IEEE Thansactioti on Nucf.eah Science, VoLUS-24, L10.3, June 7977

THE ZERO-FLUX DC CURRENT TRANSFORMER A HIGH PRECISION BIPOLAR WIDE-BAND

MEASURING DEVICE

H.C. Appelo, Cm-n Geneva

M. Groenenboom and J. Lisser FIazemeyer B.V., Hengelo (01, Holland

Summary

A current-carrying conductor is surrounded by * pair of ring cores.

A sense winding on one core provides flux rate feedback to a power amplifier which drives the ampere-turn compensating current through a common compensating winding.

The other core serves as a second-harmonic modulator to establish zero-flux operation and thus to ensure a perfect, temperature-independent current balance. A specially-developed burden resistor converts the compensating current into a voltage signal, which is amplified to give a 10 V output signal at the nominal value of the current to be measured.

A substantial number of devices, ranging from 50 to 25000 Amperes is now operational in the beam transfer and extraction power supplies for the CERR SPS.

Introduction

For beam transfer, extraction, chromaticity correction and Landau damping in the 400 GeV SPS accelerator at CERN, over 200 magnet power supplies are used with output powers from a few kilowatts to several megawatts. Many of these are bipolar, fast pulsed units with active filters. The regulation circuits for these were developed by CERN and will be the subject of a future paper. In conjunction with these regulation circuits, current measuring devices were required with a reproducibility up to 20 ppm, suitable for bipolar operation, with large bandwidth and good transient response. Hazemeyer have designed a standard range of these current measuring devices&, based on the work by Hereward and ~n~er1,2~3, who developed a wide-band beam current transformer for the Intersecting Storage Rings at. CERN.

functional diagram of DUCT

Principle of operation

As shown in the diagram, the DCCT uses two identically-wound cores, Tl and T2. The ampere- turns of the current to be measured are compensated by the secondary winding ampere-turns, through a high gain power amplifier.

On a third winding of core T2, a 50 Hz voltage signal is imposed. The phase and amplitude of the second-harmonic component of the corresponding magnetizing current are a measure for the sign and the magnitude of the ampere-turn unbalance. The 100 Hz component in the magnetizing current is therefore filtered out, passed through a synchronous rectifier and used to establish the zero-flux working point via the power amplifier. This second-harmonic modulator loop has good long- term stability and compensates effectively any slow drift of the power amplifier. Its speed of response, however, is slow.

Therefore, a third winding on core 1‘1 is connected as a magnetic integrator. Any ampere- turn unbalance in the core induces a voltage in this winding, which instantaneously modifies the compensating current via the power amplifier.

Thus, the combination of these two loops ensures that the compensating current is a perfect image of the current to be measured, combining high long-term stability with good bandwidth and transient response for bidirectional primary currents.

Via a newly developed four-terminal burden resistor, the secondary current is converted into a voltage signal, which is amplified in a precision amplifier with remote sensing to 10 V at nominal primary current with low output impedance.

Long-term stability and operational experience

The long-term stability is mainly determined by the burden resistor. To certify this stability, a 1 ohm unit has been checked weekly by the National Calibration Laboratory in the Hague, over a period of six months. Measurements were made at an ambient temperature between 22 and 27'C, with a loading between 10 and 1000 mW, i.e. between 10% and 100% of nominal current. Over the period from April 15, 1976 to October 15, 1976 a 2.5 ppm increase was measured for an initial value of 0.9999867 ohm. The uncertainty in these measurements is less than + 5 . 10-T.

Because of the factory calibration of the output signal to within + 50 ppm of the nominal 10 V value, the electronTcs modules are quickly exchangeable between units of the same type. This facilitates maintenance during machine operation; in general, no recalibration is required after exchange.

Furthermore, the nominal current to be measured can be easily adapted by adding taps to the secondary compensating windings. This permits quick adaptation of a given power supply to different magnet loads.

Since July 1976, over 350'000 unit-hours have been accumulated at the SPS, with 150 units operational since mid-November. A total of 9 DCCT failures have been recorded over this eight-month period.

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Construction

The complete system comprises a core and coil assembly and a 19-in rack mounted electronics module. The interconnection is made via a multicore cable, the length of which is not important.

The current range of 50 to 25000 Amperes is covered by six models, the dimensions of which follow from table I. Within any of these ranges I may be selected from 0.2 --- I . The rms c&rent may be different from the"n%%nal current in the case of pulsed operation.

Table I

Models used in the SPS

height of electronics 2 2 3 4 4 5 module (1%" units)

power consumption (VA) 5o 75 100 1.50 250 400

supply voltage (V) 22OV, l-p 38ov, 3-p+N

n 11

l-----4 000000

1

Performance

The following typical performance data refer to the output at the nominal current level In:

Output at 2 In : +I0 v Output impedance : I milliohm Small signal bandwidth: 10 kHz

: 25 V/ms : IO ppm of F.S. rms

Slew rate Ripple and noise Ratio error

- initial - vs. temperature - VS. time

Offset error - initial - vs. temperature - vs. time

Linearity error Warming-up time

: 100 ppm : 4 ppm/OC : 1 ppm/month

: 10 ppm of F.S. : 1 ppm of F.S./'C : 1 ppm of F.S./month : 5 wm : nil

t total err01

0

definition of errors

I I/In -

linearity

ratio

offset

These data apply for a maximum ambient temperature of 40°C for the electronics module, and 55 C for the core and coil assembly. The interpretation Of the errors as shown above is defined in the diagram.

References

1. Sharp J.B. "The induction type beam monitor for the PS". Internal report MPS/Int CO 62-15; 6-12-62 Cern.

2. Unsex- K. "Beam current transformer with DC to 200 MHz range". IEEE Trans 1969 NS16, pp 934- 938.

3. "Current transformer for the ISR". Cern Courier 1970-10 pp 980-382.

4. Groenenboom M., Lisser J., "Accurate Measurement of DC and AC by transformer". Electronics and Power Jan. '77 pp 52-55.

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