VIEWS: 13 PAGES: 10 POSTED ON: 7/21/2012
www.Fullinterview.com POWER QUALITY ABSTRACT The power quality is a term used to broadly encompass the entire scope of interaction among electrical suppliers, the environment, the system and products energized, and the uses of those systems and products. It is more than the delivery of “clean” electric power that compile with industry standards. It involves the maintainability of that power, the design, the selection, and the installation every piece of hardware and software in the electrical energy system. Stretching from the generation plant to the utility customer, power quality is a measure of how the elements affect system as a whole. www.Fullinterview.com This paper presents an overview of electric power quality with special emphasis on power quality problems, its adverse impacts on utilities and customers and the mitigation techniques. The wide spread usage of power electronic loads aimed at enhancement of energy efficiency and productivity has resulted in serious power quality problems such as voltage distortion due to current harmonics, flicker, voltage sag, voltage surges etc., which call for assessment and solution techniques. Here, we also discuss about two major power quality issues –grounding and harmonics and some power system components, which correct the harmonics problems. This paper broadly describes the above features along with the means for improvement of power quality. INTRODUCTION TO POWER QUALITY The term ‘power quality’ means different things to different people. One definition is the relative frequency and severity of deviations in the incoming power supplied to electrical equipment from the customary, steady, 50Hz sinusoidal waveform of voltage or current. These deviations may affect the safe or reliable operation of equipment such as computers and electronic instruments. It also refers to the delivery of high grade of electric service maintaining a sinusoidal load, bus voltage and current at stipulated magnitude and frequency. www.Fullinterview.com As we connect electronic devices to our power system, the “quality” after power is more important. Quality can be defined in many ways. Stable voltages and undistorted waveforms are two characteristics, which are very desirable in power systems. Thus while not having a strict basis of measurement, terms like” Poor Power Quality” generally mean there is sufficient deviation from norms in the power supply to cause equipment mis-operation or pre-mature failure. CAUSES OF POOR POWER QUALITY The causes of poor quality can be attributed to : Variations in voltage, magnitude and frequency Variations in magnitude can be due to sudden rise or fall of load, outages, repetitive varying loading pattern in rolling mills, power electronic converters, lightning etc. Variations in frequency can rise of out of system dynamics or harmonics injection. Consequently the voltage or current waveforms of a power system ceases to be purely sinusoidal in nature but consist of harmonics and other noises. IMPACT OF POOR POWER QUALITY: The effect of these aforesaid poor power quality problems has serious implication on the utilities and customers. Utility side impacts higher losses in transformers, cables etc. In conductors the neutral wires can burn due to the presence of third harmonics generated by non-linear loads. The power factor correction capacitors may puncture due to resonant conditions at resonant frequencies near lower order harmonics. The energy-meters, which are calibrated to operate under pure sinusoidal conditions, may give erroneous readings. The solid-state protective relays can maloperate due to poor power quality. There can be increased losses in cables, transformers and conductors. The customer side of the power network also experience adverse effects of poor power quality. The automatic processes employing adjustable speed drives may shut down because of nuisance tripping due to even short voltage sags.. The induction synchronous motors can have increased copper and core loses, pulsating torques and overheating with derating effect. The non-sinusoidal power supply thus reduces torque and efficiency of the motors. The computers and telecommunication equipment encounter loss of data and maloperation due to www.Fullinterview.com poor power supply quality. The domestic electronic gadgets such as digital clocks, VCRs and TVs are also affected by voltage distortions. SYMPTOMS OF POWER QUALITY PROBLEMS Electronic controlled systems that stop unexpectedly. Many systems reboots required. Abnormal failure rate of electronic systems. Transformers over heating. Motors failing. PF capacitors failing. Test results unreliable. TYPICAL POWER QUALITY PROBLEMS & THEIR SOURCES 1. Sags: A reduction in r.m.s voltage or current at the power frequency for duration of 0.5 cycles to 1 minute. Also called voltage dip. Events below the equipment: ride through capability cause load dropout. Voltage sags are originated in lighting strikes, short circuits and sudden overloads. Sags are under voltages on the power system and commonly caused by power failures, down lines, utility recloser operations and storms. They can be corrected by using backup power source such as UPSs, generators or similar voltage restoration technologies. 