Patent Application
20060001497
The present invention relates generally to electrical distribution systems. More particularly, the invention relates to apparatus and methods for tripping or closing of reclosers in electrical distribution systems.
Reclosers are sometimes referred to as auto-reclosers, auto-reclosing circuit breakers, reclosing relays, or the like. Reclosers have electrical contacts that close or open power lines in high voltage electrical distribution systems to provide electrical power to the power lines in the system. If an over-current or fault condition occurs, the recloser may open its electrical contacts at a known time delay after the occurrence of the over-current or fault condition. Actuation circuitry will sense any open condition of the recloser and one or more attempts will be made to reclose the electrical contacts of the recloser. However, if the over-current or fault condition persists, the recloser will typically go to a lock-out condition after failing to successfully reclose after about three attempts.
Reclosers typically have an actuator coil and an armature to move the electrical contacts to the closed position or to trip the electrical contacts from a previously closed position. Thus, current flowing through the actuator coil in one direction will cause the associated armature to close the electrical contacts, and current flowing through the actuator coil in the opposite direction will cause the armature to open the electrical contacts.
The prior art includes various types of circuits for applying sufficient current through the actuator coil of the recloser, as well as controlling the direction of current through the actuator coil, to selectively open or close the electrical contacts of the relay. In one such example, a capacitor of a larger value, such as greater than 1000 microfarads is charged through a diode from a variable DC voltage source of about 160 volts. At an appropriate time, a transistor is turned on to pass current from the capacitor to the actuator coil of the recloser and back to the capacitor. A resistor and second a diode, in series with the actuator coil, provide a freewheeling current path for the actuator coil when the transistor is turned off. Current through the resistor provides a voltage that opposes the voltage across the actuator coil, which causes the current flowing through the actuator coil to decrease. The speed at which the current decreases to zero is determined by the circuit design.
In order for current to be directed in either direction through the actuator coil, a double-pole, double-throw (DPDT) relay may be employed in a manner that directs current through the actuator coil in a first direction when the DPDT relay has its contacts in a first position and that directs current through the actuator coil in the opposite direction when the DPDT relay has its contacts in the opposite position. Such operation of energization of the actuator coil in either direction will allow control of the closing and of the tripping of the recloser.
Such a circuit design for controlling a magnetic actuator works adequately in most situations. However, if the recloser is closed into a high current fault condition, it is important to be able to very rapidly trip the recloser following the close. In this situation, the contacts of the DPDT relay may be damaged if the DPDT relay is switched to the opposite position to open or trip the recloser because the current flowing through the actuator coil may not yet have decayed to zero from the prior close operation. Thus, it is important for the current through the actuator coil to decrease to zero as rapidly as possible, especially after closing the recloser. A larger value of resistance will cause the actuator coil current to decrease more rapidly, but a higher value of resistance also places higher voltage stresses on the transistor.
Furthermore, to open or trip the recloser, other delays are encountered. The capacitor must adequately recharge from the DC voltage source to supply sufficient energy to the actuator coil to open or trip the recloser and the DPDT relay must also change its position to route current through the actuator coil in the opposite direction. These delays may be in addition to the delay of the current in the actuator coil decaying to zero from the prior close operation. After these conditions have been satisfied, the transistor may be turned on to supply current from the capacitor to the actuator coil in the opposite direction to open or trip the recloser. At the appropriate time when the transistor is turned off, current continuing to flow through the actuator coil begins to circulate through the resistor and second diode. As during the close operation, current flowing through the resistor creates a voltage that opposes the voltage across the actuator coil. This causes the current flowing through the actuator coil to decrease and eventually stop.
Thus, there has been a long-felt need for an effective means of controlling the current through an actuator coil of a recloser in a manner that permits rapid trip or opening of the recloser if the recloser is closed into a high current fault condition.
Accordingly, it is a general object of the present invention to provide more effective circuitry for closing and for tripping a recloser.
Another object of the present invention is to provide circuitry for closing and for tripping a recloser that places less voltage stress on the transistors in the circuit.
Yet another object of the present invention is to more rapidly decay the current through the actuator coil after a closing operation of the recloser.
A further object of the present invention is to provide circuitry for controlling the magnetic actuator of a recloser in a manner that permits rapid opening of the recloser after closing of the recloser into a high current fault condition.
