Need to design AC voltage and current measuring

[Chancy99]

Hi All -

I have a 10kW diesel generator here that feeds into a Xantrex/Trace inverter for power backup at the house. Unfortunately, the generator is fairly simplistic in that it only has a few (lousy) analogue gauges on it, with no protection/detection circuitry at all.

I would like to build a monitoring box for this setup. A microcontroller (probably an AVR) watching various sensors, and if something goes out of spec for too long, trip the shutdown relay to shut down the diesel.

Items to watch would be :

Generator voltage out
Generator current out
Diesel rpm
Output AC frequency
Oil pressure
Oil temp
Coolant temp

I can do the micro stuff, interfacing to the oil senders, the temp sensors etc, the LCD code and so on. But I'm not sure about measuring AC volts or current. I've done some internet digging, and come up with this.

It looks like I will have to read the AC voltage at an ADC input (scaled) and watch for the peak readings for simple peak-peak measurements, or do all the calculations to generate true RMS readings. So question 1 is

(1) What is the best/safest way to interface to 120VAC for voltage and frequency measurements ?

Frequency shouldn't be too hard. Feed that same signal into a peak-detector opamp configuration and count. Simple timer stuff.

For current, it looks like an open-loop hall-effect sensor is the way to go. Get a ferrite toriod, cut a notch in it for a hall-effect sensor, and put one of the generator output conductors through it, maybe with one or two loops if they will fit. Question 2 is

(2) Ummm, what exactly do I measure here ? I know a voltage will be generated proportional to the current through the primary conductor, flicking back and forth in AC like the primary. What am I looking for ? The peak V seen, or an integration across time ?

The end result of all this will be an info panel at the generator shed. An LCD will show current measurements of all sensors, and allow you to set limits for emergency shutdown. It will also have an RS485 link back into the house for a remote display panel. Maybe RS232 for logging too.

Dean.

 

[busbar]

 
Possibly some MAX133/4 DMM chips with average-responding 2-op-amp AC/DC converters. 50:5 CTs with 0.2-ohm secondary resistors on to scale/convert {50A=1V} two line currents. A couple of small 120:6V transformers to scale two L-N voltages.

The generator frequency and engine speed will be exactly proportional unless the shaft slips/breaks.

 

[dcriner]

You have a 10-kw backup diesel generator for your HOUSE? Good grief.

 

[Chancy99]

Heh, yup Well, that's what it's capable of putting out. Keep in mind, that's only really about 83A at 120V. Most houses these days are built with at least 200A service, and larger ones with more. The furnace alone pulls 9-10A. Add in the freezer, fridge, computer, a few lights, and it adds up.

The gen feeds into a Xantrex/Trace inverter, which can put out 4.5kW. This box takes in grid AC, generator AC (when called for) and 24V battery. In a power outage, it flips to inverter mode, and feeds 120V to the covered circuits from the batteries.

I know colleagues who have a pair of SW5548 inverters (that's 11kw continuous with 48V battery) and a 23kW generator. They can run the whole house as normal for about 2 days on battery alone before needing to kick on the gen.

As needed, the inverter will start and run the generator to recharge the batteries. It will also run it if the drawn load becomes too much for the batteries. Very nice, hands off system. Very clean too - the computers don't even notice the power glitching or going off completely.

But I worry about the dumb engine. Oil starvation, coolant loss, shaft breakage, etc. Damn thing would just beat itself to death.

Analog looks to have a couple of nice devices, the ADE7753/63. They do all the hard RMS work etc, with a simple SPI interface. Gotta look into those

Dean.

 

[mc5w]

Who made the diesel generator set? Most reputable manufacturers of diesel generator sets can sell you a monitoring panel. Most diesel engines have protection against oil pressure or coolant loss.

Mike Cole

 

[Chancy99]

This is a cheap China Diesel, with a 295 engine. Brought in by an importer down in NC. Hardy Diesel sells the same setup, but with better accessories -

http://www.chinadiesel.com

. Also a lot more $$.

I know I can get a simple Murphy switch setup - they have a couple of nice control panels. But they are really expensive ... Plus, I like doing and learning things on my own. The eventual final plan is for more than just a simple monitor panel.

I want a box in the gen shed that watches and displays

Generator voltage out
Generator current out
Diesel rpm (probably not needed)
Output AC frequency
Oil pressure
Oil temp
Coolant temp
Shed temp
maybe others

And will trigger the shutoff solenoid if something goes out of spec for too long. Spec and long will both be settable. In the house will be a second box acting as a remote display, plus watching the battery power usage, wattmeter type thing. Plus, it will have an rs232 output for logging.

D.

 

[ScottyUK]

Hi Chancy99,

Have a look at the AD536 and its cousins for your current and voltage measurements. Most of these ICs are well established and easy to use.

http://www.analog.com/en/subCat/0,2879,773%5F866%5F0%5F%5F0%5F,00.html

Save some effort - engine rpm and gen frequency are inextricably linked unless something has gone horribly wrong, so imply one measurement from the other one.

