Active power electronic transformer a standard building block for smart grid

1. Proceedings of the International Conference on Emerging Trends in Engineering and Management (ICETEM14)
30-31, December, 2014, Ernakulam, India
178
ACTIVE POWER ELECTRONIC TRANSFORMER A
STANDARD BUILDING BLOCK FOR SMART GRID
SHARUN JOHN1
, BRINTA N.R 2
1
P G Scholar, Department of EEE, Sree Narayana Gurukulam College of Engineering Kadayiruppu, India
2
Assistant Professor, Department of EEE, Sree Narayana Gurukulam College of Engineering Kadayiruppu, India
ABSTRACT
Nowadays distribution transformers are considered among the huge and expensive equipment because of their
massive iron core and heavy copper windings. Since the size of the transformers and the maximum power transition are
reversely related, a new family of electronic power transformer has come into existence which increases the transformer
frequency, using power electronic converters. In addition to voltage transformation and good isolation which they bring
about, these transformers are also associated with significant advantages, including considerable reduction in the size,
power quality improvement, voltage profile improvement, etc. This paper attempts to introduce a new structure for Active
Power Electronic Transformer. Active power electronic transformers, composing an important part of the smart grid,
could have many roles in that variable system. Active power electronic transformer is used later as a standard building
block in more complex systems to guarantee the power quality on the HV grid side. The performance of the base module
was verified by computer-aided simulations.
Keywords: Active Power Electronic Transformer, power electronic converters, power quality improvement, smart grid,
variable system
I. INTRODUCTION
Today’s distribution grid operators face new challenges. Distributed generation and renewable energy sources
increase the uncertainty factor in the grid and make a centralized control impossible. New operation and performance
objectives will be defined that can come under the term smart grid concept.
Transformers are widely used in electric power system to perform the primary functions, such as voltage
transformation and isolation. Transformers are one of the heaviest and most expensive devices in an electrical system
because of the large iron cores and heavy copper windings in the composition [1].
Power generation, transmission and distribution are three main parts of the modern power system, in which the
power transformers play a significant role [2]. The power electronics has offered enabling technologies for the power
quality enhancements in the transmission and distribution systems. Such systems are for example the flexible AC
transmission systems, static VAR compensators, static synchronous compensators, unified power flow controller etc. The
APET is a new type of transformer that realizes voltage transformation, galvanic isolation and power quality
enhancements in a single device. The APET is suitable for the use in the power systems that comprise renewable energy
sources, energy storage devices as well as different type of loads. Thus the bi-directional power flow is the most
important requirement that the APET has to fulfill. Additional expected characteristic is the reduction of the volumes by
means of increasing the operating frequency. Since transition power of transformer and their size are reversely related to
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2. Proceedings of the International Conference on Emerging Trends in Engineering and Management (ICETEM14)
the frequency, increasing the frequency provides for using magnetic steel core in the transformers, thereby considerably
reducing its size. Since there is no energy storage source in the conventional transformers, when there is disturbance in
the transformer input the output loads are disturbed as well. Similarly when the load is troubled with transient states,
harmonics and power disturbances the conventional transformers reflect all this problems toward the grid. In order to
overcome such problems power electronic technology could be an option, which act as energy buffer, thereby preventing
the mutual effect of grid and load on one anot
into existence which, using electronic converters, increasing the frequency of AC signals and thus reduce the transformer
sizes [3].
Different topologies have been presented for real
stage PET topology seems to be the most promising. This topology consists of the controllable input (AC/DC), the
isolation (DC/DC) and output (DC/AC) power electronic stages (see Fig. 1).
utilized to shape the input current, to correct the input power factor, and to regulate the voltage of primary DC bus.
Second stage is an isolation stage which provides the galvanic isolation between the primary an
isolation stage, the DC voltage is converted to a high
(MF) transformer and is rectified to form the DC link voltage. The output stage is a voltage source inverter w
produces the desired AC waveforms.
