Control of Power Converters for Distributed Generation

Transcription

1 Mechatronics Industrial Advisory Board 2004 Control of Power Converters for Distributed Generation Ph.D. Student: Min Dai Advisor: Prof. Ali Keyhani Department of Electrical and Computer Engineering The Ohio State University October 22, /22/2004 1

2 Agenda Introduction Problem statement System Description Power control of grid-connected inverters Single unit Multiple units Transfromerless power converter unit topology and control issues Research Plan 10/22/2004 2

3 Introduction (1) What is distributed generation (DG)? Distributed: 2-50MW2 Dispersed: <500kW Close to end user Wind, photovoltaic, fuel cells, micro gas turbines Why DG? Growth of power demand Limited transmission capability Increase of critical load Availability of new sources and new technology 10/22/2004 3

4 Introduction (2) Centralized power plants will remain the major source Role of DG standby emergency power cogeneration (heat and power) peak shaving grid support stand alone onsite power Impacts of DG on power system stability Define: Penetration = Pd/(P+Pd) 100% At small penetration: Insignificant At high penetration: Enhances system stability 10/22/2004 4

5 Introduction (3) Impacts of DG on power system protection Operation conflicts affects utility protective relaying voltage sag if disconnected prolongs fault arc and fails utility reclose Possible solutions DG unit current protection local, no utility involved Integrate protection with utility 10/22/2004 5

6 Introduction (4) How is DG going to be operated once connected to the grid? No standard solution yet in policy making stage Possibilities Small units (e.g., residential size)- no dispatch Large units run under higher level dispatch Hierarchical dispatch Networking technology Cost/profit issues 10/22/2004 6

7 Introduction (5) Solid-state power converter and distributed generation Static switch is economical only for small units Parallel operation for large capacity Local load issues High profile load requires high power quality Nonlinear load Unbalanced load Dynamic disturbances 10/22/2004 7

8 Introduction (6) Distributed generation unit the topology With an isolation transformer Transformerless Control issues Island mode: voltage control Grid-connected mode: power control Paralleled multiple units: load sharing Front-end DC bus the rectifier control 10/22/2004 8

9 Problem Statement (1) Power control of grid-connected inverters Single unit with grid-connection No load connection Disconnection Reclose Multiple units Parallel operation in island mode Connect a 2 nd unit to a grid-connected unit Disconnections Reclose 10/22/2004 9

10 Problem Statement (2) Voltage and current control of a transformerless inverter a three phase four wire system Control as a three phase system rather than three single phase systems Rectifier control to eliminate effects of unbalanced inverter load current 10/22/

11 System Description (1) Power circuit topology Single unit (3-ph 3 wire inverter plus transformer) 10/22/

12 System Description (2) Power circuit topolog y Multipl e units 10/22/

13 10/22/ /22/ System Description (3) System Description (3) State space State space modeling modeling 0, 0,, αβ αβ αβ αβ s i f inv f xfm C C dt d I T I V = αβ αβ αβ,,, 1 1 xfm f inv f inv L L dt d V V I = 0, 0, 0, 1 1 αβ αβ αβ out s s s out C C dt d I I V = 0,, 0, 0, 0, 1 1 αβ αβ αβ αβ αβ out T xfm v T s T T s L L L R dt d V V T I I =

14 System Description (4) Operation in island mode Performs voltage control Control goals: Low steady state error Low total harmonic distortion (THD) Fast response to load disturbances Challenges: Unbalanced load Nonlinear load Instantaneous load disturbance 10/22/

15 System Description (5) Existing voltage control strategies PI control State space based linear control Sliding mode control Internal model principle based control Repetitive control Servo control Our approach Robust servomechanism control for voltage loop Discrete-time time sliding mode for current loop 10/22/

16 PQ Control with Grid-Connection (1) Real & reactive power control in grid- connected mode Operating scenarios- single unit Connection with no load: Synchronization before closing the switch Control strategy change while closing the switch Insignificant transient due to low inverter output impedance Inverter Unit Local Load jx Grid 10/22/

17 PQ Control with Grid-Connection (2) Disconnection with load: Three phase opened one by one due to zero-cross switching Reclose Synchronization Control strategy switch P,Q transient exists due to non-zero X Inverter Unit Inverter Unit Local Load Local Load jx jx Grid Grid 10/22/

18 PQ Control with Grid-Connection (3) Operating scenarios-multiple units Parallel in island mode: Synchronization Load sharing after connection Harmonic load sharing Connect a 2 nd unit Synchronization Load sharing Load sharing Inverter Unit 1 Inverter Unit 2 Inverter Unit 1 Inverter Unit 2 10/22/ Local Load Local Load jx jx Grid Grid

