meeting harmonic limits using wide-spectrum passive · harmonic limits using wide-spectrum passive...

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Meetingharmoniclimits usingwide-spectrumpassiveharmonic filter

B YT O N Y H O E V E N A A R S ,I A N C . E V A N S ,& A N D Y L A W S O N

TO ADDRESS CONCERNS ASSOCIATED WITH ELECTRICAL POWER

system harmonic distortion on ships and offshore oil rigs and platforms,

marine regulating bodies have introduced strict new harmonic standards.

These standards define the acceptable level of harmonic voltage distortion

allowed on the vessels they certify. High-harmonic distortion levels are appearing as a

result of the increased use of power-electronic drive converters for electric propulsion,

drilling and pumping applications, and many other uses of variable speed-drive (VSD)

systems. To meet these new standards, methods to control harmonics must be adopted.

This article will discuss some of these methods and present an application where a unique

wide-spectrum passive harmonic filter was used to meet harmonic limits on a cable-lay-

ing ship that previously required the use of rented generators to allow operation of VSDs.

Digital Object Identifier 10.1109/MIAS.2009.934965

U.S. NAVY MILITARY SEALIF COMMAND

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Authorized licensed use limited to: Anthony Hoevenaars. Downloaded on January 7, 2010 at 15:13 from IEEE Xplore. Restrictions apply.

With the increasing use of ac and dcelectric drives in marine vessel applica-tions, such as electric propulsion and off-shore oil-drilling operations, poor powerquality due to harmonic distortion hasbecome a critical safety issue. Electricpropulsion offers significant benefits,such as greater redundancy, lower emis-sions, improved maneuverability, andreclaimed deck space for cargo and otherimportant uses. Oil drilling platformscould not function without variablespeed control of their motors. However,by drawing current in a nonlinear ornonsinusoidal manner, the VSDs canintroduce excessive levels of both cur-rent and voltage harmonics. The nonlin-ear loading on marine vessels can nowreach 80% of the onboard electrical gen-erating capacity [1] and results in total harmonic voltage dis-tortion (THDv) in excess of 20% (Figure 1).

All marine classification bodies are extremely con-cerned about harmonic voltage distortion and the possibleconsequences should some critical item of equipment mal-function or fail. Often viewed as a potential safety of life atsea (SOLAS) issue, classification bodies have imposedstrict limitations on the magnitude of harmonic voltagedistortion permitted on vessels classed under their rules[2]. Any vessels seeking classification are now beingrequired to demonstrate compliance with these standards.

The harmonic standards of the various marine classifica-tion bodies all focus on voltage harmonic distortion limits.Although the increased losses associated with high levels ofcurrent harmonic distortion can be a problem in electricaldistribution systems, it becomes only a serious issue whenthe equipment is more heavily loaded. A bigger issue withharmonic currents is that they create voltage distortion asthey flow through the power distribution system. To meetvoltage-distortion limits, current harmonics must bereduced. Therefore, by establishinglimits for voltage distortion, themarine standards indirectly limit cur-rent distortion as well.

Voltage distortion is an accumula-tion of the individual voltage dropsdetermined by Ohm’s law as each har-monic current passes through theimpedance of the power system. If thecombination of harmonic currentsand system impedance is high enough,system voltage can become severelydistorted. In marine applications, thecombination of heavy nonlinear load-ing and relatively high-source imped-ance of the supply generators results inexcessive levels of voltage distortion asshown in Figure 1.

Harmonic Standards ofMarine Classification BodiesThe Norwegian classification body,Det Norske Veritas (DNV), was one

of the first to recognize harmonics asa problem and introduce limits.These limits fall under their OffshoreStandard DNV-OS-D201—ElectricalInstallations [8].

