A 3 phase dry type transfer (15 kva)with a delta secondary (480 volts)the connected load is one 7 1/2 HP pump motor 1 1/2" miles from the transformer. Four #1's to run in PVC.(Area is very wet) Questions: 1) should the secondary of the transformer be grounded or should the secondary be remain electrical isolated from ground? 2) How should the secondary be grounded? 3)Were can I find this information in the NEC? 4)Explains the reasoning for grounding?
Thank you Jerry Flaherty
[This message has been edited by JerryF (edited 10-24-2001).]
You may choose to ground the system, or to keep it ungrounded. Either way the transformer's frame will need to be grounded and I believe there needs to be a Grounding Electrode system for the transformer - even if the system is ungrounded. Need to verify this - I know ungrounded Delta services require a Grounding Electrode system.
You may want to choose to Ground the system, since the distance to the Motor is 1.5 Miles from the Transformer. This will help stabilize the voltage to ground [so it does not "Float" from 300 VAC upto 1000 VAC!!!], also will assist with Lightning Discharges and other imposed stuff on the Motor, branch circuit and overall system.
If you choose to Ground the system, physically bond any one of the three "Phases" to the newly created Grounding Electrode system for this Transformer. Do this as you would with 4 wire Wye Transformer's "X0" terminal, or with 1 phase 3 wire "Neutral" points - the typical X2-X3 jumper. Bond one and only one "Phase" to the Grounding Electrode system. Verify which line this will be and identify it as a Grounded Conductor. Also, do not fuse this conductor, nor re-ground it at any other point than at the Transformer.
Check NEC Article 250 [mostly the first 4-5 pages], and 450-10 for more information on the NEC requirements. Also verify with the AHJ as to their conserns.
Lastly, take a look at the Transformer Schematics which are located in the 'Electrical Theory and Applications' forum area. Select to "Search From Posts Made Over 6 Months", or even longer to find them. The schematics should show Grounding Points clearer to you for better reference.
Scott " 35 " Thompson Just Say NO To Green Eggs And Ham!
Re: Grounding the secondary of a 3 phase delta transformer#78811 10/25/0112:44 AM10/25/0112:44 AM
Scott, I have heard it said but never clearly explained, why grounding "stabilizes voltages". With the exception of some sort of short between the primary & secondary windings, I don't know why secondary voltages would fluctuate on an ungrounded system under operating conditions. Could you elaborate?
Re: Grounding the secondary of a 3 phase delta transformer#78813 10/27/0111:44 AM10/27/0111:44 AM
When I was studying power generation and distribution back when, I was taught that the Earth (and little areas of earth) were simultaneously infinite sources and infinite sinks of electrons. The instructor illustrated this by talking about a generator separated by some miles from a load, one hot conductor connects the two and the return is the earth (a single wire on the pole). At high generation voltage and low utilization current the earth's impedance becomes negligible (think of a 30 A 120/240 V load on a stepdown xformer from an 8 KV xmission line - less than 0.9 A will be called for). It was quickly discovered that the area of earth around the load would develop a different charge than the area of earth around the generator, essentially static charges that would concentrate and dissipate with respect to natural phenomena, most notably, lightning. So the earth, at any given instant, becomes a second voltage source that can either add to or subtract from the generator, sometimes with deadly results.
Running a second conductor on the xmission line that was grounded at intervals does several things. When placed above the 8 KV hot conductor, it shields the hot conductor from lightning strike. This grounded conductor also Short Circuits the earth's charges from one ground connection to the next. A concentration of electrons in one area of earth will flow easily to an area of absence. The end result is, that in the immediate vicinity of the xmission line, the potential of the earth itself becomes more uniform along the length of the xmission line and the voltage at the load end is stabilized.
Re: Grounding the secondary of a 3 phase delta transformer#78814 10/27/0112:28 PM10/27/0112:28 PM
Scott those corner grounded Deltas are neat, and you know the old saying the first fault is free and the second cost double. I would definitely check voltage drop on this one. I get 18 amps full load and 36 vd. Plus at that distance you must consider the starting amps too if the drop is too big the motor might not start at all.