21-09-2012, 11:17 AM
Power Quality (PQ)
Power Quality_REPORT_JP.doc (Size: 534 KB / Downloads: 33)
ABSTRACT
The term PQ means to maintain purely sinusoidal current wave form in phase with a purely sinusoidal voltage wave form. The power generated at the generating station is purely sinusoidal in nature. The deteriorating quality of electric power is mainly because of current and voltage harmonics due to wide spread application of static power electronics converters, zero and negative sequence components originated by the use of single phase and unbalanced loads, reactive power, voltage sag, voltage swell,flicker, voltage interruption etc. To improve the power quality traditional compensation methods such as passive filters, synchronous capacitors, phase advancers, etc. were employed. However traditional controllers include many disadvantages such as fixed compensation, bulkiness, electromagnetic interference, possible resonance etc.. These disadvantages urged power system and power electronic engineers to develop adjustable and dynamic solutions using custom power devices.
INTRODUCTION
Since last 25 years there has been an increase in the use of solid state electronic technology. This new, highly efficient, electronic technology provides product quality with increased productivity. Today, we are able to produce products at costs less than in the years passed, with the introduction of automation by using the solid state electronic technology .This new technology requires clean and clear electric power.
Hence Power Quality (PQ) is an important measure of an electrical power system. The term PQ means to maintain purely sinusoidal current wave form in phase with a purely sinusoidal voltage wave form.
WHAT IS POWER QUALITY and WHY IS IT IMPORTANT?
The term power quality means different things to different people. Power quality is the interaction of electronic equipment within the electrical environment. This consists of generators, Transformers, breakers, wiring and grounding.
Adequate to superior power quality is essential for the smooth functioning of critical industrial processes. As industries expand, utilities become more interconnected and usage of electronically controlled equipment increases, power quality is jeopardized. Most large industrial and commercial sites are served by overhead lines with feeders that are subject to unpredictable and sporadic events, e.g. lightning and contact with tree limbs.
This minimizes the possibility of long-term outages but leads to a number of minor power disturbances. But it often means the overhead distribution circuit is longer, with more exposure to disturbances. And disturbances travel farther because of lower system impedances associated with higher voltage circuits. Sophisticated new systems are providing vastly increased efficiency and control in critical processes. But with their high sensitivity even to brief variations in electric power quality, today's computer-driven devices fail when power is disturbed for even a few milliseconds.
Harmonic distortion:
Power system harmonics are integer multiples of the fundamental power system frequency. Power system harmonics are created by non-linear devices connected to the power system. High levels of power system harmonics can create voltage distortion and power quality problems.
Harmonics in power systems result in increased heating in the equipment and conductors, misfiring in variable speed drives, and torque pulsations in motors.
A pure sinusoidal voltage is conceptual quantity produced by an ideal AC generator build with finely distributed stator and field windings that operate in a uniform magnetic field. Since neither the winding distribution nor the magnetic field is uniform in a working AC machine, voltage waveform distortion is created, and the voltage time relation-ship deviates from the pure sine function. The distortion at the point of generation is very small (about 1%to 2%), but nonetheless it exists.
HARMONIC FILTERS
In some cases, reactors alone will not be capable of reducing the harmonic current distortion to the desired levels. In these cases, a more sophisticated filter will be required. The common choices include shunt connected, tuned harmonic filters (harmonic traps) and series connected low pass filters (broad band suppressors). Filter is a device that removes something from whatever passes through it.
They consist of a capacitor and an inductor which are tuned to a single harmonic frequency. Since they offer very low impedance to that frequency, the specific (tuned) harmonic current is supplied to the drive by the filter rather than from the power source. If tuned harmonic filters (traps) are selected as the mitigation technique, then multiple tuned filters are needed to meet the distortion limits which are imposed.
By-pass switch:
The static switch is a versatile device for switching a new element into the circuit when the voltage support is needed. It has a dynamic response time of about one cycle. To correct quickly for voltage spikes, sags or interruptions, the static switch can used to switch one or more of devices such as capacitor, filter, alternate power line, energy storage systems etc.
Since the DVR is a series connected device, any fault current that occurs due to a fault in the downstream will flow through the inverter circuit. The power electronic components in the inverter circuit are normally rated to the load current as they are expensive to be over rated. Therefore to protect the inverter from high currents, a by-pass switch (crowbar circuit) is incorporated to by-pass the inverter. Basically the crowbar circuit senses the current flowing in the distribution circuit and if it is beyond the inverter current rating the circuit bypasses the DVR circuit components (DC Source, inverter and the filter) thus eliminating high currents flowing through the inverter side. When the supply current is in normal condition the crowbar circuit will become inactive.
Injection Transformer:
The basic function of the injection transformer is to increase the voltage supplied by the filtered VSI output to the desired level while isolating the DVR circuit from the distribution network. The transformer winding ratio is pre-determined according to the voltage required in the secondary side of the transformer (generally this is kept equal to the supply voltage to allow the DVR to compensate for full voltage sag) .A higher transformer winding ratio will increase the primary side current, which will adversely affect the performance of the power electronic devices connected in the VSI. The rating of the injection transformer is an important factor when deciding the DVR performance, since it limits the maximum compensation ability of the DVR.
Conclusion
It is atmost important that the center’s objective must be towards work with industry to improve the quality and reliability of the electricity supply to industrial, commercial and domestic users.
For best overall results when using reactors or harmonic filters, it must be ensured to install them as close as possible to the non-linear loads which they are filtering. When minimizing harmonics directly at their source, will be cleaning up the internal facility mains wiring. This will also reduce the burden on upstream electrical equipment such as circuit breakers, fuses, disconnect switches, conductors and transformers. The proper application of harmonic filtering techniques can extend equipment life and will often improve equipment reliability and facility productivity.
