OPPORTUNITIES FOR POWER ELECTRONICS DISTRIBUTED ENERGY SYSTEMS

Transcription

1 CHALLENGES AND OPPORTUNITIES FOR POWER ELECTRONICS IN THE INTEGRATION OF DISTRIBUTED ENERGY SYSTEMS Marta Molinas NTNU Seminar at University of Padova July 2, 29

2 Topics in Today s presentation Low Voltage Ride Through (LVRT) of wind energy conversion systems STATCOM based torque control in a wind energy conversion system Reactive power ancillary service provided by distributed power electronics loads High frequency direct AC link for reducing the nacelle weight in wind energy conversion systems for offshore solutions Wave energy conversion systems with all electric power take off systems: control challenges for STATCOM and Back to Back converters

3 Low Voltage Ride Through (LVRT) in Wind Energy Conversion Systems Power Electronics Interface GRID Large scale 2-5% Grid capacity. M. Molinas et.al. Robust Wind Turbine System Against Voltage Sag with Induction Generators Interfaced to the Grid by Power Electronic Converters," IEEJ 26, vol. 27D, no. 7pp

4 LVRT Profiles in Grid Codes Nordic Grid code 75% voltage drop for 25 ms Transient 95% -,5 sec. after fault 95% Small reduction of Output power (%)

5 Case-study study (): Back-to-Back Wind turbine Gear Generator side converter Grid side converter G Cage Induction Generator Power Electronics Interface DC-link Electric Gid Grid GRID

6 Experimental Investigation Wind turbine RS-232 Gear RS-232 Turbine Emulator Commercial converter M Host PC G Cage Induction Generator 55 kw M-G set-up G Set to give Constant nominal torque V gen Generator side converter I I gen CANGrid side converter DSP DSP 2 IGBT PWM Inverter V dc DC-link IGBT PWM Inverter V grid I grid mh.2mh.5mh Short circuit ms Electric Utility Bus Grid4 V

7 Line volltage (pu) Results () Line-side converter power (pu) Line vo oltage and DC-Link (pu) -.5 Line-side co onverter currents (pu) Current limit of g grid side Conv. set to pu DClink is very y stiff Time (s) P is kept relatively constant Th Id rises The i up to the h lilimit i off pu.5 -.5

8 Results (2) Current limit of grid side Conv. set to.8 pu Line voltage (pu) Line volta age and DC-Link (pu) Line-side converter pow wer (pu) L Line-side conve erter currents (pu) Excess power from generator-dc link rises above safety limit Converter protection acts And trips for overvoltage Time (s)

9 Restults (3) Current limit of grid side Conv. set to.8 pu Line voltag ge (pu) tage and DC-Link (pu) Line vol Line-side converter power (pu) Line-side con nverter currents (pu) )Excess power from generator-dc link rises-active DClink control activated on gen.side Generated power is reduced to keep DClink under control The I d rises up to the limit of.8 pu Time (s)

10 Case-study study (2): STATCOM G G Electric Grid G STATCOM Wind or Wave Farms with Asynchronous generators M. Molinas et.al. Low Voltage Ride Through of Wind Farms With Cage Generators: STATCOM Versus SVC," IEEE Trans. PE 28, vol. 23, no. 3, pp. 4-7

11 Experimental Model Set to give Reference torque 5 kw Host PC

12 Voltage regulation to wind Grid voltage [pu u] ] id power [pu] Gr [pu] Statc com current [ Uncontrolled voltage Controlled voltage Power to Grid Q from Grid (uncontrolled) STATCOM current Time [s] Controlled Q

13 LVRT test without STATCOM Gr rid voltage [p u] Grid power [p pu] Voltage collapse Power is around Zero Time [s] Gen nerator speed [pu] Generator speed accelerates Time [s]

14 LVRT-STATCOM.5 pu 25% voltage Grid voltage [pu] Terminal voltage Grid pow wer [pu] WT Power Power recovers fast Time [s]

15 LVRT-STATCOM pu 25% voltage Grid voltag ge [pu] Terminal voltage Faster Power recovery Grid po ower [pu] Time [s]

16 LVRT- STATCOM pu Gr rid voltage [pu u] tatcom curren nt [pu] St Statcom d- current Statcom q- current Generator spe eed [pu].3.2. WT generator speed Time [s]

