Distributed Cooperative Control and Stability Analysis of Multiple DC Electric Springs in a DC Microgrid

ABSTRACT:  

Recently, dc electric springs (dc-ESs) have been proposed to realize voltage regulation and power quality improvement in dc microgrids. This paper establishes a distributed cooperative control framework for multiple dc- ESs in a dc microgrid and presents the small-signal stability  analysis of the system. The primary level implements a droop control to coordinate the operations of multiple dc-ESs. The secondary control is based on a consensus algorithm to regulate the dc-bus voltage reference, incorporating  the state-of-charge (SOC) balance among dc-ESs. With the design, the cooperative control can achieve average dc-bus voltage consensus and maintain SOC balance among different dc-ESs using only neighbor-to-neighbor  information. Furthermore, a small-signal model of a four dc-ESs system with the primary and secondary controllers is developed. The eigenvalue analysis is presented to show   the effect of the communication weight on system stability. Finally, the effectiveness of the proposed control scheme and the small-signal model is verified in an islanded dc microgrid under different scenarios through simulation and  experimental studies.

KEYWORDS:

1.      Consensus

2.      Dc microgrid

3.      Distributed control

4.      Electric springs (ES)

5.      Small-signal stability

SOFTWARE: MATLAB/SIMULINK

SCHEMATIC DIAGRAM:

Fig.1.distributed network with multiple dc -ESs

 EXPECTED SIMULATION RESULTS:

Fig. 2.SEZ. Controller comparison. (a) Node bus voltage, (b) dc-ESs output

power, (c) SOC, and (d) state variables xi .

Fig. 3. Proposed controller with different aij . (a) and (d) Average  bus voltages with aij = 0.5 and aij = 10. (b) and (e) State variables with aij = 0.5 and aij = 10. (c) and (f) Bus voltages with aij = 0.5  and aij = 10.

Fig. 4. dc-ES4 failure at 5 s. (a) Node bus voltage, (b) output power,  and (c) SOC.

Fig. 5. Proposed controller with communication delay τ . (a) Node bus  average voltage, (b) SOC, and (c) state variables xi .

Fig. 6. Proposed controller with five dc-ESs. (a) Node bus voltage, (b) output power, and (c) SOC.

CONCLUSION:

A hierarchical two-level voltage control scheme was proposed for dc-ESs in a microgrid using the consensus algorithm to estimate the average dc-bus voltage and promote SOC balance among different dc-ESs. The small-signal model of four dc-ESs system incorporating the controllers was developed for eigenvalues analysis to investigate the stability of the system. The consensus of the observed average voltages and the defined state variables has been proven. Results show that the control can improve the voltage control accuracy of dc-ESs and realize power sharing in proportion to the SOC. The resilience of the system against the link failure has been improved and the system can still maintain operations as long as the remaining communication graph has a spanning tree. Simulation and experimental results also verify that the correctness and effectiveness of the proposed model and controller strategy.

REFERENCES:

[1] X. Lu, K. Sun, J. M. Guerrero, J. C. Vasquez, and L. Huang, “State-ofcharge balance using adaptive droop control for distributed energy storage systems inDCmicrogrid applications,” IEEE Trans. Ind. Electron., vol. 61, no. 6, pp. 2804–2815, Jun. 2014.

[2] Q. Shafiee, T. Dragicevic, J. C. Vasquez, and J. M. Guerrero, “Hierarchical control for multiple DC-microgrids clusters,” IEEE Trans. Energy Convers., vol. 29, no. 4, pp. 922–933, Dec. 2014.

[3] W. Yao, M. Chen, J. Matas, J. M. Guerrero, and Z. M. Qian, “Design and analysis of the droop control method for parallel inverters considering the impact of the complex impedance on the power sharing,” IEEE Trans. Ind. Electron., vol. 58, no. 2, pp. 576–588, Feb. 2011.

[4] V. Nasirian, S. Moayedi, A. Davoudi and F. Lewis, “Distributed cooperative control of DC microgrids,” IEEE Trans. Power Electron., vol. 30, no. 4, pp. 6725–6741, Dec. 2014.

