12-07-2012, 12:28 PM
POWER QUALITY IMPROVEMENT IN CONVENTIONAL ELECTRONIC LOAD CONTROLLER FOR AN ISOLATED POWER GENERATION
POWER QUALITY IMPROVEMENT .doc (Size: 1.9 MB / Downloads: 175)
ABSTRACT
This paper deals with the power quality improvement in a conventional electronic load controller (ELC) used for isolated Pico-hydropower generation based on an asynchronous generator (AG). The conventional ELC is based on a six-pulse uncontrolled Diode bridge rectifier with a chopper and an auxiliary load. It causes harmonic currents injection resulting distortion in the current and terminal voltage of the generator. The proposed ELC employs a 24-pulse rectifier with 14 diodes and a chopper. A polygon Wound autotransformer with reduced kilovolts ampere rating For 24-pulse ac–dc converter is designed and developed for harmonic Current reduction to meet the power quality requirements As prescribed by IEEE standard-519. The comparative study of
Two topologies, conventional ELC (six-pulse bridge-rectifier-based ELC) and proposed ELC (24-pulse bridge-rectifier-based ELC) is carried out in MATLAB using SIMULINK and Power System Block set toolboxes. Experimental validation is carried out for both ELCs for regulating the voltage and frequency of an isolated AG driven by uncontrolled pico-hydroturbine.
INTRODUCTION
THE SOARING use of fossil fuels and their depletion over the last two decades combined with a growing concern about pollution of environment have led to a boost for renewable energy generation. This accelerated drive has led to a tremendous progress in the field of renewable energy systems during last decade. It has also resulted in a gradual tapping of the vast mini (100 kW to 1 MW), micro (10–100 kW), and picohydro (less than 10 kW) and wind energy potential available in isolated locations (where grid supply is not accessible). In most of the cases, these generating units have to operate at remote unattended site; therefore, maintenance-free system is desirable. In view of this, the isolated asynchronous generator (IAG) with a simple controller for regulating the voltage and frequency is most prominent option for such applications.
A number of research publications are available on voltage and frequency controllers for an IAG driven by uncontrolled pico-hydroturbine for single-phase as well three phase power applications. Most of these proposed controllers are reported as electronic load controllers (ELCs) that maintain the constant power at the generator terminal, to regulate constant voltage and frequency. The value of excitation capacitor is selected to generate the rated voltage at desired power. The basic principle of controlling the constant power at the generator terminal is to employ an ELC and operate it in a way so that the total power (absorbed by the load controller and consumer load) is constant. If there is less demand by the consumer, the balance of generated power is absorbed by the ELC. The energy consumed by the ELC may be utilized for useful work like water heating, space heating, cooking, battery charging, and baking, etc.
Various types of ELCs based on controlled (thyristorized) or uncontrolled six-pulse rectifiers with a chopper and an auxiliary load are reported in the literature. These controllers provide effective control but at the cost of distorted voltage and current at the generator terminals, which, in turn, derate the machine. Moreover, the harmonic current injection at generator terminal is not within the prescribed limits by IEEE standards as (6n 1) dominant harmonics are present in such system. These harmonics cause additional losses in the system, resonance, and failure of the capacitor bank. In a phase-controlled thyristor-based ELC, the phase angle of back-to-back-connected thyristors is delayed from 0◦ to 180◦ as the consumer load is changed from zero to full load. Due to a delay in firing angle, it demands additional reactive power loading and injects harmonics in the system. In the controlled bridge rectifier type of ELC, a firing angle is changed from 0◦ to 180◦ for single phase and 0◦ to 120◦ for three phases to cover the full range of consumer load from 0% to 100%. In this scheme, six thyristors and their driving circuits are required, and hence, it is complicated, injects harmonics, and demands additional reactive power. Some of ELCs have been proposed that are having quality of the active filter and employs pulse width modulation (PWM) voltage source converter along with the chopper and auxiliary load at dc link to eliminate the harmonics and provide the functions of voltage and frequency regulation. However, such types of controllers make the system costly and complex with complicated control algorithm and simplicity requirement by the isolated system is lost.
Therefore, in this paper, a simple ELC is proposed that regulates the voltage and frequency without any harmonic distortion at the generator terminals. The proposed controller consists of a 24-pulse rectifier, a chopper, and an auxiliary load. In place of six-pulse rectifier, a 24-pulse rectifier-based ELC has negligible harmonic distortion in the generated voltage and current. A comparative study based on simulation is presented and it is also verified experimentally for both types of ELCs.
POWER QUALITY
The contemporary container crane industry, like many other industry segments, is often enamored by the bells and whistles, colorful diagnostic displays, high speed performance, and levels of automation that can be achieved. Although these features and their indirectly related computer based enhancements are key issues to an efficient terminal operation, we must not forget the foundation upon which we are building. Power quality is the mortar which bonds the
foundation blocks. Power quality also affects terminal operating economics, crane reliability, our environment, and initial investment in power distribution systems to support new crane installations. To quote the utility company newsletter which accompanied the last monthly issue of my home utility billing: ‘Using electricity wisely is a good environmental and business practice which saves you money, reduces emissions from generating plants, and conserves our
natural resources.’ As we are all aware, container crane performance requirements continue to increase at an astounding rate. Next generation container cranes, already in the bidding process, will require average power demands of 1500 to 2000 kW – almost double the total average
demand three years ago. The rapid increase in power demand levels, an increase in container crane population, SCR converter crane drive retrofits and the large AC and DC drives needed to power and control these cranes will increase awareness of the power quality issue in the very near future.
POWER QUALITY PROBLEMS
For the purpose of this article, we shall define power quality problems as:
‘Any power problem that results in failure or misoperation of customer equipment, Manifests itself as an economic burden to the user, or produces negative impacts on The environment.’ When applied to the container crane industry, the power issues which degrade power quality include:
• Power Factor
• Harmonic Distortion
• Voltage Transients
• Voltage Sags or Dips
• Voltage Swells
The AC and DC variable speed drives utilized on board container cranes are significant contributors to total harmonic current and voltage distortion. Whereas SCR phase control creates the desirable average power factor, DC SCR drives operate at less than this. In addition, line notching occurs when SCR’s commutate, creating transient peak recovery voltages that can be 3 to 4 times the nominal line voltage depending upon the system impedance and the size of the drives. The frequency and severity of these power system disturbances varies with the speed of the drive. Harmonic current injection by AC and DC drives will be highest when the drives are operating at slow speeds.