New Simplified Aalborg Inverter for Inverter Applications

IJSRD - International Journal for Scientific Research & Development| Vol. 4, Issue 04, 2016 | ISSN (online): 2321-0613

New Simplified Aalborg Inverter for Inverter Applications Joel Joy1 Sreeja C2 1 M. Tech Scholar 1,2 Department of Electrical and Electronics Application 1,2 Amal Jyothi College of Engineering, Kanjirapally Abstract— Nowadays world is facing energy crisis with shortage of power. Renewable energy sources are being utilized so as to overcome this situation. For the conversion of renewable energy to required power, new kinds of electronic converters are being used. A new dc-ac inverter is proposed by the operation of both boost and buck converter circuit. It is the simplified version of conventional Aalborg Inverter. Conventional Aalborg inverter has high switching frequency, which is reduced in the Simplified Aalborg Inverter. Over a period of AC cycle only one switch is under switching stress, with only one inductor working in the power loop. An additional natural soft switching part is added in the conventional aalborg inverter. Both advantages of VSI and CSI is achieved using buck and boost mode of operations.

Addition of soft switching components further reduces switching losses. Simulation studies show that the simplified aalborg inverter improves the advantages.

Key words: VSI, CSI, Aalborg Inverter, PI, SPWM

switching conditions. Thus switching losses and cost is reduced, thereby increasing the efficiency.[4] II. CONVENTIONAL AALBORG INVERTER The Aalborg name comes from university in Denmark, where the topology was initially tested. Compared with the inverter in [5], the main difference is that the physical position of boost stage and buck stage has been exchanged and one inductor can be saved. So theoretically, the related conduction power loss is also reduced and a higher efficiency can be achieved. Fig. 1 shows the half-bridge Aalborg inverter. As the switching frequency increases, distortions as well as component parameter values will also be reduced. The LCL filter present in the circuit plays a major role in producing an AC voltage. The main advantage is that there are less number of switches working at high frequency during an half cycle. Also only one inductor is being utilized during one half cycle.[5]

I. INTRODUCTION The world is advancing its technology to higher level, trying to reduce energy consumption. The conventional power resources are becoming extinct, with the increasing demand power loads. Renewable energy conversion and the development of distributed generation (DG), are now being researched world widely[1]. In practice, there are different types of voltage disturbances and faults in grid, which originate from various non-ideal conditions and equipment failure in power systems. Therefore, the proper control strategies are required to operate the grid converters efficiently under such imperfect grid conditions. Most of previous research work of DG system under faulty grid voltages are focused on the voltage source inverter (VSI) based grid converters [2]. Differently, the current source inverter (CSI) offers the merits of simple configuration and PWM strategy, low dv/dt, high fault-current tolerant capability. The CSI has been widely used in the mediumvoltage high-power drive systems.[3] Inverting operation is sophisticated, if the input DC is varying. Also, the output AC must be harmonics free. The DC-to-AC converter produces some impacts to distribution network. When harmonic happens, resonance problems will then occur, leading to high harmonic currents and voltages [4]. For DC-to-AC converter, multilevel inverter is a good choice for PV system application. This is because it provides quite a low harmonics as compared to other conventional inverters. But as the step size is increased harmonics is reduced, also, the circuit becomes bulkier and control becomes difficult.[3] In Simplified Aalborg Inverter, soft switching is implemented in the conventional Aalborg Inverter also the control strategy is simplified. By using DC-DC converters for the inversion operation, large amount of switching elements are reduced. Soft switching provides zero current and voltage

Fig. 1: Conventional Aalborg Inverter The output filter makes the system bulkier and also the output power is affected. In the Simplified Aalborg inverter, the output filter component L2 is removed as shown in Fig. 2. Replacing the filter inductor is compensated by the designed PI controller used in the circuit.

Fig. 2: Simplified Aalborg Inverter

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New Simplified Aalborg Inverter for Inverter Applications (IJSRD/Vol. 4/Issue 04/2016/272)

The working of Simplified Aalborg inverter is symmetrical in both positive and negative half cycle of the AC grid voltage. The operation of Aalborg inverter during positive and negative half cycle of the ac grid voltage is explained as follows.

A. During Positive Half Cycle: The positive half cycle can be divided into three time intervals as shown in Fig. 3. During T1 and T3, S3 is ON, S1 works at high frequency, and the rest of the switches are OFF. The equivalent circuits are shown in Fig. 3(a) and it can be seen that it works like a pure buck converter with an LCLfilter connected to the grid. In this case, the buck converter is a classical VSI. During T2, S1 and S3 are ON, S2 works with high frequency, and the rest of the switches are OFF. The equivalent circuit is shown in Fig. 3(b) and it can be seen that it works like a pure boost converter with a CL-filter connected to the grid. If the current of the boost inductor can be fully controlled, this equivalent circuit can be seen as a CSI.[5]

