This document discusses solid state transformers (SSTs) and their potential applications in future distribution systems. It provides background on SSTs, explaining how they use power electronics to convert AC power to high frequency AC or DC before converting it back to the desired output. The document outlines research objectives to modularly design and optimize an SST for a distribution system. Simulation results show the modular SST has higher efficiency and lower weight than a traditional low frequency transformer under daily loading profiles. The document also explores how SSTs can address issues like imbalanced loads by independently controlling positive, negative, and zero sequence components.

1. Solid State Transformers for the Future Distribution Systems
Tao Yang, Dr. Terence O'Donnell
University College Dublin
May, 2016
University of Manchester
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Conference for the Next Generation of
Researchers in Power Systems 2016

2. Content
 Introduction
 Solid State Transformer: State of the Art
 Motivations
 Research Objectives & Methodology
 Applications
 Conclusion and Future Work
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3. What are Solid State Transformers(SST)? Basic Concept
 Take 50/60 Hz input voltage
 Use power electronics switches to chop 50/60 Hz into much higher frequency
AC (e.g. 10’s kHz)
 Pass this high frequency through a transformer
 Use power electronics to “re-make” and regulate the 50 Hz voltage.
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Introduction

4. Why Solid State Transformers?
 Advantages
– Reduced size and weight
– SST is active, controllable device
• Output and input waveforms can
be decoupled
• Eliminate harmonic distortion
• Improve voltage regulation
• Control active and reactive power
– Could have a DC input/output for
connection of DG, energy storage
 Disadvantages
‒ Efficiency,
‒ Reliability
‒ Cost
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Solid State Transformers (SST)
Low-Frequency Transformer(LFT)
Motivation

5.  The high voltages and power level of distribution system transformers are
currently still beyond the rating capability of current commercial semiconductor
switches.
Source: S. Bernet, “Recent Developments of High Power Converters for Industry and Traction Applications,” IEEE Transactions on Power
Electronics, 2000, vol. 15, no. 6, pp. 1102- 1117.
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 Low Power
 High Frequency
 High Power
 Low Frequency

6. State-of-Art of Solid State Transformers
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Size/Weight Reduction of
Transformers Realized
SST Applications Research over the
Last 10 Years
What has been achieved to date?
Source: Solid State Transformer Concepts in Traction and Smart Grid Applications, J.W. Kolar, G.I. Ortiz, ETH, Zurich
 Not many examples of fully functional tested SSTs
 Modelling suggests efficiencies in the range of 90 – 95%

7. Solid State Transformers Applications
● Hugo (ABB, 2006)
- Total Power:
1.2MVA/15kV
- Module Power: 75kW
- Frequency: 400Hz
Potential applications of SST in the future
distribution system
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Source: X. She, R. Burgos, G. Y. Wang, F. Wang, and A. Q. Huang, “Review of solid state transformer in the distribution system: From implementation to filed application,” in Proc. IEEE
Energy Convers. Congr. Expo., 2012, pp. 4077–4084

8. 8/28
Research Questions & Objectives
Research Objectives?
 SST application in the distribution power system—Modular design.
 Find out the system advantages of replacing the LFT with modular SST
in the distribution network.
Research Questions?
 Modular approach to design can improve reliability and facilitate cost
reduction but:
How best to build a modular SST?
What efficiency and size advantage is possible?
 SST Efficiency can never match that of an LFT but is it possible to make
savings elsewhere in the system?

9. Modular Design of Solid State Transformer
for the Distribution system
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Interleaved (DAB)
Interleaved Inverter
Three Level Rectifier
a
b
c
n
M
N
10 kV
rms
14.1 kV
DC
14.1 kV
DC
0.8 kV
DC
0.4 kV
rms
or
The Definition of Modular Design
Built up the overall system
from a large number of lower
power sub-modules.
Methodology

10. 10/28
Modular SST Optimization
 Individual modular optimization for three stages of SST
 Three Objective Functions:
Frectifier (P, V) = f1( Pinductor+Pbridge) 2 + f2 ( Vinductor+Vbridge) 2
FDAB (P, V) = f1 ( PMFT+Pbridge+Pcapacitors) 2 +f2 ( VMFT+Vbridge+Vcapacitors) 2
Finverter (P, V) = f1 ( Pinductor+Pcapacitors+Pbridge)2 +f2 ( Vinductor+Vcapacitors+Vbridge)2
P-Power Loss, V-Volume,
f1, f2-Minimization Functions with Normalization. Specific weighting factors for
each minimized function. GRG Nonlinear Solver.
 Optimization variables:
Number of Modules, Switching Frequency, Topology, MFT Optimization Parameters,
Inductor Optimization Parameters
 Constraints:
Temperature Rise, Isaturation>Iinput (Inductor Optimization Design),
Leakage inductance Limitation (MFT).

