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Decentralized Generation In Microgrids

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Distributed generation of electric energy has become part of the current electric power system. In this context, a recent research study is arising on a new scenario in which small energy sources make up a new supply system : The Microgrid. The most recent projects show the technical difficulty of controlling the operation of Microgrids, because they are complex systems in which several subsystems interact: energy sources, power electronics converters, energy systems, linear and non-linear loads and of course, the utility grid.In next years, the electric grid will evolve from the current very centralized model toward a more distributed one.

Decentralized Generation In Microgrids

  1. 1. Juan Carlos Vásquez Quintero Advisor: Josep María Guerrero Zapata Ph.D. Thesis Dissertation 2009Ph.D. on Automatics, Robotics and Vision
  2. 2. I. Motivation: Introduction to Microgrids II. Thesis objectives III. Decentralized control: P/Q droop method IV. Droop control method applied to voltage sag mitigation V. Operation modes of a microgrid Grid-connected operation mode  Islanded operation mode Transitions between grid connected and islanded modes VI. Hierarchical control of microgrids  Primary control: - P/Q droop. Modeling and control - Virtual Impedance: hot swap operation (smooth connection)  Secondary control: - f/V Restoration (island) and Synchronization (island to grid connected mode) Tertiary control: - P/Q flow control at the PCC VII. Conclusions and publications VIII. Future workPh.D. Thesis dissertation Page 2
  3. 3. I. Motivation: Introduction to Microgrids II. Thesis objectives III. Decentralized control: P/Q droop method IV. Droop control method applied to voltage sag mitigation V. Operation modes of a microgrid Grid-connected operation mode  Islanded operation mode Transitions between grid connected and islanded modes VI. Hierarchical control of microgrids  Primary control: - P/Q droop. Modeling and control - Virtual Impedance: hot swap operation (smooth connection)  Secondary control: - f/V Restoration (island) and Synchronization (island to grid connected mode) Tertiary control: - P/Q flow control at the PCC VII. Conclusions VIII. Future workPh.D. Thesis dissertation Page 3
  4. 4. Distributed Generation (DG) Paradigm and Smart grids Nowadays problem:  Energy crisis & climate change.  Kyoto’s protocol: reduction of CO2 emission. Raise of renewable energy:  Photovoltaic.  Wind.  Hydrogen.  Micro-turbines. Small energy storage systems:  Uninterruptible Power Sources (UPSs).  Flywheels.  Super capacitors.  Compressed air devices.  Mini-hydraulics.Ph.D. Thesis dissertation Page 4
  5. 5. So, what is a Microgrid? Small energy storage Renewable energy sources systems PV Wind turbine Grid panel system UPS PCC Inverters Common ... . AC bus Intelligent Bypass Power Distribution Switch Network (IBS) Distributed loadsPh.D. Thesis dissertation Page 5
  6. 6. Microgrids are required in the next electrical grid mgrids… Avoid transmission power losses Improve the system efficiency Increase the reliabilityPh.D. Thesis dissertation Page 8
  7. 7. I. Motivation: Introduction to Microgrids II. Thesis objectives III. Decentralized control: P/Q droop method IV. Droop control method applied to voltage sag mitigation V. Operation modes of a microgrid Grid-connected operation mode  Islanded operation mode Transitions between grid connected and islanded modes VI. Hierarchical control of microgrids  Primary control: - P/Q droop. Modeling and control - Virtual Impedance: hot swap operation (smooth connection)  Secondary control: - f/V Restoration (island) and Synchronization (island to grid connected mode) Tertiary control: - P/Q flow control at the PCC VII. Conclusions VIII. Future workPh.D. Thesis dissertation Page 9
