Nanocrystalline based Mitigation Technique for Very Fast Transient over Voltages in Gas Insulated Substations

in gas insulated substations, issue of very fast transient over voltages is very familiar problem because of its effects on live ultra-high voltage equipment. During switching events of disconnectors and circuit breakers prestrikes and restrikes occur due to which voltage transients propagate through gas insulated switchgear. Reflection retraction of these transients increases voltage magnitude and generates very high frequency oscillations. Mitigation or suppression of these voltage transients is important to protect equipment and their dielectric strength and insulation. Due to very high frequency stress, they wield on the apparatus and their magnitude (up to 3.5pu), they create an important problem in the design of ultra-high voltage Gas Insulated Substations. In this paper nanocrystalline based mitigation technique for VFTOs is presented. Nanocrystalline rings can be placed around inner conductor of GIS switchgear. This method of mitigation technique is investigated with four ultra-high voltage substations simulation test setup. Simulation results are presented in MATLAB/SIMULINK ___________________________________________________________________________


1.Introduction
Less space necessity, high consistency, easy maintenance are the main reasons to prefer Gas insulated substations (GIS) over Air insulated substations (AIS) in recent years. In power transmission networks, components in gas insulated substations are highly dependable and are fit for different tenders (Ezhilarasi, 2020). Sulphur hexafluoride is the gas used as insulation medium in gas insulated switchgear as space required by it is very less compared to other insulation mediums including air. Other advantages of SF6 gas are its stability to chemical effects, a high arc interruption capability and good dielectric strength. And SF6 is advisable and feasible environmentally as it is a greenhouse gas (Seeger, M., L, 2008).
As high voltage energized live components in gas insulated substation are protected by enclosure and insulated by SF6 gas external disturbances are almost negligible (Rana, 2013). Voltage and temperature characteristics of SF6 gas is significantly even. Air can endure to very high voltages for very less time. If duration of voltage increases, the withstand voltage falls off considerably. Because of flat characteristics of SF6 gas, the ratio of basic lightning impulse level is close to unity for GIS. Compared with conventional substations like air insulated, GIS based substation needless number of lighting arresters due to its firmness .
During switching events in disconnector switches and circuit breakers of GIS, SF6 causes steep voltage rises due to prestrikes or restrikes and flashovers (Shu 2018). These voltage transients propagate through gas insulated switchgear and these electromagnetic waves reflect and refract numerous times at cut-offs due to coaxial arrangement of switchgear. VFTOs or voltage transients contain hindered impulsive form with high frequency oscillations in range of hundreds of MHz (Badinelli, 2010). magnitude of VFTOs may reach 3-3.5 times of rated voltage and their rise time will be in range of some nano seconds. And these transients will be rotten after some milli seconds. Distinctive GIS Modeling notion and generation of several over voltages because of switching events of disconnector switches with and without load is explained in (Nagarsheth, 2014).
High voltage live components and insulation system of these equipment will be affected much due to high stress created by VFTOs due to switching operations (Hoshina, Y2006). Radiation of these electromagnetic waves by VFTOs will also affect secondary equipment even without direct contact with live components due to ultrahigh voltage. Coordination of insulation and design of dielectric materials for gas insulated switchgear depends on ratio between voltage that can lighting impulse withstanding voltage to rated voltage (He, Jiaxi, 2014). For ultrahigh voltage substations range of VFTOs are becoming additional factor which decides insulation levels and dielectric strength of switchgear. Hence for ultra-high voltage gas insulated substations, suppression of VFTOs is becoming important factor in the view of cost and design (Rayati,2018).
Several suppression techniques are proposed and implemented by researchers. Suppression of voltage transients by inserting resistor with disconnector switch which switching operations caused VFTOs is proposed in (Ranjeeth, 2020). Damping voltage transients by ferrite rings is investigated in (Zhan, 2017). Reducing VFTOs magnitudes by reducing trapped charge during switching operation is explained in (Almenweer, 2019). Attenuation of VFTOs by RF resonator by placing it around inner conductor of GIS is proposed in (H. N. Scherer, 1969). Due to voltage transients and its electromagnetic waves resonator shift into resonance and then dissipates energy generated by VFTOs and hence mitigates them.
In this paper VFTOs due to disconnector switches in four different ultra-high voltage rated substations due to switching process are explained and a mitigation technique of VFTOs is proposed for GIS by considering voltage magnitudes and their frequency of oscillations. Four GIS substations with different voltage levels are modelled using MATLAB/SIMULINK by considering each apparatus equivalent circuits.

