Research On Key Issues Of Applying Hybrid On-load Tap Changer In Phase Shifting Transformer

Taking the symmetrical two-core PST and hybrid OLTC as research object,this paper carried out the research on key issues of applying hybrid OLTC in PST based on the requirements of PST for OLTC external technical parameters,the thyristor passive trigger arc extinguishing method and the working process of hybrid OLTC.The main work and innovative achievements are as follows:1.The equivalent model of symmetrical two-core PST is established and the PST operational characteristics under different operating conditions,including advance or retard operation mode,different load current and different load current and load voltage phase difference,is discussed.The equivalent model and operational characteristics have an important guiding significance to the calculation of the maximum step-voltage of OLTC,the research and design of the phase shift transformer and the establishment of corresponding control power flow strategy.2.The calculation method of OLTC maximum step-voltage is proposed.Taking the actual two-core PST as examples,the feasibility and accuracy of the calculation formulas is verified by comparing the calculation results of formulas with the MATLAB/SIMULINK simulation model.The method does not depend on the results of the system power flow calculation,which avoids the complex power flow analysis.The complexity of load change did not result in calculation difficulty of this method,which is convenient for the use of relevant engineering designers and overcome the shortcomings of inconvenient and unreliable in the existing methods.3.Based on the simulation and experimental study for thyristor passive trigger arc extinguishing method and the working process of hybrid OLTC,this paper draws the conclusion that:(1)The hybrid OLTC can play a inportant role in arc extinguishing in the switching process and has the advantage of high reliability.So it meets the requirements of phase shifting transformers;(2)When the breaking current is near zero crossing point,the thyristor trigger conditions can not be satisfied,but it do not affect the extinction performance of hybrid OLTC;(3)The overshoot voltage will formed when the current in thyristor passes through zero and it can be effectively restrained by means of adopting Zn O arresters,then forming reliable protection of thyristor.