孤岛文献

A Robust Anti-islanding Method for Grid-Connected Photovoltaic InverterByunggyu Yu1), Youngseok Jung1),Junghun So1),Hyemi Hwang1), Gwonjong Yu1)1)Korea Institute of Energy Research,71-2 Jang-Dong, Yuseong-Gu, Daejeon, South KoreaABSTRACTSIn a modern power system, Photovoltaic as distributed generated source is growing larger and it can cause a variety of problem. Most issued problem is an islanding phenomenon. In order to prevent islanding phenomenon, three kinds of active islanding detection methods have been studied. These are respectively to change magni-tude, frequency, and the start phase of inverter output current. Among them, both frequency and start phase variation anti-methods make the islanding frequency drift away from the trip window of the frequency relay if an islanding is occurred. This paper presents a robust anti-islanding method, which are consisted of a frequency variation method as AFD(Active Frequency Drift)method and a start phase variation method as SMS(Slip Mode frequency Shift) method.Clearly, the proposed anti-islanding method shows the satisfied islanding detection ability to I EEE 1547 Standard.To validate the perform-ance of the proposed method, simulation and experiment are performed. Possible islanding conditions are followed by the IEEE Standard 1547. The methodology presented in the paper can be extended to the other active anti-islanding methods.Keywords:Islanding detection methods; Grid-connected PV inverter; Slip Mode frequency Shift; Active Frequency Drift1. INTRODUCTIONIslanding phenomenon of grid connected PV inverters refers to their independent powering to a portion of the utility system even though the portion has been discon-nected from the remainder of the utility source. Recently the concern about this islanding operation has been raised seriously by the spread of the distributed resources because the islanding can cause safety problems to utility service personnel or related equipments. Consequently utility companies and PV system owners require that the grid-connected PV system include the non-islanding in-verter[1-2].To prevent this phenomenon, various anti-islanding methods have been studied, which are classified into passive and active methods. When an inverter is equipped with an over voltage relay (OVR), an under volt-age relay (UVR), an over frequency relay (OFR), and an under frequency relay (UFR), it is considered that the inverter has the basic passive anti-islanding methods. However, these passive schemes have relatively large non-detection zone of an islanding because it only moni-tors the voltage magnitude or frequency of the point of common coupling (PCC) at the PV inverter output.Unlike these passive anti-islanding methods, active anti-islanding schemes make a perturbation into the PV inverter output current by injecting an active signal. Due to the perturbation, the power balance between the PV generated power and local load power can be broken. As a result, the active anti-islanding methods(AIM)are gen-erally considered more effective than the passive ones for islanding detection[3]. On the other hand, power quality and output power generation for AIM can be impaired by the perturbation because it can change the magnitude or frequency of the output current. Therefore, it is necessary to design PV inverter to satisfy the standards for the power quality and the islanding detection capability.Active anti-islanding methods are classified into three parts with respect to what the variation parameter is. As shown in Equation (1), these are respectively to change magnitude(I m), frequency(f), and the start phase(θ)of inverter output current.)2sin(qp+=ftIIminv(1)On one hand, the magnitude variation of inverter out-put current can cause generally a change of output volt-age magnitude after islanding occurs, which makes OVR or UVR to detect an islanding. On the other hand, both frequency(f)and a start phase(θ) variation anti-methods make the islanding frequency of inverter output voltage drift away from the trip window of the frequency relay if an islanding is occurred.As a typical frequency variation method, there is the active frequency drift(AFD) method as shown in Fig.1which makes the inverter output cur-rent to drift up or down with a parameter chopping fraction (cf)in Equation (2).In addition, well-known typical start phase variation method is slip-mode frequency-shift(SMS) method as shown in Fig.2 which makes the startphase(θ) of output current be a function of output voltage frequency.2/utilzTTcf=(2)).