Voltage and Current Measurement System for
Medium Voltage Inverters
Manoel Eustáquio dos Santos
Braz de J. Cardoso Filho
Gerência de Engenharia e Utilidades
AÇOMINAS - AÇO MINAS GERAIS S/A
Ouro Branco, Minas Gerais, Brasil
mesantos@acominas.com.br
Abstract- In this paper, a low cost and high performance
current and voltage measurement system designed for application
in medium voltage inverters is proposed. The main features of the
proposed system are the use of commercially available integrated
circuits and signal transmission from sensing point to the control
board through an optic fiber link. Three single-bit oversampling
analog to digital conversion system are considered in this study.
Simulation and experimental results are presented to confirm the
high performance of the proposed measurement system.
Keywords: measurement systems; sigma-delta modulation;
voltage to frequency conversion; pulse width modulation.
I.
Flavio H.Vasconcelos
Departamento de Engenharia Elétrica
Universidade Federal de Minas Gerais
Belo Horizonte, Minas Gerais, Brasil
cardosob@cpdee.ufmg.br
to obtain a low cost and high performance measurement system
to be applied in medium voltage inverters measurements. In
section II, a brief description of the simgle bit A/D conversion
techniques and simulation results from models of the integrated
A/D converters commercially available are presented. In section
III, a prototype of the proposed measurement system consisting
of sensing boards, a 5 MBd optic fiber link and a demodulation
interface board for the TMS320LF2407, is discussed in detail.
Experimental results are presented demonstrating the
performance of the proposed system. The conclusions are
presented in section IV.
II.
INTRODUCTION
A common solution for current and voltage measurements in
power electronic converters is based on insulated Hall effect
current and voltage transducers. The analog signals are
transmitted to the DSP board through copper pairs in an
environment with high levels of electromagnetic interference.
The analog signals are then converted to digital format using
Nyquist A/D converters with either 10 or 12 bit resolution. Such
solution for current and voltage measurement turns out to be
costly and sensitive to electromagnetic interference in medium
voltage inverters. An interesting alternative based on an optic
fiber link coupled to single-bit A/D conversion techniques
allows one to obtain three important features for such data
acquisition system: low cost sensors (shunt resistors,
compensated voltage dividers); high voltage insulation between
the sensing point and DSP control board; high electromagnetic
interference immunity. Although a similar system based on
ASIC implemented multibit sigma-delta A/D converter have
been reported in the literature [1], several important issues such
as a comparative performance of suitable pulse modulation
techniques and hardware implementation details still need
further probing.
In this paper some alternatives form of implementation of an
optic fiber isolated measurement system based on oversampling
single bit A/D converters are investigated. The main purpose is
SINGLE BIT A/D CONVERSION
The optic fiber link requires the signals at sensing point to be
converted to a single-bit digital format. The transmission and
reception of signals in single-bit digital format is simpler than
serial transmission/reception with SCI or SPI protocols, since no
synchronizing clock signal is needed. Moreover, the single-bit
A/D converters can also achieve high resolution in terms of
signal-to-noise ratio [2]. The most common single-bit A/D
converters reported in literature are the Sigma-Delta converters,
the Voltage-to-Frequency converters and the PWM converters.
A. The Sigma-Delta A/D Conversion
The sigma-delta A/D conversion concept was developed in
the 19th century but only in the last three decades this technique
became more attractive [3]. The basic structure of a first order
one bit sigma-delta A/D converter is shown in fig. 1. The
converter consists of an integrator, a comparator (one-bit A/D
converter or quantizer) and a one-bit D/A converter in the
feedback path. The modulator output p[tn] is a data clocked
stream. At each instant of time, the difference (∆) between the
input u(t) and the analog version of the delayed output, which is
accumulated by an integrator (∑), is computed. The integrator
output is then quantized using a comparator (one-bit A/D
converter). Considering the noise at the output of a first order
0-7803-7420-7/02/$17.00 © 2002 IEEE
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sigma-delta modulator in the frequency range of the signal (inband noise power), the signal-to-noise ratio SNR at the
modulator output can be obtained from (1) .
