SMART DISTRIBUTION TRANSFORMER APPLIED TO SMART GRIDS
Josemar O. Quevedo*, Julian C. Giacomini*, Rafael C. Beltrame*, Fabricio E. Cazakevicius*, Cassiano
Rech*, Luciano Schuch*, Tiago B. Marchesan*, Maurício de Campos**,
Paulo S. Sausen**, Jonatas R. Kinas**
*Federal University of Santa Maria (UFSM) 97105-900, Santa Maria, RS, Brazil
**Rio Grande do Sul North-Western Regional University (Unijui) 98700-000, Ijui, RS, Brazil
josemar.oliveira.quevedo@gmail.com
Abstract – Growing demand, requirements for power
quality enhancement and distribution generation are
increasing the complexity of distribution systems. In this
scenario, voltage regulation of distribution networks also
becomes more complex, especially in long distribution
lines. In this paper, a smart distribution transformer is
proposed, which employs an electronic on load tap
changer with a bidirectional communication system. This
system enables automatic voltage regulation, telemetry
and remote control for power utilities, allowing its
application in Smart Grids.
Keywords – Electronic on load tap changer (OLTC),
smart grids (SG), smart transformer (ST).
I. INTRODUCTION
Voltage regulation is one of the most important power
quality issues in distribution systems, and it is still a problem
for power utilities. The continued growth in electric energy
consumption, the requirements for power quality
enhancement and the inclusion of distributed generation
(DG) units are increasing considerably the complexity of
distribution systems [1]. In this scenario, voltage regulation
of distribution networks also becomes more complex,
especially in rural areas, which typically experience the least
reliable power [2].
On load tap changers (OLTCs) are the most popular
voltage regulation devices, which have been employed in
medium and high power transformers. The commutation of
taps in OLTCs is done by electromechanical switches,
resulting in arcing in contacts, and consequently, their
carbonization of the contacts and degradation of the
insulation oil [3]. The high cost of these devices prevents
them from being used in distribution systems. To reduce the
cost, no load tap changers (manually commuted in loco) are
used in distribution systems, so that these devices cannot
provide automatic voltage regulation.
The development of electronic OLTCs, based on power
electronics, sensing and management systems is a potential
solution for the enhancement of the power grids [4]. These
systems can provide to power utilities OLTC devices more
stable, effective and cheaper, when compared with the
electromechanical devices.
In addition to voltage regulation, the employment of
electronic OLTC in distribution transformers enables the
development of a smart device, which can provide additional
functions to the transformer. This device is usually named as
a smart transformer [5], and can allows the online control of
978-1-4799-0272-9/13/$31.00 ©2013 IEEE
1046
the secondary voltage of the transformer [6], from the online
adjustment of the taps, including important functions for the
distribution system, such as: power dispatch control for DG
units, avoiding overvoltage in the secondary side [5];
creating a bidirectional communication path between the
power utility and the transformer, allowing the online
evaluation of the transformer loading [7]; monitoring of the
load behavior at the point of common coupling (PCC),
among others.
On the other hand, the implementation of the concept of
smart transformers in distribution systems requires an
adequate communication system. However, the existing
distribution system has limited or inexistent communication,
sensing and computing capabilities [8]. These limitations are
actually one of the main problems for the development of
smart grids.
Therefore, this paper proposes a smart distribution
transformer, which enables the automatic voltage regulation
in the PCC, allied to a bidirectional communication with the
power utility. These capabilities are complemented by the
characteristic of easy expansion of the proposed system,
which enables the creation of a network of smart
transformers, enhancing the power quality and the
management of the distribution system.
This paper is organized as follows: Section II describes
the electronic OLTC employed in the ST, and includes some
experimental results. Section III describes the proposed ST,
presenting the topology and the prospected functions of the
system. Finally, section IV presents experimental results of
proposed system.
II. ELECTRONIC ON LOAD TAP CHANGER
The use of tap changers was initially encouraged by the
generalization of ac voltage in the power systems [3]. These
devices enable the voltage regulation through the changing of
the turn ratio of the transformers. Mechanical switches are
usually employed in distribution transformers, and the
commutation of the taps is typically manual.
The development of electronic switches for higher voltage
and current levels enabled the replacement of the mechanical
switches by electronic ones. This solution allows the
transformer’s tap to be commuted electronically, making the
switching process faster and free from electric arcs.
Moreover, the application of microprocessors enables the
implementation of control strategies for the enhancement of
the commutation process, and the remote control of the
system.
