Winding Pitch Problems

8/12/2019 Winding Pitch Problems

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AC Generators with 2/3rd and 5/6thwinding pitch

Issue #WP105: Technical Information from Cummins Generator Technologies

Cummins Generator Technologies has

a wide range of AC generator products

covering many applications, from prime

power to marine, oil and gas to name

a few. The stator of the AC generator

is manufactured with either a 2/3rd

or a 5/6th winding pitch. Winding pitch

of AC generators has inuence on the

shape of waveform (harmonic contents)

and on the level of the fundamental

voltage. The winding pitch will have

an inuence on neutral circulating current

in case of parallel operation and also will

impact on the type of neutral grounding

method. Paralleling generators with

different winding pitch will require careful

consideration if interconnecting star

points of all running generators should be

necessary. Both winding pitch (2/3rd and

5/6th) have some pros and cons, correct

selection for any proposed application

is the key for ensuring a cost effective

power supply solution.

Abstract

White Paper

Dr Jawad Al-Tayie

Chris Whitworth

Dr Andreas Biebighaeuser


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The stator winding pitch of a generator is a designparameter that can be used to optimise thegenerator waveform shape and minimise thegenerator cost, because shorter pitch (lower pitchratios) use the generator stator less effectively andrequire the use of more electrical steel and copperfor the same kW output than higher pitch machines.For example:n 5/6th pitch windings suppress 5th and 7th voltage

harmonics but may enhance the triple harmonics(third, ninth etc.)

n 2/3rd pitch windings suppress the tripleharmonics but may enhance the 5th and 7th.In addition, 2/3rd pitch wound machines generallyhave lower zero sequence reactances that canincrease the single phase fault current.

When comparing generator specications, the listedvalue for output voltage total harmonic distortion(THD%) can be similar for either a 5/6th or a 2/3rdpitch winding conguration. A well designedgenerator could probably achieve very similarvoltage waveform THD% with either pitch.

Cummins Generator Technologies considers that2/3rd pitch winding is best for most electricalloads that require a three phase, four wire system,because they may include a proportion of singlephase loads. When a generator output voltagewith triplen harmonics (e.g. from a generator that isnot 2/3rd pitch) is used to power a load consistingof single phase loads with triplen harmonics,

the triplen harmonics from both sources will sumtogether in the neutral and promote high andtherefore, potentially troublesome, neutral currentsin distribution system conductors.

Thyristor based and uncontrolled rectierconverters have high harmonic currents withharmonic numbers around multiples of 61. It istherefore technically correct to consider that anoutput voltage waveform with minimal 5th and 7thharmonics would be best for such non-linear loads(NLLs). (IGBT [1] based converters and switchedmode power supplies have harmonics at muchhigher frequency order).

On most sites, there is a mixture of NLLsand normal (non-distorting) loads such as;lighting, heating, air-conditioning units, pump andfan motors, lifts, etc. What is not always appreciatedis that although many of these normal loadsappear to be benign with respect to harmonicdistortion, many now contain variable frequencydrives etc., which are harmonically distorting,and can also be susceptible to a harmonicallydistorted supply voltage waveform, as can linearloads on the system.

In addition, in the category of ordinary load it mustbe considered that most buildings including modernequipment will almost certainly have an IT networksupporting PCs or terminals. Most such modulesemploy switched mode power supplies, which arenotorious for generating high neutral currents whichcan create serious rating problems for the buildingdistribution system if there is insufcient care withthe choice of the size of neutral cables. It therefore,becomes paramount that the current waveformharmonics are considered for any generator thatmay be required to support such a mixed load toensure it provides a low enough source impedanceto ensure voltage THD% is kept acceptablylow. For a generator this starts by identifying thegenerators value of subtransient reactance (Xd)at the site load duty point kVA, V and Hz.

The no load voltage waveform does not really tellhow susceptible the generator is to NLL, it is more

about stator leakage reactance, which denes this,high values are poor, therefore in marine applicationit is often pushed for low Xd in former times formotor starting capability, reasons nowadays alsoto minimise the size of lter units.

For balanced three-phase loads, distortion iscaused by voltage drop due to the harmoniccurrents in the subtransient reactance ofthe generator. The subtransient reactanceof a generator is not a function of coil pitch.

Therefore, the coil pitch does not affect waveformdistortion. The impact of generator pitch on loadgenerated harmonic currents is highly dependenton the system conguration.

