cp interaction with power utility systemscorrosionshortcourse.com/2017 slides/2017 ucsc...february...

Special Topics

2017

February 21-23, 2017 Jay Warner Dairyland Electrical Industries, Inc. 1

Period 2

CP Interaction with Power Utility Systems

Agenda

• Utility and CP system orientation

• Utility neutral interaction

• Examples of unintended interactions (interference?) between the two

February 21-23, 2017

2017 Underground Corrosion Short Course Special Topics – Period 3

Jay Warner Dairyland Electrical Industries, Inc. 2

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…

February 21-23, 2017 Jay Warner Dairyland Electrical Industries, Inc.


3

Power/CP Proximity Case

House

Impressed anode bed

Rectifier and phone pedestal

Power feeds

Pipeline

Distribution power line



4

Power/CP Proximity Case

What CP current distribution would you expect?



5

Current Distribution Effects



What CP current distribution would you expect?

6


Resulting current flow



7


Pickup

Discharge

Image Credit: Keith Boswell, National Pipeline Service



8

Result on Phone Pedestal




9

Result on Guy Wire Anchor




10

Power Utility Systems

HV Phase

Utility Neutral

Neutral ground

Ground rod

Utility neutral is grounded once every several poles and at services.



11

Currents on Power Utility Systems

HV Phase

Utility Neutral

Grounding system and utility neutral interaction.

VOLTAGE GRADIENT



12

Currents on Power Utility Systems

HV Phase

Utility Neutral

Grounding system and utility neutral interaction, plus customer neutral.

Phone Co Bond

VOLTAGE GRADIENT



13

f

Current on all Systems

Utility and customer neutral pickup.

Utility neutral pickup/discharge

Customer neutral pickup /discharge

Customer 1

Customer 2

Bond discharge



14

Current at Discharge Points

Phone bond

Customer neutralRectifierN

G



15

Utility Transformer Connections

Utility neutral

Neutral grounding lug

X0X1 X2

Customer neutral

X0 = Customer neutral bushing

Neutral ground strap



16

Utility Transformer Connection Currents

Utility neutral

Neutral grounding lug

X0X1 X2

Customer 1 neutralCustomer 2 neutral



17

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.



18


What effect does decoupler have when two customers are served by one transformer?

Customer Neutral 1

Customer Neutral 2



19


Both customers’ neutrals, and therefore grounding systems, remain common. The decoupler separates both from the utility neutral but not from each other.



20

f

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



21

Example: Utility Proximity

House with water well

Discharge from guy anchors, power line grounds, water well. Neutrals tied at transformer, allows CP flow



22

Example: City Water Well

Well with pump

Equipment grounding conductor

N

G



23

Distribution AC Return

N

G

To AC Source

All currents shown are AC



24


AC neutral current (actually bi-directional) flows on neutral wire and all parallel grounded paths. Why? The neutral is grounded at many points.



25


N

GTo AC Source

All currents shown are AC

Corrosion?



26

Special Blocking Device Required

N

GTo AC Source

DC and AC blocking device



27

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.



28

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]



29

cp interaction with power utility systemscorrosionshortcourse.com/2017 slides/2017 ucsc...february...

Documents