MULTIPLICATION FACTOR OF ENERGY METERS 101: A COMPREHENSIVE WALK THROUGH

MULTIPLICATION FACTOR OF ENERGY METERS

We generally use a Special energy meter for keeping the count of energy flowing through the system. The meter we use has an input connection from the current transformer’s metering core’s secondary side. With current input, we also provide the voltage input to the SEM from the field’s PT secondary core as discussed in the previous article.

METERED NET ACTIVE ENERGY

The net active energy recorded by the energy meter is therefore a function of input current which is normally 1A or 5A depending on the field’s CT rating and voltage, which is generally 63.5V phase to neutral value.

Now assuming the power factor is unity.

Therefore, the power per phase recorded in the energy meter is

= V * I * cos Ø, where Ø is the phase angle.

=63.5 * 1 * 1

= 63.5 Watts.

Net active energy therefore recorded in one hour will be = 63.5 Watt-hour. Per phase.

The three-phase net energy in the above conditions will be = 3* 63.5 Wh or = 1.732*110*1*1

Which is equal to 190.5 Wh.

ACTUAL NET ACTIVE ENERGY

Now this is not the actual energy because the meter is connected to the CT and PT secondary and the actual energy is therefore proportional to this value as the primary current is not 1A and voltage is not 63.5V phase to neutral or 110V phase to phase.

Let us do the calculation using the primary current and voltage of CT and PT.

Assuming the connected CT is of 400/1A rating and PT being 132KV/110V and the power factor being unity,

Therefore, actual power considering the primary current and voltage is

= 1.732 * 132000 * 400 * 1

= 91449600 watts

And actual net active energy will be equal to 91449600 Wh.

DIFFERENCE IN ENERGIES

This difference in metered net active energy and actual net active energy is because of the reason that current and voltage have been taken in the meter from the secondaries of the transducers. However, both the energies thus calculated are proportional and the constant of proportionality is nothing but the field CT rating * PT rating.

Metered net active energy = 190.5 Wh

Actual net active energy = 91449600 Wh

MULTIPLICATION FACTOR OF ENERGY METER UNIT CONCEPT

Therefore, the multiplication factor of energy meter = actual net active energy / metered net active energy

Which is equal to in this case = 91449600 / 190.5 = 480050.

Now ideally we have energies in bulk so we convert the Wh reading to MWh. To do this we just need to multiply 10^-6 with the multiplication factor of energy meter.

Hence, the multiplication factor of energy meter becomes = 480050 * 10^-6 =0.48

Now if we multiply this multiplication factor of the energy meter by the metered value we will get the actual net active energy in MWh.

Note: if the meter’s unit is in KWh we will multiply 10^-3 for converting to Megawatt-hour MWh.

COMPARISON OF BOTH ENERGY EQUATIONS

Let us compare the metered energy equation = 1.732 * 110 * 1 * 1

With the actual energy equation = 1.732 * 132000 * 400 * 1

We will find that the multiplication factor of the energy meter is equal to (connected CT ratio * PT ratio)

or (connected CTR*PTR)

which is (400 / 1) * (132000 / 110).

SPECIAL CASES

CTR and PTR mentioned in the meter

In some cases where the energy meters have their own CT ratios and PT ratios mentioned,

Then the multiplication factor of the energy meter will be

(connected CTR * PTR) / (meter CTR * PTR)

here in the meter CTR mentioned is 200/1A and PTR is 33KV/110V. let us suppose the connected CT in the field is rated as 400/1A and PT is 33KV/110V then connected CTR * PTR = 400/1 * 33000/110 and meter CTR*PTR = 200/1 * 33000/110.

Therefore, the multiplication factor of energy meter = (connected CTR*PTR) / (Meter CTR*PTR) = 2

If the connected CTR*PTR is the same as the meter CTR*PTR then the Multiplication factor of the energy meter will be = 1.

CTR and PTR are not mentioned in the meter

When it is not mentioned we will use it as above because secondary mentioned meter ratios will get canceled out.

In the meter, it is mentioned as CTR= _/1A or _/5A in some other meter. therefore the meter CTR will get canceled because 1 / 1=1 for CT secondary rated at 1A and 5 / 5 =1 for CT secondary rated at 5A.

For PTR the secondary voltage is 110V and therefore the meter PT ratio _/110V will also get cancelled as 110 / 110 = 1

Hence in this case multiplication factor of the meter with nothing mentioned in it will be

= (connected CTR * PTR) / (meter CTR * PTR)

Here, meter CTR*PTR will be equal to 1 as explained above,

Therefore, the Multiplication factor is = connected CTR*PTR

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