  8. 8. Control of single DG units according to Microgrid requirements:  Modeling of the DG unit  Use appropriate control variables to control P and Q  Derive decentralized controllers (droops)  Propose control strategies for grid connected and islanded operation  Grid-connected mode:  Accuracy of the P and Q injected  Voltage sags and harmonics ride through  Large grid impedance variation  Islanded mode:  Voltage stability  Harmonic current sharing  Hot-swap operationPh.D. Thesis dissertation Page 10
  9. 9. Hierarchical control of microgrids  Primary control – Inside the DG units  Droop functions – Virtual inertias  Harmonic current sharing  Hot-swap operation  Secondary control – Inside the microgrid  Frequency and amplitude restoration  Synchronization loops  Tertiary control – Outside the microgrid  P and Q power flow at the PCCPh.D. Thesis dissertation Page 11
  10. 10. (IET department, AAU, Denmark)Ph.D. Thesis dissertation Page 12
  11. 11. I. Motivation: Introduction to Microgrids II. Thesis objectives III. Decentralized control: P/Q droop method IV. Droop control method applied to voltage sag mitigation V. Operation modes of a microgrid Grid-connected operation mode  Islanded operation mode Transitions between grid connected and islanded modes VI. Hierarchical control of microgrids  Primary control: - P/Q droop. Modeling and control - Virtual Impedance: hot swap operation (smooth connection)  Secondary control: - f/V Restoration (island) and Synchronization (island to grid connected mode) Tertiary control: - P/Q flow control at the PCC VII. Conclusions VIII. Future workPh.D. Thesis dissertation Page 13
  12. 12. The circulating current concept i1 Critical AC DC link bus io  i1  i2 Distributed critical i2 loads DC linkPh.D. Thesis dissertation Page 14
  13. 13. The circulating current concept E 1f1 Z o1 rL1 L1 L2 rL2 Z o2 E 2f2 V 0o i1 i2 ZL DG 1 DG #2 S 1=P 1+jQ 1 S 2=P 2+jQ 2 Circulating power analysis S L = P L +jQ S S1  S2 consequently… P P  P2 1 Q Q1  Q2 Assuming L1  (Zo1  Z L1 ), L2  (Zo 2  Z L 2 ) And total output impedance is manly inductive X T  ( L1  L2 )Ph.D. Thesis dissertation Page 15
  14. 14.  P/Q expressions being E1 E2 EE P  sin f  1 2 f XT XT sin f  f E  E1  E2 Q  V E cos f 1 f  f1  f2 XT Circulating currents: E1 E2 E i p  f iQ  VX T XT Assuming inductive output impedance, the active and reactive powers or currents can be controlled by adjusting the phase and the amplitude of the output voltage.Ph.D. Thesis dissertation Page 16
  15. 15. Conventional droop method  o1 Frequency droop o o2   m *  P    *  mP ’ m’  ’ Emax = 5% P’ max=2% P Pmax P P1 P’1 P’2 P2 Output Z=jX Z=R(q=0o) Impedance (inductiveq=90o) Amplitude droop Active P EV sin f  EV f EV cos f  V 2 E P E* power X X R E  E *  nQ Reactive EV cos f  V 2 V EV Q  (E V ) Q  sin f E power X X R Frequency droop   *  mP   *  mQ Amplitude droop E  E*  nQ E  E*  nP Qmax QPh.D. Thesis dissertation Page 17
  16. 16. Controller:    *  mP E  E *  nQ System Dynamics: ˆ ˆ ˆ ˆ s 3f  As 2f  Bsf  Cf  0 c A   2 X  nV cos   X  B  c c X  ncV cos   mVE cos   X c  1  C mVE  X cos   nV   X X   Depends on m, n, X, c System transient can not be modifiedPh.D. Thesis dissertation Page 18
  17. 17. Conventional droop method: Static restrictions 1) Static trade-off: F/V regulation and P/Q power sharing. 2) Limited transient response. System dynamic is determined by m, n and strongly determined by X. 3) Unbalancing on line impedance when P/Q is shared. 4) Synchronization problems in relation with the utility main (F and f deviation). 5) Line impedance dependence (islanded mode). Static trade-off: F/V regulationPh.D. Thesis dissertation Page 19
