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