Power quality guide
Minimum, maximum, average values! What is it, meaning, interpretation » IEC 61000-4-30 standard, class A » Time aggregation » Synchronization » Why?
What are digital measurements? Although digital measurements have already become a natural method of processing information about physical quantities, it is still difficult to visualize what this measurement is about, and, consequently, what it results from. The basis of each power quality measurement is a systematic recording (i.e. at specified intervals) of the instantaneous values of voltages and currents, assuming that they have not changed in the meantime. In this way, a set of numbers is created that are the history of the values of individual parameters over time (Fig. 1). Usually, this uninterrupted data stream is divided into longer packets and sets individual characteristics that describe each of the subsequent packets. By collecting only subsequent values of such calculated features, we significantly reduce the amount of space used, while maintaining sufficient knowledge about the essential features of the measured signals. The IEC 61000-4-30 standard for class A analyzers requires that the instantaneous (red) values are phase and frequency synchronized with the frequency of the basic signal, which is to ensure synchronization and comparability of the parameters of the same Fig. 1. Set of successive instantaneous values of the signal signal, measured by different meters. Apart from the frequency, the basic characteristic of the signal is the RMS value. As it should unambiguously represent a signal, it was assumed, in accordance with the cited standard, that the area of two adjacent half-periods will be the smallest interval for determining the rms value (Fig. 2). This is the basic quantity called RMS(1/2) and on the basis of a set of consecutive values, further characteristics of voltages and currents can be determined. With a changing signal level, when the set of successive RMS(1/2) values from which the target average value for a given averaging period is created, there will be higher and lower values. During the whole averaging period, one of them will be the highest and will be marked as the MAX value. One will be the smallest and marked as MIN, while all RMS(1/2) values collected in the averaging interval will allow to determine the average value for the entire averaging period.
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All three characteristics of each signal are recorded at the end of the averaging period. The mean values of voltages and currents are obvious as they always appear as values ≥0. Power parameters are calculated in a similar way, but the standard requires that calculations due to harmonic analyzes should be based on sets of 10-period values. The resulting numbers represent the average active and reactive powers in each of the 10 periods, which with the averaging time, e.g. for 1 s, will form a set of 5 changing values of the 10th period. As for voltages and currents, among these 5 you can find the largest, for example, PMAX, and the smallest PMIN and take the average of all as PAVG. However, there is a fundamental difference in terms of voltages and currents. The 10-period power is a positive or negative number, so the limit value search becomes more complicated, because two extreme values can be determined, but it is also possible to deterFig. 2. Half-period intervals for RMS1/2 mine the limit variability separately for P>0 and separately for P<0. In the case of calculating the average value, there is also a mutual decrease in the average value, when in the calculated period averaged there is a change in the sign of the power, i.e. a change in the flow direction. This is the basic disadvantage of most power analyzes carried out even with advanced analyzers, if they do not have extensive functionality of four-quadrant calculations. Simultaneous recording of the minimum, maximum and average values provides the possibility of reproducing the range of signal variability, allowing for a quick assessment of the occurring restless phenomena. Among the registered parameter settings, there is also a fourth value available: instantaneous. It makes practical sense when we use fast waveform recording with half-period averaging time. Only the RMS(1/2) value for voltages and currents is available. In other applications, with averaging times of 200 ms and higher, the instantaneous value recording is inactive.
Three examples of negative effects of Min, Max, Average statistics The mathematics used to calculate statistical values can sometimes give unrealistic results that do not represent physical phenomena but, if misinterpreted, lead to misunderstandings and wrong decisions. Example I Virtual voltage surge (Fig. 3) caused by prolonged no-load self-discharge of the circuit at the moment of disconnecting the voltage. Usually, the voltage sine wave in the network has an RMS value of 230 V with an amplitude of approx. 315 V. If at the moment of the peak there is a voltage loss due to a power interruption, and the voltage from 315 V drops very slowly, the RMS(1/2) may drop to values smaller than 315 V. At event thresholds of Un +10% it generates a false voltage increase event, although the sine wave shapes did not show any increase.
RMS
instantaneous
Fig. 3. Apparent voltage increase
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Example II When the active power in the averaging period is positive for a part of the period, and negative for the rest of the time (Fig. 4), then with a bit of luck the positive and negative powers may balance each other. It will result in the average active power in the averaging period =0. It is a characteristic case when the so calculated average active power placed in the denominator of the tangent coefficient will result in the calculation of unreal values and, consequently, the generation of misunderstandings. Example III It is a phenomenon similar in course, but related to reactive power. With an analyzer for recording the average reactive power, if a restless load occurs, which during conFig. 4. Change of the sign of active power (black) and tg (φ) = 1622 sumption will change the nature of reactive power many times during the averaging period, at the end of this period, the calculated value of the average reactive power will be a fiction. Further parameters calculated with the participation of such reactive power may be a source of incorrect decisions, which may lead to unnecessary costs. This is especially true for analyzers that do not have a full four-quadrant analysis. There are two solutions to this problem, especially in terms of reactive power compensation: the first is the shortest averaging times causing, however, very fast memory consumption, the second is the use of class A analyzers of the PQM-702/3/10/11 family and class S, i.e. PQM-700 and PQM-707, in which the energy counters are updated after every 10 periods, which makes the choice of averaging period independent. Simultaneously, the instruments operate in four quadrants, which further simplifies the analysis.
A few simple steps to diagnose and assess load disturbance The most important thing is to reliably record energy. Any PQM analyzer with up-to-date software can be used. In order to unambiguously register the limits of variability of a parameter (Fig. 5): 1. In the recording settings, for the measured parameters, check the Minimum, Maximum and Average fields as active. 2. After connecting the analyzer, send the modified settings. 3. Register the selected moment of work of the object using the START / STOP function. 4. Read the results with Sonel Analysis and save them to hard drive.
Fig. 5. Min., Max. and Average recording settings for phase voltages
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Analysis of energy measurement results After loading the measurement file containing energy recording using the Sonel Analysis software, you should: 1. Press the Measurements button. 2. Find on the list the parameters you are interested in and select the Average, Minimum and Maximum values. (Fig. 6). 3. Select parameter columns for the chart. 4. From the Graphs list, select the Time Graph option. Wait for the plot to be generated (Fig. 4). 5. By placing markers 1, 2, 3 in characteristic places on (as in Fig. 4) you can: a. Read the time and the value of the parameter for each of the three markers pictured with a circle icon with a number. b. Based on the results of the differences, determine the values of the parameters at individual points of this signal.
Fig. 6. Selection of parameters for the time diagram
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