Trouble Shooting open delta 1

[misteronoff]

Can somebody tell me how to trouble shoot a machine that uses a three phase open delta power configuration since you can't reference each phase ground or neutral?

 

[waross]

An open delta often has a grounded neutral connection on one of the transformers. Expect 120V, 120V, and 208V to ground from the three phases.
or
If the system is floating, refer your voltage measurements to one phase.
or
Consider grounding one phase (corner grounding).
or
Ground the the center tap of one transformer if one is available.

Bill
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"Why not the best?"
Jimmy Carter

 

[rbulsara]

What is the trouble?

Rafiq Bulsara

http://www.srengineersct.com

 

[misteronoff]

If the 3 phase Open Delta is a "floating" configuration would that mean if I am checking primary voltage at the machine internal transformer I can only check phase to phase example 240 not phase to ground example 120 or phase to neutral example 120?

 

[rbulsara]

Open delta would be 'floating' only if it is not grounded, and if ungrounded there is no reason to measure between phase and ground. If there is no neutral there nothing to measure to the neutral. If there is a neutral (from center tapped on one of the windings of the open delta) then if needs to be grounded and system won't be 'floating' any more.

I do not see anything to troubleshoot here. If you measure correct phase to phase voltages between each pair, you should be fine in normal conditions.




Rafiq Bulsara

http://www.srengineersct.com

 

[misteronoff]

Thanks guys!

 

[busbar]

An aside—voltage measurements on an open-delta system should be no different than those on a (full) delta system.

 

[PQ101]

Typically the electric utilities want to save money & install 2 bank setup (open wye-open delta) instead of 3-bank setup. So, you have 2 transfomers bank, and the 3rd leg is man-made phase. If this is the case, then phase-to- phase reading is the correct way. Depending on transformer configuration, you may read 240volts phase-to-phase, or 120Volts on 2 legs, and wild leg reads 208volts nominal voltage, which is fine. With utility voltage regulators on utlity set at 125% to support with facility voltage drop you may read slightly higher reading i.e. 215volts.

Regarding troubleshooting machine that utilizes open delta, you probaly want to monitor the incoming line voltage to establish a baseline (i.e. monitor for voltage sags, swells, transients that cause problem), and check equipment-system grounding to bad or lose connection; and to ensure compliance with NEC Article 250. Last but not least, on open wye-open delta configuration with one dummy phase or man made phase, the sensitive electronics are susseptible to voltage fluctuations, and any system fault on utility lines will cause voltage surge inside the facility. If this is the case, installing 3-phase xfmr bank would be one solution, and this modification will have other consequences or wiring changes as well. Not knowing exactly what type of problems or issues your machine is experiencing its difficult to help with solutions.

 

[rhatcher]

I would have to disagree with the statement that "if ungrounded there is no reason to measure between phase and ground." In fact, there is a good reason: to detect unintentional grounds.

Theoretically, and in practice, the phase-ground voltage on a symmetrical ungrounded delta (open or closed) should balance and should be the same as the phase-neutral voltage on a wye. This is because the capacitance and insulation resistance of the legs, if equal, cause the voltages to float at an equal level above ground.

As an example, the phase-ground voltages of a symmetrical 480V delta should be 277V each. In the case of an unintentional direct ground, the phase-ground voltages would read 480-480-0 with the zero leg being grounded. For a high impedance ground, it will be somewhere in between.

When dealing with the special case of an open delta, this still applies for a symmetrical winding, meaning the two legs are identical. For cases where the windings are not symmetrical, the phase-ground voltages will not be balanced, but it will be unbalanced in a predicatable way. Interpreting the phase-ground voltages in this case is a matter of understanding the transformer configuration.

I will add that for the type of work that I do, open delta transformers are very rare. I am speaking mostly from theory and instinct. If I am wrong someone here will (I hope) educate me.

 

[PQ101]

I agree with rhatcher. On 3-phase straight delta (ungrounded), the proper reading is phase-to-phase. However, phase to ground reading or voltage values will indicate if you have fault to ground somewhere in the system. Phase-to-ground voltage readings may vary switchgear to switchgear location, or depending on type of loads (capacitive-inductive), and the overall load current you may get i.e. 268v, 273v, 263v (A-B-C phase). It's reference to ground, not the actual phase-to-phase nominal RMS or effective voltage (an indication only). In general values should be close in range.

But if you read ZERO volts phase-to-ground it means you have a short to ground somewhere inside of the facility. First short will not shut you down on straight delta connection, but the following or second short in the system will cause voltage surge, damage equipment, and OSHA will get involved i.e. if there's personnel safety issue involved.

With respect to open-delta, on utility transformers, they are all over (anywhere you see 2 bucket transformer stations on utility poles serving 3-phase connections or facility incoming switchear-panel). When it comes to control power, we normally design for metering and protective relaying the supply voltage from open-delta, to save one transformer-basically for economics saving $$$$.

 

[racobb]

I have seen folks on many occasions think they have a problem when they check the voltage to ground on a three wire system. Good for determining if you have a grounded phase, but for nothing else that I know of so far. I usually try to explain that the first ground on a system is ok, it is the second one that causes a problem.

Alan

 

[busbar]

It may be worth noting that, for an open-delta secondary configuration, utilities will use an open-delta primary connection with three phases/cutouts, or possibly an open-wye primary with two phases/cutouts and a neutral connection.

 

[PQ101]

OpenDelta on utility transformer means: 2 pot bank (instead of 3 pot) setup, to povide 3-phase on secondary side. So, there are no 3-phase cut outs (CO's).

Only proper way to read-measure RMS voltage on open delta is to take phase-to-phase voltage readings (AB-BC-CA). No other way my friend.

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