Page 38 - Energize March 2021
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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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