ESL-IE-95-04-32 CURRENT GENERATED HARMONICS AND THEIR EFFECT UPON ELECTRICAL INDUSTRIAL SYSTEMS Harold R. Alexander, P.E. Industrial Division Electrical Section Head Black & Veatch Kansas City, MO. ABSTRACT This paper provides a general overview of harmonics and addresses the causes of current generated harmonics in electrical systems. [n addition, problems caused by current generated harmonics and their affects upon different types of electrical equipment; such as cables, meters, capacitors, motors, transformers, emergency generators, etc. are prescnted. Recommendations for solving harmonic problems are also provided. The paper discusses and analyzes two actual cases where harmonics caused problems in electrical systems, one case a computer center and the other an oil collection system using variable frequency drives (VFDs) for oil well pumps. The paper also discusses where the public utility industry appears to be headed in addressing harmonics. Daniel S. Rogge, P.E. Industrial Division Senior Electrical Engineer Black & Veatch Kansas City, MO. portions of the current or voltage wave form to pass. ODD NUMBERED HARMONICS For 6O-Hz power systems with nonlinear loads, the even numbered harmonics have bee found to be considerably less likely to occur at levels detrimental to electrical systems. This is because most nonlinear loads generate odd­ numbered harmonics, which are associated wit a current wave shape that is a distortion of the normal 6O-Hz positive and negative half cycles. This paper will concentrate on odd-numbered harmonics. Odd-numbered harmonics have positive, negative, or zero sequences that, in a balanced ­ phase system, can be defmed as follows. BACKGROUND The 3-phase, 60 Hz power provided by the electric utilities in this country produces current and voltage, under normal conditions, having an almost pure 6O-Hz sine wave. Any phenomenon that modifies this wave form on a steady basis will cause harmonic distortion. • Positive sequence harmonics consist of thr phasors, each equal in magnitude, separate fro each other by a 120° phase displacement and having the same phase sequence as phasors representing the normal 6O-Hz current. • Negative sequence harmonics also consist. f three phasors, each equal in magnitude, separa e from each other by a lWO phase displacement; however, they have a phasc sequence opposite ~o phasors representing the normal 6O-Hz current I Electrical devices or equipment having nonlinear impedance will require load currents that are not proportional to voltage. Thus, harmonics are generated thal, in turn, cause distortion to the current of an electrical system. Within a facility, the magnitude of this produced harmonic distortion will vary, depending on the size of the nonlinear loads relative to the facility's electrical system. • Zero sequence harmonics consist of three phasors equal in magnitude and having a zero phase displacement from each other. Therefor the phasors are concurrent in direction, thus I producing an amplitude that is triple of anyone phasor when they combine on the neutral of an! electrical system. These harmonics are called triplen harmonics and are symptomatic of phas~­ to-neutral nonlinear loads, such as personal computers, electronic ballasts, etc. I The most common sources of harmonics are generators and nonlinear loads. Nonlinear loads include solid state electric equipment or devices that constantly switch ON and OFF to only allow 197 Proceedings from the Seventeenth Industrial Energy Technology Conference, Houston, TX, April 5-6, 1995 ESL-IE-95-04-32 TABLE 1 shows the odd multiples of the fundamental 60-Hz current and the associated sequence (positive, negative, or zero). Note that the first harmonic is actually the fundamental 60­ Hz current (1 X 60-Hz). ELECTRIC GENERATOR AS A HARMONIC SOURCE Electric generators have always been a source of harmonics because of the way these machines arc designed and built. They do not generate a perfect sine wave. Depending on the pitch factor of windings and other design parameters, the magnitude and frequency of the harmonics generated will vary. Positive sequence harmonics from three phase nonlinear loads will cause a 3-phase motor (either induction or synchronous) to turn in the forward direction while negative sequence harmonics will try to force motors to turn in the reverse direction. Of course the rotation will depend upon the magnitude of the harmonics present compared to the normal 60- Hz current. Depending upon the harmonics present (5th, 7th, 11th, etc.) and their magnitude, the effect on the torque of the motor will vary and there will be some torsional vibrations, which may cause serious problems. The 3rd, 5th, and 7th harmonics are generated by what is considered a standard machine in this country; that is, a generator wound with 4/5 or 5/6 pitch. Generators can be purchased with specially pitched windings (usually 2/3 pitch) that will not generate 3rd