Patent Description:
Unlike typical generators, an output voltage amplitude of an ISVF generator may be based on a combination of a first magnetic flux generated by rotation of a main field winding and a second magnetic flux generated by an excitation signal applied to the main field winding. The interaction of these two magnetic fields may result in voltage behaviors that differ from traditional generators. Because of the uniqueness of ISVF generators, typical methods of power regulation may not be sufficient to meet voltage regulation standards. For example, typical methods of power regulation may not consider a frequency and a phase angle of an excitation signal relative to a shaft speed.

<NPL>, per its abstract, states: 'A computer-based two-phase excitation system for a variable-speed dual-excited synchronous generator (DESG) has been implemented. The excitation system has an automatic frequency regulation (AFR) scheme and an automatic voltage regulation (AVR) scheme. The two regulation schemes act on the excitation currents independently to enable the DESG to supply power at constant frequency and constant terminal voltage while its rotor is driven at a variable speed. This paper describes the implementation of the excitation system and describes its performance when the AFR scheme is set to the AUTO control mode and the AVR scheme is set to the MANUAL control mode. The paper also presents real-time results showing the performance of the AFR scheme when the DESG is operating under different system disturbances for an operational slip range of ±<NUM>%. The results show that the excitation system is flexible, fast acting and accurate. The stead-state error in regulated output frequency is much less than <NUM>%'.

<CIT>, per its abstract, states: 'A power generation system is provided in which, when a static frequency converter (SFC) is connected to synchronous generator's armature windings, an AC exciter performs AC excitation by allowing a d-axis winding and a q-axis winding of the AC exciter to configure d-q orthogonal axes; and, at the time of steady-state operation of the synchronous generator, an alternating current(s) supplied from an electric power source is rectified by a thyristor excitation device, and also the AC exciter thereby performs DC excitation by connecting the d-axis winding and the q-axis winding in series with each other'.

The invention is to be found in the appended claims.

The disclosed examples describe a method and system to regulate the voltage of an ISVF generator-based power system while considering load disturbances, changes in an excitation signal, and shaft speed variations. In an example, a generator control unit apparatus includes a frequency-to-voltage converter configured to generate a first excitation voltage reference component based on a shaft frequency of an ISVF generator. The apparatus further includes a proportional-integral-derivative controller (PID) configured to generate a second excitation voltage reference component based on a difference between a reference voltage and a measured output voltage of the ISVF generator. The apparatus also includes an excitation source controller configured to generate an excitation voltage control signal based on a combination of the first excitation voltage reference component and the second excitation voltage reference component, the excitation voltage control signal usable to control a voltage magnitude of an excitation signal produced by an excitation source associated with the ISVF generator.

The frequency-to-voltage converter includes a lookup table that maps shaft frequency values to excitation voltage references that, when applied to the ISVF generator, result in an output voltage at the ISVF generator that has a constant voltage magnitude for each of the shaft frequency values when the ISVF generator is in a no-load state. In some examples, the apparatus includes a difference circuit configured to generate an excitation frequency reference based on a reference frequency and the shaft frequency, where the excitation source controller is configured to generate an excitation frequency control signal based on the excitation frequency reference, the excitation frequency control signal usable to control a frequency of the excitation signal produced by the excitation source. In some examples, the excitation source controller is configured to generate an excitation phase angle control signal based on an excitation phase angle reference, the excitation phase angle control signal usable to control a phase angle of the excitation signal produced by the excitation source.

In some examples, the apparatus includes a root mean square (RMS) circuit configured to generate a magnitude of the measured output voltage of the ISVF generator and a difference circuit configured to generate the difference between the reference voltage and the measured output voltage based on the magnitude of the measured output voltage and the reference voltage. In some examples, the apparatus includes a multiplier circuit configured to generate the shaft frequency based on a shaft speed. In some examples, the ISVF generator is a multi-phase generator and the excitation signal includes multiple phases, where the multiple phases have an equal voltage magnitude that is based on the combination of the first excitation voltage reference component and the second excitation voltage reference component, where the multiple phases have an equal frequency that is based on an excitation frequency reference, and where the multiple phases have phase angles that are offset from each other by constant values.

