Patent Description:
Many modern variable-speed drives or radar power supplies on ships have little or no energy storage. Consequently, transient swings in output power exhibit correspondingly large energy swings on the main alternating current (AC) bus with consequent voltage fluctuations. When large real power fluctuations occur, these are accompanied by large swings in reactive power (kVAR) on the ship power system and affect generator operations at the highest level, including the nuclear reactor valve response. Existing ships with medium voltage AC distribution have minimal control of reactive power, and no static volt-ampere-reactive (VAR) compensators are used.

<CIT> discloses a hybrid energy storage system is configured to control pulsed power.

<CIT> discloses an AC excitation synchronous condenser and a control method, an AC-excitation induction machine, a full-controlled AC excitation converter, a grid-side converter and a controller.

<CIT> discloses a system including multiple hybrid energy storage modules (HESMs) configured to accept constant-current DC input power from a main power source.

<CIT> discloses a system including first and second sources configured to provide power to first and second medium-voltage direct current (MVDC) buses, respectively.

This disclosure provides systems for augmented bus impedance and thump control for electrical power systems.

In a first embodiment, a system comprising: a power distribution bus configured to distribute power from an electrical power source; a plurality of electrical loads configured to receive portions of the power from the electrical power source; a doubly-fed induction machine (DFIM) configured to reduce transmission impedance and voltage drop on the power distribution bus and provide energy storage capability in response to a change in power at one or more of the electrical loads; and
a flywheel coupled to the DFIM, the flywheel configured to rotate to store inertial energy that is convertible to power one or more of the electrical loads, characterized in that:.

In a second embodiment, a system comprising: a power generator configured to generate power for a plurality of electrical loads; a power distribution bus configured to receive and distribute power from the power generator; a doubly-fed induction machine (DFIM) configured to reduce transmission impedance and voltage drop on the power distribution bus in response to a change in real or reactive power at one or more of the electrical loads; and
a flywheel coupled to the DFIM, the flywheel configured to rotate to store inertial energy that is convertible to power one or more of the electrical loads, characterized in that:.

In a third embodiment, a method comprising: generating power for a plurality of electrical loads using an electrical power generator; receiving the power at a power distribution bus and distributing at least some of the power for use at the electrical loads; reducing transmission impedance and voltage drop on the power distribution bus using a doubly-fed induction machine (DFIM) in response to a demand in reactive power at one or more of the electrical loads; and
rotating a flywheel coupled to the DFIM to store inertial energy and to buffer the electrical power generator from oscillations in power associated with one of the electrical loads, so as to reduce system power surges, characterized in that:.

<FIG>, described below, and the various embodiments used to describe the principles of the present disclosure in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the disclosure. Those skilled in the art will understand that the principles of the present disclosure may be implemented in any type of suitably arranged device or system.

For simplicity and clarity, some features and components are not explicitly shown in every figure, including those illustrated in connection with other figures. It will be understood that all features illustrated in the figures may be employed in any of the embodiments described. Omission of a feature or component from a particular figure is for purposes of simplicity and clarity and is not meant to imply that the feature or component cannot be employed in the embodiments described in connection with that figure.

As discussed above, many modern industrial variable-speed drives or radar power supplies on ships have little or no energy storage. Consequently, transient swings in output power exhibit correspondingly large energy swings on the main alternating current (AC) bus with consequent voltage fluctuations. When large real power fluctuations occur, these are accompanied by large swings in reactive power on the ship power system and affect generator operations at the highest level, including the nuclear reactor valve response. Existing ships with medium voltage AC (MVAC) distribution (such as <NUM> kV to <NUM> kV) have minimal control of reactive power, and no static volt-ampere-reactive (VAR) compensators are used.

Modern ship power systems have a combination of pulsating loads of both low frequency/low pulse repetition frequency (PRF) and high frequency/high PRF combining on a common AC power distribution system. Interference between pulsating loads is a major problem on many military ships. For example, radar alternating current-to-direct current (AC/DC) power converters negatively interact with conventional variable-speed drives used for compressors and pumping equipment. To properly solve this problem on the megawatt scale, a combination of energy storage management and reactive power modulation is desired.

This disclosure provides various embodiments of augmented bus impedance control for electrical power systems. The disclosed embodiments combine real and reactive power control with active energy storage to stiffen the AC bus, reduce voltage sag, and significantly reduce "thump" (low frequency power oscillations). The disclosed embodiments use a doubly-fed multi-port induction machine to provide the reactive control and thump energy for compensation in "weak" AC power systems. Naval ships with long transmission distances from prime power generators to load sites may constitute weak systems since series electrical reactance is quite high. The disclosed embodiments improve voltage regulation and reactive power stability on these type of systems. Mitigation of thump is desirable for both land-based and ship-based power systems having fluctuating or stochastic loads.

