by the distorted waveform is what’s actually flowing on the wire. • This representation is necessary because it facilitates analysis of the power system. The effect of sinusoids on typical power system components (transformers, conductors, capacitors) is much easier to analyze than distorted signals. • Power engineers comfortable with the concept of harmonics often refer to individual harmonic components as if each really exists as a separate entity. For example, a load might be described as producing “30 A of 5th harmonic.” What’s intended is not that the load under consideration produced 30 A of current at 300 Hz, but rather that the load produced a distorted (but largely 60 Hz) current, one sinusoidal component of which has a frequency of 300 Hz with an rms magnitude of 30 A. AT313 | 6 Power system harmonic fundamental considerations • The equivalent harmonic components, while imaginary, fully and accurately represent the distorted current. As one test, try summing the instantaneous current of the harmonic components at any point in time. Compare this value to the value of the distorted waveform at the same time (see chart below). These values are the same. Graph 3 Total Harmonic Distortion The series of harmonic components that represent a distorted waveform are often described by a single number, total harmonic distortion. This number is calculated in two different ways, depending somewhat on the engineer’s geographic location. In the United States, total harmonic distortion is calculated as the sum of all the harmonic components (except the fundamental), divided by the magnitude of the fundamental. This value is represented as THD (all upper case). Or, in equation form: Note that the components are summed vectorially, not algebraically, because they have different phase angles. For a waveform represented by a fundamental current of 100 A, a 5th component of 20 A, and a 7th component of 12 A, for example, Ih would equal the square root of (202 + 122), or 23 A, not (20 + 12) = 32 A. The THD is, therefore, 23/100 = 0.23 or 23%. It is possible for the US-convention THD to exceed 1.0 or 100%, since it is possible for the magnitude of harmonic current to exceed the magnitude of fundamental current. This is the primary distinction between the US and European convention. The European convention, thd (all lower case) equals the harmonic components divided by the total rms current (harmonics plus fundamental). This thd value can never exceed 100%. AT313 | 7 Power system harmonic fundamental considerations Voltage and Current Distortion Harmonic Current Flow The current drawn by non-linear loads passes through all of the impedance between the system source and load. This current produces harmonic voltages for each harmonic as it flows through the system impedance. These harmonic voltages sum and produce a distorted voltage when combined with the fundamental. The voltage distortion magnitude is dependent on the source impedance and the harmonic voltages produced. Figure 4 illustrates how the distorted voltage is created. As illustrated, non-linear loads are typically modeled Get connected to power as a source of harmonic current. Figure 4: Creation of distorted current With low source impedance the voltage distortion will be low for a given level of harmonic current. If harmonic current increases, however, system impedance changes due to harmonic resonance (discussed below), voltage distortion can increase significantly. Circuit Impedance – Without Power Factor Correction While the preceding discussion focused on distorted current waveforms, it is important to note that ac voltage can also show the effects of harmonic distortion. The degree of distortion is determined by applying the same techniques as described earlier for current. So, what is the relationship between voltage distortion and current distortion? Higher-frequency (harmonic) components of ac voltage and current follow the same power system rules as 60 Hz voltages and currents – Kirchoff’s Voltage and Current Laws, Ohm’s Law, etc. One basic principle is that voltage and current are related by impedance. As with ac voltage and current, impedance is a complex term, consisting of resistance, capacitance, and inductance. While resistance is largely independent of frequency, impedance associated with capacitance and inductance changes as the frequency of the signal changes. AT313 | 8 Power system harmonic fundamental considerations For the typical industrial power system, the impedance as seen by the loads is dominated by inductance. Since inductive reactance is directly proportional to the frequency of the current, the system impedance approximates a straight line, as illustrated below. 