Load current
Bushing-type current trans formers located mside the power transformer are used for the
detection
of the transformer operating currents. Typically the output signal of the current transformer is an
the range ofO to 1. 2. or SA, and is transmitted to a measurement transducer in the monitoring
cubicle. The measurement transducer converts the 0 to 112/SA signal into 0 to 20mA. which is
then
transmitted to an analogue input terminal of the monitoring system.
Temperature
Transformer temperatures are typically measured by PTlOO temperature probes which are
installed in
thermometer pockets ofl" or '14" size according to EN 50216·4. Alternatively magnetic
temperature
probes can be fixed on the tank or other steel cased equipment where temperature monitoring is
of
interest. The resistance of the PT100 probe is directly processed by an RTD input terminal of the
m011itoring system.
Gas-in-oil content
Gas-in-oil sensors for protrusion into the oil are installed by means of suitable flanges, typically
sized DN2S. DN40, DNSO or DN80. Due to the fact that the outputs of these sensors are already
4
to 20mA analogue signals, no additional measuring converter is necessary and the output signal
is
directly connected to an analogue input terminal of the monitoring system.
Moisture-in-oil content
Moisture-in-oil sensors are installed with access to the oil using flanges typically of sizes Dt-
125 ,
DN40, DNSO or DNSO for mounting. These sensors, too, have 4 to 20mA analogue signal
outputs
and no additional measuring converter IS necessary, so the output signal can be directly
connected
to an analogue input terminal of the monitoring system.
On-load tap< hanger position
Inside the motor drive cabinet of the tap-changer, a resistor wheel is mechanically connected to
the
motor drive. Thus, each tapping position gives a specific resistance value which can be processed
directly by a RD (resistance detector) input terminal of the monitoring system. Alternatively the
tap-changer position can be detected by using a contact wheel which is mechanically connected
to the motor drive. In this case a BCD (binary-coded decimal) signal is used to determine the tap·
changer position. This signal is directly connected to binary input terminals of the monitoring
system.
Power consumption of tap< hanger motor drive
To measure the power consumption of the motor drive, an active power converter is installed in
the
motor drive cubicle. This converter provides a 0/4 to 20mA analogue output signal which is
directly
connected to an analogue input terminal of the monitoring system.
Operating condition of pumps and fans
Auxiliary contacts of the respective power relays are used for the detection of operating
conditions
ofthe pumps and fans. The output signal is directly connected to an analogue input terminal of
the
field bus system.
Additional features
Beside various other measuring devices, nowadays fiber optic temperature probes can be
included
into the transformer active part to monitor temperatures of windings and core. These probes
come
with special measurement converters which have 0/4 to 20mA output signals. The output signals
are directly connected to analogue input terminals of the field bus system.
Switching contacts from protection devices such as Wrl (winding temperature indicator). OTl
(oil
temperature indicator), OU (oil level indicator), OFI (oil flow indicator). PRD (pressure relieve
device).
Buchholz relay. sudden pressure relay, tap -changer protection, safety valve, etc. can be wired. in
add ition, to the binary input terminals in the monitoring module.
