Islanding detection for Low Power DPGS Marco Liserre

Islanding detection for Low Power DPGS Marco Liserre liserre@poliba. it Marco Liserre liserre@ieee. org

Islanding detection for Low Power DPGS Agenda ü International regulations ü What in the earth is anti-islanding? ü Anti-islanding requirements ü Anti-islanding methods (passive/active) ü Reference papers Marco Liserre liserre@ieee. org

Islanding detection for Low Power DPGS International regulations § In Europe the standard IEC 61727, in USA the recommendations IEEE 929 and IEEE 1574 (DG<10 MVA), in Germany the standard VDE 0126 -1 -1 §When the utility voltage is outside certain limits (typically around ± 15 %) the inverter should cease to energize within 0. 2 s to 2 s (depending on the standards). More sever under/over voltages lead to shorter intervention times §When the utility frequency is outside certain limits (typically around ± 1 %) the inverter should cease to energize the utility line within 0. 2 seconds. § Apart from voltage and frequency requirements these standards are imposing requirements also on: § harmonics (typically max 5% current THD) § reactive power (typically min 0. 9 power factor) § dc current injection (especially for tranformerless structures) § earth current § anti-islanding Marco Liserre liserre@ieee. org

Islanding detection for Low Power DPGS International regulations Conditions for reconnection after trip Marco Liserre liserre@ieee. org

Islanding detection for Low Power DPGS Understanding anti-islanding ü Islanding for a distributed power generation system (DPGS) is defined when the DPGS does not cease to operate when the grid is disconnected, i. e. is continuing to operate with local load ü Desired islanding is for example in power systems when a section containing DPGS and load is running without connection to the grid ü Undesired islanding is when it can pose danger for grid maintenance people ü In PV, WT and other distributed power generation systems (DPGS) there anti -islanding features required by standards Marco Liserre liserre@ieee. org

Islanding detection for Low Power DPGS Understanding anti-islanding ü DG grid-connected islanding may occur as a result of the following conditions: ü a fault that is detected by the grid but which is not detected by the PV inverter or protection devices; ü accidental disconnection of the normal grid supply by equipment failure; ü intentional disconnection of the line for servicing; ü human error or malicious mischief; ü an act of nature. Marco Liserre liserre@ieee. org

Islanding detection for Low Power DPGS Rationale for anti-islanding requirements ü The utility cannot control voltage and frequency in the island, creating the possibility of damage to customer equipment ü Utilities, along with the PV distributed resource owner, can be found liable for electrical damage to customer equipment connected to their lines that results from voltage/frequency excursions outside of the acceptable ranges ü Islanding may create a hazard for utility lineworkers by causing a line to remain energized that may be assumed to be disconnected from all energy sources Marco Liserre liserre@ieee. org

Islanding detection for Low Power DPGS Rationale for anti-islanding requirements ü Reclosing into an island may result in retripping the line or damaging the distributed resource equipment, or other connected equipment, because of out-of-phase closure ü Islanding may interfere with the manual or automatic restoration of normal service by the utility ü Anti-islanding requirements can be defined in terms of detecting the island condition (IEEE, IEC) or impedance (VDE Germany) ü Even if standards and recommendation will change allowing island operation, the detection of this condition will remain crucial for a safe operation Marco Liserre liserre@ieee. org

Islanding detection for Low Power DPGS IEEE 1547/IEEE 929/ IEC 61727 ü The inverter should cease to energize the utility in 10 cycles (200 ms for 50 Hz) having an RLC local load with Q = 2. 5 (Q = 2. 5 P). For the new IEEE I 547 the condition is Q=1! ü IEEE standard 1547 -2005/929 -2000 (detection time 2 seconds) ü Marco Liserre Balance of the power in the system: liserre@ieee. org