There is no unique structure for an APET. Depending on the application, it could be with multiple input/output
ports, integrating high voltage (HV) and low voltage (LV) ports, AC and dc ports, single
some of them connected to the grid or consumers, other to renewable energy plants or to energy storage systems.
The structure of the smart grid could be very complex and hardly predictable. Thus, an APET could have many
roles in that stochastic and variable system. The aim was to
as a standard building block in more complex systems. To provide flexibility and universal properties, the base module
was designed as a symmetrical topology with the transfer ratio 1:1. The power
module of the APET is guaranteed by the PFC functionality. The performance of the proposed base module of the APET
was verified by computer-aided simulations using MATLAB/SIMULINK. In this project a single phase thr
topology of APET was selected as a base module.
II. SELECTION OF THE TOPOLOGY
Active power electronic transformer (APET) can be classified into single
topologies.
Fig. 2. General Classification of Active Power
Single-stage
With LV dc
International Conference on Emerging Trends in Engineering and Management (ICETEM14)
30-31, December, 2014, Ernakulam, India
179
the frequency, increasing the frequency provides for using magnetic steel core in the transformers, thereby considerably
size. Since there is no energy storage source in the conventional transformers, when there is disturbance in
the transformer input the output loads are disturbed as well. Similarly when the load is troubled with transient states,
bances the conventional transformers reflect all this problems toward the grid. In order to
overcome such problems power electronic technology could be an option, which act as energy buffer, thereby preventing
the mutual effect of grid and load on one another. That's why the new family of electronic power transformers have come
into existence which, using electronic converters, increasing the frequency of AC signals and thus reduce the transformer
Different topologies have been presented for realizing the PET, in recent years, the research states that the three
topology seems to be the most promising. This topology consists of the controllable input (AC/DC), the
isolation (DC/DC) and output (DC/AC) power electronic stages (see Fig. 1). First stage is an AC/DC converter which is
utilized to shape the input current, to correct the input power factor, and to regulate the voltage of primary DC bus.
Second stage is an isolation stage which provides the galvanic isolation between the primary an
isolation stage, the DC voltage is converted to a high-frequency square wave voltage, coupled to the secondary of the HF
(MF) transformer and is rectified to form the DC link voltage. The output stage is a voltage source inverter w
Fig. 1. Three stage topology of PET.
There is no unique structure for an APET. Depending on the application, it could be with multiple input/output
ports, integrating high voltage (HV) and low voltage (LV) ports, AC and dc ports, single-phase and three
he grid or consumers, other to renewable energy plants or to energy storage systems.
The structure of the smart grid could be very complex and hardly predictable. Thus, an APET could have many
roles in that stochastic and variable system. The aim was to develop a base module of the APET that could be used later
as a standard building block in more complex systems. To provide flexibility and universal properties, the base module
was designed as a symmetrical topology with the transfer ratio 1:1. The power quality on the HV grid side of the base
module of the APET is guaranteed by the PFC functionality. The performance of the proposed base module of the APET
aided simulations using MATLAB/SIMULINK. In this project a single phase thr
topology of APET was selected as a base module.
SELECTION OF THE TOPOLOGY
Active power electronic transformer (APET) can be classified into single-stage, two
Classification of Active Power Electronic Transformer Topologies.
Active Power Electronic Transformer
Two-stage
With LV dc-link With HV dc link
Three
International Conference on Emerging Trends in Engineering and Management (ICETEM14)
December, 2014, Ernakulam, India
the frequency, increasing the frequency provides for using magnetic steel core in the transformers, thereby considerably
size. Since there is no energy storage source in the conventional transformers, when there is disturbance in
the transformer input the output loads are disturbed as well. Similarly when the load is troubled with transient states,
bances the conventional transformers reflect all this problems toward the grid. In order to
overcome such problems power electronic technology could be an option, which act as energy buffer, thereby preventing
her. That's why the new family of electronic power transformers have come
into existence which, using electronic converters, increasing the frequency of AC signals and thus reduce the transformer
he research states that the three
topology seems to be the most promising. This topology consists of the controllable input (AC/DC), the
irst stage is an AC/DC converter which is
utilized to shape the input current, to correct the input power factor, and to regulate the voltage of primary DC bus.