harmonic currents. However, the 5th and 7th harmonics generated by these machines are approximately double those generated by a standard machine. Harmonic Sequence Harmonic Sequence 1 Positive 19 Positive 3 Zero 21 Zero 5 Negative 23 Negative 7 Positive 25 Positive 9 Zero 27 Zero 11 Negative 29 Negative 13 Positive 31 Positive 15 Zero etc. 17 Negative Table 1. Sequence of harmonics in a 3-phase power system. At least one manufacturer of diesel-generator sets offers as standard a 2/3 ritch machine, in ratings up to 1500kW range, that eliminates the 3rd harmonic. The triplen harmonics (3rd, 9th, 15th, etc.) will add together at the neutral or ground. However, if no neutral or ground path is available, they will not flow. If a transformer's winding is a grounded wye/neutral configuration, then the triplen harmonic current will pass through this winding and combine in an additive manner with unbalanced phase current at the ground/neutral connection. The harmonics will also be transformed to, and circulate in, a primary delta winding. (1) Historically, problems caused by generator harmonics were associated with telephone interference or large 3rd harmonic currents circulating through wye connected equipment fed from the generator bus. The telephone interference was usually caused by telephone lines lying parallel to distribution feeders that were connected directly to the generator bus. 198 Proceedings from the Seventeenth Industrial Energy Technology Conference, Houston, TX, April 5-6, 1995 ESL-IE-95-04-32 NONLINEAR LOAD AS A HARMONIC NATURE OF HARMONlC CURRENT FLOW SOURCE The amount of flow of harmonic currents in an electric system is determined by the impedances of the various components and conncetcd equipment. The 60-Hz impedances a available from manufacturers and industry standards. Nonlinear loads such as inverters, solid-state rectificrs used in welders, DC power supplics, variable frequency drives, and electronic ballasts for lighting, are sources of harmonics in the electrical system that feed these loads. In most cases, there will be specific harmonics associated with each itcm of equipment. Equipment manufacturers can usually provide information on the magnitude and order of harmonics generated by their equipment. However, depending on the design of the spceific item of equipment, thc harmonics may vary in frequency and magnitude as load changes on the equipment occur. Table 2 is a summary of the magnitude and order of harmonics that have been encountered with certain loads. It's common knowledge that inductance increases and capacitance dcereases as frequenc increases. In theory, these arc linear relationships. In actual practice, however, equipment designed for 60-Hz operation often havc characteristics that causc these relationshipS to be nonlinear. Theoretically, resistance does not change with frequency. In practice, however subtle details of construction can cause the resistance of a piece of equipment to vary Harmonic Order Load Descri ption I I I 5 3 1 <) 7 I 11 lJ I I 15 I I I Six-pulse rectifier 100 - 17 11 - 5 3 - Twelve­ pulse rectifier 100 - 3 2 - 5 3 - Eighteen­ pulse rectifier 100 - 3 2 - 1 0.5 - Twenty­ four pulsc rectifier lO() - 3 2 - 1 0.5 - Electronic! computer 100 56 33 II 5 4 2 1 Lighting! electronic 100 lR 15 R :1 2 1 0.5 Office with PCs 100 51 2R <) 6 4 2 2 YFD's (range) lOO ] to 9 40 to 65 4 to 8 3 to 8 17 to 41 1 to <) o to 2 Table 2. Harmonic currents with typical magnitudes produced by various types of equipment. "YFDs" denotc variable frequency drives. The numbers under thc harmonic ordcr are expressed in percent of the fundamental 6O-Hz current. 199 Proceedings from the Seventeenth Industrial Energy Technology Conference, Houston, TX, April 5-6, 1995 ESL-IE-95-04-32 considerably with frequency. The bottom line is that without considcrable calculation elnd testing of each piece of equipment, the actual impedances of the equipment at each harmonic frequency cannot be accurately determined. And without accurate impedance valucs, it's impossible to conduct an accurate Ilow analysis. • The magnitude of current and voltage wave form distortion in a facility's electric system will be dependent upon the relative size of the nonlinear loads with respect to that system. The distortion increases as the percentage of nonlinear loads increases. (2) PROBLEMS ENCOUNTERED WITH HARMONICS However, from cxperience with harmonics and from basic knowledge of electric system characteristics, thc following guidelines generally are true. High Neutral Conductor Currcnts Perhaps the dominant harmonic problem encountered in commercial facilities and some industrial plants has been the overheating of neutral conductors of 3-phase, 4-wire branch and feeder distribution systems. • Bccause of the relatively high inductive reactance of transformers, and since most transformers arc furnishcd with delta primary windings, harmonic problems are usually limited