Modeling and Performances of an Induction Heating System with Resonant Voltage Inverter for Drying of Current Transformers from Ciungetu Power Station

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1 The 18 th National Conference on Electrical Drives CNAE 16 Modeling and Performances of an Induction Heating System with Resonant Inverter for Drying of Transformers from Ciungetu Power Station Dinu Roxan Doboșeriu 1, Mihaela Popescu, Alexandru Bitoleanu, 1 S.S.H. Hidroserv Ciungetu, Râmnicu Vâlcea, Faculty of Electric Engineering, University of Craiova, Romania Abstract The paper is dedicated to the presentation, analysis and modeling of a system used for drying of current transformers from 11 kv Ciungetu power station. The drying process is based on an induction heating system containing a fully-controlled rectifier and a resonant voltage inverter. The energetic performances of the system are determined for two control frequencies of the voltage inverter: the resonance frequency of the equivalent load and the frequency that determines the zero current switching. Keywords Induction heating, Fully-controlled rectifier, Resonant inverter. 1. INTRODUCTION The current transformers from power stations of hydropowers must meet stringent values on insulation resistance. If this resistance becomes lower than the minimum required value due to the humidity, the coils insulation drying is needed [1]. ly, at company Hidroserv Ramnicu Valcea, the drying of the current transformers used in the high power transformer stations is carried out by a combination between the induction and conduction heating at the industrial frequency of 5 Hz. Since this solution is disadvantageous in terms of energy, new technical solutions are sought. The attention is directed to a multifunction static system that could provide both DC and AC energy and, concurrently, allow for adjusting the frequency and output power [], [3]. In order to argue the feasibility of this new technical solution and to establish the concrete structure of the system, the comparative analysis of different variants and the energetic performance assessment are needed. To this end, based on the current technical solution and the experimental recording of the voltage and current, the equivalent parameters of the circuit heating coil-transformer have been determined. The entire system was modeled in Matlab/Simulink environment under conditions as close to reality as possible. The energetic performances have been determined by using the system s parameters resulted from the design calculation and the equivalent load parameters provided by processing the experimental data. Two values of the inverter control frequency have been taken in consideration to assess the energetic performances. The first one is khz and corresponds to the resonant frequency of the equivalent load, consisting of the equivalent inductor connected in parallel with the compensation capacitor. The other frequency is a higher one, in order to ensure the zero current switching of the inverter [4], [14]. Thus, in the second section, the structure of the system and its model are presented. Next, the equivalent load parameters are determined by using the waveforms of voltage and current recorded on the existing heating system. The energetic performances of the proposed solution were determined by simulating the system operation in the case of drying of current transformer. The operation for two values of the control frequency at the same active power provided to the equivalent inductor is analyzed. Finally, some conclusions are drawn and future research directions are identified.. STRUCTURE AND MODEL OF THE HEATING SYSTEM The block diagram of the induction heating system with fully controlled rectifier and resonant voltage inverter highlights the main components (Fig. 1). ACTA ELECTROTECHNICA, Volume 57, Number 3-4, 16, Special Issue, ISSN

2 46 The 18 th National Conference on Electrical Drives CNAE 16 THREE-PHASE TRANSFORMER FULLY CONTROLLED RECTIFIER CONTROL BLOCK SINGLE-PHASE VOLTAGE INVERTER Fig. 1 Block diagram of electric induction heating system. MATCHING CIRCUIT COMPENSATION CAPACITOR INDUCTOR The three-phase power transformer adjusts the voltage level to the needs of the process. In the drying process, an existing transformer from Hydroserv Ciungetu will be used. The three-phase fully-controlled rectifier is part of the indirect static converter feeding the AC load by variable voltage and frequency. It allows adjusting the active power transmitted to the load and leads to good performances from power transformer point of view. Through the voltage source resonant inverter, the conversion of electrical energy from DC to AC is performed. The adjustment of the fundamental frequency of the load voltage is allowed and high energetic performance are obtained at its output [3], [5]- [7], [1], [1], [13]. By adding a specific matching circuit at the output of the