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Surveys on neutral point treatment in medium and high voltage power grids

The development of the electrical energy supply networks, mainly caused by the continuing expansion of renewable energies, is taking place almost exclusively on the basis of three-phase cables on MV grid level. Cables are also increasingly being used in HV networks due to the current political conditions and the decreasing acceptance of new overhead lines. The consequent increase of residual currents in networks with isolated or compensated treatments requires technical measures to ensure compliance with the limits defined by DIN VDE 0228 for self-extinguishing of ground arcing or with the defined acceptable contact voltages according to DIN VDE 0101.

Accepted appropriate measures to guarantee permitted residual ground fault currents are, for example:

  • separation of the power system into clusters (use of additional HV/MV transformers or HV/HV isolating transformers)
  • active compensation of residual ground fault currents
  • change to low-resistance neutral earthing

In this context you as a grid operator have to establish the best technical and economic solution and to implement the specific measures. With regard to the change to low-resistance neutral earthing the particular difficulty arises that a large number of details are to be assessed in order to make a reliable estimate of the costs of this change.

Furthermore it is difficult, especially with large 110-kv distribution networks, to determine the sequence of implementation of the detailed measures.

P&M Power Consulting offers you in this context engineering services in the following core areas:

  • technical and economic comparison of variants of the different measures on controlling single-phase ground faults (see above)
  • technical and economic assessment of the change to low-resistance neutral earthing:
    • dimensioning of the low-resistance neutral earthing (definition of neutral earthing impedances and their positions in the grid)
    • testing of the dependability and sensitivity of grid protection
    • examination of compliance with the permitted step and contact voltages
    • examination of the galvanic and inductive influencing of telecommunications facilities
    • examination of the interaction between neutral points between different grid levels
    • assessment of introducing single-phase automatic re-start in comparison to three-phase automatic re-start
    • calculation of the numerical data and costs for the change to low-resistance neutral earthing
    • determination of the strategy or the timing/organisational process of the change to low-resistance neutral earthing