to that equipment connected to the voltage at which the harmonics arc generated. In a balanced, 3-phase, 4-wire wye system with phase-to-ncutral linear loads, the neutral current will be zero. Even with a maximum unbalance, the resulting neutral current will be no grealcr than the maximum phasc current. However, this samc system with certain nonlinear loads will generate triplcn harmonic currents (3rd, 9th 15th, ctc.), which will add in the neutral conductor. This can theoretically result in the neutral conductor carrying up to 300% of the rms current of a phase conductor, even in a balanccd system. This has caused neutral cable and neutral termination failurcs elt elcctric panels and transformcrs. • Capacitors, or equipment that have capacitive reactanec characteristics will appear as very low impedances to harmonics and will tend to allract the now of harmonic currents. • Resistive loads and resistive characteristics of connected equipment (motors, transformers, etc.) tend to dissipate harmonics. These dissipated harmonics end up as heat losses in this equipment. This can be a problem if thc equipment was marginally sized or if harmonic content is exccssive. Loads such as incandcscent lighting and resistance heating normally arc not affected by harmonic Ilow. Thc harmonics that would cause this high neutral current can be traccd back to the switchcd-mode powcr supplies in computers and other nonlinear loads which generate triplen harmonics. These devices generate large 3rd harmonic current, as well as other triplen harmonic currents because they demand current only at the peak of the voltage waveform. • On most clectric systems where harmonic currents arc present, they are usually created by nonlinear loads. Usually, most of the othcr equipment in a circuit is inductive equipment. Such equipment has inductive impedance, which provides a relatively high impedance to harmonic currents. Kirchoff's Laws will not allow these currents to disappear oncc gencrated. Therefore, they must flow into the inductive equipmcnt. Most of the energy in the positive and negative sequence harmonics are dissipated as heat in this equipment. In addition to the extra heat, the highcr frequency currents tcnd to work corc material harder and can cause prcmature corc saturation. In the case of transformers, part of the harmonics will be transmitted through the transformer. Unless filtered from the system at the computcr equipment's power supply, thc triplen harmonic currents will flow and seek the path of the least impedance; that is, through thc neutral conductors towards the transformer (source of power). With high neutral current and undersized neutrals, a facility can cxperience excessive neutral conductor heating, resulting in possible fircs, short circuits, or bus failure. 200 Proceedings from the Seventeenth Industrial Energy Technology Conference, Houston, TX, April 5-6, 1995 ESL-IE-95-04-32 Static power converters, used in adjustable speed drives and unintcrruptiblc power supplies (UPS) change the AC current waveform and thus, also contribute to the flow of harmonic currents. harmonic currents. This has resulted in blown fuses and disfigured capacitors. This is usually a result of the circuit being tuned or resonating at one of the harmonic frequencies. Heat Losses Increased losses, in the form of heat that is dissipated in electric equipment, will occur in a plant's electrical system because of harmonics. These losses are real energy power losses (kW losses). Therefore, a facility will see higher electric energy charges because of harmonic flow. Until fairly recently, these losses have typically been ignored because they are hard to define. Nevertheless, the user pays for the additional energy losses in the electrical equipment. Overeurrent Protective Device Operation Thermal overcurrent protective devices, suc as fuses and inverse-time circuit breakers, are affected by increased skin-effcct heating at the higher harmonic current levels. This excess heating can cause shifts in thc devices time­ versus-current characteristics, resulting in nuisance tripping. The magnetic trip function of older circuit breakers, whose operation depends on electro-magnetic force, is proportional to the square of peak current, not rms current. A high 1rd harmonic current, resulting in an abnormally high overall peak current, could cause these breakers to trip prematurely. Most new circuit breakers with electronic trip devices are now designed to respond only to true rms current an not peak current. Skin Effect Harmonic currents can cause overheating of conductors and insulating materials as a resull of a phenomenon callcd skin effect. This relates to the increase in AC resistance of a conductor as frequency increases. Normally, current density within a conductor is greater near its surface or skin. Skin effect, or depth of currcnt penetration, is inversely proportional to the frequency of thc