| With the rapid development of new energy power sources and the step-wise increase in the scale of DC transmission system,our country’s UHV AC/DC hybrid power grid is gradually showing the new characteristics of “strong DC and weak AC”,and the development process of the failure in the power grid is gradually showing new characteristics,too.Based on the development status of hybrid power grids and referring to related research on the problem of cascading disturbances caused by grid faults,this paper selects two types of current domestic faults that need to be studied urgently: “Sending and receiving end AC system voltage drop” and “DC-line fault recovery sequences under fault condition”,which occur in the AC and DC systems respectively,as the research objects.And then conducts in-depth research on the cascading failures that they may cause in hybrid power grid,t he specific content is as follows:Analyzing the disturbance propagation characteristics of the above two types of faults.First,according to the power exchange process of AC and DC system and the related control method of DC system,analyzing the disturb ed transient response process of the hybrid power grid after different faults and the specific propagation mechanism of fault disturbance in the system,and from this,preliminarily inferring the main impact of the cascading failures in hybrid power grid;Subsequently,building the disturbance characteristic simulation test model in the power system electromechanical transient simulation software PSD-BPA and simulating the failure conditions,after verifying and supplementing the above theoretical analysis based on the experimental results obtained,the results are as follows: The voltage drop fault that occurs in the sending end AC system will cause commutation failure on the inverter side of the DC system;Failure of the DC-line fault recovery sequences and the voltage drop fault that occurs in the receiving end AC system will cause voltage drop in the sending end AC system,and the failure caused by the latter is more serious.Proposing control measures against fault disturbance.The complete fault disturbance development process obtained from the research shows that,the DC current exceeding the steady-state value during the transient process is the main cause of the cascading failures.From the perspective of improving the control ability of the DC system,and screening by means of disturbed simulation experiments,this paper finally determined the targeted control measures as follows: Improving the current control gain coefficient of the converter stations on both sides as the main control measure to deal with “Sending and receiving end AC system voltage drop”;Increasing the initial firing angle of the restart process as the main control measure to deal with “failure of the DC-line fault recovery sequences”.All of the above control measures have passed the control effect verification based on the actual power grid model,which can prove their own practical application feasibility.Analyzing the possible threats to the safety of grid-connected operation of new energy power sources caused by the voltage drop fault that occurs in the sending end AC system after the fault.This paper selects the wind farm that equipped with doubly-fed induction generator(DFIG)as the object to analyze the disturbance characteristics,the result shows,the "callback lag" characteristic of the DFIG’s internal electric potential will cause over-voltage at the terminal of wind turbines when the voltage drop fault is eliminated,which makes DFIG face the threat of high voltage disconnection.For this reason,this paper conducts multiple simulations by changing the operating conditions and the control strategies of DFIG,proposes a countermeasure to withdraw the voltage feedback control function of it,and then combines with the disturbance simulation experiments based on actual powe r grid models of wind farms in Xinjiang region,verifies the effectiveness of the proposed countermeasures. |
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