Graetz Bridge Research Articles

Corrections of the original CFPREV control in the LCC-HVDC links and analysis of its inherent plateau effect

The most effective approach to suppressing the first commutation failure (CF) of the LCC-HVDC link at fault inception is to advance firings of the inverter, and the commutation failure prevention (CFPREV) control is the most commonly used method in practical engineering. However, it is found in this letter that there exist a few serious defects in its original scheme, and thus targeted vital corrections have been made in the first place. Furthermore, an interesting phenomenon termed the plateau effect that an excessive advancement of firings will contrarily and inevitably lead to more commutation failures is also revealed and analyzed, and it turns out that for which the inherent commutation dents of the Graetz bridge should be mostly responsible, which bridges the knowledge gap and further enhances the cognition of the limitation of CFPREV control, and it may also conduce to the design of related control parameters. Simulation results then validate the imperative of those presented corrections and confirm the existence of the plateau effect.

Porous Coatings Containing Copper and Phosphorus Obtained by Plasma Electrolytic Oxidation of Titanium

To fabricate porous copper coatings on titanium, we used the process of plasma electrolytic oxidation (PEO) with voltage control. For all experiments, the three-phase step-up transformer with six-diode Graetz bridge was used. The voltage and the amount of salt used in the electrolyte were determined so as to obtain porous coatings. Within the framework of this study, the PEO process was carried out at a voltage of 450 VRMS in four electrolytes containing the salt as copper(II) nitrate(V) trihydrate. Moreover, we showed that the content of salt in the electrolyte needed to obtain a porous PEO coating was in the range 300–600 g/dm3. After exceeding this amount of salts in the electrolyte, some inclusions on the sample surface were observed. It is worth noting that this limitation of the amount of salts in the electrolyte was not connected with the maximum solubility of copper(II) nitrate(V) trihydrate in the concentrated (85%) orthophosphoric acid. To characterize the obtained coatings, numerous techniques were used. In this work, we used scanning electron microscopy (SEM) coupled with electron-dispersive X-ray spectroscopy (EDS), conducted surface analysis using confocal laser scanning microscopy (CLSM), and studied the surface layer chemical composition of the obtained coatings by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), glow discharge of optical emission spectroscopy (GDOES), and biological tests. It was found that the higher the concentration of Cu(NO3)2∙3H2O in the electrolyte, the higher the roughness of the coatings, which may be described by 3D roughness parameters, such as Sa (1.17–1.90 μm) and Sp (7.62–13.91 μm). The thicknesses of PEO coatings obtained in the electrolyte with 300–600 g/dm3 Cu(NO3) 2∙3H2O were in the range 7.8 to 10 μm. The Cu/P ratio of the whole volume of coating measured by EDS was in the range 0.05–0.12, while the range for the top layer (measured using XPS) was 0.17–0.24. The atomic concentration of copper (0.54–0.72 at%) resulted in antibacterial and fungicidal properties in the fabricated coatings, which can be dedicated to biocompatible applications.

Experimental investigation on the piezoelectric energy harvester as the self-powered vibration sensor

The article presents an experimental study of a system consisting of a piezoelectric energy harvesting device, Graetz bridge rectifier, capacitor, voltage comparator and radio transmitter. In the presented experimental study, the recovered electrical energy is accumulated in the capacitor and is used to send signals by a radio transmitter. In the first part, the application of piezoelectric energy harvesting devices based on the cantilever beam in wireless monitoring systems is discussed. In the second part, the mathematical model of energy conversion in the piezoelectric energy harvesting devices is presented. In the third part, the characteristics obtained during laboratory research are presented.

Magnetorheological damper–based positioning system with power generation

This study investigates a newly developed positioning system based on a rotary magnetorheological damper with power generation capability, comprising an electromagnetic power generator (energy extractor) generating electrical power, an magnetorheological damper which alters the damping characteristics of the system and an electrical interface, connected in between the coil of the generator and the damper control coil, which conditions the voltage output from the generator. Structural configurations of the damper and the generator are outlined, and the results of their testing and the testing done on the entire system are summarised, covering the tests conducted with and without the interface, under the idle run and under load in the assumed velocity range. The objective of the work is to examine the performance of the proposed positioning system through experiments. Results of the system testing in the uncontrolled case (passive system) and in the controlled one (semi-active system) in a purpose-built test rig are compared and discussed. Three design options of the electrical system controlling the damper are explored. In the first option, the damper is assumed to be controlled starting from the initial moment of the motor shaft’s motion, and in the second option, the control action begins at the instant when the motor shaft assumes the present position. The third variant is similar to the first one except that an additional condenser is connected behind the Graetz bridge. The results confirm the adequacy of the developed positioning system and demonstrate an improvement in control of the system’s dynamic behaviour.

