• Journal of Power Electronics
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• v.16 no.4
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• pp.1526-1540
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• 2016

This work mainly discusses an accurate and fast islanding detection based on fractional wavelet packet transform (FRWPT)for multibus microgrid systems. The proposed protection scheme uses combined desirable features retrieved from discrete fractional Fourier transform (FRFT) and wavelet packet transform (WPT) techniques, which provides precise time-frequency information on minute perturbation signals introduced in the system. Moreover, this study focuses on the design of decoupling control with a distributed controller based on state feedback for the efficient operation of microgrid systems that are transitioning from the grid-connected mode to the islanded mode. An IEEE 9-bus test system with inverter based distributed generation (DG) units is considered for islanding assessment and smooth operation. Finally, tracking errors are greatly reduced with stability improvement based on the proposed controller. FRWPT based islanding detection is demonstrated via a time domain simulation of the system. Simulated results show an improvement in system stability with the application of the proposed controller and accurate islanding detection based on the FRWPT technique in comparison with the results obtained by applying the wavelet transform (WT) and WPT.

• Journal of the Korean Solar Energy Society
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• v.34 no.4
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• pp.1-7
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• 2014

The microgrid system is the combination of photovoltaic(PV) array, load, and battery energy storage system. The control strategies were defined as multi-modes of operation, including rest operation without use of battery, power charging, and power discharging, which enables grid connected mode or islanded mode. Photovoltaic power is a problem of the uniformity of power quality because the power generated from solar light is very sensitive to variation of insolation and duration of sunshine. As a solution to the above problem, energy storage system(ESS) is considered generally. There fore, in this study, we did basic research activities about optimization method of the amount of energy used, using a smart microgrid test-bed constructed in building. First, we analyzed the daily, monthly and period energy pattern amount of power energy used, and analyzed PV power generation level which is built on the roof. Utilizing building energy pattern analysis data, we was studied an efficient method of employing the ESS about building power consumption pattern and PV generation.

• Proceedings of the KIEE Conference
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• 2015.07a
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• pp.523-524
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• 2015

신재생 에너지는 발전 시 연료가 들지 않는다는 특징과 친환경적인 이유로 각광 받고 있다. 이러한 특징으로 인해 에너지 독립 계통을 구성하는 데 주요 역할을 한다. 풍력, 태양광, 에너지 저장 장치 등이 분산 전원으로서 계통을 구성하게 되는데, 낮은 효율과 높은 초기 설치비용으로 인해 디젤 발전기나 마이크로 터빈 등 소형 발전기를 예비 전력으로서 부착하는 경우가 대부분이다. 이러한 분산 전원들은 가격이 제각각이며 기존의 화석연료 발전기처럼 비용 함수 모델링이 어려워 경제적 분석이 어려웠다. 이 논문은 이러한 신재생 에너지를 사용한 독립 마이크로그리드를 경제적 관점에서 분석하는 것이 목적이다. 따라서 계통을 분석하는 가장 보편적인 방법인 최적 조류 계산(OPF, Optimal Power Flow)방법과 가격을 최소화하는 점을 찾는 경제급전(ED, Economic Dispatch)방법을 사용하였다. 두 가지 방법의 결과를 비교하여 발전 비용을 줄일 수 있는지에 대해 조사하였다. 7-Bus 가상 계통을 만들어 사용하였고, OPF와 ED의 발전 비용은 PowerFactory$^{(R)}$ 프로그램을 통해 시뮬레이션 하여 데이터를 얻었다.

• Journal of Power Electronics
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• v.17 no.5
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• pp.1278-1287
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• 2017

Inverters are widely used in distributed power generation and other applications. However, their lack of inertia and variable impedance may cause system instability and power transfer inaccuracy. This paper proposes a control scheme for a single phase voltage-controlled inverter with some grid-friendly characteristics. The proposed control algorithm enables the inverter to function as a voltage source with an inner output impedance in both the islanded and grid-connected modes. Virtual inertia and rotor equations are embedded in the PLL part. Thus, the frequency stability can remain. The inner output impedance can be adjusted freely, which helps to accurately decouple and transmit the output active and reactive power. The proposed inverter operates like a traditional synchronous generator. Simulations and experiments are designed and carried out to verify the proposed control strategy.