Second stage is an isolation stage which provides the galvanic isolation between the primary and secondary side. In the
frequency square wave voltage, coupled to the secondary of the HF
(MF) transformer and is rectified to form the DC link voltage. The output stage is a voltage source inverter which
There is no unique structure for an APET. Depending on the application, it could be with multiple input/output
phase and three-phase ports,
he grid or consumers, other to renewable energy plants or to energy storage systems.
The structure of the smart grid could be very complex and hardly predictable. Thus, an APET could have many
develop a base module of the APET that could be used later
as a standard building block in more complex systems. To provide flexibility and universal properties, the base module
quality on the HV grid side of the base
module of the APET is guaranteed by the PFC functionality. The performance of the proposed base module of the APET
aided simulations using MATLAB/SIMULINK. In this project a single phase three-stage
stage, two-stage and three-stage
Electronic Transformer Topologies.
Three-stage

3. Proceedings of the International Conference on Emerging Trends in Engineering and Management (ICETEM14)
Single-stage actually means a topology without a dc
of the dc-link strongly limits their functionality, integration of renewable energy sources and energy stor
Thus, single-stage topologies are not considered suitable candidates for the APET in the smart grid applications.
Two-stage topologies (Fig. 2) are divided into topologies with a HV dc
required for connection of distributed energy storages (DES). Thus, two
comply with smart grid requirements. Two
the HV side and will also have a larger
improves the output power quality and allows connection of distributed energy sources and distributed energy storage
devices. Thus, this configuration is considered a be
link.
The most feasible configuration of the APET is the three
HV and LV dc-links available [5]. The voltage or current can be sep
distributed energy storage devices or distributed energy sources. HV and LV dc
of the APET and allow power quality improvement in the input and in the output.
implemented in each stage and optimize the APET for high
was selected in the current project to build the base module.
III. PROPOSED TOPOLOGY
In general, a three-stage topology includes an input stage, an isolation stage and an output stage. In the input
stage, there is a converter, which converts the input AC voltage to DC voltage. The second part of the converter is formed
by a DC/AC converter. This part of the converter contains the MF transformer with the high insulation capability. In the
output part, the high frequency voltage is revealed as a power
introduced. It is a new configuration based on the matrix converter with new function shown in Fig. 3. It can provide
desired output voltage. In addition, it performs power quality functions, such as sag correction, reactive power
compensation and is capable to provide three
each stage can be controlled independently from the other one. Many advantages of the
quality and power factor correction depend on appropriate close
reliability of a system is indirectly proportional to the number of its components. The main purpose of proposed
reduction of the power delivery stage (AC/DC and DC/AC links) in PET with DC
Fig
IV. PROPOSED ACTIVE POWER ELECTRONIC TRANSFORMER
The electrical circuit diagram of the propose
stage (L1, H-Bridge, C1), isolation stage (inverter, medium frequency transformer, rectifier), output stage (LC filter, H
Bridge, LC filter), and load (diode rectifier with an inductive load).
International Conference on Emerging Trends in Engineering and Management (ICETEM14)
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180
stage actually means a topology without a dc-link. It includes direct ac-ac or matrix converters. The lack
link strongly limits their functionality, integration of renewable energy sources and energy stor
stage topologies are not considered suitable candidates for the APET in the smart grid applications.
) are divided into topologies with a HV dc-link and with a LV dc
r connection of distributed energy storages (DES). Thus, two-stage topologies with the HV dc
comply with smart grid requirements. Two-stage topologies with the LV dc-link cannot compensate the reactive power on
larger voltage ripple in the dc-link [4]. Nevertheless, availability of the LV dc
improves the output power quality and allows connection of distributed energy sources and distributed energy storage
devices. Thus, this configuration is considered a better choice for the APET than the two-stage topology with the HV dc
The most feasible configuration of the APET is the three-stage configuration (Fig. 1), i.e. topologies with both
]. The voltage or current can be separately controlled in each stage. It is possible to add
distributed energy storage devices or distributed energy sources. HV and LV dc-links enhance the ride
of the APET and allow power quality improvement in the input and in the output. Multilevel converter topologies can be
implemented in each stage and optimize the APET for high-voltage or high-power applications. The three
was selected in the current project to build the base module.
