The Smart Transformer: impact on the electric grid and technology challenges

Transcription

1 Marco Liserre The Smart Transformer: impact on the electric grid and technology challenges Chair of Power Electronics Christian-Albrechts-Universität zu Kiel Kaiserstraße Kiel

2 Chair of Power Electronics Head of the Chair - Associate Prof. at Politecnico di Bari, Italy - Professor Reliable Power Electronics at Aalborg University, Denmark - Professor and Head of Power Electronics Chair at Christian- Albrechts-Universität zu Kiel, September 2013 Listed in ISI-Thomson report World s Most Influential Minds Active in international scientific organization with 4 IEEE Awards (IEEE Fellow, journals, Vice-President, conferences organization) EU ERC Consolidator Grant (only one in EU in the field of power sys.) Created or contributed to the creation of several scientific laboratories Grid-connected converters (15 years) and reliability (last 5 years) Chair of Power Electronics Marco Liserre ml@tf.uni-kiel.de slide 1

3 Chair of Power Electronics Participating in the two major German initiatives regarding Energie Wende Power Electronics Laboratory Medium Voltage Laboratory under construction Several industrial Partners 50 people (every Year) Several research Partners 9 Mill Euro (3.5 Year) Chair of Power Electronics Marco Liserre ml@tf.uni-kiel.de slide 2

4 Outline From the Solid-State-Transformer (SST) to the Smart Transformer The Smart Transformer in the electric grid: identify the LV-grid, control the load/generation, offer services to MV-grid The technological challenges of the DC/DC converter The Smart Transformer: a grid-tailored Solid-State-Transformer Chair of Power Electronics Marco Liserre ml@tf.uni-kiel.de slide 3

5 From the Solid-State-Transformer (SST) to the Smart Transformer Chair of Power Electronics Marco Liserre slide 4

6 Concept and Definition of SST Definition by Mr. McMurray, 1968 : Electronic Transformer is a device based on solid state switches which behaves in the same manner as a conventional power transformer. by Mr. Brooker, 1980 : Solid State Transformer is a apparatus for providing the voltage transformation functions of a conventional electrical power transformer with waveform conditioning capability. Currently: Power electronic based solution to replace the standard LF transformer, with the features: galvanic isolation between the input and the output of the converter. active control of power flow in both directions compensation to disturbances in the power grid, such as variations of input voltage, short-term sag or swell. provide ports or interfaces to connect distributed power generators or energy storage device Smart Transformer: Solid State Transformer with control functionalities and communication. Chair of Power Electronics Marco Liserre ml@tf.uni-kiel.de slide 5

7 What is the Smart Transformer? The Smart Transformer is: a power electronics based transformer a power system management node a link to different ac or dc infrastructures a possible storage-integration technology a link to other energy sources (gas, heat, hydrogen) a support for the EV infrastructure The Smart Transformer relieves the demand on single renewable sources and increases the capacity of electrical lines Chair of Power Electronics Marco Liserre ml@tf.uni-kiel.de slide 6

8 Line miles Impact of the Smart Transformer Harbours Charging stations Wind/PV systems Data centers centralized peak decentralize d peak massive investments to modernize the electric grid: 70 s - Higher mobility of people - Electric vehicles charging infrastructure - Renewables Energies - Booming of internet -> large data centres Smart Infrastructures are also lighter infrastructures Chair of Power Electronics Marco Liserre ml@tf.uni-kiel.de slide 7

9 power electronics based transformer in traction application Traditional solution LF transformer (16 2/3 Hz) very bulky and heavy Low efficiency: 90 ~ 92 % Around 7tons Main concern: Reduce volume and weight Efficiency improvement Chair of Power Electronics Marco Liserre ml@tf.uni-kiel.de slide 8

10 power electronics based transformer in distribution application The Smart Transformer Main requirements Replace the traditional LF distribution tranformer HF/MF isolation Provide additional functionalities Functionalities Voltage sag and harmonics compensation Load voltage regulation Disturbance Rejection (available dc-link) Power Factor Correction VAR Compensation and Active filtering Overload and short-circuit protection Chair of Power Electronics Marco Liserre slide 9

11 The Smart Transformer The Smart Transformer features shall be: LV and MV DC-links available Advanced control of all the three-stages The system should be able to work even with faulty modules During partial loading conditions it should be able to fully use its rating for other services Chair of Power Electronics Marco Liserre slide 10

12 The Smart Transformer A system level optimization! Chair of Power Electronics Marco Liserre ml@tf.uni-kiel.de slide 11

