Power Quality Control Technology For Multi-system Electric Locomotive Traction Drive System

Electrified railway is a form of public transportation with s trong transportation capacity and high energy efficiency,which is an important part of the green and efficient transportation system.At present,the traction power supply system of electrified railway is divided into two types: AC and DC,among which the AC traction power supply system includes 25 k V/50 Hz and 15 k V/16.7Hz;th e DC traction power supply system includes 3k V and 1.5k V.To enable a single locomotive operates in different traction power supply systems,it is necessary to adopt multi-system electric locomotive.The traction drive system of the multi-system electric locomotive can be adapted to the above two or more traction power supply systems,so that the multi-system electric locomotive has the capability of cross-system,cross-regional and cross-border operation.This paper is supported by the National Natural Science Foundation of China(52077069),the scientific and technological project from CRRC Datong Electric Locomotive Co.,Ltd.(2017KYLX007)and the scientific and technological project from CRRC Zhuzhou Electric Co.,Ltd.(JSHT2019087).A series of research work has been carried out to solve the power quality problems existing in the actual operation of the multi-system electric locomotive traction drive system.The multi-system vehicle-grid system is a multi-converter,multi-topology combination,and multi-operating condition system,which makes its power quality problems more complicated,including: harmonic resonance problems in the AC power supply system,and vehicle-grid voltage mismatch and current ripple problems,the voltage oscillation of the traction drive system in the DC power supply system,and the structure and control mode switching of the line-side converter during the power supply system switching process.This research is based on the topology and parameters of the multi-system electric locomotive traction drive system in practical engineering,and the power quality management of multi-system traction drive system is realized by topology innovation and control method improvement,the main research work includes:(1)A high-frequency harmonic resonance suppression method based on the traction transformer integrated LCL filter system is pro posed.The proposed traction transformer integrated LCL filter system is composed of a traction tr ansformer with integrated filter winding and a filter capacitor.The line-side equivalent impedance and the traction-side equivalent impedance of the traction transformer are used as filter inductances,and an external filter capacitor is connected to the equivalent zero-impedance filter winding,which can greatly reduce the volume and space occupation of the overall filter system compared with the traditional solution.In this paper,the electromagnetic decoupling equivalent circuit model of the integrated filter winding traction transformer is established,combined with the traction network impedance model and the harmonic model of the line-side converter,the harmonic resonance suppression mechanism of the proposed method is analyzed and revealed;traction transformer with integrated filter winding and line-side converter control system are designed;prototype of traction transformer with integrated filter win ding is developed,the rationality of the impedance design is verified through electromagnetic simulation and prototype testing;good harmonic resonance suppression effect of the proposed system has been verified through field-circuit coupling simulation.(2)A vehicle-grid voltage matching and DC ripple suppression technology based on quadruple boost/buck converters is proposed.The DC link of the multi-system traction drive system adopts a voltage level of 2.8k V.To adapt to the 1.5k V and 3k V DC traction power supply systems,a switch unit is added to the original AC line-side converter to form a reconfigurable line-side converter.In the DC 1.5k V power supply system,it is reconfigured into a quadruple boost converter,and in the DC 3k V power supply system,it is reconfigured into a quadrup le buck converter.In this paper,the structural switch unit of the reconfigurable line-side converter and the variable structure topologies under the 1.5k V and 3k V traction power supply systems are designed;the vehicle-grid voltage matching and current ripple suppression principle of the quadruple boost/buck converter are analyzed.The relationship between ripple suppression effect and duty cycle is deduce;The quadruple boost/buck converter small signal model is established,based on which,its control strategy and control parameters are designed.(3)A voltage oscillation suppression method based on virtual damping control is proposed.The DC traction drive system is a cascaded system composed of line-side LC filters,quadruple boost/buck converters,traction inverters and motor systems.The resonance peak of the LC filter and the negative damping characteristics of the traction inverter and motor system in constant power mode reduce the system stability margin,resulting in voltage oscillation.In this paper,the small-signal model of line-side LC filter,quadruple boost/buck converter closed-loop system and constant power load is first established,based on which,the impedance characteristics of each part of the cascaded system are obtained.According to the stability analysis of the cascaded system,the DC voltage oscillation mechanism is revealed.For the quadruple boost converter and quadruple buck converter,the line-side series virtual impedance and line-side parallel virtual impedance control meth ods are proposed respectively.The proposed virtual impedance control effectively avoids the overlap between the output impedance of the line-side LC filter and the input impedance of the quadruple boost/buck converter at the resonant frequency,thereby improving the stability of the cascaded system and avoiding voltage oscillation.(4)A hierarchical control method of the reconfigurable line-side converter in the process of power supply system switching is proposed.Hierarchical control includes a state management layer and a converter control layer.The state manag ement layer switches the line-side converter topology and control mode according to the operating state of the locomotive,and the converter control layer sp ecifically implements the control strategy under each power supply system.The state sequence,operation mode and mode conversion mechanism of the reconfigurable line-side converter are analyzed both in the process of switching from AC to DC and from D C to AC.The switching logic of the switching unit and control mode in the state management layer is designed.The hierarchical control framework in the process of power supply system switching is designed,and the interaction between the state management layer and the converter control layer is given.The proposed hierarchical control method realizes the shock-free switching of the line-side converter structure and the stable control of the DC link voltage in the proces s of power supply system switching.