It will be understood that embodiments of this disclosure may include any one, more than one, or all of the features described here. Also, embodiments of this disclosure may additionally or alternatively include other features not listed here. While the disclosed embodiments may be described with respect to naval ships and early warning radar power systems, these embodiments are also applicable in any other suitable systems or applications.

<FIG> illustrates an example system <NUM> for augmented bus impedance control (ABIC) according to this disclosure. Some embodiments of the system <NUM> can be used for naval ship power systems, although other uses are within the scope of this disclosure. As discussed above, ABIC represents a combination of real and reactive power control with energy storage to stiffen the AC bus, reduce voltage sag, and reduce "thump.

As shown in <FIG>, the system <NUM> includes a generator set <NUM> (such as a turbine generator) that provides power at a frequency f1 to an AC power distribution bus <NUM>. The system <NUM> also includes a doubly-fed induction machine (DFIM) <NUM> that provides real power and energy storage to a fast pulsating load <NUM> (such as a laser, radar, railgun, or the like) and also provides a source of adjustable reactive power for providing voltage/VAR support for the power distribution bus <NUM>. The DFIM <NUM> is an electrical machine which has bi-directional power flow for both real and reactive power into or out of its stator or primary windings; the DFIM <NUM> is controlled by excitation AC power fed to its rotor or secondary winding.

The power distribution bus <NUM> distributes the power from the generator set <NUM> to the DFIM <NUM> and to a slow pulsating load <NUM>. The slow pulsating load <NUM> is a "housekeeping" load that can represent one or more compressors, pumps, and the like. Portions of the power distribution bus <NUM> can be characterized as a "strong" bus or a "weak" bus depending on various factors. One factor is how much voltage fluctuation occurs at the fast pulsating load <NUM> when the current fluctuates by a specified amount (such as <NUM> percent). In a weak bus, the voltage at the fast pulsating load <NUM> may fluctuate significantly (such as by <NUM> percent or more). In a strong bus, the voltage fluctuates much less (such as <NUM> percent or less). A weak bus can be due to intermediate transformers plus transmission cable inductances that are present over a long transmission distance (such as about <NUM> feet or more). Another factor that characterizes the bus strength is the short circuit rating. For example, a strong bus may have a higher short circuit rating (such as <NUM> MVA), while a weak bus may have a lower short circuit rating (such as <NUM> MVA).

The DFIM <NUM> is a rotating machine and may have about twice the capacitive energy storage density of an AC capacitor bank typically used in static VAR compensators (such as <NUM> MVAR/m<NUM> versus <NUM> MVAR/m<NUM>). The DFIM <NUM> is coupled to a high-speed flywheel <NUM>, which stores energy in the form of inertial energy, and also buffers the generator set <NUM> from oscillations in power associated with one of the electrical loads, thereby reducing system power surges. The flywheel <NUM> also helps the DFIM <NUM> reduce thump with a faster response time than standard ship turbine generators can respond. Reducing thump extends the life of all main electrical equipment.

As shown in <FIG>, the DFIM <NUM> includes three stator winding ports. Port <NUM> is the primary power input to the stator, and Ports <NUM> and <NUM> are output ports on the stator. Port <NUM> receives input apparent power (MVA) from the unstable power distribution bus <NUM>. Port <NUM> provides stable real power (on the order of megawatts) to the pulsating load <NUM> with recurrent power or current surges. The load <NUM> may be a rectified DC load or strictly AC load. Port <NUM> and Port <NUM> exchange power/energy with the flywheel <NUM>. Port <NUM> produces leading power factor reactive power Q3 (MVAR), which is modulated by the action of the rotor (or secondary) excitation polyphase power input and this reactive power Q3 is injected into the power distribution bus <NUM>. An ABIC reactive controller <NUM> adjusts the voltage level to obtain the correct VAR match to the respective load. The reactive power Q3 is generally proportional to the square of the output voltage of the DFIM <NUM>. Port <NUM> supports a "synchronous condenser" function and utilizes the stored magnetic field of the DFIM <NUM> for source energy. Polyphase rotor excitation is obtained from a separate AC excitation power supply <NUM>. This rotor excitation supply has the ability to change rotor current and voltage levels quickly on the order of one to two electrical cycles of the output power; consequently the output voltage at Ports <NUM> and <NUM> can be rapidly modulated. While <FIG> shows only one DFIM <NUM>, this is merely one example. Other embodiments could include additional DFIMs <NUM>, including one or more machines that rotate in the opposite direction.

