Research On The Risk Assessment And Restraining Measures Of The Influence Of Harmonics On The Commutation Failure Of HVDC

High voltage direct current transmission(HVDC)has been widely promoted and applied in China's power grid.Multiple large-capacity HVDC solves the power shortage problem of the load center power grid,meanwhile commutation failure of a single or multiple HVDCs even blocking causes the active and reactive power impact,which brings severe challenges to the safety and stability of the power systems.Harmonics is one of the important reasons for commutation failure.The influence of harmonics on the commutation failure of HVDC systems and suppression measures are mainly researched in this thesis.The contributions of this thesis are listed as follows:(1)Based on the commutation process of HVDC transmission,the commutation voltage-time area analysis method in this thesis is used to derive the index—the limiting harmonic influence factor,which characterizes the maximum possible influence of harmonics on commutation process.Combine the harmonic voltage and limiting harmonic influence factor to obtain the superposition of the harmonic influence,and use it as an indicator to determine whether the commutation failure will occur in the DC system.The influence of harmonics on the commutation process of HVDC is quantitatively analyzed.Finally,the method of risk assessment of commutation failure for HVDC system due to injected harmonic voltage is proposed,and the proposed index is proven to be effective by electromagnetic transient simulation.(2)By using the wide-band admittance model of system and frequency-scan,an evaluation method for the voltage distortion of the commutation bus caused by injected harmonic current at different positions in the system is proposed.Combined with the method which proposed in(1),a risk assessment of commutation failure for HVDC system due to injected harmonic current is established.The effectiveness of the proposed method is verified by electromagnetic transient simulations,and the risk of commutation failure caused by the transformer charging at different nodes in a power grid is analyzed.(3)Based on the basic control structure and control method of the DC transmission system,it shows that the voltage dependent current order limiter plays a key role in the recovery process of the commutation failure for HVDC system.However,the influence of harmonics on DC commutation is not taken into consider in the traditional voltage dependent current order limiter.Based on the detection of the harmonic voltage of the bus in the converter station,a method for suppressing continuous commutation failure of DC transmission with additional harmonic control is proposed.Compensation control components of harmonic are introduced in the original voltage dependent current order limiter.When a fault occurs,the loop of the harmonic control can reduce the harmonic components of the commutation bus to avoid subsequent commutation failures.The proposed additional control does not require additional investment of primary equipment and does not affect the steady state operation of the system.