Environmental Engineering Reference
In-Depth Information
except the fi nal distribution transformers (in the UK, the 11 kV to 400/230 V transformers as
described in Section 5.5.1).
In the network shown in Figure 6.1, the primary substation is equipped with an automatic
voltage controller, which uses a tap changer to adjust the turns-ratio of the transformer.
Closed-loop control ensures that the voltage on the transformer secondary is kept close to
11 kV. This compensates for variations on the 33 kV side and the current-dependent voltage
drops within the transformer itself.
Automatic tap changers affect the steady state
V
voltage-rise analysis presented in the
previous section. In particular, the Thévenin impedance
Z
th
must be adjusted, because its
calculation was based on the fault level, which naturally included the impedance upstream
of the voltage controller. With the automatic voltage controller active, the Thevenin voltage
is the fi xed voltage at the transformer secondary. To correct for this, the fault level and
X
-
upon -
R
ratio at the fi xed voltage node (transformer secondary) must be known so that the
upstream impedance may be calculated and deducted appropriately.
To complicate matters further some automatic voltage controllers do not simply keep the
voltage constant. For example, some provide line-drop compensation, which is a way of
estimating and allowing for the voltage drop in the downstream line. Other controllers use a
technique known as negative reactance compounding, which allows dissimilar transformers
or transformers at separate substations to be operated in parallel. Unfortunately, controllers
using these techniques can be affected by the changes in the substation power factor that can
be caused by distributed generation. To prevent such changes in the power factor, some
embedded generators are designed to operate at the same power-factor as the typical
loads. Problems of this nature could be avoided if the distributed generators are of a type that
could generate and control reactive power. In practice this option has not been fully exploited
in the low voltage distribution networks, but with the ongoing development of more sophis-
ticated and low cost power electronic interfaces it is likely to be of importance in the
future.
Δ
6.2.3 Active and Reactive Power from Renewable Energy Generators
As mentioned earlier, the large generators of conventional power systems have their indi-
vidual AVRs set to maintain the generator bus voltage virtually constant.
Generators fed from renewable energy sources are substantially smaller in rating and are,
in general, connected to the distribution rather than the transmission network. For these
reasons, conventional generator control schemes have not been considered appropriate for
small embedded renewable energy supplied generators. For example, a fi xed speed wind
turbine driving an induction generator is expected to inject into the network whatever power
is converted from the wind up to the rated wind speed, beyond which the power is limited
by aerodynamic means at the rated output. It is also expected to absorb whatever reactive
power the induction generator requires from the network, minus any locally generated
Q
from
power factor correction capacitors. The situation is similar with small hydro systems inter-
faced to the grid through induction generators.
Energy from photovoltaic, wave and tidal schemes and MW size wind turbines will invari-
ably be fed to the grid through a PWM power electronic converter. This provides the facility
of reactive power injection/extraction at the point of connection.
