Patent Description:
Three-phase medium voltage distribution power grids include single phase lateral power lines which connect to various loads. The three-phase power lines fan out from a centralized substation, initially as three-phase feeder (or "trunk"), and then commonly branch off as single phase laterals further away from the substation. An example of such a three-phase distribution network <NUM> with multiple single phase laterals is shown in <FIG>. The three-phase trunk <NUM> originates from distribution substation <NUM> and comprises three single-phase lines 16a (Phase A), 16b (Phase B), and 16c (Phase C), which are <NUM> degrees out of phase from each other. These single phase lines branch off to feed portions of the three-phase distribution network <NUM> via single-phase lateral feeds, such as lateral feeds 18a (Phase A), 18b (Phase B), and 18c (Phase C). Each lateral feed may power many different single-phase loads, including homes, businesses, and other users/producers of power.

Ideally, the three single-phase lines 16a, 16b, and 16c of three phase trunk <NUM> will be relatively balanced in terms of how much power is drawn from the loads connected to the respective single-phase lines. In extreme scenarios, however, peak loading on any single phase can be much higher than the other two phases, which can result in overloading of the entire three-phase trunk <NUM> and the three-phase assets at substation <NUM>, including substation transformers, breakers, and relays. This is an inefficient allocation of total capacity, as two of the single phases may have extra capacity to accept the excessive loading on the overly burdened single-phase line.

Currently, the way that this situation is corrected is by dispatching a line crew to manually change the source phase for the overloaded lateral feed in order to re-balance the loads on the three phases. This is done at the location where the single-phase lateral branches off from the three phase trunk. Referring to <FIG>, for example, lateral feed 18a may be removed from single-phase line 16a (phase A) at location <NUM> of the three-phase trunk <NUM> and reconnected to single-phase line 16b (phase B) at location <NUM> or single-phase line 16c (phase C) at location <NUM>, depending on which would provide the most balanced arrangement. Manual re-balancing is costly to execute and disruptive to customers. Switching a single-phase lateral from one single phase line to another from requires a line crew to de-energize these lines and an outage for the customers of at least the laterals involved, and most likely an outage for customers of all three phases. For safety reasons, this manual re-balance cannot be executed with the lines energized or "hot".

With the growth of single phase distributed generation, such as residential solar/PV and highly mobile loads such as electric vehicles, the allocation of power (current) among the three phases (A, B, C) can vary significantly on a seasonal or even daily basis, which exacerbates the load balancing problem. Utilities can no longer accurately forecast the power allocation across the three phases with much accuracy. In the long term, manual re-balancing can be prevented with an upgrade of the current (power) rating of the three phase trunk and the three- phase substation, but at fairly significant cost.

A second issue involving single-phase laterals is the compromised uptime or availability for power delivery to critical single phase loads. Faults can occur anywhere on the three-phase distribution feeder, however, most faults are single phase in nature and do not require de-energizing all three of the phases. If a fault occurs on a phase with sensitive load anywhere on the phase other than the lateral containing the sensitive load, the lateral could potentially be swapped to one of the two remaining "healthy" phases in order to keep the sensitive load energized. Again, this is only presently possible presently via manual dispatch of a line crew to swap the lateral with the sensitive load to a remaining "healthy" phase.

Therefore, there exists a need for a system and method to swap a single phase lateral line (and the electrical loads it is feeding) from the nominal "source" phase to either of the other two "destination" phases without interruption of the voltage, i.e. hot swap the phases. <CIT> discloses a load-balancing device includes a control module, and a converter for generating a voltage supplying a single-phase electrical load connected to a polyphase electrical network. The converter selectively modifies a phase shift between its output and phases of the polyphase electrical network. The control module controls synchronization of the output with a first phase of the network and its progressive phase shifting when it is synchronized with a second phase of the polyphase electrical network. It also controls connection of the load to the network and to the converter when the converter and first phase are synchronized. The control module maintains disconnection of the load from the network during the progressive phase shifting of the converter output voltage, and controls its connection the second phase of the network when its output is synchronized with the second phase.

The benefits and advantages of the present invention over existing systems will be readily apparent from the Summary of the Invention and Detailed Description to follow. One skilled in the art will appreciate that the present teachings can be practiced with embodiments other than those summarized or disclosed below.

In one aspect, the invention includes a system for load balancing on a multi-phase power line connected to a single phase lateral power line having a phase rotating transformer having a stator and a rotor. The stator includes a plurality of primary coils, each primary coil configured to be connected to one phase of the multi-phase power line. The rotor includes a secondary coil configured to be connected to the single phase lateral power line and a rotary actuator operably connected to the rotor. There is a controller configured to cause the rotary actuator to rotate the rotor to selectively electromagnetically couple the secondary coil with each of the plurality of primary coils, so as to enable electrical connection of each phase of the multi-phase power line with the single phase lateral power line.

