Does low frequency PWM create reactive power in a transformer?
Actually, there's a glimmer of truth in Linards' answer, but not for the reason he thinks.

When your PWM on-time is an odd mutliple of half the powerline's AC period, you are creating a DC component in the current through the transformer. In the worst case, if your duty cycle drops below 5%, you will be putting unipolar pulses through your transformer primary. It isn't going to like this at all.

If you're going to do this kind of AC powerline PWM, you must make sure that your PWM on-times are quantized to whole cycles of the AC waveform, so that there's never any DC component.

If you drop your PWM frequency to 1 Hz, then you can adjust the duty cycle form 0% to 100% in steps of 2% (20 ms).