1
Challenges for the Design of Wind and Solar Power
Plant Grounding System for Personal Safety
Comparison of Copper and Copper Clad Steel Conductors for WPP
Grounding Application
IEEE PES 2016 General Meeting, Boston, MA, July 17-21, 2016.
Presentation by:
Andrew Cadmore , CEng MIET, Principal Electrical Design Engineer, RES Ltd
Abdou Sana, P.Eng, Ph.D, Electrical Engineering Specialist, RES-Americas
2
1- The Use of bare Copper Clad Steel (CSS) conductor, instead of
bare Copper (Cu) conductor, as the directly buried bare
horizontal ground electrode conductor laid in all MV cable
trenches is evaluated based on a WPP project design exercise
Typical 34.5kV MV Cable Trench Cross-Section View
3
2- Copper Clad Steel (CCS)
• Composite in which a concentric copper cladding is bonded to
a steel core to provide a strong “low cost” solution for
grounding conductors
• Available Stranded CCS conductors: 40% and 30% conductivity
relative to the same size of annealed soft-drawn copper
(relative conductivity of 100%).
• Sizing of CCS conductor should be based on an equivalent
short circuit capacity to that of annealed soft-drawn copper
4
• Claims on the benefits of CCS conductors as an alternative to
copper conductors:
–
–
–
–
–
–
Cost savings compared to copper
8-12% lighter than copper conductors
105-108% higher fusing current compared to copper conductors
Highly theft resistant
Excellent fatigue properties
Extremely strong and rugged, higher breaking strength than copper
conductors
– CCS requires no special handling compared to Copper
– CCS is compatible with standard copper connectors either pressure,
bolted or exothermic welded
– CCS conductors exhibit high corrosion resistance as tested in various
soils conditions
5
3- Comparative Study copper vs CCS for a typical WPP
• Analysis of the impedance profile resulting from a 3dimensional model of the grounding system for a large WPP
– Impedance profiles as seen from the main substation and using 3
types of conductors throughout the MV cable trenches are evaluated
and compared:
• Bare copper conductors
• Equivalent 30% conductivity CCS conductors
• Equivalent 40% conductivity CCS conductors
• Parametric analysis for each of these 3 grounding conductor
systems (Copper, CCS30% & CCS40%), with logarithmic varying
soil resistivity:
•
•
•
•
10 ohm.m uniform soil model (Typ. Shoreline windfarm)
100 ohm.m uniform soil model (Typ. low-land windfarm)
1,000 ohm.m uniform soil model (Typ. high-land windfarm)
10,000 ohm.m uniform soil model (Extreme rocky mountainous windfarm)
6
WPP example in WA State, USA, Design-built in 2010/2011:
•
•
•
•
•
1x 230kV/34.5kVSubstation
83 Wind Turbines (191MW)
2 Met Masts
Approx. 5.7 mi x 3.5 mi
10- 34.5kV Collection Circuits
All UG, 5 different MV cable
sizes
7
• Equivalent Bare Grounding Conductor installed at the base of
all MV cable trenches:
– Size based on IEEE Std 80-2000 with:
•
•
•
•
Prevailing maximum 34.5kV fault level, seen at the main substation: 20,383 Amps.
Fault clearing time 0.133s
Initial Temperature : 25°C
Final Temperature: 350°C
– Results:
• 1/0 AWG stranded copper conductor, (106kcmil)
• 30% conductivity CCS conductor: 7x #6 AWG (184kcmil)
• 40% conductivity CCS conductor: 7x #7 AWG (146kcmil)
8
Minimum Conductor Cross-section Area (IEEE Std 80 -2000)
A mm2 I
1
 TC AP 10 4   K0  Tm 
 ln
 tc r r   K0  Ta 

 

A .kcmil I
197.4
K T
 TCAP   ln .0 .m
 t     K  T 
 .c .r .r   .0 .a 
I
rms current in kA
Amm2 minimum Conductor cross section in mm2
Akcmil minimum Conductor cross section in kcmil
Tm
Maximum allowable temperature in °C
Ta
Ambient temperature in °C
Tr
Reference temperature for material constants in °C
αo
Thermal coefficient of resistivity at 0 °C in 1/°C
αr
Thermal coefficient of resistivity at reference temperature Tr in 1/°C
ρr
Resistivity of the ground conductor at reference temperature Tr in μΩ-cm
Ko
1/αo or (1/αr) – Tr in °C
tc
duration of current in s
TCAP: Thermal capacity per unit volume of material in J/(cm3·°C) (see table)
αr and ρr to be evaluated at the same reference temperature of Tr °C .
