Megger PMM-1, Руководство пользователя

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The following mathematical exercises are for the purpose of coming to a better understanding of how we
might accurately account for power measurements when monitoring a system of this sort.
∆ Configuration
If we were to assume a balanced load in a closed delta circuit, then we could also assume certain
equalities within the circuit. For the sake of simplicity we will assume the following values and equalities
to use in our calculations.
E
AB
=E
BC
=E
CA
= 100 Volts
I
A
=I
B
=I
C
= 1 Amps
θ =0 ˚ , Power Factor = Cos θ = 1
Where θ represents the phase angle relationship between voltage and current
For this to be true, the voltage and current of each leg must be in phase with one another. Then single
phase power could be calculated as follows.
Watts = Voltage x Current x Power Factor
= 100 x 1 x 1
= 100 watts
Total Load Power would equal to the single phase power multiplied by the number of phases.
Total Load Power = # of phases x phase voltage x phase current x Cos θ
= 3 ( 100 )
= 300 watts
However, when using the two watt meter measurement method, Ø will become 30 degrees as a result of
using the C phase voltage for our reference in making line to line measurements. Now there are only two
legs that can have current flow in them, according to the above diagram. Power in a delta system will
now have to be based on the voltages and currents in the remaining 2 legs. We will refer to these as
P
AC
and P
BC .
The power in each of the two remaining legs can be calculated using the following formula.
P
AC
= P
BC
= E x I x √ 3 x Power Factor
Where: θ = 30°, Power Factor = Cos 30° = √ 3/2 = .866
Total Power Measured
= P
AC
+ P
BC
= ( E
AC
x I
AC
√ 3 x Cos θ ) + ( E
BC
x I
BC
√ 3 x Cos θ )
= 2 x 150
= 300 watts
Where we had previously used
Cos θ to represent power factor, θ can now be represented in terms of it’s
components. When adding the power factor values for each of the two legs of the circuit being
measured, the sum must be divided by
√ 3 . The PMM-1 only displays the individual power factor values
for each individual voltage and current being measured. Therefore, a system power factor can be
calculated by adding the power factors for each of the two legs in the system and then dividing their sum
by
√ 3
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