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Lightning protection for a OHL/UC connected GIS

Lightning protection for a OHL/UC connected GIS

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  • The system considered in this paper was the 170 kV system around the province of Aalborg in the northern Denmark in the year 2014. The special about this system is that a large part of the system is consisting of Gas Insulated Substations with underground cables. However, lightning can still enter the system through the overhead line sections in the four areas shown here. This study was a follow up from our 9th semester project, where it was investigated whether overvoltage protection was necessary at the ABØ substation. The project showed that only lightning entering Area 3 showed overvoltages at the transformer mounted at the GIS, which therefore is the only area considered in the paper.
  • The section between NVV and ABØ consist of both a cable and an overhead line section. The overhead line section has two high tower across the fjord. Here, the lightning is considered to strike the tower closest to the ABØ substation. Explain the ground wire, the 420 kV system and the 170 kV system. Surge arrester, cable section to ABØ substation. Modelling In the model, the overhead lines, cables and outdoor busbars are represented with transmission lines corresponding to their physical configuration. The GIS busbars and transformers are represented with their equivalent capacitances. Furthermore, a model for the tower, the grounding resistance and the surge arrester is needed which are explained in the following.
  • Tower model The tower model is based on the concept a shown here, where one ground wire is placed above three phase conductors. The insulators of the phase conductors are modelled by a capacitance and a breaker which represents the back flashover. The tower is represented by a surge impedance and the grounding resistance. This concept is expanded for the two high towers. The tower surge impedance is divided into three parts representing the distance between the ground wire, the 420 kV system, the 170 kV system and ground. The insulator models are modelled by capacitances and a breaker controlled by a control circuit. The grounding resistance is dependent on the current flowing to ground. The insulator model is explained in the following, followed by the model for the grounding resistance.
  • Insulator model The breakdown characteristic across the insulator is modelled with equation which represents a volt-time characteristic. Here, the value K1 represents the Lightning Impulse Withstand Level. The second part introduces the time dependency of the breakdown characteristic. This is implemented in PSCAD with the control circuit shown here. A startup circuit is determining whether a voltage difference is present between the insulator voltage and the phase-to-ground voltage. If this is the case the volt-time characteristic is stated, and the volt-time curve is calculated. The volt-time characteristic is compared with the insulator voltage, if this is exceeded a breakdown occurs and the breaker is closed.
  • Grounding resistance The dynamic grounding resistance is modelled by this equation, which is a function of the current flowing to ground. Here, R0 is the low current resistance. Ig is the critical current causing ionization of the soil. IR is the current flowing to ground. Both Ig and IR is determined by the grounding electrode and the soil resistivity. This is implemented in PSCAD with the calculation shown here. Surge arrester model The surge arrester model is based on the simplified IEEE model, where the inductances represents the frequency dependency and the two conductances represent the non-linear characteristics of the surge arrester. Furthermore, the dynamic grounding resistance is included.
  • Simulation parameters The simulation is conducted for both shielding failure and back flashover, with the general simulation parameters listed in this table. The front time and time to half for the back flashover and crest magnitude is based on recommendation from IEC. Whereas the front time for shielding failure is calculated based on the crest magnitude. The crest magnitude is found to -41,8 kA which is the maximum current that can cause a shielding failure based on a geometric evaluation. Time to half is not important for this analysis and is therefore chosen to 350 as for the BFO. The soil resistively for both the shielding failure and back flashover is based on measurement in the area. Parameter investigation As the parameters for the simulation is estimated values, it is chosen to evaluate the influence of three parameters on the overvoltage experienced at the substation. Here, the Lightning front time is only investigated for shielding failure as the BFO is of high magnitude. The soil resistivity is evaluated as the transition point is located close to the water, as well as a chalk mine. The cable length between the transformer at the GIS busbar is evaluated with a surge arrester placed at the busbar, in order to evaluate the surge arrester protection zone. In the following only the results for simulation of shielding failure are reviewed, as these gives a clearer viewing of the propagation due to lesser reflections compared with simulation of back flashover.
  • The lightning surge is entering the substations from NVV, where the measuring point for the plots is the voltage magnitude at this transformer. The overvoltages are evaluated for both open and closed busbar breaker. The lightning is striking at the time t=0, where we can se that a delay on 6 µs is present before the lightning reaches the substation. This corresponds to the travelling time across the cable and the diagonal of the overhead line. As shown reflections are experienced periodically due to reflections. This simulation of the front time showed that only a small difference in the experienced magnitude was found. This is due to the fact that the travelling time across the cable section is approximately 10 µs, letting the lightning charge to its magnitude. The overvoltages for closed breaker does not exceeds the inadmissible voltage for the transformer. For the closed breaker this is the case.
  • In the same manner as before, the voltage at this transformer is evaluated for both open and closed busbar breaker. This evaluation is conducted for different soil resistivities, varying from 10 Ohm-m to 1000 Ohm-m. We can see that the soil resistivity has a great effect on the experienced overvoltage. Though inadmissible voltage is only present for the simulation with open breaker.
  • In the same manner as before, the voltage at this transformer is evaluated for both open and closed busbar breaker.
  • God konklusion