I. Introduction


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We recommend 2/3rd pitch on all low voltage (LV) AC generators because, in general, these systemswill need to use the neutral to service loads. For lowvoltage, both 50 and 60Hz, a 2/3rd pitch windingconguration will eliminate the triplen harmoniccontribution from the generator and will not encouragean increase in the overall triplen harmonics contentof the installation.

For medium voltage (MV [2] ) and higher voltage (HV [3] )systems, it is acceptable to use other pitch machines,

since the neutrals are rarely used to directly serveloads and there are only low levels of triplen harmonicsas these are summed to zero in components suchas delta-wound transformers, etc.

Originally STAMFORD products were low voltage onlyand these were all with 2/3rd stator winding pitch.

The summing together of the triplen harmonics takesplace in the distribution system the generators werepowering and although high neutral currents couldstill occur, the generator did not contribute to theseand at generator winding level there was no currentsummation. Due to earthing methods in the LV powerplant business it was the correct pitch. This is still validtoday. An additional benet of 2/3rd pitched windingsis the prevention of circulation currents through the starpoints of our generators when connected in parallel.

When the P80 machine was designed, it was decidedto use the same pitch (2/3rd) for all LV, MV andHV machines.

II.STAMFORD generators


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n

Note that neutral point earthing andequipotential bonding are often confused.Winding pitch affects neutral point bondingrequirements only. Equipotential bonding, whereconductive parts of the installation are electrically

joined to avoid touch potential, etc., is requiredregardless of winding pitch choice. In addition,when generator star points are grounded it isimportant that neutral current is not allowed tocirculate in the ground path as this may causetelecommunication systems issues as well as

creating standing voltages in the ground.

n When star points of parallel running generatorsare connected together problems can arise withcirculating current if the generators are not 2/3rdpitch. However, if all generators have the samewinding pitch and rating, there is no problemfor generators, but might be with 3rd harmoniccurrents in ships distribution system. Stepsmust be taken to control circulating current if

generators with different winding pitches areconnected to the same busbar with the starpoints coupled. See section 6.

Cummins Generator Technologies has built LV,MV and HV AvK generators for many years [4].For AvK generators, the stator winding pitch isdependent on the application and, in some cases,the voltage level.

The pitch for AvK generators is:n 2/3 pitch for 400 V @ 50 Hz and 480 V @ 60 Hzn 5/6 pitch for 690 V @ 60 Hz and @ 50 Hzn 5/6 pitch for all MV and HVn Stationary power plants

typical voltage levels are:

400 V @ 50 Hz and 480 V @ 60 Hz => 2/3 pitch

MV and HV up to 13.8 kV => 5/6 pitchn Marine the electrical system neutral is

unearthed/high ohmic earthed. Here earth faultsare detected but because of the safety issuesassociated with tripping essential circuits toclear a single phase earth fault, an alarm is givenallowing the fault to be repaired when convenientin the ships progress and not causing a `Black

Ship`, which would occur if the system wouldbe hard earthed. This is valid for LV, MVand HV. Therefore 5/6th winding pitch is used.

Typical voltages are:n 690 V @ 60 Hz and @ 50 Hzn MV and HV:

n 6.6 kV @ 60 Hz and @ 50 Hz

n 11 kV @ 60 Hz and @ 50 Hz

III. AvK generators


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IV. What is winding pitch

A coil whose sides are separated by one pole pitch(i.e., coil span is 180 electrical) is called a full-pitchcoil. With a full-pitch coil, the emfs induced in thetwo coil sides are in phase with each other and theresultant emf is the arithmetic sum of individual emfs.

However the waveform of the resultant emf canbe improved by making the coil pitch less than a polepitch. Such a coil is called short-pitch (or fractionalor partial pitched) coil. The emf induced in ashort-pitch coil is less than that of a full pitch coil.

Resultant Waveform

Fundamental Harmonic

Third Harmonic

Fifth Harmonic

Seventh Harmonic

Figure 1: Full pitch winding machine with resultant waveform of the output phase voltage

Figure 2: Short 5/6th pitch winding machine with resultant waveform of the output phase voltage

Resultant Waveform

Fundamental Harmonic

Third Harmonic

Fifth Harmonic

Seventh Harmonic

(i) Full pitch winding

Full pitch (Figure 1) = pole centre (S) to pole centre (N), for 48 slots stator (example)= 48/4 = 12 slots (coil span 1 13)

(ii) Short 5/6th winding pitch

This winding pitch equals to 5/6th of full pitch, Figure 2.