In other aspects of the invention, one or more of the following features may be included. The multi-phase power line may comprise three phases. The phase rotating transformer may comprise a wound field synchronous generator. The controller may be configured to cause the rotary actuator to rotate the rotor to selectively electromagnetically couple the secondary coil with each of the plurality of primary coils, while the plurality of primary coils of the stator are energized. The controller may be configured to rotate the rotary actuator across a range of +/- <NUM> degrees. The rotary actuator may comprise a worm gear driven by an electric motor. The controller may be configured to rotate the rotary actuator and the secondary coil on the rotor from one of the primary coils to another of the primary coils in approximately. <NUM> seconds or less. The system may further include a current in-rush limiting circuit disposed between the multi-phase power line and the primary coils of the stator of the phase rotating transformer. The system may also include a multi-phase by-pass circuit having a multi-phase to single-phase contactor device connected electrically in parallel to the phase rotating transformer.

In another aspect, the invention includes a method for load balancing on a multi-phase power line connected to a single phase lateral power line. The method comprising providing a phase rotating transformer having a stator and a rotor; wherein the stator includes a plurality of primary coils. Each primary coil is configured to be connected to one phase of the multi-phase power line and the rotor includes a secondary coil configured to be connected to the single phase lateral power line. The method includes providing a rotary actuator operably connected to the rotor and causing the rotary actuator to rotate the rotor to selectively change the electromagnetic coupling of the secondary coil from a first of the plurality of primary coils to a second of the plurality of primary coils, so as to change the electrical connection between the multi-phase power line and the single phase lateral power line from a first phase to a second phase.

In yet other aspects of the invention, one or more of the following features may be included. The multi-phase power line may comprise three phases. The phase rotating transformer may comprise a wound field synchronous generator. When the rotary actuator is caused to rotate the rotor to selectively change the electromagnet coupling of the secondary coil from a first of the plurality of primary coils to a second of the plurality of primary coils, the plurality of primary coils of the stator may be energized. The rotary actuator may be capable of causing the rotor to rotate across a range of +/- <NUM> degrees. The rotary actuator may comprise a worm gear driven by an electric motor. The rotary actuator may cause the rotor and the secondary coil on the rotor to rotate from one of the primary coils to another of the primary coils in approximately. <NUM> seconds or less. The method may further include disposing a current in-rush limiting circuit between the multi-phase power line and the primary coils of the stator of the phase rotating transformer. The method may also include disposing a multi-phase by-pass circuit having a multi-phase to single-phase contactor device electrically in parallel to the phase rotating transformer.

In a further aspect, the invention includes a system for load balancing on a multi-phase power line connected to a single phase lateral power line, the system. There is a contactor configured to selectively connect each phase of the multi-phase power line to the single phase lateral power line. In a normal operating state, the contactor is configured to connect a first phase of the multi-phase power line to the single phase lateral power line and during a phase change state, the contactor is configured to change connection from the first phase to a second phase of the multi-phase power line. There is a power electronics device, having an input and an output, connected in parallel with the contactor between the multi-phase power line and the single phase lateral power line. During the phase change state, the input is configured to be connected to the multi-phase power line and the output is configured to be connected to the single phase lateral power line. In the normal operating state, the input is configured to be disconnected from the multi-phase power line and the output is configured to be disconnected from the single phase lateral power line. There is a controller which, during the phase change state, is configured to connect the input of the power electronics device to the multi-phase power line and connect the output of the power electronics device the single phase lateral power line. The controller is also configured to cause the power electronics device to output a voltage to the single phase lateral line aligned with the first phase and cause the power electronics device to output the voltage to the single phase lateral line aligned with the second phase. The controller is additionally configured to cause the contactor to change connection from the first phase of the multi-phase power line to the second phase of the multi-phase power line and to disconnect the input of the power electronics device from the multi-phase power line and disconnect the output of the power electronics device from the single phase lateral power line.

