isolated current voltage transducers

Fluxgate Technologies
higher frequencies. Both secondary windings of T1 and T2
also acts as the zero flux compensation winding, ensuring the
transducer works in a closed loop mode.
This design has the ability to supply a single coil (secondary of
T1) with the „Fluxgate“ current (Fig. 32 and I µ
in Fig. 34) as well
as the secondary current (I S
in Fig. 34) required for flux
compensation. The processing electronics then removes the
Fluxgate current in a „filter“ (ref. 2 in Fig. 34) to prevent noise
at the transducer output. The output current (point C of
Figure 34) is then equal to I S
= I P
• N P
/N S
. Finally, for a wide
operating temperature range, the electronics are designed to
automatically compensate the electronic offsets and voltage
drops, eliminating the need for adjustments.
„C-type“ transducers internally control the loading of the
secondary current and therefore have a voltage output.
1
Generator
3
Trigger
T1
A
N S
I S
+I µ
I µ
I P
N P
B
Figure 34: Block diagram of the C-type Fluxgate transducers
T2
2
Filter
N S
I S
4.4.2 „CT-type“ current transducers
C
4
I/V
Transducer
5
Control Loop
Please note that „CT“ is a LEM product designation and is not
used as an abbreviation for current transformer.
The LEM „CT“ current transducers measure currents up to a
maximum of 150 A. They typically have an accuracy of 0.1 %
and a remarkable 0 to 500 kHz bandwidth.
The main advantages are:
• Excellent accuracy across the entire operating temperature
range
• Wide bandwidth
• Extremely short response time
• Excellent immunity to surrounding magnetic fields
• High overload current capability (e.g. a CT 1-S transducer,
with a 1 A nominal current, can withstand a overload above
15 kA for 150 ms)
• Output short circuit protection
V M
• Very high isolation levels and excellent resistance to
partial discharge (e.g. CT 5-T/SP3 transducers have
50 kVrms isolation with a partial discharge extinction level
of 14.5 kV with < 20 pC)
The limitation of the CT transducer is the injection of a
rectangular voltage ripple on the primary line. This is caused
by the Fluxgate excitation voltage, typically at a frequency of
500 Hz, and is a function of the primary to secondary turns
ratio. This primary voltage ripple induces a current noise,
with an amplitude dependent on the primary circuit
impedance; the lower the impedance the higher the current
noise (see calculation examples in § 4.4.6b). While this
current noise is generally not a problem, it is measured by
the transducer and will be seen as an additional
measurement error.
4.4.3 „CD-type“ differential current transducers
A LEM „CD“ transducer is designed to measure differential
currents, the difference between two primary currents
flowing in opposite directions. This application takes
advantage of the very high sensitivity of this topology,
allowing measurement of a differential current that is only
0.1 % of the main current flowing in each primary conductor.
For example, a differential current of 1 A can be measured
with a main current of 1000 A, or 0.1 A with a main current of
100A. The measurement accuracy is about 5 to 10 % within
the specified operating temperature range.
The main advantages are:
• Very good differential resolution (able to measure low
differential currents with high main currents)
• Possibility to have an external adjustment by the user of
the level of the differential current to be measured (a
special design provides external terminals in the housing,
where an adjustment resistor is connected)
• Special designs allow the adjustment of the time constant
of the measured differential current, which is convenient
when the output signal of the transducer is used as a
trigger in a safety system
• Options to define several levels of differential currents to
be measured (the transducer is then designed with a
separate individual output for each current level)
• Protection against primary current overloads
The main limitations are:
• Limited frequency bandwidth, typically DC to 2kHz
• A differential current measurement should theoretically be
independent of the main current value, but the main
current must never exceed the transducer rated value to
avoid permanent local saturation affects, due to both
internal and external magnetic effects, which will reduce
the accuracy of the transducer
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