WIRELESS TECHNOLOGY SYSTEM FOR TEMPERATURE RISE VERIFICATION OF POWER TRANSFORMERS, S, USING LABVIEW PROGRAMMING ENVIRONMENT

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1 WIRELESS TECHNOLOGY SYSTEM FOR TEMPERATURE RISE VERIFICATION OF POWER TRANSFORMERS, S, USING LABVIEW PROGRAMMING ENVIRONMENT Eng. Dumitru SACERDOTIANU PhD 1, Eng. Marcel NICOLA PhD 1, Eng. Marian DUTA PhD Student 1 Eng. Claudiu NICOLA 1, Eng. Iulian HUREZEANU 1, Eng. Catalin PIRLOG 1, Eng. Aurel MELINESCU 1 1 Research, Development and Testing National Institute for Electrical Engineering - ICMET from Craiova REZUMAT. În prezent majoritatea proceselor sunt monitorizate cu echipamente inteligente, flexibile e care au la bază dezvoltarea unor algoritmi logici specifici. Aceasta imprimă procesului şi rezultatelor obţinute credibilitate şi acurateţe în evaluarea evoluţiei parametrilor tehnologici. In lucrare este prezentat un sistem de achiziţie, monitorizare, stocare si calculul al parametrilor tehnologici rezultaţi în urma procesului de evaluare a încălzirii transformatoarelor electrice de putere, in tehnologie wireless utilizand, facilitatile programarii grafice. Cuvinte cheie: transformatoare electrice de putere, sistem wireless, monitorizare, evaluare incalzire. ABSTRACT. At present, most of the processes are monitored with intelligent and flexible equipment, based on the development of some specific logic algorithms. This gives credibility and accuracy y in assessing the evolution of technological parameters, both to the process and results obtained. The paper presents a system for the acquisition, monitoring, storing and calculation of the technological parameters resulted from the process of assessing the temperature rise of power transformers, in wireless technology, by using graphical programming facilities. Keywords: power transformers, wireless system, monitoring, heating assessment. 1. INTRODUCTION In electrical networks, transformers can generate nonlinear currents even if they are supplied with sinusoidal voltage. These currents can be defined by their fundamental component and higher harmonic components. In power transformers, the main consequence of harmonic currents is the increase of losses, mainly in the windings, due to distortion of the leakage flux. Non-sinusoidal currents cause additional temperature rise in transformers as a result of the increase of losses, mainly due to eddy currents [1], [2]. Higher losses in the transformer mean that more heat is generated, so that the operating temperature increases, leading to the damage of insulation and reduction of its life. At present, the subject of harmonics has received much publicity, leading to the belief that the industry is only now beginning to understand the effects of harmonics and to calculate the increase of eddy current losses. In fact, the study of the effects is quite old, the study on eddy current losses in a magnetic field dating from Many of the previous investigations were at a higher mathematical level and the information given in these articles was detailed down to every detail, and probably as accurate as that obtained by modern computer programs. By means of computers, methods for calculating electric field and eddy current losses in the transformers were developed. These computer programs offer elegant diagrams, however their accuracy can not be proven. Hot spots, areas where the highest temperatures are found, naturally result due to non-uniform heat generation and to the fact that heat transfer to the environment is not uniform; the transformers have specific heat transfer characteristics that are not well understood. Most manufacturers of transformers simply add 30 C to the average overtemperature (calculated using empirical relationships) and declare that it is in accordance with the standards. In fact, IEEE requires that the two temperatures, both the average winding overtemperature and the hot spot temperature to be limited, for corresponding with the normalized power (MVA). The difference between these two limits happens to be 30 C, but the use of 30 C as a "rule" is not correct. It follows that, in operation, it is necessary to reduce the maximum load of the transformer, a practice known as de-rating, or to pay special attention in the design of the transformer to reduce these losses, this including also a rigorous assessment of the transformer Buletinul AGIR nr. 4/2012 octombrie-decembrie 1 115

