Modeling of Nonlinear Loads and Analysis of Harmonics in a Small Scale IT Park

Transcription

1 Proceedings of National Conference on Technological Advancements in Power and Energy 136 Modeling of Nonlinear Loads and Analysis of in a Small Scale IT Park O. Deepu and T.K. Sindhu Abstract One of the major causes of power quality issuesin electric power system is the presence of harmonics.this paper discusses the experimental study and modeling of non-linear loads used in IT park. Simulation models of non-linear loads are developed on the basis of voltage and current waveforms obtained from the laboratory testing. analysis is performed for these loads individually and it is compared with the simulation results. analysis for the whole system is done. THD is used as the index for studying the harmonics effect ofthese nonlinear loads. P Keywords--- Current, Nonliear Load, THD I. INTRODUCTION OWER quality is a term that represents a set of electrical boundaries that allows a piece of equipment to function in its intended manner without significant loss of performance or life expectancy [1]. It has become a major concern to both electric utilities and customers. The objective of the electric utility is to supply its customers with a sinusoidal voltage of fairly constant magnitude and frequency. The generators that produce the electric power generate a very close approximation to a sinusoidal signal. However, there are loads and devices on the system which have nonlinear characteristics and result in harmonic distortion of both the voltage and current signals [2]. is existing in power system for many years. But previously most of the electrical equipmentswere linear loads. But the rapid increase in the power electronics device technology such as diode, thyristors etc made industrial loads non-linear. The nonlinear loads connected to the power system distribution side generate harmonic voltage and current. A harmonic component in an ac power system is defined as a sinusoidal component of a periodic waveform that has a frequency equal to an integral multiple of the fundamental frequency of the system. The problems caused by harmonic currents are equipment malfunction, data distortion, overloading of neutrals, overheating of transformers, nuisance tripping of circuit breakers, overstressing of power factor correction capacitors and skin effects etc. O. Deepu, Department of Electrical Engineering, National Institute of Technology Calicut, Kerala, India deepuo.hfd@gmail.com T.K. Sindhu, Department of Electrical Engineering, National Institute of Technology Calicut, Kerala, India tk_sindhu@nitc.ac.in II. SOURCE OF POWER SYSTEM HARMONICS The major source of power system harmonics are mainly power electronic devices like compact fluorescent lighting, computer switch mode power supplies, inverters, dc-dc converters, variable frequency motor drive, static VAr compensator etc. In this, the harmonics producing loads, mainly coming under the IT sector are CFL load, Computer load and Uninterruptable Power Supply. Out of these, Personal computers are one of the most widely usedelectronic loads in modern life. Since IT sector contain large number of personal computers, the effect of harmonics due to them is severe in the power system. IEEE Standard 519 on Recommended Practices and Requirementsfor Harmonic Control in Electrical Power Systems, is commonly adopted for harmonics control [3]. III. MODELING OF COMPACT FLUORESCENT LAMP One of the best methods of energy saving in lighting sector is the implementation of Compact Fluorescent Lamps. CFLs have a very high luminous efficacy as compared to other lamps. The luminous efficacy of a CFL is about 34 times that of incandescent lamps and about 1.7 to 2 times that of fluorescent lamps. The CFLs have a comparatively higher life span of about 10,000 hours. It is about 8 to 10 times that of incandescent lamps. CFLs also save 75 to 80% energy as compared to that of an incandescent lamp of the same light output. The wide spread use of CFL results in harmonic injection into the system.moreover, the power factor of these lamps is around These harmonic currents on electrical networks cause distortion of waveforms and result in cables and transformers overheating and the malfunction of electrical equipments. Fig.1. Block diagram representation of CFL The commonly used CFLs employ Electronic ballast, although magnetic ballast was used during the early days. The analysis of a conventional CFL with electronic ballast is done in order to study the cause of high harmonics. The main part of the CFL consists of a rectification stage, a power factor correction stage, and a high frequency oscillator and then fluorescent tube shown in fig.1.

2 Proceedings of National Conference on Technological Advancements in Power and Energy 137 A. Design of Boost Inductor The Boost Inductance value is calculated with the followingequations; D = 1- α (1) α = Vpeak (2) V o L Boost= Where, D - dutycycle. 2 2 V peak (1 ) (3) P V - peak value of the input voltage. peak o V o - output voltage to supply the inverter stage. P o - rated power of the fluorescent lamp. B. Design of Resonant Parameter At start-up, the self-oscillating technique provides a startup resonant frequency ( ) that will be made equal to the switching frequency ( ). The equation (4) givesthe relationship between the resonant parameters and the start-up resonant frequency.the start-up resonant frequency. (4) The steady-state resonant frequency is, The resonant parameters are given by, (5) Series capacitance, (6) Fig.3. CFL output waveforms at High Frequency Fig.4. Envelope of CFL output voltage and current Fig. 5 shows the input current waveform of the CFL model developed. Fig. 6 represents the harmonic spectrum of the input current wave. 3rd, 5th and 7th are the major harmonic components present. Parallel Resonance, C = C s (7) r 15 Resonance Inductance, (8) Fig.5. CFL input current Fig.2. Simulink model of CFL C. Analysis of CFLModel With the designed values, CFL is simulated MATLAB/ SIMULINK. Fig. 3 shows the High frequency output lamp voltage and lamp current waveforms for 50% duty cycle. Fig.4 shows the zoomed version of fig.3. Fig 6. Harmonic spectrum of CFL input current D. Expermental Results and Comparison A CFL of makephilips of rating 20W, V, 75mA is taken for laboratory test. Test Location : Testing Lab, National Institute of Technology, Calicut Testing Equipment : Digital power meter CW 240 Of Yokogava Electric Corperation.

