Ground Directional Element - GE Multilin 489 Instruction Manual

A.1.4

Ground Directional Element

A–4
Courtesy of NationalSwitchgear.com
The core balance CT can be a conventional CT or a 50:0.025 Ground CT, allowing the
measurement of primary-side current levels down to 0.25 A. Using a Core Balance CT, on
the output side of the transformer will provide protection against stator ground faults in
ungrounded generators, provided that there is a source of zero-sequence current from the
grid.
Though in theory one could use this element with a zero sequence current signal obtained
from a summation of the three phase currents (neutral end or output end), by connecting it
in the star point of the phase CTs, Options 4 and 5 in the figure below, this approach is not
very useful. The main drawback, for impedance-grounded generators is that the zero-
sequence current produced by the CT ratio and phase errors could be much larger than
the zero sequence current produced by a real ground fault inside the generator.
Again the time delay on this element must be coordinated with protection elements
downstream, if the generator is grounded. Refer to Ground Directional on page 5–40 for
the range of settings of the pickup levels and the time delays. The time delay on this
element should always be longer than the longest delay on line protection downstream.
CORE
BALANCE
CT
Option 5
Option 2
(similar to
Option 4)
Option 1
Ground current input
from one of the five
FIGURE A–3: Ground Overcurrent Element with Different Current Source Signals
The 489 can detect internal stator ground faults using a Ground Directional element
implemented using the V
neutral
obtained across the grounding impedance of the generator. The ground, or zero sequence,
current is obtained from a core balance CT, as shown below (due to CT inaccuracies, it is
generally not possible to sum the outputs of the conventional phase CTs to derive the
generator high-side zero sequence current, for an impedance-grounded generator).
If correct polarities are observed in the connection of all signals to the relay, the V
signal will be in phase with the ground current signal. The element has been provided with
a setting allowing the user to change the plane of operation to cater to reactive grounding
impedances or to polarity inversions.
This element's normal 'plane of operation' for a resistor-grounded generator is the 180°
plane, as shown in FIGURE A–4: Ground Directional Element Polarities and Plane of
Operation, for an internal ground fault. That is, for an internal stator-to-ground fault, the V
signal is 180° away from the I
GENERATOR
CORE
BALANCE
CT
Option 3
489
Ground
Overcurrent
options Element
and the ground current inputs. The voltage signal is
signal, if the polarity convention is observed. If the
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489 GENERATOR MANAGEMENT RELAY – INSTRUCTION MANUAL
CHAPTER A: APPENDIX
Phase CTs
BREAKER
Breaker
Aux.
Option 4
808736A1.CDR
neutral
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