POWER QUALITY MONITORING AND POWER MEASUREMENTS BY USING VIRTUAL INSTRUMENTATION
ABSTRACT
The presented paper describes a virtual instrument used for monitoring and
analysis of the relevant power quality parameters and power measurements. The
metrological support block is realized in LabVIEW environment which uses advanced
methods for measurement and recording of the power quality parameters in
accordance with the European quality standards. In that way, a suitable
hardware solution for signal conditioning and load control is proposed. The most
important parameters (voltage, current, power) are recorded into text files
which are further used for measurement data analyses. The measurement results
are obtained by using waveform simulator METREL.
key words - Power Quality, Data Acquisition, Virtual
INTRODUCTION
The electric power is essential for running industrial production
processes, for commercial use, for transport and other purposes. In the last
years this dependency has increased and all these processes relay on the
quality of electricity supply, namely power quality. The detection of the
disturbances affecting the line voltages is one of the most qualifying points
in the estimation of the “voltage quality” or “supply quality”. The correct
assessment of the quality of the supplied voltage has become one of the key
issues in the deregulated electricity market. Ensuring a “high quality” of the supply
voltage is the main requirement for ensuring a high “power quality”. Great
attention is therefore paid to the definition of suitable indexes of voltage
quality and the definition of suitable measurement procedure to evaluate these
indexes. A large number of power quality disturbances have been reported in the
literature-some of them being transient in nature and others being related to
periodic, steady – state operation.
Some of the more common disturbances are: voltage and current harmonics,
voltage dips, electric noise, impulses, notches and flicker.
Because of these disturbances measurement of the electric quantities, such
as voltage, current and power by using equipment commonly used for measurement
of sinusoidal signals can result in errors. In this way, inclusion of the
digital signal processing techniques can be much more adequate. Anyway, a
suitable digital signal processing approach must be provided. In the recent
years adoption of personal computers (PCs) in the field of the measurement technique
offers great progress and flexibility. Step ahead for development of modern measurement
systems is achieved by adopting the concept of Virtual Instrumentation. It is a
methodology for realization of measurement instruments by using standard PC’s,
hardware data acquisition components for signal conversions and specialized
program platforms for processing and recording of the measurement results. In
this paper a Virtual Instrument for power quality monitoring is proposed.
POWER QUALITY PARAMETERS
The ideal supply voltage is pure sinusoidal voltage with nominal frequency
and nominal amplitude. Any variation from this is considered as a power quality
event or a disturbance. One important aspect in the field of power quality is monitoring
and control of the qualitative parameters of the electrical energy according to
today’s standards. In that way, a big attention is paid to define the
disturbances and determination of procedures for their measurement. A large number
of power quality disturbances have been reported in the literature. In general,
the parameters could be divided in two groups - voltage amplitude variations
and wave-form distortion. A short classification of power quality parameters is
given in Table I.
TABLE I
POWER QUALITY PARAMETERS
In the following section some theoretical analyses relied on the signal
processing are reported.
SIGNAL PROCESSING ANALYZES
Analyzing a periodic signal u(t) with angular frequency w and having
in mind the Nyquist criteria, the signal limited with it’s Nth harmonic
can be represented by 2N+1 samples over the period T.
The active power value of the voltage u(t) and current i(t) is
represented with the equation:
............................(1)
Analyzing (1), measurement of the active power demands estimation of two
time dependent components.
The p(t) spectrum is given by:
.....................(2)
According to relation (2) the spectrum of p(t) is wider than that of
u(t) and i(t) and is limited to its 2Nth harmonic. From this
analysis it can be clearly seen that if the moment value and the spectrum of
the power is required, u(t) and i(t) must be sampled with
frequency twice than the sampling theorem criteria. Theoretically, it is
possible to acquire only 2N+1 samples for the voltage and current, but
in practice the sampling frequency must be significantly increased.
The same considerations can be applied for evaluation of the RMS value of u(t)
which is expressed by the relation:
...............................(3)
The appropriate evaluation of (1) and (3) also demands for proper
definition of the observation interval. The observation interval needs to be an
integer multiply of the signal period T in order to minimize the leakage
errors in the frequency domain. Otherwise under non-synchronous sampling conditions
an interpolation algorithm must be employed.