  18. 18. Proposed strategy: Adaptive droop method Q* Droop functions ipcc Q  Qc  LPF Gq (s)  P/Q E* 90º Decoupling * v pcc P* Transformation E ipcc LPF P Pc G p ( s) f Vref * E sin(t  f ) Power Calculation Zg qg Impedance estimation algorithm f  Gp (s)Z g (P  P*)sinq g  Q  Q*  cosq g      G p ( s)  mi  mp s  md s 2 s E  E*  Gq (s)Z g (P  P*)cosq g  Q  Q*  sinq g  n n s     Gq (s)  i p sPh.D. Thesis dissertation Page 20
  19. 19. Power flow modeling  EVg cos f  Vg 2  EVg VSI Vpcc Grid P  cosq g  sin f sinq g E  Zg  Zg    EVg cos f  Vg 2  EVg i pcc Q  sinq g  sin f cosq g Ef Zo Zg Vg 0º  Zg  Zg   P/Q Decoupling transformation Pc  Z g  (P sinq g  Q cosq g ) 2000 Qc  Z g  (P cosq g  Q sinq g ) Active Power Reactive Power 1500 P[W] & Q[VAr] 1000 Pc  EVg sinf Qc  EVg cosf Vg 2  Vg  E Vg  500 Pc is mainly dependent on the phase f, and 0 Qc depends on the voltage difference 2 4 6 8 10 between the VSI and the grid (E – Vg). Time[s]Ph.D. Thesis dissertation Page 21
  20. 20. System dynamics   1  P* sin q g  Q* cosq g    tan       P* cosq g  Q* sin q g  Vg 2 Z g     Vg 2 cosq g  P*Z g E Vg  cosq g cos   sin q g sin   Characteristic equation a4 s 4  a3 s3  a2 s 2  a1s  a0  0 a4  T 2  2Tmd Vg Ecos a3  4T  4md n pVg 2 E  2Vg cos  2md E  Tn p  Tm p E  a2  2Vg cos Tmi E  Tni  2n p  2m p   4Vg 2 E  m p n p  md ni   4 a1  4Vcos  ni  mi E   4V 2 E  mi n p  m p ni  a0  4ni mi EV 2Ph.D. Thesis dissertation Page 22
  21. 21. Validating the obtained model -3 x 10 10 8 6 Phase [rd] 4 2 Model Real 0 -2 0 0.2 0.4 0.6 0.8 1 Time [s] System and model dynamic responsePh.D. Thesis dissertation Page 23
  22. 22. Root Locus System stability 30 20 3 10 Imaginary Axis b) 1 2 0 -10 4 -20 -30 -100 -80 -60 -40 -20 0 Real Axis Root Locus 1 0.8 a) 0.6 0.4 Figure Parameter Imaginary Axis 0.2 4 1 2 3 0 a) 0.00005<mp <0.0001 c) -0.2 -0.4 b) 1e-6<md <4e-5 -0.6 -0.8 c) 0.0004<np <0.01 -1 -350 -300 -250 -200 -150 -100 -50 0 Real AxisPh.D. Thesis dissertation Page 24
  23. 23.  Line impedance sensitivity 1600 1600 1400 1400 1200 1200 1000 1000 800 800 600 600 400 400 200 R=1 200 R=1 R=2 R=2 0 0 R=3 R=3 -200 -200 0 0.5 1 1.5 2 0 0.5 1 1.5 2 (a) (b) Start up of P for different line impedances, (a) with and (b) without the estimation algorithm of Zg.Ph.D. Thesis dissertation Page 25
  24. 24. VSI Local Bypass Z L load g Vg Block diagram scheme of the C proposed method v pcc Grid Voltage and Grid parameters Ident. d q current Algorithm (Estimated Driver and Monitoring Transf Values) ipcc SOGI-FLL g Vg Zg qg PWM generator *  Sine * E* P f PC P/Q  P Current Voltage Vref generator Droop QC Decoupling  Loop Loop E sin(  t  f ) E Q control Transf *  Q Inner loops PWM inverter L =713m H Local Bypass Transformer load 5kVA VDC 400V  C  2.2m F Z Grid  L =713m H 10mH Hardware ic E Vg ig Setup dSPACEPh.D. Thesis dissertation Page 26
  25. 25. Power Calculation Zg qg QC i pcc Q    LPF Gq ( s)    Q 0 * E * ES 90º GS v pcc P/Q (Freezing) E Decoupling f * PC Vref i pcc LPF P G p ( s)    E sin(  t  f ) P* fS  LPF PI (s) fS VG ( rms ) LPF PI (s) ES VG GS VC ( rms ) GS Phase Synchronization Amplitude Restoration Block diagram of the whole proposed controller using the synchronization control loopsPh.D. Thesis dissertation Page 27
  26. 26. Synchroniz. Process (grid and VSI voltages) Synchroniz. Error signal Island Grid connected mode Island P/Q load step during grid transitionsPh.D. Thesis dissertation Page 28
  27. 27. Ph.D. Thesis dissertation Page 29
  28. 28. Active power transient response for Q*=0 (P: Blue, Q:black) Active power transient during a P* step. Reactive power transient during a Q* step (absorbing)Ph.D. Thesis dissertation Page 30
