Da Xie

Research on smart grid in China

The Smart Grid is the latest direction for the future power system development. In this paper, firstly the background of Smart Grid, its meaning, as well as the concept and structure were presented. Typical diagram of Smart Grid was illustrated. Then, the current development of Smart Grid in United States and Europe were described, development ideas and the future trends in these countries were summarized and compared as well. Besides, the driving force of Smart Grid in China was analyzed, with detailed introduction of current related projects in China. The relation between the UHV Power Grid and the Smart Grid was discussed. Finally, the potential role of Smart Grid in future power grids in China was prospected and a new direction for Chinas Smart Grid development was charted.

Small-signal Modeling and Modal Analysis of Wind Turbine Based on Three-mass Shaft Model

Abstract—One critical task in wind turbine shaft torsional vibration study involves the modeling of a wind turbine and power grid. With a focus on the mechanical rotational system of wind turbine, this article provides a three-mass shaft model upon which one wind turbine to infinite bus model can be developed. The model, based on small-signal stability analysis, is used to study the wind turbine shaft torsional vibration. For this reason, this article concentrates on the union model of stall wind turbine and power grid. The small-signal stability model includes the mechanical system and electrical system. Data for the turbines blades and shaft as well as electrical generator are given to allow replication of the model in its entirety. Each of the component blocks of the wind turbine and power grid is modeled separately so that one can easily expand and modify the model to simulate variable-speed wind turbines or multi-turbines system to suit their needs. This is then followed by a case study that explains how the small-signal stability model can be used to study wind turbine shaft torsional vibration issues. Results obtained from the case study show that the model performs as expected.

A Novel Dispatching Control Strategy for EVs Intelligent Integrated Stations

In order to provide a cost-effective solution for accommodating the increasing electric vehicles (EVs) and maximizing their benefits to the grid, a novel EV intelligent integrated station (IIS) making full use of ex-service batteries, is proposed in this paper. It first presents the framework and characteristics of IISs by describing its components, including a dispatching center, multipurpose converter devices, a charge exchange system, and an echelon battery system. The grid status, batteries exchanging requests, and energy capacity of IISs are monitored timely to offer inputs for its optimal operation. The concept of generalized energy is thereby introduced to systematically understand the energy/power flow between IISs and EVs, as well as between IISs and power grids. Then, a novel charging and discharging control strategy for managing EVs is presented. Compared with existing approaches, the proposed control strategy can offer peak load shifting when meeting EV battery charging/exchanging requests. The experimental results demonstrate the effectiveness and benefits of the control strategy in terms of providing peak load shifting for the power grid. This integrated station concept can maximize the benefits of EVs, and the retired batteries more flexibly and effectively.

A New Solution for Stochastic Optimal Power Flow: Combining Limit Relaxation with Iterative Learning Control

A stochastic optimal power flow (S-OPF) model considering uncertainties of load and wind power is developed based on chance constrained programming (CCP). The difficulties in solving the model are the nonlinearity and probabilistic constraints. In this paper, a limit relaxation approach and an iterative learning control (ILC) method are implemented to solve the S-OPF model indirectly. The limit relaxation approach narrows the solution space by introducing regulatory factors, according to the relationship between the constraint equations and the optimization variables. The regulatory factors are designed by ILC method to ensure the optimality of final solution under a predefined confidence level. The optimization algorithm for S-OPF is completed based on the combination of limit relaxation and ILC and tested on the IEEE 14-bus system.

A new-style dynamic var compensation control strategy

A new dynamic reactive power compensation technique for dynamic var compensator is presented. This paper presents the DC capacitor voltage control strategy, the loss current variation control algorithm and a new reactive current calculating method. And the desired compensating current is compared with the measurement of output current in the hysteretic current feedback-loop. Based on the analysis, a new- style dynamic var compensation control strategy is realized. It also analyzes the relationship between DC capacitor voltage variation characteristic and the active and reactive power flow between AC and DC sides. A novel dynamic var compensation prototype is developed. The experimental waveforms of the prototype show that this control algorithm has the expected compensating performance, demonstrating the validity of the design scheme proposed in this article.

