Transcript gdfgg - SWEDEUS.ORG

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
[email protected]
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The Perfect Phantom Voltage
• Facilities
– CenterPoint Energy
– Everyone’s Internet (EV1)
– Diagram
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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
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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
Vo lts
500
400
300
200
100
0
-100
-200
-300
-400
-500
08:38:23.865
08:38:23.870
08:38:23.875
CH A Vo lts
08:38:23.880
CH B Vo lts
08:38:23.885
CH C Vo lts
08:38:23.890
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Test – C-phase Fuse Removed
Vo lts
750
500
250
0
-250
-500
-750
09:19:50.890
09:19:50.895
09:19:50.900
CH A Vo lts
09:19:50.905
CH B Vo lts
09:19:50.910
09:19:50.915
CH C Vo lts
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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
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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
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