13 Smart Transformer services for the electric grid Smart Transformer Voltage support (steady state and LVRT) Reactive power compensation at HV/MV substation Power quality improvements Islanding control (high DG in LV) Integration of EV-charging stations Integration of storage for dispatching Reverse Power Flow limitation Impedance identification Load identification Reverse Power Flow limitation ST overload control Soft-load reduction Damping of harmonics and resonances LV-side power quality Chair of Power Electronics Marco Liserre slide 12

14 Control of the Smart Transformer M. Liserre, G. Buticchi, M. Andresen, G. De Carne, L. F. Costa and Z. X. Zou, "The Smart Transformer: Impact on the Electric Grid and Technology Challenges," in IEEE Industrial Electronics Magazine, vol. 10, no. 2, pp , Summer Chair of Power Electronics Marco Liserre slide 13

15 The Smart Transformer in the electric grid: 1. identify the LV-grid, 2. control the load/generation, 3. offer services to MV-grid Chair of Power Electronics Marco Liserre slide 14

16 The Smart Transformer in the electric grid: 1. identify the LV-grid, 2. control the load/generation, 3. offer services to MV-grid Chair of Power Electronics Marco Liserre slide 15

17 On Line Load Identification The load can be represented with an exponential model for the voltage and with a linear dependency from the frequency P = P 0 V V 0 K p 1 + K fp f f 0 f 0 (1a) Q = Q 0 V V 0 K q 1 + K fq f f 0 f 0 (1b) V Independent of initial voltage and does not require initialization V Only one parameter is needed for active and one for reactive power. V The exponent is equal to load sensitivity to voltage G. De Carne; M. Liserre; C. Vournas, "On-line load sensitivity identification in LV distribution grids," in IEEE Transactions on Power Systems, vol.pp, no.99, pp.1-1 Chair of Power Electronics Marco Liserre ml@tf.uni-kiel.de slide 16

18 On Line Load Identification: Voltage Sensitivity Let us derive the equation for V (2) and rearranging the equations are the point V=V0 (3): dp dv = K pp 0 dq dv = K qq 0 V V 0 V V 0 K p 1 1 V 0 K q 1 1 V 0 And discretizing for the time step tk: (2) dp P0 = K dv p V0 (3) dq Q0 = K dv q V0 P t k P t k 1 Q t k Q t k 1 K p = V t k P t k K V t q = k 1 V t k V t k Q t k (4) V t k 1 V t k Chair of Power Electronics Marco Liserre ml@tf.uni-kiel.de slide 17

19 On Line Load Identification: Influence of DG on the identification Integrating eq. (9) in eq. (4) we obtain: K p = PΤP 0 P L = K VΤV p,l (10) 0 P L P G The net load reacts in different way depending on the presence of DG. Example: P L = 1, P G = 0 P 0 = 1 K p,l = 1 K p = = 1 P L = 1.5, P G = 0.5 P 0 = 1 K p,l = 1 K p = = 1.5 P = P 0 V V 0 1 Linear response P = P 0 V V More than linear response Chair of Power Electronics Marco Liserre ml@tf.uni-kiel.de slide 18

20 On Line Load Identification: algorithm implementation Start ST Voltage t k k=k+1 ST Frequency dv 0 dt? dv 0 dt Y Save V,P,Q N ST Active Power Evaluation Sensitivities K p,k q t k -1 Z t k-1 ST Reactive Power Store K p,k q t k >t kmax? N Chair of Power Electronics Marco Liserre ml@tf.uni-kiel.de slide 19 Y Average K p,k q

21 Load identification Load parameter identification with respect to Voltage and Frequency Chair of Power Electronics Marco Liserre slide 20

22 The Smart Transformer in the electric grid: 1. identify the LV-grid, 2. control the load/generation, 3. offer services to MV-grid Chair of Power Electronics Marco Liserre slide 21

23 Soft Load Reduction High load consumption can affect the system stability. The generators may not follow the load demand during grid contingencies. In case of perturbations (e.g., faults) or critical conditions (e.g., devices overload), the load shedding represents an effective, although costly, solution. The Smart Transformer can instead reduce the load consumption performing a softload reduction. Controlling the voltage amplitude in LV grid, the load power consumption can be shaped. G. De Carne, G. Buticchi, M. Liserre, C. Vournas, Load Control using Sensitivity Identification by means of Smart Transformer IEEE Transactions on Smart Grid. Chair of Power Electronics Marco Liserre ml@tf.uni-kiel.de slide 22

