Abstract:
A power electrical system is disclosed for a microturbine power generator. The invention permits the microturbine to be started using an external DC power source. The DC voltage is converted to a variable DC voltage by means of a bi-directional buck-boost circuit, DC bus and a DC-to-AC converter. The DC-to-AC converter produces at its output a fixed voltage pattern whose frequency is gradually increased in concert with the DC voltage, to accelerate the microturbine from standstill to startup speed. Once the microturbine is started, the excitation is discontinued, and the DC bus and DC-to-AC are used to produce output AC power at a voltage level and frequency to match an electrical load.

Description:
BACKGROUND OF INVENTION 
     The invention relates to the field of micro-turbines and, in particular, to starting systems for microturbines. 
     Microturbines are small turbines typically used for on-site power generation. They are generally applied as back-up or auxiliary power sources for office buildings, retail store, small manufacturing plants, homes and many other facilities. These facilities have traditionally been powered by electric utilities via a grid of power distribution lines. Microturbines enable these facilities to generate electrical power at their own sites and thereby avoid being solely dependent on conventional electrical power grids and utilities. Microturbines may also generate power at less cost and/or more reliably than the electrical power provided over the grid by electrical power utilities. 
     Microturbines require a starter. This starter usually includes a powerful electric machine and a power source for the electric machine, such as a battery. The electric machine may be configured (1) to operate as a motor to start the turbine, and (2) as a generator for a load that is driven by the microturbine once the microturbine has been started. During a start-up phase, the electric machine (operating in motor mode) rotates the microturbine until it has been accelerated to a rotational speed rapid enough to enable the microturbine to operate on its own and drive the electric machine (then operating in generator mode). 
     A conventional controller for starting a microturbine is disclosed in U.S. Pat. No. 6,020,713 (&#39;713 Patent) that describes a pulse-width modulator (PWM) inverter that operates as an adjustable speed motor drive to start the microturbine. An electrical machine coupled to the PWM inverter is configured as a motor to drive the microturbine during startup and is then configured as a generator that is driven by the microturbine. The PWM inverter described in this &#39;713 Patent varies both the frequency and amplitude of the AC power that it generates to start the microturbine. During a start-up phase of the microturbine, the PWM inverter is powered by a DC bus that is charged to a constant DC voltage. Thus, the PWM inverter is required to convert DC power at constant voltage to variable voltage and variable frequency AC power. 
     It may not be desirable to rely on a PWM inverter to generate both the variable AC voltage and variable AC frequency needed to start a microturbine. Similarly, it may not be desirable for a single power circuit component, e.g., a PWM inverter, to vary both the frequency and amplitude of the AC power generated to start a microturbine. There is a need for a power circuit for starting a microturbine that does not rely on a PWM inverter to vary both the frequency and voltage of AC power applied to start the microturbine. 
     Moreover, the voltage applied to the electric machine, when operating in motor mode, must be correlated with the frequency of the applied voltage to maintain the desired flux. This correlation is traditionally accomplished in two steps using two distinct power circuits, i.e. the battery charger and the DC-AC converter (inverter). 
     There is a need for a power circuit coupled to a microturbine that does not require distinct circuit components for starting and operating a microturbine. 
     SUMMARY OF INVENTION 
     The present invention relates to power circuits that start a microturbine and later couple a microturbine to an electrical load. A power circuit generates variable DC voltage which is subsequently converted to AC by another power circuit. The frequency of the AC power generated by the inverter (DC-AC converter) is varied in a controlled manner bearing a fixed relationship with the DC voltage, such that it leads to the acceleration of the microturbine. Accordingly, the AC power applied to start the microturbine has both variable frequency and variable voltage. 
     In an exemplary embodiment of the invention, a battery provides a source of DC power for starting a microturbine. The constant voltage of the battery is converted by a buck-boost chopper circuit to variable voltage DC power. The buck-boost chopper steps-up the voltage of the DC power through a series of voltage levels that are sequentially applied to drive the microturbine during a start-up phase via the DC-AC converter. The buck-boost chopper applies the variable DC voltage to a capacitive DC bus that distributes the variable DC voltage to other power circuit components. The variable DC power on the DC bus is converted to AC power using a DC-to-AC converter. This converter generates AC power having a variable frequency. The frequency of the AC power is sequentially increased to match (or slightly lead) the desired accelerating starting speed of the microturbine. 