2 INT. SYMPOSIUM ON ELECTRICAL ENGINEERING AND ENERGY CONVERTERS ELS 2013 temperature rise that will give the final verdict of a correctly designed and executed transformer. Evaluation of power transformer temperature rise is performed currently according to SREN [3], [4], [6]. Based on this standard, ICMET Craiova has created its own procedure for verifying the temperature rise of transformers [5]. 2. SYSTEM ARCHITECTURE Architecture for the acquisition of technological parameters is presented in Figure 1. Fig. 1 Architecture of the technological parameter acquisition system for the process of power transformer temperature rise test. M - Drive motor; G - Generator; TA - Auxiliary Transformer; VT - Voltage Transformator; T - Transformer under test ; TC - Current Transformer; R,S,T - Three-phase system; C-Contactor; I Circuit Breaker; T1, T2, T3, T4 - Thermocouples to measure ambient temperature; Tu - Oil Temperature Transducer; 1 Industrial Computer; 2 - Gateway NI WSN 9791; 3 - Wireless Module NI WSN 3212; 4 - Wireless Module NI WSN 3202; 5 - PT100/0-10V PT100/0-10V Converter; 6 - INFRATEK Analyzer 106A 2 Buletinul AGIR nr. 4/2012 octombrie-decembrie 116

3 WIRELESS TECHNOLOGY SYSTEM FOR TEMPERATURE RISE VERIFICATION OF POWER TRANSFORMERS, USING LABVIEW PROGRAMMING ENVIRONMENT 3. TECHNOLOGICAL ALGORITHM 3.1 Description of the process 1. One closes the switching devices from the main circuits (source, transformer under test test), one supplies the transformer to low values, then one rises up the voltage up to the test values, according to SREN (IEC) One measures the voltages, currents and powers and monitors them. 2. The increase of oil temperature is especially followed, and recorded in the system memory at equal time intervals. When one finds that the oil temperature has stabilized, measurements are starting to determine the winding ohmic resistance by going through the following operations: a) one disconnects the power supply by turning off the generators and opens quickly the switching devices to separate the transformer from the rest of the system; b) one turns on the DC sources by closing the switches on the measuring side of direct currents; c) after currents stabilization, one closes the switches on the measuring side of DC voltages; d) readings are performed every minute up to minute 10, to obtain the 10 points of measurement, inclusively; Measurements taken after disconnecting the transformer unit are processed according an interpolation algorithm, from which it results the winding resistance immediately after disconnection. An approached solution of this problem is "linear least squares", the straight line for which the sum of squared distances from it to the experimental points is minimal. e) afterwards, one disconnects the DC sources, first the voltmeters by means of the two switches and then the ammeters; f) calculations are performed to find out ohmic resistances; the characteristic R = f (t) is plotted and finally the value of the winding temperature rise is calculated These readings and calculations are repeated many times until it is found that the winding temperature rise is also stabilized; after that, measurements are done for finding the specific losses, the current circulation through the transformer is then interrupted. Power generators are turned off and all measuring-recording installations are deenergized and restored to original condition. Transformer connections are decoupled from the power supply and from the appropriate devices. 