Abstract:
A multilevel voltage source inverter and related control scheme in which a VSI controller operates in a low frequency mode of operation below a threshold operating frequency to more evenly distribute the duty cycle and thermal loss of the VSI switches. The technique makes use of multiple alternative switching states which achieve the same output voltage.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to voltage source inverters. More particularly, the present invention relates to control techniques for pulse width modulation (PWM) voltage source inverters. 
     BACKGROUND OF THE INVENTION 
     Multilevel pulse width modulation (PWM) voltage source inverters are useful in a variety of applications, and function generally to provide a specific voltage output on a multilevel DC bus. A typical voltage source inverter includes a number of semiconductor switches connected between the positive DC bus and the negative DC bus, and a controller is operatively coupled to control the semiconductor switches to achieve a desired output voltage on the multilevel DC bus. 
     Numerous techniques to control the semiconductor switches are known. For example, U.S. Pat. No. 5,790,396 discloses a neutral point clamped (NPC) inverter control system which includes a DC power source to output DC voltage having a neutral point, an NPC inverter to convert the DC voltage into AC voltage in three phases through PWM control, a mode selecting unit to decide a first and a second PWM modes by comparing amplitude of voltage reference with a prescribed value that is defined by a minimum pulse width, a first voltage reference conversion means to add a prescribed bias value at which a polarity changes to positive/negative within a fixed period to secure the minimum pulse width to voltage references in respective phases in a first PWM mode, a second voltage reference conversion means to fix the voltage reference in one phase by a value that secures the minimum pulse width when voltage reference in one phase is smaller than a prescribed value that is defined by the minimum pulse width in a second PWM mode and correct voltage references of other two phases so as to make line voltage to a value corresponding to the voltage reference, and a modulation frequency change-over means to lower PWM control modulation frequency in the first PWM mode and to suppress power loss caused by switching in the first PWM mode. In this control system, the PWM frequency is lowered to suppress power loss. 
     Another control technique is disclosed in U.S. Pat. No. 5,684,688. This patent discloses a three-level NPC inverter topology including two auxiliary resonant commutation circuits which are controlled to clamp the voltage across each main inverter switch to zero voltage prior to altering the state of the switch in order to achieve soft switching of all main inverter switches while reducing output voltage harmonics and gradients. In this technique, a soft switching control scheme is provided by added control circuitry to reduce power losses in the switches. 
     Other control schemes are believed to include de-rating the inverter at a low frequency by lowering the maximum current output of the inverter, and using a stall protection based on an inverter thermal model to predict device temperatures, which will generate a fault shutdown of the inverter if device temperature limits are exceeded. 
     Control schemes such as those described above do not address the problem of switch control at lower VSI frequencies. At high frequencies, switches change state frequently. At lower frequencies, the switches change state less frequently; thus, certain switches may be held in an active state for a increased period of time. This results in increased thermal losses for certain switches, and limits the inverter current capability at these lower frequencies. 
     SUMMARY OF THE INVENTION 
     It would be desirable for a voltage source inverter to be controlled in such a manner that the inverter current capability can be maximized at lower VSI frequencies. It would also be desirable to avoid an uneven distribution of thermal losses for the VSI switches. It would further be desirable to provide improved switch control without adding additional control circuitry. 
     The present inventions overcomes the disadvantages noted above, and achieves additional advantages, by providing for a voltage source inverter which includes a DC link, a plurality of switches connected in series between the positive and negative dc buses for each phase, and a controller operatively connected to a gate on each of the switches; in the embodiments described below, the controller operates to gate the switches in a substantially uniform manner such that each switch in the plurality of switches has a substantially uniform duty cycle at frequencies below a threshold frequency. The controller makes use of multiple switching states which achieve the same output voltage. The controller minimizes the dwell time of at least one (e.g., zero vector) switching state by maintaining the dwell time below a dwell time threshold. At higher frequencies, the VSI controller can operate according to a standard control technique. 
     The present invention thus provides a relatively uniform duty cycle for each switch at lower frequencies, thereby avoiding uneven thermal distribution and maximizing the inverter current capability at lower frequencies. The present invention also avoids the use of additional circuitry. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention can be more fully understood by reading the following Detailed Description of presently preferred embodiments together with the accompanying drawings, in which like reference indicators are used to designate like elements, and in which: 
     FIG. 1 is a block diagram of an exemplary voltage source inverter in which the control technique of the present invention can be implemented; and 
     FIG. 2 is a three-level space vector diagram illustrative of control vectors for three-level, three-phase systems. 
