An Overview of Micro-grid Problems and Solutions during

Autonomous Operation.

By

MOHD SAIFUZAM JAMRI

D1 (Control Systems Theory Group)

Outline

• Introduction to micro-grid• Micro-grid operating modes• The need of micro-grid• Autonomous micro-grid problems• Common solutions• Load shedding strategy problems and solutions

Introduction to micro-grid

• Before the micro-grid been constructed

• People are already invented the renewable energy• Photovoltaic module (since 1839)

• First created by Edmund Becquerel• Wind turbine (since 1891)

• First invented the electricity-generating wind turbine by James Blyth• Fuel cells (since 1839)

• First created by sir William Grove

PV & Fuel cells(1839)

Micro-grid(1882)

Wind turbine(1891)

Introduction to micro-grid

• What is micro-grid?

• First constructed on 1882 by Thomas Edison

• It generally means a collection of consumers, generators and potential energy storage entities connected together.

• Operated as a small grid which is connected to the main grid, but capable of operating a self-sufficient island.

• Excess power can be sold to the utility grid.

• Size of micro-grid may range from rural housing to municipal region.

Introduction to micro-grid

• Micro-grid components

• Distributed generation (DG)• Dispatchable DG• Non-dispatchable DG

• Loads• Frequency sensitive loads• Non-frequency sensitive loads

• Energy storage• Controllers• Point of common coupling (PCC)

Introduction to micro-grid

Structures and

applications

Utility micro-grid

Commercial micro-grid

Remote micro-grid

Introduction to micro-grid

• Utility micro-grid• Facilitate a large-scale of distributed sources• Can handle the growth of the user’s power either completely or

partially• Applied both in urban as well as rural areas• Work in island for the reduction in impact of grid faults

Introduction to micro-grid

• Commercial micro-grid• Applied for loads which requiring high power quality and reliability

• Shopping centre• Factory• Residential• Hospital

• Can be switch to island operation• In the event of grid fault• During maintenance at main grid• Periods of power quality• When grid energy prices are high

Introduction to micro-grid

• Remote micro-grid• Applied to rural areas, remote areas, island• Renewable energy is commonly chosen instead of generator• Usually operating in island mode

Micro-grid operating

modes

Grid connected

mode

Island mode

Micro-grid operating modes

Micro-grid operating modes

• Grid connected mode

• Utility grid is active• Static switch is closed• All the feeders are being supplied

by utility grid and local sources

Micro-grid operating modes

• Island mode

• Called as autonomous operation

• Utility grid is not supplying power

• Feeder A, B and C are supplied by local sources

The need of micro-grid

• The advantages of micro-grid

• Reduced dependency on main grid• Improved the energy reliability• Reduced the impact of grid faults• The excess energy can be store or sell to the electricity provider

Autonomous micro-grid problems

• The general problems

• The autonomous micro-grid operation in such cases would contribute to the unbalance issue between load demand and supply.

• The high power deficiency would contribute to the black-out condition.

• The voltage and frequency network may fluctuate due to sudden change of load demand or fluctuation of energy resource.

Autonomous micro-grid problems

• The specific problems • The problem arise from the unbalanced specifications between load and

supply:• Maximum of generation capacity is lower than maximum load capacity

• The problem comes from the distributed generator itself• Photovoltaic module – low solar irradiance [1],[2]• Wind turbine – low wind velocity [3],[4]• Generator – spinning reserve issue and device limit [5], [6]

• The load changes made the voltage and frequency fluctuated [7],[8]• The change of reactive power demand make the voltage vary and such case

going to drop• The change of real power demand make the frequency network unable to

maintain its reasonable level

Common solutions

• Power electronic interfaces [1]-[4]

• Most of non-despatchable DGs are not suitable for direct connection to the electrical network due to the characteristics of the energy produced

• Main concerned on inverter control

• Using voltage source inverter (VSI) to provide the reference voltage and frequency

Common solutions

• Battery energy storage system (BESS) [9],[10]

• Used as a backup energy• Charging during excess energy from micro-sources• Discharge energy to keep the micro-grid balance• Also been use to provide the reference voltage when sudden grid

disconnected happened

Common solutions

• Generator output power control [5],[7],[8]

• Used governor speed regulator to control the real power generation and system frequency

• Used automatic voltage regulator to control the voltage and reactive power generation

Load shedding strategy

• What is load shedding strategy?• The strategy to shut-down the unnecessary load and limit the

energy consumption• Is the last solution to keep the micro-grid balance and stable

• When we need to use this strategy?• During high power deficiency (demand exceeding supply)• When the system facing the voltage and/or frequency droop and all

the DGs are already reached its limit

Load shedding strategy problems

• The common problem arise [11]-[13]

• Slow action due to the process of identifying number of load to be shed

• Over load shedding

• Identifying the load classes (critical and non-critical load)

Load shedding solutions

• Three concepts of load shedding strategy• Traditional

• Sheds a certain amount of load when the system frequency falls below a certain threshold.

