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618 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 61, NO. 3, MARCH 2012

A Compact Remote Monitoring System fora Three-Phase 10-kVA Energy-Efficient

Switchable Distribution TransformerHari Arief Dharmawan and Sam A. M. Ali

Abstract—Remote monitoring has been implemented in manyareas. This paper introduces its specific application to a three-phase 10-kVA energy-efficient switchable distribution trans-former. A designed embedded system and embedded Ethernethave been implemented to achieve a compact remote conditionmonitoring for the transformer. The embedded system performsacquisition of voltages, currents, and temperatures, controls theswitching devices that connect the tappings of the transformer,and processes acquired data. Client and server applications weredeveloped through the use of embedded Ethernet to enable remotemonitoring through a local area network (LAN). Some protocolswere developed as parts of software development of the wholesystem. Experimentation was done by applying the remote moni-toring system to the transformer connected to three-phase variablesupply voltage and load. Results of the experimentation by usinga LAN available in the school revealed that the system can handleremote monitoring and control tasks for the transformer.

Index Terms—Embedded Ethernet, remote monitoring, switch-able transformer.

I. INTRODUCTION

R EMOTE condition monitoring of transformers has al-ready been widely known and implemented with var-

ious techniques [1]–[4]. The monitoring is mostly carriedout to reveal significant parameters that reflect conditions oftransformers, such as voltages, currents, and temperatures [5].Further suggested monitored parameters are explained in [6].In addition to applications for maintaining healthy operatingconditions of transformers, condition monitoring might be im-plemented for classroom demonstration and exercises such asdiscussed in [7]. The existence of the Internet provides furtherflexible remote monitoring in many areas, including powertransformers. Thus, many approaches are developed to performremote monitoring throughout the world by using a networkthat is connected to the Internet.

Manuscript received April 26, 2011; revised August 31, 2011; acceptedSeptember 8, 2011. Date of publication October 31, 2011; date of currentversion February 8, 2012. The Associate Editor coordinating the review processfor this paper was Dr. Carlo Muscas.

H. A. Dharmawan is with the Department of Physics, Brawijaya University,Malang 65145, Indonesia (e-mail: [email protected]).

S. A. M. Ali is with the School of Electrical and Information Engineer-ing, University of South Australia, Adelaide, S.A. 5095, Australia (e-mail:[email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TIM.2011.2170497

Fig. 1. Three-phase 10-kVA switchable transformer.

In relation to this project, remote condition monitoring wasapplied to a three-phase 10-kVA energy-efficient switchabledistribution transformer (see Fig. 1). This transformer has beenpreviously developed to show an increase in the efficiencythrough the use of special configurations of windings, calledseries and parallel. The series and parallel configurations con-secutively reduce losses of the transformer in low and highloads [8]. The configurations were made by the use of 18switching devices and controlled by three microcontrollers(AT90CAN128). These microcontrollers were chosen becausethey have an internal peripheral that supports communicationsthrough a controller area network (CAN) bus.

The remote monitoring system for the switchable trans-former is however quite unique because it includes measure-ments of switching devices’ temperature. Moreover, the systemis useful not only in monitoring the parameters of the trans-former (such as voltages and currents) but also in controllingthe switching devices and performing switching at appropriatetimings [9]. A compact monitoring system was considered tobe developed for this purpose as it requires less power andspace and it is portable. To perform remote monitoring, aclient/server system was implemented by using some embeddedEthernet boards (Ethernut 1.3 Board). The embedded Ethernethas been implemented in many areas involving data transmis-sions over a local area network (LAN) or the Internet [10]–[14].

0018-9456/$26.00 © 2011 IEEE

DHARMAWAN AND ALI: COMPACT REMOTE MONITORING SYSTEM FOR 10-kVA SWITCHABLE TRANSFORMER 619

Fig. 2. Whole system for remote monitoring of the transformer.

Client/server applications were developed using the C languagefor AVR microcontrollers and include an operating system(Nut/OS) and a Transmission Control Protocol (TCP) stack(Nut/NET). Some advantages can be seen by using these boardsand operating system.

1) A compact remote monitoring system can be designedbecause a microcontroller is implemented as a main CPU(instead of a computer) to handle client/server applica-tions through networks.

