UMG 801
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14.7 Checking individual power
If a current transformer is assigned to the wrong
phase, the corresponding power is measured and
displayed incorrectly.
The phase conductor and current transformer are
correctly assigned on the device if there is no volt-
age between the phase conductor and the associ-
ated current transformer (primary).
To ensure that a phase conductor at the voltage
measurement input is assigned to the correct cur-
rent transformer for the power measurement, the
respective current transformer can be short-cir-
cuited on the secondary side. The apparent power
displayed by the device must then be zero in this
phase conductor.
If the apparent power is displayed correctly but
the active power has a negative ("-") sign, then
the current transformer terminals are reversed or
power is being supplied to the electric utility.
14.8 Checking the power measurement
1. Short-circuit all current transformer outputs
except one and check the indicated powers.
2. The device must only display power in the
phase conductor with the current transformer
output that is not short-circuited.
3. If this is not the case, check the connections
of the measured voltage and the measured
current.
If the amount of the measured active power is
correct, but the sign is negative, this can have 2
causes:
1. Reversed connections S1(k) and S2(l) at current
transformer or
2. Active energy is being delivered back into the
grid.
14.9 Checking measurement
Correctly connected voltage and current measure-
ment inputs result in correctly calculated and dis-
played individual and summation power readings.
14.10 Checking of voltage and current inputs
by means of phasor diagram
The phasor diagram can be used to check incor-
rect connections at the voltage and current inputs.
Example 1
Primarily ohmic load.
Voltage
and
current
have
only a small deviation
in the phase position.
·The current measurement input is assigned to the
correct voltage measurement input
Example 2
Primarily ohmic load.
Voltage
and
current
have
a deviation of about 180°
in the phase position.
·The measured current input is assigned to the
correct voltage measurement input.
·In the current measurement under consideration,
the connections k and I are reversed or there is a
feedback into the supply network.
14.11 Checking the phase assignment
The assignment of the phase conductor to the cur-
rent transformer is correct if a current transformer
is short-circuited on the secondary side and the
current indicated by the device in the associated
phase conductor drops to 0 A.
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