Back-feed on primary thru banks

[PowerDawg]

Hi Guys, first post on this forum.

I work for an electric distribution utility with a primary voltage of 14.4/24.9kV.

Recently, we had an outage on a 3Ø line involving only one phase… the other two phases remained “hot”. There are several banks on this line both Wye-Delta and Wye-Wye. The lineman working the outage said that he had voltage on the “dead” phase and asked me if the banks could back-feed and re-energize the “dead” phase… I have not been able to satisfactorily answer this question. Can anyone help me with his question?

 

[davidbeach]

Yes, entirely possible.

 

[PowerDawg]

In this scenario, it seems to me that voltage would have to come through the consumers’ equipment on a Wye-Wye bank. Would the same be true for a Wye-Delta bank? Or could you see back feed on a Wye-Delta bank simply because of the secondary connections?

 

[jghrist]

You can see back feed either through delta windings on a wye-delta bank or through customer delta connected loads on a wye-wye bank. The delta load can act as a voltage divider between the two hot phases, putting voltage on the open phase.

 

[waross]

If the primary neutral is connected in the wye:delta bank, there will be a full voltage backfeed from the bank(s).
When one phase is lost, the bank becomes an open delta with the ability to support loads across the open side. The transformer on that side becomes the load and backfeeds full voltage into the missing phase. The ultimate result depends on the size of the transformer relative to connected load on the missing phase.
Possible results are:
Full voltage indefinitely.
An overload on the backfeed transformer and near full voltage on the missing phase, depending on voltage drops.
Fairly rapid fuse clearing of one or more of the fuses on the backfeed bank.
A burned out transformer. In systems that I am familiar with, wye:delta banks experience frequent clearing of the primary fuses. In frustration the repair crews resort to ever greater fuse ratings until there is no protection and the fuse passes enough current to allow the transformer to self destruct.
In the system which was under my control, I embarked on a program to convert all wye:delta banks to wye:wye banks. It took several years but in the end the frequent complaints concerning the failure of refrigerators and freezers came to an end.
Motors running single phased on wye:wye banks may also backfeed but the effect is generally much much less than the wye:delta issues.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[PowerDawg]

Waross, we float the primary neutral on our wye-delta banks. Therefore, the primary neutral of the bank is not connected to the system neutral. How does this change your response to my question?

 

[stevenal]

Please consider it live until it is isolated from all sources (including possible legal or illegal customer side sources), tested, and grounded. Even if you are serving only single phase loads, there may be induced potentials from the other phases or from other circuits.

To answer your last question, if there is no load, the open phase will assume the voltage of the floating neutral, which will assume a potential midway between that of the two remaining phases. The presence of secondary load will modify this somewhat.