See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/304411258
Flexible power distribution unit — A novel power electronic transformer
development and demonstration for distribution system
Conference Paper · November 2015
DOI: 10.1109/IECON.2015.7392155
CITATIONS
READS
9
66
7 authors, including:
Duan Qing
Jianhua Wang
China Electric Power Research Institute
Southeast University (China)
8 PUBLICATIONS 34 CITATIONS
47 PUBLICATIONS 542 CITATIONS
SEE PROFILE
All content following this page was uploaded by Jianhua Wang on 20 July 2019.
The user has requested enhancement of the downloaded file.
SEE PROFILE
Flexible Power Distribution Unit－A Novel Power
Electronic Transformer Development and
Demonstration for Distribution System
Qing Duan1, Jianhua Wang 2, Chunyan Ma1, Binshi Gu2,
1 China Electric Power Research Institute & Beijing Key
Laboratory of Distribution Transformer Energy-saving
Technology,
Baojian Ji3, Peng Qiu3, Jun You2
2 Jiangsu Provincial Key Laboratory of Smart Grid
Technology & Equipment, School of Electrical Engineering
Beijing, China
Southeast University, Nanjing, China
Email: duanqing@epri.sgcc.com.cn
3 College of Automation and Electrical Engineering
Nanjing Tech University, Nanjing, China
Abstract—Power electronic transformer (PET) is an
emerging new type of power converter in recent years. It has
not only the basic functions of power transformation and
isolation, but also the extra functions of power quality control.
A novel power electronics transformer for distribution system
named flexible power distribution unit is proposed in this
paper, and the energy exchange mechanism between the
network and load is revealed. Finally, a 100kW
600Vac/220Vac/110Vdc medium frequency isolated prototype
is developed and demonstrated in laboratory. The
experimental results show that the proposed structure and
control strategy are feasible.
Keywords—Power electronic transformer;
system; flexible power distribution unit
distribution
I. INTRODUCTION
Distribution transformer is the most important and
common equipment in power distribution network, which is
mainly responsible for voltage transformation and isolation.
Traditional distribution transformer is very reliable, but it is
bulky, cumbersome and harmonics could not be isolated
between primary and secondary sides. And extra equipment
is needed to monitor and protect itself for possible
breakdown issue. Now days these drawbacks are the real
concerns in academics and industries [1].
Therefore, the power electronics based transformer
called PET (Power electronic transformer), IUT(Intelligent
Universal Transformer)，SST(Solid State Transformer), ST
(Smart transformer) and others has gradually become an
emerging issue over the last 10 years, especially for railway
traction and smart grid[2]-[9]. ABB, Alstom, Bombardier,
Siemens have done a lot of work in traction applications.
The pilot installation from ABB was completed in mid-2011,
and homologation with the Swiss Federal Office for
Transport (FOT) was achieved by the end of the year [7]. At
the same time, scientists and researchers in the
UNIFLEX-PM (Advanced Power Converters for Universal
and Flexible Power Management in Future Electricity
Networks), FREEDM (the Future Renewable Electric
Energy Delivery and Management), MEGA Cube and
Project Supported in part by the Basic and Prospective Science and
Technology Project of State Grid Corporation of China
(EPRIYDKJ(2014)3317), by the National Natural Science Foundation of
China (51207023), by the Cooperative Innovation Fund of Jiangsu
Province--the Prospective and Joint Research Project (BY2015070-18),by
the Science and Technology Pillar Program of Zhenjiang City(GY2014029)
HEART (The Highly Efficient And Reliable smart
Transformer (HEART), a new Heart for the Electric
Distribution System) Projects lead by the leading
universities are still continuously investigating various
issues of PET for smart grid, partly because of that the
existing 50/60Hz power system is more complicate than the
16.67Hz traction electric system[8]-[9].
Besides USA EPRI IUT projects, China EPRI also starts
its PET project for distribution power system. This paper
would like to introduce its Gen-I PET project, a flexible
power distribution unit for future distribution system. A
novel PET structure for distribution system is proposed and
described here, and the prototype is also developed and
demonstrated in laboratory for detail tests. The
experimental results show that the proposed structure and
control strategy are feasible.
II. THE BASIC PRINCIPLE OF PET
PET is mainly composed of the power electronic
converter and medium/high frequency isolation transformer,
as shown in figure 1, in which the power electronic
converter mainly complete the conversion, control,
protection of electrical energy. Medium/high frequency
isolation transformer is mainly responsible for the
transformation of the galvanic isolation. Due to the
operation frequency of transformer is inversely proportional
to its volume, the high-frequency transformer can
drastically reduce the volume and weight and improve the
capacity and efficiency of the transformer.
High frequency isolation
transformer
Source
Power
electronic
converter
Power
electronic
converter
Load
Figure.1 Basic structure of PET
According to whether contains dc link can PET be
divided into two categories: one is direct AC/AC PET
which does not contain dc link in the process of
transformation, another is AC/DC/AC PET that contains dc
link. Figure 2 shows the typical structure of AC/DC/AC
PET [8]. This structure can realize the input power factor
correction and suppress the bidirectional harmonic flow.
