Power system fundamentals - Wind Power Integration: Connection and System Operational Aspects - page 30

Q is the reactive power . It is the amplitude of that component of instantaneous
power which oscillates . It must be managed carefully by power system operators,
not least because the associated current requires conductor capacity. Note that unity
power factor implies zero reactive power.
The units (and unit symbols) of active and reactive power are as follows:

(active) power watts (W)

reactive power volt-amperes reactive (VAr)

2.3.2 Phasors
So far we have described time-varying cosinusoidal (or sinusoidal) quantities in the
time domain: for example,

v ð t Þ¼ V m cos ðw t þ fÞ
This equation contains a lot of information we do not normally require. Usually
all we need are:

Magnitude the peak or r.m.s. value is sufficient

Phase the voltage (or current) leads a reference by angle f

Frequency often the same for all quantities

A shorthand version of the above equation is provided by a phasor . The phasor
is a complex quantity, in this case voltage V . This may be represented as a vector on
an Argand diagram, as depicted in Figure 2.5. The magnitude of the phasor equals

the r.m.s. value of the quantity being represented, V m = p in this case. The angle of

the phasor is the angle of the quantity relative to a reference, f in our example.
The instantaneous value may be obtained from the phasor by reversing the
above procedure. In most practical cases, the phasor quantity contains all the
information we need.

2.3.2.1 Impedance: phasor representation
Phasors provide a convenient means of analysing circuits subject to alternating
voltage and current. First, it is necessary to understand how the three passive circuit

V or V

f

0

Real axis

Figure 2.5 Representation of a phasor on an Argand diagram