HARDWARE SOLUTION
The hardware is realized by using National Instruments multifunctional data
acquisition (DAQ) card containing 32 analog input channels with resolution of
16 bits, programmable input range (±10V) and sampling rate up to 250kS/s. Two
hardware boards for voltage and current signal conditioning are realized using
six analog input channels, and three digital channels for load switching. The
current measurement signals are obtained by using three electronic transducers
incorporating current transformers and the load switching is realized by three
relay switches controlled by the DAQ card. The voltage measurement signals from
the power lines are obtained with precise resistive dividers. Block diagram of
the hardware solution is shown in Fig.1
fig. 1. Hardware block diagram
The signal conditioning circuits should provide few functions like:
galvanic isolation from supply network, attenuation or amplification of the
measured signals, protection of DAQ card and noise suppression. The main role of
the signal conditioning circuit is to adjust the sensor’s output signal span to
match the analog-to-digital converter (ADC) input range. The block diagram of
the signal conditioning circuits is shown on Fig.2
fig. 2. Signal conditioning circuit block diagram
In the absence of proper signal conditioning the signal
can exceed the ADC input range and cause saturation of its output. The signal
is first attenuated or amplified and DC level shifted with the input attenuator/amplifier.
The next block is a unity gain buffer with very high input impedance which is
used for adaptation of the impedances of the attenuator and the filter. Sixth
order active anti-aliasing filter has been designed with cut-off frequency of 6
kHz and near flat amplitude frequency and phase-frequency characteristics. The
filter is used before a signal sampler to restrict the signal’s bandwidth and
to satisfy the sampling theorem. Fast circuits for limiting the input voltage
to the ADC input range have been designed. These circuits allow signals below a
specified input level to pass unaffected while attenuating the peaks of
stronger signals that exceed this level. The used data acquisition card is with
galvanic isolated inputs. The galvanic separation eliminates all forms of
operating disturbances such as ground loop and potential separation.
LABVIEW BASED VIRTUAL INSTRUMENT
LabView is a National Instrument development software
that allows rapidly and cost-effectively interface with measurement and control
hardware, data analyzes, share results, and distribute systems. It is based on
graphical programming techniques that allow programming with visual
expressions, spatial arrangements of text and graphic symbols. The software is
based on a block diagram (intended for graphical program development) and front
panel (graphic interface formed by switches and panels intended for user interaction).The
Virtual Instrument described in this paper consists of two parts:
1) Power line voltage analyzes
2) Current and power analyzes
Fig.3 represents the voltage analyzes block diagram. This
block is identical for all three power lines.
Fig. 3. Signal conditioning circuit block diagram
Samples from three analog channels are successively taken
with sampling frequency of 2kHz per channel for sampling interval of 100ms and
are fed to a signal selection block. Every sample is multiplied by a constant
factor which indeed is the attenuation coefficient of the signal conditioning
circuits. The obtained signal is further processed and used for measurement of
the RMS, total harmonic distortion (THD), frequency and phase difference of the
input signal.
The virtual instrument contains two sub-virtual blocks
for filtration of the spectral components and data recording. The filtration
block contains sixth order Chebyshev band pass IIR filters with central
frequency at the odd spectral components and the data recording sub-virtual
block stores the results for RMS, frequency and THD of the input signal in
interval of 100ms.
The programming points are implemented as follows:
• Sample gathering;
• Voltage RMS calculation, equation (3);
• Frequency measurement;
• Phase difference calculation;
• Analyzes of the Total Harmonic Distortion, relation
(4);
100,n 2,3..N
....................(4)
• Analyze amplitude spectrum by using Amplitude
spectrum VI, equation (5);
......................(5)
• Analyze signal power spectrum using Auto Power spectrum
VI, equation (6);
...............(6)
where * is a complex conjugate.
• Display the signal waveform, amplitude and power spectrum
on a waveform graph;
• Filtration and measurement of RMS for 5 odd spectral components;
For this purpose a sixth order Chebyshev band pass IIR filters
are used with central frequency at the odd spectral components. The pass band
of the filters is 20Hz.