In an example, a system includes an independent speed variable frequency (ISVF) generator. The system further includes an excitation source configured to provide an excitation signal to rotor field windings of the ISVF generator to produce a rotating magnetic flux that is independent of a shaft speed of the ISVF generator. The system also includes a bus configured to receive an output voltage of the ISVF generator. The system includes a generator control unit configured to generate a first excitation voltage reference component based on a shaft frequency of the ISVF generator. The generator control unit is further configured to generate a second excitation voltage reference component based on a difference between a reference voltage and a measured output voltage of the ISVF generator. The generator control unit is also configured to generate an excitation voltage control signal based on a combination of the first excitation voltage reference component and the second excitation voltage reference component, the excitation voltage control signal usable to control a voltage magnitude of the excitation signal.

The generator control unit is further configured to determine the first excitation voltage reference component from the shaft frequency using a lookup table that maps shaft frequency values to excitation voltage references that, when applied to the ISVF generator, result in the output voltage at the ISVF generator having a constant magnitude for each of the shaft frequency values when the ISVF generator is in a no-load state. The generator control unit is further configured to generate an excitation frequency reference based on a reference frequency and the shaft frequency and generate an excitation frequency control signal based on the excitation frequency reference, the excitation frequency control signal usable to control a frequency of the excitation signal.

In some examples, the generator control unit is configured to generate an excitation phase angle control signal based on an excitation phase angle reference, the excitation phase angle control signal usable to control a phase angle of the excitation signal. In some examples, the generator control unit is configured to generate a magnitude of the measured output voltage of the ISVF generator and generate the difference between the reference voltage and the measured output voltage based on the magnitude of the measured output voltage and the reference voltage. In some examples, the generator control unit is configured to generate the shaft frequency based on the shaft speed of the ISVF generator. In some examples, the ISVF generator is a multi-phase generator and the excitation signal includes multiple phases, the multiple phases have an equal voltage magnitude that is based on the combination of the first excitation voltage reference component and the second excitation voltage reference component, the multiple phases have an equal frequency that is based on an excitation frequency reference, and the multiple phases have phase angles that are offset from each other by constant values.

In an example, a method includes generating a first excitation voltage reference component based on a shaft frequency of an ISVF generator using a data set that maps shaft frequency values to excitation voltage references that, when applied to the ISVF generator, result in an output voltage at the ISVF generator that has a constant magnitude for each of the shaft frequency values when the ISVF generator is in a no-load state. The method further includes generating a second excitation voltage reference component based on a difference between a reference voltage and a measured output voltage of the ISVF generator. The method also includes generating an excitation voltage control signal based on a combination of the first excitation voltage reference component and the second excitation voltage reference component, the excitation voltage control signal usable to control a voltage magnitude of an excitation signal produced by an excitation source associated with the ISVF generator.

The method includes experimentally determining the data set that maps the shaft frequency values to the excitation voltage references using the ISVF generator or a same type of ISVF generator. The data set is a lookup table. In some examples, the method includes generating an excitation frequency reference based on a reference frequency and the shaft frequency and generating an excitation frequency control signal based on the excitation frequency reference, the excitation frequency control signal usable to control a frequency of the excitation signal. In some examples, the method includes generating an excitation phase angle control signal based on an excitation phase angle reference, the excitation phase angle control signal usable to control a phase angle of the excitation signal. In some examples, the method includes generating a magnitude of the measured output voltage of the ISVF generator and generating the difference between the reference voltage and the measured output voltage based on the magnitude of the measured output voltage and the reference voltage.

While the disclosure is susceptible to various modifications and alternative forms, specific examples have been shown by way of example in the drawings and will be described in detail herein. However, it should be understood that the disclosure is not intended to be limited to the particular forms disclosed. Rather, the intention is to cover all modifications, equivalents and alternatives falling within the scope of the disclosure.

