Development of Multifunctional Inverter based Grid-tied Solar Energy System
It is already established that if we want to secure the future of our coming generation we must reduce the pace of depleting all the natural resources that Mother Nature has to offer. We need to find a way through which we can maintain the overall growth without doing any harm to our environment. One of the most important ways to achieve this is to keep a continuous & steady flow of sustainable environment friendly energy supply.
Keeping these aspects in mind govt. of India took a very futuristic & bold measure of taking its renewable energy generation capacity from only around 57GW to 175GW within next six years. Although component of solar power (around 100GW) being the highest of the targeted renewable energy generation capacity (around 175GW) by 2022, it’s present installed capacity is the lowest (only 13% of targeted 100GW) among others (wind, biomass, waste to power, small hydro power plant e.t.c). Due to his India is going to install this huge 87GW capacity of solar power which is going to make it one of the leading green energy producers in the world and hence From 2015 on wards the Ministry of New and Renewable Energy [MNRE] under National Solar Mission began laying down actionable plans for solar power sector under its ambit to make a quantum jump, building on strong foundations to realize it intentions.
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Usually Output of a pv cell is in the range of milli-volt, thousands of individual pvcells are connected in series to add up this low voltage and to get some formidable output voltage in the range of some volt.One of the other problems we face with pv cell is that the output fluctuates fairly, depending upon various reasons (environmental or constructional constraints of pv cell).
This type of fluctuating output is neither useful to convert it to ac form using inverter or to store the energy in the batteries. On the other hand most of the time at ac load terminal (may be a grid, 3phase/single-phase load) of the solar system, constant voltage at a desired level is required. So to get a stable constant output from the inverter, we need to control the output voltage of inverters. Now control of output voltage of inverters can be achieved by controlling external of ac output voltage, or by controlling external dc input voltage, but due to the presence of various power handling stages loss is high & efficiency becomes low of the overall system. Also as in these methods use of filters are evident,the cost, weight & size increases, efficiency decreases & makes the transient response sluggish.
Hence the best method of inverter output voltage control is exercising an internal control within the inverters itself. This is achieved by pulse width modulation [PWM], as this doesn’t require any additional component the loss is least.
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It should be kept in mind that the moment we use power electronics circuit [chopper or inverter]harmonics gets injected in the circuit as power electronic switches are of nonlinear nature and presence of switching circuit for these electronic switches.These harmonics have various awful effect on system like lowers the power factor hence decreases quality of power, distorts voltage, increases electrical losses, Increase in the apparent power, Damage to capacitors, occurrence of harmonic resonance, transformer and neutral conductor overload due to excessive zero-phase currents, increase in conductor losses (skin effect), negative rotation sequences in motors, nuisance tripping e.t.c. Factors like current harmonics, unbalanced non-linear load, excessive neutral current, reactive power, also effectively brings same degree of problem which further constitute to inferior power quality in the system from distribution end.[C]
To get rid of these undesirable effects we need to minimize harmonics in the system. PWM technique mentioned earlier is also useful in this aspect as by defining a certain width of pulse it eliminates or minimizes lower order harmonics, although in the process of doing so it somewhat increases amplitude of higher order harmonics. But as higher order harmonics can be filtered easily [e.ga simple low pass filter could serve the purpose] the filtering requirements are easy.
Initially various configurations of passive filters like series or shunt were constructed which provided low or high impedance to certain harmonics according to requirement with the aim of minimizing their presence. Although these filters are very cheap,very soon it became difficult to implement them industrially due to their limitations like fixed compensation, resonance, poor dynamic response & bulky size e.t.c.[A-B]. To tackle the problems associated with passive filter active filter applications emerged. Active filters are basically switch mode power electronics converter which inserts harmonic currents in phase opposition into the system in the point of common coupling so that the system needs to supply only the fundamental current.[D-G] Although due to the large VA rating of these type of converter, use of active filter is restricted.
To take advantage of both of these filters without their limitations, various kinds of hybrid filter (combining active & passive filter) design has been proposed in recent years. On various test or simulated systems these type of filter showing improved compensations of balanced and unbalanced nonlinear load currents, effective harmonic and reactive compensation and adapt itself to compensate for variations in nonlinear load currents as well.[H]
Hence various types of hybrid filters presenting excellent harmonic mitigation and overall power quality improvement properties without any constraints, incorporation of these in practical grid-tied solar energy systems resulting in inferior power quality, seems to be evident. Also, further research, analysis & improvement of the proposed system is important to check the feasibility of actually implementing it in real time solar based energy systems.
Reference:
[A1] M.R.Sindhu, Manjula. G.Nair, T.N.P.Nambiar, An ANN Controlled Three phase Auto -tuned Passive Filter for Harmonic and Reactive power compensation, J. Power Electronics, 9 (2009),p.403-409.
[B2] M. M. Abdel Aziz, E. E.-D. About El-Zahab, A. M. Ibrahim, and A. F. Zobaa, LC Compensators for Power Factor Correction of Nonlinear Loads, IEEE Trans. Power Delivery, 19(2004,), p. 331.
[C] Seema Agrawal .D. K. Palwalia, Analysis of Standalone Hybrid PV-SOFC-BatteryGeneration System Based on Shunt Hybrid Active Power Filter for Harmonics Mitigation, (2016)IEEE, 978-1-4673 89624/16
[D] Bhim Singh, Kamal Al-Haddad, and Ambrish Chandra, A review of active filters for power quality improvement, IEEE Trans. Ind. Electron., 46(1990), p.960-971.
[E] J H. Akagi, Y. Kanazawa, A. Nabae, Instantaneous reactive power compensators comprising switching devices without energy storage components, IEEE Trans. Ind. Appl., 20(1984), p. 625-630.
[F] C.L. Chen, C.E. Lin, C.L. Huang, Reactive and harmonic current compensation for unbalanced three-phase systems using the synchronous detection method, Elect. Power Syst. Res., 26(1993), p163-170.
[G] G.Bhuvaneswari ,Manjula G. Nair, Design , simulation and Analog circuit implementation of a three phase shunt active filter using the I.COSM algorithm”, IEEE Trans. Power Delivery,23(2008), p.1222-1235.
[H] M. R. Sindhua, Manjula G.Nairb, T.N.P. Nambiarc, Three Phase Auto-tuned Shunt Hybrid Filter for Harmonic and Reactive Power Compensation, Elsevier Ltd (2015) p. 482-489.
Prof. Shubhajit Pal,
Department of Electrical Engineering,
University of Engineering & Management (UEM), Jaipur