Commutation Overlap Research Articles

Commutation angle analysis of the slip energy recovery drive

A well known characteristic of the slip energy recovery drive (SERD) is the large commutation overlap of the rotor side rectifier, which compromises the accuracy of simplified dq models often used for its simulation. In this paper the variation of the rectifier commutation angle and the factors that determine its magnitude are investigated using a detailed hybrid abc-dq model of the SERD, the accuracy of which is validated by experimental results. The dependence of the commutation angle on the DC current and the operating slip is shown for a case study drive and practical relations are deduced which permit the evaluation and a-posteriori correction of the steady state error of simplified dq models, where the commutations and converter harmonics are ignored.

Modes of operation in parallel-connected 12-pulse uncontrolled bridge rectifiers without an interphase transformer

Parallel operation of two 6-n prepulse uncontrolled bridge rectifiers, to obtain a 12-pulse rectification, without an interphase transformer connected on the DC sides is investigated in this paper. The mechanism of interactions between the two six-pulse bridges is analyzed. As the load current varies from no-load to the complete short-circuit value, seven modes of operation are recognized. Equations describing the mean output voltage and various operating angles of the rectifier circuit are given in terms of a reactance factor. Voltage regulation and variations of the ignition angle and the commutation overlap angle for rectifiers with and without an interphase transformer are also compared.

Sequence impedance and equivalent circuit of a converter system

The author describes a means of deriving the sequence impedance and equivalent circuit of a power converter at fundamental frequency. The technique used is based on modulation theory, and the results can be used in the analysis of converter systems under unbalanced operation. The effect of commutation overlap is represented by a modified switching function so that an accurate calculation can be carried out. The method proposed provides a simple model for power converters which can be used in the steady-state analysis of unbalanced power systems containing several converters. Case studies using a simple converter system and a realistic power supply system have been carried out, and the results obtained confirm the validity of the method proposed.

AC-DC load flow with unit-connected generator-converter infeeds

The conventional AC-DC steady state formulation is modified to analyze HVDC load flows with unit-connected generator-converter infeeds. It is found that the steady state algorithm is restricted in its application to large power conditions due to the large commutation overlap caused by the extra reactance of the unit connection. Also, the results are shown to be in considerable error when the generators contain rotor saliency (probably the most common case for a remote generating plant). A concept called the equivalent inverter which uses unit-connection characteristics derived from time domain simulation is presented as a practical alternative for general load flow studies involving unit-connected schemes. >

New synchronization method for thyristor power converters to weak AC-systems

An improved approach to obtaining good zero-voltage-crossing signals is presented. These signals are subsequently used as synchronization signals for a phase-controlled thyristor power converter. Detection of accurate zero crossings is possible even when there are large frequency changes, sudden load changes, or large commutation overlap angles. The improved accuracy in the integrity of the zero crossing is obtained by reconstructing a voltage representing the AC source voltage. This voltage is determined from the distorted thyristor converter input voltage, the converter input current, and an online identification of the source impedance using a microcontroller-based adaptive algorithm. The improvement provided by the new zero crossing detection scheme is verified experimentally. >

Harmonic analysis of slip-power recovery drives

A method of evaluating harmonic currents and voltages associated with slip power recovery drives is presented. The rectifier commutation overlap and DC link current ripple are included in the analysis by using a hybrid between the traditional d-q and a-b-c induction motor models. The harmonic components are determined from closed-form expressions which are functions of the numerical values of the distorted waveforms. Computer simulations illustrate the nonperiodic nature of the stator currents and typical harmonic contents of rotor phase current and voltage as well as inverter current at various speed values. >

Modeling and performance of slip energy recovery induction motor drives

This program can be used to examine the transient performance of slip energy recovery drives for the proper rating of semiconductor devices or to examine the effects of faults on the associated power system on the drive performance. Current and speed controllers designed using linear techniques can be evaluated using this program as well. A computer using a fourth-order Runga-Kutta integration routine has been developed that uses a newly proposed model to predict the detailed operation of a slip energy recovery drive both in the transient and in the steady state. The hybrid model is between the well-known d, q and the phase variable models and retains the actual rotor states. This makes it possible to include the rectifier commutation impedance without any difficulty. The harmonics of the rectifier and inverter are not neglected, as was often done in the literature. Variables such as the DC link current, rotor phase current, stator current, and inverter recovered current were predicted accurately. Commutation overlap was incorporated into the model and predicted accurately, both in the steady-state and transient conditions.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Modeling, simulation, and analysis of variable-speed constant frequency power conversion scheme with a permanent magnet brushless DC generator

A variable-speed, constant-frequency (VSCF) power-conversion scheme with a permanent magnet DC (PMDC) brushless generator is proposed. The scheme is completely modeled for steady state by integrating the characteristic equations of the generator, the diode rectifier bridge and the inverter, and the harmonic characteristics for steady-state performance computation. Commutation overlap effects are also included. Key performance characteristics are experimentally verified with a laboratory prototype. The excellent correlation between the predicted and experimental results confirms the validity of the model. For the purpose of filter design, harmonics of the rectified generator voltages are analytically derived. A recommendation for overcoming the low power factor on the utility side at low speeds using the forced-commutated converters is given. >

Laboratory verification of modeling for a cycloconverter-fed microprocessor-controlled self-synchronous motor

