The extensive connection of distributed generation (DG) with the distribution network (DN) is one of the core features of a smart grid, but in case of a large number, it may result in problems concerning the DN-DG compatibility during fault isolation and service restoration, for which no efficient and economic solutions have been developed. This paper proposes a doubly-fed induction generator (DFIG) adaptive control strategy (ACS) and a coordination technology to be compatible with the typical feeder automation (FA) protection logics in the ring distribution system. First of all, an ACS simulating the inertia/damping characteristics and excitation principles of synchronous generators is developed to achieve seamless switching between DFIG grid-connection/island modes, and make distant synchronization possible. Next, a technology coordinating the DFIG islands controlled by ACS and the remote tie-switches based on local inspection of synchronization conditions for closing is developed to achieve the safety grid-connection of DFIG islands in the absence of DN-DG communication. At the last, a detailed simulation scenario with a ring DN accessed by five DFIGs is used to validate the effectiveness of ACS and coordination technology compatible with FA in various faults scenes.

To solve the problem of incompatibility and mutual repulsion between a distribution network (DN) and distributed generation (DG), this paper first analyzes the conflicts between current feeder automation (FA) and doubly-fed induction generation (DFIG). Then, a DFIG coordination control strategy compatible with FA is proposed to cause the DFIG operating continuously during fault isolation and power supply recovery. The strategy consists of three aspects: (1) a DFIG islanding active synchronization control technology to support islanding and distant presynchronization control; (2) DFIG senses the status of circuit breakers/tie-switch through generic object oriented substation event (GOOSE) messages to achieve rapid switch of control modes and distant synchronization; (3) tie-switch senses the status of DFIG through GOOSE messages for closing. Finally, the effectiveness of the strategy is validated by multiple sound and detailed time-domain simulation cases. This study indicates future possible development trends and provides a simple and effective strategy for researches in the field of smart grid DN–DG compatibility.

Original scientific paper To improve the ability of transient voltage support and that of damping system oscillation, this paper presents the additional transient control scheme of wind farm. The analysis indicates that under the condition of strong grids, relying on wind turbines' own reactive power to support the transient voltage drop leads to the significant increase of rotor current; under the condition of weak grids, transient reactive power released by wind farms not only serves to suppress transient voltage drop but also makes acceptable the increase of rotor excitation current. In addition, when double-fed induction generators are mixed with conventional power plants for power transmission, we can control the increase of system damping by adding damping through wind turbines in a bid to improve the stability of the overall system. In order to implement this scheme, we produce the experimental prototype composed of IPC, fieldbus and optical equipment and conduct the closed-loop test on the real-time digital simulator (RTDS). Simulations show that under the condition of weak grids, implementing the reactive power regulation of wind farms can suppress their voltage drop to a certain extent. Keywords: additional voltage control; doubly-fed wind farms; power system; RTDS test; simulation Doprinos dvostruko-napajanih vjetroelektrana prolaznom naponu i prigušenju energetskih mrežaIzvorni znanstveni članak Kako bi se povećala mogućnost održavanja prolaznog napona i oscilacija sustava prigušenja, u radu se predstavlja dodatna prolazna upravljačka shema vjetroelektrane. Analiza pokazuje da u uvjetima jakih mreža, oslanjanje na vlastitu reaktivnu snagu turbina na vjetar u svrhu podrške pada prolaznog napona, dovodi do značajnog povećanja struje rotora; u uvjetima slabih mreža, prolazna reaktivna snaga vjetroelektrana ne samo da služi za prigušivanje pada prolaznog napona nego i čini prihvatljivom povećanje uzbudne struje rotora. Uz to, kad se dvostruko napajani indukcioni generatori pomiješaju s konvencionalnim energanama za prijenos snage, moguće je upravljati povećanjem prigušenja sustava dodajući prigušenje preko vjetroelektrana u nastojanju poboljšanja stabilnosti cjelokupnog sustava. U svrhu implementacije ove sheme, proizvodimo eksperimentalni prototip sastavljen od IPC, glavne sabirnice i optičke opreme i provodimo ispitivanje zatvorene petlje na digitalnom simulatoru u realnom vremenu (RTDS). Simulacije pokazuju da u uvjetima slabih mreža implementacija regulacije reaktivne snage vjetreoelektrana može donekle prigušiti pad napona.

