FAULT CURRENT DISTRIBUTION IN A MESHED NETWORK USING NETS MODULE
Introduction to NETS :
NETS is a very flexible tool to solve full meshed multi-conductor and multi-phase networks and is based on the multi-phase system representation. This approach is general and overcome the classic method of symmetrical components and can be used to represents power systems as multi-conductor networks enabling the consideration of asymmetrical and/or unbalanced systems also in presence of grounding circuits or circuits with a different phases number.
The scope of the following example is the calculation of the fault current distribution in case of single phase fault current in a substation fed by a simple meshed network.
The results were compared with results provided by OpenDSS (EPRI).
The meshed network with a rated line voltage 132 kV is fed by two three-phase feeders at 400 and 230 kV respectively and includes an overhead line 40 km long, two underground cables 30 km long and a substation with a load 10 MVA.
The main scope is the calculation of the single fault to earth current and of the split factor in the faulted substation.
Feeder 400 kV
Main input data for three-phase feeder (symmetrical and balanced):
- Rated line voltage: 400 kV
- Short circuit power: 40000 MVA
- X/R = 25
- Ze = 0.1000 Ω
The equivalent internal impedance of the feeder can be calculated with the following formulas:
where:
- V (V) = rated line voltage
- Scc (VA) = short circuit power
- cos = short circuit power factor
- X/R = ratio between reactance and resistance
It follows:
- Z = 4.000 (0.03997 + j 0.9992) = 0.1599 + j 3.997 = about j4.000 Ω (for all phases)
Feeder 230 kV
Main input data for three-phase feeder (symmetrical and balanced):
- Rated line voltage: 230 kV
- Short circuit power: 20000 MVA
- X/R = 20
- Ze = 0.1000 Ω
The equivalent internal impedance of the feeder can be calculated with the previous formulas.
It follows:
Z = 2.645 (0.04994 + j 0.9988) = 0.1321 + j 2.642 Ω = about j2.645 Ω (for all phases)
Substation 400/132 kV
Main input data for transformer:
- Type: two windings transformer
- Phases numbers: 3
- Apparent rated power: 50 MVA
- Frequency = 50 Hz
- Rated line voltage in = 400 kV
- Rated line voltage out = 132 kV
- Short circuit voltage: 12%
- Short circuit losses: 0.4%
- No load current: 0.6%
- No load losses: 0.06%
- Connections and vector group: Yy0
- Neutral: grounded at both sides with a common grounding resistance: 1 Ω
Substation 230/132 kV
Main input data for transformer:
- Type: two windings transformer
- Phases numbers: 3
- Apparent rated power: 20 MVA
- Frequency = 50 Hz
- Rated line voltage in = 230 kV
- Rated line voltage out = 132 kV
- Short circuit voltage: 12%
- Short circuit losses: 0.4%
- No load current: 0.8%
- No load losses: 0.08%
- Connections and vector group: Yy0
- Neutral: grounded at both sides with a common grounding resistance: 1 Ω
Overhead Line
Figure represents the tower lattice layout and the distances between phase conductors and overhead earth wire.
Main input data:
- Low frequency soil resistivity = 100 Ωm
- High frequency soil relative permittivity = 6
- Frequency = 50 Hz
- Line length = 40 km
- Span length = 250 m (160 spans)
- Phases conductors layout: A (left - down), B (right), C (left - up)
- Maximum and minimum distance to the soil surface phase A: 15.1 and 8 m
- Maximum and minimum distance to the soil surface phase B: 17.1 and 10 m
- Maximum and minimum distance to the soil surface phase C: 19.1 and 12 m
- Maximum and minimum distance to the soil surface overhead earth wires: 24.35 and 19.4 m
- Position with respect to the tower axis: A -3.50 m, B +3.00 m, C – 2.90 m
- Phase conductors: ACSR (Aluminium Conductor Steel Reinforced the steel is inside and the aluminium is outside)
with external diameter = 31.5 mm, cross section of steel and aluminium 65.81 and 519.5 and mm2 respectively,
resistance = 0.05564 Ω/km at 20 °C
- Overhead wires: steel and aluminium conductors with external diameter = 11.5 mm, total cross section 80.66 mm2,
resistance = 1.062 Ω/km at 20 °C
- Resistance to earth of each single tower: 20 Ω
The average height above ground of phase conductors and overhead earth wire can be calculated using the formula:
For phase A h = 8+(15.1-8)/3= 10.37 m, for phase B h = 12.37 m, for phase C h = 14.37 m and for overhead earth wire h
= 21,05 m.