• Semi-adaptive• Shed a certain amount of load according to the rate of change of

frequency when the system frequency reaches a certain threshold.• Adaptive

• Used the rate of change of frequency and frequency threshold but load shedding scheme able to adapt with different types of instabilities. Able to optimize the number of load to be shed and re-close when load shed is excessive

Load shedding solutions

• Existing load shedding process [11]-[13]

• Used traditional scheme that utilize the threshold value f as a margin to trigger the first step of load shedding process

• The threshold was used to trigger the step of load shedding action according to different power deficiency.

• The threshold value is determined based on experience and simulation results.

-saifuzam-

Load shedding solutions (an expected result)

Load shedding solutions

• Proposed for study (in progress)

• Optimal load switching to avoid over load shedding problem• Target the main objective:

• Restore the network frequency• Identify the optimal number of load to be shed• increase the system response time

References

• [1] Wu, D., Tang, F., Dragicevic, T., Vasquez, J. C., & Guerrero, J. M. (2014), “Autonomous Active Power Control for Islanded AC Microgrids with Photovoltaic Generation and Energy Storage System, “IEEE Transactions on Energy Conversion, 29(4), 882-892 . 10.1109/TEC.2014.2358612

• [2] Xuemei Zheng, Qiuming Li, Peng Li, and Danmei Ding, “Cooperative Optimal Control Strategy for Microgrid under Grid-Connected and Islanded Modes,” Hindawi Publishing Corporation International Journal of Photoenergy Volume 2014, Article ID 361519, 11 pages

• [3] Durga Gautam , Lalit Goel , Raja Ayyanar , Vijay Vittal and Terry Harbour, “Control Strategy to Mitigate the Impact of Reduced Inertia Due to Doubly Fed Induction Generators on Large Power System,” IEEE Transaction on power system, Vol.26, No.1, February 2011.

• [4] L. Yang, G.Y. Yang Z. Xu, Z.Y. Dong K.P. Wong, X. Ma, “Optimal controller design of a doubly-fed induction generator wind turbine system for small signal stability enhancement,” IET Gener. Transm. Distrib., 2010, Vol. 4, Iss. 5, pp. 579–597

• [5] F. Katiraei and M. R. Iravani, “Power Management Strategies for a Microgrid With Multiple Distributed Generation Units,” IEEE Transactions on Power Systems, Vol. 21, No. 4, November 2006

References

• [6] Dobson and Liming, “ Voltage collapse precipitated by the immediate change in stability when generator reactive power limits are encountered,” IEEE Trans. Circuit and Systems, vol. 39, pp. 762-766, Sept. 1992.

• [7] Hassan Bevrani, and Shoresh Shokoohi, “An Intelligent Droop Control for SimultaneousVoltage and Frequency Regulation in Islanded Microgrids,” IEEE Transactions on Smart Grid, Vol. 4, No. 3, September 2013

• [8] Ritwik Majumder, Balarko Chaudhuri, Arindam Ghosh, Rajat Majumder, Gerard Ledwich, and Firuz Zare, “Improvement of Stability and Load Sharing in an Autonomous Microgrid Using Supplementary Droop Control Loop,” IEEE Transactions On Power Systems, Vol. 25, No. 2, May 2010

• [9] Hisham Mahmood, Dennis Michaelson and Jin Jiang, “A Power Management Strategy for PV/Battery Hybrid Systems in Islanded Microgrids,” IEEE Journal of emerging and selected topic in power electronics, Vol 2, No.4, December 2014.

• [10] Jong-Yul Kim,Jin-Hong Jeon, Seul-Ki Kim, Changhee Cho, June Ho Park, Hak-Man Kim and Kee-Young Nam, “Cooperative Control Strategy of Energy Storage System and Microsources for Stabilizing the Microgrid during Islanded Operation,” IEEE Transactions on Power Electronics, Vol. 25, No. 12, December 2010

References

• [11] X. Lin, H. Weng, Q. Zou, and P. Liu, “The Frequency Closed-Loop Control Strategy of Islanded Power Systems,” IEEE Transactions on Power Systems ,vol. 23, no. 2, pp. 796–803, 2008.

• [12] P. Mahat, Z. Chen, and B. Bak-jensen, “Underfrequency Load Shedding for an Islanded,” IEEE Transactions on Power Delivery, vol. 25, no. 2, pp. 911–918, 2010.

• [13] M. Q. Ahsan, A. H. Chowdhury, S. S. Ahmed, I. H. Bhuyan, M. A. Haque, and H. Rahman, “Technique to Develop Auto Load Shedding and Islanding Scheme to Prevent Power System Blackout,” IEEE Transactions on Power Systems, vol. 27, no. 1, pp. 198–205, Feb. 2012.