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Many steel mills in Bangladesh use electric arc furnaces (EAFs) that can impact the generating units of the country’s power system. The EAFs can cause power quality problems, such as the injection of the sub-synchronous frequencies (SSFs), which may be close to the torsional resonance (TR) frequency of the turbine generator shaft. As a result, the torsional oscillating torques may be amplified and pose a potential threat to the rotating shaft systems. This paper comprehensively investigates the torsional resonance caused by the operation of EAFs in the power grid. It simulates the power system of the Chittagong region with two steel mills with alternating current (AC) EAFs where the large power plants are represented with an equivalent generation at the Comilla-North point. The MATLAB simulation tools are used to model the three-phase EAFs. To produce realistic characteristics of the EAFs, the resistances are modulated deterministically in the range of 1.20 to 45.5 Hz. Torsional mode frequencies in the turbine generator shafts are calculated using the Fast Fourier Transform (FFT) technique. The results show that the EAF operation amplifies the torsional oscillating torques in the turbine generator shaft. The turbine generator shaft in Raozan and Sikalbaha power plants exhibits a higher potential to be damaged because they are vulnerable to the potential threat of amplified torsional torques.

This paper proposes an analytical time and frequency domains evaluation of torque harmonics produced by high-power load-commutated-inverters (LCIs) and pulsewidth-modulated (PWM) voltage-source inverters for adjustable speed applications. These drive systems are used in petrochemical and mining industries, where driven loads have multi-inertia or long shafts that are potentially subject to torsional mode excitation. Such excitation may lead to accelerated fatigue, system failure, and loss of production, and in turbine-generator sets, to system blackout. The investigated power conversion topologies are multi-pulse LCIs (6/6 and 12/12-pulse systems), parallel connected three-level neutral-clamped-converters with non-interleaved and interleaved PWM signals, as well as series-connected H-bridge multi-level inverters, single thread and four threads supplying a 12-phase machine and parallel connection of four threads converter systems with interleaved PWM commands. They are the most used variable frequency drives in the megawatt range, both with synchronous and induction machines, in the petrochemical industry. The time and frequency domains of the machine's air-gap torque is calculated from the motor's voltage and stator's current waveforms, this proven method is independent from the type of ac machine. More than 50 simulated operating points are discussed and experimental results for NPC and cascaded inverters are also presented. [ABSTRACT FROM AUTHOR]

The application of energy efficient Induction Machines (IM) is explained in reference to power consumption savings. In energy efficient IM, losses for various Horse Power (HP) ratings are summarized for bringing effective changes in design. Emphasis is laid on how load factor, speed & power quality affect machine’s efficiency. Replacement of conventional IM of higher power rating is done with required energy efficient Doubly Fed Induction Machine (DFIM) to enhance the performance at variable speeds near rated power outputs. Results of the proposed approach will give substantial savings in energy & loss reduction. The field data of Jindal Steel Rolling Mill (JSRM) at Hisar, Haryana (India) is taken into consideration. This paper proposes a non-intrusive air gap torque method for efficiency estimation of in-service IMs. This approach gives results considering stray-load and friction-windage loss according to IEC standard and IEEE112-B standard. The proposed method is validated experimentally whose effectiveness is witnessed using MATLAB/SIMULINK.

This paper analytically investigates expressions of an oscillating motor's air-gap torque resulting simultaneously from dynamic electromagnetic and dynamic mechanical components. The electromagnetic torque is generated in the air gap when the motor is supplied by variable frequency drives (VFDs), such as pulsewidth modulated voltage source inverters or load commutated inverters. Whereas, the mechanical torque components might result from dynamic mechanical processes, such as wind turbine blades, oil and gas fluid fluctuation, and, for emulating dynamic mechanical loads, a load-side VFD. Simple and straightforward analytical relationships between electrical and mechanical harmonics are developed to understand the propagation of mechanical dynamics to the electric side and vice versa. Intensive numerical simulations have been carried out to support the theoretical analysis. Time and frequency domain results confirm the robustness of the proposed relationships. Finally, laboratory experimental test results also confirm the accuracy of the proposed relationships. [ABSTRACT FROM AUTHOR]

