Neutral Overvoltage Element - GE Multilin 489 Instruction Manual

A.1.2

Neutral Overvoltage Element

A–2
Courtesy of NationalSwitchgear.com
The simplest, and one of the oldest methods to detect stator ground faults on high-
impedance-grounded generators, is to sense the voltage across the stator grounding
resistor (See References [1, 2] at the end of this section). This is illustrated, in a simplified
form in the figure below. The voltage signal is connected to the V
terminals E10 and F10. The V
neutral
protection element. This element has an alarm and a trip function, with separately
adjustable operate levels and time delays. The trip function offers a choice of timing curves
as well as a definite time delay. The neutral overvoltage function responds to fundamental
frequency voltage at the generator neutral. It provides ground fault protection for
approximately 95% of the stator winding. The limiting factor is the level of voltage signal
available for a fault in the bottom 5% of the stator winding. The element has a range of
adjustment, for the operate levels, of 2 to 100 V.
Distribution
Transformer
FIGURE A–1: Stator Ground Fault Protection
The operating time of this element should be coordinated with protective elements
downstream, such as feeder ground fault elements, since the neutral overvoltage element
will respond to external ground faults if the generator is directly connected to a power grid,
without the use of a delta-wye transformer.
In addition, the time delay should be coordinated with the ground directional element
(discussed later), if it is enabled, by using a longer delay on the neutral overvoltage element
than on the directional element.
It is recommended that an isolation transformer be used between the relay and the
grounding impedance to reduce common mode voltage problems, particularly on
installations requiring long leads between the relay and the grounding impedance.
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 common grounding
impedance (if each generator has its own grounding impedance, the magnitude of the
signal is the input signal for the 489 neutral overvoltage
Generator
R is selected for a
maximum fault current
of 10 A, typically.
Overvoltage
R
Relay
808739A1.CDR
489 GENERATOR MANAGEMENT RELAY – INSTRUCTION MANUAL
CHAPTER A: APPENDIX
input of the 489,
neutral