When medium to wide bandwidth polyphase AC to DC converters are used in the presence of high line impedance, as occurs with motor-generator sets or poorly regulated AC sources, instability problems are commonly observed. Instability at the input of a wide bandwidth polyphase power converter is caused by the interaction of the negative input impedance presented by the power converter and the positive effective source impedance. The effective source impedance may be defined as the combined impedance of the AC to DC converter input filter and the power supply line.
The real component of the input impedance of a power converter drawing constant power varies from negative at low frequencies to positive at high frequencies, crossing over at the closed loop bandwidth of the power converter. The effective source impedance is a maximum at the effective source resonant frequency of the combined input filter and line inductance and the input filter capacitance.
Unstable operation of a power converter is undesirable because it can lead to, for example, saturation of magnetic components, an increase in the level of input and output noise, failure of passive and active components due to overheating, and a reduction in the power factor due to an increase in the total amount of volt-amperes being drawn from the supply.
For a power converter to achieve a condition of stability, ordinarily the magnitude of the input impedance of the power converter must be greater than the effective source impedance for all load conditions. If this condition is violated, the sum of the real components of the input impedance of the power converter and the effective source impedance must be positive in an operating region where the condition is violated to ensure stable operation of the power converter. A more complete discussion of this requirement to achieve stability is found in Jang, Y., Erickson, R. W.: “Physical Origin of Input Filter Oscillations in Current Programmed Converters”, ”, IEEE Trans. Power Electronics, vol. 7, Iss. 4, October 1992, pp 725–733.
One prior art method commonly used to meet this stability criterion is to lower the effective source impedance by increasing the input filter shunt capacitance until stability is achieved. However, this is not practical or advisable in AC to DC converters with a requirement for near unity power factor. In providing near unity power factor there is a limit on the amount of input shunt capacitance that may be used, since shunt capacitance degrades power factor. As an alternative, the closed loop bandwidth of the AC to DC power converter must be reduced below the effective source resonant frequency to prevent instability. Reducing the bandwidth makes the input impedance of the power converter positive in the region of violation, as mentioned above, thus satisfying the stability criterion. Either a fixed or variable low bandwidth AC to DC converter may be used to achieve stability in this manner. Ordinarily, a fixed low bandwidth may be designed to be below the worse case effective source resonant frequency, while a variable bandwidth may track the actual source resonant frequency and provide improved rejection of line borne disturbances from the output of the AC to DC converter in cases other than worst case. Both of these measures result in a degree of line borne disturbance at the output and a degradation in the load transient response compared to a wide bandwidth AC to DC converter, especially in a high line impedance case.
An illustration of one prior art technique designed to address the above mentioned stability problem is shown in FIG. 1, where two converter stages are used. The first stage converter has low bandwidth and may pass some or all line frequency harmonics and line borne disturbances to a DC link capacitor positioned between the two stages forming a high noise DC link. The second stage comprises a DC to DC converter that regulates the output voltage of the converter and rejects any voltage variation appearing on the high noise DC link that may be transferred by the first stage converter. In this case, the DC link capacitor provides a low source impedance to the second stage for all loads, thereby permitting the second stage to have wide bandwidth and address the stability problem. The disadvantage of this approach is that it processes all the output power twice resulting in, for example, reduced efficiency, increased complexity and size compared to a single stage AC to DC converter.
As a possible alternative, the second stage converter might be omitted and the DC link capacitor increased in value to perform the function normally done by the second stage converter. A simple calculation based on a commonly required specification for noise and load transient response in the presence of a low bandwidth (for example 500 Hz) bandwidth first stage, reveals why this alternative is not viable: The value and therefore the size of the storage capacitor is very large, completely negating any advantage gained by simplifying the power converter circuitry in this way.
