When it comes to frequency in a power system, it is directly linked with the speed of the synchronous generator and mismatch in the mechanical input power and electrical output power accelerates or deaccelerates synchronous generator and thus frequency of the output varies, but how and why does this frequency change happen in a static inverter with load change?
As is not connected with actual power system, and it supplying only to load areas, there is no system inertia needed for stability, though could be stabilized by sufficient storage batteries backup……
Dear Vikas,
I suppose that for an only inverter-based power system, the frequency will be determined by the control of the grid-forming and grid-following inverters. This is because these two types of inverters can present the similar synchronization mechanism (eg. Droop based or PLL based) as the traditional synchronous generators based on their control systems. Thus, when the load changes in this network, different types of inverters will respond to the change and then the system frequency will accordingly vary.
To better understand such kind of process, you may refer to papers related to grid-forming and grid-following control.
Best regards,
Qian
Most inverters available today are either grid forming or grid following. With grid following inverters, they attempt to lock on to the fundamental (i.e. 50/60Hz) signal using a PLL. That depends on having a good concept of the frequency to begin with, which requires a grid with a strong short circuit ratio (the fundamental resonance is very strong, and this one inverter can't change the resonant frequency by itself very much). When looking at weaker grids, it's necessary to synthesize the resonance from scratch, and this either requires macro synchronization (say a GPS or high resolution clock) between all the inverters or grid forming such as droop style controls. But in spite of the "decision" to all be in phase, there's relatively little natural resonance going on, which is the passive physical process caused by inductors and capacitors interacting. This leads to far more fluctuations than in a high SCR grid. These come in to play at much faster timescales (there is no processing delay, so they react immediately, but they still can have large time constants). So grid following is more like a large natural resonance with a small forcing function, while grid forming is like a small natural resonance with a large forcing function.
With that said, the only thing that will keep the resonance is the ability of the forcing function, given that there is relatively little storage in the capacitance and inductance. Under loading increases, these stores of energy are the first things to discharge, and once that has been eaten up by the load, power transfer will be non-optimal and the voltage will drop at the source terminals, until it is compensated by the control loop. I don't think there's any inherent reason that frequency would "have" to change, but you would likely find the situation is unstable if the frequency deviates too much, since the power draw of motors and transformers will change with frequency.
You can refer to the paper "Operational paradigm of an all converter interfaced generation bulk power system" which discussess this topic.
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