Measurement and assessment of system perturbations
5.2 Sampling systems 1 General characteristics
With the introduction of digital technology, instruments operating in the time domain have been pushed into the background more and more. The technology used in the measuring instrument market has improved rapidly. Computers have become increasingly powerful, characterised by a growing number of computing operations per time unit. Digital signal processing is also constantly entering new fields with regard to sampling frequency and amplitude resolution. Despite this, their use remains cost effective.
The two quantities essential for digital signal processing are the sampling fre- quency and amplitude resolution.
Figure 5.2 shows how an analogue measured signal is converted to a value- continuous sampling sequence by sampling in the time range. If the amplitude is then converted to discrete amplitude values, e.g. using an analogue-digital con- verter, this produces a value sequence which can be processed by a computer or digital signal processor (DSP).
5.2.2 Basic structure of a digital measuring instrument
The basic structure of a digital measuring instrument consists of a few com- ponents (see Figure 5.3). The measured signal is decoupled by an input adapter.
Figure 5.1 Frequency range of perturbations
This assembly limits the frequency range of the measuring instrument to the working range and protects the electrically-sensitive microelectronics. The frequency band is limited by a low-pass filter, i.e. an antialiasing filter.
An A/D converter converts the continuous analogue signal to a sampling sequence which is discrete with regard to both amplitude and values. The sample and hold elements are fitted between the input adapter and converter. The pur- pose of this component is to keep the signal to be measured constant for the period of the A/D conversion.
Figure 5.2 Amplitude and time discretisation
a) continuous in time/continuous in range b) continuous in time/discrete in range c) discrete in time/continuous in range d) discrete in time/discrete in range
The sampling sequence provided by the A/D converter is then further pro- cessed by the arithmetic logic unit. Nowadays, this is a microcontroller, a digital signal processor or a complex processor system. This arithmetic logic unit also controls the display and generally any memory units.
The sampling frequency and other control signals for the arithmetic logic unit, and also for any display, are generated by a controller. This can consist essentially of a crystal generator with corresponding divider stages or a phase-locked-loop (PLL) assembly. The generation of sampling and control frequencies using a crystal generator produces sampling sequences where the sampling impulses always occur precisely at the same intervals. This means that time sections from a recorded sampling sequence can be readily determined. This type of sampling frequency generation is used on oscilloscopes and other recording instruments, such as transient recorders. If, however, a sampling sequence is required which is in a numerically-fixed relation to a dominant frequency component in a measured signal, and if the frequency of this signal is subject to certain fluctuations, only a PLL can then be considered for gener- ation of the sampling frequency. The structure of this component is shown in Figure 5.4.
In this case it should be noted that the frequency in the grid system of the UCTE can fluctuate within the 49.95 Hz to 50.05 Hz range [1] (see Figure 5.5).
A direct assignment of measured signals to specific time points is no longer possible where a PLL is used. Either the frequency of the PLL or the corres- ponding equivalent numerical value must be stored. The timing can then be reconstructed using these values. The PLL is used mainly for harmonics analysers and flicker meters.
5.2.3 Transient recorders
Recording instruments or transient recorders are used to measure voltage fluc- tuations. The voltage fluctuation must then be evaluated using the time course of the measured voltage. The principle of measuring instruments for recording voltage fluctuations is shown in Figure 5.6.
The characteristic features of a transient recorder are its amplitude resolution, its sampling frequency and its memory depth. The amplitude resolution is in the
Figure 5.4 Phase-Locked-Loop unit (PLL)
12-bit to 14-bit range (4.096 levels up to 16.384 levels). The sampling frequencies range from approximately 10 kHz up to 100 kHz. Transient recorders with sampling frequencies of a few megahertz are now available for recording very fast signals, e.g. transient overvoltages. These instruments usually have an amplitude resolution of between 8 bits and 10 bits (256 levels up to 1.024 levels). The recorded measured values can be printed directly. Special program pack- ages or general statistical or tabular calculation programs can be used for further analysis of the measured values using a computer.