<FIG> illustrates an example graph <NUM> depicting transmissible power as a function of generator reactance in the system <NUM> of <FIG> according to this disclosure. As shown in <FIG>, Pmax is the maximum normalized transmissible power from the generator set <NUM> to the load <NUM>. The power is shown versus the generator reactance Xt normalized to the transmission line characteristic impedance Zo. Even though the transmission line is mostly inductive and regardless of how long it is, the transmission line has a demand for reactive power. The curves labeled <NUM> through <NUM> in the graph <NUM> represent transmission lines of different lengths. Curve <NUM> is a transmission line with the shortest length (such as <NUM>) or the "strongest" bus. Curve <NUM> is a transmission line with the longest length (such as <NUM>) or the "weakest" bus.

The point P<NUM> represents a desired operating point for typical fixed reactance Xt/Zo = <NUM> per unit, since maximum power Pmax = <NUM> per unit. Quantity Po is the nominal output power of the source generator. For the same generator reactance Xt/Zo = <NUM>, point P<NUM> is the least desirable operating point since Pmax is only <NUM> per unit. It may be desired or necessary to have the ratio Pmax/Po be large and greater than <NUM>. The system <NUM> changes an operating point P<NUM> on a medium- to long-length transmission line to an operating point between P<NUM> and P<NUM> (by increasing permissible real power) depending on the level of VAR injection (Q3@V1 in <FIG>) by the shunt connected DFIM <NUM>. In practice, systems typically do not use Xt/Zo > <NUM> since the voltage drop is large. The disclosed embodiments also cover the need for reactive power to compensate for reactive demand of a AC/DC controlled rectifier system and a DC load further downstream from the DFIM as detailed herein.

<FIG> illustrates a range of operation for conventional doubly-fed induction generators at positive and negative slip values. In <FIG>, the real power output PG is for the stator circuit of a conventional doubly-excited wound-rotor induction machine as connected to the source, where PS is the nominal source power level. The positive slip value indicates the machine is operating as a motor, has a positive flow of power PR into the rotor and absorbing source energy. The negative slip value indicates the machine is operating as a generator, has a flow of power PR out of the rotor and giving back energy to the source. Rotor power is symmetrical about the zero slip value. The signal PGC is the control signal for estimating power to be returned to the source above and below nominal. One problem with this arrangement is that the source may not be able to absorb the excess energy developed by the rotor or its load which can be creating a thump condition or recurrent transient behavior. The disclosed embodiments solve this problem.

<FIG> illustrate example graphs <NUM>-<NUM> depicting a wide range of control for DFIM reactive power output according to this disclosure. In some embodiments, the graphs <NUM>-<NUM> can be based on a tertiary polyphase machine winding at <NUM> output with variable slip excitation control frequency. Clearly the concept can be extended to frequencies well below <NUM> and well above <NUM>. The DFIM may be represented by the DFIM <NUM> of <FIG>.

As shown in <FIG>, the graph <NUM> depicts the magnetic field quadrature density Bq corresponding to reactive power output of the Q-axis winding as a function of the stator peripheral position corresponding to Port <NUM> and Port <NUM> windings in <FIG>. The plot curve <NUM> indicates the reactive output magnetic field density Bq of the Q-axis winding between poles <NUM> and <NUM>. The plot curve <NUM> shows nearly equal positive and negative reactive power between point P<NUM> and point P<NUM> at about ±<NUM> per unit quadrature magnetic flux density for the case of <NUM>% slip. Here, reactive power is the spatial integral of the reactive output Bq with stator current loading of the winding of Port <NUM> in <FIG>.

As shown in <FIG>, the graph <NUM> depicts the reactive output radial magnetic field density Bq of the Q-axis winding as a function of stator peripheral position up to six poles. The plot curve <NUM> indicates the reactive output magnetic field density Bq of the Q-axis winding between poles <NUM> and <NUM>. The plot curve <NUM> shows reactive output from zero at point P<NUM> to highly leading capacitive output <NUM> per unit at point P<NUM> for the case of <NUM>% slip. This characteristic has an auxiliary stator excitation winding aiding the VAR generation starting at the stator peripheral position s/Tp = <NUM>. Thus, when a purely reactive output is required to satisfy the medium frequency bus for reactive power, the DFIM can be operated at a slip of <NUM>% and the output can be taken from windings located between poles <NUM> and <NUM>, as indicated by the plot curve <NUM>. The slip value is rapidly controlled by the rotor slip frequency excitation power supply in direct response to output reactive demand.

As shown in <FIG>, the graph <NUM> depicts the normalized limit of generated reactive power Q as a function of slip value (per unit) and output frequency f (Hz) for a <NUM>-pole specialized induction generator. For machines in the f = <NUM>-<NUM> range, the reactive characteristic peaks at about <NUM> per unit slip. For a machine with two or three outputs, the total Q can be divided equally among the outputs by using a proper winding design. Machines of multiples of four poles utilize multiple repeatable sections of four poles.