Another prior art technique uses damping, either active or passive, to reduce the resonant peak in the effective source impedance. Referring to FIG. 2, for power converters with marginal stability on a known maximum line impedance, several damping arrangements of the input filter are possible and this is exemplified in Vlatkovic, V., Borojevic, D., Lee, F. C., “Input Filter Design for Power Factor Correction Circuits”, IEEE Trans. Power Electronics, Vol. 11, No. 1, January 1996 pp 199–205. In FIGS. 2a and 2b, two examples of single phase passive damping circuit arrangements are shown, either a series RC in parallel with the shunt capacitance C1 as shown in FIG 2a, or a parallel LR in series with the filter inductance L1 as shown in FIG. 2b. An example of three phase active damping is shown in FIG. 2c. 
Referring to FIG. 2a, addition of a damping resistance Rd in series with a portion of the total filter shunt capacitance C1a and C1 may damp the resonance that occurs in the input filter. One of the problems with such an arrangement, however, is that the reactive line frequency current flowing in the shunt capacitance C1a produces a significant loss in the series resistance Rd, thereby reducing the converter efficiency and requiring a substantial increase in the total size of the input filter to prevent overheating. In addition, the damping capacitance C1a is required to be equal or larger than the shunt capacitance C1, resulting in a substantial increase in the size of the input filter and a degradation of the power factor.
Referring to FIG. 2b, the addition of a parallel inductance Ld and damping resistance Rd in series with the filter inductance L1 also damps the resonance that occurs in the input filter. However, for high line impedances the size of the extra inductance Ld becomes impractical and also increases the total series line impedance thereby negating any expected stability margin improvement.
Referring to FIG. 2c, the linear active circuit containing a control circuit, a power amplifier and three current transformers L, is used to damp the filter resonant peak by sensing the inductor current and injecting a canceling current. A further description of this technique is found in Vlatkovic, V., Lee, F. C., Borojevic, D.: “Damped EMI Input Filter Power Factor Correction Circuits”, and U.S. Pat. No. 5,530,396 (Vlatkovic et al) issued 25 Jun. 1996. This technique has limited success because it is commonly used to damp only the input filter resonance, and does not take into consideration the effect of high line impedance, which dominates in many cases. If the line impedance resonance were damped using this method, the damping circuit then becomes large and less efficient, reducing any advantages gained.
Passive and linear active damping schemes are usually bulky and lossy for critical damping. They provide a limited benefit to the converter stability because a practical limit occurs when critical damping is achieved and further damping will not significantly reduce the effective source impedance. This is because the impedance stability criterion described above will still be violated at some load. For high line impedance cases, instability will occur if a wide bandwidth converter is used, even with a critically damped filter.
Active damping may also be achieved in a substantially lossless manner by controlled switching by way of Pulse Width Modulation (PWM). Examples of such techniques may be found in Marques, G. D.: “A Current-type PWM Rectifier Control System with Active Damping Based in the Space Vector Technique”, IEEE Proc. of Industrial electronics ISIE 1997 pp. 318–322 vol. 2; Sato, Y., Katoka, T.: “A Current-type PWM Rectifier with Active Damping Function”, IEEE Trans. Industry Applications, Vol., 32, Iss. 3, May–June 1996, pp 533-541; and, Baumann, M., Drofenik, U., Kolar, J. W.: “New Wide Input Voltage Range Three-Phase Unity Power Factor Rectifier Formed by Integration of a Three-Switch Buck-Derived Front-End and a DC/DC Boost Converter Output Stage”, 22nd Intelec Proc., September 2000, pp. 461–470. However, these schemes pass line borne disturbances to the output of the AC to DC converter, and therefore require further measures, such as a second stage converter to remove such disturbances from the output. Generally, these schemes, including their further measures, are a variation on the prior art of FIG. 1.
