Application - ABB 615 series Technical Manual

Section 4
Protection functions
4.1.3.5
264
The Minimum trip time setting defines the minimum desired trip time for IDMT operation.
The setting is applicable only when the IDMT curves are used.
Though the Time multiplier and Minimum trip time settings are common
for different IDMT curves, the trip time essentially depends upon the type
of IDMT curve chosen.
The Timer calculates the pickup duration value PICKUP_DUR which indicates the
percentage ratio of the pickup situation and the set trip time. This output is available in the
Monitored data view.
Blocking logic
There are three operation modes in the blocking function. The operation modes are
controlled by the BLOCK input and the global setting Configuration/System/Blocking
mode which selects the blocking mode. The BLOCK input can be controlled by a binary
input, a horizontal communication input or an internal signal of the protection relay's
program. The influence of the BLOCK signal activation is preselected with the global
setting Blocking mode.
The Blocking mode setting has three blocking methods. In the "Freeze timers" mode, the
operation timer is frozen to the prevailing value. In the "Block all" mode, the whole
function is blocked and the timers are reset. In the "Block TRIP output" mode, the function
operates normally but the TRIP output is not activated.

Application

The three-phase voltage-dependent overcurrent protection is used as a backup protection
for the generators and system from damage due to the phase faults which are not cleared
by primary protection and associated breakers.
In case of a short circuit, the sustained fault current of the generator, determined by the
machine synchronous reactance, could be below the full-load current. If the generator
excitation power is fed from the generator terminals, a voltage drop caused by a short
circuit also leads to low fault current. The primary protection, like normal overcurrent
protection, might not detect this kind of fault situation. In some cases, the automatic
voltage regulator AVR can help to maintain high fault currents by controlling the
generator excitation system. If the AVR is out of service or if there is an internal fault in
the operation of AVR, the low fault currents can go unnoticed and therefore a voltage-
depended overcurrent protection should be used for backup.
Two voltage control characteristics, voltage step and voltage slope, are available in 51V.
The choice is made based on the system conditions and the level of protection to be
provided.
1MAC059074-MB A
615 series ANSI
Technical Manual