• 전기학회논문지
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• 제61권8호
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• pp.1099-1106
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• 2012

Microgrid is a new electrical energy system that composed of various generators, renewable energy, batteries and loads located near the electrical customers. When Microgrid is interconnected with large power system, Microgrid don't need to control the frequency. But in case of the outage or faults of power system, Microgrid should control the frequency to prevent the shutdown of Microgrid. This paper presents the frequency control methods using the droop function, being used by synchronous generators and EMS(Energy Management System). Using droop function, two battery systems could share the load based on locally measured signals without any communications between batteries. Also, we suggest that EMS should control the controllable distributed generators as P/Q control modes except batteries to overcome the weakness of droop function. Finally we suggest the two batteries systems to prolong the battery's life time considering the economical view. The validation of proposed methods is tested using PSCAD/EMTDC simulations and field test sites at the same time.

• Journal of Electrical Engineering and Technology
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• 제7권4호
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• pp.459-467
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• 2012

In this paper, a grid connected microgrid with multiple inverter-based distributed generators (DGs) is considered. DG in FFC mode regulates the microgrid as a controllable load from the utility point of view as long as its output is within the capacity limit. The transition mode causes a change in frequency of microgrid due to the loss of power transferred between main grid and microgrid. Frequency deviation from the nominal value can exceed the limit if the loss of power is large enough. This paper presents a coordinated control method for inverter-based DGs so that the microgrid is always regulated as a constant load from the utility viewpoint during grid connected mode, and the frequency deviation in the transition mode is minimized. DGs can share the load by changing their control modes between UPC and FFC and stabilize microgrid during transition.

• 전기학회논문지
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• 제59권4호
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• pp.707-714
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• 2010

When problems such as line fault, breakdown of a substation or a generator, etc. arise on the grid, the Microgrid is designed to be separated or isolated from the grid. Most existing DGs(Distributed Generators) in distribution system use rotating machine. However, new DGs such as micro gas turbine, fuel cell, photo voltaic, wind turbine, etc. will be interfaced with the Microgrid through an inverter. So the Microgrid may have very lower inertia than the conventional distribution system. By the way, the rate of change of frequency depends on the inertia of the power system. Moreover, frequency has a strong coupling with active power in power system. Because the frequency of the Microgrid may change rapidly and largely during transient, appropriate and fast control strategy is needed for stable operation of the Microgrid. Therefore, this paper presents a power balancing strategy in Microgrid during transient. Despite of strong power or frequency excursions, power balancing in the Microgrid can be maintained.