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MEĐUNARODNO REGIONALNO SAVETOVANJE O

ELEKTRODISTRIBUTIVNIM MREŽAMA
REGIONAL CONFERENCE AND EXHIBITION ON ELECTRICITY
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Crna Gora, Herceg Novi, 5 - 8. oktobar 2004. / Montenegro, Herceg Novi, October 5 - 8, 2004
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OPTOELECTRONIC SYSTEM FOR CURRENT AND VOLTAGE MEASUREMENT

IN HIGH-VOLTAGE SYSTEMS

J. Radunović, Z. Stojković, S. Petričević, P. Mihailović, S. Stanković, M. Barjaktarović

Faculty of Electrical Engineering, Belgrade, Serbia and Montenegro

ABSTRACT

The paper presents the operating principle and the design of an original instrument for the
simultaneous measurement of r.m.s. values of current and voltage in high voltage power
systems at industrial frequency. The measurement of current is performed using the Faraday
effect, and the measurement of voltage using a resistive voltage divider, constructed from
quality high-voltage, low capacitance resistors. The knowledge of the measured quantities
enables control and management power systems, and allows the possibility to predict the
failures of certain elements and subsystems. In that manner, besides better quality of the
delivered electric power, the reliability of the operation of the power system is also improved.

The paper presents the results of several years of research by this team, funded by the Ministry
of Science, Technologies and Development of the Republic of Serbia.

INTRODUCTION

While measuring current and voltage in an electric power system, two specific problems arise.
One of them is the problem of measurements at high voltages, and the other is connected with
electromagnetic disturbances, to which the measuring equipment is exposed. The conventional
measuring equipment, based on the direct measurement of current and voltage using
induction-based measuring transformers, requires a flawless high-voltage protection. Because
of that the conventional systems are bulky and expensive. A permanent maintenance and
control of insulator materials is necessary, especially that of the insulator oil. Besides these
special maintenance requirements, a problem occurring during measurements are distortions
caused by hysteresis and ferroresonance effects, as well as the problem of saturation.
The quick technological development of optoelectronic devices during the last decade offered a
possibility to use the theoretically well-known nonlinear optical effects to implement and certify
in practice optoelectronic measurement systems for electric power applications [1,2,9]. In this
paper it is used the Faraday effect to measure current, while it is used a voltage divider
produced of high-quality high-voltage resistors with low capacitanse to measure voltage.
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CURRENT MEASUREMENT

The measurement of current flowing through a conductor is performed indirectly, by measuring

its magnetic induction . The crystals exhibiting the magnetooptic effect (Faraday effect) are
used for that purpose. The effect is manifested by a polarization plane rotation of the plane-
polarized light propagating along the crystal if the crystal is in magnetic field. The angle of the
rotation θ is given by the relation [3,4]

θ = V·B·L, (1)
where L is the crystal length, B is the intensity of the magnetic induction oriented along the
crystal, and V is the Verdet constant. The light is guided to the crystal using an optical fiber.
Upon its emergence from the fiber a polarizer is used to perform its linear polarization. The
polarization plane is rotated in the crystal. After leaving the crystal the light traverses the
analyzer. The light intensity on the photodetector is now, according to the Malus' law

Γ(B) = k Γ0 cos2(φ-V·B·L), (2)

where Γ0 is the light intensity at the beginning of the optical fiber, k is the light attenuation
constant on the way from the source to the photodetector, and φ is the angle between the
optical axes of the analyzer and the polarizer.
Assuming that there is a linear dependence between the intensity I of the current flowing
through the conductor and the magnetic inductance due to the current, expression (2) can be
written as

Γ(I) = Γ + ∆Γ(I) = kΓ0/2 + kΓ0/2 cos(2φ-2cV·I·L), (3)

where c is a constant connecting the electric current intensity and the magnetic induction B,
and is dependent on the geometry of the sensing head.
The first term in this expression does not depend on the current we intend to measure, but it
may vary during the measurement process due to the bending of the fiber or to a decrease of
the light source intensity. To ensure measurement independent on these irrelevant but variable
parameters, it is necessary to provide in the accompanying measurement system the
measurement of the relative change of intensity at the photodetector, i.e.

∆Γ(I)/Γ = cos(2φ-2cV·I·L). (4)

The highest sensitivity and linearity of the measurement are obtained when the angle between
the optical axes of the analyzer and the polarizer is φ = 45o. In that case this equality becomes

∆Γ(I)/Γ = sin(2cV·I·L). (5)

From here it is determined the intensity of electric current by measuring the relative change of
the light intensity at the photodetector ∆Γ(I)/Γ. The measurement range is determined by the
condition

θ = cV·I·L <45o. (6)

