Question about using step down transformer in reverse
Question about using step down transformer in reverse
(OP)
Hello,
I have a 240 V 3p 4 wire delta service to a building. In one unit I have a 480V 3 phase load, so I need to step up the voltage. If I use a step down transformer, wired in reverse, I end up with a delta secondary. Is there any reason I cannot use this connection, assuming I ground a corner of the delta? What are the safety concerns?
thanks
EE
I have a 240 V 3p 4 wire delta service to a building. In one unit I have a 480V 3 phase load, so I need to step up the voltage. If I use a step down transformer, wired in reverse, I end up with a delta secondary. Is there any reason I cannot use this connection, assuming I ground a corner of the delta? What are the safety concerns?
thanks
EE






RE: Question about using step down transformer in reverse
Other than that, it works fine.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Question about using step down transformer in reverse
RE: Question about using step down transformer in reverse
A little more explanation:
The secondary vectors are directly related to the primary vectors. The angle is substantially the same and the magnitudes are proportional.
Now look at the vector diagram and simulate a voltage unbalance by shortening one primary vector about 10%. One side of the delta will be shortened by 10%. The delta will no longer close on itself. That 10% will cause a current to flow in the delta which is limited by 3 x Z or three times the PU impedance. This secondary current will be reflected by primary currents on both the phases and on the neutral. These currents will cause voltage drops in the feeder conductors and the source transformer which act to correct the voltage or phase angle error.
On long rural distribution circuits where the loads are mostly single phase and voltage regulators are used, Both conditions may occur. The voltage regulators may correct the line to neutral voltage but at the same increase the phase angle errors.
If your load can accept voltage transients of approximately 200% on energization then go ahead and float the primary wye point. As you suggested ground one corner.
Alternately you may use three small dry type transformers and a resistor to build an impedance grounding circuit. The dry type transformers must be rated for line to line voltage and be large enough to support the current through the grounding impedance. Use a wye:delta connection and connect the wye point to ground through the grounding impedance. The delta secondary is not connected to anything and the secondary voltage is unimportant. This is an instance of using the idiosyncrasy of a wye:delta connection to advantage.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Question about using step down transformer in reverse
I look at it as zero sequence current in the primary means zero sequence in the secondary. The delta cannot get rid of zero sequence. If the WYE is not grounded, there can be negative sequence currents but not zero. I think your explanation is better. Thanks.
EE
RE: Question about using step down transformer in reverse
It doesn't matter if the wye is grounded, but it does matter if the neutral is connected. In distribution circuits the terms are often used interchangeably as (almost) all neutrals are grounded and grounding grounding inherently connects the neutral, but this is often unique to distribution circuits. There is a difference in utilization circuits and improper grounding of neutrals to avoid using a dedicated neutral conductor often leads to issues with protection circuits and other issues.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Question about using step down transformer in reverse
Excuse my ignorance but how do we have a system in delta (ungrounded) with 4 wires?
RE: Question about using step down transformer in reverse
RE: Question about using step down transformer in reverse
RE: Question about using step down transformer in reverse
RE: Question about using step down transformer in reverse
RE: Question about using step down transformer in reverse