topology includes an input stage, an isolation stage and an output stage. In the input
stage, there is a converter, which converts the input AC voltage to DC voltage. The second part of the converter is formed
. This part of the converter contains the MF transformer with the high insulation capability. In the
output part, the high frequency voltage is revealed as a power-frequency voltage. In this paper, a three part design is
on based on the matrix converter with new function shown in Fig. 3. It can provide
desired output voltage. In addition, it performs power quality functions, such as sag correction, reactive power
compensation and is capable to provide three-phase power from a single phase system. The
each stage can be controlled independently from the other one. Many advantages of the A
quality and power factor correction depend on appropriate close-loop control, and correlative research is necessary. The
reliability of a system is indirectly proportional to the number of its components. The main purpose of proposed
reduction of the power delivery stage (AC/DC and DC/AC links) in PET with DC-link.
Fig. 3. Structure of a Three-Stage Topology
POWER ELECTRONIC TRANSFORMER
The electrical circuit diagram of the proposed base module is shown in Fig. 4. The model is divided into four parts: input
Bridge, C1), isolation stage (inverter, medium frequency transformer, rectifier), output stage (LC filter, H
Bridge, LC filter), and load (diode rectifier with an inductive load).
International Conference on Emerging Trends in Engineering and Management (ICETEM14)
December, 2014, Ernakulam, India
ac or matrix converters. The lack
link strongly limits their functionality, integration of renewable energy sources and energy storage devices.
stage topologies are not considered suitable candidates for the APET in the smart grid applications.
link and with a LV dc-link. The LV dc-link is
stage topologies with the HV dc-link do not
link cannot compensate the reactive power on
]. Nevertheless, availability of the LV dc-link
improves the output power quality and allows connection of distributed energy sources and distributed energy storage
stage topology with the HV dc-
stage configuration (Fig. 1), i.e. topologies with both
arately controlled in each stage. It is possible to add
links enhance the ride-through capability
Multilevel converter topologies can be
power applications. The three-stage topology
topology includes an input stage, an isolation stage and an output stage. In the input
stage, there is a converter, which converts the input AC voltage to DC voltage. The second part of the converter is formed
. This part of the converter contains the MF transformer with the high insulation capability. In the
frequency voltage. In this paper, a three part design is
on based on the matrix converter with new function shown in Fig. 3. It can provide
desired output voltage. In addition, it performs power quality functions, such as sag correction, reactive power
m a single phase system. The APET has three stages and
APET such as output power
tive research is necessary. The
reliability of a system is indirectly proportional to the number of its components. The main purpose of proposed APET is
. The model is divided into four parts: input
Bridge, C1), isolation stage (inverter, medium frequency transformer, rectifier), output stage (LC filter, H-

4. Proceedings of the International Conference on Emerging Trends in Engineering and Management (ICETEM14)
Fig. 4. Electr
In the fig 5 shows MATLAB / SIMULINK modeling of proposed Active power electronic transformer. The
proposed solution allows bidirectional energy control.
Fig 5. Mat lab / Simulink modeling of proposed active power electronic transformer
A. Input stage
The input stage is a three or single phase PWM rectifier, which is used to convert the primary low frequency
voltage into the DC voltage. The main functions asso
controlling the input power factor, and keeping the DC
are presented for control of input stage in conventional PET. Fig. 6.
A single phase PWM rectifier is used in this paper, which operates same as in
INPUT STAGE
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181
Electrical Circuit Diagram of the Base Module
In the fig 5 shows MATLAB / SIMULINK modeling of proposed Active power electronic transformer. The
proposed solution allows bidirectional energy control.
