Multi-String Five-Level Inverter with Novel PWM Control Scheme for PV Application Nasrudin A. Rahim, Senior Member, IEEE and Jeyraj Selvaraj

Transcription

1 Multi-String Five-Level Inverter with Novel PWM Control Scheme for PV Application Nasrudin A. Rahim, Senior Member, IEEE and Jeyraj Selvaraj Abstract This paper presents a single-phase multi-string five-level PV inverter topology for grid-connected photovoltaic (PV) systems with a novel PWM control scheme. Three PV strings are cascaded together in parallel configuration and connected to a five-level inverter to produce output voltage in five levels: zero, /V dc, V dc, -/V dc and -V dc. Two reference signals identical to each other with an offset equivalent to the amplitude of the triangular carrier signal were used to generate PWM signals for the switches. DSP TMS3F8 is used to implement this PWM switching scheme together with a digital PI current control algorithm. The inverter offers much less THD and can operate at near unity power factor. The validity of the proposed inverter is verified through simulation and implemented in a prototype. The experimental results are compared with conventional single-phase multi-string three-level grid-connected PWM inverter. Index Terms PWM inverter, photovoltaic (PV), PI current control, multilevel inverter, grid-connected, multi-string. A I. INTRODUCTION s the world is concerned with fossil fuel exhaustion and environmental problems caused by the conventional power generation, renewable energy sources particularly solar energy and wind energy have become very popular and demanding. PV sources are used today in many applications as they have the advantages of being maintenance and pollution free []. Solar-electric-energy demand has grown consistently by %-5% per annum over the past years, which is mainly due to the decreasing costs and prices. This decline has been driven by ) an increasing efficiency of solar cells; ) manufacturing-technology improvements; 3) economies of scale; []. PV inverter which is an important element in the PV system is used to convert DC power from the solar modules into AC power to be fed into the grid. A general overview of different types of PV inverters is given in [3]-[4]. This paper presents a multi-string five-level inverter for PV application. The multi-string inverter shown in Fig. is a further development of the string inverter, where several strings are interfaced with their own dc-dc converter to a common dc-ac inverter [5]. This is beneficial, compared with the centralized system, since every string can be controlled individually. Thus, the operator may start his/her own PV power plant with a few modules. Further enlargements are easily achieved since a new string with a dcdc converter can be plugged into the existing platform. A flexible design with high efficiency is hereby achieved [3]. In this work, a five-level inverter is used instead of a conventional three-level PWM inverter because it offers great advantages such as mproved output waveforms, smaller filter size, lower EMI, lower THD, and others [6]-[]. PV String PV String PV String 3 DC DC DC DC DC DC DC BUS Fig.. Configuration of multi-string inverters DC AC Grid This paper proposes a single-phase multi-string five-level inverter topology. It consists of three strings of PV arrays connected to their own dc-dc boost converter. An auxiliary circuit comprising of four diodes and a switch is configured together with a conventional full-bridge inverter to form this topology. A novel PWM control scheme is introduced to generate switching signals for the switches and to produce five output voltage levels: zero, /V dc, V dc, -/V dc and -V dc (assuming V dc is the supply voltage). This inverter topology uses two reference signals instead of one to generate PWM signals for the switches. Both the reference signals V ref and V ref are identical to each other except for an offset value equivalent to the amplitude of the carrier signal V carrier as shown in Fig.. V carrier V ref Manuscript received February 3, 9. Accepted for publication August 6, 9. Copyright 9 IEEE. Personal use of this material is permitted. However, permission to use this material for any other purposes must be obtained from the IEEE by sending a request to pubs-permissions@ieee.org N. A. Rahim and J. Selvaraj are with the Center of Research for Power Electronics, Drives, Automation and Control (UMPEDAC), Department of Electrical Engineering, University Malaya, 563 Kuala Lumpur, Malaysia (phone: ; fax: ; jeyraj95@um.edu.my) -V carrier Fig.. Carrier and reference signals V ref Copyright (c) 9 IEEE. Personal use is permitted. For any other purposes, Permission must be obtained from the IEEE by ing pubs-permissions@ieee.org. Authorized licensed use limited to: University of Malaya. Downloaded on November 4, 9 at 4:53 from IEEE plore. Restrictions apply.

