Simple Modulator with Voltage Balancing Control for the Hybrid Five-Level Flying-Capacitor Based ANPC Converter Jose I. Leon*, Leopoldo G. Franquelo*, Samir Kouro+ , Bin Wu+ and Sergio Vazquez* * Electronic Engineering Department University of Seville Seville, Spain 41092 Email:
[email protected] + Department of Electrical and Computer Engineering Ryerson University Toronto, Canada Email:
[email protected]
Abstract—The hybrid flying-capacitor based active-neutralpoint-clamped converter is a recent multilevel converter topology with interesting properties such as high quality output voltage and easy extension to achieve a high number of levels with reduced number of power devices. The control of the flying capacitors of the three-phase converter has been previously carried out by an elaborate control technique taking advantage of the large number of redundant switching states. This paper is focused on the introduction of a simple single-phase modulator for the single-phase five-level hybrid flying-capacitor based activeneutral-point-clamped converter. The single-phase modulator can be applied to each phase of a three-phase converter without losing generality. Using this modulation technique, the dc-link capacitors and the floating capacitor voltages are controlled to their desired values. Simulation results show the good performance of the voltage control obtaining simultaneously high quality output voltages and currents.
I. I NTRODUCTION Multilevel converters have become a very attractive solution for several industrial applications such as pumps, fans, large conveyors, HVDC systems, direct-drive converters for wind energy systems and ship propulsion among others. Since they were born in the last decades of 20th century, step by step their associated problems have been overcome. Currently, the minimization of the power losses, the balancing of the dclink capacitors, the modulation methods complexity and the accuracy of the control strategies have become mature at least for three-level converters. In this way, in the last decade a large number of commercial products can be found in the market with neutral-point-clamped (NPC), flying capacitor (FC) and cascaded H-bridge (CHB) topologies [1]–[3]. This fact shows the current and future success of multilevel converters for a wide range of medium-voltage high-power applications. Among the multilevel converters, the three-level NPC converter is nowadays the most commercialized topology all over the world achieving a nominal power up to 44 MVA [1]. One of the problems of this topology is the unequal loss distribution among the power semiconductors. This issue has been deeply
Three-level ANPC
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Fig. 1. Hybrid ANPC topology formed by the series connection of a threelevel ANPC and a floating capacitor power cell. This is a five-level topology if VC1 =VC2 =Vdc and Vf a =Vdc /2.
studied in the last years and it has been solved introducing the three-level active-NPC (ANPC) converter where active switches are used instead the clamping diodes [4]. The ANPC topology has been implemented by ABB as a power electronic building block to form IGCT based high voltage converters [5]. The extension of the ANPC converter in order to achieve a higher number of levels can be easily done. As in the three-level case, the N-level ANPC converter has the same structure than the N-level NPC converter but again active switches are used instead the clamping diodes [6]. However, the N-level ANPC converter has the same drawbacks of the Nlevel NPC converter in terms of voltage balancing control and high number of power devices. Recently, a new hybrid flyingcapacitor ANPC topology has been introduced to overcome these problems of the conventional ANPC converter [7]. This
TABLE I F IVE - LEVEL H YBRID F LYING -C APACITOR BASED ACTIVE -NPC S WITCHING S TATES . S1
S2
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Phase voltage va
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−VC2 Vf a − VC2 −Vf a 0 0 Vf a VC1 − Vf a VC1
Phase voltage va if VC1 =VC2 =2Vf a =Vdc −Vdc −Vdc /2 −Vdc /2 0 0 Vdc /2 Vdc /2 Vdc
paper is focused on the operation of this hybrid flyingcapacitor based ANPC converter. II. T HE H YBRID F LYING -C APACITOR BASED ANPC C ONVERTER The hybrid flying-capacitor based ANPC topology is formed by the series connection of a three-level ANPC and floating capacitor power cells. As an example, the single-phase hybrid ANPC converter with one floating capacitor cell is shown in Fig. 1. It has to be noticed from Fig. 1 that all the power semiconductors of the topology have the same voltage stress. The possible switching states of this topology are summarized in Table I. Usually, this converter is named five-level hybrid ANPC (5L-ANPC) because it achieves five symmetrical output voltage levels if if VC1 =VC2 =Vdc and the floating capacitor voltage Vf a is equal to Vdc /2 as can be observed in Table I. Although the 5L-ANPC topology is recent, ABB has already commercialized it as a IGBT based back-to-back topology in the ACS2000 medium voltage drive (400 kVA-1000 kVA) and it can be used for applications such as pumps, fans, conveyors, extruders, mixers, compressors and mills [5]. Recently, the 5L-ANPC topology has been proposed to be applied to wind power applications working as a 6 MVA inverter connected to a three-level ANPC active-front-end [8]. III. C OMMON M ODULATION T ECHNIQUES FOR THE 5L-ANPC C ONVERTER As can be observed in Table I, there are several switching states which obtain the same output voltage and affect to the flying capacitor voltage in opposite way. Previous publications have demonstrated that the voltage control of the flying capacitor of the 5L-ANPC can be achieved by choosing properly the redundant switching states of the switching sequence. Recently a space-vector modulation technique has been applied to a three-phase 5L-ANPC for the rectifier side of a back-to-back configuration [9]. The control region of the converter is the well-known hexagon plotted in the alpha-beta frame where the switching states are located. The switching sequence and the duty cycles are determined using the three nearest switching states to the reference vector. However, for a three-phase system, the level of flexibility and complexity is significant because of the high number of different output voltage vectors.
