Preventing circulating current in parallel generator applications

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Preventing Circulating Currents in Parallel Generator Applications

Today’s Presenter

•  Prior to joining Mirus, Tony was the Chief Facilities Electrical Engineer at an IBM manufacturing facility in Toronto

•  Tony is a professional engineer •  Tony is also a member of IEEE,

and has published multiple research papers on power quality


Preventing Circulating Current in Parallel Generator Applications


Scope of Presentation •  Generator Pitch and Harmonics •  How paralleling dissimilarly pitched generators creates

neutral circulating current •  Traditional methods used to prevent generator circulating

current •  GENLINK Dissimilar Pitch Neutral Limiter (DPNL) •  Case Studies:

–  Martin Brower Distribution Facility •  750kW and 1000kW Generators

–  Arkansas University •  2 x 800kW Generators with 1500kVA Utility Transformer

–  City of Waterloo Power Plant Expansion •  New 7200kW 13.8kV Turbine Generator

•  Summary


Generator Pitch and Harmonics Coil Pitch:

The angular distance between the two sides of an individual coil of an AC armature winding

Pole Pitch: The angular distance between centers of adjacent field poles

Winding Pitch: The ratio of Coil Pitch to Pole Pitch

Full Pitch Winding: When Coil Pitch is exactly equal to Pole Pitch Full Pitch Generator

Coil Pitch = 90°, Pole Pitch = 90°

Generator Pitch & Harmonics

Circula4ng Currents problems

Preven4ng Generator Circula4ng Current GENLINK DPNL Case Studies


Generator Pitch and Harmonics

Fractional Pitch Winding: When Coil Pitch is less than Pole Pitch

Advantages of Fractional

Pitch Generators: –  Shorter end

connections to reduce copper

–  More sinusoidal waveform and therefore, less harmonics

2/3 Pitch Generator Coil Pitch = 60°, Pole Pitch = 90°


Generator Pitch and Harmonics •  Voltage waveform produced by a generator will vary

slightly with respect to winding pitch •  Harmonic content in voltage will also vary based on

winding pitch –  Pitch factors below are used to predict harmonic

voltages based on respective harmonic fluxes

Reference: ‘Generator Winding Pitch and Harmonics, Engine Data Sheet EDS 70.4, Caterpillar, March 1993 ’

Pitch Fund. 3rd 5th 7th 9th 2/3 0.866 0.0 0.866 0.866 0.866 4/5 0.951 0.588 0.0 0.588 0.951 5/6 0.966 0.707 0.259 0.259 0.966 6/7 0.975 0.782 0.434 0.0 0.782


Generator Pitch and Zero Sequence Impedance

•  Generator impedances vary with respect to winding pitch, especially zero sequence impedance

•  Generator zero sequence impedance directly impacts the amount of circulating neutral current

•  Many claim that a 2/3rd pitch generator is better for 3rd harmonic, but this is not entirely true –  it has very low zero sequence impedance which

results in less 3rd harmonic voltage distortion under non-linear loading, but

–  the low zero sequence impedance presents little resistance to the flow of circulating currents


How Paralleling Dissimilarly Pitched Generators creates Neutral Circulating Current

•  Generators with different pitch configurations will have slightly different voltage waveshapes

•  Differences in Phase-Neutral (Ph-N) instantaneous voltages will appear as triple frequency

•  Can also apply when paralleling alternate energy sources with the Utility

5/6P Gen1

2/3P Gen2





How Paralleling Dissimilarly Pitched Generators creates Neutral Circulating Current (cont.)

•  Difference in instantaneous voltages drives neutral circulating current

•  Flow is restricted only by zero sequence impedance of generators and cables which is usually quite low


Traditional Methods for limiting Circulating Currents

•  Ensure that generators are of similar pitch –  This is not always possible or even preferred when

expanding a site with older, existing generators –  Can be difficult when sourcing from different suppliers or a

supplier that has changed their pitch design –  Even identically pitched generators can have slightly

different voltage waveshapes





Traditional Methods for limiting Circulating Currents (cont.)

•  Add impedance in the common neutral –  Neutral Grounding Resistors or Neutral Reactors can be

used but the impedance required to limit circulating current might reduce 1-ph faults to unacceptable levels

(ie. such that breakers do not trip) –  These devices must be rated for the residual steady state

circulating current –  If NGR’s are required for fault limiting, this can be an

effective method but the requirement to handle steady state current will increase size and cost


Traditional Methods for limiting Circulating Currents (cont.)

•  An ungrounded system can be used where neutrals are not connected together or grounded –  Ground fault monitoring is required and no neutral return

path is available for 1-ph loads –  Sometimes zigzag reactors are used to provide a ground

reference •  Internal generator faults may be a problem


GENLINK Dissimilar Pitch Neutral Limiter •  Multiple winding reactor installed in the common

neutral of paralleled generators or generators paralleled with Utility

•  Inserts ~ 48% impedance in neutral current circulating path at triple frequency –  Reduces neutral circulating

current by 70% or more •  Minimal effect on fault level

–  Adds < 1% saturated impedance to 1-Ph fault level

–  No impedance to 3-Ph fault level

•  Designed to meet IEEE Std 32 Requirements for Neutral Grounding Devices





•  Paralleled generators with dissimilar pitches in stand-alone or emergency stand-bye operation

•  Generator or other Alternative Energy source that is paralleled with a Utility transformer that can have its neutral-ground bond routed through the DPNL –  Cascaded DPNL’s can be used where there are more

than two connected supplies •  Examples are alternative

energy generators, standby generators, islanded supplies for drilling rigs, production platforms, pumping stations, etc.