Underground Cables
Main input data for underground cables:
- Low frequency soil resistivity = 100 Ωm
- High frequency soil relative permittivity = 6
- Rated line voltage = 132 kV
- Frequency = 50 Hz
- Line length = 30 km
- Phases conductors layout: A (left), B (center), C (right)
- Phases conductors depth = 1 m
- Phase conductor horizontal distance = 0.5 m
- Core: copper with external diameter = 34.2 mm, cross section 800 mm2, AC resistance 0.0326 Ω/km at 90 °C,
inductance 0.36 mH/km, capacitance 0.212 μF/km
- Insulating: XLPE (resistivity 1015 Ωm, relative permittivity 2.3) with thickness 19.4 mm including semi-conductive
screens
- Screen: copper wires with external diameter 74 mm, cross section 50 mm2
- Sheath: Polyethylene (resistivity 107 Ωm, relative permittivity 2.3) with thickness 4 mm
- Outer diameter: 82 mm
- Trasposition of screen: at 10 and 20 km
- Resistance to earth of grounding system at transposition points: 10 Ω
- Voltage Limiter Triggering: 1500 V
Load
Main input data for load:
- Type: three phases, balanced without neutral
- S = 10 MVA
- cos = 0.8
- Grounding resistance: 1 MΩ
The load can be represented using a transverse impedance referred to the rated voltage 132 kV with:
- Z = 1742 (0.8 + j 0.6) = 1394 + j 1045 Ω (for all phases)
- Ze = 1 MΩ
Steady state condition
The phase voltage and currents calculated with NETS and OpenDSS are displayed in the following table.
Taking into account that the system is symmetrical and the load is balanced, only the values in a the phase “A” are displayed in previous table.
The agreement between NETS and OpenDSS results is excellent.
Will update more on this example
Special Thanks to Team XGSlab for their incredible efforts and innovation in the field of electrical design engineering.
Credits to Team XGSlab: www.xgslab.com
Introduction to NETS :
NETS is a very flexible tool to solve full meshed multi-conductor and multi-phase networks and is based on the multi-phase system representation. This approach is general and overcome the classic method of symmetrical components and can be used to represents power systems as multi-conductor networks enabling the consideration of asymmetrical and/or unbalanced systems also in presence of grounding circuits or circuits with a different phases number.
The scope of the following example is the calculation of the fault current distribution in case of single phase fault current in a substation fed by a simple meshed network.
The results were compared with results provided by OpenDSS (EPRI).
The meshed network with a rated line voltage 132 kV is fed by two three-phase feeders at 400 and 230 kV respectively and includes an overhead line 40 km long, two underground cables 30 km long and a substation with a load 10 MVA.
The main scope is the calculation of the single fault to earth current and of the split factor in the faulted substation.
Feeder 400 kV
Main input data for three-phase feeder (symmetrical and balanced):
- Rated line voltage: 400 kV
- Short circuit power: 40000 MVA
- X/R = 25
- Ze = 0.1000 Ω
The equivalent internal impedance of the feeder can be calculated with the following formulas:
where:
- V (V) = rated line voltage
- Scc (VA) = short circuit power
- cos = short circuit power factor
- X/R = ratio between reactance and resistance
It follows:
- Z = 4.000 (0.03997 + j 0.9992) = 0.1599 + j 3.997 = about j4.000 Ω (for all phases)
Feeder 230 kV
Main input data for three-phase feeder (symmetrical and balanced):
- Rated line voltage: 230 kV
- Short circuit power: 20000 MVA
- X/R = 20
- Ze = 0.1000 Ω
The equivalent internal impedance of the feeder can be calculated with the previous formulas.