(Vfreqfucntion=q(3)Fig. 1 Inverter output waveform using AFD method: (a)frequency drift up, (b) frequency drift downFig. 2 Inverter output waveform using SMS method: (a) current leading phase, (b) current lagging phase Even though each variation parameter in Equation (1) can be controlled independently, the research on the combined active anti-islanding method has been little shown until now.Therefore this paper presents a combined active anti-islanding method, which is consisted of AFD method and SMS method.This proposed method will give more flexi-ble design choice for some cases by injecting signal of both frequency and phase. In the paper, operational prin-ciple of the proposed method is discussed first. Then, the performance characteristics and the practical implementa-tion will be introduced. Finally, experimental results are provided for the verification.2. Operational PrincipleBoth frequency and current start phase variation anti-islanding method are generally implemented by digitally controlled phase-locked-loop (PLL). Except for a start phase variation method, the other anti-islanding methods including AFD are usually designed to output current in phase with grid voltage for a unity power factor.When the utility is disconnected, phase difference be-tween output voltage and current are determined by the load phase angle in Equation (4).11tan 22Load R fC fL q p p -éùæö=--êúç÷èøëû(4)where: θLoadis the phase angle of the local loadFig. 3 Operating Point of OFR (Over Frequency Relay),AFD, SMS, and the Combined Method between AFD and SMSIf the inverted load line with respect to frequency is depicted like Fig. 3, the steady state operating point could be determined through PLL operation in both AFD and SMS method.For an AFD, the fundamental component of inverter output current is phase lead or phase lag by θAFD(0.5*2πf*T z =0.5π*cf)when the grid is connected[2]. After islanding, the fundamental component of the current will be in phase with voltage after all for a steady state. The steady state equation is shown as Equation (5)when islanding occurs.In SMS method, the start phase is controlled to be a function of frequency of output voltage. After islanding, the steady state equation is shown as Equation (6).For the combined method, the steady state equation is shown as equation (7).In sum, the operating point of each anti-islanding method are determined by Equation (5)~(7) as shown Fig. 3.0Load AFD q q +=(5)0Load SMS q q +=(6)0Load AFD SMS q q q ++=(7)In Fig. 3, the combined method has more fast detec-tion time due to large phase error during transient timeand less non-detection zone (NDZ) of islanding due to smaller load phase angle. In the figure, the right hand side operation from nominal line frequency is only shown for the combined anti-islanding method. Of course, the left hand side operation is also the same with the right one by changing the sign of chopping fraction.3. Practical ImplementationFig.4. shows the computational procedure. In the flow char, after the data sampling of output voltage and current, the line frequency can be calculated and then determined which mode can be applied. If the sensed line frequency is larger than the nominal frequency, the chopping fraction is positive and the start phase is also positive. That means the islanding frequency will be drift up to reach upper limit of frequency relay soon with both deviation source rather than single one. On the other hand, if thesensed line frequency is smaller than the nominal fre-quency, the islanding frequency will be also drift down toreach lower limit of frequency.Fig. 4 Flow Chart of the combined islanding detectionalgorithmFig. 5 Voltage Phase and Current Phase of AFD, SMS, and combined methodThe voltage phase and current phase in AFD, SMS, and combined method are depicted in Fig. 5.The com-bined phase waveform could have both larger displace-ment power factor (dpf) and smaller harmonic compo-nents than the single anti-islanding method. In other words, the absolute value of these factors can be smaller for the same islanding detection ability than the single one, which means the combined method could provide more flexibility in practical design case. Furthermore, combined method could make the non-detection zone (NDZ) of each method be smaller than the single one. As an example, NDZ of SMS could be happened when the slope of SMS line at the nominal line frequency is lower than that of the inverted load line. By combining AFD method, the ampli-tude of inverted load line