In another aspect, the invention includes a method for load balancing on a multi-phase power line connected to a single phase lateral power line, the method comprising. The method includes providing a contactor configured to selectively connect each phase of the multi-phase power line to the single phase lateral power line. In a normal operating state, the contactor is configured to connect a first phase of the multi-phase power line to the single phase lateral power line and during a phase change state, the contactor is configured to change connection from the first phase to a second phase of the multi-phase power line. The method also includes providing a power electronics device, having an input and an output, connected in parallel with the contactor between the multi-phase power line and the single phase lateral power line. During the phase change state, the input is configured to be connected to the multi-phase power line and the output is configured to be connected to the single phase lateral power line. In the normal operating state, the input is configured to be disconnected from the multi-phase power line and the output is configured to be disconnected from the single phase lateral power line. During the phase change state the method includes connecting the input of the power electronics device to the multi-phase power line and connecting the output of the power electronics device the single phase lateral power line. The method includes causing the power electronics device to output a voltage to the single phase lateral line aligned with the first phase and causing the power electronics device to output the voltage to the single phase lateral line rotated to align with the second phase. The method additionally includes causing the contactor to change connection from the first phase of the multi-phase power line to the second phase of the multi-phase power line and to disconnect the input of the power electronics device from the multi-phase power line and the output of the power electronics device from the single phase lateral power line.

In yet a further aspect, the invention includes a system for load balancing on a multi-phase power line connected to a single phase lateral power line. The system includes contactor configured to selectively connect each phase of the multi-phase power line to the single phase lateral power line. In a normal operating state, the contactor is configured to connect a first phase of the multi-phase power line to the single phase lateral power line and during a phase change state, the contactor is configured to change connection from the first phase to a second phase of the multi-phase power line. There is a phase change device, having an input and an output, connected in parallel with the contactor between the multi-phase power line and the single phase lateral power line. During the phase change state, the input is configured to be connected to the multi-phase power line and the output is configured to be connected to the single phase lateral power line. In the normal operating state, the input is configured to be disconnected from the multi-phase power line and the output is configured to be disconnected from the single phase lateral power line. There is a controller which is, during the phase change state, configured to connect the input of the phase change device to the multi-phase power line and connect the output of the phase change device the single phase lateral power line. The controller is also configured to cause the phase change device to output a voltage to the single phase lateral line initially aligned with the first phase and then rotated to align with the second phase. The controller is further configured to cause the contactor to change connection from the first phase of the multi-phase power line to the second phase of the multi-phase power line and to isconnect the input of the phase change device from the multi-phase power line and disconnect the output of the phase change device from the single phase lateral power line.

In other aspects of the invention, the following feature may be included. The phase change device comprises one of a power electronics device or a phase rotating transformer.

In an additional aspect, the invention includes a method for load balancing on a multi-phase power line connected to a single phase lateral power line, the system comprising. The method includes providing a contactor configured to selectively connect each phase of the multi-phase power line to the single phase lateral power line. In a normal operating state, the contactor is configured to connect a first phase of the multi-phase power line to the single phase lateral power line and during a phase change state, the contactor is configured to change connection from the first phase to a second phase of the multi-phase power line. The method includes providing a phase change device, having an input and an output, connected in parallel with the contactor between the multi-phase power line and the single phase lateral power line. During the phase change state, the input is configured to be connected to the multi-phase power line and the output is configured to be connected to the single phase lateral power line. In the normal operating state, the input is configured to be disconnected from the multi-phase power line and the output is configured to be disconnected from the single phase lateral power line. During the phase change state the includes connecting the input of the phase change device to the multi-phase power line and connecting the output of the phase change device the single phase lateral power line. The method additionally includes causing the phase change device to output a voltage to the single phase lateral line initially aligned with the first phase and then rotated to align with the second phase. The method also includes causing the contactor to change connection from the first phase of the multi-phase power line to the second phase of the multi-phase power line and disconnecting the input of the phase change device from the multi-phase power line and disconnect the output of the phase change device from the single phase lateral power line.

Embodiments of the present disclosure will now be described, by way of example only, with reference to the attached Figures, wherein:.

The disclosure and the various features and advantageous details thereof are explained more fully with reference to the non-limiting embodiments and examples that are described and/or illustrated in the accompanying drawings and detailed in the following description. It should be noted that the features illustrated in the drawings are not necessarily drawn to scale, and features of one embodiment may be employed with other embodiments as the skilled artisan would recognize, even if not explicitly stated herein. Descriptions of well-known components and processing techniques may be omitted so as to not unnecessarily obscure the embodiments of the disclosure. The examples used herein are intended merely to facilitate an understanding of ways in which the disclosure may be practiced and to further enable those of skill in the art to practice the embodiments of the disclosure. Accordingly, the examples and embodiments herein should not be construed as limiting the scope of the disclosure. Moreover, it is noted that like reference numerals represent similar parts throughout the several views of the drawings.

