NR Electric PCS-9611, Instruction Manual

Page: 85 / 300

3 Operation Theory
PCS-9611 Feeder Relay
3-49
Date: 2014-04-08
For the information about the common explanation of the settings, see Section 7.3 .
No. Menu text Explanation Range Step
1 59Q.U2_Set
The voltage setting of the negative sequence
overvoltage protection
2~120V 0.001V
2 59Q.t_Op
The time s etting of the negative sequence
overvoltage protection
0.03~100s 0.001s
3 59Q.En
The logic setting of the negative sequence
overvoltage protection
0~1 1
4 59Q.OutMap
The output matrix setting of the negative
sequence overvoltage protection
0x00000 ~
0xFFFFF
1
3.16 Frequency Protection
The frequency protection detects abnormally high and low frequencies in the power system or in
electrical machines. If the frequency is out of the allowable range, the appropriate actions are
initiated, such as load shedding or separating a generator from the system.
A decrease in system frequency occurs when the system experiences an increase in the real
power demand, or when a malfunction occurs with a generator governor or automatic generation
control (AGC) system. The frequency protection function is also used for generators, which (for a
certain time) operate to an island network. This is due to the fact that the reverse power protection
cannot operate in case of a drive power failure. The generator can be disconnected from the
power system using the frequency decrease protection.
An increase in system frequency occurs, e.g. when large blocks of load (island network) are
removed from the system, or again when a malfunction occurs with a generator governor. This
entails risk of self-excitation for generators feeding long lines under no-load conditions.
The calculation of the frequency is based on the voltage sampled values. Four cycles of the
voltage sampled values are fixedly adopted for the frequency calculation.
3.16.1 Under-frequency Protection
The feeder relay includes under-frequency protection to facilitate load shedding and subsequent
restoration. It provides a four-stage under-frequency protection with independent definite time
delay characteristics in this relay, and the four stages have same protection logics.
This protection can be enabled after 100ms only when the frequency is greater than the frequency
setting [81Ux.f_Set] (x: 1~4) and three phase-to-phase voltages are greater than the setting
[81.Upp_VCE]. Meanwhile, this protection will be blocked when the system frequency is less than
“f
n
-15.00Hz
” or greater than “f
n
+15.00Hz
” for longer than 200ms (f
n
: system rated frequency).
The logic diagram of the stage 1 under-frequency protection is shown as below.