dangerous. Designers should take into consideration the three issues given below
when designing a power distribution system that will contain harmonic current:
• The neutral wire must be of sufficient gauge.
• The distribution transformer must have an additional cooling system to continue
operating at its rated capacity when not suited to the harmonics. This is
necessary because the harmonic current in the neutral wire of the secondary
circuit circulates in the delta-connected primary circuit. The circulating harmonic
current heats up the transformer.
• Phase harmonic currents are reflected on the primary circuit and continue back
to the power source. This can cause distortion of the voltage wave so that any
power factor correction capacitors on the line can be easily overloaded.
The 5
th
and the 11
th
harmonics contrast the current flow through the motors mak-
ing its’ operation harder and shortens their average life.
In general, the higher the ordinal harmonic number, the smaller its energy is and
therefore the impact it will have on the devices (except for transformers).
15.3 POWER AND POWER FACTOR: DEFINITIONS
In a standard electric installation powered by three sine voltages the following is defined:
Where:
V
n
= RMS value of voltage
between phase n and Neutral.
I
n
= RMS value of n phase
current.
f
n
= Phase displacement
angle between voltage and
current of n phase.
51
DM-4 Power Quality Recorder
Phase Active Power:
(n=1,2,3)
P
n
= V
n
• I
n
• cos(
Θ
Θ
n
)
Phase Apparent Power:
(n=1,2,3)
S
n
= V
n
• I
n
Phase Reactive Power:
(n=1,2,3)
Q
n
= S
n
- P
n
Phase Power Factor:
(n=1,2,3)
P
F n
=
Total Active Power:
P
TOT
= P
1
+ P
2
+ P
3
Total Reactive Power:
Q
TOT
= Q
1
+ Q
2
+ Q
3
Total Apparent Power:
S
TOT
= P
TOT
2
+ Q
TOT
2
Total Power Factor:
P
F TOT
=
√
2
2
P
n
S
n
√
P
TOT
S
TOT
