Page 40 - Energize March 2021

TECHNICAL

possible with the prevailing solar resource. If the possible output
is lower than the curtailment level, the array operates at maximum
power. If the possible level is above the curtailment level, the array
will operate at the reference power level.
Curtailment of PV production is not yet applied to the South
African grid, but with increasing renewables on the grid, it might
be applied in the future. Curtailment of production during times of
overproduction or low demand is common in countries with a high
percentage of renewables.

Active power response to bulk power system contingencies (active
power reserves or headroom)
Power reserve or headroom requires the reduction of output power
during specific periods by a set amount and involves operation
at a level below that which is possible at the point in time. This is
a contracted function which allows an agreed amount of energy Figure 6: South African grid code frequency droop curve (Nersa)
generation capacity to be added to the grid within a short period, to
cater for sudden changes of demand or loss of generation elsewhere frequency. This initial reaction of the grid is known as primary
in the network. To supply active power reserves, it is necessary to frequency response. Once the system frequency is stabilised and
work in a curtailed output power mode, where the output is reduced no longer moving downward, new generation can come online to
either by a fixed amount, or by a percentage of the maximum power bring the system frequency back to the nominal level. This frequency
output possible for a given solar irradiation, to provide headroom. restoration service (also known as secondary frequency control or
The fast response of PV systems in power reserve mode makes secondary reserves) can be provided by the PV array power reserve.
it possible to implement several grid-support functions, amongst The South African grid code PV frequency response curve is shown
them frequency support and inertial response. Frequency support in Figure 6.
involves either increasing the power supplied to the grid in the case Primary and secondary frequency support is provided by power
of under-frequency or reducing power in the case of over-frequency. reserve function in the PV system. The fast response capability of
Inertial response simulates the inertia of rotating machines and uses the PV inverter makes it possible to ramp up the PV output fast
the power reserve to increase power supplied to the grid in the case enough to provide the primary response, while the ramped output
of a drop in generation. provides the secondary frequency response. The amount of power
Providing headroom results in revenue loss from curtailed which can be added will vary as the output of the array varies with
production. Where there is an ancillary services market, the decision time of day and solar irradiation levels.
to provide headroom or curtail output for frequency regulation can
be based on the price of an ancillary services product relative to Reactive power control and voltage support
the revenue lost from lower energy production. When the value of Reactive power is used to control voltages in a PV system. The
energy is low or negative (e.g., during periods of over-generation), it amount of reactive power that can be supplied depends on the
could be cost-optimal for the PV generator to curtail energy output active power output, which can depend on the solar irradiation level
in favour of providing frequency regulation. and the degree of curtailment applied to the PV array. Dynamic
Regulation-down services are generally considered more reactive power control (DRPC) varies the amount of reactive power
accessible for PV generators because they only require the according to the active power output of the array, gives faster
instantaneous curtailment of an amount of energy dictated by the response than static control, and is ideally suited for the fast-
CC. If the payment for reducing output is more than the price of the changing grid characteristics found with variable renewal energy
energy that would otherwise be provided, it can be economical for systems. DRPC can reduce the amount of reactive power required
PV plants to provide regulation-down service. Because regulation-up to maintain voltage within limits. n
services require constantly available headroom, it is less likely for PV
resources to participate in these markets because this would require References
them to incur significant reduction in revenue due to curtailment. 1. A Cobera-Tobar, et al: “Active and Reactive Power Control of a PV
Generator for Grid Code Compliance”, Energies 2019, 12(20)
Ramp rate limiting controls 2. First Solar: “First Solar power plant in Chile is world’s first to deliver grid
Due to the intermittency of the solar source, ramp rate control is also services”, First Solar, August 2020.
necessary. The aim of these ramp rates is to smooth the change 3. Greening the grid: “Automatic generation control.”
from low to high solar irradiance and vice versa, so the change 4. V Gevorgian: “Highly accurate method for real-time active power reserve
does not affect the voltage or the frequency. Ramp rate limiting is estimation for utility-scale photovoltaic power plants”, NREL/TP-
performed by the inverter controls. 5D00-73207, February 2019.
5. I Chernyakhovskiy, et al: “Grid friendly renewable energy: Solar and wind
Frequency response participation in automatic generation control systems”, NREL, June 2019,
An essential function of APC systems is returning frequency to the 6. C Loutan: “Demonstration of essential reliability services by a 300 MW
nominal level following a grid disturbance, such as a sudden loss solar photovoltaic power plant”, NREL Technical Report /TP-5D00-67799,
of a large generator, load, or transmission line. If a large generator March 2017.
trips out, generators that can react immediately (i.e., solar PV or
battery storage) can increase their output to arrest the falling system Send your comments to rogerl@nowmedia.co.za

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