Augmented bus impedance and thump control for electrical power systems

A system includes a power distribution bus configured to distribute power from an electrical power source. The system also includes a plurality of electrical loads configured to receive portions of the power from the electrical power source. The system further includes a doubly-fed induction machine (DFIM) configured to reduce transmission impedance on the power distribution bus in response to a change in real or reactive power at one or more of the electrical loads, and reduce low frequency power oscillations at the source.

TECHNICAL FIELD

This disclosure is directed in general to electrical power systems using electric machinery. More specifically, this disclosure relates to augmented bus impedance and thump control for electrical power systems having pulsating loads.

BACKGROUND

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.

SUMMARY

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

In a first embodiment, a system includes a power distribution bus configured to distribute power from an electrical power source. The system also includes a plurality of electrical loads configured to receive portions of the power from the electrical power source. The system further includes 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.

In a second embodiment, a system includes a power generator configured to generate power for a plurality of electrical loads. The system also includes a power distribution bus configured to receive and distribute power from the power generator. The system further includes a 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.

In a third embodiment, a method includes generating power for a plurality of electrical loads using an electrical power generator. The method also includes receiving the power at a power distribution bus and distributing at least some of the power for use at the electrical loads. The method further includes reducing transmission impedance and voltage drop on the power distribution bus using a DFIM in response to a change in reactive power at one or more of the electrical loads, and enhancing system stability.

DETAILED DESCRIPTION

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 4.16 kV to 13.8 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.1illustrates an example system100for augmented bus impedance control (ABIC) according to this disclosure. Some embodiments of the system100can 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 inFIG.1, the system100includes a generator set102(such as a turbine generator) that provides power at a frequency f1 to an AC power distribution bus104. The system100also includes a doubly-fed induction machine (DFIM)106that provides real power and energy storage to a fast pulsating load110(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 bus104. The DFIM106is 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 DFIM106is controlled by excitation AC power fed to its rotor or secondary winding.

The power distribution bus104distributes the power from the generator set102to the DFIM106and to a slow pulsating load112. The slow pulsating load112is a “housekeeping” load that can represent one or more compressors, pumps, and the like. Portions of the power distribution bus104can 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 load110when the current fluctuates by a specified amount (such as 80 percent). In a weak bus, the voltage at the fast pulsating load110may fluctuate significantly (such as by 20 percent or more). In a strong bus, the voltage fluctuates much less (such as 5 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 800 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 40 MVA), while a weak bus may have a lower short circuit rating (such as 6 MVA).

The DFIM106is 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 8.43 MVAR/m3versus 4 MVAR/m3). The DFIM106is coupled to a high-speed flywheel108, which stores energy in the form of inertial energy, and also buffers the generator set102from oscillations in power associated with one of the electrical loads, thereby reducing system power surges. The flywheel108also helps the DFIM106reduce 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 inFIG.1, the DFIM106includes three stator winding ports. Port 1 is the primary power input to the stator, and Ports 2 and 3 are output ports on the stator. Port 1 receives input apparent power (MVA) from the unstable power distribution bus104. Port 2 provides stable real power (on the order of megawatts) to the pulsating load110with recurrent power or current surges. The load110may be a rectified DC load or strictly AC load. Port 1 and Port 2 exchange power/energy with the flywheel108. Port 3 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 bus104. An ABIC reactive controller114adjusts 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 DFIM106. Port 3 supports a “synchronous condenser” function and utilizes the stored magnetic field of the DFIM106for source energy. Polyphase rotor excitation is obtained from a separate AC excitation power supply116. 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 2 and 3 can be rapidly modulated. WhileFIG.1shows only one DFIM106, this is merely one example. Other embodiments could include additional DFIMs106, including one or more machines that rotate in the opposite direction.

FIG.2Aillustrates an example graph200depicting transmissible power as a function of generator reactance in the system100ofFIG.1according to this disclosure. As shown inFIG.2A, Pmaxis the maximum normalized transmissible power from the generator set102to the load110. The power is shown versus the generator reactance Xtnormalized 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 1 through 5 in the graph200represent transmission lines of different lengths. Curve 1 is a transmission line with the shortest length (such as 0.5 km) or the “strongest” bus. Curve 5 is a transmission line with the longest length (such as 2.5 km) or the “weakest” bus.