For 48 slot = 5/6th of 12 slots = 10 slots (coil span 1 11)

Full pitch coil

5/6th pitch coil

Main stator(48 slots)

Main rotor(4 pole)


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Figure 3: Short 2/3rd pitch winding machine with resultant waveform of the output phase voltage

Resultant WaveformFundamental Harmonic

Third Harmonic

Fifth Harmonic

Seventh Harmonic

(iii) Short 2/3rd winding pitch

This winding pitch equals to 2/3rd of full pitch, Figure 3.For 48 slot = 2/3rd of 12 slots = 8 slots (coils span 1 - 9)

2/3rd pitch coil


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(i) Prosn Increase of power density with lower costs

and weight on the generatorn Increase of generator efciencyn Lower iron losses for 5/6th pitch due to shorter

core while same magnetic ux densityn Friction losses assumed the samen Stator copper losses about the same core length

adder but overhang length reduction for 2/3rd pitchadd up to zero, (this however depends on core

length and pole number)n Slightly lower rotor copper losses for 5/6th

pitch due to shorter core while about samecurrent density

n Additional losses about the same neglectedslightly higher losses due to damping of 5th & 7thstator winding eld harmonic

n Loss reduction is between 3% to 6% of totalalternator losses - depending on the load point

n In tendency a shorter and cheaper AC generatorwith better efciency

n Emissions better efciency leads to lower fuelconsumption and lower emissions

n THD:

n Line to line voltage THD% of 5/6th pitch haslower values as 3rd voltage harmonic addsup to 0 and 5/6th pitch keeps 5th and 7thharmonics low (IEC 60034 THD requirementsare for line to line voltage only)

n Line to neutral voltage 2/3rd pitch has lowerlevels of THD% as 3rd voltage harmonic isnot existing

n 5/6 winding pitch, more voltage (fundamental),better efciency, less eld current, less losses(copper and iron), Losses proportional to I 2.

(ii) Cons n Lower exibility in use - cannot be used for all

earthing methods. Less exible in earthing system.Hard earthed systems required 2/3 pitch, hardearthed system is from experience an exceptionon MV/HV grids

n Other design aspects of 2/3rd and 5/6thwinding pitches:

n 2/3rd pitch is better regarding grid codes(due to core length increase)

n Lower Xd valuen Higher moment of inertia

n 2/3rd pitch is better regarding motor startingcapability (lower source impedance)

n 2/3rd winding pitch, shorter coil and hencemarginally less resistance, (this is true for 4 poleand short core length but this is different for longcore lengths and high pole numbers)

V. High level Pros and Consof 5/6th pitch winding


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This section examines the parallel operation ofgenerators of different winding pitch. Can a 2/3rdand a 5/6th pitch wound generator be connectedtogether for parallel operation?

The basic requirements when operating generatorsin parallel are: waveform harmonic content,excitation system able to ensure kVAr sharing(droop or a static), engine able to share kW.During commissioning all generators must be setto have same voltage droop% for a given percentage

kVAr load relative to each generators designed rating.If they dont have this same drooping characteristic %then they will not proportionally share the kVAr.

Section 4 showed the harmonic content of generatorswith 2/3rd and 5/6th winding pitch. It thereforefollows that if a 2/3rd pitch is paralleled to a 5/6thpitch winding there will be differences in the harmonicstructure of the voltage waveform. These differencesand they are electrical potential differences, will resultin circulating currents between the two generators,in sympathy with whichever waveform - at that instant- has the highest electrical pressure.

The actual level of circulating current is a complexissue, it will depend on the interconnecting/distributionsystem impedances and will also be very susceptibleto the characteristics of the connected site load beingsupported by the generators.

Interconnecting generators stator winding star points

will also change the characteristics of the circulatingcurrents, both between the generators and the levelof neutral current within the supplied distributionsystem. However, if the star points of dissimilar windingpitch generators are not interconnected, then thereis no circulating current, but the generators may notexactly share reactive load proportionally and if theneutrals of some generators are not connected tothe system neutral, consideration must be given towhether the site single phase loads can be supportedby the generating sets with their neutrals connected,allowing for any possible maintenance conditions.

