We determine the resistance of the loading resistor using expression 4

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2. After we have determined the optimal three-phase power of the VT, we need to determine the optimal power for each phase. This is done because three single-phase additional load resistors will be installed. If you install one three-phase additional load resistor, then the meter power must be three-phase (in the passport for meters, it is usually presented as single-phase. To get three-phase power, multiply by 3. In my case, S = 3*10 VA = 30 VA). The power of the VT windings is presented as three-phase. I draw your attention to this, since this is a very important point in choosing resistors.

S A,B,C min = Sn2/3 = 112.5/3 = 37.5 VA

3. We determine the power of the loading resistors using expression 3 [L1, p.8]:

Sadgr. = S A, B, C min – Smeas.2 = 37.5 – 10 = 27.5 VA

4. We determine the resistance of the loading resistor facebook data using expression 4 [L1, p.9]:



where: Unom. = 100/√3 = 57.8 V – phase voltage of the secondary winding connected according to the “full star” scheme, line voltage is equal to – 100 V.

5. We first take three single-phase additional load resistors of the MP 3021-N-100/√3V-30VA type, with a power of Sload.1 = 30 VA, and calculate the resistance of the selected resistor.

We determine the resistance of the selected additional load resistor with a power of 30 VA

The resistance of the selected resistor is almost the same as the calculated one, which means the choice is correct. Why do we still calculate the resistance? This is because resistors are selected by resistance, not by power.

Technical characteristics of loading resistors type MP 3021-N

5. Let us determine in percentage terms how much the secondary winding of the VT will be loaded with additional resistors: