Any normal load causes a voltage drop
throughout the system. This is allowed for in the design, and the
cost associated with the losses incurred is recovered in the related
unit sales.
Unbalanced loads
Unequal loading between the phases of
the network causes an unequal displacement of the voltages. Extreme
inequality causes motors and other polyphase equipment to take
unequal current and perhaps become overloaded on one phase.
For this reason DNOs impose limits on
the extent to which they accept unbalanced loads at any particular
location in order to ensure that other consumers are not adversely
affected. Installation designers need to ensure that the same problem
does not arise due to an unbalanced voltage drop within the
consumer’s installation itself.
While most voltage unbalance is caused
by single-phase loading, the effect on a three-phase motor can best
be assessed in terms of the negative phase sequence component of the
voltage thereby created.
Providing that this is less than 2% the
inequality of current between phases should not be more than the
motor has been designed to withstand. Engineering Recommendation P29
aims to limit continuous levels of voltage unbalance to 1%.
Power factor
Many types of apparatus such as motors
and fluorescent lighting also require reactive power and thereby take
a higher current than is necessary to supply the true power alone.
This extra current is not recorded by the kWh meter but nevertheless
has to be carried by the distribution system and uses up its capacity
thereby.
It also increases the losses on the
system.
A power factor of 0.7 means that the current is 1/0.7 = 1.43
times as great as absolutely necessary and thus doubles the losses
(I2 R). If all the loads in the UK were permitted to have as low a
power factor as this, the additional cost of the losses (if the
system could stand the burden) would be in the order of £200 million
per annum.
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