The design and fabrication of electromagnetic microgenerator with integrated rectifying circuits

PurposeThe paper focuses on design, fabrication and characterization of electromagnetic microgenerators with integrated rectifying circuits to convert AC output signal to DC one. The work includes research on simulation of voltage-rectifying circuits, including charge pump, realization of the experimental printed circuit board (PCB) with selected electronic circuits and the execution of the final structure with integrated rectifying circuit. Measurements were performed on these circuits.Design/methodology/approachElectromagnetic microgenerators include multipole permanent magnets secured on rotor three-phase brushless direct current (BLDC) motor and planar multilayer multiple coils. These were fabricated using low temperature co-fired ceramics (LTCC) technology. In our experiment, six rectifying circuits were simulated and tested with a structure consisting of eight layers of coils and with an outer diameter of 50 mm fabricated earlier.FindingsThe microgenerator with Graetz bridge generates higher output power than the modified charge pump at the same rotary speed. However, it is less stable for the distance change between the structure and the magnets than the modified charge pump, which has more constant output power in a wider range of load resistance.Originality/valueThe presented electronic rectifying circuits are novel for LTCC-based electromagnetic microgenerator application. The structure with integrated rectifying circuits allows generation of electrical output power larger than 100 mW at the rotor speed of about 8,000 rpm.

Three-Stage Static Power Converter for Battery Charging Feasible for Small Wind Energy Conversion Systems

This paper presents the analysis, design, simulation, and experimental results for a three-stage static power converter for battery charging feasible to small wind energy conversion systems. The system employs a boost converter cascaded with a Graetz bridge that allows the implementation of a maximum power point (MPP) tracker and the reduction of the mechanical speed under overvoltage conditions across the batteries. Moreover, a buck converter is connected in series with the boost stage to ensure a constant voltage bus between the aforementioned topologies. Thus, it is possible to extract the maximum power over the entire wind speed range, and battery charging can be realized through conventional techniques. The complete design of the proposed battery charger including power, control, and supervisory circuits is presented and developed, considering a 300-W system, with the possibility of charging battery banks rated at 12 or 24 V. Simulation results are presented to prove the existence of MPPs in the wind generator. Finally, experimental results of the developed prototype required to validate the functionality of the proposed study are presented and discussed.

Operating characteristics of a new filter-commutated converter based on equivalent Graetz bridge circuit model

In this study, filter-commutated converter (FCC) with a new topological structure is proposed and compared with conventional high-voltage DC converters. The characteristic harmonic filters are installed at the tap of valve winding, which can implement the harmonic suppression and the reactive power compensation near the valve side. Based on multi-winding transformer theory and Kirchhoff's law, the equivalent Graetz bridge model of the FCC is established, and the concept of a reactive power compensation factor at the valve side is introduced. Furthermore, the equivalent circuit of the DC side of the FCC is presented, and the influence of the compensation factor on the FCC characteristic parameters and the system-operating parameters are analysed in detail. Finally, a test on a prototyping system equipped with the FCC is performed to verify the correctness of the theoretical analysis of the FCC.

Performance Evaluation of a Novel Hybrid Multipulse Rectifier for Utility Interface of Power Electronic Converters

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> This paper presents an improved analysis of a novel Programmable Power-factor-corrected-Based Hybrid Multipulse Power Rectifier (PFC-HMPR) for utility interface of power electronic converters. The proposed hybrid multipulse rectifier is composed of an ordinary three-phase six-pulse diode-bridge rectifier (Graetz bridge) with a parallel connection of single-phase switched converters in each three-phase rectifier leg. In this paper, the authors present a complete discussion about the controlled rectifiers' power contribution and also a complete analysis concerning the total harmonic distortion of current that can be achieved when the proposed converter operates as a conventional 12-pulse rectifier. The mathematical analysis presented in this paper corroborate, with detailed equations, the experimental results of two 6-kW prototypes implemented in a laboratory. </para>

Programmable PFC based hybrid multipulse power rectifier for ultra clean power application

