Underground Transmission Capacity Increasesfrom Dynamic Rating Analysis

Erich Schoennagel, PECenterPoint Energy

Background

In 2000, CenterPoint Energy began an ampacity study of 138kV Polk to Garrott circuitAddition of new Midtown SubstationInvestigate methods to increase capacity

Circuit Detail

138kV UndergroundHPFF Cable8 inch pipe2500 Kcmil conductorOil Dielectric Rated 257 MVA Installed in 1966

Circuit Route

Downtown Houston12,300 feet

Approach

Evaluate dielectric fluid for evidence of agingEvaluate paper insulation agingInstall DTS fiber for temperature profileEvaluate impact of crossing pipe-type cable circuit

Testing

DGA test on oil performed by Detroit EdisonField Dissipation test performed on paper insulationResult showed cables virtually un-aged from initial installation

DTS Installation

Distributed Thermal Sensing (DTS)Installed in parallel ductMulti-mode fiber optic

DTS Midtown to Polk Hot SpotHot Spot

Ampacity Considerations

Association of Edison Illuminating Companies CS2 recommends reducing max conductor temperature from 85C to 75C when environmental conditions are unknownCenterPoint Energy followed AEIC and rated circuit at 75CStudy of circuit allowed 85C uprateTypically this would increase ampacity by 20%

Unfortunately, even with a 10C increase, the result was a 3.4% decrease in ampacity due to crossing circuit hot spot

DTCR Consideration

Polk to Garrott circuit has a cyclical load patternShort duration of high loadsThe long thermal time constant of underground HPFF circuits is approximately 130 hours

Load Pattern

DTCR Model

Modified to support a real time temperature interference from a measured sourceThis was used to model heating influence from a cable crossingRun in simulated mode using summer 2000 data

DTCR Result

Using summer 2000 data, DTCR showed an increase of almost 21% in dynamic rating over the conventional book rating

DTCR Components

ThermocouplesSCADASimple PC

DTCR Output

Calculated conductor temperatureReported AmperageAllowable AmperageInput parametersTime remaining to thermal overload

0

200

400

600

800

1000

1200

1400

1600

07/02

/07 00

:00:00

CDT

07/09

/07 02

:30:00

CDT

07/16

/07 05

:00:00

CDT

07/23

/07 07

:30:00

CDT

07/30

/07 10

:00:00

CDT

09/06

/07 04

:00:00

CDT

09/13

/07 06

:30:00

CDT

09/20

/07 09

:00:00

CDT

09/27

/07 11

:30:00

CDT

10/04

/07 05

:30:00

CDT

10/11

/07 08

:00:00

CDT

10/18

/07 10

:30:00

CDT

10/25

/07 13

:00:00

CDT

11/02

/07 07

:00:00

CDT

11/09

/07 08

:30:00

CST

11/16

/07 11

:00:00

CST

11/23

/07 13

:30:00

CST

11/30

/07 16

:00:00

CST

12/07

/07 11

:00:00

CST

12/14

/07 13

:30:00

CST

12/21

/07 16

:00:00

CST

01/05

/08 22

:50:00

CST

01/13

/08 01

:20:00

CST

01/20

/08 03

:50:00

CST

01/27

/08 06

:20:00

CST

02/02

/08 18

:00:00

CST

02/06

/08 21

:00:00

CST

02/11

/08 00

:00:00

CST

02/15

/08 03

:00:00

CST

Dynamic Normal Rating Static Normal RatingActual (amps) Static 300 hr Rating

Polk to Midtown Load History

Am

pere

s

Polk to Midtown Ratings (2/29/08)

Static Normal Rating = 1035 amperes300 hour Rating = 1180 amperesDynamic Normal Rating = 1311 amperesEmergency

Thermal Limit

What have we gained?

TimeEstablish a five part upgrade plan– Dynamic rating– Shuttle oil– Additional pipe for circulating

Design is ready

– Reconductor– Add a parallel circuit

DTCR Implementation

Ampacity increaseRTO tool during high circuit loadGrid operator predictive tool

Future Use of DTCR

XLPE Underground CablesOverhead circuits

Summary

Ampacity study of Polk to Garrott showed a decrease in book rating using steady state calculationsDTCR takes advantage of cyclical load pattern and thermal response of UG cable Dynamic rating uses real-time temperature and load inputPolk to Garrott ampacity increased almost 21%System operators have a better information toolMarket predictive tool

Project Team

EPRI - Cable system assessment- DTCR developmentPower Delivery Consultants, Inc.- DTCR development- Ampacity study and condition assessmentUlrich Associates, Inc.- Software development

Questions?

DTS Correlation

Utilities Using DTCR

Southern California Edison (XLPE cable)PECO Energy (pipe type cable)BC Hydro (submarine SCFF cable)United Illuminating Co. (pipe type cable)ComEd (XLPE cable)