EE587_Ch6_sp10

EE587_Ch6_sp10 - EE 587 – Electric Power...

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Unformatted text preview: EE 587 – Electric Power Distribution/Utilization Dr. M. Safiuddin, Research Professor Department of Electrical Engineering University at Buffalo [ State University of New York ] 2/15/2010 EE 587_Sp '10 EE 587 Electric Power Distribution/Utilization TEXT BOOK: ANSI/IEEE -Std. 141-1993 [ The Red Book]; Published by Institute of Electrical & Electronics Engineers, New York. Lecture Topics: 1. 2. 3. Introduction, System Planning & Design Course orientation, Overview, Definitions, Planning guide for the supply and distribution system [Ch 2] Voltage Considerations Voltage control, Voltage selection, Effects of voltage variations on low and medium voltage equipment, voltage quality considerations.[Ch. 3] System Protection I Sources of fault currents, Fundamentals of fault current calculations, Restraints of simplified calculations, Example case studies. [Ch. 4] EE 587_Sp '10 2/15/2010 1 EE 587 Electric Power Distribution/Utilization Lecture Topics (cont’d): 4. System Protection II Analysis of system behavior and protection needs, Protective devices and their applications, Protective relays, Breakers and fuses, Protection requirements for transformers, Feeders and motors.[ Ch. 5] Surge Protection I Nature of the problem, Traveling wave behavior, Insulation voltage withstand characteristics. Arrester characteristics and ratings, Arrester selection, Application concepts.[Ch. 6] Grounding Considerations Introduction, System grounding, Equipment grounding, Static and lightning protection grounding, Connection to earth, Ground resistance measurement. [Ch. 7] EE 587_Sp '10 5. 6. 2/15/2010 EE 587 Electric Power Distribution/Utilization Lecture Topics (cont’d): System Power Factor Power factor fundamentals, Typical plant power factors, Reactive compensation techniques.[ Ch. 8] 8. Mid-term exam…………..Wednesday, March 4th 9. Power System Pollution Non-linear loads, Voltage and current harmonics, Passive and active filtering techniques.[ Ch. 9] 10. System Interfaces I Switchgear and other equipment, Transformers, Unit substations, Motor control Centers.[ Ch. 10] 7. 2/15/2010 EE 587_Sp '10 2 EE 587 Electric Power Distribution/Utilization Lecture Topics (cont’d): 11. Energy Conservation & Management I Energy audits, case studies.[Ch.14] 12. System Interfaces II Industrial substations, Plant utility interface considerations.[Ch. 15] 13. Energy Conservation & Management II Elec. Energy issues, DC & AC machine efficiency & application considerations 2/15/2010 EE 587_Sp '10 Chapter 6. Surge Protection 6.1 Nature of the problem. 6.2 Traveling-wave behavior 6.3 Insulation voltage withstand characteristics 6.4 Arrester characteristics and ratings Insulation tests & ratings, Physical properties affecting insulation strength Introduction, Metal-oxide arresters, Basis of arrester ratings, Protective characteristics, Arrester classes, Arrester discharge current withstand capability EE 587_Sp '10 Surge voltage propagation & reflection Amplification phenomena 2/15/2010 3 Chapter 6. Surge Protection 6.5 Arrester selection Maximum continuous operating voltage (MCOV) Temporary over-voltage (TOV) durability, Switching surge durability, Selection of arrester voltage rating 6.6 6.7 Selection of arrester class Application concepts Arrester location, Separation effects General considerations, Insulation coordination, Component protection 2/15/2010 EE 587_Sp '10 6.1 Nature of the problem ! # % " % 2/15/2010 " ! $ " " $ EE 587_Sp '10 ! 4 6.1 Nature of the problem ! ! & Fig. 6-1—Two traveling bodies of charges result when a quantity of charge is deposited on conducting line by lightening $ 2/15/2010 EE 587_Sp '10 6.1 Nature of the problem ( ) * ++ " + )+ ! ' $ ! # # , 2/15/2010 " ! ''$ - & EE 587_Sp '10 5 6.1 Nature of the problem Fig. 6.2– Oscillogram showing typical short-circuit current limiting action of fuse to produce transient overvoltage 2/15/2010 EE 587_Sp '10 2/15/2010 EE 587_Sp '10 6 – Surge voltage propagation – Surge voltage reflection – Amplification phenomena 2/15/2010 EE 587_Sp '10 0+1 $$ W hen wavelengths are short compared to the physical length of