Referring to <FIG>, an example of an ISVF generator-based power system <NUM> is depicted. The system <NUM> may include an ISVF generator <NUM>. The ISVF generator <NUM> may include a rotor <NUM> and a stator <NUM>. The rotor may be fixed to a shaft <NUM>. During operation of the ISVF generator <NUM>, an excitation signal <NUM> may be transmitted from the stator <NUM> to the rotor <NUM> via a set of high frequency transformers <NUM>. The excitation signal <NUM> may be used to generate a rotating magnetic flux <NUM> around the rotor <NUM> that rotates independently of a shaft speed <NUM>. In this way, a frequency of an output voltage <NUM> of the ISVF generator <NUM> may be independent of the shaft speed <NUM>. A non-limiting example of the ISVF generator <NUM> is described in <CIT>, published as <CIT>, and entitled "Independent Speed Variable Frequency Alternating Current Generator," relating to an independent speed variable frequency alternating current (AC) generator apparatus may include a rotor and a stator, the rotor configured to rotate relative to the stator. The apparatus may further include a magnetic field source attached to the rotor and configured to generate a first rotating magnetic field upon rotation of the rotor, where a rotational frequency of the first rotating magnetic field is dependent on a rotational frequency of the rotor. The apparatus may also include a main rotor winding attached to the rotor and configured to generate a second rotating magnetic field upon the rotation of the rotor, where a rotational frequency of the second rotating magnetic field is independent of the rotational frequency of the rotor.

As depicted in <FIG>, the ISVF generator <NUM> may be a multi-phase generator (e.g., a three-phase generator). In that case, the excitation signal <NUM> may include multiple three phases. Likewise, the output voltage <NUM> may include three phases. While <FIG> depicts three-phases, the ISVF generator <NUM> may be configured to produce more or fewer than three phases.

The system <NUM> may further include an excitation source <NUM> configured to provide the excitation signal <NUM> to field windings <NUM> of the rotor <NUM> of the ISVF generator <NUM> to produce the rotating magnetic flux <NUM>. A frequency and magnitude of the rotating magnetic flux <NUM> may be directly dependent on a frequency and magnitude of the excitation signal <NUM>. In order to ensure that the output voltage <NUM> of the ISVF generator <NUM> has a constant frequency, the frequency of the excitation signal <NUM> may increase and decrease inversely to a frequency of the shaft <NUM>. For example, if a desired frequency of the output voltage <NUM> is <NUM> and a frequency of the shaft <NUM> is also <NUM>, then the frequency of the rotating magnetic flux <NUM> would be zero. In other words, the shaft alone can produce the entire desired frequency independent of the rotating magnetic flux <NUM>. If, however, a frequency of the shaft <NUM> is <NUM>, then, in order to produce the output voltage <NUM> with a frequency of <NUM>, the rotating magnetic flux <NUM> and its corresponding excitation signal <NUM> may have a frequency of negative -<NUM>. The relationship between a frequency of the excitation signal <NUM> and a frequency of the shaft <NUM> is further described with reference to <FIG>.

A magnitude of the output voltage <NUM> may be based on a combination of power produced by the rotation of the shaft <NUM> and power produced by the rotating magnetic flux <NUM>. When a frequency of the shaft <NUM> is operating at the desired frequency (e.g., <NUM>), the majority of power may be generated via the rotation of the shaft <NUM>. In order to produce a desired magnitude of the output voltage <NUM>, a voltage of the excitation signal <NUM> may be comparatively low (relying on the rotation of the shaft <NUM> to produce the power). When a frequency of the shaft <NUM> is higher or lower than the desired frequency, more power may be generated via the rotating magnetic flux <NUM>. In that case, a voltage of the excitation signal may be comparatively high in order to maintain a desired output voltage level. The role that the voltage of the excitation signal plays in maintaining a constant magnitude of the output voltage <NUM> is further described with reference to <FIG>.

A bus <NUM> may be configured to receive the output voltage <NUM> of the ISVF generator <NUM>. The bus <NUM> may form part of a power distribution system. In some examples, the power distribution system may provide power to systems of a vehicle, such as an aircraft. Due to the sensitive nature of circuitry that may be coupled to the bus <NUM>, it may be desirable to regulate the output voltage <NUM> as described herein.