is smaller than before, and thenthis makes effect NDZ of SMS to be small.4. Simulated and Experimental ResultsA 3-kW PV inverter is constructed for simulated and ex-perimental results. The circuit diagram is shown in Fig. 6. The power stage consists of a boost converter, followed by a voltage-source inverter (VSI). A TMS320VC33 Digi-tal Signal Processor is used as the main controller. The boost converter provides maximum power point tracking (MPPT) function and the voltage source inverter make a stable dc link voltage from an unregulated output voltage of solar array. The PV inverter is loaded by a paralleled-tunable RLC circuit. As shown Table 1, the load is de-signed to have a resonant frequency of 60Hz by IEEE Std. 1547 with aunity quality factor.Fig. 6 Simulated and Experimental Circuit Diagram For a clear verification of operation, in the simulation the chosen chopping fraction is 5%,θM is 10degree, and f M is 63 Hz in Equation (8). The simulated results are shown Fig. 7.In practical operation, the sensed line fre-quency has always an error from measurement inaccu-racy. Therefore the chopping fraction and the θSMSchange their sign continuously shown as Fig. 7.Fig. 7 Simulated Results for a operation verification.Table I:Basic Islanding Test Condition of IEEE Std1547([1]60)[]sin 260Line SMS M M f k k f p q q éù--=êú-ëû(8)where: θSMS [k] is the starting phase of output currentf Line,[K-1]is the measured frequency of the previousvoltage cycleIn the experiments, the chopping fraction are chosen as 2%, and the θM is 10deg, and f M is 63 Hz for perform-ance of islanding detection. Using the combined method, a good power quality including total harmonic distortion (THD), and power factor is shown in Fig. 9. good islanding detection ability are shown in Fig. 10 when reactive and effective power flow to the grid are around zero. In sum, the combined method could provide more flexibility todesign islanding detection method.Fig. 8 Inverter Output voltage(100V/div) and Cur-rent(10A/div) with respect to line frequency when the grid is connectedrespect to the Line FrequencyFig. 9 Islanding test results as around zero reactive power flow to grid (CH1: Grid voltage, CH2: Inverter Out-put Voltage, CH3: Inverter Output Current, CH4 : Grid Current, CH5: Solar Voltage, CH6: Solar Current, CH7: Frequency of Inverter Output Voltage)5. ConclusionsThis paper presents a robust anti-islanding method, which are consisted of a frequency variation method as AFD (Active Frequency Drift)method and a start phase variation method as SMS (Slip Mode frequency Shift) method.This proposed method will give more flexible design choice for some cases by injecting signal of fre-quency and start phase rather than single signal injection. For better performance, the propose method are classi-fied into two modes with respect to the magnitude of line frequency. Clearly, the proposed anti-islanding method shows the satisfied islanding detection ability at the test condition which IEEE standard 1547 suggested.The methodology presented in the paper can be ex-tended to the other active anti-islanding methods.REFERENCES[1] EEE 1547 Standard for I nterconnecting Distributed Resources with Electric Power Systems, June 2003.[2]M. Ropp, “design issues for grid-connected photo-voltaic systems,” Ph.D. dissertation, Georgia Institute of Technology, Atlanta, GA, December 1998.[3] Jun Yin,Liuchen Chang,and Diduch, C., “Recent de-velopments in islanding detection for distributed power generation,” 2004 Large Engineering systems Conference of Power Engineering, pp. 124-128, July 2004.[4] Zhihong Ye, Amol Kolwalkar, Yu Zhang, Pengwei Du, Reigh Walling “Evaluation of Anti-Islanding Schemes Based on Nondetection Zone Concept” IEEE Transac-tions on Power Electronics, VOL. 19, NO. 5, September 2004-12-01[5] G.A. Smith, P.A. Onions, D.G. Infield “ Predicting islanding operation of grid connected PV inverters” IEE Proc.-Electr. Power Appl. , Vol. 147, No, 1, January 2000[6] G.K. Hung, C.C. Chang, C.L. Chen “Automatic Phase-Shift Method for Islanding Detection of Grid-Connected Photovoltaic Inverters” IEEE Transactions on Energy Conversion, VOL. 18, NO. 1, March 2003[7] M. Ropp, M. Begoviv, and A. Rohatgi, “Analysis and performance assessment of the active frequency drift method of islanding prevention”, IEEE Trans. Energy Conversion, vol. 14, pp. 810-816, Sept. 1999[8] M. Ropp, M. Begovic and A. Rohatgi, “Prevention of Islanding in Grid-connected Photovoltaic Systems”, Pro-gress in Photovoltaics. Res. Appl. 7, 39-59(1999)。