Overcurrent 51 setting for transformer 1

[budhiman]

Hello All,

I have seen typically the overcurrent pickup of the relay (51) feeding transformer is set 125% of the primary full load amperes. Is there a reason to use 125% factor from any standard or it is account for some overload (FA rating if applicable)?

 

[marks1080]

From what I've seen 125% is low. Operators in my area like to have to ability to run the transformers at 2pu in certain situations. They will know all the short term thermal limits and how far they can push things. Sometimes they may have good reason to choose to sacrifice a little bit of health of the unit.

 

[bacon4life]

If the transformer is monitored by SCADA, I agree with marks1080 that 125% of the top nameplate rating is low.

NERC PRC-023 has minimum trips setting requirements for EHV/UHV transformers of:
* 150% of the applicable maximum transformer nameplate rating (expressed in amperes), including the forced cooled ratings corresponding to all installed supplemental cooling equipment.
* 115% of the highest operator established emergency transformer rating.

 

[dpc]

Article 450 of the NEC (US) uses 125% for certain types of transformer applications. It's higher for transformers over 1000 volts. Of course utilities are not required to follow the NEC.

If there are overtemperature trips, I think the setting could be higher - 150% or even higher. Some utilities have different winter and summer ratings. For large transformers this is something that must be agreed upon between protection and operations groups. If I'm in doubt or working without guidance from the client (often), I set it just above 125%.

 

[davidbeach]

We go for 200% of top nameplate rating, but we also have LBNR (Loading Beyond Nameplate Rating) ratings that, in the winter, greatly exceed the nameplate rating. Works better than leaving customers in the dark in the middle of the winter. Settings are all based on staying out of the way of operational loading decisions and we don’t change settings seasonally.

 

[iceworm]

Disclaimer: My experience is NEC (US) Industrial - and my response is limited to that area.

budhiman -
As you can see from the responses, a bit more context will help.

Is your application utility? If so I agree, 125% is ridiculously low. Utilities like to set the OCP just short of self-sustaining incandescence. But, not my area.

However, if the application is on the customer side of the service point, then the electrons behave differently.

As dpc mentioned, NEC, Article 450 applies.

Generally speaking there are two flavors of transformer feeders, and about a million sub-categories. I'm going top stick with the two for this post.

1. MV primary/LV secondary (example 13.8KV/480V)
Primary OPC generally set 200% - 300%. NEC limits vary depending on transformer impedance, CB or fuse, and if supervised location.
Secondary OCP generally set at 125%. Again NEC limits vary.

Most important, is to set the primary (and secondary) OCP to trip inside on the transformer damage curve. IEEE C57.109 has some guidance.

2. LV primary/LV secondary (example 480V/208V)
Primary OCP generally set 125% - 250%. And as dpc mentioned, with coordinated thermal overload protection, primary OCP could be as high as 6X FLA.
Secondary OCP set to 125% or none.

My experience is 125% primary will cover inrush. Although reverse fed can be a problem. See GE XformerWhitepaper2

Is there a reason to use 125% factor from any standard or it is account for some overload (FA rating if applicable)?

From standard? - Yes
Account for some overload? - yes

the worm

Harmless flakes working together can unleash an avalanche of destruction

 

[ElecEE8]

In IEC or IEEE, practically it is recommended to set at 110% to 140% of FLA. However, the criteria to set is you must set below the transformer damage curve (always).

 

[HamburgerHelper]

175% max rating is what I have seen at plants for the pickup.

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If you can't explain it to a six year old, you don't understand it yourself.

 

[budhiman]

Thank you all for very informative responses.the transformer here is a step-down transformer for an industry application (owned by the plant) i.e. utility incoming is 12.47kV feeding customer owned MV relay which feeds a step down transformer (12.47kV to 480V).

 

[davidbeach]

So your local electrical code governs, however much sense it seems to make or not. What utilities do with their own equipment is irrelevant.

The electrical codes are based on the consumer having a relatively fixed plant, there for an indefinite life. Let's not let any of it burn down due to misuse or overload.

The utilities run a much more dynamic system and will move things around as needed, replacing a regularly overloaded transformer with a different one is reasonably routine. The utility watches (should be watching) the loading on various portions of the system and can determine when cyclical overloading becomes too much. Most consumers can't be expected to do that, so the code intends to keep them from going there in the first place. Same underlying physics, very different rules and regulations.

 

[dpc]

I'm in violent agreement with David Beach. I'll just add that the NEC requirements for transformer protection in Article 450 are fundamentally about FIRE PREVENTION, not transformer protection. This is generally true of the entire NEC. There is nothing in the NEC that requires a transformer be fully protected based on its through-fault damage curves. When fuses are used for prkmary protection, this is often impossible to achieve anyway.

 

[prc]

I think overcurrent pickup should be far below the damage curves given in C57.109-2018.As per IEC 60076-7-2018 Transformer overloading guide (for US IEEE C57.92 &115) , max pu current during normal cyclic overloading shall be limited to 1.5-1.5-1.3 irrespective of ambient temperature (for small,medium and large transformers with hotspot winding temperature reaching 120C ) and to 2-1.8-1.5 pu current under short time emergency overloading,with hot spot winding temperature reaching to 160C. To understand the extent of enhanced ageing of paper insulation under overloading, standard limits hot spot temperature under rated load to 98C at weighted annual average ambient temperature of 20C. With every 6C rise over 98 C, the life of insulation reduces by half. Transformers are designed for the above max currents under cyclic and short time loadings using ratings of bushings and tap changers accordingly. So for large transformers 125 % current setting seems reasonable for cases where emergency over loadings are not planned- a situation normally avoided by both utilities and industrial users.

 

[RRaghunath]

Budhiman,
Overcurrent protection of transformer is for short circuit faults and not for overloads.
It is transformer temperature (Oil/Winding) protection that is meant to protect the transformer against overloads.
So, please set your OCP pickup as required by your operations.
Please do remember if you overload the transformer for prolonged periods, there is a risk of trip on temperature protection.

Rompicherla Raghunath

 

[prc]

I am not a protection expert. But I find from ANSI that there is a 51 overload relay. We, transformer engineers consider the temperature indicators as indicators only and not protectors. It will act as a protection relay only in case some one load a transformer with the coolers under closed condition. We expect other types of protection against overload as time constant of OTI is 2-3 hours and WTI 6-10 minutes. To my knowledge, overloading an old transformer with lots of moisture in conductor paper covering is a risky condition. When winding temperature exceeds 100C, there is every chance the moisture turn in to steam, and release out as gas bubbles. As permittivity of gas is unity, entire electric field may concentrate in bubbles, leading to a dielectric breakdown.

 

[cranky108]

Depending on the location, and voltage ratings of the low side, a transformer could be part of the BES, and in which case there is an inference in PRC-023 that the over current is necessary to protect the transformer mechanical damage curve from through faults.

But since PRC-023 is about loadability, it would not apply to ground curves.