The system <NUM> may include a generator control unit (GCU) <NUM> including a frequency-to-voltage converter <NUM>, a proportional-integral-derivative (PID) controller <NUM>, and an excitation source controller <NUM>. The frequency-to-voltage converter <NUM> may be configured to generate a first excitation voltage reference component based on a shaft frequency derived from the shaft speed <NUM> of the ISVF generator <NUM>. The first excitation voltage reference component may be usable to ensure that the output voltage <NUM> is constant in a no-load condition. The PID controller <NUM> may be configured to generate a second excitation voltage reference component based on a difference between a reference voltage <NUM> and a measured output voltage 142a (e.g., a single phase of the output voltage <NUM> of the ISVF generator <NUM>. The second excitation component may be usable to compensate for the effect of varying loads on the output voltage <NUM>. Based on a combination of the first excitation voltage reference component and the second excitation voltage reference component, the excitation source controller <NUM> may generate an excitation voltage control signal <NUM>. The excitation voltage control signal <NUM> may be usable to control a voltage magnitude of the excitation signal <NUM>. The excitation source controller <NUM> may also generate an excitation frequency control signal <NUM>, which may be based on a reference frequency <NUM>, and an excitation phase angle control signal <NUM>. The excitation voltage control signal <NUM>, the excitation frequency control signal <NUM>, and the excitation phase angle control signal <NUM> may, together, make up an excitation source control signal <NUM>.

As described herein, the generator control unit <NUM> may be configured to determine the first excitation voltage reference component from the shaft frequency using a lookup table that maps shaft frequency values to excitation voltage references that, when applied to the ISVF generator <NUM> result in the output voltage <NUM> at the ISVF generator <NUM> having a constant magnitude for each of the shaft frequency values when the ISVF generator <NUM> is in a no-load state.

A benefit of the system <NUM> is that the frequency-to-voltage converter <NUM> may enable the generator control unit <NUM> to control an excitation signal <NUM> to maintain a constant amplitude and frequency of the output voltage <NUM> in a no-load condition. The PID controller <NUM> may then fine tune the excitation signal <NUM> to compensate for varying loads applied by devices attached to the bus <NUM>. Other benefits may exist.

Referring to <FIG>, an example of a generator control system <NUM> for an ISVF generator-based power system is depicted. The generator control system <NUM> may correspond to the generator control unit <NUM> of <FIG>. Although the system <NUM> is depicted as discrete modules, the one or more of the modules may be combined. Further, the system <NUM> may be implemented as hardware or software, depending on a particular application. In some examples, each of the functions described with reference to <FIG> may be stored in a memory device and may be performed or initiated by a processor.

The system <NUM> may include a multiplier circuit <NUM> configured to generate a shaft frequency <NUM> (i.e., fshaft) based on the shaft speed <NUM> (i.e., ωshaft). The frequency-to-voltage converter <NUM> may then convert the shaft frequency <NUM> into a first excitation voltage reference component <NUM>. In some examples, the frequency-to-voltage converter <NUM> includes a frequency-to-voltage (F/V) lookup table <NUM>. The lookup table <NUM> maps shaft frequency values to excitation voltage references that, when applied to the ISVF generator <NUM>, result in the output voltage <NUM> at the ISVF generator <NUM> having a constant voltage magnitude for each of the shaft frequency values when the ISVF generator <NUM> is in a no-load state. In other words, the lookup table <NUM> may indicate what voltage the excitation voltage control signal <NUM> should have to ensure a that the output voltage <NUM> of the ISVF generator <NUM> is constant.

The system <NUM> may include a root mean square (RMS) circuit <NUM> configured to generate a magnitude <NUM> (i.e., |Vsa|) of the measured output voltage 142a (i.e., Vsa) of the ISVF generator <NUM>. In some examples, the measured output voltage 142a (i.e., Vsa) may correspond to a single phase of a multiphase output. A first difference circuit <NUM> may be configured to generate a difference <NUM> between the reference voltage <NUM> (i.e., Vref) and the measured output voltage 142a based on the magnitude <NUM> (i.e., |Vsa|) of the measured output voltage 142a (i.e., Vsa). A PID controller <NUM> may be configured to generate a second excitation voltage reference component <NUM> based on the difference <NUM>.

