Adaptive Gain-based Stable Power Smoothing of a DFIG

Hyewon Lee,Min Hwang,Jinsik Lee,Eduard Muljadi,Hong-Ju Jung,Yong Cheol Kang 대한전기학회 2017 Journal of Electrical Engineering & Technology Vol.12 No.6

In a power system that has a high wind penetration, the output power fluctuation of a large-scale wind turbine generator (WTG) caused by the varying wind speed increases the maximum frequency deviation, which is an important metric to assess the quality of electricity, because of the reduced system inertia. This paper proposes a stable power-smoothing scheme of a doubly-fed induction generator (DFIG) that can suppress the maximum frequency deviation, particularly for a power system with a high wind penetration. To do this, the proposed scheme employs an additional control loop relying on the system frequency deviation that operates in combination with the maximum power point tracking control loop. To improve the power-smoothing capability while guaranteeing the stable operation of a DFIG, the gain of the additional loop is modified with the rotor speed and frequency deviation. The gain is set to be high if the rotor speed and/or frequency deviation is large. The simulation results based on the IEEE 14-bus system demonstrate that the proposed scheme significantly lessens the output power fluctuation of a WTG under various scenarios by modifying the gain with the rotor speed and frequency deviation, and thereby it can regulate the frequency deviation within a narrow range.

Frequency Profile Improvement of a Microgrid under Abnormal Conditions via a Demand Response Program

Muhammad Saeed uz Zaman,Raza Haider,Syed Basit Ali Bukhari,Yun-Sik Oh,Min-Sung Kim,Muhammad Mehdi,Chul-Hwan Kim 한국조명·전기설비학회 2017 조명·전기설비학회논문지 Vol.31 No.10

The frequency deviation of an electric power system from its nominal range is a significant indicator of the system’s instability. This issue is further highlighted when the system is incorporated with intermittent renewable energy resources. In this work, the frequency regulation of a microgrid is achieved by a demand response program. The proposed plan is implemented in a central controller that has two control sections. The frequency deviation is controlled by manipulating responsive loads. In the case of higher frequency deviation, the command of the first control section is proportional to the magnitude of frequency deviation and a large amount of responsive loads is manipulated to restore the frequency to its nominal range quickly. The other section of the central controller addresses minor frequency deviations. The proposed plan is implemented on a test system and outperforms the conventional methods in terms of frequency restoration. The improvement in voltage profile is also observed during abnormal conditions after the proposed plan is implemented.

Adaptive Gain-based Stable Power Smoothing of a DFIG

Lee, Hyewon,Hwang, Min,Lee, Jinsik,Muljadi, Eduard,Jung, Hong-Ju,Kang, Yong Cheol The Korean Institute of Electrical Engineers 2017 Journal of Electrical Engineering & Technology Vol.12 No.6

In a power system that has a high wind penetration, the output power fluctuation of a large-scale wind turbine generator (WTG) caused by the varying wind speed increases the maximum frequency deviation, which is an important metric to assess the quality of electricity, because of the reduced system inertia. This paper proposes a stable power-smoothing scheme of a doubly-fed induction generator (DFIG) that can suppress the maximum frequency deviation, particularly for a power system with a high wind penetration. To do this, the proposed scheme employs an additional control loop relying on the system frequency deviation that operates in combination with the maximum power point tracking control loop. To improve the power-smoothing capability while guaranteeing the stable operation of a DFIG, the gain of the additional loop is modified with the rotor speed and frequency deviation. The gain is set to be high if the rotor speed and/or frequency deviation is large. The simulation results based on the IEEE 14-bus system demonstrate that the proposed scheme significantly lessens the output power fluctuation of a WTG under various scenarios by modifying the gain with the rotor speed and frequency deviation, and thereby it can regulate the frequency deviation within a narrow range.