As shown in <FIG>, the graph <NUM> depicts the in-phase normalized airgap radial flux density Bp as a function of stator airgap peripheral location and slip value up to <NUM> per unit. The flux density Bp is the main component of the real power output of the DFIM for either Ports <NUM> or <NUM>. These curves shown in the graph <NUM>, in scalar product with stator current loading (Amp-turns per meter periphery) then integrated over spatial location, predict the limit of real power absorbed by the stator winding at Port <NUM> and transferred to flywheel acceleration in a mode when the output loads are regenerative, thereby pumping energy back into the power system. For example, in some embodiments, a slip value of <NUM> to <NUM> per unit (positive or negative) can be most desirable. The curves shown are valid for one value of magnetization factor G = Xm/R2 = <NUM>, where Xm is the magnetizing reactance and R2 is the rotor resistance.

<FIG> illustrates a graph <NUM> for an example induction machine with tertiary windings and transient conditions with the ability to provide leading reactive power to an intermediate bus, according to this disclosure. The subject DFIM tertiary windings can output both real and reactive power for currents under certain limits which are hereby defined. In a spatial transient state as defined by the winding diagrams in <FIG> and <FIG>, <FIG> shows a combination of total airgap flux density (as Bt<NUM>) and quadrature flux density (as Bq<NUM>) plotted as a function of per unit slip for different families of primary stator poles n=<NUM> to n=<NUM> in a repeatable section. After a repeatable stator section of primary windings, the stator periphery is wound with a section of polyphase tertiary windings and a spatial transient occurs. By example, a <NUM> pole machine would have <NUM> repeatable sections. The most useful characteristic is the n=<NUM> family. As illustrated by the bold line, at a sample slip value of <NUM>%, the Bq<NUM> value is <NUM> per unit (p. ) and the Bt<NUM> value is <NUM> per unit. The difference between these two points is the in-phase component of magnetic flux density Bp<NUM> = <NUM> per unit. The relation Bt<NUM> = Bp<NUM> + Bq<NUM> holds for all slip values.

The component values are consequently: Bp = <NUM> p. , Bq = <NUM> p. , and Bt= <NUM> p. The per unit base quantity is the value Us/(pr*Js) where Us is the synchronous field speed (ms), pr is the rotor surface resistivity (ohms) and Js is the stator current loading (A/m periphery) The curve in <FIG> indicates there is substantial real power (calculated as Integral Bp* Js) available from the tertiary windings in addition to the larger reactive power (calculated as Integral Bq* Js) to absorb or yield thump reactive power from/to the load. It can be seen from <FIG>, as the slip is reduced to a value such as <NUM>%, the relative level of Bp is reduced and eventually diminishes towards zero as the slip value approaches zero. This defines an operating mode for the disclosed embodiments; as the level of thump power increases, the adjustable frequency drive <NUM> to the DFIM <NUM> (described below with respect to <FIG>) momentarily increases the slip value from a low slip to a higher value to yield a higher Bp and consequent higher real power capability for at least one of the tertiary windings to absorb real power from the oscillating or unstable load. The thump energy dissipates in the machine rotor rather than being sent back to the turbine source.

When a need arises for purely real power absorption by the DFIM windings (such as to transfer load energy to the flywheel), the machine can be operated at a slip value of <NUM>-<NUM>% between poles <NUM> and <NUM> as shown in <FIG>. The <NUM>% slip curve <NUM> shows Bp increasing from <NUM> per unit to <NUM> per unit over this four-pole sector. In contrast, when the DFIM must provide both real and reactive support simultaneously, an intermediate slip value can be commanded by slip frequency regulation such as <NUM>%. In-phase flux density Bp is appreciable peaking at <NUM> per unit and quadrature flux density Bq is <NUM> per unit (see <FIG>). The slip value is regulated at any machine speed <IMG> by action of the DFIM rotor excitation circuit commanding synchronous speed <IMG> whereby slip = <IMG>. Synchronous speed (in radians/second) is in direct proportion to the applied excitation frequency fs as <IMG> = <NUM>*π fs/number of pole pairs. As modern drives can change frequency fs within a few milliseconds, the synchronous field speed of the rotor can be changed in the same short period, thereby allowing very fast control of where the DFIM output reactive power peaks.

<FIG> illustrates another example system <NUM> for augmented bus impedance control (ABIC) according to this disclosure. As discussed below, the system <NUM> includes multiple components that are the same as, or similar to, corresponding components of the system <NUM> of <FIG>.

As shown in <FIG>, the system <NUM> includes a ship power generator <NUM> that provides power to an AC power distribution bus <NUM>. In some embodiments, the generator <NUM> can be the same as, or similar to, the generator set <NUM> of <FIG>. The generator <NUM> and bus <NUM> are at medium polyphase voltage potential such as <NUM> Volts and frequency fx.

