450.3(A) MV transformer protection

[Isaiah]

NEC allows a MV CB to be set at up to 600% of transformer primary side current. But what is the risk of setting it much lower, for example 150%? Would the CB trip on energization?
Basically I want to avoid a massive feeder to the primary side by reducing the OCPD trip setting.


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[electrofelon]

NEC allows a MV CB to be set at up to 600% of transformer primary side current. But what is the risk of setting it much lower, for example 150%? Would the CB trip on energization?
Basically I want to avoid a massive feeder to the primary side by reducing the OCPD trip setting.


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You may not have to oversize the conductors even with a 6x OCPD supplying the primary feeder. See 240.101(A)

I am not sure why the 450.3 protection is allowed to be so much higher for MV than LV.

 

[Isaiah]

You may not have to oversize the conductors even with a 6x OCPD supplying the primary feeder. See 240.101(A)

I am not sure why the 450.3 protection is allowed to be so much higher for MV than LV.

I think you’re right - if we’re not worried too much about the thermal aspect can I protect more than one transformer (primary) using relays as long as I dont exceed the damage curve of the cable?


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[jim dungar]

I think you’re right - if we’re not worried too much about the thermal aspect can I protect more than one transformer (primary) using relays as long as I dont exceed the damage curve of the cable?


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Depends on what protection functions you want for the transformer(s).
Primary side protection, for MV transformers, can be much more complicated than simple conductor damage curves.

 

[Isaiah]

Depends on what protection functions you want for the transformer(s).
Primary side protection, for MV transformers, can be much more complicated than simple conductor damage curves.

Transformer protective devices include the basic 26, 63, 71 to Schweitzer SEL751 that trips the CB ie, AB 1200A with LSIG settings.


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[jim dungar]

Transformer protective devices include the basic 26, 63, 71 to Schweitzer SEL751 that trips the CB ie, AB 1200A with LSIG settings.


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But which ones are you planning to implement when you have multiple transformers fed from a single relay? Or were you planning on one relay for each transformer all tripping a single breaker?

 

[Isaiah]

But which ones are you planning to implement when you have multiple transformers fed from a single relay? Or were you planning on one relay for each transformer all tripping a single breaker?

I don’t know how you could use a single relay to protect multiple transformers.
I think it would be easier to use a separate relay for each transformer


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[David Castor]

We typically set the primary overcurrent at 125% of the transformer's maximum rating. If there is some need to allow for temporary overloading of the transformer, 150% is a reasonable value. You just have to make sure to set the instantaneous trip high enough to avoid tripping on inrush. Usually 10 times the full load current for an oil-filled transformer will work.

 

[Isaiah]

We typically set the primary overcurrent at 125% of the transformer's maximum rating. If there is some need to allow for temporary overloading of the transformer, 150% is a reasonable value. You just have to make sure to set the instantaneous trip high enough to avoid tripping on inrush. Usually 10 times the full load current for an oil-filled transformer will work.

Thanks Dave - but what percentage would you use to protect 3 transformers? Seems very problematic to do this


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[David Castor]

You have to accept some loss of protection in the overload region. The relay can protect the transformers against mechanical damage, but probably can't fully protect against overloads. It's common to have a fused switch at each transformer to improve protection against low-level faults or overloads. The NEC rules are more for fire prevention and not necessarily to protect the transformer from damage. Many industrial facilities that put multiple transformers on a single breaker also have double-ended substations so that a loss of one transformer doesn't shut them down for a long period.

It is also possible to put a relay on each transformer with each relay tripping the same breaker.

 

[Isaiah]

You have to accept some loss of protection in the overload region. The relay can protect the transformers against mechanical damage, but probably can't fully protect against overloads. It's common to have a fused switch at each transformer to improve protection against low-level faults or overloads. The NEC rules are more for fire prevention and not necessarily to protect the transformer from damage. Many industrial facilities that put multiple transformers on a single breaker also have double-ended substations so that a loss of one transformer doesn't shut them down for a long period.

It is also possible to put a relay on each transformer with each relay tripping the same breaker.

