Patent Application
20110141641
The embodiments disclosed herein relate generally to circuit breakers and more particularly to a circuit breaker for protecting electrical equipment from undesired voltages while also protecting rotating machinery, such as dynamoelectric machines, from undesired torsional transients.
Wind farms are becoming increasing popular for the generation of electricity. In a wind farm, there are a large number of wind turbines installed in locations of the country where wind is consistent and substantial. Typically, the wind turbines will include an array of blades that are coupled to a shaft. The rotation of the shaft caused by the rotation of the blades will produce electrical energy. Electrical lines will connect with the energy generator so as to deliver the energy from a particular wind turbine to a feeder line. The electrical energy from the various wind turbines in the wind farm can collectively pass energy to a substation via the feeder line.
Typically, these wind turbines can each produce between about 1.5 MW and 3.0 MW of power. The outputs of multiple wind turbines in the wind farm are often grouped into several electrical collection circuits or feeder lines. Transformers are used so as to tie the wind turbine output to the feeder line. The transformers serve to step up the output voltage of the wind turbines to a medium voltage, usually 34.5 kilovolts. The various wind turbines in a wind farm are usually paralleled into feeder lines that can deliver about 15 to 30 megawatts of power. In view of the voltage which has been stepped up to the 34.5 kilovolts, each feeder line will require a circuit breaker rated at a minimum 34.5 kilovolts capacity. The energy will pass through the circuit breaker to the 34.5 kV bus of a substation. The 34.5 kV substation bus will go into one or more main step-up transformers and then tie into a high voltage utility line or power grid, therefore it would be desirable to have a device that can interrupt the circuit if a fault occurs on the power grid
The interruption of electrical power circuits has always been an essential function, especially in cases of overloads or short circuits, when immediate interruption of the current flow becomes necessary as a protective measure. In the past, circuits could be broken only by separation of contacts in air followed by drawing the resulting electric arc out to such a length that it could no longer be maintained. This means of interruption soon became inadequate and special devices, termed “circuit breakers”, were developed. The basic problem is to control and quench the high power arc. This necessarily occurs at the separating contacts of a breaker when opening high current circuits. Since arcs generate a great deal of heat energy which is often destructive to the breaker's contacts, it is necessary to limit the duration of the arc and to develop contacts that can repeatedly withstand the effect of the arc.
A vacuum circuit breaker uses the rapid dielectric recovery and high dielectric strength of a vacuum. A pair of contacts are hermetically sealed in the vacuum envelope. An actuating motion is transmitted through bellows to a movable contact. When the electrodes are parted, an arc is produced and supported by metallic vapor boiled from the electrodes. Vapor particles expand into the vacuum and condense on solid surfaces. At a natural current zero the vapor particles disappear and the arc is extinguished.
In the past, in association with such wind farms, when circuit breakers are opened, the feeder line current would be interrupted and a temporary overvoltage situation could occur in the feeder line. This overvoltage situation can be caused by isolation of the feeder line from equipment that normally provide a reference point for the phase voltages with respect to ground potential. A short-circuit between one of the phases and ground on the feeder line will cause the voltage on that phase with respect to ground to drop to near zero, and the voltage with respect to ground on the other two phases to rise to the normal phase-to-phase value, which is about 1.73 times the normal phase-to-ground value. This overvoltage can damage equipment connected to the feeder line. Isolation of the operating wind turbines from the grid can also result in other overvoltage phenomena. The high transient voltage in the feeder line will “back up” through the circuit and to the electronics associated with the wind turbine generators. As a result, this transient overvoltage could cause damage to the circuitry or machinery associated with the wind turbines and other circuitry throughout the system. As a result, in view of the characteristics of the large energy resident within the overall wind farm, there is a need to hold within acceptable limits any overvoltage which occurs when the circuit breaker is be opened.
Typically, to avoid the over voltage situation, grounding transformers have been required to be installed. These grounding transformers would typically have a zig-zag winding or a grounded wye primary winding and either an open or closed delta with a secondary winding. If an open delta secondary winding is used, the delta is closed by an external resistor or inductor. The primary voltage of the transformer is consistent with the nominal voltage of the feeder line, typically 34.5 kV. The secondary voltage, where an open or closed delta is used, is arbitrary with regard to the application needs. This voltage is typically chosen to optimize the transformer design, or the selection of the external resistor or inductor if an open-delta secondary is used. The transformer has a core with windings therearound. In view of the core and windings, there is a continuous amount of core losses of energy associated with the use of such grounding transformers. Over time, the core losses could amount to a significant dollar amount of lost energy. Additionally, these grounding transformers have a relatively high initial cost, installation cost, and a long lead time for delivery.