Islanding detection for Low Power DPGS VDE 0126 -1 -1 (ENS) requirements for PV Having a perfect balanced local loading (Igrid=0) the PV must isolate from the grid within 5 seconds at a change of impedance of 1 ohms resistive. The test should be repeated for different grid impedance angles The automatic disconnecting facility consists in two independent mains monitoring devices (redundance) As an option the requirements for IEEE 1547 can ensure compliance Marco Liserre liserre@ieee. org

Islanding detection for Low Power DPGS Anti-islanding methods overview Marco Liserre liserre@ieee. org

Islanding detection for Low Power DPGS Under/Over Voltage and Under/Over Frequency - Description: Inverter operation is only allowed within a selected amplitude/frequency window. If the amplitude or frequency of the PCC voltage leaves the window, the PV system is disconnected from the utility. - Strengths: low cost, equivalent to utility protection, is used in conjunction with other anti-islanding methods - Weaknesses: Large NDZ, slow reaction times - NDZ: dependent on impedances, power ratings, operating point Marco Liserre liserre@ieee. org

Islanding detection for Low Power DPGS Under/Over Voltage and Under/Over Frequency ü Islanding voltage: ü Islanding pulsation: Marco Liserre liserre@ieee. org

Islanding detection for Low Power DPGS The Non Detection Zone (NDZ) ü The NDZ depends on the characteristics of the distributed generation (DG) system ü Grid disconnection: * high ∆P ∆Q => voltage amplitude or/and frequency variations * small ∆P ∆Q => NDZ ü The probability that ∆P and ∆Q fall into the NDZ of OUV/OUF can be significant Marco Liserre liserre@ieee. org

Islanding detection for Low Power DPGS Detection of voltage harmonics - Description: detect the variation of THD in the PCC voltage that can occur in islanding mode due to switching harmonics – Strengths: should not be dependent on power factor, good for multiinverter applications – Weaknesses: not always possible to select a threshold that guarantees non-islanding without causing excessive false trips. Also some inductive load could attenuate the switching harmonics below the threshold. – NDZ: difficult to evaluate! Marco Liserre liserre@ieee. org

Islanding detection for Low Power DPGS Detection of voltage harmonics mismatch ü It is based on a modified sensorless single phase controller to estimate the grid voltage harmonic distortion ü A Kalman filter allows to establish the energy mismatch between the estimated and the measured harmonics ü The proposed method belongs to the family of passive detection methods but it allows to minimize the Non Detection Zone (NDZ) Marco Liserre liserre@ieee. org

Islanding detection for Low Power DPGS Detection of power harmonics change ü A time-frequency detection algorithm based on monitoring the DPGS output power considering the influence of the PWM, the output LCL filter and of the employed current controller. ü Wavelet analysis is applied to obtain time localization of the islanding condition. Marco Liserre liserre@ieee. org

Islanding detection for Low Power DPGS Voltage phase jump detection - Description: monitor the phase difference between the inverter and the utility for a sudden jump – Strengths: easy to implement, does not affect the output power quality or system transient response – Weaknesses: difficult to choose thresholds that detect islanding without false trips – NDZ: unity power factor loads produce no phase error! Marco Liserre liserre@ieee. org

Islanding detection for Low Power DPGS Passive methods experimental comparison Marco Liserre liserre@ieee. org

Islanding detection for Low Power DPGS OUV/OUF method Grid Voltage variation with ∆P=25% Grid frequency variation with ∆Q=-15% Marco Liserre liserre@ieee. org

Islanding detection for Low Power DPGS Voltage harmonic monitoring method Grid Voltage harmonic distortion variation with ∆P=0 and ∆Q=0 Marco Liserre liserre@ieee. org

Islanding detection for Low Power DPGS Phase monitoring method Voltage phase change with ∆Q=0 Voltage phase variation with ∆Q=-15% Marco Liserre liserre@ieee. org

