Load Commutated Inverter Research Articles

Passive Harmonic Filter Design Considering Voltage Sag Performance - Applicable to Large Industries

Nowadays, nonlinear loads such as load commutated inverter (LCI) drives are being used by industrial sector throughout the world. However, despite the LCI drive uses low-cost thyristors and their corresponding controllers, which have high efficiency and low manufacturing cost, there are concerns regarding the use of LCI drives which are two-fold: (i) they generate harmonics and (ii) they are sensitive to voltage sags occurring at their PCC. In the literature, passive harmonic filters have been suggested to address the former concern and therefore various filter designing techniques have been introduced. However, the need to elaborate a cost-effective filter to address both concerns regarding LCI drive operation has been neglected. Therefore, this paper presents an innovative approach for designing passive filters which can decrease the harmonic distortions generated by LCI drives while ensuring more reliable annual operation of these equipment. Besides, other aspects of passive filter designing, such as eliminating detuning effects, resonance elimination and harmonic loading of capacitors, have been taken into account to provide a general and practical filter designing approach for real industrial applications. In order to evaluate the innovative proposed approach, a passive filter is designed for LCI drive used in an Iranian large copper mine company.

Full-Torque Starting and Low-Speed Operation of an LCI-Fed Active-Reactive Induction Motor Drive

Load commutated inverter (LCI)-based drives are quite popular in high-power, medium-voltage (MV) applications. However, the limited torque capability during starting and low-speed operation is a major drawback of these drives. This is due to the small back electromotive force (EMF) in the drive motor at low speeds, which is usually insufficient to commutate the silicon-controlled rectifiers in the LCI. A new technique to overcome this drawback for an LCI-fed active-reactive induction motor (ARIM) drive is proposed in this article. The ARIM is a dual-stator induction machine reported in literature, with one winding rated for MV and the other rated for low voltage (LV). The MV winding is fed with active power from the LCI, while the LV winding is supplied with reactive power from a lower rated LV voltage source inverter (VSI). In the proposed method, the VSI is used to apply a preselected voltage vector for a short time during each turn- <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> instant, such that a commutation voltage of sufficient magnitude is induced in the MV winding, providing safe commutation irrespective of the magnitude of the back EMF. Thus, the drawback of the low torque capability at starting and low speeds is overcome in the proposed method, which is also experimentally validated using a 75-kW, 3.3-kV prototype.

Instantaneous Electromagnetic Torque Components in Synchronous Motors Fed by Load-Commutated Inverters

This paper proposes time-domain analytical expressions of the instantaneous pulsating torque components in a synchronous machine air gap when supplied by a load-commutated-inverter (LCI) system. The LCI technology is one of the most used variable frequency drives when very high power and low speed are required in applications such as pipeline recompression and decompression, as well as liquefied natural gas compression. In such applications, synchronous motors are used because of their high efficiency resulting from a separated supply of the current to their rotor through the excitation circuit. These applications usually have long and flexible shafts, which are very sensitive to torsional vibration excitation when their natural frequencies interact with any external torque applied to the shaft. A torsional analysis is required by international standards to assess the survivability of the shaft through the overall speed range of the motor. Therefore, the magnitude and frequencies of the motor air-gap torque are needed for such evaluation. The proposed developments are supported by numerical simulations of LCI systems in a large range of operation range. From the simulation results, torque harmonic families are derived and expressed in a parametric form, which confirm the accuracy of the proposed relationships.

A Comparison of Adjustable-Speed Drive Systems: Voltage Source Inverters and Load-Commutated Inverters for High-Power Applications

The current development of adjustable-speed drives (ASDs) has made voltage source inverter (VSI) technology available to high-power applications, such as large axial compressors. Hence, the comparison of VSI technology to the traditional way of controlling large rotating equipment with load-commutated inverters (LCIs) is of interest. This article compares the ASD systems for shaft powers of 20 MW or more. A drive system includes the drive, the supply network connection, and the motor behavior. System reliability and availability are the main drivers, while system efficiency and costs are additional motivations for the technology positioning. In general, the simple scalability of the LCI leads to maximum reliability, especially for very high power. VSI technology comes with a higher degree of flexibility in terms of possible output frequencies, cable lengths, and types of motor.

