A virtual synchronous generator control in the power system with high penetration of renewable energies--weak grid!

In response to global climatic change and the accessibility of clean energy in the world,
power systems are experiencing a paradigm shift from conventional synchronous generators (SG) based systems to mixed generation systems with a large portion of renewable energies (RE). This has resulted in the erosion of the conventional power system’s frequency control. This is because, inertia of the system declines with the continual displacement of the SG by the inertia-less RE generations such as solar photovoltaic (SPV) systems and wind turbine generation (WTG) systems. Therefore, on the occurrence of a small frequency contingency in the system, a high rate of change of frequency (RoCoF) and high frequency deviations can be experienced. This results in a cascading failure and eventually a black out.
To increase the inertia of in power systems with high penetration of RES units, an
interfacing power electronic converter (inverter) can be controlled to mimic the operation
of a synchronous generator (SG). This technology is known as a virtual synchronous
generator (VSG). In this research work, VSG technology is explored in a scenario of
its connection to a weak grid. That is, a grid with high penetration of RE-generation
units. In this case, there is a strong coupling between active power(P) and active power
(Q). Therefore, a new virtual power circles based PQ decoupling scheme is presented in
chapter 3. The PQ decoupling is achieved by virtual addition of an RCL circuit in the
VSG controller.
VSGs are required to ride through faults in order to avoid cascading failure of the
system. To achieve this for a VSG connected to a weak gird, a new low voltage ride
through (LVRT) strategy using an finite control set model predictive control (FCS-MPC)
is presented in chapter 4. The strategy improves transient response of the VSG on the
occurrence of faults such as voltage sags and maximises power transfer during LVRT.
The intermittent and stochastic nature of the RES makes the system inertia to be a
time varying quantity rather than a constant quantity as in conventional strong power
systems. Therefore in weak grids, it is important to understand and track the system’s
inertia values. This provides system situational analysis such that a maximum size of the
frequency contingency can be defined. Moreover, optimisation of the available resources
for containing frequency deviations and prevent high RoCoF can be established with the
knowledge of the available inertia. Furthermore, inertia adaptive protection schemes can
be designed. Thus, a new neural network based stochastic inertia estimation method in
a weak power system is presented in chapter 5. This forms basis for designing an inertia
adaptive VSG.