VEGA74
EN - 39
12. THEORETICAL APPENDIXES
12.1. VOLTAGE ANOMALIES
The meter is capable of recording
independently by the aggregation time
all those
TRMS values as voltage anomalies (sags, swells), calculated every 20ms, beyond the
percentage thresholds of Voltage Reference (Vref) set during the programming from
±
3%
to
±
30%
with steps of
1%
. These limits remain unchanged throughout the recording period.
The reference are set to:
Nominal Voltage Phase to Neutral:
For Single Phase and 4 wires three phase system.
Nominal Voltage Phase to Phase:
For 3 wires three phase and ARON system
Example1
→
Three Phase 3 wires
Vref = 400V, LIM+= 10%, LIM-=10% =>
High Lim = 400 * [1+(10/100)] = 440V
Low Lim = 400 * [1-(10/100)] = 360V
Example2
→
Three Phase 4 wires
.
Vref = 230V, LIM+= 10%, LIM-=10% =>
High Lim = 230 * [1+(10/100)] = 253V
Low Lim = 230 * [1-(10/100)] = 207V
For each event the instrument records (
with visualization only by management
software
):
•
The number corresponding to the phase (L1, L2 or L3) where the event occurred
•
The direction of the event
: “UP” (sag) or “DN” (swell)
•
The date/time of the event beginning
•
The duration of the event, in seconds with a resolution of 20ms
•
The minimum (or maximum) value of voltage during the event.
12.2. SUPPLY VOLTAGE UNBALANCE
In normal conditions the supply voltage and the final loads are perfectly balanced.
Unbalances are possible in trouble situations (low insulation) and/or phase circuits
interruptions. Moreover, in single phase systems, the balance can be only statistic.
In order to design a correct protection installation a thorough study of anomalous
conditions on three phase systems was performed. To better understand the meaning of
an installation's parameter the theory of symmetric components is fundamental.
From the theory is always true that any tern of vectors can be decomposed in three kind of
tern: the direct (positive) symmetric tern, the reversed (negative) symmetric tern and the
omopolar (zero) tern.
On the base of this the result is that each unbalanced three phase system can be
decomposed in 3 three phase systems which can be reduced to a separate study of three
single phase circuits relative to
direct sequence
,
negative sequence
and
zero
sequence
respectively.
The EN50160 standard it declares, relative to low voltage systems that “
under
normal
operating conditions, during each period of one week, 95% of the 10 minute mean rms
values of the negative phase sequence component of the supply voltage shall be within
the range 0 to 2% of the direct phase sequence component. In some areas with partly
single phase or two phase connected customers’ installations, unbalanced up to about 3%
at three phase supply terminal occur.”
The meter permits the measure and recording of
below parameters, which are characteristics of unbalanced degree of an installations:
100
%
x
E
E
REV
d
i
=
= component at negative sequence
100
%
0
x
E
E
ZERO
d
=
= component at zero sequence
where:
E
i
= sequence of negative tern, E
d
= sequence of direct tern, E
0
= sequence of zero tern