(Tables provides data for αr and ρr at 20 °C).
9
Description
Copper, annealed softdrawn
Copper,
commercial
hard-drawn
Copper-clad steel wire
Copper-clad steel wire
Copper-clad steel rodb
Aluminum, EC grade
Aluminum, 5005 alloy
Aluminum, 6201 alloy
Aluminum-clad
steel
wire
Steel, 1020
Stainless-clad steel rodc
Zinc-coated steel rod
Stainless steel, 304
ρr 20 °C
(μΩ·cm)
234
Fusinga
temperature
Tm (°C)
1083
1.72
TCAP thermal
capacity
[J/(cm3·°C)]
3.42
0.003 81
242
1084
1.78
3.42
40.0
30.0
20.0
61.0
53.5
52.5
20.3
0.003 78
0.003 78
0.003 78
0.004 03
0.003 53
0.003 47
0.003 60
245
245
245
228
263
268
258
1084
1084
1084
657
652
654
657
4.40
5.86
8.62
2.86
3.22
3.28
8.48
3.85
3.85
3.85
2.56
2.60
2.60
3.58
10.8
9.8
8.6
2.4
0.001 60
0.001 60
0.003 20
0.001 30
605
605
293
749
1510
1400
419
1400
15.90
17.50
20.10
72.00
3.28
4.44
3.93
4.03
Material
conductivity
(%)
100.0
αr factor at
20 °C
(1/°C)
0.003 93
Ko at 0 °C
(0 °C)
97.0
a From ASTM standards.
b Copper-clad steel rods based on 0.254 mm (0.010 in) copper thickness.
c Stainless-clad steel rod based on 0.508 mm (0.020 in) No. 304 stainless steel thickness over No. 1020
steel core.
10
Ground Electrode Impedance Profile Studies Results
11
12
13
14
Ground Electrode Impedance Profile Studies Results – Table
15
4- Comments on Results
• Ground grid Impedance Zg, as seen at the Main Substation
(i.e. point of fault), increases for CCS relative to copper. Max
value 140.8% for the 30% CCS conductor, in a 10 ohm.m soil.
This results also in:
– Increase of GPR, Touch & Step Potential as seen at or near the point of
fault proportional to the increase in ground electrode impedance.
– Slightly larger Hot Zones
– Increased need for crushed rock at the WTG’s located closest to the
main sub.
For most in-land WPP (100 ohm.m soils), the various ground potential
values would all have increased by approximately 124.7%.
16
• The % deviation in Zg as seen at the Main Substation (i.e.point
of fault), decreases with CCS compared to Copper with an
increase in soil resistivity. This means that the:
– impact of CCS conductor becomes more comparable to Copper as the
soil resistivity increases in ohm.m value.
• The % deviation in Zg as seen at the farthest WTG or Met
Mast, from the Main Substation decreases with CCS compared
to Copper with an increase in the value of soil resistivity.
– This means that for transfer potentials, CSS conductors performs
better than Copper conductors due the increase in their internal
impedance.
• The difference in electrical performance between 30%CCS and
40% CCS conductor is not significant.
17
5- CONCLUSION
• Copper ground conductors offer better electrical
performance than CCS conductors.
– The difference in performance is though not so substantial (max
140.8% of increased GPR using CCS as compared to Copper during
extremely low soil resistivity conditions)
– In most cases this is manageable within the engineering design of
the windfarm grounding system.
• From an engineering design perspective, the impact of CCS
compared to Copper conductors are as follows:
– Possible need to install additional GPR control conductors, around
WTG’s, Junction Boxes or Met Masts to mitigate increased Touch &
Step Potential: Likelihood of having to install such additional
ground conductors is relatively low.
18
– Possible need to install crushed rock at more WTG’s, MV Junction
Boxes or Met Masts to mitigate increased Touch & Step Potential
– Possible need to undertake more detailed ground potential analysis
mitigate increased Touch & Step Potential.
– Increased likelihood that a project site will be subject to a “Hot”
Zone of Influence and an increase in the area size of that “Hot” Zone
of Influence. There is little that can be practically done to reduce the
size of the “Hot” Zone of Influence, beyond refining the desktop
design & analysis.