It follows:
Z = 2.645 (0.04994 + j 0.9988) = 0.1321 + j 2.642 Ω = about j2.645 Ω (for all phases)
Substation 400/132 kV
Main input data for transformer:
- Type: two windings transformer
- Phases numbers: 3
- Apparent rated power: 50 MVA
- Frequency = 50 Hz
- Rated line voltage in = 400 kV
- Rated line voltage out = 132 kV
- Short circuit voltage: 12%
- Short circuit losses: 0.4%
- No load current: 0.6%
- No load losses: 0.06%
- Connections and vector group: Yy0
- Neutral: grounded at both sides with a common grounding resistance: 1 Ω
Substation 230/132 kV
Main input data for transformer:
- Type: two windings transformer
- Phases numbers: 3
- Apparent rated power: 20 MVA
- Frequency = 50 Hz
- Rated line voltage in = 230 kV
- Rated line voltage out = 132 kV
- Short circuit voltage: 12%
- Short circuit losses: 0.4%
- No load current: 0.8%
- No load losses: 0.08%
- Connections and vector group: Yy0
- Neutral: grounded at both sides with a common grounding resistance: 1 Ω
Overhead Line
Figure represents the tower lattice layout and the distances between phase conductors and overhead earth wire.
Main input data:
- Low frequency soil resistivity = 100 Ωm
- High frequency soil relative permittivity = 6
- Frequency = 50 Hz
- Line length = 40 km
- Span length = 250 m (160 spans)
- Phases conductors layout: A (left - down), B (right), C (left - up)
- Maximum and minimum distance to the soil surface phase A: 15.1 and 8 m
- Maximum and minimum distance to the soil surface phase B: 17.1 and 10 m
- Maximum and minimum distance to the soil surface phase C: 19.1 and 12 m
- Maximum and minimum distance to the soil surface overhead earth wires: 24.35 and 19.4 m
- Position with respect to the tower axis: A -3.50 m, B +3.00 m, C – 2.90 m
- Phase conductors: ACSR (Aluminium Conductor Steel Reinforced the steel is inside and the aluminium is outside)
with external diameter = 31.5 mm, cross section of steel and aluminium 65.81 and 519.5 and mm2 respectively,
resistance = 0.05564 Ω/km at 20 °C
- Overhead wires: steel and aluminium conductors with external diameter = 11.5 mm, total cross section 80.66 mm2,
resistance = 1.062 Ω/km at 20 °C
- Resistance to earth of each single tower: 20 Ω
The average height above ground of phase conductors and overhead earth wire can be calculated using the formula:
For phase A h = 8+(15.1-8)/3= 10.37 m, for phase B h = 12.37 m, for phase C h = 14.37 m and for overhead earth wire h
= 21,05 m.
Underground Cables
Main input data for underground cables:
- Low frequency soil resistivity = 100 Ωm
- High frequency soil relative permittivity = 6
- Rated line voltage = 132 kV
- Frequency = 50 Hz
- Line length = 30 km
- Phases conductors layout: A (left), B (center), C (right)
- Phases conductors depth = 1 m
- Phase conductor horizontal distance = 0.5 m
- Core: copper with external diameter = 34.2 mm, cross section 800 mm2, AC resistance 0.0326 Ω/km at 90 °C,
inductance 0.36 mH/km, capacitance 0.212 μF/km
- Insulating: XLPE (resistivity 1015 Ωm, relative permittivity 2.3) with thickness 19.4 mm including semi-conductive
screens
- Screen: copper wires with external diameter 74 mm, cross section 50 mm2
- Sheath: Polyethylene (resistivity 107 Ωm, relative permittivity 2.3) with thickness 4 mm
- Outer diameter: 82 mm
- Trasposition of screen: at 10 and 20 km
- Resistance to earth of grounding system at transposition points: 10 Ω
- Voltage Limiter Triggering: 1500 V
Load
Main input data for load:
- Type: three phases, balanced without neutral
- S = 10 MVA
- cos = 0.8
- Grounding resistance: 1 MΩ
The load can be represented using a transverse impedance referred to the rated voltage 132 kV with:
- Z = 1742 (0.8 + j 0.6) = 1394 + j 1045 Ω (for all phases)
- Ze = 1 MΩ
Steady state condition
The phase voltage and currents calculated with NETS and OpenDSS are displayed in the following table.
Taking into account that the system is symmetrical and the load is balanced, only the values in a the phase “A” are displayed in previous table.
The agreement between NETS and OpenDSS results is excellent.
Will update more on this example
Special Thanks to Team XGSlab for their incredible efforts and innovation in the field of electrical design engineering.
Credits to Team XGSlab: www.xgslab.com
