1 Automatic Load Transfer – The Perfect Phantom Voltage SWEDE Conference Corpus Christi, TX May 3, 2006 Kenton M. Brannan, P.E. CenterPoint Energy - Distribution System Reliability

2 The Perfect Phantom Voltage Facilities –CenterPoint Energy –Everyone’s Internet (EV1) –Diagram Scenario of Events Verification & Short-Term Fix Root Cause Finding a Long-Term Solution Recommendation Closure

3 Facilities CenterPoint Energy –2–2500 kVA pad mounted transformers –35 kV–480/277 V, Grd–Wye/Grd–Wye –115’ 1/0 AL UDG cable from terminal poles –110’ # 2 AAAC OVH lateral Everyone’s Internet (EV1) –Data Center  Complete redundancy –Backup generators can support 100% facility load –3,000 A automatic transfer switch (each service) –Currently 2 services, 3 rd expected in 2006

4 Diagram of Facilities

5 Scenario of Events County maint. crew blew C-phase lateral fuse C-phase voltage went to 0.0 Volts Transfer switch rolled load to neutral position C-phase secondary voltage reappeared Adequate voltage detected, roll-back initiated C-phase voltage again went to 0.0 Volts Cycle continued every few seconds for 20 min. EV1 staff blocked roll-back

6 Verification & Short-Term Fix Test scheduled to verify voltage –All 3 fuses in place 284, 283, 284 V –C-phase lateral fuse removed 284, 284, 276 V –B-phase lateral fuse removed 284, 267, 284 V Short-Term Fix –10 minute roll-back block placed on each ATS

7 Test – All Fuses in Place

8 Test – C-phase Fuse Removed

9 Root Cause Contacted transformer manufacturer Discarded possibilities –Ferro resonance –Capacitive coupling C-phase voltage –100% load transfer –Result of vectoral sum of A & B-phase fluxes –Shifted by 180° due to nature of XFMR core –Flux induces voltage on C-phase at 240°

10 Finding a Long-Term Solution Continue with 10 minute block Replace the transformer –Delta-wye? Shift non-critical load from ATS to XFMR –How much load? Add a small load to XFMR –How much load? Future? Measure primary voltage 3-phase recloser on terminal pole –$50,000  $150,000 for all 3 services

11 Finding a Long-Term Solution Focus directed toward ATS IEEE task force for ATS Additional data acquired during test N/P sequence ratio: –All 3 fuses in place N/P – 0.4 % –C-phase lateral fuse removed N/P – 2.1 % –B-phase lateral fuse removed N/P – 6.4 %

12 Recommendation Recommendation – 47N Phase Sequence Relay –Installed at ATS controls –Set at 2.0 % (negative/positive sequence) Cummins installed relay Test of relay scheduled –Individual fuses removed –Activity of ATS monitored

13 Closure 47N relay prevented roll-back Determined successful EV1’s total installed cost –$6,000 (per ATS) –$18,000 for all 3 services Unique Situation –CenterPoint Energy –ATS controls manufacturer – Cummins