25-07-2014, 10:37 AM
Purpose and Aim of project
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. INTRODUCTION
1.1 Purpose and Aim of project
The increasing use of power electronic based loads (adjustable Speed drives, Switch mode power supplies, etc) to improve system efficiency and Controllability is increasing concern for harmonic distortion levels in end use facilities and on overall power system. The application of passive tuned filters creates new system resonances, which are dependent on specific system conditions. In addition, passive filters often need to be significantly overrated to account for possible harmonic absorption from power system. Passive filter ratings must be coordinated with reactive power requirements of the loads and it is often difficult to design the filter to avoid leading power factor Operation for some load conditions. A number of low-power electronic based appliances such as TV sets, personal computers, and adjustable speed heat pumps generate a large amount of harmonic current in power systems even though a single low power electronic based appliance, in which a single-phase diode rectifier with a dc link capacitor is used as utility interface, produces a negligible amount of harmonic current. Three-phase diode or thyristor rectifiers and cycloconverters for industry applications. Also generate a large amount of harmonic current. Voltage distortion or harmonics resulting from current harmonics produced by power electronic equipment has become a serious problem to be solved in many countries. Power system harmonics are not a new problem. Due to the widespread proliferation of nonlinear distorting loads such as power-electronic controlled devices, the problems caused by harmonics are of increasing importance. Unlike the conventional load, the power-electronic device controls the flow of power by chopping. Flattening, or shaping the waveforms of the voltage and current. Therefore, harmonics are generated during the process. These waveform distortions can cause problems for neighboring loads, and they tend to have an overall opposite effect on the quality of electric power. A concept that can improve the power quality is the active power filter. This type of filters can meet diverse load conditions. In addition to improve power factor, it also appears to be an attractive and viable method for reducing voltage and current harmonic distortion or other power quality problems such as flicker. The active power filter improves the system power quality by injecting equal-but opposite currents to compensate harmonic distortion and reactive power. Ideally, this active power filter should monitor and minimize voltage and went distortion of its connected load. In the past some active power filters were designed based on the conventional IRP theory. However, the IRP theory-based active filter can not compensate the harmonic distortion and does not function properly. In order to improve the drawbacks of the conventional IRP theory, a instantaneous active and reactive power theory-based the control strategy of the active power filter proposed. Also, for verifying the performance of this control strategy, computer simulations and experiment are made. From the simulation and experimental test results, it is found that proposed new instantaneous active and reactive current id –Iq theory-based three-phase shunt active power filter is to be an effective device to reduce harmonic current and THD.
The objective of the electric utility is to deliver sinusoidal voltage at fairly constant through out the system. The objective is complicated by the fact that there are loads on the system that produces harmonic currents.(here we are dealing with non-linear loads).
. POWER QUALITY
The power quality of power supply of an ideal power system means to supply electric energy with perfect sinusoidal waveform at a constant frequency of a specified voltage with least amount of disturbances.
Power quality is an issue that is becoming increasingly important to electricity consumers at all levels of usage. Sensitive equipment and non-linear loads are now more commonplace in both the industrial commercial sectors and the domestic environment. Because of this a heightened awareness of power quality is developing amongst electricity users. Occurrences affecting the electricity supply that were once considered acceptable by electricity companies and users are now often considered a problem to the users of everyday equipment.
How ever the harmonic is one of the major factor due to which none of condition is fulfilled in practice. The presence of harmonics, disturbs the waveform shape of voltage and current, and increases the current level and changes the power factor of supply and which in turn creates so many problems
1 Introduction:
In this part we introduces the commonly accepted definitions used in the field of power quality and discusses some of the most pertinent issues affecting end-users, equipment manufacturers and electricity suppliers relating to the field. This Special Feature contains a range of articles balanced to give the reader an overview of the current situation with representation from the electricity industry, monitoring equipment manufacturers, solution equipment manufacturers, specialist consultants and government research establishments. The term ‘power quality’ has come into the vocabulary of many industrial and commercial electricity end-users in recent years. Previously equipment was generally simpler and therefore more robust and insensitive to minor variations in supply voltage. Voltage fluctuations coming from the public supply network were therefore not even noticed. Now equipment is used which depends on a higher level of power quality and consumers expect disruption-free operation. Wide diversity of solutions to power quality problems is available to both the distribution network operator and the end-user. More sophisticated monitoring equipment is readily affordable to end-users, who empower themselves with information related to the level of power quality they receive. the following paragraphs introduce the definitions of power quality measurable quantities or occurrences
A voltage dip is a reduction in the RMS voltage in the range of 0.1 to 0.9 p.u. (retained) for duration greater than half a mains cycle and less than 1 minute. Often referred to as a ‘sag’. Caused by faults, increased load demand and transitional events such as large motor starting
A voltage swell is an increase in the RMS voltage in the range of 1.1 to 1.8 p.u. for a duration greater than half a mains cycle and less than 1 minute. Caused by system faults, load switching and capacitor switching.
