Electronics > Repair
PV inverter - Voltage reading circuit
webgiorgio:
Hi,
I am repairing a 3 KW Power One inverter for grid connected photovoltaic.
I get "error E031" when the inverter connects to the grid.
This error means that the two AC voltage measurements do not match.
It is typically caused by a bad relay, but replacing all of them didn't fix the problem.
In the block diagram you can see that there are two AC measuring points, one on the grid side, and one on the inverter side.
Soon after the inverter closes all the relays, the two voltages are compared, and if a mismatch is found, the inverter disconnects from the grid.
So, I am investigating the voltage reading circuit.
The voltage reading on the grid side is ok, because I see correct values in the inverter display when it is preparing to connect to the grid.
I was looking for a voltage divider, but instead I found 4x 10 Mohm resistors connected as shown in attached schematic.
The end of the resistors go into a flat cable to a board with 3 DSPs.
What kind of circuit is it?
It looks like the input of a high-voltage differential probe for oscilloscope (just learned it from EEVblog #932).
With a 50:1 isolated probe and oscilloscope I found 300 mVpp on one channel, and about 100 mVpp on the other one (captured with the scope in "single trigger"). The 10 Mohm resistors are within specs.
coromonadalix:
Have you tested all the 10 megs ohms resistors ? maybe a short, or bad solder ??
An grid inverter always check the line vs solar panel / battery packs etc .. and decide to switch its output on the conditions given ...
not necessarly an high voltage diff probe, but an high voltage divider
boB:
So, you measured 300mV on the AC line and 100mV on the other side of the relay at the same time while the relay was ENGAGED ? TWO channels isolated ?
The relay is probably engaged for a very short time before the error turns the relay off but maybe enough time for a reading. Or not.
If the DSP has a differential input then the resistors may go there... Otherwise, there may be a differential op-amp circuit where each of these pairs of 40 Meg Ohm resistor strings connect to and then the single output of that op-amp goes to the A/D input of the processor.
But if the relay trips too fast, you might not be able to take a good reading.
I'd check the resistors too like coromonadalix mentioned.
webgiorgio:
The 10 Mohm resistors are within specifications.
The solder joints seems ok.
I only have one differential probe for the Oscilloscope.
You see here the voltages I measured at 3 positions in the schematic, indicated with black arrows.
The green means that all the relays are closed (parallel between grid and inverter).
In the top left figure you see that the inverter gets the grid voltage for about 1.2 seconds, then the relays open (orange arrow) and the voltage decay (some filter capacitors discharge).
Bottom right is the differential voltage of the grid side. Note the amplitude.
Bottom left is the differential voltage of the inverter side. There are some oscillations when the relays switch to do the ground isolation measurement. Then all the relays close, but the amplitude of the differential voltage looks smaller than for the grid side. Hence the inverter disconnects the parallel.
Sorry for taking photos of the digital scope, I have some problems with the USB-drive (when I plug it in, the scope becomes unresponsive)
Also, the resolution is quite bad because I am using a 50:1 probe to measure a small (300 mV) signal.
Update:
The input impedance of the measuring circuit is 1.06 MOhm for the good differential channel, and 3.88 MOhm for the faulty channel. mhhh looks like some resistor is disconnected somewhere.
webgiorgio:
So, I have looked at the control board, see attachment.
I guess one 16 bit DSP is for the DC/DC, one for the inverter, and the 8 bit Atmega is for the relays and measurements.
I guess that all those op-amps are signal conditioning of the two AC voltages, 2 AC currents, 2 DC currents, and 2 DC voltages, and differential current transformer (blue one).
I see some 3.3 Mohm resistors near op-amps...
Damn, how to do without schematic with this level of complexity? :'(
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