     The combination of a buck-boost chopper circuit, capacitive DC bus, and a DC-to-AC converter provides a circuit arrangement to start a microturbine. This power circuit arrangement satisfies the need for a power circuit that does not exclusively rely on a PWM scheme to both vary the frequency and voltage of generated AC power applied to start a microturbine. 
     The invention offers several advantages over prior start-up power circuits including that a single power control circuit component is nor relied on to vary both the frequency and voltage of the AC power applied to start a microturbine. When the voltage waveforms generated at the output of the inverter are of the six-step type, the switching losses are lower. In addition, the control scheme of the present power control circuit can be simplified as compared to the PWM inverter control schemes. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     Other advantages and novel features of the present invention will become apparent upon review of the following description in conjunction with the accompanying drawings. 
     FIG. 1 is an electrical schematic diagram showing a power circuit topology of a microturbine during start-up, and 
     FIG. 2 shows a start-up timing diagram showing an external supply voltage, DC bus voltage and AC startup voltage for the power circuit shown in FIG.  1 . 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a schematic and block diagram of a power circuit  10  for coupling a microturbine  12  and an electrical machine  14  to an electrical load  16  and to a starting battery  18 . The microturbine and electrical machine may share a common rotating output shaft  20  and be formed as an integral unit. Alternatively, the microturbine and electrical machine may be separate units each having a rotating shaft output/input that is coupled to the other. The electrical machine  14  may be configured to operate as a motor to drive the microturbine during a startup phase, and then configured to operate as a generator driven by the microturbine. 
     A conventional gas microturbine, for example, generally includes a compressor (for natural gas and other fuels), recuperator, a combustion chamber, and the turbine (not shown). A compressor for the microturbine may not be required for other types of fuel, such as diesel. Air is mixed with fuel in the combustion chamber to generate hot, high pressure gases that drive the turbine. The turbine exhaust gases are ducted through the recuperator to transfer heat to the air in the compressor and thereby increase the energy of the air-fuel mixture in the combustion chamber. 
     Once started, the microturbine operates on its own and produces output power at the output power shaft  20 . This shaft drives a rotor (not shown) of the electrical machine  14 . The electrical machine may comprise a polyphase, e.g., three- or six-phase, stator winding with the winding output terminals connected to an AC-DC converter  22 , e.g. a diode rectifier. The rotor of the electrical machine generally includes a plurality of alternating magnetic poles arranged symmetrically around the axis of the electrical machine. During power generation operation of the microturbine, the magnetic fields emanating from the rotor move through a stator (not shown) of the electrical machine that surrounds the rotor. The stator has a cylindrical aperture sized to receive the rotor. The stator generally includes a plurality of windings and magnetic poles of alternating polarity. When the electrical machine is functioning as a generator, the magnetic fields rotating through the stator produce current in the stator windings. This current is output as electrical power from the electrical machine and is applied by power circuit  10  to the load  16 . 
     The current and voltage produced by the electrical machine is proportional in frequency to the rotational speed of the rotor and the turbine. The load  16  may comprise a polyphased AC electrical power source as well as other residential or commercial single or three phase loads. The power circuit  10  converts the AC power from the electrical machine to AC power that matches the electrical load  16  (typically grid frequency and voltage). For this conversion, the power circuit  10  converts the AC power from the electrical machine  14  to DC power using an AC-to-DC converter  22 . Converter  22  may have the provision for an output stage which comprises a boost circuit that converts the uncontrolled DC output voltage to a regulated voltage value higher than the unregulated DC voltage that would have been produced without the boost circuit. The DC output voltage is applied to charge a DC bus  24 . The regulated DC voltage value is matched to the AC voltage required by the electrical load  16  by a DC-to-AC converter  26  subject to the constraints imposed by the power circuit component ratings. For instance, a 480 VAC (volts alternating current) output requirement calls for a 800 or a 900 V DC regulated value. The load  16  may comprise an electrical system for a residence, retail store, other commercial establishment or other site requiring electrical power. The load may also include a public utility power grid. 
