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