Siemens SIPROTEC 7SA522 Manual page 98

2 Functions
Grading Coordina-
tion Chart
98
It is recommended to initially create a grading coordination chart for the entire galvan-
ically interconnected system. This diagram should reflect the line lengths with their
primary impedances Z in Ω/km. For the reach of the distance zones, the impedances
Z are the deciding quantities.
The first zone Z1 is usually set to cover 85°% of the protected line without any trip time
delay (i.e. T1 = 0.00 s). The protection clears faults in this range without additional time
delay, i.e. the tripping time is the relay basic operating time.
The tripping time of the higher zones is sequentially increased by one time grading in-
terval. The grading margin must take into account the circuit breaker operating time
including the spread of this time, the resetting time of the protection equipment as well
as the spread of the protection delay timers. Typical values are 0.2 s to 0.4 s. The
reach is selected to cover up to approximately 80 % of the zone with the same set time
delay on the shortest neighbouring feeder.
When using a personal computer and DIGSI
tionally entered as primary or secondary values.
In the case of parameterization with secondary quantities, the values derived from the
grading coordination chart must be converted to the secondary side of the current and
voltage transformers. In general:
Accordingly, the reach for any distance zone can be specified as follows:
with
N = Current transformer ratio
CT
N = Transformation ratio of voltage transformer
VT
On long, heavily loaded lines, the MHO circle may extend into the load impedance
range. This is of no consequence as the pick-up by overload is prevented by the load
trapezoid. Refer to margin heading "Load Area" in Subsection 2.2.1.
Calculation Example:
110 kV overhead line 150 mm
s (length) = 35 km
= 0.19 Ω/km
R /s
1
= 0.42 Ω/km
X /s
1
= 0.53 Ω/km
R /s
0
= 1.19 Ω/km
X /s
0
Current Transformer
Voltage Transformer
The following line data is calculated:
= 0.19 Ω/km · 35 km = 6.65 Ω
R
L
= 0.42 Ω/km · 35 km = 14.70 Ω
X
L
®
2
with the following data:
600 A / 5 A
110 kV / 0.1 kV
to apply the settings, these can be op-
C53000-G1176-C155-3
7SA522 Manual