PRISMIC R100 pages

PRISMIC® R10
Rotor Earth Fault Monitor

PRODUCT SPECIFICATION
Introduction
The PRISMIC R10 is an electronic unit designed to provide a brushless generator
with an alarm in the event of a rotor earth fault.
General practice on generators with brushless excitation is for the rotor (main field)
winding to be fully isolated from earth so that, in the event of a single earth fault, the
generator could continue running. However, a second earth fault would short circuit
part of the winding, necessitating an immediate shutdown and corrective action.
Detection of a single earth fault allows the generator to be taken out of service for
investigation and repair at the earliest opportunity, usually as part of a planned
maintenance programme.

Description

Fig.1 Transmitter fitted to generator rotor

To continuously monitor the condition of the rotor winding, the BRUSH PRISMIC
R10 rotor earth fault monitor (REFM) uses an electronic unit to detect a single earth
fault. This dispenses with the need for brushes and sliprings, resulting in enhanced
reliability and reduced maintenance costs.
The detector comprises a rotating transmitter, connected to the rotor winding, which
transmits an infra red signal to a stationary receiver.

Transmitter
The transmitter has three electrical connections to the rotor; to earth, rotor
positive and rotor negative. The rotor voltage provides the power supply for the
unit, from which it generates its own isolated dc source, which is used as the earth
fault detection supply. The negative of this isolated source is tied to ground, the
positive to the rotor negative. During healthy conditions, the current flowing from the
isolated dc source is negligible and the transmitter produces a high frequency signal
that is detected by the receiver aerial and the receiver indicates no fault.
Any earth fault on the rotating assembly, either on the armature or the rotor winding,
will cause a current to flow from the dc source. This current is detected and used to
remove the transmitted high frequency signal. By removing the signal the detection
system becomes “fail-to-safe”, in that a loss of transmitted signal is detected as an
earth fault condition.
Fig.2 Receiver in IP66 enclosure for
generator mounting

Receiver
The transmitted signal is sensed by an aerial and fed, via screened cable and
amplifier, into a “phase locked loop” within the receiver. This device locks on to the
transmitted signal and serves to decode the pulse signal as well as filter out any
background noise caused by external sources. The decoded signal is used to hold
off the output alarm relay after an amplification and time delay stage. In the event of
an earth fault condition the alarm relay becomes energised after a short time delay
(incorporated to suppress “nuisance” alarms) thereby giving external indication of
the fault condition.

Fig.3 Receiver mounted in IP20 enclosure
for generator control panel mounting

The power supply required by the receiver is normally provided by the excitation
controller power source (usually a permanent magnet generator). During run up on
excitation controller over flux control, or during normal off load transients, the
available rotor voltage may be too low to ensure correct transmitter operation. To
overcome this, a circuit is incorporated in the receiver to inhibit output relay
energisation on either low exciter field voltage or low auxiliary ac supply voltage. A
time delay stage within this circuit allows the rotor voltage to build up prior to giving a
permissive output.