Introduction - Control of Power Inverters in Renewable Energy and Smart Grid Integration - page 59

1.6.3 Requirements for Smart Grid Integration
As mentioned above, the integration of renewable and distributed energy sources, energy
storage and demand-side resources into smart grids is arguably the largest “new frontier” for
smart grid advancements (DOE 2009b), in particular, when these sources are connected to
smart grids via inverters. Several challenging technical problems should be addressed in order
to fully maximise the benefits of smart grids.
1.6.3.1 Synchronisation
One of the most important problems in renewable energy and smart grid integration is how
to synchronise the inverters with the grid (Blaabjerg et al . 2006; Rodriguez et al . 2007b;
Shinnaka 2008; Wildi 2005). There are two different scenarios: one is before connecting an
inverter to the grid and the other is during the operation. If an inverter is not synchronised with
the grid or another power source, to which it is to be connected, then large transient currents
may appear at the time of connection, which may cause damage. During normal operation,
the inverter needs to be synchronised with the source it is connected to so that the system can
work properly. In both scenarios, the grid information is needed accurately and in a timely
manner so that the inverter is able to synchronise with the grid voltage. Depending on the
control strategies adopted, the information needed can be any combination of the phase, the
frequency and the voltage amplitude of the grid.
1.6.3.2 Power Flow Control
A simple reason for integrating renewable energy, distributed generation and storage systems,
etc. into a grid is to inject power to the grid. This should be done in a controlled manner.
Naturally, this is done via directly controlling the current injected into the grid. Another
option is to control the voltage difference between the inverter output voltage and the grid
voltage. As a result, there are current-controlled strategies and voltage-controlled strategies.
Current-controlled strategies are easy to implement but the inverters equipped with current-
controlled strategies do not take part in the regulation of power system frequency and voltage
and, hence, they may cause problems for the system stability when the share of power fed into
the grid is significant. It is more difficult to control voltage than current but voltage-controlled
inverters can easily take part in the regulation of system frequency and voltage, which is very
important when the penetration level of renewable energy, distributed generation and storage
systems, etc. reaches a certain level. The closer to conventional synchronous generators these
sources behave, the smoother the operation of the grid is.
1.6.3.3 Power Quality Control
Power quality is a set of electrical properties that may affect the proper function of electrical
systems. It is used to describe the electric power that drives an electrical load. Without proper
power quality, an electrical device (or load) may malfunction, fail prematurely or not operate
at all. Poor power quality can be described in different ways, e.g. the continuity of power,
variations in magnitude and frequency, transient changes, harmonic contents in the waveform,
low power factor, imbalance of phases, etc.