The invention described herein provides the ability to "hot-swap" the source voltage for a given single phase lateral on a medium or low voltage power distribution grid. In one application, this capability would allow utilities to dynamically re-balance the three phase currents drawn from the three phase "trunk" and substation equipment. It would increase availability or uptime for critical single phase loads by connecting such loads to a healthy (energized) single phase source in the event of an upstream single phase fault on the phase originally feeding the lateral. Additionally, the invention may be applied where other single phase loads exist (e.g. industrial customers) that require high availability.

The ability to hot swap the entire single phase lateral line (and the electrical loads it is feeding) from the nominal "source" phase to either of the other two "destination" phases provides for correction without interruption of the voltage. The approach described herein further enables the voltage to maintain a smooth, near-sinusoidal shape during the swap event to prevent "load drop" - i.e. motors and other devices from tripping offline if the lateral voltage "jumps" very abruptly from source to destination voltage. The invention additionally provides for the continued supply (or absorption) of real power to the loads on the lateral phase during the swap event. The system described herein can be sited outside of a substation, at the point where the single phase lateral branches from the three phase trunk (i.e. along the distribution feeder), and it can be done with low capital and maintenance costs.

One potential way to implement a system and method to hot swap a single phase lateral line from the nominal "source" phase to another phase of the three-phase trunk without interruption, is described below with regard to <FIG> and <FIG>. Shown in <FIG> is a <NUM>-phase to <NUM>-phase AC-DC-AC power converter system <NUM>, with medium voltage connection transformers <NUM> and <NUM> at each AC interface. A power electronics enclosure <NUM> of power converter system <NUM> is normally bypassed, and the single-phase lateral line <NUM> is connected to the desired phase of three-phase trunk <NUM> via one of the MV contactors <NUM>. For example, single phase lines 110A, <NUM> B and 110C of three-phase trunk <NUM> are connected to contactor <NUM> and one of the single phases, in this case phase 110A, is connected to single phase lateral line <NUM>. If it is determined that single phase line 110A is overloaded and the three-phase trunk should be re-balanced, the phase connected to single phase lateral line <NUM> may be changed from phase 110A to either phase 110B or 110C, depending on which phase has sufficient capacity. The phases could be changed manually by a line crew by changing the contactor <NUM> connections but, as described above, there are significant disadvantages to this approach.

Instead, the phase change may be facilitated using the <NUM>-phase to <NUM>-phase AC-DC-AC power converter system <NUM> under the control of system controller <NUM>. In this embodiment, system controller <NUM> is positioned inside of power electronics enclosure <NUM>; however, it may alternatively be positioned next to, but outside of the enclosure, or even at a remote location; in each case, it would be in communication with and in control of the various system components, including contactor <NUM>, breakers <NUM> and <NUM>, and power electronics enclosure <NUM>. System controller <NUM> may be in communication with a network control system to instruct the controller to re-balance the loads of three-phase trunk <NUM> by changing the phase feeding the single phase lateral <NUM>. Or, controller <NUM> may operate autonomously to change the phase connected to the single phase lateral <NUM>, by assessing loading on the phases of three-phase trunk electrical loads on the single phase lateral <NUM>, based on signals provided by sensors directly to the controller <NUM>.

When a phase swap is desired/required, controller <NUM>, as shown in step <NUM> of flow chart <NUM>, initiates the phase swap sequence. In the current state (i.e. before phase swap), one of the <NUM> single phase contactors in contactor <NUM> is in the closed position, providing a nominal connection of the output line <NUM> to one of the three phases, in this example it is connected to A phase from the trunk 110A. The swap process proceeds to step <NUM> when controller <NUM> causes power electronics enclosure <NUM> to be energized by closing three phase breaker <NUM>. When breaker <NUM> is closed, <NUM> phase transformer <NUM> is energized and it steps down trunk phase voltages 110A, 110B, and 110C, from typical distribution voltage levels (e.g. <NUM>,<NUM> volts AC) to provide low voltage (e.g. <NUM> volts) phases 110a, 110b, and 110c to the <NUM> phase inverter <NUM> within power electronics enclosure <NUM>. The three phase inverter rectifies the three phase voltages and outputs DC voltage on DC bus <NUM>. It should be apparent to those skilled in the art that the three phase inverter <NUM> could be replaced by a three phase rectifier with a combination of diodes and/or thyristors.