SNR = −5.17 + 9.03r( dB )
(1)
r
2 = f a / 2 . f b , where fa is the oversampling frequency
and fb the signal frequency band.
From (1), at each time that the oversampling ratio is doubled
(r is incremented), the SNR is improved by a value close to 9
dB. This is equivalent to improve the resolution by 1.5 bit [2,4].
B. The Voltage- to- Frequency Conversion
Another one-bit A/D conversion technique is the voltage-to
frequency conversion. In this case, the converter output signal is
a data stream whose frequency is proportional to the amplitude
(voltage) of the input signal. There are two common types of
voltage-to-frequency converters: multivibrator and a chargebalance. The charge-balance type is more accurate and more
popular [6]. The block diagram of a charge-balance voltage-to
frequency converter is shown in fig. 3. This circuit is based on
an integrator, a comparator and a precision charge source.
According the comparator output, the charge source is switched
to the output or the input of the integrator, so that the frequency
of the output signal is proportional to the amplitude of the signal
at the input of the integrator. In this case, the voltage-tofrequency converter is equivalent to the first order sigma-delta
converter and its output signal-to-noise ratio is computed by
from (2) [7]. In this work, the Texas Instruments VFC110
voltage-to-frequency converter was utilized [8].
Fig. 1. First-order sigma-delta modulator block diagram.
Improvements in signal-to-noise ratio can be obtained by
using higher order modulators. The block diagram of a second
order sigma-delta modulator is presented in fig. 2. In this case,
the signal-to-noise ratio is computed as:
SNR = −12.9 + 15.05 r( dB )
(2)
With the second order sigma-delta modulation, at each time
the oversampling ratio is doubled, the SNR improves by an
amount close to 15 dB, equivalent to 2.5 bit increase in the
resolution. Considering an input signal with 1.0 kHz frequency
band and 1.0 MHz oversampling frequency, then r = 8.96 and
the SNR = 122 dB, equivalent to 20 bit resolution.
Second order sigma-delta modulators are widely used in A/D
conversion systems. In this work, the Texas Instruments ADS
1201 second order sigma-delta A/D converter was used [5].
Fig. 3. Charge-balance voltage-to-frequency converter.
C. The PWM A/D Conversion
In the PWM A/D conversion the input signal is compared to
a carrier signal and the output signal is a data stream with
variable pulsewidth and fixed frequency. In this A/D`conversion
technique the input signal is not filtered, and the output signalto-noise ratio is lower than the obtained from sigma-delta and
voltage-to-frequency converters. The one-bit PWM A/D
conversion is less used than sigma-delta and voltage-tofrequency converters. The International Rectifier IR2172 PWM
modulator is considered in this research work.
D. Simulation Results
Fig. 2. Second order one-bit sigma-delta modulator.
The performance of the three one-bit A/D conversion
techniques described above at one decimation or averaging
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period was verified by digital simulation. The results presented
are based on models developed for the ADS 1201 second order
sigma-delta modulator, VFC110 voltage-to-frequency converter
and IR2172 PWM converter. Figs. 4 to 6 show the converters
input, decimators output and decimators output averaging errors
for the three one-bit A/D converters, all operating with a
decimation frequency of 5.0 kHz. In all cases, the input signal is
sinusoidal, with an amplitude of 4.0 V and frequency of 100 Hz.
The oversampling frequency in each case is set at the upper limit
of each modulator: 0 to 4 MHz for VFC110, 1.0 MHz for
ADS1201 and 40.0 kHz for IR2172.
A comparative analyses of the results in figs. 4 to 6 indicate
the superior performance of the voltage-to-frequency converter
in terms of the decimator output averaging error. The superior
performance indicated here is mainly due to the significantly
higher oversampling frequency capability of the VFC110. The
results also confirm also the adequate selection of the
decimation frequency for the specified input signal.
