Rockwell Automation 865 Differential Protection Relay - Manual (Page 112)

5-2 Measurement
Functions
865-UM001A-EN-P – July 2009
Table 5.3 – Frequency
Measuring range 16 Hz – 75 Hz
Inaccuracy 10 mHz
In the Allen-Bradley 865, frequency is measured from current
signals.
Table 5.4 – THD and Harmonics
Inaccuracy I 0.1 PU 2 % units
Update rate Once a second
The specified frequency range is 45 Hz – 65 Hz.
The device calculates the THDs as percentage of the base
frequency for currents and voltages.
The device calculates the harmonics from the 2
nd
to the 15
th
of
phase currents and voltages. (The 17
th
harmonic component will
also be shown partly in the value of the 15 th harmonic
component. This is due to the nature of digital sampling.)
The harmonic distortion is calculated using equation
1
15
2
2
h
h
THD i
i
∑
= =
, where
h
1
= Fundamental value
h
2...15
= Harmonics
Example
h
1
= 100 A
h
3
= 10 A
h
7
= 3 A
h
11
= 8 A
% 2 . 13
100
8 3 10 2 2 2
=
+ +
= THD
For reference the RMS value is:
A RMS 9 . 100 8 3 10 100 2 2 2 2 = + + + =
Another way to calculate THD is to use the RMS value as
reference instead of the fundamental frequency value. In the
example above the result would then be 13.0 %.
Harmonics and Total
Harmonic Distortion
(THD)