POWER TRANSFORMER PEAK CURRENT 5

[josedavidch]

I just installed a power transformer of 75KVA, 480VAC on primary and 1500VAC on secondary.

We got surprised that this transformer triggers our 200A circuit breaker when energized with secondary open circuit (no load at secondary). Looking for the reason why, I figure out that when I energize the Xformer, what I am doing is apply 480VAC to a 0.2ohm bobbin, resulting on a 2400A current flow that will remain until the Xformer stabilizes it self. This is a current peak certainly, cause the open circuit current is just 2.4A.

What is the theory behind this peak current and how can I fix this issue?
How long the peak last and how can I calculate it?
Should I upgrade my circuit breaker to higher Ics one?

 

[dpc]

This is the transformer inrush current. The magnitude varies depending on the voltage phase angle when the breaker is closed. So normally, one phase will be much higher than others. Also, this will cause the breaker to trip some times but not others. The residual flux in the core also plays a role.

Is this a single-phase or three-phase transformer?

Normally, you need to allow for 10 to 12 times the normal full-load current for around 0.1 seconds.

If your primary breaker has adjustable instantaneous elements, you can try increasing the setting.

If this doesn't work, you will need a bigger breaker.

 

[alehman]

In my experience a 125A thermal-mag breaker with magnetic set at 10x (1250A) will work with a typical 75kVA 480V 3-phase transf. I'm somewhat surprised that a 200 would trip consistantly, unless it is 1-phase.

The NEC permits the primary breaker to be sized at a maximum of 125% of the primary full load current, unless secondary overcurrent protection is set <=125% of the secondary full load current.

 

[waross]

Hello dpc and alehman,
It looks like this subject is well covered. I would just add that a 200 amps looks like the proper size for single phase, and with the magnetic trips at Hi it should hold. If not the breaker may be faulty.
respectfully

 

[josedavidch]

I am glad to see how rich and fast is this forum.

This is a single phase Xformer. The breaker is not trip all the times, but let's say, 3 of every 5 times.
Right now we use a SQUARE D, KA36200 breaker. Our facilities guys advice to replace it with a KH series with a higher peak current capacity.
We tried to measure this inrush current and our meter shows 1890A before the breaker trips.

Is there any other way to absorb this inrush current than replacing the KA breaker?
The manufacturer told us that the lab test of this Xformer was made using a varactor and slowly goes from 100 VAC to 480VAC, but on the real world is does not works like this.
What if we use a kind of single phase soft starter?

 

[Warpspeed]

In theory, adding some extra resistance in series with the primary at start up would limit the peak inrush to whatever figure you wanted to make it. After less than a second the primary current will settle down to the nominal no load magnetising current. The high power dissipation in the starting resistor will only be momentary, and is not going to be significant. After a short delay, the starting resistor can be bypassed with a suitable contactor, and at the same time the secondary load can be connected.

It is a bit messy, but it may prevent voltage sags and tripping in the primary circuit.

 

[asymptote]

As a post script to dpc's comment on typical inrush currents for 3 phase transformers of 10 x FLC as the accepted norm. I have recently been advised that air cooled 1.6MVA TX will have inrush of 15XFLC.

 

[waross]

Hello
On this website you will find a down load of the breaker specs.
The maximum magnetic trip is 2000 amps.
If the breaker is tripping at 1890 amps it may be a breaker problem, but changing to a KH series will probably be a waste of money. The KH has a higher interupting rating, but the magnetic rating is the same as the KA series and this is where your problem is.
It looks like you are at the maximum breaker rating allowable by code.
You may have to energise with a limiting resistor in series and then shunt the resistor to comply with NEC.
I hope some of the VFD/Soft-Start experts give us an opinion on the suitability of a soft start for a transformer.
You can locate a down load of breaker specs. and compare the KA and KH ratings at

http://ecatalog.squared.com/techlib/docdetail.cfm?oid=0900892680195827

respectfully

 

[dpc]

You may be seeing higher than "normal" inrush because of the step-up duty. Most of these rules-of-thumb are derived from experience with step-down transformers.