• Proceedings of the KIPE Conference
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• 2018.07a
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• pp.376-377
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• 2018

A hardware-in-the-loop (HIL) platform for a power management control of islanded DC microgrids is established. In order to avoid the complexity and high costs, a decentralized control based on the DC Bus Signaling (DBS) method is applied to the HIL system. The simulation results for the HIL microgrid platform have verified the effectiveness of power management strategy.

• Journal of Power Electronics
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• v.13 no.3
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• pp.479-486
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• 2013

This paper presents a control strategy for distributed systems, which can be used in islanded microgrids. The control strategy is based on the droop method, which uses locally measured feedback to achieve load current sharing. Instead of the traditional droop method, an improved one is implemented. A virtual inductor in the synchronous frame for three-phase inverters is proposed to deal with the coupling of the frequency and the amplitude related to the active and reactive power. Compared with the traditional virtual inductor, the proposed virtual inductor is not affected by high frequency noises because it avoids differential calculations. A model is given for the distributed generation system, which is beneficial for the design of the droop coefficients and the value of the virtual inductor. The effectiveness of the proposed control strategy is verified by simulation and experiment results.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.58 no.11
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• pp.2175-2184
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• 2009

This paper describes operational analysis results of the DC micro-grid using detailed model of distributed generation. Detailed model of wind power generation, photo-voltaic generation, fuel-cell generation was implemented with the user-defined model of PSCAD/EMTDC software that is coded with C-language. The operation analysis was carried out using PSCAD/EMTDC software, in which the power circuit is implemented by built-in model and the controller is modelled by user-defined model that is also coded with C-language. Various simulation results confirm that the DC micro-grid can operate without any problem in both the interconnected mode and the islanded mode. The operation analysis result confirms that the DC micro-grid make it feasible to provide power to the load stably. And it can be utilize to develop the actual system design and building.

• Journal of Electrical Engineering and Technology
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• v.2 no.2
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• pp.165-171
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• 2007

This paper proposes a new islanding detection method for inverter-interfaced distributed generators (DG). To detect islanding conditions, this paper calculates the phase angle variation of the system voltage by using the phase-locked loop (PLL) in the inverter controllers. Because almost all inverter systems are equipped with the PLL, the implementation of this method is fairly simple and economical for inverter-interfaced DGs. The detection time can also be shortened by reducing communication delay between the relays and the DGs. The proposed method is based on the fact that islanding conditions result in the frequency and voltage variation of the islanded area. The variation depends on the amount of power mismatch. To improve the accuracy of the detection algorithm, this paper injects small low-frequency reactive power mismatch to the output power of DG.

• The Transactions of the Korean Institute of Power Electronics
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• v.24 no.5
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• pp.349-355
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• 2019

A grid-connected inverter with critical loads should be able to supply a stable voltage to critical loads at mode change and during clearing time while detecting unintentional islanding. This study proposes a mode transfer method for a grid-connected inverter with critical loads. The proposed method, which integrates the grid-connected and islanded mode control loops into one control block, provides an autonomous and seamless mode transfer from the current control to the voltage control. Therefore, the proposed scheme can supply a stable voltage to critical loads at mode change and during clearing time. Experimental results are provided to validate the proposed method.

• Proceedings of the Korea Information Processing Society Conference
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• 2011.04a
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• pp.327-328
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• 2011

마이크로그리드는 주로 신재생 전원으로 구성되는 소규모 전력시스템으로 그 관심이 고조되고 있다. 최근 멀티에이전트 기반의 마이크로그리드의 운용 및 제어 기술에 대한 연구가 진행되고 있다. 전력시스템과 연계되지 않는 독립운전의 경우는 상용주파수를 유지하기 위해서 전력의 공급과 부하의 균형을 유지시켜야 하며, 특히 전력공급이 부족한 경우는 강제적으로 전력의 부하를 차단하여야 한다. 본 논문에서는 멀티에이전트 시스템 기반의 독립운전을 하는 마이크로그리드 운용을 위한 강제적인 부하 차단을 위해서 파산문제(bankruptcy problem)와 CEA(constrained equal awards) 규칙에 근거하여 부하 차단의 기법을 제안하고 이에 대해서 그 활용 가능성을 검토하고자 한다.