Download DC Decoupling / AC Grounding and Over

DC ISOLATION & OVER-VOLTAGE
PROTECTION ON CP SYSTEMS
Mike Tachick
Dairyland Electrical Industries
Typical Problems
 AC grounding without affecting CP
 Decoupling in code-required bonds
 AC voltage mitigation
 Over-voltage protection
 Hazardous locations
Conflicting Requirements
 Structures must be cathodically protected (CP)
 CP systems require DC decoupling from ground
 All electrical equipment must be AC grounded
 The conflict:
DC Decoupling + AC Grounding
Reasons to DC Decouple From
Electrical System Ground
 If not decoupled, then:
 CP system attempts to protect grounding
system
 CP coverage area reduced
 CP current requirements increased
 CP voltage may not be adequate
Isolation problems
 Insulation strength/breakdown
 FBE coating: 5kV
 Asphalt coating: 2-3kV
 Flange insulators: 5-10kV?
 Monolithic insulators: 20-25kV
Over-Voltage Protection
 From:

Lightning (primary concern)
 Induced AC voltage
 AC power system faults
Over-Voltage Protection Goal
 Minimize voltage difference between points of
concern:




At worker contact points
Across insulated joints
From exposed pipelines to ground
Across electrical equipment
Step Potential
Touch Potential
Over-voltage Protection: Products and
Leads
 Both the protection product and the leads
have voltage across them
 Lead length can be far more significant than
the product conduction level
Effect of Lead Length
 Leads develop extremely high inductive voltage
during lighting surges
 Inductive voltage is proportional to lead length
 Leads must be kept as short as possible
 Not a significant effect seen with AC
Key Parameters of Lightning Waveform
1.0
Slope = di/dt
(Rate of rise,
Amps/µsec)
Crest Amperes
1/2 Crest Value
0
8
20
Time in microseconds
 Lightning has very high di/dt (rate of change
of current)
Amplitude
AC and Lightning Compared
Time (milliseconds)
Alternating Current
Time (microseconds)
Lightning
Over-Voltage Protection: Best
Practices
Desired characteristics:
 Lowest clamping voltage feasible
 Designed for installation with minimal lead length
 Fail-safe (fail “shorted” not “open”)
 Provide over-voltage protection for both lightning
and AC fault current
Example: Insulated Joint
Example: Insulated Joint
Example: Insulated Joint
Insulated Joint Protection
Summary
Rate for:
 AC fault current expected
 Lightning surge current
 Block CP current to DC voltage across joint
 AC induction (low AC impedance to collapse
AC voltage) – rate for available current
 Hazardous location classification
Grounding System Review
 Secondary (user) grounding system
 Primary (power co) grounding system
These systems are normally bonded
Grounding System Schematic
Primary
Secondary
Situation: Pipeline with Electrical
Equipment
 Grounded electrical equipment affects CP
system
 Code requires grounding conductor
 Pipeline in service (service disruption
undesirable)
Decoupler characteristics
 High impedance to DC current
 Low impedance to AC current
 Passes induced AC current
 Rated for lightning and AC fault current
 Fail-safe construction
 Third-party listed to meet electrical codes
Grounding System After
Decoupling
Issues Regarding Decoupling
 NEC grounding codes apply: 250.2,
250.4(A)(5), 250.6(E)
 Decoupler must be certified (UL, CSA, etc.)
 No bypass around decoupler
Rating for Equipment Decoupling
Rate for:
 AC fault current/time in that circuit
 Can rate by coordinating with ground wire
size
 Decoupler must be certified (UL, etc)
 Steady-state AC current if induction present
 DC voltage difference across device
 Hazardous area classification
Example: MOV
Decoupling Single Structures: When is it
Impractical?
 Too many bonds in a station from CP system to
ground

Bonds can’t be reasonably located
Solution: Decouple the entire facility
Decoupling from Power Utility
Decoupling From the Power Utility
 Separates user site/station from extensive utility
grounding system
 Installed by the power utility
 Decoupler then ties the two systems together
Decoupling from Power Utility
Primary
Decoupler
Secondary
Decoupling from utility
Decoupling from utility
Decoupling from utility
Decoupling from utility
 Primary and secondary have AC continuity
but DC isolation
 CP system must protect the entire secondary
grounding system
Rating for Utility Decoupling
Rate for:
 Primary (utility) phase-to-ground fault
current/time
 Ask utility for this value
 Select decoupler that exceeds this value
Case study – station decoupling
Station
Before
After
A
870mV
1130
B
800
1175
C
950
1570
D
1140
1925
P/S readings at the station before and after decoupling from the
power company grounding system
Induced AC Voltage

Pipelines near power lines develop “induced
voltage”

Can vary from a few volts to several hundred volts

Voltages over 15V should be mitigated (NACE RP0177)

Mitigation: reduction to an acceptable level
Induced AC Mitigation Concept
 Create a low impedance AC path to ground
 Have no detrimental effect on the CP system
 Provide safety during abnormal conditions
Example: Mitigating Induced AC
 Problem:
 Open-circuit induced AC on pipeline = 30 V
 Short-circuit current = 10 A
 Then, source impedance:
R(source) = 30/10 = 3 ohms
 Solution:
 Connect pipeline to ground through decoupler
Example: Mitigating Induced AC,
Continued
 Typical device impedance:
X = 0.01 ohms
0.01 ohms << 3 ohm source
10A shorted = 10A with device
 V(pipeline-to-ground) = I . X = 0.1 volts
 Result: Induced AC on pipeline reduced from 30 V to
0.1 V
Mitigation of Induced AC
Rate for:
 Induced max AC current
 DC voltage to be blocked
 AC fault current estimated to affect pipeline
Mitigation of Induced AC
 Two general approaches:

Spot mitigation
 Continuous mitigation
Spot Mitigation
 Reduces pipeline potentials at a specific point (typ.
accessible locations
 Commonly uses existing grounding systems
 Needs decoupling
Mitigation example sites
Mitigation example sites
Mitigation example sites
Mitigation example sites
Continuous Mitigation
 Reduces pipeline potentials at all locations
 Provides fairly uniform over-voltage protection
 Typically requires design by specialists
Continuous Mitigation
 Gradient control wire choices:



Zinc ribbon
Copper wire
Not tower foundations!
Hazardous Locations
 Many applications described are in Hazardous
Locations as defined by NEC Articles 500-505
 Most products presently used in these applications
are:


Not certified
Not rated for hazardous locations use
Hazardous Location Definitions
Class I = explosive gases and vapors
- Division 1: present under
normal conditions (always
present)
- Division 2: present only
under abnormal conditions
Hazardous Locations
Division 1
Division 2
CFR 192.467
(e) “An insulating device may not be installed
where combustible atmosphere is
anticipated unless precautions are taken to
prevent arcing.”
CFR 192.467, continued
(f) “Where a pipeline is located in close proximity
to electric transmission tower footings
. . . it must be provided with protection against
damage due to fault current or lightning, and
protective measures must be taken at
insulating devices.”
CFR 192 link to NEC
 CFR 192 incorporates the National Electrical
Code (NEC) “by reference”
 This classifies hazardous locations
 Defines product requirements and
installation methods
Guidance Documents (Haz Loc)




AGA XF0277 – gas facilities
API RP-500 – petroleum facilities
CFR 192.467 – gas pipeline regs
NEC section 500-505 - haz loc definitions,
requirements
 CSA C22.2 No. 213 – product requirements
 UL 1604 – product requirements
For further application questions…
Mike Tachick
Dairyland Electrical Industries
Phone:
Email:
Internet:
608-877-9900
[email protected]
www.dairyland.com