IEEE/OSA/IAPR International Conference on Informatics, Electronics & Vision
Design and Simulation of an Inverter with High Frequency Sinusoidal PWM Switching Technique for Harmonic Reduction in a Standalone/ Utility Grid Synchronized Photovoltaic System A.Z.M.Shahriar Muttalib, S.M.Ferdous Dept. of EEE American International University- Bangladesh (AIUB) Banani, Dhaka-1213 E-mail:
[email protected]
Abstract—Inverters are one of the major parts of any Photovoltaic Systems which are intended to feed power to any isolated standalone ac loads or to synchronize with the utility power grid systems. This paper discusses the design and simulation of a typical 1kW experimental precise sinusoidal Photovoltaic inverter which is intended to feed ac power at a standard 50Hz frequency to a mini grid powered by Photovoltaic solar cells. The very well-known and popular sinusoidal pulse width modulation (SPWM) technique with very high carrier frequency (in the order of kHz) has been chosen as the control scheme of the inverter by which it is also possible to synchronize the module to feed power on the 1-phase utility power grid. The high carrier frequency switching scheme enables to design a low pass smoothing filter for harmonic elimination resulting a reduction in Total Harmonic Distortion (THD) with small sized inductors and capacitors. As a prototype, the proposed circuit has been designed to supply small ac power, where the same design can be adopted and implemented to feed high power on the same grid only by increasing the device power ratings. Keywords-Inverter, Sinosudal Pulse width Modulation(SPWM), Total Harmonic Distortion (THD, Harmonic Reduction, Passive LC Filter
I.
INTRODUCTION
For any grid tied photovoltaic (PV) system, inverter is the essential piece of equipment that changes the DC power from the PV array to AC power used in the electrical grid. For high efficiency DC-AC conversion and peak power tracking, it must have low harmonic distortion along with low electromagnetic interference (EMI) and high power factor [1]. An inverter is evaluated after design by using the Inverter performance and testing standards which are IEEE 929-2000 and UL 1741 in the US EN 61727 in the EU and IEC 60364-7-712 [2]. The total harmonic distortion (THD) generated by the inverter is regulated by international standard IEC-61000-3-2 [2]. It requires that the full current THD be less than 5% and voltage THD be less than 2% for harmonic spectra up to 49th harmonic. Now designed topologies will enable the inverter to operate with near unity P.F. and THD less than 3-5% [3].
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Ahmed Mortuza Saleque, Nawjif Md. Anamul Hasan, Md. Masoom Chowdhury Dept. of EEE, (AIUB) Banani, Dhaka-1213 E-mail:
[email protected] II.
TYPES OF PV POWER SYSTEMS
PV systems can be classified according to their connection and arrangement within them or based on integration with the grid. They are A. Standalone System In general, stand-alone system is used in the rural areas where there is no sufficient facility to get an access to the main grid due to the technical problem or economical unfeasibility. Inverter of standalone system should maintain some features such as sinusoidal voltage, good voltage regulation and low harmonics in the output.
Figure 1.Schematic principle of a stand-alone PV system supplying a building
B. Hybrid In order to provide ecumenical, reliable and continuous power supply a combination of PV, wind and fossil foul generation are integrated to form a hybrid system which guarantees the same supply reliability as the public grid.
Figure 2.Schematic principle of a hybrid system with PV, wind, and diesel generators
ICIEV 2012
IEEE/OSA/IAPR International Conference on Informatics, Electronics & Vision C. Grid Concept Grid connected PV system always have a connection via a suitable inverter. Normally a highly PV integgrated network has an adverse impact as the system is going to be b introduced with number of nonlinear devices. In order to eliminate this undesirable situation properly designed filterr is required which might not be an economical solution. So appropriately designed inverter can be helpful for reducing the harmonics.
Figure 3. Block diagram of the power supply for a housse with a decentralized PV system and grid connection
III. INVERTERS FOR STAN-ALONE / GRIID CONNECTED SYSTEMS
Inverters are used to convert the DC output of PV or a storage battery in to AC electricity, in order to be fed in to the grid or to supply a standalone system. Moodern inverters are bidirectional those are capable of operatingg in both inverting and rectifying nodes. In many standalone PV installations, alternating current is needed to operate household utility appliances at 230 V, 50 Hz frequency. Gennerally stand-alone inverter operates at 12, 24, 48, 96, 120 or o even 240V dc depending on the power level [6].
For this type of inverter whhich is using SPWM scheme, a separate DC-DC converter is needed n for MPP tracking as the duty cycle is continuously vaarying. The design of a MPP tracker along with an algorithm m is a tedious and cumbersome task and that sort of discussioon is beyond the scope of this paper. ATION SCHEME IV. MODULA
In the inverter the input is dc d which is constant voltage. So in order to get an A.C voltage it requires such a controlling a well as the frequency can be system where the amplitude as controlled. For this kind of operation PWM (pulse Width modulation) inverters are beingg used. There are various kinds of PWM scheme which inclludes single pulse modulation, multiple pulse modulation and sinusoidal pulse width modulation (SPWM). But maainly we are going to focus on SPWM scheme as it improves the situation of the inverter by directly controlling the outpput voltage according to sine function. For the full bridge innverter the PWM is obtained by comparing the frequency of a high h frequency carrier with two sine waves. These two sine waaves are 180O phase shifted with each other. The control wavefoorm which is used to control the switch is achieved by comparring a sine wave at a particular frequency with a triangle wavee of fixed amplitude with much higher frequency. The comparaator gives out a pulse when sign voltage is greater than the trriangle voltage and this pulse voltage is used to trigger the resspective switches [4, 8]. The ratio between the trianggular wave and sine wave must be an integer N, the number of o voltage pulses per half-cycle, such that
2N =
fc fs
(1)
The first self-commutating inverters perrformed sine-wave modulation (SPWM – sinusoidal pulse widdth modulation) of the output current on the primary side and 500 Hz transformer to adapt the voltage level to grid voltage. In thhis paper, a similar type of inverter is designed, modeled and annalyzed to observe the performance.
