EA Technology Ltd Power System and Substation Earthing Course
Session 10.2, Example Case Study and Calculations using BS EN 50522 and S34 Formula
WJS (Bill) Rogers (Consultant)
Summary of Case Study 33/11kV Primary Substation supplied on a 33kV overhead line with no earth wire. An 11kV/LV Substation and transformer at a Farm with a PV generation array will be connected to the 33/11kV Substation with dedicated 11kV Cable. The PV array is large with extensive ground contact area and will have a low ground contact/electrode resistance Decide if the LV Supply earth system can be connected to the 11kV Substation earth system or if not what should be done?
11kV/LV PV Array Substation Directly Connected to 33/11kV Substation 33kV/11kV Substation (SSA)
PV Generation
33/11kV Substation earth grid
33/11kV 11kV
LV Neutral Earthed in LV Switchgear
11kV/LV Ground mounted Substation (SSB)
RE1
LV SWG
PV G
11kV/LV
RE3
2 km cable Connection 3 x 1 Phase 50mm2 XLPE 11kV trefoil cable 11kV Substation earth system
RE2
LV TN-C (PNB)
RE4 LV TN-S
33kV/11kV Substation (SSA) Earth Grid
Interconnection of 11kV and LV Earth Systems RE3
RE4
RE1
11kV/LV Transformer 11kV
LV
PV
SWG
SWG
G
20m
2.0 km RE2 RE2 11kV Substation Earth System
50m RE3
RE4
LV distribution board connected to PV Generation. (a) LV Neutral earthed at LV distribution board. (b) 11kV and LV earth systems interconnected
11kV/LV PV Array Substation Directly Connected to 33/11kV Substation (33kV and 11kV EF) 33kV 2 kA
PV Generation
33/11kV 11kV 11kV 1 kA
RE1
LV SWG
PV G
11kV/LV
RE3 2 km cable Connection 3 x 1 Phase 50mm2 XLPE 11kV trefoil cable
RE2
RE4
Summary of Calculations 1.
Tolerable Time/touch voltages for the conditions
2.
Primary Substation (SSA) earth system resistance
3.
11kV/LV Substation (SSB) earth system resistance
4.
Combined 11kV Substation and LV earth system resistance assuming no overlapping of resistance areas
5.
Check EPR in (SSB) for 11kV EF in SSB
6.
Decision whether 11kV EPR is acceptable
7.
Self impedance of a 2km long 11kV cable, 3 x 1 core. 2
8.
35mm copper wire screened, earthed and bonded each end Transfer of 33kV fault current and EPR to SSB 33kV earth fault at SSA
9.
Decision whether 33kV EPR in SSB is acceptable
Associated Information • Maximum EF current and duration – 33kV in Primary Substation
= 2000A
0.2sec
– 11kV in 11kV PV substation
= 1000A
0.5sec
• Average effective resistivity ( ρ) values: – At primary
= 80 -m
– Along route of 11kV cable
= 100 -m
– At 11kV Substation
= 120 -m
• Earth system resistances –
RE1
is unknown, to be calculated
–
RE2
is unknown , to be calculated
–
RE3
is 10 (declared as an intended maximum value)
–
RE4
is 1.0 (declared as an intended maximum value)
11kV Cable Circuit and Earth Systems of Substations SSA and SSB 33kV Primary Substation SSA with 33kV/11kV three phase transformers
2 km, 3x11kV, 50mm2single core 11kV Substation SSB cables with insulated sheaths bonded with earth system and earthed at SSA and SS B connected to LV system 11kV SSB LV System
33kV SSA 11kVcables trefoil laid directly in ground
RE1
RE2
RE3
Average effective resistivity along route is 100-m
RE4
Details of 3 x 1 phase, 50mm2, Copper, XLPE, Copper Wire Insulated and Sheathed Cable • • • • • •
Cable length between Substation SSA and SSB, = 2 km Cablesheathnominalscreenarea, = 35 mm2 Cable sheath AC resistance, R S1 = 0.68 Ω /km Average radius of single phase cable sheath r A = 23.2 mm AC resistance of three parallel cable sheaths, R S3 = 0.227 Ω /km Overall diameter of insulated cable D = 51 mm D
D
11kV cable sheaths
• Average buried length resistivity (ρ ) • Cable sheath self impedance to be calculated
= 100 Ω-m
Relevant Fault Voltage Limits (UTP) • Limits in BSEN50522, National Annex, Figure NA1. • Curve applies to voltage transfer into wide area LV system. – LV system with neutral point single point earthed.
