Standalone microgrids, which are mainly constructed on island areas have low system inertia, may result large frequency deviations even for small load change. Moreover, increasing penetration level of renewable energy sources (RESs) into standalone microgrids makes the frequency stability problem even worse. To overcome this problem, this paper proposes an active power sharing method with zero frequency deviations. To this end, a battery energy storage system (BESS) is operated as constant frequency (CF) control mode, whereas the other distributed generations (DGs) are operated as an active and reactive power (PQ) control mode. As a result, a state of charge (SOC) of the BESS is changed as the system load varies. Based on the SOC deviation, DGs share the load change. The SOC data is assumed to be sent via communication system, hence the communication time delay is considered. To enhance reliability, controllers of DGs are designed to take account of the failure of communication system. The simulation results show that active power can be shared among DGs according to desired ratio without frequency deviations even for large variation of output power of RESs.
This paper presents a decentralized control method for distributed generations (DGs) in an islanded direct current (DC) microgrid. In most typical DC microgrids, a decentralized control method is based on a voltage droop control method. However, the grid voltage differs from node to node due to line voltage drop, and hence the power sharing ratio among DGs cannot be matched with as desired value. Especially in an islanded DC microgrid including an energy storage system as a voltage source, it is difficult for DGs to maintain the charge state of the ESS in a decentralized way. To overcome this problem, state of charge (SOC)-voltage droop control is applied to the ESS. By using the proposed droop method, the SOC information can be assigned to the grid voltage, and hence the other DGs are able to support the SOC in a decentralized way. For DGs to enhance the accuracy of the SOC estimation, voltage drop is compensated for based on forecasting data and line impedance data. The simulation is modeled and implemented using Power System Computer Aided Design/Electromagnetic Transients for DC (PSCAD/EMTDC, version 4.2, Winnipeg, Manitoba, Canada) and the simulation results show that the capability to maintain SOC as well as the system voltage profile are improved by using the proposed method.
A wind farm is a collection of a few tens or a few hundreds of wind turbine generator installed in close vicinity. There is a need for further research to examine the performance of wind farm and wake models in difficult environments. This paper deals with the modelling of the wake effect in wind farms using real time digital simulator. There are many different models to analyse the wake effect. Jensen model is one of the widely used wake models at the moment. The model is simpler and requires a short time for calculations. Power loss of the modelled wind farm with the wake effect is analysed under the effects of wind direction and the wind speed time delay. The modelling of a wind farm for wake effect simulation is described and some exemplary simulations are also performed to show the influence of wake effect.
-In this paper, a new power control strategy for the bipolar-type low voltage direct current (LVDC) distribution system is being proposed. The dc distribution system is considered as an innovative system according to the increase of dc loads and dc output type distribution energy resources (DERs) such as photovoltaic (PV) systems and energy storage systems (ESS). Since the dc distribution system has many advantages such as feasible connection of DERs, reduction of conversion losses between dc output sources and loads, no reactive power issues, it is very suitable solution for new type buildings and residences interfaced with DERs and ESSs. In the bipolar-type, if it has each grid-interfaced converter, both sides (upper, lower-side) can be operated individually or collectively. A complementary power control strategy using two ESSs in both sides for effective and reliable operation is proposed in this paper. Detailed power control methods of the host controller and local controllers are described. To verify the performances of the proposed control strategy, simulation analysis using PSCAD/EMTDC is being performed where the results show that the proposed strategy provides efficient operations and can be applied to the bipolar-type dc distribution system.
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