Patent Description:
Electrical generators are used in a wide variety of applications. Typically, an individual electrical generator operates in a stand-by mode wherein the electrical power provided by a utility is monitored such that if the commercial electrical power from the utility fails, the engine of the electrical generator is automatically started causing the alternator to generate electrical power. When the electrical power generated by the alternator reaches a predetermined voltage and frequency desired by the customer, a transfer switch transfers the load imposed by the customer from the commercial power lines to the electrical generator. As is known, most residential electric equipment in the United States is designed to be used in connection with electrical power having a fixed frequency, namely, sixty (<NUM>) hertz (Hz).

Typically, electrical generators utilize a single driving engine coupled to a generator or alternator through a common shaft. Upon actuation of the engine, the crankshaft rotates the common shaft so as to drive the alternator that, in turn, generates electrical power. The frequency of the output power of most prior electrical generators depends on a fixed, operating speed of the engine. Typically, the predetermined operating speed of an engine for a two-pole, stand-by electrical generator is approximately <NUM> revolutions per minute to produce the rated frequency and power for which the unit is designed. However, in situations when the applied load is the less than the rated kilowatt load for which the unit is designed, the fuel-efficiency of the engine will be less than optimum. As such, it can be appreciated that it is highly desirable to vary the operating speed of the engine of an electrical generator to maximize fuel efficiency, and thus reduce CO2 emissions, of the engine for a given load. Further, operation of the engine-driven, electrical generator at its predetermined operating speed can produce unwanted noise. It can be appreciated that reducing the operating speed of the engine of an electrical generator to correspond to a given load will reduce the noise associated with operation of the engine-driven, electrical generator.

If the size of the load is substantial, it may be desirable to utilize multiple generators in parallel rather than a single generator to provide power to the load. If multiple generators are utilized and one of the generators fails, the remaining generators are still able to provide a portion of the power required by the load. Further, if the load requirements were to increase, another generator may be added in parallel to supplement the existing capacity of the generators. Replacing a single generator, which already is larger in size than paralleled generators, with an even larger generator would be a significant expense.

If two alternating current (AC) power sources, such as multiple generators, are to be connected in parallel, the AC output voltages must be synchronized otherwise the instantaneous difference in voltage potential may result in current transferred between the two voltage sources. If one of the generators is a single-phase generator, the pulsating torque produced by a single-phase generator may result in some fluctuation in frequency of the power output by the generator. Variations in the load applied to the generator may also cause fluctuation in the frequency of the power output by the generator. Historically, it has been known to mechanically couple the output shafts of the generators to ensure that the generators synchronously supply voltage to the load. However, mechanical coupling adds increased expense and complexity to connecting multiple generators in parallel.

<CIT> discloses a control system for use with a genset. The control system has a bus configured to receive power from the generator set, and a first sensor configured to generate a first signal indicative of a characteristic of power on the bus. The control system also has a second sensor configured to generate a second signal indicative of an engine parameter of the generator set, and a controller in communication with the first and second sensors. The controller is configured to synchronize an electrical output of the generator set with the power on the bus based on the first and second signals.

<CIT> discloses a startup interval to reach a synchronizing condition, wherein a phase difference and an amplitude difference between the grid voltage and the stator voltage of one phase of a winding are obtained. The difference in amplitude is decreased prior to or in parallel to synchronizing the stator voltage with the grid voltage. The calculated compensation phase compensation value is used as an initial value for synchronizing at the next synchronizing operation.

<CIT>discloses a controlling method of an engine-driven, electrical generator. The generator generates an output voltage at a frequency with the engine running at an operating speed. The method includes the steps of connecting the generator to a load and varying the operating speed of the engine to optimize fuel consumption in response to the load. Thereafter, the frequency of the output voltage is modified to a predetermined level.

Therefore, it is a primary object and feature of the present invention to provide a method for controlling a variable speed, constant frequency, stand-by electrical generator such that it may be connected in parallel with at least one other generator.

The present invention is defined in the independent claims <NUM> and <NUM>.

The drawings furnished herewith illustrate a preferred construction of the present invention in which the above advantages and features are clearly disclosed as well as others which will be readily understood from the following description of the illustrated embodiment.

