power electronics for universal and flexible power...
TRANSCRIPT
Professor Pericle Zanchetta Power, Electronics, Machines and Control
(PEMC) Research Group University of Nottingham (UK)
Power Electronics for Universal and Flexible Power Management
The Solid State Substation
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….
Traditional substations are passive:
Perform voltage step down from say 33kV to 415V, isolation point etc.
Very efficient, Very reliable
Case Study: 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
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 • .........
• Modular approach • Standardised building blocks • Scalable • Maintainability • Economy of scale
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
UNIFLEX 3
3
3 Port 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 considered Both provide bidirectional AC-AC power flow with Medium Frequency
(kHz) isolation
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 of •AC/DC/DC/AC converters •with 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
•Interleaving reduces penetration of imbalances in the loads or supplies connected to each port of the converter
Isolation Module: Topology
Isolated Dual Bridge DC/DC Converter:
Bidirectional Power Flow
Inherent soft switching ability
Simple control- drives error between E1 and E2 to zero
Transformer designed for Medium Frequency (MF) operation (2kHz)
Isolation Module: Soft switching
1. S1, S4 gated, current flowing into
transformer.
2. S1,S4 turned off. During dead-time, energy transfers from capacitors
across S2, S3 to S4, S1.
3. Capacitor charge transfer complete. Current commutates onto diodes of S2, S3. Dependent upon dead-
time and current magnitude.
4. S2, S3 turned on with zero voltage
condition- ZVS.
Isolation Module: Control
• A 2kHz square wave is generated by the H-Bridges on each side of the MF transformer
• The power flow through the transformer is determined by the voltage across the leakage inductance L
• The voltage is controlled via the phase shift, d, between the two square waves.
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 Control i.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!
1. Global Voltage Control- Ensure that the total DC link voltage is as demanded- this ensures that the global power required for the converter is drawn from the grid at port 1
2. Current control applied in the rotating frame- however stationary frame, per phase and predictive methods investigated (see later)
3. Control systems required to ensure that power is distributed amongst the ports correctly and that DC link voltages are balanced
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!
Power Converter Modulation
•Low device switching frequency modulation methods required to minimise switching losses for operation at higher power
•Many Multilevel PWM options considered including: •Carrier Based Modulation Strategies •Space Vector Modulation Strategies incorporating cell voltage balancing •Optimised Modulation Strategies
•Selective Harmonic Elimination (SHE) •Harmonic Minimisation
•Chosen method for prototyping: Phase Shifted Carrier PWM
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
• Based on Traction Transformer design
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 the University of Nottingham
•DC/DC converters (isolation module) controlled solely by the FPGA cards •Global power flow control implemented on DSP
Isolation Module: Control
• P+I Control and H-Bridge modulation signal generation is achieved using an FGPA
• Digital P+I Control implemented using logic gates and a state machine
• Square Wave generation and phase shifts controlled using counters clocked at 10MHz (accuracy to 0.072o)
Isolation Module: Results
Tracking of reference voltage
Transformer voltage and currents
Initial Hardware Setup
Module
Transducer Box
IGBT Gate Drives
dc link capacitor
Control hardware connected
Fibre Optic lines
Fibre Optic Transmitters
FPGA Cards
DSP and Comms
Card
Experimental Prototype in MV Cage
Overhead view of rig
Current Prototype Configuration
Power Flow
Ports 1 and 2 connected to grids
operated with voltages from 415V
to 3.3kV (Dependent on test).
Bidirectional power flow up to
300kW.
Two port converter
Real power flow in both directions
Power Flow from Port 1 to Port 2 Power Flow from Port 2 to Port 1
Port
1
Port
2
•Converter voltage (green) •Supply Current (red) •Supply Voltage (blue)
fsw(device)=250Hz
DC link capacitor voltage control
Step change just before 2s
4 Quadrant control of port 2 V
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 link voltage 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:
60H
z
Port
2:
50H
z
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…
More Control Challenges
•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
Advanced Control Strategies I
•Single Phase PLL tracks the supply angle on each phase of the grid voltage
•Current demands are made according to global DC link controller and reactive current regulator
•Current control is implemented per-phase using Proportional + Resonant (PR) Controllers .
*P
1
3
DC Voltage
Controller
(PI)
dcV a
dcV
*
dcV
dc ia
*
di*
aiai
2
gridV
*Q
Q
Q Controller
(PI)
Q *
qi
*
av
*
bv
*
cv
dcV b
dcV c
ib
*
bibi
ic
*
cici
(PR)
(PR)
(PR)1
3
1
3
PLLav PLL
bv PLL
sin a sin b sin c
cv
cos a cos b cos c
*
aai
*
bai
*
cai
*
ari
*
bri
*
cri
*Q
2
gridV
Natural Reference Frame Control
Advanced PLL Strategies
•Phase Lock Loops (PLL’s) are generally implemented in the rotating frame on a three phase system but may perform badly when the supply phases become unbalanced (such as under fault conditions)
•Since a single phase quantity cannot produce the orthogonal axis of the Clarke Transform- this must be made artificially. For example:
• Digital delay functions (producing a 90o phase shift) • All pass filters • Second Order Generalised Integrators (SOGIs) • Hilbert Transform Methods etc.