This invention is directed to circuitry for controlling the flow of current through an actuator coil of a recloser to selectively open or close electrical contacts of the recloser depending upon the direction of current flow through the actuator coil. The circuitry includes a source of DC voltage, a capacitor that is charged from the source of DC voltage, a first pair of transistors connected in series with the actuator coil to apply the charge from the capacitor to the actuator coil with a polarity that will energize the actuator coil to close the electrical contacts of the recloser when the first pair of transistors is rendered conductive, a first pair of diodes, one of each of the first pair of diodes in parallel with one of a second pair of transistors and poled to conduct current from the actuator coil in a direction that will recharge the capacitor when the first pair of transistors are turned off upon closure of the electrical contacts of the recloser, a second pair of transistors connected in series with the actuator coil to apply the charge from the capacitor to the actuator coil with an opposite polarity that will energize the actuator coil to open the electrical contacts of the recloser when the second pair of transistors are rendered conductive and a second pair of diodes, one of each of the second pair of diodes in parallel with one of the first pair of transistors and poled to conduct current from the actuator coil in a direction that will recharge the capacitor when the second pair of transistors are turned off upon opening of the electrical contacts of the recloser.
The voltage potential associated with the charge across the capacitor acts to oppose current flow through the actuator coil upon turn off of the first pair of transistors, as well as upon turn off of the second pair of transistors. The current flowing through the actuator coil thus rapidly decays toward zero after closing or opening of the electrical contacts. The capacitor also protects the transistors from voltage transients that may occur in the circuit.
The second pair of transistors may be biased to be conductive to trip the recloser as soon as the first pair of transistors is turned off, or while the current flow through the actuator coil from a prior closure of the electrical contacts is still decaying toward zero. The second pair of transistors then applies the charge on the capacitor to the actuator coil as soon as the current through the actuator coil from the closing of the electrical contacts by the first pair of transistors decays to zero to open the electrical contacts.
The invention also includes reclosers that include or utilize the above circuitry.
The present invention further includes methods of controlling the flow of current through an actuator coil of a recloser to selectively open or close electrical contacts of the recloser depending upon the direction of current flow through the actuator coil. The methods include the steps of charging a capacitor from a source of DC voltage, rendering a first pair of transistors conductive to apply the charge from the capacitor to the actuator coil with a polarity that will energize the actuator coil to close the electrical contacts of the recloser, providing a first pair of diodes in generally parallel circuit arrangement with a second pair of transistors, poling the first pair of diodes to conduct current in a direction that will recharge the capacitor with the current from the actuator coil when the first pair of transistors is rendered nonconductive, rendering a second pair of transistors conductive to apply the charge from the capacitor to the actuator coil with an opposite polarity that will energize the actuator coil to open the electrical contacts of the recloser, providing a second pair of diodes in generally parallel circuit arrangement with the first pair of transistors and poling the second pair of diodes to conduct current in a direction that will recharge the capacitor with current from the actuator coil when the second pair of transistors is rendered nonconductive.
The methods may further include the steps of opposing the flow of current through the actuator coil upon turn off of the first pair of transistors with a voltage potential associated with the charge on the capacitor and/or opposing the flow of current through the actuator coil upon turn off of the second pair of transistors with a voltage potential associated with the charge on the capacitor. The step of biasing the second pair of transistors to be conductive to open the recloser before the current through the actuator coil decays to zero from a prior closing of the electrical contacts of the recloser may also be included.
The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with the further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings, in the figures in which like reference numerals identify like elements, and in which:
An electronic circuit, generally designated 20, for controlling the magnetic actuator of a recloser 21 in accordance with the present invention is shown in
Circuit 20 may include a source of DC voltage 22, which may vary between 0 and 200 volts. For example, DC voltage at voltage source 22 may be supplied by half-wave or full-wave rectification of AC voltage. Alternatively, the source of DC voltage may be provided to circuit 20 by the recloser. A diode 23 in series with the DC voltage source is conductive when the potential at the DC voltage source is greater than the potential across a capacitor 24 to charge the capacitor toward the peak value of the DC voltage source. For example, capacitor 24 may charge to about 160 volts, or greater. The capacitive value of capacitor 24 is selected to supply the appropriate amount of energy to actuator coil 30. For example, capacitor 24 may have a capacitance of 1000 or more microfarads.
In the example of
When transistors 28 and 29 turn off, current flowing through actuator coil 30 in the direction of arrow 34 continues to flow through the freewheeling path comprising diodes 32 and 33. Note that current flowing in the path defined by diodes 32 and 33 acts to recharge capacitor 24. This also develops an increasing voltage across capacitor 24 that will oppose current flowing through actuator coil 30, which will cause the current to rapidly decrease toward zero.
