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Investigation into the protection of microgrids using adaptive relays

Dines, Joseph (2016) Investigation into the protection of microgrids using adaptive relays. Honours thesis, Murdoch University.

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Due to a proliferation of renewable power sources and a rise in distributed generators, the transmission network is now able to split into smaller isolated sections. These sections are often referred to as microgrids as they function similar to larger networks with power production being sufficient to power the loads. Microgrids are hampered by the lack of fault current being sufficiently high to operate traditional overcurrent protection. The lack of fault current has resulted in a need to find newer methods to provide fault protection. One such method is the use of modern relays that allow for the trip settings to be changed as the network itself changes. Relays that allow for this adaption are known as adaptive relays and provide a method to deal with the lower fault current when the microgrid is isolated from the main network by reducing their pickup current as appropriate.

The effectiveness of adaptive relays has been investigated by the use of DIgSILENT PowerFactory simulation software to model two differing networks under varying conditions. The networks chosen to be simulated, consist of a test network used in previous research and a modelling of a physical network on Hailuoto Island. The networks were simulated with the faults falling into one of the following four categories:

1. Faults occurring on the external grid
2. Faults occurring on the feeder connecting the external grid to the microgrid
3. Faults occurring on the feeders inside the microgrid
4. Faults occurring on the loads and connections inside the microgrid

The adaptive relays were found to operate correctly having discrimination between relays, once the pickup currents were calculated correctly. Each microgrid requires that a microprocessor, being informed of the status of the grid and output of each power source, calculate the pickup currents for each relay, and notify the relay of its new pickup current.

The microprocessor’s calculations were simulated in MathWorks MATLAB via a script which allows calculation of the current seen by a relay as the sum of the rated output from each distributed generator. Then multiplied by the status of the generator and its percentage contribution. This formula is an adaption from previous work by (Oudalov and Fidigatti n.d.). The current calculated by the MATLAB script is found when compared to the PowerFactory simulations as having a difference of less than 2.8 per cent and 8 per cent on the test network and Hailuoto Island respectively. Providing proof that not only does the MATLAB script and its associated formula work, so do adaptive relays for protecting microgrids.

Item Type: Thesis (Honours)
Murdoch Affiliation(s): School of Engineering and Information Technology
Supervisor(s): Crebbin, Gregory
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