The point P1represents a desired operating point for typical fixed reactance Xt/Zo=0.20 per unit, since maximum power Pmax=2.5 per unit. Quantity Pois the nominal output power of the source generator. For the same generator reactance Xt/Zo=0.20, point P5is the least desirable operating point since Pmaxis only 1.0 per unit. It may be desired or necessary to have the ratio Pmax/Pobe large and greater than 1.0. The system100changes an operating point P5on a medium- to long-length transmission line to an operating point between P1and P5(by increasing permissible real power) depending on the level of VAR injection (Q3@V1 inFIG.1) by the shunt connected DFIM106. In practice, systems typically do not use Xt/Zo>0.40 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.2Billustrates a range of operation for conventional doubly-fed induction generators at positive and negative slip values. InFIG.2B, the real power output PGis for the stator circuit of a conventional doubly-excited wound-rotor induction machine as connected to the source, where PSis the nominal source power level. The positive slip value indicates the machine is operating as a motor, has a positive flow of power PRinto the rotor and absorbing source energy. The negative slip value indicates the machine is operating as a generator, has a flow of power PRout of the rotor and giving back energy to the source. Rotor power is symmetrical about the zero slip value. The signal PGCis 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.3A through3Dillustrate example graphs301-304depicting a wide range of control for DFIM reactive power output according to this disclosure. In some embodiments, the graphs301-304can be based on a tertiary polyphase machine winding at 60 Hz output with variable slip excitation control frequency. Clearly the concept can be extended to frequencies well below 60 Hz and well above 60 Hz. The DFIM may be represented by the DFIM106ofFIG.1.

As shown inFIG.3A, the graph301depicts the magnetic field quadrature density Bqcorresponding to reactive power output of the Q-axis winding as a function of the stator peripheral position corresponding to Port 1 and Port 3 windings inFIG.1. The plot curve311indicates the reactive output magnetic field density Bqof the Q-axis winding between poles 6 and 8. The plot curve311shows nearly equal positive and negative reactive power between point P6and point P8at about ±1 per unit quadrature magnetic flux density for the case of 15% slip. Here, reactive power is the spatial integral of the reactive output Bqwith stator current loading of the winding of Port 3 inFIG.1.

As shown inFIG.3B, the graph302depicts the reactive output radial magnetic field density Bqof the Q-axis winding as a function of stator peripheral position up to six poles. The plot curve312indicates the reactive output magnetic field density Bqof the Q-axis winding between poles 4 and 6. The plot curve312shows reactive output from zero at point P4to highly leading capacitive output 5.8 per unit at point P6for the case of 19% slip. This characteristic has an auxiliary stator excitation winding aiding the VAR generation starting at the stator peripheral position s/Tp=3.0. 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 19% and the output can be taken from windings located between poles 4 and 6, as indicated by the plot curve312. The slip value is rapidly controlled by the rotor slip frequency excitation power supply in direct response to output reactive demand.

As shown inFIG.3C, the graph303depicts the normalized limit of generated reactive power Q as a function of slip value (per unit) and output frequency f (Hz) for a 4-pole specialized induction generator. For machines in the f=50-600 Hz range, the reactive characteristic peaks at about 0.05 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 inFIG.3D, the graph304depicts the in-phase normalized airgap radial flux density Bpas a function of stator airgap peripheral location and slip value up to 0.25 per unit. The flux density Bpis the main component of the real power output of the DFIM for either Ports 2 or 3. These curves shown in the graph304, 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 2 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 0.05 to 0.11 per unit (positive or negative) can be most desirable. The curves shown are valid for one value of magnetization factor G=Xm/R2=30, where Xmis the magnetizing reactance and R2 is the rotor resistance.

FIG.3Eillustrates a graph305for 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 inFIGS.8A and8B,FIG.3Eshows a combination of total airgap flux density (as Bt2) and quadrature flux density (as Bq2) plotted as a function of per unit slip for different families of primary stator poles n=1 to n=4 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 12 pole machine would have 3 repeatable sections. The most useful characteristic is the n=4 family. As illustrated by the bold line, at a sample slip value of 10%, the Bq2value is 0.27 per unit (p.u.) and the Bt2value is 0.44 per unit. The difference between these two points is the in-phase component of magnetic flux density Bp2=0.17 per unit. The relation Bt2=Bp2+Bq2holds for all slip values.