• The difference in electrical performance of 30% and 40%
conductivity CCS conductors is not significant,
– 40% CCS conductor product has a better protection of the inner
steel core against corrosion resulting from either soil chemistry, or
3rd party damage of the outer copper coating, and is recommended
over 30%CCS
1
Challenges for the Design of Wind and Solar Power
Plant Grounding System for Personal Safety
Applicability of Electrical Code (NESC/CEC) to WPP & Solar Power
Plant Grounding
IEEE PES 2016 General Meeting, Boston, MA, July 17‐21, 2016.
Presentation by:
Tracker Goree, Electrical Design Engineer, RES-Americas
Abdou Sana, P.Eng, Ph.D, Electrical Engineering Specialist, RES-Americas
2
Appx. 7 mi
Appx. 8 mi
3
• 1‐ Bare Ground Removal • 2‐ Mid Span Grounding
• 3‐ Redundant Path Requirements 4
1‐ Bare Trench Ground Requirements
NESC Rule 354 D.2.a(3) “A separate conductor in contact with the earth and in close proximity to the cable, where such cable or cables also have a grounded sheath or shield not necessarily in contact with the earth. The sheath, shield or both as well as the separate conductor, shall be adequate for the expected magnitude and duration of the fault currents that may be imposed.” This means that to remove the bare ground conductor one needs to provide either of the following:
5
• Unjacketed Cable (Concentric neutrals are bare and in contact with the soil)
or
• Semi‐conductive jacketed concentric neutral.(Outer Jacket made from semi‐conductive material (appx. 100 ohm.m)
• Unjacketed concentric neutral cable is subject to corrosion
• Semi‐conductive jacketed cable are costly items. 6
7
Ground Impedance Calculations were done using an interconnected model both including
and excluding the bare ground conductor.
Split Factor Calculations were done for each location both including and excluding the
bare ground conductor.
The GPR was calculated as a product of the short circuit current, split factor, and ground
resistance.
GPR = I_ShC * L_f * S_f * Rg
I_ShC = Actual Short-circuit current
L_f = Load Growth Factor =110%
S_f = Split factor (calculated)
Rg = Ground grid resistance
Touch and Step voltages were calculated based on simulation plots as a percentage of
the GPR.
Vtouch = Vtouch% * GPR
Vstep = Vsep% * GPR
8
Of the 42 locations evaluated: Roughly 50% of the locations considered were
unsafe without the use of crushed rock.
All Sites were safe when considered with the use of crushed rock.
9
This study was conducted using one median soil resistivity model with an average
of roughly 100 ohm-m.
If this resistivity is increased beyond 100 ohm-m what will be the effect?
Assumptions:
• Only the interconnected impedance was changed according to soil resistivity in the
calculation.
• Split factor and all other variables were considered to be the same for this exercise.
• Safety Criteria was held constant for this exercise.
10
Average Soil Resistivity Vs Touch Voltage
Touch Voltage
800
700
With Bare GND
Conductor
600
Without Bare GND
Conductor
500
Safety Criteria
Without Crushed
Rock
400
Safety Criteria With
Crushed Rock
300
200
100
0
0
200
400
600
800
1000
1200
Average Soil Resistivity
1400
1600
1800
2000
11
Main Implications of Rule for Solar and WPP
•
It has been shown in this case study that if the grounding conductor is removed the
split factor, the network impedance and subsequently the GPR will increase. This
increase in GPR will consequently increase touch and step voltages. The higher the
soil resistivity is the greater this increase will be.
•
This increase may motivate the need for mitigation measures in order to achieve
safety and this will have an associated cost impact.
•
From an NESC applicability standpoint. Rule 354-D.2.a(3) is restrictive in enforcing
unjacketed concentric neutral or semi conducting jacket in-lieu of the trench
ground, however the study proves that under certain soil conditions (low rho and low
SC current) and/or with the use of crushed rock, the safety criteria can be met
without the trench conductor or the semi-conducting jacket or unjacketed concentric
neutral.
•
For med. to high rho value soil safety can be difficult to achieve without costly
mitigation. In addition many wind turbine manufacturers local grounding or lightning
protection requirements will be difficult to meet at reasonable cost. (namely those of
IEC 61400-24).
•
Further Analysis is needed to determine the effects of soil structure on this
comparison and to evaluate the economics associated with the various code
compliant solutions.