Applications for GenLink DPNL


Application of GENLINK DPNL in 4-Wire System

•  DPNL is used when 2 or more generators of dissimilar pitch are paralleled together or a generator is paralleled with an alternate source, such as the Utility

•  DPNL is inserted in the neutral between the dissimilar groups •  Neutral should be grounded in only 1 location

–  Preferred location is neutral bus in switchboard


Application of GenLink DPNL on 4-Wire System with Utility Transformer

•  Utility transformer neutral should be grounded through DPNL

•  Ground at only 1 location


Cascaded System for Multiple Generators

•  DPNL’s can be cascaded in applications where there are more than 2 types of dissimilar pitch generators or dissimilarly pitched generators are paralleled with Utility

•  Size each DPNL based on the total generator load connected to it


DPNL Fault Current Path •  Fault current passes through the shoulder of the DPNL

(Z-X or Y-X) –  Shoulder impedance is typically 4% – 5%

•  During a fault, reactor steel saturates, significantly reducing the impedance

Fault current path


GENLINK Effect on 1-Ph Fault Level

3000A DPNL •  1-ph impedance

–  Normal Operation ~ 5% –  Above 8000A (saturated condition) < 1%


GENLINK Dissimilar Pitch Neutral Limiter •  Circulating current inductance (Y-Z) is almost 4x the

shoulder inductance •  Since the frequency of the circulating current is 3x the

fundamental current (180 Hz vs 60 Hz), circulating current impedance is 3x greater

•  Net impedance to circulating current is around 48%


Application of GENLINK DPNL in 3-Wire System

•  In a 3-Wire application where there is no return neutral current from 1-Ph loads, a smaller size DPNL can be used

•  Terminal X may or may not be grounded –  If ungrounded, Ground Fault monitoring will be required


•  Paralleled generators with dissimilar pitches that are tied to a Utility grid supplied by a transformer or multiple transformers that cannot have neutral grounded through DPNL –  DPNL prevents circulating current between the generators –  NGI helps block circulating current between the generators and

the Utility transformers –  NGI is not necessary if Neutral-Ground bond on Utility

transformers can be routed through DPNL •  Examples are Generating Stations with Alternative Energy

sources, Co-generation sites, Peak Shaving generators, etc. •  Also NGI alone can be

useful in applications with Alternative Energy sources and Utility when neutral is inaccessible

Applications for GenLink DPNL + NGI


How to Size GENLINK for a Specific Application •  Determine total capacity in

kW or kVA of all generators connected in parallel

•  Select DPNL that corresponds to this value in the appropriate system voltage column


How to Size GENLINK (cont.) •  This will size the DPNL for a return neutral current that

will be at least 50% of full phase current rating –  85% for 208V-240V or 380V-440V systems

•  It is the Users responsibility to ensure that actual return neutral current will not exceed DPNL rating –  If return neutral current is expected to exceed DPNL

rating then a larger DPNL should be selected –  A larger DPNL will be slightly less effective in reducing

circulating current •  For 3-Wire applications or where return neutral current is

known to be very low, smaller DPNLs can be selected


Total neutral current = 160A

Case Study: Martin Brower Distribution Facility Two Dissimilarly Pitched Generators





Case Study: Martin Brower Distribution Facility

Neutral Grounded at one location only


Case Study: Martin Brower Distribution Facility

•  1-Ph load return neutral current = 38.4A •  Most of the current from generators to DPNL is return

neutral current from 1- ph loads •  Almost all generator circulating current was eliminated


Case Study: Boca Hospital, S. Florida •  Generator configuration

–  2 x 1375kVA 0.8667 Pitch

–  1 x 1200kVA .0667 Pitch

•  Calculation of expected circulating neutral current provided by Caterpillar Distributor

•  Circulating 3rd harmonic current –  Ga and Gb ~ 97A –  Gc ~ 194A


Case Study: Boca Hospital, S. Florida

“Reading went from 400 amp to zero on all three generators and system has functioned with no problems since then.” Hassan Entezari, Pantropic Power Inc.

2500A GenLink DPNL


Case Study: 13.8kV System, Waterloo Illinois - Differently pitched Generators and Multiple Utility Trans.

•  New Generator different pitch than existing –  Existing 6600kW (.722P) –  New 7200kW (.814P)

•  No access to Utility transformer neutrals but Mirus was not initially informed of this


Case Study: 13.8kV System, Waterloo Illinois

•  Virtually no neutral circulating current between Generators •  Residual current to Utility transformers still too high however


Case Study: 13.8kV System, Waterloo Illinois •  Required continuously rated Neutral Grounding Resistor

(NGR) or additional Neutral Grounding Inductor (NGI) between DPNL and ground

•  NGR recommended for this application: –  60A continuous duty, Z = 19 Ohms –  36”W x 36”D x 66”H –  Residual circulating current estimated to be < 50A

•  NGI recommended for this application: –  100A continuous duty, Z = 6.6 Ohms at 60Hz –  44”W x 38”D x 66”H –  Residual circulating current estimated to be < 25A

•  Advantages of NGI over NGR –  More effective in reducing circulating current –  Less effect on fault level –  Much lower losses

•  NGR power dissipation is entirely kW –  Lower cost


Summary •  Paralleling of different power sources can cause neutral

circulating current –  Dissimilarly pitched generators –  Generator or other alternative energy source with Utility supply –  Different alternative energy sources

•  Neutral circulating current is predominantly triple frequency •  Application of GenLink DPNL can reduce triple frequency

circulating current by 70% or more without significantly affecting fault levels

•  Neutral should be grounded at only one location –  Recommended grounding at Switchboard

•  If Utility transformer neutrals are not accessible, Neutral Grounding Inductor will also be needed


Questions

Engineering

Preventing circulating current in parallel generator applications