  29. 29. Conclusions  A novel control for VSIs with the capability of flexibly operating grid- connected and islanded mode based on adaptive droop control strategy was proposed.  Active and reactive power can be decoupled from the grid impedance in order to injected the desired power to the grid.  Enhancement of the classic droop control method.  A novel control for injecting the desired active and reactive power independently into the grid for large range of impedance grid values was presented. This control is based on a parameters estimation provided by an identification algorithm.Ph.D. Thesis dissertation Page 31
  30. 30. I. Motivation: Introduction to Microgrids II. Thesis objectives III. Decentralized control: P/Q droop method IV. Droop control method applied to voltage sag mitigation V. Operation modes of a microgrid Grid-connected operation mode  Islanded operation mode Transitions between grid connected and islanded modes VI. Hierarchical control of microgrids  Primary control: - P/Q droop. Modeling and control - Virtual Impedance: hot swap operation (smooth connection)  Secondary control: - f/V Restoration (island) and Synchronization (island to grid connected mode) Tertiary control: - P/Q flow control at the PCC VII. Conclusions VIII. Future workPh.D. Thesis dissertation Page 32
  31. 31. Shunt DG inverter Control objectives: i. Enhances the stability and the dynamic QG response by damping the system. ii. Provides all the active power given by the PV V 0º source, and extracted in the previous stage by the maximum power point tracker (MPPT). LG  iii. Supports the reactive power required by the grid when voltage sag is presented RL into the grid. E IC E f IG Vg 0º IC  IG  I L I Gd I C IG  IGd  jIGq I Gq IL E Ef  V 0º IC IG  f jX I Gq Vg 0º Equivalent circuit of the power stagePh.D. Thesis dissertation Page 33
  32. 32. It can be observed that during a Q – E relationship voltage sag, the amount of reactive E current needed to maintain the load voltage at the desired value is V E E V inversely proportional to the grid V E impedance. Q Q0 Q0 Inductance value Maximum P delivered by the VSI (f=90o) Large inductance will help in mitigating EV X voltage sags. PmaxPh.D. Thesis dissertation Page 34
  33. 33. System modeling and Control design f  Gp (s)  PC  PC*  sin q   QG  QG  cosq   *  E  E*  Gq (s)  PC  PC  cos q  QG  QG  sin q   * *  s5  a4 s 4  a3 s3  a2 s 2  a1s  a0  0 a4  4o Z a3  V o cos  (n p  Em p )  2o (1  2 2 ) Z 2 2 a2  2V o  cos  n p  Em p   o VE cos mi  4o Z 2 2 3 V a1  V o cos  (n p  Em p )  VEo (2 cosmi  4 3 o n p m p )  o4 Z Z V mi  m p s a0  VEo mi (cos  n p ) 4 G p ( s)  Z s Gq (s)  n p E *a (1  a )  Qmax X V = a*E* (being a as the voltage sag percentage) np  Qmax E *aPh.D. Thesis dissertation Page 35
  34. 34. System stability a) b) c) Figure Parameter a) 20e-6<mp <1e-3 b) 2e-6<mi <1.8m c) 8.5mH<LG <5000mHPh.D. Thesis dissertation Page 36
  35. 35. Control diagram PL , QL PG , QG  LG P , QC IL Vg 0º IG C Load VC IC   Inner Repetitive  PI loop  Control I ref control VC Vref VC QG PC P IG Q Droop Outer P/Q Control P * calculation IC calculation Q*  0 ( MPPT ) control loop Q IG QG   Gq ( s) LPF   90º QG  0 * V* VC P/Q V Decoupling * Vref Vref PC  P  f IC LPF G p ( s) V sin(t  f )    Power PC * f* LPF CalculationPh.D. Thesis dissertation Page 37
  36. 36. Set-up and simulation results P/Q transient responses by the PV inverter in presence of a voltage sag of 0.15 p.u when Pc*=500W Current waveforms in case of a voltage sag of 0.15 p.u (inverter current ic , grid current Ig and load current Iload)Ph.D. Thesis dissertation Page 38
  37. 37. Simulation results Grid voltage waveform in presence of 1st, 3rd, 5th 7th and 9th voltage harmonics. P/Q transient responses and step changes provided by the PV inverterPh.D. Thesis dissertation Page 39