A. Electronic OLTC characteristics
Figure 1 shows a simplified block diagram of the
electronic OLTC under analysis. The electronic OLTC
consists of a 5 kVA single phase earth return (SWER)
transformer. Its voltage specifications are summarized in the
Table 1.
TABLE I
Voltage specifications of the transformer
Voltage characteristics
Tap
Primary
Secondary
connections
voltage
voltage
N7 – N9
7967 V*
N7 – N11
7621 V
N5 – N11
7274 V
220 V
N5 – N13
6928 V
N3 – N13
6581 V
*Nominal voltage at line frequency of 60 Hz.
From Figure 1, one can see that the primary side variables
(current and voltage) are measured through a current
transformer (CT) and a voltage transformer (VT), which
ensures the required insulation for the control system.
Furthermore, the VT feeds the instrumentation and control
systems, which ensures the operation of the system
regardless of the availability of voltage in the secondary side
of the transformer.
The commutation process must ensure that the primary side
of the transformer will not remain open circuited, preventing
the switches to support all the primary voltage of the
transformer. For this purpose, there is a need for the overlap of
two switches during the commutation. However, this process
should be as fast as possible, to limit the short circuit current
during this period. Therefore, the CT measures the primary
current and provides the reference for the zero crossing current
commutation, avoiding a huge short-circuit current in the
commutation process (limited by the tap winding inductance).
The zero cross detection is achieved by the implementation of
a Kalman filter [9] in the measured primary current, ensuring
high noise immunity in this process.
The adoption of thyristors (as presented in [10]) for the
proposed application difficulties the development of gatedriver circuits, once the latch and hold current of commercial
thyristors are higher than the nominal current through the
switches at rated power in this application. For this reason,
insulated gate bipolar transistors (IGBTs) connected with
emitter configuration, and associated with anti-parallel diodes,
can be adopted as bidirectional switches, as proposed in [6].
The secondary voltage is measured and compared to a
voltage reference in the control system, providing voltage
regulation of the secondary side of the transformer within the
acceptable range. The connection between the control system
and gate-drivers is accomplished through an optical channel.
In addition, the gate driver circuits were developed to present
the same insulation class of the distributed transformer, i.e.,
15 kV, this voltage class is obtained from a switch mode
power supply implemented through an isolated full bridge dcdc converter. Additionally, the switching command is
performed by the optical fibers.
The control of the electronic OLTC is performed by a
digital signal processor (DSP) TMS320F28335 from Texas
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Fig. 1. Simplified block diagram of the electronic OLTC.
Variable
measurements
Secondary rms voltage
unconformity detection
Rms voltage
is outside the
limits?
no
yes
no
Current
zero crossing
detection?
yes
Tap
commutation
Fig. 2. Simplified control block diagram.
Instruments. The simplified control diagram of the OLTC is
presented in Figure 2.
The electronic OLTC has a protection system designed to
handle with overvoltage in the primary side of the
transformer, possible voltage spikes existing in the
commutation process, and atmospheric discharges in both
medium and low voltage sides. Furthermore, the protection
system has a normally closed electromechanical switch in
parallel with the switch S1, which prevents the electronic
switches to support the entire primary voltage of the
transformer during the start-up process, and protects the
switches against overcurrent caused by short-circuit in the
secondary side of the transformer.
B. Experimental results of the electronic OLTC
The electronic OLTC has been implemented for nominal
parameters shown in Table 1. The tests were performed in
order to keep the rms voltage between the limits of 201 V
and 229 V in the secondary side of the transformer, which
are the acceptable voltage limits for single-phase distribution
systems according to the Brazilian standard PRODIST [11].
For the implementation of the system, a 5 kVA
distribution transformer has been used to obtain the
12
7968 V
6530 V
Voltage (kV)
10
8
6
Rms voltage
4
2
0
0
0.05
0.1
0.15
0.25
0.2
Time (s)
0.3
0.35
0.4
0.3
0.35
0.4
(a)
350
300
Voltage (V)
experimental results. The secondary side of the transformer
is connected to a 4.8 kW load, the line impedance in this case
is 0.3 pu, added to emulate a long distribution line, which
results in a primary voltage drop at the PCC of 16.3%, as can
be observed in Figure 3(a).
The electronic OLTC under analysis has an adequate
operation for disturbances until 17.4% of voltage drop,
limited by the voltage range of the tap windings. Results
presented in Figure 3 confirm the voltage regulation at the
PCC for the specified range. Figure 3(a) presents the positive
half cycles of the primary voltage with its rms voltage.