C O N V E R T E R IN P U T / D E C IM A T O R O U T P U T
4
A M P LITUD E (V O LTS )
D E C IM A TO R O U TP U T
2
0
-2
C O N V E R T E R IN P U T
-4
0
0 .0 0 5
0.01
0.015
0.02
0.025
0.03
T IM E (S )
P W M C O N V E R T E R D E C IM A TO R A V E R A G IN G E R R O R
10
E RRO R (% )
5
0
-5
-1 0
0
0 .0 0 5
0.01
0.015
0.02
0.025
0.03
T IM E (S )
Fig.6. PWM converter input and decimator output.
III.
C O N V E R T E R IN P U T / D E C IM A T O R O U T P U T
PROTOTYPE DESCRIPTION
4
A M P LITUDE (V O LTS )
D E C IM A T O R O U T P U T
2
0
-2
C O N V E R T E R IN P U T
-4
0
0.00 5
0.01
0.01 5
0.02
0.0 25
0.03
T IM E (S )
S IG M A -D E L T A D E C IM A T O R A V E R A G IN G E R R O R
10
E RRO R (% )
5
0
-5
-1 0
0
0.00 5
0.01
0.01 5
0.02
0.0 25
0.03
T IM E (S )
Fig.4. Sigma-delta converter input and decimator output.
C O N V E R T E R IN P U T / D E C IM A T O R O U T P U T
A M P LITUD E (V O LTS )
4
D E C IM A T O R O U T P U T
2
0
-2
C O N V E R T E R IN P U T
-4
0
0 .0 0 5
0 .0 1
0 .0 1 5
0 .0 2
0 .0 2 5
T IM E ( S )
C O N V E R T E R D E C I M A T O R A V E R A G IN G E R R O R
0
0 .0 0 5
0 .0 3
10
E RRO R (% )
5
0
-5
-1 0
0 .0 1
0 .0 1 5
T IP E ( S )
0 .0 2
0 .0 2 5
0 .0 3
Fig. 5. Voltage-to-frequency converter input and decimator
output.
The simulation results presented in the previous section show
that the voltage-to-frequency conversion and the second order
sigma-delta conversion offer better performance than the PWM
conversion technique, from the perspective of the averaging
errors at the decimators output. Test prototypes of the proposed
measurement system based on the sigma-delta and the voltageto-frequency converters were implemented. Fig. 7 show the
architecture of the prototype system. There are two sensing
boards (one based on the ADS1201 and the other based on the
VFC110), a demodulating interface board and the Spectrum
EVM 2407 evaluation board. The sensing boards are connected
to the demodulating interface board through a 5.0 MBd plastic
optic fiber link.
In the sensing boards the signals from sensors (shunts
resistors and/or compensated voltage dividers) are conditioned
and applied to the input of the converters. The converters output
signals are buffered in order to feed the optic fiber transmitters.
The demodulating interface board demodulates the signals from
the sensing boards to a 12 bit digital word and send these words
to the EVM2407 evaluation board at regular time intervals,
defining a 5.0 kHz decimation-averaging frequency. The
demodulator is implemented employing an EPLD chip
(ALTERA EPM7128). The EVM2407 board stores the 12 bits
digital words into a 4096 word buffer. The acquired signals are
then displayed on a PC video monitor. The output data files from
the EVM2407 board are processed using a MATLAB based
software that generates time and frequency domain plots.
The proposed measurement system was characterized in
terms of step response, frequency band and DC error. In order to
establish a reference for comparison, the same signal applied to
the input of the sigma-delta and to the voltage-to-frequency
converters was also applied directly to the input of the A/D
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converter channel zero of the EVM2407 board, with the
sampling frequency set to 5.0 kHz. The A/D conversion
technique employed in the EVM2407 A/D is successive
approximation, with 10-bit resolution. A complete set of
experimental results are plotted in figs. 8 to 12.
