Solid State Transformers (SST)

Transcription

1 Solid State Transformers (SST)

2 Classical Transformer

3 Classical Transformer

4 Classical Transformer

5 Classical Transformer Higher Frequency Lower Volume

6 Solid State Transformer The SS is one of the key elements in power electronics based microgrids systems 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. The basic idea of the SST is to achieve the voltage transformation by medium to high frequency isolation, therefore to potentially reduce the volume and weight of it compared with the traditional power transformer.

7 Solid State Transformer Key components High frequency transformers Power electronics converters AC to to to AC

8 Solid State Transformer

9 Solid State Transformer

10 Solid State Transformer

11 Solid State Transformer

12 UNIFLEX UNIFLEX was a European project with UoN as technical lead looking at power electronics structures for future European Energy Networks Lots of European leading Industry and Universities Part funded by the European Commission

13 The Solid State Substation Traditional substations are passive: Perform voltage step down from say 33kV to 415V, isolation point etc. Very efficient, Very reliable What if we improved these with power electronics? Why bother? HF Magnetics- smaller footprint Ability to carry out FACTs operations Ability to Link Renewables Ability to Link to Energy Storage More flexible control Reactive power support Link Asynchronous systems..the list continues!!!

14 Introduction UNIFLEX-PM ( Advanced Power Converters for Universal and Flexible Power Management in Future Electricity Networks ) Uniflex Project Objectives: Develop multi-cellular, modular and scalable converter architecture that can be utilised in power systems Analyse system functionalities in different operation modes Validate system functionalities with simulation and experiment

15 UNIFLEX-PM: Concept Power conversion module Controllable AC Voltage (or current) Controllable AC Voltage (or current) Isolation Barrier Isolated modules can be connected in series/parallel Configurable for many power conversion functions Three phase AC-AC power conversion Single phase AC power conversion cut-down version for traction...

16 Potential use of concept Future Electrical Network Possible layout of a future grid with UNIversal and FLEXible Power Module

17 Implementation example Modular multi-level power converter Three ports with bidirectional power flow circa 5 MW rated power Directly grid connected to the Distribution Network (10-20 kv) Incorporates Renewable Energy Systems (RES) and utilises energy storage

18 Overview of Uniflex Functionality 3 UNIFLEX Port 1 Port 2 Port Voltage ratio adjustment example: voltage at Port 1 changes, whilst voltages at Port 2 and Port 3 are maintained constant. Frequency changing Frequency at each port different connection of asynchronous systems Phase changing example: input/output voltages (ie Port 1 Port 2) are locked in frequency but maintained with (controllable) phase shift between them. Asymmetric load current cancellation example: load at port 2 unbalanced (eg unbalanced currents with balanced voltage). Current at Port 1 and Port 3 balanced. Voltage asymmetry cancellation example: voltage at Port 1 unbalanced (ie connected to unbalanced grid). Voltage at Port 2 and Port 3 maintained balanced. Reactive power control Independent control of reactive power at all ports (simultaneously) voltage support Active power control fast control of active power at each port, subject to power balance Harmonic cancellation example: harmonic pollution in currents in port 2 (for example) clean currents on port 1 and 3. Alternatively, harmonically polluted voltage fed to port 1 clean voltage produced at ports 2 and 3.

19 Modular Building Blocks Two technologies considered Both provide bidirectional AC-AC power flow with Medium Frequency (khz) isolation Building block based on / isolation module: Building block based on /AC isolation module:

20 Amplitude (Normalised) Multilevel Converters Amplitude Single H-Bridge Time /s H-Bridges in series Conventional 5 Level Amplitude Time /s Frequency /Hz The more H-Bridges the higher the voltage and the better the approximation to a sine wave (more levels)

21 Chosen prototype structure 3-phase grid/load U e1(a) U e1(a) U e2(a) U e1(b) U e3(a) U e1(c) U e4(a) U e2(a) U e1(b) U e2(b) U e2(b) U e2(c) U e3(b) U e3(a) Port 2 3-phase grid/load Cascaded structure of AC///AC converters with Medium Frequency Isolation Cascaded H-Bridge structure formed at the AC terminals (Port 1 and 2) U e4(b) U e3(b) U e1(c) U e3(c) U e2(c) U e4(a) U e3(c) U e4(b) U e4(c) U e4(c) Storage elements Structure allows the Converter to be arranged in parallel and series combinations to meet application power levels Port 1 Port 3

22 Control Challenges Assume that each / converter (isolation module) equalises the link voltage on each side of the isolation barrier. Two things need to be considered: Global Power Flow Control i.e. Power entering through one port must leave other two! through one of the Internal power flow control Energy must be distributed amongst the cells in such a way that the link capacitor voltages remain equal Evenly distributes voltage stress Ensures high quality waveforms at the AC connections Ports 2 and 3 control power for the grids/storage systems that they are connected to Port 1 is the global power flow controller since it is connected to all other ports

23 Converter control Port 1 control diagram Lots of control to cope with, lots of nested loops Need to be very careful with design The more cells, the more dc link voltages, the better the waveform: BUT- the more balancing we have to do!

