Cabernet Sauvignon

The inductance value (Lv) of a virtual impedance loop is an important parameter that can potentially affect dynamic performance of the proposed approach. In this study, the effect of Lv value is evaluated under the variation of PCC load in an islanded operation mode. The same microgrid and the specific event used in Case study 1 are adopted in this sensitivity study. Five different values of Lv are considered: Lv1= 0.01 mH, Lv2 = 0.1 mH, Lv3 = 1.2 mH, Lv4 = 2 mH and

Lv5 = 5 mH. The virtual resistance value (Rv) is chosen adaptively from the 2-D lookup table based

on the status of input parameters, as discussed before.

The dynamic response of active and reactive power supplied by the two DG units with an adaptive virtual impedance loop being incorporated in the VSG control scheme using various virtual inductance values are shown in Fig. 5.13.

177 Figure 5. 13: (a) Active and (c) reactive power supplied by the DG unit 1, (b) Active and (d)

reactive power supplied by the DG unit 2 (Sensitivity study 2)

It is found that for different values of Lv the active power is shared accurately between the two DG units when the load variation occurs. Therefore, Lv does not have significant influence on active power sharing performance. Both DG units share reactive power accurately (1 kVar equally) under the load variation when the virtual inductance value Lv3 is chosen. For a lower values of Lv, such as Lv1 and Lv2, reactive power sharing performance is deteriorated. Therefore, a sufficiently high value of Lv needs to be chosen, which is taken as 1.2 mH in this study. Any further increasing of the value of Lv does not offer significant improvement in the performance, rather it is observed that a very high value of Lv makes the system dynamic response slower. Significant effect of the

Lv value on the voltage and frequency at the PCC cannot be observed based the study results.

6. Conclusion

In this paper, an adaptive virtual impedance method is proposed for reactive power sharing enhancement in a VSG controlled microgrid. A fuzzy secondary controller based VSG control

178 scheme developed by the authors previously is adopted as the base for implementation of the proposed virtual impedance approach. Several case and sensitivity studies are carried out to verify the effectiveness and robustness of the proposed approach.

Using the proposed adaptive virtual impedance method, 2-D look up tables are developed for both islanded and grid connected modes for a microgrid. Utilizing the 2-D lookup tables, virtual resistance values can be expressed as a function using a Polynomial 22 model. Sample virtual resistance values are determined and demonstrated for the case studies.

In the proposed approach, the virtual impedance consists of a virtual resistance and a virtual inductance. The virtual resistance is chosen to be variable while the virtual inductance is kept at a fixed value. In order ensure good reactive power sharing performance, a sufficiently large value of the virtual inductance must be chosen. Based on Sensitivity study 2, the minimum virtual inductance value of the test microgrid is recommended to be 1.2 mH.

It is found that the proposed approach can accurately regulate reactive power of DG units under operating point variations for microgrids. However, it is only functioning well for a fixed electrical system configuration of a microgrid. Further research is needed to establish a more generic method, through which the virtual impedance remains valid under alteration of a microgrid configuration.

179

References

[1] H. Xu, X. Zhang, F. Liu, R. Shi, C. Yu, and R. Cao, "A reactive power sharing strategy of VSG based on virtual capacitor algorithm," IEEE Trans. Industrial Electronics, 2017.

[2] H. Zhang, S. Kim, Q. Sun, and J. Zhou, "Distributed adaptive virtual impedance control for accurate reactive power sharing based on consensus control in microgrids," IEEE Trans. Smart

Grid, vol. 8, no. 4, pp. 1749-1761, Jul 2017.

[3] A. Bouzid, P. Sicard, A. Yamane, and J. Paquin, "Simulation of droop control strategy for parallel inverters in autonomous AC microgrids," in Proc. IEEE 8th Int. Conf. Modelling, Identification and Control, 2016, pp. 701-706.

[4] N. Pogaku, M. Prodanovic, and T. C. Green, "Modeling, analysis and testing of autonomous operation of an inverter-based microgrid," IEEE Trans. Power Electronics, vol. 22, no. 2, pp. 613-625, Mar. 2007.

[5] C. Dou, Z. Zhang, D. Yue, and M. Song, "Improved droop control based on virtual impedance and virtual power source in low-voltage microgrid," IET Generation, Transmission &

Distribution, vol.11, no. 4, pp. 1046-1054, 2017.

[6] Q. Zhong, "Robust droop controller for accurate proportional load sharing among inverters operated in parallel," IEEE Trans. Industrial Electronics, vol. 60, no. 4, pp. 1281-1290, Apr. 2013.

[7] J. He, Y. W. Li, and F. Blaabjerg, "An enhanced islanding microgrid reactive power, imbalance power, and harmonic power sharing scheme," IEEE Trans. Power Electronics, vol. 30, no. 6, pp. 3389-3401, Jun. 2015.

180 [8] J. Matas, M. Castilla, L. G. Vicuna, J. Miret, and J. C. Vasquez, "Virtual impedance loop for droop-controlled single-phase parallel inverters using a second-order general-integrator scheme," IEEE Trans. Power Electronics, vol. 25, no. 12, pp. 2993-3002, Dec 2010.

