HARMONIC ANALYSIS OF [NDUSTRIAL POWER SYSTEMS

Robert Ellis, P Eng , Member, IEEE

Allen-Bradley Canada Ltd 135 Diindas Street

Cambridge, Ontario N I R 5x1

:lbstmcl- When large harmonic producing loads are added to an industrial plant power system it is good engineering practice to analyze the impact on the power system by performing harmonic modeling analysis of the system at the design stage. Such a study can identify any potentially hannful resonances or other harmonic levels that are predicted to be in excess of IEEE 5 19 recommended limits and suggest corrective measures (if necessary). This paper discusses the impact of the harmonic limits of IEEE 519-1992 on the: industrial power consumer and addresses the differences between the 1992 and 1981 versions of the standard. Harmonics produced by variable frequency drives are discussed The data required to conduct a harmonic study, the types of analyses that can be performed, and some of the mitigating measures that can be taken to alleviate a potential harmonic problem are detailed. A case study is presented basetd on a typical paper mill where a large variable frequency drive was added to the power system.

INTRODlJCTlON

Power system harmonics is an area that is receiving a great deal of attention recently This is primarily due to the fact that non-line,ar loads are comprising a larger and larger portion of the total connected load for a typical industrial plant. If power factor correction capacitors are applied to a power systerm the results coluld be disastrous in some instances without due consideration to natural frequencies of the system and harmonic producing loads. If a reso:nanc:e occurs there is a potential for capacitor hsle blowing or premature equipment failure, or transformer or motor overheatling.

Harmonics are a mathematical way of describing distortion to a voltage or current waveform, are a continuous, steady state phenomenon, and should not be confused with spikes. surges or other forms of power system transients. Fourier theory tells us thai. any repetetive waveform can be esprcssed as the summation 01 a number of sinusoids of various frequencies. Harmonics, by definition, are components of a waleform which are integer multiples of the fundamental frequency.

to address harmonic concems prior to the addition of large non-linear loads by performing harmonic modeling analysis. IEEE 5 19- 1092, the major standard that governs harmonic limits is also interpreted for the reader.

The: intent of this paper is to give an overview of how

IEEE 519 AND ITS IMPACT ON THE INDUSTRIAL POWER CONSUMER

IEEE Standard 519-1992 entitled "IEEE Recommended Practices and Requirements for Harmonic Control in Electric Power Systems" has been officially released and put into effect. There have been some significant changes made to this document since it was first published in I98 1. The main emphasis i n these changes has been to establish an approach to set harmonic limits on individual power consumers that will result in reasonable harmonic voltage distortion on the utility power system. An oveniew of the major changes to this harmonic standard follows [ 11.

Response Characteristics." The section addresses. in some detail, the factors that influencc the response o f a power system to harmonics. Some of the factors discussed arc: short circuit capacity, capacitors, cables, system loads, system unbalances, and parallel and series resonance. Some useful assumptions for typical distribution, transmission. and industrial systems are also outlined.

A separate section has been added that gives information on the harmonics produced by sis pulse rectifiers, arc furnaces, static VAR compensators, three phase inverters, electronic phase control, cycloconverters. switch-mode power supplies, and pulse-width modulated drives. The 198 1 standard quoted typical harmonic magnitudes that were :;upposed to be representative of any six-pulse converter. The 1992 standard contains curves which take into account the impedance of the power system and the amount of DC ripple in the current to more accurately characterize the harmonic currents that will be generated by particular converter on a given power system.

More information has been added regarding the effects of harmonics on various types of equipment such as motors, cables, capacitors, electronic equipment, metering, switchgear and relays, telephones, and static converters.

The portion of IEEE-5 19 that has the most impact on an industrial plant is the "Recommended Pract.ices for Individual Consumers." Substantial changes have been made in this section compared to the 1981 version. IEEE 519-1981 stated 5% total harmonic distortion as a recommended limit for voltage distortion on a general power system. 'The 198 1 standard did not recommend limits for individual consumers. This left some gray areas between utility and consumer responsibilities. The new version makes the responsibilities of the individual consumer and the utility clearer The guiding philosophy has not changed much in the 1992 standard in that the goal is to limit voltage distortion at the point of common coupling (PCC) to 5%. The PCC is generally defined as the

A new section has been created entitled "System

0-7803-2(p28-X-6/94 $4.00 Q 1994 IEEE 116

utilit!-/customer connection point. On a smaller scale, the PCC within ;in industrial plant can tie used. for example, as an artificial interface between mill di\.isions to address specific concerns about harmonic distoilion.

