Let's say we have an ordinary radial distribution system. Now we want this network to be able to run intentional islanding with diesel generators connected to the end of the network. What are the procedures and requirements to do this?
A very interesting question!
Based on my understanding, the most critical issues in island operation are related to the voltage and frequency control, protection of the island system, and a possible re-synchronization with the grid (if ever needed). The generators in the island system should in principle be very fast to respond to the variations in the system so that the frequency does not become out of the acceptable range. Sufficient source of reactive power sources should be present in the system to support voltage.
This answer is rather brief. Practically, there are a lot more issues that need to be considered. More experienced researchers will probably say more about this.
With an increasing penetration of distributed energy sources, we will probably see more distribution networks operating in an island mode.
Cheers!
Desta
A very interesting question!
Based on my understanding, the most critical issues in island operation are related to the voltage and frequency control, protection of the island system, and a possible re-synchronization with the grid (if ever needed). The generators in the island system should in principle be very fast to respond to the variations in the system so that the frequency does not become out of the acceptable range. Sufficient source of reactive power sources should be present in the system to support voltage.
This answer is rather brief. Practically, there are a lot more issues that need to be considered. More experienced researchers will probably say more about this.
With an increasing penetration of distributed energy sources, we will probably see more distribution networks operating in an island mode.
Cheers!
Desta
Frequency and voltage are key issues related to your problem. Large generators with their high inertia will not be able to provide fast response to these two quantities. In case if you are not using any renewable source in your islanded distribution grid system, then I would suggest the use of micro-turbines which have considerably low inertia and can handle transients in your system. In other words, we need dispatchable sources somehow in your system. As you very well know, generators have a ramp up or ramp down time whenever there is a requirement of an increase or decrease in real power in the system. The system that you are looking into is a a broader picture to what I am looking into for my research. I myself is looking into islanded distribution grid but I am dealing with single phase microgrids at residential level.
There are many networks that run in this way. most isolated infrastructure and island communities are supplied by such networks. Mostly, the diesel generators are all located together in a single power station, which allows them to be controlled to share the load easily within the group, and to be individually cycled in and out, as power demand fluctuates. If they are far from each other, then they can be made to share the load without any other communication except the system frequency, which is the same everywhere in the system. This is done by arranging the throttles of all the diesels to respond to decreasing frequency by injecting more power, and vice-versa. This is known as operating in governor "droop" mode, because the frequency droops slightly with increasing system demand.
There must be a constant balance between power demand and power supply, as loads are switched in and out anywhere on the network. This is achieved by controlling the throttles of the machines with a frequency governor so that power supply and demand is matched by keeping the frequency steady. I think there is a need for significant inertia, to provide a momentary source of energy when new loads are switched on, and before the governing mechanisms can act to apply more power. The reverse is also the case; when loads are switched off, there is an excess of power and inertia is required to absorb this excess and prevent the system from over-speeding. Frequency stabilising is essential, because the characteristics of network components are very dependent on frequency and so frequency must stay in a narrow band in order to keep their performance within a workable range. Traditionally, inertia is provided by the rotating mechanical masses of electrical machines and their connected rotating burdens. Fast-acting electronic inverters can also provide stabilising that is similar to inertia, by their power output being controlled in inverse proportion to frequency error.
The biggest impediment reliable operations is system control and visibility. How does one put the network together after an incident, which has resulted in all of the generators shutting down? How does one maintain operations reliably, when everything is dependent on everything else? While anything is possible with modern controls and communications, it comes down to economics. How much can one afford to spend on control and communications, and on retaining the highly skilled staff required to operate and to trouble-shoot such installations, when there is only the revenue from a few GWh of energy coming in?
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