[9] U. Borup, F. Blaabjerg, and P. N. Enjeti, "Sharing of nonlinear load in parallel-connected three- phase converters," IEEE Trans. Industry Applications, vol. 37, no. 6, 1817-1823, Nov. 2001. [10] X. Wu, C. Shen, and R. Iravani, "Feasible range and optimal value of the virtual impedance

for droop-based control of microgrids," IEEE Trans. Smart Grid, vol. 8, no. 3, pp. 1242-1251, May 2017.

[11] Y. Zhu, F. Zhuo, F. Wang, B. Liu, R. Gou, and Y. Zhao, "A virtual impedance optimization method for reactive power sharing in networked microgrid," IEEE Trans. Power Electronics, vol. 31, no. 4, pp. 2890-2904, Apr 2016.

[12] Z. Liu, S. Ouyang, and W. Bao, "An improved droop control based on complex virtual impedance in medium voltage micro-grid," in Proc. IEEE PES Asia-Pacific Power and Energy

Engineering Conference, 2013, pp. 1-6.

[13] M. C. Chandorkar, D. M. Divan, and R. Adapa, "Control of parallel connected inverters in standalone ac supply systems," IEEE Trans. Industry Applications, vol. 29, no. 1, pp. 136-143, Jan. 1993.

[14] Y. Sun, X. Hou, J. Yang, H. Han, M. Su, and J. M. Guerrero, "New Perspectives on Droop Control in AC Microgrid," IEEE Trans. Industrial Electronics, vol. 64, no. 7, pp. 5741-5745, Jul. 2017.

[15] J. M. Guerrero, L. G. Vicuna, J. Matas, M. Castilla, and J. Miret, "Output impedance design of parallel-connected UPS inverters with wireless load-sharing control," IEEE Trans.

181 [16] J. M. Guerrero, J. Matas, L. G. Vicuna, M. Castilla, and J. Miret, "Decentralized control for parallel operation of distributed generation inverters using resistive output impedance,"

IEEE Trans. Industrial Electronics, vol. 54, no. 2 pp. 994-1004, Apr 2007.

[17] J. He, Y. W. Li, J. M. Guerrero, F. Blaabjerg, and J. C. Vasquez, "An islanding microgrid power sharing approach using enhanced virtual impedance control scheme," IEEE Trans.

Power Electronics, vol. 28, no. 11, pp. 5272-5282, Nov 2013.

[18] C. Sao and P. W. Lehn, "Autonomous load sharing of voltage source converters," IEEE

Trans. Power Delivery, vol. 20, no. 2, pp. 1009-1016, Apr 2005.

[19] H. Mahmood, D. Michaelson, and J. Jiang, "Accurate reactive power sharing in an islanded microgrid using adaptive virtual impedances," IEEE Trans. Power Electronics, vol. 30, no. 3, pp. 1605-1617, Mar 2015.

[20] C. Andalib-Bin-Karim, X. Liang, and H. Zhang, “Fuzzy Secondary Controller Based Virtual Synchronous Generator Control Scheme for Microgrids”, in Proc IEEE Industry

Applications Society Annual Meeting, 2017, pp. 1-14.

[21] H. P. Beck and R. Hesse, "Virtual synchronous machine," in Proc. 9th Int. Conf. Electrical

Power Quality and Utilisation, 2007, pp. 1-6.

[22] S. D'Arco and J. A. Suul, "Equivalence of virtual synchronous machines and frequency- droops for converter-based microgrids," IEEE Trans. Smart Grid, vol. 5, no. 1, pp. 394-395, Jan. 2014.

[23] X. Liang, "Emerging power quality challenges due to integration of renewable energy sources," IEEE Trans. Industry Applications, vol. 53, no. 2, pp. 855-866, Mar. 2017.

182 [24] X. Liang and C. Andalib-Bin-Karim, "Virtual synchronous machine method in renewable energy integration," in Proc. IEEE PES Asia-Pacific Power and Energy Engineering

Conference, 2016, pp. 364-368.

[25] Y. Chen, R. Hesse, D. Turschner, and H. P. Beck, "Dynamic properties of the virtual synchronous machine (VISMA)," in Proc. Int. Conf. Renewable Energies and Power Quality, 2011, pp. 1-6.

[26] H. Zhao, Q. Yang and H. Zeng, "Multi-loop virtual synchronous generator control of inverter-based DGs under microgrid dynamics," IET Generation, Transmission &

Distribution, vol. 11, no. 3, pp. 795-803, 2017.

[27] Y. Chen, R. Hesse, D. Turschner, and H. P. Beck, "Improving the grid power quality using virtual synchronous machines," in Proc. IEEE Int. Conf. Power engineering, energy and

electrical drives, 2011, pp. 1-6.

[28] Q. C. Zhong and G. Weiss, “Synchronverters: Inverters that mimic synchronous generators,” IEEE Trans. Industrial Electronics, vol. 58, no. 4, pp. 1259–1267, April 2011. [29] J. Alipoor, Y. Miura and T. Ise, "Power system stabilization using virtual synchronous

generator with alternating moment of inertia," IEEE Journal of Emerging and Selected Topics

in Power Electronics, vol. 3, no. 2, pp. 451-458, June 2015.

[30] Y. W. Li and C. Kao, "An accurate power control strategy for power-electronics-interfaced distributed generation units operating in a low-voltage multibus microgrid," IEEE Trans.