A transient is an undesirable momentary deviation of the supply voltage or load current. Transients are generally classified into two categories: impulsive and oscillatory.
Introduction
Power electronic devices are non-linear loads that create harmonic distortion and can be susceptible to voltage dips if not adequately protected. The most common ‘economically damaging’ power quality problem encountered involves the use of variable-speed drives.
Variable-speed motor drives or inverters are highly susceptible to voltage dip disturbances and cause particular problems in industrial processes where loss of mechanical synchronism is an issue. The ideal solution to problems of this nature would be for systems engineers to specify equipment that has a ‘reasonable level’ of susceptibility to voltage clips from the outset.
Power electronics has two faces in power distribution: (a) that consists of controllable industrial and domestic equipment to match the appliance with the power supply and (b) that helps to solve those power quality problems created by the controllers. Modern semiconductor switching devices are being utilized more and more in a wide range of applications in distribution networks, particularly in domestic and industrial loads. Examples of such applications widely used are adjustable-speed motor drives, diode and thyristor rectifiers, uninterruptible power supplies (UPSs), computers and their peripherals, consumer electronics appliances (TV sets for example), among others. Those power electronics devices offer economical and reliable solutions to better manage and control the use of
Transformers
A transformer’s harmonic generating effect is based on the magnetic non-linearity of iron. The magnetic flux density does not grow linearly with increase of magnetic field strength. Due to the hysteresis of the transformer’s iron core the magnetic flux does not follow the curve form of supply voltage. Due to hysteresis of the transformer’s iron core the magnetization current does not follow the curve form of the supply voltage. Transformer connection, structure of the core and way of earthing has a remarkable effect on the generation and spreading of harmonics.
2.4 Adverse effects of harmonic currents
Excessive heating of conductor due to circulating harmonic currents through the system.
Over heating of transformer due to harmonic current resulting in insulation damage and failure.
Increase in losses and consequent heating of rotating machines, reduced life, frequent maintenance and repair of system equipments.
Intermittent electrical noise from connections loosened by thermal cycling.
Malfunctioning of protective relays due to distortion in voltage and current.
Interference with the adjacent communication network, thereby corrupting the message transmitted.
3.2 Shunt active power filters [3,4,5,9]
This class of filters constitutes the most important and widely used filter configuration in industrial process. It is connected in parallel to the main power circuit as shown in Fig.4.1. The concept of the shunt active power filter is based on harmonic cancellation through the act of injecting equal and opposite harmonic currents into the supply line by means of solid-state converter circuits. These filters are voltage source type or current source type as shown in Fig. 4.2, respectively. Normally these filters are connected in parallel with the load, and carry only a fraction of the fundamental current. Furthermore, they can be designed to provide compensation for all of the system non-linearties at the point of common coupling (PCC) under distorted and non-distorted supply.