     To produce power, the microturbine must first be started by driving the electrical machine  14  as a motor which accelerates the microturbine to a startup speed. Once the turbine reaches the start-up speed, rotation of the microturbine can be sustained without external power. Until the turbine is accelerated to start-up speed, external power is required to rotate the turbine. During start-up, electrical power flows in direction of the arrows  28  to drive the electrical machine  14  as a motor. Power may flow in opposite directions after the microturbine has been started and the electrical machine is producing power. 
     To start the microturbine, the battery  18  provides a source of direct current (DC) at a substantially constant voltage to the power circuit  10 . The battery may comprise a conventional storage cell battery having deep discharge capability to sustain prolonged and repetitive starts. The constant voltage DC power from the battery is converted to variable voltage DC power by a buck-boost chopper  30  connected to the battery. The buck-boost chopper operates as a power stage circuit that steps-up or steps-down, i.e., increases or decreases, the battery voltage level to match the variable voltage requirements for starting the microturbine and permits power flow in and out of the battery. The buck boost chopper  30  may comprise a conventional buck boost chopper circuit component that generally includes a microcontroller that controls the voltage levels output from the chopper circuit. The buck boost chopper may be controlled to sequentially produce DC power at a series of selected voltage levels. The controls for the buck boost chopper circuit may be included in the startup control program that is included with the microturbine. 
     The sequence of voltage levels generated by the buck boost chopper  30  is selected to match the turbine startup sequence. Specifically, the buck boost chopper  30  may gradually increase the DC voltage through a series of step-wise voltage levels during the startup period of the turbine. The increase in voltage levels is used to drive the electrical machine as it accelerates the microturbine to startup speed. Once the turbine has been started and is generating power, the buck boost chopper  30  converts DC power from the DC bus  24  to a DC voltage level suitable to charge the battery  18 . 
     During the startup phase, the capacitive DC bus  24  distributes the variable voltage DC power produced from the buck boost chopper  30  to DC-to-AC (alternating current) converter  26 . The DC bus may comprise a conventional capacitive device commonly used for distributing DC power within a power circuit. The DC bus  24  also distributes DC power from AC-to-DC converter  22 , when the electrical machine  14  is producing power. However, during start-up, the AC-to-DC converter  22  is isolated (due to switch contactor C 3   32 ) and performs no function. 
     During start-up, the DC-to-AC converter  26  converts the variable voltage DC power from the bus  24  to AC power. In one embodiment, the DC-to-AC converter  26  comprises a modified pulse width modulated (PWM) inverter that has been configured to operated as a variable frequency converter rather than as a PWM inverter. During turbine startup phase, the DC-to-AC converter  26  gradually increases the frequency of its output AC power. The converter  26  may include a microcontroller that gradually increases the frequency of the AC power during the startup phase. The rate of increase of the AC frequency is selected to accelerate the electrical machine  14  and microturbine  12  to startup speed. Typically microturbine  12  includes a controller (not shown) with a start up control program loaded therein. The rate of frequency increase produced by DC-to-AC converter  26  may be controlled by the startup control program that is included with the microturbine. 
     The AC power produced by the DC-to-AC converter  26  may optionally be passed through an output filter  34  to remove extraneous noise or other unwanted fluctuations in the AC power. The AC power from the DC-to-AC converter  26  is routed to the electrical machine  14  which is then driven as a motor. To apply the AC power to the electrical machine excitation contactor  36  (C 1 ) is closed during startup phase, but is open while the microturbine is generating power. During startup, an output contactor switch  38  (C 2 ) is held open to isolate the power circuit  10  and microturbine from the electrical load  16 , but is closed while the microturbine  12  generates power so that the AC power produced by the electrical machine  14  can be applied to the load  16 . Contactor (C 3 )  32  is kept open during start up and closed during normal operation. 
     FIG. 2 shows a timing diagram of the conversion of battery power to drive the electrical machine in motor mode, during the microturbine startup phase. The timing diagram shows voltage level (V) as a function of time (t) for the battery  18  (DC voltage  40 ), the output of the buck-boost chopper  30  (DC voltage  42 ) and the output of the DC-to-AC converter  32  (AC voltage  44 ). The timing diagram shown in FIG. 2 relates to the startup phase of the microturbine and, thus, primarily shows the voltage levels that correspond to the startup phase. The voltage levels generated by the microturbine during its power production phase are not generally shown in FIG.  2 . 
