Ground fault detection system for ungrounded power systems
AssigneeSchweitzer Engineering Laboratories, Inc.
InventorsJeffrey B. Roberts, David E. Whitehead
Life of this patent
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Abstract
The fault detection system includes at least one valid zero sequence voltage source, either from a local relay or a remote relay on a protected line. Zero sequence current is selected between total (vector sum from both ends of the protected line) measured zero sequence current or calculated zero sequence current. A zero sequence impedance-based calculation is then made and the result is compared against a threshold value to produce a trip decision. The trip decision is then applied through a normal tripping circuit or a slow tripping circuit, depending upon selected circuit conditions, involving the local and remote ends of the line.
Claims11
A ground fault detection system for use in a local protective relay for ungrounded power systems, comprising:
- a selected one of (a) a zero sequence voltage value from a local relay on a protected power line and (b) a zero sequence voltage value from a remote relay on the power line;
- total zero sequence current values from the local relay and from the remote relay;
- a circuit for calculating zero sequence impedance from the selected zero sequence voltage value and the total zero sequence current value;
- and
- a first tripping circuit for tripping a circuit breaker associated with the protected power line at a selected time interval following determination that the calculated zero sequence impedance value exceeds a selected threshold, the first tripping circuit being subject to a second tripping circuit for tripping the circuit breaker at a time interval greater than the selected time interval when the calculated zero sequence impedance exceeds a preselected value and when selected other circuit conditions exist.
Dependencies are inferred from the claim text.