Transformer xo bonding

[mbrooke]

:thumbsup:Sounds like design issues to me.

That it is- and not the scope of the NEC.


I know people might argue but what if it damages the trafo? Well, so can continuous overloading which the NEC does not directly address.

 

[LarryFine]

I remember reading something about loss compensation being designed into transformers, which would mean that the output of a reversed unit would be slightly lower than nominal. Anyone?

 

[kwired]

I remember reading something about loss compensation being designed into transformers, which would mean that the output of a reversed unit would be slightly lower than nominal. Anyone?

My thoughts are it will try to pass the energy asked for on through, but may result in more heating losses. I guess you would get different results with an inductive load compared to resistance load, both still have increased losses in the transformer though.

 

[GoldDigger]

I remember reading something about loss compensation being designed into transformers, which would mean that the output of a reversed unit would be slightly lower than nominal. Anyone?

In the US control transformer are often (usually?) constructed with compensated windings (the turn ratio being slightly different from the nominal voltage ratio to cancel out resistive voltage drops in primary and secondary under full load conditions.) But power transformers traditionally have an exact turn ratio. Any needed compensation as well as allowance for off-nominal supply voltage is done by changing taps.

In EU (or specifically Great Britain) common practice is to use a compensated turn ratio on power transformers too. That causes the problem you refer to when using them in reverse mode.

In either case, the voltage changing taps, any, end up being on the secondary rather than the primary, which in extreme cases can drive the transformer core too close to saturation for good performance.

 

[MTW]

From the link provided earlier, Hammond Power Solutions.

HPS strongly discourages back-feeding Industrial Control transformers, and Distribution transformers smaller than 6kVA. Due to the use of compensated windings in these products, the resulting voltage will vary from the rating with as much as 10%.

HPS does not recommend back-feeding HPS distribution transformers. Users that decide to use a distribution transformer in reverse feed have to make sure they comply with all applicable Codes and Standards, and consider the following aspects:
– The effect of the resulting high inrush current on all protective devices (breakers and fuses) feeding the transformer. Back-feeding a transformer causes very high excitation inrush, making the coordination with all protective devices extremely challenging within the limits set by the local and national electrical codes. A higher than normal inrush current may also stress the insulation system and produce premature product failures.
– The use and effectiveness of transformer taps is negatively affected by back-feeding. Typically the taps are located in the primary and allow the adjustment in +/- 2.5% increments of the output voltage as needed based on the available primary voltage. When back-feeding a transformer, the taps will be located in the secondary and would not provide the appropriate flux levels in the transformer core as they are designed to do. The use of taps in this case is effectively unreliable, not matching the nameplate voltage variations.
– A typical distribution transformer has a Delta primary and a Wye secondary connection with the system neutral connected in the secondary. If a transformer is back-fed, the neutral terminal X0 cannot be connected to the primary system neutral, it has to be isolated. Furthermore the X0 terminal should not be grounded, but provisions should be made to properly ground the enclosure. In this case the grounding strap between the X0 terminal and enclosure (where provided) should be removed.
On the load side, the transformer winding connection being a Delta would not provide the necessary neutral that most distribution systems require. This may prevent the use of loads that require a neutral connection (ex: single phase loads).
The above described situation may cause serious issues for the whole electrical distribution equipment system in the case of a short circuit, practically altering the path of the fault current and negatively affecting the reliability and efficiency of the upstream and downstream protection devices.
Also, due to the higher mechanical stresses that could occur during such a fault within a back-fed transformer, mechanical damage may occur.
– Sound levels of the back-fed transformers may increase as well outside of the noise levels defined by standards.
– Back-feeding an HPS transformer may void the standard warranty.

Other manufactures state that their small transformers are compensated up to 3 KVA, it's best if you check the specifications of the model you are considering using. Not all are considered equal.

From the SqD link. Most of their units are listed for backfeeding.

1. The practice of backfeeding a general purpose transformer is NOT recommended, especially in transformers smaller than 3Kva. Backfeeding is not allowed for any Industrial Control Transformers of any size, because windings are compensated and backfeeding will result in lower than expected output voltage.
2. Backfeeding causes very high excitation inrush, making coordination to breakers or fuses difficult without exceeding the limits set by the National Electrical Code. Avoid backfeeding wherever possible for this reason.
3. If a Delta-Wye transformer is to be backfed so that the Wye side is the input, do not connect the neutral terminal to the primary system neutral, nor should the neutral terminal be connected to ground.
4. If the transformer has taps, when backfed the taps do not help compensate for poor (other than nominal) source voltage to provide the appropriate magnetic flux levels in the core as they are designed to do.

MTW