Figure 5. SPWM modulation m scheme
Figure 4.GeneralOverview of the designeed system
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According to this scheme iff the peak of sine voltage is less than the half the triangular volttage harmonics less than the 2N are eliminated. For higher valuues of sinusoidal voltage, higher
ICIEV 2012
IEEE/OSA/IAPR International Conference on Informatics, Electronics & Vision order harmonics appear with amplitude of 15% less the fundamental [9]. The following equations are used in the sinusoidal PWM scheme: i. Percentage of individual harmonics is calculated by the equation,
%
⎛ 4 Mf ⎞ rms(n) (−1) i +1 cos nα i ⎟⎟ = 100× ⎜⎜ ∑ Vdc ⎝ nπ 2 p=1 ⎠
For this inverter an unipolar switching scheme is used rather than bipolar switching to achieve a better performance from the inverter. The simulated sine wave and triangular wave obtained from the two oscillator circuit are shown in Fig. 7.
(2)
n = nth harmonics ii.
Total Harmonic Distortion (THD) is given by
THD = where, V h =
Vh V1
(3) ∞
∑
V n2
n = 2 , 3 ,..
A. Generation of SPWM Signal Sinusoidal pulse width modulation(SPWM) means that, the output voltage is controlled according to a sine function. The control signal is achieved by comparing a reference sine wave produced by an oscillator of a particular frequency fS with a triangular wave of fixed amplitude with a much higher frequency fC (generally in kHz range). Both the sine and triangular waves are compared with each other by using a simple comparator circuit to generate the switching waveform S1 and S4 (i.e. for positive cycle switching). The comparator detects the crossing points of the two signals and produces there from the required control signal with desired duty cycle. This signal is applied to the gates of the switching devices of the inverter and thus producing the chopped PWM voltage at the output. An inverted version of the reference sine wave is again compared with the carrier wave to generate the switching waveform S2 and S3 (i.e. for negative cycle). The total circuit arrangement is shown in detail in Fig. 6.
Figure 7.Simulated sinusoidal reference signal (50Hz) and triangular carrier signal (1.6 kHz) generated by the switching waveform genetation circuit of Figure 6.
B. Reduction of harmonics of the inverter output Inverter output waveform may vary to a large extent depending upon the application and the circuit used. In most cases, an AC load requires sinusoidal output but the majority of the inverters produce square wave voltages. Therefore, appropriate means are adopted to alter the waveform of the inverter output to a more or less sinusoidal waveshape. Harmonic attenuation can be achieved by several methods such as by resonating the load, by an LC filter, pulse width modulation, sinewave synthesis, selected harmonic reduction and by polyphase inveters [9]. C. Harmonic Attenuation by Pulse Width Modulation Generally several PWM scheme is employed to reduce harmonics. Among them sinusoidal pulse width modulation is used for this inverter. Use of SPWM reduces further the harmonic content at the inverter output, the reduction being more with large number of pulses per half cycle. For the modulation scheme used here the value of N is equal to 1600/50 = 32; which implies that all harmonic voltages below the 64th harmonics should be absent [8, 9]. But as a bipolar triangular wave is used as carrier signal instead of unipolar signal, the number of eliminated harmonics will be upto 32 because of the presence of the negative cycles in the signal. D. Harmonic Attenuation by LC filter In this filter, attenuation of the harmonic comonents increases with frequency. The phase shift through the filter is also a function of frequency and is nearly zero at low frequencies and approaches 180º at higher frequencies. The filter should be designed to make a good compromisation between maximum inverter current and voltage regulation. The LC resonance frequency should be less than the lowest harmonic to be attenuated. At the same time the load power factor should be considered in selecting the individual values of L and C [9].
Figure 6. Complete circuit diagram to generate the switching wave form (S1, S4) and (S2, S3) for the SPWM scheme.
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To get a pure sine wave output or an output with very low THD (Total Harmonic Distortion)from a pure square wave, a
ICIEV 2012
IEEE/OSA/IAPR International Conference on Informatics, Electronics & Vision Low pass filter with very sharpe response characteristic is required. At the same time the size of the filter components will be large to filter out the low frequency harmonic components specially the third and fifth harmonics. Modulation with a very high frequency signal shifts the harmonic components to a higher frequency points which enablethe filtering process much easier using a LC filter built with small sized inductor and capacitor with moderate frequency response characteristics as a very sharpe response is not that much needed. The value of inductor is calculated such that the voltage drop across the inductor is less than 3% of the inverter output voltage, V0 so that it satisfies the following equation [6]Iload max x 2πfL < 0.03V0
this case an ideal transformer is being used to step up the voltage to meet the specified output. In practical case there may be some sort of unpredictable change in the output depending on the time constant of the system. The transformer is placed before the filter for scaling down the filter size as the transformer it self takes part in filtering process. C. Operation and simulation of the inverter circuit (without filtering) As per design, the control scheme the circuit arrangement is simulated in the Pspice (version-16.3). The control signal of the (S1, S4) and (S2, S3) are shown in Figure 8 (a) and (b) respectively.
(4)
where Iload max is the maximum RMS load current and f is output voltage frequency (50Hz). V.
SEQUENTIAL APPROACH OF DESIGN AND SIMULATION OF THE INVERTER
The design of the inverter is done based on the following output specifications: Power 1kW Input voltage (d.c) 12V Output voltage (a.c) 240V Output frequency 50Hz Total harmonic distortion
