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Digital Measurement System for Power Quality Evaluation

ABOUT THE AUTHORS

Giovanni Bucci received a
degree in electrical engineering in 1985 from the University of
L'Aquila. He is now Assistant Professor in Electrical Measurement
at the University of L'Aquila. His current research interests
include ADC and wireless device testing, multiprocessor-based
measuring systems, digital algorithms for real-time measuring
instruments, and power measurements. He is the author of more than
70 scientific papers.

More…

Current electronic devices, such as
microprocessors, are very sensitive to power quality. Power
disturbances compromise product quality, increase downtime, and
reduce customer satisfaction. As a result, many power utilities
perform power monitoring as an essential service for their main
customers. Essential capabilities of a power-quality
measurement system are reduced cost and remote data transmission
capability.

This article proposes a PC-based Virtual Instrument (VI) that
offers flexibility due to a high-level programming language, a
familiar Windows environment, and the low cost of a personal
computer. A LAN or a modem card allows users to control the
measurement system via a TCP/IP network, allowing them to implement
a distributed power-quality evaluation system.

The Measured Power Quality Indices

A lack of quality results from supply interruptions and
deviations of the actual voltage from the nominal voltage. For
example, power supplies based on switch-based regulators or
variable-speed drives introduce higher-order harmonics on the power
line. Voltage fluctuations from brightness regulators
that produce large voltage variations can also compromise power
quality. In addition, the frequency and width of these voltage
variations can be physiologically irritating, due to incandescent
lighting fluctuations.

Power-quality disturbances can range from short interruptions to
long-duration magnitude variations and flicker. The first step in
the implementation of a power-quality measuring instrument is the
identification and definition of the power-quality indexes. The
power supply system can only control voltage quality and has no
control over the current drawn by a particular load.

The primary international organizations working on power-quality
issues are the International Electrotechnical Commission (IEC) and
the Institute of Electrical and Electronic Engineers (IEEE).
According to the specifications produced by these organizations, it
is possible to identify different low-voltage waveform disturbances
(Table 1 ).

IEEE Categories
Std 1159-1995
IEC Categories
EN 50160-2000
Short Duration Variations
Typical
Duration
Instantaneous Sag
0.5 – 30 cycles
Supply voltage dip

A sudden reduction of the supply voltage to a value between 90% and
1% of the declared voltage, followed by a voltage recovery after a
short period of time. The duration is 10 ms-1 min.

Momentary Sag
30 cycles – 3 s
Temporary Sag
3 s – 1 min.
Instantaneous Swell
0.5 – 30 cycles
Temporary power frequency overvoltage

An overvoltage, at a given location, of relatively long
duration.

Momentary Swell
30 cycles – 3 s
Temporary Swell
3 s – 1 min.
Momentary Interruption
0.5 cycle – 3 s
Supply interruption

A short interruption (up to three minutes) caused by a
transient fault.

Temporary Interruption
3 s – 1 min.

Table 1:  Comparison of IEC and IEEE power quality
parameters

Brief (<1/2 cycle) voltage disturbances are classified as impulsive or oscillatory transients. Sag and swell refer to momentary (from 1/2 cycle to 1 minute) decreases and increases, respectively, in the AC voltage amplitude at the power frequency.

The voltage waveform under steady-state conditions can have a
harmonic distortion problem. Transient disturbances can also affect
this distortion.

You can implement the evaluation of non-steady-state events
(Figure 1 ) with both time-domain and frequency-domain
analyses. In a time-domain analysis, the amplitude, duration time,
rise and fall times, and slew rate (dV/dt) peak of disturbances are
particularly important. Those indices quantify the stress that the
disturbance causes to dielectric materials.


Figure 1:  Transient disturbances for a voltage
waveform

Sags and swells that describe voltage variations, with typical
durations of 1/2 cycle to 1 minute, are rms events. If the rms
value of the voltage wave is below 10% of the voltage's nominal
value (Vnom ), then a sag or swell occurs. Any rms
voltages greater than or equal to 10% above or below nominal value
and last longer than 1 minute are overvoltages or undervoltages.
The effective (rms) value of voltage (v) is calculated as:


The overload on the electric and electronic components due to
the disturbance is expressed by the Specific Energy:


The frequency-domain analysis is based on the FFT of the sampled
disturbance waveform. The most significant information for this
kind of analysis are bandwidth, maximum frequency value, and
spectrum area of the disturbance waveform.

You can evaluate permanent harmonic distortion by means of a
global index. The Voltage Total Harmonic Distortion (VTHD) index is:


where n is the order of the harmonic. The VTHD is widely used to
compare differently distorted waveforms, when amplitude and phase
spectra of the acquired waveforms are not directly comparable.

The Telephone Influence Factor (TIF) is defined as:


where pI is the i-th element of a weighting
coefficient array. The TIF evaluates the effect of power harmonics
on telephone lines and, in general, the degree of distortion of the
power signal (voltage and current). The VTHD and TIF indices don't
express information about the amplitude spectrum diagram of each
acquired waveform; therefore, the IEC standards additionally set
the maximum values for the first 25 harmonics of the
voltage-amplitude spectrum.