Oil and coolant sensors: it might be easiest to use automotive parts if you can get calibration data for them. Many have a three terminal power/signal/ground connection and give a voltage or current output which would be readily compatible with a monitoring system. Each manufacturer has their own standard, although it should not be too hard to calibrate them against a known gauge or thermocouple


You have a FURNACE in a house? Why?! Are you an undertaker?




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If we learn from our mistakes,
I'm getting a great education!

 

[Chancy99]

ScottyUK -

Thanks for the tip - that series looks to be pretty easy to use. Just puts out a DC voltage that correspoonds to the RMS value of the AC being measured. Hmm, doesn't look like it does current measurement though - will still need something for that.

I've been looking around at oil pressure senders - getting the curves for them is tough. Easiest to use I think is the VDO ones - they publish the resistance ranges for them right on the web.

Furnace, sure. Plain old ordinary forced hot-air house oil furnace for heating. Coupled with an AC. Winter gets cold here, and power outages are a real pain. Few years ago we had a bad ice storm that knocked out the lines for over a week. Had to go stay with my parents for a while - not fun with a wife and kids

The burner/oil pump takes about 2A, and the blower about 9A or so.

D.

 

[ScottyUK]

Hi -

You're right, they don't measure current, only voltage. It's pretty simple to pass the secondary current of a CT through a low value resistance - think in terms of a few tens of milliohms or so - to develop a volt-drop proportional to current. Measure the volt-drop across the resistor using a differential-connected op-amp and use a second op-amp to boost the single-ended output of the diff amp to a more useable level. Ground one side of the CT secondary. The other possibility is to use a Hall-effect sensor from LEM-HEME or whoever else makes them in the US. They are cheap these days at about £30, use standard supply voltages (+/-15V or +24V), and give either a voltage or current output.

Ah, realization dawns! What you know as a furnace, we know as a boiler. Most of our homes in the UK use closed loop low pressure hot water with radiators in the rooms. The UK pretty much gave up on ducted warm air heating for domestic use by the beginning of the 1970's. Your furnace sounds a bit more powerful than the 24kW and 28kW natural gas units which are common over here. The electronics and small I.D. fan runs happily on a standard computer UPS fitted with distinctly non-standard lead-acid batterys to get longer endurance.



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If we learn from our mistakes,
I'm getting a great education!

 

[buzzp]

I would not go cutting gaps in torroids on your own. This leads to much error in your readings (perhaps even too much for a look up table). I would go buy a standard torroid with a 1A or 5A secondary and hook this across a resistor to measure the voltage (diff amp and LP filter recommended). If you really want RMS voltages and currents the AD536 (others similar too, AD636, 736?) is a good choice. If you do not want rms then you can just measure the peak with your AD and use the scaling factor to get an averaging rms value (any distortion will not be measured). However, if it were me I would want rms values for sure. You could do all of this in your AVR too (square AD reading, then average, then square root) although it is a bit more time consuming to develop and calibrate.
Or if you really don't want to make one then simply buy one with a 4-20 out or RS485 or whatever your heart desires. They are a dime a dozen in averaging relays. If rms is desired it will cost you more and reduce your selection greatly (still out there though).

 

[GOTWW]

Just a tid-bit of advice. Make sure that you install a universal bypass switch, as your wire-wrapped wonder could maybe less reliable that the item it is to protect. Say 5% here or there and the very reason for going to so much trouble is wasted due to some temperature issue or loose connection. It is easy to outsmart your self, especially, in the middle of the night, when all those darn lousy analog guages look good, yet your computerized brain says NO WAY BABY! Worst case yet, you are happly vacationing in tha Caribbean, when you get call from your dog or house sitter. There is a problem NASA!. And why have a generator drive an invertor. Unless you are trying to charge batteries, In which case a seperate voltage-frequency intolerent rectifier may be more robust in the charging mode. And penny-wise, let the POC burn up. Get something better. BEWARE of new generators that come new with spare parts.

 

[Chancy99]

Hi Guys -

Thanks for all the advice I think I'll go with an ADE7753 type for the voltage and current monitoring, using a plain old current transformer. I can get an appropriate one from Digikey for about $11.00. I'll write up the code for doing RMS first, to see if I really need the ADE7753.

The blower in this furnace has about a 15A start current (as far as I recall) and uses about 7-9A when running. According to our service company, that's fairly typical.

The Trace/Xantrex inverter/charger takes 3 power inputs :

AC in 1 (normally grid)
AC in 2 (generator)
24V DC from battery bank

and puts out a single 120V feed to a subpanel. In normal use, it feeds power from AC1 to the output, and steals some to charge and maintain the batteries. In a power outage, it flips over to inverter mode, and starts using the batteries to feed the 120V output.

As needed, it will kick on the generator to recharge the batteries, or even to supplement them in the event of a large load. This is the most efficient way to use a generator, as it utilizes as much as possible the rated generator output. The energy from AC2/gen goes to the output as required, and the rest goes to recharge the batteries. When they're "full" it flips back to inverter mode and stops the gen.

D.