Siemens 7SJ82 - Application Example: Tank Leakage Protection; Description

Observe that with the selection of a higher pickup value I
pick
, the resistor value must be lowered and therefore
the dissipation rises sharply.
The varistor (see following figure) must be sized such that it remains high impedance up to the knee-point
voltage, for example:
•
Approx. 100 V with 5 A transformer
•
Approx. 500 V with 1 A transformer
[dwanedif-310113-01.tif, 2, en_US]
Figure 6-108 Connection Diagram of the Restricted Ground-Fault Protection According to the High-Impe-
dance Principle
Even with unfavorable wiring, the maximum occurring voltage peaks do not exceed 2 kV for safety reasons.
When for performance reasons, several varistors must be connected in parallel, give preference to types with
flat characteristic curves, in order to avoid an unbalanced load. Siemens therefore recommends the following
types by METROSIL:
600A/S1/S256 (k = 450, β = 0.25)
600A/S1/S1088 (k = 900, β = 0.25)
In the example, set the pickup value of the first Definite-time overcurrent protection stage (setting
Threshold) to 0.1 A for 5-A transformers or 0.05 A for 1-A transformers. No further protection stages are
needed. Delete these or switch them off. Set the Operate delay setting to 0 s.
If several current transformers are connected in series, for example, with use as busbar protection with several
feeders, the magnetization currents of the transformers switched in parallel can no longer be neglected. In
this case, add up the magnetization currents at half of the knee-point voltage (corresponds to the set
Threshold). These magnetization currents reduce the current through the resistor R. Thus, the actual pickup
value is correspondingly higher.
Application Example: Tank Leakage Protection
Description
Tank leakage protection records short-circuits to ground – including high-impedance ones – between a phase
and the tank of a transformer. The tank is thus insulated, or at least grounded with high impedance. The tank
must be connected with a line to ground. The current that flows through this line is fed to the protection
device. If a short-circuit to ground occurs in a tank, a fault current (tank current) flows to substation ground
via the ground connection.
The function Overcurrent protection, 1-phase detects the tank current. If the tank current exceeds the set
Threshold, the function Overcurrent protection, 1-phase generates an operate indication. Depending on
the set Operate delay, the transformer is tripped immediately or time-delayed on all sides.
For tank protection, a sensitive, 1-phase current measuring input is used.
6.12.8
6.12.8.1
Protection and Automation Functions
6.12 Overcurrent Protection, 1-Phase
SIPROTEC 5, Overcurrent Protection, Manual 541
C53000-G5040-C017-8, Edition 07.2017