User Manual TOPACC-HC
1. INTRODUCTION
The bipolar Zero-fluxcurrent transformer System TOPACC-HC, developed by PM SMS for scientific
research, epitomise the concept of a
galvanic ally separated system for
measurement of direct and
alternating high currents up to 30 kA
with exceptionally high accuracy and
stability.
1.1 The basic principle
The principle of measurement is
based on obtaining a perfect balance
between the magnetic flux generated
by the current in the primary current
carrier and that generated by the
current in the secondary winding
situated in the measuring head. This
balance is known as the condition of
zero flux.
The current to be measured is sensed by a toroidal ring core which is mounted around the primary
conductor. A magnetic field is generated by the primary current Ip. The electronics module of the
TOPACC-HC produces a current in the secondary winding of the core in the measuring head, which
creates a counteracting magnetic field. When the magnetic fields balance each other this is known as the
condition of zero-flux. The TOPACC-HC has a magnetic modulator with a patented peak detector. It
continuously checks whether the secondary ampere-turns are in perfect balance with the primary. The
secondary current, which is an exact image of the primary current, is fed trough an external burden
resistor to make the signal available for further use. The TOPACC-HC’s unique design provides high
accuracy and stability without the need for special temperature control devices.
1.2 The burden resistor
Extremely high requirements hold for the burden resistor into which the secondary current is fed. After
amplification in the output circuit of the TOPACC-HC Zero-fluxcurrent transformer the voltage across
this resistor must be suitable to allow very
accurate readings to be made, whether they are
ultimately produced in analogue or digital form. In
view of the required measurement precision a
four-wire resistor is the best. Despite the fact that
high-quality resistors can be purchased on the
industrial market, for technical reasons, PM SMS
much prefers to make its own for the TOPACC-HC.
A special coating contributes to the long-term stability. To achieve a high bandwidth the current and
voltage conductors are put close together in a special way. These measures enable a bandwidth of 1 MHz
to be achieved. To reduce any thermocouple effects which might arise between the alloy of the resis-
tance wire and the copper of the voltage conductors good thermal coupling is made between the two
voltage pick-offs. The thermal stability of the burden resistor under nominal load conditions is, even over
the long term, ensured without the need of resorting to heating elements, Peltier elements or constant
temperature chambers.
1.3 The precision amplifier
The precision amplifier is a very stable differential amplifier, which delivers a highly accurate output
voltage of 10V when the secondary current through the burden resistor meets the rated value. To ensure
that the amplification factor remains constant, the most important point is that the temperature
coefficients of the four amplifier resistors are matched (TCR tracking). The offset error is minimised by
careful choice of the operational amplifier. The precision amplifier is fitted with sensing outputs to
compensate for voltage losses in the externally connected conductors upgrading the measurement
precision of the TOPACC-HC Zero-fluxcurrent transformer. Furthermore, sensitivity to HF interference is
much reduced by capacitive coupling of these outputs.
Fig.1 Basic diagram of the Zero-fluxcurrent transformer
Fig.2 Construction of burden resistor
