International Journal of Electrical Engineering and Technology (IJEET), ISSN (Print), INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING &

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1 INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING & TECHNOLOGY (IJEET) ISSN (Print) ISSN (Online) Volume 5, Issue 2, February (2014), pp IAEME: Journal Impact Factor (2014): (Calculated by GISI) IJEET I A E M E A COMPARATIVE STUDY OF HARMONIC CURRENTS ON POWER SUPPLY SYSTEM DUE TO 24 PULSE AC-DC DIODE CONVERTERS AND THYRISTOR CONVERTERS IN ALUMINUM SMELTERS Prof. Sharavan kumar Jhajharia Manipal University Jaipur ABSTRACT This paper will explain a basic understanding of how current harmonics are produced by the AC-DC converters of Aluminum smelters. The effects of AC-DC converter harmonics on power supply system are being discussed. In this paper the comparison has been carried out on the basis of practical data collected from the field at Hindalco smelters Renukoot, when both type of converters were in operations without compensation and with compensation. Keywords: Harmonic Analysis, Aluminum, Converters, Smelter, Compensation. INTRODUCTION Harmonics were always present in power systems right from the beginning. Earlier harmonics were minor and had no detrimental effects. Present day the advent of power electronics in every field and proliferation of non-linear loads in industrial power applications has given rise to the problem of harmonics. The use of AC-DC converters in the industry non- ferrous metals and caustic soda industries is too much. These harmonic current due to AC-DC converters produce harmonics. We cannot wish away harmonics from the power system. In this paper AC-DC harmonic current due to converters of Aluminum smelters has been highlighted and their effects on power quality are a topic of concern. HARMONICS It is known that electricity generation is normally produced at constant frequencies of Hz and the generator s e.m.f is considered practically sinusoidal. Let the generated an instantaneous voltage (v) represented by v = Vm sin 2πft volts. This is the wave form which varies sinusoidally 60

2 with time (t), has a frequency (f) and maximum value (Vm). Alternatively voltages as assumed to have wave shape sinusoidal only when one frequency is present. However, when a source of sinusoidal voltage is applied to AC-DC converters or non-linear devices or load then the resulting current is known perfectively sinusoidal or contains harmonics which gets injected into the power supply system because the source impedance is low and causing distortion of original power supply wave form. When a wave is not sinusoidal then it is called a complex wave whatever may be its shape. If the complex wave is being splitted mathematically into the components called the fundamental and a multiple integer of fundamental, then this process is called harmonic analysis. The fundamental (or the first harmonic) is sinusoidal and has the supply frequency (f). The other harmonics are also sine waves having frequencies which are integer multiples of (f). If the supply frequency is 50 Hz then 3 rd harmonic frequency 150 Hz, the 5 th harmonic frequencies is 250 Hz and so on. HOW AC-DC CONVERTERS PRODUCE HARMONICS The Aluminum smelters use only two types of AC-DC converters i.e. diode rectifiers & thyristor rectifiers. A diode rectifier is a device for converting an AC to DC into a unidirectional or approximate direct current. It is known fact that diode has a low impedance in forward direction to current flow and in reverse direction a nearly infinity impedance to oppose the flow of current. So when an AC voltage is applied to a diode current will flow through its positive half cycles only and current will be zero during negative half cycles. A typical voltage and current wave of half wave rectification is shown in Fig 1 is produced by using single diode. Figure 1 In thyristor rectifier the amount of power delivered to a load can be controlled. A basic circuit is used to produce necessary gate current to turn the thyristor on. If the thyristor is fired at angle α then the current flowing in the load register has a wave form and the sharp rise shown as in figure 2, however gives rise to harmonics. 61