2) Complications in making a connection from the micro-controller to a network can be minimized because theboard contains an Ethernet controller as a bridge betweenthe microcontroller and a LAN.

3) Client/server applications can be built with leastattempt, as most primitive commands and proceduresrelated to networking are handled by the Nut/OS and Nut/NET [15].

4) Free development applications for the implemented mi-crocontroller can be found from the Internet to buildclient/server applications.

II. SYSTEM ARCHITECTURE

Fig. 2 shows the architecture of the whole system for remotemonitoring of the transformer. The system contains a three-phase 10-kVA energy-efficient switchable distribution trans-former with its switching devices, three-phase variable powersupply and load, an embedded system, embedded Ethernetunits, and a computer. The embedded system is made up ofsome modules to acquire as well as to control the parametersof the transformer and developed using a modular approachas presented in [9]. The power supply provides voltages of415 V on the primary terminals of the transformer while theload draws currents of up to a maximum rating (24 A) on thesecondary sides of the transformer.

Voltages and currents on each of the primary and secondarysides of the transformer are acquired by using some D3600 volt-age transformers and LTS 25-NP current transducers; accordingto [16], LTS 25-NP current transducers have an accuracy of±0.7% (with a 50-Ω internal measuring resistance, a 25-A pri-mary nominal RMS current, and a 25 ◦C ambient temperature)and frequency bandwidth from dc to 100 kHz (for 0–0.5 dB).In this case, waveforms of voltages and currents instead of just


Fig. 3. Acquisition of voltage and current signals.

RMS values are acquired to get further significant parametersof the signals, such as harmonic contents. Temperatures of thetransformer as well as of the switching devices are acquiredby using digital temperature sensors (DS18S20) with a 1-Wirebus. The winding configurations on all phases are chosen bythe acquisition modules based on the variations of the load andset by turning on or off the switching devices through the useof some drivers. In the embedded system, a CAN bus is usedto enable exchange of information or parameters between themodules.

The embedded Ethernet units contain a microcontrollerATMEGA128 which was programmed to execute client/serverapplications. The embedded Ethernet unit which is functionedas a server is connected to the embedded system through theremote terminal module using an RS232 interface. Connectionsto the LAN were done through the use of LAN controllers(RTL8019AS). The computer in this system is used for ex-perimentation purposes and eliminated when the most compact

remote monitoring system is needed. In addition, this computeris used for debugging developed programs and for writing thecodes of firmware to the microcontrollers.

III. MEASUREMENTS

Measurements in the system can be divided into two groups.The first group is intended to get parameters from signals ofvoltages and currents. Fig. 3 shows the method of acquiringthese signals. The sensors (VTp, VTs, CTp, and CTs) providelow-voltage output signals (VVTp, VVTs, VCTp, and VCTs)which are fed to signal conditioning circuits. Outputs of thesecircuits contain dc offsets and vary in the range of 0–5 V.These outputs are then sampled by a 10-b analog-to-digitalconverter (ADC) in the AT90CAN128. With only one ADC inthe AT90CAN128, the signals need to be acquired through amultiplexing mechanism, as shown in Fig. 3(b). In other words,the samplings of these signals cannot be performed at the same


Fig. 4. Measurements of temperatures [17].

time. Furthermore, the next sampling can only be initiatedafter the analog-to-digital conversion of the previous samplehas ended. Fig. 3(c) shows the sequence of the samplings.To handle errors in the calculations of powers and powerfactors due to sampling delays between voltage and currentsignals, a technique, as shown in Fig. 3(d), is implemented.In this technique, amplitudes of a voltage signal at the timeswhen samplings of a current signal occur are first calculated.Calculations of powers and power factors are then performedby entering values as results of this calculation. Each intendedamplitude is determined by knowing two consecutive ampli-tudes of the voltage signal as results from sampling processes;as an example, at t2 (where a current signal is sampled), Vn

and Vn+1 are used in the calculation to get V ′n. By knowing

that Δt = t2 − t1 = t3 − t2 = 234.4 µs (corresponding to an

angle of 4.22◦), V ′n can be calculated using (1). This equa-

tion is attained by considering that the implemented line fre-quency is 50 Hz and by assuming that the voltage signal issinusoidal

