The transient at the change of active power from an inverter is simply the control loop responding to the change and altering the inverter's transfer function to compensate. When you say you change the reactive power from the inverter exactly what is it you are doing? Changing the phase of the current in the load to lag or lead? Speeding up the response depends on the usual factors such as loop gain and frequency of switching of the inverter. In your diagrams I don't think the response is particularly bad though it could probably be improved somewhat. A 200 mS response time from 0 to 100% full power is pretty good.

Hi Bhargav

In electrical circuits the major contributors of transientsare L&C

The transient at the change of active power from an inverter is simply the control loop responding to the change and altering the inverter's transfer function to compensate. When you say you change the reactive power from the inverter exactly what is it you are doing? Changing the phase of the current in the load to lag or lead? Speeding up the response depends on the usual factors such as loop gain and frequency of switching of the inverter. In your diagrams I don't think the response is particularly bad though it could probably be improved somewhat. A 200 mS response time from 0 to 100% full power is pretty good.

There is something wrong here, unless I am missing something!

In a controlled inverter, why should you change either power reference ? Isn't your objective to control the output voltage and frequency, or at best: power factor ??

The active and reactive power should be decided by the DC side power flow and the AC side load - isn't that the case ?

Anyway, assuming that you have a capacitor supported DC side, and that you truly are changing the "power references" in the two figure, I would first suspect the transient energy that would naturally flow out of the DC capacitor in response to the reference changes ! 

The dynamics themselves, as suggested by Dr. Maine, look reasonable. Of course there are various details as included in his reply. But if you are seeking the "physical reason" for the power dynamics in response to reference changes, my first answer will still be the DC cap...

Try to design correctly the controller!  If it is a VSC-based power inverter; the time constant of the closed-current control-loop system should be selected small for a fast current-control response, but adequate such that, the bandwidth of the closed-current loop control is 10 times smaller than the switching frequency of the power inverter.  Also, the time constant of the closed-voltage control-loop should be selected higher than the time constant of the closed-current control-loop system; because the current-loop response is faster than the voltage-loop response.

This is the cause of charging batteries, do not use the system while charging the batteries.

I recommend using batteries with high capacity and initially charged , the transient behavior is almost eliminated.

The model used in this simulation is coupled, there are a EMF between the two loops which are not decoupled by the control, this is due to the magnetizing current of the inductance parameter

To excite more discussion it is advisable to add the unit step stimulus. or at least to mansion when do you applied  the unit step function for the reactive and active power.

The comments are very useful. However the most effective comment is that the control parameters determines the dynamic process. I would estimate that your loop is a second order and subsequently you have three main parameters: The natural frequency of oscillation of the loop wn, the damping factor and loop gain..

The most influencing parameters of the time response is the natural frequency of oscillation of the loop. It is clear that your natural frequency is relatively small.

It seems me that the loop is underdamped, the best damping factor is 0.7.

The reactive  power is coupled with the active power while the active power while the reactive power has small effect on the active power.

I would propose that you parameteric study for the system dynamic response.

wish you success

This seems to be a simulation only. If you changed the actual circuitry, or the timing which input current / voltage was applied, be clear what the change is. neither of your two jpgs specify the exact change was "from x voltage or current applied at time 1 second" to whatever the change was.

When ever there are energy storage components like inductor and capacitor, definitely there will be transients in the voltage and currents for any change in system. Moreover in inverter based system the controller parameters will also contribute in the transients. In you case if the system is a closed loop system then transient is because of effect of both LC components and controller parameters (especially gains) 

The parameters that are sometimes neglected, particularly when using a closed loop feedback control system is the equivalent series resistance of any capacitance or capacitors, and the winding resistance of any inductance. Just by changing the capacitance to having a different equivalent series resistance, or inductance to a different winding resistance can dramatically modify the risk of unwanted oscillation. I have witnessed on system becoming unstable due to changing the capacitance to a different capacitor, not in terms of capacitance, but its equivalent series resistance. Whenever a negative feedback loop has been applied, just changing one reactive component can make a dramatic difference.