Figure 3(b) presents the positive half cycles of the secondary
voltage with its rms voltage, where it is possible to see the
adequate regulation of voltage. The specified commutation
period (from one tap to next one) is defined as ten line
voltage cycles. However, this period was considered only for
better visualization of results. It must be highlighted that the
system is able to commutate the taps every half cycle of the
line frequency, as shown in Figure 4(a) and 4(b), for the
same conditions of Figure 3.
Figure 5 presents the currents through the switches S1 and
S2 during the commutation process, confirming the
mitigation of the short-circuit current due to the reduced
overlap time between the switches, which is ensured by the
fast response of the electronic switches and adequate control
strategy. In addition, the commutation is realized at zerocrossing instant to reduce the current efforts and possible
voltage spikes over semiconductor switches.
Figure 6 presents the proposed electronic OLTC.
Extensive laboratory tests have been successfully performed,
demonstrating the applicability of this electronic OLTC.
250
200
150
100
50
0
0
0.05
0.1
0.15
0.25
0.2
Time (s)
(b)
Fig. 4. Voltage regulation of the electronic OLTC for each half
cycle of line frequency, (a) primary voltage, (b) secondary
voltage.
Primarycurrent
current
Primary
11
Current
Current through
across S1S1
2
12
7969V
6872V
6771V
6667V
6535V
10
Voltage (kV)
3
3
6394V
Current
Current through
across S2S2
1
2
1
8
6
Rms voltage
3
3
Fig. 5. Zero crossing current commutation with nominal load.
4
2
0
0
0.1
0.2
0.3
0.4
0.6
0.5
Time (s)
0.7
0.8
0.9
1
0.7
0.8
0.9
1
(a)
350
Voltage (V)
300
250
200
150
100
50
0
0
0.1
0.2
0.3
0.4
0.6
0.5
Tempo (s)
3
(b)
Fig. 3. Voltage regulation of the electronic OLTC for ten cycles of
line frequency, (a) primary voltage, (b) secondary voltage.
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Fig. 6. Electronic OLTC transformer.
PLC channel
PLC channel
Wireless channel
Short distance communication
Long distance communication
Primary feeder
Smart
transformer N
Power Utility
Smart
transformer 1
wireless
module
Gateway and
wireless module
Control system
and PLC interface
Data center and
grid management
Fig. 7. Schematic of proposed smart transformer communication system.
III. TOPOLOGY FOR SMART TRANSFORMER
APPLIED IN SMART GRIDS
In future electrical grids, the voltage regulation will be
affected especially due to the growth of demand for
electricity and the integration of distributed generation in the
distribution systems. Moreover, power quality problems such
as sags and swells will be less acceptable with the
requirements for better power quality delivered to end users.
Therefore, automatic voltage regulation devices shall be
developed to manage these problems.
The concept of smart transformer aims to mitigate these
voltage problems in the electrical grids, providing voltage
regulation and creating a bidirectional communication path
with the power utility. In addition to automatic voltage
regulation, the smart transformer can enhance the
management of the distribution systems, enabling, for
instance: direct actuation over transformer taps; monitoring
of transformer variables; evaluation of transformer loading;
planning of grid expansion, maintenance, and replacement of
overloaded transformer; among others.
However, the application of smart transformers requires a
communication system able to support a set of functions not
provided by the current distribution systems. In this way, this
communication system should be compatible with smart grid
applications, being capable of communication with other
appliances along the distribution system.
The communication is one of the most important roles in a
smart grid, providing advanced control and monitoring,
improvements in security, efficiency, reliability, redundancy,
self-healing, interactivity, etc. [2],[12]-[14]. It is essential to
communication system work in a bidirectional way, this
capability is required for interaction between consumer and
power utility, and also it enables the enhancement in the
management of the power system [15]-[16].
It is worth mentioning that two information infrastructures
are basically needed in a smart grid: sensors and electrical
appliances (such as smart meters), and communication
between the electrical devices and the data center [8]. As
suggested by [17], the first set of data can be accomplished
by power line communication (PLC), or wireless
communications, such as: ZigBee, 6LowPAN and Z-wave.
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The communication with the data center can be done by
cellular and/or internet technologies.
The integration of the smart metering with the proposed
communication system may allow the voltage regulation in
the most critical part of the secondary distribution system,
depending on its characteristics, e.g., right after the
secondary terminals of the transformer, in the middle or in
the end of secondary grid [5]. This approach may help the
power utilities to have online voltage regulation, and
enhances the power quality of the distribution system at its
critical points.
