Energy Meters are one of the important instrument to account for energy imported / exported in an industry. In Power Generating Stations, the cumulative reading of such energy meters are used for the calculation of total Million Units (MUs) of energy exported. On the basis of MUs, total revenue generated by the Station is calculated. Thus reading of Energy Meter is quite important to be recorded, accounted and audited.
As we know that,
Power = VICosØ where Ø is power factor.
Therefore, an Energy Meter shall take current and voltage as an input. Current and voltage input is extended through metering core of Current Transformer (CT) and metering core of Potential Transformer (PT). Based on this input, the energy meter calculates and display amount of energy imported / exported.
Let us take an example. Suppose the energy meter is connected in STAR connected system having CT ratio of 200/1 A & PT ratio of 220 kV / 110 V (phase to phase value). Assume power factor to be unity for the sake of easy understanding of Multiplication Factor.
So, the current and voltage input to meter will be 1 A (phase to neutral current) and 110 V (phase to phase). Let us convert the input current and voltage to phase to neutral value.
Phase Voltage = 110/1.732 = 63.5 V
Phase Current = 1 A
Per phase power calculated by the Energy Meter assuming power factor to be unity
= 63.5 x 1 x 1 Watt = 63.5 Watt
Hence, total power as displayed in Meter = 3xPer Pahse Power
= 3×63.5 Watt
= 190.5 Watt
Energy Meter displays the cumulative energy imported / exported in Wh. Hence, energy displayed by meter assuming 190.5 Watt of power is supplied for 1 hour
= 190.5 Wh
Is this the actual power? Definitely not.
Actual power = 1.732x220x200x1 kW (Three phase power)
= 76208 kW
Actual energy for 1 hour in watt hour = 76208 kWh
This discrepancy in the actual power and power displayed in energy meter is due to the fact that meter is taking CT and PT secondary value as input for its calculation. To overcome this discrepancy, we need to have a Multiplication Factor of Energy Meter.
Calculation of Multiplication Factor:
Multiplication Factor of Energy Meter is calculated as below
Multiplication Factor = (External CT Ratio x External PT Ratio) / (Meter CT Ratio x Meter PT Ratio)
You can find the meter CT and PT ratio in the manufacturer manual. Usually both the ratios are 1.
Let us calculate the Multiplication Factor for our example.
External CT Ratio = 200/1
External PT Ratio = 220 kV / 110 V
Multiplication Factor (MF) = (200×220 kV) / (1×110)
Thus the difference of meter reading must be multiplied by the above Multiplication Factor to get the actual amount of energy imported / exported for our example.
To conclude, as an energy meter shows the cumulative energy imported / exported in Watt Hour, therefore the difference of meter reading taken at two different point of time gives the net energy imported / exported. Let this difference in meter reading is n Wh for the given period.
Then the actual energy imported / exported during that period
= n x MF Wh
= n x MFx10-3 kWh
= n x MFx10-3 Units