Conducted EMI Measurement and Analysis for VFD.

Transcription

1 INTERNATIONAL JOURNAL OF ELECTRICAL SYSTEMS AND CONTROL (IJESC) Vol. 3, No. 1, Jan-June 2011, pp Conducted EMI Measurement and Analysis for VFD. Tejas Y. Dalal 1 and M.N. Bhusavalwala 2 Abstract: This paper gives an analytical study of harmonics generated due to non linear load. Measurement of harmonic current is carried out on VFD installed in a process plant by using power analyzer. Measurements are taken on harmonic source (VFD) and electrical distribution system (MCCs + PCCs) of process plant. During measurements operating frequency of VFD is varied. Analysis is done on waveforms and numerical results obtained by measurement. From analysis, results were derived on harmonic generation and it s effects on electrical distribution system. Keywords: Total Harmonic Distortion, Non-Linear load, IEEE STD , Power Factor. 1. INTRODUCTION Drives are inevitable in today s industry. It is used for controlling speed of motor to meet various industrial applications. Drive being nonlinear load injects harmonics to electrical distribution system. Main effect of harmonic on electrical system is: (1) Increases neutral current and damages neutral bus bar due to overheat. (2) Disturbs power factor which leads to increase in power consumption. (3) Damages capacitor bank by burning dielectric of capacitor. (4) Creates malfunctioning of protective devices. (5) THD goes above specified limits at MCC and PCC. (6) Burns transformer oil and motor winding by overheating. The paper explains in detail generation of current harmonic by VFD, it s relation with operating frequency of VFD and it s effects on electrical distribution system. 2. CONDUCTED EMI HARMONICS Electrical load is generally of two types 1. Linear Load and 2. Non- linear load. Harmonics are generated by non linear load as explained below. Rectifiers, electronic ballast, computers, etc draw sudden current due to it s switching operation at peak of voltage waveform. Current waveform is non sinusoidal, not in phase to voltage waveform. Non linear load current waveform consists additional frequency including 1 B.E. (I & C Engg.), M. Tech Research Student Electrical Engg Dept., SVNIT, Surat , India, tydalal_ic@yahoo.co.in 2 M. E., (Electrical Engg.), Electrical Engineering Dept., SVNIT, Surat , India, mnb@eed.svnit. ac.in fundamental frequency resulting to distortion of current waveform as shown in Fig. 1. Fig. 1: Distorted Current Waveform. Waveform consists of fundamental frequency and other frequencies that is n th multiple of fundamental frequency. nth multiple frequencies of voltage and current are said as Harmonics. Harmonics in a waveform is measured in terms of Total Harmonic Distortion (THD): Total Harmonic Distortion (THD): It is defined as the ratio of RMS value of total harmonic current to RMS value of the fundamental current & expressed as % of fundamental current drawn by load. THDi = n 2 Ih h= 2 I 1...(1) Distorted periodic non sinusoidal voltage and current waveform is expressed by Fourier series as:

2 3 4 International Journal of Electrical Systems and Control (IJESC) V(t) = V 0 + Σ(h = 1 to n) V h (Sin hωt + θ h )...(2) I(t) = I 0 + Σ(h = 1 to n) I h (Sin hωt + θ h )...(3) Waveform is symmetrical to X-axis, it is represented as f(t) = f( t). Even functions i.e. cosine terms equals to zero and so cosine terms are eliminated from Eq. (2) and Eq. (3). 3. FORMULA FOR FUNDAMENTAL CURRENT RMS Current: Irms = (I I I I I n 2 )...(4) as: Using Eq. (4) in Eq. (1), Eq. (1) can be rewritten THD = ((Irms 2 I 1 2 )/I 1 )...(5) Model: ALM 10. C. T. Range: Amp. Type: 3 phase single C.T. instrument. For measurement at individual load, power analyzer is connected between source of electrical supply and load. Measurements on MCCs and PCCs is done by connecting instrument at it s input bus bar. Power Distribution from electricity supply company to individual load at process plant is shown in Fig. 2. From Eq. (5), Fundamental current I 1 = ((Σ (Irms 2 )/(1 + THD 2 ))....(6) 4. IEEE STANDARD OF LIMITS ON HARMONICS IEEE has established limits on harmonics in An IEEE standard on power quality and linked with harmonic control is recognized as IEEE STD IEEE STD is named as Recommended practices and requirements for harmonic control in electrical power system. IEEE suggests PCC as Within an industrial plant, PCC is the point between nonlinear load and other load IEEE Std layed down limits for harmonic current distortion at PCC as a function of Ratio of maximum short circuit current at the PCC to the maximum demand load current at the PCC. Table 1 Limits of THD Current Maximum Harmonic Current Distortion in % of I L Individual Harmonic Order (Odd Harmonics) I SC /I L <11 11<h<17 17<h<23 23<h< 35 35<h < < < < > THD values of current shall be less than mentioned limits in Table 1 for electrical distribution system. 5. STUDY OF HARMONIC GENERATION IN PROCESS PLANT Measurement of Harmonic Current is done by Power Analyzer Instrument with specifications given below: Make: KRYCARD, Fig. 2: Power Distribution at Process Plant Total plant load connected at PCC is 4500KW. VFD Load is 1300 KW and UPS Load is 50 KW. VFD contributes to 30% of total plant load. So VFD is selected to study harmonic generation and it s effects on electrical distribution system. Measurement 1: 180 HP VFD at operating frequency of 41 Hz. ABB make 180 HP VFD is a 3 phase 6 pulse VFD with operating frequency of 41 Hz. Fig. 1A: Voltage/Current Waveform

