ABB RELION REX640 Technical Manual page 1126

Protection functions
Level detector 2
The calculated compensated unbalance current I_AMPL_COMP is compared to the
set
detector 2 sends enabling signal to the Alarm control module.
Alarm control
Depending on the
"Normal mode" or "Element counter mode".
In the "Normal mode" the time characteristic is according to DT. When the
alarm timer has reached the value set by
activated. If the fault disappears before the alarm activates, the alarm timer is reset
immediately.
The "Element counter mode" is used to detect faulty elements of the capacitor bank
and count the number of element failures in each branch and line. On activation, this
module increments the corresponding element failure counters after the set
delay time has elapsed.
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 IED
program. The influence of the BLOCK signal activation is preselected with the global
setting
The
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
OPERATE output" mode, CUBPTOC operates normally but the OPERATE output is
not activated.
4.12.2.6 Application
CUBPTOC is designed for the protection against internal faults in double-Y-
connected capacitor banks. This unbalance protection detects an asymmetry in the
capacitor bank caused by blown fuses or short circuits across bushings or between
capacitor units and the racks in which they are mounted.
Normally, the capacitor units are designed to withstand 110 percent of the nominal
voltage continuously. When an element inside a capacitor bank fails, the remaining
healthier elements experience an increase in voltage across them. If the voltage
exceeds the 110 percent value of the nominal voltage, it can lead to a failure of
the healthier elements of the bank and in turn fail the entire capacitor bank. Since
the capacitor unbalance current is directly proportional to the element failures,
unbalance protection is an effective way of detecting capacitor element failures.
The current unbalance protection function is usually used with the three-phase
capacitor bank overload protection function to increase the sensitivity of protection
for capacitor banks.
Due to the two-stage (operation and alarm stage) unbalance protection and
the natural unbalance compensation facility, the protection of capacitor banks
with internal fuses can be implemented with a very high degree of sensitivity.
1126
Alarm start value . If the I_AMPL_COMP exceeds the set Alarm value the Level
Alarm mode setting, the alarm stage operation is according to
Blocking mode .
Blocking mode setting has three blocking methods. In the "Freeze timers"
Alarm delay time , the ALARM output is
1MRS759142 F
Alarm
REX640
Technical Manual