The component values are consequently: Bp=0.412 p.u., Bq=0.519 p.u., and Bt=0.663 p.u. The per unit base quantity is the value Us/(pr*Js) where Us is the synchronous field speed (ins), pris the rotor surface resistivity (ohms) and Js is the stator current loading (A/m periphery). The curve inFIG.3Eindicates 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 fromFIG.3E, as the slip is reduced to a value such as 5%, 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 drive414to the DFIM406(described below with respect toFIG.4) 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 5-11% between poles 4 and 8 as shown inFIG.3D. The 8% slip curve313shows Bpincreasing from 4.7 per unit to 12 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 16%. In-phase flux density Bpis appreciable peaking at 9.8 per unit and quadrature flux density Bqis 3.0 per unit (seeFIG.3B). The slip value is regulated at any machine speed ω̨rby action of the DFIM rotor excitation circuit commanding synchronous speed ω̨swhereby slip=(ω̨s−ω̨r)/ω̨s. Synchronous speed (in radians/second) is in direct proportion to the applied excitation frequency fsas ω̨s=2*π fs/number of pole pairs. As modern drives can change frequency fswithin 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.4illustrates another example system400for augmented bus impedance control (ABIC) according to this disclosure. As discussed below, the system400includes multiple components that are the same as, or similar to, corresponding components of the system100ofFIG.1.

As shown inFIG.4, the system400includes a ship power generator402that provides power to an AC power distribution bus404. In some embodiments, the generator402can be the same as, or similar to, the generator set102ofFIG.1. The generator402and bus404are at medium polyphase voltage potential such as 4160 Volts and frequency fx.

The system400also includes a DFIM406with multiple (such as three) polyphase tertiary winding ports 1-3, each compensating for a distinct pulsed load410-412(such as a radar, a jammer, an electromagnetic effector requiring a higher voltage input, and the like). The DFIM406comprises 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 loads410-412. The DFIM406is coupled to an energy storage inertial flywheel408and is brought up to speed by an adjustable-speed variable-voltage variable-frequency (VVVF) drive414having source frequency fx at the input and frequency fo at the output. Real power Po is provided to the primary winding of the DFIM406by the VVVF drive414to compensate for acceleration power, friction losses, windage losses and primary I2R losses. Once the rotor and flywheel are up to rated speed, the DFIM406operates as a rotating condenser with adjustable output kVAR and kW characteristics. In some embodiments, the DFIM406includes an 8-pole machine and has an output range of 500-733 Hz based on a practical operating speed range of 7,500-11,000 rpm. Of course, other pole counts, output ranges, and operating speed ranges are possible and within the scope of this disclosure. Also, whileFIG.4shows only one DFIM406, this is merely one example. Other embodiments could include additional DFIMs406, including one or more that rotate in the opposite direction.

The circuit for each load410-412has an AC-to-AC frequency converter416-418and a step-up or step-down transformer plus AC/DC rectifier420-422as appropriate to the desired input voltage to the load. The transformers420-422are provided to galvanically isolate the loads410-412from the source power. Each AC-to-AC frequency converter416-418converts the source frequency fx to an intermediate frequency f1, f2, f3 selected for the corresponding load410-412. The advantage of a medium frequency intermediate link at f1, f2, f3 is the reduction in size of the transformer420-422and the reduction in filter component size, including the size of the DFIM406. The architecture of the system400allows the rotor damper cage of the DFIM406to absorb higher harmonics generated by the AC/DC rectifiers420-422feeding the pulsating loads410-412.

