THD function measures constantly phase and residual current magnitudes and the harmonic content of
the monitored signals up to 31.st harmonic component. When the THD function is activated the THD
measurements are available for displays also. User has possibility to set also the alarming limits for
each measured channels if required by the application.
THD of the measured signals can be selected either amplitude- or power ratio THD. The difference is in
the calculation formula:
Power THD ratio is the sum of harmonic components squared divided by the fundamental component
squared.
Figure. 5.6.6. - 158. THD calculation formula
Amplitude THD (percentage) is otherwise similar in difference of that the result is square root of the
Power THD:
Both of these mentioned ways to calculate THD exist, while power THD is known by IEEE and IEC
de nes the amplitude ratio.
Blocking signal and setting group selection controls the operating characteristics of the function during
normal operation if the alarming is selected to be active.
Outputs of the function are Start and Alarm act signals for phase current THD, I01 THD, I02 THD and
Blocked signals. Setting parameters are static inputs for the function which are changed only by user
input in the setup phase of the function. THD function utilizes total of eight separate setting groups
which can be selected from one common source.
The operational logic consists of input magnitude processing, threshold comparator, block signal
check, time delay characteristics and output processing.
Inputs for the function are setting parameters and measured and pre-processed current magnitudes
and binary input signals. Function outputs THD Alarm act and BLOCKED signals which can be used for
direct IO controlling and for user logic programming. The function registers its operation into 12 last
time-stamped registers and also generates general time stamped ON/OFF events to the common
event buffer from each of the two output signals. Time stamp resolution is 1ms. Function provides also
cumulative counters for THD Start and Alarm act and BLOCKED events.
In the following gure is presented the simpli ed function block diagram of the THD function.
AQ-T257
Instruction manual
Version: 2.00
283
© Arcteq Relays Ltd
