2 Functions
50
7SD5 Manual
C53000-G1176-C169-1
conversion of the setting values which depend on this distance unit. They have to be
re-entered into their corresponding valid addresses.
Mode of Earth Im-
pedance (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 compen-
sation is determined. It is possible to either use 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).
Operating 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-synchro-
nous the instant the breaker poles make contact.
Trip Command
Duration
In address
240
the minimum trip command duration
TMin TRIP CMD
is set. This
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 trip test is initiated via
the device. This parameter can only be altered with DIGSI
®
under
Additional Set-
tings
.
In address
241
the maximum close command duration
TMax CLOSE CMD
is set. This
applies to all close commands 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 set 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 ter-
minated following a new trip command from a protection function. This setting is only
possible via DIGSI
®
at "
Additional Settings
".
Circuit Breaker
Test
7SD5 allows a circuit breaker test during operation by means of a tripping and a
closing command entered on the front panel or via 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 0.1s.
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 sum of the sec-
ondary currents can reach considerable peaks due to the saturation of the transform-
ers when a short-circuit current flows through the line. These peaks may feign an in-
ternal fault. The features included in the 7SD5 to prevent errors in case of current
transformer saturation work completely satisfying if the protection knows the response
characteristic of the current transformers.
For this, the characteristic data of the current transformers and of their secondary cir-
cuits are set (see also Figure 2-19 in Section 2.3). The default setting are adequate in
most cases; they correspond to usual current transformers for protection purposes.
The nominal accuracy limit factor n of the current transformers and the nominal power
P
N
are usually stated on the rating plate of the current transformers. The information
stated refers to reference conditions (nominal current, nominal burden). For example
(according to VDE 0414 / Part 1 or IEC 60044)
Current transformer 10P10; 30 VA
→
n = 10; P
N
= 30 VA
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