Electric power conversion systems – Current conversion – With condition responsive means to control the output...
Reexamination Certificate
2003-02-19
2004-03-09
Vu, Bao Q. (Department: 2838)
Electric power conversion systems
Current conversion
With condition responsive means to control the output...
C363S054000, C363S070000
Reexamination Certificate
active
06704214
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention pertains generally to excitation control systems for rotating electrical apparatus and, more particularly, to such systems which provide excitation voltage to a generator. The invention also pertains to methods for providing an excitation voltage for a rotating electrical apparatus.
2. Background Information
Three-phase rectifier circuits are commonly employed to convert AC signals to DC signals. These circuits often use SCRs disposed in one or more bridge segments, with typically one SCR for each polarity of each AC phase. Typically, a bridge firing control circuit controls the firing point for each rectifier in each AC cycle. Examples of such circuits are disclosed in U.S. Pat. Nos. 5,963,440; 5,963,441; 6,046,917; 6,208,120; and 6,232,751.
SCR bridges are commonly employed in an excitation control system to provide field excitation for a rotating electrical apparatus (e.g., large synchronous generators and motors, utility synchronous generators and motors, industrial synchronous generators and motors, synchronous generators and motors for naval or other shipping applications, synchronous generators and motors for oil well drilling rigs).
A typical excitation control system includes a transformer, one or more controlled rectifier bridges, one or more bridge control modules and/or a firing control module for plural bridge control modules with a communication network between the modules, and a circuit breaker (e.g., a “41 breaker”) normally feeding AC power from the transformer to the controlled rectifier bridge(s). See Pat. Nos. 6,046,917; and 6,208,120.
One type of transformer is a three-phase power potential transformer (PPT) including three primary windings in a WYE-configuration and three corresponding secondary windings in a delta-configuration, although other transformer configurations may be employed (e.g., a delta-configuration in the primary and a delta-configuration in the secondary). See U.S. Pat. No. 6,232,751. The primary windings are interconnected with the AC phases from the generator, while the corresponding secondary windings are interconnected with the AC inputs of the bridge(s) by the circuit breaker. The PPT may be electrically interconnected with the terminals of the generator and, thus, that system is commonly referred to as a “terminal fed excitation system”. Alternatively, the PPT may be electrically interconnected with any other suitable source of three-phase voltages. This alternative system is commonly referred to as an “auxiliary bus fed excitation system”.
In a static excitation system, the PPT is electrically interconnected with a suitable (e.g., reliable) power source, such as, for example, the station power source or the generator terminals. When this power source is present and available, it may be employed to remove energy from the generator field relatively very quickly by a process known as “rapid de-excitation”. Otherwise, when this power source is not available, energy is removed from the generator field at a relatively slower rate. “Rapid de-excitation” is accomplished by phasing back the firing angle of the one or more rectifier bridges, in order that a suitable negative field excitation voltage is applied to the generator field.
As an alternative to employing a negative field excitation voltage to accomplish “rapid de-excitation”, a similar result may be accomplished by electrically inserting (e.g., by employing a DC contactor or DC circuit breaker) a suitable discharge resistor in series with the field excitation voltage to the generator field.
When the generator is tripped, it is beneficial to remove the energy in the field excitation circuit as quickly as possible. A suitable way to accomplish this is through “rapid de-excitation”. However, if the voltage of the PPT power source is too low, such as would happen with a high side fault (e.g., a relatively low impedance and, thus, a relatively low voltage short to ground) on the generator terminals, then the circuit breaker or “41 breaker” between the transformer and the controlled rectifier bridge(s) should be immediately tripped. Known excitation control systems handle this by providing two trip inputs: (1) emergency trip, which opens the circuit breaker or “41 breaker”; and (2) normal trip, which initiates “rapid de-excitation” before the circuit breaker opens. Because it is not known whether rapid de-excitation can or cannot be successfully implemented, emergency trips result in an immediate opening of the “41 breaker”.
Accordingly, there is room for improvement in excitation control systems and methods.
SUMMARY OF THE INVENTION
These needs and others are met by the present invention, which provides improvements in an excitation control system for a rotating electrical apparatus. In accordance with the invention, the ability to rapidly de-excite determines what happens after a trip signal is input by the excitation control system. When the excitation control system receives the trip signal, it attempts “rapid de-excitation” and, also, verifies that “rapid de-excitation” is occurring. If “rapid de-excitation” is not occurring, then the circuit breaker or “41 breaker” sourcing AC power to the controlled rectifier bridge is immediately opened.
The excitation control system initially attempts “rapid de-excitation” in response to the trip signal. This may be accomplished by phasing back the firing angle of the rectifier bridge, in order to attempt to apply a negative field excitation voltage to the generator field. However, if a suitable power source voltage, such as a PPT voltage, is not present, then the system immediately opens the circuit breaker or “41 breaker”. Otherwise, if the suitable PPT voltage is present, then after a suitable time delay, the excitation control system verifies that the generator field voltage is sufficiently negative. If so, then “rapid de-excitation” continues to be employed. Otherwise, if the generator field voltage is insufficiently negative, then “rapid de-excitation” is not occurring and the system opens the circuit breaker or “41 breaker”.
In accordance with one aspect of the invention, an excitation control system for outputting an excitation voltage for a rotating electrical apparatus comprises: an electrical switching apparatus comprising a plurality of input terminals including a plurality of alternating current phases, a plurality of output terminals, a plurality of separable contacts electrically connected between the input and output terminals, and an input to open the separable contacts, the electrical switching apparatus providing the alternating current phases at the output terminals when the separable contacts are closed; a rectifier bridge comprising a plurality of inputs electrically interconnected with the output terminals of the electrical switching apparatus, a plurality of segments to convert the alternating current phases to the excitation voltage, and an output having the excitation voltage, each of the segments including an element having an input responsive to one of a plurality of firing signals; and a controller comprising: an input including a first signal, an output including a second signal, the output of the controller being electrically interconnected with the input of the electrical switching apparatus, a first value, a second value, a first sensor inputting at least one voltage corresponding to the alternating current phases of the electrical switching apparatus and outputting a sensed voltage, a second sensor inputting the excitation voltage of the output of the rectifier bridge and outputting a sensed excitation voltage, a plurality of outputs, each of the outputs electrically interconnected with the input of the element of a corresponding one of the segments of the rectifier bridge to provide a corresponding one of the firing signals thereto, a first function detecting the first signal and responsively controlling the firing signals of the outputs of the controller in order to provide the excitation voltage having a negative value, a second function,
Gibbs Irving A.
Mummert Charles R.
Eaton Corporation
Moran Martin J.
Vu Bao Q.
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