Influence on Vf a if ia > 0 − ↓ ↑ − − ↓ ↑ −
Influence on VC1 if ia > 0 − − ↑ ↑ ↑ ↑ − −
Influence on VC2 if ia > 0 − − ↓ ↓ ↓ ↓ − −
On the other hand, several multi-carrier based pulse widthmodulation (PWM) technique have been introduced to be applied to the hybrid flying-capacitor based ANPC converter. The multi-carrier PWM techniques are level-shifted [8] or phaseshifted techniques [10]. Specially interesting is the phaseshifted solution which applies triangular carriers in a similar way compared with the multilevel cascaded converter case. The phase shifting effect leads to a natural balance of the flying capacitors of the converter but losing dynamic performance when a large load step is present as happens when it is applied to the conventional multilevel flying-capacitor converter. This switching states redundancy has been also used applying other pre-programmed modulation techniques such as the selective harmonic elimination (SHE). In this case, the switching of the converter is determined offline in order to eliminate the harmonic distortion of some low order harmonics. The voltage level to be generated by the converter is finally obtained online choosing carefully the best switching achieving the floating voltage control simultaneously [11]– [13]. A similar technique has been also introduced to minimize online the overall total harmonic distortion [14]. IV. P ROPOSED M ODULATOR FOR THE S INGLE -P HASE 5L-ANPC C ONVERTER In this paper, a simple modulation technique is proposed considering the modulation concept introduced in [15], and called 1DM. The 1DM technique can be applied to any singlephase multilevel converter and only the final trigger signals depend on the multilevel converter topology. The 1DM technique is based on the generation of the reference phase voltage as an average of the nearest voltage levels. The single-phase modulation problem is reduced to very simple calculations determining easily the switching sequence (formed by two switching states) and the corresponding switching times. The 1DM modulator only determines the output voltage levels to be used to generate the phase voltage with low distortion. However, it does not include any additional control objective such as the dc voltage balance, the power losses minimization, etcetera. In this way, the 1DM modulator can be called a raw modulation method and it needs an auxiliary post-processing if other extra targets have to be achieved. This concept is shown in Fig. 2 where the diagram of a typical controller plus modulator of any converter is represented.
CONTROLLER TARGETS - Current tracking - Total DC-Link voltage control
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Fig. 2. General diagram of the control of a power converter including the controller, the modulator, additional controller for auxiliary control targets, the gate drivers and the converters
In this paper, an additional controller is added to the raw modulation technique in order to balance the flying-capacitors of the hybrid ANPC converter. As can be observed from Table I, there is always a switching state in the switching sequence which tends to balance the floating capacitors of the converter. The balancing control algorithm is based on choosing the proper redundant switching state taking into account the instantaneous values of the direction of the phase current and the flying capacitor imbalances. Using the proposed modulator, a maximum of only one switching is present during the sampling time Ts in each power semiconductor couple commanded by variables S1 , S2 and S3 . The flow diagram of the single-phase modulator for the 5LANPC converter is shown in Fig. 3. The computational cost of the final modulation technique is very low only including simple mathematical expressions. In the flow diagram, variable s takes values ±1 because it is defined as s = sign(Vref ).