current. Thus, the higher the frequency, the less skin depth available in the conductor. When this occurs in transformers, the result is cxtra losses in the windings. When protcctive relays are subjected to system harmonics, relay misoperation is possible, possibly resulting in undesircd pick-up values, changes in voltage and current operating characleristics, and falsc tripping. Metering Errors Harmonics can causc errors in induction wall-hour meter readings. Since the induction disks are designed to monitor non-distorted fundamental current, harmonics will cause measurement errors. This may result in the end user paying more for electricity than if the same rms current being drawn was sinusoidal. Harmonic voltages and currents will increase the rotor winding and stator winding losses in motors. Since these losses are fR losses, increased heating due to skin effect can be expected at thc higher harmonic frequencies. Transformer Derating Waveform distortion will cause increased heating in all types of transformers. This heating is due to an increase in the frequcncy-depcndent eddy current and hysteresis losses. Increased heating can also be expected from skin effect heating in thc windings. For transformers experiencing large harmonic current flow, derating of transformers may be required. Elcctronic Equipment Malfunction When the system voltage waveform become distorted, electronic equipment also can malfunction. For instance, electronic clocks that count zero-crossings in the waveform may not operate correctly because the distorted waveforn provides more zcro-crossings t han a nondistorte waveform. Thus, these clocks will run fast, causing the equipment they control to incorrectl operatc. Capacitor Failure Since the impedance of a capacitor is frequency dcpendent (decreasing reactance with increasing frequency), capacitors will be negatively affected by harmonics. Thus, power factor correction capacitors will appear as very low impedances paths and tend to altract Some generator voltage regulators measure peak voltage in controlling the generator's outpU\ voltage. A distorted voltage waveform results in a high peak-to-rms ratio. Thus, a generator wouldl 201 Proceedings from the Seventeenth Industrial Energy Technology Conference, Houston, TX, April 5-6, 1995 ESL-IE-95-04-32 "sec" higher peak voltages, causing the generator voltage regulator to malfunction. particularly effective when the 3rd harmonic is presenl. With communication systems, cross talk caused by harmonic pickup on communication links between sensitive electronic equipment can occur, resulting in erroneous data transmission. NEUTRAL AND PHASE CURRENT COMPARISON Another method to identify the presence of triplen harmonics is to measure the phase and neutral currents with a true rms meter. If the neutral current is greater than what would normally be the unbalance of the phase currents, 3rd or triplen harmonics are presenl. MEASURING VOLTAGE OR CURRENT HARMONICS The presence of harmonics (i.e., waveform distortion, particularly from the 3rd harmonic) may be detected by taking rms and instantaneous peak readings of either current or voltage. Since all harmonics generated by nonlinear loads are current generated, the current reading usually will be more sensitive than voltage readings. Special note: current readings must be taken at the power source of the nonlinear loads while voltage readings can be taken almost anywhere on the bus. In measuring current and voltage where harmonics are present, true rms reading instruments must be used. These meters are usually calibrated in rms amps or volts, based on a crest factor of 1.4] 4. Conventional meters (analog and digital) measure either the average value or the peak value of a waveform, and then are calibrated to read the equivalent rms value. The average-value measuring meter is usually termed an average­ responding or average-calibrated meter. The peak-value measuring meter is usually termed a peak-sensing meter. These meters will give mis­ leading readings when harmonic distortion is present. For example, on a square wave, the average-calibrated meter will read rms values about 11% high while the peak-calibrated unit will read about 30% low. For pulses, the errors can be tremendous, depending on the height of the peak and the off-time between pulses. The average-calibrated meler will read very low, (as much as 50%) while the peak-calibrated unit will read very high (sometimes more than 100%). VIEW THE WAVEFORM Another effective way to identify harmonics is to look at the voltage and current waveforms with an oscilloscope. Current transformers (CTs) will be required to view the current waveforms. These CTs must be of high quality (with a very high band width) to sense the high frequencies accurately, if the rms reading is to be accurate for high-order harmonics and pulsed currents. (This is not a problem for pure 60-Hz sine waves. SPECTRUM ANALYZER MEASUREMENTS A