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Automatic Load Transfer –The Perfect Phantom Voltage

SWEDE ConferenceCorpus Christi, TX

May 3, 2006

Kenton M. Brannan, P.E.CenterPoint Energy - Distribution System Reliability

kenton.brannan@centerpointenergy.com

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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

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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, 3rd expected in 2006

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Diagram of Facilities

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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

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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

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Test – All Fuses in Place

CHA Volts CHB Volts CHC Volts

08:38:23.865 08:38:23.870 08:38:23.875 08:38:23.880 08:38:23.885 08:38:23.890

Volts

-500

-400

-300

-200

-100

0

100

200

300

400

500

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Test – C-phase Fuse Removed

CHA Volts CHB Volts CHC Volts

09:19:50.890 09:19:50.895 09:19:50.900 09:19:50.905 09:19:50.910 09:19:50.915

Volts

-750

-500

-250

0

250

500

750

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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°

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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

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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 %

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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

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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