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Grid synchronization for power converters Marco Liserre Grid synchronization for power converters Marco Liserre

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Grid synchronization for power converters Marco Liserre Grid requirements for DG inverters PLL Basics, PLL in power systems Design of PLL PLL for single-phase systems –Methods to create the orthogonal component –Methods using adaptive filters PLL for three-phase systems Conclusions Reference papers Outline

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Grid synchronization for power converters Marco Liserre Grid Distrurbances Thomsen,1999; CIGRE WG14-31, 1999 Grid disturbances are not at all a new issue, and the utilities are aware of them. However, they have to take a new look because of the rapidly changing customers needs and the nature of loads (CIGRE WG14-31, 1999)

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Grid synchronization for power converters Marco Liserre Grid requirements for DG inverters The following conditions should be met, with voltages in RMS and measured at the point of utility connection. When the utility frequency is outside the range of +/- 1 Hz the inverter should cease to energize the utility line within 0.2 seconds. The PV system shall have an average lagging power factor greater than 0,9 when the output is greater than 50% rated. Thus the grid voltage and frequency should be estimated and monitored fast and accurate enough in order to cope with the standard

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Grid synchronization for power converters Marco Liserre Grid synchronization requirements A good synchronization of the current with the grid voltage is necessary as : the standards require a high power factor (> 0.9) a clean reference for the current is necesarry in order to cope with the harmonic requirements of grid standards and codes grid connection transients needs to be minimized in order not to trip the inverter Distributed Generation systems of higher power have also requirements in terms of voltage support or reactive power injection capability and of frequency support or active power droop Micro-grid distributed generation systems have wider range of voltage and frequency and the estimated grid voltage parameters are often involved in control loops

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Grid synchronization for power converters Marco Liserre Grid synchronization options and challenges There are two basical synchronization methods: Filtered Zero Cross Detection (ZCD) PLL Single-phase systems: The classical solution for single-phase systems was Filtered ZCD as for the PLL two orthogonal voltages are required. The trend now is to use the PLL technique also by creating virtual orthogonal components using different techniques! Three-phase systems: Three-phase PLL should deal with unbalnace hence with negative sequence Moreover in three-phase systems dynamics would be better if synchronizing to all three phase voltages, i.e. based on space vectors rather then on a scalar voltage

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Grid synchronization for power converters Marco Liserre Zero Cross Detection (ZCD) circuits Resistive feedback hysteresis circuit Dual point interpolation circuit Dynamic hysteresis comparator circuit Source: R.W. Wall, Simple methods for detecting zero crossing, IEEE IECON03, pp

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Grid synchronization for power converters Marco Liserre Filtered Zero Cross Detection (ZCD) based monitoring and synchronization Filtering introduces delay. There are digital predictive FIR filters without delay bu with high complexity (very high order!) The RMS voltage and frequency are calculated once in a period poor detection of changes (sags, dips, etc.)

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Grid synchronization for power converters Marco Liserre Basic idea of synchronization based on a phase-locked loop: Phase-locked technology is broadly used in military, aerospace, consumer electronics systems where some kind of feedback is used to synchronize some local periodic event with some recognizable external event Many biological processes are synchronized to environmental events. Actually, most of us schedule our daily activities phase-locking timing information supplied by a clock. A grid connected power converter should phase-lock its internal oscillator to the grid voltage (or current), i.e., an amplitude and phase coherent internal signal should be generated. Event based synchronization (simple, discontinuous, …) Phase-locked synchronization (continuous, predictive,…) PLL basis

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Grid synchronization for power converters Marco Liserre Basic blocks: Phase Detector (PD). This block generates an output signal proportional to the phase difference between its two input signals. Depending on the type of PD, high frequency ac components appear together the dc phase difference signal. Loop Filter (LF). This block exhibits low pass characteristic and filters out the high frequency ac components from the PD output. Typically this is a 1-st order LPF or PI controller. Voltage Controlled Oscillator (VCO). This block generates at its output an ac signal whose frequency varies respect a central frequency as a function of the input voltage. PLL basis

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Grid synchronization for power converters Marco Liserre PLL in power systems In 1968 Ainsworth proposed to use a voltage controlled oscillator (VCO) inside the control loop of a High Voltage Direct Current (HVDC) transmission system to deal with the novel, at that time, harmonic instability problem. Subsequently, analog phase locked loops (PLL) were proposed to be used as measurement blocks, which provide frequency adaptation in motor drives.

