Wye Grounded High Voltage Primary Transformers – Code File, May 2022

May 26, 2022 – Utilities typically provide high voltage (HV) power service to manufacturing facilities, hospitals, and other large electricity consumers. Some Ontario utilities have required their customers to use a customer-owned wye primary winding transformer (XFMR) for 27.6/16 kV feeder services, in addition to various other specifications.

Historically, delta primary XFMRs were the norm for these scenarios. So, when the HV wye primary XFMR is not utility owned, questions arise regarding the CE code requirements for the primary side connection.

The code does not explicitly address star-grounded primary XFMR connections; however, we may apply the rules in Section 10 (unless modified by the grounding and bonding requirements of Section 36).

The main neutral point (HO terminal) on the XFMR must be connected to the incoming utility neutral, which must also be grounded and connected to the equipment bonding terminal with a system bonding jumper to the junction box. serve (Rule 10-210). The XFMR primary neutral must also be grounded to the station ground electrode (Rule 36-308 6a).

CE Code Rule 10-210 also prohibits any connection between the neutral and the system grounding line side or load side of the system grounding jumper installed on the service entrance equipment. This supports the requirements of the 10-100 and 10-500 rules, which prohibit unwanted currents in the ground and bond paths.

Magnetizing current from the primary windings—as well as any imbalance of line-to-neutral loads on the secondary—can result in a primary neutral current. When the neutral is also grounded at the XFMR rather than just the service switch, we could potentially get current on the ground and bond paths.

These requirements raise several valid questions:

• Is the charger feeding a customer-owned XFMR that has a 3-phase grounded wye primary winding, to be a 4-wire charger that includes a neutral conductor and a separate bonding conductor?

• Should the neutral supplying the primary neutral (HO) terminal of a 3-phase earthed wye XFMR be isolated from conductive non-current-carrying (grounded metal) parts of the electrical equipment on the load side of the service, or when the ground connection is made? After all, Rules 10-210 and 10-212 do not permit a neutral connection to non-current-carrying parts of electrical equipment on either side of the system bonding jumper. Meanwhile, the 10-100 and 10-500 rules do not allow unwanted current on 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 supply neutral conductor which is grounded at the terminal box. connection or other source equipment? Because the station ground electrode may be at (or remote from) the HV service box, HV XFMRs inside a building are rarely located near an electrode. mass of the local station.

• Should the distribution equipment (eg main service or switchgear) on the supply side of a grounded wye primary transformer be a 4-wire equipment with a neutral bus and a ground bus separated?

One of the biggest challenges when considering the above questions is finding HV switchgear that includes a neutral bus.

After speaking with various manufacturers, it would appear that the higher the voltage class, the harder it is to find equipment marked as 4-wire with a separate neutral bus, primarily because clearances can be difficult to adhere to when added an extra bar inside. the apparatus. The space occupied by the neutral bar could also reduce the availability of cable conduit space.

Another important factor is the limitation of heating, as required by the relevant product standards. Having a distributed neutral could introduce additional heat inside the switchgear, possibly causing the need to derate the equipment.

To consider the above issues, one must weigh the safety risk with the added burden of providing the fourth wire and having HV equipment that includes a neutral bar. There are two viable options here: avoid the use of HV star primary XFMRs and use HV delta primary XFMRs; or, in cases where it is utility critical that the customer uses a HV wye primary XFMR, allow 3-wire equipment and multiple grounding of the neutral after the service box.

Another issue to consider is that HV distribution is usually owned by utilities, where they have a multi-earth neutral and try 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 result in such an increased cost (with a negligible increase in security).

Nansy Hanna, P.Eng., is Senior Manager, Engineering and Regulatory, at the Electrical Safety Authority of Ontario (ESA). She is also Chair of the Canadian Advisory Council on Electrical Safety (CACES) and a member of the ULC Advisory Council, the CSA Technical Committee on Industrial, Consumer and Commercial Products and CE Code Part I of the CSA Sections 24, 32 and 46. She can be reached at [email protected]

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