The construction of this sensor for the measurement of electric current intensity at industrial
frequency consists of a sensing head which is actually a magnetic field concentrator (Fig. 1). It
is a magnetic circuit with a gap. The conductor is placed inside the magnetic circuit, and the
crystal with the polarizer, the analyzer and the necessary optical system are placed within the
gap with a length l. The light is guided by a fiber from the optical source to the polarizer, and
the other fiber is used to guide the light from the analyzer to the photodetector. For the
concentrators designed to envelop conductors with a diameter up to several centimeters and
with a magnetic permeability µr>1000, it is theoretically possible to increase the sensitivity, i.e.
the modulation depth, up to several tens of times if the length of the gap is decreased.
Regretfully, the degree of modulation depends on two parameters, the crystal length and the
width of the gap in the magnetic circuit. It increases with the increase of the length of the crystal
L, but also increases with the decrease of the gap width l. It follows that it is necessary to find
the optimum values for a given geometry of the concentrator and the dimensions of the crystal.
Practical calculations show that in that case the degree of sensitivity can be increased not more
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than 2,5 times [5]. However, the importance of the concentrator is not only in this, but also in
the fact that its use practically ensures that position of the conductor inside the sensing head
does not influence measurement accuracy, as well as an elimination of the influence of the
surrounding conductors.

µr
I
r

l
Fig.1. Representation of the sensing head.

The block diagram of the electronic circuit of the measuring system is shown in Fig. 2. The
photocurrent from the photodetector is converted to voltage by a transimpedance amplifier
stage (TrA in Fig. 2). According to (5), this signal contains a component ∆Γ varying in time, and
a component Γ which practically only weakly changes in time, and whose changes only
disturbs the measurement.
The new implementation of the sensing head improves the level of the useful signal several
times, so that the method utilized in this work, in contrast to the method used in [6,7,8,9], does
not require two channels of an AD converter for separate measurements of ∆Γ and Γ , but only
a single channel instead. Now after the AD conversion of the signal these two measured
components of the signal are subsequently divided in microprocessor by software processing.
In this way the value of the current intensity I(t) at the given instant is obtained in the digital
form. Further processing gives the effective value of the current to be shown in the LCD
display. By the RS232 interface it is possible to send the complete signal to a PC for advanced
signal processing.
Contrary to the papers [6,7,8,9], the second channel can be now utilized for voltage
measurement. The idea is to do it using a high-voltage resistor-type divider whose role is to
linearly scale the high voltage present in the conductor and to adapt it to the measurement
over the second channel. In this manner a combined method was realized which measures
current in optical way and voltage in the conventional electric way.

VOLTAGE MEASUREMENT

The measurement of high voltage is done utilizing a resistor-based voltage divider produced
from high-quality resistors. The resistance of the resistor R1 (Fig. 2) which is in contact with
high voltage conductor, is 10 MΩ (model SGP 148S, manufacturer EBG). It is designed for the
maximum voltage of 50 kV DC, which allows its operation at 35 kV of effective steady-state
voltage. The resistance of the resistor R2 is 10 kΩ, and its steady-state voltage, with a value of
35 V. The capacitive current here is negligible.
The voltage signal from the R2 is fed by a BNC cable to the electronic measurement circuitry.
The signal arrives first to the input inverter amplifier (InA, Fig. 2) which adapts the level of the
measured signal voltage to the input voltage range of the AD converter, which performs its
digital conversion. The signal processing is done by a microprocessor. The measured value is
displayed at the LCD display, and the RS232 interface allows to send the complete signal to a
PC for advanced signal processing.
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ELECTRONIC MEASUREMENT SYSTEM

A block drawing of the electronic measurement system is given in Fig. 2. The measured signal
for the current I(t) is the amplitude modulated photocurrent from the photodetector. This
photocurrent is converted in the transimpedance stage (TrA, Fig.2) to a voltage signal to be
introduced into the first channel of the AD converter. The measured signal for the voltage U(t) is
scaled down to low voltage with an effective value of 35 V using the voltage divider. Thus
obtained signal is led to the input inverter amplifier (InA, Fig. 2) which adapts the voltage level
to 3.5 V effective value, corresponding to the input level of the AD converter. In this point an
overvoltage protection to 90 V is implemented using a gas discharge tube. This voltage signal
is introduced to the AD converter through the second channel. The AD converter is 20-bit
sigma-delta converter which performs signal digitalization in a frequency range 0-10 kHz. The
digitized values from the AD converter are collected by the AT89S8252 microcontroller and
stored into a static memory (SRAM). The sampling frequency of the AD converter is 28800 Hz,
a total of 576 points are stored during one period at 50 Hz. From thus obtained time series the
effective values of the current and the voltage are calculated in the microcontroller and shown
at the display of the instrument. The time series of both measured signals can be led by an
RS232 interface to a PC.

