Application - ABB RELION REX640 Technical Manual

Section 4
Protection functions
4.12.4.6
1048
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, SRCPTOC operates normally but the OPERATE output is not activated.

Application

Switched shunt capacitor banks are widely used by utilities and customers in
industrial distribution systems to provide voltage support and to improve the power
factor of a load. Capacitor steps may be switched in and out of circuits routinely as the
demand for capacitive VAR compensation of a load fluctuates. Normally, automatic
power factor controllers are employed which automatically switch on or off the
capacitors of the capacitor bank, depending upon the prevalent reactive power
requirement in the system.
One potential problem for the application of automatic power factor controllers is that
it may cause harmonic resonance under certain system conditions. Capacitor
switching changes the parameters of the system, which may cause the resonance
frequency of the circuit to be equal to one of the frequencies of the harmonic sources
prevalent in the system. Harmonic resonance, when it occurs, may result in severe
voltage and current distortions, which increases losses and causes overheating of other
equipment in the circuit.
A traditional way of solving the problem is to conduct a detailed system study for each
individual installation and use the results to properly size the capacitors and determine
the right operating range of capacitors to avoid harmonic resonance with other system
components. However, this method is not economical but more time-consuming.
The capacitor switching-resonance protection function can be used as a solution to the
above mentioned problem. The basis for the harmonic resonance protection is the
detection of a current harmonic resonance condition caused by capacitor switching. A
prolonged increase of the harmonic distortion level after a switching operation is a
clear indication of such condition. When a resonant condition caused by capacitor
switching occurs in a circuit, SRCPTOC detunes the circuit by taking the reverse
action, that is, switching the capacitor bank off if switching it on causes resonance. If
the resonance situation has been detected and SRCPTOC has switched off a capacitor
bank, power factor controller should not try to switch on the capacitor bank until the
switching resonance function reset.
The capacitor switching-resonance protection function can also be used to protect
harmonic filters. In harmonic filter bank applications, the SRCPTOC function can be
tuned to harmonic frequency for which the harmonic filter is designed to ensure that
the function does not include thr tuned harmonic frequency current into the
calculation of the resonance current. If there is more than one harmonic filter bank
involved, each SRCPTOC tunes to the harmonic frequency of its corresponding filter
bank. The interlinking between the functions can be done in such a way that if
resonance occurs in a higher harmonic frequency filter bank, all the lower harmonic
frequency filter banks can be tripped immediately by activating the function input
RESO_IN.
1MRS759142 C
REX640
Technical Manual