15 years of experience stator ground fault protection
TRANSCRIPT
15 YEARS OF EXPERIENCEWITH
100% GENERATOR STATOR GROUND FAULT PROTECTIONWHAT WORKS, WHAT DOESN’T AND WHY
15 YEARS OF EXPERIENCEWITH
100% GENERATOR STATOR GROUND FAULT PROTECTIONWHAT WORKS, WHAT DOESN’T AND WHY
Chuck MozinaConsultant
Beckwith Electric
BACKGROUND:
Until the Late ’70’s 100% Stator Ground Schemes Were Not Widely Used in the US.
Users Accepted Ground Fault Coverage of Only 90-95% of the Stator Winding.
Conventional 59G Neutral Overvoltage Protection Used.
Stator Ground Fault Protection
100% Schemes Using 3rd Harmonic Neutral Undervoltage Introduced in the Late ’70’s.
Early ’80’s 100% 3rd Harmonic Ratio Scheme was Introduced.
During this Period 100% Subharmonic Injection Schemes Were Being Used in Europe
BACKGROUND:
Stator Ground Fault Protection
Conventional 59G Stator Ground Fault Application and Settings.
Undervoltage Neutral 3rd Harmonic scheme for 100% Stator Ground Fault Protection.
3rd Harmonic Ratio Scheme for 100% Stator Ground Fault Protection.
Limitation of These 3rd Harmonic Schemes and Some Methods of Overcoming these Limitations.
Subharmonic Injection Method for 100% Stator Ground Fault Protection.
PAPER ADDRESSES:
Stator Ground Fault Protection
CONVENTIONAL (59G) STATOR GROUND FAULT
PROTECTION
CONVENTIONAL (59G) STATOR GROUND FAULT
PROTECTION
Stator Ground Fault Protection
Conventional Stator Ground Fault Protection
Fault Position
Volta
ge a
t Neu
tral
(60
Hz)
0% 50% 100%0
0.5pu
1.0pu
N T
90-95%
59G
Stator Ground Fault Protection
59G Coordination Issues
F1 – TRANSMISSION SYSTEM GROUND FAULTF2 – VT SECONDARY GROUND FAULT
CH-L
F1
F2
59G
Stator Ground Fault Protection
Calculation of Generator Neutral Voltage for a Transmission System Ground Fault
C0
CH-L
VHO
Where:
= High side zero sequence voltage for a transmissionsystem ground fault.
= Generator step-up transformer high-to-low windingcapacitance
CH-L
VHO
C0= Generator system zero sequence capacitance perphase
3RN
3RN = Equivalent primary resistance in generator neutral
VR
Stator Ground Fault Protection
VT Connection to Avoid 59G Coordination with VT Fuses
59G
Stator Ground Fault Protection
3rd HARMONICSTATOR GROUNDFAULT DETECTION
SCHEMES
3rd HARMONICSTATOR GROUNDFAULT DETECTION
SCHEMES
Stator Ground Fault Protection
100% SCHEMES USING 3RD HARMONIC
Undervoltage 3rd Harmonic Scheme
3rd Harmonic Ratio Scheme
Stator Ground Fault Protection
Level of 3rd Harmonic Voltage Depends on:
The Construction of the Generator-- The Pitch of the Stator Windings
The MW and MVAR Output of the Generator
The Generator Terminal Capacitance
Stator Ground Fault Protection
General 3rd Harmonic Behavior
Stator Ground Fault Protection
3rd HARMONIC NEUTRAL
UNDERVOLTAGESCHEME
3rd HARMONIC NEUTRAL
UNDERVOLTAGESCHEME
Stator Ground Fault Protection
Third Harmonic Neutral Undervoltage Scheme
59G
59G
Stator Ground Fault Protection
Overlapping of 59G and 27TN to Provide 100% Stator Ground Fault Coverage
59G
59G
Stator Ground Fault Protection
Field Measurements of the Magnitude of Third Harmonic Neutral Voltage
Stator Ground Fault Protection
Advantages / Disadvantages:
Low Cost and Simple.
No New Equipment.
Requires Field Testing to Determine Minimum 3rd
Harmonic.
May Have to be Blocked During Some Operating Conditions.
Will not Work at all if 3rd Harmonic is Low – False Tripping.
