Comment on dpc (Electrical) Mar 2, 2004 marked ///\\jbartos said:
"1. The protective relay settings will be protecting better on the line to ground faults when the Y-Y transformer connections with solidly grounded neutrals are applied. The Y-Y network transformer standardization leads to numerous advantages."
To my mind, protective relaying is MUCH easier on a delta-wye transformer because ground faults on the secondary side are isolated from the primary, making coordination much easier. If there is upstream relaying on a delta-wye transformer, any zero-sequence current can be assumed to be from a primary ground fault, allowing very sensitive ground fault protection. On a wye-wye, a low-side ground fault causes primary ground fault current, making coordination more difficult. Actually, ground fault protection is one of the primary advantages of delta-wye units.
///I think that you are referring to the power distribution system from substation to Clients/Consumers. In medium and high voltage power distribution networks, the delta connections are less used since the bushing would experience higher stresses, the wye connection solidly grounded provides lower impedance ground return path for the line to ground faults, causing the protective relay more positively trip. The high voltage and extra high voltage power distributions mostly use wye connections\\\
Utilities like wye-wye transformers, as near as I can tell, because they are cheaper, it allows them to standardize and they provide protection against that old boogeyman ferro-resonance.
If I am designing an industrial installation, I will always try to negotiate for a delta-wye unit.
///Please, notice a distinction between distribution transformer and network transformer.
Reference: IEEE Std 100 Dictionary defines:
Distribution Transformer (power and distribution transformers) A transformer for transferring electrical energy from a primary distribution circuit to a secondary distribution circuit or consumer's service circuit. Note: Distribution transformers are usually rated in the order of 5-500kVA.
Network Transformer (power and distribution transformers) A transformer designed for use in a vault to feed a viable capacity system of interconnected secondaries.
Please notice that I referred to Y-Y network transformers.\\Old literature will also mention wye-wye units have problems with telephone interference due to third harmonics passing through the transformer, but I'm not sure if this is really much of an issue anymore.
///When it comes to transmission lines terminated via Y-Y connections, the telephone lines are usually far away from the medium and high voltage transmission lines.\\