Advanced Control Strategies II
Per Phase Deadbeat Control
•Model based Deadbeat/Predictive current control
•Control law derived from above equation calculated over a sampling period
•During each sampling period interrupt, the required converter action is calculated which will result in zero current error in the next interrupt
)V -(V convertersupplysupply dt
dIL
dt VT
1)ii(
T
LV
sk
k
Tt
t
supply
s
1)(k(k)
s
supplyk)converter(
Model Predictive Control
Per Phase Modeling
)V -(Vdt
diL jNjf
dt (t)VT
1)]T(ti)(ti[
T
L)t(v
sk
k
Tt
t
j
sSkjkj
s
f
kjN
ij f
vj vjN
Port 1 Model Predictive control (including DC-link voltage regulation)
The currents at the next sampling period:
Integrals are calculated on previous sample period
cb,a,jfor )t(vL
Tdt)t(v
L
1)t(i)Tt(i kjN
f
STt
tj
f
kjSkj
Sk
k
cb,a,jfor )Tt(v)t(vL
1dt)t(v
L
1SkINTjkINTj
f
Tt
tj
f
Sk
k
The voltages at the next sampling period:
Angles and RMS voltage value are provided by a PLL
cb,a,jfor )Tcos(2VT)t(v)Tt(v jSRMSjSkjSkj
Model Predictive Control
Active, reactive and zero-sequence power at the next sampling period are calculated from the predicted voltages and currents in a b 0 frame:
Each phase sinusoidal current references is:
where:
cb,a,jfor )Tsin(Ii *
jjS
*
j
*
j
cb,a,jfor P
Qtan
*
j
*
j1*
j
cb,a,jfor )cos(
P
V
2I
*
j
*
j
RMSj
*
j
cb,a,jfor 3
PVDCP REF
REGj
*
j 3
**
j
Model Predictive Control
At every sample period one of the following three possible voltage values on each phase can be applied by the converter:
Hence the control algorithm chooses among all 27 possible output configurations the one that minimize the cost function G:
Model Predictive Control
-1,0,1i Vi)Tt(v)t(v DCSkjNkjN
d
d
2
0
*
0
2*
2*
2
c
*
c
2
b
*
b
2
a
*
a PPQQPP)1(iiiiiiG
5.0 0P case our In
10 where
0 d
d
Simulation and Experimental Implementation
Power Flow
Ports 1 and 2 connected to grids
operated with voltages from 415V
to 3.3kV (Dependent on test).
Two port converter
Parameters
Units Value
Rated voltage (line-to-line) Vn
[kV] 3.3
Rated apparent power / converter Snc
[kVA] 300
Power Supply inductance Ls
[mH] 1.7
Input filter inductance Lf
[mH] 16
Rated output current / H-brigde Inm
[A] 52.63
DC-link voltage / H-brigde VDC
[V] 1100
DC-link capacitor / H-brigde C
[mF] 6.2
Sampling frequency for control fs
[kHz] 3.6
Simulation Results for bidirectional power flow
Simulation Results for grid voltage excursions
Simulation Results for grid voltage unbalances (3%)
Simulation Results Harmonic content
Experimental Results: Converter waveforms at Port 1
Converter voltage (blue); Supply Current (green); Supply Voltage (red) Fsw=10kHz
Experimental Results: Real power flow from port 2 to Port 1
Experimental results: Reactive power flow from port 2 to Port 1
One phase short circuit
Two phase with ground short circuit
Conclusions
UNIFLEX-PM was a European project aiming at investigating multi-cellular power converter structures for future grid applications
Consortium consisted of several universities and companies
Project ended in August 2009 with the successful testing of a 3.3kV, 300kW prototype power converter
Further testing under way (advanced controllers, three port configuration)
Conclusions: Consortium
ABB Secheron SA Switzerland
AREVA T&D Centre U.K.
Project Coordinator (Now Alstom Grid)
Dynex Semiconductor Ltd. U.K.
Aalborg University Denmark
École Polytechnique Fédérale de Lausanne
Switzerland
European Power Electronics and Drives Association
Belgium
Industry
Professional Association
Academia
Nottingham University U.K.
Technical Lead
Università degli Studi di Genoa
Italy
Thanks to everybody for these days
It has been a very good event and expericnce.
Conclusions
Future work