  38. 38. Simulation results (detail) Inverter output voltage, active and reactive power transient responses under a nonlinear load (detail) Current waveforms in case of a nonlinear load. Vg (upper), Inverter current (middle), grid currentPh.D. Thesis dissertation Page 40
  39. 39. Laboratory set-up (Electrical Engineering department, Polytechnic of Bari, Italy)Ph.D. Thesis dissertation Page 41
  40. 40. Experimental results Waveform of the grid voltage and the grid current during the sag. Case of a voltage sag of 0.15 p.u (voltage controlled with droop control) grid voltage, load voltage and grid current. Voltage waveform during the sag. Grid voltage and current (upper). Grid voltage, capacitor voltage and load voltage (lower)Ph.D. Thesis dissertation Page 42
  41. 41. Conclusions  A detailed analysis has shown that the novel adaptive droop control strategy has superior behavior in comparison with the existing droop control methods, ensuring efficient control of frequency and voltage even in presence of voltage sags  The converter provides fast dynamic response and active power to local loads by injecting reactive power into the grid providing voltage support at fundamental frequency. Using the basis of the droop characteristic, a new control strategy was proposed, providing all the desired active power given by the PV source. The validity of the proposed control showed ride-through capability to the system, even if a voltage sag is presented in the grid.PhD Thesis dissertation Page 43
  42. 42. I. Motivation: Introduction to Microgrids II. Thesis objectives III. Decentralized control: P/Q droop method IV. Droop control method applied to voltage sag mitigation V. Operation modes of a microgrid Grid-connected operation mode  Islanded operation mode Transitions between grid connected and islanded modes VI. Hierarchical control of microgrids  Primary control: - P/Q droop. Modeling and control - Virtual Impedance: hot swap operation (smooth connection)  Secondary control: - f/V Restoration (island) and Synchronization (island to grid connected mode) Tertiary control: - P/Q flow control at the PCC VII. Conclusions VIII. Future workPh.D. Thesis dissertation Page 44
  43. 43. PV Wind turbine panel system UPS PCC Inverters Common AC bus . . . Static Transfer switch Distributed loads Micro-grid (IBS) Typical structure of inverter microgrid based on renewable energy resources Synchronization Islanded mode Grid -connected Operation mode restoration Grid failure Islanded mode Grid -connected IBS disconnection Operation modePh.D. Thesis dissertation Page 45
  44. 44. Primary control: droop Control (P/Q Sharing, softstart). Secondary Control: Synchronization and Tertiary Restoration (Set-points assignation to the DGs ). Control Tertiary Control : Power Import/export from/to the grid. Secondary Control Primary Control Bypass off E= V* P= P* ; Q=Q* Import/export * P/Q Grid Islanding Connected Operation E= Vg Bypass on gPh.D. Thesis dissertation Page 46
  45. 45. Primary control: Droop Control (P/Q Sharing, softstart). The inverters are programmed to act as generators by including virtual inertias through the droop method. Grid-connected mode: Microgrid is connected to the grid through an IBS. In this case, all DG units must be programmed with the same droop function Normally, P∗ should coincide with the nominal active power of each inverter, and Q∗ = 0. Two possibilities: 1) Importing energy from the grid: the IBS must adjust P∗ by using low bandwidth communications to absorb the nominal power from the grid in the PCC. 2) Exporting energy to the gr. the IBS may enforce to inject the rest of the power to the grid. Moreover, it must adjust the power references.idPh.D. Thesis dissertation Page 47
