Operation and Control of Smart Transformer for Improving Performance of Medium Voltage Power Distribution System

 ABSTRACT:  

Smart transformer (ST) is a power electronic based transformer equipped with effective control and communication. It is expected to play a significant role in future power distribution system, however, their operational features in the medium voltage (MV) power distribution systems are yet not explored. In this paper, operation and control of ST are presented for improving its performance and operational range in a power distribution system consisting of two radial feeders in a city center. For investigating the performance of ST in above system, one conventional power transformer (CPT) is replaced by the ST whereas other feeder is continued to be supplied through the CPT. In this scheme, the ST is operated such that it makes total MV grid currents of the combined system balanced sinusoidal with unity power factor. Therefore, in addition to providing continuous and reliable operation of ST based loads, the ST can also improve the performance of the loads which are supplied by the CPT in a different feeder. Moreover, the proposed scheme eliminates the need of power quality improvement devices at the other feeder. Therefore, the scheme also makes the application of ST in the distribution system cost effective. Simulation results validate the suitability of ST in improving the performance of multiple feeder medium voltage power distribution system.

KEYWORDS:

1.      Smart transformer (ST)

2.      Medium voltage rectifier

3.      Power distribution system

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Fig. 1. A schematic of conventional power distribution system consisting of

two radial feeders in a city center.

 EXPECTED SIMULATION RESULTS:

Fig. 2. Simulated waveforms when ST is not compensating for feeder II. (a) PCC voltages. (b) Grid currents. (c) MV rectifier currents (d) Feeder II currents.

Fig. 3. Simulated waveforms when ST is compensating for feeder II. (a) PCC voltages. (b) Grid currents. (c) MV rectifier currents (d) Feeder II currents.

Fig. 4. Simulated waveforms during transient conditions when load in feeder II is changed. (a) PCC voltages. (b) Grid currents. (c) MV rectifier currents (d) Feeder II currents.

CONCLUSION:

In this paper, the operation and control of a futuristic power distribution system consisting of two radial feeders with one CPT replaced by an ST is presented. It is shown that the ST can compensate for the loads connected at the feeder supplied by the CPTs, in addition to supplying their own loads. This scheme has potential to eliminate the requirement of power quality improvement devices such as STATCOM, power factor correcting capacitors, etc., connected at the second feeder. This ancillary feature in the medium voltage power distribution systems has potential to make application of ST more attractive and cost effective.

REFERENCES:

[1] S. Bifaretti, P. Zanchetta, A. Watson, L. Tarisciotti, and J. Clare, “Advanced power electronic conversion and control system for universal and flexible power management,” Smart Grid, IEEE Transactions on, vol. 2, no. 2, pp. 231–243, Jun. 2011.

[2] X. She, R. Burgos, G. Wang, F. Wang, and A. Huang, “Review of solid state transformer in the distribution system: From components to field application,” in Energy Conversion Congress and Exposition (ECCE),

2012 IEEE, Sep. 2012, pp. 4077–4084.

[3] S. Alepuz, F. Gonzalez, J. Martin-Arnedo, and J. Martinez, “Solid state transformer with low-voltage ride-through and current unbalance management capabilities,” in Industrial Electronics Society, IECON 2013 - 39th Annual Conference of the IEEE, Nov. 2013, pp. 1278–1283.

[4] S.-H. Hwang, X. Liu, J.-M. Kim, and H. Li, “Distributed digital control of modular-based solid-state transformer using dsp+fpga,” Industrial Electronics, IEEE Transactions on, vol. 60, no. 2, pp. 670–680, Feb. 2013.

A Unity Power Factor Converter with Isolation for Electric Vehicle Battery Charger

ABSTRACT:  

This paper deals with a unity power factor (UPF) Cuk converter EV (Electric Vehicle) battery charger having a high frequency transformer isolation instead of only a single phase front end converter used in vehicle's conventional battery chargers. The operation of the proposed converter is defined in various modes of the converter components i.e. DCM (Discontinuous Conduction Mode) or CCM (Continuous Conduction Mode) along with the optimum design equations. In this way, this isolated PFC converter makes the input current sinusoidal in shape and improves input power factor to unity. Simulation results for the proposed converter are shown for charging a lead acid EV battery in constant current constant voltage (CC-CV) mode. The rated full load and varying input supply conditions have been considered to show the improved power quality indices as compared to conventional battery chargers. These indices follow the international IEC 61000-3-2 standard to give harmonic free input parameters for the proposed  circuit.

