Abstract
Microgrids are small scale electrical power systems that comprise distributed energy resources (DER), loads, and storage devices. The integration of DER into the electrical power system basically allows the clustering of small parts of the main grid into Microgrids. Due to the increasing amount of renewable energy, which is integrated into the main grid, high power fluctuations are expected to become common in the next years. This carries the risk of blackouts to be also more likely in the future. Microgrids hold the potential of increasing reliability of supply, since they are capable of providing a backup supply during a blackout of the main grid. This paper investigates the black-starting and islanding capabilities of a battery energy storage system (BESS) in order to provide a possible backup supply for a small part of the main grid. Based on field tests in a real Microgrid, the backup supply of a residential medium voltage grid is tested. Whereas local wind turbines within this grid section are integrated into this Microgrid during the field test, the supply of households is reproduced by artificial loads consisting of impedance- and motor loads, since a supply of real households carries a high risk of safety issues and open questions regarding legal responsibility. To operate other DER during the island operation of such a Microgrid, control mechanisms have to ensure the power capabilities and energy reserves of the BESS to be respected. Since the operation during a backup supply of such a Microgrid requires a simple implementation, this paper presents a simple master–slave control approach, which influences the power output of other DER based on frequency characteristics without the need for further communication. Besides the operation of other DER, the capability to handle load changes during island operation while ensuring acceptable power quality is crucial for such a Microgrid. With the help of artificial loads, significant load changes of the residential grid section are reproduced and their influence on power quality is investigated during the field tests. Besides these load changes, the implementation and behavior of the master–slave control approach presented in this paper is tested. To prepare these field tests, simulations in Matlab/Simulink are performed to select appropriate sizes for the artificial loads and to estimate the expected behavior during the field tests. The field tests prove that a backup supply of a grid section during a blackout of the main grid by a BESS is possible. By creating the possibility of operating other DER during this backup supply, based on the master–slave control approach presented in this paper, the maximum duration for this backup supply can be increased.
Highlights
In order to mitigate global warming, the integration of renewable energy will increase dramatically in the coming years
The application of a frequency control characteristic based on a master–slave control approach allows the integration of distributed energy resources (DER) with nominal powers higher than the Battery Energy Storage Systems (BESS) into the Microgrid, while carrying out an appropriate energy management by guaranteeing the state of charge (SoC) of the BESS within allowable limits
The BESS is capable of controlling active- and reactive load changes, which are considered to be representative for significant load changes in a residential grid section, with an acceptable level of power quality, indicated by voltage drops not to fall below a value of
Summary
In order to mitigate global warming, the integration of renewable energy will increase dramatically in the coming years. Most of the DER, which will be integrated into the main grid in the future are based on variable renewable generation They are not fully capable of contributing to power balancing during a backup supply in an islanded Microgrid because they are connected via grid-following converters and may only cover a share of the energy demand of the loads in an intermittent way. Against the background of value-stacking, islanding is considered as an additional service BESS can provide to increase their profitability, while at the same time increasing the reliability of supply in the case of blackouts of the interconnected power grid by building local Microgrids Assuming such Microgrids to comprise a high share of variable renewable energy, an easy approach for ensuring their functionality during island operation is desired in order to lower the requirements of the energy reserves of the BESS and to increase the maximum duration of island operation.
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