Therefore, consider if the site load can be correctlysupported and the sites electrical protection systemwill still function correctly, without the two dissimilargenerators having their star points interconnected.

It follows that if the site loads single phase equipmentcan be supported by the fully rated neutral of justone generator then dont interconnect more neutralstogether if this is likely to promote unnecessarycirculating currents around the neutral conductorsystem. However it must be remembered thatif the generator that is intended to supply neutralcurrent is off-line for any reason, no single phaseloads can be supported and if these are connected,the system line-neutral voltages may become severelyunbalanced. Consideration should be given to a

changeover arrangement to allow other machinesto become source for line-neutral current in the eventof maintenance being required.

However, if the schemes protection philosophyis such that all star points must be interconnected,then 5/6th generators need neutral reactors and thenmeasurements have to be made as to the extent ofthe circulating currents to ensure that the generatorstator windings are not overloaded and that thesite distribution systems neutral cables are ratedaccordingly. Once generators have to supply real load(active and reactive) the degree of circulating currentchanges (reduces).

Many generators of 2/3rd, or 5/6th pitch, are usedfor embedded/co-generation applications with localmains/utility systems. In most cases such generatorsdo not have their star points interconnected tothe mains neutral or earth connection. In such anapplication the pitch factor is not an important issue.

Also methods to restrict the circulating current inthe common neutral may be implemented in caseof dissimilar pitched generators are parallel together.Standard reactors may be used for this purposehowever consideration must be given to the singlephase fault level in the system.

When dealing with generators operating in parallelwith a mains supply, then understand the protectionphilosophy of the mains network and ensure that thebonding of any generator star points to earth will notprovide a parallel path for fault current outside theprotections systems monitored fault-path.

VI. Parallel Operationof Generators


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It is important to understand the differences betweenneutral star point reference earthing and equipotentialbonding. The latter applies no matter what the windingpitch or neutral conguration and is an electrical safetyconsideration. The former is an electrical systemdesign function that may have safety implicationsif not performed correctly. In addition, when generatorneutrals are grounded it is important that neutralcurrent is not allowed to circulate in the ground pathas this may cause telecommunication systems issuesas well as creating standing voltages in the ground.

There are many congurations of the neutral star pointconnection to earth. Figure 4 highlights some of thesemethods. The method of star point connection to earthdepends on type of applications, voltage level andgenerator winding pitch.

For paralleling generators with similar winding pitch,their star neutral points can be interconnected.Depending on their winding pitch and voltage level,their star may be connected to earth directly or viacurrent limiting impedance (reactor). Figures 5 to 7illustrate some of the neutral and earth connectionmethods of parallel generators.

In some applications, an unreferenced system isemployed and the generator neutral points are notconnected together. In this case, there will be nopath for the circulating current to ow. Also therewill be no path for single phase fault currents soground fault detection system must be utilised for

such installations as stated in the requirements of theelectrical code. Figure 5 shows three wire system withungrounded neutral.

There should only be minimal circulating currentsbetween generators when their star points beconnected together regardless of their applications,voltage level and winding pitch. Connecting generatorswith similar winding pitch will limit the circulatingcurrent. Connecting generators with similar pitch isnot always possible especially when expanding a sitethat has existing older generators. Adding a reactorin the common neutral limits the circulating current.

The neutral circulating current can be attenuated byimplementing current limiting impedance (reactor)

as shown in Figure 6.

Neutral current limiting reactor would also substantiallyreduce the single phase fault level in the system.

A fault level that is too low can be a serious safetyconcern since it can prevent over current protectionfrom operating and lead to serious damages andre hazards.

Figure 7 shows that a multi winding reactor can beused to reduce the neutral circuiting current and alsoto provide a path for the single phase fault current.

This should not be confused with neutral earthingresistors where generator neutrals are normallyconnected together (with or without attenuatingreactors) and the common point is grounded viaa resistor with the purpose of attenuating groundfault current. Not often is each generator individuallygrounded through impedance to earth as this set uptoo many parallel paths and so difcult to get circuit

breakers to trip.

