Chengxiong Mao

An Improved Optimal Sizing Method for Wind-Solar-Battery Hybrid Power System

This paper proposes an improved optimal sizing method for wind-solar-battery hybrid power system (WSB-HPS), considering the system working in stand-alone and grid-connected modes. The proposed method is based on the following principles: a) high power supply reliability; b) full utilization of the complementary characteristics of wind and solar; c) small fluctuation of power injected into the grid; d) optimization of the batterys charge and discharge state; e) minimization of the total cost of system. Compared with the traditional methods, the proposed method can achieve a higher power supply reliability while require less battery capacity in stand-alone mode. And in grid-connected mode, the optimization strategy based on energy filter is further utilized to achieve the optimal battery capacity. Thus, the proposed method can achieve a much smaller fluctuation of power injected into the grid. In addition, the batterys charge and discharge state can be optimized thanks to the consideration of the batterys depth of discharge (DOD), the charge/discharge current, rate and cycles, which will prolong the batterys lifetime. A case study of WSB-HPS located in Hohhot, China is presented to verify the advantages of the proposed optimal sizing method.

Use of Battery Energy Storage System to Improve the Power Quality and Stability of Wind Farms

This paper presents the use of battery energy storage system for power smoothing in generation systems in which power flow variations can occur. These variations are the norm in wind energy generation. A simulation model for a battery energy storage system and a wind farm using squirrel cage induction generators magnetized with fixed capacitor banks are presented. And a decoupling and feed forward compensating control strategy is proposed. Simulations have been carried out and demonstrate that the BESS has the potential to significantly improve the wind farms performance. Through control of the charging and discharging of the BESS using a power electronic interface, it is shown that the wind farms output power can be controlled to a constant value. In addition, there is an overall improvement in the transient and dynamic response of the system.

Coordinated Control of EPT and Generator Excitation System for Multidouble-Circuit Transmission-Lines System

A new optimal coordinated control strategy between an electronic power transformer (EPT) and generator excitation is proposed to improve power system dynamic performance and stability. The studied system is based on a single-machine multidouble-circuit transmission-lines (SMTL) system. In the SMTL system, the generator is connected to an infinite bus through a conventional power transformer with a double-circuit transmission line, and to another infinite bus with an EPT with a double-circuit transmission line. The mathematical model of the SMTL system is established, and the proposed optimal coordinated control strategy is upon linear optimal control theory. Numerical simulations were carried out for the SMTL system, and the simulation results showed that the proposed control strategy has good dynamical performance for a variety of disturbances, and can improve the system damping and voltage performance effectively.

Design and Implementation of an Energy Feedback Digital Device Used in Elevator

Reducing energy consumption is very important in solving the problem of energy crisis and environmental problem. Usually, an elevator with a diode rectifier wastes regenerated power from its electric motor as heat on breaking resistors. This paper proposes an energy feedback digital system used in an elevator of 18.5 kW which is capable of recycling the regenerated power: obtaining near-unity power factor, sinusoid output current, and low total harmonic distortion in generator-operation state. It has reliable protection circuits and cute man-machine interface, and the feedback point of the dc voltage can be set arbitrarily in a certain extent, meeting the large-range variation of power system voltage. Field operation results showed good performance and energy-saving effect.

Peak load forecasting using the self-organizing map

This paper aims to study the short-term load forecasting of electricity by using an extended self-organizing map. We first adopt a traditional Kohonen self-organizing map (SOM) to learn time-series load data with weather information as parameters. Then, in order to improve the accuracy of the prediction, an extension of SOM algorithm based on error-correction learning rule is used, and the estimation of the peak load is achieved by averaging the output of all the neurons. Finally, as an implementation example, data of electricity demand from New York Independent System Operator (ISO) are used to verify the effectiveness of the learning and prediction for the proposed methods.