The limits for harmonic distortionare found in Section 2, A207. THDv, indistribution systems, is not to exceed5%, nor shall any single-order harmonicexceed 3%. These limits are allowed tobe exceeded, provided that all consumersand distribution equipment subjected tothe increased distortion level shall bedocumented to withstand the actual lev-els. A guidance note lists some consider-ations for this documentation as:

n additional heat losses in ma-chines, transformers, and coilsof switchgear and control gear

n additional heat losses in capacitors, e.g., in com-pensated fluorescent lighting

n resonance effects in the networkn functioning of instruments and control systems

subjected to the distortionn distortion of the accuracy of measuring instru-

ments and protective gear (relays)n interference of electronic equipment of all kinds,

e.g., regulators, communication and control sys-tems, position-finding systems, and radar and nav-igation systems.

The American Bureau of Shipping (ABS) introducednew harmonic limits in May 2006. Their publication,Guidance Notes for the Control of Harmonics in Electrical PowerSystems [2], provides an extensive overview of harmonics,including various treatment methods. The limits arelisted in Section 11. In general systems, THDv shall notexceed 5% with no individual harmonic greater than 3%of the fundamental voltage value. Measurements are to betaken at least up to the 50th harmonic.

0

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3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49Harmonic Voltage Order

Har

mon

ic V

olta

ges

as %

of F

unda

men

tal

L1 Vh L2 Vh L3 Vh

1Voltage harmonic spectrum of offshore oil production platform at 600 V. AverageTHDv/phase was 24.7%. Higher harmonics above 21st were due to large numberof 18- and 24-pulse VFDs.

HARMONICSCAN BE A

PARTICULARLYSERIOUS

PROBLEM INZONES 1 AND 2

EXPLOSION-PROOF MOTORINSTALLATIONS.

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ABS does allow a higher limit of 10% on dedicated sys-tems. However, all connected equipment on a dedicatedsystem must have the manufacturer’s guarantee that it canoperate trouble-free at this higher voltage-distortion level.

The ABS rule was relaxed somewhat for mobile offshoredrilling units (MODUs) in January 2009. The harmonicinformation sheet for MODUs, Ref: 4-3-2/7.9, nowallows THDv levels up to 8 and 5% for any individual har-monic provided the parties concerned (owners, shipyard,designer, and system integrator) have no objections.

Most of the marine classification organizations haveadopted voltage-distortion limits similar to DNV andABS. Lloyd’s Register (LR), for example, uses the morerelaxed level of 8%. Harmonic limits can be found in Part6, Section 2, Item 1.7.3 of their Rules and Regulations for theClassification of Ships [9]. THDv is allowed to reach an 8%limit at the switchboard or sectionboard measured for har-monics up to the 50th. LR does recognize, however, thateven relatively low levels of higher harmonic frequenciescan cause problems and, therefore, establishes a limit of1.5% for any individual voltage harmonic above the 25th.

VSDs and HarmonicsMost power systems can accommodate a certain level ofharmonic current but will experience problems when thenonlinear loading becomes a significant component of the

overall load. This is certainly the case in many marineapplications due to the extensive use of VSDs.

A VSD is a solid-state device that converts supply volt-age to a variable voltage and frequency to control the speedof a three-phase induction motor [also referred to as avariable frequency drive (VFD)] or to dc for dc motorapplications. By controlling the motor’s speed, both energysavings and better motor control can be achieved. TypicalVSD marine and offshore applications include main pro-pulsion drives, thrusters, cableship remotely operatedtrenching vehicle (ROV) drives, top-drives, draw-works,winches, mud pumps, compressors, and general purposefan and pump drives (Figure 2).

VSDs generate harmonic currents because their front-end or input rectifiers do not draw current in a sinusoidalmanner. Instead, they draw discontinuous, pulsed currentsthat can be broken down into harmonic components byapplying Fourier analysis. For a typical three-phase recti-fier bridge, the predominant harmonic currents that willbe generated are fifth, seventh, 11th, and 13th. Typicalcurrent distortion levels range from 35% to more than80%, depending on the supply impedance and whether ornot an ac or dc reactor is applied to the drive.