17 Consequences of STATCOM for LVRT Voltage Vmains [pu] I STATCOM =.8 pu I STATCOM = pu I STATCOM =.5 pu No control.5.5 a).8 NO STATCOM - UNSTABLE Speed IG spe eed [pu] No-control I STATCOM = pu I STATCOM =.5 pu I STATCOM =.8 pu.5.5 b) STATCOM current I statcom [pu u] Time [s] M. Molinas et.al. Extending the Life of Gear Box in Wind Generators c) by Smoothing Transient Torque with STATCOM," under review process in IEEE Trans. IE, 29

18 Influence of STATCOM operation on generator torque NO STATCOM - UNSTABLE IG Torque [pu].5 Accelerating torque Higher Iq gives: -.5 No control I STATCOM =.5 pu Faster recovery - I STATCOM = 8pu.8 I STATCOM = pu STATCOM More stable system -.5 But higher peak torque Time [s]

19 STATCOM based Torque Control DC link L f STATCOM v a, b i a, b PWM v Clark, Clark i i, V f ( T, n ) ref ref gen n g T ref ITC V * dc V dc + - PI * d i Voltage Oriented Park Vector Current Control v i Park-inv. v d d * v * d v q + L Park i q V ref pu V ref PI * i q ITC V d Normal STATCOM

20 Torque Control Effect

21 STATCOM In a Wind Park Wind turbine Gear Box G A PCC Three line to ground fault Cage Induction Generator Wind turbine 2 Gear Box STATCOM Transformer Electric Grid Grid G B Cage Induction Generator 2 STATCOM

22 Results: Grid side ] Terminal volta age [pu] Volta age at PCC [pu O * X X O * Generator - Normal STATCOM Generator - ITC Generator 2 - Normal STATCOM Generator 2 - ITC Time [s] X.2 Normal STATCOM X ITC Time [s] Grid Power [MW W] X X Normal STATCOM ITC Time [s]

23 Reactive Power Ancillary Service by Distributed Responsive Loads Power Electronics dominated power systems Grid P P P Q P Q P Q P Three-phase line P Distributed Generation Loads Loads Loads M. Molinas et.al. Investigation on the role of power electronic controlled constant power loads for voltage support in distributed AC systems," IEEE PESC28, Rhodes 28.

24 Active Rectifier Interfaced Load to AC distributed system Typical examples of CPL load motor drives power supplies interface with diode/thyristor rectifier large rectifiers for DC loads aluminum plants, paper mills Pref Vref P V CPL controller PI PI Iq,ref Id,ref Vector current control Vpwm Induction motor drive system with active rectifier CPL R Induction motor Load

25 System Investigated Point of voltage measurement Asynchronous Generator L PCC L L A Lg Distribution System Fixed Capacitor Line to ground fault Grid RP RP RP RP L,2 pu LP LP LP LP L g,2 pu = R C = R C = R C STATCOM CPL CPL2 CPL3 One CPL=25% of generated power

26 CPLs operate with Negative incremental Resistance 2 dv 2 P V i P R di i P 2 L

27 Incremental Current Rating q I d d Voltage drop at the VSC terminals..6 Incremental current rating moderate I t I q I d P I t I d V d I I I 2 2 t d q I It I * t I * t In this region it is beneficial to have it in a CPL than in a STATCOM Total current when I q is disabled

28 Required Total Current.6.55 i d (i q )forv=. g i tot (i q ) for v g =. i d (i q ) for v g =.8 Total current rating as function of grid parameters.5.45 i tot (i ) for v =.8 q g i d (i q ) for v g =.8 and reduced reactance i tot (i q ) for v g =.8 and reduced reactance Minimum required Iq for reducing stresses in the grid Minimum total current does not apper at PF= i [pu] x g CPL Reduced x g.8 pu.25 pu.4 pu iq [pu] V g R g X g P const

29 Distributed Iq versus STATCOM distributed reactive current support by CPL less than with STATCOM > 3 ms fault with 2 CPLs more convenient than STATCOM 3 CPLs with I q always more convenient Total power drawn by CPLs is kept constant = 8% of generated power

