Medium Voltage AC Research Articles

Estimation of the Fundamental Current in Low-Switching-Frequency High Dynamic Medium-Voltage Drives

The switching frequency of medium-voltage ac drives is limited to low values to restrain the dynamic losses of the power devices. This favors the use of synchronous optimal pulsewidth-modulation schemes that minimize the harmonic current. It is a drawback, though, that optimal algorithms do not have a means to extract the fundamental component of the load current. High-performance torque control is therefore difficult to obtain. This paper proposes a method to identify the instantaneous fundamental component of the stator currents. A novel observer is developed for this purpose. The approach enables fast torque control at very low switching frequency. Experimental results from a 30-kW induction motor drive are presented.

Fast Dynamic Control of Medium Voltage Drives Operating at Very Low Switching Frequency—An Overview

Medium voltage AC machines fed by high-power inverters operate at a low switching frequency to restrain the switching losses of power semiconductor devices. Particular care is thus required in the design of the drive control system. The signal delay caused by low switching frequency operation increases undesired cross-coupling effects in vector-controlled schemes. These are not sufficiently compensated by established methods like feedforward control. Improvements are achieved by a more accurate modeling of the machine and the inverter. An adequate controller is introduced, having a transfer function with complex coefficients. The high harmonic distortion due to the low switching frequency is a tradeoff. Using synchronous optimal pulsewidth as an alternative permits reducing the switching frequency without increasing the harmonics. The detrimental effects of conventional control methods are eliminated by forcing the harmonic components on an optimal spatial trajectory. Deadbeat behavior and complete decoupling are thus achieved. The performance of the aforementioned schemes is compared based on mathematical analyses and experimental results.

Research on a Novel Capacitor Clamped Multilevel Matrix Converter

A new multilevel matrix converter that can be applied to medium or high voltage ac drives is presented to alleviate harmonic components in the output voltage. The proposed converter contains six flying capacitors to balance the voltage distribution of series connection bidirectional switch modules and provide middle voltage levels. Stable flying capacitors voltage must be maintained to facilitate the operation of the converter. When the converter is working, the voltage of flying capacitors can be controlled by swapping two switching modes with opposite charging current corresponding to each middle voltage levels. A simple output voltage vector synthesis method is described and utilized. The operation and commutation strategies are discussed. Simulations and experiments are carried out to validate the proposed converter. Comparisons are made between proposed converter and conventional matrix converter.

Regenerative Medium-Voltage AC Drive Based on a Multicell Arrangement With Reduced Energy Storage Requirements

Multicell power circuit topologies have proved to be an effective alternative to medium-voltage ac drives. The main advantage is the improved power quality at both the ac system and the motor sides. However, several drawbacks are present in these configurations, such as a lack of sustained regenerative operating mode, uncontrolled input reactive power, and a large second current harmonic that is injected by the load into the dc link of each cell, which leads to a bulky electrolytic capacitor. This paper proposes to replace the input diode-based front-end rectifier with an active front-end rectifier in all cells of the topology and a novel control strategy in order to overcome the aforementioned drawbacks. In fact, the active front-end rectifier allows the topology to regenerate and the control strategy handles the reactive input power and reduces the large second current harmonic from the dc-link capacitor, thus reducing its size. These features are achieved without any penalties in the quality of both the ac input current and the motor voltage waveforms. Experimental results confirm the theoretical considerations.

AC Motors in the Recent Pulp & Paper Plants

There are a large number of and a wide range capacity of AC motors and AC drives applied in pulp and paper industries. Constant speed motors are used for pumps, fans and blowers. However, recently adjustable speed drives can be applied for medium voltage motors.This article describes the energy savings effect, introducing high efficiency motors and medium voltage inverters, for saving energy and operation cost reduction.It is also important to check the insulation condition of medium voltage AC motors to maintain the motor characteristics. The latest technology of dielectric diagnosis for the motor insulation is also described.

Statistical And Probabilistic Methods Applied To Dc Traction System Supply

Considering a DC traction system connected to the medium voltage AC supply network by means of Electric Supply Substations (ESSs) equipped with power static converters, it is worth in general to adopt statistical analysis for both interfaces to power distribution (AC supply and DC distribution). The structure of the system and the values of the parameters are variable (random variables), with a specific range of values and a probabilistic distribution. Attention is focused onto two problems: the power quality (low frequency conducted harmonics and resonance issues) of the AC supply interface and the DC distribution interface. Some results are obtained by means of statistical and probabilistic approaches. Software tools and code used to this aim are described.

