Jesus A. Guerra, P.E. Distribution System Reliability CenterPoint Energy SWEDE 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 /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 slightly higher than the ANSI standard TRV curve) 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 1 st 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 1 st Qtr 2010 Increase capacity for 138 kV lines 2.5 miles from KDL Re-evaluation of Switching Procedures

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 installation 2.Post installation of transmission sub 3.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