Unbalanced Load on Delta-Delta Transformer 2

[iitm70]

I would appreciate if someone could discuss the disadvantages of unbalanced loads on delta-delta power transformer. Assuming the loading is within the transformer rating, are there any undesirable consequences of this? Any effect on power system harmonics, delta circulating currents, or whatever else?

 

[cuky2000]

Delta-Delta connections can tolerate large unbalance load satisfactorily.
Circulating current is not a major issue unless the winding turn ratios of each single-phase unit are not equal. (ANSI/IEEE C57.105 provides example and other details in this matter).
Delta-delta connection is suitable for harmonic suppression. Particularly the third harmonic exciting currents will circulate and damped within the closed delta and will not appear on the line side.

One of the major draw back of the delta-delta connection is the susceptibility to ferroresonance.

 

[fsmyth]

Can you go into a bit more detail in the tendency
towards ferroresonance?
<als>

 

[cuky2000]

Hi FSmyth,

The susceptibility to ferroresonance of delta-connected transformers was frequently observed in the 1960’s with the introduction of URD cables and pad mounted transformers. This problem forces the utility industry to shift to other connections less prone to ferroresonance.

Although this is a random phenomenon, often is observed during the single-phase energization of a transformer bank without its neutral grounded, such as the delta connection. This usually is done in the field by closing single-phase switches or fused cutouts, one at a time, energizing the compounded transformer and cable inductance in series with a the capacitance.

Enclose is an excerpt from the IEEE, indicating that delta-delta configuration is one of the connection susceptible to ferroresonance

 

[fsmyth]

Thanks, cuky.
Have a star. I'd a give you two if you had included some values.
<als>

 

[cuky2000]

Fsmyth,

It is difficult to give an specific figure of a possible risk of ferroresonance since this is a random phenomenon to be analyzed case-by-case and generally depend on many variables such as the transformer design, core material, transformer connection, configuration of line, length of cable, type of installation (OH or UG), loading conditions, etc.
I see cases of overvoltage caused by ferroresonance that in most cases are around 2.5 PU. However, in particular conditions, ferroresonance overvoltage could reach up to 5 PU for long duration with potential capability to create a lot of damage in the system.

Here is a generic data from Cooper Power that indicates the Minimum Distribution Transformer Capacitance Limits for ferroresonance.

http://cuky2000.250free.com/Ferroresonance-3.jpg