12
2‐ Mid Span Grounding Requirements
NESC Rule 096C : Multi Grounded Systems “The neutral, which shall be of sufficient size and ampacity for the duty involved, shall be connected to a made or existing electrode at each transformer location and at a sufficient number of additional points with the made or existing electrodes to total not less than four grounds in each 1.6 km (1 mi) of the entire line, not including grounds at individual services.” 13
Mid Span Grounding Requirements
NESC Rule 354‐D.3.c : Random separation (<12in) between Insulating jacketed grounded neutral supply cables and communication cables “Grounded in accordance with Rule 314 except that the grounding interval required by Rule 96C shall be not less than eight in each 1.6 km (1 mile) of the random buried section, not including grounds at individual services” 14
NESC Rule 96C when Applied to Wind Power Plant collection Systems:
‐ Distances in‐between WTG are less or slightly greater than 400m (1/4 mile)
‐ Feeders (home runs) length may >5miles and Each additional grounding point is a an additional potential point of failure
‐ WPP and SPP MV collection systems are generally run as Balanced networks, with Delta/Wye MV/LV transformers, and are therefore not generally subject to standing Neutral load current flow.
Cable Shield/sheath Standing Voltages Installation
3 Phase – Cables Trefoil
Typical : d=2.2in; S=d; S/d~1
3 Phase – Cable Flat Formation
- Transposed Cross-bonded
(T&XB)
- Single Point Bonded (SPB)
Typical: d=2.2in; S=12in
IEEE Std 575‐2014
‐ Trefoil –Cables Touching (S/d=1) : E= 60V/1000m/1000A
‐ Flat ‐ Cable Transposed & Screen Cross‐Bonded (T&XB ‐ S/d=5.5): E= 180V/1000m/1000A ‐ Flat ‐ Single Point Bonded (SPB – S/d=5.5): E= 180V/1000m/1000A
Sheath Induced Voltage Based on Loading In accordance with IEEE 575-2014 - Concentric Neutral Shielded Cable
Continous Loading
Max Acceptable
Max Length for Max
Full-Load
Max Continuous
Acceptable Sheath
Standing Sheath
Loading
Standing Voltage
Voltage
Installation Installation
Method
Configuration
V/km/kA
(V)
Direct Buried
Trefoil
60.000
65
Direct Buried
Flat - T&XB
180.000
65
Direct Buried
Flat - SPB
180.000
25
(A)
300
350
400
450
500
550
600
300
350
400
450
500
550
600
300
350
400
450
500
550
600
(ft)
NESC 096C
- 4xGnd/Mile
(ft)
11847
10155
8886
7898
7108
6462
5924
3949
3385
2962
2633
2369
2154
1975
1519
1302
1139
1013
911
828
759
Assumed per-unit sheath voltage values (IEEE 575-2014- Figure 1):
- Trefoil - Cables Touching: 60V/1000m/1000A
- Flat - Single Point Bonded (Flat-SPB): 180V/1000m/1000A
- Flat - Cable Transposed & Screen Cross-Bonded (Flat-T&XB): 180V/1000m/1000A for "Minor" Section Lengths
Maximum acceptable full-load standing sheath voltage (IEEE 575-2014 Annex C1):
- 65V for Trefoil or Flat formation Transposed and cross-bonded
- 25V for Flat formation and Shield Single Point Bonded SPB cable terminations
1320
1320
1320
18
Maximum Cable Length for Sheath Induced Voltage <65V vs Loading In accordance with IEEE 575‐2014 ‐
12000
11000
10000
Trefoil ‐
V_Sheath<65V
Cable Section Length (ft)
9000
8000
Flat ‐ T&XB ‐
V_Sheath<65V
7000
6000
Flat ‐ SPB ‐
V_Sheath<25V
5000
4000
NESC
4xGnd/Mile
3000
2000
1000
0
300
350
400
450
500
Maximum Continuous Loading Current (Amp)
550
600
19
Main Implications of Rule for Solar and WPP
‐ For cable in trefoil touching and cables in flat formation transposed and cross‐bonded: ‐ Acceptable shield standing voltages of Max 65V can be achieved.