The system <NUM> also includes a DFIM <NUM> with multiple (such as three) polyphase tertiary winding ports <NUM>-<NUM>, each compensating for a distinct pulsed load <NUM>-<NUM> (such as a radar, a jammer, an electromagnetic effector requiring a higher voltage input, and the like). The DFIM <NUM> comprises a non-symmetrical, space-transient winding that generates leading reactive power in the tertiary windings, that operates at a voltage level different from the primary or secondary windings, and compensates for at least some of the reactive power demanded by the pulsed loads <NUM>-<NUM>. The DFIM <NUM> is coupled to an energy storage inertial flywheel <NUM> and is brought up to speed by an adjustable-speed variable-voltage variable-frequency (VVVF) drive <NUM> having source frequency fx at the input and frequency fo at the output. Real power Po is provided to the primary winding of the DFIM <NUM> by the VVVF drive <NUM> to compensate for acceleration power, friction losses, windage losses and primary I<NUM>R losses. Once the rotor and flywheel are up to rated speed, the DFIM <NUM> operates as a rotating condenser with adjustable output kVAR and kW characteristics. In some embodiments, the DFIM <NUM> includes an <NUM>-pole machine and has an output range of <NUM>-<NUM> based on a practical operating speed range of <NUM>,<NUM> -<NUM>,<NUM> rpm. Of course, other pole counts, output ranges, and operating speed ranges are possible and within the scope of this disclosure. Also, while <FIG> shows only one DFIM <NUM>, this is merely one example. Other embodiments could include additional DFIMs <NUM>, including one or more that rotate in the opposite direction.

The circuit for each load <NUM>-<NUM> has an AC-to-AC frequency converter <NUM>-<NUM> and a step-up or step-down transformer plus AC/DC rectifier <NUM>-<NUM> as appropriate to the desired input voltage to the load. The transformers <NUM>-<NUM> are provided to galvanically isolate the loads <NUM>-<NUM> from the source power. Each AC-to-AC frequency converter <NUM>-<NUM> converts the source frequency fx to an intermediate frequency f1, f2, f3 selected for the corresponding load <NUM>-<NUM>. The advantage of a medium frequency intermediate link at f1, f2, f3 is the reduction in size of the transformer <NUM>-<NUM> and the reduction in filter component size, including the size of the DFIM <NUM>. The architecture of the system <NUM> allows the rotor damper cage of the DFIM <NUM> to absorb higher harmonics generated by the AC/DC rectifiers <NUM>-<NUM> feeding the pulsating loads <NUM>-<NUM>.

Each frequency converter <NUM>-<NUM> outputs a frequency f1, f2 or f3, which is substantially higher (such as 10x) than the source frequency fx. By using a DFIM exciter <NUM> to vary the excitation current Ie and frequency fr on the secondary (rotor) winding of the DFIM <NUM> to be inversely proportional to shaft speed, the output frequencies f1, f2 and f3 on the tertiary windings can remain substantially constant over wide speed variations, as energy is extracted or returned to the flywheel <NUM>. Due to the magnetics design of the machine windings of the DFIM <NUM>, frequencies f1, f2 and f3 are preferably equal and also of the same frequency as the output fo from the VVVF drive <NUM>, which is injected into the main stator winding of the DFIM <NUM>. The DFIM exciter <NUM> enables the tertiary winding output to provide leading or lagging reactive power to compensate for oscillating load condition power factor on a recurrent or transient basis.

Each pulsed load <NUM>-<NUM> has an equivalent reactance Xqq, as reflected to the input to the transformers <NUM>-<NUM>. Three separate output tertiary windings at the DFIM <NUM> provide reactive currents I1, I2, I3 and reactive power Q1, Q2, Q3 (as Q=I<NUM>Xqq) and associated reactive energy on a corresponding high frequency bus, albeit these can be at different line voltages V1, V2, V3 without restriction. As shown in <FIG>, two loads <NUM>-<NUM> are matched with a step-down transformer <NUM>-<NUM>, while the third load <NUM> has a step-up transformer <NUM>. The frequency converters <NUM>-<NUM> have the ability to boost or buck the output voltage V1, V2, V3 above or below the source voltage Vx. The three outputs of the DFIM <NUM> have a common frequency, yet the output power/energy can be at substantially different rates (MW/s or MJ/s) and magnitudes.