3 Conducted EMI Measurement and Analysis for VFD. 3 5 Measurement of harmonic is done to observe the effects of reduced frequency on Harmonic Generation. Fig. 1B: Current Waveform w.r.t. Time Fig. 2A: Bar Graph of Harmonic Current Fig. 1C: Individual Harmonic Current in Percentage From Fig. 1A, Irms = Amp. Fundamental load current I 1 = 130 Amp (As calculated by Eq.(6)). I 1 produces Active Power. Harmonics draws additional current which dissipates in form of heat equals to I 2 R loss. Fig. 1C indicates harmonic current in % of fundamental current H01. H01 is considered as 100%. Hence, individual harmonic current is calculated as per equation given below: I h = (I h (%)/100) * I 1....(7) For above Measurement 1, I 1 = 130 Amp is considered as 100%. Individual odd current harmonics calculated using Eq. (7) and Fig. 1C are: I 3 = 5.2A. I 7 = 26A I 11 = 9.75A I 5 = 59.8A I 9 = 1.3A I 13 = 5.2A I 15 = 0.5A Power Factor of System is Measurement 2: 180 HP VFD at operating frequency of 27 Hz. The operating frequency of VFD used in Measurement 1 is reduced from 41 Hz to 27 Hz. Fig. 2B: Individual Harmonic Current in Percentage Individual odd current harmonics calculated using Eq. (7) and Fig. 2B are: I 3 = 5.4A. I 7 = 16.2A I 11 = 3.8A I 15 = 0.38A I 5 = 24.5A I 9 = 1.0A I 13 = 3.8A Power Factor of System is Comparative Study of Measurement 1 & 2. THD of current at 27Hz = 80.63% and at 41 Hz = 51.3%. So as frequency is reduced current harmonic increases. Power Factor at 41Hz = It reduced to 0.78 at 27 Hz. So as frequency is reduced, power factor deteriorates. Measurement 3: Measurement at MCC with VFD at operating frequency of 40 Hz. Measurement is carried out on MCC input bus bar whose feeder is used to power 180 HP VFD used in Measurement 1 & 2.

4 3 6 International Journal of Electrical Systems and Control (IJESC) This measurement will prove that THD reduces as it travels from load (VFD) to MCC Bus Bar. Details of MCC: MCC Rating: 1000 Amp. MCC Impedance: 6.5%. Measurement 4: Measurement at MCC with VFD at operating frequency of 45 Hz. Measurement at same point on MCC input bus bar after increasing frequency of drive to 45 Hz. Fig. 3A: Voltage/Current Waveform Fig. 4A: Voltage/Current Waveform Fig. 3B: Individual Harmonic Current in Percentage Using Eq. (6), I 1 = 243A. Individual odd current harmonics calculated using Eq. (7) and Fig. 3B are: I 5 = 79.46A I 7 = 28.43A I 11 = 3.3A I 13 = 6.0A Current THD was 51 % on drive which reduced to 35% on MCC. THD reduces as it travels from load to MCC. Short Circuit Current I SC = (I 1 ) / Impedance...(8) I SC = 243/0.065 = A. Ratio of Isc /I 1 = For above ratio, referring to Table 1, THD shall be less than 4% upto 11 th Harmonic and 2% above 11 th Harmonic. From Fig. 3B measured THD for odd harmonics is very high compared to IEEE Standards. These indicate that harmonics affects electrical distribution system. Fig. 4B: Individual Harmonic Current in Percentage Comparing Fig. 3A with Fig. 4A and Fig. 3B with Fig. 4B, as operating frequency of VFD is increased, THD is decreased from 36% to 34%. Individual harmonics has also decreased. Neutral current measured at MCC neutral bus bar = 2A. I Crest Factor = I PEAK / I RMS = 415/254 = Crest factor shall be 2 = Crest factor is above 1.41 which gives indication that current harmonic is affecting voltage waveform. Flat topped voltage waveform is observed in Fig. 4A indicating disturbance in voltage by current harmonic. Measurement 5: Measurement on PCC: After measuring on MCC and drive, measurement is finally carried out on PCC.