‐ Section lengths far greater than ¼ mile are acceptable without shield grounding for typical feeder maximum current loading of 600A. ‐ Sections length without need for shield grounding are even greater for lower current loading <600A. ‐ NESC rule 96C requires max 1320ft (1/4mile)
20
3‐ Redundant Ground Conductor for Substation Remote Ground Electrodes
36‐302 (3)(a) Ontario Safety Code “ … Two grounding conductors of a minimum of No. 2/0 AWG copper shall connect the ground electrode to the station equipment in such a way that should one grounding conductor or ground electrode be damaged, no single metal structure or equipment frame may become isolated;…” This Rule is enforced by the Ontario Electrical Safety Authority as follows:
21
• Each remote grounding station has to be connected to the substation by redundant paths which is either: – a double ground conductor (bare or OHG) or
– a looped ground connection to the substation through 2 or more circuits)
• If underground shielded cables (Concentric Neutral, tape shield etc..) are used, the cable shield shall not be considered a current return path (don’t count on it). • If OHL are used, the neutral cannot be considered a sufficient path for fault current return • In addition, the grounding system shall exhibit a GPR < 5000V and be safe (V_Step ≤ Safe V_Step; and V_Touch ≤ Safe V_Touch)
22
• If any of the above conditions cannot be achieved, then the remote ground station and the substation have to be safe as standalone.
– Full short‐circuit applicable,
– No allowance for ground fault current split factor
– No allowance to consider an interconnected ground grid resistance
This rule is understandable for a substation with remote electrodes (counterpoise terminated by a ground rod). However, for WTG and Solar PP it’s a big challenge 23
For Wind PP, any WTG grounding station is considered remote to the main substation and needs to comply with the above rule (safe as standalone), or use redundant ground conductors (double or looped ground or both)
A Typical WPP Plant Feeder Ground has to be doubled
Single Ground
Conductor System
Double Ground
Conductors 1
24
Fully looped or mixed lopped and double ground conductors may be used. Mix of Double and Looped
Ground Conductors
Looped Ground
Conductors
25
For Solar PP., each inverter station is considered remote to the main substation and needs to comply with the above rule (safe as standalone, or use redundant ground conductors (double or looped ground or both)
26
Main Implications of OSC Rule 36‐302 (3)(a) for Solar & WPP
• Extraordinary cost increase especially on difficult soils (500Ohm.m and above) up to 15,000$ per WTG
• Improved Personnel Safety ? – Safety is achieved with standard calculations without need to apply this Rule
– UG and OH systems layout are such that the ground conductor cannot be severed without cutting the communication cable and hence cannot be unnoticed. The intent of redundancy is thus achieved through a careful and methodic trench and OHL layout.
– Even if the ground conductor is voluntarily severed without touching the communication system, part of phase to ground current will still flow in the concentric neutral and hence a split factor should be allowed to be applied for the standalone cases.
– Due to inductive coupling and outer jacket insulation the split factor component of the concentric neutral is more stable than that of the bare ground conductor. 27
1
Challenges for the Design of Wind and Solar Power Plant Grounding System for Personal Safety
WPP Grounding System Design Challenges on High Resistivity Ground
IEEE PES 2016 General Meeting Boston MA July 17‐21 2016
IEEE PES 2016 General Meeting, Boston, MA, July 17‐21, 2016.
Presentation by:
Presentation by:
Andrew Cadmore CEng MIET, RES Ltd., Principal Electrical Design Engineer
2
Project 'A'
Project
A Windfarm,
Windfarm
Ontario, Canada (2010/11)
,
(
/ )
98.9MW (43x 2.3MW turbines) project ~60km north‐east of Thunder Bay. Project required 10.3km (49 spans) of new build d l i it 230kV t
dual circuit 230kV transmission line from the new Point Of i i li f
th
P i t Of
Interconnection Switching Station (POI) to the new Windfarm 230/34.5kV Main Substation (WF Sub). RES were responsible for
230/34.5kV Main Substation (WF Sub). RES were responsible for all project electrical design works, inc. 230kV POI through to LV terminals at base of each wind turbine.
3
230kV Grid Connection
230kV Grid Connection
Existing
Remote Sub Remote
Sub ‘B’
B
Sub
183km
New POI 230kV Sw.Sta.
