Power Quality basics
Multi-point measurements Requirements, causes and mechanisms
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Power Quality basics
Multi-point measurements Increasingly, power quality analyses and further identification of the causes and sources of interference are possible only based on the comparison of parameters in characteristic network points. Nevertheless, reliable conclusions based on simultaneous measurements from several analysers, even of the top A-Class, are not at all obvious. In some cases, they are simply insufficient to be certain of the full compatibility of connections and, consequently, of the reasonableness of the results of comparisons.
Requirements needed to avoid mistakes To ensure clarity of the compared information and one hundred percent certainty that such clarity has just been achieved, the fulfilment of specific conditions is required: 1. Synchronising the sampling with the measured signal in typical A-class analysers 2. Accurate and consistent time aggregation of measurement results to less than 100 ¾s, 3. Unambiguous identification of the compatibility of analysers’ connections to the same phases in tested multiphase networks. Point 3, in particular, is very time-consuming, and even difficult or simply impossible to be achieved in typical A-Class analysers. However, based
Power Quality basics
on the dedicated feature supporting remote identification of the compatibility of matching connections, available in A-Class PQM power quality analysers, this process is very simple, fast and effective for one person, even over a long distance.
Why so many requirements are necessary? To perform a measurement in many places at the same time (Fig. 1), while maintaining the possibility of reliable comparison of the results, it is necessary to ensure perfect compatibility of moments when the values of voltages and currents are recorded in all analysers. It should be noted that simultaneity is the foundation for calculations in electrical engineering. An example may be the Ohm’s law, applicable to a long a power cable. When you know the voltage on both ends
and the current in the cable, you can determine the cable impedance. However, it is not until we use in the calculations the results measured on the same conductor at the same time, that, for fundamentals, we arrive at the actual impedance value of such conductor for the fundamental. Similar unambiguity will be achieved by calculating energy balances, voltage drops along the line, interference distribution along the line or by directly comparing waveforms.
Load Load Load
Load
Load Load
Fig. 1 Example of multi-point GPS-synchronised diagnostics in an extensive distribution network.
Power Quality basics
Analysers PQM-702(T)/703/710/711, compliant with EN-61000-4-30 in A-class These analysers guarantee, by using the PLL loop, the hardware tracking of fundamental frequency of the tested network and detection of its periods to ensure that blocks consisting of a 10/12 periods include precisely 2048 instantaneous values. As a result, all analysers gather instantaneous values of voltages and currents at the same sampling times, as they are synchronised by the measured network itself. This is a requirement met by all analysers in accordance with A-class requirements.
The precision of time aggregation required from A-class equipment is significantly below 20 ms The basic requirement of time uncertainty below 20 ms in A-class devices, synchronised by an external GPS signal, is not a sufficient condition. Even if an analyser with GPS ensures accuracy of 1ms, the comparability of effective values RMS1/2 is maintained, which is used for statistical analyses. Nevertheless, when the phase shift of 30⁰ corresponds to the time of 1.66 ms, even the uncertainty of 1ms is insufficient for comparisons of waveforms. The sources of time synchronisation: DCF77, GPS, read by a serial port without additional hardware support and SNTP synchronisation through Ethernet or Wi-Fi are also insufficient. Only the use a built-in GPS receiver with the uncertainty of time readout significantly below 100 µs ensured by A-Class PQM analysers, provide full compatibility of waveforms with accuracy to the sample. This feature and mechanisms described above provide compliance of time aggregation, every RMS1/2 value and waveforms, i.e. unambiguous compliance of time and transient values of results gathered in individual measuring points.
Identification of the compatibility of analysers’ connections to the same phases As the experience in real objects shows, matching the compatibility of analysers’ connections to the same conductors of a cable line or an overhead line based only on the description of the cables is highly uncertain due to frequent errors and mistakes. An effective solution to this problem is the dedicated innovative feature of remote support for phase adjustment, available only in A-class PQM analysers. The standard equipment of analysers in the form of GPS synchronisation module and a GSM communication module allows for quite easy, clear and quick verification of the compatibility of analysers’ connections to the same conductors of a cable, even at a considerable distance from one another. It is sufficient to compare simultaneously the observed current synchronised waveforms in tested points. In the absence of a SIM card for remote reading, it is even possible to adjust observations via direct telephone communication.