[5] J. M. Guerrero, J. C. Vasquez, J. Matas, L. G. de Vicu˜na, and M. Castilla, “Hierarchical control of droop-controlledAC andDCmicrogrids—A general  approach toward standardization,” IEEE Trans. Ind. Electron., vol. 58, no. 1, pp. 158–172, Jan. 2011.

Mitigating Distribution Power Loss of DC Microgrids with DC Electric Springs

 ABSTRACT:  

 DC microgrids fed with substantial intermittent renewable energy sources (RES) face the immediate problem of power imbalance and the subsequent DC bus voltage fluctuation problem (that can easily breach power system standards). It has recently been demonstrated that DC electric springs (DCES), when connected with series non-critical loads, are capable of stabilizing the voltage of local nodes and improving the power quality of DC microgrids without large energy storage. In this paper, two centralized model predictive control (CMPC) schemes with (i) non-adaptive weighting factors and (ii) adaptive weighting factors are proposed to extend the existing functions of the DCES in the microgrid. The control schemes coordinate the DCES to mitigate the distribution power loss in the DC microgrids, while simultaneously providing their original function of DC bus voltage regulation. Using the DCES model that was previously validated with experiments, simulations based on MATLAB/SIMULINK platform are conducted to validate the control schemes. The results show that with the proposed CMPC schemes, the DCES are capable of eliminating the bus voltage offsets as well as reducing the distribution power loss of the DC microgrid.

KEYWORDS:

1.      DC microgrids

2.      DC electric springs (DCES)

3.      Centralized model predictive control (CMPC)

4.      Non-adaptive weighting factors

5.      Adaptive weighting factors

6.      Distribution power loss

SOFTWARE: MATLAB/SIMULINK

CIRCUIT DIAGRAM:

Fig. 1. An m-bus DC microgrid with n RES units.

EXPECTED SIMULATION RESULTS:

Fig. 2. Waveforms of the power supply by RES and the bus voltages of the DC microgrid without DCES.

Fig. 3. Waveforms of the bus voltages of the DC microgrid when the DCES is installed at the five buses.

    

Fig. 4. Waveforms of the bus voltages of the DC microgrid with three DCES installed at bus 1, bus 4 and bus 5.

Fig. 5. The comparisons of the power loss on the distribution lines between α=1 and α=0.9 when three DCES are installed.

Fig. 6. Waveforms of the bus voltages of the DC microgrid with four DCES installed at bus 1, bus 2, bus 4 and bus 5.

Fig. 7. Comparisons of the power loss on the distribution lines for different values of α when four DCES are installed.

CONCLUSION:

DC electric springs (DCES) is an emerging technology that can be used to stabilize and improve the power quality of DC microgrids. In this paper, a centralized model predictive control (CMPC) with both non-adaptive weighting factors and adaptive weighting factors is proposed for multiple DCES to further mitigate the power loss on the distribution lines of a DC microgrid. Using a DCES model previously verified with experiments, simulation studies have been conducted for a DC microgrid setup. Simulation results on a 48 V five-bus DC microgrid show that the energy is saved about 49.4% in the 5 seconds when three DCES are controlled by the CMPC with non-adaptive weighting factors and is saved about 58.5% in the 5 seconds when four DCES are controlled by the CMPC with non-adaptive weighting factors. It is also demonstrated that the power loss on the distribution lines of the DC microgrid can be further reduced by the CMPC with adaptive weighting factors, as compared to the CMPC with non-adaptive weighting factors.

REFERENCES:

[1] B. C. Beaudreau, World Trade: A Network Approach, iUniverse, 2004.

[2] G. Neidhofer, “Early three-phase power,” IEEE Power and Energy Magazine, vol. 5, no. 5, pp.88−100, Sep. 2007.

[3] R. H. Lasseter and P. Paigi, “Microgrid: a conceptual solution,” in Proc. IEEE Power Electron. Spec. Conf., 2004, pp. 4285−4290.

[4] S. Anand, B. Fernandes, and J. Guerrero, “Distributed control to ensure proportional load sharing and improve voltage regulation in low-voltage dc microgrids,” IEEE Tran. Pow. Elect., vol. 28, no. 4, Aug. 2012.