Medium-voltage standard AC drives launched

Medium-voltage standard AC drives launched

A new approach to enhance power quality for medium voltage AC drives

A new approach to medium-voltage variable-frequency static AC motor drives offers improvements in power quality. Harmonic current injection into the power lines is below the most severe requirements of IEEE Standard 519-1992. The power factor of this new type of drive exceeds 94% at full load and is above 90% at 10% load. Motor voltage and current waveforms are improved so that torque pulsations are reduced. Peak voltage stress on motor insulation does not exceed peak input line voltage, and no zero sequence voltage is imposed. Drive efficiency exceeds 96%. This paper describes the new approach and some of the results achieved.

A multilevel thyristor rectifier with improved power factor

The thyristor bridge rectifier is a reliable and cost-effective topology commonly used in medium-voltage applications. Low and load-varying input displacement factor (IDF) and poor input harmonic current profile are the main drawbacks of this topology and are gaining more and more importance. This paper proposes modification of a conventional (18-pulse) thyristor rectifier to achieve higher power factor. The proposed topology employs one gate-turn-off (GTO) device across each upper bridge to short them out when necessary, hence, lowering the voltage level, decreasing the delay angle, and improving the power factor. In medium voltage (4160-V) AC drives, usually three devices are connected in series to withstand the voltage; therefore, the 18-pulse configuration has the same number of SCRs as compared to a six-pulse bridge. This paper includes analysis, simulation, and a comparison of the proposed topology with the conventional 6-, 12-, and 18-pulse thyristor bridges. Experimental results from a 10-kVA 12-pulse system verify the feasibility of the proposed topology.

Interaction of helical arc devices with interconnected networks

Rotary arcs driven electromagnetically by a fault or load current are used for interrupting duties at distribution level and as preheaters for high temperature chemical reactors. One such form of rotary arc device is the helical arc unit. The paper investigates the electrical interaction between repeated arc short circuiting and the connected network or power supply. The results are relevant to medium and high voltage AC interrupters and chemical processing preheaters.

Vacuum-fused circuit breakers for low and medium voltage AC trailing cable protection

Since the enactment of the Federal Coal Mine Safety Act of 1969, the use of fuses has not been allowed for protection for three phase AC trailing cables in underground coal mines. In 1981 a mining equipment manufacturer approached the Mine Safety and Health Administration with a combination fuse-contactor device requesting that it be permitted as a means of protection for three phase AC trailing cables of 1000 volts or less. The concept behind the design of the vacuum contactor-fuse device was to use a vacuum contactor to provide protection to a specific current level during abnormal circuit conditions. If fault currents exceeded this level, a current limiting fuse would be used to interrupt the current before the vacuum contactor could open. The Mine Electrical Systems Division located in the Mine Safety and Health Administration's Pittsburgh Safety and Health Technology Center was called upon to make an evaluation of this device. This paper details the tests and criteria used to ensure that the device provided no less protection than a molded case circuit breaker. Details of the laboratory tests conducted and the results of extensive field tests are presented. >

DC breaking tests up to 55 kA in a single vacuum interrupter

DC current breaking tests were carried out on several vacuum circuit-breakers during the design of the energy transfer units for the RFX fusion experiment. The tests were performed with a synthetic circuit able to reproduce the expected stresses on the switching element during the breaking phase. The performance of a prototype for a medium-voltage AC switch with a self-produced axial magnetic field is discussed. The interrupter was found capable of breaking currents up to 55 kA with a recovery voltage greater than 35 kV and high values of di/dt in a large range of arcing times. >

Electrical Power Distribution System for Clinch River Breeder Reactor Plant

This paper describes the auxiliary electric power system for the Clinch River Breeder Reactor Plant (CRBRP). The power systems covered are the utility grid interconnections, plant power supply systems medium and low voltage AC systems, 125/250V DC system, 120V vital AC system, and safety related AC distribution system. The paper also describes the mode of connection of non-Class 1E loads to Class 1E power systems, and a cross-connect feature between Division 1 and Division 2 Class 1E AC Power Systems.