ABB RELION 620 Series Technical Manual page 669

1MRS757644 F
4.5.6.5
620 series
Technical Manual
Level detector
The calculated negative-sequence voltage is compared to the set Start value setting. If
the value exceeds the set Start value, the level detector enables the timer.
Timer
Once activated, the timer activates the START output. The time characteristic is
according to DT. When the operation timer has reached the value set by Operate delay
time, the OPERATE output is activated if the overvoltage condition persists. If the
negative-sequence voltage normalizes before the module operates, the reset timer is
activated. If the reset timer reaches the value set by Reset delay time, the operate timer
resets and the START output is deactivated.
The timer calculates the start duration value START_DUR, which indicates the
percentage ratio of the start situation and the set operation time. The value 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 in 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, but the OPERATE output is not
deactivated when blocking is activated. In the "Block all" mode, the whole function is
blocked and the timers are reset. In the "Block OPERATE output" mode, the function
operates normally but the OPERATE output is not activated.
Application
A continuous or temporary voltage unbalance can appear in the network for various
reasons. The voltage unbalance mainly occurs due to broken conductors or
asymmetrical loads and is characterized by the appearance of a negative-sequence
component of the voltage. In rotating machines, the voltage unbalance results in a
current unbalance, which heats the rotors of the machines. The rotating machines,
therefore, do not tolerate a continuous negative-sequence voltage higher than
typically 1-2 percent x U
The negative-sequence component current I
synchronous machine, is linearly proportional to the negative-sequence component
voltage U . When U is P% of U
2 2
The negative-sequence overcurrent NSPTOC blocks are used to accomplish a
selective protection against the voltage and current unbalance for each machine
.
n
, drawn by an asynchronous or a
2
, I is typically about 5 x P% x I
n 2
Section 4
Protection functions
.
n
663