• Also EPR is tolerable up to 2 times curve values – With a basic earth system. – LV common to the site. – No transfer voltages.
• EPR is tolerable up to 4 times curve values with specific measures of mitigation. • EPR above 4 times curve values subject to detailed assessment of touch and step voltages
BS EN 50522 Figure 4 and UK NA4 BS EN 50522 Touch Voltage Limits with no Additional Resistances
80 0 e
70 0
a g t l 60 0 o V e 50 0 c u o 40 0 T e l b 30 0 a r e l o 20 0 T
Fig 4 UK NA4
Potential hand-hand transfer voltage
10 0 0 0
0 .2
0 .4
0 .6
0 .8
Time (seconds)
1
1 .2
BS EN 50522:2012 Figure NA.2 – in National Annex NA (UK Permissible touch voltages with additional resistances) 5000 Curve 1 (No additional Impedances) Curve 2 (RA = 2150 ohms) Curve 3 (RA = 2500 ohms) Curve 4 (RA = 3000 ohms)
Voltage (V) 4000 3500 3000
Curve 4 Curve 3 Curve 2
2500 2000 1500 1000 Curve 1
500 0 10 msec
100 msec
1sec
Time
BSEN50522 Voltage limits for Fault Durations Fault Duration
0.1 (s)
0.15 (s)
0.2 (s)
0.5 (s)
1.0 (s)
2.0 (s)
3.0 (s)
5.0 (s)
10 (s)
tp V Fig(V) 4 (0Ω)
654
-
537
220
117
96
-
86
85
Vtp (V) NA1 (0Ω)
405
362
320
135
68
-
52
-
-
-
162 -
-
(Additional protective resistances … Ω) 2150 Ω (Ground)
2070
1570
578
233
(Inside Substation)
2500 Ω 3000 Ω
1808
2341
2043
1773
650
259
-
180 -
-
2728
2379
2064
753
298
-
205 -
-
Information on Primary Substation and Earth Grid 11kV neutral point is resistance earthed for 1000A maximum
30m Concrete Reinforcing is not connected to earth system
Fence independently earthed. Un-insulated busbars and 33kV connections. Surface layer of stone chippings.
Earth grid layout with 40mm x 3mm Strip electrode Resistivity (ρ) = 80 Ω-m
33/11kV Primary Substation Earth Grid (RE1) • Buried earth grid within fenced area. • No concrete reinforcing and fence independently earthed. • Grid size 30m x 30m with 15m x 15m square meshes. • Eight 2.4 m earth rods connected at corners and mid point of peripheral earth conductors. • Calculate earth resistance (R E1) earth grid. For simplicity use formula Area Ω r Re
4r L plate with same = radius of equivalent circular area as earth grid. • (ρ) = effective ground resistivity = 80 Ω –m
• r
Calculate Resistance of 33/11kV Primary Substation Earth Grid (RE1) • Calculate earth resistance (R E1) earth grid. For simplicity use formula Area Re
4r
Ω
r
L
• (r) = radius of equivalent circular plate = ………….m • Length of perimeter electrode = ………….m • Length of infill electrode = ………….m • Length of rods = .. * 2.4m = ………….m • Total length of buried electrode (L) = …………..m • Resistance of earth grid (R E1) = …………………………………………
Ω
Calculate Resistance of 33/11kV Primary Substation Earth Grid (RE1) • Calculate earth resistance (R E1) earth grid. For simplicity use formula Re
4r
Ω
L
• (r) = radius of equivalent circular plate = 16.93m • Length of perimeter electrode = 120m • Length of infill electrode = 60m • Length of rods = 8 * 2.4m = 19.2m • Total length of buried electrode (L) = 199.2m • Resistance of earth grid (R E1) = (80/4*16.92) + 80/(199.2) = 1.182 + 0.402
= 1.584 Ω
11kV Substation and LV Supply Earth Systems
11kV/LV Substation and earth grid. Concrete Reinforcing is connected to earth system H
ER
Cable to photo voltaic array control and collector system and with earth electrode
ER
I
MV/LV Transformer H
ER
ER
I H
I
H
Earth Bar
I Control &
11kV
Meters
Switch
I
LV Board
I H
ER
Controls
H
I I
H
ER
Separated LV Substation and earth grid.