VII. Star Points Connectionand Equipotential Bonding

PH A

PH CPH B

BondingEarth

Figure 4a: No star point connection(there is always a reference earth point connection)

Figure 4b: Solid star point connection(unrestricted fault current)

PH A

PH CPH B

BondingEarth

Neutral

Enclosure/Chassis/ Exposed Conductive

Surfaces


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Figure 4c: Star point connection via impedance(restricted fault current)

Figure 5: Neutral connection of similar and dissimilar winding pitch generators(no connection between dissimilar pitch generators, neutral not connected to earth)

Figure 7: Neutral connection of similar and dissimilar winding pitch generators(connection via multi core reactor of dissimilar pitch generators)

Figure 6: Neutral connection of similar and dissimilar winding pitch generators(connection via inductive reactor of dissimilar pitch generators, neutral not connected to earth)

Figure 4d: Neutral star point connection via switching device

PH A

PH CPH B

BondingEarth

Neutral

PH A

PH CPH B

BondingEarth

Neutral

ImpedanceSwitchingDevice

PH A

PH CPH B

BondingEarth

2/3 pitch 5/6 pitch

PH A

PH CPH B

BondingEarth

2/3 pitch 5/6 pitchReactor

PH A

PH CPH B

BondingEarth

2/3 pitch 5/6 pitchReactor

Neutral


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The winding pitch of AC generators has inuenceon the shape of waveform (harmonic contents) andon the level of the fundamental voltage.

2/3rd winding pitch is mainly used with three-phasefour wire low voltage generators due to the fact thatthe load is directly connected to the generator withouta transformer and most likely that single phase andnon-linear loads are present.

5/6th winding pitch machines are more suitable for

three-phase three wire MV and HV systems as theload will not be directly connected and hence thetriplen harmonics are not present as third harmonicswill sum to zero in the line to line voltages. 5/6th pitchis suitable for application where the neural point isnot connected to earth solidly. Directly connectedsingle phase loads are not present in most MV orHV systems.

5/6th winding pitch reduces the quantity of activematerials and it is a smart way to increase thefundamental generated voltage within stator.Historically 5/6th pitch was preferred with NLLsto reduce the harmonics in the load currents.However recent developments in IGBT technologieshave negated the need for 5/6th with NLLas harmonics occur as much higher frequencies.

The 2/3 pitch winding does have a low zero sequencereactance and while at low voltage, the advantagesassociated with distributed neutral outweigh the

disadvantages, at MV/HV this is not the case and thelow reactance causes higher than necessary groundfault currents which cause greater stress on machinewindings, cables and load components.

The fact that a 2/3rd pitch generator has a lowpitch factor for the triplen harmonics does notmean that it will perform any better in parallelingoperations. In fact, a 2/3 pitch generator has verylow zero sequence reactance and therefore, has lessimpedance to reduce the ow of circulating neutralcurrent. Circulating currents can result with anywinding pitch.

Usually MV and HV generators are connected todelta/star transformers and this is an importantadvantage of the 5/6th winding pitch for MV and HVmachines as the transformer will act as barrier for thetriplen harmonics.

Paralleling generators with different winding pitch willrequire careful consideration if interconnecting starpoints of all running generators should be necessary.Circulating 3rd harmonics current in the systemneutral cables must be considered at project outset,

and steps taken to control.

Both the 2/3rd and 5/6th winding pitch have benets.Correct selection for the proposed applicationis the key for ensuring a cost effective powersupply scheme.

Conclusion

AppendixKp (winding pitch) = Sin (VP/2)

= 0.866 for 2/3

= 0.966 for 5/6 for fundamental frequency.

Where:

V is Harmonic, 1, 3, 5, 7, ...

P is Angle, for 2/3 = 120, 5/6 = 150

[1] IGBT: Insulated Gate Bipolar Transistor

[2] Cummins Generator Technologies designationfor MV is 1kV up to 4.16kV

[3] Cummins Generator Technologies designationfor HV is 6kV up to 15kV

[4] We do understand that in electrical powerdistribution, the term medium voltage (MV) coversthe range from 1kV up to 62kV (in some areasup to 72.5kV).

Footnote


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Cummins Generator Technologies LtdBarnack Road, StamfordLincolnshire, PE9 2NB, UK

Tel: +44 (0) 1780 484000Fax: +44 (0) 1780 484100email: [email protected]

Copyright 2013 Cummins Generator Technologies. All rights reserved.

Cummins and the Cummins logo are registered trade marks of Cummins Inc.

STAMFORD d A K i d d k f C i G T h l i L d

www.cumminsgeneratortechnologies.com

Documents

Winding Pitch Problems