Jesus A. Guerra, P.E.Distribution System Reliability

CenterPoint EnergySWEDE 2011

Agenda

• CNP T&D Facts• Background of Kuykendahl Substation• PTS Failure\New Switching Procedures• Addition of Transmission Substation• Conclusion• Questions

T&D Facts

• CNP T&D serves a 5,000-square-mile area that includes Houston, the nation’s fourth largest city.

• CNP delivers electricity on behalf of 79 retail electric providers. • 2+ Million Metered Delivery Customers• 27,726 miles of Overhead Distribution Lines• 3,729 miles of Overhead Transmission Lines • 20,080 miles of Underground Distribution Lines• 26 miles of Underground Transmission Lines• 1521 Distribution circuits• 230 Substations (Transmission and Distribution)• 13 Service Centers

CNP Service Area Map

Background

Kuykendahl (KDL) Substation •CNP’s only 345/35 kV Sub•Built in 2001 •2-60/80/100 MVA Transformers•Initially serving four circuits

(Currently 5 circuits)•Constructed to serve increased load growth that would result in voltage design criteria violations.• Built on existing CNP property

Location

Substation Site (Cow Pasture)

Substation Site

New Feeder Location

New Feeders

PTS Failures

• From 7/31/01 – 10/19/01 three PTS failed during attempts to separate a KDL Feeder from Feeders served by other substations (138/35 kV).

• Some failed PTS had successful switching operations. (Crews reported excessive arcing)

• Due to unique sub design failures resulted in an open transition switching only until further investigated.

PTS Failures• Identified 3 possible causes for PTS failure:

1. Excessive currents• PTS Rated for 600A and 900A, breaker setting was lower.

2. Transient Recovery Voltage (TRV)• Difficult to measure in the field.• TRV model was constructed using ATP (Alternative Transient

Program).– (Model showed results were lower than the manufacturer’s TRV

ratings)

3. Switch (Mechanical) Failure• Misalignment, contamination, broken insulators, operator

error, etc

PTS Failures

• Only the last switch was saved for analysis– Arcing was present.– Manufacturer concluded that interrupters were not

properly aligned.– Without the other switches it’s impossible to

determine what effect switch failure played in these events.

New Switching Procedure• Load Flow Model simulations revealed large flows

of current at the breaker could cause it to trip during routine switching or service restoration.

• A new closed transition switching procedure was created.– KDL to KDL switching as 1st option (tie bus at sub)– Other procedures required when rolling the circuit to

a particular transformer or adding load to it and performing the operation.

– Closed transition would not be possible if one TRF is out of service.

Addition of Transmission Sub

New Transmission Substation •345/138 kV Sub•Energized 1st Qtr 2010 •Increase capacity for 138 kV lines•2.5 miles from KDL•Re-evaluation of Switching Procedures

New Transmission Sub

New Transmission Sub

345 Ckt A

345 Ckt B

345/35 kV Sub

New 345/138 kV Sub138 kV Ckts

Re-Evaluation of Switching Procedure

• Load Flow study conducted using PSS/E software– L-F studies help determine the steady-state operation

on an electrical power system.• 3 models were analyzed to evaluate voltage and

current changes at various normally open PTS.1. Prior to transmission sub installation2. Post installation of transmission sub3. Post installation including future load growth

Results

• 37 Open PTS studied• Results showed no significant increase in

voltage levels prior to and post new transmission sub installation.

• Current Changes– 2/3 scenarios showed less than 5% change in

current levels– 1/3 scenarios showed larger than 5% change:

• 8 PTS showed decrease in current up to 33%• 5 PTS showed increase in current (6 – 7%)

Conclusion

• The addition of the new transmission sub will allow CNP to continue with the current switching procedures.

• As the dynamics of the system changes CNP will have to revisit the switching procedures.

Questions