HT instruments SIRIUS89N - Manual (Page 125)

SIRIUS89N
EN - 123
16.11.4. Presence of harmonics: consequences
In general, even harmonics, i.e. the 2
nd
, 4
th
etc., do not cause problems. Triple harmonics,
odd multiples of three, are added on the neutral (instead of cancelling each other) thus
creating a condition of overheating of the wire which is extremely 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. This 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
harmonic contrast the current flow through the motors making its
operation harder and shortening 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).
16.12. POWER AND POWER FACTOR DEFINITION
In a standard electric installation powered by three sine voltages the following is defined:
Phase Active Power:
(n=1,2,3)
) cos( I V P
n n nN n
ϕ ⋅ ⋅ =
Phase Apparent Power:
(n=1,2,3)
n nN n
I V S
⋅ =
Phase Reactive Power:
(n=1,2,3)
2 2
n n n
P S Q
− =
Phase Power Factor:
(n=1,2,3)
n
n
n F
S
P
P
=
Total Active Power:
3 2 1
P P P P
TOT
+ + =
Total Reactive Power:
3 2 1
Q Q Q Q
TOT
+ + =
Total Apparent Power:
2 2
TOT TOT TOT
Q P S
+ =
Total Power Factor:
TOT
TOT
TOT F
S
P
P
=
where:
V
nN
= 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.