The various features and advantageous details of the subject matter disclosed herein are explained more fully with reference to the non-limiting, embodiments described in detail in the following description. Referring to <FIG> an engine-driven, electrical generator system for performing the methodology of the present invention is generally identified by the reference numeral <NUM>. The generator system <NUM> includes an alternator <NUM> defined by a cylindrical rotor <NUM> rotatably received within a stator <NUM>. By way of example, rotor <NUM> includes three-phase windings 31a-31c supplied by an inverter <NUM>, as hereinafter described. The stator <NUM> includes a plurality of electrical windings (e.g. main winding <NUM>) wound in coils over an iron core and an excitation winding <NUM> shifted <NUM> degrees from the main winding <NUM>. Rotation of the rotor <NUM> generates a moving magnetic field around the stator <NUM> which, in turn, induces a voltage difference between the windings of the stator <NUM>, As a result, alternating current (AC) power is provided across outputs 33a-33c of the stator <NUM>. The outputs 33a-33c of the stator <NUM> are configured to connect to a load <NUM> for supplying AC power thereto. Referring also to <FIG>, the inverter <NUM> includes a processor <NUM> and memory <NUM>. The processor <NUM> may be a single processor or multiple processors operating in parallel. The memory <NUM> may be a single device or multiple devices and may include volatile memory, nonvolatile memory, or a combination thereof. The processor <NUM> is configured to execute instructions stored in the memory <NUM> to control operation of the inverter <NUM>. The inverter <NUM> includes a power input <NUM> configured to receive an input voltage <NUM>, <NUM> from a power source. According to the embodiment illustrated in <FIG>, the power source is the excitation winding <NUM>. The inverter <NUM> also includes a rectifier <NUM> which converts the AC voltage from the excitation winding <NUM> to a DC voltage on the DC link <NUM>. An inverter section <NUM> converts the DC voltage from the DC link <NUM> to a regulated output voltage at the power output <NUM>. The inverter section <NUM> includes multiple switches, such as insulated gate bipolar transistors (IGBTs), metal-oxide semiconductor field effect transistors (MOSFETs), silicon controlled rectifiers (SCRs), or the like. The switches are controlled by a modulation routine stored in memory <NUM> and executed by the processor <NUM> to selectively connect and disconnect the DC link <NUM> to the power output <NUM>. The resultant output
voltage is a modulated waveform having a fundamental AC component at a desired amplitude and frequency.

The inverter <NUM> may also include inputs configured to receive control signals. A first input <NUM> is configured to receive a position signal <NUM>. It is contemplated that the position signal <NUM> may be generated by a position sensor operatively coupled to a rotating machine. The position sensor may be, for example, an encoder or a resolver, and the rotating machine may be an external generator <NUM>, as shown in <FIG>. Optionally, the position signal <NUM> may be one or more voltage and/or current signals corresponding to a voltage and/or current at the output 110a-110c of the external generator <NUM>. The processor <NUM> uses the position signal <NUM> to generate gating signals <NUM> for the inverter section <NUM> as discussed in more detail below. The gating signals <NUM> are used to enable and disable the switches and are controlled to generate the desired AC output voltage. The inverter <NUM> may also include one or more sensors connected to the output of the inverter section <NUM> with each sensor generating a signal to the processor <NUM> corresponding to a magnitude of voltage or current output from the inverter section <NUM>. Additional inputs 35a and 35b may be configured to receive a voltage or current signal from a sensor operatively connected to one of the outputs 33a-33c of the stator <NUM> via lines <NUM> and <NUM>. It is contemplated that the voltage and/or current feedback signal may be utilized by the processor <NUM> to determine the angular position of the rotor <NUM>.

The generator system <NUM> further includes an engine <NUM>. As is conventional, the engine <NUM> receives fuel such as diesel, natural gas, or liquid propane vapor through an intake. The fuel provided to the engine <NUM> is compressed and ignited within each of the cylinders responsive to a firing signal so as to generate reciprocating motion of the pistons of the engine <NUM>. The reciprocating motion of the pistons of the engine <NUM> is converted to rotary motion by a crankshaft. The crankshaft is operatively coupled to the rotor <NUM> of the alternator <NUM> through a shaft <NUM> such that as the crankshaft is rotated by operation of the engine <NUM>, the shaft <NUM> drives the rotor <NUM> of the alternator <NUM>. As is known, the frequency of the AC power at outputs 33a-33c of stator <NUM> is dependent upon the number of poles and the rotational speed of rotor <NUM> which corresponds, in turn, to the speed of engine <NUM>. The engine speed corresponding to a particular frequency of the AC power is called the synchronous speed (Ns) for that frequency. By way of example, the synchronous speed for a two pole rotor producing AC power at <NUM> hertz at outputs 33a-33c of stator <NUM> is <NUM> revolutions per minute.

