Review and Comparison of Step-Up Transformerless Topologies for Photovoltaic AC-Module Application

ABSTRACT:

This paper presents a comprehensive review of step-up single phase non isolated inverters suitable for ac-module applications. In order to compare the most feasible solutions of the reviewed topologies, a benchmark is set. This benchmark is based on a typical ac-module application considering the requirements for the solar panels and the grid. The selected solutions are designed and simulated complying with the benchmark obtaining passive and semiconductor components ratings in order to perform a comparison in terms of size and cost. A discussion of the analyzed topologies regarding the obtained ratings as well as ground currents is presented. Recommendations for topological solutions complying with the application benchmark are provided.

KEYWORDS:

1.      AC-module

2.      Photovoltaic(PV)

3.      Step-up Inverter

4.      Transformerless

SOFTWARE: MATLAB/SIMULINK

 BLOCK DIAGRAM:

Fig.1 Block diagram of a two stage topology for an ac module

STEP-UP TRANSFORMERLESS INVERTERS:

Fig 2 Boost converter and full bridge inverter

Fig 3 Time sharing boost converter with full bridge inverter

Fig 4 Parallel resonant soft switched boost converter and full bridge inverter

Fig 5 Parallel input series-output bipolar dc output converter and full bridge inverter

Fig 6 Boost converter and half bridge inverter

Fig 7 Boost converter and neutral point clamped inverter

Fig 8 Series combined boost and buck boost and half bridge inverter

Fig  9 Center-tapped coupled inductor converter with bipolar output and half bridge inverter

Fig 10 Single inductor bipolar output buck-boost converter and half bridge inverter

Fig  11 Boost + FB integrated and dual grounded

Fig 12 Block diagram of a pseudo-dc-link topology for an ac module

Fig 13 Buck-boost DCM converter and unfolding stage

Fig 14 Noninverting buck-boost DCM converter and unfolding stage

Fig 15 Switched inductor buck boost DCM converter and unfolding stage

Fig 16 Boost buck time sharing converter and unfolding stage

Fig 17 Block diagram of a single stage topology for an ac module

Fig 18 Universal single stage grid connected inverter

Fig 19 Integrated boost converter

Fig 20 Differential boost converter

Fig 21 Boost inverter with improved zero crossing.

Fig 22 Integrated Buck boost inverter

Fig 23 Buck Boost inverter with extended input voltage range

Fig 24 Differential buck boost inverter

Fig 25 Two sourced anti parallel buck boost inverter

Fig 26 Single stage full bridge buck boost inverter

Fig 27 Buck boost based single stage inverter

Fig 28 Switched inductor buck boost based single stage inverter

Fig 29 Single inductor buck boost based inverter

Fig 30 Doubly grounded single inductor buck boost based inverter

Fig 31 Single inductor  buck boost based inverter with dual ground

Fig 32 Three switch buck boost inverter

Fig 33 Coupled inductor buck boost inverter

Fig 34 Impedance-admittance conversion theory based inverter

Fig 35 Single phase Z source inverter

Fig 36 Semi quasi Z source inverter with continuous voltage gain

CONCLUSION:

In this paper, a comprehensive review of single phase non isolated inverters for ac module applications is presented. Both the grid connection and the solar panel requirements are analyzed emphasizing the leakage current regulation as it is a main concern in non isolated PV grid connected inverters. In order to compare the most suitable solutions of the reviewed topologies under the same specifications, a benchmark of a typical ac module application is set. These solutions have been designed and simulated, obtaining ratings for the passive and the semiconductor components. These ratings are used for the topology comparison in terms of size and cost. Furthermore, detailed simulations of representative topologies have been performed using semiconductor and inductor models to estimate the efficiency of the reviewed solutions. As a result of the comparison, the required voltage boost necessary for the connection to the European grid is difficult to achieve with transformerless topologies, but it is adequate for U.S. requirements. Two stage topologies, including the solution with dual grounding capability that theoretically avoids the ground leakage currents, are the preferred option for the set benchmark in which switching frequency for the dc-dc stage is set twice than for the dc-ac one. The two stage combination of a step-up dc-dc converter and a step-up inverter should be considered. In addition, the analyzed pseudo-dc-link approaches are an alternative solution in terms of size and cost. Furthermore, ground currents are expected to be low in these solutions because of the line frequency interface and weighted efficiency is the highest due to the flat behavior of the efficiency with the output power. The analyzed single stage topologies have higher cost than the other analyzed solutions and control is expected to be more complex to avoid dc current injection. In addition, DCM operation mode allows smaller solutions, including a solution with dual ground capability, but efficiency is lower due to the high RMS currents.