2 Since the inverter is used in a PV system, a PI current control scheme is employed to keep the output current sinusoidal and to have high dynamic performance under rapidly changing atmospheric conditions and to maintain the power factor at near unity. Simulation and experimental results are presented to validate the proposed inverter configuration. II. MULTI-STRING FIVE-LEVEL INVERTER TOPOLOGY The proposed single-phase multi-string five-level inverter topology is shown in Fig. 3. It consists of three dc-dc boost converters connected to a common dc bus, an auxiliary circuit and a full-bridge inverter configuration. Input sources, PV string, PV string and PV string 3 are connected to the inverter via the dc-dc boost converters. Since the proposed inverter is used in a grid-connected PV system, the utility grid is used instead of a load. The dc-dc boost converters are used to track the maximum power point (MPP) independently as well as to step-up the inverter output voltage V inv to be more than of the grid voltage V g to ensure power flow from the PV arrays into the grid [3]-[4]. As a step-up transformer with a ratio of : is used, V inv should be: V g Vinv > or () Vg Vinv > () Therefore, the dc bus voltage is assumed to be approximately V. In this work, multi-string approach is adopted since each dc-dc-converter can independently perform maximum power point tracking (MPPT) for its PV strings. This will compensate for mismatches in panels of like manufacture, which can be up to.5% [5]. It offers the further advantage of allowing panels to be given different orientations and so open up new possibilities in architectural applications. Furthermore, a greater tolerance to localized shading of panels can be achieved. Another advantage of multi-string configuration is the mixing of different sources becomes possible i.e. existing PV panel strings could be extended by adding new higher output panels without compromising overall string reliability or performance. Besides that, greater safety during installation and maintenance adds to the advantages of multi-string configuration. Depending on the design, each converter module may be able to isolate its connected power source, so that the wiring of series or parallel connection of these strings can be performed safely. The power-source-converter connection is a safe low-voltage connection [6]. The dc-dc boost converters are connected in parallel to avoid high dc bus voltage which eventually will increase the size of the capacitors and the inverter s cost. Therefore, only two capacitors with equal capacitance rating are used as the dc bus and the other dc-dc boost converters is connected to this dc bus as shown in Fig. 3. A filtering inductance L f is used to filter the current injected into the grid. The injected current must be sinusoidal with low harmonics distortion. In order to generate sinusoidal current, sinusoidal PWM is used since it is one of the most effective methods. Sinusoidal PWM is obtained by comparing a high-frequency carrier signal with a low-frequency sinusoid signal, which is the modulating signal or reference signal. The carrier has a constant period; therefore the switches have constant switching frequency. The switching instant is determined from the crossing of the carrier and the modulating signal. PV String 3 PV String PV String DC-DC Boost Converter L D L L3 S S S3 D D3 DC Bus Auxiliary Circuit Full-Bridge Inverter Vpv/ C Vpv/ C Fig. 3. Single-phase multi-string five-level inverter topology. D4 S4 D5 D6 D7 III. PWM MODULATION Modulation index M a for five-level PWM inverter is given as [7] Am M a = (3) Ac where A c is the peak-to-peak value of carrier and A m is the peak value of voltage reference V ref. Since in this work two reference signals identical to each other are used, equation (3) can be expressed in terms of amplitude of carrier signal V c by replacing A c with V c. and A = V = V = V. m Vref M = (4) Vc If M>, higher harmonics in the phase waveform is obtained. Therefore, M is maintained between and. If the amplitude of the reference signal is increased higher than the amplitude of the carrier signal, i.e. M>, this will lead to overmodulation. Large values of M in sinusoidal PWM techniques lead to full overmodulation [8]. Fig. 4 shows the carrier and reference signals for different values of M. V carrier V carrier V ref V ref (a) ref S5 S6 ref Lf Ig ref S7 S8 Vinv : Vg Copyright (c) 9 IEEE. Personal use is permitted. For any other purposes, Permission must be obtained from the IEEE by ing pubs-permissions@ieee.org. Authorized licensed use limited to: University of Malaya. Downloaded on November 4, 9 at 4:53 from IEEE plore. Restrictions apply.

3 3 V carrier V ref P m [( ) sin( nαm )] 4Vdc A n = (8) nπ m= V carrier V ref IV. OPERATIONAL PRINCIPLES OF MULTI-STRING FIVE- LEVEL INVERTER V carrier V carrier V carrier V carrier V ref V ref (b) (c) V ref V ref Combinations of PV strings are used as the input voltage sources. The voltage across the strings are known as V pv, V vp and V pv3. Referring to equations () and (), V pv, V vp and V vp3 are boosted by the dc-dc boost converters to exceed the grid voltage V g and the voltage across the dc bus is known as V pv. The operational principle of the proposed inverter is to generate five-level output voltage i.e., V pv /, V pv, -V pv /, - V pv as in Fig. 5. As illustrated in Fig 3, an auxiliary circuit which consists of four diodes and a switch S 4 is used between the dc bus capacitors and the full-bridge inverter. Proper switching control of the auxiliary circuit can generate halflevel of PV supply voltage i.e. V pv / and -V pv /. [7]. Two reference signals V ref and V ref will take turns to be compared with the carrier signal at a time. If V ref exceeds the peak amplitude of the carrier signal V carrier, V ref will be compared with the carrier signal until it reaches. At this point onwards, V ref takes over the comparison process until it exceeds V carrier. This will lead to a switching pattern as shown in Fig. 5. Switches S 4 -S 6 will be switching at the rate of the carrier signal frequency while S 7 and S 8 will operate at a frequency equivalent to the fundamental frequency. Table illustrates the level of V inv during S 4 -S 8 switch on and off. (d) V ref Fig. 4. Carrier and reference signals for different values of modulation index, M (a) M=.3. (b) M=.5. (c) M=.7. (d) M=.. From the PWM modulation, the analysis of harmonic components in the proposed inverter can be preformed. The output voltage produced by comparison of the two reference signals and the carrier signal can be expressed as [7] V ( ) = A (A cos nθ B sin nθ ) o θ n= n If there are P pulses per quarter period, and it is an odd number, the coefficients B n and A o would be a zero where n is an even number. Therefore, the equation (5) can be rewritten as n n=,3... ( θ) = A cos nθ n (5) Vo (6) P 4 int( i/) [ ( ) sin( nαm i )] Vdc A n = (7) nπ m= i= where m is a pulse number and α is the phase displacement angle. The Fourier series coefficients of the conventional single-phase full-bridge inverter by sinusoidal PWM is given as Vpv Vpv/ -Vpv/ -Vpv S 4 S 5 S 6 S 7 S 8 Vinv α α α3 α4 π Fig. 5. Inverter output voltage, V inv and switching pattern for single-phase five-level inverter π V ref Copyright (c) 9 IEEE. Personal use is permitted. For any other purposes, Permission must be obtained from the IEEE by ing pubs-permissions@ieee.org. Authorized licensed use limited to: University of Malaya. Downloaded on November 4, 9 at 4:53 from IEEE plore. Restrictions apply.