Referring next to <FIG>, one generator system 10a may be connected in parallel with another generator system 10b or one or more other external generators 100a, 100b. Paralleling generators provides certain advantages. If a single generator system 10a, 10b or external generator 100a, 100b fails, the remaining generators are still able to generate a portion of the power while the failed generator is replaced. Further, if the size of the load <NUM> increases, additional generators may be added to increase the total power capacity from the paralleled generators. As each generator system 10a, 10b or external generator 100a, 100b is started up, there is a period of time required for the engine of the generator to accelerate up to speed and to begin driving the alternator <NUM> at the desired speed to generate power at the desired frequency. A switch 25a-25d is placed between each generator and the load <NUM> to allow the generator to remain disconnected during the initial start up period and for a control signal 17a-17d to be generated to connect the generator to the load <NUM> when the generator is producing power at the desired frequency. It is contemplated that separate switches 25a 25d may be utilized to selectively connect each generator to the load <NUM>. Optionally, a single switch such as a transfer switch that alternately connects utility power and the generators to the load <NUM> may be utilized. According to still another embodiment, a combination of switches, including, for example, a transfer switch, as well as, one or more individual switches 25a-25d may be utilized to connect the generators to the load <NUM>.

As discussed in more detail below, the generator systems 10a, 10b are controlled to synchronize the power output with an external signal, such as the position signal <NUM>. A controller <NUM> may be used to generate the control signal 17a-17d and only connect the generator system 10a, 10b to the load when the output voltage is at the desired output frequency and synchronized to the external signal. If a transfer switch is included, a sensor monitors the status of the utility grid. The sensor may be a separate sensor or integral, for example, to the transfer switch or to a separate system controller. When the utility grid is operating normally, the transfer switch may connect the utility grid to the load <NUM>, and when the utility grid has failed, the transfer switch may connect the generators to the load <NUM>.

It is noted that the engine <NUM> of the generator system <NUM> does not operate at a fixed, constant speed, but rather, operates at a speed that varies in accordance with the load magnitude. In other words, at low loads, where relatively little current is required by the load <NUM> from the alternator <NUM>, the engine speed is relatively low. At higher loads, where greater current is drawn from the alternator <NUM>, the engine speed is higher. While it can be appreciated that the speed of the engine <NUM> can be readily adjusted to optimize the fuel consumption and reduce the noise level associated with operation of the engine <NUM>, these changes in the engine speed, in turn, cause the frequency and voltage at the output 33a-33c of the alternator <NUM> to change. However, even when operating in a stand-alone application, the frequency and voltage of the AC power produced at outputs 33a-33c of stator <NUM> must remain relatively constant and substantially within preestablished upper and lower limits (e.g., <NUM>-<NUM>, and <NUM>-<NUM> Vrms). When operating in parallel with another AC power source, the AC power produced at outputs 33a-33c of stator <NUM> must typically remain within an even narrower range and must also be synchronized with the other AC power source.

The generator system <NUM> includes the controller <NUM> operatively connected to a current transformer (not shown) and to the throttle actuator of engine <NUM>. The current transformer measures a magnitude of the load <NUM> and supplies the same to the controller <NUM>. It is intended for the controller <NUM> to calculate the optimum fuel consumption for the engine <NUM> for a given load <NUM>. It can be appreciated that minimum fuel consumption typically occurs at approximately <NUM>/<NUM> of the synchronous speed (Ns) of the engine <NUM>. As such, for a two pole rotor producing <NUM> hertz AC power at the outputs 33a-33c of the stator <NUM>, the minimum fuel consumption occurs at an engine speed of <NUM> revolutions per minute. In response to instructions received from the controller <NUM>, the throttle actuator coupled to engine <NUM> increases or decreases the speed of the engine <NUM> to optimize the fuel consumption of the engine <NUM>. It is also contemplated for controller <NUM> to receive various additional inputs indicative of the engine operating conditions and provides additional control commands (e.g., an engine shutdown command in the event oil pressure is lost) to the engine <NUM>.