A damping method for torsional vibrations in a DFIG wind turbine system based on small-signal analysis

Abstract Torsional vibrations present in the drivetrain excited by turbulent winds or grid disturbances (often manifesting as voltage sags) can produce severe stresses on the components of wind turbines. This study presents a damping method for the rotor side converter of a doubly fed induction generator (DFIG)-based wind turbine to improve system damping during small grid disturbances. Within the damping strategy, torsional damper is included into the power loop of the rotor side converter to provide damping for the system. During vibrations, a well-designed low-pass filter (LPF) is employed by the torsional damper to extract the dynamics of the generator speed and to generate a damping signal. To demonstrate the proposed damping approach, the small-signal stability model of a wind turbine connected to an infinite bus system is presented, and the eigenvalue analysis is conducted to verify the damping effect introduced by the torsional damper. In the case study, grid voltage sag at the point of common coupling (PCC) is applied to excite the torsional vibration of the wind turbine shaft. The validity and effectiveness of the proposed damping approach are testified by using the simulation results against theory analysis.

Black-start scheme based on EV's intelligent integrated station

With electric power network getting larger and more complex, it is unprecedented vulnerable and modern society is faced with higher blackout risk The stable and reliable operation of a power system is necessary after major emergencies (or blackouts) following a major system event. The intelligent integrated station (IIS), providing energy and changing batteries for EVs, has the ability to separate and isolate itself from the power grid seamlessly with little or no disruption to loads in the station. Then, when the utility grid returns to normal, the IIS resynchronizes and reconnects itself to the grid in an equally seamless manner. According to conventional system restoration after a black-out, the control principles of black-start for intelligent integrated station are put forward. The build-up strategy is used to restore power system with IIS, which includes the process of restoring separated parts (islands) in the power system and interconnecting them afterwards. The control of IIS with energy storage system will deal with building isolated network, forming and operating island, and connecting island and re-synchronization with upstream network or grid system. A model based on the black-start principle proposed above is developed and the model is applied in a regional power grid.

Black-start control for EV's intelligent integrated station

After a blackout, power needs to be restored as quickly and reliably as possible to reduce economic loss, and consequently, detailed restoration plans are necessary. According to the basic black-start method, the control principles of black-start for intelligent integrated station (IIS) are put forward. To speed up the restoration process, the whole system containing intelligent integrated stations is partitioned into several subsystems which are restored in parallel. These independent subsystems will later be synchronized to restore the original power system seamlessly through virtual connection control. A model based on the black-start principle proposed above is developed. At last, a regional power system containing 16 intelligent integrated stations is employed as an example to verify the validity of the proposed approach.

Adaptive multi-variable coordinated control and safe operating characteristics for MMC under unbalanced AC grid

In the grid-connected modular multilevel converter (MMC), grid faults not only cause the current to be higher than the rated current, but also bring the unbalanced issues to the inner unbalanced currents and arm voltages. In this paper, a multi-variable coordinated control method which is employed in the d-q coordinate is presented. First, a parameter is introduced in the calculation of the grid currents references. Afterwards, the relationships between the system operating parameters and the parameter are derived. Furthermore, the influences of parameter on the safe operation areas are analyzed in details. In addition, the P-Q diagrams of the MMC are deduced in terms of the safe operating areas under unbalanced conditions, and an adaptive multi-variable coordinated control is proposed. Finally, the simulating results from MATLAB/Simulink are presented to verify the theoretical analysis.

Torsional vibration characteristic study of the grid-connected DFIG wind turbine

This paper studies the torsional vibration characteristics of the grid-connected doubly-fed induction generator (DFIG) wind turbine by small signal analysis method. Firstly a detailed small-signal stability union model of the grid-connected DFIG wind turbine is developed, including the mechanical system and electrical system. To study the dynamic characteristic of the blade, gearbox, low speed and high speed shafts, a three mass shaft model for the mechanical system is adopted. At the same time, small signal models of DFIG, the voltage source converter (VSC) and the transmission line of the electrical system are developed respectively. Then, through calculating the eigenvalues of the state matrix A and the corresponding participation factors, the modal analysis is conducted in the shaft torsional vibration issues. And the impact of the system parameters including the series compensation capacitor, the flat-wave reactor, the PI parameters, especially the speed controller of generator rotor on shaft torsional vibration are discussed. The results show that the speed controller strengthens association between the mechanical system and the electrical system, and also produces a low-frequency oscillation mode.