HT instruments VEGA78 - Manual (Page 119)

VEGA78
EN - 117
10.3. 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 as shown in below picture:
Fig. 189: Decomposition of a vectors 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
NEG
d
r

= component at negative sequence
100 % 0 x
E
E
ZERO
d

= component at zero sequence
where:
E
r
= sequence of negative tern.
E
d
= sequence of direct tern.
E
0
= sequence of zero tern.