Control and Design of Microgrid Components - Power Systems ...

Chapter 2. Static Switch
The static switch has the task of disconnecting all the sensitive loads from the grid once the
quality of power delivered starts deteriorating. The static switch does not disconnect the local
system from the grid, but it disconnects only the sensitive loads.
There are two main reasons to adopt a static switch to implement the connection and
disconnection from the grid: first a static switch does no have mechanical moving parts,
therefore its operating life will be extensively elongated compared to a traditional contactor with
moving parts. The second reason to use a dedicated switch is because during reconnection with
the grid a complex series of synchronization checks need to be performed. A normal interrupting
breaker would be able to perform the function of disconnecting to the grid, but a sophisticated
static switch is required to properly reconnect to the utility system without creating hazardous
electrical transients across the microgrid.
The static switch plays a key role in the interface between the microgrid and the utility system.
This device needs to be controlled by a logic that verifies some constraints at the terminals of the
switch before allowing for synchronization. The same logic applies to the circuitry that controls
the action of the contactor, the device used to physically connect a microsource to the feeder.
Disconnection at the static switch is regulated differently than at the contactor. Disconnection at
the static switch takes place because of deterioration of quality of electric power delivery from
the utility system. More in particular, there are at least five conditions that will enable the
disconnection logic and command the transfer to intentional island:
i) poor voltage quality from the utility, like unbalances due to nearby asymmetrical loads
ii) frequency of the utility falls below a threshold, indicating lack of generation on the utility
side
iii) voltage dips that last longer than the local sensitive loads can tolerate
iv) faults in the system that keep a sustained high current injection from the grid
v) any current that is detected flowing from the microgrid to the utility system for a certain
period of time
Synchronization conditions are detected by verifying two constraints: the first is that the voltage
across the switch has to be very small (ideally zero), and the second is that the resulting current
after the switch is closed must be inbound from the utility system towards the microgrid. The
second condition needs to be re-spelled for the case of a contactor connecting two microsources
in island mode: in this scenario the resulting current must always be from the highest frequency
source to the lower one (which is by the way the same constraint that is enforced when
connecting to the grid, but there it was described using more familiar terms).
2.1 Direction of Current at Synchronization
The first thing that needs to be taken into consideration is that the switch is going to close on a
R-L circuit that has no current previously flowing into it. Furthermore, the voltages at each of the
two ends of the switch rotate at different frequencies. This implies that the relative phase angle
between the voltage of the grid, E, and the voltage on the microgrid side, V, is constantly
changing from a minimum value of zero degrees to a maximum of 180 and back to zero again.
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