International Conference on Magnetics, Machines & Drives (AICERA-2014 iCMMD)
A Novel Technique for Shaping Source Current Waveform Using Electronic Circuit Mr.Ganesh M. SAINGITS College of Engg. ganesh.mahade
[email protected] m
Ms. Anju R.K
anju5urk@gmai l.com
Ms.Darsana M. Nair
darsanamnair@ gmail.com
Mr. Achu S.
achu477@gmail .com
Abstract— All over the world, utilities of electricity and end users of electricity are becoming increasingly concerned about the quality of power. Power quality has become one of the most prolific buzzwords in the electric power industry since the late 1980s because widespread studies on harmonics and its impact. Most of the electrical devices are will fail or will be malfunctioned when exposed to one or more power quality issues. In this paper various reasons for power quality issues are discussed also specially focusing on a new method to minimize distortion with the help of a non critical load .During the entire cycle of operation, the critical load and non critical load are triggered alternatively to compensate the imbalance in the current waveform. Simulation is done using PowerSIM simulation environment. In this paper various reasons for power quality issues are discussed with particular focus on a new method to minimize one of the major causes of distortion. Traditional methods to reduce the causes for power quality issues are mentioned. A new control technique is used to control critical loads. Simulation is done using PowerSIM simulation environment Keywords—Power Quality(PQ), distortion, critical loads.
I.
INTRODUCTION
Power Quality problems in the distribution system are given in the literature [1] due to the increased use of sensitive and critical equipment pieces such as communication network, process industries, digital computers, power electronic devices, SMPS and precise manufacturing processes. Major power quality problems such as sags, swells, transients and other distortions to the pure sinusoidal waveform of the supply voltage will surely affect the performance of these equipment pieces adversely. The causes of power quality problems are generally not simple to understand and are difficult to detect. When we speak technically, the ideal ac power supply by the utility system should be a pure sinusoidal waveform of fundamental frequency (50 Hz). Also in addition to that, the peak value of the voltage should match with the rated value (230V for single phase).But the voltage that we receive from the utility is far from the ideal specifications. So we are lacking standard quality power, which can cause production loss, various equipments may be damaged or can even be detrimental to health of users. It is therefore imperative that a
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Ms. Deepthy Thomas
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Mr. Polly Thomas
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Mr. Deepu E. Koshy koshy.deepu @gmail.com
high standard of power quality is maintained. Voltage Quality relates to the purity of the characteristics of the voltage waveform including the absolute voltage level and frequency. Quality of supply means Un-interrupted supply of power with sinusoidal voltage and current waveform at acceptable frequency and magnitude. Voltage or power quality occurs due to Steady State Variations e.g. nonlinear characteristics of loads, furnace/induction heating loads, switching of converters etc.(resulting in harmonics, notching and noise). Main causes of poor power quality are nonlinear loads , adjustable-speed drives, Traction drives Start of large motor loads, Arc furnaces, Intermittent loads transients , Lightning, Switching, transients, Faults etc. Harmonics are carried through the system from the source and can nearly double the amount of current on the neutral conductor in three phase four wire distribution systems. Distorted currents from harmonic-producing loads also distort the voltage, which appear to other end users on the system. Overall electrical system performance and power quality is affected by the introduction of harmonics. Actually the PQ refers to the quality of voltage. Sources provide some-what perfect sinusoidal voltage. The alternating current flowing through the impedance (resistance and reactance) of the system can cause disturbances to this voltage. Different classes of PQ problems are Transients, voltage variation (Long duration), voltage variation (Short duration), Waveform Distortion, Voltage Flickering, Voltage imbalances. Distortion is a qualitative term, which is indicating the deviation of voltage waveform from its ideal sine wave characteristics. Harmonic distortion refers to the continuous deformation of the waveforms (voltage or current). Harmonic distortion is generally caused by the power electronics equipments, usually that converts AC to DC, namely rectifiers (controlled). These rectifiers instead of drawing continuous and current waveform of purely sinusoidal shape from the power source draws rather in short, rapid bursts. These loads which are non linear in nature draws what appears to be a discontinuous current waveform or a chopped current through the entire electrical system all the way till the power source end(transformer, utility system, generator). As more power electronics or electronics loads are added to the existing
International Conference on Magnetics, Machines & Drives (AICERA-2014 iCMMD) electrical system, due to distortions produced by each load, the magnitude of harmonic distortion increases.
critical load respectively. S represents the master switch which controls the non critical load.
Nonlinear loads produce impact on the entire system as they can create harmonic currents which can move to many locations in the electrical system and it can also come back to the power source itself[2]-[5]. Harmonic voltages and currents can produce a many effects that are harmful to the utility equipments and to entire power system. Harmonic currents and voltages are produced as a result of load characteristics. Since we are dealing with these types of loads, we must understand that harmonic voltages and currents will depends on the loads. One way to avoid the impact of harmonics is by using harmonic filters [6].
Load current through the critical load can be controlled by varying the firing angle of thyristors Tc1 and Tc2.
