12-pulse Converter Research Articles

Generalization of the Y-differential autotransformer for 12- and 18-pulse converters

This work proposes a methodology to generalize the Y-connections for 12- and 18-pulse autotransformers. A single mathematical expression, obtained through simple trigonometric operations, represents all the connections. The proposed methodology allows choosing any ratio between the input and the output voltages. The converters can operate either as step-up or as step-down voltage. To simplify the design of the windings, graphics are generated to calculate the turn-ratio and the polarity of each secondary winding, with respect to the primary winding. A design example, followed by digital simulations, illustrates the presented steps. Experimental results of two prototypes (12 and 18 pulses) are presented. The results also show that high power factor is an inherent characteristic of multi-pulse converters, without any active or passive power factor pre-regulators needs.

Mathematical Model of 4-Quadrant DC Electric Drive with 6-Pulse Dual Converter

Mathematical model simulating the operation of DC electric drive hoisting mechanism has been developed. The study illustrates the capabilities of the developed model for investigation both normal and abnormal modes of operation of the converter and the supplying voltage. Large attention was given to the thyristor converter and its firing system so that fast-proceed micro-processes occurring in the converter can be simulated with a high degree of accuracy.

The effect of converter configurations of HVDC links on sub- and super-synchronous disturbances to turbine units

In this paper, the rotor torque disturbances to turbine–generator units arising from harmonic interactions between converters of a HVDC system are studied. It is shown that a distinct-pulse converter configuration will be a better choice for avoiding sub- or super-synchronous resonance. For an asynchronous HVDC link, the risk of electromechanical resonance could be completely got rid of on either side of the link, no matter what directions the powers flow, if an 18-pulse converter were used on the 60 Hz side and a 12-pulse converter on the 50 Hz side. Such a configuration could still perform well even under the large frequency deviations. Furthermore, it shows that power system type plays a significant role on the possible excitation of resonance and on the probably damaging sections of a turbine–generator unit. Good match of the converter configuration and power system type is significant for a HVDC link to exempt from sub- and super-synchronous resonance.

Application of a Series-Connected Hybrid Multi-Converter System to STATCOM

This paper gives an analysis and design of a static synchronous compensator (STATCOM) using a series-connected hybrid multi-converter system. The hybrid multi-converter system consists of two GTO converters and an IGBT converter connected in series by transformers. The proposed STATCOM has advantages that the harmonic components of the output voltage/current would be less than a conventional configuration STATCOM composed of a 12-, 24-, or 48-pulse converter; the total switching losses would be less than a PWM STATCOM; and the response would be as rapid as a PWM STATCOM. One problem of the hybrid configuration STATCOM is that it tends to operate in an over-modulation range if a considerable compensating reference change occurs. To solve this problem, this paper also proposes a variable DC voltage control strategy, where the DC voltage is adaptively adjusted in accordance with the required reactive power. The validity of the proposed configuration STATCOM and control strategies are verified by simulation and experimental results.

Determining pulsed alternator thyristor converter firing angles to produce a desired launcher current

Proposed electromagnetic launch systems using pulsed alternators usually employ a phase-angle-controlled thyristor converter to supply dc to the railgun launcher. By carefully choosing the firing angles of the individual thyristors, the system designers can control shape of the current pulse delivered to the launcher, thereby controlling projectile acceleration and magnetic energy recovery from the launcher after muzzle exit. This paper describes a simple method for determining the required phase angles for a specified launch current waveform, using a simplified equivalent controlled dc source model for the pulsed alternator and converter system. Once a desired current profile is chosen to deliver the required action to the projectile, the railgun equations are used to determine the required breech voltage profile. The thyristor firing angle versus time to produce this breech voltage is then calculated from the equivalent dc source model. Improvements to the basic model incorporating field current decay and rotor speed reduction with time are discussed. The method has been embedded in a Mathcad worksheet. Results from the simplified model show good agreement when compared to a detailed Saber system simulation.

Effects of magnetic saturation on induction machines driven by static converters

The effects of magnetic saturation on the operation of induction motors driven by static converters are analyzed. A mathematical model based on magnetic harmonic functions is used to account for saturation. Distortions on the air gap flux due to non-linear magnetic characteristics lead to appearance of space harmonics in the resultant flux density distribution. This causes specific distortions in stator and rotor time quantities, which are different from those due to static converter. Operation with six pulse and sinusoidal PWM converters under saturated conditions is considered. Comparisons of experimental and simulated results are presented and found to be in very good agreement.

Comparative study of pulsed DC-link voltage converters

This paper classifies the pulse pulsed dc-link voltage (PDCLV) topologies in six types, depending on both the shape of the pulsed DC-link voltage waveform and the circuit ability to synchronize, or not, with external pulse width modulation (PWM) signals. Main features are compared for a large number of topologies in which the bridge devices commutate under zero-voltage-switching (ZVS) technique. The paper also presents a comparative study of the losses produced by these converters and discusses the principles they employ causing generation such losses. An overview of the control and PWM possibilities for these converters is given and an extensive bibliography is included.

