Research On Integrated Compensation Technology Of Harmonic And Negative Sequence Current For Electrified Railway Power Network Based On MMC

As a green transportation way with low energy loss and low pollution, railway transportation has been developed rapidly in China, which makes its transportation density and unit-power at the forefront of the world. However, with comparatively slow development of China grid, it faces great challenge to electrified railway on power quality improvement. Currently, the negative-sequence current and wide-band frequency harmonic are coming as the main problems in power supply with the increasing of AC-DC-AC locomotives, which frequently leading to the damage of RC and circuit resonance, especially in the situation of two kinds of locomotive being mixed run. It not only threatens locomotives safe operating, but also influences the production and life of people who sharing the same grid. As there are a large number of AC-DC locomotives, the phenomenon of mixed-running will exist for a long time, as well as the problem of power quality. That’s why it has been a hotspot for many scholars in this field to research on. It mainly depends on the combination mode of railway static power conditioner (RPC, which is comprised by converter based on back to back parallel H-Bridge and step-up transformer) and Static Var Compensator (SVC). However multi winding transformer has the shortages of high loss and big volume, meanwhile with the current capacity limit of single switching device, it is hard to realize a large capacity power regulation and harmonic suppression. Therefore, based on the summary of current technologies, it analyzed the harmonic characteristic of two kinds of locomotive mixed run, and researched the operation principles and control techniques of the modular multi-level railway static power conditioner (MRPC).The main innovative results are listed as follows.(1) A new MRPC topology is proposed, which can realize the direct application of low voltage devices in high voltage system by adopting half-bridge cascaded technology, and the problems which brought by transformers such as time delay, phase shift, room occupation and cost can be resolved. Furthermore, because the energy transmission of MRPC is realized through high voltage DC bus, and its DC current is low and the energy of bi-directional transmission is almost unrestricted, the difficult problems of heat dissipation and low-inductance design caused by high DC current in traditional RPC are avoided. In addition, equivalent high frequency is obtained with the cascaded design of multiple converters, and utilization of low frequency device which is beneficial to realize compensation for harmonic and reactive power. Then the operating principles of two bridge-arm, three bridge-arm and four bridge-arm MRPC topologies are analyzed, and their advantages and disadvantages, applicable fields are comparatively studied in detail.(2) Taking V/V traction substation as a research object, the relationship between current amplitude and phase Angle of RPC and negative sequence current is analyzed, and the mathematic model of the minimum negative sequence current is established under the restraining conditions of voltage fluctuation, total power factor, device capacity, transformer capacity and active power balance. In order to resolve the multidimensional nonlinear constrained optimization problem, a self-adaptive optimization algorithm based on the sequence quadratic programming traction power supply negative sequence current is proposed. According to the load condition, the optimal value of RPC instruction current amplitude and phase angle is achieved based on the real-time optimization. The calculating speed was nearly to5ms every time, the efficiency of precise operations was satisfied for the need of real-time optimization.(3) Analysis of three MRPC engineering application focus on voltage, current, power, electrical mathematic relationships and make a research into its key control technology. Raised out detection methods suitable for grid applications based on SOGI system negative sequence and reactive power, also two-phase stationary coordinate of PR regulator control proposal has been introduced to actively compensate traction negative sequence and active power in guarantee of DC voltage stable, to reduce the MRPC leg circulation current. (4) Aiming at the problem of wide-band frequency harmonic, the combined harmonic suppression method based on MRPC and HPF is proposed, the former is used to suppress the low-order harmonic,and the latter is used to absorb the high-order harmonic. For the sake of improving the harmonic extraction accuracy based on traditional second-order generalized integrator, the upgrade method is consisting of the second-order generalized integrator and PR adjustor is proposed, which is beneficial to separate the specific form characteristic harmonic, and guarantee the precision of compensation.(5) According to the parameters of a certain traction substation, the parameters of three bridge-arm MRPC is calculated, including DC-link capacitance, sub-module capacitance, bridge-arm inductance, output inductance and switching devices, etc. The system parameter and control strategy are verified by the RT-LAB real-time simulation platform. The simulation results show that the mixed system of MRPC and HPF can realize the integrated compensation of harmonic and negative sequence current, together with a better dynamic performance.