The DC on bus <NUM> is then chopped by the single phase inverter <NUM> to create the single phase AC voltage on output line <NUM>. At step <NUM>, the controller causes the output the output to be initially aligned with the nominal "source" voltage (i.e. single phase line 110A) on the single phase lateral line <NUM>. Thus, on output line <NUM> may be produced low ac voltage 110a, which is then stepped up using single-phase transformer <NUM> to produce single phase voltage 110A on single phase lateral line <NUM>, when single phase breaker <NUM> connects the output of transformer <NUM> to lateral line <NUM>, step <NUM>. Then, at step <NUM>, the single phase voltage on output <NUM> may be rotated by single phase inverter <NUM> to produce the "destination" phase, i.e. single phase 110b or 110c. At step <NUM> the controller causes the contactor <NUM> to connect the destination phase (either 110B or 110C) of the three phase trunk to lateral single-phase line <NUM>. The power electronics enclosure <NUM> may be bypassed by opening the three phase breaker <NUM> and the single phase breaker <NUM> at step <NUM>, thus completing the phase swap sequence at step <NUM>.

It should be noted that system <NUM> could also be realized with Medium Voltage AC-DC-AC converter topologies, many of which would allow for the elimination of the transformers in exchange for more or higher voltage rated IGBTs and diodes.

While better than the manual approach of the prior art, there may be disadvantages of using system <NUM>. A principal disadvantage is that it is a solid state system (i.e. power is processed via power electronics), which has a multitude of components and subsystems (logic circuits, power supplies, gate drivers, etc.). Moreover, the packaging of power electronics systems for siting and installation outside of a substation is challenging.

Another embodiment of a system and method to hot swap a single phase lateral line from the nominal "source" phase to another phase of the three-phase trunk without interruption, is described below with regard to <FIG>. Shown in <FIG> is a <NUM>-phase to <NUM>-phase phase rotating transformer system <NUM>, with circuit breakers <NUM> and <NUM> at the three-phase trunk line <NUM> interface and the single-phase lateral <NUM> interface, respectively. A phase rotating transformer enclosure <NUM> of phase rotating transformer system <NUM> is normally bypassed by placing breakers <NUM> and <NUM> in the normally open position, and the single-phase lateral line <NUM> connected to the proper phase of three-phase trunk <NUM> via one of the MV contactors <NUM>. For example, single phase lines 210A, <NUM> B and 210C of three-phase trunk <NUM> are connected to contactor <NUM> and one of the single phases, in this case phase 210A, is connected to single phase lateral line <NUM>. If it is determined that single phase line 210A is overloaded and the three-phase trunk should be re-balanced, the phase connected to single phase lateral line <NUM> may be changed from phase 210A to either phase 210B or 210C, depending on which phase has sufficient capacity.

A central component of the embodiment shown on <FIG> is an electromechanical device, referred to as a phase rotating transformer <NUM>. The phase rotating transformer <NUM> is connected between three-phase breaker <NUM> and a single-phase breaker <NUM>. An optional in-rush current limiting circuit <NUM>, may be used to prevent high three phase current draw on the stator winding <NUM> and rotor winding <NUM> of phase rotating transformer <NUM> when breaker <NUM> is closed. Phase rotating transformer <NUM> may be implemented as a wound field synchronous generator, for example, whose rotor angle is controlled to vary the coupling between one of the three phases of the stator winding <NUM> to the single phase rotor winding <NUM>, while the stator and rotor windings are energized (i.e. the lateral phase may be "hot" swapped). Overall operation of <NUM>-phase to <NUM>-phase phase rotating transformer system <NUM> is controlled by controller <NUM> in a manner similar to that of controller <NUM> of <NUM>-phase to <NUM>-phase AC-DC-AC power converter system <NUM>, <FIG> and <FIG>.

As the mechanical shaft angle, θ, (and the electrical phase angle) of phase rotating transformer <NUM> is changed by a rotary actuator (not shown), the field winding, F, and the single-phase lateral <NUM>, are coupled controllably to phases A, B, and to C of three-phase trunk <NUM>. The transformer coupling properties of a wound field rotor synchronous generator are well established. What is uniquely recognized herein is the control of the rotor angle to provide a smooth and continuous coupling of power from the source voltages to the lateral during a hot-swapping process. It should be noted that synchronous generators are commonly manufactured with <NUM>, <NUM>, <NUM>, or <NUM> poles. For each these designs, <NUM> degrees of electrical rotation respectively corresponds to <NUM>, <NUM>, <NUM>, <NUM> degrees of rotor mechanical rotation.

As is well known to those skilled in the art of design or selection of rotary electromagnetic machines (motors and generators), the selection of the number of poles allows the machine designer to exchange, at the "mechanical port" (i.e. the shaft), in an inverse proportional manner, lowering the required angular displacement in exchange for higher required torque, assuming the required mechanical power is constant. This is akin to selecting a different gear ratio in an automotive transmission or a different turns ratio in an electrical transformer. At the "mechanical port" (the shaft) of an electric machine, the two quantities being exchanged are torque and angular displacement, as opposed to voltage in a fixed transformer, though the product of the two quantities remains constant. For example, with a <NUM> pole synchronous generator, the rotor would only need to rotate <NUM> mechanical degrees (i.e. <NUM>/<NUM> of a full <NUM> degree rotation) to change the field coupling, F, from phase A to phase B. However, the required torque increases by a factor of twelve (<NUM>).