2. Surges: Voltage variations are another common source of problems to home computers and other sensitive electronic equipment. Voltage variations can be positive (higher than normal) or negative (lower). Positive voltage variations can be even more troubling than negative ones. If powerful enough they can destroy components in sensitive electronic equipment. Lighting striking power lines is a frequent cause, as is load switching (re-routing power around the grid), by utility. Voltage surges can also be caused by equipment in our home Refrigerator motors, air conditioners, vacuum cleaners and other electrical loads can generate voltage surges and electrical noise. www.Fullinterview.com Voltage Sag Voltage Swell 3. Transients: The main difficulty with transients is in detection, since they manifest only as a short duration change in voltage. The switching on and off of the electric motors that power air conditioners, power tools, furnace ignitions, electrostatic copiers, arc welders and elevators causes low energy swells. Lighting usually causes larger swells. Electrical noise is another, milder transient power irregularity that often manifests as a computer glitch rather than an equipment failure. Essentially, electrical noise is created when one piece of equipment interacts negative with another, or with building grounding or wiring. Loose connections or the equipment itself can be responsible for noise. Known noise-generating equipment includes everything from computers, radios and fluorescent lights to fax machines, welders and light sockets. 4. Voltage Fluctuations: Flickering lights can be an indication of voltage fluctuations in your building’s or facilities electrical system. Left unchecked, high and low-voltage conditions can result in equipment damage, data loss and erroneous readings on monitoring systems. Overloaded power circuits are typically the cause behind under voltage conditions. Heavily loaded motors such as air conditioners can result in intermittent low voltages. Less common but more damaging, facilities with rapidly varying loads can cause over voltage conditions. 5. Two modern power quality issues – Harmonics & Grounding Voltage Surge Harmonic Penetration Harmonics: A sinusoidal component of a periodic wave of quality having a frequency that is an integral multiple of the fundamental frequency. It is a mathematical model, which is used to analyse distorted waveforms and the current drawn by computers, electronic ballasts; variable frequency drives and other equipment, which have modern “transformer-less” power supplies. www.Fullinterview.com The dynamic power system loads produce a time varying amplitude in current waveforms depending on the load characteristics which consists of the fundamental and harmonics components. These harmonic components distort the voltage or current waveforms thereby deteriorating the power quality. The non-linear loads such as inverter fed adjustable speed drives. UPS (uninterrupted power supply system), rectifiers and furnaces, cyclo-converters etc., which form the major chunk of industrial loads, contribute to the severe fluctuations in power quality The industrial load also consist of large percentage of power factor improvement capacitors which often create resonance conditions at particular harmonic frequencies generated by non- linear loads fed from the load bus, producing high oscillating currents at resonant frequency and there by induces harmonic voltages distorting the pure sinusoidal voltage waveform.. For assessing power quality it is important to know the total harmonic distortion i.e. the voltage and current distortion factors V THD = VK & I THD = IK V1 I1 Vk = Voltage of Kth harmonic, Ik = Current of Kth harmonic Where V1 and I1 are the r.m.s values of fundamental components of voltage and current waveforms. The power quality deteriorates if the source has significant impedance causing the distortion of voltage of the load bus supplying combination of linear and non-linear loads. Harmonics problems often can be corrected by filtering or resizing power system components like: Harmonic Filters Filters are sometimes most cost effective in an existing structure where rewiring is difficult or costly. The filters are used to block or trap the offending currents, lessening the harmonic loads on the wiring. But the filter design is dependent on the equipment on which it is installed, and may be ineffective if the particular piece of equipment is changed. Filtering characteristics need to be carefully designed for a given installation, and seeking professional design advice is recommended. Filters are also fairly expensive on a per-kVA basis. Shielded Isolation Transformers www.Fullinterview.com Shielded isolation transformers are filtering devices that lessen feed-through of harmonic frequencies from the source or the load. They are a plausible retrofit technique where power problems have already been encountered, but are also quite expensive per-kVA. K-Rated Transformers K-rated transformers have beefed-up conductors and sometimes cooling to safely handle harmonic loads. Alternatively, standard transformers are sometimes de-rated to allow for the extra heating due to harmonics. Depending on the conditions encountered, a load limit of as little as 50% of the nameplate rating is observed. This may be adequate to handle harmonics, but lowers effective transformer efficiency. A careful comparison of the relative costs of K-rated vs. de-rated standard transformers should be made. Harmonic-Rated Circuit Breakers and Panels Overheating due to harmonics is the danger here, and beefed-up components used in these elements offer protection. Neutral buses should be rated for double the phase current. Grounding: The primary purpose of grounding electrical systems is to protect personnel and property if a fault (short circuit) were to occur. Grounding conductors connect all of the non-current carrying parts of the electrical system, or any metallic parts in the vicinity of the electrical system together. This part includes conduits, enclosures, supports and other metallic objects. This grounding system has two purposes: 1. Safety. The grounding conductor system provides a low impedance path for fault currents to flow. This allows the full current to be detected by over current protective devices (fuses and circuit breakers), safely clearing the fault quickly. 