Dynamic Modelling of Photovoltaic Units in Response to Voltage Disturbances

The objective of this work was to build a simplified mathematical model of
distributed photovoltaic (PV) units representing small-scale systems accounting for
residential installations. These installations are generally connected to the Low
Voltage (LV) network but can be aggregated to a Medium Voltage (MV) bus
through equivalent impedance accounting for the distribution feeder and the
transformer. The model represents reliably the dynamic behavior of small-scale PV
units in response to grid voltage disturbances according to new connection
requirements. It focuses on grid interactions through the electronic interface and
does not propose a detailed mathematical representation of each physical
component of the system. Therefore, the mathematical model is based on
recent/near future grid requirements and ancillary services that should be
provided by PV units to support the grid during a fault. The proceedings of this
work can be divided in three parts. First, one third of the time has consisted in the
review of the literature and grid codes to make out a list of PV model
specifications. The most important specifications are the Low Voltage Ride-
Through (LVRT) capability, the voltage support through reactive current injection
and the active-reactive strategy during a fault. During the second third of the
time, the dynamic model of PV units has been built based on existing elaborated
models that have been improved in order to implement the PV model
specifications. Finally, the remaining time has been devoted to dynamic
simulations using a 75-bus MV test network where PV units have been attached to
each node next to static and dynamic loads. The external transmission system has
been represented by its Thevenin equivalent defined by its short circuit power.
First, simulations have been made in order to validate the model. Then different
scenarios interesting from a System Operator point of view have been
investigated in order to see how the units react in particular fault/low voltage
situations. The figure 1 shows the fast increase in PV units installed capacity
around the world that clearly illustrates the needs in the introduction of new PV
units grid requirements and in new models implementing these requirements.