The mechanical shaft angle, θ, may be adjusted (over a range of +/- θ) by means of a mechanical rotary actuator <NUM>, described with regard to <FIG>. The mechanical rotary actuator preferably has the following features: <NUM>) it is controllable over a range of +/- <NUM> electrical degrees, and <NUM>) it has zero power holding force (torque). Mechanical rotary actuator <NUM> may be configured in various ways known to those skilled in the art. One such implementation is depicted to include a linear actuator <NUM> (ball screw, worm drive or the like) having an actuator arm <NUM>, which may be extended and retracted under the control of control device <NUM> based on control signals from the system level controller <NUM> (<FIG>). Actuator arm <NUM> is mechanically connected to a first end of lever arm <NUM> and the second end of lever arm <NUM> is fixed to rotor shaft <NUM> of rotor <NUM>. When the actuator arm <NUM> is extended, it causes lever arm <NUM> to move in a counter-clockwise direction (as indicated by arrow <NUM>) and when it is retracted it causes lever arm <NUM> to move in the clockwise direction. When lever arm <NUM> is rotated, it in turn rotates rotor shaft <NUM> and rotor <NUM> over a full range of +/- θ, where <NUM> degrees, + θ degrees, and - θ degrees, each correspond to one of electrical phases A, B, and C of the three phase trunk. In these three positions, the respective phase of the primary windings of the stator <NUM> are magnetically coupled to the rotor field winding in rotor <NUM> and the single phase electrical current from the rotor field winding is output via flexible electrical connections <NUM> to single phase breaker <NUM> and lateral <NUM> of <FIG>.

The phase rotating transformer in combination with the mechanical rotary actuator according to an aspect of the invention, has the ability to hot swap an entire single phase lateral line (and the electrical loads it is feeding) from a nominal "source" phase to either of the other two "destination" phases of a three-phase trunk, without interruption of the voltage. In other words, the voltage of the single lateral phase would maintain a smooth, near-sinusoidal shape during the swap event to prevent "load drop" - i.e. motors and other devices from tripping offline if the lateral voltage were to "jump" very abruptly from source to destination voltage. Moreover, the system would continue to supply (or absorb) real power to the load on that lateral during the swap event.

The operation of phase rotating transformer <NUM> of <FIG> is depicted by way of a simplified coupled winding model <NUM> in <FIG>. In this simplified model the phase windings are combined in one angular location on the stator or rotor. In practical realization in motors and generators, these windings are distributed around each of these structures, and the magnetic coupling is not binary (i.e. not on/off), but rather it progresses. However, this model captures the behavior of the machine when viewed externally as operating in a binary manner.

In model <NUM>, stator windings <NUM> are shown to include phase A winding 220A, phase B winding 220B, and phase C winding 220C, each having a voltage across the windings and a current passing through the windings, respectively, VA/IA, VB/IB, and Vc/Ic. The stator windings are displaced about the full circumference of the stator and are mechanically spaced approximately <NUM> degrees between each phase, which would be the arrangement for a <NUM>-pole machine, as described above. Rotor field winding <NUM> comprises single phase lateral windings 222F, which has a voltage across the windings and a current passing through the windings, VF/IF.

If, for example, the single phase lateral windings 222F were magnetically coupled to phase A winding 220A, the lateral windings 22F would be mechanically aligned with the phase A winding. And, phase A 220A which is coupled to Phase A of three-phase trunk <NUM> would be powering single-phase lateral <NUM>. If the phase A trunk loading were to become unbalanced with phases B and/or C of three-phase trunk <NUM>, then phase rotating transformer <NUM> could be activated to hot swap the single phase lateral windings 222F to phase B or C by changing the coupling of single phase lateral windings 222F to either phase B winding 220B or phase C winding 220C. Continuing to refer to <FIG>, lateral windings 222F are shown to be mechanically displaced from phase A winding 220A by mechanical shaft angle, θ1elec, in the rotational direction via a mechanical rotary actuator (not shown) toward phase B winding 220B, depicting the hot swapping of the phase lateral windings 222F from phase A winding 220A to the phase B winding 220B, in mid-process.