Hence, it has been proposed, as shown in Figure 7, a
communication topology for employment in smart
transformers for distribution systems, especially thought for
smart grid applications. Specifically, this system is designed
for applications in rural distribution networks, where voltage
regulation is usually critical, and signals of cell phone and
internet are poorly efficient. The system is composed of a
distribution transformer, equipped with an electronic OLTC
for automatic voltage regulation, and a communication
system, which is a hybrid composition of two stages: a PLC
system over the medium voltage line; and a wireless
technology, based on cell phone, internet technology or
radio.
The PLC channel is used to establish a path between the
distant locations, where the transformer is installed, and the
gateway that concentrates the data from other smart
transformers installed in the region. The gateway also
converts the data into a wireless communication channel.
The amount of data transferred and received by the smart
transformer makes the PLC suitable for this application,
since it does not need a high data transmission rate. On the
other hand, the wireless channel needs a higher data
transmission rate, since it concentrates the data from a set of
smart transformers, which prompts cell phone and internet
technologies a good choice.
Figure 8 presents the block diagram that represents the
operation of the proposed Smart Transformer. The set of
blocks “Transformer → Instrumentation → Control → Gatedriver → Electronic OLTC → Transformer,” builds the
voltage regulation block, which is responsible for the
secondary voltage regulation. The “Interface module” blocks
are data converters, which convert variables and events of the
Transformer
Smart
transformer 1
Smart
transformer n
Electronic
OLTC
Gate
driver
Instrumentation
Control
Interface
module 1
A. Interface and intelligent sensor modules
The Intelligent Sensor Modules (ISM) [18] are responsible
for the acquisition transformer’s variables. The ISM is
equipped with four sensing inputs, two digital and two
analog. The analog inputs are designed to operate with
signals in the range of 0 to 5 V or 4 to 20 mA, complying
with most commercial sensor characteristics. If the sensor
connected to the ISM requires power, a power supply can be
implemented with the signal connector. In addition to the
magnitudes of voltages and currents, the ISM can acquire:
temperature (transformer and ambient), humidity, state of
protection devices, and others.
The number of ISMs to be used for transformer depends
on the amount of data required of each measured variable,
and the transmission data rate available. Furthermore the
ISM has a digital output, which allows it to actuate on the
system. The physical aspect of the ISM module is shown in
Figure 10.
Temperature,
humidity,
fault...
Interface
module n
Master
module
PLC
Interface
Gateway
Wireless
Interface
Power utility
Fig. 8. Block diagram of the Smart Transformer.
Transformer
Smart
transformer 1
Smart
transformer n
Electronic
OLTC
Gate
driver
Instrumentation
Control
Interface
module 1
only the wireless interface. This option may be suitable for
urban applications, where radio, cellphone and internet
technologies can be directly applied.
Temperature,
humidity,
fault...
Interface
module n
Master
module
Wireless
interface
Power utility
Fig. 9. Alternative block diagram of the Smart Transformer.
control system and send them to the “Master module.” In the
“Master module,” the data of all the interface modules are
packed and sent to the “PLC interface,” which converts the
data to be sent through the medium voltage line.
The data coming from a set of transformers are received
by the “Gateway” block, which concentrates and sends them
to the “Wireless Interface.” From this block, the data are sent
to the “Power utility,” where it is received and processed.
The analysis of the data, and the bidirectional
communication created, enables the power utility to actuate
over the smart transformer in order to enhance the power
quality, through the online operation of the taps. Also, the
evaluation of data received allows the detection of critical
points along the grid, while still allowing loss and
maintenance analysis.
An alternative communication system for smart
transformers is presented in Figure 9. The block diagram
presented suggests the removal of the PLC interface, using
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B. PLC interface
The PLC modem incorporates a CPU, and application
memory of 4 kbytes and 2 kbytes of RAM. The CPU
executes routines for node protocol interconnections in a
network, PLC, Interoperable Self Installation (ISI), and
communication protocols, with the option to comply or not
with the CENELECTM standard. All these protocols are
proprietary and are stored in ROM memory on the device.
The modem can operate in bands A and C defined in
CENELECTM standard, which are selected from the crystal
used to trigger the modem. The selection of the CENELECTM
band also defines the rate of data transmission on the
network. By selecting the A band, the communication will
occur at a rate of 3.6 kbps.
C. Gateway
The implemented Gateway is responsible for
interconnecting the set of sensors (ISMs) and the
transmission system PLC. The main difference from the
Gateway to the actual ISM is that there is an additional
RS232 serial communication port used to accomplish the
interconnection with the PLC. The physical aspect of the
Gateway is shown in Figure 11. The communication among
the Gateway and ISMs uses the MODBUS protocol.
