Research on VSC–HVDC control strategy based on large offshore wind power system

Based on the port-controlled dissipative Hamiltonian (PCDH) system, a novel nonlinear (NN) control is designed for the wind farm side (WFS) voltage source converter (VSC) of VSC-based high-voltage direct current (VSC-HVDC) transmission system connected large wind farm. Firstly, the uncertain nonlinear dynamical model of WFS VSC in the VSC-HVDC is established by taking external disturbances and parameter variations into account. Then, the dissipative Hamiltonian structure of the WFS VSC system is constructed by means of variable transformation. Thirdly, on the basis of the obtained dissipative Hamiltonian structure, a NN control is put forward. Finally, the simulation results show that the proposed NN control gains the advantage over the interconnection and damping assignment passivity-based (IDA-PB) control.

The microgrid containing a large amount of high frequency power switches and nonlinear components has put forward high requirements for power system real-time simulation technology. Multivalued coefficient prestorage can reduce the calculation steps in real-time simulation. In order to reduce the storage pressure of the multivalued coefficients, the whole network is divided into multiple subnetworks that can be simulated in parallel, and only the parameters for computing input variables and internal variables are prestored. The multiport hybrid equivalent is performed to reduce the number of simultaneous network equations. The input variables are tied to state variables of the circuit so that the iterative calculation is limited to the local network. The devised methodology is validated through simulation of a low-voltage microgrid on a field programmable gate array (FPGA)-based real-time digital simulation (FRTDS) platform at a 5 μs time step. Comparison with a power systems computer aided design (PSCAD)/electromagnetic transients including DC (EMTDC) model shows that the proposed method is effective.

A diode-based rectifier (DR) is an attractive transmission technology for offshore wind farms, which reduces the volume of large bulk platforms. A novel parallel–series DC wind farm based on a distributed DR is proposed, which meets the requirements of high voltage and high power with an isolation capability from other units. The coupling mechanism between a modular multilevel converter (MMC) and a DR has been built, and the coordinate control strategy for the whole system has been proposed based on the MMC triple control targets with intermediate variables. Under the proposed control strategy, the system automatically operates at maximum power point tracking (MPPT). The feasibility of topology and the effectiveness of the control strategy are verified under start-up, power fluctuation, onshore alternating current (AC) fault, and direct current (DC) fault based on the power systems computer-aided design (PSCAD)/electromagnetic transients including direct current (EMTDC) simulation.

A conventional dynamic voltage restorer with fault current limiting capability

Wind power is also known as junk. This is because wind power fluctuations affect the security and stability operation. Wind power wind turbines created is mainly concerned with the speed of wind. Because of the wind direction uncertain, intermittent, and wake effects between each unit wind farm, wind turbines cannot make that kind of power according to the demand for energy as conventional generators. Due to the lack of experimental data, assess the volatility of wind power is still a lack of effective methods. This article studies the sample in a northeast wind farm power, and based on a sliding differential algorithm, distribution fitting and quantitative calculations describe the characteristics of wind power fluctuations. This article studies the sample in a northeast wind farm power, and based on a sliding difference algorithm, through the analysis showed that wind power fluctuations obey t location scale distribution. And it is affected by factors such as spatial and temporal distribution, there is a big difference between the output power fluctuation characteristics of wind farm output power and single wind turbine. This is due to the wind turbine suffered varying differences, and wake effects between field units, making the distribution of frequent power fluctuations; relative to a single unit, the fluctuation of the whole wind farm is more gentle, that is to say with the spatial distribution increased scale, wind power fluctuations presents certain "gentle effect."

This paper proposes a control algorithm for permanent magnet synchronous generators with a back-to-back three-level neutral-point clamped voltage source converter in a medium-voltage off-shore wind power system under unbalanced grid conditions. Specifically, the proposed control algorithm compensates for unbalanced grid voltage at the PCC (Point of Common Coupling) in a collector bus of an off-shore wind power system. This control algorithm has been formulated based on symmetrical components in positive and negative synchronous rotating reference frames under generalized unbalanced operating conditions. Instantaneous active and reactive power is described in terms of symmetrical components of measured grid input voltages and currents. Negative sequential component of AC input current is injected into the PCC in the proposed control strategy. The amplitude of negative sequential component is calculated to minimize the negative sequential component of grid voltage under the limitation of current capability in a voltage source converter. The proposed control algorithm enables the provision of balanced voltage at the PCC resulting in the high quality generated power from off-shore wind power systems under unbalanced network conditions.