Operation of Series and Shunt Converters with 48-pulse Series Connected Three-level NPC Converter for UPFC

ABSTRACT:

The 48-pulse series connected 3-level Neutral Point Clamped (NPC) converter approach has been used in Unified Power Flow Controller (UPFC) application due to its near sinusoidal voltage quality. This paper investigates the control and operation of series and shunt converters with 48-pulse Voltage Source Converters (VSC) for UPFC application. A novel controller for series converter is designed based on the “angle control” of the 48-pulse voltage source converter. The complete simulation model of shunt and series converters for UPFC application is implemented in Matlab/Simulink. The practical real and reactive power operation boundary of UPFC in a 3-bus power system is specifically investigated. The performance of UPFC connected to the 500-kV grid with the proposed controller is evaluated. The simulation results validate the proposed control scheme under both steady state and dynamic operating conditions.

KEYWORDS:

1.      48-pulse converter

2.      Neutral Point Clamped (NPC) converter

3.      Angle control

4.      Unified Power Flow Controller (UPFC)

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Fig.1 Schematic of Unified Power Flow Controller (UPFC)

Fig. 2. 48-pulse VSC based +100 MVA UPFC in a 3-bus power system

EXPECTED SIMULATION RESULTS:

Fig.3 Line real power (top) and reactive power (bottom) references (MVA)

Fig. 4 Measured real and reactive power, DC link voltage and converter angles (Top trace: measured line real power (MW); second top trace: measured line reactive power, (MVar ); third top trace: DC bus voltage; fourth top trace: shunt converter angle α ; fifth top trace: series converter angle α ; bottom trace: series converter angle σ ).

Fig.5 Shunt converter output voltage (blue), Line voltage (green) and shunt

converter current (red) (5.42s-5.48s)

Fig.6 Shunt converter real power (blue, p.u.), reactive power (green, p.u.).

Fig.7 Current (p.u.) of transmission line L1.

Fig.8 Series converter 48 pulse converter voltage (blue, p.u.) and current

(black, p.u.) during time 2~2.03s (when real power reference is increased)

Fig.9 Series converter angle σ vs. DC bus voltage (Top trace: line real

power and reactive power; second top trace: shunt converter injected reactive

power; third top trace: DC bus voltage; bottom trace: series converter

angle σ )

 CONCLUSION:

In this paper, the control and operation of series and shunt converters with 48-pulse series connected 3-level NPC converter for UPFC application are investigated. A new angle controller for 48-pulse series converter is proposed to control the series injection voltage, and therefore the real and reactive power flow on the compensated line. The practical UPFC real and reactive power operation boundary in a 3-bus system is investigated; this provides a benchmark to set the system P and Q references. The simulation of UPFC connected to the 500-kV grid verifies the proposed controller and the independent real power and reactive power control of UPFC with series connected transformer based 48-pulse converter.

REFERENCES:

[1] N. G. Hingorani, "Power electronics in electric utilities: role of power electronics in future power systems," Proceedings of the IEEE, vol. 76, pp. 481, 1988.

[2] N. G. Hingorani and L. Gyugyi, Understanding FACTS: concepts and technology of flexible AC transmission systems: IEEE Press, 2000.