3. Interpretation of temperature rise and overload test results is made according to SR EN (IEC) section 2 or EN (IEC) p.3. After the accomplishment of temperature rise test, the values of measured temperatures or the values of the resulted temperature rise are compared to the values allowed in the reference documents 3.2. Used formulas Correction of temperature rise for oilimmersed transformers For calculating the winding temperature rise, if the test was done at the total losses and rated voltage R2 θ = ( T1 ) 235 T a R1 + ( θ us θ uc ) (1) where: R1 winding resistance under cold condition; T1 temperature at which R1 has been measured; Ta environment temperature (during the last quarter of the test); θus oil temperature rise after stabilization, at the sum of losses; θuc oil temperature rise after the hour of rated current ; If the test was done at the current: and losses, the formula for temperature rise calculation is: R2 θ = ( T1 ) 235 T a, (2) R1 and the winding and oil temperature rise is calculated by the following formulas: - for windings: ' I N y θ = θ ( ) (3) I ' y = 1.6 for transformers with ONAN, ONAF and OFAF cooling y = 2 for transformers with ODAF cooling - for oil: ' P ( T ) x θ = θ (4) ' P x = 0.8 for distribution transformers with natural cooling, maximum power 2500 kva x = 0.9 for transformers with power higher than 2500 kva, ONAN, ONAF cooling x = 1.0 for transformers with ONAF or ODAF cooling,where: θ winding temperature rise for current value IN θ winding temperature rise for current value IN IN rated current of winding I current at which the test was performed Buletinul AGIR nr. 4/2012 octombrie-decembrie 3 117

4 INT. SYMPOSIUM ON ELECTRICAL ENGINEERING AND ENERGY CONVERTERS ELS 2013 θ oil temperature rise calculated for total losses PT θ oil temperature rise calculated corresponding to the losses P P losses at which the stabilization was performed PT total losses PT = Pk750C + P0 (5) Determination of the winding hot spot The notations from are used, and additionally: - Tpc - winding hot spot temperature - θpc - winding hot spot overtemperature - Tu oil temperature - Fpc hot spot factor is: 1.1 -for distribution transformers and for medium power transformers - θs temperature rise for oil at the cooler (sink) inlet and stabilization (top) - θj temperature rise for oil at the cooler (sink) outlet and stabilization (bottom) One uses the formula: Ts T j Tpc = Tu + Fpc ( θ + Ta ( Tu ) (6) 2 or, for overtemperature: θ pc θ s θ j = θu + Fpc ( θ ( θu )) (7) 2 Acceptance criteria: The oil overtemperature measured when ending the test and the winding overtemperature of the respective windings should be equal with or lower than the overtemperatures imposed in standards or contracts. 4. PRESENTATION OF THE DEVELOPMENT SOFTWARE The Software developed in this paper uses LabVIEW programming environment and is used for signals acquisition, measurement analysis and presentation of data, at the same time providing a friendly interface for users The program running on the PC consists in a chart using blocks of arithmetic and logic functions available in the library of virtual instruments (VI's) Acquisition of voltages, currents and active powers. INFRATEK 106A network analyzer is used to acquire voltages, currents and active powers. Electrical quantities acquired from INFRATEK 106A analyzer and the controls corresponding to these quantities are: - voltages of the three phases (R, S, T), and the control for voltage is of the type: VOLTage:RMS:AC ; - currents of the three phases (R, S, T), and the control for current is of the type CURRent:RMS:AC ; - active powers of the three phases (R, S, T) and their sum, and the control for the power is POWer:ACTive:AC ; - the control corresponding to the choice of the phase according to which the acquisition of the electrical quantities is done has the form "FORM: PH L1'; L1, L2, L3, in accordance with to R, S, T phases. Example: Block diagram of software procedure for the R phase voltage acquisition is shown in Figure 2. Fig. 2. Explanation. Block diagram of the procedure for the R phase voltage acquisition 4 Buletinul AGIR nr. 4/2012 octombrie-decembrie 118

