With the wide application of non-linear loads and the large-scale access of distributed energy generations based on power electronics equipments, power quality problems in the distribution network are increasingly serious with new characteristics. Further in-depth research is of great significance in theory and practice. This paper provides an overview of power quality analysis, compensators, and control technologies under the new situation of smart grid. It focuses on the topologies and control methods for power quality conditioners, especially new characteristics of power quality and applicable control technologies in microgrids and distributed power plants. Finally, trends and prospects of power quality control technology are introduced, which is important to achieve security and efficient operation of the smart grid.

With the rapid development of modern electrified railway, negative-sequence current minimization is one of the most important considerations in the high-speed railway traction system. In the past, many multiple or multilevel topologies with high compensation capacity have been introduced for railway power conditioner (RPC). This paper presents a simplified quantitative comparison of five previous modular RPC topologies for negative sequence compensation in V/V and SCOTT traction systems, aiming for an optimal selection of the compensators. Performance criteria such as transformer requirement, voltage stress and current stress of a power switch, numbers of the power switches and capacitor are derived by analytical methods. Moreover, the numerical comparison of operating controllers is completed for modular RPCs. In addition, power losses of five modular RPCs are obtained by theoretical analysis, IPOSIM calculation as well as PSIM simulation. These calculations are validated via simulations results in PSIM. The main conclusion is that presented modular RPCs can be divided into general purpose RPC and special purpose RPC in terms of the behavior and efficiency. It is helpful to choose the appropriate topology for specific applications.

In the rapid development process of a high-speed electrified railway, power quality problems in traction power grid have become increasingly deteriorative. In order to ensure a three-phase balanced traction power grid, a modular multilevel converter based railway traction power conditioner (RTPC) is presented. The RTPC consists of four H-Bridge clusters and filter inductors, and it can be directly connected to traction feeders in a co-phase traction system without insulation transformers. According to the equivalent circuit analysis of RTPC system, it can be considered as four single-phase inversion systems. Based on the equivalent control model of each single-phase cluster, the relationship between the multilevel output voltage and current slopes in a control period is analyzed, and an improved model predictive control is proposed. Moreover, a linear combination of two different output levels is proposed to improve tracking performance of cluster current control, and enhance the waveform quality of ac current. Finally, both simulation and experiment results are presented to verify the effectiveness of the structure and its control method.

In this paper, a railway power conditioner (RPC) based on a modular multilevel converter (MMC) with a split supercapacitor energy storage system (SCESS) is studied. In this case, the MMC-SCESS-based RPC could not only provide normal negative-sequence current compensation, but also reduce the impact of power fluctuations caused by the locomotive braking or startup on the electric grid. First, this paper analyzes the power flow patterns and deduces the reference circulating current under different operation modes of the MMC-SCESS-based RPC. Then, the control objectives of the MMC-SCESS-based RPC are divided into two categories—the balance control and the current tracking control. The balance control methods are developed for the submodule capacitor voltages and the state of charge of the supercapacitor, which are associated with the operation modes. To ensure the current tracking performance, a model-predictive direct current control method is presented for the MMC and the bidirectional energy converter. Finally, the effectiveness of the proposed control methods is verified by the experimental results of a downscaled prototype.

To reduce the sensitivity of static synchronous compensator (STATCOM) to power or voltage fluctuation induced by renewable energy generation or grid itself, it can add inertia for reactive compensation. Analogizing the virtual synchronous generator (VSG), a virtual synchronous compensator (VSCOM) is proposed in this article, which adds virtual inertia in reactive power regulation. According to the differences and the respective characteristics of VSG and VSCOM, a proportional-differential (PD) controller is proposed to achieve the fast control of dc-side voltage and cooperates with a series integrator to achieve the stable control of dc voltage. Meanwhile, an integral controller is designed for realizing the inertial control of reactive power. The small-signal model of the proposed VSCOM is built, and the design method of the control parameters is analyzed. Meanwhile, the inertial performance of the proposed VSCOM with different control parameters is analyzed from the unit-step response. In weak grid or multiple parallel devices, the proposed VSCOM is more stable than the typical STATCOM in direct reactive power control mode. Finally, experimental results validate the proposed control strategy and parameter design method.