3.4 Combination of series-shunt active power filters [8,9]
Fig.4.6 shows the line diagram of the combination of the shunt active and series active filter. The major functions of the series filter are to provide voltage harmonic isolation between the supply side and the load side, voltage regulation, voltage flicker and/or imbalance compensation at the point of common coupling (PCC). The main functions of the shunt active filter is to act as harmonic sink, however this may also be used to provide reactive power compensation and dc link voltage regulation between the filters. This combination is generally known as “Unified Power Flow Controller (UPFC)” or “Unified Power Quality Conditioner (UPQC)”. Active power filters can be used with passive filters improving compensation characteristics of the passive filter,
and avoiding the possibility of the generation of series or parallel resonance. One example of this combination is the series active power filter shown in section V. In this scheme, (Fig.4.5), if the passive filters are not connected, the series active power filter can compensate only voltage regulation, and voltage unbalance. If passive filters are not used in Fig. 4.5, the topology cannot compensate current harmonic components. Another possibility to combine the compensation characteristics of passive and active power filters is by connecting the active passive filter in series with the passive one, as shown in Fig. 4.5. In this way, the compensation characteristics of the passive filter is significantly improved, since the active scheme generated voltage harmonic components across the terminal of the primary windings of the series transformer, forcing current harmonics generated by the load to circulate through the passive filter instead of the power distribution system.
Fig (3.5) The hybrid active power filter configuration.
By controlling the amplitude of the voltage fundamental component across the coupling transformer, the power factor of the power distribution system can be adjusted. However, the control of the load power factor imposed a higher voltage across the filter capacitor. This consideration has to be considered when the filter capacitor is specified. This type of configuration is very convenient for compensation of high power medium voltage non linear loads, such as large power ac drives with cycloconverters or high power medium voltage rectifiers for application in electro wining process or for compensation of arc furnace. In all these applications passive filters do not have enough compensation capability to reduce current harmonics in order to satisfy IEEE Std.519. Simulated waveforms for this type of compensation are shown in Figs. 4.5
3.5 The following factors were considered for active filter topology selection [3]
o Ability to meet IEEE 519 harmonic standards, including for ‘stiff supply systems and in presence of maximum allowable 5% supply voltage distortion.
o Impact of voltage notching due to rectifier front-end at the active filter terminal on the performance and rating of the active filter.
o KVA rating and cost of active and passive filter components.
o Overall displacement factor from 125% to 105% load rating.
o Losses of active filter system, including switching ripple filter.
o Ability to provide value-added features such as harmonic isolation between supply and load, voltage regulation in presence of ±1O% supply voltage swells/sags, immunity to supply and load transients and ride-through capability.
o Ability for maximum integration with the ASD- i.e., capability of using the same ASD dc bus, one unit packaging to minimize cost, filter topology selection size, weight and floor space.
o Sensor requirement and control complexity.
o Affect of supply voltage distortion, supply voltage sags/swells and unbalances, and sup- ply side or ASD load induced sub harmonics in the supply current, on the performance and rating of the active filter to meet IEEE 519 harmonic standards Bandwidth and switching frequency of active filter inverter.
o Damping/mitigation of supply/load side resonances, including additional cost and bandwidth requirement for providing active damping by the active filter inverter.
o Start-up, sequencing and active filter system protection issues, including cost of isolation and protection switchgear.
3.6 Parallel active filter systems have the following advantages [3]
o Viable and cost-effective.for low to medium LVA industrial loads where system engineering effort is a large part of overall cost.
o Do not create displacement power factor problems and utility loading.
o Supply side inductance L, does not affect the harmonic compensation capability of parallel active filter system.
o Controlled as a harmonic current source and its controller implementation is simple.
o Can damp harmonic propagation in a distribution feeder or between two distribution feeders.
o Performance, controlled as a harmonic current source, is not affected by supply voltage harmonics.
o Provides immunity from ambient harmonic loads.
o Protection and sequencing is relatively easy and does not require expensive isolation switchgear.
o Has a high possibility of system integration with various harmonic front-ends with common functionalities.
o Can be installed as a ‘black box’ solution with minimal system level design expense and provides viable retrofit options. And Is scalable for higher load KVA rating by paralleling units.
Conclusion
This control method is applied for non – linear loads which eliminate high distorted current harmonics to allowable range.This is very much applicable even when the load on the system frequently changes. Thus it provides good results for dynamic load changes.
The harmonic currents, which need to be compensated, are directly calculated from the supply voltage thus this system is independent of the frequency. Hence it can be applied for any system irrespective of the frequency changes.
The Instantaneous Active and Reactive current component (Id - Iq) control method provides efficient compensation of current harmonics of supply current waveform and uses a simple control strategy.