Power Quality_REPORT_JP.doc (Size: 534 KB / Downloads: 33)
ABSTRACT
The term PQ means to maintain purely sinusoidal current wave form in phase with a purely sinusoidal voltage wave form. The power generated at the generating station is purely sinusoidal in nature. The deteriorating quality of electric power is mainly because of current and voltage harmonics due to wide spread application of static power electronics converters, zero and negative sequence components originated by the use of single phase and unbalanced loads, reactive power, voltage sag, voltage swell,flicker, voltage interruption etc. To improve the power quality traditional compensation methods such as passive filters, synchronous capacitors, phase advancers, etc. were employed. However traditional controllers include many disadvantages such as fixed compensation, bulkiness, electromagnetic interference, possible resonance etc.. These disadvantages urged power system and power electronic engineers to develop adjustable and dynamic solutions using custom power devices.
INTRODUCTION
Since last 25 years there has been an increase in the use of solid state electronic technology. This new, highly efficient, electronic technology provides product quality with increased productivity. Today, we are able to produce products at costs less than in the years passed, with the introduction of automation by using the solid state electronic technology .This new technology requires clean and clear electric power.
Hence Power Quality (PQ) is an important measure of an electrical power system. The term PQ means to maintain purely sinusoidal current wave form in phase with a purely sinusoidal voltage wave form.
WHAT IS POWER QUALITY and WHY IS IT IMPORTANT?
The term power quality means different things to different people. Power quality is the interaction of electronic equipment within the electrical environment. This consists of generators, Transformers, breakers, wiring and grounding.
Adequate to superior power quality is essential for the smooth functioning of critical industrial processes. As industries expand, utilities become more interconnected and usage of electronically controlled equipment increases, power quality is jeopardized. Most large industrial and commercial sites are served by overhead lines with feeders that are subject to unpredictable and sporadic events, e.g. lightning and contact with tree limbs.
This minimizes the possibility of long-term outages but leads to a number of minor power disturbances. But it often means the overhead distribution circuit is longer, with more exposure to disturbances. And disturbances travel farther because of lower system impedances associated with higher voltage circuits. Sophisticated new systems are providing vastly increased efficiency and control in critical processes. But with their high sensitivity even to brief variations in electric power quality, today's computer-driven devices fail when power is disturbed for even a few milliseconds.
Harmonic distortion:
Power system harmonics are integer multiples of the fundamental power system frequency. Power system harmonics are created by non-linear devices connected to the power system. High levels of power system harmonics can create voltage distortion and power quality problems.
Harmonics in power systems result in increased heating in the equipment and conductors, misfiring in variable speed drives, and torque pulsations in motors.
A pure sinusoidal voltage is conceptual quantity produced by an ideal AC generator build with finely distributed stator and field windings that operate in a uniform magnetic field. Since neither the winding distribution nor the magnetic field is uniform in a working AC machine, voltage waveform distortion is created, and the voltage time relation-ship deviates from the pure sine function. The distortion at the point of generation is very small (about 1%to 2%), but nonetheless it exists.
HARMONIC FILTERS
In some cases, reactors alone will not be capable of reducing the harmonic current distortion to the desired levels. In these cases, a more sophisticated filter will be required. The common choices include shunt connected, tuned harmonic filters (harmonic traps) and series connected low pass filters (broad band suppressors). Filter is a device that removes something from whatever passes through it.
They consist of a capacitor and an inductor which are tuned to a single harmonic frequency. Since they offer very low impedance to that frequency, the specific (tuned) harmonic current is supplied to the drive by the filter rather than from the power source. If tuned harmonic filters (traps) are selected as the mitigation technique, then multiple tuned filters are needed to meet the distortion limits which are imposed.
By-pass switch:
The static switch is a versatile device for switching a new element into the circuit when the voltage support is needed. It has a dynamic response time of about one cycle. To correct quickly for voltage spikes, sags or interruptions, the static switch can used to switch one or more of devices such as capacitor, filter, alternate power line, energy storage systems etc.
Since the DVR is a series connected device, any fault current that occurs due to a fault in the downstream will flow through the inverter circuit. The power electronic components in the inverter circuit are normally rated to the load current as they are expensive to be over rated. Therefore to protect the inverter from high currents, a by-pass switch (crowbar circuit) is incorporated to by-pass the inverter. Basically the crowbar circuit senses the current flowing in the distribution circuit and if it is beyond the inverter current rating the circuit bypasses the DVR circuit components (DC Source, inverter and the filter) thus eliminating high currents flowing through the inverter side. When the supply current is in normal condition the crowbar circuit will become inactive.
Injection Transformer:
The basic function of the injection transformer is to increase the voltage supplied by the filtered VSI output to the desired level while isolating the DVR circuit from the distribution network. The transformer winding ratio is pre-determined according to the voltage required in the secondary side of the transformer (generally this is kept equal to the supply voltage to allow the DVR to compensate for full voltage sag) .A higher transformer winding ratio will increase the primary side current, which will adversely affect the performance of the power electronic devices connected in the VSI. The rating of the injection transformer is an important factor when deciding the DVR performance, since it limits the maximum compensation ability of the DVR.
Conclusion
It is atmost important that the center’s objective must be towards work with industry to improve the quality and reliability of the electricity supply to industrial, commercial and domestic users.
For best overall results when using reactors or harmonic filters, it must be ensured to install them as close as possible to the non-linear loads which they are filtering. When minimizing harmonics directly at their source, will be cleaning up the internal facility mains wiring. This will also reduce the burden on upstream electrical equipment such as circuit breakers, fuses, disconnect switches, conductors and transformers. The proper application of harmonic filtering techniques can extend equipment life and will often improve equipment reliability and facility productivity.