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H Bridge Grid Tied Inverter is good despite non-sinusoidal current?

Flyback

Well-Known Member
Thread starter #1
Hello,

I have done this “poor-person’s” Grid Tied Inverter simulation in LTspice. (Schematic and LTspice simulation are attached, waveform of current supplied back into the mains is also attached)

It puts power back into the mains.

Admittedly, the current waveform is not sinusoidal, though it does have a decent power factor.

The thing is, surely it is not so absolutely massively important for a Grid Tied Inverter to have a purely sinusoidal output waveform? After all, the duty of drawing sinusoidal, in-phase current from the mains rests with the loads, not the inverters. ..And a great many of the loads on the mains draw non-sinusoidal mains current (eg sub 75W SMPS’s in the EU). If all of the loads on a particular mains phase are drawing non-sinusoidal current, then a pure-sinusoidal inverter for that particular phase is a waste of time…….because it simply will not be able to deliver sinusoidal current into that particular mains phase.

So do you agree, that the attached H Bridge inverter is of some good (admittedly not perfect) and can play a role in energy saving by pushing power back into the mains from eg a renewable source?
 

Attachments

Flyback

Well-Known Member
Thread starter #2
Also,

If you do object to the previous version of the H Bridge grid tied inverter,(due to its lack of sinusoidal current) then does the following one please you?...

This H bridge grid tied inverter (attached) pumps a sinusoidal current back into the mains….however it simply does this by having a sinusoidal reference voltage into its pwm controller….there is absolutely none of the complicated control software which exists in all the microcontroller solutions for grid tied inverters. This is an extremely simple grid tied inverter. Admittedly, it is devoid of the rather faster dynamic feedback loop of the usual software based GTI solutions…but in all truth, as long as somebody on the same mains phase is drawing more power than this simple H bridge is pumping back into the mains, then it will operate perfectly well.

So what’s wrong with it?

Why is there a need for GTI’s with reams of complex software control algorithms?

Attached please find the LTspice schematic, schematic and current waveform of this “simple sinusoidal grid tied inverter”
 

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gophert

Active Member
#5
How would you keep the reference or sine source "in sync" with the mains in the real world?

And avoid any possible out-of-sync startup?
Doesn't your power company (and government regulations) have some documentation on the equipment, process, quality of putting energy into the grid. How to get paid for your power, etc.?

If there are repairs in your area, how do you communicate with authorities / linecrews to disconnect your feed-in?
 

rjenkinsgb

Active Member
#6
i would derive the reference sine from the mains itself
... Which you are affecting by the action of supplying energy to the mains.

It's not the waveform accuracy that bothers me, it's the safety and [power] regulation aspects.

You need to be perfectly in sync before starting to activate the power bridge and you then need current sensing to measure and control the voltage & phase so you are keeping the power output to within limits for your source, without exceeding voltage tolerances if the external load is low.
Either increasing voltage or a very small phase lead will increase the output power sent to the mains.

It also needs to allow for interruptions to the mains, both isolation and power outage types, so it does not get out of sync then do unpleasant things if the external power is temporarily disconnected, or try to supply the whole street if there is an area power failure.


It's not the basic inverter that is particularly complex, it's the essential regulation, safety and interlock controls of that that takes most of the design and testing work.
 

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