The frequency of the AC voltage at the output 33a-33c of the stator <NUM> is a function of both the rotor speed (Nr) and the frequency of the voltage applied to the rotor windings 31a-31c. As previously indicated, it is desirable to maintain a relatively constant frequency and voltage of the AC power produced at the outputs 33a-33c of the stator <NUM>. Therefore, if the controller <NUM> varies the rotor speed (Nr) to achieve improved fuel consumption and/or noise reduction in the generator system <NUM>, the frequency of the voltage applied to the rotor windings <NUM> a-31c must also vary to maintain the relatively constant output frequency. Lines 40a-40c are operatively connected to rotor windings 31a-31c, respectively, of rotor <NUM> via, e.g. slip rings, to supply three phase currents thereto. Given the rotor speed (Nr), the traveling wave of magnetic flux produced by the three phase currents supplied by the inverter <NUM> relative to the rotor <NUM> is equal to the difference between the synchronous speed (Ns) and the rotor speed (Nr). As such, the stator <NUM> "sees" the magnetic flux wave travelling at the synchronous speed (Ns) independent of the rotor speed (Nr) and will produce a constant frequency at outputs 33a-33c thereof. For a rotor <NUM> having two poles, the required frequency for the AC power supplied by inverter <NUM> to rotor windings 31a-31c to produce a traveling wave of magnetic flux that causes the outputs of stator <NUM> to have a constant frequency may be calculated according to the equation: <MAT> wherein: finverter is the frequency of the AC power supplied by the inverter <NUM> to rotor windings 31a-31c; Ns is the synchronous speed; and Nr is the rotor speed.

In order to deliver constant voltage and current at outputs 33a-33c of stator <NUM>, the AC power supplied by the inverter <NUM> may be calculated according to the equation: <MAT> wherein: P inverter is the AC power supplied by the inverter <NUM> or slip power; P stator is the AC power at the outputs 33a-33c and the quadrature winding <NUM> of the stator <NUM>; Ns is the synchronous speed; and Nr is the rotor speed.

In view of the foregoing, it can be appreciated that by controlling the magnitude and the frequency of the AC power supplied to the rotor windings 31a-31c by the inverter <NUM>, the frequency and voltage of the AC power produced by the generator system <NUM> at the outputs 33a-33c of the stator <NUM> is controlled. In operation, the engine <NUM> is started such that the alternator <NUM> generates electrical power at the outputs 33a-33c of the stator <NUM>, as heretofore described. The controller <NUM> monitors the magnitude of the load <NUM> and calculates the optimum fuel consumption fur the engine <NUM>. In response to instructions received from the controller <NUM>, the throttle actuator coupled to the engine <NUM> increases or decreases the engine speed (up to a maximum of <NUM> revolutions for a two pole) to optimize the fuel consumption of the engine <NUM>. Independent of the load <NUM>, the controller <NUM> maintains the speed of the engine <NUM> at a minimum <NUM> revolutions per minute since the minimum fuel consumption of the engine <NUM> occurs at an engine speed of <NUM> evolutions per minute,
When the rotor <NUM> is rotating at synchronous speed (Ns), the inverter <NUM> must provide a stationary wave relative to the rotor <NUM> in order to produce the same magnetomotive force as produced by a normal constant speed generator. In this manner, the inverter <NUM> behaves as an automatic voltage regulator behaves in a conventional alternator which has to provide a magnetizing magnetomotive force, as well as, a component to oppose the armature reaction. Further, it can be appreciated that by utilizing the excitation winding <NUM> of the stator <NUM> to power the DC link <NUM> of the inverter <NUM>, the main windings of the stator <NUM> are kept free of harmonics which occur as a natural result of DC link <NUM>. This, in turn, eliminates the need for additional filtering or for power factor correction upstream of the DC link <NUM>. When the generator system <NUM> is operating independently of another power source, it is desirable to maintain the frequency and voltage of the AC power produced at the relatively constant frequency and voltage. In order to maintain the frequency and voltage of the AC power produced by the generator system <NUM> at outputs 33a-33c of stator <NUM> within the preestablished upper and lower limits, the controller <NUM> determines the frequency and magnitude of the slip power to be supplied to the rotor windings 31a-31c by the inverter <NUM>, The frequency output by the inverter, referred to herein as an adjustment frequency, is the difference between the frequency of the voltage at the outputs 33a-33c of the stator <NUM> generated as a result of the operating speed of the engine <NUM> and the desired frequency (e.g., <NUM>). Thus, under the control of the controller <NUM>, the inverter <NUM> generates an AC voltage having the desired magnitude at the adjustment frequency to provide the necessary slip power to the rotor windings 31a-31c.