The major difference in the recommended practices is the introduction of current distortion limits. There are differenl current distortion limits depending on which one of f i~c categories the particular power system falls; into. On the one extreme of this spectrum is a power system that serves 1 or 2 large customers. The other end of the spcctrum is a power systeni that sen'es many small custorners. Different current distortion limits are assigned based on which category a particular power system falls into. The idea behind this is to allou individual power consumers their fair share of harnioiiic current distortion while assuring that voltage distortion at the PCC does not esceed 5% THD (barring a significant parallel resonance at ;I harmonic frequency). The a~ctual basis for categorizing a power system is the ratio of maximum short circuit current to maximum demand load current at the P U ' .

Rat10 = Isc/Il where. Isc = maximum short-circuit current at PCC 11 = niawnum demand load current at PCC

Indi\.idual and total currelit harmonic distortion limits are expressed as 21 percentage of thc maximum demand load current (not the fundamental current of a particular harmonic producing load) and is rcferrl:d to as total demand distortion (TDD). Most large industrial power systems seem to fall into the categor) that limits current distortion (TDD) to 5% with additional, lower limits on individual harmonic currents. One of the main factors that will decide whether the limits will be esceeded is the relative current of the harmonic producing loads in comparison to the total load for the plant.

The harmonic current limits in thc tables of this scction applj. for six-pulse rectifiers. The benefit of implementing rectifiers wiih higher pulse numbers has been recognimd and the limits have been relaxed on the characteristic harmonics as. long as tlie non-characteristic harmonics are 25% or less of ihe six-pulse limits. Using a twelvc-pulse rectifier as an example. the lirnits on the 1 lth. 13th. 23rd. 25th. 35th. 37111, -17th. & 49th harmonics are increased b), a factor of 1.414 This :issum(:s that the 5th. 7111, 17th. 19th. 29th. 1lst. -1lst. & 43rd harmonics do not esceed 25Y0 of the limits in the tables What this means is that a larger twelve-pulse rectifier could be used compared to a six-pulse rectifier v,hilc niaintaining thc limits specilied.

reached on the point of coninion coupling. The closer the PCC is to the input terminals of the non-linear loads the more difficult 11 uill be to nieet these reconiniendations without adding cost to the installation of the system.

Befor-e applying 1EE:E 5 19 an agreement must be

HARMONlCS P R O D U C E B Y V A R l A m FRE 0 I J EN C Y DRIVES

The pulse number of the rectifier is the determining factor in nhat the characteristic power system harmonics will be for a pwticular tinkc The harrtionics produced by a six-pulse

rectifier will be the 5th, 7th, 11 th, 13th, 23rd. 25th. etc. Their magnitudes are roughly the in\ erse of the harmonic order tinics the magnitude of the hndamental (e.g., the 5th harmonic is about one fifth of the fundamental current). A Iwelvc-pulse drive will exhibit harmonics at the 1 ltli. 13th. 2.3rd. 25th. etc. Twelve-pulse drives will produce small amount:; of 5th. 7th. 17th. and 19th harmonics (typically on the order of 1 0 perccnt of the levels for a six-pulse drive).

the inverter to the motor. These harmonics are typically multiples of the inverter operating frequency (not the power supply frequency) but no generalization can be made about their magnitude since this varies greatly with the typc of drive and the switching algorithm for the inverter semiconductors.

and the output waveforms. Interharnionics do rlot f i t tlie classical definition of harmonics since they are not strictly integer multiples of the fundamental frequency, Intcrliarnionics can be a result of rectifier harmonics showing u p on the output or inverter harmonics appearing at the input. Harmonics can occur on the input that are at the pouer system frequency plus or minus the inverter operating frequency but are normally small in magnitude. The inverter output can contain harmonics that are the rectifier pulse number tinies the power sj stem frequency plus or minus the inverter operating frequency anid can be significant in magnitude. These output interharmonics are related to the ripple in the DC link current c'r voltage

Drives also produce harmonic currents on the output of

Some interharmonics may also be prescnt in the input

KEY ELEMENTS IN HARMONIC MODEL= ANALYSIS Comuonents which need to be included in the model

All but the simplest of power systems will require a computer simulation program to perform a ineaiiingful harmonic study in a reasonable amount of time. Most information that is required to model a n industrial plant power system is available from the overall one-line diagram for the system 12],[3].

equivalent (i.e., short circuit MVA, and per unil 60 Hz inductance, and resistance). It is generally sufficient to iissunie that the harmonic impedance of the utility systein will be the harmonic number times the furidamental impedmce and that the X/R ratio of the system is constant for all frt,quencies. In some cases, the utility may have measured and documented the impedance of the power system at a number of frequencies which is referred to as the harnioiiic envelope. 'The utility's harmonic envelope should be used when available instead of making assumptions about the harmonic imped;inces as stated above.

