Environmental Engineering Reference
In-Depth Information
pu
Last
customer
±6%
Tolerance
1st Customer
km
Distance of primary feeder
Substation
with OLTC
Service
Service
Figure 4.1
Voltage drop along a distribution primary feeder [178]
the substation to the final service transformers. Besides, voltage drops can also be
influenced by the amount of connected load and its nature (
i.e.
power factor), this
is because a greater power demand requires a greater current. In other words, while
the line impedance value remains constant, the voltage drop varies in proportion to
the load. To address this issue, the voltage can be regulated by various automatic
mechanisms, the most common being OLTC units [177].
OLTCs are mechanisms that represent the final point of voltage control for low-
voltage consumers at the primary substation. Therefore, these devices operate with
the purpose of maintaining voltage values within operational boundaries, which in
turn guarantee a good quality in the delivery of energy and hence can attenuate power
losses. The DNOs and regulators work towards establishing the voltage variation
permitted in the network, generally a value around
±
6% of the nominal operating
voltage [179].
The amount of electrical losses in distribution networks is influenced their by
both technical and operational features; on average in the UK losses are in the order
of 7%. However, a thorough survey in 2001 showed DNO loss indicators can vary
considerably from region to region; this data is illustrated in Table 4.1 [180].
To reduce the losses in the networks, which tend to increase the cost of operating
the grid, DNOs popularly use control devices such as OLTCs when necessary. As the
name implies, the OLTC can change its tapping position with load current flowing
through the transformer. In practical terms, distribution systems use a tap-changing
mechanism within a power transformer to enable a range of voltage magnitude regula-
tion at one of their terminals. This modulation is achieved by varying the transformer
ratios, which is the equivalent of producing a variable voltage magnitude with respect
to the voltage magnitude at the output winding [181].
Today, some basic requirements expected in OLTC devices include [182]:
Low investment cost and minimal maintenance costs;
●
High reliability and dielectric strength;
●
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