Design Considerations for an Active–Reactive Induction Motor for Medium-Voltage Applications

The active–reactive induction motor (ARIM) has been introduced in the literature as a low-cost and high-efficiency alternative to the synchronous motor in load commutated inverter (LCI) fed drives. ARIM is a squirrel cage machine with two sets of dissimilar three-phase stator windings. While one winding is rated for high voltage and power, the other winding is rated for a lower voltage and power. The low-voltage winding fed from a voltage source inverter (VSI) supplies only reactive power to the machine such that a leading power factor appears at the high-voltage terminals, enabling LCI-fed operation at this winding. Since the ARIM is fundamentally an induction motor, the conventional induction machine design process is applicable. However, due to the nonconventional arrangement of stator windings and being intended specifically for an LCI-fed topology, this new class of induction machines requires some critical design considerations. It is shown in this article that the leakage inductances play a pivotal role in determining the ratings of the low-voltage windings and the VSI. Therefore, an iterative design procedure to obtain the correct ratings for the flux winding and the low-voltage components is proposed. It is also required to know the correct value of the leakage inductance in order to design the control loops for reliable operation. This article proposes experimental methods to determine these leakage inductances. The proposed methods are validated by designing a 3.3-kV, 75-kW prototype and experimental results are presented.

An Envelope-Prediction-Based Sensorless Rotor Position Observation Scheme for LCI-Fed EESM at Zero and Low Speed

An envelope-prediction-based sensorless rotor position observation scheme is proposed for a load commutated inverter (LCI)-fed electrically excited synchronous motor (EESM) at zero and low speed. The proposed rotor position observation scheme does not need to utilize the full controlled power device, nor to modify the LCI drive topology. An interval trigger mode for the exciter control is proposed to generate the appropriate fixed-frequency injected signal. The bandpass filters (BPFs) are optimized to extract the target signal, and the observation error due to the BPF is analyzed and compensated. A specific signal sampling method and an envelope-prediction algorithm are presented to simplify calculation and improve the observation accuracy. A normalized phase-locked loop is designed such that the observer has a better observational performance. Finally, the proposed rotor position observation scheme is verified by a 15-kW LCI-fed EESM experimental platform.

An Induction Machine With Tapped Stator Windings for LCI-Fed Medium Voltage Drive Applications

The search for thyristor-based load commutated inverter (LCI) fed induction motor drives for medium voltage (MV) applications has been ongoing since the past few decades, and several new topologies have been proposed. Low cost, better efficiency, and greater ruggedness are the definite advantages of these drives. Since leading power factor is an absolute necessity for load commutation, most of these drives supply additional auxiliary reactive power at the motor terminals. However, in MV applications, this would require the auxiliary source of reactive power (typically a capacitor bank or voltage source inverter (VSI)), also to be rated for MV. To overcome this issue, a new topology using a tapped stator winding induction machine is proposed in this article. Here, a low voltage tapping on the stator winding is used to connect a low voltage VSI which supplies reactive power to the machine. The LCI is used to provide all the active power required from the main stator terminals rated for the MV operation. This article describes the proposed machine and drive configuration and the vector control method for the LCI fed drive. Experimental verification is done using a 75-kW, 3.3-kV prototype machine with a 350 V tapping on the stator winding.

An Asymmetric Nine-Phase Induction Motor for LCI-Fed Medium Voltage Drive Applications

This article presents a new asymmetric nine-phase induction motor-based load commutated inverter (LCI)-fed drive configuration as an alternative to the popular LCI-based synchronous motor drive topology in medium voltage (MV), high power applications. The proposed machine has three sets of three phase windings, two of which are designed to carry the rated active power from the LCI at MV levels. The other three phase winding is designed to handle all the reactive power requirement of the machine at a standard low voltage level from an insulated gate bipolar transistor (IGBT)-based voltage source inverter (VSI). The VSI also assists the LCI operation by supplying some additional reactive power required for safe commutation of the thyristors of the LCI. A smooth torque profile is also ensured by using the VSI to cancel out the low frequency harmonics in the quasi-square-shaped LCI currents. A laboratory prototype of the machine rated 75 kW, 1650/1650/400 V is designed and built to verify the proposed drive and the experimental results obtained are presented in this article.

LCIs and PWM-VSIs for the Petroleum Industry: A Torque Oriented Evaluation for Torsional Analysis Purposes

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.