[5] T. Gragicevic, X. Lu, J. C. Vasquez, and J. M. Guerrero, “DC microgrids−part I: a review of control strategies and stabilization techniques,” IEEE Trans. Pow. Elect., vol. 31, no. 7, Jul. 2016.

An Improved Beatless Control Method of AC Drives for Railway Traction Converters

ABSTRACT:  

The traction converter consists of a single phase AC-DC rectifier and a three-phase DC-AC inverter. Due to special structural characteristics of single phase rectifier, a fluctuating voltage component with the frequency twice of the grid’s, exists in DC-link voltage. Fed by fluctuating DC-link voltage, a beat phenomenon occurs in traction motor, and harmonic components appear in both stator current and electromagnetic torque, especially when motor operates near the ripple frequency. In this paper, the mechanism and influence of fluctuating voltage are analyzed in detail. Based on modeling analysis of motor and switching function of inverter, a frequency compensation factor is derived in vector control of induction motor. Then an improved frequency compensation control method is proposed to suppress beat phenomenon without LC resonant circuit. Finally the modified scheme is verified by simulation and experiment.

KEYWORDS:

1.      Fluctuating DC voltage

2.      Beat phenomenon

3.      Vector control

4.      Beatless control

SOFTWARE: MATLAB/SIMULINK

 BLOCK DIAGRAM:

Fig. 1. FOC with frequency compensation for Induction Motor

 EXPECTED SIMULATION RESULTS:

Fig. 2. Waves of stator current and electromagnetic torque of traction Motor

Fig. 3. FFT of stator current and electromagnetic torque before adding frequency compensation method

Fig. 4. FFT of stator current and electromagnetic torque after adding traditional frequency compensation method

Fig. 5. FFT of stator current and electromagnetic torque after adding improved frequency compensation method

 CONCLUSION:

In high-power traction converters, without LC filter circuit paralleled in DC-link, a fluctuating voltage twice of the grid frequency contains in DC-link voltage. This paper aims at adopting software control method to suppress beat phenomenon in traction motor caused byDC ripple voltage. According to theoretical analysis, DC ripple voltage is influenced by output power of motor, DC-link capacitor and power factor. Then, influences of fluctuating voltage are analyzed in detailed from aspect of switching function and motor model. Based on above analysis, combining with rotor field oriented control of traction motor, the frequency of switching function is modified to suppress beat phenomenon. An improved frequency compensation control method is proposed. Simulation model is built to verify the proposed scheme. Finally, the proposed control method is verified by drag experiment on a dynamometer test platform.

REFERENCES:

[1] J. Klima, M. Chomat, L. Schreier, “Analytical Closed-form Investigation of PWM Inverter Induction Motor Drive Performance under DC Bus Voltage Pulsation,” IET Electric Power Application, Vol. 2, No. 6, pp. 341–352, Nov, 2008.

[2] H. W. van der Broeck and H. C. Skudelny, "Analytical analysis of the harmonic effects of a PWM AC drive," inIEEE Transactions on Power Electronics, vol. 3, no. 2, pp. 216-223, Apr 1988.

[3] K Nakata, T Nakamachi , K Nakamura, “A beatless control of inverter-induction motor system driven by a rippled DC power source,” Electrical Engineering in Japan, Vol.109, No.5, pp.122-131,1989.

[4] Z Salam, C.J. Goodman, “Compensation of fluctuating DC link voltage for traction inverter driver,” Power Electronics and Variable Speed Drives, 1996. Sixth International Conference on (Conf. Publ. No. 429), pp. 390-395, 1996.

[5] S. Kouro, P. Lezana, M. Angulo and J. Rodriguez, "Multicarrier PWM With DC-Link Ripple Feedforward Compensation for Multilevel Inverters," IEEE Transactions on Power Electronics, vol. 23, no. 1, pp. 52- 59, Jan. 2008.

Integrated Photovoltaic and Dynamic Voltage Restorer System Configuration

ABSTRACT:  

This paper presents a new system configuration for integrating a grid-connected photovoltaic (PV) system together with a self-supported dynamic voltage restorer (DVR). The proposed system termed as a “six-port converter,” consists of nine semiconductor switches in total. The proposed configuration retains all the essential features of normal PV and DVR systems while reducing the overall switch count from twelve to nine. In addition, the dual functionality feature significantly enhances the system robustness against severe symmetrical/asymmetrical grid faults and voltage dips. A detailed study on all the possible operational modes of six-port converter is presented. An appropriate control algorithm is developed and the validity of the proposed configuration is verified through extensive simulation as well as experimental studies under different operating conditions.

