Load Commutated Inverter Research Articles

Multilevel Converter Topology Using Two Types of Current-Source Inverters

A multilevel power-converter topology based on two types of current-source inverters (CSIs) has been proposed for large induction motor drives. The topology utilizes the combination of the load-commutated inverter (LCI) using thyristors and the CSI using gate turn-off thyristors (GTOs). As a result, the multilevel-inverter operation takes advantage of a soft switching of the LCI and hard switching of the CSI. The output capacitor is required to generate the leading power factor for load commutation of the LCI. It is shown that the proposed multilevel operation contributes to the leading-power-factor generation by providing the effective phase shift. Thus, the leading power factor required for the load commutation of the LCI is obtained with significantly smaller capacitors at the inverter's output terminals. As a result, the proposed approach can employ the LCI and utilize its soft-switching operation, yielding a cost-effective solution compared with the conventional multilevel CSI using two GTO CSIs. The switching losses are curtailed due to the natural commutation of the LCI and the six-step operation of the GTO CSI. Therefore, the proposed multilevel inverter can both increase the output power level and decrease the output capacitor size for LCI. The simulation and experimental results have been shown to verify the feasibility of the proposed multilevel topology and its operation

A Hybrid Converter System for High-Performance Large Induction Motor Drives

A hybrid converter system employing a combination of a load-commutated inverter (LCI), a dc-dc buck converter, and a voltage-source inverter (VSI) is proposed for the large induction motor drives. The VSI ensures the safe commutation of the LCI with active commutation angle control over all speed regions. By replacing capacitor banks and a forced dc-commutation circuit, this system can eliminate all drawbacks related to these circuits in the conventional LCI-based induction motor drives. Sinusoidal motor current and voltage waveforms are achieved with the VSI providing the reactive and harmonic power to the motor, resulting in high-performance drives. The buck converter enables both the VSI and the LCI to be fed from the single-diode rectifier. As a result, the dc-link inductor size can be reduced and the LCI is operated without the controlled rectifier. In addition, faster dynamic response can be obtained through the VSI and the buck converter operation. Finally, the buck converter performs the dc-link current control to ensure minimum VSI rating. The feasibility of the proposed hybrid circuit for the high-power drive systems is verified by computer simulation for a 500-hp induction motor. Experimental results are also included for a 1-hp induction motor laboratory setup controlled by the proposed hybrid system.

A Hybrid Solution for Load-Commutated-Inverter-Fed Induction Motor Drives

A novel, hybrid solution employing a combination of a load-commutated inverter (LCI) and a voltage-source inverter (VSI) is proposed for induction motor drives. By avoiding the use of output capacitors and a forced dc-commutation circuit, this solution can eliminate all disadvantages related with these circuits in the conventional LCI-based induction motor drives. In addition, improved quality of output current waveforms and faster dynamic response can be achieved. The proposed hybrid scheme features the following tasks: 1) the safe commutation angle for the LCI, controlled by the VSI in the entire speed region of the induction motor and 2) a dc-link current control loop to ensure minimum VSI rating. Advantages of the proposed solution over the conventional LCI-based induction motor drives include the following: 1) sinusoidal motor phase current and voltage based on the instantaneous motor speed control; 2) fast dynamic response by the VSI operation; and 3) elimination of motor circuit resonance and motor torque pulsation. The feasibility of the proposed hybrid circuit for the high-power drive system is verified by computer simulation for a 500-hp induction motor. Experimental results to support the use of the proposed system are also included for a 1-hp induction motor laboratory setup.

Harmonic losses in LCI-fed synchronous motors

Load-commutated inverters produce considerable current harmonies, which can cause losses and heating inside the machine. As drive outputs increase, it is important to be able to accurately predict not only the magnitude of these losses, but also the precise location of the losses within the motor. Recent experience and modern calculation techniques have shown that the effects of these losses in large motors can be quite dramatic. If not considered at the design stage, they can lead to high-temperature hot spots. These potential hot spots need eliminating to prevent surface temperatures in excess of gas ignition temperatures and also possible early machine failure. New detailed calculations have been performed which allowed a novel technique to be determined to reduce these losses and allow larger ratings of drives to be safely engineered.

Simplified steady-state model of an SCR-inverter-based load-commutated commutatorless series motor

This paper presents a simplified analytical model of a silicon-controlled-rectifier based, load-commutated inverter driven, commutatorless series motor to predict its speed-current, torque-current and torque-speed characteristics. The model has been formulated, keeping in view the steady state equivalent circuit of a conventional DC series motor. The proposed analytical expressions relating torque and current with speed are found to follow an inverse law and that relating torque and current is found to be parabolic, as is expected in a "series" machine. The drive has been implemented, loaded with a DC generator and experimentally similar characteristics have been obtained, as predicted by the analytical model. The characteristics obtained experimentally are seen to follow the ones, predicted by the analytical model closely.

