I energy meters We sell them all MID certified and therefore suitable for energy pricing, therefore it is necessary to pay attention to the choice and wiring of the CT.
The amperometric transformer or TA, in its simplest form, consists of two windings (primary and secondary) which are isolated from each other and of a magnetic core on which the windings are wound. The primary winding is traversed by the current to be measured and then connected in series in the power circuit, while the secondary winding feeds the measuring instruments.
For the measurement of the powers, in particular the active one, it is essential that the current value and the secondary phase angle of the CT are correct so that the power measurements are also correct. An improper power rating of the CT and / or a cable of the CT connection under dimensioned, can generate considerable distortions and attenuations of the signal and consequently distort the measurement.
The TA is characterized by nominal sizes that define its functionality. The nominal frequency, the primary current to which we normally assign integer values (for example 10A, 20A, 100A, ...) and the secondary current that is chosen in relation to the characteristics of the measuring devices are usually normalized the values of 1A, 2A, 5A. The transformation ratio is given by the ratio between the primary current and the secondary current.
For three-phase circuits, the absorbed active power, che è quella considerata al fine della fatturazione, is obtained by adding the powers measured on the individual phases. For each phase, considering the voltage Vf between phase and neutral, the current If and the phase shift angle (between voltage and current) φ, the following applies: Pf = If * Vf * cos φ.
If for simplicity we assume 0 phase shift the total power absorbed is given by the sum of the three phase powers therefore Wtl = (IFR + + IfS IfT) * Vf. For the calculation of the CT to be combined with our meter we must identify the phase current to be measured. If, for example, the power consumption is 60kW, the phase / neutral voltage is 220V we will have a total current of 60000 / 220 = 273A which is equivalent to 91A for each phase.
L'SDM630 allows you to select between two current values (1A, 5A), so we will have to choose a CT with a transformation ratio such that the maximum current value to be measured on the primary corresponds to a current on the secondary that is as close as possible to one of the two values accepted.
So I should choose a TA from 100 / 5 then with Ip / Is ratio of 20. Now I have to set the TA data in the counter. Select the CT parameter, set Ct2 (Secondary current to 5A) and the parameter Ct Rate (Transformation report to 20).
Warning! On MID certified meters the setting of these two parameters is possible only once. This is to avoid the user being able to reset them by distorting the measurement and therefore the energy billing.