Harmonics can be a particularly serious problem inzones 1 and 2 explosion-proof motor installations suchas those encountered on oil refineries and oil production

G G G G

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M M M M

MMMM

Drilling Package–Mud Pumps, Draw Works, Top Drive

M M M M MM

11 kV/690 V

11 kV/690 V

11 kV/60 Hz

11 kV/690 V

5,000 kVA/11 kV/660 V

5,200 kW 5,200 kW 5,200 kW 5,200 kW

3,200 kW740 r/min

3,200 kW740 r/min

3,200 kW740 r/min

3,200 kW740 r/min

3,200 kW740 r/min

3,200 kW740 r/min

3,200 kW740 r/min

3,200 kW740 r/min

11 kV/690 V

≈=

≈=

≈=

≈=

≈=

≈=

2Typical power system single-line diagram for dynamically positioned Class 3 drilling rig.

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platforms. For example, an increasedrisk of explosion can result from thedegradation of shaft seals on double-cage flameproof motors. In addition,rotors that are overheated by har-monics can degrade bearing lubrica-tion, resulting in frictional sparking.In an attempt to protect against this,certifications on standard Interna-tional Electrotechnical Commission(IEC) explosion-proof motors are nowconditional upon voltage-distortionlevels of 2 or 3% depending on theirspecific protection concept [3]–[5].The motor will lose certification asan explosion proof type, if these lim-its are exceeded.

The problem of harmonic voltagedistortion on explosion-proof motorsis not without foundation. Some con-sideration could be given to thepossibility that the world’s largestoffshore oil-field disaster in 1988,the Piper Alpha platform in the Scottish North Sea, mayhave been caused by harmonics on the power system dueto large electrical submersible pumps and other VSDs. Itis feasible that fixed-speed ExN motors (nonsparking forzone 2 use only) with double-cage rotors connected to asupply with high levels of voltage distortion may havecaused ignition of escaping gas or vapor.

In addition, a large fire on another oil platform in theNorth Sea occurred in March 2006 and took more than 4 hto extinguish. The platform had high levels of voltage dis-tortion, and the fire was attributed to the failure of an explo-sion-proof motor. Although no direct correlation has beenmade, questions still exist whether harmonics could havebeen a contributing factor. In 2007, two more fires occurredon North Sea platforms with causes possibly unrelated toharmonics; nevertheless, harmonics have become an issue,which authorities are starting to consider very seriously.

Phase Shifting and Multipulse Drive SystemsTo mitigate the harmonic current and voltage distortionintroduced by VSDs, a number of techniques are being usedto varying degrees of effectiveness. Multipulse drive systemshave been one of the most common forms of harmonic miti-gation used on marine vessels to date. In this technique,transformers with multiple secondary windings are used tophase-shift multiple VSD rectifiers against each other.

A drive system’s pulse number is determined by thenumber of discrete converters used and the phase-shiftangles between these converters. The characteristic har-monics generated by the diode-bridge rectifier of a VSDwill follow the relationship below:

h ¼ np�1,

where h is the harmonic number, n is any integer, and p isthe pulse number of the rectifier.

Most VSDs incorporate a three-phase, six-pulse ( p ¼ 6)diode-bridge rectifier, which results in currents of harmonicnumber fifth, seventh, 11th, 13th, etc. being generated.

When dual rectifiers are used and phase shifted by 30�, a12-pulse scheme is created. Twelve-pulse VSDs will onlyhave residual amounts of fifth and seventh harmonics assubstituting p ¼ 12 in the earlier equation leaves harmon-ics 11th, 13th, 23rd, 25th, etc. Similarly, an 18-pulse driveconsists of three input rectifiers with 20� phase shiftsbetween them (Figure 3).

Configurations up to 48-pulse are possible for very largesystems, but the effectiveness of phase shifting at high-pulse numbers becomes questionable because the phaseangles of harmonic currents at higher frequencies are typi-cally not similar enough to produce sufficient cancellation.

Although they are theoretically an effective means ofharmonic treatment, all phase-shift drive systems can per-form rather poorly under real-world conditions. Toleran-ces in the manufacture of the transformer windings,applied voltage imbalances, preexisting voltage distor-tion, and light loading levels will have a detrimental effecton the drive’s ability to cancel harmonic currents at thehigher frequencies. Figure 4 illustrates the effect of 2%voltage imbalance on a typical 18-pulse drive system, andFigure 5 shows the performance degradation when back-ground voltage distortion is present.