    
    
     DETAILED DESCRIPTION 
     Referring now to FIG. 1, a circuit diagram of a conventional three-phase, three-level, neutral-point-clamped (NPC) voltage source inverter is shown. It will be appreciated that the circuit of FIG. 1 is but one example of a circuit in which the control techniques of the present invention can be implemented. In the circuit of FIG. 1, a constant voltage is maintained on a bus  10  connected to a DC link, and this voltage is equally divided between upper and lower capacitances  12  and  14 , respectively. The circuit includes a plurality of switches (S 1 , S 2 , S 3 , S 4 ) connected in series between the positive DC bus  10  and the negative DC bus  11 . Each switch has a corresponding diode (D 1 , D 2 , D 3 , D 4 ) associated with it. It will be appreciated that in the circuit of FIG. 1, there is such an arrangement of switches and switching diodes for each phase (in this example, the three phases are designated as A, B, and C; only the circuit elements for phase A are described for purposes of explanation). There are also provided clamping diodes D 5  and D 6  connected in series between a first node between switches S 1  and S 2 , and a second node between switches S 3  and S 4 . A clamping node between diodes D 5  and D 6  is connected to a node between the capacitances  12  and  14 , and to a grounding resistance  15 . The gate of each switch is operatively coupled to receive a control signal from a PWM controller  16 ; it will be appreciated that the connections between the controller  16  and the switches is not shown for ease of illustration and explanation. 
     In the arrangement of FIG. 1, each output phase (A, B, C) can have three possible voltage states: 1) a positive (+) state when the upper two switches S 1  &amp; S 2  are on (and the other switches are off); 2) a negative (−) state when the lower two switches S 3 , S 4  are on (and the other switches are off); 3) a neutral (0) state when the middle two switches S 2 , S 3  are on (and the other switches are off). To achieve a required three-phase output voltage, switch states in all three phases are coordinated properly by control signals generated and supplied by the PWM controller  16 . Thus, an output state for the three phases can be represented as xyz, where x, y, and z are one of (0), (+), or (−). 
     FIG. 2 shows a space vector diagram which illustrates a modulator scheme for the three-level three-phase system of FIG.  1 . Each vector on the diagram of FIG. 2 represents a balanced three-phase voltage, and each node of the diagram represents an available switching vector. Many switching vectors can be achieved by multiple switching states (that is, multiple combinations of open and closed switches); for example, vector 0 can be achieved by three different switching states, 000, +++, or −−−. In other words, any of these switching combinations will result in an identical output state on phase A. An optimal way to synthesize an arbitrary voltage vector such as V 1  is to use the three adjacent vectors, in this case, vectors 0, 1 and 2 (states 000, 00+, and 0+0). The scheme is optimal because it produces the smallest possible voltage errors, harmonics, and switching losses. The actual dwell times in each vector are preferably determined by a modulator algorithm designed to ensure the smallest voltage error over a PWM cycle. Obviously, a vector close to switching vector 0 will have a long dwell time in vector 0. In a conventional mode of operation, at typical operating frequencies that are not very low, the voltage vector will rotate through the vector space of FIG. 2 quickly, and the switches change state quickly. This results in a relatively even distribution of the duty cycles for the switches. At relatively low frequencies however, voltage vectors tend to change very slowly, such that certain switches have much heavier duty cycles during a given period of time. The extreme case is a Direct Current (DC) voltage, which corresponds to a stationary voltage vector. 
     Assuming that the magnitude of the voltage vector is near zero, often the case in inverter applications requiring constant volts per hertz ratio, the inverter will dwell mostly at vector 0. Using the conventional optimal centered space vector modulation scheme described above, the switching sequence for the near zero vector will be from (000) to (+00) to (++0) to (+++) for the first PWM cycle, and then from (+++) to (++0) to (+00) to (000) for the second cycle, and then from (000) to (00−) to (0−−) to (−−−) for the third cycle, and then from (−−−) to (0−−) to (00−) to (000) for the fourth cycle. The dwell times in each of these states in every transition are substantially the same. In this control scheme, the inner switches S 2  and S 3  (and diodes associated with them), which are conductive and open in (000) as well as (+++) or (−−−), have a duty cycle up to 75%. The outer switches S 1  &amp; S 4  have a duty cycle on the order of 25%. This uneven distribution contributes to the inverter rating problem at the low or zero frequencies. 
     According to an embodiment of the present invention, the controller  16  is suitably programmed to skew the dwell time distribution in the switching sequence at frequencies below a threshold frequency. Continuing with the example described above, the controller  16  can determine when the operating frequency of the VSI is below the threshold, and enter an alternative mode of operation in which the controller will favor the (+++) and (−−−) states, and minimize the dwell times for the (000) state. As a result, all four switches can have substantially uniform (about 50%) duty cycles, a much more uniform distribution and 50% improvement from conventional control techniques. It will be appreciated that the duty cycles of clamping diodes (D 5  &amp; D 6 ) are reduced even more. When the controller  16  determines that the operating frequency has exceeded the threshold frequency value, the controller  16  can operate in a default mode which can be an otherwise conventional control technique. 
     According to one embodiment of the present invention, the threshold frequency is approximately the reciprocal of three bridge thermal time constants. Typically this frequency will be about 5 hertz. 
     A voltage source inverter according to one embodiment of the present invention can control the switches such that a dwell time in at least one switching state can be less than a maximum dwell time. One exemplary maximum dwell time can be defined as the larger of the minimum on time and the minimum off time for the device. Such times can be on the order of 30 μsec or less. 
     The foregoing description includes numerous details that have been provided for purposes of explanation only. These details are not to be construed as limitations of the invention. The details and examples presented above can be readily modified without departing from the spirit and scope of the invention, as defined by the following claims and their legal equivalents.