The invention relates generally to DC to AC inverters. More specifically, the invention relates to a 5-level single-phase inverter that comprises a 3-level leg and a 2-level leg having a reduced number of switching devices, which leads to lower losses and increased efficiency. The control method developed for the single-phase inverter is then used to build a three-phase inverter comprised of three single-phase hybrid inverters in order to achieve a line-to-neutral voltage having five levels and a line-to-line voltage having nine levels.
Today, the power industry has revived and entered a new age using renewable energy, and high efficiency power generation, transmission and distribution where multilevel power converters can assume significant roles.
Multilevel inverters offer a distinct advantage over their 2-level counterparts due to their ability to synthesize AC waveforms with lower Total Harmonic Distortion (THD), and smaller
and common-mode voltage. Traditionally, multilevel power conversions are dominantly used in the applications of medium voltage AC drives, flexible AC transmission systems (FACTS), and High-Voltage DC (HVDC) transmission systems, because single power semiconductor devices cannot handle high voltage. However, the higher cost of multilevel inverters has restricted its presence in low voltage applications.
A multilevel structure can be considered as an AC voltage synthesizer realized from multiple discrete DC voltage sources. Multiple, equal DC sources are required. Multilevel inverters provide an AC output waveform at discrete voltage levels. The more steps or levels generate a smoother sinusoidal waveform and reduce the amount of output filtering. Practically, it is a trade-off to select the number of levels considering the converter complexity and filter requirements. By optimizing the angles and heights of steps, certain lower order harmonics can be cancelled. In addition, the harmonic spectrum can be reduced by using Pulse-Width Modulation (PWM) techniques at each level.
Recently, there is a trend towards implementing multilevel solutions in low voltage renewable energy applications, e.g., in a solar farm where grid connected inverters are required to feed high quality current into the electric distribution system. Also, interests revive multilevel topologies for their ability to reduce the mass and size of LC filters and to eliminate line-frequency transformers.
Numerous multilevel topologies have been proposed and studied for utility and motor drive applications.
There is a need for a simplified 5-level, low THD inverter.
The inventor has discovered that it would be desirable to have a single-phase hybrid multilevel inverter that combines a 3-level leg and a 2-level leg to reduce the number of overall switching devices for a 5-level inverter. A three-phase inverter embodiment comprised of three single-phase hybrid inverters results in a line-to-neutral voltage having five levels and a line-to-line voltage having nine levels. Embodiments use a smaller number of switching devices and are viable in applications where galvanic isolation is required, e.g., in solar power systems and UPS applications.
Embodiments provide a single-phase 5-level inverter topology that combines a 3-level flying capacitor leg with a 2-level inverter leg. The 2-level inverter leg switches at a fundamental frequency and the 3-level leg switches at a higher frequency. Embodiments achieve an optimum single-phase voltage inverter with automatic capacitor balancing using a minimum number of switching devices.
Embodiments employ a PWM method that provides a low THD output voltage spectrum when compared to phase-shifted PWM that is typically used for flying capacitor topologies. These single-phase embodiments are combined to form a three-phase 5-level inverter.
One aspect of the invention provides a hybrid inverter. Inverters according to this aspect of the invention comprise a topology comprising a 3-level flying capacitor leg coupled in parallel with a half-bridge 2-level leg, the 3-level flying capacitor leg comprising four unidirectional controlled switches coupled together in series that define a positive node (+) beginning at a first switch S1, a node C between the first switch S1 and a second switch S2, a node A between the second switch S2 and a third switch S3, a node D between the third switch S3 and a fourth switch S4 and a negative node (−) after the fourth switch S4, and a capacitor C1 coupled to nodes C and D, and the half-bridge 2-level leg comprising two unidirectional controlled switches coupled together in series that define a positive node (+) beginning at a fifth switch S5, a node B between the fifth switch S5 and a sixth switch S6 and a negative node (−) after the sixth switch S6, and an alternating current output defined between nodes A and B.
