To determine what kind of metering system is required to measure a specific service, we have to give a look on Blondel's Theorem. In a further simplification, It provides the information about the number of wattmeters required in a polyphase system for the measurement of total power consumptions.
So, what does Blondel’s Theorem say and what does it do for us?
If the power is provided to a system through N number conductors, The total power is measured by adding the readings of individual N wattmeters.
The wattmeter connection in such a way that the current transformer(CT) of wattmeter is in each line and and the corresponding voltage transformer(PT) is connected between that line and a common point.
If the common point is located on one of the lines, Then the total power consumption could be measured by N-1 wattmeters.
- The theorem relates to power measurement in a polyphase circuit.
- According to this theorem, the minimum number of watt-meters required to measure the power of a polyphase system is one less than the number of wires in the system.
- Thus for 3-phase,4-wire system, three wattmeters are required. But for a 3-phase,3-wire system, only two wattmeters are required
Sources: Learnmetering ElectricalVoice
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