The use of integral modularized transformation method
creates the concise structure. Furthermore, the proposed
PET topology in this paper improves the structure with a
muti-winding transformer.
AC/DC
High frequency
transformer
AC/DC
DC/AC
Lg
es
ed
e
a
r
ao
h
h
l
T
p
ea
DC/AC
eb
ec
Input
Isolation
Output
Figure.2 AC/DC/AC topology of PET
The working principle of the PET is accessible. Firstly,
input AC/DC converter converts high voltage power
frequency alternating current (ac) to high voltage direct
current (dc). Secondly, the isolation DC/AC inverter
converts direct current into high frequency alternating
current square wave. Then through high frequency isolation
transformer, the square wave is coupled to the transformer
vice side. The isolation AC/DC rectifier converts alternating
current square wave to low voltage direct current. Finally,
the output DC/AC inverter convers direct current to
required alternating current.
III. THE SYSTEM STRUCTURE OF PET
Figure 3 shows the main circuit topology of a
three-phase four-wire PET which is suitable for the smart
distribution network. This is a typical AC/DC/AC type
tertiary structure that contains input, isolation and output. In
figure 3, ea,eb,ec are three-phase grid voltage; iga,igb,igc are
three-phase grid current; Lg is filter inductance for the grid
side; udcH is the high voltage dc bus input voltage;
udcL1,udcL2,udcL3 are the isolation low voltage dc bus output
Input
voltage; iLa,iLb,iLc are the inverter bridge arm output current;
Lf is inverter output filter inductance; Cf is inverter output
filter capacitor; ioa、iob、ioc are three-phase load current.
The input of PET adopts three-phase PWM full
controlled rectifier which has mature technology, can run
with high power factor and is widely used by medium
power capacity PWM rectifier. The isolation single phase
full bridge inverter convers high-voltage direct current to
high frequency square wave that is access to the primary
side of the high-frequency transformer. Transformer's
viceside is composed of three single phase full bridge
rectifier which convert high frequency square wave to
direct current. The output is made up of three two-level
single phase full bridge inverter and LC filter, and uses YN
connection to form the three-phase four-wire system. This
topology is flexible to control, simple and easy to realize
three phase independent control.
Figure 4 shows the input control block diagram. θ is
theoutput power grid voltage vector angle of phase-locked
loop, and θ=ωt. ω is the angular frequency of the power
grid voltage, in this paper ω=100π rad/s. ed、eq are d and q
axis components of power grid voltage. igd、igq are d and q
axis components of power grid current. udcH* is the
reference value of dc bus voltage udcH. igd*、igq* are the
reference values of igd、igq. In order to achieve the unit
power factor of input, the reference value of q axis current
igq* is set to 0.
The isolation module uses open loop control to convert
input dc to high frequency square wave whose duty ratio is
50%. Then the square wave is coupled to the vice side of
the high-frequency transformer and changes into dc signal
through a rectifier. The high frequency transformer mainly
Isolation
Output
iLa Lf
udcL1
ea
iga
Lg
eb
igb
Lg
ec
igc
Lg
ioa
iLb Lf
udcH
udcL2
iob
b
Cf
iLc Lf
udcL3
a
Cf
ioc
c
Cf
n
Figure.3 A novel Power electronic transformer for distribution system
(a) Input
(b) Isolation
Figure.6 Some photos of the PET
Figure.4 control block diagram of input stage
has two functions: one is to realize the electrical isolation
between input and output, another is voltage grade
transformation.
The output module whose main function is transform
the output direct current of the isolation module to
constant-voltage and constant-frequency alternating current
is made up of three single phase inverter and LC filter. In
order to ensure the output phase voltage of the inverter has
constant root mean square (RMS) and sinusoidal waveform,
double loop control is used. The inner loop is phase voltage
instantaneous value control loop while outer loop is the
phase voltage RMS control loop. Inner loop can acquire fast
dynamic performance through instantaneous value control
to ensure good sinusoidal output waveform. Outer loop
ensures the output voltage RMS constant and has high
precision by controlling RMS. The control block diagram of
output module is shown in figure 5. Firstly, the RMS of
output voltage uo is compared with a given reference urms*
to get error signal. Then after the PI regulator the signal is
multiplied by unit sine wave to be the reference
instantaneous value of inner loop. Finally, the value is
compared with output voltage instantaneous value to get a
new error signal which is sent to SPWM generator through
a PI regulator.
