Angelo Jose Junqueira Rezek

Energy conservation with use of "soft-starter"

The authors describe how energy saving has been obtained by using the soft-starter to reduce the voltage applied to the motor. Iron losses are then reduced and energy saving is achieved. The use of an autotransformer instead of a soft-starter has enabled a high energy saving, because the additional losses due to the harmonic currents are not present in this case. Energy saving with no load motor has been about 11% by using the soft-starter and 25% when the autotransformer is used, for load torque proportional to speed and for constant torque loads. Energy saving is decreased when the load torque is increased. Energy saving has been obtained until a load torque of about 42% of the rated torque; for higher torques no energy conservation is achieved. So the results of this paper are useful only for low load conditions. When the autotransformer is used the power factor and efficiency are better than in the other cases.

Ultracapacitor-based ride-through system for adjustable speed drives applied to critical process

Many industrial plants rely on adjustable speed drives (ASD) for critical process applications. However, most ASD are susceptible to power disturbances, specially voltage sags and momentary interruptions. ASD tripping in industrial process can be very costly due to defective units produced as well as production down time. This paper presents an ultracapacitor-based ride-through system for adjustable speed drives applied to critical process. The complete system is composed by an unifield power quality conditioner (UPQC) and a two-quadrant chopper.

The capacitor commutated converter (CCC) as an alternative for application in HVDC projects

Series compensated commutation is obtained by inserting a capacitor between the inverter transformer and the six pulse thyristorized Graetz bridge. This arrangement will provide a reduction in the impedance of the commutation circuit. This can permit grater efficiency of commutation, reducing the reactive power consumption of the converter and the consequent necessity of large shunt compensation banks. An improvement in the susceptibility of the converter to commutation failure is achieved, when the converter is operating in the inverter mode. In this paper, the valve commutation process as well as experimental and simulated results of current and voltage waveforms of the CCC, is studied and discussed.

Small-signal modeling of a single-phase DVR for voltage sag mitigation

Voltage sags are sudden voltage reduction that may be present among power systems. It persists from milliseconds to a minute of duration. Different causes may produce voltage sags such as lightning, presence of non-linear equipment connected to the power grid or presence of short circuit faults. Sensitive loads, like electronic equipments, may be damaged or present malfunctioning during the voltage sag, generating economical losses. A suitable solution to mitigate this voltage disturbance is the Dynamic Voltage Restorer (DVR). It is a power electronics-based solution that provides power compensation to sensitive loads during the sag occurrence. The DVR must operate with control loops, monitoring the load voltage and generate the compensation during the sag. Those control loops must be tuned with aid of a mathematical model, a performance criteria and a proper design procedure of the controller. This paper presents a small-signal modeling for a single-phase transformerless DVR. A small-signal equivalent circuit will be obtained and its analysis will produce the control-to-output transfer function. Using the control-to-output transfer function a PID voltage controller will be designed. This PID design methods will be described and the proper operation will be verified through computer simulation.

Small-signal model of a single-phase series active filter

Electrical power supplies or networks containing voltage harmonics cause harmful effects to the electrical equipments and thus economic losses to their users. Series active filters are an alternative to reduce or even eliminate these voltage distortions. They produce opposing voltages to the harmonic voltages from the power supply and therefore loads are fed with an almost sine wave signal. This paper presents a development of a small-signal model of a single-phase series active filter. The model takes in account parasitic elements of energy storage components and semiconductors devices of the series active filter. Due to this fact, that model makes possible an investigation of their influence on the dynamic behavior of the series active filter and on its controllers design. Simulation results are presented to validate the proposed small-signal model, control scheme and series active filter effectiveness.

Teaching voltage regulation using a TSC TYPE SVC

In this study, we present a laboratory scale to implement a Static VAr Compensation (SVC) solution. It was implemented by using a Thyristor Switched Capacitors (TSC) SVC, which was implemented with a binary weight capacitors banks. This equipment was designed to be used in power quality and in power electronics didactical laboratories, especially to train students from electrical engineering and from control engineering. We also exposed a SVC implemented application.

Analysis of synchronous and induction generators in parallel operation mode in an isolated electric system

This paper presents first an analysis of a parallel connection of one synchronous generator and one self-excited induction generator, each coupled to its dc machine as well as a capacitor bank with a resistive load connected to the system. In this stage, the voltage and frequency adjustments were done manually. In the second portion, a similar circuit was implemented considering a voltage loop control and a speed loop control to the synchronous generator. These controls kept on the system voltage and frequency stables. The induction generator supplies the part of active power required by load. The synchronous generator controls the frequency and supplies the reactive power and the additional active power. The experiment demonstrated the feasibility and stable operational performance for this alternative and creative parallel arrangement of generators.

Winding turns calculus methodology for a new 48 pulse multiconverter system employing lower cost three winding special transformers

The objective of this paper is to present a calculation methodology regarding a new 48 pulse multiconverter system (AC/DC converter). The paper proposes an implementing for application in the aluminum industry which requires very high currents (on the order of hundreds of [kA]). Due to the rectification process harmonic mitigation is necessary for reducing harmonic distortion and subsequent problems for other customers. Identical units of three-winding special transformers, with special extended delta primary connection, secondary star and tertiary delta will be presented and described. The proposed topology considers the feasibility of the application and for simplicity of maintenance, lower cost and physical space required. A special autotransformer is also presented in order to propitiate the implementation of the 48 pulse multiconverter. It is expected that in a future implementation, the prototype will present very good performance results.

A controlled dynamic braking of a separate excitation DC machine

This paper aims at demonstrating that a separate excitation for a DC machine can have its dynamic braking optimized via a significant reduction of its elapsed time. Dynamic braking is accomplished when a DC motor is led to operate as DC generator. Its kinetic energy is almost totally converted into electric energy. This energy is dissipated on a resistor connected to the machine armature. The machine field circuit is fed by a battery set via a chopper, produces an excitation current capable of making the e.m.f. constant, until the machine field current reaches its maximum value. After this, the e.m.f decreases, and therefore the braking current is also decreased. Thus, much more energy can be dissipated per unity of time in such an interval. In being so, the necessary time for braking is substantially lesser. Other variables such as braking current, and speed are also analyzed. It is possible to conclude that the machine and its load have not been subjected to any kind of abnormalities or even prohibitive efforts. An industrial application of the project is presented.

Power factor improvement of line-commutated graetz converters by increasing their number of pulses: modeling and experimental results

The alternating current (AC) of line commutated converters is nonsinusoidal, generating, therefore, harmonics in the electric system. Six-pulse graetz line-commutated converters are widely employed in industry. Twelve-pulse converter can be achieved by associating two six-pulse converters in series or in parallel. However it is necessary to use supply transformers. These transformers are connected in such a way that the secondary voltage phase angles are displaced thirty degrees in relation to each other. Lower order characteristic harmonics are eliminated in this case (5/spl deg/ , 7/spl deg/), rendering these transformers as adequate for the proposed application. A very good harmonic mitigation can be obtained when 24 or 48-pulse multiconverters are employed, by associating four or eight graetz six-pulse converters, respectively. When the converter pulse number is increased there will be also, a system power factor improvement. Experimental results are presented and discussed.