Each frequency converter416-418outputs a frequency f1, f2 or f3, which is substantially higher (such as 10×) than the source frequency fx. By using a DFIM exciter424to vary the excitation current Ie and frequency fr on the secondary (rotor) winding of the DFIM406to 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 flywheel408. Due to the magnetics design of the machine windings of the DFIM406, frequencies f1, f2 and f3 are preferably equal and also of the same frequency as the output fo from the VVVF drive414, which is injected into the main stator winding of the DFIM406. The DFIM exciter424enables 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 load410-412has an equivalent reactance Xqq, as reflected to the input to the transformers420-422. Three separate output tertiary windings at the DFIM406provide reactive currents I1, I2, I3 and reactive power Q1, Q2, Q3 (as Q=I2Xqq) 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 inFIG.4, two loads410-411are matched with a step-down transformer420-421, while the third load412has a step-up transformer422. The frequency converters416-418have the ability to boost or buck the output voltage V1, V2, V3 above or below the source voltage Vx. The three outputs of the DFIM406have 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 DFIM406, each associated with one of the output ports 1-3, provide VAR support for the loads410-412. The tertiary windings, responsive to the slip value operating range, also provide real power output or absorb real power (ref.FIG.3D). Each output port 1-3 can provide leading reactive current and power to the corresponding load410-412, which has a reactive demand or reactive power oscillation. This consequently reduces the reactive power demand on and physical size/weight of the frequency converters416-418providing mainly real power from the ship power generator402. 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 flywheel408, rather than adversely affecting the ship power generator402. 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 DFIM406reduces 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 system400, the effective bus impedance is adjustable by slip excitation control. The transformers420-422have an input reactive kVAR demand that is variable depending on conditions of the loads410-412. The higher the pulsing rate (pulses/s) of each load410-412, the higher is the reactive demand of the fundamental power and of the harmonic power at the input to the corresponding transformer/rectifier pair420-422. The DFIM406controls 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 withFIGS.5A through5D. The DFIM exciter424has 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 bus404depends on various factors. The DFIM406comprises a negative AC resistance at any output frequency when controlled in the low slip mode (such as 1.5-2.5%). The DFIM406is 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 DFIM406does not rely upon a converter for this characteristic, although the three frequency converters416-418are employed to match the bus404(e.g., 60 Hz) to a DFIM output frequency (such as 1000 Hz) required to obtain high power density for both kW and kVAR output. In some embodiments, the system400can exhibit a reduction in bus impedance, such as from 0.63 Ohms to 0.12 Ohms at 60 Hz, although other values are possible and within the scope of this disclosure.

The DFIM406attains a high power density (such as 6 kVA/kg) when high shaft speeds are used. The frequency converters416-418are very compact, efficient, and lightweight. The reactive power (kVAR) output of the DFIM406is independent of the real power (kW) output within the overall kVA machine rating. The output port 3 operates on a substantially quadrature axis magnetic circuit, while the real power output of the output port 2 operates on a substantially direct axis magnetic circuit.

FIGS.5A through5Dillustrate example phasor diagrams501-504showing reactive power control by the DFIM406according to this disclosure. InFIG.5A, the phasor diagram501shows three pulsed loads (load 1, load 2, load 3) representing the three pulsed loads410-412ofFIG.4, respectively. In the phasor diagram501, real power P is indicated by the X axis, and reactive power Q is indicated by the Y axis. As shown inFIG.5A, the lines510-512represent 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 DFIM406(indicated by the line513) in multiple windings/ports fully compensates for the three reactive powers Q1L>Q2L>Q3L, and real power is drawn through the frequency converters416-418exclusively.

InFIG.5B, the phasor diagram502shows a sudden change in load 2 (line511), which now exceeds load 1 and load 3 (i.e., P2>P1>P3) and thus Q2L>Q1L>Q3L. The situation is stable and the DFIM406compensates for the change within 2-5 cycles, and the real power P2 of load 2 is increased by the frequency converter417. The DFIM406operates at a phase angle of 88-89 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 2 may be oscillating with large real and reactive power swings between the conditions ofFIG.5AandFIG.5Bon a periodic basis at multiple times per second, or at a low frequency (such as 2 Hz), thus creating a thump condition. It is noted that in most cases, the frequency converters416-418are unidirectional in power flow, since the generator402(such as a gas turbine) cannot readily accept regenerative power from a large load.

InFIG.5C, the phasor diagram503shows a case where load 3 (line512) 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 converter418is unidirectional, the system400can absorb the load energy E3 as real power by having the DFIM406absorb this power/energy P3/E3 and use this energy E3 to recharge the flywheel408coupled to the DFIM406. This mode is shown inFIG.5C, where the DFIM406(line513) now operates in Quadrant IV, with phasor ST3 providing all reactive power for the three loads410-412and absorbing real power/energy from load 3 on a recurrent basis until such time as the flywheel408can no longer accept further energy increase and must discharge its energy to any of the loads410-412.

The design of the DFIM406and the DFIM exciter424allows 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 flywheel408, this eliminates thump energy that would otherwise be distributed throughout the system400(other than a prime mover source), which could result in undesirable over-voltage and transient effects. The system400allows for the DFIM406to have each output port 1-3 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.

