Solid State Transformers (SST)

Solid State Transformers (SST)

Classical Transformer




Higher Frequency Lower Volume

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.

Solid State Transformer

Key components

• High frequency transformers

• Power electronics converters

AC to DCDC to DCDC to AC






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

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!!!

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

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• .........

Possible layout of a future grid with UNIversal and FLEXible Power Module

Future Electrical Network

Potential use of concept

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

Overview of Uniflex Functionality

UNIFLEX3

3

3Port 1 Port 2

Port 3

• 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.

Modular Building Blocks

Building block based on DC/DC isolation module:

Building block based on DC/AC isolation module:

Two technologies consideredBoth provide bidirectional AC-AC power flow with Medium Frequency (kHz) isolation

Multilevel Converters

0 0.005 0.01 0.015 0.02

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

Time /s

Am

plit

ude

0 0.005 0.01 0.015 0.02

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

Time /s

Am

plit

ude

0 1000 2000 3000 4000 50000

0.2

0.4

0.6

0.8

1

Frequency /Hz

Am

plit

ude

(N

orm

alis

ed)

5 Level

Conventional

The more H-Bridges… the higher the voltage and the better the approximation to a sine wave (more levels)

Single H-Bridge

2 H-Bridges in series

Chosen prototype structure

H-bridge

H-bridge

H-bridge

H-bridge

DCDC

DCDC

DCDC

DCDC

H-bridge

H-bridge

H-bridge

H-bridge

DCDC

DCDC

DCDC

DCDC

H-bridge

H-bridge

H-bridge

H-bridge

DCDC

DCDC

DCDC

DCDC

Port 1

H-bridge

H-bridge

H-bridge

H-bridge

H-bridge

H-bridge

H-bridge

H-bridge

H-bridge

H-bridge

H-bridge

H-bridge

Port 2

Port 3

3-p

hase

grid

/load

Sto

rag

e e

lem

ents

3-p

hase

grid

/load

Ue1(A)

Ue2(A)

Ue3(A)

Ue4(A)

Ue1(B)

Ue2(B)

Ue3(B)

Ue4(B)

Ue1(C)

Ue2(C)

Ue3(C)

Ue4(C)

Ue1(A)

Ue1(B)

Ue1(C)

Ue2(A)

Ue2(B)

Ue2(C)

Ue3(A)

Ue3(B)

Ue3(C)

Ue4(A)

Ue4(B)

Ue4(C)

•Cascaded structure ofAC/DC/DC/AC converterswith Medium Frequency Isolation

•Cascaded H-Bridge structure formed at the AC terminals (Port 1 and 2)

• Structure allows the Converter to be arranged in parallel and series combinations to meet application power levels

Control Challenges

•Assume that each DC/DC converter (isolation module) equalises the DC link voltage on each side of the isolation barrier. Two things need to be considered:

•Global Power Flow Controli.e. Power entering through one port must leave through one of the

other two!

•Internal power flow control•Energy must be distributed amongst the cells in such a way that the DC 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

Converter control

Port 1 control diagram

1 2 3

• 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!

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!

Converter Prototype

•Converter designed for operation at 3.3kV with a power rating up to 300-500kW

•Each UNIFLEX-PM module rated at around 25kW with a DC 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

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

AC/DC/DC/AC Module

• Two H-Bridges and a DC link connected on either side of the transformer

• H-bridges consist of:

• DYNEX 1700V, 200A modules

• Forced air cooling

• Gate drives isolated for several kV

• DC Link Capacitance on each side of the transformer:

• 1350V, 3.3mF

Implementation of Control

•Control of entire converter implemented using

•TI6713 DSK board•5 Actel ProAsic 3 FPGA

boards designed at theUniversity of Nottingham

•DC/DC converters (isolation module) controlled solely by the FPGA cards

•Global power flow control implemented on DSP

Initial Hardware Setup

Module

Transducer Box

IGBT GateDrives

dc link capacitor

Control hardware connected

Fibre Opticlines

Fibre OpticTransmitters

FPGACards

DSP andComms

Card

Experimental Prototype in MV Cage

Overhead view of rig

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.

Real power flow in both directions

Power Flow from Port 1 to Port 2 Power Flow from Port 2 to Port 1

Port

1Po

rt 2

•Converter voltage (green)•Supply Current (red)•Supply Voltage (blue)

fsw(device)=250Hz

4 Quadrant control of port 2V

oltage (

V),

Curr

ent

(A*1

0) Port 2

-P, +/- Q

Port 2

P, +/- Q

Imbalanced cell power flow control

•Imbalanced power drawn from port 2 cells resulting in DC linkvoltage divergence. Corrected by balancing control scheme

Asynchronous systems 60Hz/50Hz: Experimental Setup

Supplied by Chroma

61705 Variable Frequency

Power Supply @415V

Power Flow

Asynchronous systems 60Hz/50Hz: Experimental result

Port

1: 6

0H

zPo

rt 2

: 50

Hz

Medium Voltage Testing

•Vs=3.3kV approx., fsw=250Hz, 205kW power flow

4 Quadrant Power Flow @3.3kV

-P,-Q

-P,Q

P,-Q

P, Q

4 Quadrant Test Video

Video of four quadrant transients for LV testing…

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.

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 HVDC 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

THANK YOU!

Documents

Solid State Transformers (SST)