Electric network monitoring and control
The list of functionality of modern power supply network monitoring systems includes the following: power monitoring on generators, network load balancing and distribution, protection and data collection. The systems provide protection for transformers, fuses and other components, perform diagnostics of the power supply network and detect failure conditions. In order to save energy, the system dynamically aligns the load and monitors power quality. Standardized accuracy is dynamically required to monitor power delivery in real time, detect failures, and balance the network load. Thus, according to IEC 62053 (European Union standard) for equipment of class 0.2 the minimum required accuracy of measurements is 0.2% of the nominal values of current and voltage. In order for the power factor indicators to be sufficiently accurate, a phase coordination factor of at least 0.1% is required at the time of measurement.
Typically, when distributing multiphase power (e.g., three-phase), energy supplier organizations use a star or so-called Y-shaped circuit, due to the peculiarity of connecting windings of three transformers to one point in the form of Latin letter Y. The line voltages are phase shifted by one third of a turn, i.e. by 120°. In the case of an equal load in each of the three phases, the system is considered to be balanced, therefore there is no need for current through the neutral conductor, which is connected to the point of connection of the windings of the three transformers and allows the unbalanced loads on the connection lines to be coordinated.
Voltage and current readings at different phases in power supply monitoring systems are monitored using analogue-to-digital converters. For each phase in the classical scheme, the power readings are measured using current transformers and voltage transformers (4 pairs in total for each phase and the neutral pair).
The digital signal processor calculates the power factor values based on the measurement results and also evaluates the parameters of the system itself (active, reactive, total energy). The frequency of sampling is regulated by regional and international standards. Minimum requirements include a wide dynamic range (at least 90 dB) and sampling rate of 16 thousand samples per second. These parameters allow analysis of multiple AC network harmonics, and identify short-term failures such as throws or power failures.
To solve the problem, nowadays precision analog-to-digital converters with the function of simultaneous multi-channel sampling are used, which are synchronized within the information system itself, which reduces the volume of system costs.
16-bit analog-to-digital converters with several channels (4, 6 or 8), with a minimum signal-to-noise ratio of 92 dB are used. 24-bit inverters with 4 channels and a minimum signal-to-noise ratio of 117 dB can be used.
If the input impedance of an analog-to-digital converter is sufficiently high, the transformers can be connected directly to the converters themselves. If the input impedance level is low, a precision low-noise amplifier is used.
Due to the fact that environmental factors (temperature, humidity, drift, etc.) exist at the place of operation, the system performs self-test and correction at certain intervals to compensate for them.
The excessive adjustment range in this case is eliminated by randomly setting the upper and lower voltage of the digital-to-analogue converter using calibrated meters and digitally controlled potentiometers. For example, with an upper value of 2 V and a lower value of 1 V, it is sufficient to use an 8-bit device to implement step 0.0039 V in this range. For automatic recovery of the calibration values, digitally controlled calibration transducers and potentiometers are used, which have an internal non-volatile memory from which the data are downloaded.
References
Della Giustina, D.; Andersson, L.; Casirati, C.; Zanini, S.; Cremaschini, L. Testing the Broadband Power Line Communication for the Distribution GridManagement in a Real Operational Environment // Proceedings of the International Symposium on Electronics, Electrical Drives, Automation and Motion. – Sorrento, 2012. – P. 785–789.
105 Machine-to-Machine communications (M2M); Applicability of M2M architecture to Smart Grid Networks; Impact of Smart Grids on M2M platform. - ETSI TR 102 935 v2.1.1. – 2012. -Vol. 9. – 58 p.
To be continued
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