Conventional Synchronisation Techniques - Control of Power Inverters in Renewable Energy and Smart Grid Integration - page 346

22
Conventional Synchronisation
Techniques
One of the most important problems in renewable energy and smart grid integration is how
to synchronise the inverters with the grid. There are two different scenarios: one is before
connecting an inverter to the grid and the other is during the operation. If the inverter is not
synchronised with the grid or another power source it is to be connected to, then large transient
currents may appear at the time of connection, which may damage the equipment. During the
normal operation, the inverter needs to be synchronised with the source it is connected to so
that the system can work properly. In both cases, the grid information is needed accurately and
in a timely manner. Depending on the control strategies adopted, the grid information needed
can be any combination of the phase, the frequency and the voltage amplitude.
In this chapter, some synchronisation methods widely available in the literature are dis-
cussed. Simulation and experimental results are provided for two widely adopted methods,
SOGI-PLL and STA.
22.1 Introduction
There are two categories of methods for the purpose of synchronisation: open-loop methods
and close-loop methods. Typical open-loop methods include detecting the zero crossing of
the grid voltage and directly filtering the grid voltage, etc. An example of the filtering method
is the one used in Chapters 3, 15 and 16, where a phase-lead low-pass filter is adopted to
provide the information of the grid voltage. Other filtering methods include the space-vector
filter (SVF)-based method and the extended Kalman filter (EKF) method (Svensson 2001), etc.
These methods are often found to be sluggish with high sensitivity to frequency deviations,
voltage distortions and voltage imbalance (Timbus et al . 2005).
Closed-loop methods introduce a mechanism to make sure that the information obtained is
accurate. Typical examples of closed-loop methods include the conventional PLL, which is
widely used in single-phase applications, and the synchronously rotating reference frame PLL
(SRF-PLL), which is widely used in three-phase applications. PLLs have been adopted as part