This document discusses protection schemes for inverter-based microgrids. It begins by defining system protection and apparatus protection in conventional grids. It then defines microgrids and their operating modes of grid-connected and islanded. Challenges in microgrid protection include bidirectional power flow, different operating modes and fault current levels. Various proposed protection schemes are described for the islanded and grid-connected modes, including adaptive directional overcurrent, travelling wave, multi-agent, and pattern recognition schemes. Future microgrid protection is expected to involve communication between relays, machine learning techniques, complex signal processing and fast response times.

1. Protection of Inverter Based
Microgrids
EN 406 Seminar
Department of Energy Science and Engineering
IIT Bombay
Satya Sahoo
16D170026

2. System protection
Protection of the grid, and their constituents from abnormal conditions generated
artificially or naturally.
System Protection
● Under-frequency
● Out of step protection
● Islanding protection
● Rate of change of frequency
● Reverse Power Flow
● Voltage Surge
Apparatus Protection
● Transmission line
● Transformer
● Motor
● Busbar
● Generator

3. Protection techniques
● Overcurrent Protection
● Directional Overcurrent Protection
● Distance Overcurrent Protection
● Differential Protection

4. Microgrids
A microgrid consists of a group of interconnected loads and DERs with clearly
defined electrical boundaries which can be operated as a single standalone
controllable entity and can be operated either grid-interconnected or grid-isolated
● Decentralized generation
● Interconnected system
● Multiple operating modes
Operation
● Grid-connected
● Islanded

5. Purple - 380kV
Green - Transformers
Blue - 220kV
Electrical Grid vs Microgrids
Expected interconnection of MG
components
Justo, Jackson & Mwasilu, Francis & Lee, Ju & Jung, Jin-Woo. (2013). AC-microgrids versus
DC-microgrids with distributed energy resources: A review. Renewable and Sustainable
Energy Reviews. 24. 387-405. 10.1016/j.rser.2013.03.067.

6. Challenges in Microgrid Protection
● Bidirectional power flow
● Operating modes & corresponding behaviour
● Estimation of fault current levels
● Blinding of protection
● Sympathetic Tripping
● Dynamic nature of microgrids
● Grounding topologies

7. Operating Modes
Grid Connected
● MG configuration
● Feeder current
Islanded
● Variable fault current levels
Transition
● Ride Through
● Coordination of MG protection with PCC
● Re synchronization

8. Grounding
● of Sources
○ Synchronous / Asynchronous Generators
○ Doubly Fed Induction Generator
○ Inverter
● of Transformers
○ Y connected winding
○ Δ connected winding
● of Microgrid
○ Three wire ungrounded
○ Three wire uni-grounded
○ Four wire uni-grounded
○ Four wire multi-grounded

9. Proposed Schemes
For Islanded Mode
● Harmonic content based protection scheme
○ THD and frequency measurement of converter voltages, communication link between relays
○ Difficult to assess THD threshold values
● Voltage based protection scheme
○ abc-dq0 transformation of DGs output voltages and communication link between relays
○ Single-pole tripping and high impedance faults are not considered
● Symmetrical component and residual current based scheme
○ OC relays, static switch at PCC, zoning principle
○ three phase and high impedance faults and single pole tripping not considered
● Adaptive protection scheme
○ IEDs (voltage, current, directional OC measurements with interlocking), high speed
communication link, zoning.
○ Complex

10. Proposed Schemes
For both islanded and grid connected mode
● Adaptive directional overcurrent protection
○ Numerical directional OC relays with directional interlocking capability
● Current travelling waves based scheme
○ Busbar voltages for faults detection and current travelling waves for fault location
● Multi-agent protection schemes
○ Network zoning, IEDs, communication link, wavelet coefficients of transient current for fault
location
● Inverse-time admittance- based protection scheme
○ Inverse–time admittance relay, directional element, zoning
● Pattern recognition based scheme
○ Time-frequency transform (S-transform), spectral energy contours, calculation of differential
energy

11. Future
By expectation, the protection systems of microgrid interfaced grids, which will
mostly be dominated by inverter based systems will most likely have one or more
of the below features:
● Communication b/w relays and other system elements
● ML based adaptive protection schemes
● Complex signal processing and analysis
● Fast response times

12. References
[1] IEEE recommended practice for grounding of industrial and commercial power systems. IEEE Std 142-2007 (Revision of IEEE Std
142-1991), pages 1–225, Nov 2007.
[2] H Al-Nasseri and MA Redfern. A new voltage based relay scheme to protect microgrids dominated by embedded generation using
solid state converters. In 19th International Conference Electricity Distribution, pages 1–4, 2007.
[3] Belwin J Brearley and R Raja Prabu. A review on issues and approaches for microgrid protection. Renewable and Sustainable
Energy Reviews, 67:988–997, 2017.
[4] G Buigues, A Dysko, V Valverde, I Zamora, and E Fern´andez. Microgrid protection: Technical challenges and existing techniques.
In International Conference on Renewable Energies and Power Quality, volume 1, pages 222–227, 2013.
[5] E. Casagrande, W. L. Woon, H. H. Zeineldin, and D. Svetinovic. A differential sequence component protection scheme for
microgrids with inverter-based distributed generators. IEEE Transactions on Smart Grid, 5(1):29–37, Jan 2014.
[6] E. A. A. Coelho, P. C. Cortizo, and P. F. D. Garcia. Small-signal stability for parallel-connected inverters in stand-alone ac supply
systems. IEEE Transactions on Industry Applications, 38(2):533–542, March 2002.
[7] Y. Han, X. Hu, and D. Zhang. Study of adaptive fault current algorithm for microgrid dominated by inverter based distributed
generators. In The 2nd International Symposium on Power Electronics for Distributed Generation Systems, pages 852– 854, June
2010.
[8] Seyed Amir Hosseini, Hossein Askarian Abyaneh, Seyed Hossein Hesamedin Sadeghi, Farzad Razavi, and Adel Nasiri. An
overview of microgrid protection methods and the factors involved. Renewable and Sustainable Energy Reviews, 64:174–186, 2016.

13. References (cotd.)
[9] Patrick Tendayi Manditereza and Ramesh Bansal. Renewable distributed generation: The hidden challenges–a review from the
protection perspective. Renewable and Sustainable Energy Reviews, 58:1457–1465, 2016.
[10] Aushiq Ali Memon and Kimmo Kauhaniemi. A critical review of ac microgrid protection issues and available solutions. Electric
Power Systems Research, 129:23–31, 2015. 22
[11] D. P. Mishra, S. R. Samantaray, and G. Joos. A combined wavelet and data-mining based intelligent protection scheme for
microgrid. IEEE Transactions on Smart Grid, 7(5):2295–2304, Sep. 2016.
[12] J. Mohammadi, F. Badrkhani Ajaei, and G. Stevens. Grounding the ac microgrid. IEEE Transactions on Industry Applications,
55(1):98–105, Jan 2019.
[13] H. Nikkhajoei and R. H. Lasseter. Microgrid protection. In 2007 IEEE Power Engineering Society General Meeting, pages 1–6,
June 2007.
[14] Nagaraju Pogaku, Milan Prodanovic, and Timothy C Green. Modeling, analysis and testing of autonomous operation of an inverter-
based microgrid. IEEE Transactions on power electronics, 22(2):613–625, 2007.
[15] Power System Relaying and Control Committee Subcommittee C Working Group C30. Microgrid Protection Systems. IEEE Power
Energy Society, July 2019.
[16] S.A. Soman. Digital protection of power systems. https://nptel.ac.in/courses/ 108101039/.