Harmonic_Effect_in_Power_System_1604488338674.pptx

Powesystem

1. Harmonic Effect in Power System

2. Effect of Harmonics on Power system Harmonic currents produced by nonlinear loads are injected back into the supply systems These currents can interact adversely with a wide range of power system equipment, most notably capacitors, transformers, and motors, causing additional losses, overheating, and overloading These harmonic currents can also cause interference with telecommunication lines and errors in power metering

3. Impact on capacitors A capacitor bank experiences high voltage distortion during resonance The current flowing in the capacitor bank is also significantly large and rich in a monotonic harmonic Under resonance condition, the rms current is typically higher than the capacitor rms current rating.

4. IEEE Standard for Shunt Power Capacitors IEEE Standard 18-1992- specifies the following continuous capacitor ratings : 135 percent of nameplate kvar 110 percent of rated rms voltage (including harmonics but excluding transients ) 180 percent of rated rms current (including fundamental and harmonic current ) 120 percent of peak voltage (including harmonics)

5. The fundamental full-load current for the 1200-kvar capacitor bank is determined from

6. The capacitor is subjected principally to two harmonics: the fifth and the seventh. The voltage distortion consists of 4 percent fifth and 3 percent seventh . This results in 20 percent fifth harmonic current and 21 percent seventh harmonic current.

7. Impact on transformers Transformers are designed to deliver the required power to the connected loads with minimum losses at fundamental frequency . Harmonic Current- Additional heating in Transformer As a general rule , a transformer in which the current distortion exceeds 5 percent is a candidate for derating for harmonics. The effects of harmonics are not known until failure occurs.

8. There are three effects that result in increased transformer heating when the load current includes harmonic components RMS current . If the transformer is sized only for the kVA requirements of the load, harmonic currents may result in the transformer rms current being higher than its capacity. The increased total rms current results in increased conductor losses.

9. There are three effects that result in increased transformer heating when the load current includes harmonic components 2. Eddy current losses. These are induced currents in a transformer caused by the magnetic fluxes . These induced currents flow in the windings , in the core, and in other conducting bodies subjected to the magnetic field of the transformer and cause additional heating . This component of the transformer losses increases with the square of the frequency of the current causing the eddy currents

10. There are three effects that result in increased transformer heating when the load current includes harmonic components 3. Core losses. The increase in core losses in the presence of harmonics will be dependent on the effect of the harmonics on the applied voltage and the design of the transformer core. Increasing the voltage distortion may increase the eddy currents in the core laminations. The net impact that this will have depends on the thickness of the core laminations and the quality of the core steel. The increase in these losses due to harmonics is generally not as critical as the previous two.

11.   The load loss P LL can be considered to have two components: I 2 R loss and eddy current loss P EC : P LL = I 2 R + P EC W 12

12. ANSI/ IEEE Standard C57.110-1998, Recommended Practice for Establishing Transformer Capability When Supplying Non sinusoidal Load Currents . The common factor used in the power quality field for transformer derating is K-factor

13. This derating factor is based on the percentage of the harmonic currents in the load and the rated winding eddy current losses k factor One method by which transformers may be rated for suitability to handle harmonic loads is by k factor ratings. The k factor is equal to the sum of the square of the harmonic frequency currents (expressed as a ratio of the total RMS current) multiplied by the square of the harmonic frequency numbers 14

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16. Derating of transformer k factor concept is derived from ANSI/IEEE C57.110 standard, Recommended Practices for Establishing Transformer Capability When Supplying Non-Sinusoidal Load Currents 18

17. Derating a transformer when supplying harmonic loads. 19

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19. AC MOTORS Distorted voltage to a motor results in additional losses in the magnetic core of the motor. Hysteresis and eddy current losses in the core increase as higher frequency harmonic voltages are impressed on the motor windings. Hysteresis losses increase with frequency and eddy current losses increase as the square of the frequency . Also, harmonic currents produce additional I2R losses in the motor windings Excessive vibration and noise in a motor operating in a harmonic environment should be investigated to prevent failures 22

20. Another effect, and perhaps a more serious one, is torsional oscillations due to harmonics .

21. Two of the more prominent harmonics found in a typical power system are the fifth and seventh harmonics. The fifth harmonic is a negative sequence harmonic , and the resulting magnetic field revolves in a direction opposite to that of the fundamental field at a speed five times the fundamental . The seventh harmonic is a positive sequence harmonic with a resulting magnetic field revolving in the same direction as the fundamental field at a speed seven times the fundamental