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Grid synchronization for power converters Marco Liserre Phase Locked Loop tuning Reference : VCO output: PD/Mixer output: VCO angle: if, then, Small signal analysis: The average value is if, then,

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Grid synchronization for power converters Marco Liserre Phase Locked Loop tuning assuming that can be written as with The PLL can be tuned as function of the damping and of the settling time then

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Grid synchronization for power converters Marco Liserre The hold range H is the frequency range at which a PLL is able to maintain lock statically. Key parameters of the PLL The lock range L is the frequency range within which a PLL locks within one-single beat note between the reference frequency and the output frequency. For the PI, LF(0)= and the hold range is only limited by the frequency range of the VCO Pull-in time: The pull-in range P is the frequency range at which a PLL will always became locked, but the process can become rather slow. For the PI loop filter this range trends to infinite. Lock-in time:

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Grid synchronization for power converters Marco Liserre Phase Locked Loop: the need of the orthogonal component To eliminate the 2° harmonic oscillation from and obtain it should be considered that

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Grid synchronization for power converters Marco Liserre Park transformation in the PD Park transformation: Assuming in = out :

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Grid synchronization for power converters Marco Liserre Park transformation in the PD PI on v d PI on v q From here on, it will be considered: and PI on v q,, i.e., Therefore:

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Grid synchronization for power converters Marco Liserre Methods to create the orthogonal component Transport Delay T/4 The transport delay block is easily implemented through the use of a first-in-first-out (FIFO) buffer, with size set to one fourth the number of samples contained in one cycle of the fundamental frequency. This method works fine for fixed grid frequency. If the grid frequency is changing with for ex +/-1 Hz, then the PLL will produce an error If input voltage consists of several frequency components, orthogonal signals generation will produce errors because each of the components should be delayed one fourth of its fundamental period.

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Grid synchronization for power converters Marco Liserre Methods to create the orthogonal component Inverse Park Transformation A single phase voltage ( v ) and an internally generated signal ( v ) are used as inputs to a Park transformation block (αβ-dq). The d axis output of the Park transformation is used in a control loop to obtain phase and frequency information of the input signal. v is obtained through the use of an inverse Park transformation, where the inputs are the d and q- axis outputs of the Park transformation (dq-αβ). fed through first-order low pass filters. Although the algorithm of the PLL based on the inverse Park transformation is easily implemented, requiring only an inverse Park and two first-order low-pass filters

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Grid synchronization for power converters Marco Liserre Methods to create the orthogonal component Second Order Generalized Integrator

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Grid synchronization for power converters Marco Liserre Methods using adaptive filters Adaptive Notch Filter (ANF) v out =0 when: v out can not be directly used as PD in the PLL v out =0 when: v out can be used as PD in the PLL

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Grid synchronization for power converters Marco Liserre Methods using adaptive filters ANF-based PLL Very sensible to frequency variation ANF+PLL EPLL More robust Faster dynamic response Combination of an ANF with a conventional PLL gives rise to the Enhanced PLL (EPLL)

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Grid synchronization for power converters Marco Liserre Enhanced PLL (EPLL) Original structure of the EPLL Methods using adaptive filters

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Grid synchronization for power converters Marco Liserre SOGI-PLL Methods using adaptive filters Adaptive band-pass filter: Damping factor is a function of the detected frequency value Second order generalized integrator follower: If can change, SOGI follower can be seen as an adaptive band-pass filter with damping factor set by k and unitary gain As in the EPLL, a standard PLL can be used to detect grid frequency and angle ju is 90º-leading v when the PLL is synchronized in steady state ju=-qu and qu qv It seems intuitive to use -qu (instead ju ) as the feedback signal for the PD of the PLL

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Grid synchronization for power converters Marco Liserre SOGI-based Frequency Locked Loop (SOGI-FLL) Methods using adaptive filters Does not need any trigonometric function since neither synchronous reference frame nor voltage controlled oscillator are used in its algorithm. Is frequency-adaptive by using a FLL and not a PLL. Is highly robust in front of transient events since grid frequency is more stable than voltage phase-angle. Attenuates high-order harmonics of the grid voltage. Entails light computational burden, using only five integrators for detection of both sequence components.

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Grid synchronization for power converters Marco Liserre Distorted and unbalanced voltage vector Three-phase grid synchronization Neither constant amplitude nor rotation speed

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Grid synchronization for power converters Marco Liserre Characterization of voltage dips Phase-voltages from characteristic parameters Sequence components from characteristic parameters

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Grid synchronization for power converters Marco Liserre Three-phase Synchronous Reference Frame PLL Three-phase grid synchronization Balanced voltage Unbalanced voltage

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Grid synchronization for power converters Marco Liserre Three-phase Synchronous Reference Frame PLL Three-phase grid synchronization The SRF is not able to track instantaneous evolution of the voltage vector when the PLL bandwidth is low Near of synchronization:

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Grid synchronization for power converters Marco Liserre Three-phase Synchronous Reference Frame PLL Three-phase grid synchronization Setting a low PLL bandwidth and using a low-pass filter it is possible to obtain a reasonable approximation of the positive sequence voltage but the dynamic is too slow. Advanced filtering strategies can be used to cancel out the double frequency oscillation keeping high locking dynamics, e.g., a repetitive controller based on a DFT algorithm. Additional improvements are added to these filters to make them frequency adaptive.