A novel hybrid three-phase rectifier is proposed. It is capable to achieve high input power factor (PF) and low total harmonic input currents distortion (THD/sub I/). The proposed hybrid high power rectifier is composed by a standard three-phase six-pulse diode rectifier (Graetz bridge) with a parallel connection of single-phase Sepic rectifiers in each three-phase rectifier leg. Such topology results in a structure capable of programming the input current waveform and providing conditions for obtaining high input power factor and low harmonic current distortion. In order to validate the proposed hybrid rectifier, this work describes its principles, with detailed operation, simulation, experimental results, and discussions on power rating of the required Sepic converters as related to the desired total harmonic current distortion. It is demonstrated that only a fraction of the output power is processed through the Sepic converters, making the proposed solution economically viable for very high power installations, with fast investment payback. Moreover, retrofitting to existing installations is also feasible since the parallel path can be easily controlled by integration with the existing dc-link. A prototype has been implemented in the laboratory and it was fully demonstrated to both operate with excellent performance and be feasibly implemented in higher power applications.

Capacitor commutated converters for long-cable HVDC transmission

An overview of the capacitor commutated converter is given, emphasising its applications in long-cable HVDC transmission schemes. Its advantages and disadvantages in comparison with the conventional HVDC Graetz bridge converter are identified, and a critical assessment of its performance is presented using a developed steady-state formulation as well as a transient simulation based analysis.

Static VAr compensator (STATCOM) based on single-phase chain circuit converters

Voltage-sourced converter circuits using gate turn-off (GTO) thyristors have been proposed as an alternative means of providing static VAr compensation (SVC). Such ‘advanced SVCs’ are classified under the nomenclature STATCOM (STATic COMpensator). The published converter circuits suitable for power system application include the classical Graetz bridge and the multilevel converter. An alternative circuit arrangement has been developed with significant advantages in terms of cost and performance; this is in the form of a number of GTO converter links connected in series to form a chain and is referred to as a chain circuit converter. The paper discusses the chain circuit converter and its application to VAr compensation, followed by the design and simulated performance of a 3-phase STATCOM under both steady state and transient operating conditions. The technical advantages of the chain circuit converter compared with other known arrangements of STATCOM are given. These include good harmonic performance, fast response and improved output at low voltage compared with conventional SVCs. The paper includes comments on the successful testing of two prototype chain links for both leading and lagging reactive currents up to 2000 A peak and concludes with a description of the equipment now being designed for commercial service.

Apparatus for supplying an isolated DC load from a variable-speed self-excited induction generator

This paper describes the design and laboratory testing of novel generation apparatus for supplying an isolated DC load from a self-excited induction generator operable at variable speed. The variable-speed generating apparatus consists of a self-excited induction machine, a controlled Graetz bridge rectifier, a voltage-boost power converter, and a control system. The induction generator supplies the rectifier. The voltage-boost power converter interfaces the variable output voltage of the rectifier to the fixed DC voltage required for the load. The rectifier is operated at levels of average DC current and voltage which control machine voltage to the rated AC voltage and which also draw the necessary power to supply the DC load. Performance is enhanced with respect to earlier apparatus in that both the DC voltage supplied to the load and the AC voltage on the machine are simultaneously controlled to fixed reference levels over broad operating ranges of load and speed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Power conditioning systems for superconductive magnetic energy storage

Two power conditioning systems for superconductive magnetic energy storage (SMES) are presented. One power conditioning system is based on a hybrid current sourced inverter (CSI), the second is a combination of a DC chopper with a voltage sourced inverter (VSI). Both of these systems have independent control of real and reactive power. These systems have a significant reduction in MVA rating levels as related to the more traditional Graetz bridge. >

Application of a superconducting coil for transient stability enhancement

Abstract Transient stability is an important part of power system design and operation. This paper presents a method to augment the transient stability by using a superconducting magnetic energy storage system. A conventional Graetz bridge converter provides an ideal interface between the three-phase power system and the superconducting coil. This makes power exchange in the superconducting coil fast and controllable. With appropriate gating to the thyristors of the converter, power can be drawn and stored in the magnetic field or vice versa. A mathematical model for the superconducting magnetic energy storage coil with a converter interface is presented. The developed model is integrated with an AC power system by using diakoptical techniques. Two types of control schemes with the generator rotor speed deviation as an input signal to the controllers have been studied on three different power systems. Finally, a comparative study between the dynamic shunt braking resistor method and the superconducting magnetic energy storage method was carried out on a 4-generator, 6-bus system. The computer simulation results reveal that the transient stability enhancement with the superconducting coil is superior to that of the dynamic braking resistor.