  46. 46. Primary control: Droop Control (P/Q Sharing, softstart). This is done with small increments and decrements of P∗ as a function of the measured grid power, by using a slow PI controller: P*  k p ( Pg *  Pg )  ki  ( Pg *  Pg )dt  Pi* where Pg and Pg∗ are the measured and the reference active powers of the grid, respectively, and Pi∗ is the nominal power of the inverter i. Similarly, reactive-power control law can be defined as Q*  k p (Qg *  Qg )  k i  ( Pg *  Pg )dt  Qi* where Qg and Qg∗ are the measured and the reference reactive powers of the grid, respectively, and Q i ∗ is the nominal reactive power.Ph.D. Thesis dissertation Page 48
  47. 47. Primary control: Droop Control (P/Q Sharing, softstart). Islanded mode : When the grid is not present, the IBS disconnects the microgrid from the main grid, starting the autonomous operation:  Droop method is enough to guarantee proper power sharing between the DG units.  Energetic storage: Power sharing should take into account the batteries charging level of each module.  Level of charge of the batteries mmin  * m a sist a 1 Batteries Batteries Fully charge a  0.01 Empty 30% charged fully charged P 30% Pmax Pmax Droop characteristic as a function of the batteries charge level.Ph.D. Thesis dissertation Page 49
  48. 48. Primary control analysis :  P  c V  cos   E sin   e        Q  s  c X  sin  E cos   f  c V e  (n  nd s ) cos   e  E sin   f  s  c Rd m  V  c f     md  sin   e  E sin   f  s  Rd ( s  c ) s3  As 2  Bs  C  0 c   V  A 2 RD  nV cos   nd cV cos   mdVE  cos   nd c   X   X  c  V  V  B  c  ncV cos   mVE cos   nd c  md cVE  cos   n   X  X  X  c  V C  mVE  cos   n  X  XPh.D. Thesis dissertation Page 50
  49. 49. Primary control: Droop Control (P/Q Sharing, softstart). Stability for all battery charge conditions Root locus plot in function of the batteries charge level (arrows indicate decreasing value from 1 to 0.01).Ph.D. Thesis dissertation Page 51
  50. 50. Power scheme of the primary control Virtual output impedance concept Zo Vref  vo  io Zo (s) * t Inner loops  Vref v Voltage Current PWM +UPS 0.1  loop loop Inverter io Zo (H) 0.05 ZO(s) 0 0.6 10 0.8 1 1.2 1.4 1.6 1.8 2 Virtual output impedance loop P Io1 (A) 0 -10 Voltage  P* 10 0.6 0.8 1 1.2 1.4 1.6 1.8 2 * Vo Reference P/Q Q* Calculation E sin (t) E Io2 (A) 0 Q -10 0.6 0.8 1 1.2 1.4 1.6 1.8 2 P/Q control outer loops time (s)Ph.D. Thesis dissertation Page 52
  51. 51. Secondary control: Restoration and synchronization In order to restore the microgrid voltage to nominal values, the supervisor sends proper signals by using low bandwidth communications. This control also can be used to synchronize the microgrid with the main grid before they have to be interconnected, facilitating the transition from islanded to grid-connected mode. Grid Freq. measurement PLL _ PI Primary  * control control Dg units + Supervisory control Primary control Dg unitsPh.D. Thesis dissertation Page 53
  52. 52. Secondary control analysis : c V e  (n  nd s ) cos   e  E sin   f  s  c Rd m  V  c f     md  sin   e  E sin   f  s  Rd ( s  c ) m  ( P*  Pi* ) EV c Pi  X ( s  c ) Pi (ki  k p s)mEV c / N  2 Pg * Xs  c [ X  mEV (nk p  1)]s  NmEV c kiPh.D. Thesis dissertation Page 54
  53. 53. Stability analysis: Root Locus 1 0.8 0.6 0.4 0.2 4 5 Root-locus plot for Imaginary Axis 0 a) 0.1 ≤ ki ≤ 0.8. -0.2 b) 0.7 ≤ kp ≤ 0.8. -0.4 -0.6 -0.8 -1 Root Locus -80 -70 -60 -50 -40 -30 -20 -10 0 25 Real Axis 20 a) 15 10 5 4 Imaginary Axis 0 -5 -10 5 -15 -20 b) -25 -70 -60 -50 -40 -30 Real Axis -20 -10 0Ph.D. Thesis dissertation Page 55