Harmonic experiences with a large load-commutated inverter drive

Harmonic experiences with a large load-commutated inverter drive

Brushless permanent magnet (BPM) motor drive system using load-commutated inverter

The goal of this work is to develop a brushless permanent magnet (BPM) motor drive system with low total system cost, high reliability and adequate performance for high-volume production and application to commercial appliances. The power converter used is a low-cost thyristor-based load-commutated inverter (LCI). Although LCIs have been used to supply sinusoidally excited permanent magnet motors, their application to BPM motors is a key contribution of this work. A detailed digital computer model capable of predicting the steady state as well as the transient performance of a BPM motor driven by an LCI has been developed. The utility-side phase-controlled rectifier, as well as the motor-side inverter-including the DC-link inductor, are modeled. A load-commutated inverter specifically designed to supply the BPM motor has been fabricated in the laboratory. The developed control strategy has been implemented on an INTEL 80C196KD microcontroller board. Simulation and experimental results to support the use of an LCI to drive a BPM motor are included in the paper.

Current-forced neutral injection in a three-phase rectifier/converter

A new three-phase rectifier/converter with low harmonic input currents and minimal DC voltage ripple is described. The device achieves continuous input line currents by injecting a triangular third harmonic current, derived from the DC link current, into the neutral point of a delta–star transformer at the supply. It uses a conventional six-pulse phase controlled converter, with an additional bridge of four devices in the DC link. The additional bridge also cancels much of the DC voltage ripple, allowing a much smaller DC link inductance to be used. A prototype 10 kW system, suitable for such applications as DC motor drives, load-commutated inverters, exciters for AC generators, and low-power DC links is described. Experimental results obtained from this system are compared with ideal theoretical predictions.

Start up performance of load-commutated inverter fed synchronous machine drives

Load-commutated inverter (LCI) synchronous machine drives are common in the high power range. However, the starting performance of these systems is a concern since load-commutation cannot occur at arbitrarily low speeds. This paper compares three different starting strategies including one apparently neglected method-normal load commutation. In particular, it is shown that, depending on system parameters, it may be possible to start the system with no special provision for start up. A method to determine whether a given system can be started using load commutation is presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

EXPERIMENTAL INVESTIGATIONS ON STARTING OF INVERTER FED CAGE MOTOR

Starting and low speed operation of an inverter fed cage motor is considered, which is not possible with load commutated inverter (LCI) fed induction motor due to lack of sufficient induced emf in the stator winding at zero and low speeds. To overcome this drawback of starting with LCI, three starting schemes are developed. Principle of operation of these schemes is described. Experimental performance of all the three schemes are discussed and their comparative features are also given

Application, design, and specification of harmonic filters for variable frequency drives

The proliferation of variable frequency drives in industry has led to serious concerns about power quality in electrical distribution systems. This paper presents tables and charts to help the system design engineers address some basic concerns such as the need for harmonic filters, their effect on displacement power factor, etc., without going into a full-scale harmonic analysis of the distribution system. Relative harmonic performance of the different types of drives such as the load commutated inverter (LCI), the current source inverter (CSI), and PWM drives is discussed. A factor called the harmonic factor is defined which can characterize the performance of a drive by a single number. The relation between harmonic factor and the total harmonic voltage distortion is derived. Attenuation of the harmonic voltages by the filters is calculated and presented in the form of generalized curves. >

Application of a 15000-HP, 6000-r/min adjustable-speed drive in a petrochemical facility

The improved reliability of power electronics and the development of motors capable of operating directly at high speeds have made electric motors a viable alternative to steam turbines in process plants. This paper discusses the application of a large high-speed, adjustable-speed synchronous motor with a load commutated inverter as a recycle compressor driver in a petrochemical facility.

Fundamental Design and Electrical Characteristics of Superconducting Commutatorless Motor.

Since there are no iron cores in the rotor and stator windings of superconducting synchronous motors, some reactances are intrinsically smaller than the ones of conventional motors. So the low synchronous reactance would be expected to improve the torque-ampere ratio stability and the overload capacity in case of the variable speed drive system of a load commutated inverter due to the small armature reaction. Moreover, the low sub-transient reactance may contribute to lessen the voltage stress during the shortened commutating period, especially in current source type inverter (CSI) systems.In this paper, after presenting analytical formulas needed for a machine design, one design algorithm is described, citing one example of an experimental CSI fed superconducting motor of 30 kVA which will be fabricated. Further, the distinguished features in its operation, as expected just above, are verified by the computer simulation study in comparison with a conventional machine in terms of a high specific power density, an extension of stable power limit and so on.