11. Results: Modular SST Optimization
11/28
Total SST(Optimization)
PT(w
)
VT(d
m3)
Efficiency
(%)
Power
Density(kW/L)
1459
8.4
110.8
5
96.48 3.61
AC/DC 3
AC/DC 2
AC/DC 1
DAB1-1
DC/AC
1
DC/AC
2
DC/AC
n
10 kV 5.3 kV
0.8 kV 0.4 kV
13.33 A
3.33 kV
3.33 kV
3.33 kV
1 kV
166.67
A
0.4 kV
0.4 kV
0.4 kVDAB1-2
DAB2-1
DAB2-2
DAB3-1
DAB3-2
DAB4-1
DAB4-2
1 kV
1 kV
1 kV
0.4 kV
0.4 kV
0.4 kV
0.4 kV
0.4 kV
n=3 n=7, m=1&2 n=30
Increase or Reduce Number of
Modules depending on Loading
Flexible Number Modules
Fix Number
Modules

12. Application to a 400 kVA, 10 kV/0.4 kV Distribution Network
10 kV Bus
0.4 kV Bus
10 kV/0.4 kV
400 kVA
External Grid
AC
DC
DC
AC
DualActiveBridge
(DAB)
Loads
LFT
SST
Distribution system with SST and LFT.
Winter and summer daily loading profile for a
400 kVA distribution system
Source: McKenna, K.; Keane, A., "Discrete Elastic Residential Load Response under Variable Pricing Schemes," Innovative Smart Grid Technologies Europe (ISGT EUROPE),
2014 5th IEEE/PES, 12-15 Oct. 2014
 A 400 kVA, 10 kV/400 V distribution system with 144 residential customers
has been modelled.
 A winter and summer day loading profile with 1 minute resolution based on
the average yearly energy consumption is used
12/28

13. Peak Loading Modular SST 50 HZ LFT
Efficiency[%] 95 98.11
Weight[kg] 773.18
(x0.58)
1350
Volume[m3]
Factor
0.568
(x0.333)
1.7061
Comparison Results under daily loading profile
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14. Imbalanced Loading in the Distribution System
Modern Distribution Grid Three Phase
Imbalanced Loads
In an SST-fed distribution system, Can
SST ensure that certain tolerances on
phase loads imbalance at its output
terminals.(few publications have
addressed the issue )
• Unevenly distributed
single-phase load.
• Balanced three-
phase load running
at a fault
condition.
0 500 1000 1500
0
1
2
3
4
5
6
7
8
x 10
4
Time (min)(a Day)
ApparentPower(VA)
Load A
Load B
Load C
Applications
14/28

15. 15/28
Positive
Sequence
Negative
Sequence
Zero
Sequence
Imbalance
• Imbalanced phase loading causes negative
sequence and zero sequence to flow into the
power system.
SST Solutions to the Imbalanced Load