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Grid synchronization for power converters Marco Liserre Decoupled Doubled SRF-PLL. Decoupling Three-phase grid synchronization Near of synchronization: Generic decoupling cell: This terms act as interferences on the SRF dq n rotating at n frequency and viceversa

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Grid synchronization for power converters Marco Liserre Three-phase grid synchronization y. Decoupled Doubled SRF-PLL PLL input normalization

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Grid synchronization for power converters Marco Liserre Conclusions PLL is a very useful method that enable the grid inverters to: Create a "clean" current reference synchronized with the grid Comply with the grid monitoring standards The PLL generate is able to track the frequency and phase of the input signal in a designed settling time By setting a higher settling time a "filtering" effect can be achieved in order to obtain a "clean" reference even with a polluted grid. Some PLLs need two signals in quadrature at the input. For single-phase systems as there is only one signal available, the orthogonal signal needs to be created artificially. Transport Delay, Inverse Park Transformation, or Second Order Generalized Integrators are some the methods used for quadrature signal generation. Adaptive notch filters canceling fundamental utility frequency are used as phase detectors in PLLs FLL based on a SOGI is a very effective method for single phase synchronization

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Grid synchronization for power converters Marco Liserre References 1.J. D. Ainsworth, The phase-locked oscillator-a new control system for controlled static convertors, IEEE Transactions on Power Apparatus and Systems, vol. 87, no. 3, pp , Mar G. C. Hsieh, J. C. Hung, Phase-locked loop techniques – A survey, IEEE Trans. On Ind. Electronics, vol.43, pp , Dec F. M. Gardner, Phase Lock Techniques. New York: Wiley, L. D. Zhang, M. H. J. Bollen Characteristic of voltage dips (sags) in power systems, IEEE Trans. Power Delivery, vol.15, pp , April F. Blaabjerg, R. Teodorescu, M. Liserre, and A. V. Timbus, Overview of Control and Grid Synchronization for Distributed Power Generation Systems, IEEE Trans. on Ind. Electronics, Vol. 53, Oct Page(s):1398 – M. K. Ghartemani, M.R. Iravani, A method for synchronization of power electronic converters in polluted and variable-frequency environments, IEEE Trans. Power Systems, vol. 19, pp , Aug M.K. Ghartemani, M.R. Iravani, A Method for Synchronization of Power Electronic Converters in Polluted and Variable-Frequency Environments, IEEE Trans. Power Systems, vol. 19, Aug. 2004, pp H.-S. Song and K. Nam, Dual current control scheme for PWM converter under unbalanced input voltage conditions, IEEE Trans. On Industrial Electronics, vol. 46, no. 5, pp. 953–959, 1999.

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Grid synchronization for power converters Marco Liserre References 1.P. Rodríguez, A. Luna, I. Candela, R. Teodorescu, and F. Blaabjerg, Grid Synchronization of Power Converters using Multiple Second Order Generalized Integrators, IECON08, Nov P. Rodríguez, J. Pou, J. Bergas, J.I. Candela, R. Burgos and D. Boroyevich, Decoupled Double Synchronous Reference Frame PLL for Power Converters Control, IEEE Trans. on Power Electronics, March P. Rodriguez, R. Teodorescu, R.; I. Candela, I.; A.V. Timbus, M. Liserre, F. Blaabjerg, New Positive-sequence Voltage Detector for Grid Synchronization of Power Converters under Faulty Grid Conditions, PESC '06, June M Ciubotaru, Teodorescu, R., Blaabjerg, F., A New Single-Phase PLL Structure Based on Second Order Generalized Integrator, PESC06, June P. Rodríguez, A. Luna, M. Ciobotaru, R. Teodorescu, and F. Blaabjerg, Advanced Grid Synchronization System for Power Converters under Unbalanced and Distorted Operating Conditions, IECON06, Nov S.-K. Chung, Phase-Locked Loop for grid-connected three-phase power conversion systems, IEE Proceedings on Electronic Power Applications, vol. 147, no. 3, pp. 213–219, Francisco Daniel Freijedo Fernández, Contributions to Grid-Synchronization Techniques for Power Electronic Converters, PhD Thesis, Vigo University, Spain, 2009

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Grid synchronization for power converters Marco Liserre Acknowledgment Part of the material is or was included in the present and/or past editions of the Industrial/Ph.D. Course in Power Electronics for Renewable Energy Systems – in theory and practice Speakers: R. Teodorescu, P. Rodriguez, M. Liserre, J. M. Guerrero, Place: Aalborg University, Denmark The course is held twice (May and November) every year

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