30 Critical Clearing Time and Iq CCT CCT.5.5 V CPL [p.u.] Time [s] Voltage measured at PCC Time [s] CCTs for different loading types and regulation of CPLs Type of loading Regulation CCT Case : 8% CPL P constant and I q = 62 ms Case 2: 2% CPL, 6% induction motor P constant and I q 87 ms Case 3: 4% CPL, P constant t and I q 238 ms 4% induction motor Case 4: 8% CPL P constant and I q =38% 5 ms

31 A Reactive Power Investigation: the System

32 Reactive Power Characteristic 2.5 Distribution line with z=.257+j.4 pu, X/R=.56 Transmission line with z=.869+j.7726 pu, X/R=9.48 Subsea cable with z=.5+j.4 pu, X/R=8.2 [pu] wer compensation Qc Reactive po u] e power compensation [pu Reactive -4-2 Conditions:r=.294, x=.247 in pu (very long distribution lines from -5 UMIST source) Q=.2; P=.2; Vs=; n=3 (for distributed compensation) centralized compensation distributed compensation Voltage at the point of load connection [pu] Voltage at point of load connection [pu] Type of line Impedance [pu] X/R ratio Very long distribution line.257+j.4.56 Transmission line.869+j Sub-sea cable.5+j M. Molinas, J. Kondoh, Reactive Power Ancillary Service with Power Electronic Loads: Analytical and Experimental Investigation," EPE 29, Barcelona.

33 Converter control influence Reactive po ower compensation, Active power, DC link voltage Vdc [VAR, W, V] React tive power compensatio on [VAR], Active power P[W], DC link voltage Vdc[V] Measured reactive power compensation Qc Measured active power P Analytically obtained Qc curve with measured values of impedance Measured converter DC link voltage for controlling Qc Measured Qc curve obtained in the lab Measured active power P -3 Analytically obtained Qc curve with measured values of impedance Measured DC link voltage during control of Qc Voltage at point of compensation [Volts] Voltage at point of compensation[volts]

34 High Frequency Direct AC Link for Wind Energy Conversion Offshore A. B. Mogstad, M. Molinas, A Power Conversion System for Offshore Wind Parks," IEEE IECON28, Florida 28.

35 Standard Solution: offshore AC grid with centralized converter

36 Proposed Series Connection

37 Direct AC link Full Bridge Direct AC-AC converter Bi-direction Switches

38 Comparing the Losses

39 Wave Energy Conversion Systems: Control Challenges for Power Electronics WEC G WEC G Interface Technology Electric Grid WEC G Case : Induction generator + STATCOM Case 2: Doubly fed induction generator with rotor converter Case 3: Induction generator wiht full converter Molinas et.al., Power electronics as grid interface for actively controlled wave energy converters," IEEE ICCEP, Capri 27, pp

40 Challenges Cost-effective: ti active control for increased extraction ti Active control for Grid Code compliance Power electronics for both: Active control of WEC and Power quality

41 Power Electronic Interfaces: lessons from wind Case: Induction generator +STATCOM IG Grid AC DC Case 2: Doubly fed induction generator with rotor converter IG AC DC DC AC Grid Case 3: Induction generator with full converter in series IG AC DC DC AC Grid Energy Storage (Batt/Supercap )

42 Power extraction traces for irregular waves 6 5 Passive control 35 Pinst Pavg P peak /P av =7 P 3 peak /P av =7 Latching control Pinst Pavg 25 4 Pow wer (kw) 3 Po ower (kw) Time (seconds) Time (seconds) P av-latching /P av-passive =5 Highly fluctuating power poses difficulties for voltage stability in case of large scale wave power penetration

43 Induction Generator+STATCOM Platform High pressure accumulator IG IG Grid AC DC Buoy Hydraulic PTO with the induction generator- Induction generator with a shunt connected STATCOM as the grid interface technology STATCOM as grid interface

