MI 2792 PowerQ4 Plus
Theory and internal operation
109
where
0
120
1
3
2
1
2
1
j
e
j
a
.
For unbalance calculus, instrument use the fundamental component of the voltage input
signals (U
1
, U
2
, U
3
), measured over a 10-cycle time interval.
The negative sequence ratio u
-
, expressed as a percentage, is evaluated by:
100
(%)
U
U
u
(26)
The zero sequence ratio u
0
, expressed as a percentage, is evaluated by:
100
(%)
0
0
U
U
u
(27)
Note:
In 3W systems zero sequence components U
0
and I
0
are by definition zero.
The supply current unbalance is evaluated in same fashion.
5.1.12 Voltage events
Voltage dips (U
Dip
), swells (U
Swell
), minimum (U
Rms(1/2)Min
) and maximum (U
Rms(1/2)Max
)
measurement method
Standard compliance: IEC 61000-4-30 Class A& S (Section 5.4.1)
The basic measurement for event is U
Rms(1/2)
.
U
Rms(1/2)
is value of the RMS voltage measured over 1 cycle, commencing at a
fundamental zero crossing and refreshed each half-cycle.
The cycle duration for U
Rms(1/2)
depends on the frequency, which is determined by the
last 10-cycle frequency measurement. The U
Rms(1/2)
value includes, by definition,
harmonics, interharmonics, mains signalling voltage, etc.
Voltage dip
Standard compliance: IEC 61000-4-30 Class S (Section 5.4.2)
The dip threshold is a percentage of Nominal voltage defined in EVENT SETUP menu.
The dip threshold can be set by the user according to the use. Instrument event
evaluation depends on Connection type:
On single-phase systems, a voltage dip begins when the U
Rms(1/2)
voltage falls
below the dip threshold, and ends when the U
Rms(1/2)
voltage is equal to or above
the dip threshold plus the 2% of hysteresis voltage (see Figure 5.8)
On three-phase systems two different evaluation techniques can be used for
evaluation simultaneously:
o
a dip begins when the U
Rms(1/2)
voltage of one or more channels is below
the dip threshold and ends when the U
Rms(1/2)
voltage on all measured
channels is equal to or above the dip threshold plus the 2% of hysteresis
voltage.
o
a voltage dip begins when the U
Rms(1/2)
voltage of one channel falls below
the dip threshold, and ends when the U
Rms(1/2)
voltage is equal to or above
the dip threshold plus the 2% of hysteresis voltage, on the same phase.
A voltage dip is characterized by a pair of data: residual voltage U
Dip
and dip duration:
U
Dip
is the residual voltage, the lowest U
Rms(1/2)
value measured on any channel
during the dip.
The start time of a dip is time stamped with the time of the start of the U
Rms(1/2)
of
the channel that initiated the event, and the end time of the dip is time stamped
