Renewable Energy Generation in Power Systems - Renewable Energy in Power Systems - page 180

To summarize, for relatively small embedded renewable energy generators connected to
strong networks the P injection depends solely on the renewable energy source (wind, sun,
water) level at the time and the Q injection either on the natural generator Q / P characteristics
or on the control characteristics of the power electronic converter. In the latter case the con-
verter could be regulated to inject active power at unity power factor, thus avoiding any Q
exchanges with the network, and charges for reactive power. It is also possible that a mutu-
ally benefi cial formula could be agreed between the owner of the renewable generator and
the utility so that the Q generated/consumed by the converter is adjusted to suit the local
network.
Generators that are very small individually have negligible infl uence on system frequency.
However, when large numbers of such generators are connected their aggregate impact can
be signifi cant. In fact, as penetration levels of wind power increase, as for example in
Denmark where the annual level exceeds 20%, and much higher levels will occur at times,
utilities will require that renewable generators are designed to contribute positively to
power system frequency and voltage stability during contingencies, but more of this later.
In a similar manner, it might be expected that in future scenarios with substantial PV genera-
tion, that the PV converters might be required to be controlled in relation to system
frequency.
6.2.4 Example Load Flow
As discussed already, renewable energy generation can affect both line loadings and voltages
throughout the system. Load fl ow is a technique that allows the fl ows of real and reactive
current throughout the network to be calculated, based on the location of the loads and sources
and the line impedances.
The network of Figure 6.1 is used here to illustrate the way in which load fl ow analysis is
applied at a distribution level to assess the effect of connecting a renewable energy generator
at a node. This network is redrawn in Figure 6.3 to include node numbering and a proposed
embedded renewable energy generator at node 10. It is necessary to know whether such an
embedded generator is likely to affect adversely the network voltage profi le.
At the outset a fault analysis is undertaken to provide the short-circuit levels at all nodes
and so indicate the acceptable rating of the proposed renewable energy generator. To embark
on a load fl ow it is necessary to specify the parameters of the lines, transformers and the
known node variables. Table 6.2 gives the line data in terms of line position, length and line
type. Table 6.3 provides additional information for each line type in terms of impedance and
current rating.
Section 5.6.5 described the analysis required to determine network fault levels. For the
network of Figure 6.3 the results are shown in Figure 6.4. As expected, the fault level is
highest close to the secondary of the distribution transformer (node 1) and due to the cumula-
tive impedance of the transmission lines declines moving away from this point. At node 10,
where the renewable energy generator is to be connected, the fault level is 42 MVA. A wind
turbine rated at 500 kW resulting in a short-circuit ratio of 0.5/42 = 12% is within the accept-
able range.
In this example it is assumed that the embedded generator is a wind turbine consisting of
a directly connected induction generator operating at a power factor of 0.9 at full output. At