Step-Up Transformers fed from Autotransforer

[rockman7892]

I'm looking at an application for a Test Lab in an electronics manufacturing facility that requires step-up transformation from utility service 480V to both 4.16kV and 13.8kV using 200kVA step-up transformers for each. The MV secondary's of these transformers will be distributed through MV fused contactors to allow customer to connect MV source to new equipment prototypes they are developing (Power supply & energy storage type equipment)

For testing purposes the test voltage needs to be varied so in order to accomplish this the customer is requesting a 200kVA Autotransformer on the 480V side to vary the voltage to the primary of the step-up transformers in order to achieve a varied voltage at MV equipment under test. Are there any issues or potential pitfalls with having this autotransformer on the primary of the step-up transformers in order to vary voltage at the secondary?

Some questions I have are:

1) How are the step-up transformers impacted by supplying them with a reduced voltage on the primary when the autotransformer reduces the voltage. Is there a minimum voltage that the transformers would be capable of tolerating for a continuous duration?

2) Are thee any potential issues with increased inrush current with these step-up transformers (I've heard in the past that inrush on step-up transformers is much more significant than step-down)? Is there a potential to nuisance trip breakers that will warrant installing a resistor w/bypass or can inrush simply be carefully evaluated vs breaker tripping characteristics with breaker applied correctly?

3)Is it recommended to provide a LRG on the secondary neutrals of the step-up transformers (wye secondary's). I know these are typically provided in most applications with MV transformers but not sure if it would be suited here were system is connecting to electronic power supplies?

Thanks

 

[MatthewDB]

I did exactly as you propose years ago. We used a standard commercial variac (Variable auto-transformer) with a motor operator and a transformer. We had a custom transformer built that had many voltage taps available so we could just use one transformer and get all of the desired voltage outputs by adjusting the taps.
To answer your specific questions:
1: The transformer does not care one bit about lower than normal voltage operation.

2: We avoided the inrush by always starting the variac at zero voltage setpoint. As I mentioned above, we used a motor operated variac. We placed a contactor in the feed to the variac. The contractor was wired to be held closed by an auxiliary contact with three wire control (a normally open "on" button to close it and a normally closed "off" button to open the contactor). We used an existing cam switch in the variac to only permit close when the variac was set for zero voltage. We also used an auxiliary contact set on the contactor to break the "raise" command and send a "lower" command so even without operator intervention, the variac returned to zero voltage in preparation for the next use.
For safety we also had an E-stop circuit with double safety contactors upstream and an E-stop loop around the lab. If that was tripped the E-stop had to be reset, and then the system could be turned back on using the normal contactor.

3: The auto-transformer / variac is always supplied in wye-wye. If fed three wire, they are somewhat prone to ferroresonance, particularly if there is ground capacitance on their output. Nearly all are three ganged single phase units so there is no phantom delta effect you would see in a three phase magnetic core. If the variac is fed a four wire source then ferroresonance of the transformer is eliminated because that will stabilize the line to neutral voltage in the variac.
If you want to have a resistance grounded secondary, your only option is to use a delta-wye transformer. That's not going to be available as a standard transformer. In North America utility practice, there are wye-wye transformers available. In North American industrial use along with the rest of the world, the standard is delta-wye, but the HV winding is the delta and you need the delta on the LV side.

 

[waross]

Another option is a motor generator set and control the voltage by varying the excitation.
In an old industrial plant there may be a suitable motor in storage.
If you have to buy everything new, then Matthew's solution may be more cost effective.

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

 

[rockman7892]

MatthewDB/waross

Thanks for the responses. Based on your responses it appears that perhaps it was unclear that there are actually (2) transformers in the circuit which consist of of the auto-transformer (480-480 varying) followed buy a step-up transformer to transformer 480V to 4.16kV for energizing test equipment. (There is also a 2nd step-up transformer in parallel with first for stepping up 480V to 13.8kV). The secondary of the transformers will contain fused contactors for supplying power to prototype test equipment.

My question was mainly focused on what affect the autotransformer would have on the step-up transformer located after the autotransformer? For instance if you vary the output of the autotransformer is it possible to under excite or overexcite the transformer potentially causing damage to transformer or current transients?

