Test&Measurement

A novel high-current sensing technique developed by PM Special Measuring Systems (PM SMS) in the Netherlands is being used by Yokogawa Europe to boost the current measuring capacity of its industry-leading WT3000 precision power analyser.

With its SC1000 product, PM SMS is a pioneer of the zero-flux split-core measurement technique, which is ideally suited to applications demanding non-intrusive, high-accuracy and contactless current measurement. From

Yokogawa adds laboratory class split-core current sensor for accurate high-current measurements

Yokogawa’s point of view, these sensors are used when the input current to be measured exceeds the current input capacity of the power analyser. For example, in the case of the Yokogawa WT3000, the power analyser’s input module has a maximum capacity of 30 A, so an external current sensor is needed to handle higher currents. Adding the split-core current sensor allows currents up to 700 A RMS to be measured, with the further benefit that the user does not have to disconnect any cables in the test environment.

The new technique also detects and measures any possible DC component in the current up to a nominal

Precision Making

Precision Making

peak value of the primary current (AC plus DC) of ±1000 A.

Features and benefitsThe SC1000 offers significant benefits over conventional current transformers for AC measurements, which lack the capability to measure current at low frequencies (for example 5 Hz, as found in frequency inverter drives). Moreover, DC current will not be transformed at all, as it saturates the transformer. AC with some DC might saturate a conventional transformer, or at least strongly distort the current shape.

A direct-current transformer based on the Zero-flux™ principle, on the other hand, is able to measure currents over a wide bandwidth from DC to several kilohertz with a very high accuracy, eliminating measuring errors which may arise with conventional AC sensors (Fig.2).

Basic principleRegular current transformers suffer from core losses. The Zero-flux principle brings the core losses close to zero by putting the transformer in an active feedback configuration. A sense winding detects any excursion from zero flux in the core and compensates this through driving an equal and opposite current in the secondary winding and hence balancing any flux change induced by the primary winding.

The basic principle of operation of the SC1000 is illustrated in Fig.1. The primary current Ip generates a magnetic flux that is actively counteracted by the current Is in the secondary winding (Ns) of the measuring head. Any remaining flux is sensed by three toroidal-wound ring

cores located within the secondary winding volume. Two of these cores (Nl and N2) are used to sense the DC part of the remaining flux, while N3 senses the AC part. An oscillator drives the two DC flux-sensing cores into saturation in opposite directions. The resulting current peaks are equal in both directions if the remaining DC flux is zero. If not zero, their difference is proportional to the residual DC flux.

The Zero-flux™ current transformer has a double peak detector to find this DC flux. After adding the AC component (N3), a

control loop is set up to generate the secondary current for creating flux balance. A power amplifier provides this current Is to the secondary winding Ns. The secondary current, which is a scaled image (l/Ns) of the primary current, is fed to the load resistor to convert the signal into a voltage. The signal across the load is amplified to make the signal available for further use.

The gapless and non-intrusive sensing of the primary flux, the active compensation of the primary flux with the unique PM SMS electronics and, last but not least, over 30 years of recognition by many well-known research institutes all over the world ensure that zero-flux measuring systems provide high accuracy and stability without the need for any temperature control devices.

Above several kilohertz, the power amplifier no longer has active control over its output current, but merely forms a short circuit. The Zero-flux™ current transformer still performs as a wideband current measuring device, but now with the measuring head as a passive current transformer. The final bandwidth is limited by the stray reactance and capacitance in the head and interconnecting cable.

By default, the zero-flux current transformer provides a current output. For some applications such as analogue to digital conversion, a voltage output is preferred and is optionally available.

If the core saturates due to primary overload, the zero-flux condition is lost, and a search cycle is started automatically.

Basic diagram of a Zero-flux™CT with voltage output

Precision Making

Yokogawa Test & Measurement are the ‘Precision Makers’, and the company’s instruments are renowned for maintaining high levels of precision and for continuing to deliver value for far longer than oth-er instruments. Yokogawa believes that precise and effective measurement lies at the heart of successful innovation – and has focused its own R&D on providing the tools that researchers and engineers need to address challenges great and small.

Yokogawa Europe B.V.

Euroweg 2, 3825 HD,

Amersfoort,

The Netherlands

Tel. +31 88 464 1000

Fax +31 88 464 1111

tmi@nl.yokogawa.com

This means that the secondary current is slowly swept between the negative and positive current limits until zero flux is detected and normal tracking continues. The same happens when the auxiliary power is switched on with the primary current present. The search cycle lasts 1-2 seconds and is repeated until the zero-flux condition can be established.

The load resistorTo achieve the required measurement precision a 4-wire resistor is the best choice for the load resistor. The power dissipation is kept very low, because the voltage drop across the resistor (usually 0.5 V at rated current) is low. The thermal stability of the load resistor under normal load conditions is ensured, even over long periods.

The precision amplifierThe precision amplifier is a very stable differential amplifier which delivers a highly accurate output voltage that is proportional to the secondary current through the load resistor. To ensure that the gain factor remains constant, the temperature coefficients of the gain-setting resistors are matched. The offset error is minimised by careful selection of the operational amplifier and fine tuning during adjustment. The gain, usually 20×, is factory adjusted in

order to compensate for tolerances in the load and gain-setting resistors. The output usually delivers 10 V at the rated current and may be loaded by up to 5 mA.

Power meterHitec DCCTRogowski

ACCT

1 10 100 1k 10k 100k 1M f (Hz)

1

0.1

0.01

0.750Hz

(-3dB)

Output signal (at constant input)

The load resistor and precision amplifier are used to achieve a voltage output. However, they are omitted for a system with current output, which is preferable in order to achieve the best accuracy for precision power measurement applications.

The SC1000 from PM SMS incorporates its own built-in electronics with a current or voltage output, which presents a precision, scaled copy of the primary current over a high bandwidth. The unit also provides automatic re-establishment of zero-flux conditions after temporary loss of supply voltage or at overcurrent conditions. An extra open-collector output indicates proper operation of the system.

Transfer ratio of several current sensors types related to power meter