Generally, a wind turbine may have inferior reactive power dynamic performance where constant power control is adopted by a wind farm equipped with doubly fed induction generators (DFIGs)

The wind-thermal hybrid power transmission will someday be the main form of transmitting wind power in China but such transmission mode is poor in system stability. In this paper, a coordinated stability control strategy is proposed to improve the system stability. Firstly, the mathematical model of doubly fed wind farms and DC power transmission system is established. The rapid power controllability of large-scale wind farms is discussed based on DFIG model and wide-field optical fiber delay feature. Secondly, low frequency oscillation and power-angle stability are analyzed and discussed under the hybrid transmission mode of a conventional power plant with wind farms. A coordinated control strategy for the wind-thermal hybrid AC/DC power system is proposed and an experimental prototype is made. Finally, real time simulation modeling is set up through Real Time Digital Simulator (RTDS), including wind power system and synchronous generator system and DC power transmission system. The experimental prototype is connected with RTDS for joint debugging. Joint debugging result shows that, under the coordinated control strategy, the experimental prototype is conductive to enhance the grid damping and effectively prevents the grid from occurring low frequency oscillation. It can also increase the transient power-angle stability of a power system.

Aiming at the problem of power coupling and complicated decoupling in the d-q coordinate system of a three-phase grid-connected inverter, a current closed-loop control strategy based on an improved QPIR (quasi-proportional integral resonant) controller in the α-β two-phase static coordinate system is proposed. Firstly, the mathematical model of an LCL three-phase grid-connected inverter is established, and its instantaneous power calculation equation is deduced. Secondly, the frequency method is applied to compare and analyze the proportional resonant, quasi-proportional resonant, and improved current controller, and the appropriate improved controller parameters are obtained according to the traditional proportional integral controller parameter design method and the weight coefficient. Finally, the improved controller is compared with the traditional controller in the simulation model of the LCL three-phase grid-connected inverter based on active damping. The results show that the proposed improved current control strategy has good dynamic response characteristics, can realize the non-static error control of grid-connected current, and realizes the decoupling control of active power and reactive power when the load jumps. At the same time, the results also prove the superiority of the proposed control strategy and verify its effectiveness.

Real-time online simulation based on a real-time workshop (RTW) plays a vital role in the study and application of power electronics. However, restricted by the performance of equipment and hardware, the simulators so far available in the market mainly support simulation steps over 50 μs, while large step simulation may result in the action delay of pulse-width modulating (PWM), numerical oscillation and high-level non-characteristic harmonic distortion. In view of these problems, this paper puts forward a modeling method based on integral prediction and interpolation compensation. First of all, prediction is performed one step in advance by the implicit trapezoidal method to find out the accurate time when the triangle carrier wave intersects with the modulation wave. At the same time, a mathematic model is built for the insulated gate bipolar transistor (IGBT) to output equivalent voltage waveform according to the principle of area equivalent. Next, in MATLAB/Simulink, offline simulation is performed with the three-phase AC-DC-AC converter as the subject. By comparing the control accuracy, the content of harmonic wave and the simulation time, the simulation effects of the 50 μs fixed-step interpolation prediction model are the same as that for a 5 μs fixed-step standard model. Finally, the effectiveness and high efficiency of this algorithm are verified on a real-time simulator, marking the application of offline models on real-time simulators.

A multi-rate parallel real-time simulation method based on FPGA–CPU is studied to realize the asynchronous co-simulation of the converter of doubly fed wind power systems with the wind turbine and external power grid. The doubly fed wind power system is partitioned by simulation step length, and the partitioned small-step-length data are processed using integral homogenization. For large-step data, an improved delay-compensated linear interpolation method combined with Newton interpolation is proposed for processing. The general small time-step (GST) model method is used to implement the FPGA modeling of the small-step converter, and resource optimization is achieved through timing time-division multiplexing. Asynchronous parallel co-simulation of a doubly fed wind power system is implemented on an FPGA–CPU co-simulation platform. Among them, the FPGA realizes the development of the converter HDL with a small step of 1 μs, while the CPU completes the simulation of the wind turbine and power grid synchronously with a large step of 50 μs. Finally, by comparing with MATLAB/Simulink offline simulation and analyzing the error, it is concluded that the simulation accuracy of the improved method in this paper is higher than that of the un-interpolated parallel simulation, which verifies the real-time performance and accuracy of the modeling and improved method in this paper.