The system is intended to be used in transient duty or short durations (~<NUM> to <NUM> seconds for "hot swap" depending on requirements for rate of frequency change to keep load from dropping). However, unlike the solid state power converter of <NUM>, the electrical power flow path in the phase rotating transformer of <NUM> is comprised of copper and iron and has no semiconductor devices. As such, it can tolerate very high currents (5X nominal or more) for several seconds or more. This is important for the design of a (phase rotating) transformer, as it allows the sizing of the unit to take advantage of the high ratio of peak current/nominal current allowable for transformers and generators and reduces the overall cost of this approach.

The hot swapping process of changing the single phase lateral 18b of <FIG>, which is initially connected to trunk phase B, to trunk phase A, using the <NUM>-phase to <NUM>-phase phase rotating transformer system <NUM>, is depicted in table <NUM> shown in <FIG>. Waveforms 400a depict trunk voltages (A, B, C) and lateral or field winding voltage F during time period <NUM> (before the hot swap), time period <NUM> (during the hot swap), and time period <NUM> (swap completed). Before hot swap <NUM>, within the set of voltages in 400a, the dashed line showing the field winding voltage F tracks with the phase B voltage. During hot swap <NUM>, within the set of voltages in 400a, the field winding voltage F processes from overlapping with phase B voltage at the start of <NUM> to overlapping with the phase A voltage at the end of <NUM>. At the swap completion <NUM>, within the set of voltages in 400a, the field winding voltage F tracks with the phase A voltage.

Waveforms 400b depict the trunk currents (A, B, C) which start in time period <NUM> in an unbalanced state, with an excessive amount of B phase current <NUM> and a lesser amount of A phase current <NUM> as compared to the amount of C phase current <NUM>, which is at a desired level. In time period <NUM>, during the hot swap, the C phase current increases at <NUM>, while the A phase current remains at about the same magnitude, but has a short time frame when it is not oscillating and is in a DC state at <NUM> and then returns to an oscillating state at <NUM>. In time period <NUM>, when the swap is completed all three phases are shown to have approximately the same amount of current as was originally on C phase in period <NUM>. Clearly evident from this process is that the currents are now more balanced among the three phases, with no one phase having excessive loading compared with either of the other two.

Waveform 400c shows the mechanical shaft power, which in periods <NUM> and <NUM>, before and after hot swapping, respectively, are shown to be zero in regions <NUM> and <NUM>. During the hot swapping process, time period <NUM>, there is shown a negative average torque and power applied to the mechanical shaft (in region <NUM>) in order to rotate the shaft in the clockwise direction, so that the field winding is moved from being aligned with phase B to being aligned with phase A.

Waveforms 400d depict the three phase trunk electrical power being initially unbalanced (oscillatory) in regions <NUM> and <NUM> during time periods <NUM> and <NUM> (before and during hot swapping) and balanced (nearly constant) in region <NUM> during time period <NUM> when the hot swap is complete. The single phase lateral power <NUM> is oscillatory at all times (which is an essential property of single phase power systems) but has nearly constant average power and remains uninterrupted throughout the swap sequence, proceeding through <NUM>, to <NUM> and through <NUM>.

In each of time periods <NUM>, <NUM>, and <NUM>, machine winding models are shown in portion 400e of table <NUM> and lateral phase connections in the distribution network are shown in portion 400f of table <NUM>. As shown in model <NUM> and distribution network <NUM>, the field winding of the phase rotating transformer and the single phase lateral 18b (<FIG>) are connected to trunk phase B during time period <NUM>. During time period <NUM> (hot swapping), the single-phase lateral of the distribution network <NUM> is connected exclusively to the field winding of the phase rotating transformer, which provide any and all current to these loads. The field winding of model <NUM> is mechanically rotated in alignment from phase B to phase A, as described previously. And, during time period <NUM>, model <NUM> and distribution network <NUM>, the field winding of the phase rotating transformer and the single phase lateral 18b (<FIG>) are connected to trunk phase A and the hot swap is complete.

In <FIG> there is shown flow chart <NUM>, which describes the operation of controller <NUM> of phase rotating transformer system <NUM>, <FIG>, when a phase swap is desired/required, from phase B to phase A, as shown in <FIG>. Controller <NUM>, as shown in step <NUM>, initiates the phase swap sequence. In the current state (i.e. before phase swap), one of the <NUM> single phase contactors in contactor <NUM> is in the closed position, providing a nominal connection of the output line <NUM> to one of the three phases, in this example it is connected to B phase from the trunk 210B. The swap process proceeds to step <NUM> when controller <NUM> causes phase rotating transformer enclosure <NUM> to be energized and connected to single phase lateral <NUM> by closing three phase breaker <NUM> and single phase breaker <NUM>. In step <NUM>, rotary actuator <NUM> causes rotor field winding to rotate to <NUM> degrees to change magnetic coupling from phase B to phase A (destination phase). In step <NUM>, controller <NUM> causes contactor <NUM> to switch from nominal phase (phase B) and connect to the destination phase (either 210A) of the three phase trunk to lateral single-phase line <NUM>. Then, in step <NUM> the three phase breaker <NUM> and single phase breaker <NUM> are opened and the phase rotating transformer enclosure <NUM> is de-energized and disconnected, thus completing the phase swap sequence at step <NUM>.