SecondariesPrimary

+20°

–20°

0°

3 × 6 PulseInput Bridges

18-Pulse ac PWM Drive

dc Bus

M3~

InverterBridge

3Typical 18-pulse drive system. Three six-pulse converters and three secondarywindings, 20� displaced.

100

80

60

ITH

D (

%)

40

20

00 20 40 60

Load (%)80 100

18-Pulse (Balanced)18-Pulse (2% Imbalance)

4Effect of 2% voltage imbalance on an 18-pulse drive.

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It should be noted that, in addition to increasing thelevels of characteristic harmonics, voltage imbalance and/or excessive levels of background voltage distortion will

also result in the appearance of uncharacteristic harmonics,including triplens and even order [1]. Background voltagedistortion on ships and offshore oil platforms often reachlevels above 10%, and measurements above 20% are notuncommon as illustrated in Figure 1. Therefore, whenusing phase-shift-based drive systems, it is important toconsider if an acceptable level of mitigation will beachieved under the very likely scenario of high levels ofbackground distortion and voltage imbalance.

Active FiltersActive filters are current injection devices and are catego-rized into two distinct types: selective fast Fourier trans-form (FFT) and broadband. Selective FFTs allow for theselection of the harmonic orders being mitigated up to amaximum number, usually 15 or 25 harmonic orders.Broadband systems, on the other hand, can treat all thenonfundamental currents not just integer harmonics.This includes noncharacteristic harmonics, interhar-monics, and load-generated disturbances. In addition,

the selective FFTs respond ratherslowly 40�50 msð Þ, whereas broad-band systems respond at 40�100 lsand are therefore more suited todynamic loads. In this article, onlythe broadband type of active filter isconsidered (Figure 6).

An active filter is connected in par-allel with the loads (nonlinear and anylinear loads) and monitors the load cur-rent through two current transformers(CTs) located on the load side of theactive filter connection (Figure 7).Measurements from the CTs, whichinclude the current waveshapes, are fedinto a notch filter (i.e., band-stop fil-ter), which removes the fundamentalcomponent of current (i.e., 50 or 60 Hzcomponent). The remaining signal isthen classed as distortion current (i.e.,all nonfundamental currents). The sig-nal is processed and used to construct acurrent waveform, which when addedin real time to the load current wave-form, results in a near sinusoidalcurrent upstream of the filter.

If the filter’s cancelation current rat-ing is properly sized, it will producethe harmonic currents, which the loadrequires to function, and the source willonly be asked to provide the fundamen-tal current. An active filter cannot beoverloaded. At 100% cancelation rat-ing, the device will current limit, andany additional harmonic currents willspill over into the source as distortioncurrent, increasing the total harmoniccurrent distortion (THDi) and associ-ated THDv on the source side.

If applied correctly, active filters canoffer very high levels of performance(i.e., <5% THDi), but their high cost

30

25

20

15

10

5

00 1 2 3 4 5

Background VTHD (%)

ITH

D (

%)

18-Pulse (50% Load)18-Pulse (100% Load)

5Effect of background voltage distortion on an 18-pulse drive.

Fuses

PrechargeResistor

IGBT Modules

dcBusCapacitors

PrechargeContactor

InputTerminals

ExternalInductor

InternalInductor

EMI andCarrier

FrequencyFilters

C C C

EEES1 S3 S5

C C C

EEES2 S4 S6

6Simplified power circuit of broadband active filter.

System Busbarsor PCC

IGBT Inverter Bridgeand

Interface Components

dc Bus

IGBT Gatingand Control

Signals

Source Current(Fund)

LoadCurrent

Active Filter Current(Harm)

CurrentTransformers

IS IL

If

NonlinearLoad

(e.g., PWMVFD)

Active Filter

7Block diagram of typical shunt connected broadband active filter withassociated current waveforms.