16. 16/28
A
B
C
20 kV RMS
16.3kV
Four Single-
Phase Rectifier
with series
connection
Four Single-
Phase Rectifier
with series
connection
Four Single-
Phase Rectifier
with series
connection
Four DAB
with
parallel
connection
Four DAB
with
parallel
connection
Four DAB
with
parallel
connection
………
ILa
ILb
ILc
L
LnIn
C
Load
6 kV
800 V
VA VB VC
Sn1
Sn2
Sa1
Sa2
Sb1
Sb2
Sc1
Sc2
+
380 V
IOa
IOc
IOb
dq
abcVA
VB
VC
dq
abcVA
VB
VC
Notch Filter
Notch Filter
Notch Filter
Notch Filter
dq
abcILa
ILb
ILc
dq
abcILa
ILb
ILc
Notch Filter
Notch Filter
Notch Filter
Notch Filter
dq
abcIOa
IOb
IOc
dq
abcIOa
IOb
IOc
Notch Filter
Notch Filter
Notch Filter
Notch Filter
Vd
Vq
V-d
V-q
ILd
ILd
IL-d
IL-q
IOd
IOq
IO-d
IO-q
Σ/3 V0
- --
Σ/3 IO_0
PLL
abc
dq
VA
VB
VC
PLL
Transformation
0
Vore
f
+
xPI
Vd
C
0xPI
VqC
x
x
IOq
IOd
ILd
ILq
L
L
x
x
PI
PI
x
x
-
++
-
+
+
+
+
-
+
-
+
-
+
+
-
+
+
+
ILrefd
ILrefq
0+
xPI
V-d
C
0xPI
V-qC
x
x
IO-q
IO-d
IL-d
IL-q
L
L
x
x
PI
PI
x
x
-
++
-
+
+
+
+
-
+
-
+
-
+
+
-
+
+
+
Ilref-d
Ilref-q
xPIxPI
x
x
Sa
Sb
Sc
Sn
dq
αβ
dq
αβ
αβ
dq
dq0
αβγ
Vα*
Vβ*
Vγ*
V0In
+
-
+
-
+
+
+
+
Vd*
V0*
Vq*
Positive
Sequence
Control
Negative
Sequence
Control
Zero
Sequence
Control
Controller for
3-Phase 4-Leg
Inverter +
IO_0
Ic*
x
+
+
VL0*
V0
3-D
SVPWM
Balancing the output voltage
3-phase 4-leg SST
 Control all three sequence
components separately, by
regulating the positive
sequence to the correct value,
while eliminating the negative
and zero-sequence.

17. 17/28
Balanced Loads Imbalanced Loads
In Neutral Current
Va VbVc
Phase C Open Fault
Sometimes, the large neutral current is not allowed.

18. 18/28
Neutral Current Elimination under the Imbalanced loads
𝑖 𝑁 = 𝐼 𝑎 cos 𝜃 + 𝜃 𝑎 + 𝐼 𝑏 cos 𝜃 + 𝜃 𝑏 + 𝐼𝑐 cos 𝜃 + 𝜃𝑐
Where, Ia,Ib,Ic are the amplitude of output phase current,
θa,θb,θcare the current phase angle.
Phase Shifting Control
Voltage Amplitude Control
Phase Shifting and Voltage Amplitude Combination Control

19. 19/28
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1
-400
-300
-200
-100
0
100
200
300
400
Time (s)
Voltage(V)
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1
-5
-4
-3
-2
-1
0
1
2
3
4
5
Time (s)
NeutralCurrent(A)
Balanced Output Control Phase Shifting Control
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1
-400
-300
-200
-100
0
100
200
300
400
Time (s)
Voltage(V)
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1
-6
-4
-2
0
2
4
6
Time (s)
NeutralCurrent(A)
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1
-300
-200
-100
0
100
200
300
400
Time (s)
Voltage(V)
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1
-8
-6
-4
-2
0
2
4
6
8
10
12
Time (s)
NeutralCurrent(A)
Combination Control
Voltage Amplitude Control
Simulation Results under the Imbalanced loads
Neutral Current
Output Voltage
Do these output voltage imbalance
degree meet the standard document
requirements?

20. Optimal Minimum Losses Control
• The EN 50160 standard: imbalanced voltage degree in the distribution system
should be below 2% for 95% of the time.
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Voltage Imbalanced % = PVUR =
𝑉𝑚𝑎𝑥 − 𝑉 𝑚𝑖𝑛
𝑉𝑎𝑣𝑔
× 100%
Voltage Imbalanced % = UBF =
𝑉𝑛
𝑉𝑝
× 100%
Objective Function: 𝐹 𝑀𝑖𝑛 𝑡𝑜𝑡𝑎𝑙 𝑙𝑜𝑠𝑠𝑒𝑠 = (𝐼 𝑎∠𝜃a)2+(𝐼 𝑏∠𝜃b)2+(𝐼𝑐∠𝜃c)2+ 𝑖 𝑛
2
Constrains:
•
𝑉 𝑚𝑎𝑥−𝑉 𝑚𝑖𝑛
𝑉𝑎𝑣𝑔
≤ 2%
•
𝑉𝑛
𝑉𝑝
≤ 2%

