Weak Distribution Grid Research Articles

A Novel Approach for Ramp-Rate Control of Solar PV Using Energy Storage to Mitigate Output Fluctuations Caused by Cloud Passing

The variability of solar irradiance with a high ramp-rate, caused by cloud passing, can create fluctuation in the PV output. In a weak distribution grid with a high PV penetration, this can create significant voltage fluctuations. Energy storage devices are used to smooth out the fluctuation using traditional moving average control. However, moving average does not control the ramp-rate directly; rather the ramp-rate depends on previous values of PV output. This paper proposes a strategy where the ramp-rate of PV panel output is used to control the PV inverter ramp-rate to a desired level by deploying energy storage (which can be available for other purposes, such as storing surplus power, countering voltage rise, etc.). During the ramping event, the desired ramp-rate is governed by controlling the energy storage based on an inverse relationship with the PV panel output ramp-rate to improve the fluctuation mitigation performance. In contrast to the moving average method, the proposed strategy is able to control the desired ramp-rate independent of the past history of the PV panel output. A dynamic model of the PV-storage integrated system is developed to verify the proposed strategy in the presence of physical device time lags. The proposed strategy is verified using simulation results based on an Australian distribution system. A laboratory experiment is also conducted to validate the concept of the proposed control strategy.

Integration of a hybrid fuel cell-battery system to a distribution grid

In order to integrate a proton exchange membrane type (PEM) fuel cell system (FCS) combined with a battery bank to a distribution grid; this paper proposes a local controller based on fuzzy logic. The proposed system provides primary frequency control and local bus voltage support to the local grid. This opposes the passive distributed generation of the present that do not provide auxiliary services, such as back-up power, voltage support and reliability of supply as they operate under constant power factor equal to 1 at all times. During network disturbances, the distributed generations of the present are disconnected until normal operation is reestablished. When the distributed generation penetration is high this may lead to system instability. The microgrid concept is the effective solution for the control and quality improvement of grids with high level of DG penetration. So, the proposed system, also, can be an active controllable microsource of a microgrid in the future that cooperates with other microsources in order to cover the local load demands for active and reactive power either under grid-connected mode or under islanding operating mode. In cases where the distribution grid (working as microgrid) is forced to operate in islanded mode, the hybrid system provides the demanded active and reactive power. The FCS is connected to a weak distribution grid so that the system performance is studied under the worst conditions. The simulation results are obtained using MATLAB software under a severe step load change where the grid is still connected and under islanded operation. In both cases the system presents a good performance.

Static VAR compensation of a fixed speed stall control wind turbine during start-up

An installed 900 kW fixed speed stall controlled soft-started wind turbine connected to a weak distribution grid was modeled under start-up conditions. Generator and soft-start control and design parameters were not available, so a modeling process independent of this information was developed. Field measured transients were closely reproduced in simulation using a full-order generator model with generic parameters and a thyristor-based graduated interconnection to the grid. A Static VAR Compensator model was then added to the wind turbine model to explore a method of reducing start-up transients. It was found that for the specific transients modeled, a 300 kVAR SVC supplied sufficient reactive power to limit line voltage variations to within 3% of steady state values. Model details and simulation output are presented in this paper.

Opportunities for hydrogen production in connection with wind power in weak grids

This paper gives an overview of the opportunities that exist for combining wind power and hydrogen ( H 2 ) production in weak grids. It is described how H 2 storage can be applied in both isolated and grid-connected systems, and how the produced H 2 can be utilized for stationary energy supply and/or as a fuel for transportation. The paper discusses the benefits and limitations of the different H 2 storage applications, and presents a logistic simulation model for performance evaluation of wind- H 2 plants. A case study simulating the use of excess wind power in a weak distribution grid to produce H 2 for vehicles has been presented. It is shown that the penetration of wind power can be significantly increased by introducing electrolytic H 2 production as a controllable load. The results also indicate that there are large benefits of using the grid as backup for H 2 production in periods with low wind speed, regarding the H 2 storage sizing and the electrolyser operating conditions.