Research On Improvement Of CFPREV In HVDC Transmission System

The commutation failure is an important constraint that restricts the safe operation of line-commutated converter based high voltage direct current(LCC-HVDC)transmission system.The methods of suppressing commutation failure in practical projects mainly include: improving devices and improving corresponding control.The former is limited by device characteristics and process level.The latter has obvious economic advantages,so it has become a hot spot in academia and engineering field.From the perspective of pre-prevention,practical projects are mainly using the commutation failure prevention control(CFPREV)up to now.The CFPREV consists of two parts: detecting AC fault and outputting command value.Detecting AC fault is the basis of the CFPREV.It is found in actual operation and simulation analysis that the sensitivity of the current fault detection algorithm has time discreteness.In addition,whether the command value of the CFPREV is reasonable determines its ability to prevent commutation failure and the degree of influence on the AC and DC system,and the command value is closely related to the control parameters.Setting unreasonable control parameters may cause commutation failure by excessive control.And setting more aggressive control parameters can improve the ability to prevent commutation failure,but it may affect the recovery characteristic of voltage in AC system,and then voltage stability problems come up.Therefore,it is necessary to carry out a comprehensive evaluation of CFPREV and improve CFPREV's performance to reduce the impact on the stable operation of AC and DC systems on the premise of reducing the risk of commutation failure as much as possible.For the problem that the sensitivity of the detection algorithm using voltage's zero-sequence component and ?? component has time discreteness under different fault closing angles,this paper uses a voltage RMS algorithm based on the three-phase sampling values at the same time to improve the current algorithm.The results show that under certain faults of single-phase grounding or two-phase short-circuit grounding,the detection sensitivity of the improved CFPREV is higher than the CFPREV,and the improved CFPREV reduces the risk of commutation failure to a certain extent.For the problem that the excessive pre-triggering of the CFPREV causes commutation failure,this paper firstly analyzes the mechanism and process that the excessive pre-triggering leads to commutation failure.The analysis shows that the problem is essentially the result of the interaction between the two bridges caused by the adjustment of CFPREV.In the two adjacent commutation processes,when the next commutation process is triggered in advance,the voltage's amplitude of the current commutation process will drop and the zero crossing point will move forward.If the next commutation process triggers excessively in advance,it may result in insufficient commutation area of the current commutation process and cause commutation failure.On this basis,this paper proposes the corresponding optimization schemes.The simulation results show that appropriately reducing the threshold value,decreasing the gain coefficient,or increasing the rising-filtering time constant can reduce the occurrence of the problem about excessive pre-triggering.For the problem that the CFPREV affects the stable operation of AC and DC system,this paper firstly analyzes the transient characteristics of AC and DC system under the adjustment of CFPREV,and studies the mechanism of CFPREV's impact on AC and DC system.The analysis shows that the adjustment of CFPREV may cause a big decrease in active power and a big demand on reactive power,which may cause a big reduction in the voltage of the converter bus,and then seriously affects the stable operation of the AC and DC system.On this basis,this paper proposes the corresponding optimization measures.The results show that appropriately reducing the gain coefficient and the upper limit value,or increasing the rising-filtering time constant can reduce the CFPREV's impact on AC and DC system.In addition,adding the auxiliary control strategy in conjunction with the above optimization measures can further reduce the impact of CFPREV on AC and DC system.