Renewable Energy Generation in Power Systems - Renewable Energy in Power Systems - page 177

Table 6.1 Design rules. (Reproduced from Jenkins, N., Allan, R., Crossley, P., Kirchen, D. and Strbac,
G., Embedded Generation , IEE Power and Engineering Series 31, Institution of Electrical Engineers,
London, 2000. Reproduced with permission of IET)

Network location Maximum capacity of embedded generator

Out on 400 V network 50 kVA

At 400 V busbars 200 - 250 kVA

Out on 11 kV or 11.5 kV network 2 - 3 MVA

At 11 kV or 11.5 kV busbars 8 MVA

On 15 kV or 20 kV network and at busbars 6.5 - 10 MVA

On 63 kV to 90 kV network 10 - 40 MVA

at the PCC caused by extraction or injection of active or reactive power. The impact of a
renewable energy generator on the network is therefore very dependent on the fault level at
the point of connection as well as on the size of the proposed generator.
The appropriate voltage at which to connect a distributed generator is largely dependent
on its rated capacity. There are many other factors, as will be seen, and so a range of indica-
tive fi gures are used as guidelines. Of course, whether a network is weak or not is entirely
in relation to the size of the generator being considered. It is therefore common to express a
proposed renewable energy source capacity (in MW) as a percentage of the fault level (in
MVA) that can be labelled 'short circuit ratio'. This can provide a rough guide to accept-
ability. Typical fi gures for wind farms range from 2 to 24%.
Table 6.1 gives rough fi gures for the maximum capacity of a generator that can be con-
nected at a particular voltage level. These rules should ensure that the infl uence of the genera-
tor on the voltage at the point of connection is acceptable. Connecting at a higher voltage is
usually more expensive because of the increased costs of transformers and switchgear and
most likely because of the longer line required to make connection with the existing network.
Connecting at too low a voltage may not be allowed if the generator were to result in an
excessive effect on the local network. This can lead to a situation where the developer of the
renewable energy system wishes to connect at one voltage level for economic reasons, while
the network operator suggests connection at the next level up.

6.2 Voltage Effects
6.2.1 Steady State Voltage Rise
The connection of a distributed generator usually has the effect of raising the voltage at the
PCC and this can lead to overvoltages for nearby customers. The need to limit this voltage
rise, rather than exceeding the thermal capacity of the line, often determines the limiting size
of generator that may be connected to a particular location.
An initial estimate of the voltage rise caused by connection of a generator can be obtained
from analysis of the system as represented in a simplifi ed form by Figure 6.2. The network
up to the point of common coupling can be represented as a Thévenin equivalent with the
Th é venin impedance Z th estimated from the fault level and X - upon - R ratio at the PCC.