Islanding detection for Low Power DPGS Harmonic based methods Grid-disconnection transient in case of harmonically distorted grid voltage. Channel 1 is the measured grid voltage (400 V/div). Channel 3 is the PCC voltage (400 V/div). Channel A shows the difference between channel 1 and 3 (400 V/div). Channel 4 is the inverter output current (5 A/div). Channel 2 shows voltage across the switch which disconnects the isolation transformer. Marco Liserre Test to verify immunity of the method (no false trip) to frequency variation liserre@ieee. org

Islanding detection for Low Power DPGS Passive methods comparison Method Trip time OUV -17 %≤ΔP≤ 24 % Not applicable OUF -5%≤ΔQ≤ 5% Not applicable Phase monitoring -5%≤ΔQ≤ 5% Not applicable Harmonic monitoring Marco Liserre NDZ Absent 0, 1 – 0, 2 s liserre@ieee. org

Islanding detection for Low Power DPGS Positive feedback methods RLC load characteristics: Two mechanisms Voltage feedback – when inverter output voltage is increasing, the feedback will command the inverter output power to increase. In order to balance the power, the voltage will keep increase. As a result the voltage will be eventually out of nominal ranges so the islanding can be detected! Similar but opposite destabilization occurs when the sensed voltage is decreasing Frequency feedback – when the inverter-sensed frequency is increasing, the feedback will command the inverter reactive-power output to increase. Due to load characteristic, the frequency will keep increasing in order to balance the reactive power. The increased frequency will further drive the inverter reactive power up, and as result, the frequency will increase more. Eventually the frequency will be driven out of range and the inverter will trip. Similar results occurs when the frequency is initially decreasing. Marco Liserre liserre@ieee. org

Islanding detection for Low Power DPGS Positive feedback methods It is possible to vary the PV inverter active power to carry the amplitude and the frequency of voltage out from the normal operation range if the grid is disconnected ü ü Positive feedback ü Marco Liserre Frequency and voltage are measured The control of the speed is made by kv and kf liserre@ieee. org

Islanding detection for Low Power DPGS Positive feedback methods Marco Liserre liserre@ieee. org

Islanding detection for Low Power DPGS Frequency positive feedback implementation Concept DQ implementation Marco Liserre liserre@ieee. org

Islanding detection for Low Power DPGS Frequency positive feedback DQ pll using Inverse Park Transformation can be used in single-phase systems to create the dq components! BPF 1 -10 Hz needed to remove noise and dc-offset! Gain should be high enough to make the system unstable in islanding but stable in non-islanding mode For Voltage positive feedback, Vd (amplitude og Vgrid) is feedforwarded to id_ref Marco Liserre liserre@ieee. org

Islanding detection for Low Power DPGS Impedance measurement - external device Classical solution - using external device that produces a transient For ex. A capacitor is switched in and the grid voltage zero-crossing shift is measured. Thus the grid impedance is estimated The disadvantage is that it requires extra hardware! Marco Liserre liserre@ieee. org

Islanding detection for Low Power DPGS Impedance measurement - harmonic injection ü Special case of the harmonic monitoring method ü Detection of islanding using only a monitoring PLL synchronized with the specific harmonic This method results in a measure of impedance at a specific frequency at inverter terminals: Marco Liserre liserre@ieee. org

Islanding detection for Low Power DPGS Impedance measurement - harmonic injection A non-characteristic harmonic current is injected in the grid by through the PWM. The resulting harmonic voltage will depend on the grid impedance. The voltage and current at this frequency are computed using DFT. The impedance is calculated by dividing the voltage to the current. The real part is Rg and the imaginary part is Lg Marco Liserre liserre@ieee. org

Islanding detection for Low Power DPGS Simulation results Impedance measurement - harmonic injection 75 Hz Comparison of spectra with and without harmonic injection. a) Grid voltage. b) Grid current. The PV power is 3 k. W. A current transient in the current is introduced together with the injection resulting in impedance estimation error! Marco Liserre liserre@ieee. org