47km
Existing
Remote Sub ‘C’
S b
Sub
Grid Point of
Interconnection
10.3km
New Windfarm 230/34.5kV Sub
4
230kV Transmission Line Design
230kV Transmission Line Design
OPGW 1
129mm²
OPGW 2
A1
C2
B1
B2
C1
400mm²
ACSR
A2
5
Soil Resistivity
Soil Resistivity
Electrical soil resistivity ‘ρ’ increased significantly between POI and WF Sub, as windfarm is located on the Canadian Granite Cap. Grounding analysis software was used to determine equivalent multi‐layer soil models along OHL route, terminal substations and throughout the windfarm collection system
substations and throughout the windfarm collection system. OEB/HONI Transmission Code design criteria required the connected facility grounding be designed for a ground return yg
g
g
g
current of ‘Ig = 25kA’ (actual max 3.9kA) at the POI, reducing to 10.7kA (actual max 3.2kA) at WF Sub. 6
Wide Range of Soil Resistivity
Wide Range of Soil Resistivity
Project 'A'
Range of Electrical Soil Resistivity:
From 230kV POI Switching Station to Windfarm 230/34.5kV Main Substation
g
/
100,000
Windfarm Median
WF 230/34.5kV SUB
200m from SUB
10,000
Electrical Soil Resiistivity
(ohm‐m)
400m from SUB
600m from SUB
800m from SUB
800m from SUB
1000m from SUB
3300m from SUB
Midway SWY/SUB
900m from SWY
1,000
790m from SWY
600m from SWY
420m from SWY
140m from SWY
60m from SWY
POI 230kV SWY
100
0.1
1.0
10.0
Average Electrode Spacing
(m)
100.0
7
Ground Electrodes
Ground Electrodes
WF Sub stand‐alone ground grid design = ‘R
b
d l
d dd
‘ g = 29.79Ω’ even with 6x 100m ’
h
vertical electrodes, reducing to ‘Zg+wf = 3.25 + j0.73Ω’ upon connection to windfarm grounding system. WF Sub grid required to be designed stand‐alone sufficient.
Grounding analysis software was used to model all ground electrode systems, g
y
g
y
,
allowing full consideration of internal impedances.
RES specified installation of 2x low resistance OPGW sky wires, and control of RES
specified installation of 2x low resistance OPGW sky wires and control of
OHL pole ground electrode ‘Rtg’ values, to reduce current split factor (Sf) as seen from both ends.
Rtg ≤95Ω ‐ except initial 7x poles out from POI Rtg ≤9Ω
8
Windfarm Sub Ground Grid
Windfarm Sub Ground Grid
-30
0
SOIL SURFACE
Z AX
XIS (METERS)
30
60
90
120
114
Y
AX
IS
84
(M
ET
54
ER
S)
24
WF Sub Ground Grid – 3D View
-6
-36
-30
0
30
XAXIS
X
AXIS (METERS)
3-D View of Conductors
60
90
120
9
Windfarm Grounding System
Windfarm Grounding System
3000
0
-3000
WF Sub
Windfarm Grounding System – Plan View
-6000
-7500
-4500
-1500
1500
4500
7500
10
230kV T L Pole Ground Electrodes
230kV T.L. Pole Ground Electrodes
Project 'A'
230kV Terminal & Pole Earth Electrode Resistances
100
90
80
Earth Electrod
de Resistance
(ohm
ms)
70
60
50
40
30
20
10
0
0
5
10
15
20
25
30
Earth Resistance (Design)
Earth Resistance (Test)
35
40
45
50
WF Sub
POI Sw.Stn.
Overhead Line Structure
(No.)
11
Ground Potential Rise
Ground Potential Rise
OESC GPR limit of ≤5kV was not practically achievable. Therefore, as an OESC permitted deviation, RES recommended p
the IEEE 367 value of 25kV be the max GPR limit. IEEE 80: 2000, Table C.1, covers many transmission line scenarios for calculation of ‘S
f ‘Sf ’, but did not cater for the Project 'A' scenario. Grounding ’ b did
f h
j ' '
i G
di
analysis software was used to analyse and calculate bespoke values of ‘SSf ’ allowing for: 2x parallel low resistance OPGW values of allowing for: 2x parallel low resistance OPGW
conductors; variable span lengths; variable pole ‘Rtg‘ values; variable soil resistivity ‘ρ’ along OHL route.
