Research On Novel Arc-suppression Coil Grounding System Based On Transformer With Load Controllable

On the basis of comprehensively summarizing the current development and unsolved problems in neutral resonant grounding power system, this dissertation focus on following three parts to carry out studying, these are: new structure and control method of variable reactor, automatic tuning of resonant grounding system and fault line detection in single phase to ground faults.The existing structures and control strategies of variable reactor have some drawbacks, such as the inductance can not be regulated rapidly and continuously or generate serious harmonics. These shortages take negative influence on arc-suppression coil, which limit the compensation effect in practical application. Aim at these problems, a novel variable reactor structure with its control method has been proposed in this dissertation. The reactance tuning principle of this reactor is: using voltage source PWM inverter to be the secondary load of linear transformer, by controlling the inverter's output current, which is also the transformer's load current, the equivalent impedance of transformer's primary winding can be regulated arbitrarily. The key point of this method is real-time control of inverter's output current, which should trace transformer's primary current rapidly and accurately. The holistic control model concerning the inverter with the transformer is founded. By analyses on the model, the criterion of system stabilization and design principle of the inverter's DC link voltage are derived. Furthermore, the steady-state performance and dynamic performance of current tracing are analyzed by simulation tools, on the basis of which an effective arithmetic for this control system is presented. Large capacity variable reactor can be realized by adopting the structure of transformer with multiple secondary windings. Experimental results verified the excellent characteristic of this new type reactor, such as reactance linear and continuous tuning, fast response and few harmonic currents, which are also the ideal performance pursuing in arc-suppression coil application.Accurately detection of distribution network line-to-earth capacitance is the fundamental of effective compensation in neutral resonant grounding system. Injecting variable-frequency signal method has many advantages such as no need to adjust inductance of arc-suppression coil, unchanging running status of power utility, secondary measurement, etc, which is a widely used method in capacitive current detection. However, the existence of neutral point displacement voltage could take influence on the injected signal, which may make the phase judgement inaccurate. These problems have been analyzed basing on the presented novel arc-suppression coil. The equivalent circuit and control model of injecting signal method in unbalanced power networks are given out for the first time. With analysis on the control model, an improved automatic tuning method for arc-suppression coil is proposed, that is: adopting voltage feed forward control to restrain interfere of 50Hz displacement voltage, which can improve the characteristic of the injected signal significantly, make the injected signal without reference to unbalanced network and neutral displaced system. Another innovation is detecting the magnitude of neutral point signal-frequency voltage to judge the resonant state, which improves the sensitivity and accuracy of resonant judgement effectively. The experiments verified that the proposed method faithfully enhanced the capacitance detection capability and precision in unbalanced network or neutral with high displacement voltage network, which proved to have excellent adaptability in all type distribution networks.The fault line selection problem in neutral resonant grounding distribution network has not been well settled until now. On summarizing current fault line selection techniques and state-of-art developments in resonant grounding system, the technique based on zero-sequence active current is mainly researched in this dissertation. By analyzing the influence on the zero-sequence active current polarity while line-to-earth capacitance is unbalance in distribution network, a new fault-line selection method based on zero-sequence active power have been proposed. This method uses the DC component in line instantaneous power to be the criterion, which effectively solved the problem of precise detection of small active signal. To eliminate the interfere of inherent unbalanced current in the feeder, an auxiliary criterion is presented, which used the angle between zero-sequence varied current with zero-sequence varied voltage to judge the direction of active power. Additionally, arithmetic to calculate the phase angle has been proposed in the dissertation.A trend in the resonant grounding system is to make an integrated and coordinated control between the compensation of arc-suppression coil and the single phase to ground fault line selection. A new control strategy is presented by combining the proposed fault line selection method with the unique feature of resistance tuning capability of the novel arc-suppression coil. The fundamental principle is that properly varying the equivalent resistance of the arc-suppression coil to increase the active component in the fault line, which make the character of the fault line more evident, lead to the advance of high selection accuracy. Simulation results proved the validity and practicability of the presented method, the accuracy is satisfactory even in high-resistance grounding situation and in unbalanced system. The fault line selection is synchronous with capacitive current compensation, which fulfills an integrated compensation and protection equipment.A prototype based on the transformer with load controllable theory has been constructed, the capacity of which is up to 800kVA. A full digital control system based on DSP and CPLD microchip has been developed. The pivotal aspects in hardware and software design have been researched and discussed. Manifold optimized techniques in system design have been proposed, e.g. anti-jamming of PWM trigger signal, suppression of switching noise, design of snubber circuit and some kernel control arithmetic. The actual waveforms verified the validity of the proposed improvements.Three phase simulative experiments have been carried out on the 800kVA prototype. Experimental results proved that the novel arc-suppression apparatus has the characteristic of high precision in capacitance detection, wide compensation range, fast response and few harmonics. Authorized by national professional identify institution, the performances of this new type arc-suppression coil satify national standards. Now the 10kV series products have been well running in the power utility, it can be anticipated that this new arc-suppression coil grounding system have expectable future in medium-level distribution networks.