24 Soft-load reduction For each phase a,b,c P A = P A V V A K pa (1) Differ. The power variation in each phase gives: P A = P A V A K pa V V A P A + P B + P C = P (2) (3) Including (2) in (3) the general equation (4) is obtained V = 1 + P+ P AK pa + P B K pb + P C K pc V 0 P A VA K pa + P B V K pb + P C B V K pc C (4) Chair of Power Electronics Marco Liserre ml@tf.uni-kiel.de slide 23

25 Soft-load reduction Simplifying (4) in (5), the voltage variation to impose in order to get the desired power variation ΔP is obtained: V V 0 = 1 + P P A K pa + P B K pb + P C K pc (5) Example in Figure: the reduction power request is 5%. The load has voltage sensitivity coefficients varying between 0.5 and 0.9 pu (plot above). The Soft Reduction algorithm decides to decrease the voltage of 0.08 pu (central plot). The load shed during the considered time window is about 5% (plot below). Chair of Power Electronics Marco Liserre ml@tf.uni-kiel.de slide 24

26 The Smart Transformer in the electric grid: 1. identify the LV-grid, 2. control the load/generation, 3. offer services to MV-grid Chair of Power Electronics Marco Liserre slide 25

27 Increasing DG hosting capacity Typical DG penetration limits in MV feeders Voltage rise during light load Compensation of sudden loss of RES power If at least some MV feeder loads are supplied through STs ST MV converter can apply voltage control Either locally or with remote measurement ST can also provide emergency P control Acting on the LV connected load or DG Gao, X., G. De Carne, M. Liserre, C. Vournas. "Increasing Integration of Wind Power in Medium Voltage Grid by voltage support of Smart Transformer." EWEA Chair of Power Electronics Marco Liserre ml@tf.uni-kiel.de slide 26

28 Feeder with Wind Gen Typical Distribution feeder with Distributed Generation Case studies for voltage regulation and DG hosting capacity Chair of Power Electronics Marco Liserre slide 27

29 MV feeder Test results Without ST Consider allowed overvoltage ΔV up to 2,5% Max penetration limit 7.5 MW Chair of Power Electronics Marco Liserre slide 28

30 MV feeder Test results With ST ΔV limit +2,5% Max penetration limit 12 MW Increase of 4.5MW (60%) Chair of Power Electronics Marco Liserre slide 29

31 Dispatching by means of Smart Transformer-based storage Voltage control in distribution systems is normally performed by on-load tap changer transformer and/or controllable DG. We consider a smart transformer where battery-based storage capacity is added on the DC bus (ST + storage) to extend the class of ancillary service it is possible to provide. A control strategy for a ST + storage to: 1. dispatch the operation of the underneath distribution system; 2. control the voltage of the LV and MV grids on a best effort basis by exploiting smart meters and remote terminal units measurements and/or state estimation processes. Fabrizio Sossan, Kostantina Christakou, Mario Paolone, Xiang Gao, Marco Liserre Enhancing the Provision of Ancillary Services from Storage Systems using Smart Transformer and Smart Meters, ISIE 2017 Chair of Power Electronics Marco Liserre ml@tf.uni-kiel.de slide 30

32 Dispatching the operation of the LV network The power flow at the MV/LV interface follows a dispatch plan (average power flow at 5 minutes resolution) established the day before the operation. A two-stage procedure based on [1]: 1. Day-ahead procedure: The dispatch plan is computed. 2. Real-time operation: The battery charge/discharge is controlled to compensate the mismatch between the dispatch plan and realization. When the control strategy is actuated (-- line), the power flow at the MV interface is as the dispatch plan (shaded profile), while it would be stochastic otherwise (- line). Dispatch error from 350 to < 0.1 kwh/day. [1] Sossan, E. Namor, R. Cherkaoui and M. Paolone, "Achieving the Dispatchability of Distribution Feeders Through Prosumers Data Driven Forecasting and Model Predictive Control of Electrochemical Storage," in IEEE Transactions on Sustainable Energy, Chair of Power Electronics Marco Liserre ml@tf.uni-kiel.de slide 31

33 Real-time voltage regulation of the MV network The MV converter reactive power flow is regulated to control the voltage of the MV grid. Let Without control be the voltage sensitivity coefficients of each node with respect to the ST reactive power injection, the real-time control problem is: ref Minimize deviation between voltage and reference value over all the buses. With control Converter apparent power constraint. Improved MV voltage levels. Average deviation reduced from to -0.11%. Max. abs. deviation from 4.06 to 2.51% Chair of Power Electronics Marco Liserre slide 32