spectrum analyzer can record the current and voltage waveforms, determine the magnitude and types of harmonics present, and provide a printout of these data. If equipment under evaluation has its power supplied from on-site emergency generators, readings of the current and voltage waveforms at the equipment should be taken with the generators on-line. This will probably be the worst case because on-site emergency generators typically represent a greater source impedance than the utility system. As a result, higher voltage distortion on the facility's electrical system can be expected when the emergency generators are running. ACTUAL CASES WITH HARMONICS Computer Center Background An existing computer center had been operating without an emergency generator for several years. However; because of an expansion in the computer center and its increased criticality to company operations, a decision was made to install an emergency diesel-engine generator to supply all of the computer center's operations during loss of normal (utility company) AC power. The computer center loads were served PHASE CURRENT MEASUREMENTS If the phase current readings of a true rms sensing meter are significantly different than those of an average responding meter, it's likely that harmonics are present and arc distorting the current waveform. The difference in readings is a function of how the two types of meters measure, as previously discussed. This method is 202 Proceedings from the Seventeenth Industrial Energy Technology Conference, Houston, TX, April 5-6, 1995 ESL-IE-95-04-32 unit did not experience any problems because of barmonics. power via a 12.5 kV to 4R0/277 volt step-down transformer and 480/277 volt switchboard. The emergency diesel-engine generator was installed and connected at 12.5 kV to thc computer center's step-down transformer using a ] 2.5 kV automatic circuit breaker transfer scheme between the emergency generator and utility power. Oil Collection System Using VFDs for Oil Well Pumps Background An electric power system was installed to support an oil field installation. Because of the location of the oil field, a remote location, the power system was not connected to a utility company grid. Power was produced by using multiple 13.8 kV diesel-engine powered generators and then transported to various areas using two 69 kV transmission lines. For reliability, the 69 kV system could be operated as a closed loop system. Several distribution substations were connected to the 69 kV transmission system and in turn distributed l3.R kV power to the oil wells. At each oil well, the power was transformed down to 480 volts and then connected to a variable frequency drive (VFD) and the well pump motor. Approximatel 80 percent of the systcm load was the VFDs at the oil wells. Problem During testing of the emergency generator, it was discovered that the generating unit's controls were not stable when serving the computer center's loads. An oscilloscope showed that the system voltage waveform at the emergency generator was very distorted. Investigation An investigation revealed that most of the computer center's loads were supplied via uninterruptable power supply (UPS) modules that utilizcd twelve-pulse rectifiers. The UPS modules had been installed without input filters to limit harmonic distortion on the electrical system. Measurements were taken of each UPS module's input voltage and input current waveforms. Typical values measured for total harmonic distortion (THO) were 5.5 percent of the 60-Hz voltage and ]6.2 percent of the 60-Hz current when operating on the utility company's source of power. Though not measured, we would expect the THD values to be higher when operating on the emergency generator because of the generator being a higher impedance source when compared to thc utility company grid. Problem During starting and checkout of the system, the manufacturer of the diesel-generators measured the voltage waveform at their equipment and expressed concern that an overvoltage condition existed. Using an oscilloscope, the manufacturer measured 11 percent peak-to-peak ovcrvoltage in the waveform. Depending on system loading, 25 percent peak-to-peak overvoltage had been detected. The manufacturer was concerned that insulation material would deteriorate at a higher rate than normal. RMS metering did not dcteet the overvoltage peaks and plant operators were unaware of the problem. Solution Input filters were installed on each of the UPS modules and THO measurements retaken. Typical THO valves measured were 2.7 percent of the 60-Hz voltage and 6.3 percent of the 60-Hz current when operating on the utility company's source of power. Investigation In addition, it was discovered that the generating unit's voltage regulator was sensitive to harmonics because the regulator measured peak voltage. The unit's controls and metering circuits were also filtered to avoid harmonic problems. A study