1.50 Typical system impedance (without power factor correction capacitors) Impedance, Ω 1.20 0.90 0.60 0.30 0.00 0 300 600 900 1200 1500 Frequency (Hz) Graph 4 For this typical power system, the impedance encountered by the 300 Hz (5th harmonic) component of current is approximately five times the impedance encountered by the 60 Hz (fundamental) component. With this type of power system, the amount of voltage distortion can be estimated by summing the voltage drop at each harmonic component, as summarized in the following table. The table assumes that the circuit load is represented by the single harmonic source shown earlier, with a total Irms = 102.7 A, and ITHD = 23%, with a nominal system voltage of 480 Vrms. Table 1 Harmonic Current, Irms Impedance, Ohms Voltage Drop, Vrms 1 100 0.01 1.00 5 20 0.05 1.00 7 12 0.07 0.84 Total Vh drop 1.3 Resulting VTHD 0.27% Resulting Vrms 478 Circuit Impedance – With Power Factor Correction Power factor correction capacitors are often utilized in industrial and commercial power systems to reduce power factor penalties, release circuit capacity, improve voltage regulation and reduce resistive heating losses in circuit conductors. While power factor correction capacitors do not inject harmonic distortion (that is, PFC’s are linear loads – they produce a sinusoidal current waveform when energized from a sinusoidal voltage source), their presence on a power system dramatically changes the circuit impedance. These impedance changes can adversely affect power system components, and worsen harmonic distortion concerns. AT313 | 9 Power system harmonic fundamental considerations Graph 5 When power factor correction capacitors are installed, a frequency of high impedance known as the resonance point results from the new combination of inductive and capacitive reactance. This resonance point is limited in magnitude only by the amount of resistance in the circuit, and is often many times the value of the inductive impedance at that frequency. The more capacitance added to the circuit, the lower the frequency at which this resonance point occurs. This high-impedance point, coupled with the operation of harmonic-producing loads, can result in much higher levels of voltage distortion than the circuit without capacitors. That’s why it is so important to closely evaluate the addition of power factor correction capacitors on a power circuit serving harmonic loads. The example estimate below shows the distortion estimate associated with the same 103 A, 23% THD load shown earlier, except that the impedance at the 7th harmonic is assumed to be ten times its non-PFC value (a resonance point at or near 420 Hz). Note that this change results in nearly a tenfold increase in voltage distortion. Table 2 Harmonic Current, Irms Impedance, Ohms Voltage Drop, Vrms 1 100 0.01 1.00 5 20 0.05 1.00 7 12 0.7 8.40 Total Vh drop 8.5 Resulting VTHD 1.26% Resulting Vrms 471 AT313 | 10 Power system harmonic fundamental considerations Harmonics and Power Factor – Displacement and Total As discussed, harmonic distortion and power factor correction are seldom considered as separate topics. This is due to the dramatic effect on system impedance at harmonic frequencies that can result from the addition of conventional power factor correction capacitors. The relationship, unfortunately, does not end there. Due to their non-linear nature, the presence of harmonic loads can sometimes fool the power engineer into considering unnecessary power factor correction in the first place! Get connected to power Power Factor of a PWM Drive – An Extreme Example? The pulse-width-modulated (PWM) variable frequency drive (VFD) produces a characteristic current waveform when energized from a sinusoidal voltage source. This three-phase device produces a voltage and current waveform for one phase that resembles the following graphic: Graph 6 If the power parameters (real, reactive, and apparent) associated with this PWM device are measured with a true-rms meter, the typical values would show a relationship of real (kW) to apparent power (kVA) of approximately 0.6. The engineer might conclude from this knowledge that the power factor of the device is poor, and that a circuit containing many of these PWM drives (not uncommon) would require power factor correction capacitors. Unfortunately, this line of reasoning is incorrect and can lead to disastrous results. While the kW/kVA relationship indicated above is accurate, 0.6 is not the power factor of the device. At least, it is not the complete picture of the power factor. Further measurements would reveal that the displacement angle between voltage and current for this device is 0. That is, the current and voltage are in phase with each other. Or, more accurately, the fundamental (60 Hz) component of voltage and the