Easiest way to provide constant 250 ARMS for junction box thermal testing? 2

[BigMotorGuy]

I am looking for the best/easiest way to perform a temperature rise test (i.e. measure box temperature with 250amps flowing through the 3 phases - need a T6 rating) on a new junction box which is rated at 6.6kv and 250amps. Will simply finding a constant current power supply rated to 250amps or higher be good? I would assume that some sort of external resistor bank would be needed (not for current control) in order for the power supply to not see basically a dead short (which is what the load would be without the resistive load in series with it).

Any suggestions?

I don't want to have to buy a power supply, just rent since I only need to certify a single junction box for a project.

 

[davidbeach]

Wire it up in a closed loop and use a CT to couple the current into it.

 

[waross]

To expand on David's good suggestion:
Power the CT with a Variac.
If the current is too hard to control with a direct connection, put some resistance in the primary circuit between the Variac and the CT.
It's difficult to suggest a resistance value without knowing the impedance of the various components and the CT ratio. The resistance may be a couple of incandescent light bulbs or an old toaster.
This application is a good candidate for cut and try.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[FreddyNurk]

An interesting suggestion indeed. How high a current can you go (without running into safety or other issues) with the CT?

 

[waross]

What ratio CT?? 5 amps on the 5 Amp winding.
Example: 1000:5 CT Pass 5 Amps through the 5 Amp winding and if the impedance of the closed loop secondary is low enough the CT will induce 1000 Amps.
There are others on this site who have much more experience than I with CTs.
Hopefully one of the Gurus will post in with some information as to the calculations as to how to determine the suitability of a given CT to drive the required current through the impedance of the circuit under test.
Hint: You can gain secondary voltage by winding the secondary conductor through the window more than once.
For example, it is found that a 500:5 CT cannot drive sufficient current through the impedance of the load. Pass the load conductor through the window one more time. The CT will now at as a 250:5 CT but with twice the secondary voltage and able to handle twice the load impedance.
Spot welding transformers have a lot in common with a CT used backwards to induce a high current.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[FreddyNurk]

waross, I was thinking that due to overall CT restrictions, you might not get 2kA out of a 2000:5 as an example. I'd also expect that putting 10A through the secondary wouldn't be smart either. But for a 250A requirement? I've learnt something new here.

 

[waross]

Here's the catch Freddy.
For an example apply 120 Volts to the 5 Amp winding of a 2000:5 CT. The CT ratio is 2000 / 5 = 400:1
With 120 Volts applied, the secondary voltage will be 0.3 Volts per turn. That may be enough voltage to drive the required current through the impedance of the load.
The knee point voltage may be several times 120 Volts so you can use a higher voltage and multiple turns to get a high enough secondary voltage to drive enough current through the load.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[DTR2011]

This may also be accomplished easily and safely with a primary injection test set, such as those used for LV circuit breaker testing.
Keep in mind that most primary test sets utilize an output transformer and have thermal / time limitations. Choosing a test set
rated for much higher current ~2kA, will generally allow for a long term test run. These test sets generally output their current at a low
voltage (3-10 V) and have built in ammeters, timers, etc and can run on a 120V, single phase supply. Higher currents / times are available with higher input power / voltage. Some units also have reporting software, if that makes a difference.

Nearly all of the test equipment rental companies would have something available to suit your needs.

Programma/Megger make an ODEN test set, Omicron has CPC, there are others as well. It would be a good idea to have some kind of estimate of the time you need
to run the test as the test equipment manufacturers specifications generally detail some kind of output current / compliance voltage / time curves.

I don't see anything wrong with the CT method, however be sure to build in some operator safety to the test set up.

Does your test specification allow for a single phase current to be utilized in series with the three phases? Is true three phase current required?

If three phase current is required, another option is to use a three phase distribution transformer at reduced voltage with the LV side short circuited. This method is often utilized to commission differential relaying schemes and once understood is quite easy to perform, provided there is a suitably sized transformer and power source available. SEL has several papers on how to perform this test. Google "Proper Testing of Protection Systems
Ensures Against False Tripping and
Unnecessary Outages" You may have to register to gain access.

 

[ScottyUK]

As this is a temperature rise test it could be performed using a DC source just as effectively as an AC source. The power supply can be operated as a constant voltage source or a constant current source, and for this task you would choose the latter. HP / Agilent and Xantrex make supplies in this output range, as do a few others. Agilent's 668x series can certainly muster a good 600A or so, maybe a little more, with ample voltage to drive current around the loop. They're available to rent from the usual test equipment rental houses, and they crop up at bargain prices on the surplus market or even ebay from time to time.

 

[BigMotorGuy]

Thanks for all the help. I think I will look at renting a unit per ScottyUK and see what I can get for cheap. this is a one off thing for me, so buying equipment is not in the budget!

I do like the CT suggestion though....I will see what our test guys have and maybe do a scaled down test to confirm based on CT availability.

 

[LionelHutz]

The big trick with cheap will be keeping the current constant. Someone will likely have to keep tweaking the current vs the better test sets which will control the current.

Have you looked for a test facility? The advantage with a decent test facility is that they can do certified tests.

I'd use a soft-starter setup for current control but we build them so the parts are easy to acquire. I worked with a customer who is using 3 single phase units to control a step-down transformer for current testing. I think their setup could provide up to about 5kA of test current and by controlling each phase it keeps all 3 phases balanced.