34 Real-time voltage regulation of the LV network The voltage at the root of the feeder is set such that the voltage levels along the buses are as close as possible to the reference value V*ref. Without control Let be the voltage sensitivity coefficients of each LV node with respect to the voltage at the root of the feeder V*. The real-time control problem is: Minimize deviation between voltage and reference value over all LV buses. ref With control Improved LV voltage levels. Average deviation reduced from to -0.03%. Max. abs. deviation from to 8.45% Chair of Power Electronics Marco Liserre slide 33

35 Conclusions regarding integration of storage through ST A control strategy for a smart transformer with integrated storage to stack the following ancillary services: dispatch the operation of the underneath distribution system; voltage control of the MV network; voltage control of the LV network. Simulations on the 34-bus IEEE test feeder and the CIGRE reference network for LV systems. Dispatched operation is attained with an energy error < 0.1 kwh per day, the average voltage deviation from the reference is reduced from 4.0% to 2.5% on the MV side, and 16.0% to 8.5% on the LV. Noncomplex architecture and IT infrastructure. All the control is localized at substation level, only smart meters measurements are required from remote units. Chair of Power Electronics Marco Liserre ml@tf.uni-kiel.de slide 34

36 The technological challenges of the DC/DC converter Chair of Power Electronics Marco Liserre slide 35

37 Challenges of the DC-DC Stage DC-DC Stage: The most challenge stage Isolation Efficiency Cost Deserves more attention High voltage Isolation High Input voltage High output current Galvanic Isolation in Medium/High frequency Power flow control dc link control Dc breacker feature (short circuit current proctection) Chair of Power Electronics Marco Liserre slide 36

38 DC-DC Stage: Implementation Concept Non-Modular Vs Modular Fewer number of components High Voltage WBG devices Simple control/communication system Low voltage/current rating semiconductors Scalability in voltage/power Fault tolerance capability Reduced dv/dt and di/dt Chair of Power Electronics Marco Liserre slide 37

39 Challenges of the DC-DC Stage DC-DC Stage: Building Block Converter High Voltage Isolation Bidirectional power flow Galvanic Isolation in Medium/High frequency Power flow control dc link control Dc breacker feature (short circuit current proctection) Efficiency Chair of Power Electronics Marco Liserre slide 38

40 Review on high efficiency dc-dc converter Relevant converters: Dual-Active-Bridge Converter Power converter: DAB Cell topology: FB, 3L Semiconductor: Mosfet, SiC Chair of Power Electronics Marco Liserre slide 39

41 Review on high efficiency dc-dc converter Relevant converters: 3 Phase - Dual-Active-Bridge Converter Power converter: 3P-DAB Cell topology: FB-3Legs/-6Legs Semiconductor: Mosfet, IGBT Chair of Power Electronics Marco Liserre ml@tf.uni-kiel.de slide 40

42 Review on high efficiency dc-dc converter Relevant converters: Triple-Active-Bridge Converter Power converter: TAB Cell topology: FB Semiconductor: Mosfet, IGBT Chair of Power Electronics Marco Liserre slide 41

43 Review on high efficiency dc-dc converter Relevant converters: Series-Resonant Converter Power converter: SRC Cell topology: FB, HB, 3L, NPC Semiconductor: Mosfet, IGBT, SiC, GaN Chair of Power Electronics Marco Liserre slide 42

44 Review on high efficiency dc-dc converter Relevant converters: Phase-shift Full-Bridge Series-Resonant Converter Dual-Active-Bridge Multiple-Active-Bridge Chair of Power Electronics Marco Liserre slide 43

45 Series-Resonant Converter Target: Efficiency Reliability Accurate losses modeling Automatic design - (optimum parameter selection) Wideband gap devices Fault tolerant topology Lifetime devices considerations Chair of Power Electronics Marco Liserre ml@tf.uni-kiel.de slide 44

46 Series-Resonant Converter Overview of basic dc-dc topologies suitable to be used as a building block of the ST dc-dc stage Influence on efficiency: Wideband-gap devices plays an important role Design: correct parameters selection CAU Kiel dc-dc converter Max Eff = 98.61% Eff (@P max ) = 98.1% L. F. Costa, G. Buticchi, M. Liserre, Highly Efficient and Reliable SiC-based DC-DC Converter for Smart Transformer, in IEEE Transactions on Industrial Electronics Chair of Power Electronics Marco Liserre ml@tf.uni-kiel.de slide 45

47 Quadrupole Active Bridge Extension of the DAB with 2 additonal ports Operation is similar to DAB Phase shift modulation for power transfer Power transfer between all ports possible: Chair of Power Electronics Marco Liserre ml@tf.uni-kiel.de slide 46