was performed Lo evaluate and mak recommendations addressing the diesel-engine generator manufacturer's concerns. Field measurements of current and voltage distortion were taken at all operating oil wells using a spectrum analyzer. Voltage readings were taken at power plant and substation buses. The VFDs were six pulse without any input filtering provideq Aftcr the above modifications were made, the generating unit was retested. The generating 203 Proceedings from the Seventeenth Industrial Energy Technology Conference, Houston, TX, April 5-6, 1995 ESL-IE-95-04-32 on the drives. Each VFO was fed by a delta-wye pad-mounted transformer. THO of voltage measured at the VFDs input varied between 19 percent and 28 percent, while the THO of current varied between 24 percent and 59 percent, depending on loading of VFO. In addition, the largest harmonics measured, were the 5th and 7th harmonics. The 5th harmonic current measured varied between lR perccnt and 49 percent of fundamental current, while the 7th harmonic current varied between 3 percent and 37 percent of fundamental current. The THO voltage measured at the 13.8 kV power plant bus was approximately 10 percent. The HI-WAVE software used considers the complete electric system model whcn analyzing harmonics. U nfortunatcly, it will only provide complete information on up to ten pre-selected buses and branches within the model. TheoretiealJy, a complete analysis of one operating condition for a 300 buslbranch system would require approximately ]5 computer runs. Then, if you eonsidcr the number of combinations of on-off operation of thirty wells, it becomcs obvious that it is not practical to analyze a system this large. rr we consider analyzing one substation with eight wells, there would be 256 well on-off operating combinations to be analyzed. If another well is added, there would be 5]2 operating combinations to analyze. With experience, it may be possible to eliminate some of these cases. However, the analysis required to provide proper operation for all operating combinations of wells with bus filters is substantial. Thc addition of one well would require substantial additional analysis. The cost of these studies and the inaccuracies caused by not having known device impedances at higher frequencies make the application of bus filters in this application im practical. The system was computer modeled using field mcasurements taken for harmonic distortion and system impedance data. Software used was SKM Systems Analysis, Inc. Power * Tools HI­ WAVES. From the model developed, alternative plans were investigated, including future load growth. The alternative plans considered VFO input versus distribution bus filters. The plans using the existing delta-wye transformers used filters tuned to the fifth harmonic. Thc plans which considered replacing half of the pad-mounted transformers with delta-della transformers used filters tuned to the 11th harmonic. The HI­ WA YES program includes a filter design feature. This was used to size the filters considered. In general, the vaJue of capacitance was selected and the computer calculated the resistance and reactance to tune the filter to the required harmonic. When an effective filter with reasonable values of capacitance, resistance, and reactance was found, it was lIsed in this study. Time did not permit optimization of filter design. Solution Jt was recommended to apply filters at the line side of each variable frequency drive. These filters essentially trap the harmonics at their source. By doing this, you essentially eliminate thc need for robust equipment designed to tolerate the presence of harmonics. Also, the losses due to harmonics heating electrical equipment are minimized. These filters minimize the harmonics on the distribution system, therefore, minimize the exposure to resonance at the various harmonic frequencies. Field data taken with most of the wells in service, did not indicate resonance at any of the wells. Originally, it was thought that passive distributed bus filters would provide thc best and least expensive solution. However, the requirement to have complete operating and expansion capabilities proved to be a problem for the bus filter. The computer analysis indicated that an effective bus filter for one operating condition could cause resonance at other operating conditions. Also, the analysis indicated that it was possible to have resonant conditions on the distribution system, depending on the configuration of lines, transformers, and loads. ADDRESSING HARMONICS Equipment Derating A generally accepted method of coping with harmonics is the derating of certain electrical equipment to compensate for the resultant additional heating. A 20 to 25% derating [actor for transformers and generators is typically employcd. However, this solution can still result in equipment failures because the full extent of losses and resultant heating may not he known. 