Fig. 4: Negative Half Cycle Operation during a) T4 and T6 b) T5 C. Control Method: When the dc voltage is higher than the amplitude of the grid voltage, the buck control loop works and boost switches are OFF. The reference current can be derived as follows: V g (t ) i Buck  I0 (1) V0 where Rf is the equivalent resistor for calculating the generated power. When the input dc voltage is lower than the amplitude of the grid voltage, boost control loop works. The given current should be calculated through the energy balance equation as follows:

i Boost 

Fig. 3: Positive Half Cycle Operation during a) T1 and T3 b) T2 B. During Negative Half Cycle: The negative half cycle can be divided into three time intervals as shown in Fig. 4. During T4 and T6, S6 is ON, S4 works at high frequency, and the rest of the switches are OFF. The equivalent circuit is shown in Fig. 4(a) and it can be seen that it also works like a pure buck circuit with an LCL-filter connected to the grid. During T5, S4 and S6 are ON, S5 works at high frequency, and the rest of the switches are OFF. The equivalent circuit is shown in Fig. 4(b) and it can also be seen as a CSI with a CL-filter connected to the grid, if the current of the boost inductor can be fully controlled. The most outstanding character of this type inverter is that the minimum filtering inductance in the power loop is adopted and over filtering will not take place; so, theoretically, the proposed inverter has the merit to achieve higher efficiency than other inverters under the same condition of the input dc voltage.[5]

| V g (t ) |

2

(2) I0 V 0 E1 By comparing the reference currents for both boost and buck with the instantaneous inductor currents, the corresponding errors are detected. Based on these error PI based controllers are designed to reduce them to zero. The output of the PI is compared with a triangular waveform of 5kHz which is the required switching frequency, providing the switching pulses for the switches. Thus SPWM technique is implemented in the Simplified Aalborg Inverter. Either voltage feedback or current feedback can be used. Only boost switch operation need to be controlled when the current feedback based PI controller is adopted.

III. MODIFIED AALBORG INVERTER The proposed modified Aalborg Inverter is shown in Fig. 5.

In the modified aalborg inverter a quasi state soft switching stage is added. The switch Sd in series with the capacitance Cd performs the soft switching. With proper control scheme, the inverter can alternate between the voltage source inverter and the current source inverter to reduce the reverse recovery power losses of power stage, thus achieving more soft and quasi-soft switching transitions. Furthermore, unlike other traditional resonant soft- switching inverters, the soft- and quasi-switching states are independent of the variation of the parameters at the inverter side.

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New Simplified Aalborg Inverter for Inverter Applications (IJSRD/Vol. 4/Issue 04/2016/272)

Fig. 5: Modified Aalborg Inverter Two control strategies are to be used in the Modified Aalborg Inverter. PI controller is required for limiting the inductor current in the circuit for safe operation. Soft switching control is given for getting zero current and voltage switching conditions.

Simulation of the proposed topology was done in MATLAB software. The open loop control was implemented to simulate the working of Aalborg Inverter. The circuit is designed for 30V DC input to 50Hz 60V AC grid supply. The FFT analysis show 12.5% of Total Harmonic Distortion (THD).

[3] J. Li, J. Liu, and Z. Liu, "Comparison of Z-source inverter and traditional two-stage boost-buck inverter in grid-tied renewable energy generation,"IEEE Proceedings on International Power Electronics, Motion Control Conference, pp. 1493-1497,May 2009.

[4] W. Wu, Y. He, and F. Blaabjerg, "An LLCL-power filter for single-phase grid-tied inverter," IEEE Transactions on Power Electronics, volume 27, no. 2, pp. 782- 789, February 2012.

[5] W Wu, Junhao Ji, and F Blaabjerg," Aalborg InverterA New Type of "Buck in Buck, Boost in Boost Grid-Tied Inverter" IEEE Transactions on Power Electronics., volume 30, no. 9, pp. 4784-4793, September 2012.

[6] Muhammad H. Rashid, "Power Electronics Handbook Devices, Circuits, And Applications", ButterworthHeinemann Press, Oxford, UK, 2011.

[7] A Tere,I Feo,P pnik, "DC/DC Converters with Soft (ZVS) Switching" IEEE Transaction on Power Electronics, volume 28, no. 11, pp. 5063-5073, November 2013.

[8] C Carriero ,F Rains ,G Volpi , "Comparison Between Hard and Soft Switching Topologies for Low Voltage Low Power DC-DC Converter in Space Application," IEEE Transaction on Power Electronics, volume 28, no.

1, pp. 11-18, January 1999.

Fig. 6: Output Waveforms- Voltage and Current IV. CONCLUSION Modified aalborg inverter was designed and simulated in MATLAB Simulink software. In the simulation work, it was seen that the modified aalborg inverter works as per the theoretical approach. The high current owing through the inductor during the boost operation was removed. The PI controller improves the boost operation.

REFERENCES [1] J.W. Kolar, T. Friedli, J. Rodriguez, and P.W.Wheeler, "Review of three phase PWM ac-ac converter topologies," IEEE Transactions on Industrial Electronics, volume 58,no. 11, pp. 4988-5006, November 2011 [2] F. Z. Peng, "Z-source inverter," IEEE Transactions on Industrial Applications, volume 39, no. 2, pp. 504-510, March-April 2003.

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