Mitigation of Very Fast Transient Overvoltages in Gas Insulated UHV Substations

SUMMARY Before starting the design of a UHV substation, it has to be clarified whether very fast transient overvoltages (VFTO) have significant influence on the insulation co-ordination. The decision shall be based on the maximum VFTO peak value that occurs with reference to the rated lightning impulse withstand voltage (LIWV) of the equipment. If the maximum VFTO is below the LIWV, no measures need to be taken. Otherwise it is necessary to design considering the VFTO level as dimensioning criteria or to suppress VFTO by damping devices. For the different sources of VFTO and for the different equipment many mitigation methods are known. The damping of VFTO by integration of a damping resistor is one proven technology. The way to overcome the drawback of such unwieldy design is to use other internal damping measures such as ferrite or high frequency (HF) resonators. The VFTO damping solution utilizing ferrite rings has been analyzed and tested and is described here. The measurements show that the damping effect can be achieved, but with an important drawback: the magnetic material goes easily into saturation, which complicates the design and reduces its general applicability and robustness. A new approach for damping is to implement compact electromagnetic high-frequency resonators with low quality factor specially designed to cover a wider frequency range. The novelty of this idea is not only in designing the resonators but also in dissipating the received VFTO energy by intentionally allowing sparking to occur within the electrically shielded gap of the resonator. The time-domain and frequency-domain method for the analysis of the HF resonators was used for the simulation. The VFTO damping effect of the developed HF resonator tuned to the dominant harmonic component was confirmed by experiments. The maximum value of the VFTO depends on the voltage drop at the disconnector switch (DS) just before striking along with the specific location. Trapped charge remaining on the load side of the DS must be taken into consideration. By using a new VFT disconnector model, it is possible to evaluate the trapped charge characteristic and the entire switching process. The trapped charge voltage (TCV) behaviour strongly depends on the contact speed. A lower contact speed results in a lower TCV and provides an additional safety margin compared to the calculation based on a trapped charge voltage of -1 pu. For the insulation co-ordination this additional margin has to be considered.