Operation Principle - ABB RELION 620 Series Technical Manual

1MAC504801-IB E
4.1.4.4
620 series ANSI
Technical Manual
The function picks up and trips when the measured (IG) or calculated (IN) ground current
exceeds the set limit. The trip time characteristic for low stage 51N/G and high stage 50N/
G-1/2 can be selected to be either definite time (DT) or inverse definite minimum time
(IDMT). The instantaneous stage 50N/G-3 always trips with the DT characteristic.
In the DT mode, the function trips after a predefined trip time and resets when the fault
current disappears. The IDMT mode provides current-dependent timer characteristics.
The function contains a blocking functionality. It is possible to block function outputs,
timers or the function itself, if desired.

Operation principle

The function can be enabled and disabled with the Operation setting. The corresponding
parameter values are "Enable" and "Disable".
The operation of non-directional ground-fault protection can be described by using a
module diagram. All the modules in the diagram are explained in the next sections.
A070437-ANSI V1 EN
Figure 86: Functional module diagram. Io represents the residual current.
Level detector
The measured ground current is compared to the set Pickup value. If the measured value
exceeds the set Pickup value, the level detector sends an enable-signal to the timer module.
If the ENA_MULT input is active, the Pickup value setting is multiplied by the Pickup
value Mult setting.
Do not set the multiplier setting Pickup value Mult higher than necessary.
If the value is too high, the function may not trip at all during an inrush
followed by a fault, no matter how severe the fault is.
The pickup value multiplication is normally done when the inrush detection function
(INR) is connected to the ENA_MULT input.
Section 4
Protection functions
189