• Write the amplitude RMS, signal frequency and THD into
text files;
All measurement data with time and date of recording are recorded
into separate text files. These data can be further used for data storage and
analysis by using some graphical presentation software such as DIAdem or MS
Excel.
Fig.4 represents the front panel of the virtual
instrument
Fig. 4. Front panel of the virtual instrument
CURRENT AND POWER ANALYZES
Three current channels are sampled with frequency of 2kHz per channel and a
sampling interval of 100ms. Every sample is multiplied by constant factor
corresponding to the transducer attenuation. The obtained signal is further processed
and used for measurement of the RMS, total harmonic distortion (THD), and the
active and reactive power of the input signal.
Fig.5 shows the LabView programming block diagram for current and power
measurements for one measurement channel.
Fig. 5. Current and power measurements block diagram
The programming points are implemented as follows:
•Sample gathering;
•Current RMS calculation;
•Analyzes of the Total Harmonic Distortion;
•Analyze amplitude spectrum;
•Current phase measurement and current-voltage phase difference
calculation;
• Active (7) and reactive (8) power calculation;
• Display the signal waveform and amplitude spectrum on a waveform graph;
• Write the current RMS, active and reactive power into a text file;
The front panel corresponding to the LabView programming sequence is shown
in (in) Fig.6
Fig. 6. Front panel of the virtual instrument for current
and power measurements
MEASUREMENT RESULTS
Measurement of the power quality is usually defined as a measurement of low
frequency conducted disturbance with the addition of transient phenomena. The
ideal single phase supply voltage is a pure sine wave with nominal frequency and
voltage amplitude. Any variation of this is considered as a power quality
disturbance.
The following parameters of supply voltage are influenced by disturbances:
• Frequency
• Voltage level
• Wave shape
• Symmetry of three phase system
In the experimental tests one phase power simulator Metrel MI 2191 is used.
The instrument is able to simulate typical voltage and current shapes, such as
voltage and current harmonics, flickers, transients, voltage interruptions etc.
Three examples for measurement of transients, flickers and harmonics are shown
in the results.
• Transient is a term for short, highly damped momentary voltage or current
disturbance Fig.7 and Fig.8;
• Flicker is a visual sense caused by unsteadiness of a light. The level of
the sense depends on the frequency and magnitude of a light change and the
observer itself Fig.9;
• Harmonics are any periodic deviation of a pure sinusoidal voltage Fig.10;
Fig.11.a, Fig.11.b and Fig.11.c represent the recorded values for the RMS
voltage, frequency and THD during 10 hour interval by using the text files from
the virtual instrument. In the second experiment measurement of current and
power of a 100W light is presented (Fig.6).
Switching of the light is controlled by the DAQ card.
Fig. 7. Transients caused by SRC switching
Fig. 8. High transient pulse caused by lightning
Fig. 9. Flicker with square distribution
Fig. 10. Highly distorted signal of a simple chopper
voltage converter
Fig. 11.a Current and power measurements block diagram
Fig. 11.b. Current and power measurements block diagram
Fig. 11.c. Current and power measurements block diagram
In Fig.11.a, Fig.11.b and Fig.11.c recorded values for
the RMS voltage, frequency and THD during 10 hour interval are
presented. The graphs are obtained by using the data records from text files
presented in MS Excel. The virtual instrument detected appearance of short
voltage interruption, as it can be seen from the results.
CONCLUSIONS
This paper has summarizes theoretical and practical facts
concerning the monitoring and analysis of power quality parameters. One
possible hardware solution for signal conditioning in combination with DAQ card
is implemented. This system is used for measurement and analyzes of different
power quality disturbances. The signal conditioning module is developed in a
way so it can be used for measurement of all power quality parameters. The voltage,
current and power analyses are completely developed using virtual
instrumentation techniques implemented in LabView software. Measurement data
are recorded in text files for further analysis by using some graphical
presentation software such as DIAdem or MS Excel. The performance of the
proposed equipment is good enough for an effective application to test the
power quality parameters.
The implemented system worked correctly in real time and
detected and stored different types of disturbances.
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