The Research on Characteristics of Electronic Power Transformer for Distribution System

The 3-phase and 4-wire distribution electronic power transformer (D-EPT) is composed of the input three phase PWM rectifier, medium frequency transformer and output single phase inverters. In this paper, the operation principle of D-EPT is analyzed and its control scheme is designed. For the PWM rectifier, an outer loop control of the DC voltage and an inner loop control of AC current are adopted. Then, a dual-loop control is implemented for the output inverters based on the RMS value and instantaneous value of output voltage respectively. The simulation results show that D-EPT has good steady-state and dynamic load characteristics under the proposed control scheme

Parallel operation of electronic power transformer and conventional transformer

The idea that the secondary voltage of the conventional transformer is selected as the reference voltage of electronic power transformer - EPTs output voltage is proposed on the base of the analysis of parallel principle of EPT and conventional transformer in this paper, in order to eliminate the circulation current and the power unbalance resulted from the parallel operation of EPT and conventional transformer. A novel parallel control scheme based on the proposed idea is proposed and its control performance is also analyzed in detail. The proposed scheme includes three control loops, one is the reference voltage feedforward loop and the other two are the instantaneous output voltage feedback loop and the instantaneous inductance current feedback loop. The theory analysis indicates the proposed scheme has good steady- state accuracy and a fast dynamic response compared with the conventional PI control scheme. Detailed computer simulations based on MATLAB/SIMULINK for one EPT and one conventional transformer parallel operation is conducted, and this parallel system is also implemented in laboratory based on DSP TMS320F2812. Simulation results and experimental results show that the proposed control scheme has excellent power sharing and current-sharing regulation characteristic, a fast dynamic response.

400 V/1000 kVA Hybrid Automatic Transfer Switch

A lot of large-scale industrial critical loads suffer from voltage interruptions and sags which can cause a significant financial loss. The thyristor-based solid-state transfer switch (STS) and mechanical-based automatic transfer switch (ATS) are used to protect loads against these power quality problems. However, the STS has a great deal of loss consumption because of the forward voltage drop of the thyristors, and ATS can hardly satisfy the voltage-sensitive load due to the long transfer time. This paper proposes a 400 V/1000 kVA hybrid ATS (thyristor switch in parallel with mechanical switch) (HATS) to realize low power consumption under normal situation and transfer fast when required. The proposed HATS has four transfer strategies which can be chosen according to different applications, it realizes the overlapping transfer of the neutral wire, and it can make a decision to transfer or not according to the different fault locations (transfer on the source side and not transfer on the load side). The design details of the prototype are presented, and laboratory tests and field experiments show the prototype good performance.

Forecasting electricity demand by hybrid machine learning model

This paper proposes a hybrid machine learning model for electricity demand forecasting, based on Bayesian Clustering by Dynamics (BCD) and Support Vector Machine (SVM). In the proposed model, a BCD classifier is firstly applied to cluster the input data set into several subsets by the dynamics of load series in an unsupervised manner, and then, groups of 24 SVMs for the next days electricity demand curve are used to fit the training data of each subset. In the numerical experiment, the proposed model has been trained and tested on the data of the historical load from New York City.

A 10-kV/400-V 500-kVA Electronic Power Transformer

This paper presents the design and development of a three-phase 10-kV/400-V 500-kVA electronic power transformer (EPT). The power circuit is designed in a modular fashion, i.e., the main circuit consists of many identical ac-dc-dc-ac modules (abbreviated as power modules). Each power module consists of a high-voltage power cell, a low-voltage power cell (LVPC), a medium-frequency isolation transformer, and a filter. The corresponding control and protection system is developed. A special three-stage startup strategy is designed to shorten the startup time and reduce the startup inrush current. The negative-sequence current compensation is introduced in the input stage to handle the unbalanced loads. To keep the dc-link voltages balanced, an individual dc voltage balancing controller based on regulating the output power of each parallel LVPC is proposed. The detailed control hardware design and software implementation are discussed. The functions of this 10-kV EPT prototype are verified through the laboratory and field tests. The results are shown in this paper. Currently, the prototype is operating in the industrial power grid.