3 Figure 2 The problem of current harmonics due to AC-DC converter of aluminum in the input current of AC-DC converter s load can be explained with the help of fig. 3a Ps G R S +jx S Transmission Lines Pg Fig 3 PCC Static converters P C R L P l Where (G) feeds a resistive load of the smelter through a line with impedance (Rs + j Xs) and a static converter. The generator supplies power (Pg) to the point of common coupling (PCC) of the load with other consumers. Here the maximum power pl is transferred to the load, some of power of its, power (pc) is converted to power at different frequencies in the static converter. Besides there is some loss of power (ps) in the impedance of transmission and generation system. 62

4 Ih R Sh +jx Sh Plh Psh Rgh + jxgh R L Pgh Fig 3b How the harmonic power flows from AC-DC converter has been shown through the fig. 3b by short circuit generators and AC transmission line and generator are represented by their harmonic impedances (Rsh+jXsh) and (Rgh+jXgh) respectively. In the figure the static converter acts as source of harmonic currents. A small part of fundamental power (pc) is transferred in the harmonic power and some of this power (psh+pgh) is consumed in the system and generator resistances and the rest power (plh) in the load. Thus the total power consists of fundamental frequency component (ps) and harmonic power caused by the presence of the converter as (psh+pgh+plh). It is clear from fig. 1 that the angle between voltage source and fundamental component of current is zero. Then the displacement power factor (DPF) for a diode rectifier is unity. But for thyristor Idc = Im/π (1+cosα) Irms = π/2 Idc (1+cosα) So, in case of fully controlled AC to DC thyristor converter, the DPF will be same as cosine of the delay angle α resulting further detoriation of power factor. From above it is clear that AC-DC converters introduces large amount of harmonics in to the source current. If the source is having inductance then the voltage distorted at point of common coupling shall be substantial. To see the actual behavior of AC-DC converters of Aluminum smelters, the measurements of 24 pulse rectifiers were taken on the operating smelters of Hindalco without compensation. In fig. 4 and 5 a typical 24-pulse converter s front end of bridge rectifier circuit using 24 diodes and thyristors has been shown. In figure 4 only one set of 35 KA of 12 pulse has been shown while at site there are two sets of 35KA each and the primaries of both the transformers are having phase shifts of and to make their 24 pulse. The advantage of this 24 pulse configuration is to have elimination of 5 th, 7 th and 11 th harmonics. This is minimizing the magnitude of these harmonics but not eliminating them fully in actual. At Hindalco smelters harmonic filters had been provided for 5 th, 7 th and 11 th harmonics with denuted inductors in order to improve the power factor and avoid resonance. For port line 7 the rating of the filter banks is as given. For 5 th harmonic 515 KVR, for 7 th harmonics 371 KVR and for 63

5 E 11 th 374 KVR. In the same manner for the pot line number 9 the capacity of filters for 5 th harmonic is 486 KVR and for 11 th harmonic 287 KVR. Fig 4. SLD of potline number 7 Fig 5. SLD of potline number 9 64

6 From the measurements it is clear that 23 rd and 25 th harmonic components are the dominant harmonics. And in diode rectifiers the magnitude of harmonics is lower than the thyristor rectifier because of the smoother current wave forms due to voltage controlled by saturated reactors. Theoretically a 24 pulse converter does not produce 5 th, 7 th, 11 th and 13 th harmonics but due to unbalanced voltage or impedance and differences in transformer binding ratios of (wye) and delta connections, difference in thyristor converter pulse angles between multiple circuit and variations in converter turn of times etc. gives the presence of these order harmonics. Harmonic orders Unit.9 Diode based with load 71.6 KA and 861V DC Current Current harmonics harmonics with without compensation compensation (% of (% of fundamental) fundamental) Unit. 7 Thyristor based load 71.6 KA and 857 V DC Current Current harmonics harmonics with without compensation compensation (% of (% of fundamental) fundamental) Standard value as IEEE Table 1: Measured parameters of harmonics on both type of converters with compensation and without compensation Diode Converter Thyristor Converter Graph 1: Comparison of harmonics without compensation of both type of converter. 65

