Switching on electrical measuring instruments by means of measuring transformers

In wattmeters, meters, phase meters and some other devices, the deflection of the moving part (in counters — the direction of rotation of the disk) depends on the direction of the currents in their circuits. Hence their inclusion through current and voltage measuring transformers it is necessary to make it so that the currents in the circuits of the devices have the same direction as when turning on devices without transformers.

For the correct connection of the devices, the terminals of the windings of the measuring transformers are marked with special signs. The terminals of the primary winding of the current transformer are marked with signs L1 and L2 (line) and the corresponding terminals of the secondary winding I1 and I2 (measuring device). The terminals of the primary winding of a single-phase voltage transformer are labeled A and X, and the secondary winding is labeled a and x.

When wattmeters and other devices are switched through measuring transformers whose readings are affected by phase shifts between currents and voltages, the angular errors of the transformers affect the readings of the devices.

Always remember the following when switching instruments with instrument transformers:

1. The generator clamps of wattmeters and other devices must be connected to the terminal «a» of the voltage transformer (parallel circuits) and to the terminal «I1» of the current transformer (current circuits), and when the current circuits of the devices are connected in series, as shown in the figure.

Connection scheme of electromechanical devices by measuring current and voltage transformers

2. In the presence of primary current the secondary circuit of the current transformers must not be opened… The secondary winding of the voltage transformer must not be short-circuited.

For the safety of the operating personnel and the protection of the devices, the secondary circuits of the measuring transformers must be grounded as shown in the figure. The grounding of the secondary circuits of the transformers eliminates the possibility of the appearance of high voltage in the circuits of the devices relative to the ground in case of damage (breakdown) of the insulation between the windings of the measuring transformer.