(1)
In the flow diagram represented in Fig. 3, the switching variable Si (i = 1, 2, 3) takes the value Si1 during t1 /2, Si2 during 1 − t1 and finally again Si1 during t1 /2. The switching signal S1 is simply generated by comparing the reference phase voltage Vref with zero. Therefore, the modulator forces a fundamental switching frequency in the power semiconductors commanded by S1 leading to a reduction of the switching losses of the system. This reduction is due to the fact that S1 is the gate signal of eight power semiconductors while S2 and S3 command the state of a couple of them respectively. V. R ESULTS OF THE M ODULATOR FOR THE S INGLE -P HASE 5L-ANPC C ONVERTER The proposed modulator has been applied to the 5L-ANPC topology where the dc-link capacitors C1 and C2 are equal to 7mF, the floating capacitor Cf a is equal to 2mF and the total voltage of the dc-link 2Vdc is equal to 3000 volts. The total dc-link voltage is kept constant by an active front end. The desired floating capacitor voltage Vf a is equal to 750 volts. The 5L-ANPC is connected to a resistive-inductive load formed by the series connection of R=10Ω and L=3mH. The reference voltage Vref is a pure sinusoidal voltage with an amplitude equal to 1500 volts. The floating capacitor is initially discharged starting from zero volts. The sampling frequency fs of the modulator is equal to 800 Hz.
Vref>0 YES
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S22=S32=(1+s)/2 t1=2(Vdc-|Vref|)/Vdc
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Fig. 3. Flow diagram of the single-phase modulator for the 5L-ANPC converter with flying capacitor voltage control.
The flow diagram introduced in Fig. 3 is applied to the single-phase 5L-ANPC converter and the obtained results are represented in Fig. 4 where the phase voltage va , the phase current ia , the flying capacitor voltage Vf a and a half of the dc-link voltage VC1 are depicted. A detail of the phase voltage and current, the floating capacitor voltage and a half of the dc-link voltage in steady state conditions is shown in Fig. 5. It can be observed that the floating capacitor voltage achieves its desired value while the phase voltage and current have high quality. The switching frequency of power semiconductors commanded by S1 is 50Hz while the average switching frequency of S2 and S3 is around 675Hz. As can be observed in Fig. 4 and Fig. 5, the dc-link capacitor voltages are naturally balanced because the voltage of capacitor C1 (VC1 ) remains around 1500 volts (the half of the total dc-link voltage). This phenomenon can be explained considering the experiment shown in Fig. 6 and Fig. 7. In
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Fig. 4. Results of the proposed modulation technique applied to the singlephase 5L-ANPC. From top to bottom: Phase voltage, phase current, floating capacitor voltage and half of the dc-link voltage.
Fig. 5. Detailed results of the proposed modulation technique applied to the single-phase 5L-ANPC. From top to bottom: Phase voltage, phase current, floating capacitor voltage and half of the dc-link voltage.
this new test, the power converter starts the operation with an unbalanced situation in the dc-link (VC1 and VC2 are equal to 1000 and 2000 volts respectively). The floating voltage Vf a is 750 volts which is its desired voltage. In this case, in order to show clearly the natural balance effect of the dc-link capacitors, the dc-link capacitors C1 and C2 are equal to 2mF, the floating capacitor Cf a is equal to 1mF. The RL load is R=10Ω and L=35mH. From Fig. 6, it can be observed that the dc-link imbalance decreases achieving the desired operation point where both dc-link capacitor voltages are equal to 1500 volts. A detail of the waveforms of this experiment is shown in Fig. 7. Gate signals S1 and S2 are plotted in both graphics because they affect directly to the dc-link voltages VC1 and VC2 . The dc-link capacitor voltages are naturally balanced because the actual dc-link voltage imbalance creates an offset in the phase voltage when the proposed modulator is applied. As can be observed in the zoomed part of Fig. 7, the zero voltage is not generated when the dc-link is unbalanced. This fact leads to an offset in the phase current which directly affects to the dc-link capacitor voltages as was introduced in Table I. In this way, if the dc-link voltage VC1 is less than 1500 volts, a positive offset appears in the phase voltage and the phase current. This fact, in average, tends to increase the voltage of capacitor C1 reducing the dc-link voltage imbalance.