D. Wireless interface
After collecting the data on the server, they can be viewed
in real time on web application (Android or iOS systems),
enabling in the future the remote control of the system. The
system can also evaluate fault conditions based on this data.
IV. EXPERIMENTAL RESULTS
The smart transformer topology proposed has been
implemented for the configuration presented in Figure 9. It
was used a wireless interface communication with two
900 MHz radio modules model IPWR from Landis Gyr,
which enables the communication in distances up to 17 km.
Fig 10. ISM – Intelligent Sensor Module.
Fig. 11. Developed gateway.
The data communication and transformer’s control was
performed efficiently. Figure 12 presents the interface of
supervisory system developed for data receiving and manual
transformer’s control, which enables two operation modes,
i.e., “automatic regulation mode”, performed by the
electronic OLTC itself, and a “management operation
mode”, which permits the power utility to set a specific tap
of transformer despite of the secondary terminal voltage. The
supervisory system also runs on iOS® appliances, enabling
mobility to the system analysis and control, Figure 13
presents the representation of the system in an iPAD®.
As shown in Figure 3, the automatic voltage regulation
mode regulates the voltage right after the secondary
terminals of the transformer. This mode of operation is
interesting for concentrated consumers, and can help in the
power dispatch control of DG. However, depending on the
characteristics of the secondary distribution grid, especially
the line impedance, it may be more interesting to regulate the
voltage in different points of the grid. This can be performed
by the “management operation mode” of the smart
transformer, giving to the power utility total control over
transformer taps, what can be an important enhancement for
the grids, since it permits to regulate the voltage in different
places along the secondary system.
Figure 14 presents the comparison between the data
received from the supervisory system, with the voltage
measured right at the secondary terminals of the transformer.
This situation represents a possible operation of the power
utility in order to regulate grid voltage level during different
periods of day, raising the secondary voltage in periods of
higher load, and reducing the voltage level in periods of
lower load. Figure 14(a) presents the data received from the
supervisory system, and Figure 14(b) presents the oscilogram
of instantaneous voltage level measured in the secondary side
of the transformer.
Fig. 12. Developed supervisory system.
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proposed to be employed in urban locations, where wireless
technologies themselves can perform the communication
between the smart transformer and the power utility.
The communication system based on the wireless
technology has being successfully implemented. A
supervisory system was developed, enabling the operation of
the transformer in both automatic and management modes,
still proving data logger capability.
The main characteristic of the proposed system is the easy
expansion, enabling the management of a set of smart
transformers with the same communication path. It also
allows forming a network of smart transformers. These
features make the smart transformer a promising alternative
for applications in distribution systems, in that it enables a
voltage self-regulation and grid management, being suitable
for distributed generation and smart grid applications.
It is worth mentioning that the incorporation of the smart
metering with the management operation mode of the smart
transformer can enhance the voltage characteristics along the
grid, providing greater amount of data about the grid’s
behavior and, in this way, enabling programmed actions in
order to improve aspects of the grid performance, as voltage
regulation in the critical parts of the grid.
Fig. 13. Supervisory system for iOS appliances.
ACKNOWLEDGEMENT
The authors would like to express their gratitude to
“Conselho Nacional de Desenvolvimento Científico e
Tecnológico (CNPq),” “Coordenação de Aperfeiçoamento de
Pessoal de Nível Superior (CAPES),” “Fundação de Amparo
à Pesquisa do Estado do Rio Grande do Sul (FAPERGS)”
and “Centrais Elétricas de Carazinho S/A” (P&D Aneel) for
financial support.
(a)
350
Voltage (V)
300
250
216 V
225 V
S1
S2
235 V
225 V
S3
S2
216 V
REFERENCES
200
150
S1
100
50
0
0
5
10
15
20
Time (s)
25
30
35
40
(b)
Fig 14. Comparison between the data received from (a) the
supervisory system, (b) voltage measured in the secondary
terminals of the transformer and switches under operation.
V. CONCLUSION
The smart transformer proposed and implemented in this
paper employs an electronic OLTC with two communication
systems: (i) an hybrid solution, which is composed of PLC
and wireless technologies, and (ii) just wireless
communication. The first system is initially proposed to
communicate the power utilities with rural and sparsely
populated areas, where the cell phone technology is usually
less efficient or inexistent. These two technologies are
complementary, combining the benefits of PLC (for
applications using the power cables and communication in
islanded regions) and the large capability of communication
of cell phone and internet technologies. The second system is
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