5.3.3 Non-directional earth fault protection (I0>; 50N/51N)
The non-directional earth fault function is used for instant and time-delayed earth fault protection. The
number of stages in the function depend on the device model. The operating characteristics are based
on the selected neutral current magnitudes which the function measures constantly. The available
analog measurement channels are I01 and I02 (residual current measurement) and I0Calc (residual
current calculated from phase current). The user can select these channels to use RMS values, TRMS
values (including harmonics up to 32
nd
), or peak-to-peak values. The blocking signal and the setting
group selection control the operating characteristics of the function during normal operation, i.e. the
user or user-defined logic can change function parameters while the function is running.
The outputs of the function are the START, TRIP and BLOCKED signals. The non-directional earth fault
function uses a total of eight (8) separate setting groups which can be selected from one common
source.
The function can operate on instant or time-delayed mode. In the time-delayed mode the operation can
be selected for definite time (DT) or for inverse definite minimum time (IDMT); the IDMT operation
supports both IEC and ANSI standard time delays as well as custom parameters. The function includes
the checking of CT saturation which allows the function to start and operate accurately even during CT
saturation.
The operational logic consists of the following:
• input magnitude selection
• input magnitude processing
• saturation check
• threshold comparator
• block signal check
• time delay characteristics
• output processing.
The inputs for the function are the following:
• operating mode selections
• setting parameters
• digital inputs and logic signals
• measured and pre-processed current magnitudes.
The function outputs the START, TRIP and BLOCKED signals which can be used for direct I/O
controlling and user logic programming. The function generates general time-stamped ON/OFF events
to the common event buffer from each of the three (3) output signals. In the instant operating mode the
function outputs START and TRIP events simultaneously with an equivalent time stamp. The time stamp
resolution is 1 ms. The function also provides a resettable cumulative counter for the START, TRIP and
BLOCKED events.
The following figure presents a simplified function block diagram of the non-directional earth fault
function.
A
AQ
Q-T215
-T215
Instruction manual
Version: 2.04
104
