Available online at ScienceDirect. Procedia Engineering 150 (2016 ) Conference on Industrial Engineering, ICIE 2016

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1 Available online at ScienceDirect Procedia Engineering 150 (2016 ) 3 10 Conference on Industrial Engineering, ICIE 2016 Conceptual Area of Development of Power Saving Thyristor Electric Drives of Rolling Mills A.S. Karandaev a, V.R. Khramshin b,*, R.R. Khramshin b, I.I. Barankova b a South Ural State University, 76, Lenin Avenue, Chelyabinsk, , Russia b Nosov Magnitogorsk State Technical University, Lenina Av. 38, Magnitogorsk, , Russia Abstract Thyristor electric drives of the rolling mills are known to be the most power-intensive consumers at the iron and steel works. Obviously, significant losses of electric power are connected with consumption of reactive power that depends on the value of the reserve of rectified EDV of the thyristor converter. The paper mentions characteristic dynamic modes providing a maximum reserve of the rectified EDV. It estimates the influence of the rectified EDV on the value of reserve for overcorrection in the impact-loading mode. Experiments have proven the impact of the network voltage deviation in amount of 10 12% decreasingly. The reserve of the rectified EDV has been divided into components. The paper provides a defined concept for generation of the power-saving systems of two-region speed control based on the principles of rearranging the reserve of the rectified EDV in the steady state and dynamic modes. It considers the method and system of two-region dependent speed control as a function of the rectified EDV of the thyristor converter. The system is proved to provide a constant reserve of the rectified EDV of the thyristor converter within the load range of the electric drive that is lower than the rated one. It is reasonable to develop the system of tworegion speed control providing power saving due to reduction of the rectified EDV reserve in all dynamic modes during rolling cycle The Authors. Published by Elsevier by Elsevier Ltd. Ltd. This is an open access article under the CC BY-NC-ND license ( Peer-review under responsibility of the organizing committee of ICIE Peer-review under responsibility of the organizing committee of ICIE 2016 Keywords: wide-strip hot-rolling mill; thyristor electric drive; two-region speed control; energy indicators; reactive power; reserve of the rectified EDV; analysis; constituents; power saving concept; power-saving electric drive. * Corresponding author. Tel.: address: hvrmgn@gmail.com The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ( Peer-review under responsibility of the organizing committee of ICIE 2016 doi: /j.proeng

