Single Phase Transformer Zero Sequence Diagram 1

[ThePunisher]

I have been trying to convince my fellows here at work but rather posting this to get second opinion.

Say we have a single phase 13.2 kV - 240/120 V pole type transformer with secondary 240/120 V single phase 3 wire (X1-X2-X3). The H2 is connected to a grounded primary conductor (typical to this type of service)and the transformer tank is bonded to the primary grounded conductor.

HOWEVER, the X1-X2 terminals are connected to the UNGROUNDED service drop conductors and X3 terminal is floating.

My concern is that this violates NEC as there is no grounded conductor in the service drop where the customer can make the necessary ground return path via main bonding jumper at the service equipment. My fellow argued that since the transformer tank is bonded to the primary grounded conductor, it should be ok.

I informed him that the transformer primary and secondary windings are isolated grounding-wise and a ground fault at the secondary MUST FLOW BACK TO THE SECONDARY winding and if one of the secondary terminals (X2 if 240V is only required OR X3 if 120 V is required) is not solidly grounded, the equivalent zero-sequence is OPEN-CIRCUITED at the secondary side and hence will be forced to flow back through the soil ground resistance which is not an effective ground return path.

Is my explanation correct and accurate?

Regards,

 

[wroggent]

If the NESC applies to you, then as I understand it, 92.B.1 would require you to ground this 240V service. NEC 250.24(C) requires that IF the system is grounded, the grounded conductor must be run to the service equipment.

 

[ThePunisher]

Thanks wroggent. Do you have any idea on what would be the equivalent zero sequence diagram for this single phase transformer. All zero sequence diagram I have known are all three phase but I know that if a winding is grounded, it creates a path for the ground fault current to flow back to the source winding.

 

[stevenal]

Sequence components are applicable only to multi-phase systems, so there can be no zero sequence diagram for a single phase transformer.

 

[waross]

Or, if you refer back to the three phase system feeding this, this will be all zero sequence current.

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

 

[HamburgerHelper]

I believe if referred back to the three phase system feeding it, it would be IA=I1+I2+I0 for phase A with I1=I2=I0. To me, I think a single phase load should look similar to a high impedance slg fault.

 

[jghrist]

The whole idea of symmetrical components is to allow you to analyze an unbalanced three phase system like a balanced system and use single phase diagrams. It would be far simpler to analyze a single phase system as a single phase system and forget about symmetrical components.

 

[cuky2000]

Zero sequence of single phase: Stevenal is right, for a single phase isolated system there is only one component, the positive sequence. The source could be a three phase system with (+), (-) & (0) sequences. However, the source zero sequence impedance has a negligible impact in the single phase transformer and primary & secondary feeders.
NEC Code Compliance: Not necessarily utilities need to comply with the NEC since they are exempt per Article 90.2.B (5). For 1 kV and above the NEC allow per Section 250.180 (D) to use multi-grounded neutral (MGN). Utility is only responsible up to the interconnection point at the service entrance. Beyond that point, the installation will under the jurisdiction of the NEC. Should be noted that for up to 600V systems, under the NEC jurisdiction a single point grounding it is required.
Ground Fault Return Path: In the of event hazardous stray current or a SC ground fault, the current will return to the source by a combination path of the one injected into the ground and a metallic return (neutral, shield wire, piping, etc.). Depend on the magnitude and duration of the current travelling in the earth, a ground potential might rise and safety assessment should be performed.
I hope this help.

 

[waross]

Forget grounding. You have to have a neutral. The neutral is the main return path for fault currents. The ground serves to limit voltages to ground and carries some of the fault current.
In distribution work, it is common to refer to a neutral conductor as a "ground" conductor. When you get down to utilization voltages and particularly at 120:240 Volts, there is a definite difference between the use and installation of a neutral conductor and a ground conductor.
Even for a 240 Volt service with no 120 Volt circuits, you must have a neutral from the transformer to the service entrance to return any ground fault currents.
The Canadian code has in recent years reduced the minimum size of some grounding conductors in recognition that the greater portion of a ground fault current returns to the transformer via the neutral conductor, not the ground path.

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

 

[ThePunisher]

Thank you everyone for posting all your comments.

Waross, would you agree with me that the distribution utility, even with 240 V line-line (120 V not used by customer), must extend the "neutral" (from X2 terminal of a X1-X2-X3 240/120 V 3 wire) up to the service drop for customer connection to the service disconnecting means with utilization equipment grounding via main bonding jumper?

My concern if this is not extended, the customer side ground fault will rely on the mutual ground resistance coupling between the customer ground electrode and the DU ground electrode which is highly variable and depend on soil resistivity

 

[waross]

Yes I agree and the Canadian Electrical Code, Part 1 agrees.
I suspect that the NEC has similar rules.
If you are in Canada, look at the CEC Part 1, Section 10, and Appendix B Notes on rules.
In all cases, whether a neutral is used and whether the installation is directly grounded or high resistance grounded, a conductor is required between the supply transformer and the service entrance box.
Do not overlook appendix B, Notes on rules. This includes 7 figures showing different possible service arrangements.
I note with interest that fig. 7 shows an ungrounded Delta system. Even with NO neutral, a grounded conductor is required between the case of the supply transformer and the service entrance box.
Here are some excerpts in case you do not have the 2015 CEC Part 1 on your desk:
CEC 2015
10-106 AC systems
(1) Except as otherwise provided for in this Code, ac systems shall be grounded if
(a) by so doing, their maximum voltage-to-ground does not exceed 150 V; or
(b) the system incorporates a neutral conductor.

10-204 Grounding connections for ac systems (see Appendices B and I)
(1) When a consumer’s service is supplied by an ac system that is required to be grounded in accordance with
Rule 10-106(1), the system shall
(a) be connected to a grounding conductor at the transformer or other source of supply;
(b) be connected to a grounding conductor at each individual service, with the connection made on the
supply side of the service disconnecting means either in the service box or in other service
equipment; and
(c) except as provided for in Rule 10-208, have no connection between the grounded circuit conductor
on the load side of the service disconnecting means and the grounding electrode.
(2) Where the system is grounded at any point, the grounded conductor shall
(a) be run to each individual service;
(b) have a minimum size as specified for bonding conductors in Rule 10-814;
(c) also comply with Rule 4-024 where it serves as the neutral; and
(d) be included in each parallel run where the service conductors are run in parallel.
(3) Notwithstanding Rule 12-108, the size of the system grounded conductors in each parallel run shall be
permitted to be smaller than No. 1/0 AWG.

Appendix B notes on rules:
Definitions:
Grounding conductor
It is intended that the grounding conductor will terminate on the enclosure for the service box or protective
devices supplying the system in cases where the system is not grounded, and at the internal bus for the grounded
conductor where the system is grounded.

Rule 10-1102
Unless the installation is exceptional in some way, the ground grid system for the supply authority should always
be interconnected with the consumer’s ground grid system as outlined in Rule 10-204(2) for a grounded system.
Where neutral grounding devices are employed, a grounded service conductor will not be available for this
purpose and a separate conductor may need to be installed to interconnect the two ground grid systems.

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