Measuring Energy - ABB A43 User Manual

Measurement Methods
7.1 Measuring
Energy
Active energy
It is easy to understand the need for a utility to measure active energy, since the
information is necessary to bill the customer correctly. Usually the more energy
the customer consumes the higher the accuracy of the meter needs to be.
Normally 4 accuracy classes are used: 2%- (small consumers, e.g. households),
1%-, 0.5%-and 0.2%-meters with defined power levels for each class.
Also from a customer point of view it is easy to understand the need to measure
the active energy as it can give him information about where and when energy
is consumed. This information can then be used to take measures to decrease
the consumption.
In many cases it is desired to simplify the measurement. In such cases
simplified methods can be used of which the most common are described in this
chapter. These methods most often require a balanced load, which means that
the imped-ance is the same in all phases giving the same current amplitude and
power factor in all phases.
Note –
accuracy will be decreased if the incoming voltages are not the same on all phases.
Reactive energy
Sometimes there is also a need to measure the reactive energy. Consumer
equip-ment often introduces a phase shift between current and voltage due to
the fact that the load has a more or less reactive component, e.g. motors that
have an inductive component, etc. A reactive load will increase the current
which means that the power source generator and the size of the power lines
have to increase which in turn means higher cost for the utility. A higher
current also means that the line losses increase.
Because of that, the maximum permissible phase shift is sometimes governed in
the terms of the contract that the consumer have with the power supplier. If the
consumer exceeds a specified maximum reactive load, he will be liable for an
extra charge. This type of contract will require a utility meter that measures
reac-tive energy and/or power.
Also, from the customer's point of view, it may be of some interest to measure
reactive energy/power since it gives him knowledge about the nature of the
load. That is, how big the different loads are and how they vary over time. This
knowl-edge can be used in the planning how to decrease the reactive
power/energy to decrease the electricity bill.
Resistive, inductive and capacitive loads
Resistive loads don't give rise to any phase shifts. Inductive loads have phase shift in
one direction with the current lagging the voltage, while capacitive loads pro-duces
a phase shift in the opposite direction with the current leading the voltage. As a result,
inductive and capacitive loads can be used to compensate each other
2CMC48001M0201
Revision: C
It should be mentioned that even if the load is perfectly balanced the
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