7 12 10 Diode Converter Thyristor Converter Graph 2: Comparison of harmonics with compensation of both type of converter. Effects and negative consequences of the harmonics Harmonics generated by AC-DC converters of aluminum smelters have measure effects on the following: 1) Transmission lines: The flow of harmonic currents in transmission system has two main effects. One is the additional power loss caused by increased r.m.s value of the current wave form. Due to harmonics the system resistance increases at the harmonic frequency because Skeen effects and proximity effects are function of frequency and is similar to a centrifugal force. As the AC resistance increases then I 2 R losses will also increase. Second effect of harmonic currents flow is creation of harmonic voltage drops across the various circuit impedances. The system will be weak than a strong system. In case of cables, harmonic voltage increases the dielectric stress and this shortens the life of cables. 2) Capacitors: Capacitors are affected due to heat rise which increases the power loss and reduces the life of the capacitors. Overloading of capacitor, which offers lower impedance to higher harmonic order currents. Capacitors acts like sink. X = 1/j2πfC C Possibility of resonance with system impedance causing high circulating currents. Can damage capacitors and other equipments. nx = 1/nX sys C Harmonics cause higher THD (total harmonic distortion) at the point of common coupling. 3) Fuses and Circuit Breakers: harmonics can cause false or spurious operations and trips, damaging or blowing components for no apparent reason. 66

8 4) Transformers/Rectiformers: The effect of harmonics generated by AC-DC converters on the transformers/rectiformers can be seen as the generation of extra heat generated by the losses caused by harmonic content of the load current due to harmonic voltage, the hysteresis and eddy current losses increases the core losses. In the case of rectifier transformers the flow of harmonic current increases the copper losses and rectifier transformers often have hot spots in their tanks. Typically, the uses of appropriate K factor rated units are recommended for non-linear loads. 5) Generators: have similar problems to transformers. Sizing and coordination is critical to the operation of the voltage regulator and controls. Excessive harmonic voltage distortion will cause multiple zero crossings of the current waveform. Multiple zero crossings affect the timing of the voltage regulator, causing interference and operation instability. 6) Utility Meters: It may record measurements incorrectly, resulting in higher billings to consumers. 7) Drives/Power Supplies: can be affected by misoperation due to multiple zero crossings. Harmonics can cause failure of the commutation circuits, found in DC drives and AC drives with silicon controlled rectifiers (SCRs). 8) Computers: It may experience interference or failures. 9) Interference with communications: The power transmission lines with harmonic currents can create noise on communication circuits and degrade the transmission quality and can interfere with signaling. CONCLUSION The purpose of this research was to understand the nature and characteristic of harmonics produced by the two types of rectifiers used in port line 7 and 9 at the Hindalco smelters. According to the practical measurements it was observed that thyristor based rectifiers have higher harmonics than diode based rectifiers. By using measurement results available for 24 pulse operation low order non characteristics harmonics (3 rd, 5 th and 7 th ) have been also recorded which have been caused by unbalance in the supply and asymmetries in the transformers are very much important in this study. REFERENCES [1] L Hu, and R Yacamine, Harmonic transfer through converters and HVDC links. IEEE Trans. Power Electronics, 7(4), 1992, [2] J Arvillage and R Neville Watson, Power system harmonics, John Wiley and Sons Ltd, [3] G Goldberg, Behaviour of apparatus under the influence of voltage and current harmonics. Bull. Soc. R. Belg. Electr. 91, 1975, [4] J Bird, Electrical circuit Theory and Technology, Elsevier Ltd, [5] Tanay Rastogi, Mohd. Tabish Siddiqui Prof. R.Sudha and Prof. K. Govardhan, Analysis of THYRISTOR Based HVDC Transmission System, International Journal of Electrical Engineering & Technology (IJEET), Volume 3, Issue 2, 2012, pp , ISSN Print: , ISSN Online: [6] Prof. Sharwan Kumar Jhajharia, A Comparative Study of Diode and Thyristor Converters used in the Aluminum Smelters, International Journal of Electrical Engineering & Technology (IJEET), Volume 4, Issue 6, 2013, pp. 1-13, ISSN Print: , ISSN Online: