CP Interaction with Power Utility Systemscorrosionshortcourse.com/2017 slides/2017 UCSC...February...
Transcript of CP Interaction with Power Utility Systemscorrosionshortcourse.com/2017 slides/2017 UCSC...February...
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Special Topics
2017
February 21-23, 2017 Jay Warner Dairyland Electrical Industries, Inc. 1
Period 2
CP Interaction with Power Utility Systems
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Agenda
• Utility and CP system orientation
• Utility neutral interaction
• Examples of unintended interactions (interference?) between the two
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System Orientation
• Power utility and customer grounding systems are bonded together by default.
• Your facility and all other power co customers are connected together.
• CP systems and utility systems exist in the same environment.
• Let the fun begin…
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Power/CP Proximity Case
House
Impressed anode bed
Rectifier and phone pedestal
Power feeds
Pipeline
Distribution power line
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Power/CP Proximity Case
What CP current distribution would you expect?
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Current Distribution Effects
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What CP current distribution would you expect?
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Current Distribution Effects
Resulting current flow
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Current Distribution Effects
Pickup
Discharge
Image Credit: Keith Boswell, National Pipeline Service
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Result on Phone Pedestal
Image Credit: Keith Boswell, National Pipeline Service
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Result on Guy Wire Anchor
Image Credit: Keith Boswell, National Pipeline Service
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Power Utility Systems
HV Phase
Utility Neutral
Neutral ground
Ground rod
Utility neutral is grounded once every several poles and at services.
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Currents on Power Utility Systems
HV Phase
Utility Neutral
Grounding system and utility neutral interaction.
VOLTAGE GRADIENT
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Currents on Power Utility Systems
HV Phase
Utility Neutral
Grounding system and utility neutral interaction, plus customer neutral.
Phone Co Bond
VOLTAGE GRADIENT
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Current on all Systems
Utility and customer neutral pickup.
Utility neutral pickup/discharge
Customer neutral pickup /discharge
Customer 1
Customer 2
Bond discharge
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Current at Discharge Points
Phone bond
Customer neutralRectifierN
G
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Utility Transformer Connections
Utility neutral
Neutral grounding lug
X0X1 X2
Customer neutral
X0 = Customer neutral bushing
Neutral ground strap
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Utility Transformer Connection Currents
Utility neutral
Neutral grounding lug
X0X1 X2
Customer 1 neutralCustomer 2 neutral
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Decoupling at Transformer
Neutral ground strap removed
Decoupler
Decoupler connected between isolated X0 (customer neutral) bushing and tank ground (utility neutral). Negative to customer side and positive to utility side.
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Decoupling at Transformer
What effect does decoupler have when two customers are served by one transformer?
Customer Neutral 1
Customer Neutral 2
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Decoupling at Transformer
Both customers’ neutrals, and therefore grounding systems, remain common. The decoupler separates both from the utility neutral but not from each other.
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Current Split with DecouplingEven with decoupler at transformer, current flow separates into two paths, but not eliminated.
Utility neutral pickup/discharge
Customer neutral pickup /discharge
Customer 1
Customer 2
Bond discharge
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Example: Utility Proximity
House with water well
Discharge from guy anchors, power line grounds, water well. Neutrals tied at transformer, allows CP flow
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Example: City Water Well
Well with pump
Equipment grounding conductor
N
G
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Distribution AC Return
N
G
To AC Source
All currents shown are AC
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Distribution AC Return
AC neutral current (actually bi-directional) flows on neutral wire and all parallel grounded paths. Why? The neutral is grounded at many points.
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Distribution AC Return
N
GTo AC Source
All currents shown are AC
Corrosion?
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Special Blocking Device Required
N
GTo AC Source
DC and AC blocking device
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Special Blocking Device Required
• Normal decouplers block DC, pass AC to accomplish induction mitigation and continuity.
• Corrosion related to AC return current requires AC (and DC) blocking device. Standard decoupler would have no effect.
• Structure may have CP and need DC isolation (or prevent galvanic cell creation).
• AC discharge may prevent proper polarization film on well pump surface, allowing active corrosion and shortened life of only months to several years.
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Summary
• Utility structures and CP systems live in the same space.
• There will always be interaction between the two.
• Non-obvious connections between utility systems and other systems.
• Consider if orientation, bonding, current magnitudes/concentration are a concern.
Questions? Contact [email protected]
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