Logic of the Transient Ground-Fault Functionality
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Figure 6-131
Logic Diagram of the Directional Transient Ground-Fault Stage
Measured Values, Method of Measurement
The zero-sequence values of zero-sequence voltage and zero-sequence current are measured directly or calcu-
lated from the phase variables. When measuring directly, the following is detected:
•
Zero-sequence voltage on the broken-delta winding
•
Zero-sequence current via Holmgreen connection or via core balance current transformer
The voltage measured on the broken-delta winding will be converted to the zero-sequence voltage V
0
.
The instantaneous values of the zero-sequence voltage V0(t) that are sampled at a high frequency (8 kHz)
serve to determine the point in time of the ground fault occurrence T0.
The instantaneous values of the zero-sequence voltage V0(t) and the ground current 3I0(t), which are
sampled at a high frequency (8 kHz), are the basis for direction determination.
The fundamental-component values of the zero-sequence voltage V0 serve to release the directional result
and the pickup as well as a criterion for the stabilization against switching operations.
The positive-sequence system (if it exists as a measured value) serves as an additional criterion for the stabili-
zation against switching operations.
The fundamental-component values V0 and 3I0 will be used in the optional trip logic.
Operational, meaning circulating zero-sequence currents, can occur in closed loops or meshed systems. This
type of zero-sequence current is also present in case of a failure and can falsify the directional result. There-
fore, an operational zero-sequence current is eliminated.
Protection and Automation Functions
6.15 Sensitive Ground-Fault Detection
SIPROTEC 5, Overcurrent Protection, Manual 579
C53000-G5040-C017-8, Edition 07.2017
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