The flicker effect is due to a periodic amplitude modulation of
the voltage sinusoidal waveform in the 0.5-25 Hz frequency range.
This modulation produces annoying luminescence fluctuation in an
incandescent lamp. Flicker-effect evaluation is based on a
simulation of the lamp-eye-brain response to the voltage amplitude
modulation, done to obtain an instantaneous perception function. You obtain the short-time Flicker Severity index,
Pst , from the cumulative probability function of the
instantaneous perception, for signals acquired over 10-minute time
intervals.

The Power Quality Measurement System

The Hardware Architecture
The new system's goal is the implementation of a cost-effective and
flexible digital system with data processing and remote
communication capabilities, and a suitable user interface. System
hardware consists of a PC that hosts a data acquisition (DAQ)
board, linked to the power system by a voltage transducer
(Figure 2 ).


Figure 2:  Architecture of the measurement
station

The computer controls the acquisition task, processes data, and
communicates with a remote server. The voltage transducer is a
Hall-effect closed-loop device (Figure 3 ). The transducer
guarantees galvanic insulation from the power network, frequency
bandwidth of DC-150 kHz, accuracy of ±0.5%, and a response
time less than 1 µs. The main features of the DAQ
multifunction board are eight differential analog input channels
with 16-bit resolution, maximum sampling and data transfer
frequencies of 333 kS/s, and input ranges of ±5 V and ±10
V. The host computer is a low-cost PC with LAN board, operating
under Windows 98.


Figure 3:  Hall-effect closed-loop voltage
transducer

The Measurement Software
The instrument uses digital signal processing to perform power
quality analysis of the voltage waveform. Different VIs,
implemented using the National Instruments LabVIEW graphical
programming language, processes the signal obtained through
periodic sampling. With the instrument, you can distinguish the
disturbance simulation VIs, the measurement VIs, the acquisition
VIs, and the data server VI.


Figure 4:  User interface for the transient
disturbance simulation VI

The simulation VIs generate voltage waveforms affected by
transient disturbances (Figure 4 ), amplitude modulation
(Figure 5 ), or permanent harmonic distortion (Figure 6 ). In this way we provide the reference signals to
evaluate the performance of the measurement VIs.


Figure 5:  User interface for the amplitude
modulation simulation VI

The measurement VIs include an Amplitude Spectrum Analyzer
(Figure 7 ), a Transient Disturbances Analyzer (Figure
8
) , a voltage rms (Vrms) Statistic Analyzer
(Figure 9 ), and an instrument to display flicker (Figure 10 ). The time required for the data processing is
about 1 s on the adopted computer.


Figure 7:  User interface panel for the spectrum
analyzer


Figure 8:  User interface panel for the transient
disturbances analyzer


Figure 9:  User interface panel for the Vrms
statistic analyzer

The acquisition VIs are the Multiscan Acquisition Board
Controller (Figure 11 ) and the Continuous Acquisition
Controller (Figure 12 ). The Multiscan Acquisition Board
Controller acquires voltage waveforms simultaneously with a
particular event, such as load transitions, the start of a
high-power induction motor, or a change in the electric
supply-network configuration. The Continuous Acquisition Controller
acquires waveforms continuously for off-line measurements.


Figure 11:  Multiscan Acquisition Board
Controller


Figure 12:  Continuous Acquisition Controller

The Integrated Acquisition and Measurement VI controls the main
parameters of the VIs (Figure 13 ). This VI allows the operator to acquire a
voltage waveform from one input channel, modify the sampling
frequency and length of the sampled waveform for the different
analysis requirements, execute the measurement algorithms of the
power quality indexes, and store only the waveforms whose
disturbance exceed fixed values.

The Integrated Acquisition and Measurement VI can operate both
in local and remote modes. In local mode, the DAQ board installed
on the same PC acquires the voltage waveform. In remote mode, the
measurement PC operates as a client, processing the waveform
acquired by a data server PC, which runs the Data Server VI
(Figure 14 ).


Figure 14:  Data Server VI

The Distributed Measurement System Via TCP/IP Network
Many commercial and industrial companies need to install power
monitors on a large scale. With the aim of managing the remote
communication, we can connect the instrument both to a World Area
Network (WAN) with a modem card or to a Local Area Network (LAN)
with a LAN card. In the second case, a PC Gateway (Figure 15 ) assures the connection to a WAN.

The described measurement system makes it possible to monitor a
large plant and to verify the functioning of each single station
from a remote location by means of high-level user interfaces. The
choice of the TCP/IP Internet connection makes possible low-cost
data communication, in order to evaluate the power quality indexes
in wide electric power plant or in industrial zones. Of particular
importance is the use of the distributed measurement system for the
disturbances source individuation.

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