Restricted © Siemens AG 2013 All rights reserved.

Voting for Enhanced Security – How Far is Far Enough

Roy Moxley, Siemens

• About 1500 False trips

in 16 months (NERC)

• 16 Times more False

Trips than Failure to

Trip

• 41% of False trips are

during an external fault

Unpleasant Facts

From NERC Report, Jan 2011 – April 2012

From NERC Report, Jan 2011 – April 2012

Restricted © Siemens AG 2013 All rights reserved.

Cost of Relays has come down an inflation adjusted 90% since 1980

Pleasant Fact

Why Do We Apply Redundant Protection ?

It Costs More

Extra Engineering

Extra Maintenance

Extra Wiring

Hole Filling

So what Constitutes a Hole ?

Arc Resistance Coverage

Communication Failure Coverage

Power Swing Response

Fuse Failure (loss of potential) Response

It’s only a holeIf you hit it.

Maintenance Backup

Relay Failure Backup

OR

A+B A+C B+C

Two of Three Logic Improves Dependability and Security

Let’s Define (A,C,B):1% chance of False Trip95% chance of Correct Trip

A+B = 1%2 = .01% Chance of False Trip(A+B)*(A+C)*(B+C)= 0.03% Chance of False Trip

A+B = 95%2 = 90.25% Chance of Correct Trip(A+B)*(A+C)*(B+C)= 99.9% Chance of Correct Trip

OR

AND AND AND

Page 12

Implementing 2 of 3 is Not difficult

A

C

B C

B A

Details: On Loss of Relay, Revert to 1/2 or 2/2? Use Contacts or Logical Derivation? Use Same or Different Relays?

What Does It Mean to be Different ?

Voting Must be Different to Count

Human Error and Training Considerations

• Settings and Calculations

• Different Schemes

• Operations

• Maintenance

• Relay Out-of-Service

Testing Changes

• Test of Relay Modifies

Scheme Logic

• Voting Logic is Part of

System Test

Questions to Ask

How Many False Trips per Year are Acceptable?

What is the System Cost of a False Trip Coupled with a Correct Trip? Is This Condition Modeled

Conclusions

• Two of Three Voting can Reduce False Trips Significantly

• Costs of Voting Schemes are 15% of Dual Schemes 35 Years Ago

“Sometimes the questions are complicated and the

answers are simple”

Dr. Suess