A Sensorless Rotor Position Estimation Method Based on the Field Current Harmonic for an LCI-Fed EESM

In this paper, a sensorless rotor position estimation method based on the field current harmonic for a load-commutated inverter (LCI)-fed electrically excited synchronous motor (EESM) operating at zero or low speed is proposed. A detailed theoretical analysis and mathematical deduction are presented. The field current harmonic is used instead of the extra high-frequency signal injection. A reasonable harmonic frequency is selected, and a realizable field current control method is given. To simplify the complex signal processing and improve the measurement accuracy, a specific bandpass filter and a normalized quadrature phase-locked loop are designed. The phase change caused by the filters is compensated properly. The proposed approach is demonstrated through the simulation and experiment with a 15-kW LCI-fed EESM drive platform, confirming its feasibility and accuracy.

Sensorless Stator Flux Oriented Control for Startup Gas Turbine

This paper proposes a hybrid position estimation method for load commutated inverter (LCI) fed wound field synchronous motor (WFSM) at zero speed up to turbine ignition speed. In this method injection based position estimation method is used at low speed. Then a stator flux observer is applied to firing pulse generation and position estimation.With use of proposed algorithm the problems of other position estimation are reduced and thyristor's commutations at zero up to final speed are well done. In this paper a new structure has been introduced for stator flux oriented control of LCI fed WFSM which improves power factor and dynamic response of speed controller. Moreover, manufacturing cost is reduced. Finally proposed algorithm and structure are simulated in MATLAB Simulink.

Busbar earthfault protection methods on LCI static starter turbogenerators using generator protection relays

The purpose of this document is to highlight the special treatment or requirements of protection functions when applied to load commutated inverter (LCI) static starter turbogenerators. Static starting of TG's is accomplished by using an LCI, adjustable speed drive system, to motor the synchronous machine and coupled turbine. More specifically, the focus will be on 64B function (busbar earthfault protection) when achieved through 59BN function (neutral overvoltage).

Increasing the Robustness of Large Electric Driven Compressor Systems During Voltage Dips

Grid voltage disturbances pose serious problems to gas compression stations powered by drives such as load commutated inverters (LCI). Until recently, drive control systems used in industrial practice were not capable of handling reduced grid voltage situations appropriately, and instead interrupted the supply of drive torque. Without drive torque, compressors may quickly enter surge conditions, under which the gas flows rapidly back and forth, causing wear and risking damage to the equipment. In this paper, experimental evidence is provided showing that a combination of a revised control scheme for LCIs based on model predictive control and an advanced protection scheme for compressors, the so-called dynamic time to surge improves robustness of the system during voltage dips. We share successful voltage dip ride-throughs of two 41.2 MW LCI-fed synchronous machines, each powering a gas compressor at a large industrial site in Norway. We further analyze the impact of partial torque during voltage dips on the availability and safety of the gas compression station.

A Novel Thyristor-Based CSI Topology With Multilevel Current Waveform for Improved Drive Performance

Load-commutated inverters (LCIs), combined with wound-field synchronous machines (WFSMs), can be an excellent solution for high power drives, but their present technology suffers from important drawbacks related to low power factor, large torque pulsations, and poor starting performance. This paper presents a new LCI design intended to overcome the mentioned limitations. An SCR-based forced-commutation circuit is added to the common inverter topology to obtain a five-level waveform for the stator current. This leads to significantly reduced current harmonics and torque pulsations, in addition to bringing benefits in terms of lower additional losses. As a further advantage, the proposed design allows for a significant power factor enhancement. Finally, it enables the WFSM to be started with a much smoother torque compared to the traditional pulsed operating mode of conventional LCI drives. Simulation studies are conducted on a high-power drive scheme to show the aforementioned improvements. Also, a reduced-scale laboratory prototype of a WFSM drive system is tested to verify the feasibility of the proposed converter.

Torque Harmonic Minimization for Load Commutated Inverters in Pulse Mode

This paper considers the operation of a load commutated inverter (LCI) fed synchronous machine when the stator voltage is too low to ensure commutation of the machine side thyristors. Under such low-voltage conditions, the LCI is operated in a so-called pulse mode, accompanied by higher drive torque harmonics than during standard operation. Following a theoretical analysis, an alternative pulse mode is proposed, describing how to operate the LCI during pulse mode in order to keep the excitation of the natural frequencies of the drive shaft at a minimum.