The multiple tertiary output windings of the DFIM <NUM>, each associated with one of the output ports <NUM>-<NUM>, provide VAR support for the loads <NUM>-<NUM>. The tertiary windings, responsive to the slip value operating range, also provide real power output or absorb real power (ref. Each output port <NUM>-<NUM> can provide leading reactive current and power to the corresponding load <NUM>-<NUM>, which has a reactive demand or reactive power oscillation. This consequently reduces the reactive power demand on and physical size/weight of the frequency converters <NUM>-<NUM> providing mainly real power from the ship power generator <NUM>. When a thump condition develops (which can be typical for radar and similar applications), the thump real-power energy is extracted from or returned to the flywheel <NUM>, rather than adversely affecting the ship power generator <NUM>. This reduces overall ship power modulation, which is especially important for nuclear powered aircraft carriers, which are sensitive to adverse power modulation.

The shunt connection of the DFIM <NUM> reduces the medium frequency bus impedance to a value below that which would exist without the DFIM connection, in much the same fashion as a static shunt capacitor reduces AC bus impedance, except that in the system <NUM>, the effective bus impedance is adjustable by slip excitation control. The transformers <NUM>-<NUM> have an input reactive kVAR demand that is variable depending on conditions of the loads <NUM>-<NUM>. The higher the pulsing rate (pulses/s) of each load <NUM>-<NUM>, the higher is the reactive demand of the fundamental power and of the harmonic power at the input to the corresponding transformer/rectifier pair <NUM>-<NUM>. The DFIM <NUM> controls the output of reactive power Q1, Q2, or Q3 by a combination of regulating the slip excitation current "Ie" and regulating the slip excitation frequency "fr" as explained below in conjunction with <FIG>. The DFIM exciter <NUM> has a real power output Pr that compensates for winding losses in the DFIM rotor circuit.

The principles of lowering the impedance of the AC power distribution bus <NUM> depends on various factors. The DFIM <NUM> comprises a negative AC resistance at any output frequency when controlled in the low slip mode (such as <NUM>-<NUM>%). The DFIM <NUM> is kept in the negative resistance region by use of a fast active rotor frequency controller (such as a variable frequency Insulated Gate Bipolar Transistor or IGBT drive) over wide speed/energy range. The negative resistance of the DFIM <NUM> does not rely upon a converter for this characteristic, although the three frequency converters <NUM>-<NUM> are employed to match the bus <NUM> (e.g., <NUM>) to a DFIM output frequency (such as <NUM>) required to obtain high power density for both kW and kVAR output. In some embodiments, the system <NUM> can exhibit a reduction in bus impedance, such as from <NUM> Ohms to <NUM> Ohms at <NUM>, although other values are possible and within the scope of this disclosure.

The DFIM <NUM> attains a high power density (such as <NUM> kVA/kg) when high shaft speeds are used. The frequency converters <NUM>-<NUM> are very compact, efficient, and lightweight. The reactive power (kVAR) output of the DFIM <NUM> is independent of the real power (kW) output within the overall kVA machine rating. The output port <NUM> operates on a substantially quadrature axis magnetic circuit, while the real power output of the output port <NUM> operates on a substantially direct axis magnetic circuit.

<FIG> illustrate example phasor diagrams <NUM>-<NUM> showing reactive power control by the DFIM <NUM> according to this disclosure. In <FIG>, the phasor diagram <NUM> shows three pulsed loads (load <NUM>, load <NUM>, load <NUM>) representing the three pulsed loads <NUM>-<NUM> of <FIG>, respectively. In the phasor diagram <NUM>, real power P is indicated by the X axis, and reactive power Q is indicated by the Y axis. As shown in <FIG>, the lines <NUM>-<NUM> represent the apparent powers S1, S2, S3 for the three pulsed loads, whereby apparent powers S1 > S2 > S3 and real powers P1 > P2 > P3. The output of the DFIM <NUM> (indicated by the line <NUM>) in multiple windings/ports fully compensates for the three reactive powers Q1L > Q2L > Q3L, and real power is drawn through the frequency converters <NUM>-<NUM> exclusively.

In <FIG>, the phasor diagram <NUM> shows a sudden change in load <NUM> (line <NUM>), which now exceeds load <NUM> and load <NUM> (i.e., P2 > P1 > P3) and thus Q2L > Q1L > Q3L. The situation is stable and the DFIM <NUM> compensates for the change within <NUM>-<NUM> cycles, and the real power P2 of load <NUM> is increased by the frequency converter <NUM>. The DFIM <NUM> operates at a phase angle of <NUM>-<NUM> degrees, since most of its output is reactive power and only a small amount of real power Po is used for friction and windage loss. In actual pulsed power implementations, load <NUM> may be oscillating with large real and reactive power swings between the conditions of <FIG> and <FIG> on a periodic basis at multiple times per second, or at a low frequency (such as <NUM>), thus creating a thump condition. It is noted that in most cases, the frequency converters <NUM>-<NUM> are unidirectional in power flow, since the generator <NUM> (such as a gas turbine) cannot readily accept regenerative power from a large load.