. Mat lab / Simulink modeling of proposed active power electronic transformer
The input stage is a three or single phase PWM rectifier, which is used to convert the primary low frequency
voltage into the DC voltage. The main functions associated with the rectifier control are shaping the input current,
controlling the input power factor, and keeping the DC-link voltage at the desired reference value. Many control methods
tage in conventional PET. Fig. 6. Shows single phase rectifier
phase PWM rectifier is used in this paper, which operates same as input stage of conventional PET [6
ISOLATION STAGE OUTPUT
International Conference on Emerging Trends in Engineering and Management (ICETEM14)
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In the fig 5 shows MATLAB / SIMULINK modeling of proposed Active power electronic transformer. The
. Mat lab / Simulink modeling of proposed active power electronic transformer
The input stage is a three or single phase PWM rectifier, which is used to convert the primary low frequency
ciated with the rectifier control are shaping the input current,
link voltage at the desired reference value. Many control methods
phase rectifier with input inductances.
put stage of conventional PET [6]-[7].
OUTPUT STAGE

5. Proceedings of the International Conference on Emerging Trends in Engineering and Management (ICETEM14)
Figure
B. Isolation Stage
The isolation stage consists of the inverter, MF transformer and rectifier (Fig. 7).
single-phase high frequency voltage source converter (VSC), which converts the input DC voltage to AC square voltage
with high (or medium) frequency and HF (MF) transformer. The main functions of the HF (MF) transformer are such as
voltage transformation and isolation between source and load.
Figure
C. Output Stage
The output stage is represented in Fig. 8.
shape and frequency. The output stage consists of LC filter 1, sine wave modulator; H
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182
Figure 6 Structure of the Proposed Input Stage
The isolation stage consists of the inverter, MF transformer and rectifier (Fig. 7). Isolation stage is contained a
phase high frequency voltage source converter (VSC), which converts the input DC voltage to AC square voltage
requency and HF (MF) transformer. The main functions of the HF (MF) transformer are such as
voltage transformation and isolation between source and load.
Figure 7 Structure of the Proposed Isolation Stage
The output stage is represented in Fig. 8. The proposed converter generates desired output voltage with suitable
shape and frequency. The output stage consists of LC filter 1, sine wave modulator; H-Bridge inverter and LC filter 2.
Figure 8 Circuit of Output Stage
International Conference on Emerging Trends in Engineering and Management (ICETEM14)
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Isolation stage is contained a
phase high frequency voltage source converter (VSC), which converts the input DC voltage to AC square voltage
requency and HF (MF) transformer. The main functions of the HF (MF) transformer are such as
The proposed converter generates desired output voltage with suitable
Bridge inverter and LC filter 2.

6. Proceedings of the International Conference on Emerging Trends in Engineering and Management (ICETEM14)
D. Modeling Load
To simulate a nonlinear load, a diode rectifier was used with an induct
resistive load, the inductive load was replaced by a resistor.
V. SIMULATION RESULTS
To evaluate the expected performance of the APET, the design was simulated to predict steady state
performance. A prototype based on the proposed topology is simulated using MATLAB/SIMULINK. Operation of
proposed APET is described by Fig. 9. Fig. 9(a) shows input line vol
output voltage of the input rectifier. Fig. 9(c) depicts the output voltage of inverter stage that transforms DC voltage to
medium frequency (2 KHz) AC voltage as the transformer primary voltage. In the outp
voltage is revealed as a 50 Hz waveform followed by a rectifier & inverter stage. Fig. 9(d) depicts the output of
transformer. Fig. 9(e) & Fig. 9(f) implies the output
Figure 9 (a) Input voltage (b) DC
Transformer secondary voltage (e) Secondary rectifier output voltage
TABLE 1
Symbol
Uin Input voltage (RMS)
L1 Input inductor
rL1 Series resistor of the inductor
C1 HV dc-link capacitor
rC1 Shunting resistor of the capacitor
N Turns ratio of
L2 Inductance of the LV dc
rL2 Series resistor of the inductor
C2 LV dc-link capacitor
rC2 Shunting resistor of the capacitor
Udc1 HV dc-link voltage
L3 Output filter inductor
rL3 Series resistor of the inductor
C3 Output filter capacitor
rC3 Shunting resistor of the capacitor
Lload Load inductor
rload Load resistor
Uout Output voltage of APET (RMS)
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183
To simulate a nonlinear load, a diode rectifier was used with an inductive load in the output.