A further prior art technique is shown in FIG. 3, comprising a polyphase shunt active filter connected to the input of a polyphase AC to DC converter, which operates to compensate for load reactive power and line harmonics. Further description of such compensation techniques is found in Rastogi, M., Mohan, N., Edris, A.: “Filtering of Harmonic Currents and Damping of Resonances in Power Systems with a Hybrid-Active Filter”, IEEE APEC Proc., March 1995, pp. 607–612 (Rastogi et al); and, Newman, M. J., Zmood, D. N.: “Stationary Frame Harmonic Reference Generation for Active Filter Systems”, IEEE Trans. Industry Applications, Vol. 38, No. 6, November 2002, pp 1591–1599 (Newman et al). As a by-product of performing their primary function of filtering harmonic currents, such active filters may also lower the source impedance in the frequency band where instability would otherwise occur. However, the primary function of these filters is to filter harmonic currents and as a result, the design of these circuits, as described in Rastogi et al and Newman et al involves large complex circuitry directed at targeting harmonic currents specifically and is not applied to the instability problem resulting from high line impedance. Accordingly, there is no real discussion of the instability problem in either of Rastogi et al or Newman et al. The main disadvantages of these prior art techniques are the high degree of complexity and large size of the shunt active filters.
Further prior art schemes for power conversion are disclosed in U.S. Pat. No. 6,115,267 (Herbert) issued 5 Sep. 2000 and, U.S. Pat. No. 5,144,222 (Herbert) issued 1 Sep. 1992. U.S. Pat. No. 6,115,267 is directed to a transformer isolated, Power Factor Corrected (PFC) AC-DC power converter comprising a main power path which is buck derived, and most of the power passes through a single power stage to the output. A parallel path in the secondary circuit shunts current to a storage capacitor during the times when the input AC current is at its peak, and returns current to the circuit when the input AC current is low. In one embodiment, the shunt stage comprises a secondary side boost converter. In another, the shunt stage comprises a buck converter. U.S. Pat. No. 6,115,267 addresses the problem of input rectifiers contributing significantly to losses in the converter by a single phase power converter circuit in which the input stage may operate without input rectifiers if AC switches, such as back to back MOSFET's are used, because the output of the transformer and secondary rectifiers is the same regardless of the polarity of the instantaneous input voltage. Although U.S. Pat. No. 6,115,267 discloses a single power stage that may be operated as a low bandwidth converter, this reference like Rastogi et al and Newman et al, does not discuss or set out to address the instability problem. There is no discussion or attempt to address the increase in line borne disturbances and degradation in the load transient response that results from using a low bandwidth converter, compared to a wide bandwidth converter.
U.S. Pat. No. 5,144,222 discloses techniques for the polyphase power conversion case. However, this reference again has no discussion of the stability problem associated with high input line impedances and discloses dual stage power converters similar to those of FIG. 1. Furthermore, the converter stages disclosed in U.S. Pat. No. 5,144,222 comprise complex combinations of buck or boost converters with additional complex control circuitry requiring more than one controller in its stages.
The prior art single phase AC to DC converters, do not suffer from the stability problem since they use and usually require a very low bandwidth AC to DC converter stage in some form for power factor correction. In addition, a large energy storage element is used to provide power to the DC output during the region where the AC supply voltage crosses zero and this capacitor also acts to smooth the DC output. A discussion of single phase systems is found in Tse, C. K., Chow, M. H. L., Chung, M. K. H.: “A Family of PFC Voltage Regulator Configurations with Reduced Redundant Power Processing”, IEEE Trans. Power Electronics, vol. 16, No. 6, November 2001, pp 794–802.
In general, the prior art techniques discussed above increase the size and limit the efficiency of polyphase AC to DC power converters when there is a necessity to operate on sources with high line impedance.
Any discussion of documents, devices, acts or knowledge in this specification is included to explain the context of the invention. It should not be taken as an admission that any of the material formed part of the prior art base or the common general knowledge in the relevant art in Australia or elsewhere on or before the priority date of the claims herein.
It is an object of the present invention to provide a power converter, which alleviates at least one disadvantage of the prior art arrangements. Another object of the present invention is to provide a wide bandwidth polyphase AC to DC converter capable of operating on high line impedance with minimal energy storage elements and conversion stages to enable a reduction in converter size while increasing the conversion efficiency.