2 4 A.S. Karandaev et al. / Procedia Engineering 150 ( 2016 ) Introduction High power consumption of the iron and steel production accompanied by continuous cost escalation of power resources stipulates primary importance of power saving for its all process stages [1]. The main consumer of the electric power at the iron and steel facilities is an electric drive (ED). The total capacity of electric motors at the iron and steel facilities amounts about 87% of power of all equipment, while their power consumption makes up about 65% of the total one [2, 3]. Thus, highest power savings may be obtained due to improved energy indexes of electric drives, in the first turn that of thyristor EDs of the rolling mills. Energy indexes of thyristor EDs are mainly deteriorated due to the reactive power consumption caused by phase control of the rectified voltage (rectified EDV). Improvement of energy indexes of electric drives of the rolling mills is a challenging issue due to their high power consumption. It should be noted that the total power of the equipment of the 2,000 mm wide-strip hot-rolling mill at OJSC Magnitogorsk Iron and Steel Works is 362 MVA [4]. The power of the stands' electric drives is 139 MW, of which over 96b MW (26.6%) falls to the share of the main EDs of the finishing train. Unit output of an electric drive of the finishing stand is over 14 MW. Energy indexes of similar drives are far from the optimal ones. It follows that search for power saving reserves in such powerful consumers is an important scientific and practical task. The task of power loss reduction is even more relevant because of the switching wide-strip mills to production of thick strips of difficult-to-form steels [5, 6]. Thus, hollow bullet (up to 18 mm thick strip of up to 300 mm thick slab) is rolled at the 2,000 mm mill. Transition to production of up to 25 mm thick hot-rolled strips at modern WSHRMs is a world-wide trend. Hollow bullet is rolled at low speeds and high slabbing, that is, high non-uniform load of electric drives. The studies of modes of rolling these strips are provided in [7, 8]. This rolling is connected with risk of loss of electric drive control because of opening the speed control circuit. 2. Problem statement The most national wide-strip hot-rolling mills (WSHRMs) are equipped with DC electric drives with two-region speed control. The highest demands relative to high-speed response and stable operation at processing dynamic modes during rolling are imposed upon them. These requirements are met if they provide the required dynamic margin of the control system, primarily, reserve of the rectified EDV of the thyristor converter (TS). This may be explained with the fact that loss of control over an electric drive (for ED of the stand loss of speed control because of the thyristor converter saturation) is an emergency mode for continuous process lines. An additional margin increase provides improvement of ED stability but may lead to deterioration of energy indexes. This is caused by increased consumption of reactive power depending on the extent of control of the rectified EDV. Peculiarities of dynamic loads of the main WSHRM electric drives are directly connected to maintenance of process flow. The heaviest duty dynamic modes are: shock loading mode at the strip gripping; drive under load at rolling with acceleration. The impact-loading mode is accompanied by overcorrection of the rectified EDV of the thyristor converter (TC). The overcorrection value depends on speed and current loop settings under similar conditions of strip gripping. For single-integrating system of speed control, it is (16 18)%, for double-integrating one (19 22)% [9]. The maximum rectified EDV E d0 of the TC is selected with due regard to mode optimization without opening the speed control circuit, that is, without TC saturation. The relation between E d0 and the actual rectified EDV E d defines the value of reserve of the rectified EDV. Margin increase results in boosting reactive power consumption Q, which depends at constant load current I d on the extent of control C P of the rectified EDV E d [10, 11]: Q I U d d sin P C P E 2 d d 2 1 CP 1 CP CP I, where delay angle; P active power;

3 A.S. Karandaev et al. / Procedia Engineering 150 ( 2016 ) C p E d E d 0 1. For thyristor electric drive of the rolling stand with output of MW, the rectified EDV reserve of V, that is, 22 25% of the maximum rectified EDV of the TC, and rated load current, the value of reactive power exceeds 5 MVAr. It results in power losses within the range of 5 7 million kwh/year. These losses determine the need for measures to reduce the rectified EDV reserve if high EC reliability indexes are maintained. 3. Main part 3.1. Consumption of reactive power by thyristor EDs The term reserve of the rectified EDV means the difference between the value of the rectified EDV of the TC at zero delay angle and that of the rectified EDV in the steady-state mode at the maximum static load ( E d = E d0 - E d ). Fig. 1 shows ocyllographs of transient processes of speed, current of the armature circuit I, excitation flow F (relative units), motor EDV E and rectified EDV E d of the TC for a rolling cycle. At the moment t 1, impact loading is applied to the strip gripping. In the period t 2 t 3 (for about 30 s), a transient process appears that is caused by rolling with acceleration (relatively smooth load changing). At the known EDs with two-region speed control, the latter mode is accompanied by the rise of the rectified TC EDV E d form 900 to 930 V (by 3.3%) at the constant motor EDV E of 860 V. Fig. 1. Transient processes of coordinates of stand's electric drive for a rolling cycle