5 WIRELESS TECHNOLOGY SYSTEM FOR TEMPERATURE RISE VERIFICATION OF POWER TRANSFORMERS, USING LABVIEW PROGRAMMING ENVIRONMENT 4.2 Temperatures acquisition Temperature measurement during the temperature rise test is performed using the scheme in Figure 1. Temperatures of cooling medium (air) are acquired through thermocouples and a wireless module NI WSN 3212, and transmitted wirelessly to NI WSN Module 9791 that communicates with the computer, on which the software for acquisition and processing will be installed [7], [8]. Oil temperature is acquired through a PT100 thermal resistor via a PT100/0-10V converter and a NI WSN 3202 wireless module and transmitted wirelessly to NI WSN Module that communicates with the computer, on which the acquisition and processing software will be installed. Block diagram and front panel for temperatures monitoring are presented in Figure 3 and Figure 4. Fig. 3. Block diagram for the temperature acquisition and monitoring during the temperature rise test Buletinul AGIR nr. 4/2012 octombrie-decembrie 5 119

6 INT. SYMPOSIUM ON ELECTRICAL ENGINEERING AND ENERGY CONVERTERS ELS 2013 Fig. 4. Front panel for temperature monitoring 4.3. Measurement of winding ohmic resistance For winding resistances measurement it is used the voltmeter ammeter method and the following algorithm: - internal resistances of the ammeter and voltmeter are measured and inserted then into the database; - unknown resistance values are calculated with the processed by an interpolation algorithm from which the winding resistance immediately after disconnection results. equations: Fig.5. Front panel for determination of R2 winding resistance. Upstream Mounting Downstream Mounting 4.4 Calculation of temperature rise - measurements performed after disconnecting the transformer unit are entered into the database and The software whose interface is shown in Figure 6 is customized for a calculation example of a transformer whose cooling is ONAN [3]. Fig. 6. Front panel for monitoring the temperature rise process 6 Buletinul AGIR nr. 4/2012 octombrie-decembrie 120

7 WIRELESS TECHNOLOGY SYSTEM FOR TEMPERATURE RISE VERIFICATION OF POWER TRANSFORMERS, USING LABVIEW PROGRAMMING ENVIRONMENT Data Management Writing in the data base. The program sequence dedicated to writing in MySQL database is shown in Figure 7 It performs the following operations: Connection opening - connection information - connection information specifies the string called "Claudiu" for connecting to the database. Data insertion: - date - it specifies data that you want to insert into the database; - table - it specifies the name of the table from the database in which to enter data. Connection closing - connection reference - it specifies a reference to an object Connection ADO Fig.7 Block diagram. Writing in the database Reading in the database (Figure 8) Fig.8 Block Diagram. Reading in the database The following operations are performed: connection opening: connection information - connection information specifies the string called "Claudiu" to connect to the database. data selection - selects data from the table of the database, identified by the connection reference using the columns provided in the columns array. - table - is the name of the table "data" in the database from which to select data. You can specify multiple tables using a comma as a delimiter; - columns - specifies columns Ambient_ temperature 1, Ambient_ temperature 2 Ambient_ temperature 3, Ambient_ temperature 4 Oil _temperature ; in the table from which data are selected converting data from the format supported by MySQL into the format supported by LabVIEW. connection closing - once the reading in database has finished, the connection must always be closed 5. The practical realization A prototype of the system was developed consisting of wireless system enclosure - acquisition unit assembly (Figure 9) and wireless system enclosure - central unit assembly (Figure 10.). Fig. 9 Overall system Fig.10 Overall system - acquisition unit - central unit Buletinul AGIR nr. 4/2012 octombrie-decembrie 7 121

