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Zero Crossing Detector

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Well, what else does it need? A way to measure the grid voltage I guess...
Click to expand...
It may use e.g. a grid voltage feed forward in output voltage generation, or even use the grid voltage waveform as current model.
Depends compeletely on your inverter's operation principle. A digital controlller would most likely use AD converted grid voltages UV and VW.
 

FvM said:
A digital controlller would most likely use AD converted grid voltages UV and VW.
Click to expand...

Yes, I am using a DSP to control the inverter. I am going to connect the signal for the grid voltage to the ADC converter of the DSP.

I am not sure on how to condition the signal in order to put it in the ADC (from 220V to <3V) ?

I have to get signal for each of the three phases right ?

Again - We are talking about the grid voltage level not the phase of the voltage.
 

Again - We are talking about the grid voltage level not the phase of the voltage.
Click to expand...
Actually, I'm talking of both. The analog waveform contains both informations. As said, you need to acquire two interphase voltages and get the third by a simple arithmetic operation. Most DSP based three-phase controllers use αβ-transformed voltage and current quantities, which automatically removes the redundancy of a third voltage measurement.
 

Okay thanks. That's good news for me.

Could you please suggest me a circuit for this task which would stand between mains and DSP ?
 

There are various ways to achieve this. If you want to operate the ADC respectively the DSP chip isolated from grid voltage, measurement transformers are one option, differential amplifiers with good common mode rejection another.

Perhaps you know the differential probes used for oscilloscope measurements in power electronics. They use symmetrical voltage dividers and instrumentation amplifiers. Scaling of measured voltage to 0-3V range shouldn't be a problem.

For good common mode rejection, the voltage dividers should either use high precision (e.g. 0.1%) resistors or provide an CMR adjustment (as the said probes usually have). According to my experience, you can achieve about 0.5-1 MHz bandwidth with uncompensated dividers, which is more than sufficient for grid voltage measurements.
 

FvM said:
differential amplifiers with good common mode rejection another.
Click to expand...

I am sorry but I don't understand. Which differential amplifier will be able to receive 220 V on its input ???
 

FvM said:
...They use symmetrical voltage dividers and instrumentation amplifiers. Scaling of measured voltage to 0-3V range shouldn't be a problem....
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Greeting!
K.
 

Thank you. I understand now. Please excuse my incompetence. I'm having a really hard time understanding these things step by step.
 

Hi,
:) No problem!
Good progress!
K.
 

However if I use resistors at the input of a circuit that will measure both voltage and phase of the voltage the zero-cross detection capability of the circuit suffers because it will detect the zero-crossing point with a delay...

Having this in mind is it convenient to use one circuit for both phase synchronization and voltage level measurement ?
 

Thanks karesz for assistance.

the zero-cross detection capability of the circuit suffers because it will detect the zero-crossing point with a delay
Click to expand...
As said: "Depends completely on your inverter's operation principle". My suggestions are assuming a usual space vector based controller. Your design possibly works different.

It's unclear to me, what's the role of "synchronization" to grid voltage zero crossing in your controller. You have been talking of a DSP based controller. Except for possible ADC aquisition time, a digital controller adds processing delay, also the pwm output generation creates a small systematic delay. To achieve exactly unitiy power factor, a phase adjustment is needed anyway. I already mentioned it previously. In a vectorial αβ signal representation, a rotation is a simple arithmetic operation.
 

Hi Frank,
:) Your welcome!
Good time!
K.
 

Well it has an explanation. Here is the funny part:
I have not designed any control scheme or program.

My zero-crossing detector(s) only gives to the DSP the moment in time when the inverter must start it's 6 period cycle (by driving first set of 3 transistors)... or at least this is how I have been able to understand the principle.

And you are perfectly correct with what you said in your last post but I am not able to think of these things by myself (yet). I am glad that I understand it once it has been explained to me though.

I am just selecting the electronic elements for the inverter. I know that an inverter is a simple electronic device with a complex control but that's just the direction that my project went - selecting elements. It's my first project of any kind so I hope I am forgiven.

1. I selected IGBT module 6 pack:
**broken link removed**

2. Calculated losses in IGBT module:
**broken link removed**

3. Made a thermal model to determine heat sinking:
**broken link removed**

4. Selected this "cooler" (liquid cooled cold plate):
**broken link removed**
(Although it does not match the size of the module I am going to mount on it, so the thermal resistance will be different from what I have used :) )

5. Selected the following DSP:
http://www.freescale.com/webapp/sps/site/prod_summary.jsp?code=MC56F800x&tid=mDHp

6. I liked a lot this zero-crossing detector because it is a completed circuit which accepts directly my 220V:
**broken link removed**

7. And finally found this "voltage sensor" which I found in one thesis for design of 1ph grid-tied inverter:
(see the attached image)

Just printed it and going to present it tomorrow. All I have to do now is pray that no one is ever going to look seriously at this paper. I hope when I am making my next one it will be far more sophisticated.

Sorry for all the bothering and also thank you FvM for the time.
 

All we have to do now is pray that no one is ever going to look seriously at this paper.
Click to expand...
A proverb says "Paper doesn't blush".

The individual parts are fine. Also a transformer coupled voltage measurement is generally O.K.. (We may want to see the connection of the transformer to OP07 somewhat more detailed).

I hope, you got more basic insights from the said thesis on grid connected inverters. I guess, the most serious problem may arise, if someone's asking, how they actually work, what's done with the voltage measurement, how a sine current waveform and unity power factor can be achieved, which parameters are important to choose a pwm frequency, inductor dimensioning and ...

Good luck!
 

This is the said thesis (Mr.Cool from this forum mentioned it in another topic about grid-tied inverter.):
**broken link removed**

Look at page 52 for the "voltage sensor" but I do not see any more details on how the diff amp. is connected to the transformer. If you see it please share.


You got me interested here and if you could enlighten me on some of these:

1. Which are the important parameters for choosing PWM frequency ?

actually I chose mine blindfolded - 10kHz. I know that some will hear this inverter working.

i know - it's better not to hear it, high enough to produce a nice form of the sine but also low because of the switching losses

2. Which inductor are you referring to ("inductor dimensioning")?
At each output phase of the inverter or at the input (DC bus) ? Actually I am making voltage-fed three-phase inverter.

The voltage measurement is needed so that inverter produces always a little more than grid voltage. If grid voltage drops the inverter must not continue to produce lets say 220 AC but it must also fall respectively (a 10 % + difference in voltages is what I've heard in order for power to flow to the grid). Right ?

3. Could you refer me to some basic paper which focuses on "how unity power factor is achieved" ? - although I have read such but there might be some particular one that you will suggest.

By the way where have you been studying and what programme/discipline. If you owe a part of your knowledge to an institution it's a good attestation for it. One day I may send my kids to this university so they can help me with my lousy project afterwards :D
 


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