Environmental Engineering Reference
In-Depth Information
8.6.3 Problems Associated with Distributed Generation
The introduction of embedded generation into distribution networks will have a number of
technical implications in the following areas.
Fault Levels
In urban areas the distribution networks are designed to have as high a short-circuit level as
possible. This is helpful in minimizing the effect that one consumer has on another and in
maintaining the consumer voltage as close as possible to the nominal level. Economics dictate
that distribution transformers, circuit breakers and cables are rated as near to their maximum
load as possible. This small margin between operation and rating implies that if embedded
generation is installed, the short circuit level may increase beyond the plant capabilities.
Voltage Levels
One feature of distribution of radial circuits is that they supply a number of distributed con-
sumers; therefore economics dictate that they taper along their length. Embedded generation
connected to the end of a long rural circuit is likely to increase the local voltage above statu-
tory levels.
Network Security
Connection of embedded generation has to comply with planning standards that aim to main-
tain the supply security at the pre-connection level. This may adversely limit the size and
type of the embedded generator. The possibility exists that under fault conditions where the
supply from the grid is lost, the local system is run in island mode fed solely from the em-
bedded generator(s). In this case, embedded generation will enhance security. This mode of
operation requires local active control and is addressed later.
Network Stability
Under certain fault conditions, the system dynamics are excited and it is possible that the
characteristics of an embedded generator are such that the ensuing oscillations may cause
tripping of the local network. If the dynamics of the generator are known, a stability study
can be carried out before connection and if instabilities are detected, control systems theory
can devise stabilizing networks.
8.6.4 Options to Resolve Technical Diffi culties
Planning Standards
Present planning standards are largely deterministic and therefore conservative. New stan-
dards could be developed that rely on the probabilistic nature of renewable energy and of
demand patterns. Such standards would be more forgiving and reduce the connection costs
and penalties imposed on embedded generation.
