Dynamic structure of microgrids
By Taha Selim Ustun, School of Electrical and Computer Engineering, Carnegie-Mellon University, PA, USA
Electricity + Control, September 2015 (pages 34 – 39)
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One of the key features of microgrids is their dynamic behaviour. The connection or disconnection of a relay, load or generator at any given instance impacts the operation . Connection of a load or a generator changes the load flow and generation settings. Therefore, the generation settings of the generators shall be updated, accordingly. Connection or disconnection of a relay changes the structure of a network and it requires adjustments. To further elaborate the challenges, as an example, we shall focus on the protection challenges due to dynamic behaviour of microgrids. The challenges from other aspects can be detailed in a similar fashion.
Selectivity is a well known protection concept which means isolating the fault with the relay in an effort to minimise its effect on the rest of the system. This requires that in case of a fault, the relays should react according to a hierarchy. In conventional protection systems designed for passive networks, the relays which are downstream and closer to the fault point are required to operate first. However, if the fault current is very large and downstream relays are not capable of interrupting it, then other relays with larger capacities are expected to operate and isolate the fault. Implementation of selectivity is not that straightforward with the introduction of Distribution Generators (DGs). The very concepts of downstream and upstream relays are prone to change according to the status of the microgrid. The operating mode, i.e. grid-connected or islanded-mode, changing network structure with alternative paths and new deployments are some of the factors that would alter the selectivity parameters.
- The lessons learned in distributed control systems are now being applied in power network control.
- New algorithms of control are continually evolving as we require more and more ‘íntelligence’ in our power networks.
- Dijkstra’s algorithm has been shown to be applicable to determining the path from point of common coupling to different parts of the network – enhancing our ability to understand the network operation.