Islanding detection for Low Power DPGS Impedance measurement – LCL-resonance ü It is based on the controlled excitation of the LCL-filter resonance, for example increasing the current control proportional gain the following systems resonate 0 m. H 0. 06 0. 04 0. 02 0 0 40 60 80 0 1. 5 m. H 20 20 40 60 80 100 0. 04 0. 03 0. 02 0. 01 0 Marco Liserre 100 liserre@ieee. org

Islanding detection for Low Power DPGS Active frequency drift - Description: output waveform is slightly distorted so islanding causes a drift in frequency – Strengths: very easy to implement with microprocessor based inverters – Weaknesses: small degradation in output power quality – NDZ: relatively large relative to other active methods, depends on the value of the chopping fraction used, small (<1% then same as SMS), larger causes NDZ to shift toward capacitive. Ipv goes to zero before or after the PCC voltage. Marco Liserre liserre@ieee. org

Islanding detection for Low Power DPGS Slip-mode Phase Shift (SMS) - Description: the current-voltage phase is changed as function of frequency so islanding causes a drift in frequency – Strengths: very easy to implement with microprocessor based inverters, works with multi-inverter applications – Weaknesses: some RLC loads (high-Q loads with resonant frequencies very near the line frequency) have phase response curves such that the phase of the load increases faster than the phase of the PV inverter – NDZ: relatively small. Marco Liserre liserre@ieee. org

Islanding detection for Low Power DPGS Active methods experimental comparison Marco Liserre liserre@ieee. org

Islanding detection for Low Power DPGS Active and reactive power variation method Marco Liserre liserre@ieee. org

Islanding detection for Low Power DPGS Experimental results Test setup 3 k. W IGBT full-bridge inverter, TMS 430 F 24 xx DSP controller (40 ms computation time) LCL filter resonance freq = 4. 25 k. Hz, Ud = 370 V, Ug = 230 V rms, Kp=2, Ki=300, fs=8. 5 k. Hz Marco Liserre liserre@ieee. org

Islanding detection for Low Power DPGS Experimental results 3 k. W PV inverter Grid impedance value (1 V eqv. 1 W) Non-continuous injection of the harmonic into the grid. Impedance estimation when the PV current is changing from P=0. 3 k. W to P=3 k. W. The result is filtered every 1 s Non-continuous injection of the harmonic. Estimation is checked for different power levels Marco Liserre liserre@ieee. org

Islanding detection for Low Power DPGS Experimental results 3 k. W PV inverter Detection of both resistive and inductive impedance jump! Grid impedance value (1 V eqv. 1 W) Grid impedance estimation after 0. 25 W increase due Detection time to Ltest Initial grid impedance estimation value of 1. 2 W Voltage over Ltest Dynamic response of the method detecting 0. 5 W a 0. 8 m. H inductive increase of the grid impedance. resistive increase. Marco Liserre liserre@ieee. org

Islanding detection for Low Power DPGS Active methods comparison Method NDZ Trip time Current harmonic injection Absent 0, 1 – 0, 2 s in function of set threshold Active power variation Absent 0, 45 s 0, 3 s kv=5 kv=10 Reactive power variation Absent 0, 75 s 0, 65 s kf=50 kf=60 Capacitor insertion Absent Marco Liserre 0, 3 s with Qc=100 VAr 0, 4 s with Qc=50 VAr liserre@ieee. org

Islanding detection for Low Power DPGS Islanding detection methods at the grid level: impedance connection method ü Marco Liserre A low-value impedance, usually a capacitor bank is installed on the grid system inside the potential island liserre@ieee. org

Islanding detection for Low Power DPGS Methods based on the communication between the grid and photovoltaic inverter ü A transmitter (T) is installed near the line protection switch and a receiver (R) is positioned in the PCC in proximity of the inverter. ü There are two kind of communication: ü PLCC (power line carrier communication) ü SPD (Signal produced by disconnected) Marco Liserre liserre@ieee. org