This paper explains a hybrid control approach to model a controller for voltage source converters (VSCs) in offshore wind farm (OSWF) applications, where the OSWFs are integrated to AC grid through the multi-terminal high voltage direct current (MT-HVDC) transmission system. Proper DC link voltage provides the effective power dispatch. So, a constant DC link voltage should be maintained to achieve the effective power dispatch between OSWFs and AC grid. A new control approach is a compound of proportional-integral and sliding mode control to regulate the DC link voltage on the grid side VSC. Also, the proposed control approach can control AC voltage and reactive power on wind farm and grid side VSCs respectively. Time- domain simulations executed in MATLAB/Simulink software are used to verify the proposed and conventional control approaches. Transient stability analysis is carried out for a case study. The three-terminal VSC-HVDC system has taken for study, where two OSWFs are fed to AC grid.

The anti-peak shaving characteristics of wind power is an important factor that limits the consumption of wind power. The use of the space-time translation capability of a battery energy storage system is one of the effective means for promoting wind power consumption. Thus, this study proposes an energy storage system smoothing wind power fluctuation control strategy considering wind power consumption to improve the utilization level and economy of an energy storage system. First, the statistical characteristics of wind power are analyzed, and the scene division principle of a multiscenario regulation of an energy storage system is formulated using a correlation coefficient. On this basis, the charging and discharging intervals of the energy storage system in each scenario are determined by comprehensively considering the wind power and the time domain distribution law of load. Then, the control strategy of energy storage system based on inter-zone control is proposed based on the division of charging and discharging intervals. During the charging interval, smoothing the upward fluctuation of wind power or reducing the amount of abandoned wind power is aimed at storing electricity. During the discharge interval, electricity is released to smooth the downward fluctuation of wind power. Finally, considering the real-time state of charge and wind power consumption of the energy storage system, a method of charging and discharging power correction based on fuzzy control is proposed. Taking the actual data of a wind farm as an example, a test is performed on a simulation platform of a combined wind storage power generation system to verify the feasibility and superiority of the proposed control strategy.

In the light of the practical project experience, the multi-terminal DC (MTDC) is regarded as one of the preferable solutions to solve the grid interconnection issue of wind generation. This paper mainly focuses on the application of the voltage source converter(VSC) based MTDC technology to integrate large scale wind farms to the electric power grid. A radial MTDC system is explored as the best choice for wind power integration, due to that it can mitigate the fluctuation of the aggregated wind power. Based on the analysis of the VSC model and control, the coordinated control strategy for the proposed MTDC system is designed. The operation performance of a four-terminal MTDC system connecting two DFIG-based wind farms, the local and remote grids is also studied, and the proposed control strategy is adopted to achieve a constant power for long distant transmission to the load center under wind speed variations and faults on DC line.

This work advocates an effective way of energy management in a microgrid consisting of a synchronous reluctance generator (SynRG) based wind energy conversion system (WECS) connected to a DC bus, which voltage is maintained by a battery energy storage system (BESS). The system is capable of feeding a fairly constant value of power to the grid apart from supplying the local loads. Apart from maintaining the DC link voltage, the BESS compensates for the fluctuations in the wind power by channelizing the power in and out of the BESS as per the power generation and power demand. The SynRG is operated in direct torque control (DTC) mode corresponding to maximum power point (MPP) of the WECS by adjusting the control inputs to the machine side converter. The battery in BESS, is integrated to the DC bus using a bidirectional converter (BDC), which helps in charging and discharging of the battery. A voltage source converter (VSC) connected on the grid side evacuates the power from the common DC link and feeds it to the three-phase grid. This VSC is controlled in such a way as to maintain the required power factor on the grid side as per the reference values of the direct and quadrature axes components of currents specified. The proposed system is designed, modeled and simulated in Simulink/ MATLAB environment.