19 PQ Control with Grid-Connection (4) Disconnections with load: Three phase opened one by one due to zero-cross switching Inverter Unit 1 Inverter Unit 2 Local Load jx Grid Inverter Unit 1 jx Local Load Grid Inverter Unit 2 10/22/

20 PQ Control with Grid-Connection (5) Reclose Synchronization in parallel Control strategy switch Fundamental and harmonic load sharing before and after switching P,Q transient exists due to non-zero X Inverter Unit 1 Inverter Unit 2 Local Load jx Grid 10/22/

21 PQ Control with Grid-Connection (6) Control goals Low steady state PQ tracking error Relatively fast transient response Steady state decoupling between P and Q controls Small PQ transient at reclose Load sharing in island mode including harmonic load sharing Load sharing in grid-connected mode As less control interconnections between the units as possible 10/22/

22 PQ Control with Grid-Connection (7) Power control principle Power flow between two souces P = V out X E sinδ V out /_δ jx I Line E/_0 Q = V 2 out V out X E cosδ 10/22/

23 PQ Control with Grid-Connection (8) V, δ impacts on P, Q δ -> > P, V -> Q 10/22/

24 PQ Control with Grid-Connection (9) Basic methodology Add one more feedback control loop the power loop The power controller generates voltage command 10/22/

25 PQ Control with Grid-Connection (10) Existing approaches Integral control based technique Parameter based feedforward V, δ,, E, X P, Q P ref, Q ref V ref, δ ref 10/22/

26 PQ Control with Grid-Connection (11) The problems The integral control Slow when P, Q error is large Parameter based feedforward Without coupling inductor Line impedance X unknown With a coupling inductor Extra component Local load voltage ripple To utilize the filter inductor/transformer Existence of capacitors causes complexity in analytical solution Parametric inaccuracy Unknown power of local load Control plant structure change Multi-units units power control problems not reported 10/22/

27 PQ Control with Grid-Connection (12) Proposed research direction Single unit case Integral control plus a good inner voltage loop Robust servomechanism plus discrete-time time sliding mode A new feedforward power controller to eliminate PQ transients at reclose Multi-unit unit case Load sharing between multiple units with gird-connection and local load, on the basis of technique for load sharing in island mode Synchronization before reclose Feedforward issue 10/22/

28 PQ Control with Grid-Connection (13) Simulation results single unit Island Mode, V-I control Grid connected, P,Q step response 10/22/

29 PQ Control with Grid-Connection (14) Simulation results single unit Grid connected, Line current conditioning Grid connected, Reclose 10/22/

30 The 3-Ph 3 4-Wire 4 Inverter Control (1) The topology 10/22/

31 The 3-Ph 3 4-Wire 4 Inverter Control (2) Control goals voltage control Low steady state error Low THD Robust to load disturbances Fast transient response Challenges Nonlinear load Unbalanced load Transient load disturbances 10/22/

32 The 3-Ph 3 4-Wire 4 Inverter Control (3) Existing approaches Treat it as 3 independent single phase inverter (half( half-bridge) Control conducted in ABC reference frame Or synchronous reference frame for PI controllers Performs Sine-triangle PWM (SPWM) 10/22/

33 The 3-Ph 3 4-Wire 4 Inverter Control (4) Problems PI control in synchronous frame Designed for fundamental, poor in harmonics Any control algorithm SPWM Higher THD compared to space vector PWM (SVPWM) Over-modulation causes pulse drop out If the control is not done in ABC frame, extra reference frame transformation is needed 10/22/

34 The 3-Ph 3 4-Wire 4 Inverter Control (5) If the front-end is powered by a controlled rectifier, unbalanced inverter load causes Unbalanced input current Ripple on DC bus voltage 10/22/

35 The 3-Ph 3 4-Wire 4 Inverter Control (6) Proposed research direction Utilize SVPWM It has to be modified Add 0-sequence 0 control capability Perform control in stationary αβ0 reference frame Eliminate impacts of unbalanced inverter load on front-end rectifier control and make the input current balanced. 10/22/

36 Research Plan (1) 1. Analyze the traditional three phase three wire inverter plant in theoretical domain. Choose an effective existing voltage control technique to be the lower level controller for power control. 2. Analyze the load sharing problem in island mode in theoretical domain. Choose an effective existing load sharing control technique for parallel operation of multiple DG units in island mode and the harmonic load sharing part should be used in grid-connected mode. 3. Implementation of the single unit voltage control technique in simulation. 4. Implementation of the multi-unit unit load sharing control technique in simulation. 10/22/