fundamental (60 Hz) component of current are in phase, as shown below. AT313 | 11 Power system harmonic fundamental considerations Graph 7 Since harmonic loads like PWM drives are able to consume power in a non-linear fashion; that is, by turning on and off in a manner not proportional to the applied instantaneous voltage, their kW/kVA relationship is not equal to the phase angle between fundamental voltage and current. This peculiarity, in fact, has required the establishment of two power factor definitions. These two power factors are equal for undistorted (sinusoidal) voltages and currents. Displacement Power Factor (dPF) – Cosine of the phase angle between fundamental voltage and fundamental current. Total (sometimes referred to as “True”) Power Factor (tPF) – Real power (kW) divided by apparent power (kVA). Power factor correction capacitors primarily affect the displacement power factor for a circuit. If PFC’s are applied on a circuit that already has a high dPF, then the fundamental current component could be shifted into a leading relationship to fundamental voltage. This situation can result in voltage regulation and distortion problems for the circuit. In addition, the addition of large PFCs on a PWM circuit can also increase the likelihood of harmonic resonance problems, and the resulting excessive voltage distortion issues introduced earlier. AT313 | 12 Power system harmonic fundamental considerations Harmonic Mitigation – Two Passive Techniques Harmonics Attenuation The earlier voltage and current distortion discussion, and voltage distortion estimates assume that the current distortion remains unchanged regardless of the circuit impedance, but this is not entirely true. Harmonic current distortion is affected by the amount of circuit impedance. In fact, an engineer will discover that placing the same harmonic producing load at two different nodes in a power system will result in two different levels of load current distortion. Get connected to power Power system designers can utilize this effect, called attenuation, as one method of passive harmonic mitigation. The current waveforms below show the effects of introducing a series line reactor (“choke”) at the terminals of a 100 hp pulse-width-modulated (PWM) adjustable-speed drive (ASD). The current total harmonic distortion associated with the ASD drops from about 81% to 38%. Graph 8 Graph 9 AT313 | 13 Power system harmonic fundamental considerations This attenuation effect is often employed to reduce the harmonic distortion associated with three-phase ASD’s. The ASD operation is not adversely affected, provided the line reactor chosen for the application does not exceed about 5% impedance (relative to the drive base). Harmonics Cancellation In addition to attenuation, harmonic current distortion can be reduced by cancellation. Cancellation occurs because individual harmonic components of a distorted current are affected differently when passing through normal power system transformers. The magnitude of harmonic currents, like the 60 Hz component, increases or decreases consistent with the transformer turns ratio. The phase angle of harmonic components, however, is influenced by the type of connection of the three phase transformer. The 5th and 7th components, for example, experience a 30° phase angle shift through a power system transformer connected delta-wye, as compared with the same current components transmitted through a wye-wye or delta-delta connected transformer. This phase-angle effect can be used with multiple ASD’s to reduce the current distortion on the circuit feeding the drives. As demonstrated in the diagram below, the alternating combination of delta-wye and wye-wye connections can produce much lower harmonic distortion for similarly-sized and similarly-loaded drives. The combination of line reactors and delta-wye transformers produces a similar cancellation effect. Graph 10 AT313 | 14 Power system harmonic fundamental considerations Common Harmonic Current Signatures IEEE 519a (Draft) Table Despite the preponderance of electronic loads, there are surprisingly few categories required to characterize the major harmonic-producing devices in industrial and commercial facilities. Electronic machines that share similar rectifier configurations create similar characteristic harmonic current signatures, as the table below demonstrates. Get connected to power The first column of the table describes the type of electronic device. A “single-phase power supply,” for example, indicates the typical switch-mode power supply inside a conventional personal computer. The second column shows the typical current signature, or waveform, that the device produces when energized from a low-impedance, sinusoidal voltage source. For