All power factor correction capacitors must be incl~uded in the model. This applies to large capacitor baitlks applied to a bus as well as capacitors that are switched with individual motors. Capacitors in combination with the sys'lem inductance will determine the resonant frequencies of the system.

from tlie nameplate. The actual X / R ratio of tlir: transformer should be used if readily available. An X/R rati3 of 1 0 can Ibe assumed for most distribution transformers if th.: actual ratilo is

The utility should be modeled as its short-circuit

Transformers are modeled using percentage impedance

not available. The resistance of a transformer will change with 117

frequency and often the X / R ratio is assumed to be constant as frequency changes for the purposes of harmonic modeling. The winding connection configuration (delta or wye) should also be taken into account since a 30 degree phase shift will occur between delta and s q e windings.

Cable impedances within a plant are not usually significant for harmonic modeling purposes. Cables will have the effect of slightly dampening the system response at or near a resonant frequency. Analyzing ithe system without modeling the cables is the worst case. It is usually not practical or worthwhile to collect data on exact lengths and sizes of all cables in the system.

Significant motor loads, should be modeled by their subtransient reactance which cain be approximated based on locked rotor current if reactance is not readily available. Large motors should be modeled individually while smaller motors may be lumped together as a single impedance. Motors have the effect of raising the parallel resonant frequency of the power system since the inductance is in parallel with the system inductance and tlie resonant frequency is inversely proportional to the system inductance.

multiple current sources (one for each characteristic harmonic frequency). As mentioned previ,ously, IEEE 5 19 contains some very useful information to deterimine analytically the harmonic currents generated by a number of different types of non-linear loads.

inductive and resistive component. The inductive component of the load will have the effect of raising the natural frequency of the system. The resistive component will lower the peak of a resonance.

Harmonic producing loads are generally modeled as

Other loads on the system should be modeled as an

Types of analyses which can be performed Voltape and current distortiianalvsis

The computer mocleling software used must be able to provide predicted total harrnonic distortion of voltage and current for each bus or branch of the system. The calculations are usually performed by creating a system of “n“ equations of ‘‘n’’ unknowns, applying Ohm’s and Kirchoff s Laws, and using sparse matrix techniques to sol\.e for the unknown voltages and currents.

compared to the recommended practices of IEEE 5 19. A high distortion levcl in a particular portion of the system may indicate a resonance condition. The software package should also calculate the magnitude of individual voltage and current harmonics. Analysis of individual harmonics can help in predicting a resonance pricir to energizing the system.

The voltage and current distortion figures can be

Impedance ainalpsis

Impedance versus frequencj for each bus of the system in a tabular and/or graphical format IS another useful feature in a modeling system. A self impedance plot gives a quick visual indication of the natural frequencies at a specific bus. A peak in the impedance plot indicates a parallel resonant frequency and a valley indicates a series resonant frequency.

The effect of a parallel resonance is to amplifq one or more current harmonics if they fall at or near a natural frequency of the system. Similarly, a series resotrance can cause amplification of harmonic voltages. The height of the peak or depth of the valley gives an indication of the expccted amplification of a harmonic that is close to a resonant frequency. The resistive impedances i n the system \+ill deterniine the amount of amplification that OCCUI’S in ii resonant condition. The greater the amount of resistance. the lower the amplification of the harmonics. Therefore. if analysis of harmonic amplification is of great importance in the study, particular attention should be paid to accurately iinodel resistive elements.

Telephone Influence (I. T produca

In some cases it may be beneficial to have ;in

indication of the expected level of telephone influence in the branch of the system that connects to the utilitl. Telephone interference is not generally a problem within a plant ;I long as appropriate segregation of wiring classes is impleniented. Telephone influence can occur where there are Icing, parallel runs of utility power cables and telephone cables The most common way to express telephone interference is the I T product. The 1.T product takes into account the rms value of each injected harmonic current as well as a weighting factor for each frequency since certain frequencies 1iaL.e a greater effect on telephone circuits than others. IEEE 5 19 contains some rough guidelines on 1.T levels that are likely or not likcly to cause interference.

CASE STUDY - PAPER MILL - APPLICATION OF LARGE HORSEPOWER MEDIUM VOLTAGE DRIVE

Backmound

A harmonic modeling study was performed i n

conjunction with the addition of a 1250HP, 2300V. medium voltage. variable frequency drive system. The utilit) feed to the mill was at 69KV and the main transformer for the system was 20 MVA. The primary elements in the power s>.stein model were transformers, large induction motors, power factor correction capacitors, and the drive as a harmonic current source (see Fig. 1). For the purposes of the stud) it was assumed that all motors operated continuously. and simultaneously. I n reality, there niay be times when only some of the motors are in operation.

