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Introduction
Summary
Quiz
Glossary
Online Learning Center
Beckwith Electric Home
Impact of Voltage Transformer Connection and Grounding
on Generator Protection Security and Selectivity
A "quick guide" and summary:
1) If you are concerned about not tripping on 24, 27 and 59 elements for in-zone generator ground faults, either use delta VT secondaries, or use software derived line-to-line quantities for these elements. The benefit of using the software derived quantities is the ability to use line-to-ground quantities for other elements, and for metering, oscillography, the ability to use the 59D element, and other positive attributes.
2) If you are concerned about not tripping the generator ground fault
protection (59N) on VT secondary ground faults, you can elect to ground
the VTs on a phase versus the neutral. In this manner, if any phase is
grounded, the fault appears on the primary side as a phase-to-phase fault.
Caution should be exercised, however, because a ground fault on the VT
secondary neutral will appear as a ground fault on the primary side.
| VT Connection | Relay Connections | 24, 27 and 59 elements are secure for primary phase to ground faults | 59N element is secure for VT secondary phase to ground faults | Can use directional ground elements (zero-sequence polarized) | Can use single phase power elements | Can view discrete phase voltages |
 |
Vag, Vbg, Vcg (L-G voltages) |
No
|
No
|
Yes
|
Yes
|
Yes
|
 |
Van, Vbn, Vcn (L-N voltages) |
No
|
Yes
|
Yes
|
Yes
|
Yes
|
 |
Vab, Vbc, Vca (L-L voltages) Neutral not wired |
Yes
|
Yes
|
No
|
No
|
No
|
 |
Vab, Vbc, Vca (L-L voltages) |
Yes
|
Yes
|
No
|
No
|
No
|
 |
Vag, Vbg, Vcg (L-G voltages) L-L quantities derived in relay software |
Yes
|
No
|
Yes
|
Yes
|
Yes
|
 |
Vab, Vbc, Vca (L-L voltages) |
Yes
|
Yes
|
No
|
No
|
No
|
 |
Van, Vbn, Vcn (L-N voltages) L-L quantities derived in relay software |
Yes
|
Yes
|
Yes
|
Yes
|
Yes
|
Page 8 of 9
© 2002 Beckwith Electric Co, Inc. All rights reserved.