It sounds like any concerns for nuisance tripping can be mitigated by ensuring that the autotransformer is set for zero voltage before allowing closing of breaker to energize autotransformer and step-up transformer?

With the step-up transformers having the secondary winding as a wye configuration would you still provide an NGR on the secondary neutral with transformers being small (200kVA) and the secondary being connected to electronic equipment prototypes (not sure of possible internal L-N connections in electronic equipment)

 

[crshears]

My gut tells me that if it were up to me I'd use the two variacs idea, one each to supply 0-480 to the 4 kV output and 13.8 kV output transformers respectively. This would give virtually stepless voltage regulation rather than supplies in voltage increments while also avoiding the arcy-sparky stuff of switching between taps.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[waross]

The voltage must be great enough to exceed the hysteresis of the transformer core.
Transformer cores are designed to reduce the hysteresis as much as possible.
You may need to apply a few volts before you see an output on the step up transformer.
This will probably not be an issue.
In regards to cr's concern, Are you using a Variac to feed a tapped transformer?
It is not completely clear.
If you are using a Variac to feed a tapped transformer then I suggest picking a tap that lets you used the Variac at as high a voltage as possible.
eg:
You are testing a device at 1000 Volts, the Current out of the Variac is 10 Amps with the Variac set at 480 Volts, and the step up transformer on the 1000 Volt tap.
Put the transformer on the 2000 Volt tap and the Variac at 240 Volts. The Current out of the Variac will now be 20 Amps.
Put the transformer on the 4000 Volt tap and the Variac at 120 Volts. The Current out of the Variac will now be 40 Amps.
You see where this is going in regards to low voltages.
This is a matter of tap selection rather than ramping the voltage up on the Variac.
I hope that this is what you are looking for.

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

 

[rockman7892]

Circling back with an update on this one.

The customer had decided on the concept of a Variable transformer (Variac) feeding a 225kVA 480V-13.8kV step-up transformer which their test equipment will be connected to. So the Variac will take 480V utility power and can vary it in a range of 0-480V to primary of transformer (fixed taps) to vary the secondary voltage. The rating of the proposed Variac is 0-480V, 245 Max Amps and 203.4 Max KVA indicating Constant Current Load for both current and kVA. There is a graph that shows that the Variac has a constant 100% rated output current across the full voltage range (0-100%) of the transformer. The same graph applies to the 240V input model at which at aprox 120% of rated voltage the output current capability starts to decay.

Does this mean that the output current will be kept the same (245A) as the output voltage is lowered from 480V down to 0V? Is the output kVA capacity then a function of the maximum current output and the reduced output voltage. For example if the output of the Variac is dialed down to 144V does that mean that the max output is 144V*245A*1.732= 61kVA?

The example that the customer portrayed is wanting to test a piece of equipment at 4.16kV and 110kVA on the secondary of the step-up transformer. To get 4.16kV on the secondary of the 225kVA step-up transformer the Variac would have to be adjusted to supply 144.7V to the primary of this transformer to get 4.16kV on secondary. With the maximum Varicac output of 245A at 144.7V this would reflect a current of 8.5A at 4.16kV secondary and thus only be capable of producing 61kVA, and thus the Variac would be limiting factor in this case and need to be sized larger?

Am I looking at this right or am I missing something?

 

[waross]

The maximum KVA output of the Variac will be the rated current times the output voltage.
At 50% voltage you may draw full current, but of course rated Amps times 50% voltage gives 50% KVA.
If you dial the 13.8 kV transformer down to 4.16 kV, you will limit your capacity to 4.14/13.8 or 30%.
You figured this out when you calculated the KVA available when the 13.8 kV was dialed down to 4.16 kV.
Two suggestions:
1. Order a three winding transformer, 4.8 kV, 4.16 kV and 13.8 kV.
2. Order a transformer with a 4.16 kV winding in series with a 9.64 kV winding for a total of 13.8 kV with full KVA available from either winding (but not simultaneously).
Consider using MatthewDB's control scheme.

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