Environmental Engineering Reference
In-Depth Information
where
i
1
2
3
4
5
a
i
0,0314
0,0525
0,0657
0,028
0,080
P
i
%
P
(0,1%)
P
(1%)
P
(3%)
P
(10%)
P (50%)
The level values are recorded in a 10 min test series, as determined from mea-
sured quantiles of the relevant cumulative value, e.g.
P
10%
. The long-time flicker
strength follows with N = 12 for 12
·
10 min by cubic smoothing:
3
1
12
12
1
P
st
,
i
P
lt
=
(7.8)
An example of a cumulative power curve, taken from an arc-furnace (which is
the main source of flicker) is shown in Fig. 7.11. The power levels
P
i
can be read
from the curve
F
(
P
), and the algorithm to determine
P
lt
applied.
Grid regulations require the long term flicker strength not to exceed
P
lt
= 0
,
65
on the low voltage side. The code [VDEW98] for medium voltage level requires a
tighter maximum of
P
lt
= 0
,
46, corresponding to
A
lt
≈
0
,
1.
To assess the quality of one or more generating units relating to flicker-relevant
voltage variations, a dimensionless flicker coefficient
c
is defined. It can be deter-
mined from tests under realistic operation conditions, alternatively declared by the
manufacturer or a test institute. When
c
is known, the long term flicker strength is
calculated by:
P
lt
=
c
S
nG
S
kV
Hence
c
is connected with the long-term flicker strength
P
lt
, and the ratio of the
rated apparent power
S
nG
of a generating unit, and
S
kV
the short circuit power at
the point of common connection. At further inspection, determination of relative
voltage variations requires to take the relevant phase angle into account; in this case
ψ
kV
of the short-circuit impedance and
ϕ
f
, a flicker-relevant angle of the generator
unit. Coefficient
c
is then determined from measured values of
P
lt
and
ϕ
f
of the
generator:
S
kV
S
nG
·
cos
(
ϕ
f
)
c
=
P
lt
·
(7.9)
ψ
kV
+
where
ϕ
f
= atan
Δ
Q
Δ
P
Note that phase angles are counted from current to voltage, so that values are
>
0 for inductive impedances. Fig. 7.14 illustrates the case of an asynchronous gen-
erator, connected to an infinite bus via a short-circuit impedance
Z
k
; the circuit is
similar to that in Fig. 7.2a. The current vector
I
G
for generator operation ends on the
machine's current-locus circle of which a part is shown. The tangent in the working
point is used for linearization; hence the phase angle of small current variations
Δ
I
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