Power System Analysis - Renewable Energy in Power Systems - page 172

Z th

V th

Figure 5.15 Th é venin equivalent circuit

2

V
S

Z th = (5.11)

k

where V is the nominal line-to-line voltage. The angle of Z th could also have been found from
the fault level, if only I sc and S k had been calculated and stored as complex numbers. Instead
of this, normal practice is to express the fault level as a scalar and to express the associated
angle as an X / R ratio, which can be used in Z th = R th + j X th . The Th é venin source voltage can
often be taken as the nominal voltage at the point of interest, being careful to use the phase-
to - neutral or line - to - line value consistent with the calculation.
The fault level is an important design parameter, not only for predicting currents under
fault conditions but also for predicting performance under normal operating conditions. It
defi nes the strength of the network at a particular point. A weak grid is a network or part of
a network where fault levels are low; i.e. the Thévenin or source impedance Z th is high. A
high Z th implies that the node voltage would be sensitive to active and/or reactive power
extraction or injection at the node.
The impact of a renewable energy generator on a network is very dependent on the fault
level at the point of connection. Considering a proposed wind farm capacity (in MW) as a
percentage of the fault level (in MVA) can provide a rough guide to acceptability. This topic
is dealt in greater detail in Section 6.1.3.

5.8 Time Varying and Dynamic Simulations
One load fl ow calculation (after it has iterated and converged) provides the results for one
snapshot of power system operating conditions. Many power system simulation packages
allow the user to defi ne load and generation profi les: time series data at daily, hourly or shorter
intervals. The load fl ow is then run repeatedly to provide voltage profi les, loading profi les,
etc., on the same time-base.
Reducing the time-step to milliseconds allows dynamic simulations to be performed, in
which the dynamic characteristics of the generators and their associated control systems are
included. This type of dynamic simulation is used for stability studies, particularly in the
design of transmission systems. The behaviour of the network is modelled through repeated
load fl ow calculations similar in principle to those discussed throughout this section. Load
fl ow uses phasor representations of voltages and current and therefore implicitly assumes
pure sinusoidal operation.
More sophisticated simulation packages offer transient analysis , in which the components
are represented by differential equations. This allows nonsinusoidal operation to be modelled