The first excitation voltage reference component <NUM> may be usable to ensure that the output voltage <NUM> is constant in a no-load condition. The second excitation voltage reference component <NUM> may be usable to compensate for the effect of varying loads. A summing circuit <NUM> may be configured to generate a combination <NUM> (i.e., Vref-EX) of the first excitation voltage reference component <NUM> and the second excitation voltage reference component <NUM>. The excitation source controller <NUM> may use the combination <NUM> to generate an excitation voltage control signal <NUM> that is usable to control a voltage magnitude of the excitation signal <NUM> depicted in <FIG>. As shown in <FIG>, for a three-phase system, the magnitude of each phase |VEX-a|, |VEX-b|,|VEX-c| of the excitation voltage control signal <NUM> may be equal to the combination <NUM> (i.e., |Vref-EX|).

The system <NUM> may include a second difference circuit <NUM> configured to receive the shaft frequency <NUM> and the reference frequency <NUM> and to calculate a difference between them, resulting in an excitation frequency reference <NUM> (i.e., fEX). An excitation frequency control signal <NUM> may be generated by the excitation source controller <NUM> based on the excitation frequency reference <NUM>. The excitation frequency control signal <NUM> may be usable to control a frequency of the excitation signal <NUM>. As shown in <FIG>, for a three-phase system, the frequency of each phase |fEX-a|, |fEX-b|,/fEX-c| of the excitation frequency control signal <NUM> may be equal to the excitation frequency reference <NUM> (i.e., fEX).

The excitation source controller <NUM> may be configured to generate an excitation phase angle control signal <NUM> based on an excitation phase angle reference <NUM> (i.e., θEX). The excitation phase angle control signal <NUM> may be usable to control a phase angle of the excitation signal <NUM> produced by the excitation source <NUM>. For a multiphase system, the multiple phases (θEX-a, θEX-b, θEX-c) may have phase angles that are offset from each other by constant values (i.e., <NUM>°, <NUM>°).

Referring to <FIG>, a graph <NUM> depicts shaft frequency values <NUM> along an x-axis and excitation voltage reference values <NUM> along a y-axis. A function <NUM> maps the shaft frequency values <NUM> to the excitation voltage reference values <NUM> such that the resultant excitation voltage reference values <NUM>, when applied to the ISVF generator <NUM>, result in an output voltage at the ISVF generator <NUM> that has a constant voltage magnitude for each of the shaft frequency values <NUM> when the ISVF generator <NUM> is in a no-load state. The data in the graph <NUM> may be determined experimentally and may vary between different ISVF generators. Further, the data may correspond to a particular desired output frequency. In the example of <FIG>, the desired output frequency is <NUM>.

The data shown in the graph <NUM> may be incorporated into the lookup table <NUM> of <FIG>, or otherwise used to determine the first excitation voltage reference component <NUM>. While the PID controller <NUM> may be sufficient to make minor adjustments to the excitation voltage control signal <NUM>, as the data in the graph <NUM> shows, major adjustments may be made in the excitation voltage control signal <NUM> based on the shaft frequency <NUM> depicted in <FIG>. Because these changes are predictable, it may be more efficient to use the lookup table <NUM> to generate the first excitation voltage reference component <NUM> rather than relying on the PID controller <NUM>. The PID controller <NUM> may then be used to make minor adjustments to compensate for loaded conditions.

Referring to <FIG>, a graph <NUM> depicts a zoomed portion of the graph <NUM>. For example, the graph <NUM> depicts shaft frequency values 302a along an x-axis and excitation voltage reference values 304a along a y-axis, with a function 306a that maps the shaft frequency values 302a to the excitation voltage reference values 304a. Applying FIG. <NUM> to FIGS. <NUM> and <NUM>, when the shaft frequency <NUM> is at <NUM>, the excitation signal <NUM> supplied to the ISVF generator may be zero. In this state, the majority of power may be generated by the rotation of the shaft <NUM> as opposed to the excitation signal <NUM>. Thus, in order to maintain a constant output voltage <NUM>, a voltage of the excitation signal <NUM> may be minimal at <NUM>. As the shaft frequency <NUM> moves away from <NUM>, more of the output voltage of the ISVF generator <NUM> may be derived from the excitation signal <NUM>. Thus, in order to maintain a constant output voltage <NUM>, a voltage of the excitation signal <NUM> is increased.