I
Resistivity (ρ) = 120 Ω-m ER
H
ER
11kV Substation Earth System (RE2) • Compact brick substation mounted on concrete base with reinforcing connected to earth system (reinforcing assumed as 25m per m2). • Substation base ring outline dimensions are 51m mx4m with peripheral of electrode buried outside concrete base. • Four 2.4m earth rods connected to peripheral earth conductor at corners. • Calculate earth resistance (R E2) earth grid. For simplicity use formula
Re
Ω r Area 4r L • Effective Ground resistivity = ρ = 120 Ω -m
Calculate Resistance of 11kV Substation Earth Grid (RE2) • Calculate earth resistance (R E1) earth grid. For simplicity use formula Area Re
4r
Ω
L
r
• (r) = radius of equivalent circular plate = ………….m • Length of perimeter electrode = ………….m • Length of infill electrode = ………….m • Length of rods = .. * 2.4m = ………….m • Total length of buried electrode (L) = …………..m • Resistance of earth grid (R E1) = …………………………………………
Ω
Calculate Resistance of 11kV Substation Earth System (RE2) • Calculate earth resistance (R E2) of 11kV earth grid. For simplicity use formula Area Re
4r
L
Ω
r
• r = radius of equivalent circular plate with same area as earth system • Area = 7m * 6m = 42 m 2 (r) = 3.656 m • Length of perimeter electrode = 26m • Length of concrete reinforcing = 5*4*25 = 500m • Length of rods = 4 * 2.4m = 9.6m • Total buried length of electrode (L) = 535.6m • Resistance of earth grid (R E2) = 120/(4*3.656) + 120/(535.6) =8.206+0.224 = 8.430 Ω
Resistance of Combined 11kV Substation and LV Earth Systems • Calculated 11kV Substation earth system resistance = …….………….Ω • Declared maximum LV switchboard electrode resistance = 10 Ω • Declared maximum LV PV Array earth electroderesistance
=1.0 Ω
• Calculate combined resistance of all earth systems assuming no overlapping of resistance areas RE 11kV and LV TOTAL (RE2T ) = …………………. Ω
Resistance of Combined 11kV Substation and LV Earth Systems • Calculated 11kV Substation earth system resistance = 8.430 Ω • Given LV switchboard electrode resistance = 10 Ω • Given LV PV Array earth electrodes
= 1.0 Ω
• Calculate combined resistance of all earth systems assuming no overlapping of resistance areas RE1 11kV and LV TOTAL (RE2T ) = (8.431-1 +10-1 +1.0-1)-1 = 0.821 Ω
Options to Assess EPR at SSB for an 11kV Earth Fault at Substation (SSB) The most accurate calculation of ground current and EPR is achieved with calculations using cable sheath self reactance and the earth electrodes at SSA and SSB. Before this stage, a pessimistic EPR at SSB may be calculated using resistance calculation only for speed and simplicity. If the EPR does not exceed the allowable touch voltage limit then there is no need to proceed further. RCS
IEF
ICS
SS
11kV 1 kA
SS IG
A
REA
Earth
B
REB
Remember the UK Earthing Design Methodology in Figure NA7 BSEN 50522.
Stage 1 and Stage 2 Resistive Calculation for Maximum EPR at SSB for an 11kV Earth Fault Stages to calculate worst case EPR taking account of the 11kV cable sheath impedance with increasing accuracy and reduced pessimism in the calculated EPR are as follows: Stage 1, (the most pessimistic) calculates the total fault voltage between SSA and SSB ignoring electrodes: = maximum 11kV EF current * cable sheath resistance (RCS) This is the maximum fault voltage between SSA – SSB is assumed maximum possible EPR Stage 2, has improved accuracy for ground current by considering parallel path of).cable sheath (RCS) and the earth electrodes (Rresistances ) and (R E1 E2T Calculation of EPR at SSB will still be pessimistic and worst case
Assess Maximum EPR for 11kV Earth Fault at Substation SSB without Electrodes Given that maximum 11kV earth fault current in SSB (IEF) is limited by the 11kV NER in SSA to: = 1000A For this assignment and speed and simplicity calculate (a pessimistic) EPR at SSB using resistance of cable sheath only Cable sheath resistance
= 2*0.68 Ω/3 RCS
SSA
= 0.445Ω IEF
ICS SSB
Earth
Maximum possible EPRB
