# Split range control structure for the inverter

I have been working on the grid forming inverter control. I have started with the cascaded voltage and current control loop i.e. control structure like that

As far as I understand correctly this control structure its inherent feature is that the supplied current is limited by reducing the output voltage.

Let's say I would like to further limit the supplied current via reducing the output frequency (especially in case that one of the grid connected devices is the squirell cage induction motor). Current limiting via frequency reduction should occur independently to the current limiting via output voltage magnitude and I would like to have following sequencing where at first the current is limited via voltage magnitude and then via frequency reduction.

I have noticed that I have basically two action variables for current control

• output voltage magnitude
• output frequency

This fact drove me to the idea that I can use the well know split-range control and modify the above mentioned control structure in that way

As far as the splitter block its responsibility is to split the common action variable into the output voltage magnitude according to the following characteristic

and into the output frequency (indirectly via modification of the angle step) according to the following characteristic

I am not sure whether that idea could work. I have especially doubts regarding the modification of the output frequency via the angle step because in that case different transform angle is used for the actual values of the grid voltage calculation and different transform angle is used for the calculation of the reference voltage for the modulator.

I can address only the anticipated response of loads with the proposed control strategy. I leave it to others to address the method for achieving the strategy.

If your load includes one or more induction motors, voltage reduction may not achieve much load shedding without frequency reduction. However, if the motor load is a centrifugal pump or fan reducing only the voltage would reduce the load.

If you reduce frequency, you probably have three options.

1. Reduce voltage and frequency in tandem with each other at the point where limiting become necessary.
2. Start by reducing only voltage, and add frequency reduction at some selected load level.
3. Start by reducing only voltage, then discontinue voltage reduction and start frequency reduction at some selected load level.

Motor's driving centrifugal fans or pumps will operate reasonably well with reduced V/Hz, but a motor driving a load like a refrigeration compressor may stall. Many electronic loads may not consume less power with reduced input voltage, but will malfunction or shut down at some voltage level. I suspect that dimmable LED lighting will consume less power. The V/Hz probably doesn't affect most electronic loads.

Any motor or transformer load should not be subjected to V/Hz increase above rated. Frequency must not be reduced without proportional voltage reduction.