Referring to <FIG>, simulation results of an ISVF generator-based power system are depicted over time. <FIG> depicts a simulated generator output voltage <NUM> over time. <FIG> depicts a simulated shaft speed <NUM> and an excitation frequency reference <NUM> over time. <FIG> depicts a simulated excitation voltage reference <NUM> and an excitation voltage reference value <NUM> that is based on an output of the frequency-to-voltage converter <NUM>.

The simulation of <FIG> may correspond to a generator output frequency of <NUM>, and a line-to-neutral voltage root-mean square (RMS) value of 220V. As shown in <FIG>, the simulated shaft speed changes from <NUM> at t=<NUM> to <NUM> at t=<NUM>, then back to <NUM> at t=<NUM>. At t=<NUM>, <NUM>% of a rated load is applied as can be seen by the small disturbance <NUM> in the output voltage <NUM> of <FIG>.

<FIG> shows a generator output voltage <NUM> RMS value. The generator output voltage <NUM> increased from <NUM> and stabilizes at <NUM> V at about <NUM> second. At T=<NUM>, a <NUM>% load is applied, and the output voltage <NUM> passes through a dynamic transient and keeps at 220V. At T =<NUM> and T=<NUM>, the output voltage <NUM> shows a small amplitude variation. This variation is due to the shaft speed equaling <NUM> at those times. When the shaft speed is at <NUM>, a small voltage amplitude change in an excitation signal may cause a large deviation at a generator output voltage.

<FIG> shows the shaft speed <NUM> and the excitation frequency reference <NUM> for the excitation source. The sum of the two values equals <NUM>, which corresponds to the desired generator output frequency.

<FIG> shows a simulated excitation voltage reference <NUM> and an excitation voltage reference value <NUM> that is based on an output of the frequency-to-voltage converter <NUM>. Applying FIG. <NUM> to FIG. <NUM>, the excitation voltage reference value <NUM> may correspond to the first excitation voltage reference component <NUM>, while the excitation voltage reference value <NUM> may correspond to the combination <NUM> of the first excitation voltage reference component <NUM> and the second excitation voltage reference component <NUM>. The difference between the two curves may be due to the PID output, which compensates the voltage drops due to a load change.

Referring to <FIG>, a flow chart depicts an example of a method <NUM> for voltage regulation. The method <NUM> may include generating a first excitation voltage reference component based on a shaft frequency of an ISVF generator using a data set that maps shaft frequency values to excitation voltage references that, when applied to the ISVF generator, result in an output voltage at the ISVF generator that has a constant magnitude for each of the shaft frequency values when the ISVF generator is in a no-load state, at <NUM>. For example, the first excitation voltage reference component <NUM> may be generated by the frequency-to-voltage converter <NUM> using the lookup table <NUM>.

The method <NUM> may further include generating a magnitude of a measured output voltage of the ISVF generator, at <NUM>. For example, the magnitude <NUM> may be generated by the RMS circuit <NUM>.

The method <NUM> may also include generating a difference between a reference voltage and the measured output voltage based on the magnitude of the measured output voltage and the reference voltage, at <NUM>. For example, the first difference circuit <NUM> may generate the difference <NUM> between the reference voltage <NUM> and the measured output voltage 142a based on the magnitude <NUM>.

The method <NUM> may include generating a second excitation voltage reference component based on the difference between the reference voltage and the measured output voltage of the ISVF generator, at <NUM>. For example, the PID controller <NUM> may generate the second excitation voltage reference component <NUM>.

The method <NUM> may further include generating an excitation voltage control signal based on a combination of the first excitation voltage reference component and the second excitation voltage reference component, the excitation voltage control signal usable to control a voltage magnitude of an excitation signal produced by an excitation source associated with the ISVF generator, at <NUM>. For example, the excitation source controller <NUM> may generate the excitation voltage control signal <NUM> based on the combination <NUM> of the first excitation voltage reference component <NUM> and the second excitation voltage reference component <NUM>.

The method <NUM> may also include generating an excitation frequency reference based on a reference frequency and the shaft frequency, at <NUM>. For example, the second difference circuit <NUM> may generate the excitation frequency reference <NUM> based on the reference frequency <NUM> and the shaft frequency <NUM>.

The method <NUM> may include generating an excitation frequency control signal based on the excitation frequency reference, the excitation frequency control signal usable to control a frequency of the excitation signal, at <NUM>. For example, the excitation source controller <NUM> may generate the excitation frequency control signal <NUM> based on the excitation frequency reference <NUM>.