4 4 TABLE INVERTER OUTPUT VOLTAGE DURING S 4 -S 8 SWITCH ON AND OFF S 4 S 5 S 6 S 7 S 8 V inv ON OFF OFF OFF ON V pv / OFF ON OFF OFF ON V pv OFF ON or OFF OFF or ON ON or OFF OFF or ON ON OFF OFF ON OFF -V pv / OFF OFF ON ON OFF -V pv If one of the PV strings is disconnected from the dc bus, the operation of the other dc-dc boost converters will not be affected as they are connected in parallel. As the dc-dc boost converters is used to track the maximum power point tracking (MPPT) point, it can be concluded that the MPPT tracking of the PV strings is done independently. Later expansion of the PV strings is also possible by adding a dc-dc boost converter as shown in Fig. 6. PV String PV String PV String 3 PV String n L L L 3 L n S S S 3 S n D D D 3 D n PV String Extention Fig. 6. PV string extension for existing configuration. V. CONTROL SYSTEM ALGORITHM AND IMPLEMENTATION e of the problems in the PV generation systems is the amount of the electric power generated by the solar arrays is always changing with weather conditions, i.e., the intensity of the solar radiation. A MPPT method or algorithm, which has quick response characteristics and is able to make good use of the electric power generated in any weather, is needed to solve the above problem [9]. Various MPPT control methods have been discussed in detail in []-[]. In this paper, Perturb and Observe (P&O) algorithm is used to extract maximum power from the PV arrays and deliver it to the inverter. The feedback controller used for the inverter is the proportional integral (PI) algorithm. As shown in Fig. 7, C C - the current injected into the grid also known as grid current I g is sensed and fed back to a comparator which compares it with the reference current I ref. I ref is obtained by sensing the utility grid voltage V g. The sensed V g signal is converted into the reference signal before it is multiplied with variable m. Therefore I ref = V g x m (9) Variable m is the sum of m, m and m 3 i.e, m = m m m 3 () Variables m, m and m 3 are obtained from the MPPT algorithm as illustrated in the flowchart in Fig. 8. Variables m, m and m 3 correspond to MPPT algorithm for string, string and string 3 respectively. The values of m, m and m 3 change with respect to irradiance level. If irradiance level is high, the corresponding values of m, m and m 3 are also high. Thus, by referring to equation (), it will lead to high value of m. Since I ref is proportional to m, high value of I ref is obtained. As a result, the inverter s output current I g will be high as it follows I ref to minimize the instantaneous error between I g and I ref. The instantaneous current error is fed to a PI controller. The integral term in the PI controller improves the tracking by reducing the instantaneous error between the reference and the actual current. The resulting error signal u which forms V ref and V ref is compared with a triangular carrier signal and intersections are sought to produce PWM signals for the inverter switches. This is to ensure I g to be in phase with grid voltage V g and always at near unity power factor. A. Mathematical Formulation The PI algorithm can be expressed in the continuous time domain as: u(t) = K pe(t) Ki e( τ )dτ where: u (t) is the control signal t τ = e (t) is the error signal t is the continuous-time domain time variable τ is the calculus variable of integration K is the proportional mode control gain p K i is the integral mode control gain () Implementing this algorithm by using a DSP requires one to transform it into the discrete time domain. Trapezoidal sum approximation is used to transform the integral term into the discrete time domain since it is the most straightforward technique. The proportional term is directly used without approximation. P term: K p e(t) = K pe(k) () t k τ = i= h I term: K i e() τ dτ K i [e(i) e(i )] (3) Time relationship : t = k * h Copyright (c) 9 IEEE. Personal use is permitted. For any other purposes, Permission must be obtained from the IEEE by ing pubs-permissions@ieee.org. Authorized licensed use limited to: University of Malaya. Downloaded on November 4, 9 at 4:53 from IEEE plore. Restrictions apply.

