| Commutation failure(CF)is one of the most frequent problems in high voltage direct current transmission(HVDC)system.Commutation failure will result in the decrease of DC system voltage and the increase of DC system current.In severe cases,it may cause the valve control group to lock.Generally,one commutation failure occurs more frequently,but the system will continue to operate normally in two commutation cycles.However,when the system problems are serious or the control system operation is not ideal,multiple commutation failures are prone to occur.It is not a single factor that affects commutation failure,and the main reason is the change of harmonic voltage and DC current caused by inverter side fault.In order to detect the commutation failure as soon as possible and avoid more harm and loss,it is very important to detect the commutation failure timely and accurately.In order to study the influence of harmonics on commutation failure in practical engineering,based on the commutation voltage time area method,this paper studies the influence of harmonics on commutation area and the influence of harmonic influence factors on commutation failure.By analyzing the relationship between harmonic times,commutation angle and harmonic influence factors,it is understood that low-order harmonics play a key role in harmonic influence factors.Then,combined with the harmonic influence factor,the calculation method of turn-off angle considering harmonic is deduced.Finally,taking a 500 k V DC line of a power grid as the research object,the influence of harmonic on commutation failure is simulated on PSCAD simulation platform,which has good engineering practicability.In order to solve the problem that the detection accuracy of commutation failure needs to be improved under the influence of DC current factors,this paper improves the prediction method of DC current based on the Newton quadratic interpolation method of equidistant nodes,thereby improving the prediction accuracy of DC current.On this basis,a commutation failure detection method based on the influence of the improved DC factor is given.The simulation comparison verifies that the improved DC current prediction method is more feasible than the conventional DC current prediction method.And the effectiveness of the proposed commutation failure detection method to a certain extent improves the detection accuracy of commutation failure.In order to solve the problem of high-voltage DC commutation failure detection under the influence of harmonics and DC current factors,this paper studies the commutation process of the inverter side during the AC system fault transient.The commutation failure judgment method of the angle comparison method,harmonics and DC current can be obtained more conveniently,and has good engineering practicability.On this basis,the method of commutation failure detection is given,and the commutation failure detection flow chart is designed according to the electrical parameters such as the commutation voltage after the fault and the calculated value of the improved turn-off angle.The CIGRE HVDC standard test model was established on PSCAD/EMTDC power simulation software,and it was simulated under the influence of single harmonic,single DC current factor,and harmonics and DC current at the same time.Through comparison simulation.As a result,the accuracy of the improved commutation failure judgment method and the validity of the detection method and the detection flow chart are verified.And harmonic voltage,DC current and turn-off angle,according to the relevant research content of this paper,can be more convenient to obtain,has good engineering practicability. |
