Hello,

The instantaneous active and reactive powers can be calculated as P = Vd Id + Vq Iq and Q = Vq Id - Vd Iq. In a balanced grid voltages case, when the synchronous frame is aligned to voltage, it is assumed that Vq = 0, which implies, P = Vd Id and Q = - Vd Iq. Thus, independent control of active and reactive power is possible by controlling the currents at the inner loop and voltages at the outer loop.

I look for better answers, Best.

S.K.

Dear Multan,

I think the selection shall be made depending on what you want to achieve (what control goals you aim at). In the end, it all comes down to leading to active and reactive power exchange between grid and converter. This ist just because of physics and not tied to any control structure. If you use a control structure, the question left is: How to determine the desired values? Exemplary control goals are:

Active power

- keeping the DC-side voltage constant (if the DC-side is modelled)

- keeping the active power exchange with the grid constant (if the DC-side is not modelled).

Reactive power

- keeping the reactive power exchange with the grid constant

- keeping the voltage at a specific grid node (close to the converter) constant

Of course, it does not need to be a constant value, it also can be a setpoint changing over time or changing in dependency of other electrical values (e.g. Q(U)).

To sustain a desired output voltage then control the voltage, same goes to power. But most converter would be depending on voltage/current.

Thanks, everyone for your time and suggestions.

From other colleagues, I have received a high-level view of the answer. The selection of power/voltage controller as the outer loop depends on control objectives and the role of power converters. I have known two terminologies that define the role of power converters: 1) grid following converter and 2) grid forming converter. If I have a strong grid with stable and regulated voltage and frequency and need to operate a power converter to inject whatever power is available, then it is necessary to select a power controller as the outer loop. In this case, the converter would be grid following converter and the outer loop would control P and Q. On the other hand, if I have a local grid or small weak grid and the control objective is to regulate the bus voltage and frequency, then it is necessary to select a voltage controller as the outer loop. In this case, the converter would be grid forming converter and the outer loop control V and f.

Now, this is a grid-connected converter example, but the principles would be similar if it was for other applications, such as internal DC buses of back-to-back converters, etc.