In <FIG>, the phasor diagram <NUM> shows a case where load <NUM> (line <NUM>) suddenly changes from being a non-regenerative Quadrant II load to a Quadrant III load, which is regenerative. To protect the prime mover, if the frequency converter <NUM> is unidirectional, the system <NUM> can absorb the load energy E3 as real power by having the DFIM <NUM> absorb this power/energy P3/E3 and use this energy E3 to recharge the flywheel <NUM> coupled to the DFIM <NUM>. This mode is shown in <FIG>, where the DFIM <NUM> (line <NUM>) now operates in Quadrant IV, with phasor ST<NUM> providing all reactive power for the three loads <NUM>-<NUM> and absorbing real power/energy from load <NUM> on a recurrent basis until such time as the flywheel <NUM> can no longer accept further energy increase and must discharge its energy to any of the loads <NUM>-<NUM>.

The design of the DFIM <NUM> and the DFIM exciter <NUM> allows the swing of a load from Quadrant II to Quadrant III to occur on a stochastic basis or alternately on a periodic basis with rapid response. By absorbing the load energy into the flywheel <NUM>, this eliminates thump energy that would otherwise be distributed throughout the system <NUM> (other than a prime mover source), which could result in undesirable over-voltage and transient effects. The system <NUM> allows for the DFIM <NUM> to have each output port <NUM>-<NUM> absorb thump energy independent of the adjacent ports and return each segment of thump energy to its attached energy storage unit while simultaneously providing reactive power compensation to each load.

In <FIG>, the phasor diagram <NUM> is illustrative of a standard high power <NUM>-pulse controlled AC-DC rectifier, such as the rectifiers <NUM>-<NUM> of <FIG>. A circuit diagram <NUM> in <FIG> illustrates representative circuitry of such a high power <NUM>-pulse controlled AC-DC rectifier. In the figure, Q<NUM> represents fundamental reactive power and QH represents harmonic reactive power.

<FIG> shows an example equivalent circuit <NUM> of one of the three output branches shown in <FIG> according to this disclosure. As shown in <FIG>, the circuit <NUM> models the generator <NUM> as a source <NUM> with source voltage Vx, and models the frequency converter <NUM>-<NUM> as an input frequency converter <NUM> with combined impedance Zx = Rx. The input frequency converter <NUM> feeds a node <NUM> at the medium frequency bus. The node <NUM> also has power injection from the DFIM <NUM>, which is modeled in the circuit <NUM> as a DFIM <NUM> with source voltage V4 and series impedance Z4=R4 - jX4. The load <NUM>-<NUM> and the transformer with rectifier <NUM>-<NUM> are modeled as a shunt branch <NUM> having magnetizing reactance Xm and a series branch <NUM> having impedance ZL= RL + jXL. The reactive power developed by the DFIM <NUM> for each of its ports fully compensates for reactive power consumed in Xm and XL.

If the load rectifier <NUM>-<NUM> is a phase-controlled bridge device such as a thyristor, when this device has a gate delay (ά angle) and a phase back switching of currents, an equivalent reactive demand occurs on the input to the rectifier <NUM>-<NUM>, even when the load <NUM>-<NUM> is purely resistive. This reactive demand is especially large in acceleration or transient swings of the load <NUM>-<NUM>; it is represented in the circuit <NUM> by the reactance Xc in series with the load. The consequent reactive demand is a combination of the transformer magnetizing reactance, the transformer leakage reactance and the AC-to-DC converter reactive demand, as explained below. Commutation overlap present in high power thyristor based AC-to-DC converters also adds to the reactive demand. This AC-to-DC converter reactive demand increases as a function of output DC current. The negative reactance -jX4 is a design feature of the DFIM output windings and can match the output reactances Xl + Xc in parallel to Xm, so the input frequency converter has no effective reactive demand.

<FIG> illustrate properties of large conventional power converters. The illustrations in <FIG> show graphs from the textbook "Basic Guide to Power Electronics" by Albert Kloss-Brown (Boveri & Cie, <NUM>). <FIG> illustrates Figure <NUM> from the textbook. The power circle diagram on the right side of <FIG> shows normalized reactive power as a function of thyristor gating delay angle α. It is noted that AC-DC converter reactive power demand peaks at about α = <NUM> degrees at <NUM> per unit. <FIG> illustrates Figure <NUM> from the textbook. The lower graph of <FIG> shows normalized reactive power as a function of DC load current id and α angle. In <FIG> and <FIG>, lines <NUM>-<NUM> have been added to the figures to illustrate reactive power demand (lagging PF) on the input side of the AC-DC controlled rectifier after the transformer.

