US 5,436,786 A · Isolator surge protector for DC isolation and AC grounding of cathodically protected systems

Abstract

An isolator surge protector for use with corrosion protection systems which are an inherent part of or coupled to an electrical power system, for power transformers, and for DC transmission systems, where it is necessary to present a high impedance to DC and a low impedance to AC up to a predetermined voltage level. Anti-paralleled thyristors in the isolator surge protector turn on during power surges. A bypass circuit insures turn off of the thyristors after a triggering event such as lightning or a 60 Hz surge has ended. The bypass circuit shunts DC bias currents, such as that which may be provided by a DC bias voltage source for cathodic protection, from the thyristors to ensure that the voltage across each thyristor falls below its holding voltage or current, and thus prevents the thyristor from being stuck in a continuous conduction mode. Power to operate the bypass circuit may be tapped from the surge which caused the thyristor to trigger so that the bypass circuit operates autonomously. The circuit may also operate with an auxiliary power supply to provide the required operating power and to turn off the thyristors.

Claims40

Dependent claims

A bypass circuit for an isolator surge protector in an electrical system, the isolator surge protector of the type having two terminals and at least one thyristor which provides a low impedance conduction path between the terminals of the isolator surge protector in response to a surge condition, and the electrical system of the type having an external DC bias circuit, the bypass circuit comprising:

(a) switch means responsive to a control signal for providing a low impedance bypass path across the at least one thyristor;
(b) control means for generating the control signal to provide the bypass path a selected period of time after the voltage across the at least one thyristor falls below and remains below a first predetermined level after exceeding that level, the first predetermined level indicative of a surge condition, and for removing the control signal after a limited period of time so that the switch means no longer provides a low impedance bypass path across the at least one thyristor.

An isolator surge protector, comprising:

(a) two terminals and a pair of thyristors connected in anti-parallel across the two terminals;
(b) means for triggering conduction of the thyristor which has a forward voltage across it which exceeds a predetermined level indicative of a surge condition;
(c) switch means responsive to a control signal for providing a low impedance bypass path across at least one of the thyristors;
(d) control means for generating the control signal to provide the bypass path when the current in the at least one thyristor becomes continuous DC current which falls below and remains below a selected level to turn off the at least one thyristor, and for removing the control signal after a limited period of time so that the switch means no longer provides a low impedance bypass path across the at least one thyristor.

1 more element

Dependencies are inferred from the claim text.

Isolator surge protector for DC isolation and AC grounding of cathodically protected systems

Life of this patent

Isolator surge protector for DC isolation and AC grounding of cathodically protected systems

Life of this patent

Abstract

Claims40