A lower cost approach is favored by some wind farm owners/operators, and this approach is illustrated in
According to one aspect of the present invention, a circuit breaker is provided for protecting dynamoelectric machinery. The circuit breaker includes a feeder input connection connected to a feeder line. The feeder line is connected to a dynamoelectric machine. A substation connection is connected to a substation bus. An interrupting breaker is connected between the feeder input connection and the substation connection. A shorting switch is connected to the feeder input connection, and an impedance device is connected to the shorting switch and a ground or neutral. The impedance device, shorting switch and ground or neutral can reduce excessive voltages on the feeder line when the feeder line is isolated from the substation by the circuit breaker, and the impedance device is selected to reduce a torque transient experienced by the dynamoelectric machine.
According to another aspect of the present invention, a substation having a circuit breaker is provided for protecting dynamoelectric machinery. The substation includes a feeder input connection connected to a feeder line. The feeder line is connected to a dynamoelectric machine. A substation connection is connected to a substation bus. An interrupting breaker is connected between the feeder input connection and the substation connection. A shorting switch is connected to the feeder input connection, and an impedance device is connected to the shorting switch and a ground or neutral. The impedance device, shorting switch and ground or neutral reduces excessive voltages on the feeder line when the feeder line is isolated from the substation by the circuit breaker, and the impedance device is selected to reduce a torque transient experienced by the dynamoelectric machine.
According to a still further aspect of the present invention, a circuit breaker is provided for protecting dynamoelectric machinery. The circuit breaker includes a feeder input connection connected to a feeder line. The feeder line is connected to a dynamoelectric machine. A substation connection is connected to a substation bus. An interrupting breaker is connected between the feeder input connection and the substation connection. A shorting switch is connected to the feeder input connection, and an impedance device is connected to the shorting switch and a ground or neutral. The impedance device, shorting switch and ground/neutral reduces excessive voltages on the feeder line when the feeder line is isolated from the substation by the circuit breaker, and the impedance device is selected to reduce a torque transient experienced by the dynamoelectric machine. The dynamoelectric machine is a wind turbine, an electrical generator and/or a motor
In circuit breakers, the circuit to the substation can be broken upon the application of a manual force to a button or lever of the circuit breaker or by an automatic relay which opens the circuit. Typically, the current is measured to the substation. If any relay senses a problem, then a signal is transmitted to the circuit breaker so as to open the breaker. Typically, the relays will be maintained within the substation. The opening of the circuit breaker will prevent the energy from being transmitted from the feeder line to the substation. Sometimes, the circuit breaker is open so as to allow maintenance personnel to work on the wind farm system, on the circuit breaker, or on the substation. Typically, the relays will operate if the sensors sense an excessive amount of current, or a component of current having a common-mode component indicative of a fault to ground.
The feeder line 426 is connected to a circuit breaker 434 and substation 438. In normal operation, energy generated by the dynamoelectric machines is transmitted via feeder line 426 through circuit breaker 434 and substation 438, then through line 432 to the power grid. Line 432 can be referred to as a substation bus. Circuit breaker 434 includes a connection to ground 446.
The electrical interrupting circuit (530, 520) could be a vacuum interrupter switch, a sulfur hexafluoride interrupter switch or any other suitable interrupting and switching device. The electrical shorting switch 520 is designed to be capable of handling fault current connected to the feeder side of circuit breaker 434. The actuator 540 in common to the interrupting breaker 530 and shorting switch 520 maintains the shorting switch 520 open when interrupting breaker 530 is closed. Conversely, when interrupting breaker 530 is opened, shorting switch 520 is closed with a minimum of delay time after electrical circuit interruption is accomplished by interrupting breaker 530. The impedance 510 is connected between the shorting switch 520 and ground 446 or system electrical neutral.
The circuit breaker 434 operates as a combination of electrical circuit breaker and shorting, or “crowbar” device. When the interrupting contacts in the interrupting breaker 530 are opened, as for example, when an electrical fault is detected on the feeder 426, the shorting switch 520 closes and inserts the impedance 510 to load the circuit and provide a ground reference. The value for impedance 510, which may be resistive or reactive, is chosen to be sufficiently small such that voltages on the feeder are not excessive when the feeder is isolated from the substation 438 by interrupting breaker 530, particularly voltages on an unfaulted phase when one phase is faulted to ground, but sufficiently large such that the torque transient placed on the generators (410-416) by the shorting action is acceptable. In other words, the impedance value is selected to reduce excessive voltages on the feeder line, and to reduce torque transients experienced by the dynamo electric machines.
The actuator 540 can be any suitable actuating device (electrical and/or mechanical) that controls the “ON” and “OFF” state of switches 520 and 530. The actuator can be mechanically and/or electrically connected to the switches 520 and 530. In addition, the connection between the actuator 540 and switches 520, 530 can be implemented via wired or wireless communication.
While the invention has been described in connection with what is presently considered to be one of the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.