In <FIG>, the line <NUM> shows normalized reactive power input q<NUM> to the AC-DC rectifier during acceleration or transient change of load conditions as being very high in the first three cycles. In <FIG>, the line <NUM> shows mean reactive power of about <NUM> per unit during the first three cycles during acceleration region. Thus during rapid load changes, additional reactive power in needed beyond a steady-state requirement.

<FIG> and <FIG> illustrate example winding diagrams <NUM>-<NUM> for an ABIC machine, such as the DFIM <NUM>, according to this disclosure. As shown in <FIG> and <FIG>, the ABIC machine is a <NUM>-pole ABIC machine with <NUM> stator slots and <NUM> reactive output windings. In <FIG>, the winding diagram <NUM> shows windings for the ABIC main input "M". The winding diagram <NUM> shows windings for the reactive output Q1 group. In <FIG>, the winding diagram <NUM> shows windings for the reactive output Q2 group. The winding diagram <NUM> shows windings for the reactive output Q3 group. The blocks M, Q1, M, Q2, etc., shown at the bottom of <FIG> are functional blocks illustrating a main winding, which provides the excitation for the machine, followed by a reactive winding. This is then followed by another main winding and another reactive winding. This sequence of windings creates a transient in the machine magnetic circuit, which generates additional leading reactive power.

Although <FIG> illustrate example systems for augmented bus impedance control and related details, various changes may be made to <FIG>. For example, while the figures show systems with only one DFIM, this is merely one example. In other embodiments, the systems could include additional DFIMs, including one or more that rotate in the opposite direction. In general, electrical power systems come in a wide variety of configurations, and <FIG> do not limit this disclosure to any particular configuration.

<FIG> illustrates an example method <NUM> for augmented bus impedance control according to this disclosure. For ease of explanation, the method <NUM> is described as being performed using the system <NUM> of <FIG> or the system <NUM> of <FIG>. However, the method <NUM> could be used with any other suitable device or system.

As shown in <FIG>, power for a plurality of loads is generated using a power generator at step <NUM>. This may include, for example, the power generator <NUM> generating power for use in the system <NUM>. The power is received at a power distribution bus and at least some of the power is distributed for use at the loads at step <NUM>. This may include, for example, the power distribution bus <NUM> distributing power, some of which is used at each of the loads <NUM>-<NUM>. Impedance is reduced on the power distribution bus using a shunt connected DFIM in response to a change in power at one or more of the loads at step <NUM>. This may include, for example, the DFIM <NUM> reducing the impedance on the bus <NUM> in response to a change in power at one or more of the loads <NUM>-<NUM>.

Pulsating power from load ripple or thump is absorbed into tertiary windings of the DFIM <NUM> at step <NUM>. Load ripple or thump energy is transferred into recharging power or acceleration of flywheel energy storage at step <NUM>. Energy from the flywheel energy storage is released into the DFIM <NUM> and into load at step <NUM> when the load real power demand is high. Voltage is modulated, at step <NUM>, on intermediate loads by DFIM reactive power output and injection into the bus to compensate for large load swings which use reactive power in the output converter.

Although <FIG> illustrates one example of a method <NUM> for augmented bus impedance control, various changes may be made to <FIG>. For example, while shown as a series of steps, various steps shown in <FIG> could overlap, occur in parallel, occur in a different order, or occur multiple times. Moreover, some steps could be combined or removed and additional steps could be added according to particular needs.

Claim 1:
A system (<NUM>) comprising:
a power distribution bus (<NUM>) configured to distribute power from an electrical power source;
a plurality of electrical loads configured to receive portions of the power from the electrical power source;
a doubly-fed induction machine (DFIM) (<NUM>) configured to reduce transmission impedance and reduce voltage drop on the power distribution bus in response to a change in power at one or more of the electrical loads; and
a flywheel (<NUM>) coupled to the DFIM (<NUM>), the flywheel (<NUM>) configured to rotate to store inertial energy that is convertible to power one or more of the electrical loads,
characterized in that:
power associated with one of the electrical loads oscillates at a low frequency associated with a thump condition;
the DFIM (<NUM>) is configured to extract energy associated with the thump condition from, or return energy associated with the thump condition to, the flywheel (<NUM>) in order to minimize an impact of the thump condition on the electrical power source or the system (<NUM>);
the system further comprises a plurality of frequency converters (<NUM>-<NUM>) each corresponding to one of the electrical loads, each frequency converter (<NUM>-<NUM>) configured to convert a frequency of the power from the electrical power source to an output frequency associated with the corresponding electrical load; and
the DFIM (<NUM>) comprises a plurality of DFIM tertiary winding ports, each DFIM tertiary winding port configured to provide leading reactive power to a corresponding one of the electrical loads having a reactive power demand or reactive power oscillation and consequently reduce the reactive power demand on the plurality of frequency converters (<NUM>-<NUM>), which provide mainly real power from the electrical power source.