My standard is to use a 225A breaker for a 75 kVA 480V primary (single-phase).

It will be much simpler and less expensive to replace the breaker than attempt any scheme to insert resistance in the circuit for startup or use a soft starter. You may to need to increase the size of the conductors as well, in order to meet NEC.

Another consideration is that these molded case circuit breakers are not high accuracy devices. You could try replacing your breaker with another 200A breaker. There is a lot of variability in these low-end breakers.

 

[Warpspeed]

Agree fully that a suitably rated circuit breaker is the obvious solution. Provided that the source is stiff enough to supply the peak inrush without creating other problems.

If genuine soft startup is required, then something a bit more complicated may be needed.

 

[waross]

I would probably do the same as you, dpc. I was under the impression that the 125% rule for transformers less than 600 volts referred to the primary voltage. If we don't have to comply with this rule with a secondary over 600 volts, then I wouldn't hesitate to do as you suggest and use the 225 amp breaker.
respectfully

 

[josedavidch]

Waross

Why NEC refers to that arrange on the limiting resistor? That actually implies that I need 2 big contactor to do the limited current startup.

What If I set a the resistence in series with a secondary contactor. This series will be in parallel with the main contactor. When we startup the Xformer, we power up the secondary contactor allow reduced current to flow to the Xformer.Then, 1s later, the main contact is energized, shunting the series of the secondary contactor and the resistor. Next step will be open the secondary contactor and then, all the current will flow tru the main contactor contacts. This, in my opinion, allow me to use a smaller and cheaper contactor to enable the resitor.

Plesae advcce

 

[waross]

Hello josedavidch
Do you have any over current protection on the secondary of the transformer? If you have a fuse or a breaker in the secondary of the transformer you can go up to 250% of rated current for your primary breaker. You may have to increase the size of the conductors.
There are several suggestions to increase the breaker size to 225 amps and increase the size of the wire if needed.
I am inclined to agree with them.
Can you check with the local inspector as to what he will accept?
Re the contactors; The contactors would be sized for the normal transformer current, not for the 1800 amp inrush.
In regards to the resistor, you may be able to wind an inductor to use instead of a resistor.
I am not sure about sizing contactors for transformers.
dpc, can we use motor current ratings for contactors for transformer duty?
respectfully

 

[Warpspeed]

A choke is going to cost a lot more than a simple wire wound resistor. And yes, the contactor to initially switch in the resistor can be lighter duty than the main contactor.

 

[josedavidch]

Hi All


Still no certainly sure what solution is better, no considering $$$ as an issue. What is better for the electrical network of the facilitie:

- Reduce the inrush at the source using resistance in series? or
- Using a bigger inrush capabilitie on a circuit breaker?

 

[waross]

Hello josedavidch
Do you have lights dimming when the transformer is energized? What size is your panel that the transformer is fed from? What size is your main service?
I would suggest that if there are lights dimming or any other sign that the transformer is being energised then consider a series resistor.
If there are no problems being caused by the transformer being energised go for the larger breaker.
How do you energise the transformer now?
Whether you are using just the breaker or a switch or a contactor, you can add a resistor and just one contactor and a timer.
You are comparing the price of a replacement circuit breaker and possibly larger wires on the one hand to the price of a contactor, a resistor, and a timer on the other hand.
respectfully

 

[josedavidch]

Hello Waross

There is no dimming lights, since the main panel where the Xformers fed from is a large one, matter fact, is one of the main panel of the facility.
I am more afraid of things no one sees or are not visible enough.

Our plant has many many test equipments and we have expirienced problems due to noise on the electrical power network. What I think is if reducing as much as possible the inrush from Xformer, will help keeping the noise lower and no sudden spikes from the newtwork affect the equipment.

The cost of replacinf bkeaker is quite high, about $2K, Installing a secondary contactor, a time controller and resistor would be nearby $400.

Regards

 

[waross]

Hello josedavidch
I think you have just answered your own question. Cost and performance are both on the side of the resistor.
respectfully