AIS spol. s r.o. Measuring and information technology for power engineering

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Distributed synchronous measurement in electrical networks:

The quality of data obtained from measurement in electric networks of all levels is affected, apart from other influences, by time diversity of individual information. Result of this is limit of exactness of such data sets analysis. Synchronous measurement, namely the measurement of synchrophasors, represents possibility how to reduce or eliminate these errors. Data obtained by synchronous measurement represent significant input for the new trends in dispatcher control of distribution networks.

The need of reliable electricity supply poses high claims to modern systems of network control, to support systems and to software tools for secure, effective and cost optimal network control. The control is based on status vector of electric network. Available redundant measurements are used for estimation of this vector, i.e. voltages, currents, real and reactive power. However, the calculation algorithms are valid only for simultaneously measured data. The inexactness of measured quantities comes from errors of individual parts of measuring chain in the switching station control system. Another source of errors is asynchronous measurement of measured quantities and data delay in communication lines. All these errors mean aggravation of exactness of calculations.

Relative phasors between network nodes are specific variables. Methods of their measurement are synchronous by principle and they are not dependent on the error of amplitude. The demands on their quality differ according to the way of their utilization.

Optimization of utilization of distribution networks with high load raises need of real time knowledge of actual steady operation and dynamic transitions. Asynchronous real time measurement brings considerable errors to subsequent control processes and control system calculations. Devices for synchronous measurement of voltage and current phasors, improved communication between objects and dispatching centre and powerful computer hardware represent the solution of these problems.

Synchronous measurement plays key role for the estimation of high voltage network. Practical experience shows that delay of information concerning changed measurement can significantly damage the results of estimation. The benefit of expanding the measured group by voltage synchrophasors is possible to evaluate by comparative calculation of estimation criterion. Adding of several number of phasor measurements can significantly reduce consequences of traditional asynchronous measurement.

Wide Area Measurement Systems (WAMS) represent new way how to solve power transfer and distribution problems. These systems are intended for monitoring of wide networks by extensive measurement of synchronous phasors in crucial network points. WAMS consist of network of GPS synchronized Phasor Measurement Units (PMUs), system of data transfer and collection using various types of communication similar to that of SCADA systems, data storage and processing and collection of applications performing on-line and off-line data presentation or consequent processing.

Particular applications can perform important tasks like monitoring of network stability, early fault warning, support of switching, evaluation of power quality etc.

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