Ground-fault protection for three phase motors supplied by ungrounded delta source 1

[Owameng]

Hi to everyone!

An industrial facility privately owns a 34.5 kV overhead distribution line that supplies several motors for their main operation. Now, they are planning to add 2 large water pumps (440V, 100 hp, 3-phase each) and also want to provide single phase power for premise lighting and small office.

I have drafted my design and decided that I will propose a bank of 34.5 kV - 440V, step down transformers which are wye-connected on the secondary to energize a panel with 3 branches - two 3 pole breakers for water pumps and one 2 pole breaker that will feed a 440V - 230V small, single phase, step-down transformer for lighting and office loads. Finished.

However, as part of learning, I am curious about something. Considering that single phase loads are pretty small compared to three phase pump loads, I am thinking what if instead of a wye-connected secondary transformer bank, I propose a delta-connected secondary with high-leg? This will give me 440V for the pumps and 220V for the single phase loads.

I have few issues though. First, delta high-leg is not a common transformer connection from where I live. I think I've never heard it yet on the field though I am still a young engineer with limited experience. Second, in wye-connected, I can easily have ground-fault protection for the water pump. I can run an EGC to bond the motor metal casing with branch circuit enclosure and with the GEC at the main service disconnect. I don't think I have this liberty on delta high leg. As I see it, these three phase water pumps will be fed by an ungrounded delta source. Thus, I believe that an intricate and dedicated ground fault detection and isolation is recommended on each of these pumps.

Have you experienced the same situation? What did you do? Do you think it is better to stick with my drafted design (and purchase a separate small single phase step down transformer for single phase loads)? Or connecting to a high leg delta plus individual ground fault protection is worth it? What are your proposed ground fault protection and detection on this scenario?

I want to learn from your experiences. Consider me your youngest brother on this family and my question is solely out of curiosity and knowledge sharing. I don't intend to replace my current design without proper training.

Thank you for your time and knowledge.

 

[waross]

I would go with the wye connection and ground the wye point either directly or through a neutral grounding resistor to limit ground fault current.
The High leg systems were mostly used for 115:230 Volt secondaries. The center tap of the 115:230 Volt transformer was grounded.
The high leg was also used and is still used with an open delta bank when a small three phase load is to be added to a large single phase load.
Often three 19.9 kV transformers in wye are used on 34.5 kV systems.
Do not use a full delta secondary with a grounded wye primary. This connection is prone to unintended back-feeds, overloads due to unbalanced primary voltages and/or phase angle errors and possible burnouts on the loss of a primary phase.
Delta secondaries are best avoided on distribution circuits (With the exception of open delta circuits.)

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[bacon4life]

If this is in the USA, I suspect the office needs a 120/240 V center grounded service rather than just a 240 V single phase service.

My utility long ago stopped allowing the installation of new 240/480 V high leg delta services. I assume there were some hard learned lessons.

The Whys of Wyes explains many considerations of use a wye connected primary, and in particular, the danger of wye-grounded-delta connections during with unbalanced voltage on the primary side.

 

[waross]

My utility long ago stopped allowing the installation of new 240/480 V high leg delta services. I assume there were some hard learned lessons.

That would be more an issue with the wye/delta than with the high leg.
When I was system engineer for the little island utility, we banned all full delta connections. (Primary connections were always wye.
Open delta was allowed.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[jraef]

Before embarking on any option journey, start with how you will be controlling the new 100HP 440V pumps. If you will be, now or in the future, using VFDs, you absolutely do NOT want any delta power configuration. Your decision is made for you, solidly grounded Wye only.

If these pumps will always be Across-The-Line (DOL), then either is fine. Then you have to consider this: what happens if some day that transformer is damaged and needs replacement? Non-standard transformer configurations do not offer “off the shelf” availability, so how much down time of these pumps is tolerable?

I would opt for as close to standard available options as possible and leaving open the possibilities of someone in the future wanting to change the control methods if necessary by using standard grounded Wye 3 phase system and a standard single phase service transformer.

Side note; if this is for 60Hz environments, 440V has not been a standard motor voltage for a long time, it has been 460V (for being fed by a 480V distribution transformer) for over half a century. That’s not to say that 440V motors are not available, but they are the odd man out now.
If this is for some of the few countries that use 440V 50Hz, never mind this statement.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[waross]

Feeding motors from an ungrounded source may result in motor burnout from a "Discontinuous Ground Fault".
Or an arcing ground fault.
When I was young, the old timers remembered instances in ungrounded plants when an arcing ground fault took out multiple motors across the plant.
An arcing ground fault discharging the system capacitance may act as a spark-gap transmitter and superimpose a high frequency, high voltage on the system.
Never hard of it?
That's how long it has been since someone fed motors from an ungrounded system.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Tony Tonon]

I agree with waross
I had a situation while working on a train system at a major airport where the control transformers on the ground detection system burnt up. The multiple damage was discovered in (5) substations. It was found that a single car had a traction motor brush that had dislodged from its holder, coming in contact with the motor casing. This produced a striking ground fault on a resistance grounding system.
I can attach the field report if anyone is interested.