21. 21/28
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1
-400
-300
-200
-100
0
100
200
300
400
Time (s)
Voltage(V)
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1
-6
-4
-2
0
2
4
6
Time (s)
NeutralCurrent(A)
Balanced Output Control Optimal Control
Balanced Output Optimal Control
Voltage Amplitude (V) 300;300;300 294.08;294.08;300
Phase shifting angle (degree) 0;0;0 2.4788;-2.1937;2.0328
Current Amplitude (A) 15;12;10 14.70 11.76 10
Neutral Current Amplitude (A) 4.36 3.23
Total Losses (W) 243.2 231.8
Total Output Power (W) 5543 5379
PVUR 0% 2%
UBF 0% 2%
Results for the Optimal Control
Neutral Current

22. 22/28
Experimental Validation
Rated Power 2 kVA
DC Bus
voltage
440 V
Load
Resistors
25/39/98Ω
Output
Voltage
220 V
Switching
Frequency
3 kHz LC Filter
80µF
65mH
Inductors
OPAL-RT
Control Board
Inverter
Bridges
Power
Supply
Prototype parameters

23. 23/28
0 500 1000 1500
0
1
2
3
4
5
6
7
8
x 10
4
Time (min)(a Day)
ApparentPower(VA)
Load A
Load B
Load C
The Distribution Network with three phase Imbalanced Daily Load
Source: McKenna, K.; Keane, A., "Discrete Elastic Residential Load Response under Variable Pricing Schemes," Innovative Smart Grid Technologies Europe (ISGT EUROPE),
2014 5th IEEE/PES, 12-15 Oct. 2014
0 500 1000 1500
0
20
40
60
80
100
120
140
Time (min)LoadImbalanceDegree(%)
Imbalance Degree VS Time
Three Phase Loading Imbalance DegreeThree Phase Imbalance Daily Loading

24. 24/28
0 20 40 60 80 100 120
0
20
40
60
80
100
Load Imbalance Degree(%)
OutputVoltageImbalanceDegree(%)
Balanced Output Control
Phase Shifting Control
Voltage Amplitude Control
Combination Control
Optimal Control
IEEE Standard Limitation
Application Results
0 20 40 60 80 100 120
0
10
20
30
40
50
60
70
80
90
100
Load Imbalanced Degree (%)
NeutralCurrent(A)
Balanced Output Control
Phase Shifting Control
Votlage Amplitude Control
Combination Control
Optimal Control
Apply the Optimal Control for a Typical Distribution Network Under the three phase
imbalanced daily load

25. Conclusion
 SST System benefits
 SSTs can ensure distribution level feeder voltage balance in the face of significant
loads imbalance.
 Can both balance the output voltage within the standard requirement and reduce the
neutral current somehow.
 Modular SST’s Distribution Level Application(Daily Loading)
 The efficiencies of SSTs significantly lower
 Different numbers of modules switch on for the different
loading is available for reducing the power loss.
 70% reduction of volume compared to LFT
 Further investigate for efficiency gains and size reduction
25/28

26. Future Work
 Further investigate potential efficiency gains and size reduction of the SSTs
 Full SST vs Transformers + PE (e.g. FACTS Devices) vs. Hybrid Solutions
 Impact of new switch technologies
 Protection
 Reliability
26/28

27. 27/28
Acknowledgements
This work was conducted in the Electricity Research Centre, University College
Dublin, Ireland, which is supported by the Commission for Energy Regulation,
Bord Gais Energy, Bord na Mona Energy, Cylon Controls, EirGrid, Electric
Ireland, EPRI, ESB International, ESB Networks, Gaelectric, Intel, SSE
Renewables and Energia. T. Yang is partly funded by the UCD-Chinese
Scholarship Council Scheme. T. Yang and T. O'Donnell are funded through the
Sustainable Electrical Energy Systems Strategic Research Cluster (SEES Cluster)
under grant number 09/SRC/E1780.

28. Thank you for listening!
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