Islanding detection for Low Power DPGS Conclusions ü The anti-islanding requirement prevents islanding for safety reasons and are very different across the world. ü Software Anti-islanding methods are possible and the algorithm can be easily implemented in the ”existing” DSP control platform and is not requiring extra hardware ü Especially in Germany as required by VDE 0126 leaded to complex and expensive solutions. The new VDE 0126 -1 -1 bringsd good news. ü In US UL 1471 and IEE 1547 (2005) have been consolidated ü In Germany VDE 0126 -1 -1 (2006) has ”softened” the ENS requirements by increasing the detectable griud impedance detection threshold from 0. 5 ohms to 1 ohms and by introducing the option of a requirement similar to IEEE 1547. Redundancy requirements still remians! Marco Liserre liserre@ieee. org

Islanding detection for Low Power DPGS Conclusions ü IEC 61727 tries to get worldwide ”unified” standard (based on IEEE 1547). It has been recently approved and will be published soon (exp. 2008). ü Thus the trend is to use active anti-islanding methods based on drifting the frequency/voltage in combination with passive methods to obtain minimum NDZ. ü The harmonic injection was used a lot for the old ENS but now is possible to comply with “softer” methods like AFD ü Islaning detection will be useful even if anti-islanding wil not be required or standards will change ü Islanding detection helps in managing connection/disconnection to the main grid in a soft way Marco Liserre liserre@ieee. org

Islanding detection for Low Power DPGS References [1] Dugan, R. C. ; Key, T. S. ; Ball, G. J. , "Distributed resources standards, " Industry Applications Magazine, IEEE , vol. 12, no. 1, pp. 27 -34, Jan. -Feb. 2006 [2] IEEE Std 929 -2000 – “IEEE Recommended Practice for Utility Interface of Photovoltaic (PV) Systems, ", ISBN 0 -7381 -1934 -2 SH 94811, April 2000. [3] UL standard 1741, “Inverters, Converters, and controllers for Use in Independent Power Systems”, Underwriters Laboratories Inc. US, 2001 [4] IEEE Std 1547 -2003 – “Standard for Interconnecting Distributed Resources with Electric Power Systems, " ISBN 0 -7381 -3720 -0 SH 95144, IEEE, June 2003 [5] IEEE Std 1547. 1 -2005 – “Standard Conformance Test Procedures for Equipment Interconnecting Distributed Resources with Electric Power Systems” ISBN 0 -7381 -4736 -2 SH 95346, IEEE, July 2005 [6] IEC 61727 Ed. 2 – “Photovoltaic (PV) Systems - Characteristics of the Utility Interface”, December, 2004 [7] IEC 62116 CDV Ed. 1 – “Test procedure of islanding prevention measures for utilityinterconnected photovoltaic inverters”, IEC 82/402/CD: 2005 [8] VDE V 0126 -1 -1 “Automatic disconnection device between a generator and the public lowvoltage grid”, VDE Verlag, Doc nr. 0126003, 2006 Marco Liserre liserre@ieee. org

Islanding detection for Low Power DPGS References [9] IEC 61000 -3 -2, Ed. 3. 0 – “Electromagnetic compatibility (EMC) –Part 3 -2: Limits –Limits for harmonic current emissions (equipment input current ≤ 16 A per phase)”, ISBN 2 -8318 -8353 -9, November 2005 [10] EN 61000 -3 -3, Ed. 1. 2 —“Electromagnetic compatibility (EMC) –Part 3 -3: Limits – Limitation of voltage changes, voltage fluctuations and flicker in public low-voltage supply systems, for equipment with rated current ≤ 16 A per phase and not subject to conditional connection”, ISBN 2 -8318 -8209 -5, November 2005 [11] Standard EN 50160 – “Voltage Characteristics of Public Distribution System”, CENELEC: European Committee for Electrotechnical Standardization, Brussels, Belgium, November 1999. [12] IEC 61000 -3 -12, Ed. 1 – “Electromagnetic compatibility (EMC) –Part 3 -12: Limits – Limits for harmonic currents produced by equipment connected to public low-voltage systems with input current >16 A and ≤ 75 A per phase” , November 2004 [13] IEC 61000 -3 -11, Ed. 1 —“ Electromagnetic compatibility (EMC) – Part 3 -11: Limits – Limitation of voltage changes, voltage fluctuations and flicker in public low-voltage supply systems – Equipment with rated current ≤ 75 A and subject to conditional connection” , August 2000. Marco Liserre liserre@ieee. org

