GE 489 - Manual (Page 230)

A-2 489 Generator Management Relay GE Multilin
A.1 STATOR GROUND FAULT APPENDIX A
A
When several small generators are operated in parallel with a single step-up transformer, all generators may be grounded
through the same impedance (the impedance normally consists of a distribution transformer and a properly sized resistor).
It is possible that only one generator is grounded while the others have a floating neutral point when connected to the
power grid (see the figure below). This operating mode is often adopted to prevent circulation of third-harmonic currents
through the generators, if the installation is such that all the star points would end up connected together ahead of the com-
mon grounding impedance (if each generator has its own grounding impedance, the magnitude of the circulating third har-
monic current will be quite small). With a common ground point, the same V
neutral
signal is brought to all the relays but only
the one which is grounded should have the neutral overvoltage element in service.
For these cases, the neutral overvoltage element has been provided with a supervising signal obtained from an auxiliary
contact off the grounding switch. When the grounding switch is opened, the element is disabled. The grounding switch aux-
iliary contact is also used in the ground directional element, as is the breaker auxiliary contact, as discussed later.
If all the generators are left grounded through the same impedance, the neutral overvoltage element in each relay will
respond to a ground fault in any of the generators. For this reason, the ground directional element should be used in each
relay, in addition to the neutral overvoltage element.
Figure A–2: PARALLEL GENERATORS WITH COMMON GROUNDING IMPEDANCE
A.1.3 GROUND OVERCURRENT ELEMENT
The ground overcurrent element can be used as a direct replacement or a backup for the neutral overvoltage element, with
the appropriate current signal from the generator neutral point, for grounded generators. This element can also be used
with a Core Balance CT, either in the neutral end or the output end of the generator, as shown below. The use of the special
CT, with its dedicated input to the relay, offers very sensitive current detection, but still does not offer protection for the full
stator. The setting of this element must be above the maximum unbalance current that normally flows in the neutral circuit.
Having the element respond only to the fundamental frequency component allows an increase in sensitivity.
The core balance CT can be a conventional CT or a 50:0.025 Ground CT, allowing the measurement of primary-side cur-
rent 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.
808737A1.CDR
Other Generators,
as the case may be
489
Relay
V neutral
Aux.
Contact
G2
Aux.
Contact
Breaker Grounding
Switch
Grounding
Switch
Trans. & R
Aux.
Contact
V neutral
G1
Common
Grounding
Impedance
Isolating
Trans.
489
Relay
Breaker
Aux.
Contact