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(purchase, spares, commissioning, andmaintenance) and complexity often lim-its their use. Other concerns include thepossibility of system resonance at the car-rier frequency of the filter’s insulated-gate bipolar transistors (IGBTs) and thefilter’s own susceptibility to backgroundvoltage distortion.

Case Study 1: Active FilterApplication on an OffshoreProduction PlatformAn offshore oil production platformoperating 6 3 800 hp, six-pulse dcdrives produced current distortion inexcess of 30% (Figure 8) and voltage-distortion levels greater than 10%. Toprevent operational problems thatcould arise from these high levels ofTHDv, 2 3 300 A broadband active filters were installedon each side of the main switchboard.

Current distortion was reduced from 34 to 3.6% (Fig-ure 9). By removing the current harmonics, the active

filter lowered voltage-distortion levelsto less than 4%. To demonstrate this,Figure 10 shows a trend line of THDvwhile switching the filter on and thenoff again.

Active Front-End DrivesAn active front end (AFE) drive con-sists of an ac pulsewidth modulation(PWM) VFD in which the standardsix-pulse, diode-input rectifier isreplaced by a bidirectional IGBT recti-fier (Figure 11). The input rectifierdraws current that is closer to sinusoi-dal but usually contains fairly high lev-els of higher frequency harmonics. Toreduce these, a passive L-C-L filtertuned near the switching or carrierfrequency of the IGBTs (usually be-

tween 2 and 3.5 kHz) must be used.AFE drives are often promoted as the best technical solu-

tion for marine drive applications because of their low-harmonic current-distortion profile (typically <5% THDi).

910 A

0

–910<t = 5.0 ms l1 = +712 l2 = –366 l3 = –340 >

1 23.4% 3.6% 3.8%3 1 2100.0%760.4 A

+000°

100.0%749.6 A

+000°

100.0%746.9 A

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3

49.91 Hz 90% 60%14/07/06 16:23 14/07/06 14:25

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RMS THD CFmaxmin

%100

50

V A VA U

1 3 5 7 9 11 13 15 17 19 21 23 25

Ah 01

49.92 Hz

–.+3LL1L2L3

(a) (b)9

(a) Current waveform and (b) spectrum with active filter. THDi = 3:6%.

MOST OF THEMARINE

CLASSIFICATIONORGANIZATIONSHAVE ADOPTED

VOLTAGE-DISTORTION

LIMITS SIMILAR TODNV AND ABS.

1,240A

0

–1,240<t = 5.0 ms l1 = +1,083 l2 = –1,054 l3 = –32 >

1 234.0 % 33.6 % 34.0 %3 1 2100.0%723.7 A

+000°

100.0%722.3 A

+000°

100.0%712.8 A

+000°

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49.93 Hz 60 % 60 %11/07/06 14:11 14/07/06 14:25

3U3V3AL1L2L3

RMS THD CFmaxmin

%100

50

V A VA U

1 3 5 7 9 11 13 15 17 19 21 23 25

Ah 01

49.95 Hz

–.+3LL1L2L3

(a) (b)8

(a) Current waveform and (b) spectrum without active filter. THDi = 34%.

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Although harmonic performance below the 50th harmonicis far superior to that produced by a standard six-pulseVFD, there are growing concerns regarding the harmonicperformance of AFE drives above the 50th harmonic, whichare directly attributed to the carrier switching frequency ofthe input IGBT rectifier. Figure 12 provides a high-speedoscilloscope trace of a typical current waveform at the inputterminals of an AFE drive with the high-frequency compo-nents clearly visible [6].

Recent measurements taken by the Polish Register ofShipping (PRS) on a research vessel with 2 3 90 kW and2 3 315 kW AFE drives used for propulsion also showthat current distortion can be >5% even with harmonics

up to the 50th only being measured [7]. Voltage spectrummeasurements above the 50th harmonic taken on the sameship show a significant increase in voltage distortionoccurring near the 67th harmonic (Figure 13).