Identification of Converter Faults Using Sequence Detectors as Signature Analyzers

High-voltage direct current (HVDC) links are increasingly used throughout the world for long distance transmission and asynchronous interconnections. Conversion is accomplished through 12-pulse bridge converters employing thyristor-valves. The operation of the bridge is affected by the disturbance in the AC or DC system or mal-operation of the valves. Normally the valves are fired in a particular sequence and conduct for a period of one-third of a cycle. Any discrepancy in the conduction sequence is the indication of the misbehavior. This paper presents a method of identifying the misbehavior of the bridge converter based on the discrepancy in the conduction sequence. The technique of signature analysis is used for the identification. The proposed scheme uses the firing pulses and the valve ON-OFF states for identifying misbehavior of the valves. The signature analyzer (sequence detector circuits) consisting of R_S flip-flops and AND gates is used in this scheme to identify the misbehavior. Simulation results with PSCAD/EMTDC confirm the success of the proposed scheme.

All-Optical Two-Dimensional Serial-to-Parallel Pulse Converter Using an Organic Film with Femtosecond Optical Response

In this study, we introduce a new concept of all-optical two-dimensional serial-to-parallel pulse converters. Femtosecond optical pulses can be understood as thin plates of light traveling in space. When a femtosecond signal-pulse train and a single gate pulse were fed onto a material with a finite incident angle, each signal-pulse plate met the gate-pulse plate at different locations in the material due to the time-of-flight effect. Meeting points can be made two-dimensional by adding a partial time delay to the gate pulse. By placing a nonlinear optical material at an appropriate position, two-dimensional serial-to-parallel conversion of a signal-pulse train can be achieved with a single gate pulse. We demonstrated the detection of parallel outputs from a 1-Tb/s optical-pulse train through the use of a BaB2O4 crystal. We also succeeded in demonstrating 1-Tb/s serial-to-parallel operation through the use of a novel organic nonlinear optical material, squarylium-dye J-aggregate film, which exhibits ultrafast recovery of bleached absorption.

SMES system for study on utility and customer power applications

A grid model SMES system was developed for study on utility and customer power applications. The system consists of a 20 kJ NbTi magnet, a 15 kW 12-pulse IGBT current source converter and a DSP-based controller. To reduce the heat leakage, a couple of HTS current leads were designed and fabricated which are composed of Bi-2223 tapes and have a maximum operating current of 150 A. The 12-pulse current source topology was chosen for the converter not only owing to its low total harmonic distortion (THD) but also to its high capability of injecting capacitive reactive power into network. In the controller of the system two very fast DSPs (TMS320C32) are used to implement the control algorithms and perform the generation of the firing signals, giving a high accuracy and a high speed of response.

SMES control algorithms for improving customer power quality

Control algorithms of SMES based on a 12-pulse IGBT current source converter were developed for improving customer power quality. The control performances include uninterruptible power supplies (UPS) and load fluctuation leveling. To demonstrate the control algorithms, both simulations and experiments were carried out on a 20 kJ/15 kW grid model SMES system. The results of the simulations and experiments are presented and compared in this paper.

Femtosecond two-dimensional serial-to-parallel pulse converter using a squarylium dye J-aggregate film

Based on a time-to-space conversion technique, a two-dimensional serial-to-parallel pulse converter using a squarylium dye J-aggregate film is proposed. The squarylium dye J-aggregate film, which exhibits an ultrafast recovery of bleached absorption, operates as a large-area femtosecond all-optical shutter. The system is composed of a few simple optical components and enables conversion of temporally serial signals into multiple parallel outputs in one- or two-dimensional space. The serial-to-parallel conversion of 1 Tb/s optical pulses was demonstrated and different 4-bit signal patterns were successfully discriminated in one dimension. Also, 2 × 2 outputs were clearly observed as two-dimensional outputs using this converter.

Small signal frequency domain model of an HVDC converter

A small-signal analytic frequency domain model of a 6-pulse HVDC converter is presented. The model consists of a set of explicit algebraic equations which relate the transfer of distortion from AC voltage, DC current and firing angle modulation, to AC current and DC voltage. The equations represent the linearisation of the transfers around a base operating point, and are derived from a piecewise linear description of the AC current and DC voltage waveforms. The model provides an understanding of the transfer of distortion by the converter and is in excellent agreement with time domain simulations.

Self-Learning Control of Pulse Converter Non-Linearities

A new selp-learning control algorithm is presented, designed for plants with unknown cyclic or state-dependem distur-bances. Exemplified by the compensation of pulse converter non-linearities the control structure is explained. The control perfimnance is illustrated by measurements from a real-rime experimental set-up.