5 5 where : h is the sampling period k is the discrete-time index: k =,,,... For simplification, it is convenient to define the new controller gains as: ' h K i = K i (4) From which one can construct the discrete-time PI control law as: k ' i i= u(k) = K pe(t) K [e(i) e(i )] (5) To eliminate the need to calculate the full summation at each time step (which would require an ever increasing amount of computation as time goes on), the summation is expressed as a running sum: sum(k) = sum(k ) [e(k) e(k )] (6) u( k ) ' i = K e( k ) K sum( k ) (7) p These two equations, which represent the discrete-time PI control law, are implemented in the DSP TMS3F8 to control the overall operation of the inverter. I PV L D PV String 3 PV String PV String I PV I PV3 S V PV L S V PV L 3 S 3 V PV3 D D 3 C C D 4 S 4 D 5 D 6 D 7 S 5 S 6 I g L f S 7 S 8 Vinv : V g S 4-S 8 Gate Drivers I PV V PV P PV MPPT m u PI Controller I PV V PV P PV MPPT m m I ref error - I g I PV3 P PV3 MPPT3 m 3 V g V PV3 DSP TMS3F8 Fig. 7. Multi-string five-level inverter with control algorithm implemented in DSP TMS3F8. Copyright (c) 9 IEEE. Personal use is permitted. For any other purposes, Permission must be obtained from the IEEE by ing pubs-permissions@ieee.org. Authorized licensed use limited to: University of Malaya. Downloaded on November 4, 9 at 4:53 from IEEE plore. Restrictions apply.

6 6 START K P Sense V(k) & I(k) I g - error u u Yes P(k)-P(k-)= K i Anti-windup Anti-windup No No P(k)-P(k-)> Yes Fig. 9. PI control algorithm implemented in DSP TMS3F8 V(k)-V(k-)> V(k)-V(k-)> VI. SIMULATION AND EPERIMENTAL RESULTS Yes Increase mx x=,,3,. Fig. 8. MPPT flowchart No Decrease mx RETURN No Increase mx Yes Decrease mx B. Algorithm Implementation Control signal saturation and integral mode anti-windup limiting are easily implemented in software. In this work, the control signal itself takes the form of pulse-width modulated (PWM) outputs from the DSP. Therefore, the control signal is saturated at the value that corresponds to % duty cycle for the PWM. An undesirable side-effect of saturating the controller output is the integral mode windup. When the control output saturates, the integral mode control term (i.e., the summation) will continue to increase, but will not produce a corresponding increase in the controller output (and hence will not produce any additional increase in the plant response). The integral can become quite large, and it can take a long time before the controller is able to reduce it once the error signal changes sign. The effect of windup on the closed-loop output is larger transient overshoot and undershoot, and longer settling times. e approach for overcoming the integral-mode windup is to simply limit in the software the maximum absolute value allowed for the integral, independent of the controller output saturation [3], as shown in Fig. 9. A. Simulation Results Simulations were performed by using MATLAB SIMULINK to verify that the proposed inverter can be practically implemented in a PV system. It helps to confirm the PWM switching strategy for the multi-string five-level inverter. Then, this strategy is implemented in a real-time environment i.e. the DSP to produce PWM switching signals for the switches. Fig. (a) shows the way the PWM switching signals are generated by using two reference signals and a triangular carrier signal. The resulting PWM signals for switches S 4 to S 8 are shown in Fig. (b)-(f). Note that one leg of the inverter is operating at a high switching rate equivalent to the frequency of the carrier signal while the other leg is operating at the rate of fundamental frequency (i.e. 5Hz). The switch at the auxiliary circuit S 4 also operates at the rate of the carrier signal. As mentioned earlier, the modulation index M will determine the shape of the inverter output voltage V inv and the grid current I g. Fig. shows simulation results of V inv and I g for different values of M. Referring to equations () and (), the dc bus voltage is set to V ( > V g /, in this case V g is 4V) to inject current into the grid. Fig. (a) shows V inv is less than V g / due to M being less than.5. The inverter should not operate at this condition because the current will be injected from the grid 4 PWM Signal Generation PWM Switching Signal for Switch S4 3 V ref.5 Amplitude S4.5 - V carrier V ref (a) (b) Copyright (c) 9 IEEE. Personal use is permitted. For any other purposes, Permission must be obtained from the IEEE by ing pubs-permissions@ieee.org. Authorized licensed use limited to: University of Malaya. Downloaded on November 4, 9 at 4:53 from IEEE plore. Restrictions apply.

7 7 PWM Switching Signal for S5 PWM Switching Signal for S6.5.5 S5.5 S (c) (d) PWM Switching Signal for S7 PWM Switching Signal for S8.5.5 S7.5 S (e) (f) Fig.. PWM switching strategy and PWM signal for S 4 -S 8 5 Inverter Output Voltage (Vinv) 3 Grid Current (Ig) 5 Voltage (V) -5 Current (A) (a) (b) 5 Inverter Output Voltage (Vinv) 5 Grid Current (Ig) 5 5 Voltage (V) 5-5 Current (A) (c) Inverter Output Voltage (V) (d) Grid Current (Ig) Voltage (V) 5-5 Current (A) (e) (f) Fig.. Inverter output voltage (V inv ) and grid current (I g ) for different values of M (a) V inv for M <.5. (b) I g for M <.5. (c) V inv for M >.. (d) I g for M >.. (e) V inv for.5 M.. (f) I g for.5 M.. Copyright (c) 9 IEEE. Personal use is permitted. For any other purposes, Permission must be obtained from the IEEE by ing pubs-permissions@ieee.org. Authorized licensed use limited to: University of Malaya. Downloaded on November 4, 9 at 4:53 from IEEE plore. Restrictions apply.