44 Power and Voltage Quality P-No STATCOM Q-No STATCOM P(higher storage) Q(higher storage) P(lower storage) Q(lower storage) VA] P, Q [W, 5 5 P, Q [W,VA] PCC Voltage [pu] Time [s] PCC Voltage [p pu] com currents [pu] Statc Id(lower storage) Iq(higher storage) Id(higher storage) Iq(lower storage) Time [s] Active and reactive powers, and PCC voltage without reactive support by the STATCOM Active and reactive powers, PCC voltage and STATCOM currents for the Case Study with lower and higher energy buffering capacities

45 Induction Generator+ series Back-to- Back converteres IG AC DC DC AC Grid Energy Storage (Batt/Supercap ) Generator system Hydrodynamic forces Power quality easier to handle Complete decoupling between WEC and grid

46 Power Extraction with Active Control active control of WEC with Power Electronics Power quality not a big issue because of grid side converter P av-latching /P av-passive = Wave elevation [m] Buoy position [m] Power extraction with passive control [ kw] Average power [ kw] 2 Wave elevation [m] Buoy position [m] Power extraction with latching control [kw] Average power [kw].5 Power (kw) Time (seconds) Time (seconds) Power extraction ti with passive control for the configuration of full converter in series Power extraction with latching control for the configuration of full converter in series

47 Concluding Remarks Power Electronic components are going to dominate the future electric power systemstem Transient and dynamic interactions of these components with the power system is not yet well understood But it appears clear that control structure and strategy will have a dominant role in a system with a large share of power electronics

48 Future work Influence of modeling approaches for stability investigations of grid dominated by power electronics Detailed mathematical model versus software implemented models for investigating small signal stability Multi domain design approach for energy conversion systems

49 Current trayectory...

50 Concluding Remarks

51 High Frequency Transformer

52 Effect of I q on the Nose Curve ISIE 28 Loading can be increased at the expense of a flat P-V curve Control structure and tuning have an important t influence Voltage,4,2,8,6 Resistive load,4 non-controlled CPL 2 controlled CPLs with Vdc control,2 controlled CPL STATCOM+ non-controlled CPL 2,2 4,4 6,6 8,8 2,2 4,4 6,6 8,8 2 x g CPL.8 pu.25 pu Power

53 Hybrid Thyristor-Transistor Based HVDC Link for Wind Energy

54 Influence of the Droop Control Total reactive current injection with droop control is lower Simple to implement, no need of communication and good result It will influence the nose curve by allowing for increased loading,2 Ip_q [p.u.] CPL reactiv ve current,8,6,4,2 WI TH DROOP WI T HOUT DROOP L [ km]

55 Discussions and future work ISIE 28 Results CPLs increases the chances of voltage instability (voltage collapse) Voltage source converters as preferable interface for loads (controllability, flexibility, ability of I q control) Transient stability improved by I q and distributed I q lower than STATCOM Steady state stability influenced by control structure and tuning Required increase of current rating of converters depend on grid parameters Droop control reduces needed amount of I q and therefore rating of converter Future work Role of the control structure on overall stability Thorough analytical investigation of small signal stability Cii Critical share of CPLs in the system with ih reactive current support Customized design of converters for CPLs Influence of several CPLs control in the system stability

56 4 3 Transient Behavior Voltage at point of converter coonnection Converter DC link voltage Vdc 5 Voltage at point of converter connection Vuv Converter DC link voltage Vdc 4 voltage Vdc [V] oltage Vdc [V] ter connection Vuv, Converter DC link v erter connection Vuv, Converter DC link vo Time [s] Time [s] Voltage at point of convert Voltage at point of conve

57 Doubly Fed Induction Generator IG Grid AC DC DC AC Non suitable for direct drive (speed Non suitable for direct drive (speed variation ) Together with hydraulics PTO Limited LVRT (similar to case )

58 Results 5 [pu] STAT TCOM current O * X X O * Unit - Normal STATCOM Unit - ITC Unit 2 - Normal STATCOM Unit 2 - ITC Time [s] 2 PCC Reactive Power [MVAr] -2-4 O * X Turbine - Normal STATCOM Turbine - ITC Turbine 2 - Normal STATCOM Turbine 2 - ITC Time [s] X O * MVAr] Grid Rea active Power [ -5 - X X Normal STATCOM ITC Time [s]