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4 Recommended Recording Practice and Instrument Connection
68
Figure 4.8: Voltage ratio for 11kV/110kV transformer example
Instrument should be connected to the network according to figure bellow.
LN L3 C
B
A
L1
L1
L3C
N
A
L2
L2
B
L2
L1
high
voltage
power plant
measuring instruments
A
A
A
L3
xA / 5A
xA / 5A
xA / 5A
Figure 4.9: Connecting instrument to the existing current transformers in medium
voltage system
4.2.3 Current clamp selection and transformation ratio setting
Clamp selection can be explained by two typical use cases:
direct current
measurement
and
indirect current measurement.
In next section recommended
practice for both cases is shown.
Direct current measurement with clamp-on current transformer
In this type of measurement load/generator current is measured directly with one of
clap-on current transformer. Current to voltage conversion is
performed
directly
by the
clamps.
Direct current measurement can be
performed
by any clamp-on current transformer. We
particularly recommend: flex clamps A 1227 and iron clamps A 1281. Also older Metrel
models A 1033 (1000A), A1069 (100A), A1120 (3000A), A1099 (3000A), etc.. can be
used.
In the case of large loads there can be few parallel feeders which can’t be
embraced by single clamps. In this case we can measure current only through
one feeder as shown on figure bellow.