Transcript

  • 1.
    • Lightning Simulation of a Combined Overhead Line/Cable Connected GIS
    • by
    • Jakob Kessel, Ví ðir Már Atlason and Claus Leth Bak
    • Aalborg University,
    • Institute of Energy Technology
    • and
    • Jesper Lund
    • NV Net A/S
  • 2. Introduction
    • 170 kV transmission system for year 2014
      • Mainly underground cable and GIS
    • Follow up on 9th semester project
  • 3. Introduction
    • Only Area 3 showed overvoltages
    • Modelling
      • Lines, cables and outdoor busbars
        • Transmission lines
      • GIS busbars and transformers
        • Equivalent capacitances
      • Tower model
      • Grounding resistance
      • Surge arrester
    SA
  • 4. Modelling
    • Tower model
      • Tower surge impedance
      • Insulator model
      • Grounding resistance
    Cite: Fast Front Task Force of the IEEE, and Sargent et al.
  • 5. Modelling
    • Insulator model
      • Voltage-time characteristic
    Cite: Fast Front Task Force of the IEEE, and Yadee et al.
  • 6. Modelling
    • Grounding resistance
      • Dynamic grounding resistance
      • Where:
        • R 0 is low current grounding resistance
        • I g is the critical current causing ionization of the soil
        • I R is the current to ground
    • Surge arrester model
      • Simplified IEEE model
    Cite: Fast Front Task Force of the IEEE, and Crisholm et al. Cite: Pinceti et al.
  • 7. Simulation
    • Simulation parameters
      • Shielding failure
      • Back flashover
      • The lightning surge is estimated with double exponential function
    • Parameter investigation
      • Lightning front time
        • Only for shielding failure
      • Soil resistivity (at the overhead line/cable transition point)
      • Cable length (between transformer and surge arrester in GIS)
    Soil resitivity Crest magnitude Time to half Front time [ Ω m] [kA] [µs] [µs] 92,5 -200 350 10 Back flashover 92,5 -41,8 350 1,4 Shielding failure
  • 8. Results
    • Simulation results
      • Varying lightning
      • front time, SF,
      • closed breaker
      • Varying lightning
      • front time, SF,
      • open breaker
    U tf
  • 9. Results
    • Simulation results
      • Varying soil
      • resistivity, SF,
      • closed breaker
      • Varying soil
      • resistivity, SF,
      • open breaker
    U tf
  • 10. Results
    • Simulation results
      • Varying cable
      • length, SF,
      • closed breaker
      • Varying cable
      • length, SF,
      • open breaker
    U tf
  • 11. Results
    • Evaluation of critical lightning current
      • Varying front time
        • Only evaluated for shielding failure
        • No overvoltages with closed breaker
      • Varying soil resistivity
        • No overvoltage for SF with closed breaker
  • 12. Results
    • Evaluation of critical lightning current
      • Cable length
        • No overvoltages with closed breaker
      • The MTBF is found based on the lightning current
  • 13. Modelling
    • Risk assesment
      • Back flashover
        • MTBF closed = ( P (closed) P (current) N flashes ) - 1
        • MTBF open = ( P (open) P (current) N flashes ) -1
        • P (open) ≈ 1/365
        • P (closed) = 1 - P (open)
      • Shielding failure
        • MTBF closed = ( P (closed) P (sf) P (current) N flashes ) -1
        • MTBF open = ( P (open) P (sf) P (current) N flashes ) -1
    • Mean Time Between Failure
  • 14. Conclusion & Discussion
    • Conclusion
      • The steepness of the lightning surge
      • Limited effect on the overvoltages.
      • The soil resistivity at the overhead line/cable transition point
      • Great effect on the overvoltages.
      • The cable length between the transformer and the surge arrester in the GIS
      • Increased cable length yielding increased voltage magnitude at the transformer.
      • MTBF > 2000 years
      • The surge arrester at the overhead line/cable transition point provides adequate protection for the substation.
      • Further protection in form of a surge arrester at the GIS busbar is not necessary.