EXPERIENCE OF DETERMINING ELECTRIC ENERGY FAILURE
Energy accounting is an integral part of any enterprise. First of all, it is necessary for the implementation of the cash settlement between the energy supplying organization and consumers. The main purpose of commercial electricity metering is to obtain reliable information on the amount of consumed (issued) electrical energy. In three-phase four-wire systems, three-element electricity meters are used to measure consumed electricity, which are a measuring watt-meter system and belong to integrating devices. An electricity meter is a device that reacts not only to the energy value, but also to the direction of its transmission, therefore, when the meter is connected, it is necessary to ensure both the correct directions of the currents and their correct combinations with the voltages in each element of the meter. Active energy measured by the meter in time t 2 - t 1 , is defined as:
where the index “f” denotes phase voltages and currents. Let us determine the magnitude of under-counting of electricity with some of the most common faults in the elements of the metering metering complex and errors in the connection of electricity meters.
Break of current circuit or secondary winding of current transformer (CT). The current in the secondary circuit is zero, suppose in the phase indicated by the index "1" (I 1f = 0). With a symmetrical load, the active energy measured by the meter, according to (1):
those. will be 2/3 of the actual value. The same magnitude of undercounting is observed in the absence of voltage on one of the elements of the meter (on the parallel winding of the meter). The labeling in current circuits was reversed, as a result of which the wires going from the TT were reversed, for example, the phases with the index “2”. In this case, the current flows in all phases, but in phase "2" has the opposite direction. In other words, on the vector diagram (Fig. 1), the current of phase “2” is turned through 180 0 , therefore, the energy taken into account by the counter:
Figure 1. Vector diagram of currents and voltages on the meter when connecting the TT phase "2" with reverse polarity with active-inductive load.
those. with a symmetrical load will be 1/3 of the actual value.
To check the correctness of the meter connection, it is enough to remove the vector diagram and build it, compare it with the expected one, which depends on the nature of the load. Vector diagrams are most conveniently taken with VAF-85M or VAF-A instruments. To get a reliable measurement result, the vector diagram should be taken directly from the meter terminals, which are usually under the seal of the energy supplying organization, so you can use a simplified method of checking what you need: measure all phase voltages, as well as currents passing through the primary windings of the CT of all phases, determine the total power by the formula:
- measure the speed of the disk of the active electricity meter or the number of pulses of the light indicator (for electronic or microprocessor meters) N during T (enough 30-60 seconds), then determine the active power using the formula:
where K TT - the transformation ratio of TT; n is the gear ratio of the counter. In the presence of a reactive energy meter, make measurements of reactive power Q and calculate it using a formula similar to (5), as well as determine the total power "by counters"
- compare the values of the total power, determined by the formulas (4) and (6), which must coincide with the correct connection of the meter and the operational elements of the measuring complex of electricity metering. In the absence of a reactive energy counter, the values of S and P are compared, which, if the meter is properly connected, should differ from each other by no more than Cos ( f ) times. The smallest error introduced by the change in load is achieved by “simultaneous” measurement of phase currents and the frequency of rotation of the meter's disk. Using the proposed simple measurements and calculations, it is possible to conclude that the meter is connected correctly and that the metering metering complex (current transformers, measuring circuits from the CT to the meter, parallel meter windings, voltage circuits) and the amount of underestimated electricity are correct.