static converter supplying an inductor for induction heating, the operation at resonance frequency of the group inductor-compensation capacitor is facilitated and the energetic performance is much improved [11]. The assembly consisting of inductor-heated bodycompensation capacitor, which is the heating system load, together with the matching circuit, makes up the load of the resonant inverter. In the study of the energetic transfer performance, the losses on the matching circuit and compensation capacitor can be neglected, so that the active power at the inverter output can be considered equal to the active power supplied to the ensemble inductor - heated piece [6], [7], [9]. Basically, the inverter control can be performed either at the resonance frequency of the equivalent output circuit, or to obtain zero current switching, as well as at the needed frequency in order to adjust the power transmitted to the load. Following the developments in the Isolated Gate Bipolar Transistors (IGBTs) technology, the performances of the voltage source inverters are now reconsidered [4], [5], [7], [8], [1]-[1]. Thus, the use of an IGBT-based single-phase voltage source inverter is the adopted solution for this practical application. The Simulink model created for the whole induction heating system is presented in Fig.. It is a high detail model which is achieved mainly with specific blocks from Sim Power Systems library and includes all the electrical components of the system. To avoid using multiple Simulink models or the tandem use of the model and other MATLAB programs such as those of script type, all the needed calculation is included in the developed model. Thus, after simulation, all parameters are provided (e.g. rms and average values of the quantities which are of interest, active and apparent powers, power factor, performance indicators). It is noted that high accuracy results are prefigured by adopting a small simulation step in the discretized model with forced-commutated electronic switches. By adding some digital display and scope blocks to the model, various data are displayed and captured during simulation and the proper operation of the system is monitored. The main parameters of the power transformer, rectifier and inverter in the Simulink model are given in Table PARAMETERS OF THE EQUIVALENT LOAD In the current technology, the current transformer to be dried is covered with an insulating film of textolit, and a coil of 38 turns is made over it (Fig. 3). The obtained coil has the shape of a truncated cone with base diameters of about 68 mm and 45 mm respectively, and height of 8 mm. The conductor used is made of flexible copper class 5 according to EN 68, with stranded profile (wire diameter of.51mm) and outer diameter of 15.8 mm. Thus, the coil parameter variation depending on the frequency can be neglected. The power supply is an autotransformer associated to a welding source able to provide 1 Amperes DC. In order to determine the load parameters, the current and voltage across the inductor were recorded using an oscilloscope Tektronix TDS3. The current was recorded by means of a.5 mω/1 mv shunt, with the precision class of.%. Table 1 The main parameters of the Simulink model Transformer Rectifier Inverter R1.55 Ω Th T6- IGBT BSMGB 1DN R.55 Ω D 1N374 Vf V Ls1 5.3 µh Vf 1.38 V Ron.1 Ω Ls 5.3 µh Ron.5 Ω Fall time 1-7 s M.1749 H Rsnubb 5 Ω Tail time 1-7 s Csnubb.5 µf Rsnubb 15 Ω Csnubb 47 µf ACTA ELECTROTECHNICA, Volume 57, Number 3-4, 16, Special Issue, ISSN

3 The 18 th National Conference on Electrical Drives CNAE Fig. Simulink model of induction heating system with fully controlled rectifier and resonant voltage inverter Fig. 3 Detail about the drying of current transformers in the actual technology The acquisition frequency was 1 khz. For use in the calculation of the parameters, the waveform of the current and voltage have been filtered with first order filters having the period of 1-4 seconds, which eliminated the high order harmonics. The total harmonic distortion factor (THD) resulted for the measured current is 3.49%, whereas the partial harmonic distortion factor (PHD) corresponding to the first 31 harmonics is 3.3%. It was found that PHD of the filtered current is slightly lower than the value corresponding to the unfiltered wave, which is 3.36%. It means that the filtering process does not affect the harmonics of low order, which have impact on the energetic aspects. The equivalent load, consisting of the coil and the current transformer as heated piece, can be seen as an R- L circuit. In order to determine the two parameters (R and L), two ways to proceed have been identified. W1. The regime is approximated as sinusoidal and the filtered waveforms are used. The phase shift between current and voltage (ϕ) is determined and the following expressions are used: L cos R / R ; (1) L U I R /. () Next, the resistance and inductance are obtained as: U / I R U / I cos ; L 1/ R.. (3) ACTA ELECTROTECHNICA, Volume 57, Number 3-4, 16, Special Issue, ISSN