Figure 5.5 Relative frequency of UCTE network frequency
(measuring time: one year)
5.2.4 Harmonics analysers
Various kinds of measuring instruments have long been in use for measuring harmonics. Harmonics analysers can, for instance, be designed on the basis of selective filters coupled with r.m.s. value measurement. Such instruments are now rarely found in use. Because of the technical developments in computer technology, instruments consisting of sampling systems and which calculate harmonic components using Fourier transformation, or discrete Fourier trans- formation (DFT), are more commonly used.
A harmonics analyser which determines the harmonic components using Fourier transformation consists of the following components:
•
measured signal coupling/amplifier,•
antialiasing filter,•
sample and hold elements,•
multiplexer (if required),•
A/D converter•
computer unit,•
display unit,•
storage medium,•
unit for generating the sampling frequency and a controller.The named components are combined in principle as shown in Figure 5.7. The measured signal can be input either galvanically separated or galvanically coupled. It is then amplified for the individual measurement ranges so that the best possible control of the A/D converter results. These components can also compensate for the fundamental component. The measured signals are then applied to the antialiasing filter and band-limited. After this initial processing, the signals are then passed to the sample and hold elements.
The assemblies described up to now are provided for each measurement
Figure 5.7 General block-structure of a measurement system for harmonics
channel. Depending on the design of the instrument, the measured signals of the individual channels are either passed via a multiplexer to a central A/D converter, or each measurement channel may have its own converter. A/D con- verters in use today mainly have a resolution of 12 to 16 bits (4,096 to 65,536 quantisation levels). The digitalised measured values are applied to the com- puter unit where they are analysed. From here, the measurement results are passed to the display, statistically processed and stored as necessary. The meas- uring instrument has a controller which contains as an essential component, a unit for generation of the sampling frequency. This is generally a precision timebase combined with a PLL.
5.2.5 Flicker meter
Either a transient recorder or flicker meter can be used to measure flicker levels. The flicker meter shows the instantaneous flicker impression pf and flicker level
Pst or Ast relative to an adjustable measuring interval (1 minute, 5 minutes or 10
minutes) as a direct measured value. The principle of construction of a flicker meter using digital technology is shown in Figure 5.8 [2].
The flicker meter consists of various functional blocks [3]. The first block regulates the amplification of the measured voltage. The measured voltage is corrected to 100% via a first order low-pass filter with a correction time of 60 s. This step enables the voltage changes to be considered as relative quantities.
Block 2 considers the squaring in the lamp transmission function (Φ = U2).
The signal is demodulated in a low-pass filter in block 2. This is a Butterworth low-pass filter of the sixth order. At the same time it suppresses the signal components with a double modulation frequency produced by the squaring. In block 3 the low-pass characteristic of lamps is simulated and this block also shows the form filter with a band pass characteristic for simulating the transmission function of the human eye.
F(s)= kω1s s2+ 2λsω2 1 + 1+ s ω2
冢
1+ s ω3冣 冢
1+ s ω4冣
(5.1)The parameters are:
k = 1.74802 λ = 2π 4.05981 ω1 = 2π 9.15454 ω2 = 2π 2.27979 ω3 = 2π 1.22535 ω4 = 2π 21.9
Block 4 contains a variance estimator which is achieved by squaring with first order low-pass filtering (τ = 300 ms). The signal of the instantaneous flicker level
Figure 5.8 General block-structure of a flicker meter with signals
pf is present at the output of this block. This level is statistically evaluated in
block 5 using the Pst disturbance evaluation method (see section 3.4.2).
5.2.6 Combination instruments
Various instruments can be used for measuring and investigating the voltage quality and determining the system perturbations, depending on the individual aspects to be considered. Table 5.1 is a summary of the assignment of instru- ments to the individual aspects of the determination of the voltage quality by measurement.
Because of the high degree of integration that can be achieved with today’s technology, measuring instruments are available that are a combination of tran- sient recorders, harmonics analysers, flicker meters and oscilloscopes. These instruments are sometimes able to perform the individual measurement func- tions simultaneously. Furthermore, these instruments are fitted with special analysis functions and suitable software to evaluate the measurements.