Islanding detection for Low Power DPGS References [14] F. De Mango, M. Liserre, A. Dell’Aquila and A. Pigazo “Overview of anti-islanding algorithms for PV systems. Part I: passive methods ” EPE-PEMC 2006. [15] Blaabjerg, F. ; Teodorescu, R. ; Liserre, M. ; Timbus, A. V. , "Overview of Control and Grid Synchronization for Distributed Power Generation Systems, " Industrial Electronics, IEEE Transactions on , vol. 53, no. 5, pp. 1398 -1409, Oct. 2006 [16] Rodriguez, P. ; Luna, A. ; Ciobotaru, M. ; Teodorescu, R. ; Blaabjerg, F. , "Advanced Grid Synchronization System for Power Converters under Unbalanced and Distorted Operating Conditions, " IEEE Industrial Electronics, IECON 2006 - 32 nd Annual Conference on , vol. , no. , pp. 5173 -5178, Nov. 2006 [17] Liserre, M. ; Pigazo, A. ; Dell'Aquila, A. ; Moreno, V. M. , "An Anti-Islanding Method for Single-Phase Inverters Based on a Grid Voltage Sensorless Control, " Industrial Electronics, IEEE Transactions on , vol. 53, no. 5, pp. 1418 -1426, Oct. 2006 [18] W. Bower, M. Ropp- Evaluation of Islanding Detection Methods for Utility-Interactive Inverters in Photovoltaic Systems, SAND 2002 -3591 Nov. 2002 [19] Evaluation of Islanding Detection Methods for Photovoltaic Utility Interactive Power Systems - Task V, Report IEA-PVPS T 5 -09: 2002 March 2002 [20] F. De Mango, M. Liserre, A. Dell’Aquila, “Overview of anti-islanding algorithms for PV systems. Part II: active methods ” EPE-PEMC 2006. Marco Liserre liserre@ieee. org

Islanding detection for Low Power DPGS References [21] Ropp, M. E. ; Begovic, M. ; Rohatgi, A. , "Analysis and performance assessment of the active frequency drift method of islanding prevention, " Energy Conversion, IEEE Transaction on , vol. 14, no. 3, pp. 810 -816, Sep 1999 [22] Lopes, L. A. C. ; Huili Sun, "Performance assessment of active frequency drifting islanding detection methods, " Energy Conversion, IEEE Transaction on , vol. 21, no. 1, pp. 171 -180, March 2006 [23] Stevens, J. , Bonn, R. , Ginn, J. , Gonzalez, S. , Kern, G. , Development and Testing of an Approach to Anti-Islanding in Utility-Interconnected Photovoltaic Systems, Sandia National Laboratories report SAND 2000 -1939, Albuquerque, NM, Aug 2000. [24] Ye, Z. ; Walling, R. ; Garces, L. ; Zhou, R. ; Li, L. ; Wang, T. "Study and Development of Anti-Islanding Control for Grid-Connected Inverters" NREL/SR-560 -36243. Golden, CO: National Renewable Energy Laboratory. May 2004 [25] Koizumi, H. ; Mizuno, T. ; Kaito, T. ; Noda, Y. ; Goshima, N. ; Kawasaki, M. ; Nagasaka, K. ; Kurokawa, K. , "A Novel Microcontroller for Grid-Connected Photovoltaic Systems, " Industrial Electronics, IEEE Transactions on , vol. 53, no. 6, pp. 1889 -1897, Dec. 2006 [26] Istvan Varjasi; Attila Balogh; Sandor Halasz, "Sensorless Control of a Grid-Connected PV Converter, " Power Electronics and Motion Control Conference, 2006. EPE-PEMC 2006. 12 th International , vol. , no. , pp. 901 -906, Aug. 2006 Marco Liserre liserre@ieee. org