4 46 The 18 th National Conference on Electrical Drives CNAE 16 In the above expressions, ω is the angular frequency corresponding to the supply frequency. W. The regime is approximated as sinusoidal and expressions resulted from the active and apparent powers are used: R P / I ; (4) R L S / I. (5) From equation (5), the equivalent inductance is obtained as: S / I L 1/ R. (6) Two sets of the two parameters have been obtained by the second way, which correspond to the use of filtered waveforms (a) and unfiltered waveforms (b), respectively. Thus, from the filtered waveforms (Fig. 4), it can be seen that t=3.48 ms and the period is. seconds. It results that the phase shift is ϕ=6.64, cos φ =.4596 and f = 5 Hz. [A]; [Vx3] ms Fig. 4 Filtered waveforms of voltage and current Table The numerical values of the energetic parameters and the equivalent resistance and inductance WI WII(a) WII(b) U [V] I [A] cos(ϕ) P [W] S [VA] R [Ω] L [H] f [Hz] 5 error_r [%] error _L [%].9.39 The values resulted for the equivalent resistance and inductance are shown in Table. Taking as reference the results in W1, the errors in W are below % for resistance and below.4% for inductance. Consequently, any of the resulting values can be used. 4. ENERGETIC PERFORMANCES First, for an active power provided to the equivalent inductor of about 15 kw, the operation of the system has been analyzed when the control frequency of the inverter is equal to the resonant frequency of the equivalent inductor and compensation capacitor. Then, the operation with the same active power and an increased control frequency in order to achieve the zero current switching for the inverter s IGBTs is taken into consideration. The values obtained for the energetic quantities are summarized in Table 3 and their significance is as follows: f sw - the switching frequency of the inverter; P s and S s - the active and apparent powers in the transformer secondary (rectifier input); PF s - the power factor in the transformer secondary; P d - the active power at the inverter input; η R - the rectifier efficiency; P I and S 1 - the active and apparent powers at the inverter output; PF I - the power factor at the inverter output; η I - the inverter efficiency; P ind - the active power across the equivalent inductor; S ind - the apparent power across the equivalent inductor; PF ind - the power factor across the equivalent inductor; η ind - the efficiency of the equivalent inductor; η t - the total efficiency (P ind/p s). The numerical results presented in Table 3 in correlation with the waveforms (Fig. 5 and Fig. 6) highlight some specific qualitative and quantitative aspects of the system. 1. In the transformer secondary, the voltage is little affected by the rectifier switching. The waveform of the current is rectangular and its shape does not depend on the control angle (Fig. 5(a) and Fig. 6(a)).. The DC-link voltage is practically constant and the the inverter s input current is pulsed (Fig. 5(b) and Fig. 6(b)). 3. The inverter output voltage is rectangular, and the current is symmetrical and nonsinusoidal (Fig. 5(c) and Fig. 6(c)). Table 3 Energy performances of the induction heating system fsw [khz] Ps[kW] Ss[kVA] PFs[%] Pd[kW] ηr[%] PI[kW] SI[kVA] PFI[%] ηi[%] Pind Sind PFind [%] ηind [%] ηt[%] ACTA ELECTROTECHNICA, Volume 57, Number 3-4, 16, Special Issue, ISSN

5 The 18 th National Conference on Electrical Drives CNAE [V]; [Ax5] (c) [V]; [Ax5] (a (d) 4 Fig. 5 Waveforms of currents and voltages when the inverter is controlled at the resonance frequency: (a) voltage and current in the transformer secondary; (b) voltage and current at the inverter input; (c) voltage and current at the inverter output; (d) voltage across the inductor (in black), the equivalent inductor current (in blue) and the current through the compensation capacitor (in red) [v]; [A] [V]; Curent [Ax5] (b) [V]; [Ax5] [V]; [A] 4 (a) (c) [V]; [Ax5] 6 5 (b) (d) [V]; [A] Fig. 6 Waveforms of currents and voltages when the inverter switches at zero current: (a) voltage and current in the transformer secondary; (b) voltage and current at the inverter input; (c) voltage and current at the inverter output; (d) voltage across the inductor (in black), the equivalent inductor current (in blue) and the current through the compensation capacitor (in red) 4. The current and voltage across the inductor, as well as the current through the compensation capacitor are practically sinusoidal (their rms values are equal to those of the fundamental components), (Fig. 5(d) and Fig. 6(d)). As regards the influence of the control frequency, the following aspects are highlighted. 1. To obtain the same active power on the equivalent inductor, the DC voltage at the inverter input is by about V lower in the case of zero current switching (Fig. 5(b) and Fig. 6(b)).. The transformer secondary current is practically symmetric in the both cases (Fig. 5(a) and Fig. (6a)). 3. In the case of resonance frequency, the inverter input current has negative values, showing that, for short time intervals, the compensating capacitor is charging from the load (Fig. 5(b)). ACTA ELECTROTECHNICA, Volume 57, Number 3-4, 16, Special Issue, ISSN