resistive load, the inductive load was replaced by a resistor.
expected performance of the APET, the design was simulated to predict steady state
performance. A prototype based on the proposed topology is simulated using MATLAB/SIMULINK. Operation of
proposed APET is described by Fig. 9. Fig. 9(a) shows input line voltage of APET. As it can be seen in Fig.9(b), the
output voltage of the input rectifier. Fig. 9(c) depicts the output voltage of inverter stage that transforms DC voltage to
medium frequency (2 KHz) AC voltage as the transformer primary voltage. In the output stage, the medium frequency
voltage is revealed as a 50 Hz waveform followed by a rectifier & inverter stage. Fig. 9(d) depicts the output of
implies the output of, the AC/DC and DC/AC converters respectively.
Figure 9 (a) Input voltage (b) DC-link voltage (c) MF transformer primary voltage (d)
Transformer secondary voltage (e) Secondary rectifier output voltage and (f) output v
TABLE 1 : SIMULATION PARAMETERS
Parameter Value
nput voltage (RMS) 230V,
nput inductor 100 mH
eries resistor of the inductor 10 µ
link capacitor 10 mF
hunting resistor of the capacitor 100 k
Turns ratio of isolation transformer
nductance of the LV dc-link inductor 30 mH
eries resistor of the inductor 200 m
link capacitor 500
hunting resistor of the capacitor 10 k
link voltage 400 V dc
utput filter inductor 50 mH
eries resistor of the inductor 500 m
utput filter capacitor 100
hunting resistor of the capacitor 10 k
oad inductor 500 mH
oad resistor 100
utput voltage of APET (RMS) 20V, 50 Hz
International Conference on Emerging Trends in Engineering and Management (ICETEM14)
December, 2014, Ernakulam, India
ive load in the output. To simulate a
expected performance of the APET, the design was simulated to predict steady state
performance. A prototype based on the proposed topology is simulated using MATLAB/SIMULINK. Operation of
tage of APET. As it can be seen in Fig.9(b), the
output voltage of the input rectifier. Fig. 9(c) depicts the output voltage of inverter stage that transforms DC voltage to
ut stage, the medium frequency
voltage is revealed as a 50 Hz waveform followed by a rectifier & inverter stage. Fig. 9(d) depicts the output of step down
respectively.
transformer primary voltage (d) MF
and (f) output voltage
Value
230V,50 Hz
100 mH
10 µΩ
10 mF
100 kΩ
1:1
30 mH
200 mΩ
500 µF
10 kΩ
400 V dc
50 mH
500 mΩ
100 µF
10 kΩ
500 mH
100 Ω
V, 50 Hz

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VI. HARDWARE IMPLEMENTATION
The system parameters are as follows: ① Utility grid side: input voltage 12 V AC, 50Hz ① primary dc link side:
capacitor 1000 µf, ① high-frequency transformer: ratio of transformation is 1:1, frequency 10 kHz, ① secondary DC link
side: filter capacitance 1000 µF, ① load side: filter inductance 14 mH, filter capacitor 2.5 µF, reference value of load
voltage is 12 V AC, 50Hz.
The DC voltage from the input stage is modulated into the high-frequency square wave by the full-bridge
inverter, and the amplitude of high-frequency square wave voltage is half of DC high voltage.
Figure 10 Hardware circuit model
VII. CONCLUSION
An active power electronic transformer (APET) could have many roles in the smart grid. The base module of the APET
that could be used later as a standard building block in more complex systems was proposed and evaluated. The base
module is a single-phase system consisting of different converters. In addition to providing galvanic isolation between the
input and the output, it isolates input from output distortions. Thus, the input voltage disturbances have no effect on the
output ones, and vice versa. In general, the power quality is improved in both ports: input and output. The performance of
the proposed base module of the APET was verified by computer-aided simulations using MATLAB/SIMULINK.
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