Islanding detection for Low Power DPGS References [27] G. Hernandez-Gonzalez; R. Iravani, "Current injection for active islanding detection of electronically-interfaced distributed resources, " Power Delivery, IEEE Transactions on , vol. 21, no. 3, pp. 1698 -1705, July 2006 [28] Jin Beom Jeong; Hee Jun Kim; Kang Soon Ahn; Chan Ho Kang, "A Novel Method for Anti- Islanding using Reactive Power, " Telecommunications Conference, 2005. INTELEC '05. Twenty. Seventh International , vol. , no. , pp. 101 -106, Sept. 2005 [29] Gyu-Ha Choe; Hong-Sung Kim; Han-Goo Kim; Young-Ho Choi; Jae-Chul Kim, "The Characteristic Analysis of Grid Frequency Variation under Islanding Mode for Utility Interactive PV System with Reactive Power Variation Scheme for Anti-Islanding, " Power Electronics Specialists Conference, 2006. PESC '06. 37 th IEEE , vol. , no. , pp. 1 -5, 18 -22 June 2006 [30] United States Patent, “Method and apparatus for measuring the impedance of an electrical energy supply system”, Patent No. US 6, 933, 714 B 2, Aug. 23, 2005 [31] Ciobotaru, M, Teodorescu, R. , Blaabjerg, F. , “On-line grid impedance estimation based on harmonic injection for grid-connected PV inverters” – Proceedings of ISIE 2007 International Symposium on Industrial Electronics, pp-2437 – 2442 [32] Bertling, F. ; Soter, S. , "A novel converter integrable impedance measuring method for islanding detection in grids with widespread use of decentral generation, " Power Electronics, Electrical Drives, Automation and Motion, 2006. SPEEDAM 2006. International Symposium on , vol. , no. , pp. 503 -507, May, 23 rd - 26 th, 2006 Marco Liserre liserre@ieee. org

Islanding detection for Low Power DPGS References [33] Liserre, M. ; Blaabjerg, F. ; Teodorescu, R. , "Grid Impedance Estimation via Excitation of LCL -Filter Resonance, " Industry Applications, IEEE Transactions on , vol. 43, no. 5, pp. 14011407, Sept. -oct. 2007 [34] Ciobotaru, Mihai; Teodorescu, Remus; Rodriguez, Pedro; Timbus, Adrian; Blaabjerg, Frede, "Online grid impedance estimation for single-phase grid-connected systems using PQ variations, " Power Electronics Specialists Conference, 2007. PESC 2007. IEEE , vol. , no. , pp. 2306 -2312, 17 -21 June 2007 [35] Timbus, A. V. ; Teodorescu, R. ; Rodriguez, P. , “Grid Impedance Identification Based on Active Power Variations and Grid Voltage Control” – Proceedings of IAS 2007, IEEE Industry Application Annual Meeting, 2007 [36] Ropp, M. ; Larson, D. ; Meendering, S. ; Mcmahon, D. ; Ginn, J. ; Stevens, J. ; Bower, W. ; Gonzalez, S. ; Fennell, K. ; Brusseau, L. , "Discussion of a Power Line Carrier Communications. Based Anti-Islanding Scheme using a Commercial Automatic Meter Reading System, " Photovoltaic Energy Conversion, Conference Record of the 2006 IEEE 4 th World Conference on , vol. 2, no. , pp. 2351 -2354, May 2006. [37] A. Pigazo, V. Moreno, M. Liserre, R. Mastromauro, A. Dell’Aquila, “Wavelet-based Islanding Detection in Grid-Connected PV Systems” to be published in the Special Section on Photovoltaic Power Processing Systems IEEE Transactions on Industrial Electronics. Marco Liserre liserre@ieee. org