A Three-Phase Controlled-Current PWM Converter with Leading Power Factor

Experimental tests performed on a three-phase bipolartransistor controlled-current PWM power modulator show that it can operate with near-sinusoidal currents at 60 Hz with a 360-degree power angle range. Because of its capability to operate with leading power factor and good waveform, the PWM converter is a promising alternative to the thyristor Graetz bridge. A theoretical method based on the concept of ``local average'' is presented.

Experimental studies of current sharing in parallel driven Graetz bridge units for diurnal superconductive magnetic energy storage

Superconductive Magnetic Energy Storage (SMES) coils for diurnal load leveling and system peaking are envisioned to operate at hundreds of thousands of amperes and a few kilovolts. The interface between the SMES coil and the electric utility is envisioned to be Graetz bridges using SCR switches. Many parallel SCR switches or bridge units will have to operate in parallel because of the high operating current of the coil. Current balancing on parallel Graetz bridges driving a single 8-hy superconducting coil has been achieved on a laboratory model using delay-angle control with an LSI 11/2 microprocessor and external digital control hardware.

Analysis of Thyristor-Controlled Shunt SSR Countermeasures

This paper presents the results of a study to compare the effectiveness of three SSR countermeasures. Both frequency and time domain techniques were used to insure the integrity of the investigation. Reactive power modulation with generator speed feedback will damp all SSR modes if phase lags associated with thyristor firing controls are compensated. It was also found that Graetz bridge devices can damp SSR oscillations in addition to their primary functions.

Application of superconducting coils to reactive power control in electric power systems

A novel type of static thyristor-controlled reactive power compensator, called SAVAR, consisting of a multipulse Graetz bridge controlled in a variable free wheeling mode and connected to a superconducting coil is described. Each thyristor of the Graetz bridge is fired twice during a cycle in a cyclic order that results in a constant average coil current which flows through the line during part of a cycle and free wheels through the bridge during the rest of the cycle. By varying the fraction of a cycle during which the coil current is free wheeling, the line current can be varied continuously between a minimum and maximum limit according to the required reactive current demand. The control algorithm for the variable free wheeling mode of the bridge and the methods to produce a low loss superconducting coil subjected to high voltage harmonics are described. Experimental results of a prototype unit with an existing non-optimized superconducting coil are given, providing the proof-of-principle of the SAVAR system. Preliminary cost and performance data for a 40 MVAR SAVAR reference design are compared with those obtained from a conventional static reactive power compensator.

Superconductive magnetic energy storage

Technical and economic aspects of large scale superconductive magnetic energy storage are discussed. This paper is a review of a program which has been under way at the University of Wisconsin since 1970. Early work between 1970 and 1976 was primarily involved in proving economic and technical feasibility of the concept The present program deals with component development and detailed design ultimately leading to construction of a large superconducting magnet capable of storing 1000–10,000 MWh. The magnet is a single-layered segmented solenoid of approx. 100 m radius. Energy containment is achieved economically by burying the magnet underground in bedrock tunnels. Magnetic loads are transmitted from the conductor to bedrock through glass fiber reinforced composite struts. The conductor consists of a composite of aluminum and NbTi and is designed for full cryogenic stability in 1.8 K superfluid helium. The dewar-conductor assembly will be rippled in a l m radius of curvature to reduce the hoop stress tension. A Graetz bridge is required to convert the d.c. superconducting current into a.c. current in the three-phase power system. Economic analysis indicates that superconductive magnetic energy storage is competitive with alternative large scale storage schemes for units greater than 1000 MWh size.

Economical Equidistant Pulse Firing Scheme for Thyristorized DC Drives

From the literature published, six different gate firing schemes are available for three-phase thyristor Graetz bridge converters employing equidistant pulses. All these thyristor firing schemes have been developed with particular attention to the special control requirements of converters used in a high voltage dc transmission scheme. Herein is described a new simple equidistant pulse firing scheme intended to be economically quite competitive to the presently existing individual phase control circuits used in three-phase converter circuits for industrial applications. The princidple of operation and the circuit details of the firing circuit are described. Experimental results obtained on a laboratory dc drive of capacity 3 KVA are discussed.