hello
I have work on microgrid area and microgrid voltage and frequency control. there is a contradiction for me about selecting the line impedance type (R>>X or X
Hi Mehdi,
I do tests and measurements with artificial mains network. The resistive line impedances in these test setups are 1-2 ohms for each line and 12 ohms between lines and ground. For higher frequencies you can expect about 50 ohms between cables in an installation and 200 to 350 ohms characteristic impedance between cables and cable trays of metal.
In cases where DC-DC converters are used, you can experience a situation with negative impedance, this can be explained by decreasing current when the (supply) voltage increases, when the power consumption is constant. This is due to fast reacting pulse width regulation of the individual DC-DC converters. I assume the same phenomenon can be seen in grids with AC-AC and DC-AC converters. In some cases this will prevent a converter from powering up, if the series inductances of the supply cables and filters are very high. So it will just start and stop again and again.
thanks Mr. Kofoed
but I dont give my asnwer. my question exactly is that, is there a microgrid with inductive line impedanc?
Yes, every grid system will have some form of inductive line impedance. For any wire, the self induction L can be expected to be 1 nH/mm.
Reactance XL = 2 pi f L (ohm)
For high frequencies, the distance to the ground plane will determine the characteristic impedances.
I cannot refer to any real life grid systems, sorry. I hope someone else will.
Microgrids are usually low voltage networks with limited geographical stretch. They are essentially distribution networks, in which distributed generation and the loads coexist. The distribution networks are characterised by high R/X ratio. The ratio is very high, such that the X component is neglected.
Thanks Mr. Kumar
I know about distribution networks and also I know that the line impedance in distribution networks is resistive. but as I before mentioned in my question (first post) I was read a credible IEEE papres that refers about inductive microgrids. I wana know that, really can we consider an inductive microgrid for my research?
Hello Mr. Mehdi,
I have worked on hybrid microgrid. As per definition of microgrid we are defining microgrid as a short range of area withing which we are generating power and distribute it. So according my opinion R>X.
A small power network (or microgrid) with internal AC grid has a dominant line reactance (Xl=omega*L>>>>R). For example, a small wind farm supplying AC loads. Each wind conversion units (WCUs) within the wind farm includes a wind-turbine, a generator + electronics circuits, and a 50-or 60- Hz transformer. The WCUs are interconnected either in radial, ring, star, etc.using AC internal cables. A short AC cable links the wind farm to the AC platform. The AC platform includes a main 50Hz transformer which step up the AC voltage output across the wind farm. The platform links to the AC transmission line then to the AC loads. Such microgrid system has a dominant inductive line impedance.
The parameters of the power transmission and distribution line in microgrid are small and generally presented high resistance-to-inductance (R/X) ratio which will seriously influence the performance of the traditional power droop controller, even leading to instability
In actual the microgrid generation and distribution are tightly coupled, the line with your condition (X>>R) may not exist, better you consider the multi micro grid, where the output level can be brought to medium Voltage and in that you can observe or predict the line reactance much greater than impedance.
No, microgrid is either connected to distribution network at close to load or islanded.
thanks Mr. Elsayed
your answer is very good. I think that your answer solves my problem. but there is other question about your answer. is there a system with several DG's with long lines? most DG's have the low power and if we wana in interfacing DG's in the system with long lines, this can not be Affordable.
As K. P. Kumar stated X/R ratio of line is increased with the nominal voltage. Both X and R are in a unit per km, thus independent from line length. Characteristic of R trends to reduce with higher nominal voltage due to its higher cross section area or its bundle. In contrast, L trends to increase due to its line span between phase. As a result for a low voltage distribution line model has R>>X and medium voltage it appear that X>R. For a high voltage transmission level has X>>R. However, this seem so trivial issue for simulating dynamic behavior of voltage and frequency deviation under micro grid scenario.
In general MG line impedance is said to be mostly resistive. But however with firmly coupled generation and distribution some times the MG impedance behavior can be changed with sudden switching of inductive loads. then the impedance condition of the MG due to load will be changed. This feature is more predominant in islanded condition. where as in interconnected mode, there is support from main grid. So MG in islanded mode with sudden switching of reactive load can be approximated as the line with the condition, X>>R
- Badges
- Science topic