Xfrmr Primary/Sec OCPD Help

[jim dungar]

By this I assume you mean protecting the secondary conductors as well as the ability to divide the secondary into multiple different branch circuits?

No. Your question was specific to ground fault protection of transformers. Secondary side GF protection almost never has any impact on the primary side protection requirements. For the majority of installations the primary side sees a secondary side GF as if it was a load.

 

[jim dungar]

Understood. But if the secondary winding shorted to the grounded core, despite obviously demanding more from the primary and thus tripping the primary OCPD, would a secondary OCPD not theoretically "react" more quickly?

Where would you locate a secondary GF device so that it could actually see that a secondary winding is shorted to ground?

Complicated transformer protection involves differential relaying in order to 'see' internal transformer faults.

 

[Jerramundi]

Where would you locate a secondary GF device so that it could actually see that a secondary winding is shorted to ground?

At the first disconnect location on the secondary side of the SDS, in like a main lug panel w/ a GFCI breaker.

 

[jim dungar]

At the first disconnect location on the secondary side of the SDS, in like a main lug panel w/ a GFCI breaker.

GF devices look downstream not upstream.
The only thing upstream from a transformer secondary winding is the transformer primary.

 

[Jerramundi]

GF devices look downstream not upstream.
The only thing upstream from a transformer secondary winding is the transformer primary.

You're right. I'm jumping around between (1) the secondary winding shorting to the grounded core and (2) a downstream ground fault. Reprise my question in regards to downstream GF's. Would a secondary OCPD not theoretically react more quickly?

Obviously time is a factor, even if it is relatively minute to us beings up on here on the level of classical physics as opposed the subatomic level.

 

[jim dungar]

You're right. I'm jumping around between (1) the secondary winding shorting to the grounded core and (2) a downstream ground fault. Reprise my question in regards to downstream GF's. Would a secondary OCPD not theoretically react more quickly?

As I have said: primaries of delta-wye transformers do not see a secondary GF as anything other than a load. You cannot use a primary side device to sense a secondary GF.

 

[Jerramundi]

As I have said: primaries of delta-wye transformers do not see a secondary GF as anything other than a load. You cannot use a primary side device to sense a secondary GF.

I understand that the primary side OCPD won't see the secondary side GF as anything but increased current. So when installing an SDS in which the entire system requires GFCI protection, a secondary side OCPD with GF would be required?

 

[jim dungar]

If it is an SDS the secondary side protection must stand alone, its GF is completely independent of the primary. Likewise the primary side ground fault protection would stand alone.

I do not know what you mean by the 'entire system' requiring GF protection.

 

[paulengr]

A couple details here.

First off in industrial/commercial we are so conditioned to the idea that going even a little above the design limit results in total destruction. Not true with transformers. In fact utilities routinely overload them. As an example oil filled transformers are typically rated for 65 C temperature rise over a 24 hour ambient temperature of 40 C or 104 F. It would be hard to find anywhere in North America that hot as a 24 hour average, even Death Valley which gets pretty cool at night. So recalculating for the actual ambient maximum where I live in coastal North Carolina for instance allows at least 25% over name plate with no damage. Smaller transformers are less forgiving but you get the idea. Every 10 degrees C doubles the thermal limit. Remember it’s the square of current.

So this should give you some idea how resilient transformers are.

The primary protection also has to dodge the magnetization current. At startup magnetization current can reach 20-100 times rated current. So typically you can’t size it right at name plate. Primary protection is realistically for short circuits. You can’t really expect secondary protection. The only way I know to pull this off is if you use a breaker with a multifunction protection relay. The relay is set up for the higher (250% of FLA) level when the breaker closes. Once the current drops down below name plate the relay switches over to trip at 100%.

Another issue with delta wyes is the phase shift. You basically split a single phase fault over two phases making primary side protection even harder. This is not a problem with OPs example of what sounds like a single phase case.

There is an exception called a virtual secondary. In this arrangement you put bushing CTs on the secondary side tied to a protection relay that shunt trips a breaker on the primary side. A big benefit is primary side breakers see the primary side current (much lower) and the high fault currents (and arc flash) on the primary side of a breaker on the secondary side are eliminated. This is more of a large transformer trick.

 

[Jerramundi]

If it is an SDS the secondary side protection must stand alone, its GF is completely independent of the primary. Likewise the primary side ground fault protection would stand alone.

I do not know what you mean by the 'entire system' requiring GF protection.

Entire system meaning primary and secondary sides. You answered my question. If it's an SDS the GF on the secondary side is independent of the primary.

I'm just debating the merits of doing additional protection on the secondary side when not required to do so. I don't have any receptacles in areas that would require GF protection, but there are accessible openings quite a ways away from the primary OCPD, so I'm thinking about a small main lug panel with a GFI breaker on the secondary side.

 

[Jerramundi]

One last question regarding the GF protection. What about the load side of an autotransformer? Will a GF there also just be seen as additional load by the primary protection as opposed to an imbalance between L1 and N?

Or is just in the case of an SDS that GF protection on the primary does not equate to GF protection on the secondary?

 

[synchro]

How are you proposing to connect an autotransformer?

 

[Jerramundi]

How are you proposing to connect an autotransformer?

Not sure what you're asking by this. Obviously non-isolation, non-SDS as an autotransformer so just wondering if primary GFCI protection would extend to the secondary...

GFCI breaker, 120V, to a buckboost near the panel, boosted to ~136V for a longer run, which at the end features a receptacle requiring GFCI protection.

Seems to me that, with an Autotransformer and a GF on the secondary, the current "leaving" on L1 would NOT match the current "returning" on N and thus a GFCI breaker on the primary side would be sufficient for a GF on the secondary, unlike with an isolation transformer, SDS where said difference in current on the secondary due to a GF would just be seen as an overall increase in current across the primary OCPD.

 

[synchro]

GFCI breaker, 120V, to a buckboost near the panel, boosted to ~136V for a longer run, which at the end features a receptacle requiring GFCI protection.

Seems to me that, with an Autotransformer and a GF on the secondary, the current "leaving" on L1 would NOT match the current "returning" on N and thus a GFCI breaker on the primary side would be sufficient for a GF on the secondary

That is correct. A GFCI breaker on the input to the buckboost will provide GF protection on the buckboost output, and it will trip at the same GF current level as without the buckboost.

For example if you had a 10mA ground fault on the buckboost output then the current on the L1 input to the autotransformer would be (1 + 16/120) x 10mA = 11.33 mA.
But the current on the neutral input to the autotransformer would not be zero as it would be without the transformer. Instead it will be the current flowing through the winding with 120V across it, which is the ground fault current scaled by a factor of 16/120: (16/120) x 10 mA = 1.33 mA. And so the difference in line and neutral currents sensed by the toroid of the GFCI is 11.33 mA - 1.33 mA = 10 mA, which is the same as without the autotransformer. Therefore the buckboost does not change the sensitivity to a ground fault. This result makes sense because the ground fault is still the only thing drawing "common mode" current (i.e., current in the same direction) on the line and neutral outputs even when the buckboost is present.

 

[GoldDigger]

But the current level seen by the primary side GFCI will be multiplied by the inverse of the turns ratio of primary and secondary taps. So if a boost the GFCI will be more sensitive, if a buck the GFCI will be less sensitive. A small percentage change for a typical buck boost, but autotransformers in general can easily have a 2:1 ratio.