PLL 주파수 합성기를 이용한 새로운 주파수 변조 회로 설계 및 제작

양승식,이종환,염경환 한국전자파학회 2004 한국전자파학회논문지 Vol.15 No.6

이 논문은 PLL주파수 합성기의 루프 대역폭보다 높은 주파수에서 낮은 주파수까지 변화하는 신호에 대한 주파수 변조가 일정한 최대 주파수 편이를 갖도록 하는 단순하면서도 저가의 새로운 주파수 변조회로를 소개하였다. 이 주파수 변조회로는 PLL 안에서의 주파수에 따른 루프 필터의 궤환량을 보상하도록 설계되었고 최대주파수 편이값 조절과 루프와의 상호 간섭을 제거할 수 있도록 설계되었다. 또한 기존의 스펙트럼 분석기로 $\Delta$f(최대 주파수 편이)또는 $\beta$(변조 지수)를 측정하는 방법은 협대역 주파수 변조에서만 유용하여 광대역 주파수에서 측정할 수 있도록 새로운 측정방법을 제안하고 변조 신호 발생기를 이용하여 정확성을 확인하였다. 이런 한 방법으로 설계하여 제작한 회로를 측정하여 기대한 일정한 최대 주파수 편이를 가지는 것을 확인하였다. In this paper, using phase locked loop(PLL) synthesizer, we introduce a novel but simple and low cost frequency modulation(FM) circuit of a flat peak frequency deviation fur modulation signal whose frequency covers from outside to inside of the loop-bandwidth of PLL. The FM circuit was basically designed to compensate an amount of feedback of the loop filter in PLL. The circuit also includes the capability of the adjustment of peak frequency deviation and of blocking the intereference with the loop filter. The designed circuit was successfully implemented and showed the flat frequency deviation as expected in the design. In addition, the novel measurement method of the wideband FM modulation index is suggested verified With the suggest measurement, it can be successfully shown the designed circuit has the expected frequency deviation.

Study on Effect Mechanism of Flow Characteristics Deviation on Low Frequency Oscillations

Jin, Tan,Ge, Jin,Sun, Cai 대한전기학회 2014 The Journal of International Council on Electrical Vol.4 No.1

This paper focus on low frequency oscillations induced by the flow characteristics deviation of steam turbine. The causes and features of flow characteristics deviation are analyzed and described first. The coupling model between thermodynamic system and power system was established for time-domain simulation analysis, in an attempt to find out the effect mechanism of flow characteristics deviation on low frequency oscillations. The effect of flow characteristics derivation on the responses characteristics of turbo-generator is analyzed through simulation. Results indicate that the flow characteristics deviation is apt to cause low frequency oscillations. Both the amplitude and the speed of oscillations are related to the extent of flow characteristics deviation.

Load Power Estimation Based Secondary Control for Microgrids

Teng Wu,Jinjun Liu,Zeng Liu,Shike Wang,Baojin Liu 전력전자학회 2015 ICPE(ISPE)논문집 Vol.2015 No.6

The well-known active power-frequency and reactive power-voltage amplitude droop control is widely used in the coordinative control of parallel inverters in microgrids. However, this conventional droop method may cause frequency and voltage deviation, which affects the accuracy of power supply. This paper proposes a novel secondary control strategy to compensate this deviation. This strategy mimics the Master-Slave control but requires no communication lines among the parallel inverters. The master inverter adopts conventional droop method (using power to control frequency and voltage amplitude) and is controlled as a voltage source while the slave inverters adopt reversed droop method (using frequency and voltage amplitude to control power) and are controlled as current sources. The droop characteristic bias of slave inverters is designed online based on the estimation of load power demand. Through this method, frequency and voltage deviation can be eliminated and power sharing can be realized among all the slave inverters. Simulation and experimental results are provided to prove the effectiveness of the proposed control strategy.

디지털주파수계전기의 고속 샘플링추정기법

남시복(S. B. Nam),이훈구(H. G. Lee),마석범(S. B. Ma),김진수(J. S. Kim) 대한전기학회 2006 대한전기학회 학술대회 논문집 Vol.2006 No.10

Frequency and phasor are the most important quantities in power system operation because they can reflect the whole power system situation. Frequency reflects the dynamic energy balance between load and generating power, while operators use phasor to constitute the state of system and, moreover, phasor based line relays are currently used in most power systems. So frequency and phasor are regarded as indices for the operating power systems in practice. The proposed technique is suitable for estimation near-nominal, nominal, and off-nominal frequencies. It is useful in designing microprocessor- based relays and meters that need to measure power system frequency. Performance test results, using signals from EMTP source and Excel program, indicate that the proposed technique can provide accurate estimates within 16ms. Maximum estimation errors observed during testing are of the order of 0.006㎐ for nominal, near-nominal, and off-nominal frequencies. The proposed technique provides accurate estimates in presence of noise and harmonics and in case ground fault. The rate change of the phase angle is used for estimation. To confirm the validity of the proposed algorithm, the simulation studies carried out on a typical 154[KV] double T/L system by using EMTP software. Some test results are presented in the paper.