204 Proceedings from the Seventeenth Industrial Energy Technology Conference, Houston, TX, April 5-6, 1995 ESL-IE-95-04-32 extra charge for a "beefed-up" design to handle the harmonics. Nevertheless, harmonics will still he present. With higher content of harmonic currents, additional derating may he required. Generator manufacturers are recommending thc derating of their units hy as much as 50%, depending upon the magnitude of harmonic currents present in the load. This would rcsult in generators sized as much as 200% of anticipated load: a very expensive way of coping with harmonics. The key to eliminaling harmonic problems is the use of filters at the respective nonlinear load. Since a filler is designed to reduce the amplitude of one or more fLXed frequency currents, a harmonic filter is one means of reducing waveform distortion. Since the harmonic frequencies are very close to the fundamental power frequency (60 Hz), a very selective filter is required. This specially tuned filter will tune out the dominant harmonic heing generated. K-Factor Transformers In the past few years, k-factor transformers have been marketed for systems with triplen harmonics. The design of these units eliminates the harmful effects within the transformer caused by harmonic problems. Such a transformer has a special core, double sized neutral lug, and special windings to handle the triplen currents and their negative effects. These featurcs result in a more expensive unit. A frequency selective harmonic filter can be used as either a series filler or a sh unt filter. A series filter uses a high impedance to block the power source from the now of harmonic current being generated by the nonlinear load. A shunt filter, on the other hand, provides a low impedance path so that the harmonic current will divert to ground. Although either method of filtering will reduce harmonics, the shunt filter is usually prefcrred, since the series filter must he designed ror full line current and insulated for line voltage. Transformers with Multiple Secondary Outputs Morc reccntly, the introduction of a transformer with multiple secondary outputs has been offered as a solution. [I lIses a wye connected secondary winding with multiple outputs phase shifted 15 electrical degrees from each other. As a result, the positive and negative sequence harmonics will tend to cancel in the transformer. However, unless these harmonics are equally balanced on the outputs, cancellation will not be complete. Theoretical data shows that for a 50% unbalanced load, the total harmonic current distortion can he cut in half. Our recommendation for most applications i. to filter the harmonics at their sources (at the nonlinear loads). By filtering at the harmonics' source, the magnitude of harmonics nowing on the electric system will be minimized. In today's market, manufacturers can provide filters that limit their equipment's current distortion. Large solid state electric equipment should he purchased with a filter which will filter its input to a reasonable level. SOLVlNG HARMONIC PROBLEMS Derating equipment, using k-factor transformers and/or transformers with multiple secondary outputs does not resolve harmonic problems. WHAT WlLL BE PUBLIC UTILITY COMPANIES RESPONSE TO INDUSTRY GENERATED HARMONICS'? The question today is, "How will public uti lit companies respond to industry generated harmonics?" To-date, most utility companies hav not enforced penalties associated with industry generated harmonics on their distribution system unless the utility company is experiencing problems with other customers. However, in the future, we can expect this to change. IEEE Standard 5] 9 has been rcvised to estahlish recommended practices and requirements for Harmonic currents still now in the system, and possibly on higher voltage level systems through transformers' windings. Additional real power losses also are incurred in cahles and motors. Additionally, more equipment failures can he expected, especially for electric equipment that is sized without any consideration for additional heating from harmonic currents that continue to now. And, specifying specialized equipment means that this equipment will have an 205 Proceedings from the Seventeenth Industrial Energy Technology Conference, Houston, TX, April 5-6, 1995 ESL-IE-95-04-32 harmonic control in electrical power systems for both voltage and current distortion. We believe utility companies will use this standard as the basis for establishing guidelines concerning allowable levels of harmonics generated by industrial customers. REFERENCES 1. Alexander, H.R., and D.S. Rogge, "Harmonics: Causes, Problems, Solutions ­ Parl 1," EC&M, January, ]994, pp. 35-38, 42. 2. Alexander, H.R. and D.S. Rogge, "Harmonics: Causes, Problems, Solutions ­ Part 2," EC&M, February, ]994, pp. 47-55. 206 Proceedings from the Seventeenth Industrial Energy Technology Conference, Houston, TX, April 5-6, 1995