distribution question(s)

[urbanxer]

I work in the line department for a utility and also have some technical education (ASEE). Our company’s primary distribution scheme is 13.8 kv wye. The lighting transformers (25 &37.5 kva) that I have encountered in the field are single bushing. The primary connections are to one phase of the primary and the other to the system neutral. It seams that the system neutral is made common with the center tap of the secondary (autotransformer). It appears to me the neutral that runs in the secondaries is also the system neutral because subsequent transformers down the line use this neutral as its other primary side connection. I guess some of my questions are, wont this ‘neutral’ have to carry a lot of current? What if another lighting pot is connected to a different phase, wouldn’t his centertap be a different potential (angle?)? Can someone give a basic explanation of this type of distribution scheme or point me to a link or resource that does. Thanks.

 

[alehman]

Using a common primary/secondary neutral is normal practice. Your statement about different angles is the key to answering your question. On a line with several transformers, the primaries should be connected to different phases. The three phases are 120 degrees apart so the sum of the primary return currents (in the neutral) will be small if the loads are approximately balanced.

The secondary works the same way. For single-phase pots, the two phases are 180 degrees apart. If the secondary phase loads are balanced, the secondary neutral current is small.

 

[busbar]

 
Do the 3ø transformer banks use two or three 1ø units? Grounded-wye—delta is not a recommended configuration for its ability to source 13.8kV zero-sequence current. Or are they open-wye primary—open-delta secondary?

Multigrounded-neutral distribution and its tradeoffs are discussed in ANSI/IEEE C62.92.4-1991 …Application of Neutral Grounding in Electrical Utility Systems, Part IV—Distribution or IEEE C57.105-1978 …Application of Transformer Connections in Three-Phase Distribution Systems

TOCs at

http://standards.ieee.org/reading/ieee/std_public/description/surge/C62.92.4-1991_desc.html

and

http://standards.ieee.org/reading/ieee/std_public/description/dtransformers/C57.105-1978_desc.html

[Common-grounded primary/secondary connections do not “qualify” the bank as an autotransformer.]

 

[tinfoil]

As alehman stated, utilities expect and count on a statistical 'cancelling effect' to take place on a common neutral.

(Ignoring currents flowing through ground for this discussion

Even if a home's two 120 'legs' are not well balanced at any given moment, the resulting neutral return current will be 'summed' with the other services from that distribution transformer, which will also have imbalances in their resultant neutral currents. About half of these currents will be 180deg out of phase with the other half, and tend to cancel out.

The transformer's overall aggregate imbalance will then return to the 'primary' system neutral, which should be bonded to the xmfr's secondary neutral.

This neutral current may or may not be cancelled somewhat by the transformer's primary neutral current, but will be added to the VECTOR sum of all of the other transformer's neutral return currents, as well as all of the primary return currents sharing the common neutral.

If long one-phase primary taps are taken from the 3-phase main feeder, then the primary connections of the transformers on the rest of the circuit are usually connected to favor the remaining two phases. Sometimes, the connections of these 1-phase taps are moved to another phase to assist in phase balancing.

As long as the transformers are connected in a ratio of 'about' 1/3 to each of the three primary phases, the net neutral current is surprisingly low throughout any given load period. It is not uncommon to have 1000-5000 customers on a single three-phase feeder, which may represent 200-1000 separate distribution transformers.

At this utility where I work, neutral currents returning to the substations are usually in the range of 5-10% of the average phase currents, for all hours of all days (ignoring harmonics).