Restricted by experimental conditions, the load power
tested in the lab is only 15KW which means system is at
light load condition. Figure 7 shows the PET’s experiment
waveforms. Figure (a) shows three-phase input line voltage
waveform measured by Fluke434 power quality analyzer.
uab is 597.1V, ubc is 595.6 V, uca is 593.5V. Figure (b) shows
system operation parameters. The load power is 14.83kW
and system power factor is 0.95 which shows that the input
runs in high power factor. Figure (c) shows the input dc bus
voltage 1050V which achieve the value expected, so input
can realize the floating control. Figure (d) shows the dc bus
voltage characteristic curve when the load changes
suddenly and the voltage fluctuation is small. After 300ms
system is stable and the robustness is good. The C phase
voltage is almost the same which proves the topological
structure has good resistance to the load imbalance. Figure
(e) and (f) shows the primary side and vice side voltage of
high frequency isolation transformer. The sharp peak is
caused by the leakage inductance of the transformer. Figure
(g) shows the low voltage dc bus voltage. The single phase
full bridge rectifier converts ac square wave into 350V
direct current. Figure (h) shows the output three-phase ac
voltage whose waveforms are symmetrical and sinusoidal
and the RMS is 221V. Compared with the reference 220V,
the error is only 0.4%. Figure (I) shows that the output A
phase voltage THD is only 2.5% which conforms to the
national standard that the impedance load THD must be
within 3%.
Figure.5 control block diagram of output stage
(a) Three phase line voltage input
(b) Power and energy
udcH
Udcl(100V/grid)
us(400V/grid)
IV. EXPERIMENT
To validate the performance of the PET, a 100KW
600V/220V three-phase four-wire PET experiment
prototype is built based on the DSP TMS320F2812. Main
parameters of the system are as follows: (1) input: line
voltage 600V, frequency 50Hz, filter inductance 1.5mH,
high voltage dc side capacitor 2160uF, switching frequency
4.8kHz; (2) isolation: working frequency 2kHz, transformer
change ratio 3:1:1:1, low voltage dc side capacitor 3000uF;
(3) output: filter inductance 0.4mH, filter capacitor 50uF,
switching frequency 10kHz; three-phase resistance load of
low voltage side 10.67Ω.Figure 6 shows the 100 KW PET
prototype’s photos of input and isolation module.
t(200μs/grid)
uc
t(200μs/grid)
(c) High voltage dc bus voltage(d) high voltage dc bus and c phase
voltage with load change
us(400V/grid)
up(200V/grid)
network is set up. The experimental results prove that the
PET topology and its control strategy are feasible.
REFERENCE
t(200μs/grid)
t(200μs/grid)
(e) primary side voltage
(f) vice side voltage
ub
uc
u/(100V/grid)
Udcl(100V/grid)
ua
t(200μs/grid)
(g) Low voltage dc bus voltage
[1]. J. Wang, A. Huang, W. Sung, Y. Liu, and B. Baliga, “Smart Grid
[2].
[3].
t/(4ms/grid)
[4].
(h) Three-phase ac output voltage
[5].
[6].
(i)Output A phase voltage THD
Figure.7 Experimental waveforms
V. CONCLUSION
This paper proposes a novel power electronic
transformer for distribution system named flexible power
distribution unit. Its topology structure, working principle
and control strategies are presented. The isolation module
adopts open loop control to reduce the control complexity
of system. Output module adopts three single-phase
inverters to ensure the independence. A 600V/ 220V PET
prototype which is suitable for the smart distribution
View publication stats
[7].
[8].
[9].
Technologies,” IEEE Industrial Electronics Magazine, 2009, 3(2):
16–23.
J. Zhao, “Simulation Study of a Power electronic transformer with
Constant Output Voltage,” Automation of Electric Power Systems,
2003, 27(18):30-34, 46.
D. Wang, C. Mao, J. Li, “Auto Balancing Electronic Power
Transformer,” Proceedings of the CSEE, 2007, 27(6):77-83.
G. Ortiz, J. Biela, and J. W. Kolar, “Optimized Design of Medium
Frequency Transformers with High Isolation Requirements,” in
Annual Conference of the IEEE Industrial Electronics Society
(IECON), Glandale, 2010, pp. 631–638.
Z. Li, P. Wang, Z. Chu, et al, “Research on Medium and High Voltage
Smart Distribution Grid Oriented Power Electronic Transformer,”
Power System Technology, 2013, 37(9):2592-2601.
G. Wang, A. Huang, J. Wang, et al, “Comparisons of 6.5 kV 25A Si
IGBT and 10-kV SiC MOSFET in Solid-State Transformer
Application,” IEEE Energy Conversion Congress and Exposition
(ECCE), 2010: 100-104.
C. Zhao, D. Dujic, A. Mester, et al, “Power Electronic Traction
Transformer—Medium Voltage Prototype,” IEEE Transactions on
Industrial Electronics, 2014, 61(7): 3257-3268.
D. Siemaszko, F. Zurkinden, L. Fleischli, et al, “Description and
Efficiency Comparison of Two 25 kVA DC/AC Isolation Modules,”
EPE Journal: European Power Electronics and Drives Association
Journal, 2009, 19(LEI-ARTICLE-2010-002): 17-24.
G. Quartarone, M. Liserre, F. Fuchs, et al, “Impact of the Modularity
on the Efficiency of Smart Transformer Solutions,” Annual
Conference on IEEE Industrial Electronics Society (IECON), 2014:
1512-1518.