Steady-State Simulation of LCI-Fed Synchronous Motor Drives Through a Computationally Efficient Algebraic Method

Wound-field synchronous motors (WFSMs) fed by load-commutated inverters (LCIs) are widely used for high-power applications in many fields like ship propulsion, oil and gas industry, and pumped-storage hydropower generation. Several design architectures exist for LCI drives, depending on the number of LCIs and their dc-link connection as well as on the number of WFSM phase count. The prediction of LCI drive performance at steady state is important in the design stage, especially in regard to the prediction of the torque pulsations, which can give rise to serious mechanical resonance issues. This paper proposes an algebraic method to simulate the steady-state behavior of LCI drives in all their configurations of practical interest. Compared to conventional dynamic simulation approaches based on differential equation solution, the method is much more computationally efficient and requires a very limited knowledge of system parameters. Its accuracy is experimentally assessed by comparison against measurements taken on a real LCI drive arranged according to various possible schemes. Furthermore, the advantages of the proposed algebraic method over the dynamic simulations are highlighted by comparison against the simulation results on a high-power LCI-fed WFSM drive in MATLAB/Simulink environment.

A Five-Level Current-Source Inverter for Grid-Connected or High-Power Three-Phase Wound-Field Synchronous Motor Drives

Simple converter structure, inherent short-circuit protection and regenerative capability are the most important advantages of current-source inverters (CSI’s) which have made them suitable for medium-voltage high-power drives. Usually in grid-connected gas turbine generators or pumped storage hydro power plants, efficient and reliable current-source load-commutated inverters (LCI’s) with thyristor switches are employed. Also, this type of CSI is widely used in very large drives with power ratings of tens of megawatts to supply wound-field synchronous motors (WFSM’s). However, LCI’s suffer from some disadvantages such as large torque pulsations, poor power factor, and start-up criticalities. In this paper, a novel multilevel-based CSI is proposed. The proposed converter consists of one LCI and one CSI bridge with self-turn-off switches along with a voltage clamping circuit. The CSI switches are forced commutated; hence, a voltage clamping circuit is employed to limit voltage spikes caused by current variations in inductive paths during commutation transients. Drastic reduction in harmonic distortion of stator current and improved fundamental power factor are achieved by the proposed topology. In addition, torque pulsations are reduced remarkably for normal and starting operating conditions. Comprehensive analysis of the proposed structure is presented and the design of converter components is elaborated.

Model Predictive Control in the Multi-Megawatt Range

This paper presents an application of model predictive control to a variable-speed drive system operating in the multi-megawatt range. The variable-speed drive system comprises a synchronous machine fed by a line-commutated rectifier and a load-commutated inverter. The control task is to regulate the dc-link current, and hence the machine torque, to ensure the machine speed follows a given reference. The proposed control approach is model predictive control, where both the rectifier- and inverter-firing angles are considered as control inputs. The nonlinear model predictive torque controller has been implemented on an embedded system and applied in an industrial-scale pilot plant installation. The experiments show the successful operation of model predictive control on a plant with more than 48 MW power.

An Algebraic Algorithm for Motor Voltage Waveform Prediction in Dual-LCI Drives With Interconnected DC-Links

Load-commutated inverters (LCI's) are often used to supply dual-three-phase synchronous motors in high-power variable-speed applications. A pair of LCIs is used in this arrangement to feed the two motor three-phase winding sets. In order to cope with inter-harmonic issues, a drive configuration with an interconnection of the two LCI dc-links has been proposed. In this paper, such a drive design is shown to produce an increased voltage stress on motor windings compared with traditional configurations. The problem is investigated in the paper by proposing an algebraic algorithm capable of predicting the steady-state voltage waveform applied to the motor terminals and arising between the star points of the two winding sets. Unlike conventional dynamic simulations, the proposed approach gives practically instantaneous results, making it possible to quickly investigate a wide number of possible operating conditions. Furthermore, it requires a limited knowledge of system parameters, which are often hardly available. Its reliability and accuracy are assessed by comparison with measurements on a test drive system and examples are given of the method application to the sizing of motor insulation system.

Synchronous Motors on Grinding Mills: The Different Excitation Types and Resulting Performance Characteristics with VFD Control for New or Retrofit Installations

Synchronous motors (SMS) continue to be a viable alternative to induction motors because of efficiency advantages. Since SMs have long been used to provide leading kvar for power factor (PF) improvement and to help with the device switching in load-commutated inverter (LCI) drives, these machines are efficient prime movers for large mill applications. Both in motor efficiency and in low-base-speed configurations, the SM has advantages of lower current and higher efficiency. Often used in large mills in the cement and mining industry, these motors were driven by LCI variable-frequency drives (VFDs) or used with direct start with a clutch system that would synchronize the motor first to overcome low starting torques. This article reviews various mill drive configurations and improved SM characteristics when used with advanced drives. It will also review the different SM excitation types and resulting performance characteristics with VFD control for new or retrofit installations.