Having all three transformers energize at the same time seems like a lot of inrush current - wouldn’t the CB trip?
I like the last option of using relays for overcurrent, but the transformers have no CTs (for 87 differential) AND there is no smart relay at the substation to connect a CT for protection It seems we are sort of ‘boxed in’ to adding fused switches for each transformer primary.


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[jim dungar]

Having all three transformers energize at the same time seems like a lot of inrush current - wouldn’t the CB trip?
I like the last option of using relays for overcurrent, but the transformers have no CTs (for 87 differential) AND there is no smart relay at the substation to connect a CT for protection It seems we are sort of ‘boxed in’ to adding fused switches for each transformer primary.


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What size transformers are these? What type of loads? Many transformers in the kVA size are installed without differential protection.

 

[Isaiah]

What size transformers are these? What type of loads? Many transformers in the kVA size are installed without differential protection.

These are only 2500kVA, 6.9kV-480/277V
Lightly loaded. The main concern is there is no overcurrent protection other than the CB which is mainly for ground fault


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[Isaiah]

You have to accept some loss of protection in the overload region. The relay can protect the transformers against mechanical damage, but probably can't fully protect against overloads. It's common to have a fused switch at each transformer to improve protection against low-level faults or overloads. The NEC rules are more for fire prevention and not necessarily to protect the transformer from damage. Many industrial facilities that put multiple transformers on a single breaker also have double-ended substations so that a loss of one transformer doesn't shut them down for a long period.

It is also possible to put a relay on each transformer with each relay tripping the same breaker.

So we could add a CT at each XFMR and relay at the switchgear to trip a common MCB, correct? Would this be an 87 function?


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[jim dungar]

These are only 2500kVA, 6.9kV-480/277V
Lightly loaded. The main concern is there is no overcurrent protection other than the CB which is mainly for ground fault


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It is not uncommon to find MV primary transformers, in qualified industrial installations, with only short circuit and ground fault primary side protection. The NEC allows this probably because the secondary side devices provide overload protection in situation these situations.

 

[David Castor]

You would need a separate overcurrent relay at each transformer connected to CTs at (or in) the transformer. It's best to keep the CT leads short. In addition you would need control power for the relays. Each relay would be wired to trip the commo breaker. It's not differential (87) protection - that's a different beast. In theory, you could install differential relays for each transformer, but that requires additional CTs on the secondary side. Most people will want a disconnecting means at the transformer anyway, so commonly a fused switched in installed ahead of the transformer primary. Another consideration these days is the arc-flash incident energy at the transformer (primary and secondary).

 

[Isaiah]

You would need a separate overcurrent relay at each transformer connected to CTs at (or in) the transformer. It's best to keep the CT leads short. In addition you would need control power for the relays. Each relay would be wired to trip the commo breaker. It's not differential (87) protection - that's a different beast. In theory, you could install differential relays for each transformer, but that requires additional CTs on the secondary side. Most people will want a disconnecting means at the transformer anyway, so commonly a fused switched in installed ahead of the transformer primary. Another consideration these days is the arc-flash incident energy at the transformer (primary and secondary).

Ok but does this overcurrent relay respond to one of the ANSI functions? Like 50/51 - for the primary side?


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[Isaiah]

It is not uncommon to find MV primary transformers, in qualified industrial installations, with only short circuit and ground fault primary side protection. The NEC allows this probably because the secondary side devices provide overload protection in situation these situations.

Where does NEC state this?


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[jim dungar]

Where does NEC state this?


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450.3 allows breakers to be up to 600% of the primary full load amps, how would this value provide any level of 'overload' protection?

In particular see table 450.3(A) footnote 3.

 

[Isaiah]

450.3 allows breakers to be up to 600% of the primary full load amps, how would this value provide any level of 'overload' protection?

In particular see table 450.3(A) footnote 3.

Makes sense.
But I think I still need a fused disconnect - maybe cutouts? - at least something to isolate each primary against inrush current. Without this all 3 2500kVA transformers will energize at the same time. Surely a single CB can’t handle this amount of current without nuisance trips.


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