5 RMS Values of current when measured through power analyzer is Irms = 4.0A. C. T. Ratio is 2000/5. So Irms = 1600A. Fig. 5A: Vthd and Ithd graph w.r.t. time. As per Fig 5A, V THD = 1.7%. and I THD = 10.4%. As per Fig. 2, 22 KVA electric supply is step down to 415V by transformer of 2500 KVA having Impedance of 6%. Using Eq. (7), full load current I 1 = 1525 A. Using Eq. (8), Isc = 1525/0.06 = Ratio Isc/I l = 25425/1525 = Referring to Table 1, for ratio of Isc/I 1 = <20, THD for current shall be maximum 4%. In above measurement I THD = 10.4%. It concludes that conducted EMI-Harmonics generated by non linear load is impacting electrical system significantly on PCC. 6. DIODE CONDUCTION COMPARATIVE STUDY To find why harmonic current increases as frequency is decreased, comparative study of diode conduction at two different frequencies is done. Fig. 7: Diode Conduction at 27 Hz. In Fig. 6, diode conduction is more, diode switching is less and diode current pulse width is more as compared to Fig. 7. At frequency of 27Hz, case is vice versa. Above comparison concludes that at less frequency harmonic increases due to more switching of diodes. 7. RESULTS DERIVED BY ANALYSIS OF ABOVE MEASUREMENT OF HARMONIC 1. THD of Current is Affecting Electrical Distribution System As per the measurement 3 to 5, THD measured is higher than limits as per Table-2 at both MCCs and PCCs. 2. For VFD, as Frequency is Decreased, THD Increases and Vice Versa THD depends upon the conduction time and switching of diodes. As shown in Fig. 6 (Drive frequency 41Hz), diode switching is less and conduction time is more. In Fig. 7 (Drive frequency 27Hz), diode switching is more and conduction is less. So THD increases at low frequency due to more switching of diodes. 3. Harmonic Leads to Disturbance in Power Factor Diodes conduct at peak of voltage waveform. So current waveform lags voltage waveform. 4. Harmonic Current Distortion Leads to Voltage Distortion Non linear load draw current at peak of sine wave. As per Fig. 3A & Fig. 4A, current harmonic flat topped voltage waveform. Fig. 6: Diode Conduction at 41 Hz. 5. Current Harmonic Distortion Reduces as it Goes from Load to MCC to PCC MCC is used to power up multiple (linear + non linear) loads. Current distortion of non linear and linear load

6 3 8 International Journal of Electrical Systems and Control (IJESC) travels to MCC. Highly distorted harmonic spectrum of non linear load and less distorted harmonic spectrum of linear load results to final spectrum on MCC/PCC with less THD. Where H = p * n + 1,...(9) H = Harmonic Order, p = pulse per cycle and 6. THD Does Not Consists Even and Triplen Harmonic Sinusoidal waveform is symmetrical to X-Axis. Sine wave is odd function so even harmonics does not exist. In 3 phase 6 pulse system, waveform symmetry leads to cancellation of triplen (3 rd, 9 th, 15 th,..) harmonics. Harmonic spectrum can be expressed in equation as: n = integer 7. Considerable Neutral Current Observed at Bus Bar of MCCs and PCC For single phase circuit, return current (harmonic affected) flows through neutral. Waveform of 3 rd harmonic (triplen) is shown in Fig.8. Fig. 8: 3 rd Harmonic Current Waveform For triplen harmonic, current waveform of all three phase comes in line and pass through neutral. 8. CONCLUSION VFD is a harmonic generating source mainly producing 5 th, 7 th, 11 th, 13 th harmonic. THD of current at MCCs and PCC is above limits specified by IEEE. THD increases as frequency is reduced. It pollutes quality of power by reducing power factor, affecting voltage waveform and increasing neutral current. Further work may be carried out for reducing harmonic generated through VFD. REFERENCES [1] L M Tolbert, H D Hollis and P. S. Hale Jr., Survey of Harmonic Measurements in Electrical Distribution System, Presented at IEEE IAS Annular Meeting Oct, 6-10, 1993, San Diego, CA PP: [2] J. Gosbell, S. Perara and V. Smith., Harmonic Distortion in the Electric Supply System, Presented at Power Quality Centre, University of Wollongong. [3] Rockwell Automation Straight Talk About PWM A C Drive Harmonic Problems and Solutions, Published on website: com. [4] ABB Drives Guide to Harmonics with A. C. Drives, Technical Guide No. 6 Published on Website: