Page 289 - Offshore Electrical Engineering Manual

276 CHAPTER 9 Harmonics

H is generally taken equal to 50 but can be limited, in most cases, to 25.
Note that THD can exceed 1 and is generally expressed as a percentage.

CURRENT OR VOLTAGE TOTAL HARMONIC DISTORTION

For current harmonics the equation is
�
� h= H ( ) 2
�
I h
∑
THD i = �
h= 2 I 1
�
h= H
�
� ∑ 2
I rms = � I h
h= 1
By introducing the total rms value of the current, we obtain the following relation:
√
( ) 2
I rms
THD i = − 1
I 1
equivalent to
√ 2
I rms = I 1 1 + THD i
Example: For THD = 40%, we get
i
√ 2 √
I rms = I 1 1 + (0.4) = I 1 1 + 0.16 ≈ I 1 × 1.08

For voltage harmonics, the equation is
�
� ) 2
h= H (
� ∑ U h
THD u = �
h= 2 U 1