7SG12 DAD N Applications Guide
Note that where high quality CTs are being used, with a high V
k
, the value of V
s
chosen must not be so low
that the CT will be operating at the very low, non-linear part of its magnetising curve.
It is important that R
stab
is suitably rated to withstand the current levels expected during an internal (in-
zone) fault. The continuous rating of R
stab
should be:
Continuous Rating of stab
2
sstab R x )(I (W) R ≥
Similarly, R
stab
must have a short time rating large enough to withstand the fault current levels before the
fault is cleared. This is usually for 0.5 seconds to allow for a failure in the main protection system or
switchgear and so considers longer fault clearance times through operation of the back-up protection.
Short Time Rating of
R
V
(W)R
stab
2
aultMaxSecIntF
stab
≥
1.3 x )I x R x (V (V)V
1/4
aultMaxSecIntFstab
3
K aultMaxSecIntF
≥
Where:
V
k
= Kneepoint voltage of the CT
V
MaxSecIntFault
= Maximum secondary internal fault voltage
I
MaxSecIntFault
= Maximum secondary internal fault current = I
MaxPriIntFault
x CT Ratio
Once again, if I
MaxPriIntFault
is not known, the breaking capacity current of the Circuit Breaker can be used.
Note that the stabilising resistor will have no effect until CT saturation occurs. The CT secondaries act as
current sources and any imbalance must go through the relay circuit despite the presence of R
stab
.
3.2 Non-Linear Resistor
For safety reasons, overvoltages within a protection panel must not be allowed to go above 3kV. For the
protection system:
)
) V- R x I( Vx 2 x 2 (V)V KstabaultMaxSecIntF x K Peak =
If V
peak
can go above 3kV, a Metrosil (non-linear resistor) must be fitted in parallel with the Relay circuit to
limit its maximum level. However, it is considered good practice to fit a Metrosil for all installations.
Relay
R
stab
Figure 6 – Non-Linear Resistor
Metrosils are specified by 3 figures – their diameter and the fixed, device-specific constants C (thickness)
and β (chemical composition). The diameter relates to the Power Rating of the device, C and B to the
current which will flow through the Metrosil for a given voltage:
V = C x I
β
The voltage characteristic of the Metrosil must be;
Large enough so that negligible current flows through the Metosil at the relay operating voltage.
C x I
s
β
» R
stab
x I
s
©2010 Siemens Protection Devices Limited Chapter 5 Page 6 of 16
To Next Page
Loading...
