PROTECT ION TR I P UNI TS
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S(V) - Voltage controlled overcurrent protection (ANSI 51V) This provide protection from a maximum current with a constant trip time (t = k) that is sensitive to the voltage value. Following a voltage drop, the current set threshold decreases in steps or linearly. It is possi- ble to set the operating mode to: active, alarm only, or deactivated. The protection operates also with the circuit-breaker open, thus allowing fault identification before circuit-breaker closing.
t
1
Ks
t20
I
KsxI20
I20
In step mode (controlled mode) the protection is tripped at a set threshold (I20) if the voltage is above U, whereas it is tripped at the lower threshold of the factor Ks (I20 * Ks) if the voltage is below U.
K
1
Ks
V
UI
t
Extremely Inverse Very Inverse Standard Inverse
8 In linear mode (restrained mode) two voltage limits are selected within which the protection is tripped at the set threshold (I20) reduced by a factor of K corresponding to the measured voltage. The variation of the factor K is proportional to the voltage, and for voltages greater than the upper threshold (Uh) the threshold I20 works, whereas for voltages below the lower threshold (UI) the minimum threshold (I20 * Ks) applies.
K
1
Ks
I
V
UI
Uh
t
UV S2(V) – 2nd protection against voltage-controlled overcurrent protection (ANSI 51V) Available in addition to the protection S(V), this enables total selectivity to be achieved in all installations. It is possible to set the operating mode to: active, alarm only, or deactivated. The protection also operates with the circuit-breaker open, thus allowing fault identification before circuit-breaker closing. Residual overvoltage (ANSI 59N) With a constant trip time (t = k), this protects against insulation loss in systems with insulated neutral or with neutral earthed with impedance. It is possible to set the operating mode to: active, alarm only, or de- activated. The protection also operates with the circuit-breaker open, thus allowing fault identification before circuit-breaker closing. OV
U8
t8
V
t
2
V23
V12
U9
α α
t8 9
1
3
V31
V
UF Underfrequency (ANSI 81L) With a constant trip time (t = k), this trips when the network frequency falls below a set threshold.
t
f12
UF
t12
f
t
OF
f13
t13
f
mean
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