The first known numerical simulation is presented of a cycloconverter-link self-synchronous motor drive operated by a discrete-time speed controller. Theoretically predicted and laboratory-measured transient and steady-state performances are compared to assess the accuracy of the model. The correlation between predictions made by the average value of motor phase current base simulation and the laboratory-measured performance shows that the model is quite adequate for steady-state analysis. Furthermore, the model offers close prediction of speed during transient conditions, indicating that the associated half-cycle average values of current and torque were reasonably accurate, although the capability of measuring the variables was not available. In addition, the general describing equations of motor phase currents during the commutation overlap interval are derived and numerically solved. Measured and predicted commutation overlap intervals for steady-state conditions are compared.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A generalized approach to steady-state analysis of a current-source inverter with induction motor load including commutation overlap

A generalized state-space approach is presented for the steady-state commutation analysis of an autosequential commuted current-source inverter-fed induction motor drive covering all possible modes of operation using a digital computer. The analysis adopts the basic per-phase equivalent circuit for the induction motor model, includes the effect of DC link filter impedance and the variation of back EMF during the commutation interval, and extends the inverter model to cover the complex commutation overlap mode occurring during high frequency operation. A digital computer program is evolved, to identify automatically the relevant sets of equations applicable to a particular mode from a generalized set of equations and to solve them. >

Effect of rotor rectifier on motor performance in slip recovery drives

Despite its simplicity, the static Kramer drive presents a formidable analytical challenge. Rigorous analysis is particularly difficult and there is need for a simpler form of analysis when calculating ratings and steady-state performance. One aspect which is not well documented is the influence of commutation overlap within the rotor rectifier upon motor power factor and peak attainable torque. This point is dealt with and it is also shown how the system performance can be predicted by largely analytical means, allowing fully for motor resistances, diode voltage drops, and the several possible rectifier overlap modes.

A State Space Analysis of LCI Fed Synchronous Motor Drives in the Steady State

The steady-state performance of a load-commutated in-verter (LCI) fed synchronous motor drive is computed by means of a digital computer-based state space solution of the synchronous machine equations. The computer solution permits accurate calculation of current, voltage, and torque waveforms under the assumptions of constant shaft speed and constant dc link current. Steady-state operating points are characterized by the firing angle, commutation overlap angle, and the ratio of average field current to dc link current. The two angles are specified as input to the program, and the ratio of field current to dc link current is calculated. In this way, a solution is guaranteed to exist for a given operating point. The symmetry of steady-state operation is exploited to calculate the appropriate boundary conditions for a given operating point directly, without the need to perform iterative calculations . Sample calculations for a 4500-hp LCI drive are presented, showing voltage, current, and torque waveforms, and steady-state performance characteristics with constant commutation margin angle. Consortium (WEMPEC) and in part by General Electric.

Sub- and Super-Synchronous Induction Motor-Thyristor Cascade Control

Normally, thyristor controlled converter circuits are used on the rotor side of slip-ring induction motor for the purpose of sub-synchronous speed control. With possible reversibility of power flow in the rotor circuit by using reversible controlled converters, super-synchronous as well as sub-synchronous speed control of induction motor drive is obtained. By proper design of such a system, it is possible to obtain the control and signal processing for current and speed feed-back over a very wide speed range with either driving or braking torque.This paper presents a modified speed control scheme of sub-and super-synchronous induction motor-static converter cascade using a current source converter on the slip-ring side and a modified bridge converter on the line side, thus the energy being returned or taken from the supply, A thorough analysis of the steady state performance between and during commutation of the current source converter is provided. The analysis also gives the three different modes of operation of the modified bridge converter taken into account the effect of commutation overlap. The control firing strategy for the two converters is formulated in order to reduce reactive power requirements. Power factor and harmonic content are identified for the purpose of obtaining optimal power factor operation. The results are found by computer programming over a very wide range from standstill up to about twice synchronous speed and a comparative discussion with other types of cascade is outlined.

Operating mode of converter without commutation overlap angle

Operating mode of converter without commutation overlap angle

Accurate representation of commutation overlap effects in steady-state multi AC/HVDC link studies

Most steady-state HVDC link representations, such as multi AC/HVDC loadflow analysis and state estimation, do not model commutation overlap angle effects; instead they assume convertor switching to be instantaneous. A simple technique is presented and tested which allows the commutation overlap to be explicitly included in conventional HVDC link representation with only a slight modification. The resulting algorithms are shown to accurately model the commutation overlap process under normal HVDC link operating conditions (overlap angle &lt;60°) and convergence of the algorithm is not degraded

Commutation phenomena in a static power convertor

A systematic experimental and theoretical study was made of the effect of various inherent and extraneous circuit parameters on the commutation oscillations and on the period of commutation overlap in a static power convertor. The initiation of oscillations by a voltage jump occurring across a convertor device at the start and at the end of commutation overlap was analysed for a variety of conditions, and the theoretical deductions were experimentally verified. The period of commutation overlap in a convertor without extraneous capacitances and in steady-state operation was shown to be determined by the sum of the instantaneous values of direct current prevailing at the start and the end of the overlap. Enforced reductions of the period of commutation overlap were found to be obtained by an appropriate adjustment of the magnitudes of the relevant parameters in a controlled-rectifier circuit either with capacitors connected between the anodes of the convertor devices or with capacitors connected between the input terminals of the convertor transformer in the presence of series inductances in the a.c. supply lines.