Toshiba GRD150 Series Instruction Manual page 83

2.2.3 Zero Phase Sequence Overvoltage Protection
The zero phase sequence overvoltage protection is applied to earth fault detection on unearthed,
resistance-earthed system or on ac generators.
The zero phase sequence overvoltage (ZOV) element is available for the following models and
their [APPL-VT] settings:
Model
[APPL-VT] setting
ZOV
Note: (∗1) V is calculated from the three measured phase voltages.
0
(∗2) V
0
The low voltage settings which may be applied make the ZOV element susceptible to any 3
harmonic component which may be superimposed on the input signal. Therefore, a 3
filter is provided to suppress such superimposed components.
For the earth fault detection, following two methods are in general use.
• Measuring the zero sequence voltage produced by VT residual connection (broken-delta
connection) as shown in Figure 2.2.9.
• Measuring the residual voltage across the earthing transformers as shown in Figure 2.2.10.
A B C
Figure 2.2.9
A
100 and 200 series
3PN 3PV
(∗1) (∗2)
is measured directly in the form of the system residual voltage.
Earth Fault Detection on Unearthed System
B
G
Resistor
Figure 2.2.10 Earth Fault Detection on Generator
⎯
82
300 and 400 series
3PP 2PP 3PN
(∗2) (∗2) (∗1)
V
0
GRD150
V
0
⎯
6 F 2 S 0 8 4 2
3PP 2PP
－ －
rd
harmonic
GRD150
rd