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This invention relates to AC to DC rectifiers and, more particularly, to a boost converter to achieve unity power factor with low input current harmonics.
Typical voltage source inverter based variable frequency drives (VFDs) have an AC to DC rectifier unit with a large DC bus capacitor to smooth the voltage ripple. The DC bus capacitor draws charging current only when it is discharged in to the motor load. The charging current flows into the DC bus capacitor when the input rectifier is forward biased, which occurs when the instantaneous input voltage is higher than the DC voltage across the DC bus capacitor. The pulsed current drawn from the AC source is rich in harmonics because it is discontinuous. This type of nonlinear current flow is associated with poor input power factor. The power delivery equipment is also subject to unnecessary power loss and the overall system efficiency suffers. In light of the above mentioned problems, it is worthwhile to suggest power topologies to help improve the input power factor and lower the input current distortion thereby increasing the system efficiency.
There are many techniques, both passive and active, that are employed to improve the current waveform and reduce the overall current harmonics. The active techniques have an advantage over the passive techniques from size and performance viewpoints. However, the cost of certain type of active techniques can be higher than passive techniques.
The present invention, as disclosed in the detailed description below, employs boost converters in a manner that lends itself to easy integration and for applying to high power applications. A basic prior circuit for a single-phase boost converter application is shown in
The boost converter circuits shown in
An advantage of the bridgeless topology is that at any given instant of time only two semiconductors conduct current—a switch and a diode or two diodes; whereas in the traditional boost, converter of
In the standard bridgeless topology, the control requires sensing the input current through both of the boost inductors. This can be an involved process. An intelligent way of sensing both of the input currents using only one current sensor has also been used in product offerings.
The operation of a standard single phase boost converter is explained here using an equivalent circuit shown in
When the switch S1 is turned off, Mode 2 begins. Since the current through the inductor LDC cannot abruptly cease to flow, it maintains its flow by forward biasing the blocking diode D. The capacitor CDC gets charged since the energy in the inductor LDC transfers to the capacitor CDC and so the capacitor voltage starts to increase. A part of the inductor current flows into the load as well. The voltage across the inductor LDC is negative during this period since the input voltage is lower than the DC bus voltage. This has to be true since the average voltage across an inductor is always zero.
Based on the discussions thus far, the waveforms at important points in the boost converter can now be confirmed, as shown in
The output voltage is a function of the duty cycle ‘α’ of the switch S1. The relationship between the average output voltage VDC and the average input voltage Vrec in terms of the duty cycle α is derived next. If the switch S1 is ON for duration tON, and has a cycle duration of T, then duty cycle α is defined as:
Since the average voltage across the inductor is zero, the following expression is true:
The average output voltage VDC in terms of the average input voltage Vrec and duty ratio α is given as:
When the duty cycle α is zero, then the average output voltage is the same as the average input voltage because the switch S1 is not being utilized. If the duty ratio α is 1, then the output voltage can theoretically go to infinity, which is impractical. Since the output DC voltage VDC is greater than the input rectified voltage Vrec, for all practical values of α, the circuit of
Because of the boost action, during loaded operating condition, current flows through the input terminals irrespective of the fact that there exists a DC bus capacitor. By modulating the switching duty cycle a, to follow the input AC voltage, the input current can be made sinusoidal. In other words, the input power factor can be significantly improved and the input current distortion can be reduced.
Average current control is a popular method to regulate the output DC voltage in a boost converter and simultaneously achieve low input current harmonics. Problems of crossover distortion in complete sensing schemes such as current programmed control can be avoided. The current ILDC through the inductor LDC, is considered to be the best choice for average current control. The reasons are as follows: the phase of ILDC with respect to the rectified output voltage has information necessary to achieve unity power factor; and the amplitude of ILDC has information pertaining to the operating power condition.
Hence, the current ILDC is considered to be the choice parameter for average current control. The reference current equation can be derived from the power balance condition. The average input power should be equal to the average output power neglecting the loss in the DC link inductor LDC and the switch S1. Using this condition, the following relationship is obtained:
In the above equations, Re is the effective and programmable load resistance. The load current ILOAD is assumed constant so that the reference current generation I*LDC requires knowledge of the desired regulated DC bus voltage (A), the instantaneous value of the input rectified voltage Vrec that has the phase information (B) and the square of the rms value of the input AC voltage (C). The importance of the C term is visible from the point of power. For a given power rating, if the input AC voltage is high, then the input current amplitude should automatically be reduced as square, of the input ac voltage.