Two types of commonly used harmonic filters are: passive and active filters. Filters that employ usage of passive components such as inductors, capacitors and resistors are called passive filters. An arrangement of passive components incorporated to the power electronic devices, are tuned to the harmonic frequency component that is to be filtered. Inductor and capacitor ratings required are chosen in such a way that, it offers only a lower impedance to the harmonic frequency that is to be filtered out. So due to the low impedance offered by the filter to harmonic frequency when compared to the impedance of the power source, the harmonic frequency current will be circulating between the filter and the load[7]. This helps to keeps the components of harmonic current of the desired frequency away from the source and other loads in the system. To filter out other harmonic frequencies, additional tuned filters can be connected in parallel. Application and development of harmonic filters according to desired specification requires careful consideration. II.
SOURCE
REGULATOR
FIRING CIRCUIT FOR CRITICAL LOAD
CRITICAL LOAD
FIRING CIRCUIT FOR NON CRITICAL LOAD
NON CRITICAL LOAD
Fig.1 Block Diagram of the proposed method
SYSTEM CONFIGURATION
Fig.1. shows the block diagram of the proposed method. It consists of a critical load and a non critical load of same rating. The regulator circuit of block diagram controls the shape of the input waveform given to the critical load which produces distortion in the current waveform drawn from the source. Regulator uses switching devices connected in anti-parallel whose switching is controlled by the firing circuit for the critical load. Firing circuit for non critical load generates control signals by taking input from source, from the output of firing circuit of critical load and also from the regulated output of the critical load. Hence non critical load is triggered whenever the critical load is not drawing any current. Thus, the sum of currents drawn by the critical load and non critical load together preserves the overall current drawn from the source. A. Circuit Diagram Fig.2. shows the schematic circuit diagram of the proposed method. Critical and non critical loads are represented as resistors for simplicity. In actual practice the load can be of any type. But the proposed method is for resistive load. Vs represents the ac source, Is is the source current, Ic and Inc represents the current flowing through the critical and non
Fig.2. Schematic circuit Diagram of the proposed method
Similarly, current through the non critical load can be controlled by varying the firing angle of thyristors Tnc1 and Tnc2. B. Triggering of critical load Triggering circuitry for controlling the critical load is shown in fig.3.To adjust the firing angle of the thyristors Tc1 and Tc2, DC input voltage, Vdc to the comparator can be varied.
International Conference on Magnetics, Machines & Drives (AICERA-2014 iCMMD) V.
SIMULATION RESULTS
Proposed method was simulated in PSIM simulation software environment. The waveforms of gating pulses of critical load are shown in fig.5.The waveforms of gating pulses of non critical load are shown in fig.6. The waveforms of gating pulses to master switch are shown in fig.7. Fig.8 shows the current waveforms for critical load (Ic), non critical load (Inc) and source (Is) respectively. From fig.8, it was observed that the current drawn by critical and non critical load are complementary and hence the resultant current drawn from the source was continuous and nearly sinusoidal in shape. Thus the distortion of the source current waveform created by regulator circuit is completely eliminated here. Fig.3 Triggering circuitry for controlling the critical load III.
TRIGGERING OF NON CRITICAL LOAD
Triggering circuitry for controlling the non critical load is shown in fig.4. Thyristor Tnc1 should be triggered only during positive half cycle and meanwhile the thyristor Tc1 is not conducting. Thyristor Tnc2 should be triggered only during negative half cycle and meanwhile the thyristor Tc2 is not conducting.
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Fig.5 Gating pulses of critical load Vgnc1 1
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Fig.4 Triggering circuitry for controlling the non critical load IV.
NEED FOR MASTER SWITCH
If the switching pulse given to Tc1 and Tc2 are of short duration, multiple triggering of non critical load may occur during same half cycle, which is unnecessary and may cause waveform distortion. In order to avoid such undesired conduction, a master switch is introduced. MOSFET switch is used here. Triggering circuitry for controlling the master switch of non critical load is also shown in fig.4.
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Fig.6 Gating pulses of non critical load
International Conference on Magnetics, Machines & Drives (AICERA-2014 iCMMD) [6]
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Fig.7 Gating pulses to master switch
Fig.8. Current waveforms of critical load, non critical load and resultant source current respectively. VI.
CONCLUSION
A new method to improve the shape of input waveform has been demonstrated using electronic circuits. Simulation studies have been performed. It is concluded that desired results were obtained from the discussed circuitry. Simulation is done only for resistive loads. Control circuits can also be designed for other types of loads. REFERENCES [1] [2] [3] [4] [5]
A. Ghosh and G. Ledwich, Power quality enhancement using custom power devices. London, U.K.: Kluwer, 2002.C. Sankaran, Power Quality. CRC Press LLC 2002. G. T. Heydt, Electric power quality. West Lafayette, IN:Stars in a Circle, 1991. Muhammad H. Rashid, Power electronics handbook. ELSEVIER 2004. M.D. Singh, K.B. Khanchandani, Power electronics, McGraw Hill Education (India) Private Limited. Bhim Singh, Kamal Al-Haddad, and Ambrish Chandra, "A review of active filters for power quality improvement", IEEE Transactions On Industrial Electronics, Vol. 46, No. 5, October 1999.
IEEE Working Group on Non sinusoidal Situations, "Practical definitions for powers in systems with non sinusoidal waveforms, unbalanced loads: A discussion," IEEE Trans. Power Delivery, vol. 11, pp. 79-101, Jan.1996. Wang Kui, Guan Shuhua, Hou Qian, Hou Yuanhong, "Investigation of harmonic distortion and losses in distribution systems with non-linear loads” China International Conference on Electricity Distribution 2008.