Femto Cell 전파환경에서의 전송가능 대역폭 평가방법에 관한 연구

하덕호(Deock-Ho Ha) 한국정보기술학회 2017 한국정보기술학회논문지 Vol.15 No.4

It is very important thing to search a practicable transmission bandwidth which can aim the upper limit of code rate in urban mobile communications. Usually, the measurement method of analysing the practicable transmission bandwidth can be conducted by Fourier transform analysis for the measured time delay distribution data. But this measuring method is very difficult because of the requirement of synchronization between the transmit and receiving end. Another method of deciding the practicable transmission bandwidth is to utilize the frequency correlation coefficient calculated from the two field strength signals with each different frequencies. This calculating method of frequency correlation coefficients is also not so good recommendable method because of its incorrectness due to the characteristic of field strength distribution. In this paper, we proposed and analyzed the standard deviation value calculated from difference decibel value between the two field strength signals with each different frequencies. From the simulation results, it can be seen that the dB standard deviation value of received signal strength is suitable estimation parameter because the dB standard deviation values can be correctly indicated a proportional value from 7.18 to 0.37 due to the variation ratio of directed wave components(-30~+20dB).

Power System Harmonic Estimation Based on Park Transform

Ya Chen,Tianyao Ji,Mengshi Li,Qinghua Wu,Xuejian Wang 대한전기학회 2016 Journal of Electrical Engineering & Technology Vol.11 No.3

This paper presents a novel method for power system harmonic estimation based on the Park transform. The proposed method firstly extends the signal to a group of three-phase signals in a-b-c coordinate. Then, a linear fitting based method is adopted to estimate the fundamental frequency. Afterwards, the Park transform is utilized to convert the three-phase signals from a-b-c coordinate to d-q- 0 coordinate. Finally, the amplitude and phase of a harmonic component of interest can be calculated using the d-axis and q-axis components obtained. Simulation studies have been conducted using matrix laboratory (MATLAB) and power system computer aided design/electromagnetic transients including direct current (PSCAD/EMTDC). Simulation studies in MATLAB have considered three scenarios, i.e., no-frequency-deviation scenario, frequency-deviation scenario and the scenario in the presence of inter-harminics. The results have demonstrated that the proposed method achieves very high accuracy in frequency, phase and amplitude estimation under noisy conditions, and suffers little influence of the inter-harmonics. Moreover, comparison studies have proved that the proposed method is superior to FFT and Interpolated FFT with the Hanning Window (IpFFTHW). Finally, a popular case in PSCAD/EMTDC has been employed to further verify the effectiveness of the proposed method.

Power System Harmonic Estimation Based on Park Transform

Chen, Ya,Ji, Tianyao,Li, Mengshi,Wu, Qinghua,Wang, Xuejian The Korean Institute of Electrical Engineers 2016 Journal of Electrical Engineering & Technology Vol.11 No.3

This paper presents a novel method for power system harmonic estimation based on the Park transform. The proposed method firstly extends the signal to a group of three-phase signals in a-b-c coordinate. Then, a linear fitting based method is adopted to estimate the fundamental frequency. Afterwards, the Park transform is utilized to convert the three-phase signals from a-b-c coordinate to d-q-0 coordinate. Finally, the amplitude and phase of a harmonic component of interest can be calculated using the d-axis and q-axis components obtained. Simulation studies have been conducted using matrix laboratory (MATLAB) and power system computer aided design/electromagnetic transients including direct current (PSCAD/EMTDC). Simulation studies in MATLAB have considered three scenarios, i.e., no-frequency-deviation scenario, frequency-deviation scenario and the scenario in the presence of inter-harminics. The results have demonstrated that the proposed method achieves very high accuracy in frequency, phase and amplitude estimation under noisy conditions, and suffers little influence of the inter-harmonics. Moreover, comparison studies have proved that the proposed method is superior to FFT and Interpolated FFT with the Hanning Window (IpFFTHW). Finally, a popular case in PSCAD/EMTDC has been employed to further verify the effectiveness of the proposed method.