This would be the result of excessive current harmon-ics drawn by the AFE drives near their carrier frequency.The THDv at harmonics below the 50th was 3.32% andabove the 50th was 3.25%. The total THDv (<100thharmonic) was 4.65%. It is not surprising that somemarine classification bodies, including the ABS and thePRS, now stipulate in their respective rules that ‘‘On ves-sels with AFE drives, the harmonics up to the 100th shallbe measured’’ [2].

There are other issues associatedwith AFE drives, which must beaddressed to ensure a problem-freeinstallation:

1) The electromagnetic interfer-ence (EMI) emissions, bothconducted and radiated, ema-nating from AFE drives arealmost double that from astandard six-pulse VFD withdiode bridge rectifier. A ves-sel’s power distribution sys-tem is usually based on ITnetworks (i.e., isolated neu-tral) where the vessel’s cablingprovides a high-frequencyconducted emissions path toground. These high-frequencyemissions, which circulatethroughout the power system,can seriously disrupt sensitiveequipment. Standard EMCfilters cannot be applied in ITnetworks because of high-leakage current through theircapacitors to ground and thelikely occurrence of damageto the filter in the event of aground fault.

2) The capacitors in the L-C-L fil-ter used to attenuate the rippleassociated with the switchingof the AFE IGBT rectifier maycause resonance with the net-work. This may require adjust-ment of the filter’s capacitorsto resolve, likely reducing itsfiltering effectiveness.

3) One advantage of an AFEdrive is its ability to regener-ate power (i.e., they are fullfour-quadrant drives). Whilethis may be applicable to cranes,hoists, etc., it is of no benefitto pumps, fans, and othersingle-quadrant applications.For main propulsion applica-tions, the regenerative capabil-ity may be of limited value

600

400

2000

0 0.005–0.005–0.01–0.015–0.02 0.01 0.015 0.02–200

–400

–600

800

–800

0.025–0.025

12AFE drive typical input current waveform [6].

2.000

4.000

6.000

8.000

10.00

%

14/07/200616:30:00.000

14/07/200616:51:20.000

21:20.000 (M:S)4 min/div

10THDv on 33800 hp dc SCR drives without-with-without broadband active filters.THDv is reduced from 11.2 to 3.7%.

IGBT Carrier Filterand Two Sets ofLine Reactors

IGBT Input Bridge(aka Active Front End) IGBT Inverter Bridge

dc Bus

Motor

ac Supply

3~

11The ac PWM drive with IGBT AFE.

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depending on the ability of the generators toaccept the regenerated power without tripping.The British Royal Navy recently decided againstAFE drives for their new Type 45 destroyers,which have 2 3 20 MW, 4:16 kV, 15-phase, mul-tilevel PWM drives with six-pulse SCR rectifiersand dynamic braking, after considering the damageto the gas turbine generators that could result fromexcessive regeneration from the AFE drives duringcrash stop and other violent maneuvers.

4) On systems where AFE drives and standard six-pulse VFDs have been applied to the same bus,there may be problems due to the AFE rectifier’shigh-frequency currents being impressed on thedc bus of the standard six-pulse VFDs [5]. Recentstudies have suggested that the six-pulse dc busvoltage can rise by up to 15%. If the six-pulseconverter is running at light loads, it may run therisk of tripping out on over-voltage dc bus.

5) On marine applications, reliability is paramount,especially on equipment for propulsion where theability to get me home is crucial. As a much morecomplex device, an AFE drive is arguably less reli-able than a standard six-pulse drive. In addition,if the IGBT rectifier being used to reduce harmon-ics were to fail, the entire drive would be out ofservice. A failure of the phase-shift transformerson multipulse drive systems would also causethese drives to shutdown. However, with input fil-ters, such as the wide-spectrum filter presentednext, the device may simply be bypassed in theevent of failure, allowing the drive to operate.Harmonics would be generated, but the drivewould likely be capable of operating at a reducedoutput without problems.