V ′n =

Vn + Vn+1

2 cos(4.22◦). (1)

A sampling frequency of 8.533 kHz was chosen (based onthe line frequency of 50 Hz) for the four sequenced channels.With the same time intervals between the samples, the samplingfrequency of each channel will be 8.533/4 = 2.133 kHz. Thiswill lead to acquiring approximately 42.66 samples of eachchannel per waveform period or a total of 512 samples ofthe four channels within three waveform periods. The integer


number of waveform periods in one acquisition was preferredto avoid spectral leakage in harmonic contents as results ofa fast Fourier transform calculation in the processing section.The digital data from the ADC are temporarily saved as 16-bnumbers in an array and processed through some subroutinesto get output parameters, i.e., RMS voltages/currents, powers,power factors, efficiency, and harmonic contents.

The second group involves measurements of temperatures;see Fig. 4. Six sensors are used to measure temperatures of theswitching devices on each phase (18 sensors are installed forthe whole switching devices). Furthermore, eight sensors areattached on the surfaces of the core and windings to measuretemperatures of the transformers. These sensors provide 9-bresolution outputs which are read directly by the AT90CAN128through a digital input pin. One sensor is read at one time byfirst sending a 64-b serial code of the sensor.

IV. REMOTE MONITORING MODULES

In the system, the Ethernet boards with client/server applica-tions are the main parts which support the remote monitoringand control facility. A 240 × 230 graphic liquid crystal display(LCD) and a 4 × 4 keypad are added as user interface. TheEthernut boards and user interface form an interactive modulewhich is then functioned as a client as well as a server. Theblock diagram of the remote monitoring module is shown inFig. 5. Connections to a network are supported by the useof an onboard Ethernet controller. This controller combinedwith the ATMEGA128 enables a reliable application for re-mote monitoring of the transformer to be easily developed. Anexternal 32-kB static random access memory is provided fortemporary data storage. The amount of this memory is quitemassive for embedded applications with an operating system.The board is connected to a computer serially through anRS232 interface. To reprogram the ATMEGA128, an in-systemprogramming (ISP) programmer for AVR microcontrollers isused and connected to the ten-pin ISP connector.

The same hardware elements and connections were used forclient and server modules. For the server module, the user in-terface provides facilities to modify server settings and displayserver status as well as information about connected clients.The user interface on the client module, on the other hand,enables users to modify client settings and control parameters(such as winding configurations and fan status) in the embeddedsystem. More importantly, the display will show significantparameters of the transformer as well as waveforms of voltagesand currents.

V. DEVELOPMENT OF APPLICATIONS

Developed applications for the whole monitoring system in-clude applications for microcontrollers in the embedded systemand client/server applications for both microcontrollers andcomputers. Applications for computers were used for testingpurposes. Protocols were developed to enable these applica-tions to communicate through a CAN bus, an RS232 interface,and a LAN. As can be seen from Fig. 6, these protocols were

Fig. 5. Embedded Ethernet as a remote monitoring module.

divided into three zones based on the implemented physicallayers:

1) zone 1 protocol: using CAN bus;2) zone 2 protocol: using RS232 interface;3) zone 3 protocol: using LAN.In this figure, below the client/server application layers, TCP

is used. This protocol is preferred instead of User DatagramProtocol because it is more reliable. In addition, the transmittedinformation will be received by the receiver in a sequential way.

Data from modules in the embedded system are sent to theclient module through the server module, as shown in Fig. 7.In zone 1, CAN data frames containing 8-B message data and a29-b identifier from acquisition modules or temperature moduleare sent through a CAN bus to the remote terminal module.The frames contain information about identifiers of a sendingmodule, target module, and parameters. The remote terminalmodule then removes some parts of the frames, adds start markas well as checksum characters, and forwards it to the servermodule through an RS232 interface (see zone 2 in Fig. 7).The server collects the received frames in a buffer by initiallytruncating the start mark and checksum characters and addinga separator character. As the next step, anytime the data arerequested by the client module, the server module will transmitthem in a block of frames (see zone 3 in Fig. 7).


Fig. 6. Protocols for zones 1, 2, and 3.