Power Quality basics
Mechanism for remote phase adjustment
Fig. 2 Markers confirming waveform synchronisation with GPS in a current reading
Analyser software 1.34 or later and Sonel Analysis 4.4.0. or later are required. The basis for the clear adjustment of phases is to connect analysers to any branch of the same network and to ensure that the time in the used analysers is synchronised with the GPS time (Fig. 1). An indicator for the analyser synchronisation is the green colour of the text of date and time displayed on the LCD screen of each analyser. Waveform images of current readings with Sonel Analysis software constitute the basis for comparison. When it sends waveforms, each analyser freezes the phases of the observed records of voltages and currents with respect to the left edge at the time of passage through every 30 s in a minute acc. to the GPS time, i.e. by xx:xx:00 and xx:xx:30 of every minute. Until the next synchronisation, the images of voltages and currents will only change their shapes. The visual confirmation of this synchronisation status on the PC is the visible
vertical marker at the beginning of each signal, near the left edge of the screen (Fig. 2). Assuming that one of the analysers connected in accordance with the description of line conductors is considered as the reference analyser, then the second analyser synchronised basing on the comparison of indications identifies unambiguously any errors in connections or descriptions. Only after adjusting the connections and obtaining full compliance of waveform images, the full compatibility of the connection of the two analysers to the same phases may be guaranteed. This is the last prerequisite for simultaneous multi-point diagnostics. It enables the user to compare values of parameters on individual conductors of the overhead line or cable, the distribution of parameter changes along the lines, in order to facilitate finding the location of sources of certain interferences.
Power Quality basics
Example of complete compatibility of connections
L1
L2
L3
Fig. 3 Full compatibility of connections – UL1, UL2, UL3 voltage waveforms have the same positions on the screens
Necessary steps of waveforms verification (Fig. 3): 1. The necessary occurrence of vertical synchronisation indicators as left edges of individual waveforms. 2. The direction of sequences (delays) of phases of each analyser is correct, which means that the correct direction of phase rotation is maintained. 3. U L2 test leads of both analysers are connected to the same phase of the power line. 4. Other leads: U L3 and U L1 of both analysers are also connected to the same phases of the power line. 5. Due to the potential changes in the actual network frequency, every 30 seconds, step movements of signals with respect to the left edge will be visible.
Power Quality basics
Example of incompatibilities of analyser connections to individual cables of the power line
L3
L1
L2
Fig. 4 Compatibility of connections for phase L1 and incompatibility of L2 and L3 – positions of UL2 and UL3 are crossed
Necessary steps of waveforms verification (Fig. 4): 1. The necessary occurrence of vertical synchronisation indicators as left edges of individual waveforms. 2. The top (reference) waveform indicates the correct sequence of delays of phases L2, L3 with respect to L1, the rotation direction is correct. 3. The bottom waveform indicates the opposite direction of phase sequences (delays) with respect to L1 and the connection needs to be adjusted. 4. Between the top (reference) and the bottom waveform there is compatibility of connections of U L1 leads to the same phase. 5. Other leads: U L2 and U L3 of the bottom chart clearly require the replacement of lead connection to the line due to the opposite rotation direction as stated above and the incompatibility of moments when maximum values occurred in comparable time points. 6. After making the necessary changes, the images on the analysers should present full compatibility of the connection (Fig. 3).
Power Quality basics
Final conclusion To compare the values of parameters during multi-point distributed diagnostics, for instance along a loaded power line, we need to be certain that the measured values in both points refer to the same conductor, i.e. the same phase. The application of standard A-class analysers requires accurate GPS synchronisation and performance of additional preliminary registrations and a complex comparative analysis of results. With analysers PQM-702(T)/703/710/711 by SONEL S.A., owing to a GPS receiver as standard, which guarantees precise synchronisation of waveforms, and owing to an innovative visualisation mechanism for the status of reliable synchronisation of instantaneous voltage current images, as soon as after 1 minute it is known if the compatibility of phase adjustment in both measurement points is maintained. Additionally, when remote reading via GSM is used, one person is sufficient to adjust phases in an accurate manner. Only after completion of this important activity, the multipoint diagnostics is sensible and the interpretation of results is substantive
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