Measurement of currents and voltages in three-phase circuits

When measuring currents and voltages in three-phase current circuits, in most cases they are satisfied by measuring one of the line currents and one of the line voltages. In this case, the measurements are carried out in the same way as in single-phase current circuits. In low-voltage circuits, a single switch voltmeter is sometimes used to measure three line voltages (Fig. 1).

If it is necessary to measure three line currents in a three-wire three-phase circuit using current transformers, then two current transformers are sufficient to measure three currents. This follows directly from the property of the sum of line currents, according to which the sum of three line currents is zero:

and therefore the sum of two line currents is equal to the third-order current taken with the opposite sign.

One of the possible connection schemes is shown in fig. 2.

Rice. 1. Circuit diagram of a voltmeter with a switch.

Rice. 2. Connection diagram of three ammeters through two current transformers.

As can be seen from the diagram, current ia flows through the first ammeter, ib through the second, therefore, the current in the third ammeter, equal to the sum of two linear currents ia and ib, is equal to the current of the third order -

If it is necessary to measure three line voltages of a three-phase three-wire circuit using voltage transformers, then two voltage transformers are sufficient for measurement, which directly follows from the properties of the sum of line voltages. The sum of the three line voltages is zero:

therefore the sum of two line voltages is equal to the third line voltage taken with the opposite sign.

One of the possible schemes for connecting three voltmeters through two voltage transformers to measure three line voltages is given in fig. 3.

Rice. 3. Connection diagram of three voltmeters through two voltage transformers