Stator Ground Fault Protection
3rd HARMONICRATIO SCHEME 3rd HARMONICRATIO SCHEME
Stator Ground Fault Protection
3rd Harmonic Ratio Scheme
5 9 G
5 9 D T h ir d H a rm o n icR a t io R e la y
C o n v e n t io n G r o u n dO v e r v o lta g e R e la y
3 V 0
Stator Ground Fault Protection
Field Measurements of the Magnitude of Third Harmonic Voltages
Stator Ground Fault Protection
Advantages / Disadvantages:
More Secure Than Using Only Neutral 3rd Voltage.Requires Measurement of 3rd Harmonic Terminal Voltage.Requires Wye Grounded Primary VT with Broken Delta Secondary.This Generally Requires VT Additions or Modifications and Additional Wiring.Requires Extensive Field Testing to Determine Maximum 3rd Harmonic Ratio.Will Not Work at all if 3rd Harmonic is Low – But No False Tripping.
Stator Ground Fault Protection
LIMITATIONS OF
3rd HARMONICSCHEMES
LIMITATIONS OF
3rd HARMONICSCHEMES
Stator Ground Fault Protection
Hydro Plant Applications:
False Tripping during Load Rejection
Synchronous Condenser Operation
Performance of Third Harmonic Scheme on Parallel Generators
Stator Ground Fault Protection
Parallel Generators on a Common GSU
BUS
G1
G2
59G
27TN
59G
27TN
GSU
Stator Ground Fault Protection
Cross Compound and Large Generators Applications
27TN
W inding 1W inding 2
W inding not covered by27TN
G S U
Stator Ground Fault Protection
Gas Turbine Third Harmonic Voltage Profile
Stator Ground Fault Protection
Third Harmonic Neutral Undervoltage(27TN) Blocking
Stator Ground Fault Protection
SUBHARMONIC VOLTAGEINJECTIONSCHEME
SUBHARMONIC VOLTAGEINJECTIONSCHEME
Stator Ground Fault Protection
Subharmonic Voltage Injection Scheme
*
C o u p lin gF ilte r
V o lta g eIn je c to r
R e la yM e a s u re m e n ts
I
V
N a tu ra l C a p a c ita n c e
5 9 GN o te s :
S u b h a rm o n ic in je c t io n ty p ic a lly a t 1 5 -2 0 H zC o u p lin g f i l te r lo w p a s s o r n o tc h tu n e d fo r s u b h a rm o n ic f re q u e n c yM e a s u re m e n t in p u ts tu n e d to re s p o n d to s u b h a rm o n ic fre q u e n c y
Stator Ground Fault Protection
Coupling Filter and Voltage Injector Stator Ground Fault Protection
Advantages / Disadvantages:
Does Not Use 3rd Harmonic Voltage. Independent of Gen. MW and MVAR Loading.Can Detect Stator Ground Fault When Gen. is Off-Line.Detects Stator Ground Faults Over the Entire Winding–No Blind Spots. Requires Field Measurement of Subharmonic Charging Current – Off-Line Measurement.Requires Additional Equipment in Grounding Transformer Secondary – Added Cost.Cost have Reduced With Digital Technology.
Stator Ground Fault Protection
CONCLUSIONSIt’s Important to Upgrade to 100% Stator Ground Fault Protection.If 3rd Harmonic Schemes are to Being Considered-- It is Important to First Determine 3rd Harmonic “Signature”of the Generator.Recognize that 3rd Harmonic Schemes Have Limitations. Subharmonic Injection Provides an Viable Alternative to 3rd Harmonic Schemes.Injection Scheme Cost have Reduced with Digital Technology – Good Choice for Important Generators.Injection Scheme can Detect Stator Grounds Over the Entire Stator Winding and in an Off-line Generator.
Stator Ground Fault Protection
THE END
Questions ?
THE END
Questions ?
15 YEARS OF EXPERIENCEWITH
100% GENERATOR STATOR GROUND FAULT PROTECTIONWHAT WORKS, WHAT DOESN’T AND WHY
15 YEARS OF EXPERIENCEWITH
100% GENERATOR STATOR GROUND FAULT PROTECTIONWHAT WORKS, WHAT DOESN’T AND WHY
Stator Ground Fault Protection
©2008 Beckwith Electric Co., Inc.