Application - ABB REG670 Applications Manual

Section 8
Impedance protection
8.2.2
8.2.2.1
188

Application

The fast distance protection function ZMFPDIS in the IED is designed to provide sub-
cycle, down to half-cycle operating time for basic faults. At the same time, it is
specifically designed for extra care during difficult conditions in high-voltage
transmission networks, like faults on long heavily loaded lines and faults generating
heavily distorted signals. These faults are handled with utmost security and
dependability, although sometimes with a reduced operating speed.
System earthing
The type of system earthing plays an important role when designing the protection
system. Some hints with respect to distance protection are highlighted below.
Solidly earthed networks
In solidly earthed systems, the transformer neutrals are connected directly to earth
without any impedance between the transformer neutral and earth.
IEC05000215 V2 EN
Figure 80: Solidly earthed network
The earth-fault current is as high or even higher than the short-circuit current. The
series impedances determine the magnitude of the fault current. The shunt admittance
has very limited influence on the earth-fault current. The shunt admittance may,
however, have some marginal influence on the earth-fault current in networks with
long transmission lines.
The earth-fault current at single phase-to-earth in phase L1 can be calculated as
equation 33:
×
3 U
=
3I L1
0
+ + +
Z Z Z 3Z
1 2 0 f
EQUATION1267 V3 EN
Where:
U is the phase-to-earth voltage (kV) in the faulty phase before fault
L1
Z is the positive sequence impedance (Ω/phase)
1
Z is the negative sequence impedance (Ω/phase)
2
Table continues on next page
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
U
=
L1
+ +
Z Z Z
1 N f
1MRK 502 071-UEN -
(Equation 33)
Application manual