4 6 A.S. Karandaev et al. / Procedia Engineering 150 ( 2016 ) 3 10 Furthermore, the provided oscyllographs show that the mode of optimization of impact loading is essential for the rectified EDV at the recorded acceleration of the electric drive (about 0.14 s -2 ). The maximum value of the rectified TC EDV is 968 V; it exceeds the steady value by about 10 %. It should be noted that overcorrection actions of the rectified EDV (20 22% of the rated value) have been detected during oscillography testing the modes of strip gripping Estimation of components of rectified EDV reserve Shock increase of electric drive loading at metal gripping by rolls results in speed reduction. Early recovery of the speed mode is of primary importance for continuous mills, as a per-second flow of metal passing each stand shall be supported. The time of speed recovery depends of the rate of current rise as it determines the motor torque. The second conditions to define the reserve value are voltage deviations of the supplying network [12, 13]. Fig. 2 shows oscyllograps of load currents of electric drives of two WSHRM finishing stands and voltage deviations of 10 kv section. At moments t 1, t 2 stand rolls grip metal, which is followed by shock rise of load current and step down of the network voltage (by 2% and 1.5%, correspondingly). Experiments prove that voltage level is decreased by 5 7% after metal gripping by the last stand at the maximum ED load. Fig. 2. Deviations of network voltage caused by loading EDs of finishing stands Fig. 3 shows deviations of network voltage at rolling three strips that do not involve loading electric drives of the stand. These deviations are caused by loading other consumers connected to the same 10 kv bus section; they are occasional for EDs of the stands under consideration. As we can see, their level is rather high (5 8% of the rated value). The provided oscyllographs confirm the information known that voltage deviation of the network supplying EDs of the hot-rolling mills make up 10 12%, decreasingly, and shall not be taken into consideration at calculation of the TC voltage margin.

5 A.S. Karandaev et al. / Procedia Engineering 150 ( 2016 ) Fig. 3. Deviations of network voltage that do not involve ED loading Fig. 4 shows diagrams reflecting distribution of the rectified EDV reserve required for failure-free optimization of impact loading electric drive of the WSHRM stand [14]. The following components are shown: 1. Voltage drop at resistance of the rectified current circuit. I max R (I max maximum current of static load, R equivalent resistance). 2. Margin for dynamic increment of the rectified EDV of the converter at the load increase (voltage drop at inductive reactance L of the rectified current circuit). 3. Component related to voltage deviations of the supply network. 4. Component E d0 (1-cos min ) to be determined by the limit of the minimum delay angle min. Fig. 4. Components of rectified EDV reserve As we can see on the diagram, the margin value is 23.6% of the maximum rectified TC EDV or 27% of the rated rectified voltage. Accepted division of the rectified EDV reserve into components enabled development of the concept for the tworegion speed control system (with improved energy indexes). It is based on the principle of rearranging reserve of

6 8 A.S. Karandaev et al. / Procedia Engineering 150 ( 2016 ) 3 10 the rectified EDV in steady state and dynamic modes related to changing load of the electric drive. To limit the reserve value in the dynamic drive mode at metal located between the rolls (that is, at relative smooth increase of load current), the authors have developed the method and system of two-region dependent speed control as a function of the rectified EDV of the thyristor converter. Their distinctive feature is in generation of setting action on the external controller of the excitation circuit proportionally to the rated rectified TC EDV [11]. The function chart of the control system implementing the proposed method is provided in Fig. 5. Feedback signals are supplied from the output of the current controller of the armature circuit. Thus, the transducer of the rectified EDV may be eliminated. In this way, the device is simplified. Fig. 5. Function chart of the two-region speed control system as a function of the rectified TC EDV: TC thyristor converter; A motor armature; CC current controller of the armature circuit; CT current transducer; SC speed controller; PuS power-up sensor; TG tachometer-generator; EC EDV controller; ESD rectified EDV set-point device; ECR excitation current regulator; ECT excitation current transducer; EW excitation winding; TE thyristor exciter Fig. 6 explains the control principle. It provides idealized dependencies of voltage u M of the motor armature circuit, motor EDV e, rectified TC EDV e d and maximum value of TC EDV overcorrection e d max at impact loading plotted as a load current function i[15]. Fig. 6. Changes of voltages and EDV within dependent systems: as a motor EDV function at e 0 1 ; b as a rectified TC EDV function The curves in Fig. 6, a corresponds to the known system of two-region dependent control system as a motor EDV function, those of Fig. 6, b comply with the proposed two-region control system as a rectified TC EDV function. Curves are plotted in relative units; the rated current I r and rated rectified TC EDV E dr are taken as basis values.

