Environmental Engineering Reference
In-Depth Information
Using Power Electronics Technology
High power electronics have made enormous advances over the last decade. These advances
have resulted in new devices capable of increasing the fl exibility of AC power systems. Such
devices, known collectively as fl exible AC transmission systems (FACTS), offer considerable
opportunities and challenges to power systems engineers. The challenges stem from the
necessity of extending as far as possible the capacity of existing transmission lines without
affecting adversely the voltage profi le and jeopardizing stability. Building additional trans-
mission facilities would have solved some of the problems but these are often limited by
economic and environmental considerations. FACTS covers a range of electronic based
systems such as Static Var compensators, thyristor controlled capacitors, phase-shift trans-
formers, interphase power controllers and unifi ed power fl ow controllers. Some of these
devices have been used to assist in the integration of RE sources and are likely to facilitate
future connections.
The nature of RE sources are such that they often require a power electronic interface,
usually a PWM converter, for connection to the grid. This interface could be regulated, not
only to utilize optimally the renewable energy resource, but also to make the generator 'con-
nection friendly'. The local or some other vital neighbouring voltage can be monitored and
regulated by reactive power injection/extraction. The active power injected into the node may
be limited at times to safeguard other local operating restrictions or requirements. The inter-
face could be made sensitive to local faults so that it limits the contribution made by the local
generator but does not limit it to the extent of endangering network stability. Finally, it could
sense the loss of mains and arrange that the local network operates as an island arranging
synchronization and reconnection when the mains is re-established.
Such a powerful local control activity will require substantial information processing and
transfer. Interactive local control systems could be developed so that the embedded generator
interface is in communication with other similar local units such as FACTS and with the
local tap-change transformers, so that the most appropriate control actions are taken. The
required communications may be provided through the Internet.
Islanding
An actively managed distribution network with locally generated power that approximately
balances the demand could theoretically be run autonomously if connection to the mains were
to be lost due to a fault. Such operation would require some sophisticated local control actions
to fi rst maintain the frequency close to the nominal value and then to reconnect the islanded
system back to the mains when the fault is cleared. The capability to island could increase
the reliability of supply to the local consumers but at the cost of more complex software and
hardware. The desirability of such operation depends crucially on the typical reliability of
supply experienced by the consumers. If the reliability is high, say one short interruption per
year on average, the benefi ts of islanding may be doubtful.
Dynamic Loads
In Section 3.8.3 an application of a dynamic load was discussed as a means of regulating
frequency, i.e. providing power balance in a power system. In such an application the fi rst