UHVDC link is one of the appropriate solutions to transmit large-scale renewable energy resources. An advanced real-time digital/analogue hybrid simulation platform is applied, which can accurately simulate both large-scale wind farms with digital models and UHVDC link with physical models. The UHVDC coordinated control for transmitting large scale wind power is researched in depth. According to the characteristics of the active power adjustments and equipment operating characteristics of UHVDC link, a novel type of active power control strategy is proposed to track the fluctuation of wind power. Combined with dynamic characteristics of power grid stability recovery, a reactive power coordinated control strategy of UHVDC link is put forward to improve the fault recovery and enhance the system stability when the wind farms are tripped out from power grid. The proposed coordinated control strategies are testified and verified by real-time digital/analogue hybrid simulation test. The study results provide an indispensable technical support to the application of large scale wind power transmission via UHVDC link.

In this paper, a comparison of simulation between Power Systems Computer Aided Design (PSCAD) and Real Time Digital Power System Simulator (RTDS) Hardware In-the-Loop (HIL) is presented under the same conditions to express their corresponding results for other researchers reference the differences and choice the suitable testing system. Both RTDS and PSCAD are applied widely and trusted in power electronics researching field. However, RTDS provides a real time simulation process to get close to real experimental results. Further, RTDS support HIL and permit Digital Signal Processor (DSP) control inside simulation components. Therefore, the results differences produced by RTDS and PSCAD becomes one important reference for a system or algorithm. In this study, one power compensator simulation system is built in both PSCAD and RSCAD. The set-up process is introduced, and their simulation results are analyzed.

With the integration of large scale wind power, the characteristics of power system operation has been changing, and, some problems come forth. On-line security assessment and control decision support system provides the stability and security state message for dispatcher in time, and, some suggestions to improve the level of power system stability and security. To suit the requirements of large amount wind power integrated power system, the technology of on-line security assessment and control decision support must be modified. This paper focuses on the technology of on-line security assessment and control decision for operation of power system with large scale wind power and designing the support system. In the paper, the study and application of on-line security assessment and control decision support technology in China are introduced. Considering the wind power in China having some distinct characteristics such as large scale, centralized development, long-distance and high voltage transportation, the new requirements to on-line security assessment and control decision support are analyzed. They includes need for new information, on-line modelling with large scale wind farm, transfer limitation calculation under the constraint condition of wind power integration requirement, auxiliary preventive control decision based on the forecasting and adjustment properties of wind power output, coordinated emergency control wind power with hydraulic and thermal power. On the basis of technical rule for connecting wind farm into power grid and the interaction between wind power and power grid, the advices on engineering application of on-line security assessment and control decision support technology for large amount wind power integrated power system, are proposed, for example, with respect to the standard of security assessment and selection of pre-contingency sets. (7 pages)

This paper describes the use of voltage source converter (VSC)-based HVDC transmission system (VSC transmission) technology for connecting large doubly fed induction generator (DFIG)-based wind farms over long distance. The operation principles of the proposed system are described, and new control strategies for normal and grid fault conditions are proposed. To obtain smooth operation, the wind farm side VSC (WFVSC) is controlled as an infinite voltage source that automatically absorbs power generated by the wind farm and maintains a stable local ac network. Fault ride through of the system during grid ac faults is achieved by ensuring automatic power balancing through frequency modulation using WFVSC and frequency control using DFIG. PSCAD/EMTDC simulations are presented to demonstrate robust performance during wind speed and power variations and to validate the fault ride through capability of the proposed system.

This paper proposes a coordinated operational strategy of energy storage system (ESS) and wind farm to reduce wind power spillage and alleviate wind power fluctuation. Firstly, power curve of a wind farm, in the form of an equivalent wind turbine, is determined by least square fitting. Based on the power output characteristic of the wind farm and historic operating data, the statistical properties of wind power spillage and fluctuation can be obtained. Secondly, the target for wind farm and ESS power output is calculated. Power generation/absorption capabilities of ESS and available power output reducing range of the wind farm are quantified as well, considering the operational constraints of ESS and dispatched wind power output. Finally, a coordinated operational strategy is developed not only to adjust the combined ESS and wind farm to meet the dispatch requirement, but also to ensure ESS and the wind farm to satisfy operational constraints. Case studies are conducted on a practical wind farm with battery energy storage system installed, which demonstrate that the coordination of ESS can greatly boost the revenue and improve power quality of wind farm.