By way of example only, phase rotating transformer <NUM> sizing may be determined as follows. As noted previously, the iron and copper based phase rotating transformer used in a phase re-balancer can operate and withstand currents approximately <NUM> times nominal (nameplate) ratings for up to a minute or more required for swap sequence. As such, for an example application servicing a single-phase lateral having a 1200kW loading, a three-phase synchronous generator (i.e. phase rotating transformer) with a nameplate rating of 400kW may be used. A <NUM> kW <NUM>-pole squirrel cage induction motor (generator) <NUM>, <FIG>, with <NUM> kV medium voltage stator may be used a proxy for a physical size of three-phase synchronous generator (i.e. phase rotating transformer <NUM>). Generator <NUM> would have roughly the equivalent magnetic material (iron) and size needed for this application, however, as shown it often includes a forced air fan <NUM>, which would not be needed for the phase rotating transformer according to this invention. The dimensions of generator <NUM> are approximately 3ft, in width by 7ft, in length by 5ft in height with the fan. Without the fan, the height would be reduced by approximately two feet.

Given the above dimensions for generator <NUM>, the system could be desirably configured as a pad mount system which would be sited outside of a substation, at the point where the single phase lateral branches from the three phase trunk (i.e. along the distribution feeder where space will likely be limited). As noted above, the forced air fan <NUM> would ideally not be included to reduce the overall size of the phase rotating transformer. In addition, three-phase synchronous generator <NUM>, would need to be modified to support a greater stator voltage, up to <NUM> kV class for distribution feeders. And, rotor field winding, should be further modified to operate at medium voltage, specifically the line-to-neutral equivalent for <NUM> kV LL class system to enable direction connection, and include multiple taps. If a low voltage rotor field were to be used (say 277V or 400V LN), the system would require a one- phase low voltage to medium voltage step up transformer for stepping up the low voltage output of the field winding to the medium voltage of the single phase lateral branch. Direct (flexible) connections to field winding terminals, rather than slip-rings that are common in motors and generators, would be beneficial.

A concept drawing of a load balancing system <NUM> according to an aspect of the invention is shown in <FIG> to include phase rotating transformer <NUM> of <FIG>, rotary actuator <NUM>, which would be interconnector to the shaft of phase rotating transformer <NUM>, in order to rotate the rotor and field windings to hot swap phases connected to the stator. There is also a breaker unit <NUM> which includes a current inrush limiter. The foregoing components in addition to other components, such as controller circuitry, may be included within enclosure <NUM> which may be mounted on pad <NUM>.

While the foregoing description enables one of ordinary skill to make and use what is considered presently to be the best mode of the system and method for load balancing on a multi-phase power line connected to a single phase lateral power line, those of ordinary skill will understand and appreciate the existence of variations, combinations, and equivalents of the specific embodiments and examples herein. The above-described embodiments of the present invention are intended to be examples only. Alterations, modifications and variations may be effected to the particular embodiments by those of skill in the art without departing from the scope of the invention, which is defined solely by the claims appended hereto.

Claim 1:
A system (<NUM>) for load balancing on a multi-phase power line connected to a single phase lateral power line, the system (<NUM>) comprising:
a phase rotating transformer (<NUM>) having a stator and a rotor; wherein the stator includes a plurality of primary coils, each primary coil configured to be connected to one phase of the multi-phase power line and wherein the rotor includes a secondary coil configured to be connected to the single phase lateral power line;
a rotary actuator (<NUM>) operably connected to the rotor; and
a controller (<NUM>) configured to cause the rotary actuator (<NUM>) to rotate the rotor to selectively electromagnetically couple the secondary coil with each of the plurality of primary coils, so as to enable electrical connection of each phase of the multi-phase power line with the single phase lateral power line; characterised in that the controller (<NUM>) is configured to cause the rotary actuator (<NUM>) to rotate the rotor from alignment with a first primary coil to alignment with a second primary coil and to stop rotation of the rotor when alignment with the second primary coil is achieved.