24 Modulation Challenges Since the target application is for high power, switching frequency must be minimised. In this case: Switching Frequency of each AC side H-Bridge =250Hz Switching Frequency of isolation modules =2kHz (soft switched- phew!) Fortunately for the AC side, if we have lots switching at low frequency, we still get a good waveform!

25 Converter Prototype Converter designed for operation at 3.3kV with a power rating up to kW Each UNIFLEX-PM module rated at around 25kW with a link voltage of 1.1kV approx. Construction: Transformers designed and constructed by ABB Secheron Cells designed by EPFL, Switzerland- single cell tested in lab at EPFL Control design, construction of full 3.3kV converter and peripherals (measurement, gate drives etc.)- PEMC group UoN

26 Isolation Module: Transformer MF transformer design by ABB Secheron, Switzerland Designed for operation at 2kHz- Amorphous core, Litz wire etc. Oil immersed for insulation and cooling

27 AC///AC Module Two H-Bridges and a link connected on either side of the transformer s consist of: DYNEX 1700V, 200A modules Forced air cooling Gate drives isolated for several kv Link Capacitance on each side of the transformer: 1350V, 3.3mF

28 Implementation of Control Control of entire converter implemented using TI6713 DSK board 5 Actel ProAsic 3 FPGA boards designed at the University of Nottingham / converters (isolation module) controlled solely by the FPGA cards Global power flow control implemented on DSP

29 Initial Hardware Setup Module Transducer Box IGBT Gate Drives dc link capacitor

30 Control hardware connected Fibre Optic lines Fibre Optic Transmitters FPGA Cards DSP and Comms Card

31 Experimental Prototype in MV Cage

32 Overhead view of rig

33 Experimental Work: 3 Phase Y-Y two ports Two port converter Ports 1 and 2 connected to grids operated with voltages from 415V to 3.3kV (Dependent on test). Bidirectional power flow up to 300kW.

34 Port 2 Port 1 Real power flow in both directions Power Flow from Port 1 to Port 2 Power Flow from Port 2 to Port 1 Converter voltage (green) Supply Current (red) Supply Voltage (blue) f sw(device) =250Hz

35 Voltage (V), Current (A*10) 4 Quadrant control of port 2 Port 2 -P, +/- Q Port 2 P, +/- Q

36 Imbalanced cell power flow control Imbalanced power drawn from port 2 cells resulting in link voltage divergence. Corrected by balancing control scheme

37 Asynchronous systems 60Hz/50Hz: Experimental Setup Supplied by Chroma Variable Frequency Power Power Flow

38 Port 2: 50Hz Port 1: 60Hz Asynchronous systems 60Hz/50Hz: Experimental result

39 Medium Voltage Testing Vs=3.3kV approx., fsw=250hz, 205kW power flow

40 4 Quadrant Power -P,-Q -P,Q P,-Q P, Q

41 4 Quadrant Test Video Video of four quadrant transients for LV testing

42 Further work Advanced Control Strategy to control the converter on a per-phase basis that enables the converter to: Monitor each phase of the supply and track the gird angle of each phase independently Control the power flow in each phase independently. Operate under conditions of grid disturbances such as: Phase Jumps Voltage sags and swells Fault Conditions Frequency excursions Low device switching frequency modulation is required to minimise the switching losses for operation at higher power. Power Flow control using three ports within a system emulating a real grid.

43 Power Electronics Challenges High Voltage - sharing between series devices etc. This has been a problem for many years getting it wrong can mean huge cascading failures High Power - largest current HV systems 6300MW (Itaipu, Brazil) - At these powers, efficiency is vital- 90% efficiency for a 10MW converter means we have 1MW of loss!!! - Power Electronics losses Switching Loss (Imperfect operation of semiconductors) Conduction Loss (Voltage drop) Magnetics (Eddy Currents+ Hysteresis especially at High Frequency) High Frequency- Size of magnetic components reduces as we increase the frequency - Also- Size of filtering components for PWM reduces as we increase the frequency - Unfortunately, our switching losses will increase with frequency (generally) Soft Switching important

44 THANK YOU!