The method <NUM> may further include generating an excitation phase angle control signal based on an excitation phase angle reference, the excitation phase angle control signal usable to control a phase angle of the excitation signal, at <NUM>. For example, the excitation source controller <NUM> may generate the excitation phase angle control signal <NUM> based on the excitation phase angle reference <NUM>.

Referring to <FIG>, a flow chart depicts an example of a method <NUM> for generating a lookup table for voltage regulation. The method <NUM> may include experimentally determining a data set that maps shaft frequency values to excitation voltage references using an ISVF generator, at <NUM>. For example, the data depicted in <FIG> may be experimentally determined.

Further, the disclosure comprises illustrative, non-exclusive examples of embodiments of the invention according to the following examples, whereby it is noted that the scope of protection is provided by the claims.

In a first example, a generator control unit apparatus comprises: a frequency-to-voltage converter configured to generate a first excitation voltage reference component based on a shaft frequency of an independent speed variable frequency (ISVF) generator; a proportional-integral-derivative (PID) controller configured to generate a second excitation voltage reference component based on a difference between a reference voltage and a measured output voltage of the ISVF generator; and an excitation source controller configured to generate an excitation voltage control signal based on a combination of the first excitation voltage reference component and the second excitation voltage reference component, the excitation voltage control signal usable to control a voltage magnitude of an excitation signal produced by an excitation source associated with the ISVF generator.

Optionally, the frequency-to-voltage converter comprises a lookup table that maps shaft frequency values to excitation voltage reference values that, when applied to the ISVF generator, result in an output voltage at the ISVF generator that has a constant voltage magnitude for each of the shaft frequency values when the ISVF generator is in a no-load state.

Optionally, the first example further comprises: a difference circuit configured to generate an excitation frequency reference based on a reference frequency and the shaft frequency, wherein the excitation source controller is configured to generate an excitation frequency control signal based on the excitation frequency reference, the excitation frequency control signal usable to control a frequency of the excitation signal produced by the excitation source.

Optionally, the excitation source controller is configured to generate an excitation phase angle control signal based on an excitation phase angle reference, the excitation phase angle control signal usable to control a phase angle of the excitation signal produced by the excitation source.

Optionally, the first example further comprises: a root mean square (RMS) circuit configured to generate a magnitude of the measured output voltage of the ISVF generator; and a difference circuit configured to generate the difference between the reference voltage and the measured output voltage based on the magnitude of the measured output voltage and the reference voltage.

Optionally, the first example further comprises: a multiplier circuit configured to generate the shaft frequency based on a shaft speed.

Optionally, the ISVF generator is a multi-phase generator and the excitation signal includes multiple phases, wherein the multiple phases have an equal voltage magnitude that is based on the combination of the first excitation voltage reference component and the second excitation voltage reference component, wherein the multiple phases have an equal frequency that is based on an excitation frequency reference, and wherein the multiple phases have phase angles that are offset from each other by constant values.

In a second example, a system comprises: an independent speed variable frequency (ISVF) generator; an excitation source configured to provide an excitation signal to field windings of a rotor of the ISVF generator to produce a rotating magnetic flux that is independent of a shaft speed of the ISVF generator; a bus configured to receive an output voltage of the ISVF generator; and a generator control unit configured to: generate a first excitation voltage reference component based on a shaft frequency of the ISVF generator; generate a second excitation voltage reference component based on a difference between a reference voltage and a measured output voltage of the ISVF generator; and generate an excitation voltage control signal based on a combination of the first excitation voltage reference component and the second excitation voltage reference component, the excitation voltage control signal usable to control a voltage magnitude of the excitation signal.

Optionally, the generator control unit is further configured to: determine the first excitation voltage reference component from the shaft frequency using a lookup table that maps shaft frequency values to excitation voltage reference values that, when applied to the ISVF generator, result in the output voltage at the ISVF generator having a constant magnitude for each of the shaft frequency values when the ISVF generator is in a no-load state.