KEYWORDS:

1.      Bidirectional power flow

2.      Distributed power generation

3.      Photovoltaic (PV) systems

4.      Power quality

5.      Voltage control

SOFTWARE: MATLAB/SIMULINK

 CIRCUIT DIAGRAM:

Fig. 1. Proposed integrated PV and DVR system configuration.

EXPECTED SIMULATION RESULTS:

Fig. 2. Simulation results: operation of proposed system during health grid mode (PV-VSI: active and DVR-VSI: inactive). (a) Vpcc; (b) PQload; (c) PQgrid; (d) PQpv-VSI; and (e) PQdvr-VSI.

Fig. 3. Simulation results: operation of proposed system during fault mode (PV-VSI: inactive and DVR-VSI: active). (a) Vpcc; (b) Vdvr; (c) Vload; (d) PQload; (e) PQgrid; (f) PQpv-VSI; and (g) PQdvr-VSI.

Fig. 4. Simulation results: operation of proposed system during balance three phase sag mode (PV-VSI: active and DVR-VSI: active). (a) Vpcc; (b) Vdvr-VSI; (c) Vload; (d) PQgrid; (e) PQpv-VSI; and (f) PQdvr-VSI.

Fig. 5. Simulation results: operation of proposed system during unbalanced sag mode (PV-VSI: active and DVR-VSI: active). (a) Vpcc; (b) Vdvr-vsi; (c) Vload; (d) PQgrid; (e) PQpv-VSI; and (f) PQdvr-VSI.

Fig. 6. Simulation results: operation of proposed system during inactive PV plantmode (PV-VSI: active and DVR-VSI: active). (a) Vpcc; (b) Vload; (c) Vdc; (d) PQload; (e) PQdvr-VSI; and (f) PQpv-VSI.

 CONCLUSION:

In this paper, a new system configuration for integrating a conventional grid-connected PV system and self supported DVR is proposed. The proposed configuration not only exhibits all the functionalities of existing PV and DVR system, but also enhances the DVR operating range. It allows DVR to utilize active power of PV plant and thus improves the system robustness against sever grid faults. The proposed configuration can operate in different modes based on the grid condition and PV power generation. The discussed modes are healthy grid mode, fault mode, sag mode, and PV inactive mode. The comprehensive simulation study and experimental validation demonstrate the effectiveness of the proposed configuration and its practical feasibility to perform under different operating conditions. The proposed configuration could be very useful for modern load centers where on-site PV generation and strict voltage regulation are required.

REFERENCES:

[1] R. A. Walling, R. Saint, R. C. Dugan, J. Burke, and L. A. Kojovic, “Summary of distributed resources impact on power delivery systems,” IEEE Trans. Power Del., vol. 23, no. 3, pp. 1636–1644, Jul. 2008.

[2] C. Meza, J. J. Negroni, D. Biel, and F. Guinjoan, “Energy-balance modeling and discrete control for single-phase grid-connected PV central inverters,” IEEE Trans. Ind. Electron., vol. 55, no. 7, pp. 2734–2743, Jul.2008.

[3] T. Shimizu, O. Hashimoto, and G. Kimura, “A novel high-performance utility-interactive photovoltaic inverter system,” IEEE Trans. Power Electron., vol. 18, no. 2, pp. 704–711, Mar. 2003.

[4] S. B. Kjaer, J. K. Pedersen, and F. Blaabjerg, “A review of single-phase grid-connected inverters for photovoltaic modules,” IEEE Trans. Ind.Appl., vol. 41, no. 5, pp. 1292–1306, Sep./Oct. 2005.

[5] T. Esram, J. W. Kimball, P. T. Krein, P. L. Chapman, and P. Midya, m“Dynamic maximum power point tracking of photovoltaic arrays using ripple correlation control,” IEEE Trans. Power Electron., vol. 21, no. 5, pp. 1282–1291, Sep. 2006.