48 Quadrupole Active Bridge Phase shift affects power transfer between bridges Demonstration for: Phase shift modulation affects additional reactive currents -> additional losses Schematic voltages and currents for the QAB. Chair of Power Electronics Marco Liserre slide 47

49 Quadrupole Active Bridge ZVS range of DAB and QAB are similar under symmetrical loading Reactive currents are also similar under symmetrical loading Zero voltage switching range of the DAB [Alonso, 2010]. Zero voltage switching range of the QAB and reactive currents. Chair of Power Electronics Marco Liserre slide 48

50 Quadrupole Active Bridge SiC-based Input voltage (MV side): 1.8 kv Voltage of the MV cells: 600 V Output voltage (LV side): 700 V Power: 10 kw Efficiency 97.5 % L. Costa, G. Buticchi, M. Liserre "Quad-Active-Bridge DC-DC Converter as Cross-Link for Medium Voltage Modular Inverters" IEEE Transactions on Industry Applications. Chair of Power Electronics Marco Liserre ml@tf.uni-kiel.de slide 49

51 Quadruple Active Bridge Overview of basic dc-dc topologies suitable to be used as a building block of the ST dc-dc stage Influence on efficiency: Wideband-gap devices plays an important role Design: correct parameters selection CAU Kiel dc-dc converter Max Eff = 97.5% (SiC) Highest efficiency of a MAB converter Chair of Power Electronics Marco Liserre ml@tf.uni-kiel.de slide 50

52 DC/DC for the Smart Transformer Dual/Quad Active Bridge Serie Resonant Converter DAB controlability Voltage and current sensors simplicity LVDC link control VSI Chair of Power Electronics Marco Liserre slide 51

53 DC/DC for the Smart Transformer Dual/Quab Active Bridge DAB controls the LVDC link Serie Resonant Converter CHB controls the MVDC link and, consequenlty the LVDC link Chair of Power Electronics Marco Liserre slide 52

54 Avergae Model of DAB and SRC Dual Active Bridge MV DC link Series Resonant Converter LV DC link DAB SRC Chair of Power Electronics Marco Liserre slide 53

55 DAB or QAB? Nº of Units Unit power level Nº of CHB MV dclink IGBT voltage Mean IGBT current (kw) cells (kv) rating (kv) current rating (A) Cost QAB DAB QAB DAB (A) (U$) Chair of Power Electronics Marco Liserre ml@tf.uni-kiel.de slide 54

56 DAB or QAB? Chair of Power Electronics Marco Liserre slide 55

57 Scaled Prototype: Architecture Peak power: 100kW Rated power of a phase unit: 30kW Three-phase converter Each phase contains: QAB converter 3-Cell CHB converter Chair of Power Electronics Marco Liserre ml@tf.uni-kiel.de slide 56

58 Scaled Prototype: Parameters Parameter Value Peak power 100 kw MV DC link voltage 800 V LV DC link voltage 800 V AS rms voltage 1500 V DC link capacitance 750 μf Number of FB cells 21 Number of MF transfromers 3 Transformer rated power 30 kw DAB switching frequency 20 khz Number of levels in CHB 7 CHB carrier frequency 7 khz CHB effective switching frequency 21 khz Chair of Power Electronics Marco Liserre ml@tf.uni-kiel.de slide 57

59 The Smart Transformer: a gridtailored Solid-State-Transformer Chair of Power Electronics Marco Liserre ml@tf.uni-kiel.de slide 58

60 How to rate the ST? L. Ferreira Costa, G. De Carne, G. Buticchi and M. Liserre, "The Smart Transformer: A solid-state transformer tailored to provide ancillary services to the distribution grid," in IEEE Power Electronics Magazine, vol. 4, no. 2, pp , June Chair of Power Electronics Marco Liserre ml@tf.uni-kiel.de slide 59

61 How to rate the ST? Chair of Power Electronics Marco Liserre slide 60

62 How to rate the ST? Chair of Power Electronics Marco Liserre slide 61

63 Summary Difference between SST and ST is in functionalities The virtuous flow: identify, regulate, use the capacity for services SRC is for efficiency but for ST DAB/QAB are needed to decouple MV and LV A grid-tailored design means ST is not rated equally in all its stages Chair of Power Electronics Marco Liserre ml@tf.uni-kiel.de slide 62

64 Join the PhD Course Feb 2018 Half time in Lab! Chair of Power Electronics Marco Liserre slide 63