A controller IC, such as the model UC3854, that has all the features described thus far to facilitate unity power-factor operation with low input current distortion is popularly used in many commercially available boost converters. The theoretical waveforms on a supply frequency time scale are shown in
In the schemes discussed thus far, the topology assumes that the input AC source is of single phase type. Hence, in all cases, the load level is typically less than or equal to a maximum value of 3.7 kW. When the load power increases beyond this value, it is customary to adopt three-phase input AC supply. Many local and municipal codes do not allow operation of larger power loads via a single phase AC source. A three phase version of the standard boost topology has been studied. There is need for a neutral point source. In other words, a three phase, four-wire system is needed for proper operation of such known circuits. Such circuits also attempt to achieve zero voltage transition (ZVT) to reduce the overall loss in the power stage.
One known topology uses two single-phase boost converters in an asymmetrical manner to achieve high power factor and low input current distortion at the input 3-phase AC supply. By converting the input three phases to just two phases using an autotransformer low input current and high input power factor may be achieved using only two boost converters. One of the drawbacks is that there is unbalanced current flowing in the input of the rectifier modules since the gains of the two boost converters are modified such that the power being supplied from each of the two boost converters is exactly one-half of the total load power. Since the current flowing in the two boost converters are different because the power balance needs to be maintained, there is need for two independent current sensors for independently controlling the two boost converters. The two boost converters can share the same voltage feedback information since both these units need to control the same dc bus voltage.
The present invention addresses the need for a three phase boost converter in a three phase 4-wire system,
This application describes a low cost active circuit that is based on employing three single phase boost converters. Since the need to improve input power factor and reduce input current harmonic distortion is more important in high power AC to DC rectifiers typically forming the front end of high power Variable Frequency Drives (VFDs), it is necessary to address power factor and harmonic problems in such applications.
In one aspect there is disclosed a voltage source inverter comprising a zigzag transformer for receiving three phase AC power from an AC source and providing a neutral point. A rectifier unit comprises three single phase boost converters each connected to one of the three phases of AC power and the neutral point and comprising a bridge rectifier converting single phase AC power to DC power and using a boost inductor with a boost switch and blocking diode to develop a DC output. An inverter receives DC power and converts DC power to AC power. A link circuit is connected between each boost converter DC output and the inverter circuit and comprises a DC bus having first and second rails to provide a relatively fixed DC voltage for the inverter and a DC bus capacitance across the first and second rails to smooth voltage ripple. A switch controller controls operation of the boost switches using average current control for each of the boost converters.
There is disclosed in accordance with another aspect of the invention a three phase boost converter system comprising a zigzag transformer for receiving three phase AC power from an AC source and providing a neutral point. A rectifier unit comprises three single phase boost converters each connected to one of the three phases of AC power and the neutral point and comprising a bridge rectifier converting single phase AC power to DC power and using a boost inductor with a boost switch and blocking diode to develop a DC output. A link circuit is connected to each boost converter DC output and comprises a DC bus having first and second rails to provide a relatively fixed DC voltage for a load comprising DC/AC converters fed from a common DC voltage source with each DC/AC converter having a DC bus capacitor to smooth voltage ripple. A switch controller controls operation of the boost switches using average current control for each of the boost converters.
It is a feature of the invention that the switch controller senses current through the boost Conductor of each boost converter.
Three current sensors may be provided, one for each boost, converter, and the sensed currents may be summed as an average current input of the switch controller. Alternatively, a single current sensor may sense current through each boost inductor as an average current input to the switch controller.
It is a further feature that the switch controller is connected to the DC link to sense DC voltage for each, of the boost converters.
It is another feature that the switch controller senses bridge rectifier DC output for each boost converter.
It is still a further feature that the switch controller controls duty cycle of each boost switch to maintain desired average current.
It is yet another feature that the switch controller monitors ripple current and reduces the conduction duration if the ripple current increases.
Other features and advantages will be apparent from a review of the entire specification, including the appended claims and drawings.
As described below, a VFD uses three single phase boost converters that are balanced under steady state conditions and share the power equally among them. In other words, if the load power is PLOAD, then each boost converter will be required to process power corresponding to only PLOAD/3. The disclosed topology will not require use of a 3 phase, 4-wire source. Instead, the neutral point is obtained from an existing 3 phase supply using a zigzag transformer. Since the neutral currents are balanced and they cancel each other out, the actual neutral current flowing into the zigzag neutral is zero. This reduces the size of the zigzag transformer dramatically. A zigzag transformer rated to handle only a low magnetizing current will suffice. Typically, the VA rating of the zigzag transformer in the disclosed topology will be less than or equal to 0.08 pu.
