Transformer Power Factor and Transformer Setting

Hai friend, As you told tranformer didn't have power factor, because the power factor is depend upon the load you are taking from transformer.If you are using total resistive load, your pf is 1. Means if you are having 100kva transformer you can supply 100kw to resistive load. And If you are having some induction motors of 0.8 pf load you can supply only 80kw.And you know the Transformer losses does not depend on Power Factor.

And for setting of your relay You have not mentioned the voltage you are using.

Any way you can calculate the current from the formuale I = KVA/√3.V.And you can set the current depend upon your relay(DMT,IDMT).

If we use I = KVA/√3.V. It means assumtion by cos phi = 1

KW = power = V.I (Vector dot product) = VICosΦ

KVA = V*I = Arithmatic multiplication = VI

The transformers (please look at innumerable previous posts) works to convert the Voltage (and current) by its ratio (in fact it is the current- magnetic flux- linkage with secondary- induced emf- current in secondary)

As you can see it is the quantity of the current that matters and not the phase angle with voltage (from which the active, reactive, power factor etc arrive)

This is the reason the transformers are rated in KVA (Kilo Volt Ampere), MVA or simple VA but never in W, KW, MW... Since it is least worried about the power transmitted by it, and only by what current (hence copper) and Voltage (hence insulation) it has to handle.

Put your protections as the prev posts said at 1.25 times FLA and go ahead. Only since it is a bit old, think whether it is needed to derate it.

If you base on your 6.250KVA with 13.8KV then your maximum primary current will be at 261.488 Amps

The formula is as follow:

I (current) = 6.250 KVA/(13.8KV*1.732) =261.488 Amps

For over current protection, (for my protection of my transformer) I always set at 100% of the transformer current at the primary side.

Over Current = 100%

Relay delay time (TMS) = 0.3 sec

Earth Faults = 0.1 sec

For your secondary side of the transformer, the maximum secondary current is about 859.177 amps. It is the same formula to calculate the secondary current.

I (current) = 6.250 KVA/(4.2KV*1.732) =859.177Amps

For over current protection, (for my protection of my transformer) I always set at 80% of the total transformer secondary current.

Over Current = 80%

Relay delay time (TMS) = 0.3 sec

Earth Faults = 0.1 sec

But the above may not be relevant to your country requirement. You have to check your electrical regulation and law of your country on all these setting. Each country setting are not the same.

Dear Simon,

Can I ask more question about you stated:

1. What is your consideration in primary you use 100% over current protection and in the secondary use 80% Why does it differences/

2. Do you have a book reference or electrical standarization about the decrease transformer capacity caused by the transformer put on high altitude level (i.e. put on 3000 metres above sea level)

Thanks,

kg

First of all, the transformer is rated by the KVA and not KW. This is because, the transformer does not consume the total maximum current, once you have energized it. If your transformer is energized and you do not have any load at the secondary side (Free Load), the transformer will only consume few ampere only, for the circulation in the transformer winding. That what we call copper loss and heat loss.

Over Current setting on the Primary side of the transformer (for me) will be at 100%. It usually depended on each individual, how and what is the percentage of protection setting needed at the primary side of the transformer. You can set your primary side of your transformer for the over current protection to either 100% or 90% or 80%. Of course, but one thing you must take in to consideration is your future extra connected load.

For example; If I set my primary side of the transformer at 70% and the maximum current that I allow to have, from this transformer is about 183 Amp i.e 70% of your 261.488 Amp at the Primary Side. So, at your secondary side of your transformer, you can have maximum about 601.43 Amp i.e 70% of your 859.177 Amps, for your consumption.

In reality, you don't have maximum of 601.43 Amps to be used. You need to distinguish the difference between your primary over current setting and your secondary over current setting. You cannot have both setting at 100% other wise, when there is an over current detected in your network due to 'Line Faults' or 'Earth Faults', maybe both the circuit breaker for the primary side and the secondary side will trip simultaneously and thus causing problem to other utility user.

Your setting for the over current for the secondary side of your transformer is always 15% to 20% higher than your total load consumption. This is also to help to protect your transformer form discharge too high current to the network due to over current cause by 'Line Faults' or 'Earth Faults'. For example; if your total connected load is consume about 350 Amps (about 41% of your total current of 859.177 Amps) then your over current protection at the secondary side of your breakers should be set at 56% or 481.14 Amps.

The other consideration is, few years down the road, when I need to increase my connected load (Extra load for new plant), I don't have to reset my over current setting at my primary side of the transformer. All I have to do is to set my secondary side of the breakers to the require setting I needed, but of course, the setting at the secondary side will never-never over or same setting as the primary side setting. There is always distinguish the difference between the primary over current setting and the secondary over current setting and the secondary side is always 10 to 15 % lower than the primary side.

Most 'FAULTS" always occurs at the secondary side of the distribution networks and not on the primary side of the transformer.

transformer rating is always in KVA or MVA...

this is because the core loses depends on the voltage u applied to it and copper losses depends on the load current. This load current depend on the load you are connected to the transformer. because of this only the transformer is rated in KVA or MVA. this is the reason.

for relay setting u can take it as P=1.732*V*I

so I=P/(1.732*V)

All,

Thank a heaps for four explanation. Clear understood

Rgds,

kg