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

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°

•  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
Generator
Pitch
&

Harmonics

Circula4ng
Currents

problems

Preven4ng
Generator

Circula4ng
Current

GENLINK
DPNL
Case
Studies

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
Generator
Pitch
&

Harmonics

Circula4ng
Currents

problems

Preven4ng
Generator

Circula4ng
Current

GENLINK
DPNL
Case
Studies

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

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
Generator
Pitch
&

Harmonics

Circula4ng
Currents

problems

Preven4ng
Generator

Circula4ng
Current

GENLINK
DPNL
Case
Studies

•  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
Generator
Pitch
&

Harmonics

Circula4ng
Currents

problems

Preven4ng
Generator

Circula4ng
Current

GENLINK
DPNL
Case
Studies

Neutral Grounded at one location only

•  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

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

•  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

Preventing circulating current in parallel generator applications

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