[3] L. Gyugyi, "Dynamic compensation of AC transmission lines by solid-state synchronous voltage sources," IEEE Transactions on Power Delivery, vol. 9, pp. 904, 1994.

[4] C. D. Schauder, L. Gyugyi etc. “Operation of the unified power flow controller (UPFC) under practical constraints,” IEEE Transactions on Power Delivery, vol. 13, pp. 630-639, April 1998.

[5] L. Gyugyi. “Unified power-flow control concept for flexible AC transmission systems,” IEE Proceedings - Generation, Transmission and Distribution, vo. 139, pp. 323-331, July 1992.

Analysis of 12 Pulse Phase Control AC/DC Converter

ABSTRACT:

In this paper, the unbalanced current in the 12- pulse phase control AC/DC converters was studied. The 12- pulse A-Y type AClDC converter will keep a balanced voltage with 30" phase shifted at the low coupling coefficient condition. But an unbalanced current will be obtained in the 12-pulse autotransformer phase shift AClDC converter at the low coupling coefficient condition. The theoretical phasor analysis of the unbalanced current was presented and a feedback controller was designed to overcome this problem. Finally, a 3 kW 12-pulse autotransformer phase shifted AClDC converter was implemented to demonstrate the theoretical analysis.

KEYWORDS:

1.      12 Pulse AClDC Converter

2.      Phase Controller

3.      Autotransformer

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Fig. 1. Conventional 12-pulse AClDC converter.

(a) A-Y isolated transformer.

(b) Autotransformer phase shifted.

Fig. 2. 12-pulse phase control A-Y connected AC/DC converter.

Fig. 3. I;!-pulse phase control autotransformer connected AC/DC

converter.

EXPECTED SIMULATION RESULTS:

Fig. 4 The output current io, and io, of 12-pulse phase control A-Y

typ: transformer ACDC converter with K=0.96 and a = 30"

Fig 5 The output current io, and io, of 12-pulse autotransformer

phase shift ACDC converter with K=0.96 and a = 30".

Fig. 6 The output current of 12-pulse autotransformer connected

AC/DC converter with the controller

Fig. 7 Experimental results for a resistive load without controller

Fig. 8 Experimental results for a resistive load with controller

CONCLUSION:

In this paper, the 12-pulse phase control ACDC converters with A-Y type and autotransformer type are analyzed and studied. The theoretical analysis is presented and the computer simulation results are performed. The 12- pulse A-Y type ACDC converter can function well under any firing condition. However, a serious unbalanced circulation current exists in the autotransformer connected ACDC converter at the non-unity coupling coefficient conditions. Finally, a 3 kW 12-pulse autotransformer phase controlled ACDC converter was implemented to demonstrate the theoretical analysis.

REFERENCES:

1. S. Choi, A. Jouanne, P. Enjeti and 1. Pitel, “New Polyphase Transformer Arrangements with Reduced kVA Capacities for Harmonic Current Reduction in Rectifier Type Utility Interface,“ IEEE PESC, 1995.

2. S. Choi, P. N. Enjeti, H. Lee and I. J. Pitel, “A New Active Interphase Reactor for 12-Pulse Rectifiers Provides Clean Power Utility Interface,” IEEE IAS, pp.2468-2474, 1995.

3. G. Oliver, G. E. April, E. Ngandui and C. Guimaraes, “Novel Transformer Connection to Improve Current Sharing on High Current DC Rectifier,” IEEE IAS, pp.986-962, 1993.

4. S. Miyairi, etc.al, “New Method for Reducing Harmonic Involved in Input and Output of Rectifier with Interphase Transformer,” IEEE Trans. On Industry Applications, Vol. IA-22, No.5, pp.790- 797, SepIOct, 1986.

5. A .R. Prasad, P. D. Ziogas, and S. Manias, “An Active Power Factor Correction Technique for Three-phase Diode Rectifier,” IEEE Trans. on Power Electronics, Vo1.6, No.1, pp.83-92, 1991