8 8 into the inverter as shown in Fig. (b). Over modulation condition, which happens when M >., is shown in Fig. (c). It has a flat top at the peak of the positive and the negative cycles because both the reference signals exceed the maximum amplitude of the carrier signal. This will cause I g to have a flat portion at the peak of the sine waveform as shown in Fig (d). To optimize the power transferred from the PV arrays to the grid, it is recommended to operate at.5 M.. V inv and I g for optimal operating condition are shown in Fig. (e) and (f). As I g is almost a pure sinewave, the total harmonic distortion (THD) can be reduced compared with that under other values of M. B. Experimental Results The simulation results are verified experimentally by using a DSP TMS3F8. Three PV strings with different types of solar modules and locations are connected to the five-level inverter via a common dc bus. Table illustrates the PV modules characteristics and their location while Table 3 shows the multi-string five-level inverter s specifications and its controller parameters. The prototype inverter is shown in Fig.. PWM switching signals for the switches is generated by comparing a triangular carrier signal with two reference signals as shown in Fig. 3. TABLE 3 PV MULTI-STRING FIVE-LEVEL INVERTER SPECIFICATIONS AND CONTROLLER PARAMETERS S -S 8 : IGBT IRG4PC4UDPBF V CE =6, I C =A D -D 7 : RHRP3 V RR =V, I=3A L -L 3 :.mh L f : 3mH C -C : uf V DC =5V Aluminium Electrolytic K p : K i :. Switching Frequency : khz Sampling Frequency : 78kHz TABLE CHARACTERISTICS OF PV MODULES PV STRING Model : SIEMENS SP75 No. of Panels : 6 in series Max Power : 75W Short circuit current, I SC : 4.8A MPPT current, I MPPT : 4.4 A Open Circuit voltage, V OC :.7V MPPT voltage, V MPPT :7.V Location : Roof Top PV STRING Model : SIEMENS SP85 No. of Panels : 4 in series Max Power : 85W Short circuit current, I SC : 5.45A MPPT current, I MPPT : 4.95 A Open Circuit voltage, V OC :.V MPPT voltage, V MPPT :7.V Location : Gnd Floor Fig.. Prototype of the multi-string five-level PWM inverter. (a) PV STRING 3 Model : MITSUBISHI PV-AE5MF5N No. of Panels : 4 in series Max Power : 5W Short circuit current, I SC : 7.9 A MPPT current, I MPPT : 7.3 A Open Circuit voltage, V OC :.8V MPPT voltage, V MPPT :7.3 V Location : Roof Top Copyright (c) 9 IEEE. Personal use is permitted. For any other purposes, Permission must be obtained from the IEEE by ing pubs-permissions@ieee.org. Authorized licensed use limited to: University of Malaya. Downloaded on November 4, 9 at 4:53 from IEEE plore. Restrictions apply.

