Page 324 - A Comprehensive Guide to Solar Energy Systems

Chapter 15 • Integration of PV Generated Electricity into National Grids 329

network’s recovery from a disturbance. The growth in solar PV capacity means that this is
being reconsidered. Changes are underway to implement European Network rules (see
reference [13]), which could place requirements to withstand network disturbances on
generators as small as 1 MW due to the increasing proportion of electricity being supplied
by small scale generators.
Inverters can also be configured to operate with a phase angle, which is intentionally
different to the local voltage it observes and is either fixed or varies in response to a need.
This has the effect of raising or lowering local grid voltage. This feature is uncommon at
the moment but is increasingly being asked for where lots of solar PV are connected and
local voltages have the potential to rise to a level that network equipment is not designed
to withstand: in other words, the “problem” created by the solar PV is being solved by the
solar PV itself.
It is possible to use solar PV installations to support the networks beyond their lo-
cal connection. distributed energy resources such as solar PV could help stabilize the
transmission network through a coordinated voltage control approach. This has been
proposed by the National Grid and the electricity distribution company for london
and the South East of England, UK Power Networks, through the Power Potential proj-
ect [14]. The project proposes to investigate the implementation of a novel voltage
control arrangement. The benefit here is that it is potentially cheaper and more com-
petitive than conventional network reinforcement. For further details see references
[6] and [14].

15.7 Converter Technology

The current generation of converter technology typically locks on to existing voltage refer-
ence signals using phase lock loops (Pll). This works fine while there is a strong system
to connect to, acting as a reference. The idea is explored conceptually in Fig. 15.7 (see
Fig. 4.21 reference [6]) using the analogy of a driver going through a tunnel. Further infor-
mation on phase-locked loop technology can be found in references: [15–19].
As the electricity system becomes increasingly converter dominated, the system
strength decreases and the reference is increasingly more susceptible to disturbances. It is
possible to bolster system strength using either conventional “synchronous” generators or
equipment known as synchronous condensers. These come at a cost, in terms of upfront
investment, operating costs, and if fuel is burnt, carbon emissions. Consequently there is
a growing hope and conviction among many engineers that the future is inverters, which
behave much more like conventional generators. Virtual synchronous machine (VSM)
technology is one way to approach this and the possibility has been discussed in various
papers and industrial working groups. Other options using more sophisticated monitoring
and control techniques which are capable at looking at and reacting to what is going on
across the network are also being considered.
More information on VSM technology can be found in reference [20].