Results

Most of the predicted harmonic levels’ were within the recommendations of IEEE 5 19 except for tlie vciltage distortion at Bus 8 (Fig. 1) which was expected to be about G.8?4, THD. After reviewing the impedance plot for Bus 8 and re\iewing the individual harmonic levels for this portion of the system it was apparent that resonances were at the root of tlie excessive voltage distortion. The 7th. 1 Ith. and 13th harmonic currents were predicted to be higher in this branch of thc system than the levels produced at the input to the drive. I t was evident from the impedance plot that parallel resonances around the 8th and

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12th harmonics were causing amplification of the nearby harmonics.

combination of the system inductance and the 4.8 MVar capacitor bank on the 12.47 KV bus. The 12th harmonic resonance was due to the system inductance in parallel with the 550 KVar power factor correction capacitors on Bus 8. The capacitance on this bus actually consisted of 2-250 KVar capacitors and 1-50 KVar capacitor switched with 3 different motors that added up to 2750 HP. Some analysis was done to investigate the effect removing the capacitors on this bus. With the 550 KVar of capacitance omitted from the power system model the voltage distortion at Bus 8 was predicted to be 3.9%. Th~s represented a significant improvement over the 6.8% distortion expected with the capacitors in the circuit. It was also noted that harmonic levels at most other busses in the system were predcted to decline. The recommendation was made to disconnect the capacitors from the three induction

The 8th harmonic resonance was due to the parallel

PLANT POWER FACTOR CORRECTION STRATEGIES

There are tradeoffs in planning a power system for acceptable power factor while avoiding harmonic resonance problems [4]. If a single large capacitor bank is implemented at the main bus on the system the number of resonances is minimized but if a resonance does occur near a harmonic frequency, the amplification can be lugh since there is very little resistance to provide dampening. If multiple capacitors are switched with individual motors, then power system natural frequencies will constantly be changing, malung it d~fEicult to analyze a harmonic problem if one should occur. The resistance between the individual capacitors and the system inductance will be lugher, however, which means that less harmonic amplification would occur compared to using a single, larger capacitor bank on an upstream bus. A tlurd possible power factor correction strategy would be to implement tuned capacitors. By adding a reactor in series with the

SUB I 4 41s-E64 I

FIGURE I: POWER SYSTEM MODEL

motor starters in question. The customer decided that the benefit of avoiding possible harmonic resonance problems outweighed the drawback of losing a small portion of the plant power factor correction and disconnected these capacitors. Harmonic filtering is another option that could have been considered in this case. Adding one or more series LC filter legs at the input to the drive would limit the amount of harmonic current injected back into the power system. The filter would also provide some power factor correction. A medium voltage vacuum contactor would be necessary to switch the filter with starting or stopping of the dnve so that a leading power factor would not occuf with the drive energzed but not running. The benefit of improving the power factor would have to be weighed against the cost of designing and manufacturing a filter.

power system since the addition of this medium voltage drive. The drive has been in operation for about two years.

It is difficult to perform analysis of every possible resonant condition on a system such as this because of the fact that a new set of resonances OCCUT every time an induction

No harmonic related problems have arisen on the

capacitor, the bank may be tuned to a specific frequency (typically just below the first s igdicant harmonic frequency). This approach has the benefits of nailing down the resonant frequencies in the system where no harm can be done as well as providing some harmonic filtering effect. Where: synchronous generators or motors are present in the plant it is sometimes feasible to correct the plant power factor without the use of capacitors.

CONCLUSIONS

Harmonics is an issue that is not going io go away any time in the near future. IEEE 5 19-1992 imposes more rigid harmonic limitations than in the past. Although the likelihood of harmonic problems is low, the instances in wbch they do occur can result in reducing the reliability of the power system and potentially affect plant output. When adding large non- linear loads to a power system, performing analysis up front may reduce any surprises during commissioning.

was presented that allows one to determine at the design stage if An approach to harmonic modeling of a power system

motodpower factor correction capacitor is switched on or off, hannonic mitigation techmques may be required to avoid a 119

resonance or keep harmonic levels within those imposed by the utility and/or IEEE 5 19.

A case study was presented that gives an example of hartnonic modeling and one approach to harmonic reduction

Alternative plant power factor correction strategies were presented with their pros and cons. It is not feasible to single out one strategy as being superior in all cases.

REFERENCES

LUDBROOK, A., "IEEE Standard 5 19, Its Effect on Equipment Manufacturers, Users, and Utilities", Canadian ElectriciQ Forum, I9913 Power QualityA'ower Harmonics Forum, Toronto.

DEWINTER F.D.. "A Practical Approach to Solving Large Drive Harnionic Problems at the Design Stage", IEEE Paper NO. PCIC-89-42 (1989).

HUNEAULT, DR. M.. "Comiputer Methods for Harmonic Analysis". ~Canadian Electricity Forum. 1993 Power QualityPower Harmonics Forum, Toronto.

LOWENSTEIN, M.Z., "Improving Power Factor in the Presence of Harmonics IJsing Low-Voltage Tuned Filters", IEEE Transactions on Induslry Applications, (May/June 1993).

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