Application - ABB RELION REX640 Technical Manual

1MRS759142 C
4.12.2.6
REX640
Technical Manual
Alarm control
Depending on the Alarm mode setting, the alarm stage operation is according to
"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 Alarm delay time, the ALARM output is 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 Alarm
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
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 OPERATE output" mode,
CUBPTOC operates normally but the OPERATE output is not activated.

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. Furthermore, CUBPTOC
Section 4
Protection functions
1027