Siemens Reyrolle 7SR5, Руководство пользователя

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Application and Setting Notes
A high-impedance current differential scheme provides protection stability (does not operate) during through
faults with or without coincident saturated CT conditions. The scheme allows the user to select a suitable
operate current to detect all internal fault conditions.
Basic Principles of Operation
The CT locations define the limits of the protected zone. CT secondary windings are connected in parallel with
the relay so that all currents are summated and only unbalance current flows through the protection relay.
Typically 1 protection relay is provided per protected zone. On important circuits relays may be duplicated i.e.
2 relays are wired in parallel.
[dw_7SR5_function87GHApplicationAndSettingNotes1, 1, en_US]
Figure 5-44 Balanced Circulating Current Protection System
The relay measures the difference in current “entering” the protected zone with that “leaving” the protected
zone. Where no internal fault occurs and the CTs transform perfectly the differential current is zero.
High-impedance differential protection must:
•
Guarantee stability for all load and through fault conditions. Note that due to transient CT errors (e.g. CT
saturation) the CTs may not transform perfectly
•
Guarantee operation for all internal fault conditions
Stability Requirement
The use of class PX CTs (IEC 61869-2) ensures steady state CT errors are minimized. Transient CT errors are
caused by CT saturation e.g. due to high currents flowing at times of through faults. Where CT saturation
conditions are different in each CT this will cause differential current to flow in the CT secondary circuit wiring.
The highest level of differential current will flow when one set of CTs is fully saturated, providing zero output
and all other CTs transform normally.
When fully saturated the CT secondary provides no current and it behaves as a resistance in the secondary
circuit. Differential current in the secondary circuit will flow either through this resistance or through the relay.
A stabilizing resistance is added in series with the relay input to ensured that the operate voltage at the
current setting is greater than the maximum voltage which can appear across the element/stabilizing resistor
during the maximum assigned through fault conditions. It is assumed that any earthing resistor can become
short-circuit.
This maximum voltage that can appear across the relay circuit can be determined by a simple calculation
which makes the following assumptions:
•
One current transformer is fully saturated making its excitation current negligible
•
The remaining current transformers maintain their ratio
The resistance of the secondary winding of the saturated CT together with the leads connecting it to the relay
circuit terminals constitute the only burden in parallel with the relay.
The minimum required relay operate voltage setting (V
set
) is given by:
[fo_function87GHApplication&SettingsFunction1, 1, en_US]
To ensure high speed relay operation the relay circuit operating voltage should be selected in accordance with
the stability requirement above, also, the operate voltage should not exceed 0.5 ⋅ CT knee point voltage (Vk).
5.21.4
Protection and Automation Functions
5.21 87GH Restricted Earth-Fault Protection – High-Impedance
Reyrolle 7SR5, Transformer Protection Device, Device Manual 297
C53000-G7040-C015-1, Edition 11.2019