2. Power quality. The grounding system allows all equipment to have the same reference voltage. This helps the facility electronic equipments operation and helps prevent the flowing of objectionable currents on communication lines, seals and other connections. 6. Wiring: www.Fullinterview.com Generally, wiring and grounding problems come in the form of intermittent network failures, buzzing sounds (corona effect), scorched insulation, intermittent voltages at equipment, and burned panel or junction boxes. The table below illustrates some of the new wiring practices recommended to achieve a high level of power quality. Many of the "before" practices are still reflected in building codes today. Receptacle Before: 13 Recommended: 3 to 6 Outlets per 20 Amp Circuit Neutrals Before: Full size or downsized Recommended: Use double size neutral (on 3-phase systems) neutral (CBEMA) or larger Before: One neutral shared among Recommended: Separate neutrals equipment (on 1-phase branch) or upsized neutral back to panel Phase Before: Standard phase conductor Recommended: Use upsized phase Conductors sizing per code conductors to minimize heating for harmonics Circuits Before: Can shared among many Recommended: Use separate outlets and uses circuits for harmonic-sensitive loads Grounding Before: Can use metal conduit as Recommended: Use separate grounding conductor insulated wire as grounding conductor Before: Downsized grounding Recommended: Use full size or conductor over size grounding conductor Before: (Commercial/Industrial) Recommended: Use a copper Must use metal water pipe and a ground ring and multiple concrete-encased electrode (if interconnected ground rods available) Before: Use a second ground rod if Recommended: Use multiple rods first is over 25 ohms (no resistance or ring and measure to ensure very measurement required) low resistance to ground Before: Access floor for Recommended: Use copper system equipotential grid in computer for equipotential grid mainframe room Before: No lightning or surge Recommended: Use lightning and protection surge protection Courtesy: Copper org. 7. Lightning: Lightning Protection Systems In simple terms, if part of the "path of least resistance" to ground the lightning sees is through your wiring or equipment that is where it will flow. Lightning produces very high currents, for a short time interval, but enough to cause fires or to destroy microcircuits even miles away. The idea of air terminals, or lightning rods as commonly known, goes back to www.Fullinterview.com Benjamin Franklin. The purpose is to provide a convenient, controlled point for lightning to strike, and then be safely conducted to ground. To provide the least resistive path, heavy-gage copper wire should be employed in the leaders and down conductors. Grounding of Lightning Systems The down conductors tie directly to the ring ground described above, or other grounding electrode system, along with all building steel and electric service grounds. Use heavy-gage copper conductors to minimize impedance. Techniques to mitigate power quality problems: The increasing application of sensitive loads in the power networks has necessitated the need to mitigate the serious power quality problems. The compensation techniques can be broadly classified into two main categories viz.: passive and active techniques. Passive techniques: these techniques employ following devices: Passive shunt L-C Filters Power Factor Correction Capacitors. Active techniques: these techniques employ the following devices: PWM (VSI/CSI ) Active Filters. Thus, finally the following steps may prevent most of the power quality problems from occurring: Use double-size neutral conductors or separate neutrals for each phase. Specify a separate, insulated full-size grounding conductor, rather than relying on the conduit alone. Use an isolated grounding conductor for sensitive equipment. Segregate sensitive loads on separate branch circuits, fed from a separate panelboard, fed from separate feeders (and even separate transformers if possible). Run a separate branch circuit for every 4 to 6 duplex outlets. www.Fullinterview.com Use an outside copper ground ring and multiple ground rods as part of the grounding electrode to achieve lowest practical resistance to ground. Measure ground resistance. Use harmonic-rated circuit breakers, panelboards, and transformers. Use surge and lightning protection. Oversize phase conductors to minimize voltage drop. (This will save energy too, and may even pay for itself through lower I2R losses.) Choose materials based on superior connect ability. Poor quality connections are a major consideration. This is where all-copper wiring excels over other materials. Design these features into new construction or renovation work. Reduced downtime or data loss will more than pay for these measures. CONCLUSION: The wide spread applications of non-linear power electronics loads nave brought but degradation of power quality in the electric network. This paper has focused broadly on the power quality issues, the implications on the utilities and customers in the power system. At the same time, the paper has discussed in brief the assessment of power quality. The effective means of compensation through various techniques have also been highlighted. REFERENCES: 1. Power quality and Harmonics: JOHN H. WAGGONER. 2. Inside PQ: MAX McGRANAGHAN. 3. Grounding and lightening protection: ROBISON, M.D. 4. Electric Power Quality: problems and means to improve them: S.MOHANTY, B.R.MISHRA, Dr. D.S. CHAUHAN.
Pages to are hidden for
"CAUSES OF POOR POWER QUALITY Projects"Please download to view full document