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Impact of Voltage Transformer Connection and Grounding on Generator Protection Security and Selectivity

Example One: Wye primary / wye secondary, neutral grounded

To source generator protective relaying, several VT arrangements may be used, based on application and cost considerations.

A) Wye primary / wye secondary, neutral grounded (Figure 1)

Figure 1, Wye Primary / Wye Secondary, Neutral Grounded

Pros:

• This arrangement offers the ability to use single-phase power elements
• This arrangement offers the ability to zero-sequence directionalize ground elements from phase-to-neutral voltages and employ the 59D element
• This arrangement offers the ability to view discrete phase voltages on the waveform capture

Cons:

• This arrangement may allow the 24, 27, and 59 elements to operate for phase to ground faults in the generator zone. This is due to a having ground fault on an essentially ungrounded system (the generator zone is high impedance grounded and terminated in the ungrounded delta winding of the generator step up transformer) causes a neutral shift with attendant high voltages (approaching VLL) on the unaffected phases, and a near zero voltage on the affected phase. See Figure 2.

Figure 2, Ground Fault with Neutral Shift and Resultant Voltages on High Impedance Grounded Generator
(High Impedance Grounded System approximates Ungrounded System)

• This arrangement may allow the ground overvoltage protection (59N) to operate on VT secondary side ground faults (time coordination dependent). This is because a phase to ground fault on the secondary side as a phase-to-ground fault on the primary (See Figure 3).
• This arrangement is typically the most expensive (compared to open delta), as three VTs are required.

Figure 3, Wye Primary / Wye Secondary, Neutral Grounded Ground Fault on Secondary Side, Appears as Ground Fault on Primary Side

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