InFIG.5D, the phasor diagram504is illustrative of a standard high power 6-pulse controlled AC-DC rectifier, such as the rectifiers420-422ofFIG.4. A circuit diagram505inFIG.5Dillustrates representative circuitry of such a high power 6-pulse controlled AC-DC rectifier. In the figure, Q1represents fundamental reactive power and QHrepresents harmonic reactive power.

FIG.6shows an example equivalent circuit600of one of the three output branches shown inFIG.4according to this disclosure. As shown inFIG.6, the circuit600models the generator402as a source602with source voltage Vx, and models the frequency converter416-418as an input frequency converter604with combined impedance Zx=Rx. The input frequency converter604feeds a node605at the medium frequency bus. The node605also has power injection from the DFIM406, which is modeled in the circuit600as a DFIM606with source voltage V4 and series impedance Z4=R4−jX4. The load410-412and the transformer with rectifier420-422are modeled as a shunt branch608having magnetizing reactance Xm and a series branch610having impedance ZL=RL+jXL. The reactive power developed by the DFIM406for each of its ports fully compensates for reactive power consumed in Xm and XL.

If the load rectifier420-422is a phase-controlled bridge device such as a thyristor, when this device has a gate delay ({acute over (α)} angle) and a phase back switching of currents, an equivalent reactive demand occurs on the input to the rectifier420-422, even when the load410-412is purely resistive. This reactive demand is especially large in acceleration or transient swings of the load410-412; it is represented in the circuit600by 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.

FIGS.7A through7Dillustrate properties of large conventional power converters. The illustrations inFIGS.7A through7Dshow graphs from the textbook “Basic Guide to Power Electronics” by Albert Kloss-Brown (Boveri & Cie, 1984).FIG.7AillustratesFIG.77from the textbook. The power circle diagram on the right side ofFIG.7Ashows 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=87 degrees at 1.0 per unit.FIG.7BillustratesFigure81from the textbook. The lower graph ofFIG.7Bshows normalized reactive power as a function of DC load current idand α angle. InFIGS.7A and7B, lines701-702have 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.

InFIG.7C, the line703shows normalized reactive power input qi to the AC-DC rectifier during acceleration or transient change of load conditions as being very high in the first three cycles. InFIG.7D, the line704shows mean reactive power of about 0.75 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.

FIGS.8A and8Billustrate example winding diagrams801-804for an ABIC machine, such as the DFIM804, according to this disclosure. As shown inFIGS.8A and8B, the ABIC machine is a 2-pole ABIC machine with108stator slots and 3 reactive output windings. InFIG.8A, the winding diagram801shows windings for the ABIC main input “M”. The winding diagram802shows windings for the reactive output Q1 group. InFIG.8B, the winding diagram803shows windings for the reactive output Q2 group. The winding diagram804shows windings for the reactive output Q3 group. The blocks M, Q1, M, Q2, etc., shown at the bottom ofFIG.8Bare 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.

AlthoughFIGS.1through8Billustrate example systems for augmented bus impedance control and related details, various changes may be made toFIGS.1through8B. 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, andFIGS.1through8Bdo not limit this disclosure to any particular configuration.

FIG.9illustrates an example method900for augmented bus impedance control according to this disclosure. For ease of explanation, the method900is described as being performed using the system100ofFIG.1or the system400ofFIG.4. However, the method900could be used with any other suitable device or system.

As shown inFIG.9, power for a plurality of loads is generated using a power generator at step902. This may include, for example, the power generator402generating power for use in the system400. The power is received at a power distribution bus and at least some of the power is distributed for use at the loads at step904. This may include, for example, the power distribution bus404distributing power, some of which is used at each of the loads410-412. 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 step906. This may include, for example, the DFIM406reducing the impedance on the bus404in response to a change in power at one or more of the loads410-412.

Pulsating power from load ripple or thump is absorbed into tertiary windings of the DFIM406at step908. Load ripple or thump energy is transferred into recharging power or acceleration of flywheel energy storage at step910. Energy from the flywheel energy storage is released into the DFIM406and into load at step912when the load real power demand is high. Voltage is modulated, at step914, 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.

AlthoughFIG.9illustrates one example of a method900for augmented bus impedance control, various changes may be made toFIG.9. For example, while shown as a series of steps, various steps shown inFIG.9could 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.