circuitry, then it may be necessary to use “Distributed-constant” representation 2 $ $ 4, 5 03 ! , ./ ! (# 2/15/2010 EE 587_Sp '10 7 0+1 $$ Inductance L and capacitance C are expressed in per unit length. Stored energy in inductance is ½ LI 2 Stored energy in capacitance is ½ CE 2 Equating the two, we get Z0 = E = I L C The wave propagation velocity is expressed as 1/√LC It approximates speed of light [1000 ft/µs] Typical values of Z0: 200-400 for overhead Lines 20-50 2/15/2010 for insulated cables EE 587_Sp '10 0++ $$ , * 2/15/2010 EE 587_Sp '10 8 0++ $$ , * ( E )( Z1 − Z 2 ) ( Z1 + Z 2 ) 2/15/2010 EE 587_Sp '10 Figure 6-6– Relative wave magnitudes along traveling medium for given changes of surge impedance Voltage at the point of refraction = (E)(2Z2)/(Z2 + Z1) 2/15/2010 EE 587_Sp '10 9 6.2.3 Amplification Phenomena + ( 6 9 ; : <+ (= 78 <8 + ( 6 6 ; 2/15/2010 78 : < 8= EE 587_Sp '10 < + 78 (> 6.2.3 Amplification Phenomena + ( 6 78 ; 78 # ; < (= : < (>8 7 EE 587_Sp '10 ; 2/15/2010 10 6.2.3 Amplification Phenomena A traveling surge-voltage, encountering in succession junctions with higher surge impedance, may have its voltage magnitude elevated to a value in excess of twice the magnitude of the initial voltage. ( 18Ω +Ω 8 3Ω 8 ?Ω 8 ( 1$@ @( 1$A ?( ++ ( $? 3B ( $3 Voltage at the point of refraction = (E)(2Z2)/(Z2 + Z1) 2/15/2010 EE 587_Sp '10 6.3 Insulation voltage withstand characteristics Insulation tests and ratings: Standards have been developed that recognize the need for electrical equipment to withstand a limited amount of temporary excess voltage stresses above and beyond the normal operating voltages. Standardized factory tests: • 1 minute high potential [ hi-pot] test at power frequency • 1.2/50 full-wave voltage impulse test • For low voltage [<1000 V] equipment, additional wave shapes are prescribed in IEEE stnd. C62.41.2-2002 2/15/2010 EE 587_Sp '10 11 IEEE C62.41.2-2002 2/15/2010 EE 587_Sp '10 IEEE C62.41.2-2002 2/15/2010 EE 587_Sp '10 12 IEEE C62.41.2-2002 2/15/2010 EE 587_Sp '10 Full-wave voltage impulse test C C Industrial Power Systems Handbook; McGraw-Hill, NY; 1955 2/15/2010 EE 587_Sp '10 13 IEEE C62.41.2-2002 2/15/2010 EE 587_Sp '10 Transformer insulation system overvoltage tests !0 ,1 D 2/15/2010 EE 587_Sp '10 14 2/15/2010 EE 587_Sp '10 2/15/2010 EE 587_Sp '10 15 2/15/2010 EE 587_Sp '10 Rotating machines have relatively low impulse strength and no standardized BILs. 2/15/2010 EE 587_Sp '10 16 ((( E % 6 < 1$BF,+ 2/15/2010 EE 587_Sp '10 Physical properties affecting insulation strength Aging4 Over-testing4 E ,@ 8G # Moisture4 Heat F $ $ 2/15/2010 EE 587_Sp '10 17 Non uniform turn-to-turn overvoltage distribution-In electrical machines )6 ) 2 04) ,13 % , 2/15/2010 EE 587_Sp '10 Some fundamentals ) " , " 2/15/2010 EE 587_Sp '10 18 Zener Diodes 2/15/2010 EE 587_Sp '10 !" 6.4 Arrester characteristics and ratings Introduction Non-linear resistor characteristics-- I = k V α α = 10 for silicon carbide, 50 for metal oxide For decades valve elements were made of silicon carbide Mid 1970s metal oxide valve elements were introduced Mid 1980s polymer housings appeared to replace porcelain housings- reduces the risk of injuries and/or equipment damage due to arrester failures IEEE std. C62.11-2005 and C62.22-1997 are two arrester standards of particular interest for medium and HV protection. 2/15/2010 EE 587_Sp '10 19 α≈B 8 α ≈ 18 2/15/2010 EE 587_Sp '10 03@ D $$ Two performance ratingsFundamental frequency, duty cycle voltage rating Maximum continuous operating voltage rating [MCOV] Duty-cycle test establishes the ability of the arrester to discharge impulse current while energized at duty-cycle voltage and thermally recover at MCOV. Test involves twenty high current impulses (5,000 or 10,000 A, 8/20 µs) for medium and HV up to 550kV systems. 2/15/2010 EE 587_Sp '10 20 6.4.4 Protective characteristics ) 2 ,, .9 2 E/ , ((( .- / .