  54. 54. Power scheme of the secondary control Io Current V* dV control Driver and Frequency restoration Voltage restoration loop level PWM Inverter level Voltage Generator ref Secondary Gwr(s) control control Inner o loop loops Vo Vref Gvr(s) Primary control (droop method) vo Voltage restoration Io Current level V* dV control Driver and Low bandwidth loop PWM Inverter communications Voltage Generator Secondary control control Inner loop loops Vo Primary control (droop method)Ph.D. Thesis dissertation Page 56
  55. 55. Tertiary Control : Power Import/export from/to the grid This energy production level controls the power flow between the microgrid and the grid. Power flow can be controlled by adjusting the frequency (changing the phase in steady state) and amplitude of the voltage inside the microgrid by measuring the P/Q through the static bypass switch. (Grid-connected mode).   *  m(P  P*)  PG and PG they can be compared with   *  mP  mP* *  * the desired PG and QG . The control laws can be expressed as following: *   grid d v  k pQ  Q  QG   kiQ   Q  QG  dt * G * G d  k pP  P*  P   kiP   P*  P  dt G G G G P0 P0 PPh.D. Thesis dissertation Page 57
  56. 56. Power scheme of the tertiary control Bypass Main AC grid P,Q Microgrid PLL RMS P/Q calculation df Q Emax _ _ Q* Vref ++ + Gq Gse dv Emin df Secondary P _ max control P* + fref + Gp + Gsw d min Tertiary control Synchronization loopPh.D. Thesis dissertation Page 58
  57. 57. Simulation results Inverter_I 500 P* Io1 Vload P Q* Vo1 PQ MAIN P* calculation Q ILoad P* Io2 Iload pm_sense Q* Vo2 50 1 RL qm_sense Q* Inverter_II RLoad Switch IGrid Grid connected Inverter I , Inverter II and Operation Vgrid ouput voltage P grid Q grid P* grid status Validation vgrid 2000 P Nominal Inv Q* (Sg) Vgrid Vg P Nominal Inv P* grid V.sin (Wt) L2 Iogrid Pid_Pout [Vload] VL Pid_Pin 1000 phi_g Q* grid Terminator R+L P*grid 1000e-6 model Vgrid Grid 0 PID 1000e-6 Status default Q*grid I_PgridPh.D. Thesis dissertation Page 59
  58. 58. Simulation results 4000 3000 UPS # 2 2000 disconnected P(W) P1 1000 P2 Pg 0 Grid connected Islanded -1000 0 1 2 3 4 5 6 7 8 9 t(s) 4000 Active power transients between grid connected and islanded modes for 2 DG units 3000 2000PhD Thesis dissertation (W) Page 60
  59. 59. Simulation resultsPh.D. Thesis dissertation Page 61
  60. 60. Experimental results Hot-swap capability Currents of the UPS#1, UPS#2, UPS#3 and UPS#4 in a) Soft-start operation, and b) Disconnection scenarioPh.D. Thesis dissertation Page 62
  61. 61. Experimental results Output current 1 Circulating current Output current 2 X: 100ms/div, Y: 20 A/divPh.D. Thesis dissertation Page 63
  62. 62. Experimental results Non-linear loads Waveforms of output voltage and load current sharing a nonlinear load. (X-axis: 5 ms, Y -axis: 20 A/div).Ph.D. Thesis dissertation Page 64
  63. 63. From the analysis, system stability and power management control starting from a single voltage source inverter to a number of interconnected DG units forming flexible Microgrids: A dynamic model design and a small signal analysis. Control-oriented modeling based on active and reactive power analysis. A control strategy in order to inject both the desired active and the reactive power independently into the grid for large range of impedance grid values.  Control synthesis based on enhanced droop control technique capable of decouple active and reactive power flows. Voltage ride-through capability, by controlling the injection of reactive power to solve grid voltage sags.Ph.D. Thesis dissertation Page 65
