Environmental Engineering Reference
In-Depth Information
Z
th
V
th
V
pcc
P Q
+
j
Thévenin equivalent
of existing network
Proposed generator
Figure 6.2
Equivalent circuit for estimating voltage rise
From Equation (5.11) the magnitude of
Z
th
is given by |
Z
th
| =
V
2
/
S
k
where
V
is the nominal
line - to - line voltage. The
X
- upon -
R
ratio is then used to fi nd the resistance and reactance from
Z
th
= R + j
X
The voltage rise
(
PR
+
QX
)/
V
, where
P
and
Q
are positive if the active and reactive powers are positive, i.e. have the directions shown
in Figure 6.2. If an induction generator is connected directly to the network,
Q
has a negative
value.
The allowable voltage rise is dependent on how the network is currently operated i.e. how
close the existing voltages get to the allowable maximum. Typically, a rise of 1% would be
a concern to the network operator. Voltage rise is often the main consideration for wind farms,
which tend to be in rural areas, connected by long and relatively high impedance lines.
Voltage rise often puts a limit on the amount of generation that can be connected in a particu-
lar rural area. This can occur long before there is any chance of the power fl ow actually
reversing or of the thermal limits of the lines being reached.
In some situations, calculations (or load fl ow modelling) show that voltages will exceed
acceptable limits for only a few hours in a typical year. In this case, it may be cost effective
to constrain generation during those hours. The lost revenue may be small in comparison to
the cost of installing a stronger line.
Voltage rise can be mitigated through the extraction of reactive power at the PCC. With
induction generators (the norm for smaller wind turbines), this can sometimes be achieved
by removing some or all power-factor correction capacitors. With synchronous generators
(more common in hydro and biomass fuelled systems), the excitation can be adjusted.
However, an embedded generator will normally be charged for any reactive power that it
consumes except in low voltage networks (400/230 V) where reactive power is not normally
metered. Thus, the normal initial design objective is to operate with a power factor near to
unity.
Δ
V
can then be found using Equation (5.6):
Δ
V
≈
6.2.2 Automatic Voltage Control - Tap Changers
The simple picture presented so far is complicated in practice by the existence of automatic
voltage-control mechanisms in distribution networks. The most common mechanisms are
automatic on-load tap changers, which are fi tted to most transformers throughout the system,