37 Research Plan (2) 5. Apply basic power control on single unit with grid connection and implement the control technique in simulation. 6. Develop a feedforward power controller which can estimate the Thevenin parameters of the utility grid and help to find the correct operating point of the grid connected DG. In presence of the knowledge of the operating point, seamless transition should be able to achieved in reclose operation. Demonstrate the effectiveness of the technique in simulation. 7. Conduct case study for grid-connected single unit in simulation. 8. Connect a second inverter to a grid-connected inverter and achieve harmonic load sharing. Demonstrate the result in simulation. 10/22/

38 Research Plan (3) 9. Conduct case study for grid-connected multiple units based on the proposal in simulation. 10. For a three phase four wire system, develop an αβ0 reference frame based control technique plus a modified space vector PWM which can perform 0-sequence 0 control. Demonstrate the result in simulation under different types of load. 11. For a three phase four wire system, analyze the impact on the DC bus voltage by unbalanced inverter load. Develop a rectifier control technique based on the analysis result to yield balanced front-end input current. Demonstrate the performance in simulation. 12. Get familiar with existing code of the DSP controllers in the laboratory setup. 10/22/

39 Research Plan (4) 13. Experimental test for single unit power control with gird-connection. Go through the case studies. 14. Experimental test for multi-unit unit power control with gird-connection. Go through the case studies. 15. Experimental test for the proposed control technique for three phase four wire inverter under different types of load. 16. Experimental test for the proposed control technique for three phase four wire rectifier under unbalanced inverter load. 10/22/

40 The Experimental Setup (1) Measurements: A: 2C, 2V; A : 2C, 2V; B: 1C, 1V; B : 1C, 1V; C: 2C, 2V; C : 2C, 2V; D: 3C, 3V; D : 3C, 3V; E: 3C, 3V; E : 3C, 3V; Total: 22C + 22V = 44 Channels E Circuit Breaker M2 208V Main Circuit Breaker M1 Unit 1 A B C D Contactor M2 Circuit Breaker L1 Contactor L1 240V Main Load E Circuit Breaker M3 Unit 2 A B C D Contactor L2 Circuit Breaker M4 Contactor M4 Circuit Breaker L2 240V Main Load 10/22/

41 The Experimental Setup (2) Signal Conditioning Module 240V L-L Main CB A in B in C in Input A out B out C out 4.2mH Inductor 4.2mH Inductor Rectifier + - Inverter 1.8mH Inductor - in DC - out + in + out A in B in C in Inverter & Transformer Primary A out B out C out /Y Transformer 120V L-N Main CB A in B in C in N in A in B in C in N in Load & Transformer Secondary Bypass A out B out C out N out A out B out C out N out CB CB 55µF Capacitor Bank A B Load C ABCN N 5µF Y Cap Bank Legend Thick wire with big banana plugs at both ends Thin wire with a big banana plug at one end and a small banana plug at the other end Thin wire with a capacitor connector at one end Testing wire with small banana plugs at both ends Signal Conditioning Module 240V L-L Main CB A in B in C in Input A out B out C out 4.2mH Inductor 4.2mH Inductor Rectifier + - Inverter 1.8mH Inductor - in DC - out + in + out A in B in C in Inverter & Transformer Primary A out B out C out /Y Transformer A in B in C in N in Load & Transformer Secondary A out B out C out N out CB 55µF Capacitor Bank 5µF Y Cap Bank 120V L-N Main CB A in B in C in Bypass A out B out C out CB N in N out 10/22/

42 The Experimental Setup (3) Voltage Divider Signal Conditioning TMS320 LF2407A DSP PWM Optical Isolation To Gate Drive Hall Effect Current Sensor Host PC 10/22/

43 Recent Publications Min Dai, Mohammad N. Marwali, Jin-Woo Jung, and Ali Keyhani, A PWM rectifier control technique for three phase double-conversion UPS under unbalanced load, IEEE APEC 05, accepted for oral presentation. Jin-Woo Jung, Min Dai, and Ali Keyhani, "Modeling and control of a fuel cell based Z-source converter, IEEE APEC'05, Austin, TX, USA, March 6-10, 2005, accepted for oral presentation. Min Dai, Mohammad N. Marwali, Jin-Woo Jung, and Ali Keyhani, Power flow control of a single distributed generation unit with nonlinear local load, IEEE PSCE 04, Oct. 2004, New York, NY, PSCE pdf. J. W. Jung, M. Dai, and A. Keyhani, "Optimal control of three-phase PWM inverter for UPS systems," IEEE Power Electronics Specialist Conference (PESC'04), pp , Aachen Germany, June 20-24, /22/