Electricity: electrical systems and devices – Safety and protection of systems and devices – Capacitor protection
Reexamination Certificate
2001-06-18
2002-11-19
Tso, Edward H. (Department: 2830)
Electricity: electrical systems and devices
Safety and protection of systems and devices
Capacitor protection
Reexamination Certificate
active
06483679
ABSTRACT:
TECHNICAL FIELD
The present invention relates to switches for connecting and disconnecting high voltage devices to electric power circuits and, more particularly, to a switch with external resistors and a high speed whip and drive mechanism for staged insertion of the resistors when connecting a capacitor bank to a circuit.
BACKGROUND OF THE INVENTION
Electric power delivery systems such as those operated by electric utilities, large industrial facilities, military bases, and airports typically include a number of high voltage devices such as capacitor banks, voltage regulators, transformers, reclosers, surge arrestors, circuit breakers, and so forth. These devices are used in the operation of the system to maintain the quality of the electric power delivered at a power factor close to 1.0, to deliver the electric power at a certain voltage, to increase system reliability, and/or for other functions. Typically, each of these devices is connectable to the power circuit by a switch.
Conventional electric power switches have a male and a female contact that can be moved between a “closed” position with the contacts in physical contact and an “open” position with the contacts physically separated. For an electric power line that carries a high voltage and/or high current, it is desirable to open and close the male and female contacts very quickly in order to avoid a pre-strike, in which the electric current arcs across a physical gap between the contacts. Pre-strikes impose high current spikes and serious voltage disturbances on the power line, and can also physically degrade the components of the switch, especially the contacts. These current spikes and voltage disturbances can also damage other pieces of equipment connected to the circuit.
Pre-strikes occur when the switch's contacts are not yet in physical contact in the closing operation, but are still close enough to each other to permit electric current to arc through the air or other media between the contacts. When the contacts of a properly designed switch are fully separated in the open position, the distance between the contacts is sufficient to minimize pre-strikes. However, a pre-strike can occur as the contacts are moved through a “closing stroke” from the fully separate, open position to the fully connected, closed position. Likewise, an arc can occur across a gap between the contacts as the contacts are moved through an “opening stroke” from the closed position to the open position.
In order to minimize the occurrence of pre-strikes and their associated problems, “interrupter” switches are often provided with high-speed mechanisms for closing the contacts either at voltage zero or after the voltage is minimized by a pre-insertion impedance which minimizes the closing transients. Such mechanisms are sometimes provided by spring-loaded mechanisms. Also, the contacts of interrupter switches are sometimes provided in a sealed housing with a dielectric gas, vacuum, or other media for quenching the arc. Additionally, interrupter switches are sometimes provided with a linkage connected to an actuator having an electric motor, fluid cylinder, or the like. Such linkages and actuators are designed for generating a large force to increase the velocity of the opening and closing strokes, operating the contacts of a three-phase switch simultaneously, and/or operating the switch remotely.
Typically, interrupter switches are designed to prevent restrikes when “opening” the switch under a load. However, when “closed” switching a charged capacitor bank into the power circuit, conventional interrupter switches often are not able to prevent pre-strikes.
Charged capacitor banks are switched into the power circuit to correct the power factor during high-load periods, and later switched out of the circuit when the load drops. Capacitor banks store a charge, for example, 1 “per unit” (PU), and electric power systems operate at a system voltage, plus or minus 1 PU. Therefore, a conventional system-rated capacitor switch for connecting a 1 PU capacitor to the power circuit will be subjected to a 0 to 2 PU voltage surge when closing to connect the charged capacitor bank to the circuit, often resulting in intense high overvoltages. Additionally, capacitor banks are often connected and disconnected to the power circuit several times a day as the system load varies, resulting in multiple overvoltages each day.
Specialized capacitor switches have been developed in an effort to address this problem. One such type of capacitor switch has a series of sacrificial contacts that are designed to deteriorate over time as a result of current surges. However, these contacts must be regularly monitored and replaced as they deteriorate, thereby increasing the cost of using this type of switch. Because capacitor banks are often connected and disconnected to the power circuit more than once a day, the contacts must be monitored and replaced on a very strict basis. These switches do not prevent pre-strikes when connecting a charged capacitor bank to the power line, so the electric power system is still subjected to damaging current spikes and voltage disturbances. This is in part because these switches are generally based on conventional interrupter switch designs which prevent restrikes upon opening the switch, but for capacitor switching the potential for pre-strikes is greatest upon closing of the switch.
Another type of known capacitor switch includes a resistor in series with the capacitor when the capacitor is first connected into the circuit in order to reduce the current spikes. However, these devices tend to be unwieldy, bulky, and very difficult to time so that they are introduced into the circuit just as the capacitor is connected to reduce these inrush currents.
Accordingly, there is a need in the art for a capacitor switch for connecting a charged capacitor bank to an electric power circuit with controlled current spikes, that is easy to adjust for properly timing the switching operation, that is durable and reliable over thousands of operations, and that can be made and used at an affordable cost.
SUMMARY OF THE INVENTION
The present invention satisfies the aforementioned needs by providing a switch for gradually stepping a capacitor bank into an electric power circuit to compensate for power factor deviations. This is accomplished by providing two (or another number of resistors or other current limiting devices) and a conventional interrupter switch mechanism configured in series with the capacitor bank. Current flow is initiated in a staged sequence through the first resistor, then the second resistor, then the switch mechanism. The first resistor has a significantly higher resistance than the second resistor, which in turn has a significantly higher resistance than the switch mechanism. In this fashion, current flow from the charged capacitor bank is gradually stepped into the circuit, thereby significantly reducing electrical disturbances in the capacitor bank, the switch, and the circuit.
Additionally, the staged sequence of introducing the resistors and the switch mechanism is accomplished by the provision of a drive mechanism for operating an engagement arm to introduce the first and second resistors into the circuit, and an actuator for operating the switch mechanism. The drive mechanism and the actuator are operatively coupled together by a drive shaft or another linkage, with the drive mechanism, the actuator, and/or the drive shaft being readily adjustable to accomplish the desired timing of the sequence. The drive mechanism and the actuator are operable by the drive shaft to sequentially introduce the resistors into the circuit, then to introduce the switch mechanism and remove the resistors from the circuit so that the circuit has the full benefit of the capacitor bank for achieving power factor correction. Furthermore, the drive mechanism, the actuator, the switch mechanism, and the drive shaft can be provided by or made of relatively simple, readily available components, so that the switch is reliable, durable, and cost eff
Ahrano Cary J.
Anand Raj
Rostron Joseph R.
Gardner Groff & Mehrman P.C.
Mehrman Michael J.
Southern States, Inc.
Tso Edward H.
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