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Distance Unit
Address 236 Distance Unit determines the distance unit (km or Miles) for the fault location indications. If
the compounding function of the voltage protection is used, the overall line capacitance is calculated from the
line length and the capacitance per unit length. If compounding is not used and fault location is not available,
this parameter is of no consequence. Changing the distance unit will not result in an automatic conversion of
the setting values which depend on this distance unit. They have to be re-entered into their corresponding valid
addresses.
Mode of the earth impedance (residual) compensation
Matching of the earth to line impedance is an essential prerequisite for the accurate measurement of the fault
distance (distance protection, fault locator) during earth faults. In address 237 Format Z0/Z1 the format for
entering the residual compensation is determined. It is possible to use either the ratio RE/RL, XE/XL or to
enter the complex earth (residual) impedance factor K0. The setting of the earth (residual) impedance factors
is done in the power system data 2 (refer to Section 2.1.4).
Closing time of the circuit breaker
The circuit breaker closing time T-CB close at address 239 is required if the device is to close also under
asynchronous system conditions, no matter whether for manual closing, for automatic reclosing after three-pole
tripping, or both. The device will then calculate the time for the close command such that the voltages are
phase-synchronous the instant the breaker poles make contact.
Trip command duration
In address 240 the minimum trip command duration TMin TRIP CMD is set. It applies to all protection and
control functions which may issue a trip command. It also determines the duration of the trip pulse when a circuit
breaker test is initiated via the device. This parameter can only be altered using DIGSI under Additional Set-
tings.
In address 241 the maximum close command duration TMax CLOSE CMD is set. It applies to all close com-
mands issued by the device. It also determines the length of the close command pulse when a circuit breaker
test cycle is issued via the device. It must be long enough to ensure that the circuit breaker has securely closed.
There is no risk in setting this time too long, as the close command will in any event be terminated following a
new trip command from a protection function. This parameter can only be altered using DIGSI under Addition-
al Settings.
Circuit breaker test
The 7SD5 allows a circuit breaker test during operation by means of a tripping and a closing command entered
on the front panel or using DIGSI. The duration of the trip command is set as explained above. Address 242
T-CBtest-dead determines the duration from the end of the trip command until the start of the close
command for this test. It should not be less than 0.1 s.
Current transformer characteristic
The basic principle of the differential protection assumes that all currents flowing into a healthy protected
section add up to zero. If the current transformer sets at the line ends have different transformation errors in
the overcurrent range, the total of the secondary currents can reach considerable peaks when a short-circuit
current flows through the line. These peaks may feign an internal fault. The measures to prevent errors in case
of current transformer saturation included in 7SD5 work completely satisfying if the protection knows the trans-
mission behaviour of the current transformers.
For this, the characteristic data of the current transformers and of their secondary circuits are set (see also
Figure 2-27 in Section 2.3). The default setting is adequate in most cases; it considers the data of the worst-
case protective current transformers.
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
Functions
2.1 General
49
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