EXPERIENCE OF DETERMINING ELECTRIC ENERGY DISCIPLINE
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 about 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 meter element. 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 frequently encountered malfunctions of 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 when there is no voltage on one of the meter elements (on the parallel winding of the meter). The labeling in current circuits has been confused , as a result of which the wires going from the TT have been 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. The vector diagram of currents and voltages on the counter 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 suffices 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 counters) N during T (enough 30-60 seconds), then determine the active power by 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, measure the 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 intact elements of the electricity metering measuring complex. 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, we can conclude about the correctness of the electrical meter connection and the health of the metering metering complex (current transformers, measuring circuits from the CT to the meter, parallel meter winding, voltage circuits), as well as the amount of underestimated electricity.