Fig. 7. Block of data from the embedded system to the client module.

Fig. 8. Block of data from the client module to the embedded system.

A slightly different mechanism is used to send data fromthe client module to the modules in the embedded system. Inthis case, the block of data from the client module containsidentifiers of the modules that will be used as targets in additionto the data and identifier of parameters; see Fig. 8. From theclient module, the data will be forwarded by the server moduleto the remote terminal module by first adding start mark andchecksum characters. Finally, these data are sent to one or moreintended modules through the CAN bus.

Client/server applications for the microcontrollerATMEGA128 are much supported by the use of operating sys-tem for microcontrollers (Nut/OS) and TCP stack (Nut/NET).Functions to handle socket connections and send data usingTCP are provided in the Nut/Net. Furthermore, the use ofoperating system had provided easiness in creating threads.Threads in the applications were created to perform endlessspecific tiny tasks such as scanning the keypad and readingcharacters from serial buffer.


Fig. 9. (a) Client application. (b) Server application.

Fig. 10. (a) Subroutine in the client application. (b) Thread in the client application.

Fig. 9 shows flowcharts of the client and server applica-tions for the microcontroller. As can be seen in Fig. 9(a), theclient application will first initialize implemented hardware

such as the pins of the microcontroller, universal asynchronousreceiver/transmitter (UART), and LCD, and it is then followedby providing an initial message to users. As the next step,


TABLE IMAIN FUNCTIONS AVAILABLE IN Nut/NET [18]

some threads are created, and hardware of the Ethernet isinitiated. The application then attempts to automatically makea connection to the server program using a specific port numberand an Internet Protocol (IP) address of the server module. Ifthe connection is unsuccessful, it will automatically retry inthe next several seconds. Once connected to the server, thefollowing main repetitive procedure will be executed.

1) Request data containing parameters of the transformersusing a NutTCPSend function.

2) Read the data from the server using a NutTCPReceivefunction.

3) Parse string of data, and display parameters to thedisplay.

The processes of requesting and reading data from the serverare done through a subroutine, as shown in Fig. 10(a). Anexample of an implemented thread is shown through a flow-chart in Fig. 10(b). Routines in this thread will repetitivelycheck the keypad in case it is pressed by a user. A menuis given through the display, and a corresponding subroutine(such as changing settings or controlling parameters of theembedded system) will be executed once an item in the menu isselected.

The server application has a first responsibility to activatethe TCP server and listen to client requests of making a TCPconnection. When a connection is made, the server will waitor serve the requested data from the client. The data from theremote terminal module are read and collected by the servermodule through a separated thread. The server application willterminate the connection in case it is required by the client ap-plication. Table I shows the list of functions available in the Nut/NET and mainly used in the client/server applications for com-munication in zone 3. Both client and server applications usethe NutTcpCreateSocket, NutTcpSend, and NutTcpReceivefunctions to consecutively create a TCP socket and send andreceive information in the form of strings. Functions of NutTc-pAccept and NutTcpCloseSocket are however only used in theserver application for accepting and closing TCP connectionpurposes.

Fig. 11 shows an example of communication steps imple-mented in zone 3. In step 1, the client requests a connectionfrom the server. As a response (step 2), the server opens aconnection and sends a welcoming message. A request of data

Fig. 11. Example of communication steps in zone 3.

Fig. 12. Configurations of the system for testing purposes.

Fig. 13. Displayed parameters in the client module.

is made in step 3 and served by the server by sending a block ofdata in the next step. The client sends parameters to the modulesin the embedded system though the server in the form of a datablock (step 5), and the data are acknowledged by the server


Fig. 14. Application for computers as a client showing parameters of the transformer.

in step 6. The client may also send an instruction instead ofparameters to the module, as shown in steps 7 and 8. Two finalsteps in this figure show how a request of disconnection is sentby the client and responded by the server by closing the TCPconnection.