  64. 64.  Droop-controlled microgrids can be used in islanded mode.  Improvements to the conventional droop method are required for integrate inverter-based energy resources:  Improvement of the transient response  Virtual impedance: harmonic power sharing and hot-swapping  Adaptive droop control laws  The hierarchical control is required for a AC microgrid:  Primary control is based on the droop method allowing the connection of different AC sources without any intercommunication.  Secondary control avoids the voltage and frequency deviation produced by the primary control. Only low bandwidth communications are needed to perform this control level. A synchronization loop can be add in this level to transfer from islanding to grid connected modes.  Tertiary control allows to import/export active and reactive power to the grid.Ph.D. Thesis dissertation Page 66
  65. 65. Journal Publications Control Strategy for Flexible Microgrid Based on Parallel Line-Interactive UPS Systems. Guerrero, J.M.; Vasquez, J.C.; Matas, J.; Castilla, M.; de Vicuna, L.G. Industrial Electronics, IEEE Transactions on, Volume 56, Issue 3, March 2009 Page(s):726 – 736. Voltage Support Provided by a Droop-Controlled Multifunctional Inverter Vasquez, J.C.; Mastromauro, R.A.; Guerrero, J.M.; Liserre, M. Industrial electronics, IEEE Transactions on , Volume 56, Issue 11, Nov. 2009 Page(s):4510 – 4519 Adaptive Droop Control Applied to Voltage-Source Inverters Operating in Grid- Connected and Islanded Modes. Vasquez, J.C.; Guerrero, J.M.; Luna, A.; Rodriguez, P.; Teodorescu, R. Industrial Electronics, IEEE Transactions on Volume 56, Issue 10, Oct. 2009 Page(s):4088 – 4096 Book chapters Josep M. Guerrero and Juan. C. Vasquez, Uninterruptible Power Supplies, The Industrial Electronics Handbook, Second edition, Irwin, J. David (ed.).Ph.D. Thesis dissertation Page 67
  66. 66. Conference Publications Adaptive Droop Control Applied to Distributed Generation Inverters Connected to the Grid, J. C. Vasquez, J. M. Guerrero, E. Gregorio, P. Rodriguez, R. Teodorescu and F.Blaabjerg, IEEE International Symposium on Industrial Electronics (ISIE’08). Pages 2420-2425. Jun. 30, 2008. Droop Control of a Multifunctional PV Inverter. R. A.Mastromauro,M. Liserre, A. dell’Aquila, J.M. Guerrero and J. C. Vasquez, IEEE International Symposium on Industrial Electronics (ISIE’08), pages 2396-2400. Jun. 30, 2008. Ride-Through Improvement of Wind-Turbines Via Feedback Linearization, J.Matas, P. Rodriguez, J.M. Guerrero, J. C. Vasquez, In IEEE International Symposium on Industrial Electronics (ISIE’08), Pages 2377-2382, Jun. 30, 2008. Parallel Operation of Uninterruptible Power Supply Systems in Microgrids, J. M. Guerrero, J. C. Vasquez, J. Matas, J. L. Sosa and L. Garcia de Vicuña, 12th European Conference on Power Electronics and Applications (EPE’07), sept.2007.Page(s): 1-9. Hierarchical control of droop-controlled DC and AC microgrids−a general approach towards standardization, J. M. Guerrero, J. C. Vasquez and R. Teodorescu, “,” in Proc. IEEE IECON, 2009, pp. 4341-4346.Ph.D. Thesis dissertation Page 68
  67. 67. Control and power management of DC coupling microgrids. Addresses an open field for new renewable energy power-management control strategies. (Power quality, electrical power quality, reliability and flexible operation in Microgrids). Hierarchical control. Integrated energy storage systems.  Capabilities and limitations of each storage technology. The final goal is to develop and integrate energy storage systems specifically designed and optimized for grid-tied PV applications. Improving the hierarchical control strategy. DPS applications such as telecom voltage networks. Hard power quality standards, demand new control strategies imposed for energy management of Microgrids, based on intelligent/adaptive algorithms. Enhancing of tertiary control for harmonics injection to the grid (power quality) Active filters functionalities.Ph.D. Thesis dissertation Page 69
  68. 68. Thanks for your attentionPh.D. Thesis dissertation Page 70

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