9 9 (b) (c) Fig. 3. PWM switching signals for S 4 - S 8, (a) S 4. (b) S 5 and S 6 (c) S 7 and S 8. Code Composer Studio (CCS), the programming platform for DSP TMS3F8, programs the control algorithm for the proposed multi-string five-level inverter. CCS offers the advantage of graph displaying, which can be used to investigate the results as in Figs. 4 to 8. As the input voltage of each string is converted into floating point values for DSP manipulation, the values corresponding to Figs. 4 to 8 do not represent the actual values of the input voltage but are good enough for investigation and analysis. Fig. 4 shows the input of PV strings, and 3. Here, PV string is on while PV string 3 is off. When PV string is turned from off to on, m increases to increase the amplitude of I ref because more power is generated at the input, subsequently increasing the current injected into the grid. As PV string shuts down, its voltage goes to zero while PV string maintain its MPP as illustrated in Fig. 5. m decreases to decrease the amplitude of I ref as the current injected into the grid is less because PV string stops producing power. The same phenomena happen when PV string is on, PV string is off and PV string 3 is turned from off to on as shown in Fig. 6. When PV string 3 is turned off, m decreases as in Fig. 7 to decrease I ref. As a result, less current is injected into the grid compared with the previous condition when PV string 3 was on. Fig. 8 is captured when all three strings are on initially. Then PV string 3 is turned off followed by string. m decreases when PV string 3 is turned off and it decreases further when PV string is turned off. This shows that the strings are working independently and later expansion of the strings is possible. Fig. 9 shows experiment results for the grid voltage V g and the inverter s output voltage, V inv. As the grid voltage had been stepped down to half the actual voltage by using a : ratio transformer, the magnitude of V g is now V. To inject current into the grid, V inv > V g ; V inv is thus set at V. Fig. - illustrates the experiment results for V inv and I g for 8A, 5A, and 3A, respectively. It can be seen that V inv consists of five levels of output voltage, and I g has been filtered to resemble a pure sinewave. The magnitude of V inv did not change, but maintained at V as the current injected into the grid decreased when irradiance level decreased. The modulation index M is.8. For M less than.5, V inv is less than V g /. Therefore, current will be injected from the grid into the inverter as shown in Fig. 3. This condition should be avoided to protect the PV system from damage. In the case of M being more than., the results are not shown since the PV system is designed to operate at conditions of M being less than. This is done by calculating the input current and input voltage corresponding to the output voltage and output current. Then, M is varied accordingly, for the inverter to operate at minimum and maximum power conditions. Below the minimum power condition (for example, during heavy clouds or night time) or above the maximum power condition (for example, over rating of PV arrays which exceeds the rating of the inverter) the inverter should not operate to ensure the safety of the PV system and the environment. To prove that the proposed multi-string five-level inverter has advantages over the conventional multi-sting three-level inverter in terms of THD and power factor, the corresponding measurements were made on both inverters. FLUKE 43B Power Quality Analyzer was used for this purpose. The conventional multi-string three-level inverter for gridconnected PV application is shown in Fig. 4. The same current control techniques were used to control the overall performance of the inverter. The only difference between both inverters is the elimination of the auxiliary circuit, and therefore only one dc bus capacitor is used. Fig. 5 shows the THD measurement for the multi-string five-level inverter while Fig. 6 shows the THD measurement for the multi-string threelevel inverter. The %THD for five-level inverter is recorded as 5.7% while the %THD for three-level inverter is 9.5%. From both illustrations the THD measurement for multi-string fivelevel inverter is much lower than that of the multi-string threelevel inverter. The power factor measurement is shown in Fig. 7. It is notable that both the grid voltage V g and the current injected into the grid I g are in phase with a power factor of.99. Fig. 8 illustrates the relationship between I g and the THD measurement. It shows that as I g increases, the THD decreases. Since I g is increased by increasing the modulation index M to force more current injected into the grid, it can be concluded that M is proportional to I g. Efficiency measurements were carried out to compare the efficiency of the multi-string three-level PWM inverter with the multi-string five-level PWM inverter for PV application. Copyright (c) 9 IEEE. Personal use is permitted. For any other purposes, Permission must be obtained from the IEEE by ing pubs-permissions@ieee.org. Authorized licensed use limited to: University of Malaya. Downloaded on November 4, 9 at 4:53 from IEEE plore. Restrictions apply.

10 Table 4 illustrates the measured efficiency of both inverters operating at different output powers. At 96W and 6W operating conditions, the measured efficiency of the multistring three-level PWM inverter was approximately 9% while the measured efficiency for the multi-string five-level PWM inverter was 86%. For 36W operating condition, the efficiency decreased to 89%, and to 84%, for the three-level PWM inverter, and the five-level PWM inverter, respectively. As expected, the efficiency of the multi-string five-level PWM inverter is lower compared to the conventional multistring three-level PWM inverter. The main reason is the addition of the auxiliary circuit between the dc-dc boost TABLE 4 MEASURED EFFICIENCY OF THREE-LEVEL AND FIVE-LEVEL PWM INVERTER AT DIFFERENT OUTPUT POWER Power (W) Efficiency, Three-level PWM inverter (%) Efficiency, Five-level PWM inverter (%) I g =8A 9 86 I g =5A 9 86 I g =3A PV string PV string 3 Off PV string V MPPT PV string Fig. 4. Conditions during PV string s being switched on and PV string s being switched from off to on. Copyright (c) 9 IEEE. Personal use is permitted. For any other purposes, Permission must be obtained from the IEEE by ing pubs-permissions@ieee.org. Authorized licensed use limited to: University of Malaya. Downloaded on November 4, 9 at 4:53 from IEEE plore. Restrictions apply.

11 This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. P V s trin g P V s trin g 3 P V s trin g P V s trin g Fig. 5. Conditions during PV string s being switched on and PV string s being switched off. V MPPT P V s trin g P V s trin g 3 P V s trin g P V s trin g 3 Fig. 6. Conditions during PV string s being switched on and PV string 3 s being switched from off to on. Copyright (c) 9 IEEE. Personal use is permitted. For any other purposes, Permission must be obtained from the IEEE by ing pubs-permissions@ieee.org. Authorized licensed use limited to: University of Malaya. Downloaded on November 4, 9 at 4:53 from IEEE plore. Restrictions apply.

12 This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. P V s tr in g 3 P V s tr in g P V s tr in g 3 P V s tr in g Fig. 7. Conditions during PV string s being switched on and PV string 3 s being switched off. P V s t r in g P V s tr in g 3 P V s tr in g 3 P V s tr in g P V s t r in g Fig. 8. Conditions during PV string s being switched on, and PV strings s and 3 s being switched off. Copyright (c) 9 IEEE. Personal use is permitted. For any other purposes, Permission must be obtained from the IEEE by ing pubs-permissions@ieee.org. Authorized licensed use limited to: University of Malaya. Downloaded on November 4, 9 at 4:53 from IEEE plore. Restrictions apply.