LED

Optical
fibers Photo TrA
detector SRAM
ADC Micro
Sensing controller
head
U(t)
I(t)

InA

Conductor PC LCD
R1
High voltage
divider

R2 Scaled
voltage

Fig. 2 Block-representation of the electronic circuit

In this way the most important requirement of the measurement is fulfilled, that for a
synchronous measurement of current and voltage. The obtained digital values of the current
and the voltage after AD conversion originate from the identical moments of time and thus it is
possible to determine accurately the current voltage phase relationship. Also, the measurement
allows storing in 576 points the values of the current and the voltage during a single period at
50 Hz, while up to 7 periods of voltage and current can be stored in the memory of the
instrument. It is possible to decrease the number of points per period, to encompass more
periods and thus achieve a better accuracy of harmonic analysis.
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CALIBRATION

The method of current measurement described here is nonlinear, thus it is necessary to

calibrate the instrument in the whole measurement range, while the method of voltage
measurement is linear and the calibration is done for it only in two points of the measurement
range (zero and maximum voltage). The calibration of the instrument for the current
measurement requires the calculation of a conversion table containing all possible digital
values of the measured signal which may occur during measurement (depending on the
required resolution of the device) and the corresponding effective values of the current. To
calculate this table, it is necessary first to perform calibration measurements for several current
standards, i.e. to form a calibration table containing the measured values of the effective
current in these standards and the corresponding digital signatures of the measured signal
values. Now the interpolation polynomial is calculated using this calibration table and used to
generate a fixed conversion table and store it to the memory. In this manner the results on the
LCD are displayed by directly reading the conversion table, i.e. the interpolation polynomial is
not calculated over and over again, which significantly accelerates the measurement
procedure.

CONCLUSION

In this paper it is implemented an optoelectronic system for the measurement of current and
voltage in high-voltage systems. It is possible to use it to measure currents in the range from
several A to 1000 A, and voltages from several kV to 35 kV. The achieved accuracy is ± 1% of
the measured value. The measurement system has a high resolution, which enables a high
dynamic range of the measurement of current and voltage. Thus there is no need to change the
instrument range while performing measurements on different lines with various nominal
voltages and currents. In contrast to the method used in [6,7,8,9] where it was necessary to
utilize a two-channel AD converter to measure current, the improve design of the magnetic
concentrator in this case enabled conversion of the current signal by using only one channel of
the AD converter. In this way it was possible to use the second channel for the signal led from
the voltage divider. This ensured the fulfillment of the most important requirement of the
measurement, i.e. the synchronous measurement of current and voltage. The time series of the
both measured signals can be sent by an RS232 interface to a PC for a further and more
complex processing. Let us remind that all of these measurements are performed at high
voltage. Both of the sensors, for the current and the voltage measurements, can be mounted to
an insulator po1e and the complete design can be implemented to use the instrument as a
portable device and to perform measurement without any interruptions in the operation of the
power system.

REFERENCES

1. Ulmer A.E., 1990, “A high-accuracy optical current tranducer for electric power systems”,
IEEE Trans. Power Delivery, Vol. 5, No.2.
2. Katsukawa H., Ishikawa H., Okajima H., Cease T., 1996, “Development of an optical
current transducer with a bulk type Faraday sensors for metering”, IEEE Trans. Power
Delivery, Vol.11, No.2.
3. Mašanović G., Radunović J., 1997, “Magnetic field sensor based on Faraday effect”, J.
Electrotech. Math., No.1, pp.21-30.
4. Radunović J., Mašanović G., Petričević S., Elazar J., Mihailović P., 1998,: “Fiber-optic
sensor for contactless measurement of electric current intensity”, Proc. 1st Conf. JUKO-
CIRED, Zlatibor. (in Serbian)
5. Zlatanović S., Mihailović P., Mašanović G., Radunović J., 1999,: “Optican current sensor
with ferromagnetic ring”, Proc. XLIII Conf. ETRAN, pp.186-188. (in Serbian)
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6. Mihailović P., Petričević S., Stanković S., 2000,: “Frequency characteristic of measurement
probe of a fiber optic sensor for assessment of quality of electric current ”, Proc. XLIV Conf.
ETRAN, pp.199-202. (in Serbian)
7. Mihailović P., Petričević S., Radunović J., 2002,: ”Fiber optic system for measurement of
current intensity at high voltage”, Proc. III Conf. JUKO-CIRED, Vrnjačka Banja. (in Serbian)
8. Radunović J., Petričević S., Mihailović P., Mašanović G., Stanković S., Barjaktarović M.,
2003,: ”Optoelectronic measurement systems in power engineering”, (invited paper) Proc.
XLVII Conf. ETRAN, Herceg Novi. (in Serbian)
9. Mihailović P., Petričević S., Stojković Z, Radunović J., 2004,: ” Development of a Portable
Fiber Optic Current Sensor for Power Systems Monitoring”, IEEE Transactions on
Instrumentation and Measurement, Vol. 53, No.1, pp. 24-30.

Contact: prof. Dr Jovan Radunović, Faculty of Electrical Engineering, 11000 Belgrade,

Bul. Kralja Aleksandra 73, radun@Infosky.net, radunovic@etf.bg.ac.yu