Measurement Transformers in three-phase networks (Part 2)

Info Letter No. 2

Performance measurement in a balanced three-wire three-phase network with a two-pole insulated voltage transformer

In a symmetrically loaded three-wire three-phase network, a two-pole insulated voltage transformer can only be used for measuring performance if in the measuring equipment, the phase shift of the delta voltage versus the star voltage corresponding to the current is removed by a phase-rotating member. The required phase shift depends on the selected voltages.

For φ = 0:

Connection U12 and I1 → U12 leads I1 by 30°;
Connection U13 and I1 → U13 leads I1 by 30°;
Connection U32 and I1 → U32 leads I1 by 90°;

By using a single-pole insulated voltage transformer that is connected to the conductor and earth the star voltage can be measured. With the use of a two-pole voltage transformer, the difficulties associated with certain operating states are avoided.

Cyclic commutation with current connections in other external conductors

Normally the measuring equipment are provided for the connection of I1. If, however, the measuring equipment is supplied with the current I2 or I3 instead of the current I1, the voltage must also be changed cyclically.

Measurement of the currents in all three external conductors with two current transformers

From the two currents of any two external conductors, in a three-wire three-phase network

i_{1} + i_{2} +i_{3} = 0

the current in the third external conductor can be determined exactly. The required circuit is shown in Figure 2.2.

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