13 3 converters and the full-bridge inverter configuration. Switching losses of switch S 4 in the auxiliary circuit caused the efficiency of the multi-string five-level PWM inverter to be approximately 4% less than the multi-string three-level PWM inverter. However, simulation and experimental results show that the THD of the proposed inverter is lower as compared with the conventional three-level PWM inverter which is an important element for grid-connected PV systems. Fig.. Experimental result of V inv and I g at I g =3A for M=.8. Fig. 9 Experimental result of V g and V inv for M=.8. Fig. 3. Experimental result of V inv and I g for M =.. IPV L D Fig.. Experimental result of V inv and I g at I g =8A for M=.8. PV String 3 PV String PV String S VPV L IPV S VPV IPV3 L3 S3 VPV3 D D3 C S4 S5 Ig Lf S6 S7 Vinv : Vg S4-S7 Gate Drivers IPV VPV PPV MPPT m u PI Controller IPV VPV PPV MPPT m m Iref error - Ig IPV3 PPV3 MPPT3 m3 Vg VPV3 DSP TMS3F8 Fig.. Experimental result of V inv and I g at I g =5A for M=.8. Fig. 4. Conventional multi-string three-level PWM inverter for PV application Copyright (c) 9 IEEE. Personal use is permitted. For any other purposes, Permission must be obtained from the IEEE by ing pubs-permissions@ieee.org. Authorized licensed use limited to: University of Malaya. Downloaded on November 4, 9 at 4:53 from IEEE plore. Restrictions apply.

14 4 THD vs Ig %THD Ig (A) Fig. 8. Relationship between Ig and THD measurement Fig. 5. THD result of multi-string five-level PV inverter VII. CONCLUSION This paper presented a single-phase multi-string five-level inverter for PV application. A novel PWM control scheme with two reference signals and a carrier signal were used to generate the PWM switching signals. The circuit topology, control algorithm, and operational principle of the proposed inverter were analyzed in detail. The configuration is suitable for PV application as the PV strings operate independently and later expansion is possible. Furthermore, experimental results indicate that the THD of the multi-string five-level inverter is much less than that of the conventional multi-string three-level inverter. In addition, both the grid voltage and the grid current are in-phase at near-unity power factor. Fig. 6. THD result of multi-string three-level PV inverter Fig. 7. Grid Voltage V g and Grid Current I g at near -unity power factor REFERENCES [] N. A. Rahim, Saad Mekhilef, Implementation of Three- Phase grid Connected Inverter for Photovoltaic Solar Power Generation System Proceedings IEEE. PowerCon. Vol, pp , Oct. [] J. M. Carrasco, L. G. Franquelo, J. T. Bialasiewicz, E. Galvan, R. C. PortilloGuisado, M. A. M. Prats, J. I. Leon, N. Moreno-Alfonso, "Power- Electronic Systems for the Grid Integration of Renewable Energy Sources: A Survey," IEEE Trans. on Industrial Electronics, vol. 53, no. 4, pp. - 6, Aug 6. [3] S. B. Kjaer, J. K. Pedersen, F. Blaabjerg, A Review of Single-Phase Grid Connected Inverters for Photovoltaic Modules, IEEE Trans. Ind. Appl., vol. 4, no. 5, pp. 9 36, Sept/Oct. 5. [4] S. Daher, J. Schmid, F. L. M. Antunes, Multilevel Inverter Topologies for Stand-Alone PV Systems, IEEE Trans. on Industrial Electronic, vol. 55, no. 7, pp. 73-7, July. 8. [5] M. Meinhardt, G. Cramer, Past, Present and Future of Grid-Connected Photovoltaic and Hybrid-Power-Systems, in Proc. IEEE-PES Summer Meeting, vol., pp , July. [6] S. Kouro, J. Rebolledo, J. Rodriguez, "Reduced Switching-Frequency- Modulation Algorithm for High-Power Multilevel Inverters," IEEE Trans. on Industrial Electronics, vol. 54, no. 5, pp , Oct. 7. [7] S. J. Park, F. S. Kang, M. H. Lee, C. U. Kim, A New Single-Phase Five- Level PWM Inverter Employing a Deadbeat Control Scheme, IEEE Trans. Power Electronics., vol. 8, no. 8, pp , May. 3. [8] L. M. Tolbert, T. G. Habetler, Novel Multilevel Inverter Carrier-Based PWM Method, IEEE Trans. Industry Application., vol. 35, no.5, pp , Sept/Oct 999. [9] Y. Liu, H. Hong, A. Q. Huang, Real-Time Calculation of Switching Angles Minimizing THD for Multilevel Inverters with Step Modulation, IEEE Trans. on Industrial Electronic, vol. 56, no., pp , Feb. 9. [] N. S. Choi, J. G. Cho, G. H. Cho, A General Circuit Topology of Multilevel Inverter, Power Electronics Specialists Conference, 99. SC IEEE th Annual Volume, 4-7 June 99 pp Copyright (c) 9 IEEE. Personal use is permitted. For any other purposes, Permission must be obtained from the IEEE by ing pubs-permissions@ieee.org. Authorized licensed use limited to: University of Malaya. Downloaded on November 4, 9 at 4:53 from IEEE plore. Restrictions apply.

