User manual
5 Power quality - a guide
65
which is determined once every 10 minutes and long-term P
lt
, which is calculated on the basis of
12 consecutive P
st
values, i.e. every 2 hours. Long time of measurement results directly from the
slow-changing nature of this phenomenon - to collect sample data the measurement must be long.
P
st
equal to 1 is considered to be a value on the border of annoyance – certainly sensitivity to flicker
is different for different persons; this threshold has been assumed basing on tests carried out on a
representative group of people.
What causes flicker? Most frequently, the reason is the voltage drop as a result of connecting
and disconnecting large loads and some level of flicker is present in the majority of mains systems.
In addition to the previously described adverse impact on human health, flicker does not need to be
(and usually it isn't) a symptom of malfunctioning of our installation. However, if a rather abrupt and
unexplainable flicker increase is observed in the mains (increased P
st
and P
lt
parameters) it should
not be ignored under any circumstances. It may turn out that the flicker is caused by poor connec-
tions in the installation – increased voltage drops on connections in the distribution panel (for ex-
ample) will result in higher voltage fluctuations on the receivers, such as light bulbs. The voltage
drops on connections also cause their heating, and finally sparking and possibly a fire. Periodical
mains tests and described symptoms may turn our attention and help find the source of hazard.
5.3 Power measurement
Power is one of the most important parameters determining the properties of electrical circuits.
The basic unit used in financial settlements between the electricity supplier and consumer is electric
energy calculated as the product of power and time.
In electrical engineering, several different power types are distinguished:
Active Power - marked with P and measured in Watts,
Reactive Power - marked with Q, unit: var,
Apparent Power) S, unit: VA.
These three types of power are the most known, but there are also other types.
At school we are taught that these powers form the so-called 'power triangle' with properties
expressed in the equation:
This equation, however, is valid only for systems with sinusoidal voltage and current waveforms.
Before moving to a more detailed discussion concerning power measurement, individual types
of power should be defined.
5.3.1 Active power
Active power P is a magnitude with precise physical meaning and it expresses the ability of a
system to perform a particular work. It is the power most desired by the energy consumers and it is
for this supplied power that the consumer pays the supplier in a given settlement period (the problem
of fees for additional reactive power is discussed separately – see below). It is the active power
(and consequently, the active energy) which is measured by electric energy meters in each house-
hold.
The basic formula for calculating the active power is as follows:
where: u(t) – instantaneous voltage value, i(t) - instantaneous current value, T - period for which the
power is calculated.
In sinusoidal systems, the active power may be calculated as: