which angle to use in svpwm switching timings generation

for grid connected inverter should i use the angle from pll as theta reference or the angle which is generated from the voltages

[halleyhit]

what does " the angle which is generated from the voltages" mean? Usually, you are using pll to measure angle from voltage data.

[PARADESI]

usually we use θref​=atan2(vβ​,vα​) . so i have doubt to you which one among the both. i'm getting the switching timings patter which is increasing . what might be the reason for this?

Answer 1

[Shivam Gothi]

Hello,

Simplified schematics of the three phase PLL block is shown below:

Fig -1

Brief summary:

We can see that the voltages are first transformed from “abc” frame to “α-β” frame, according to following relation:

Now, we assume an arbitrary rotating reference frame (rotating at synchronous speeds) and name it as “d-q” frame. The voltages in “α-β” frame are then transformed to “d-q” frame using the following transformation :

Refer to fig-1. We now have "Vd" and "Vq",

Vd = the component of revolving voltage space vector along the d-axis.

Vq = the component of revolving voltage space vector along the q-axis.

We are passing “Vq” as an input to the “PI” controller block. Ultimately, at steady state , “PI” block is trying to make “Vq” = 0. Now, here is a catch!!

Refer to eq(2). We can write:

At steady state, “Vq”=0. This implies:

Therefore, at steady state, the angle returned by PLL is same as "atan2(vβ​,vα​) ".

Conclusion:

In the case of balanced three phase voltage, both of the approaches will give the same angle at steady state. But, under transient conditions, both of the approaches gives different angles. Therefore, if there are transients in your application, it is good to use PLL block, because, it acts as a low pass filter, thus filtering out the unwanted high frequency component from the “angle” output.

I hope this helps !