When the generator system <NUM> is operating in parallel with another power source, such as the illustrated generator <NUM>, it is desirable to maintain the frequency and voltage of the AC power produced by the generator system <NUM> synchronized with the power produced by the other generator <NUM>. The other generator <NUM> may be a standard generator and, therefore, susceptible to variations in the output frequency and/or voltage as a function of the magnitude of the load <NUM> applied. Therefore, it is desirable to synchronize the generator system <NUM> to the other generator <NUM> prior to connecting the two in parallel. The switch <NUM> may be controlled to alternately connect/disconnect the generator system <NUM> in parallel with the other generator system <NUM>. Once the controller <NUM> has synchronized the generator system <NUM> with the other generator system <NUM>, it generates a control signal <NUM> to connect the outputs (33a 33c and 110a-110c) in parallel.

According to the illustrated embodiment, a position signal <NUM> corresponding to the phase angle of the voltage output from the other generator <NUM> is provided to the inverter <NUM>. The other generator <NUM> may include an angular position sensor, such as a resolver or an encoder, operatively connected to the rotor of the other generator <NUM> and generating a measured position signal <NUM> corresponding to the angular position of the rotor. Optionally, a voltage and/or current sensor may be connected to the outputs 110a-<NUM>0c of the other generator <NUM> and the generator system <NUM> determines the angular position of the output voltage and/or current, respectively. It is further contemplated that the other generator <NUM> may internally measure operating parameters such as the voltage and/or current generated therein and provide an estimated position signal <NUM> based on the measured values. The inverter <NUM> may use the position signal <NUM> to determine the frequency of the voltage output by the other generator <NUM>. The inverter <NUM> determines the difference between the frequency of the voltage at the outputs 33a-33c of the stator <NUM> generated as a result of the operating speed of the engine <NUM> and the frequency of the voltage output by the other generator <NUM>. The difference between the frequencies is used as the adjustment frequency such that the inverter <NUM> generates an AC voltage having the desired magnitude at the adjustment frequency to provide the necessary slip power to the rotor windings 31a-31c. The inverter <NUM> monitors the position signal <NUM> to adjust the phase angle of the AC voltage at the outputs 33a-33c of the stator <NUM> such that it remains synchronized with the fluctuations from the AC voltage generated by the other generator <NUM>. The phase angle of the AC voltage at the outputs 33a-33c of the stator <NUM> is adjusted by varying the adjustment frequency in the inverter <NUM> and, in turn, varying the frequency of the slip power provided to the rotor windings 31a-31c. It is further contemplated that the inverter <NUM> may be controlled to offset the angle of the AC voltage at the outputs 33a-33c of the stator <NUM> to control sharing of the load <NUM> between the other generator <NUM> and the generator system <NUM>.

Optionally, the position signal <NUM> may be provided to the controller <NUM>, and the controller <NUM> may be configured to generate a frequency reference signal. The frequency reference signal corresponds to the difference between the frequency of the voltage at the outputs 33a-33c of the stator <NUM> generated as a result of the operating speed of the engine <NUM> and the frequency of the voltage output by the other generator <NUM> and is provide to the inverter <NUM>. The inverter <NUM> sets the adjustment frequency equal to the frequency reference signal and then generates an AC voltage having the desired magnitude at the adjustment frequency to provide the necessary slip power to the rotor windings <NUM> a-31c. With reference to <FIG>, the first AC voltage <NUM> from the generator system <NUM> and the second AC voltage <NUM> from the other generator <NUM> are synchronized with each other and may be provided in parallel to the load <NUM>.

Claim 1:
A method of controlling an engine-driven, electrical generator system (<NUM>) configured to be connected in parallel with a second power source, the generator system (<NUM>) generating a first alternating current (AC) voltage at a first frequency with the engine (<NUM>) running at an operating speed, the second power source providing a second AC voltage at a second frequency, the method comprising the steps of:
configuring an inverter (<NUM>) to receive an input signal corresponding to a phase angle of the second AC voltage;
determining the second frequency as a function of the input signal;
varying the operating speed of the engine (<NUM>) in response to a load (<NUM>) thereon;
calculating a difference between the first frequency generated responsive to the operating speed of the engine (<NUM>) and the second frequency and providing the difference as an adjustment frequency; characterized by monitoring a positional signal (<NUM>) to adjust a phase angle of the first AC voltage at outputs (33a-33c) of a stator (<NUM>) to remain synchronized with fluctuations with the second AC voltage, the phase angle of the first AC voltage at outputs (33a-33e) of a stator (<NUM>) is adjusted by varying the adjustment frequency in the inverter (<NUM>) and, in turn, varying a frequency of slip power provided to rotor windings (31a-31c) of a rotor (<NUM>) independent of the engine speed.