(4.0 Volts), variable frequency sinusoidal input signal. The Total
Harmonic Distortion computed from the FFT of each output file
is calculated as:
THD =
n = np
100
.
V( f )
∑V
2
(n)
(%)
(3)
n =1
where V is the amplitude of harmonic voltage, n the order of the
element in harmonic vector, np the harmonic vector length and f
the fundamental frequency.
The output waveforms and their harmonic spectrum are
plotted in figs. 9 and 10, respectively, for a 4 V/100 Hertz input
signal. Fig. 11 show the frequency response for the three
conversion system with 5.0 kHz decimation frequency.
A M P LITUDE (V )
E V M 2 4 0 7 A / D C O N V E R TE R
A M P LITUDE (V )
Fig. 7. Prototype of the measurement system.
A. Step Response Results
A M P LITUDE (V )
Fig. 8 shows the demodulated outputs from the voltage-tofrequency, sigma-delta and EVM2407 A/D converter, obtained
from 4096 word length data sets and a 4 V/100 Hz square wave
input signal. The three conversion techniques lead to similar
results. The voltage-to-frequency and sigma-delta output
decimators are delayed by 200 microseconds (one decimation
period) with respect to the input signal.
4
2
0
-2
-4
4
0
0.005
0.01
0 . 0 15
V O L TA G E TO F R E Q U E N C Y C O N V E R TE R
0. 0 2
0 .0 2 5
0.03
TIM E (S )
0
0 . 0 15
0. 0 2
0 .0 2 5
0.03
TIM E (S )
0 . 0 15
0. 0 2
0 .0 2 5
2
0
-2
-4
4
0.005
0.01
S IG M A -D E L TA C O N V E R TE R
2
0
-2
-4
0
0.005
0.01
0.03
TIM E (S )
Fig. 9. Converters output waveforms.
E V M 2 4 0 7 A /D C O N V E R TE R
E V M . 2 4 0 7 A / D C O N V E R TE R : H A R M O N IC S P E C TR U M
A M P LITUDE (V )
A M P LITU DE (V )
5
0
-5
0
0.004
0 .0 0 6
0.008
0 .0 1
0.01 2
0 .0 1 4
0 .0 1 6
0 .0 1 8
0 .0 2
T IM E ( S )
0
A M P LITUDE (V )
A M P LITUD E (V )
0 .0 0 2
V O L TA G E TO F R E Q U E N C Y C O N V E R TE R
5
-5
0
0 .0 0 2
0.004
0 .0 0 6
0.008
0 .0 1
0.01 2
S IG M A - D E L T A C O N V E R T E R
0 .0 1 4
0 .0 1 6
0 .0 1 8
0 .0 2
T IM E ( S )
3 . 6 1 V O L TS , 1 0 2 H E R TZ
2
TH D = 3 4 , 8 8 %
0
4
0
20
40
60
80
100
120
140
V O L TA G E TO F R E Q U E N C Y C O N V E R TE R : H A R M O N IC S P E C TR U M
-5
0
0 .0 0 2
0.004
0 .0 0 6
0.008
0 .0 1
0.01 2
0 .0 1 4
0 .0 1 6
0 .0 1 8
0 .0 2
T IM E ( S )
Fig. 8. Step Response Experimental Results
160
3 . 6 3 V O L TS , 1 0 2 H E R TZ
2
TH D = 3 4 . 8 8 %
0
0
0
A M P LITUDE (V )
A M P LITUDE (V )
5
4
4
20
40
60
80
100
120
S IG M A -D E L TA C O N V E R TE R : H A R M O N IC S P E C TR U M
140
160
140
160
3 .5 4 V O L TS , 1 0 2 H E R TZ
2
TH D = 3 4 . 8 2 %
0
0
20
40
60
80
100
120
H A R M O N IC O R D E R
Fig. 10. Harmonic spectrum.
