Environmental Engineering Reference
In-Depth Information
with the square of the voltage
V
. The downside of operating at higher voltages is that costs
of insulation and other power system equipment increase substantially. Thus, for bulk trans-
mission of power over long distances, higher voltages are most economic whereas, for local
distribution of modest power to numerous connection points, lower voltages are most eco-
nomic. The economics also dictate that it is worthwhile to have several intermediate voltages.
This multiple voltage arrangement results in network transmission losses confi ned to within
5-10% of the throughput power.
The bulk of global electricity is generated in large (
500 MW) power stations at around
20 kV. This is then stepped up by transformers to an extra high voltage (EHV) level such as
400 kV and carried by the transmission system to the bulk supply points, where it is stepped
down to a high voltage (HV) level of around 100 kV. Some very large industrial consumers
are connected at this level but most power is transformed down again to medium voltage
(MV) levels such as 30 kV, then to 10 kV and fi nally to the low voltage (LV) level of 400 V,
also referred to as the distribution system, which provides 230 V, when the connection is
single-phase. In the USA and a few other countries, the LV level is 200 V three-phase, 115 V
single-phase. The voltages used vary from country to country but the power system structure
follows closely the layout of Figure 1.12.
In Figure 1.12, a circuit-breaker is shown after the generator step-up transformer. This is
a component part of an extensive protection network which permeates all levels of the power
system. Faults on the network may result in low resistance paths that cause excessive currents
capable of damaging equipment. The protection devices, circuit breakers at high voltage
levels and fuses at domestic distribution level, operate to isolate the faulty part of the network
and prevent equipment damage. The effect on the protection system of introducing renewable
energy sources will be discussed in Chapter 6.
>
1.4.2 Integrating Renewables into Power Systems
The term
grid
is often used loosely to describe the totality of the network. In particular,
grid
connected
means connected to any part of the network The term
national grid
usually means
the EHV transmission network.
Integration
specifi cally means the physical connection of the generator to the network with
due regard to the secure and safe operation of the system and the control of the generator so
that the energy resource is exploited optimally. The proper integration of any electrical gen-
erator into an electrical power system requires knowledge of the well-established principles
of electrical engineering. The integration of generators powered from renewable energy
sources is fundamentally similar to that of fossil fuelled powered generators and is based on
the same principles, but, renewable energy sources are often variable and geographically
dispersed.
A renewable energy generator may be described either as
standalone
or
grid - connected
.
In a standalone system a renewable energy generator (with or without other back-up genera-
tors or storage) supplies the greater part of the demand. In a grid-connected system, the
renewable energy generator feeds power to a large interconnected grid, also fed by a variety
of other generators. The crucial distinction here is that the power injected by the renewable
energy generator is only a small fraction of that generated by the totality of generators on the
grid. The distinction between standalone and grid-connected generators is a useful one but