KEYWORDS:

1.      UPF Cuk Converter

2.      Battery Charger

3.      Front end converter

4.      CC-CV mode

5.      IEC 61000-3-2 standard

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Fig. 1 General Schematic of an EV Battery Charger with PFC CUK Converter

 EXPECTED SIMULATION RESULTS:

(a)

(b)

(c)

Fig.2 Simulated performance of the isolated Cuk converter in rated condition (a) rated input side and output side quantities (b-c) harmonic analysis of the current at source end

(a)

(b)

(c)

Fig.3 Simulated performance of the isolated Cuk converter while input is varied to 270V (a) rated input side and output side quantities (b-c) harmonic analysis of the current at source end

(a)

(b)

(c)

Fig.4 Simulated performance of the isolated Cuk converter while input is reduced to 270V (a) rated input side and output side quantities (b-c) harmonic analysis of the current at source end

(a)

(b)

(c)

Fig.5 Simulated performance of the isolated Cuk converter at light load condition (a) rated input side and output side quantities (b-c) harmonic analysis of the current at source end

CONCLUSION:

An isolated Cuk converter based battery charger for EV with remarkably improved PQ indices along with well regulated battery charging voltage and current has been designed and simulated. The converter performance has been found satisfactory and well within standard for rated as well as different varying input rms value of supply voltages. The considerably improved THD in the current at the source end makes the proposed system an attractive solution for efficient charging of EVs at low cost. The proposed UPF converter performance has been tested to show its suitability for improved power quality based charging of an EV battery in CC-CV mode. Moreover, the cascaded dual loop PI controllers are tuned to have the smooth charging characteristics along with maintaining the low THD in mains current. The proposed UPF converter topology have the inherent advantage of low ripples in input and output side due to the added input and output side inductors. Therefore, the life cycle of the battery is increased. MATLAB based simulation shows the performance assessment of the proposed charger for the steady state and dynamics condition which clearly state that the proposed charger can sustain the sudden disturbances in supply for charging the rated EV battery load. Moreover, during whole disturbances in supply voltage, the power quality parameters at the input side, are maintained within the IEC 61000-3-2 standard and THD is also very low.

REFERENCES:

[1] Limits for Harmonics Current Emissions (Equipment current ≤ 16A per Phase), International standards IEC 61000-3-2, 2000.

[2] Muhammad H. Rashid, “Power Electronics Handbook, Devices, Circuits, and Applications”, Butterworth-Heinemann, third edition, 2011.

[3] N. Mohan, T. M. Undeland, and W. P. Robbins, Power Electronics: Converters, Applications and Design. Hoboken, NJ, USA: Wiley, 2009.

[4] B. Singh, S. Singh, A. Chandra and K. Al-Haddad, “Comprehensive Study of Single-Phase AC-DC Power Factor Corrected Converters With High-Frequency Isolation”, IEEE Trans. Industrial Informatics, vol. 7, no. 4, pp. 540-556, Nov. 2011.

[5] A. Abramovitz K. M. Smedley "Analysis and design of a tapped-inductor buck–boost PFC rectifier with low bus voltage" IEEE Trans. Power Electron., vol. 26 no. 9 pp. 2637-2649 Sep. 2011.

The Application of Electric Spring in Grid-Connected Photovoltaic System

ABSTRACT:  

The characteristics of distributed photovoltaic system power generation system is intermittent and instability. Under the weak grid conditions, when the active power of the PV system injected into the grid is fluctuant, the voltage of supply feeder will increase or decrease, thus affecting the normal use of sensitive load. The electric spring can transfer the energy injected into the supply feeder to the wide-voltage load, which is in series with the ES, to ensure the voltage stability of the sensitive load in the system. In this paper, a grid-connected photovoltaic simulation model with electric spring is built in Matlab / simulink. The voltage waveforms on the ES and sensitive load is obtained under the condition of changing the active power injected into the supply feeder by the grid-connected photovoltaic system. Thought the analysis of the waveforms, we can find that the Electric spring is a kind of effective method to solve the voltage fluctuation of the supply feeder in the grid-connected PV system.

KEYWORDS:

1.      Electric spring

2.      Grid-Connected Photovoltaic System

3.      Voltage Regulation

4.      Photovoltaic Consumption

SOFTWARE: MATLAB/SIMULINK

 BLOCK DIAGRAM:

Figure 1. The photovoltaic system model with Electric spring

 EXPECTED SIMULATION RESULTS:

Figure 2. The effective value of line voltage when the active power of PV system decreases

Figure 3. The line voltage when the active power of PV system increases (with ES)

CONCLUSION:

This paper applies the electric spring to the PV system to solve the problem that the bus voltage fluctuates due to the power fluctuation during the PV power injected into the bus. By building a simulation model in Matlab /Simulink, it is proved that the voltage on the bus can be effectively stabilized after adding the electric spring in the grid-connected photovoltaic system. When the active power of the PV fluctuates, the electric spring can transfer the voltage fluctuation on the bus to the wide-voltage load, in order to ensure that the bus voltage stability in the vicinity of the given value. Therefore, this is an effective method to solve the fluctuation of the bus voltage in PV grid connected system.

