Open-delta vs wye-connected potential transformers

In medium-voltage (MV) switchgear, potential transformers (PT) are typically designed as open-delta (image left) or wye-connected (image right) configuration. Open-delta is two PTs connected line-to-line and wye-connected is three PTs connected line-to-neutral.

The line-to-line vs line-to-neutral connection of the two is the difference that results in the benefits for the different applications.

When to use open-delta:

• Use two open-delta potential transformers on simple applications where cost savings are desired. Simple applications are defined as applications that:
  • Don't use voltage for ground fault sensing or tripping (as you need the line-to-neutral voltage sensing for this)
  • Are not 4-wire systems where the individual phase voltage to neutral are desired to be measured
  • Don't require line-to-neutral voltages for measurement or monitoring

When to use three wye-connected potential transformers:

• Use three wye-connected potential transformers on four-wire systems where there are phase to neutral loads. These loads, if significant, will affect the line-to-neutral voltage on the system. As three wye-connected PTs measure line-to-neutral voltage, they are a good selection for these types of applications.
• Use three wye-connected potential transformers when using voltage as part of the ground fault protection. The line-to-neutral wye-connection on all three phases give the protection relay positive, negative and zero-sequence voltages.
• Use three wye-connected potential transformers when it is desired for the voltage angles on the secondary to be in phase as the primary for all three phases. Conversely, on an open-delta PT, the secondary 'B' phase is a calculated value and 180 degrees out of phase.

Three wye-connected PTs are not required for synchronization schemes where two voltage sources are synced. However, it is important that the same type of PT (delta vs wye-connected) is used to check for voltage phase and magnitude.

A word of caution on wye-connected potential transformers - be cognizant of potential grounding issues caused by the primary wye-connection to ground. You are creating a neutral point terminated to ground, which can create multiple grounded neutrals on the same system. While this grounded PT neutral is commonly done, if the system did have ground fault issues, the grounded neutral on the PT may be a contributor.