- / $ +11,+ )0 $ 88B 2E 9 1$ ) ((( $4 8$ µ " B +6 $ 2/15/2010 % , EE 587_Sp '10 $ 6.4.4 Protective characteristics Lightening impulse protective level [LPL] 1. Classifying current ranges between 1.5 kA and 20 kA 2. IR is the voltage that appears across the arrester when a standard 8/20 current wave is conducted through the arrester [Tables 6-5 and 6-6 list protective characteristics] Switching impulse level [SPL]- the higher of 1. The discharge voltage with a current wave through the arrester of switching impulse classifying current magnitude [500A-2,000A] and a time of actual current crest of 3-2,000 µs, or 2. Gap sparkover on similar wave shape 2/15/2010 EE 587_Sp '10 21 6.4.5 Arrester classes Four classes of valve-type arresters are recognized in order of decreasing cost and overall performance: 1. Station class 2. Intermediate class 3. Distribution class – heavy duty & normal duty 4. Secondary Tables 6-5 and 6-6 list protective characteristics of the metal oxide arresters and representative of several makes. 2/15/2010 EE 587_Sp '10 6.4.6 Arrester discharge-current withstand capability Discharge-current withstand tests are specified by the standards. Two of these tests relate to 1. High-current, short-duration: Two discharges of a surge current having a (4-6)/(10-15) s wave shape 2. Low-current, long-duration: To display capability to discharge charged capacitance equivalent to specified transmission line lengths. 2/15/2010 EE 587_Sp '10 22 6.5 Arrester selection Arrester voltage rating should be tentatively selected on the basis of 1. 2. 3. Maximum continuous operating voltage Temporary over-voltage (TOV) durability Switching surge durability (MCOV) Proper application requires that the system configuration, system grounding, and the arrester connection be evaluated 2/15/2010 EE 587_Sp '10 00 $ ! /' !/ /5 # # ! , ! ! ! ! $ , $ # 2/15/2010 EE 587_Sp '10 ! ! 0 " ,0 ,B 0 " ! , # # ! H 0 " ,0 ,B 0 " , , , $ 23 6.6 Selection of arrester class Duty cycle voltage/class 3-72 kV station arresters >72 kV station arresters All voltages intermediate Symm. rms 40 kA- 65 kA 40 k- 65 kA 16,100 A Duration (sec) 0.2 0.1 0.2 Pressure-relief capabilities are not standardized for distribution arresters, but IEEE std C62.22-1997 does require that all distribution arresters for which a faultcurrent withstand rating is claimed shall be tested in accordance with procedures similar to those for station and intermediate arresters 2/15/2010 EE 587_Sp '10 6.6.1 Arrester location A major factor in locating arresters within a station or substation is the line and equipment shielding 6.6.2 Separation effects The voltage at the protected insulation will be usually be higher that at the arrester terminals due to the L di/dt of the connecting leads. The rise in voltage is called the separation effect (SE) 2/15/2010 EE 587_Sp '10 24 6.7 Application concepts – General considerations – Insulation coordination – Component protection Outdoor substations Effectively shielded substations Non-effectively shielded substations Metal-clad switchgear Dry-type transformer Overhead line protection Aerial cable 2/15/2010 EE 587_Sp '10 2/15/2010 EE 587_Sp '10 25 Figure 6-17 Curves showing maximum permissible length of cable for which are arresters are not required in metal-clad switchgear versus linecable junction arrester clamping voltage 2/15/2010 EE 587_Sp '10 2/15/2010 EE 587_Sp '10 26 Figure 6-19 Curves showing maximum surge permissible at supply transformer without requiring arrester at standard dry-type transformer 2/15/2010 EE 587_Sp '10 Rated motor voltage 650 V 2400-6900 V 11,500 V Capacitance, 2/15/2010 1.0 µF 0.5 µF 0.25 µF Table 6-7 Capacitance of surge-protective capacitors per line terminal connected line to ground EE 587_Sp '10 27 Solving the first order differential equation, we get Ec = 2 Es ( 1 – e (-t/t’ ) ) ; where t’ = RC Also, maximum value of dEc/dt = I/C (max) Example: Es = 16 kV, Z 0 = 50 Ω; C = 0.5 (10 –6 ) Time constant, RC = 25 µ Sec. 2/15/2010 EE 587_Sp '10 At t = 0, E c = 0; so initial charging current is maximum That is, I max = 2Es /Z0 = 32,000/50 = 640 A. Max. dEc/dt = Imax /C = 640A/0.5µF = 1280 V/µs For a surge-voltage duration of t = 5 µs, t/t’ = 5/25 = 0.2 and e (-t/t’) = e –0.2 = 0.8187 ( 1 – e-0.2 ) = 0.1813 and Ec = 2 Es (0.1813) = 5.81 kV In comparison with the 13 kV crest value of the motor high-potential test, the capacitor voltage is way below it. It is also below the protective level of the surge arrester 2/15/2010 EE 587_Sp '10 28 ...
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