6 464 The 18 th National Conference on Electrical Drives CNAE At the resonance frequency, the inverter switching occurs when the current reaches the maximal value (Fig. 5(c)). 5. In the case of zero current switching, the equivalent load has a capacitive behavior. Consequently, the current flow through the compensation capacitor is higher (Fig. 6(d)). 6. The efficiencies of the rectifier and the equivalent inductor depend slightly on the control frequency and their values are high (Table 3). 7. The inverter efficiency and the overall efficiency depend significantly on the control frequency (Table 3). 8. The global efficiency is by about 4 % higher in the case of the zero current switching. 9. The power factor in the transformer secondary is very low (Table 3). 5. CONCLUSIONS 1. In the both variants of control, the current and the voltage across the inductor are practically sinusoidal.. In the both variants of control, the energetic performances are superior, compared to those of the current solution [8] (the total efficiency is higher by more than 1 %). 3. When the inverter is controlled for zero current switching, the total efficiency is by about 4 % higher than in the case of the operation at resonance frequency. It is due to the inverter efficiency, which is by about 5 % higher. 4. The next stage for further research is the closed loop control of the system. Even if, for our applications, the power could be adjusted in open loop, the adjustment of the frequency leading to zero current switching can be implemented only in a closed loop. The acquisition of such a system is also taken into consideration. REFERENCES 1. Anghel Sprânceană Felicia, Anghel D., Metode şi procedee tehnologice, vol. II. Tehnologii moderne, Ed. Printech, Bucuresti, 6.. Bitoleanu A., Ivanov S., Popescu Mihaela, Convertoare statice, Ed. Infomed, Craiova, Bitoleanu A., Mihai D., Popescu Mihaela, Constantinescu C., Convertoare statice şi structuri de conducere performante pentru acţionări electrice, Ed. SITECH, Craiova,. 4. Bitoleanu A., Popescu Mihaela, Suru V., Maximizing power transfer in induction heating system with voltage source inverter, The nd International Conference on Nonlinear Dynamics of Electronic Systems (NDES 14), Albena, Bulgaria. 5. Chudjuarjeen S., Sangswang A., Koompai C., An improved LLC resonant inverter for induction-heating applications with asymmetrical control, IEEE Transactions on Industrial Electronics, vol. 58, issue 7, July, 11, pp Dawson F.P., Jain P., A comparison of load commutated inverter systems for induction heating and melting applications, IEEE Transactions on Power Electronics, vol. 6, no. 3. July 1991, pp Dieckerhoff S., Ruan M.J., De Doncker R.W., Design of an IGBT-based LCL-resonant inverter for high-frequency induction heating, The 34th Industry Applications Conference, vol. 3, Oct. 3-7, 1999, pp Doboşeriu D.R., Bitoleanu A., Popescu Mihaela, Energetic analysis of drying process of current transformer from 11 kv Ciunget power station, The 1th International Workshop of Electromagnetic Compatibility (CEM 16), Craiova, Romania, accepted paper. 9. Esteve V., Pardo J., Jordan J., Dede E., Sanchis-Kilders E., Maset E., High power resonant inverter with simultaneous dualfrequency output, 36th Power Electronics Specialists Conference (PESC '5), 5, pp Sreenivas P., Vaddi R., Ranganayakulu J.S., Full bridge resonant inverter for induction heating applications, International Journal of Engineering Research and Applications (IJERA), vol. 3, issue 1, Jan.-Feb. 13, pp Popescu Mihaela, Bitoleanu A., Dobriceanu M., Analysis and optimal design of matching inductance for induction heating system with voltage inverter, The 8th International Symposium on. Advanced Topics in Electrical Engineering (ATEE 13), Bucharest, Romania, 3-5 May, Popescu Mihaela, Bitoleanu A., Subţirelu E., Design and performance of the voltage control loop in induction heating systems with L-LC resonant inverters, Annals of the University of Craiova, Electrical Engineering series, no. 37, 13, pp Suresh A., Rama R.S., Parallel resonance based current source inverter for induction heating, European Journal of Scientific Research, vol.58, no., 11, pp Suru V., Popescu Mihaela, Bitoleanu A., Energetic performances of induction heating systems with voltage resonant inverter, Proceedings of International Symposium on Electrical and Electronics Engineering, Galaţi, România, October 11-13, 13. Mihaela Popescu Faculty of Electrical Engineering, University of Craiova, Decebal Bd. 17, Craiova, Romania mpopescu@em.ucv.ro ACTA ELECTROTECHNICA, Volume 57, Number 3-4, 16, Special Issue, ISSN