Research On Novel Fast Adjustable Peterson Coil Based Grounding System

With the large-scale reconstruction of city power system in recent years, the power supply network is becoming larger and larger. In addition, the enormous use of cables in city power system increases the current through the grounded capacitor. These make the single-phase fault current get greater, which lead to more and more instantaneous grounding faults unable to eliminate naturally. To solve these problems, the compensating method of neutral point grounding through Peterson coil is commonly used in our country. After the neutral point is grounded through Peterson coil, it can decrease the capacitor current through fault point and the over-voltage caused by the grounding fault, which is advantage for the extinguishments of electric arc.This dissertation is based on a novel Peterson coil based grounding method, which is temporarily named novel quickly adjustable Peterson coil. With this grounding method, the inductance value of the Peterson coil can be adjusted quickly and continuously in real time. To achieve this function, this dissertation proposes an adjusting method by using a master-slave of computers control system with CAN Bus.The quick adjustment of the inductance value of the Peterson coil is realized by using a method, which can control the trigger angle of the thyristor with power electronics technology. The advantages of introducing this technique into the Peterson coil grounded system and the requirements of decreasing harmonics currents components and reducing the voltage on the thyristor are studied in detail in this dissertation. A novel structure of Peterson coil with particular design can fully satisfy above requirements and allow us to use the lower critical voltage-level thyristor in the 6-10kV and higher voltage power system. The novel Peterson coil design resolves key problems of using thyristor to control the Peterson coil, and becomes the foundation of realizing quick adjustment of the inductance value of the Peterson coil. According to the quickly adjustable characteristic of the Peterson coil grounding system, this dissertation proposes a new control strategy. When the system is in operation, the strategy can limit the series resonance over-voltage by using the off-resonance method. When fault occurs, the strategy can conveniently suppressing arc by rapidly adjusting the inductance value of the Peterson coil to the point, which the resonance with grounding capacitance of the power network is occurred. This new method overcomes the serious disadvantages that the system is easy to generate resonant over-voltage because of the traditional Peterson coil with slower adjustment speed so that it has to be operated close to the resonant point. To realize quick adjustmentof the novel Peterson coil and the protection by selecting fault line, a set of computers control system is proposed in this dissertation. This control system resolves synchronous sampling, the trace and lock phase of frequency and digital thyristor triggering system etc, computerizes the controlling scheme, and meets the requirement of power substation without watchman. The novel quickly adjustable Peterson coil grounding system and its control theory of this dissertation have a great prospect in the reformation of power network in the cities of our country.