Due to industrial development and urban population concentration, traffic volume and traffic congestion have reached a serious level, and measures to solve the transportation problems are emphasized. Considering this situation, electric railways have been proposed as the best alternative to solve transportation problems due to the advantages of being environmental-friendly, energy-efficient, safe, fast, and convenient. At the same time, the demand for high-speed trains continues to increase due to their fast and convenient advantages. Therefore, railway companies are increasing the number of trains to meet increasing demand. However, if the number of trains is increased, it will cause power margin and peak poer problems for power supply in high-speed railway substations (HSRSs). This peak power not only increases the substation electricity rates, but also adversely affects the stability of power supply, such as the voltage drop on feeder line.
In this dissertation, the peak power reduction system (PPRS) for HSRSs was applied to solve the problem of power instability caused by peak power. The PPRS for HSRSs consists of a power management system (PMS) for generating power tracking values, a PCS for power control, and batteries for energy storage. Among them, the PCS is a device that is in charge of power control of PPRS, which converts the energy stored in the battery into AC power of the feeder line. For this reason, HSRSs require the supply of high-quality and high-efficiency power from PCS. Since the existing PCS was installed in the three-phase general distribution system, a three-phase two-level inverter method was applied to supply an output voltage of up to 440[V] from the battery voltage. However, this traditional method is difficult to apply to the feeder line of a high-speed railway using single-phase high voltage and large capacity due to problems of efficiency and THD characteristics.
A cascade type multi-level inverter method was proposed in this dissertation to supply high-quality and high-efficiency power to high-speed railway feeder line using single-phase high voltage. The proposed cascade type multi-level inverter has the advantage of increasing the AC output voltage despite the limitation of the series voltage of the battery. In addition, the AC output voltage has the advantage of very good THD characteristics because the voltage of each cascade type cell inverter is stacked and output. However, it is necessary to keep the voltage balance of batteries independently installed in the cell inverter constituting each layer.
The proposed PCS consists of a single-phase cascade type 13-level inverter using six cell inverters. In addition, the controllers for performing power control of the multi-level inverter are composed of a phase detection (PLL: Phase locked loop) controller, a power controller, and a battery voltage balancing controller.
The PLL controller and the power controller applied to the single-phase feeder line were able to use the control method by synchronous coordinate conversion in the three-phase system. So, a virtual 90 degree delayed voltage was generated from the single-phase voltage so that the synchronous coordinate conversion was possible. In order to obtain a virtually 90-degree delayed voltage, an all-pass filter (APF) was used. The same method was applied to the output current of PCS so that it could be composed of d-axis and q-axis currents in the synchronous coordinate system. By designing the controller from the d-axis and q-axis voltages and currents separated from the synchronous coordinate system, it was designed to be a controller with fast response characteristics and robust against external disturbances.
In the case of the voltage balancing controller applied to the existing cascade method, it was difficult to select the controller gain value and there was a problem in using the average value. In order to solve these problems, a voltage balancing controller that operated in dual modes according the SOC (State of Charge) variation of the battery has been proposed. When the SOC deviation is small, the RPR (Reference Period Rotation) mode is operated to maintain the current deviation value, and when the SOC deviation is large, the operation id performed in the ASS (Adapted Selected Switch) mode to reduce the current deviation value. Also, when the two modes are changed, the transient characteristics can be minimized by the hysteresis control.
The validity of the proposed system, including the dynamic characteristics of the PLL controller and power controller, the performance of the battery balancing controller, and the THD characteristics of the voltage and current, was verified through PSiM simulations. Furthermore, the effectiveness and the feasibility of the proposed system were verified by installing a demonstration system of 6[MW] PCS and 2.68[MWh] batteries at one of the Gyeongbu high-speed line substations.
The simulation and demonstration system test results shows that the response characteristic of transient state in power control has 7.5[ms] response time, and the error of steady state shows a high accuracy of less than 1[%]. In addition, the maximum SOC deviation of the battery banks is managed below 5[%] by the battery voltage balancing controller. The proposed PCS has more than 6 times the THD characteristics with the application of the 13 level multi-level method. And in terms of efficiency, the average is 97.67[%] with an improvement of more than 2[%]. The installed PPRS contributes to reduce the peak power of the substation by more than 11.4[%] on average.
As the future, work research on the use of regenerative energy generated by high-speed trains and the analysis of the energy savings and the effects of voltage stabilization is necessary.