Help:Achieving Mains Sync with Solar inverter

[abicash]

Hello

I have already posted it here
I will explain it again here..

I am running a H-bridge inverter with sine weighted PWM with a microcontroller.The DC voltage is from Solar PV.
The inverter generated sine frequency is 50 Hz.
I now take the Mains/grid rectified wave to an input capture channel of the MCU.
The inverter is indexed to zero at every rising edge of the input capture event.

Now with this scheme i have a problem of understanding the phase of the input wave, so sometimes the generated wave is 180 deg shifted to i/p mains.
For this i took a half wave rectifier of the mains, so i have confirmed positive wave of the mains sine.
But with this , sometimes a half cycle misses sync.

Is there a simpler way of doing this?
Rather , what is the best method of doing this

All help appreciated

 

[alec_t]

Unless you are hoping to pump power back into the mains to get payment from your utility company (in which case you'd have to have an officially approved setup anyway) does it really matter if there is exact sync with mains at all times? What will you be powering with this?

 

[abicash]

Hello alec_t

Thanks for replying
This would be used in a sine inverter.
A relay connects the i/p AC wave to the o/p of the inverter.

I normally will check for sync, and when that happens, transfer the relay and then i switch off inverter PWM
With this the load does not see an off cycle.
Am i on the right track here?

 

[alec_t]

Understood. How fast can your relay change over the connections (bearing in mind the mains cycle lasts <= 20ms)?

 

[abicash]

close to 5 ms

 

[alec_t]

With this the load does not see an off cycle.

I don't see how you can guarantee that . Suppose the mains fails immediately after a rising edge? That will go undetected until the following cycle, and then there will be a 5mS interval before the inverter connects to the load. In addition, the start-up time of the inverter needs to be factored in.

 

[abicash]

I will explain another scenario where it is worthwhile

Mains was absent and inverter was ON.
Mains resumes , yet i haven't transferred the relay.Now i lock both phases.Once locked, i transfer the relay.
Once Mains fails again, do not wait for anything, start the inverter right away.
Does this make any sense? or am out of line

 

[abicash]

I see a bit of phase shift (yellow channel). I have filtered the PWM channel with an RC.
Blue channel is mains from a stepped down transformer.
I am a bit surprised right now, since i see not a single half cycle missing sync , which was happening earlier, which prompted me to post this thread.

 

[alec_t]

Once Mains fails again, do not wait for anything, start the inverter right away.

I don't see how that is different? As I explained in post #6, there will inevitably be delay in detecting mains absence, relay switching and inverter start-up (reaching target voltage from zero).

I am a bit surprised right now, since i see not a single half cycle missing sync

Perhaps the missing half cycles were due to mains transients.

 

[savvej]

Hi abicash and altec,
Just wondering as to how much adverse effect can it have on the PV inverter or the load, in case if there is non-synchronized abrupt switch-over?
Is it really that important for the PV inverter to be in sync with the mains while switching over?
Regards,
SAV