Analysis of LCI synchronous motor drives with finite DC link inductance

Steady-state operation of a load commutated inverter (LCI) fed synchronous motor drive is analysed by means of a digital computer based solution of the synchronous machine equations. The solution permits calculation of current, voltage, and torque waveforms and includes the effects of finite DC link inductance, saliency of the rotor and all machine resistances. The continuity constraints for steady-state operation are exploited to enable the proper initial conditions to be calculated directly, without the need for iteration. Operating points are described by the firing angle yo, and the overlap angle μ, and are computed for an assumed DC supply voltage of 1.0 per unit. The linearity of the state equations allows the normalised solutions to be scaled to obtain any desired value of torque, DC link current etc. Sample calculations are provided, showing the influence of the DC link inductance on the torque, DC link current and voltage waveforms. These are then compared with experimentally measured data taken from a 3 kVA LCI fed synchronous motor drive, and a good level of agreement is achieved.

Control loop study of a load commutated CSI-fed induction motor

The authors present a systematic study for the induction motor drive system using a load commutated current source inverter (LCCSI). The steady state characteristics of a LCCSI-fed induction motor are shown to be the intermediate characteristics between the voltage source inverter (VSI) and the autosequentially commutated current source inverter (ASCI) systems. The dynamic characteristics of the LCCSI-fed induction motor are also investigated using a linearised model and the system control loops without a speed sensor are discussed. A study through the pole/zero locations of the linearised system shows that a V/Hz ratio feedback loop plays an essential role in the system stabilisation.

Calculation of neutral voltage shift in LCI driven induction motors

Load commutated inverter (LCI) drives are being used for adjustable speed control of induction motors and synchronous motors. It has been found that the motors and transformers connected to these drives are subjected to higher than normal voltage stresses to ground and premature failures of motors have been reported. This has been shown to be due to a voltage difference between the source side neutral voltage and the motor neutral voltage. This voltage is known as the neutral shift. This neutral shift voltage is added to the normal line to neutral or phase voltage causing a much higher than normal line or terminal to ground voltage stress in the ungrounded side of any DC linked AC converter. Voltages as high as 3.3 pu normal phase voltage may be seen for certain types of inverters. A method of determining the neutral shift voltage is presented. Results of a computer simulation of an LCI circuit and test results are presented to verify this analysis.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

PERFORMANCE ANALYSIS OF LOAD COMMUTATED INVERTER FED CAGE INDUCTION MOTOR DRIVE

ABSTRACT An approach, to analyse the performance of a variable speed cage induction motor drive fed from load commutated inverter (LCI), is presented. Induction motor with an appropriate capacitor across its stator terminals constitutes the load on the inverter. A fully controlled bridge converter, supplied by a variable dc voltage source, commutated with the back emf of the motor, acts as an inverter. An algorithm to compute the motor performance, is developed using equivalent circuit representation of the system. Strategies for the selection of control variables, in order to achieve the desired speed range for satisfactory performance of the drive are given. The performance of a 3-hp, cage induction motor fed from LCI is computed, using the developed algorithm for wide range of speed; and is verfied experimentally.

A case study of replacing steam turbines with LCI-type variable-speed drives

The problems and experiences associated with the installation, start-up, and operation of 7000 HP 1800 r/min and 3600 HP 6000 r/min synchronous motors using load commutated inverter (LCI) variable-speed controllers are examined. The application of new technology in existing process units can present some unusual and sometimes perplexing challenges. The steep learning curve, and the fact that most projects are time and money driven, all contribute to the hurdles that must be overcome. The described application of LCI variable-speed drives covered all of the factors. Some of the problems and concerns encountered with this project resulted from equipment previously installed, coupled with the dynamics of the electrical, instrumentation, and process systems. When planning the application of variable-speed drives, factors such as electrical system disturbances, impact of harmonics on the existing equipment, training, commissioning, operator acceptance, craft training, and spare parts all need to be taken into consideration. >

Power measurement and harmonic analysis of large adjustable speed drives

Details are given of two power electronics adjustable speed drives, (ASDs) that were installed on utility power-plant boiler-feed-pump motors for conversion to speed control. One is a 2250 HP modified load commutated inverter with six pulse input and six pulse output. The other is a current-source inverter with 12 pulse input rectifier and 12 pulse output inverter. The authors show electrical system configuration, input and output current, and voltage wave shapes for each system. Means for measuring power and harmonics in power plant ASD installations using a real-time spectrum analyzer are discussed. ASD efficiency and measured additional motor losses from harmonics as determined in the field are presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Voltages to ground in load-commutated inverters

Load-commutated inverter (LCI) drives are well suited to the starting and variable-speed operation of large fans, pumps, and compressors used in the petroleum industry. The origin and magnitude of the voltages to ground found in these types of drives are introduced and explained. The voltage to ground in most of the LCI circuit depends on bridge firing angles and can reach a maximum value as high as 2.4 p.u. (per unit) of rated. An analysis of this phenomenon and an algorithm for predicting the magnitude of the voltage as a function of machine voltage, and bridge firing angles is presented. Test results are included for validation of the analysis. How system grounding and DC link reactance can be used to reduce the magnitude of the elevated voltage to ground and transfer it to less devices is discussed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>