15 5 [] G. Carrara, S. Gardella, M. Marchesoni, R. Salutari, G. Sciutto, A New Multilevel PWM method: A Theoretical Analysis, IEEE Trans. Power Electronics., vol. 7, no. 3, pp , July. 99. [] R. Gonzalez, E. Gubia, J. Lopez, L. Marroyo, Transformerless Siingle- Phase Multilevel-Based Photovoltaic Inverter, IEEE Trans. on Industrial Electronic, vol. 55, no. 7, pp , July. 8. [3] M. Calais A Cascaded Inverter for Transformerlss Single Phase Grid- Connnected Power Electronics Specialists Conference,. PESC. IEEE 3th Annual Volume 3, 8-3 June pp: Vol.3. [4] J. Selvaraj, N. A. Rahim, Multilevel Inverter for Grid-Connected PV System Employing Digital PI Controller, IEEE Trans. on Industrial Electronics, vol. 56, no., pp , Jan 9. [5] T. Noguchi, S. Togashi, R. Nakamoto, Short-Circuit Pulse-Based Maximum-Power-Point Tracking Method for Multiple Photovoltaic-and- Converter System, IEEE Trans. on Industrial Electronics, vol. 49, pp. 7-3, Feb. [6] G. R. Walker, P. C. Sernia, Cascaded DC-DC Converter Connection of Photovoltaic Modules, IEEE Trans. Power Electronics., vol. 9, no. 4, pp. 3-39, July. 4. [7] V. G. Agelidis, D. M. Baker, W. B. Lawrance, C. V. Nayar, A Multilevel PWM Inverter Topology for Photovoltaic Application, in Proc. IEEE ISIE 97, Guimaraes, Portugal, 997, pp [8] Muhammad H. Rashid, Power electronics: Circuits, Devices, and Applications, 3 rd ed. Pearson Prentice Hall, 4, pp.67. [9] Nobuyoshi Mutoh, Takayoshi Inoue, A Control Method to Charge Series-Connected Ultraelectric Double-Layer Capacitors Suitable for Photovoltaic Generation Systems Combining MPPT Control Method, IEEE Trans. on Industrial Electronics, vol. 54, no., pp , Feb. 7. [] Esram T, Chapman P.L, Comparison of Photovoltaic Array Maximum Power Point Tracking Techniques, IEEE Trans. Energy Conversion, vol., no., pp , June 7. [] Femia N., Petrone G., Spagnuolo G., Vitelli M.: Optimizing duty-cycle perturbation of P&O MPPT technique Power Electronics Specialists Conference, 4. PESC 4. 4 IEEE 35th Annual Volume 3,-5 June 4 pp Vol.3. [] Liu., Lopes L.A.C.: An improved perturbation and observation maximum power point tracking algorithm for PV arrays Power Electronics Specialists Conference, 4. PESC 4. 4 IEEE 35th Annual Volume 3, -5 June 4 pp. 5 - Vol.3. [3] David M.Aletr, Thermoelectric cooler control using TMS3F8 DSP and a DRV59 power amplifier Texas Instruments Application Report. Nasrudin A. Rahim (M 89-SM 8) was born in Johor, Malaysia, in 96. He received the B.Sc. (Hons.) and M.Sc. degrees from the University of Strathclyde, Glasgow, U.K., and the Ph.D. degree in 995 from Heriot- Watt University, Edinburgh, U.K. He is currently a Professor in the Department of Electrical Engineering, University of Malaya, Kuala Lumpur, Malaysia and Director of Center of Research for Power Electronics, Drives, Automation and Control (UMPEDAC).. Prof. Rahim is the Chairman of the Working Group WG-8 Covering Reluctance Motor of the IEEE Motor Subcommittee under IEEE-PES Electric Machinery Committee His research interests includes power electronics, real-time control systems, and electrical drives. Jeyraj Selvaraj was born in Kedah, Malaysia in 98. He received the B.Eng(Hons) from Multimedia University, Malaysia and M.Sc in Power Electronics and Drives from University of Birmingham and University of Nottingham, U.K in and 4 respectively. He also obtained his PhD degree from University Malaya, Malaysia in 9. He is currently pursuing his carrier at the Center of Research for Power Electronics, Drives, Automation and Control (UMPEDAC), Department of Electrical Engineering, University Malaya, Malaysia. His research interests are single-phase and three-phase multi-level inverters, digital PI current control techniques, photovoltaic inverters and dc-dc converters. Copyright (c) 9 IEEE. Personal use is permitted. For any other purposes, Permission must be obtained from the IEEE by ing pubs-permissions@ieee.org. Authorized licensed use limited to: University of Malaya. Downloaded on November 4, 9 at 4:53 from IEEE plore. Restrictions apply.