In accordance with one aspect of the present invention, a trip command can quickly follow a close command since the circuit in
The trip or open operation for the circuit illustrated in
When transistors 40 and 41 turn off, current flowing through actuator coil 30 in the direction of arrow 35 continues to flow through another freewheeling path comprising diodes 42 and 43. Note that current flowing in the path defined by diodes 42 and 43 also acts to recharge capacitor 24. This also develops an increasing voltage across capacitor 24 that will oppose current flowing through actuator coil 30, which will cause the current through the actuator coil to rapidly decrease toward zero.
Diodes 26-27 and 38-39, which are in series with transistors 28-29 and 40-41, respectively, operate to block flyback or transient currents from flowing through the respective transistors. For example, when transistor 41 stops conducting, the voltage reverses on actuator coil 30 which provides a reverse potential across transistor 28 and diode 26 when line 44 is positive with respect to line 36. However, diode 26 will then be reverse-biased and will prevent reverse current from flowing through transistor 28. Under these circumstances, diode 43 will become conductive and will typically limit the reverse bias to less than one volt. Diodes 27 and 38-39 provide similar protection for their respective transistors.
In accordance with another aspect of the present invention, a trip command can be issued before the close operation is complete. For example, if transistors 40 and 41 are biased on and transistors 28 and 29 are biased off simultaneously, circuit 20 would operate as previously described until the closing current (in the direction of arrow 34) through actuator coil 30 decreases to zero. At that time, trip current begins flowing from capacitor 24 through actuator coil 30 in the direction of arrow 35, causing the recloser 21 to reopen its electrical contacts. Circuit 20 thus allows the fastest possible trip time following a close into a high current fault.
In accordance with yet another aspect of the present invention, capacitor 24 protects transistors 28-29 and 40-41 and diodes 32-33 and 42-43 during transient events. For example, such transient events may be caused by lightning induced voltage, power system faults and the like. During any such events, any high voltages that may occur on lines 44 and 45 are clamped by capacitor 24, thus protecting the semiconductors from potentially destructive over voltages. Any voltage surges tend to charge capacitor 24 to a higher voltage, or to discharge capacitor 24 to a lower voltage. Since capacitor 24 is of a relatively high capacitance, capacitor 24 will effectively filter any voltage transients that may occur, such as on lines 44-45 and/or in actuator coil 30. Transistors 28-29 and 40-41 will therefore not be subjected to the peak voltages of any such transients.
Moreover, if transistors 28-29 and 40-41 are of the MOSFET type, each of such transistors is usually provided with an internal protective metal-oxide varistor (MOV) that is electrically in parallel with the transistor. For example, in
If a positive-going transient occurs on line 44 (to the left of actuator coil 30 in
It will be appreciated that transistors 28-29 and 40-41 can be any type of semiconductor switching element, such as the MOSFET type of transistors indicated by the symbols in
In view of the above presentation of the circuit 20, it will be appreciated that the present invention also includes methods of controlling the flow of current through an actuator coil 30 of a recloser 21 to selectively open or close electrical contacts of the recloser depending upon the direction of current flow through the actuator coil 30. The methods include the steps of charging a capacitor 24 from a source of DC voltage 22, rendering a first pair of transistors 28 and 29 conductive to apply the charge from the capacitor 24 to the actuator coil 30 with a polarity that will energize the actuator coil to close the electrical contacts of the recloser 21, providing a first pair of diodes 32 and 33 in generally parallel circuit arrangement with a second pair of transistors 40 and 41, poling the first pair of diodes to conduct current in a direction that will recharge the capacitor 24 with the current from the actuator coil 30 when the first pair of transistors 28 and 29 is rendered nonconductive, rendering a second pair of transistors 40 and 41 conductive to apply the charge from the capacitor 24 to the actuator coil 30 with an opposite polarity that will energize the actuator coil to open the electrical contacts of the recloser 21, providing a second pair of diodes 42 and 43 in generally parallel circuit arrangement with the first pair of transistors 28 and 29 and poling the second pair of diodes 42 and 43 to conduct current in a direction that will recharge capacitor 24 with current from the actuator coil 30 when the second pair of transistors 40 and 41 is rendered nonconductive.
The methods may further include the steps of opposing the flow of current through the actuator coil 30 upon turn off of the first pair of transistors 28 and 29 with a voltage potential associated with the charge on capacitor 24 and/or opposing the flow of current through the actuator coil 30 upon turn off of the second pair of transistors 40 and 41 with a voltage potential associated with the charge on the capacitor. The step of biasing the second pair of transistors 40 and 41 to be conductive to open the recloser before the current through the actuator coil decays to zero from a prior closing of the electrical contacts of the recloser may also be included.
While particular embodiments of the invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made therein without departing from the invention in its broader aspects.
