Page 67 - Energize January 2022

TECHNICAL

Under light loads the voltage at the
transformer end can exceed the limits and
under heavy loads the far end voltage can
fall below the lower limit.
This situation is usually catered for by
using a distribution transformer with tap
changers, that can alter the voltage at the
transformer end in accordance with load.
This works for the situation where load
distribution is smooth and does not change,
that is, all loads increase or decrease
equally. The tap-changer transformer does
not work well with varying individual loads
as there is no way of knowing how the end Figure 2: Supplementary voltage control devices
of line voltage is affected.
In addition, a situation can arise where Voltage regulators
volt drop is not smooth, for example the Voltage regulators are autotransformers with tap changers. They provide fine-tuning
presence of a heavy load along the route, capability for voltage at specific points on the feeders. Voltage regulators are constructed
which results in a higher-than-average from three basic parts:
voltage drop in that section, affecting the • Autotransformer: A transformer with part of one winding common to both the primary
voltage on the balance of the route. and secondary windings
These problems are solved by • Load tap changer: The switch is designed to work under load to change the configuration
adding voltage control devices such as of a transformer coil, providing greater regulator versatility
voltage regulating transformers and VAR • Voltage regulator control: The control senses the system and automatically controls the
compensating devices at interval along the tap changer
route. This allows a much closer control of
voltage and a better response to changing
loads (see Figure 2).

Voltage control devices
Smart distribution transformer (SDT)
Substation transformers are normally
equipped with tap changers which can
adjust the feeder voltage at the substation
in predetermined steps, depending on the
loading on the transformer, and provide
a rough control over the feeder voltage.
A recent development is the smart
distribution transformer, which allows Figure 3: Smart distribution transformer 2
continuous control over output voltage.
This device consists of a fixed transformer
coupled with an electronic voltage
regulator, which features continuous
voltage regulation (Figure 3).
The effectiveness of the voltage
regulator depends on how its output
voltage is set, based on line and load
conditions, and how the regulator’s
output voltage compensates for the line
voltage drop (or rise), ensuring that the
voltage at all customers is within the
statutory requirements. For the SDT to be
effective, full network voltage data should
be available to the regulator, and smart-
metering data could provide this. Figure 4: Voltage regulator

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