Codes & Standards
High-voltage primary wye-grounded transformers – Code File, May 2022
May 26, 2022 By Nansy Hanna, P.Eng.
May 26, 2022 – Utilities typically provide high-voltage (HV) supply services to manufacturing facilities, hospitals and other heavy electricity users. Some Ontario utilities have requested their customers use a wye primary winding, customer-owned transformer (XFMR) for 27.6/16-kV supply services, in addition to other various specifications.
Historically, delta primary XFMRs were the norm for these scenarios so, when the HV wye primary XFMR is not utility-owned, questions arise about CE Code requirements for the primary-side connection.
The code does not explicitly address wye-grounded primary XFMR connections; however, we can apply rules from Section 10 (unless amended by the grounding and bonding requirements of Section 36).
The primary neutral point (HO terminal) on the XFMR is required to be connected to the incoming utility neutral, which is also required to be grounded and connected to the equipment bonding terminal with a system bonding jumper at the service box (Rule 10-210). The primary neutral of the XFMR is also required to be grounded to the station ground electrode (Rule 36-308 6a).
CE Code Rule 10-210 also prohibits any connection between the neutral and bonding system on the line or load side of the system bonding jumper installed at the service entrance equipment. This supports the requirements of Rules 10-100 and 10-500, which prohibit objectionable current over the grounding and bonding paths.
The magnetizing current of the primary windings—as well as any imbalance of line to neutral loads on the secondary—may result in primary neutral current. Where the neutral also grounded at the XFMR rather than at the service switch only, we could possibly get current over grounding and bonding paths.
These requirements raise several valid questions:
• Is the feeder supplying a customer-owned XFMR that has a wye-grounded 3-phase primary winding, required to be a 4-wire feeder that includes a neutral conductor and a separate bond conductor?
• Is the neutral supplying the primary neutral terminal (HO) of a wye-grounded 3-phase XFMR required to be isolated from non-current-carrying conductive parts (grounded metal) of electrical equipment on the load side of the service, or where the grounding connection is made? After all, Rules 10-210 and 10-212 do not permit a neutral connection to non-current-carrying conductive parts of electrical equipment on either side of the system bonding jumper. Meantime, Rules 10-100 and 10-500 do not permit objectionable current over grounding or bonding conductors.
• Can the HV primary neutral of a wye-grounded XFMR instead be connected to the station ground electrode via the primary feeder neutral conductor that is grounded at the service box or other source equipment? Knowing that the station ground electrode can be at the HV service box (or remote from it), HV XFMRs within a building are seldom located in proximity to a local station ground electrode.
• Is distribution equipment (e.g. main service or switchgear) on the supply side of a grounded wye primary transformer required to be 4-wire equipment with a separate neutral and ground bus?
One of the biggest challenges when considering the above questions is finding HV switchgear that includes a neutral bus.
After speaking with different manufacturers, it would seem that the higher the voltage class, the more challenging it is to find equipment marked as 4-wire with a separate neutral bus—mainly because the clearances may be hard to keep when adding an additional bar inside the switchgear. The space taken by the neutral bar could also decrease cable conduit space availability.
Another important factor is the heat rise limitation, as required by relevant product standards. Having a distributed neutral could introduce additional heat inside the switchgear, possibly resulting in the need to de-rate the equipment.
To consider the above questions, one needs to weigh the safety risk with the added burden of providing the fourth wire and having HV equipment that includes a neutral bar. There two viable options here: avoid the use of HV wye primary XFMRs and use HV delta primary XFMRs; or, in cases where it is critical for the utility that the customer use an HV wye primary XFMR, permit 3-wire equipment and multi-grounding of the neutral after the service box.
Another issue to consider: HV distribution is typically owned by the utilities, where they have a multi-grounded neutral and attempt to keep its potential as close to zero as possible. Therefore, moving the demarcation point and having the XFMR owned by the customer should not introduce such a huge increase to cost (with negligible increase to safety).
Nansy Hanna, P.Eng., is senior director, Engineering & Regulations, at Ontario’s Electrical Safety Authority (ESA). She is also chair of the Canadian Advisory Council on Electrical Safety (CACES) and a member of the ULC Advisory Council, CSA Technical Committee on Industrial, Consumer and Commercial Products and CSA CE Code-Part I, Sections 24, 32, and 46. She can be reached at firstname.lastname@example.org.
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