three phase loads, only one phase current is shown. The other phases are similar in form, and separated in phase angle by 120°. The third column shows the typical ITHD associated with the waveform. Note that third and fourth entries represent the PWM drive – with and without line reactor – discussed earlier. The fourth and final column requires a bit more explanation. This column represents a weighting factor associated with each load type intended to facilitate simple harmonic assessments. The weighting factor for individual loads can be used to estimate the total weighted power requirement of all the harmonic loads in the facility. This weighted power requirement could then be compared against the system short circuit capacity to evaluate the likelihood of adverse effects associated with harmonic distortion. If the weighted power requirement of the harmonic loads, for example, could be shown to be less than 0.1% of the short-circuit capacity, then the likelihood of harmonic problems is low. A fuller discussion of this weighting factor is included in IEEE 519a, and in IEC Standard 61000-3-6. AT313 | 15 Power system harmonic fundamental considerations Table 3 Type of Load Typical Waveform Current Distortion Weighing Factor (W i ) Single-phase power supply 80% (high 3rd) 2.5 Semiconverter High 2nd, 3rd, 4th at partial loads 2.5 Six pulse converter, capacitive smoothing, no series inductance 80% 2 Six pulse converter, capacitive smoothing, with series inductance > 3%, or DC drive 40% 1 Six pulse converter with large inductor for current smoothing 28% 0.8 12 pulse converter 15% 0.5 AC voltage regulator Varies with firing angle 0.7 Fluorescent lighting 0.05 0.5 AT313 | 16 Power system harmonic fundamental considerations Harmonic Distortion Simulation Methods The weighting factor method for evaluating the likelihood of harmonic problems is applicable to only a small power system with few harmonic loads and no power factor correction capacitors. Most power systems do not fall into this category, so other, more sophisticated methods must be employed to evaluate harmonic concerns. Computer Techniques Get connected to power Simple radial networks can sometimes be analyzed utilizing hand calculations (as performed earlier for the small 103-A harmonic load). In most cases, however, these calculations quickly become tedious as the circuit size increases beyond a few nodes and devices. The most common computer techniques are based on nodal admittance equations for the network. These equations are usually stated in the form: I = Y * E, where I is the injected current at each node, Y is the circuit admittance matrix, and E are the node voltages. This Y matrix is built at each frequency, and the resulting equations are solved for the node voltages, E. The solution is carried out by either matrix inversion, Gaussian Elimination, or some other technique. Computers are especially suited for this solution task. Commercially-available software tools, like the HI_WAVE module of Power*Tools for Windows® (SKM Systems Analysis, Inc.; www.skm.com), facilitate harmonic analysis of complex systems and loads. These tools provide graphical interface to build a variety of circuit types, including radial, loop systems, and multiple independent systems of different voltage levels. They also contain a large library of conductor, transformer, capacitor, motor, and harmonic load types. The library eliminates the need to enter individual harmonic waveforms, for example, by offering the ability to use characteristic models already listed. Variable-Frequency Drive Applications Another software tool, called HarmCalc,™ has been developed to facilitate a common harmonic evaluation task: Application of variable frequency drives (VFD) to an existing low-voltage radial power system. While this tool is not accurate for complex systems, or for systems with power factor correction capacitors or harmonic filters, it is widely applicable for evaluating VFD applications. Many consulting engineers and end-users who specify VFD designs accept this tool as a suitable means of estimating VFD effect on a power system, and as a means of evaluating certain mitigating devices like line reactors and drive-isolation transformers, delta-wye transformer connections and broadband filters. VFD’s and IEEE 519 IEEE Standard 519 is frequently quoted in consulting engineer specifications associated with VFD installations. The gist of these specifications is that the VFD vendor assumes responsibility for supplying VFD’s that “comply with IEEE 519.” This specification requirement is clearly outside the original intent of the standard, and often unnecessarily increases the cost of a VFD installation. AT313 | 17 Power system harmonic fundamental considerations The