3 Proceedings of National Conference on Technological Advancements in Power and Energy 138 Test Conditions : 240 V AC, 50 Hz Table 1: Experimental Results of Harmonic Analysis on CFL Voltage Current V rms =230.0 V I rms = 72mA Oder Magnitude Oder Magnitude 3 0.8V 3 72% 5 1.5V 5 41% 7 0.7V 7, 9 etc. 9%, 5% THD = 3.5% THD = 72.0% (1 D) S r L f Capacitance C = 2 (8) Where r is the voltage ripple and f is the switching frequency. The buck converter transfer function is, Vg V0 d 2 L LCS S 1 (9) R The model of CFL is developed and it is compared with the experimental results. of 72% THD is present in the input current wave form and it is above the IEEE 519 limits. Table 2: IEEE 519 Current Limits I sc / I L h<11 11 to to to 35 35< h THD < > IV. MODELING OF COMPUTER LOAD Computer loads are the main nonlinear load which are used in the IT sector. The main part of the Computer load is the SMPS, which supplies the constant DC voltage for the other parts and is the source of harmonics current in computer load. A typical PC load model uses SMPS and comprises of a full wave rectifier, a DC storage capacitor, a diode bridge resistance and a series RFI choke which is represented by an inductor. Fig.8. Simulink model of computer SMPS A PI controller is tuned for the above buck converter transfer function. The integral constant is obtained as C. Analysis of Computer SMPS With the designed values of buck converter parameters the SMPS for computer load is modeled for 5V constant supply. The fig.9 and10 shown below represents the input current waveform and its harmonic spectrum respectively. Fig.9.Input current of Computer load Fig.7. Block diagram representation of SMPS 230 V, 50Hz ac supply is given to the input of the computer load. First it undergoes a voltage step-down stage to 12 V ac. Then the rectification is done. The varying output voltage of the uncontrolled rectifier is given to the buck converter[5]. Buck converter will produce a constant dc voltage of 5V. A. Design Specification of Buck Converter Output voltage, V o = 5V Input voltage, V in = 10 13Vdc Load current = 2-4 Amps Voltage ripple = 1 % B. Buck Convrter Design Duty cycle, (6) Inductance (7) Fig.10. Harmonic spectrum of SMPS input current D. Expermental Results and Comparison An SMPS of rating 180W, V, 8A is taken for laboratory test. Table 4: Experimental results of harmonic analysis on SMPS Voltage Current V rms = V Total = 0.54A Oder Magnitude Oder Magnitude 3 2.2V 3 88% V 5 66% 7 1.9V 7,11, etc. 50%,27%.. THD = 1.4% THD = 125.0%

4 Proceedings of National Conference on Technological Advancements in Power and Energy 139 The model of SMPS is developed and it is compared with the experimental results. of 125% THD is present in the input current waveform and it is above the IEEE 519 limits. Third and fifth harmonics are present in the order of 88% and 66% of fundamental. V. MODELING OF UNINTERRUPTIBLE POWER SUPPLY Uninterruptible power supply is an electricalequipment, that provides power to a load when the input power fails. It is also one of the main electrical nonlinear loads in IT sector. In this the three phase-six pulse diode rectifier is the main harmonic current source. 5 th and 7 th order harmonics components are dominant [6]. Fig.11. Block Diagram Representation of UPS A three phase supply of 415V, 50Hz is given to a three phase controlled rectifier which is used for the rectification purpose.the DC bus voltage is 400V.It is obtained by connecting 32 numbers of 12V,7Ahbatteries in series. SVPWM based inverter is used for producing the three phase voltage. A three phase filter is designed for the inverter circuit. The design specifications are given by the following equations. Filter inductance, L = 1 Vdc 1 (9) 8 Ir fs 1 1 Resonance frequency, f o (10) 2 LC Where v dc - Input voltage to inverter, f s - Switching Frequency, I r - 20% of load current. Fig.13. Filtered output voltage of UPS VI. CASE STUDY The case study on the harmonic effect of nonlinear loads of a small scale IT Park is carried out in the computer lab of electrical department at NIT Calicut, Kerala, India. Large number of non-linear loads like Computer, CFL and linear loads like fan load etc. are present. A three phase UPS is Fig.12. Simulink Model of UPS supplying the power to the lab.the table 5 shown below represents the total load connected to the system. Table.5 Loads connected in the computer lab Load Connected Quantity Total Power CompactFluorescent Lamp Watt Computer Watt Fan 5 200Watt The harmonics analysis is carried out in the supply side of the computer lab.i.e., at the input side of the UPS load. UPS Rating Input : 400V, 22A, 50Hz, 3 phase Battery : 12V, 7Ah ( 32 no in series) Output : 400V, 50Hz, 15kVA, 13.5kW At the time of measurements taken, only 17% of UPS is loaded. The table shown below represents the data obtained by the measurements taken in the input side of the UPS with Digital Power Meter CW 240 of YOKOGAWA Electrical Cooperation.

