Weihao Hu

Flicker Mitigation by Active Power Control of Variable-Speed Wind Turbines With Full-Scale Back-to-Back Power Converters

Grid-connected wind turbines are fluctuating power sources that may produce flicker during continuous operation. This paper presents a simulation model of a megawatt-level variable-speed wind turbine with a full-scale back-to-back power converter developed in the simulation tool of PSCAD/EMTDC. Flicker emission of this system is investigated. Reactive power compensation is mostly adopted for flicker mitigation. However, the flicker mitigation technique shows its limits, when the grid impedance angle is low in some distribution networks. A new method of flicker mitigation by controlling active power is proposed. It smoothes the 3p active power oscillations from wind shear and tower shadow effects of the wind turbine by varying the dc-link voltage of the full-scale converter. Simulation results show that damping the 3p active power oscillation by using the flicker mitigation controller is an effective means for flicker mitigation of variable-speed wind turbines with full-scale back-to-back power converters during continuous operation.

Optimal Operation of Plug-In Electric Vehicles in Power Systems With High Wind Power Penetrations

The Danish power system has a large penetration of wind power. The wind fluctuation causes a high variation in the power generation, which must be balanced by other sources. The battery storage-based Plug-In Electric Vehicles (PEVs) may be a possible solution to balance the wind power variations in the power systems with high wind power penetrations. In this paper, the integration of plug-in electric vehicles in the power systems with high wind power penetrations is proposed and discussed. Optimal operation strategies of PEV in the spot market are proposed in order to decrease the energy cost for PEV owners. Furthermore, the application of battery storage based aggregated PEV is analyzed as a regulation services provider in the power system with high wind power penetrations. The western Danish power system where the total share of annual wind power production is more than 27% of the electrical energy demand is chosen as the studied case. The results show that an optimal operation of PEV in both spot market and regulation market can not only decrease the energy costs for PEV owners, but also significantly decrease the power deviations between West Denmark and Union for the Coordination of Electricity Transmission (UCTE) control areas.

Optimal operation strategy of battery energy storage system to real-time electricity price in Denmark

Since the hourly spot market price is available one day ahead, the price could be transferred to the consumers and they may have some motivations to install an energy storage system in order to save their energy costs. This paper presents an optimal operation strategy for a battery energy storage system (BESS) in relation to the real-time electricity price in order to achieve the maximum profits of the BESS. The western Danish power system, which is currently the grid area in the world that has the largest share of wind power in its generation profiles and may represent the future of electricity markets in some ways, is chosen as the studied power system in this paper. Two kinds of BESS, based on polysulfide-bromine (PSB) and vanadium redox (VRB) battery technologies, are studies in the paper. Simulation results show, that the proposed optimal operation strategy is an effective measure to achieve maximum profits of the BESS. The results also show that it will take longer time to return the original investment of the VRB battery. So the PSB battery is the better investment choice for the time being.

Flicker Mitigation by Individual Pitch Control of Variable Speed Wind Turbines With DFIG

Due to the wind speed variation, wind shear and tower shadow effects, grid connected wind turbines are the sources of power fluctuations which may produce flicker during continuous operation. This paper presents a model of an MW-level variable-speed wind turbine with a doubly fed induction generatorto investigate the flicker emission and mitigation issues. An individual pitch control (IPC) strategy is proposed to reduce the flicker emission at different wind speed conditions. The IPC scheme is proposed and the individual pitch controller is designed according to the generator active power and the azimuth angle of the wind turbine. The simulations are performed on the NREL (National Renewable Energy Laboratory) 1.5-MW upwind reference wind turbine model. Simulation results show that damping the generator active power by IPC is an effective means for flicker mitigation of variable speed wind turbines during continuous operation.

Optimized Placement of Wind Turbines in Large-Scale Offshore Wind Farm Using Particle Swarm Optimization Algorithm

With the increasing size of wind farms, the impact of the wake effect on wind farm energy yields become more and more evident. The arrangement of locations of the wind turbines (WTs) will influence the capital investment and contribute to the wake losses, which incur the reduction of energy production. As a consequence, the optimized placement of the WTs may be done by considering the wake effect as well as the components cost within the wind farm. In this paper, a mathematical model which includes the variation of both wind direction and wake deficit is proposed. The problem is formulated by using levelized production cost (LPC) as the objective function. The optimization procedure is performed by a particle swarm optimization (PSO) algorithm with the purpose of maximizing the energy yields while minimizing the total investment. The simulation results indicate that the proposed method is effective to find the optimized layout, which minimizes the LPC. The optimization procedure is applicable for optimized placement of WTs within wind farms and extendible for different wind conditions and capacity of wind farms.