original intention of IEEE Standard 519 was to introduce harmonic current and voltage distortion guidelines for electric utilities and their customers. The objective was to establish acceptable levels of current distortion that an individual customer could generate without adversely affecting other electric utility customers sharing the same distribution system. Further, this standard provided recommended limits for electric utility control of voltage distortion that could result from customer harmonic current injection Despite the misapplication, this specification requirement has become so widespread that IEEE 519 has effectively become the consensus equipment standard for VFD applications. Given that backdrop, use of HarmCalc and other tools in evaluating VFD installations requires further exploration, and definition of terms associated with IEEE 519. Point-of-Common Coupling As discussed, the circuit node at which harmonic current and voltage limits were to be evaluated was that point on the electric utility system at which other customers could be served. This so-called point-of-commoncoupling, or PCC is described graphically, as shown above. Often, VFD specifications that require 519 compliance will also designate the PCC. Generally, the closer this PCC is to the VFD terminals, the more costly the compliance requirements will be. Graph 11 Voltage Distortion Limits The voltage distortion limits in IEEE 519 are fairly straightforward, as reproduced below. There are only three levels recommended, the first intending to address sub-transmission and distribution circuits on the electric utility system. The voltage distortion, by the way, is calculated using the nominal voltage as the “fundamental” component (in the denominator of the THD calculation. Table 4 Bus Voltage at PCC (Vn ) Individual Harmonic Voltage Distortion (%) Total Voltage Distortion – THDVn (%) Vn ≤ 69 kV 3.0 5.0 69 kV < Vn ≤ 161 kV 1.5 2.5 Vn > 161 kV 1.0 1.5 AT313 | 18 Power system harmonic fundamental considerations The assumptions behind establishment of the voltage distortion limits may be useful in determining whether or not to invest in harmonic mitigation at low-voltage circuits. The 519 Working Group established 5% voltage distortion as the limit at electric utility distribution circuits under the assumption that customer harmonic loads would drive the VTHD higher at customer low voltage busses. The 5% THD limit was selected to allow lowvoltage busses to be maintained at 8% VTHD; a value that is acceptable to most linear and non-linear loads. Current Distortion Limits Determining current distortion limits is a more involved than voltage. This is due to the fact that the degree to which one customer’s harmonic loads might affect another’s is dependent on the utility’s system impedance at the PCC node. A relatively weak source impedance point in a utility’s system would reach the voltage distortion limit at a much lower harmonic current injection level than a stiffer point. That’s why the current distortion table requires a little more information in order to apply it correctly. Specifically, the short-circuit value at the PCC, ISC, needs to be determined. For PCCs on the electric utility circuit, the utility usually provides this number. The second required value is the estimated or measured demand current of the customer’s total load, IL. TDD – A Valuable Quantity to Learn and Apply Another useful term needs to be defined at this juncture – TDD, or, more accurately, ITDD. Note that the current distortion limits below are given in TDD, not THD. ITDD is similar to the definition of ITHD introduced earlier, with one important difference: The denominator is IL; that is, the total customer 60 Hz demand current. Recall that the ITHD denominator is the fundamental current of the distorted waveform, whether small or large relative to other loads on the circuit. The difference between the two quantities boils down to this: ITDD more accurately reflects the amount of harmonic current a power system can absorb than ITHD. Since ITDD is the amount of harmonic current compared to the customer’s total fundamental load current, it is a better predictor of the circuit’s ability to transmit the harmonic current without adverse effects. TDD is also an extremely valuable term to utilize throughout harmonic distortion evaluation and analysis, not just IEEE 519 assessment. ITHD, on the other hand, is often a misleading value. Suppose, for example, that a harmonic measurement indicates ITHD = 47%. Will this circuit experience harmonic problems? The ITHD value alone cannot be used to answer this question. If the 47% THD measurement is actually 0.47 A of harmonic current and 1.0 A of fundamental current, and the circuit in question has