Page 38 - Energize March 2021
P. 38
TECHNICAL
Flexible PV systems provide support
services for future grids
by Mike Rycroft, Now Media
PV arrays have customarily been designed to produce a fixed maximum active power output and sized to
give exactly the required power output only. As the amount of solar generation increases, there is a need for
innovation and increased operational flexibility to accommodate the requirements of the grid. Flexibility is a
key factor in the move towards greater renewable penetration and reduces the need for PV plant storage.
s more large-scale photovoltaic power plants (LS-PV) are Active power control
being installed, there is a requirement for renewable energy Active power control involves controlling the output of the PV
A(RE) systems to take over a bigger part of the grid support plant directly from the CC, in order to achieve the power balancing
functions which are currently supplied by conventional generation. In objectives of the system operator. Active power control (APC)
a 100% renewable grid, these functions would have to be supplied capabilities include:
completely by the renewable generators. This would require more • Automatic generation control (AGC), the ability to follow dispatch
flexibility of PV systems, both in active power and reactive power, set points.
and systems will need to provide such features as spinning reserve, • Ramp-rate-limiting controls
artificial inertia, reactive power production, flexibility of output and • Active power response to bulk power system contingencies
others as yet unthought of. (spinning reserve or headroom)
Developments in the use of flexible PV have shown that the • Inertial response
required functionality can be achieved without the use of storage, • Primary frequency response (PFR)
and although the process is in its infancy, several pilot projects are in • Secondary frequency response
operation. Preliminary results show that PV power plant can perform
better than fast gas turbine technologies, which are typically used by Many countries have updated their grid codes to permit a smooth
grid operators to balance the grid. This approach involves a move interaction between PV power plants and the grid. The South African
away from the priority-despatch fixed price systems currently used grid code requires that the active power management for US-PV
in renewable energy, to a more flexible market type system. 1 (when applied) should consider power curtailment, ramp rate control
Grid support functions fall into two key areas: active power and active power reserves (Figure 2).
control, and reactive power control. Both operate under control of
the PV plant controller, which in turn responds to signals from the Automatic generation control (AGC)
system operator control centre (CC), which is established to manage A basic principle of power system operation is that production and
the balancing of power within an area. Operation of these systems consumption of electric power must be balanced at every instant. In
requires a reliable, fast speed communication link between the CC a generation system, sufficient generating capacity must be installed
and the PV plant (Figure 1). The CC handles the power balancing to meet the maximum demand, which can vary considerably. This
function for the area and will exchange information with the PV plant means that during periods of lower demand, some of the capacity
on a short time scale. must be taken out of service or diverted to storage or other functions.
Figure 1: Grid friendly PV power plant (NREL) Figure 2: Active power control 2
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