= 1000A * 0.445 Ω
= 445V
Stage 2 Calculation of Maximum EPR for 11kV Earth Fault at Substation SSB Given that maximum 11kV earth fault current in SSB (IEF) is limited by the 11kV NER in SSA to: 1000A R ICS
CS
SSA
IEF SSB
IG REA
Earth
REB
Return path impedance between SSA and SSB = RA-B R * ( REA REB ) 0.454 * (1.584 0.821) R A B CS R A B RCS ( REA REB ) 0.454 (1.584 0.821)
= 0.382 Ω
VA-B = 1000A * 0.382Ω
= 382V
IG
0.382 *1000 A (1.584 0.882)
159 A
EPRB 159 A * 0.821 130.4V
Suitability of Basic SSB 11kV Earth System for 11kV Earth Fault at Substation SSB Given that duration of 11kV earth fault current is
……
Given that maximum tolerable 11kV EPR with a basic earth system in SSB (from BSEN50522): (2*Utp) = 2 * 135V …………
0.5sec
370V
The maximum possible EPR using cable sheath resistance only = ………… 445V The maximum EPR using cable sheath and electrode resistances = ………… 130.4V Is the maximum EPR less than the voltage limit (2*Utp)? ………… Yes The basic earth system is acceptable for an 11kV earth fault (Subject to normal bonding and no external transfer voltages)
Discussion Points • Is it reasonable to use combined 11kV and LV earth systems for EPR of SSB for an 11kV fault. • DNO owned/operated 11kV Substation and consumer 11kV connection • DNO owned/operated 11kV Substation and consumer LV connection • Consumer 33kV connection
Rigorous Calculation of SSB EPR for 11kV Earth Fault • More detailed and accurate calculation of 11kV cable sheath self impedance to calculate the 11kV earth fault ground component current and the earth potential rise (EPR) is not expected. • However the following calculations demonstrate the procedure. • Self impedance of the cable sheath must be calculated. This will take account of the combined resistance of all three cable sheaths and their Geometric Mean Radius (GMR).
Self Impedance of 11kV Cable Circuit between SSA and SSB • Average buried length resistivity
( ρ)
= 100 Ω-m
• Self impedance of three parallel cable sheaths (Z
3 10 .. / km GMR 3 Se
) Ω/km SA-B
Z CS , E RCS 49.4 j 62.8 * log e
• Ground return distance =
Se 93.2
.( m)
• GMR3 is the GMR of three parallel cable sheaths
GMD and GMR of Cable Core and Sheaths Three cable sheaths as single conductor = GMR3sheaths GMR1 of single cable sheath is assumed the radius
S
r = average sheath diameter d/2 = 46.4/2 = 23.2 mm S = 51 mm GMD of three cable sheaths = GMD3
GMD 3 sheaths
S 1 2 * S 2 3 * S 3 1 S
3
GMR 3 sheaths
9
r3 * S 6
3
r*S2
GMD3 = 51 mm GMR3 = 39.22 mm
Self Impedance of 11kV Cable Circuit between SSA and SSB • Average buried length resistivity • Ground return distance =
Se 93.2
( ρ) .( cm)
= 100 Ω-m = 932 m
• Self impedance of three parallel cable sheaths (Z CS,E) Ω/km
3 10 .. / km GMR 3 Se
Z CS , E RCS 49.4 j 62.8 * log e
932 3 10 .. / km 0.0392
Z CS , E 0.227 49.4 j 62.8 log e
• (ZCS,E) Ω/km = 0.276 + j0.633 Ω/km
Ground Current at Substation B for 11kV Earth Fault • Length of cable circuit (L)
= 2 km
• Self impedance of three parallel cable sheaths = 0.2764 + j0.633 Ω/km
(Z CS,E)
• Earth system resistances R E1 = 1.584 Ω and RE2 = 0.821 Ω IG IF
RCS R A RB L
• IF/IG =
Z CS
IG =
Ω/km impedance values
EPR1 =
EPR2 =
More Accurate Ground Current and EPR at 11kV Substation (SSB) for 11kV Earth Fault IG
I EF
RCS R A RB L
IG IF
IG IF
IG
IF
Z CS
0.227
IG
1.479 j 0.633
IF
0.130 j 0.0556
0.227 1.584 0.821 0.2764 j 0.633 2
IG IF
IG
EPRA EPRB
0.227(1.479 j 0.633)
IG
1.479 2 0.6332
IF
0.3357 j 0.144
0.0912 165O
= IF *(0.0913 165O) = 1000A * 0.0912 165O = 91.3A 165O = 91.3 * 1.584 Ω = 91.3 * 0.821 Ω
= 145V = 75V
2.588
Summary of 11kV EPR Calculation
V
Cable sheath resistance only Cable sheath resistance and electrodes Cable sheath self impedance and electrodes
EPRA
EPRB
445V
-
445V
382V
252V
130V
220V
145V
75V
A-B
More detailed and accurate calculation of 11kV cable sheath self impedance to calculate the 11kV earth fault ground component current and the earth potential rise (EPR) is not expected.