an ampacity of 1600 A, harmonic problems are unlikely. If, on the other hand, the 47% ITHD represents 470 A of harmonic and 1000 A of fundamental current on the same 1600 A circuit, harmonic problems are highly probable and should be investigated at once. For this reason, many power engineers prefer to calculate ITDD for a circuit in question, even if IEEE 519 compliance is not an issue. For low-voltage circuits within a facility, for example, they will often utilize the circuit ampacity, or some other reasonable estimate of the circuit’s capacity, for the denominator. AT313 | 19 Power system harmonic fundamental considerations HarmCalc Background To facilitate VFD application into power systems that utilize IEEE 519 guidelines, Square D and Electrotek Concepts co-developed an executable software program called HarmCalc. This software tool is available free of charge at Square D’s Design Resource Center; follow the “Consulting Engineer” link under the “Customers and Markets” listing at www.squared.com/us to “software tools” and download the file. The HarmCalc tool includes an electronic tutorial and help documentation, but the following example can illustrate its usefulness in evaluating VFD applications and some basic mitigating options. Note that HarmCalc also includes capabilities for exporting the solution information to two third-party software tools intended to facilitate additional analysis. Web links to these tools, SuperHarm™ and TOP,™ are available with the HarmCalc installation. Example The simple power system shown below was configured to serve as an example of the program and its use. There is a 1000 kVA transformer serving an MCC at 480 V. The electric utility system has been defined by its available-fault-current and X/R ratio (values typically available from the utility). The MCC already serves about 300 hp in induction motor load (“linear load”). Two Square D® VFDs are being added, and IEEE 519 compliance has been specified. The VFD’s specified are identical Square D model Altivar 66 VT, 100 hp each. These VFD’s (as described in the “Help” screen associated with VFD selection) are variable-torque devices, meaning that they are intended for application on variable-torque motor loads like centrifugal pumps or fans. The VFD’s chosen here are configured with 6-pulse diode rectifiers, although these VFDs are also available in 12- and 18-pulse versions. Three options have been considered; as summarized in the table below. Note that the unmitigated addition of the VFD’s would result in an ITDD slightly above the IEEE 519 current distortion limit, although the VTHD value is slightly below 5%. The 519 limits can be readily met if the VFD’s are equipped with 3%-impedance line reactors. While the delta-wye configuration for one drive results in the lowest harmonic distortion, this option is likely to be less cost effective. In addition, line reactors provide other benefits, including reducing the tendency of nuisance VFD tripping on electric utility power factor correction capacitor switching transients. Table 5 Harmonic Mitigation VTHD at PCC2 ITHD at PCC2 IEEE 519 Compliance? None 4.85% 8.92% No 3% line reactors only 2.23% 4.06% Yes Delta-wye/delta-delta only 1.97% 1.69% Yes AT313 | 20 Power system harmonic fundamental considerations Screen Capture 1 ReactiVar AccuSine® PCS Another useful software tool for the harmonic-mitigation designer is available on the Square D website. This spreadsheet-based tool facilitates sizing and application of the Square D ReactiVar AccuSine active harmonic filter. The active filter is the state-of-the-art method for harmonic current mitigation. It works by sensing the amount and type of harmonic current being required by the loads (through the “CT,” or current transformer, shown below), then injects harmonic current of the proper magnitude and frequency to cancel the load harmonics. The resulting current returning to the “source” is low in current distortion. This and other harmonic mitigation, power quality and power factor correction products and services are also available. Graph 12 AT313 | 21 Power system harmonic fundamental considerations References 1.IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems, ANSI/IEEE Std. 519-1992. 2.IEEE Guide for Applying Harmonic Limits on Power Systems – Unpublished Draft , IEEE Std P519.1™/D9a, January, 2004. 3.Electrical Power System Harmonics Design Guide, R.C. Dugan, M.F. McGranaghan, E.W. Gunther, Electrotek Concepts, Inc., Knoxville, TN, 1992. Get connected to power AT313 | 22 ©2011 Schneider Electric Industries SAS, All Rights Reserved. Schneider Electric USA, Inc. Data Center Solutions 1010 Airpark Center Drive Nashville, TN 37217 615-844-8300 sedatacenters.com Document Number AT313 October 2011 tk