Electrical transmission or interconnection systems – Switching systems – Condition responsive
Reexamination Certificate
2002-12-18
2004-06-29
Riley, Shawn (Department: 2838)
Electrical transmission or interconnection systems
Switching systems
Condition responsive
C363S095000, C363S040000, C318S805000
Reexamination Certificate
active
06756702
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention generally relates to power generation systems, and, more particularly, to power generation systems that utilize digitally or processor controlled inverters to generate AC power.
Inherent within the design of power generation systems that utilize digitally or processor controlled inverters to generate AC power are time intervals that must pass between the sampling of the system state, and the action required as a result of computation in the processor. These time intervals, commonly referred to as the update rate, can be represented as a transport time delay, or transport lag.
Transport lag, i.e. the time delay required to propagate a signal through a physical system, is an inherent property of many physical systems. The transport lag time delay can cause deterioration of performance, or limit performance, of control systems used to operate the physical system. Transport lags can be continuous or discrete in form. For example, a continuous transport lag is exemplified by thickness measurement and control in the process of rolling sheets in steel mills. If the rollers and the measurement sensors have to be separated by a significant distance, due, for example, to the environment near the rollers being too hostile for the measurement sensors, a considerable delay, relative to the amount of steel processed through the rollers, results between the thickness measurement of the rolled sheets and the control of the rollers.
A discrete transport lag may be exemplified by the discrete nature in which a microprocessor samples the state of a system at time zero, and then computes over a period of time (the update rate) what must be done to make the output equal to the commanded reference. Clearly, as the computational time becomes longer (the lower the update rate), the transport lag becomes longer, which in turn limits the response of the control system. A second example of a discrete transport lag is illustrated by the discrete updates of a microprocessor-based control system. The update rate, or the time between updates, is, in essence, a transport lag time delay which is a function of the speed of the microprocessor and the amount of computation demanded by the control system.
Transport lags impact control systems differently than first order lags. While both produce a phase shift between their respective inputs and outputs, the major difference is the attenuation associated with them. For example, in the case of a first order lag at a frequency with a 60 degree delay, there is an approximately 6 decibel (dB) attenuation, while there is no attenuation associated with a corresponding transport lag.
An important feature of a control system is its capability to minimize the effects of external disturbances. The time delay associated with the transport lag deteriorates the performance of the control system by limiting its transient response. Conventional frequency domain based compensation techniques, such as pole-zero cancellations, for example, can provide only first-order approximation in the case of transport lags.
An electrical power distribution system representative of the type used on aircraft may supply electricity in the form of 3-phase power at 400 Hertz (Hz) and 115 Volts alternating current (VAC). Power for such an aircraft electrical power distribution system may be supplied from a 270 Volt direct current (VDC) power source, for example, the alternating current (AC) power output of a generator may be rectified, i.e. converted to DC, and passed through a solid state power converter, also commonly referred to as an inverter, to provide 3-phase alternating current at 400 Hertz and 115 VAC. The electrical power distribution system may be used to power various subsystems and components, for example, electric motors, which can inject noise or power fluctuations into the electrical power distribution system. The power quality of the AC voltage at the interface of the inverter with the electrical power distribution system may be subject to certain requirements and constraints. For military aircraft, for example, the power quality of the AC voltage at the interface of the inverter with the electrical power distribution system is typically specified by a military standard such as Mil-Std 704.
Electrical distribution systems on aircraft are also subject to requirements limiting the amount of electromagnetic energy conducted to the distribution system, which may interfere with other electronics systems on the aircraft, and is referred to as electromagnetic interference (EMI). To meet EMI requirements, which are stringent for military aircraft in particular, electrical distribution systems contain LC-type filters, comprised of inductances and capacitances, such as coils and capacitors, to filter out fluctuations, such as harmonics, in the current and voltage. For example, the electrical power distribution system described above may require an EMI filter at the output of the inverter or may at least contain a 3-phase capacitor bank at the output of the inverter. The LC filter circuits are prone, however, to harmonic resonance, i.e., such circuits may resonate at certain frequencies.
For example, an electric motor powered by the electric power distribution system may inject some amount of current harmonics into the distribution system, despite interfacing with its feeder through appropriate EMI filters. The frequency of the harmonics may vary with the speed of the electric motor. At some intermediate speeds, the current harmonics injected by the electric motor resonate with some of the LC filter components situated at or near the electric motor. The amplitude of the resonant currents circulating throughout the electrical power distribution system may become so large as to create unacceptable voltage fluctuation, or ripple, at the output of the inverter. Such large voltage ripples are unacceptable because they interfere with voltage control of the electric power distribution system, and may even interfere with voltage control to the extent of creating limit-cycle conditions, and because they exceed allowable power quality limits.
The conventional, wideband voltage controller used to regulate voltage in an electric power distribution system, such as described above, has insufficient selectivity to minimize the effects of external disturbances, such as voltage and current fluctuations due to EMI and resonance, in electric power distribution systems with significant transport lag. By merely increasing the selectivity of the voltage controller in a system with significant transport lag, voltage control to minimize the effects of external disturbances can be achieved over a narrow range of frequencies of a disturbance, but by merely increasing the selectivity of the voltage controller, the necessary voltage control for disturbances over the broad range of frequencies outside the narrow range is lost.
As can be seen, there is a need, in electrical power distribution systems, for a voltage controller with sufficient selectivity to control voltage fluctuations over a narrow range of frequencies of a disturbance, which can operate independently of, but in conjunction with, a wide band voltage controller. There is also a need, in electrical power distribution systems with significant transport lag, for a transport lag compensation technique which can eliminate performance deterioration of the voltage control system due to transport lags.
SUMMARY OF THE INVENTION
The present invention provides, in electrical power distribution systems, a transport lag compensator for voltage regulation, with sufficient selectivity to minimize the effects of external disturbances, and which can operate independently of, but in conjunction with, a wide band voltage controller. The present invention also provides, in electrical power distribution systems with significant transport lag, a transport lag compensation technique which can eliminate performance deterioration of the voltage control system due to transport lags.
In one aspect of the present invention, an electri
Huggett Colin
Kalman Gabor
Cagler, Esq. Oral
Honeywell International , Inc.
Riley Shawn
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