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18 Electrical power systems ± an overview

Other types of loads are less susceptible to voltage and frequency variations and
exhibit a constant resistance characteristic:
. incandescent lighting
. heating.
Large clusters of end user loads based on power electronics technology are capable of
injecting significant harmonic currents back into the network. Examples of these are:
. colour TV sets
. microwave ovens
. energy saving lamps
. computer equipment
. industrial variable speed motor drives
. battery recharging stations.

Electric energy storage is an area of great research activity, which over the last decade
has experienced some very significant breakthroughs, particularly with the use of
superconductivity and hydrogen related technologies. Nevertheless, for the purpose
of industrial applications it is reasonable to say that, apart from pumped hydro
storage, there is very little energy storage in the system. Thus, at any time the
following basic relation must be met:

Generation ˆ Demand ‡ Transmission Losses
Power engineers have no direct control over the electricity demand. Load shedding may
be used as a last resort but this is not applicable to normal system control. It is normally
carried out only under extreme pressure when serious faults or overloads persist.

1.4 An overview of the dynamic response of electrical
power networks

Electrical power systems aim to provide a reliable service to all consumers and should
be designed to cope with a wide range of normal, i.e. expected, operating conditions,
such as:
. connection and disconnection of both large and small loads in any part of the
network
. connection and disconnection of generating units to meet system demand
. scheduled topology changes in the transmission system.
They must also cope with a range of abnormal operating conditions resulting from
faulty connections in the network, such as sudden loss of generation, phase con-
ductors falling to the ground and phase conductors coming into direct contact with
each other.
The ensuing transient phenomena that follow both planned and unplanned events
bring the network into dynamic operation. In practice, the system load and the
generation are changing continuously and the electrical network is never in a truly
steady state condition, but in a perpetual dynamic state. The dynamic performance of