SSB Substation EPR for 33kV Earth Fault • A 33kV earth fault in SSA and transfer voltage to SSB must also be considered. • Because of the type of 33kV overhead line all 33kV fault current must flow to earth in the earth system of SSA combined via the cable sheath with remote SSB. • This will cause some 33kV fault current to flow to SSB and a 33kV EPR will be impressed on SSA earth system and supplied LV system if the HV and LV earth systems are connected. • The transferred 33kV EPR to SSB for combined earth systems must be calculated using the previouslycalculated value for cable sheath self impedance (ZCS,E,).
Calculation of 33kV EPR at Primary Substation (SSA) and Transfer of EPR to SSB Total 33kV earth fault current 33kV 2 kA
Substation SSA
IB ZCS,E,
Substation SSB
IA RE1
IB ZSSB = ZS,A-B + RE2
RE2
ZCS,E
= Self impedance of 11kV cable sheaths
ZSSB
= Earth impedance of cable sheaths from SSA to SSB (ZCS,E + RE2 )
ZT
= Combined SSA & SSB earth system Z T impedance
RE1 * Z SSB RE1 Z SSB
Calculation of 33kV EPR at Primary Substation IF = 2000A Length of 11kV cable circuit (L)
= 2 km
Ω/km) Self impedance of three parallel cable=sheaths 0.2764(+ j0.6328 Ω/km
Self impedance of 2 km parallel cable sheaths (Ω/km) (ZCS,E) = 0.5528 + j1.2656 Ω/km SSA earth system resistance RE1
=
1.584 Ω
SSB earth system resistance RE2
=
0.821 Ω
SS
connection impedance to earth from SS (Z ) = A-B A C ………………See following sheet
Calculation of 33kV EPR at Primary Substation (Continued) • SSA-B connection impedance to earth from SSA (ZSSB) = ZC ZC = ZCS,E + RE2 = 1.3738 + j1.2656 Ω = 1.868 42.652O • SSA earth system resistance RE1
=
1.584 Ω
• Combined earth impedance at SS A (ZT): ZT
ZT
RE 1 * Z C RE 1 Z C
ZT
1.584 * (1.3738 j1.2656) 1.584 1.3738 j1.2656
2.95942.652O 3.217423.164O
• EPRA (at SSA) = IF * ZT
2.1761 j 2.0047 2.958 j1.2656
O Z 0.9197 19.488 T
= 2000A * 0.9197 19.488O = 1839.4V 19.488O
Summary of 33kV and 11kV EPR Calculation
V
Cable sheath resistance only (11kV Fault) Cable sheath resistance and electrodes(11kV Fault) Cable sheath self impedance and electrodes (11kV Fault) Cable sheath self impedance and electrodes (33kV Fault)
EPRA
EPRB
445V
-
445V
382V
252V
130V
220V
145V
75V
-
1839V
808V
A-B
Calculation of 33kV EPR Transferred from SSA to SSB • EPRA (at SSA) = IF * ZT
= 2000A * 0.9197 19.488O = 1839.4V 19.488O
• Current in R E1 at SSA
= 1839.4V 19.488O /1.584Ω = 1161.2A 19.488O
• Current in R E2 at SSB (IC) = EPR A / ZC = 1839.4V 19.488O / 1.868 Ω 42.652O = 984.7A -23.164O A • EPRB (at SSB) = IC * RE2 = 984.7A -23.164O A * 0.821 Ω O
= 808.4 -23.164 V
Suitability of SSB Basic Earth System for 33kV Earth Fault at Substation SSA Given that duration of 33kV earth fault current is
0.2sec
Given that maximum tolerable 11kV EPR with a basic earth system in SSB (from BSEN50522): (2*Utp) is 2*320V=
640V
The maximum 33kV EPR transferred to SSB
808V
=
Is the maximum EPR less than the voltage limit (2*Utp)? …………….
No
The LV system earth resistance may be reduced.
Is the maximum EPR less than the voltage limit (4*Utp)? ……………. Yes The basic earth system is acceptable (subject to the specified measures for additional protection)
Additional Measures for 33kV Earth Fault at Substation SSA Since the maximum EPR will not exceed the voltage limit (4*Utp) the following additional measures may be taken to finalise the earth systems • General application of surface covering to increase touch voltage limit to the required value • The basic earth systems resistances R E2 , RE3 ,and RE4 may be reduced • General disconnection of extended cross site and external metallic paths or installation of barriers to prevent hand to hand contact with transfer voltage • Risk assessment
Copy of BS EN 50522, Figure NA.7 – Earthing design methodology including the option for probabilistic risk assessment
End of calculations