Induction motors consume a great portion of the generated electrical energy. Moreover, most of them work at underloaded conditions, so they have low efficiencies and waste a lot of energy. Because of this, the efficiency estimation of in-service induction motors is a matter of great importance. This efficiency estimation is usually performed through indirect methods, which do not require invasive measurements of torque or speed. One of these methods is the modified Air-Gap Torque (AGT) method, which only requires voltage and current data, the stator resistance value, and the mechanical losses. This paper approaches the computation of a modified stator resistance including the mechanical losses effect to be applied in the AGT method for torque and efficiency estimation of induction motors. Some improvements are proposed in the computation of this resistance by using a direct method, as well as the possibility to estimate this parameter directly from the nameplate data of the induction motor. The proposed methodology only relies on line voltages, currents, and nameplate data and is not intrusive. The proposed methodology is analyzed through simulation and validated through experimental results with three-phase induction motors. Also, a comparison of methods for in-service induction motors efficiency estimation is presented for the tested motors.

A robust method to monitor the operating conditions of induction motors is presented. This method utilizes the data analysis of the air-gap torque profile in conjunction with a Bayesian classifier to determine the operating condition of an induction motor as either healthy or faulty. This method is trained offline with datasets generated either from an induction motor modeled by a time-stepping finite-element (TSFE) method or experimental data. This method can effectively monitor the operating conditions of induction motors that are different in frame/class, ratings, or design from the motor used in the training stage. Such differences can include the level of load torque and operating frequency. This is due to a novel air-gap torque normalization method introduced here, which leads to a motor fault classification process independent of these parameters and with no need for prior information about the motor being monitored. The experimental results given in this paper validate the robustness and efficacy of this method. Additionally, this method relies exclusively on data analysis of motor terminal operating voltages and currents, without relying on complex motor modeling or internal performance parameters not readily available. [ABSTRACT FROM PUBLISHER]

In large-drive applications, load-commutated inverters (LCIs) are one of the most used technologies mainly because of their excellent reliability records. However, LCIs are known to generate interharmonics. They can interact with the mechanical system at torsional natural frequencies of the rotating train, on both the inverter and rectifier sides in weakly connected power systems, such as offshore oil and gas platforms. These interactions can lead to accelerated shaft fatigue, lifetime reduction, gear damage, and system blackouts. On the other hand, pulsewidth-modulated (PWM) voltage-source inverters (VSIs) are known to produce less torque ripple compared to LCIs. Consequently, VSIs are supposed to be less subject to exciting torsional resonances in mechanical shaft systems. Even with reduced torque ripple, mechanical failures have been consistently reported due to motor air-gap torques supplied by PWM drives. This paper is focused on solving torsional vibration issues. Due to system uncertainties, these issues cannot be excluded in the design phase of LCIs and VSIs for high-power applications. With regard to LCIs, the dc-link inductance is used as an integrated energy-storage unit. Motor and generator interactions are therefore decoupled. Excited eigenmodes on the grid side are damped with the rectifier; the inverter is driving the variable-speed motor. This approach is also successfully applied to damping resonance modes in the motor side. Simulation and selected experimental results on a 30-MW LCI system are provided to validate the proposed design approach. As for VSIs, a more general approach of damping or controlling excited shafts is proposed. This method is successfully applied using the dc-link capacitor as an integrated energy-storage source. This approach is used to optimize the design of new systems; otherwise, it is used to improve the performance of existing systems with minor modifications. [ABSTRACT FROM AUTHOR]

Induction motors consume a great portion of the generated electrical energy. Moreover, most of them work at underloaded conditions, so they have low efficiencies and waste a lot of energy. Because of this, the efficiency estimation of in-service induction motors is a matter of great importance. This efficiency estimation is usually performed through indirect methods, which do not require invasive measurements of torque or speed. One of these methods is the modified Air-Gap Torque (AGT) method, which only requires voltage and current data, the stator resistance value, and the mechanical losses. This paper approaches the computation of a modified stator resistance including the mechanical losses effect to be applied in the AGT method for torque and efficiency estimation of induction motors. Some improvements are proposed in the computation of this resistance by using a direct method, as well as the possibility to estimate this parameter directly from the nameplate data of the induction motor. The proposed methodology only relies on line voltages, currents, and nameplate data and is not intrusive. The proposed methodology is analyzed through simulation and validated through experimental results with three-phase induction motors. Also, a comparison of methods for in-service induction motors efficiency estimation is presented for the tested motors. [ABSTRACT FROM AUTHOR]