Selective Stator Ground Protection
Expo Presentation
Team SyncGEN
• Sponsor – Larry Gross
Relay Application Innovation
• Advisors – Dr. Brian Johnson & Dr. Joe Law
• Webmaster / Client Contact – John Trombetta
• Poster / Presentations – Robert Schloss
• Documentation Manager – Jason Panos
Acknowledgments
• Relay Application Innovation – Larry Gross
• Idaho Power
• Schweitzer Engineering Laboratories
• University of Idaho - ECE Department
Dr. Brian Johnson & Dr. Joe Law
Background
• RAI customer has multiple machines on common high impedance grounded bus
• Using traditional overvoltage protection scheme
• Interested in a faster and more robust protection scheme for their system
Standard Protection
• Faults detected from neutral overvoltage
a
b
c
V Meter(Relay)
3 phase Synchronous Machine
Stator Ground Fault Detection from neutral
overvoltage
The Problem
• Faults seen on all machines
• Impossible to directly isolate the faulted machine from system measurements
• 3rd Harmonic protection cannot isolate faulted machine
Multiple machines on a common high impedance bus
The Problem
• Units must be sequentially tripped until the faulted machine is isolated
• Non-faulted machines must be restarted if tripped in the detection process
• Leads to large disturbances in the system
Proposed Protection
• Selective stator ground protection using microprocessor relay directional algorithms for ungrounded distribution systems
• Sensitive CT’s in SEL-351 for detecting zero sequence current flow in the neutral
• Zero sequence current flows toward faulted machine
Proposed Protection
• Fast isolation of faulted units
• Selective stator ground protection in addition to standard overvoltage protection
• Standard sequential delays apply, but trip immediately when faulted unit is isolated
System Oneline
System Configuration
Protection Constraints
• Standard protection must be used in addition
• 32NF indicates unit is faulted
• 32NR indicates the unit is not faulted
• 32NF enables immediate tripping, bypassing standard delay
Testing Constraints
• Faults must be seen across the bus
• For bus faults, 32NR shows on all machines
• Fault isolation needed for faults down to lowest stator turn
Relay Elements
• SEL-351
• 50 Overcurrent
• 32 Directional
• SEL-300G
• 64G Overvoltage
• 59 Overvoltage
• Trip: IN101*64G1T (IN101 asserts for 32NF)
Event Report
Evaluation
• Method works for isolated machines down to lower 5% of stator windings
• Passed our tests for isolated machines
• Worked intermittently for multiple machines due to issues with the model power system
Issues Encountered
• PT’s of the model power system solidly grounded to neutral
• No CT in neutral line
• No neutral line from Avista
• System ground was floating
• Unable to elevate neutral impedance
• Model power system schematics were incomplete
Economic Analysis
• Setup Cost
• SEL-351 $4,530
• Engineering design
• Additional hardware & wiring
• Repair costs
• Stator rewinding $500,000
• Loss of Operation
Conclusions & Recommendations
• Selective stator ground protection works in lower 5% of stator windings on isolated machines
• Address all issues with the model power system
• Test further to prove the method works for multiple machine configurations
• Use this method as supervisory protection with traditional overvoltage protection
Questions?
Generator Specifications
Lab Connections
Distribution Box
Sub-coils of Stator Windings