8 INT. SYMPOSIUM ON ELECTRICAL ENGINEERING AND ENERGY CONVERTERS ELS CONCLUSIONS National Instruments used solutions offered: infinite capacity for acquiring and storing the data according to the power transformer temperature rise test; existence of multiple options for data display configurable options for analysis; simplicity of interfacing tool - user; automated measurements; time and measured values recording; Internet access for sending data; communication with the database; automatic generation of reports. BIBLIOGRAPHY [1] Cioc I., s.a. - "Maşini electrice. Îndrumar de proiectare", vol.ii, Ed. ScrisulRomânesc, Craiova, [2] Cioc I., s.a. - "Transformatorul electric. Construcţie teorie, proiectare, fabricare, exploatare". Ed. Scrisul Românesc, Craiova, [3] *** SR EN :2002 Transformatoare de putere. Partea 2: Încãlzirea [4] *** SR EN A11:2001/A12:2003 : Transformatoare de putere. [5] *** PT (Procedura Tehnica ICMET + LMP) -Verificarea încercărilor individuale şi a încălzirii transformatoarelor de putere [6] *** CEI : 2005 : Transformatoare de putere. Partea 7: Ghid pentru încărcarea transformatoarelor de putere imersate în ulei [7] *** 104/lang/ro/fmid/2038/ - NI LabVIEW Wireless Sensor Network (WSN) Module. [8] *** Wireless Standards. IEEE , WiFi. IEEE /ZigBee About the authors Eng. Dumitru SACERDOTIANU, PhD; dumitru_sacerdotianu@yahoo.com Dumitru SACERDOTIANU graduated in 1985 the University of Craiova, Electrical Engineering Faculty, and got the Ph.D. degree at the same University, in At present the Research Development and Testing National Institute for Electrical Engineering ICMET Craiova. He is a researcher of third degree, authors of patents who reached gold and silver rewards at the International Fairs of Inventions from Brussels, Budapest, Zagreb, Cluj Napoca and Bucharest. Eng. Marcel NICOLA, PhD; marcel_nicola@yahoo.com Marcel NICOLA graduated in 1995 the University of Craiova, Automation, Computers and Electronics Faculty, got the MS degree in 1997 and Ph.D. degree at the same University in After graduation, he began to work in research field at ICEMENERG Bucharest, branch from Craiova, and at present he is a researcher of second degree at ICMET Craiova, and associate professor of Automation Department from University of Craiova. Interest fields: systems theory, automatic control, SCADA and industrial software. Eng. Marian DUTA, PhD Student; general_manager@icmet.ro Marian DUŢĂ graduated in 1984 the University of Craiova, Electrical Engineering Faculty. Author of 11 patents, awarded gold, silver and bronze medals at International Exhibitions of Inventions in Geneva, Brussels, London. Since 2008 General Manager INCDIE-ICMET Craiova Eng. Nicola Claudiu; nicola_claudiu@yahoo.com Claudiu Ionel NICOLA graduated in 2003 the University of Craiova, Automation, Computers and Electronics faculty, got the Master degree at the same University in He began to work in research field at Research Development and Testing National Institute for Electrical Engineering ICMET Criova from 1985 up to present. He is a researcher, participant in various projects from the National Research Plan and European Projects. Eng. Iulian HUREZEANU; iulian_hurezeanu@yahoo.com Iulian HUREZEANU graduated in 1983 the University of Craiova, Electrical Engineering faculty, Electric Drives section. He began to work in research field at Research Development and Testing National Institute for Electrical Engineering ICMET Criova from 1983 up to present. He is a researcher of third degree, authors of patents, leader and participant in various projects from the National Research Plan. Interest fields: electric drives, electric equipment operation monitoring Eng. Pirlog Catalin; icmet@icmet.ro Catalin PIRLOG graduated in 2003 the University of Craiova, Faculty of Automation, Computers and Electronics, Software division. After finishing his university studies he worked at ICMET Craiova. His responsibilities were SOFTWARE DEVELOPER AND NETWORK ADMINISTRATOR. At present he is working at ICMET Craiova and my responsibility is Software Developer. Interest fields: Microcontroller, Data acquisition, Monitoring system, Industrial software. Eng. Melinescu Aurel; icmet@icmet.ro Aurel MELINESCU graduated in 2005 the University of Craiova, Automation, Computers and Electronics faculty.he began to work in research field at Research Development and Testing National Institute for Electrical Engineering ICMET Craiova from 2005 up to present. He is a researcher of third degree, participant in various projects from the National Research Plan and European Projects. Interest fields: Microcontroller, Data acquisition, Monitoring system, Industrial software. 8 Buletinul AGIR nr. 4/2012 octombrie-decembrie 122