5 Proceedings of National Conference on Technological Advancements in Power and Energy 140 Table 6. Input data of UPS from the Power Analyser RMS Voltage(V) RMS Current(A) Magnitude THD Magnitude THD V R = % I R = % V R = % I Y = % V B = % I B = % From the analysis it is observed that, in the supply voltage side the total harmonic distortion comes around 1 to 1.5%,and it is within the IEEE standard limit. But in the case of input current, which is got highly distorted by the us e of these nonlinear loads. The THD is around 34% in each phase, and it is above the limits of IEEE Standard 519. VII. SIMULATION STUDY OF THE SYSTEM The system which is taken for case study is simulated by the models which are developed. The fan of 40W ratingis represented by an RL load. Fig.16. Input current in the three phases Fig.14.Block diagram representation of system model In the system only the computer loads are connected to the UPS. Other linear and nonlinear loads are directly connected to the supply side. The fig.15 shows the Simulink model of the system taken for case study. Fig.15. Simulink Model of the System Fig.17. Harmonic spectrum of inputcurrent in phase R The input current and the harmonics analysis of the simulated system are shown in figures 15 and 16 respectively. Itis found that THD is around 35% and is dominated by the 5 th harmonics component. The triplet harmonic components are absent in the balanced three phase system. It is clear that the THD and individual harmonics ofthe load current exceed the IEEE standard for currentharmonics. Thelosses due to harmonics can be reduced by introducingharmonic filters in the systemand the efficiency of the system can be increased. VIII. DESIGN OF HARMONIC FILTER Harmonic filters are used to limit the harmonic currentsflowing into the upstream network. A single tuned passive filter is designed to bypass the 5th order harmonic component[7].the LC filter is designed using the following equations, The filter reactance, X filter = kv 2 (10) MVAr X cap = h 2 X filter (11) X filter = X cap X Indu (12) Q factor is selected as 45. Capacitor of rating 7kVArand inductance of 2.84mH is designed.

6 Proceedings of National Conference on Technological Advancements in Power and Energy 141 Fig.18. Filtered input current in phase R The figure, fig.18 shown above represents the filtered input current by using the designed filter values. And the THD is obtained as 5.75%. IX. CONCLUSION Harmonic analysis of different non linear loads present in the IT Park has been taken for study. Laboratory test on harmonic analysis has been done on different non linear loads such as CFL, Personal Computer and UPS. Matlab/Simulink model of nonlinear load such as CFL, Personal Computer and UPS has been developed and the harmonic effect is compared with the experimentally obtained values. A case study has been conducted on small scale IT sector. The same system is simulated in Matlab/Simulink. A single tuned harmonic filter is also designed to reduce the harmonic effect. APPENDIX For the CFL Cp = 1.24nF, Cs = 18.57nF, Lr = 8.74mH; L boost =10.717mH, For buck converter L= 5.5mH,C=9.6mF, f s =300kHz. For inverter filter L=10.8mH, C=9.6µF. REFERENCES [1] IEEE Working Group on Power System, Power System : An overview, IEEE Trans. on Power Apparatus and Systems, Vol. PAS-102, No. 8, August 1983 [2] C. Venkatesh, D. Srikanth Kumar, D.V.S.S. Siva Sarma, IEEE and M. Sydulu, Member, IEEE. Modelling of Nonlinear Loads and Estimation of in Industrial Distribution System. Fifteenth National Power Systems Conference (NPSC), IIT Bombay, December 2008 [3] Walter A. Malsowski, Allen Bradley, in Power Systems, Electrical Electronics Insulation Conf., Sept. 1995, pp [4] V.Jagannathan, B. Adhavan, P. Deepthi. A Single Stage Electronic Ballast With Duty Ratio Controlled Current Fed Resonant Inverter. International Conference on Process Automation, Control and Computing (PACC), Coimbatore, July 2011 [5] J.B.V Reddy, G Bhuvaneswari Singh, B. A single DC-DC Converter Based Multiple Output SMPS With Fully Regulated and Isolated Outputs, IEEE Transactions on Power Delivery, vol. 19, pp ,11-13 Dec [6] D.Rathnakumar, J.LakshmanaPerumal, T.Srinivasan. A New Software Implementation of Space Vector PWM. Southeast IEEE Conference,pp ,8-10 April 2005 [7] Najwa Mahamad, C. M. Hadzer and Syafrudin Masri, Application of LC Filter in Reduction Article in a Proceedings of National Power & Energy Conference, Kuala Lumpur, Malaysia..pp , Nov