Page 53 - Energize September 2022

TECHNICAL

Feed in tariffs: The impact on the

distribution network of large

PV penetration

The recent announcement that feed-in tariffs, for behind-the-meter renewable energy systems, would

be introduced has raised concerns over the effect on the distribution network of a large number of
distributed solar PV systems feeding energy into the grid.

By Mike Rycroft, Energise

he voltage on modern distribution circuits is becoming effects of FIT on existing distribution networks is likely to be small.
increasing difficult to manage. New distributed energy However, if there is an uptake the effect could be significant, and
Tresources, such as solar, combined with the growth of may require adaption of the grid.
sophisticated loads, creates voltage challenges on distribution
network systems. High penetration level of rooftop small-scale Effect of feed-in of renewable energy on the
renewable energy generation (REG) such as solar and wind power distribution network
into the existing low voltage (LV) network would cause the flow of Feed-in of energy can cause the operation of the distribution network
power in reverse direction. and the grid to deviate from the design state. Some of the effects are:
The effect of renewable energy systems, mainly rooftop PV, • Varying voltage levels and misoperation of voltage controls
on the performance of distribution systems has been studied • Reverse power flow through the transformer and in segments of
extensively since the early 2000s, and numerous articles have the distribution feeder.
been published covering both theoretical studies and practical • Phase unbalance
experience. This article attempts to provide an overview of the • Maloperation of protection operation
current status of theoretical knowledge and practical solutions • Increased power losses
to problems arising from a high penetration of PV systems in
distribution networks. Distributed generation (DG) systems in general reduce system
losses as they bring generation closer to the load and decrease
Current situation in South Africa the load on distribution transformers. This is true until reverse
Feed-in of surplus energy from rooftop PV systems to the grid is power flow starts to occur. At low penetration levels distribution
allowed in this country, but only on a net-metered basis. Under system losses reach a minimum value, but as the penetration level
this system, production is limited to the level of consumption, increases, the losses also increase and may exceed the no-DG case.
ie a consumer must feed in less energy than is consumed from Of the above, voltage regulation and reverse power flow
the grid over a period of time. Payment for energy supplied to through the transformer are seen to be the most significant.
the grid is either directly in terms of metered unit credits, or at This article will only consider the effect of solar PV as small-scale
an agreed rate. Rates can vary from very low (equal to what the wind generation is limited in application and is not common in
distributor pays for bulk electricity) to higher than the rate charged distribution networks.
by the distributor. The consumer’s motivation for feeding surplus
electricity to the grid is currently a reduction in nett consumption There are two situations to consider:
and hence reduction in total energy costs. This disadvantages 1. Normal power flow – total PV production is less than
customers such as schools that shut down for periods and cannot consumption and power flows in the normal direction through
sell energy produced during the shut-down period to the grid. the transformer. PV production, however, affects the voltage
This restriction limits the size of the installation to that required on the feeder and the operation of voltage regulation and
to meet the energy demands of the consumer. The introduction of protection devices. Current flow may be reversed on sections of
feed-in tariffs could remove the restriction and allow consumers to the feeder.
become nett producers of electricity. This could range from small 2. Reverse power flow – PV production is greater than consumption
residential customers to large commercial and industrial customers. and power flows in the reverse direction through the
With the exception of residential estates, rooftop solar is transformer. This affects line voltages, voltage regulation devices,
currently sparsely distributed in residential areas, and the initial protection devices and the transformer.

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