An efficient experimental method for high power direct drive wind energy conversion systems

Because of energy shortage and environment pollution, the renewable energy, especially wind energy has become more and more considerable all over the world. Direct drive wind energy conversion systems based on multipole permanent magnet synchronous generator (PMSG) have some advantages such as no gearbox, high power density, high precision and easy to control. In our research project, a 2MW direct drive wind energy conversion system is developed. In this paper, an efficient experimental method for high power converter is presented. A large circulating current flows in the converter, but only a small part of the current caused by the losses of the converter flows into the grid. The method can save a lot of energy when the converter is tested and the experiment can be done in the micro grid. The method can test the main stage, drive circuit, protect circuit and some parts of control circuit. Modeling and control scheme of the efficient experimental method are introduced in this paper, as well as the control scheme of the grid side converter. Some differences caused by sinusoidal pulse width modulation (SPWM) and space vector modulation (SVM) in the method have been analyzed. Simulation and experimental results verify the analysis and the conclusion.

Three Reference Frame Control Scheme of 4 wire Grid-connected Inverter for Micro Grid Under Unbalanced Grid Voltage Conditions

For 4 wire VSI, Under unbalanced grid voltage conditions, neutral currents that flow through the fourth wire will distort the symmetrical output voltage. And dc link voltage ripple will be produced. We introduced a three reference frame scheme to eliminate the dc link voltage ripple and output voltage distortion. Unbalanced grid voltage and ac link current are decomposed into positive, negative and zero sequence components. Positive sequence components is regulated in a positive synchronous reference frame (SRF), negative sequence components is regulated in a negative SRF, and zero sequence components is regulated in a zero SRF. In positive SRF, positive components appear as dc, while negative components appear as 2¿ rad/s. In contrast, in negative SRF, negative components appear as dc, while positive sequence appear as 2¿ rad/s. Zero sequence composents will not have any impace on dq, it appears as a disturbance in the o-variable at the fundamental frequency. In order to eleminated the dc link voltage ripple and output voltage distortion, positive, negative and zero sequence components shoud be controlled simultaneously. Simulation and experimental results are presented to verify the validity of the proposed control scheme.

Optimal Load Response to Time-of-Use Power Price for Demand Side Management in Denmark

Since the hourly spot market price is available one day ahead in Denmark, the price could be transferred to the consumers and they may shift their loads from high price periods to the low price periods in order to save their energy costs. This paper presents a load optimization method to time-of-use power price for demand side management in order to save the energy costs as much as possible. 3 typical different kinds of loads (industrial load, residential load and commercial load) in Denmark are chosen as study cases. The energy costs decrease up to 9.6% with optimal load response to time-of-use power price for different loads. Simulation results show that the optimal load response to time-of-use power price for demand side management generates different load profiles and reduces the load peaks. This kind of load patterns may also have significant effects on the power system normal operation.

The Relationship Between Electricity Price and Wind Power Generation in Danish Electricity Markets

The western Danish power system is currently the grid area in the world that has the largest share of wind power in its generation profiles, with more than 20% of its annual consumption generated by wind turbines. In this paper, the western Danish power system, which may represent the future of competitive electricity markets in some ways, is chosen as the studied power system. The relationship between the electricity price (both the spot price and the regulation price) and the wind power generation in an electricity market is investigated in this paper. The spot price, the down regulation price and the up regulation price generally decreases when the wind power penetration in the power system increases. The statistical characteristics of the spot price for different wind power penetration are analyzed. The findings of this paper may be useful for wind power generation companies to make the optimal bidding strategy and may be also useful for the optimal operation of modern power systems with high wind power penetrations.

A Reactive Power Dispatch Strategy With Loss Minimization for a DFIG-Based Wind Farm

An optimal reactive power dispatch strategy is proposed to minimize the total electrical losses of a wind farm (WF), including not only losses in the transmission cables and wind turbine (WT) transformers, but also losses inside wind energy generation systems. The reactive power dispatch inside a WT uses optimal splitting strategy over the stator and the grid side converter (GSC), which aims to minimize the total loss of the wind energy generation system, including the generator, the converters, and the filters. Optimization problems are formulated based on established loss models and WT reactive power limits. A WF is carefully designed and used for case studies. Wake effect is considered when calculating the active power at each WT. The total losses of the WF are calculated by implementing the proposed strategy at different wind speeds and reactive power references. The simulation results show the effectiveness of the proposed strategy.