12
Standard v Optimised Design
Standard v Optimised Design
Applicable Standard and Code Requirements
Applicable Standard and Code Requirements
Ontario Electrical Safety Code (OESC) 24th Ed.: 2009 ‐ Rule 36‐304:
Standard GPR Limit
Ontario Electrical Safety Code (OESC) 24th Ed.: 2009 ‐ Rules 36‐304 & 2‐030:
E t d d GPR Li it P
Extended GPR Limit ‐ Permitted deviation with written OESA approval
itt d d i ti
ith itt OESA
l
IEEE 367: 1996 ‐ Clause 4.2 ‐ Possible high GPR under 'unusual circumstances'
Calculation Methods
IEEE 80: 2000 ‐ Table C.1: 1x Transmission Line; 0x Distribution Lines 1x 7x #10 AWG (36 8mm²) Alumoweld (ACS) Shield Wire: R
1x 7x #10 AWG (36.8mm²) Alumoweld (ACS)
Shield Wire Rac = 2.94Ω/km @ 20°C
2 94Ω/km @ 20°C
IEEE 80: 2000 ‐ Table C.1: 2x Transmission Lines; 0x Distribution Lines 2x 7x #10 AWG (36.8mm²) Alumoweld (ACS) Shield Wire: Rac = 2.94Ω/km @ 20°C
Calculated: 1x Single Circuit Transmission Line (Check against IEEE 80, Table C.1)
1x 7x #10 AWG (36.8mm²) Alumoweld (ACS) Shield Wire: Rac = 2.94Ω/km @ 20°C
(
²) l
ld (
) h ld
/k
°
Calculated: 1x Dual Circuit Transmission Line
2x 7x #10 AWG (36.8mm²) Alumoweld (ACS) Shield Wire: Rac = 2.94Ω/km @ 20°C
Calculated: 1x Dual Circuit Transmission Line ‐ Design approved by HONI & OESA
2x OPGW (129mm²) ACS/AA Shield Wire Rac 0 423Ω/km @ 20°C
2x OPGW (129mm²) ACS/AA Shield Wire: Rac = 0.423Ω/km @ 20°C
Calculated: 1x Dual Circuit Transmission Line ‐ As‐Built approved by HONI & OESA
2x OPGW (129mm²) ACS/AA Shield Wire: Rac = 0.423Ω/km @ 20°C
If
(A)
Rg
(ohm)
Sf
(%)
Ig
(A)
GPR
(V)
≤5,000
>5,000
,
≤25000
29.79
10,667
21.40
18.20%
1,941
57,837
10.00%
1,067
31,779
18.22%
8 %
1,943
,9 3
57,885
5
,885
13.10%
1,398
41,636
5.82%
620
18,486
6.17%
658
14,090
13
Design Validation
Design Validation
Following site testing of as‐built works, and using the same g
g
,
g
grounding analysis software, RES undertook validation checks of design calculations based on the as‐built ground electrode resistance test data taken from the terminal substations and each line pole. Both Design and As‐Built installation were accepted and approved by HONI & OESA
accepted and approved by HONI & OESA.
14
Design Validation
Design Validation
Project 'A'
Current Split Factor Curve ‐ 230/34.5kV Windfarm Sub
3.25
21.4
3.30
50%
29.8
18000
45%
16000
40%
14000
35%
12000
30%
10000
25%
8000
20%
6000
15%
4000
10%
2000
5%
0
0.1
1.0
Current Split Factor
(%)
Ground Potential Rise
((volts)
20000
0%
100.0
10.0
Earth Electrode Resistance seen at Windfarm Main Sub
(ohms)
OEB/HONI GPR [10.67kA] (Design)
OEB/HONI GPR [10.67kA] (As‐Built)
Max Actual GPR [3.16kA] (Design)
Max Actual GPR [3.16kA] (As‐Built)
OESC 5000V (Design)
Current Split Factor (Design)
Current Split Factor (As‐Built)
R Grid (Design)
R Grid (As‐Built)
R Windfarm (Design)
R Windfarm (As‐Built)
15
Conclusion
The grounding design criteria at Project 'A' WPP proved to be g
g
g
j
p
very challenging throughout, particularly at the 230kV terminal substations. However, in‐depth modelling and analysis of the 230kV transmission line configuration, to accurately calculate ground fault current split factors, demonstrated: excellent correlation between IEEE 80 2000 Table C 1 values and analysis
correlation between IEEE 80‐2000, Table C.1 values and analysis results; opportunity to improve on standard split factor values through in‐depth modelling of an enhanced OPGW conductor g
p
g
installation; very good validation of design values through extensive on‐site testing prior to approval and commissioning.
All helping to provide a confident design solution, for a very difficult grounding environment.