Control and protection scheme of HVDC system with self-commutated converter in system fault conditions

For extending self-commutated converter application to future trunk power systems, it is important to develop a stable operation scheme as well as to realize substantial cost reduction through coordinated system and control design. Suppression controls of converter overcurrent and dc overvoltage in various system fault conditions are essential in order to ensure stable operation and cost reduction of HVDC systems with voltage source type self-commutated converters. Converter control and protection schemes which include such suppression controls have been developed, employing CRIEPI's ac/dc Power System Simulator test and EMTP analysis. This paper first discusses the cause of converter overcurrent at ac system faults, considering the effect of PWM pulse number and converter control speed. Continued operation has been achieved by adding a new overcurrent suppression scheme to the converter control. In the case of a dc line grounding fault, the selection of the grounding circuit constant and the adoption of a high-speed converter control practically ensure the reduction of dc overvoltage while suppressing converter overcurrent. The converter block and restart sequence after a dc fault, which is coordinated with dc circuit breaker operation, enables stable recovery of HVDC transmission as fast as the usual line-commutated HVDC system. © 2000 Scripta Technica, Electr Eng Jpn, 132(2): 6–18, 2000

A new passive 28-step current shaper for three-phase rectification

A new passive 28-step current shaper for three-phase rectification is proposed in this paper. With a phase-shifting transformer on the AC side and six interphase transformers on the DC side, per-phase input current can be shaped into a 28-step sinusoidal waveform. The total harmonic distortion of AC input currents obtained is 6.53%, lower than one-half of that in a conventional 12-pulse converter. The transformer voltampere rating is also lowered down to one-fifth of that in a 12-pulse converter. A 2 kW experiment is performed to verify the proposed circuit.

Efficient HVDC converter model for real time transients simulation

An efficient model has been developed to simulate high voltage direct current (HVDC) power converters in real-time transient simulators. Real time performance of 45 /spl mu/s per solution step has been achieved for a 12-pulse monopolar converter and 81 /spl mu/s for the bipolar converter using a single processor Pentium Pro 200 MHz desktop computer. The HVDC power converter model is solved simultaneously with the power network. Simultaneous solution makes the model capable of sustained long-time simulation for real time testing of system performance and control functions.

Reducing supply current harmonics

In this article, a simple and effective method for reducing the supply current harmonics has been proposed for a parallel connected 12-pulse thyristor converter with an interphase reactor. The proposed method requires only an interphase reactor with an optimal inductance value being much smaller than conventional inductance values, without any additional devices or components. The supply current waveform of the 12-pulse thyristor converter with the optimal interphase reactor is more sinusoidal than that of a 36-pulse thyristor converter. The proposed design concept of the interphase reactor has been discussed, and confirmed by using digital computer simulation. An experimental system has been constructed and tested, and the experimental results have verified the practicability and effectiveness. The authors conclude that the proposed concept provides a clean power utility interface for various industrial/utility applications including superconducting energy storage systems (SMES).

Transient and steady-state short-circuit currents in rectifiers for DC traction supply

This paper presents some significant extension and analytical formalization of the knowledge on transient and steady-state short-circuit currents in DC traction systems. Electrical supply substations (ESSs) in modern metrorail and railway DC systems are equipped with 6- and 12-pulse diode bridge rectifiers. Large existing plants are equipped with 6-pulse converters, but the 12-pulse option is preferred in the design of new plants in the parallel configuration. Such a converter presents a good compromise between number and voltage/current rating of semiconductor devices, complexity and rating of transformers, DC-side voltage ripple, and AC-side current harmonic content. In this paper, the author considers the features of 6- and 12-pulse conversion units in terms of transient and steady-state short-circuit currents on zero fault impedance, accurate determination of the transient DC short-circuit current is important for correct rating and tuning of protection devices and power equipment (high-speed circuit breakers, isolator switches, switchgears, DC-busbars, etc.). An analytical solution of the short-circuit behavior is given for zero fault impedance currents: phase currents of the transformer, transient short-circuit DC current, approximate expression of the DC short-circuit current, and accurate expression of the steady-state DC short-circuit current. The general calculation method used in the paper is extensible to any conversion unit with a double secondary transformer.

自励式直流送電システムにおける系統事故時の制御保護方式

For extending self-commutated converter application to future trunk power systems, it is important to develop a stable operation scheme as well as to realize substantial cost reduction through coordinated system and control design. Suppression controls of converter overcurrent and dc overvoltage in various system fault conditions are essential in order to ensure stable operation and cost reduction of HVDC systems with voltage source type self-commutated converters. Converter control and protection schemes which include such suppression controls have been developed, employing CRIEPI's ac/dc Power System Simulator test and EMTP analysis. This paper first discusses the cause of converter overcurrent at ac system faults, considering the effect of PWM pulse number and converter control speed. Continued operation has been achieved by adding a new overcurrent suppression scheme to the converter control. In the case of a dc line grounding fault, the selection of the grounding circuit constant and the adoption of a high-speed converter control practically ensure the reduction of dc overvoltage while suppressing converter overcurrent. The converter block and restart sequence after a dc fault, which is coordinated with dc circuit breaker operation, enables stable recovery of HVDC transmission as fast as the usual line-commutated HVDC system. © 2000 Scripta Technica, Electr Eng Jpn, 132(2): 6–18, 2000