phase than using a complex space vector modulation technique which takes into account all the phases simultaneously. This concept highlights that a complex three-phase modulator can be substituted with three simple single-phase modulators making easy the determination of the switching sequence of the converter. In this way, the proposed single-phase modulator has been applied to each phase of a three-phase 5L-ANPC topology with the same dc voltage and capacitance values than in the single-phase converter case. The three-phase 5L-ANPC is connected to a star configuration resistive-inductive load formed by the series connection of R=10Ω and L=3mH. The sampling frequency fs of the modulator is equal to 800 Hz. The reference voltage Vref is again a pure sinusoidal voltage with an amplitude equal to 1500 volts. The floating capacitors are initially discharged starting from zero volts. The obtained results are represented in Fig. 8 where one of the phase voltages, the phase currents, the flying capacitor voltages and a half of the dc-link voltage are depicted. A detail of the phase voltage and phase currents, the floating capacitor voltages and a half of the dc-link voltage in steady state conditions is shown in Fig. 9. It can be observed that the floating capacitor voltages achieve their desired values while the phase voltages and currents have high quality. As in the single-phase case, the dc-link capacitor voltages are naturally balanced.
VI. R ESULTS OF THE M ODULATOR FOR THE T HREE -P HASE 5L-ANPC C ONVERTER The proposed single-phase modulator can be directly applied to each phase of a three-phase 5L-ANPC. In this way, the modulator could be implemented in the commercial ACS2000 by ABB. The use of single-phase modulators applied to multiple phases of a converter has been successfully introduced in recent publications. In [16], it is demonstrated that the obtained results are the same applying a single-phase modulator to each
VII. C ONCLUSIONS The active neutral-point-clamped converter (ANPC) is focusing increasing attention by the industry and the academia. In this paper, a five-level hybrid flying-capacitor based ANPC topology formed by the series connection of a three-level ANPC with a floating capacitor cell, called 5L-ANPC, has been studied. The aim of this paper is to introduce a simple
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Fig. 6. Results of the proposed modulation technique applied to the single-phase 5L-ANPC starting from an unbalanced situation in the dc-link. From top to bottom: Phase voltage, phase current, gate signals S1 and S2 , half of the dc-link voltage VC1 and floating capacitor voltage.
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Fig. 7. Detailed Results of the proposed modulation technique applied to the single-phase 5L-ANPC starting from an unbalanced situation in the dc-link. From top to bottom: Phase voltage, phase current, gate signals S1 and S2 , half of the dc-link voltage VC1 and floating capacitor voltage.
modulation technique to obtain high performance output waveforms with voltage balance control of the flying capacitors and the dc-link capacitors. The proposed modulation method is based on a simple single-phase modulator previously introduced to be applied to other multilevel converter topologies. Auxiliary calculations are added to this simple modulation to achieve the control of floating capacitor and the dc-link voltages. The resulting modulation technique has very low computational cost only including simple equations and com-
parisons. Applying the proposed modulation strategy to the singlephase 5L-ANPC, the phase capacitor voltage is controlled to their desired value which is 25% of the total dc-link voltage in order to generate five equally distributed output voltage values. This voltage balancing control is achieved by carrying out a proper selection of the switching signals of the converter taking into account the actual values of the floating capacitor voltage and the phase current. In addition,
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Fig. 8. Results of the proposed modulation technique applied to the threephase 5L-ANPC. From top to bottom: Phase a voltage, phase currents, floating capacitor voltages and half of the dc-link voltage. 2000 0 −2000 0.13 200
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The authors gratefully acknowledge the financial support provided by the Ministry of Education under grant PR20100162 and by the Chilean research fund Fondecyt (No. 1110783).
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Fig. 9. Detailed results of the proposed modulation technique applied to the three-phase 5L-ANPC. From top to bottom: Phase a voltage, phase currents, floating capacitor voltages and half of the dc-link voltage.
the dc-link capacitor voltages are naturally balanced due to the offset effect introduced by the modulator if a dc-link voltage imbalance is present in the converter. The proposed modulation method can be applied with very low switching frequency. In the presented experiments, the sampling frequency of the modulator is 800Hz leading to a switching frequency of the power devices (except S1 which has fundamental switching frequency) around 675Hz. The commercial 5L-ANPC converter is a three-phase system and the proposed modulator can be independently applied to each phase of the converter achieving good results as well. The obtained waveforms of the three-phase system are also included in this paper. The results show the good performance of the proposed modulation method.
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