INVESTIGATION OF CAPACITANCE IN A DIRECT CURRENT

Description

CHAPTER 1

INTRODUCTION  

• Background to the study

Induction motors are widely used in industrial power plants due to their robustness, reliability and high performance under variable operating conditions in the electrical power system. Modern industrial progress is dependent on these ruggedly constructed induction motors. Almost every sophisticated process of the industry is based on induction motors. Most of these motors are controlled by means of inverters that change the line frequency. The change in parameters of inverter makes it possible to control the motor according to the design requirements. The reliability of inverter based motor control is an important issue for industrial applications and therefore, it becomes very vital for design engineers to have comprehensive analysis of the inverter fed induction machine. This paper investigates one of the faults that may occur on the DC link of an inverter fed induction motor.

The effect of the capacitor short circuit is presented in this paper. It also deals with the effects of short circuited capacitor on freewheeling diode. DC link capacitors are well designed and even the probability of capacitor failure is high, it is always a rare case if they puncture, however this analysis will add to the reliability of the induction machine under variable operating condition.

Currently our planet faces huge energy challenges from the scarcity of fossil fuels and the simultaneous greenhouse effects. Burning of fossil fuels with large anthropogenic CO2 emissions outpaces nature’s recycling capability, resulting in significant environmental harm, such as global warming and oceans acidification. Thus, energy insecurity, rising prices of fossil fuels and climate changes threat remarkably our health and political stability. Consequently, we are observing an increase in renewable energy technologies from solar, wind as well as the advent of hybrid electric vehicles with low emissions. However, due to the intermittent character of renewable, reliable electrical energy storage systems are required for adapting these technologies to the demand in electricity.

Among the various realistic solutions, energy can be stored electrochemically in accumulators (batteries) and supercapacitors. Although batteries currently present much higher energy density, their relatively low power density and poor life cycle hinders the high power demanding applications such as regenerative braking and load leveling systems. By contrast, supercapacitors store larger amounts of energy than the traditional dielectric capacitors and provide energy far faster than batteries.15-20 Therefore, they are particularly adapted for applications requiring energy pulses in short periods of time, e.g., seconds or tens of seconds.

Such exceptional properties arise from the Nano metric scale capacitors formed by the polarized electrode material and a layer of attracted ions from the electrolyte on its surface. The thickness of the electrode-electrolyte interface is directly controlled by the size of ions. Recently, supercapacitors have been proposed and widely marketed for various applications.

Coupled, for example, with a battery/internal combustion engine in hybrid vehicles, supercapacitors improve the battery lifetime/fuel economy and the energy recovery efficiency in braking. They can also stabilize current when intermittent renewable energies are introduced in the energetic mix. Although supercapacitors are now commercially available, they still require improvements, especially for enhancing their energy density and cut the cost at the same time. It requires a fundamental understanding of their properties and exact operating principles, in addition to improving electrode materials, electrolytes and integration in systems.

All these aspects led to a very strong motivation for academics and industry. The energy density of supercapacitors can be enhanced by increasing the voltage and/or the capacitance. To attain these objectives, various strategies have been proposed in the literature, involving the development of new materials, new geometries and new electrolytes However, the performance and the cost of the supercapacitor device are the main parameters which guide the choices of industry. For these reasons, most of the supercapacitors presently available on the market are based on activated carbon electrodes and an organic electrolyte. The main advantages of activated carbon are related to its high versatility of structure/texture, low cost and high surface area. More important, its high electrical conductivity allows the realization of high power systems without requiring complicated design, and consequently expensive, electrode materials. Although generally unfriendly, due to the use of acetonitrile as solvent, the organic electrolytes are preferred by industry to the aqueous ones because of their high stability window, e.g., 2.7 – 2.8 V, allowing high energy density to be reached.

• Statement of the Problem

Ripples in DC link of an inverter are smoothed by inserting a capacitor between the rectifier and inverter. In this paper, two cases are analyzed that are related to short circuit fault of the DC link capacitor.

(1) The effect of short circuiting DC link capacitor on motor performance.

(2) Effect of back e.m.f. on freewheeling diodes of inverter.

Former case, being an analysis of system on DC link capacitor short-circuiting, is simpler. However the latter case is interesting and requires detailed explanation presented below.

For inductive load, it is mandatory to control stress on an inverter.

Diodes and MOSFETs connected in the anti-parallel direction are used to counter this stress. Freewheeling diodes allow the current to flow in the same direction when the motor inductance changes its polarity. Under normal conditions, state of switches and generation of back e.m.f control the amount of current drawn by the motor. Under circumstances when back e.m.f exceeds input voltage, the motor will start acting as a generator. If the motor is connected to an inverter, there can be several reasons for the termination of voltage.

For example:

_ Inverter output terminal open circuit.

_ Shut down of input supply.

_ DC link capacitor short circuit.

Inverter output terminal open circuit may occur due to loose connections but it only hampers normal operation and the motor slows down to zero speed. Shutdown of input supply obviously makes output voltage zero. For a short duration freewheeling diode conducts and the motor stop eventually depend upon load inertia. While the last case of DC link capacitor short circuit is quite interesting in this case, the output of inverter drops to zero thus making back e.m.f greater than applied voltage and induction motor starts behaving like an induction generator. The point of interest is short circuit occurring at terminals of motor via faulty DC link capacitor which draws current from motor. This short circuit current passes through a switch or a freewheeling diode depending upon the direction of flow of current. In this paper, we would present an analysis of capacitor short circuit on the freewheeling diode.

• Aim of the Study

The aim of this study is to investigate the effect of capacitance on a direct current (D.C) supply.