22. The net effect is a magnetic field that revolves at a relative speed of six times the speed of the rotor. This induces currents in the rotor bars at a frequency of six times the fundamental frequency. The resulting interaction between the magnetic fields and the rotor-induced currents produces torsional oscillations of the motor shaft. If the frequency of the oscillation coincides with the natural frequency of the motor rotating members, severe damage to the motor can result. Excessive vibration and noise in a motor operating in a harmonic environment should be investigated to prevent failure

23. IEEE Standard 519-1992 There is usually no need to derate motors if the voltage distortion remains within IEEE Standard 519-1992 limits of 5 percent THD and 3 percent for any individual harmonic . Excessive heating problems begin when the voltage distortion reaches 8 to 10 percent and higher. Such distortion should be corrected for long motor life

24. If the harmonic levels become excessive, filters may be applied at the motor terminals to keep the harmonic currents from the motor windings . Large motors supplied from ASDs are usually provided with harmonic filters to prevent motor damage due to harmonics

25. CABLES Current flowing in a cable produces I2R losses When the load current contains harmonic content, additional losses are introduced The effective resistance of the cable increases with frequency because of the phenomenon known as skin effect. 28

26. Skin Effect 29

27. Skin Effect Skin effect is due to unequal flux linkage across the cross section of the conductor which causes AC currents to flow only on the outer periphery of the conductor. 30

28. This has the effect of increasing the resistance of the conductor for AC currents Higher the frequency of the current, the greater the tendency of the current to crowd at the outer periphery of the conductor and the greater the effective resistance for that frequency 31

29. The skin effect factor depends on the skin depth, which is an indicator of the penetration of the current in a conductor Skin depth (δ) is inversely proportional to the square root of the frequency S is a proportionality constant based on the physical characteristics of the cable and its magnetic permeability f is the frequency of the current 32

30. Skin effect factor R dc is the DC resistance of the cable The AC resistance at frequency f, R f = K × R dc K is determined from lookup table The magnetic permeability of a nonmagnetic material such as copper is approximately equal to 1.0. 33

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32. skin effect ratio The ratio of the resistance of the cable at a given frequency to its resistance at fundamental is defined as the skin effect ratio E E = resistance at second harmonic frequency ÷ resistance at the fundamental frequency = R 100 ÷ R 50 35

33. Find the 60-Hz and 420-Hz resistance of a copper cable with a DC resistance of 0.276 Ω per mile 36

34. current rating factor (q) The current rating factor (q) is the equivalent fundamental frequency current at which the cable should be rated for carrying nonlinear loads containing harmonic frequency components I 1 , I 2 , I 3 are the ratios of the harmonic frequency currents to the fundamental current, and E 1 , E 2 , E 3 are the skin effect ratios 37

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36. CAPACITOR BANKS Capacitor banks are commonly found in commercial and industrial power systems to correct for low power factor conditions Capacitor banks are designed to operate at a maximum voltage of 110% of their rated voltages and at 135% of their rated kVARS When large levels of voltage and current harmonics are present, the ratings are quite often exceeded, resulting in failures 39

37. the reactance of a capacitor bank is inversely proportional to frequency , harmonic currents can find their way into a capacitor bank. The capacitor bank acts as a sink , absorbing stray harmonic currents and causing overloads and subsequent failure of the bank 40

38. More serious condition with potential for substantial damage occurs due to a phenomenon called harmonic resonance. Resonance conditions are created when the inductive and capacitive reactances become equal at one of the harmonic frequencies. Two types of resonances are series and parallel. series resonance produces voltage amplification and parallel resonance results in current multiplication. 41

39. A 2000-kVA, 13.8-kV to 480/277-V transformer with a leakage reactance of 6.0% feeding a bus containing two 500-hp adjustable speed drives. A 750-kVAR Y-connected capacitor bank is installed on the 480-V bus for power factor correction. Perform an analysis to determine the conditions for resonance 42

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41. PROTECTIVE DEVICES Harmonic currents influence the operation of protective devices. Fuses and motor thermal overload devices are prone to nuisance operation when subjected to nonlinear currents. This factor should be given due consideration when sizing protective devices for use in a harmonic environment. 44

42. Impact on Telecommunications Harmonic currents flowing on the utility distribution system can create interference in communication circuits sharing a common path. The induced voltage per ampere of current increases with frequency. Triplen harmonics (3rd, 9th, 15th) create problems in four-wire systems because they are in phase in all conductors of a three-phase circuit and, therefore, add directly in the neutral circuit, which has the greatest exposure with the communications circuit. Figure 4.30 Inductive coupling of power system residual current to telephone circuit. 45

43. IR drop in cable shield resulting in potential differences in ground references at ends of cable . cable   Inductive coupling of power system residual current to telephone circuit. 46

44. The K factor commonly found in power quality literature concerning transformer derating can be defined solely in terms of the harmonic currents as follows : The rms of the distorted current is