Application Research Of Cascaded H-bridge Based Hybrid Active Power Filter In LCC-HVDC

With a large number of Line Commutated Converter based High Voltage Direct Current(LCC-HVDC)projects put into operation and the DC transmission power greatly improved,the safety,stability and power quality problem of DC connected to the AC grid have attracted extensive attention.At present,the LCC-HVDC systems are all equipped with passive power filters on the AC side to filter harmonics,but passive power filters have defects such as large footprint,easy resonance with the AC system impedance and easy detuning.Hybrid Active Power Filter(HAPF),which is composed of passive power filter and active power filter,provides a new type of active filtering scheme for the AC side filtering of the LCC-HVDC system.If HAPF can be applied to the LCC-HVDC system,it will effectively reduce the footprint of the converter station and greatly improve the filtering performance and stability of the system.To this end,the paper proposes a cascaded H-bridge based HAPF suitable for the LCC-HVDC system,and carries out related research on it.The specific contents are as follows:Firstly,by analyzing the four access schemes of the active power filter,the cascaded H-bridge based HAPF topology suitable for the LCC-HVDC system is determined,that is,the 12th-and 24th-order single-tuned filter and the cascaded H-bridge based active power filter is directly connected in series.The working principle of HAPF is analyzed.The current controller adopts the quasi-proportional resonant controller,which ensures that HAPF has good filtering performance.And taking the rectifier side of the Cigre model as the application object,the parameters of the passive part and the active part in HAPF are designed.Secondly,a cooperative control strategy that takes into account both filtering and dynamic reactive power compensation is proposed for the cascaded H-bridge based HAPF,which mainly includes four links:harmonic current detection,harmonic current compensation,dynamic reactive power compensation,and DC side voltage control.The simulation results show that the proposed control strategy makes the cascade H-bridge based HAPF have a good filtering effect,and the reactive power can be output dynamically while filtering,so the voltage fluctuation caused by the switching of capacitors can be compensated.Which can increase the capacity of each group of parallel capacitors,reduce the number of groups,and save the footprint of the converter station.Finally,the impedance analysis method is used to quantitatively analyze the relationship between the stability of the HAPF grid-connected system and the control delay.It is pointed out that the stability of the system will decrease with the increase of the control delay.On this basis,a resonance suppression method based on the HAPF discrete state space model is proposed,and resonance suppression is achieved by compensating the control delay.At the same time,a state observer is designed to predict the state variables in the discrete state equation,so that the state variables are no longer detected by sensors,which can avoid adding additional sensors to the system.The simulation results verify the relationship between the stability of the HAPF grid-connected system and the control delay,and verify the effectiveness of the proposed resonance suppression method.