Referring particularly to
The AC source 12 may comprise a drive or the like developing three phase AC power using three wires which are connected to terminals H1, H2 and H3 of a zigzag auto transformer 20. The zigzag auto transformer 20 may be of conventional construction and provides a neutral point N. As indicated, the current flowing into the zigzag neutral N is zero.
The boost converter system 18 is in the form of a rectifier unit comprising three single phase boost converters 22, individually labeled as 22-1, 22-2 and 22-3. Each single phase boost converter 22 is of identical construction.
The first single phase boost converter 22-1 comprises a bridge rectifier 24-1 connected to the first phase terminal. H1 and the neutral point N for converting AC power to DC power across a high side node P1 and a low side node N1. Boost inductors LDC1+ and LDC1− are connected to the respective points P1 and N1 and to blocking diodes D1+ and D1−. A switch S1 is connected across the junction between the respective inductors LDC1+ and diode D1+ and LDC1− and diode D1−. The switch S1 includes terminals G1 and E1 for controlling operation thereof. The single phase boost converter 22-1 develops DC power at a DC output formed by output nodes 26-1. A current sensor CT1 senses current through the boost inductor LDC1+.
The second and third single phase boost converters 22-2 and 22-3 are of identical construction and are not described in detail herein.
The DC outputs of the three single phase boost converters 22 are connected by a DC link circuit 28 to an inverter 30. The DC link circuit 28 comprises a DC bus 32 having first and second rails + and − to provide a relatively fixed DC voltage for the inverter 30. A DC bus capacitance formed by capacitors CDC1 and CDC2 is across the first and second rails to smooth voltage ripple.
An active switch controller 34 controls operation of the boost switches S1, S2 and S3 using average current control. The inputs to the active switch controller comprise the input DC voltages at terminals P1 and N1 for the first boost converter 22-1, and similar terminals P2 and N2 for the second boost converter 22-2 and P3 and N3 for the third boost converter 22-3. The active switch controller 34 has a single voltage input comprising voltage across the DC bus 32. The current through the three current transformers CT1, CT2 and CT3 are summed at a summer 36 to provide an average current input to the active switch controller 34.
The active switch controller 34 comprises three separate control integrated circuits to generate the necessary gating pulses at terminals G1 and E1, G2 and E2, G3 and E3, for the three single phase boost converters 22-1, 22-2 and 22-3, respectively. The integrated circuits may be, for example, UC1854 type high power factor preregulators. These implement average current control operation to regulate output DC voltage, as discussed above.
The salient features of the disclosed topology are listed below:
Since the current is equally shared by the three single phase boast converters 22, only one current sensor DCCT, see
By providing an ability to feed DC power to the VFD load, common DC bus configuration is easily achievable as shown in the system configuration in
From the discussions put forth above, the advantages of the disclosed system, include the system achieves low input current distortion and improves the true input power factor by employing three single phase boost converters without the need for a three phase, 4-wire system. The disclosed system employs a zigzag transformer to provide a stable neutral point needed for satisfactory operation of the three single phase boost converters. The use of a zigzag transformer also helps reduce preexisting imbalance in the three phase AC system by providing a path for the imbalance components to flow into the autotransformer and cancel each other out. The balanced neutral currents from the three single phase boost converters are balanced and phase shifted by 120 electrical degrees, thereby cancelling each other out. This fact reduces the VA rating of the zigzag autotransformer, which is needed to only accommodate the magnetizing current. A typical VA rating of the zigzag autotransformer is proposed to be 0.08 pu or less. Since the three boost converters feed a common load, the power and current rating of the boost converters are balanced and of the same value. This allows the currents of each boost converter to go through a common single current sensor for processing and control. This feature saves cost, space, and improves reliability. Since the outputs of the boost converter feed into a common load, it is easy to implement this topology for common DC bus application as has been disclosed here.
It will be appreciated by those skilled in the art that there are many possible modifications to be made to the specific forms of the features and components of the disclosed embodiments while keeping within the spirit of the concepts disclosed herein. Accordingly, no limitations to the specific forms of the embodiments disclosed herein should be read into the claims unless expressly recited in the claims. Although a few embodiments have been described in detail above, other modifications are possible. For example, the logic flows depicted in the figures do not require the particular order shown, or sequential order, to achieve desirable results. Other steps may be provided, or steps may be eliminated, from the described flows, and other components may be added to, or removed from, the described systems. Other embodiments may be within the scope of the following claims.