B. Frequency Response
The frequency response of the implemented one-bit
conversion techniques was performed from a constant amplitude
The frequency response plot for a 5.0 kHz decimation
frequency is depicted in fig. 11. As observed in the step response
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test, the three conversion techniques present similar results. For
a decimation frequency of 5 kHz, all three techniques frequency
led to a bandwidth of about 300 Hertz.
F UNDA M E NTA L A M P LITUDE (V OLTS )
10
10
10
10
F RE Q UE NCY RE S P ON S E
1
0
-1
-2
10
0
10
1
10
2
10
3
FRE QU E NCY (HE RTZ)
Fig. 11. Frequency response plot.
bit A/D conversion were studied. The second order sigma-delta
and the voltage-to-frequency converters presented superior
performance and were implemented for further probing
employing commercially available integrated circuits.
Simulation and experimental results demonstrated the similar
performance of these alternatives. From the hardware
perspective, the sensing board with the VFC110 voltage-tofrequency converter is simpler than the ADS1201 second order
sigma-delta converter one because not need calibration
procedures and clock signal. Fig. 13 shows the proposed
measurement system applied to medium voltage inverters. The
analog signals from the sensors (shunt resistors or compensated
voltage dividers) are converted to digital format by the VFC110
converter and transmitted to the DSP board control by a 5 MBd
plastic optic fiber link. In the DSP control board the signals are
both decoded to a 12 bit digital word. A 200 µs averaging period
is employed on the signals acquired for control purposes.
Another VFC110 is used to allow conversion of the protection
signals back to analog format. Important features of the
proposed measurement system are low cost, high performance
and high EMI immunity.
B. DC Errors Tests Results
These results were obtained by applying DC voltage at
various levels to the devices. The lowest level was set to –4 volts
and increased in steps of one volt until +4 volts. The reference
was given by a 6½ digits meter with a valid calibration
certificate. Three readings were taken in each step and this
procedure was repeated three times in order to assure
significance of the results.
0.6
Error (volts)
0.4
Fig. 13. Proposed measurement system for medium voltage
inverters.
* Sensors: shunt resistors and/or compensated voltage dividers.
0.2
0.0
-0.2
-0.4
-4.0
-3.0
-2.0
-1.0
A/D
V/F
0.0
1.0
2.0
3.0
4.0
REFERENCES
Sigma/Delta
-0.6
Input (volts)
Fig. 12. Calibration curves.
The test results for each converter are plotted in fig. 12 for a
200 microseconds averaging period. It is interesting to point out
that each plotted point is actually the average of three sets of
three points taken apart on time. The (calibration) curves show
that all the converters are affected by systematic effects such as
offset, non-linearity and calibration.
In particular the
Sigma/Delta converter unexpectedly presented the worst results.
IV.
CONCLUSIONS
Three alternative techniques for the implementation an optic
fiber isolated measurement system based on oversampling single
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Converters Using Sigma-Delta A/D Conversion” in IEEE Industry
Applications Annual Meeting Conference Records, pp. 1513-1519, 1999.
[2] Pervez M. Aziz, Henrik V. Sorensen and Jan Van der Spiegel, “An
Overview of Sigma Delta Converters” , in IEEE Signal Processing Magazine
January, 1996.
[3] J. Candy and G. Temes, “ Oversampling methods for A/D and D/A
Conversion “, in Oversampling Delta-Sigma Data Converters, pp.1-25, IEEE
Press, 1992.
[4] Sanjit K. Mitra, “Digital Signal Processing: A computer-Based Approach”,
McGraw-Hill International Editions, 1998.
[5] Bur-Brown Application Bulletin SBAS081:” High Dynamic Range Delta
Sigma Modulator”.
[6] James Bryant: “Ask the Applications Engineer-3”, Analog devices Products
& Datasheets;
[7] Bur-Brown Application Bulletin SBVA009: “ Voltage-to-Frequency
Converters Offer Useful Options in A/D conversion” .
[8] Bur-Brown Application Bulletin SBVS021: ” VFC110: High-Frequency
Voltage-to-frequency converter”.
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