Hi everyone,
I am doing research on small signal stability/ dynamic stability in a microgrid. As you know, it's the droop parameters of power controller which are mainly responsible for the dynamic stability of a microgrid. I am thus, considering research on optimization of these parameters. I have however few questions regarding it.
1) The load modeling plays a vital role in small signal stability of a large grid. However, I have seen most microgrid loads are represented as only impedance (R-L load). Can I expect a much different behavior if I model the load also as constant power or constant current type?
2) The dc side voltage is considered constant in most microgrid analysis. If I model the PV or some source, the dc reference changes. Do you think this may effect the dynamic stability?
3) Thirdly, are there any specific suggestions in dynamic stability domain of a microgrid which I can use in my research?
Thank you in advance
For 3rd point:
Dear Samundra sir,
I reckon, your research is oriented towards the inter-connection of multiple non-inertial distributed generating stations complemented with some micro hydos, probably. In that case, as you yourself are well aware of, penetration level would be a major guiding factor which in turn is a major clog for defining transient as well as steady state stability of the system.
To summarize,you can consider mitigating the problems of dynamic stability via Virtual Synchronous Machine (VSM) topology (though not suggested for large and high inertial grids). It is well suited for low inertial grid like you have mentioned. Also, its relatively high response time make it ideal amour to take control of transient stability. Try pairing it up with Ultra capacitors instead of batteries for optimum result.
Hope it helps.
Regards,
Bishal
Dear Bishal, I was actually thinking about optimizing parameters related to virtual inertia and obtaining an acceptable performance in a microgrid
For the second point:
Consider a PV source supplying power at a particular irradiance level. The PV power may be reduced by the movement of clouds over the panels. This movement is noted to be in seconds rather than in milliseconds. Hence affect on the transients will be minimal even if you try to model the PV source. Also usually these sources are connected through boost converter topologies, which is isolated from the AC side by the DC link voltage, hence whatever you do at the DC side (be it modeling or no modeling at all) will not affect the AC side dynamics if you keeping the DC link voltage constant.
Loads with switching power supply will effect the (sub)transient dynamic stability.
For the first point:
As you stated, the types of load and the share of these types within a grid will influence the stability of the system. You can see in the IEEE Std C37.117‐2007, the dependency of the load with system frequency and voltage. The typical share of the load types in the system is 70% constant power loads, 20% constant current loads and 10% constant impedance. This share can change depending upon the type of the feeder, e.g. industrial feeder might have more constant power loads such as asynchronous motors. Thus, changing the system behavior when subjected to a disturbance.
1-Constant loads decrease microgrid damping and so decrease the system stability.
2- DC side dynamics are very slow in comparison to the AC side dynamics. So it can be neglected for stability studies.
3- The droop coefficients are very important stability factors if you use grid-support grid forming strategy for most of ec-ders.
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