     The battery voltage level  40  (V batt ), e.g., 24 volts, remains substantially constant during the startup phase of the microturbine. The battery voltage level and the battery current are sufficient to drive the electrical machine  14  as a motor in order to start the microturbine. Preferably, the battery is recharged by the electrical machine  14  after the microturbine has been started and produces power. 
     The buck boost chopper circuit  30  steps-down (or steps up) the constant battery voltage to output a gradually increasing voltage level  42  that is applied to charge the DC bus  24 . The voltage level applied by the buck boost chopper circuit  30  to the DC bus gradually increases in accordance with a ramp function that is applied to control the output voltage level of the chopper circuit. The ramp function may be expressed in equation (1) below: 
     
       
           VDC=k ( t−t   start ), when 0 &lt;t&lt;t   ramp   (Equation 1) 
       
     
     Where V DC  is the voltage level of the DC bus  24 , “k” is a constant setting the slope of the ramp function, “t” is time, “t start ” is time at the beginning of the microturbine startup phase  46 , and t ramp    48  is end period during which the DC voltage is being increased during the startup phase. 
     In the first period of the microturbine startup phase, voltage is applied to drive the electrical machine  14  as a motor. During this first period of the startup phase, the buck boost chopper circuit  30  gradually increases the voltage of the DC bus  24  from zero volts (or some other relatively low starting voltage level) to the DC rated voltage of the ramp  50 . This gradual increase in voltage level is performed to match the drive voltage requirements of the electrical machine  14 , which is being operated as a motor during the startup phase. During the first period of the startup phase and when the startup time (t) exceeds t ramp , the voltage outputs of the buck boost chopper and the DC bus remain at a steady machine  50  that corresponds to the rated voltage of the bus  24 . The voltage level on the DC bus remains at this rated machine  50  until the microturbine is started and, thus, ending the first period of the startup phase. 
     Also during the first period of the startup phase, the DC-to-AC converter  26  (which is functioning as an alternator) converts the DC power from the DC bus to AC power  44  having a gradually increasing frequency. The DC-to-AC converter alternatively inverts the voltage level of the DC bus to produce a series of voltage pulses, where adjacent pulses are inverted. The effect of this series of alternatively inverted pulses is AC power. In addition, the converter is polyphase and will, for example, generate three phases of the AC power that are each identical in voltage level and frequency and off-set in their phases. 
     The frequency of the AC power  44  is increased in proportion to the increase in the voltage level on the DC bus. Accordingly, the frequency output by the DC-to-AC converter can be controlled by the voltage level on the DC bus, such that as the voltage level on the bus increases the AC frequency will increase proportionally. Accordingly, as the DC voltage increases, the periods (T 1 , T 2 , T 3 . . . T k ) of the AC voltage cycle are sequentially shortened by the DC-to-AC converter. At t ramp ,the AC frequency reaches a steady state (T SS )  52 . 
     At the end of the first period of the startup phase and when the microturbine has started and is self-sustaining, the excitation contactor  36  is opened to cease operating the electrical machine as a motor. During phase two  54  of the startup phase, the microturbine  12  is operating but is not coupled to the load  16 . In phase two, the contactor  32  remains open and the DC bus  24  is still powered by the battery  18 . 
     Shortly after the excitation contactor  36  is opened (during Phase II  54 ), contactor  32  is closed so that AC power from the electrical machine  14  is applied to converter  22 , which converts that AC power to DC power applied to the DC bus. The power from the electrical machine and converted by converter  22  further charges the DC bus to a voltage level  56  that corresponds to the desired no-load operating voltage of the DC-to-AC converter  26 . The second period  54  of the startup phase ends, when the DC bus has been charged to the non-load operating voltage level and the output contactor  38  is closed to connect the load  16  to the electrical machine  14 . 
     In order to abort a start-up, in one embodiment, DC-AC converter  26  is disabled and contactor  36  (C 1 ) is opened. Buck-boost chopper  30  then transfers power from DC bus  24  to battery  18 . 
     Although the invention has been described and illustrated in detail, it is clearly understood that the same is by way of example and illustration only and is not to be taken by way of limitation. The terms of the present invention are to be limited only by the terms of the appended claims.