7 A.S. Karandaev et al. / Procedia Engineering 150 ( 2016 ) The known system (Fig. 6, ) features the following: the motor EDV is maintained at the level e 0 = e r = 1- e, while the rectified TC EDV rises with load increase and at i=1 reaches the rated value e d =1 ( e relative voltage drop at equivalent resistance R e of the rectified current circuit e = R e I r / E dr ). Consequently, e d <1 within the whole load changing range except for the mode corresponding to i=1. It results in increase of the armature current and reduction of the power factor within the thyristor ED systems. In the proposed system (Fig. 6, b) the rectified EDV controller supports its stability at the rated level e dr =1 regardless of the load current in the steady-state load mode. Voltage at the motor armature (curve u M ) and motor EDV (curve e) are reduced at the current growth due to the increased number of voltage drops at corresponding internal resistances. At this, the reserve of the rectified TC EDV to be determined as a difference between the maximum EDV e d0 and its current value e d max = e dr remains constant regardless of the load current. Based on the comparison of Fig. 6, and Fig. 6, b, we may conclude that this system provides the lower reserve of the rectified TC EDV at all current values i from zero to the rated one and, correspondingly, better energy indexes of ED operation. 4. Results discussing With the proposed method implemented, the reserve of the rectified EDV may be reduced at a relative smooth load changing. Thus, this system is recommended for EDs of the continuous and reverse cold rolling mills operating within a wide range of load changing. This method enables elimination of the rectified EDV exceeding the values set for the main WSHRM electric drives at acceleration during rolling. In such a way, it ensures decreased reserve of the rectified EDV. Unfortunately, performance of the EDV control system (in the considered case rectified TC EDV) is about in order of magnitude less than that of the speed control system at optimum controller settings. So, limitation of the rectified EDV cannot be ensured at impact loading. Consequently, the authors propose the method of dependent control of excitation flow as a function of the rectified TC EDV with automatic resetting rectified EDV [14]. In addition, the systems of two-region control with coordinate switching controlled with the excitation circuit have been developed [10]. Theoretical and experimental studies of the developed systems prove the possibility to reduce the reserve of the rectified TC EDV practically without deterioration of dynamic indexes and reliability of the electric drive [3, 16 26]. Improvement of energy indexes of newly designed electric drives is provided by rational selection of secondary voltage of the rectifier transformer. Compared with the rated voltage of the known electric drive of a similar class, it should be reduced by 10 12%. It helps to reduce consumption of reactive power by 7 12%, that is, significantly decrease power losses practically without any capital costs. Virtually the same effect may be obtained for main electric drives of the operated WSHTMs by means of switching branch lines (or installation of additional ones) of the secondary transformer windings. Acknowledgements This research was performed with financial support in the form of a grant from the President of the Russian Federation (Grant No. MD ) References [1].S. Karandaev, Improvement of Automatic Electric Drives for Rolling Machinery, Russian Internet Journal of Industrial Engineering. 1 (2014) [2] I.A. Selivanov, A.S. Karandaev, S.A. Evdokimov, V.R. Khramshin, Improving Diagnosis and Automated Electric Power Electric, Izvestiya Vysshikh Uchebnykh Zavedenii, Elektromekhanika, Russian Electromechanics. 1 (2009) [3] A.S. Karandaev, V.R. Khramshin, I.Ju. Andrjushin, V.V. Golovin, New Technical Solutions in Electric Drives and Control Systems of Technological Parameters of Hot Rolling Mills, Izvestija Tulskogo gosudarstvennogo universiteta. 3 (2010) [4] V.R. Khramshin, Energy-saving Thyristor Electric Rolling Mills, MSTU, Magnitogorsk, 2013.

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