Nanocrystalline based Mitigation of VFTO
Effective suppression of fast voltage transients is possible by using Nanocrystalline material rings. These rings can be placed around inner conductor of GIS switch gear. Silicon, iron and boron are the main metals used to for alloy of nanocrystalline state. Using a speedy hardening technology, these metals in liquid form can be processed into very tinny ribbons of 20µm thickness. These ribbons can be converted into rings. These tapes with amorphous microstructure will be converted into required nanocrystalline state with fine crystalline grains embedded in an amorphous residual phase using heat treatment with temperatures between 500-600oC. Diameter of these crystalline grains is in the range of 10-40 nm. Specific properties of these materials can be adjusted by using external magnetic fields during heat treatment.
Nanocrystalline materials will be saturated magnetically by some amount of magnetic field. In gas insulated substations magnetic field strength is because of current in inner conductors is very much higher than the saturation field strength. Hence in low frequency range, there is no energy loss as the rings are magnetically saturated. If the frequency is in range of hundreds of MHz due to voltage transients, energy loss will occur and hence mitigation of VFTO can be achieved.
By placing nanocrystalline rings around inner conductor of gas insulated switchgear there is possibility of three types of losses may occur which leads to damping of voltage transients. By alignment change in existed magnetic domain due to external alternating magnetic field of nanocrystalline device hysteresis losses will occur. The rotating process of the magnetic domains converts the rotating energy into thermal energy. These losses are very less and inconsequential and not much contribute to damping of transients. Very fast rotating magnetic field is created by voltage transients around the inner conductor in the direction of the nanocrystalline ring winding. Interaction between these two magnetic fields generates steep field variation due to which eddy currents can be induced. Power loss due to these eddy currents suppress voltage transients and this process escalates with increase in frequency.
Transverse anisotropy behaviour of nanocrystalline rings generated micro eddy currents due to magnetization process of magnetic domains. Losses due to these micro eddy currents will contribute for suppression of VFTOs. Equivalent circuit of nanocrystalline is shown in fig 1. Resistance, inductance, and capacitance , and can be optimized by simulating the selected GIS substations with number of values and the values which are giving better mitigation effect of voltage transients can be chosen.

Figure 1. Equivalent Circuit of nanocrystalline ring 3.GIS Substations
Four ultra-high voltage rated substations are chosen to study GIS Modeling and impacts of VFTOs and their mitigation using nanocrystalline rings.     . Important factor to be considered for UHV substation is coordination insulation levels of various equipment in point of withstanding capability of lighting impulse and switching impulse voltage. CB1-CB15 are the disconnector switches which are operated to study VFTOs and their suppression using mitigating device.

4.Simulation Results
Effect of nanocrystalline rings on damping of voltage transients due to switching events is studied on four ultra-   1 and resistance, inductance and capacitance in the circuit is equal to 135ohms, 0.82mH and 2.56µF. These values are optimized by comparing damping effect of VFTOs by nanocrystalline rings with various values. The peak value of voltage transient using ferrite ring as mitigating device is in and around 1.6p.u. and this peak value is decreased to 1.1p.u. after replacing resistor with nanocrystalline rings due to effective dissipation of energy by eddy currents and hysteresis currents.      9. VFTOs at disconnector switches (CB1-CB6) and at potential transformers (PT1-PT6) of 1000 KV substation due to switching operation of CB1 disconnector switch with Ferrite Rings as mitigating device Figure 10. VFTOs at disconnector switches (CB1-CB6) and at potential transformers (PT1-PT6) of 1000 KV substation due to switching operation of CB1 disconnector switch with Nanocrystalline as mitigating device   VFTOs at disconnector switches (CB1-CB15) and at potential transformers (PT1-PT11) of 1200 KV substation due to switching operation of CB1 disconnector switch with Ferrite Rings as mitigating device Figure 13. VFTOs at disconnector switches (CB1-CB15) and at potential transformers (PT1-PT11) of 1200 KV substation due to switching operation of CB1 disconnector switch with Nanocrystalline as mitigating device Table 3. VFTOs magnitude, settling time and rise time for 1200KV substation with ferrite ring and nanocrystalline as mitigating device

Conclusion
A new approach for suppression of very fast transient over voltages is presented in this paper. Response of nanocrystalline materials to other magnetic fields and their specific properties can be adjusted by heat treatment. By some amount of magnetic field nanocrystalline materials will be saturated. In gas insulated substations magnetic field strength is because of current in inner conductors is very much higher than the saturation field strength. Hence in low frequency range, there is no energy loss as the rings are magnetically saturated. If the frequency is in range of hundreds of MHz due to voltage transients, energy loss will occur and hence mitigation of VFTO can be achieved. These nanocrystalline rings can be placed around the inner conductor of gas insulated switchgear. By increasing number of rings damping effect on voltage transients can be increased. Very low diameter and high permeability of the rings increase effect of damping. Nanocrystalline rings are modelled as parallel connected resistance, inductance, and capacitance with disconnector switch.