REFERENCES:

1. Hui S Y R, Lee C K, Wu F. Electric springs—A new smart grid technology[J]. IEEE Transactions on Smart Grid, 2012, 3(3): 1552-1561.

2. F. Kienzle, P. Ahein, and G. Andersson, “Valuing investments in multi-energy conversion, storage, and demand-Side management systems under uncertainty,” IEEE Trans Sustain. Energy, vol. 2, no. 2, pp. 194–202,Apr. 2011.

3. C. K. Lee and S. Y. R. Hui, “Input voltage control bidirectional power converters,” US patent application, US2013/0322139, May 31, 2013.

4. CHEN Xu, ZHANG Yongjun, HUANG Xiangmin. Review of Reactive Power and Voltage Control Method in the Background of Active Distribution Network[J]. Automation of Electric Power Systems,2016,40(01):143-

5. Lee S C, Kim S J, Kim S H. Demand side management with air conditioner loads based on the queuing system model[J]. IEEE Transactions on Power Systems, 2010, 26 (2): 661-668.

Control for Grid-Connected and Stand-Alone Operations of Three-Phase Grid-Connected Inverter

ABSTRACT:    

This paper describes a simple grid current control method for the grid-connected operation, and inverter voltage control method based on the phase locked loop (PLL) for the intentional islanding operation at the three-phase grid-connected inverter. The PLL controller based on the pq theory with a simple P-controller is used to synchronize the phase of inverter output voltage with a grid voltage at the grid-connected operation or generate a desired inverter output voltage at the islanding operation. The outputs of current controller are connected together to those of voltage controller, in order to prevent a sudden change of the outputs of both controllers during the transfer instant. The simulation and experimental results are carried out to verify the effectiveness of the proposed control strategies.

KEYWORDS:

1.      Distributed generation (DG)

2.      Grid-connected operation

3.      Islanding operation

4.      Phase locked loop (PLL)

5.      Three phase inverter.

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Figure 1. A control structure of three-phase grid-connected inverter.

EXPECTED SIMULATION RESULTS:

Figure 2. Simulation result for grid current control at grid-connected operation

Figure 3. Simulation result for inverter voltage control at the islanding operation.

CONCLUSION:

This paper described a simple grid current control method for the grid-connected operation, and output voltage control method based on the PLL for the intentional islanding operation at the three-phase grid-connected inverter. The PLL controller based on the pq theory with a simple P-controller which has no steady-state phase error, was used to synchronize the phase of inverter output voltage with a grid voltage or generate a desired voltage. As the outputs of current controller are connected together to those of voltage controller, the grid connected inverter was able to change smoothly from the grid connected operation to islanding operation. The experimental results showed that the proposed control schemes are capable of obtaining the good grid current response and also maintaining the inverter voltage within the desired level. The measured THDs of grid current and output voltage of inverter are only 1.92% and 1.89%, respectively.

REFERENCES:

[1] B. Kroposki, R. Lasseter, T. Ise, S. Morozumi, S. Papathanassiou, and N. Hatziargyriou, “Making Microgrids Work.” IEEE Power & Energy Mag., vol.6, no.3, pp.41-53, May/June, 2008.

[2] H. M. Kojabadi, B. Yu, I. A. Gadoura, L. Chang, and M. Ghribi, “A Novel DSP-Based Current-Controller PWM Strategy for Single Phase Grid Connected Inverters,” IEEE Trans. Power Electron., vol.21, no.4, pp.985-993, July 2006.

[3] I. J. Gabe, V. F. Montagner, and H. Pinheiro, “Design and Implementation of a Robust Current Controller for VSI Connected to the Grid Through an LCL Filter,” IEEE Trans. Power Electron., vol.24, no.6, pp.1444-1452, June 2009.

[4] J. C. Moreno, J. M. Espi. Huerta, P. G. Gil, and S. A. Geonzalez, “A Robust Predictive Current Control for Three-Phase Grid-Connected Inverters,” IEEE Trans. Ind. Electron, vol.56, no.6, pp.1993-2004, June 2009.

[5] K. J. Lee, B. G. Park, R. Y. Kim, and D. S. Hyun, “Robust Predictive Current Control Based on a Disturbance Estimation in a Three-Phase Grid-Connected Inverter,” IEEE Trans. Power Electron., vol.27, no.1, pp.276-283, Jan. 2012.