Optionally, the generator control unit is further configured to: generate an excitation frequency reference based on a reference frequency and the shaft frequency; and generate an excitation frequency control signal based on the excitation frequency reference, the excitation frequency control signal usable to control a frequency of the excitation signal.

Optionally, the generator control unit is further configured to: generate an excitation phase angle control signal based on an excitation phase angle reference, the excitation phase angle control signal usable to control a phase angle of the excitation signal.

Optionally, the generator control unit is further configured to: generate a magnitude of the measured output voltage of the ISVF generator; and generate the difference between the reference voltage and the measured output voltage based on the magnitude of the measured output voltage and the reference voltage.

Optionally, the generator control unit is further configured to: generate the shaft frequency based on the shaft speed of the ISVF generator.

In a third example, a method comprises: generating a first excitation voltage reference component based on a shaft frequency of an independent speed variable frequency (ISVF) generator using a data set that maps shaft frequency values to excitation voltage reference values that, when applied to the ISVF generator, result in an output voltage at the ISVF generator that has a constant magnitude for each of the shaft frequency values when the ISVF generator is in a no-load state; generating a second excitation voltage reference component based on a difference between a reference voltage and a measured output voltage of the ISVF generator; and generating an excitation voltage control signal based on a combination of the first excitation voltage reference component and the second excitation voltage reference component, the excitation voltage control signal usable to control a voltage magnitude of an excitation signal produced by an excitation source associated with the ISVF generator.

Optionally, the third example further comprises: experimentally determining the data set that maps the shaft frequency values to the excitation voltage references using the ISVF generator or a same type of ISVF generator.

Optionally, the data set is a lookup table.

Optionally, the third example further comprises: generating an excitation frequency reference based on a reference frequency and the shaft frequency; and generating an excitation frequency control signal based on the excitation frequency reference, the excitation frequency control signal usable to control a frequency of the excitation signal.

Optionally, the third example further comprises: generating an excitation phase angle control signal based on an excitation phase angle reference, the excitation phase angle control signal usable to control a phase angle of the excitation signal.

Optionally, the third example further comprises: generating a magnitude of the measured output voltage of the ISVF generator; and generating the difference between the reference voltage and the measured output voltage based on the magnitude of the measured output voltage and the reference voltage.

Optionally, the third example further comprises: generating the shaft frequency based on the shaft speed of the ISVF generator.

Claim 1:
A system comprising:
an independent speed variable frequency (ISVF) generator (<NUM>);
an excitation source (<NUM>) configured to provide an excitation signal (<NUM>) to field windings (<NUM>) of a rotor (<NUM>) of the ISVF generator (<NUM>) to produce a rotating magnetic flux (<NUM>) that is independent of a shaft speed (<NUM>) of the ISVF generator (<NUM>);
a bus (<NUM>) configured to receive an output voltage (<NUM>) of the ISVF generator (<NUM>); and
a generator control unit (<NUM>) configured to:
generate a first excitation voltage reference component (<NUM>) based on a shaft frequency (<NUM>) of the ISVF generator (<NUM>);
generate a second excitation voltage reference component (<NUM>) based on a difference (<NUM>) between a reference voltage (<NUM>) and a measured output voltage (142a) of the ISVF generator (<NUM>);
generate an excitation voltage control signal (<NUM>) based on a combination (<NUM>) of the first excitation voltage reference component (<NUM>) and the second excitation voltage reference component (<NUM>), the excitation voltage control signal (<NUM>) usable to control a voltage magnitude of the excitation signal (<NUM>); and
generate an excitation phase angle control signal (<NUM>) based on an excitation phase angle reference (<NUM>), the excitation phase angle control signal (<NUM>) usable to control a phase angle of the excitation signal (<NUM>), wherein the generator control unit (<NUM>) is further configured to:
determine the first excitation voltage reference component (<NUM>) from the shaft frequency (<NUM>) using a lookup table (<NUM>) that maps shaft frequency values (<NUM>) to excitation voltage reference values (<NUM>) that, when applied to the ISVF generator (<NUM>), result in the output voltage (<NUM>) at the ISVF generator (<NUM>) having a constant magnitude for each of the shaft frequency values when the ISVF generator (<NUM>) is in a no-load state, and is further configured to determine the second excitation voltage reference component (<NUM>) to compensate for loaded conditions.