Wide-Spectrum Passive FilterThe wide-spectrum filter consists of a reactor with multi-ple windings on a common core and a relatively smallcapacitor bank. This design exploits the mutual couplingbetween the windings to improve performance. Figure 14shows a configuration of this filter. A high-impedancewinding, L1, is used as the main blocking inductance andis sized to prevent the importation of upstream harmonics.A filtering winding, L3, combines with the capacitor bankto provide a low-impedance path to filter out the harmoniccurrents generated by the downstream load. To decreasethrough impedance and reduce voltage drop across the filter,a compensating winding, L2, can be used and is wound inopposite polarity to the blocking winding.

One key advantage of the unique reactor design of thewide-spectrum filter is that it allows for the use of a signifi-cantly smaller capacitor bank (typically <15% reactivepower as a percent of full-load rating). This will reducevoltage boost and reactive power at no load to ensure com-patibility with generators. Capacitors introduce a source ofreactive current (i.e., leading power factor) to the genera-tor(s), which will tend to boost the generator voltage. Gen-erators are designed to adjust for voltage drop as their loadincreases and typically have minimal ability to lower volt-age when raised by a leading power-factor load. It is there-fore important that any filtering device that incorporates

capacitors is designed to ensure that the filter’s maximumcapacitive reactive current (which occurs under no-loadconditions) is comfortably below the maximum allowed bythe generator.

The filter is connected in series between the main sup-ply and drive. THDi is typically reduced to <6% whenapplied to a six-pulse ac PWM drive (Figure 15), regard-less of whether the drive is equipped with an ac or dc reac-tor or not.

3

2

Frequency (Hz)Courtesy: Polish Register of Shipping

1

0

Con

tent

s (%

)

2,53

53,

361

4,18

75,

013

5,83

96,

666

7,49

28,

318

9,14

4

10,0

00

13Voltage frequency spectrum on vessel with AFE propulsiondrives. THDv attributed to voltage harmonics above 50th(i.e., 3 kHz) was 3.26%. Below the 50th harmonic, the THDvwas 3.23%, and the total THDv was 4.65% [7].

L3

A3 B3 C3

L2L1

C1

B1

A1

C2

B2

A2

C (Patented Design)14

Wide-spectrum harmonic filter schematic.

300 A

0

Current

Voltage

15Input waveforms with wide-spectrum filter. THDi = 4:8%.

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The wide-spectrum filters can be applied to ac driveswith diode or SCR precharge input rectifiers ranging insize from 5 hp=4 kW to 3; 500 hp=2; 600 kW at present.They can be applied to single ormultiple drives, but only drive loadsshould be connected as the filter isdesigned specifically for rectifier oper-ation. The filter can usually be retro-fitted to the existing drives withoutthe requirement for drive modifica-tions, whether for single- or multiple-drive applications. The filter can alsooperate on fully controlled SCR brid-ges as used in dc drives but with aslight reduction in performance.

Case Study 2: CableshipApplication of theWide-Spectrum FilterHarmonic distortion resulting fromthe operation of numerous ac VFDson a Norwegian cable laying vesselknown as the Ocean Challenger (Fig-ure 16) was preventing the ship fromsailing without the use of rented gen-erators. When connected to the ship’s

main electrical distribution, voltage distortion signifi-cantly exceeded the 5% level permitted by the classifica-tion body, DNV. By supplying all harmonic generatingVSDs from the rented generators, the voltage distortioncreated was isolated from the rest of the equipment on theship. The ship’s operators wanted to reduce costs andreclaim deck space by finding a cost-effective solution fortreating the VSD harmonics, so that the drives could bereturned to the ship’s main distribution supply and therented generators could be eliminated.

The Ocean Challenger’s trenching operation is performedby a remotely operated pipe-line trenching vehicle or ROV(Figure 17). The ROV is equipped with ten 30-kW electricthrusters for maneuvering and four 300-kW high-volume,flow-rate electric pumps.

The electric thrusters and pumps are independentlyspeed controlled via ac PWM VSDs mounted in the sur-face module. Each VSD was equipped with a 3% ac linereactor to partially attenuate the harmonic currents theygenerate. When connected to the ship’s normal powersupply, however, the 1.5 MW of ac drives produced high-harmonic voltage distortion on the two 2,800 kVA shaftgenerators, preventing operation in this mode.