VI. EXPERIMENTATION RESULTS

Experimentations were done by connecting the 10-kVAtransformer to three-phase supply voltage and variable load.Four alternative configurations, as shown in Fig. 12, were im-plemented to perform remote monitoring and control while thetransformer was on. Results of the experimentations by usingLAN in the school revealed that the system could work properlywith all of these configurations. For instance, the embeddedEthernet as a server could serve client applications in eitherthe embedded Ethernet or computers. The embedded Ethernetapplication for clients could also be implemented to performmonitoring and control when a computer (instead of embeddedEthernet) was used a server. The most compact system could beachieved by implementing the embedded Ethernet as the clientand server modules.

Fig. 13 provides an example of display provided in the clientmodule when remote monitoring was carried out. The displayshows parameters of the transformer on all phases such as RMSvoltages, RMS currents, powers, efficiency, and temperatures.Furthermore, winding configurations (whether in series or inparallel) as well as their modes are also shown. Users need tochoose the next page of the display using the keypad to viewother parameters or waveforms of voltages and currents. Thetext on the bottom of the display shows useful tips for users inrelation to the current display.

The client application for computers however provideseasiness in displaying, controlling, and recording parameters ofthe transformer. Fig. 14 shows a screen capture of the developedapplication for computer as a client when remote monitoringwas performed. This application also uses a TCP connectionto accomplish communication in zone 3. The connection wasmade by first entering the IP address of the server and theport. Once connected, this application automatically maderepetitive requests of parameters from the server. Further ex-ample is given in Fig. 15 to show captured waveforms ofvoltages and currents on phase A using the client application.Note that the currents in this figure have been multiplied byten to make it clearer to view. With this form, users can selectwaveforms that will be captured and save them into a textfile.

Based on the implemented settings during the exper-imentations, it can be noted that the compact monitor-ing system could work with the following communicationrequirements:

1) through the CAN bus (zone 1): speed of 500 Mb/s;2) through the RS232 interface (zone 2): baud rate of 57 600;3) through the LAN (zone 3): speed of 10 Mb/s.

This suggests that the requirements to use the system canbe fulfilled by standard communication speed. Unlike remotemonitoring by using computers, the compact system providesthe following advantages.

1) It is easy to operate, to maintain, and to reproduce mas-sively with low cost.

2) The client and server modules can be easily moved.3) The start-up of the modules is fast.


Fig. 15. Application for computers as a client showing signals of voltages and currents.

VII. CONCLUSION

The remote monitoring system that had been developed wasvery useful in understanding conditions of the transformer. Italso enables operators to monitor the parameters away from thetransformer.

The result of the experimentations showed that the systemcould handle remote monitoring control tasks for the 10-kVAswitchable transformer. The system may contain either embed-ded Ethernet units or computers or a combination of them.The use of embedded Ethernet units as a client and a serverhowever provides the most compact system. This system is easyto operate, to maintain, and to reproduce massively with lowcost for applications in the fields.

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Hari Arief Dharmawan received the B.Sc. degreein physics from Brawijaya University, Malang, In-donesia in 1992, and the M.Eng degree from theSchool of Electrical and Information Engineering,University of South Australia, Adelaide, Australia,in 2001.

Few years after he got the B.Sc. degree, he startedteaching in the Department of Physics, BrawijayaUniversity, where is currently a Lecturer.

Sam A. M. Ali received the Ph.D. degree in elec-trical engineering from the University of Salford,Manchester, U.K., in 1987. His thesis dealt withpower factor and efficiency improvement in three-phase power-controlled drives.

He is a Senior Lecturer and the Program Directorwith the School of Electrical and Information En-gineering, University of South Australia, Adelaide,Australia. His career spans 27 years, 6 years of en-gineering practical experience in various companiesas a Quality Control Engineer with GEC Radio and

Allied Industries, U.K., and a Design Engineer with Crompton ParkinsonMotors Ltd., Leeds, U.K., as well as 21 years of full-time teaching andresearch experience with the University of South Australia. He has published45 papers. The majority of these deal with energy savings in three-phase in-duction machines as well as three-phase transformers. He currently supervisesresearchers related to this project. One group concentrates on the research topicof “Remote monitoring, data acquisition and control of 3 phase switchableenergy efficient distribution transformers using embedded systems,” and theother concentrates on “Direct torque control of 3 phase induction motorsusing embedded systems,” in addition to “Design optimisation of switchabletransformers using 3-D finite elements methods.”