The ship’s operator, CTC Marine Projects, installed2 3 750 kW wide-spectrum filters—one for each of twogroups of 5 3 30 kW thrusters and 2 3 300 kW pumpdrives. Each 750-kW filter (Figure 18) was connected toone of the 2,880 kVA main power-shaft generators.

In sea trials, the THDi measured at the terminals of thefilter under the ROV’s maximum loading of about 85%was 6.1 and 6.4% for the port and starboard filters, respec-tively (Figure 19). It should be noted that if the VSDscould have been run up to their full-rated load, the THDiwould have approached levels of<5%.

The THDv at the filter terminals was measured at 2.2and 2.7% (Figure 20) at the 85% loading. Ship staff moni-tored both the operation of the two shaft generators andthe THDv on the main switchboards. The ship’s electricalengineer reported that the generators operated flawlessly

and at no time did the THDv riseabove 1.4 and 1.6% on their respec-tive switchboards. Figure 20 showsthe display from a Fluke 43 measuredat the switchboard connected to theport filter.

The installation of the two 750-kW filters allowed the vessel to meetthe 5% voltage-distortion limit ofthe DNV without the need for therented generators and additional deckspace. Two years after installation,CTC Marine Projects were able toreport that the filters had workedfully to specification without anyincident, cost effectively resolvingthe high-voltage distortion issue.

ConclusionAs the marine industry has movedtoward electric propulsion and otheruses for VSDs, harmonics have becomea major power-quality concern. This

18One of two 750-kW wide-spectrumharmonic filters installed on each groupof combined thruster and pump load.

17Remotely operated trenching vehicle with 10330 kWthrusters and 43300 kW pumps onboard.

16Ocean Challenger cableship.

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has led classification bodies to demand that voltage distor-tion remain below acceptable levels. A few of the technolo-gies being adopted to mitigate the effects of harmonics havebeen reviewed, and a case study was presented where aunique passive, wide-spectrum filter was successfullyinstalled on a cable laying ship to meet the limits requiredby one of these classification bodies, DNV.

References[1] S. Bourguet, P. Guerin, and R. Le Doeuff, Non-Characteristic Harmonics

Generated by Frequency Converter, GE44, Laboratory, Saint-Nazaire,

France, All Electric Ship 2003, Edinburgh.

[2] ABS Guidance Notes on Control of Harmonics in Electrical Power Systems,American Bureau of Shipping, May 2006.

[3] Rotating Electrical Machines—Part 1: Rating and Performance, IEC

60034-1, 2004.

[4] Rotating Electrical Machines—Part 12: Starting Performance of Single-Speed Three-Phase Cage Induction Motors, IEC 60034-12, 2002–2004.

[5] I. C. Evans, ‘‘Are we playing with fire,’’ Elect. Rev., (U.K.), vol. 221,

no. 3, p. 32, Feb. 1988.

[6] L. Moran, J. Espinoza, M. Ortiz, J. Rodrique, and J. Dixon, ‘‘Practical

problems associated with the operation of ASDs based on active front

end converters in power distribution systems,’’ in Proc. IndustrialApplications Conf., 2004, vol. 4, pp. 2568–2572.

[7] J. Mindykowski, T. Tarasiuk, M. Szweda, and I. C. Evans, ‘‘Electric

power quality measurements on an all electric ship with active front

end propulsion drives,’’ Polish Register of Shipping, Gdynia Maritime

Univ., May–June 2007.

[8] Electrical Installations, Offshore Standards DNV-OS-D201, Jan. 2005.

[9] Rules and Regulations for the Classification of Ships, Lloyd’s Register, July 2001.

Tony Hoevenaars ([email protected]) is withMirus International Inc. in Mississauga, Ontario, Canada.Ian C. Evans is with Harmonic Solutions Co., Scotland.Andy Lawson is with Parkburn Precision Handling SystemsInc. in Hamilton, Scotland. Hoevenaars is a Member of theIEEE. This article first appeared as ‘‘Meeting New MarineHarmonic Standards’’ at the 2008 Petroleum and ChemicalIndustry Conference.

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