Another aspect of the inverter is an inverter switch waveform synthesizer configured to generate switch signals S1PULSE, S2PULSE, S3PULSE, S4PULSE, S5PULSE and S6PULSE that control the first switch S1, the second switch S2, the third switch S3, the fourth switch S4, the fifth switch S5, and the sixth switch S6 respectively comprising a reference sine wave generator configured to output a reference sine wave f(t) at a fundamental frequency ff, amplitude m, phase angle φ and time t, a first comparator configured to receive the reference sine wave f(t) and compare the reference sine wave f(t) with zero to generate the switch signal waveform S5PULSE wherein if the reference sine wave is greater than 0 switch S5 is off and if the reference sine wave is less than or equal to 0 switch S5 is on, a not function configured to receive the switch signal waveform S5PULSE and output the switch signal waveform S6PULSE, a first frequency divider configured to receive the switch signal waveform S5PULSE and divide the switch signal waveform S5PULSE by 2 to generate an SQF signal, a mapping function configured to map the reference sine wave f(t) wherein the discrete time value of the reference sine wave is M and if M>0, M is mapped according to f(M)=2M−1 and if M≦0, M is mapped according to f(M)=2M+1, a second comparator configured to receive the mapped f(M) values and compare the mapped f(M) values with a positive triangle carrier waveform TC1 wherein the second comparator outputs a signal VAOP that is 1 when f(M)>TC1(t) and 0 when f(M) is not greater than TC1(t), a third comparator configured to receive the mapped f(M) values and compare the mapped f(M) values with a negative triangle carrier waveform TC2 wherein the third comparator outputs a signal VAON that is 1 when f(M)>TC2(t) and 0 when f(M) is not greater than TC2(t), a positive square pulse generator with a frequency fs configured to output a signal SQP based on the positive triangle carrier TC1 period Ts wherein if
the signal SQP is 1 and if
SQP is 0, a negative square pulse generator with a frequency fs configured to output a signal SQN based on the negative triangle carrier TC2 period Ts wherein if
the signal SQN is 0 and if
the signal SQN is 1, a second frequency divider configured to receive the signal SQP, divide the signal SQP by 2 and output a signal SQPO2 that has a frequency
wherein if 0<t<Ts, the signal SQPO2 is 1 and if Ts<t<2·Ts, the signal SQPO2 is 0, a third frequency divider configured to receive the signal SQN, divide the signal SQN by 2 and output a signal SQNO2 that has a frequency
wherein if 0<t<Ts, the signal SQNO2 is 0 and if Ts<t<2·Ts, the signal SQNO2 is 1, a fourth frequency divider configured to receive the signal SQPO2, divide the signal SQPO2 by 2 and output a signal SQPO4 that has a frequency
wherein if 0<t<2 T, the signal SQPO4 is 1 and if 2·Ts<t<4·Ts, the signal SQPO4 is 0, a fifth frequency divider configured to receive the signal SQNO2, divide the signal SQNO2 by 2 and output a signal SQNO4 that has a frequency
wherein if 0<t<2·Ts, the signal SQNO4 is 0 and if 2·Ts<t<4·Ts, the signal SQNO4 is 1, a signal S1P generated from signals SQP, SQN, SQPO2, SQNO2, SQPO4, SQNO4 and VAOP defined as
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.
Embodiments of the invention will be described with reference to the accompanying drawing figures wherein like numbers represent like elements throughout. Before embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of the examples set forth in the following description or illustrated in the figures. The invention is capable of other embodiments and of being practiced or carried out in a variety of applications and in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having,” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items.
The terms “connected” and “coupled” are used broadly and encompass both direct and indirect connecting, and coupling. Further, “connected” and “coupled” are not restricted to physical or mechanical connections or couplings.
It should be noted that the invention is not limited to any particular software language described or that is implied in the figures. One of ordinary skill in the art will understand that a variety of software languages may be used for implementation of the invention. It should also be understood that some of the components and items are illustrated and described as if they were hardware elements, as is common practice within the art. However, one of ordinary skill in the art, and based on a reading of this detailed description, would understand that, in at least one embodiment, components may be implemented as software or hardware.
Embodiments of the invention provide 5-level, hybrid inverter topologies, switching methods, and computer-usable media storing computer-readable instructions for the switching methods. The switching methods may be deployed as software as an application program tangibly embodied on a program storage device. The application code for execution can reside on a plurality of different types of computer readable media known to those skilled in the art.
The single-phase, 5-level inverter topology 201 comprises a VDC source input across positive (+) and negative (−) nodes, a 3-level flying capacitor PWM switching leg 203, a half-bridge 2-level fundamental switching leg 205 and output nodes A and B.
The 3-level flying capacitor leg 203 comprises four unidirectional controlled switches S1, S2, S3, S4 coupled together in series and a flying capacitor C1. A positive node (+) is defined beginning at the first switch S1, a node C is defined between the first switch S1 and the second switch S2, a node A is defined between the second switch S2 and the third switch S3, a node D is defined between the third switch S3 and the fourth switch S4 and a negative node (−) is defined after the fourth switch S4. The flying capacitor C1 is coupled in parallel across the second S2 and third S3 switches to nodes C and D.
The half-bridge 2-level fundamental switching leg 205 is coupled in parallel with the 3-level flying capacitor leg 203 and comprises two unidirectional controlled switches S5, S6 coupled together in series. A positive node (+) is defined beginning at the fifth switch S5, a node B is defined between the fifth switch S5 and the sixth switch S6 and a negative node (−) is defined after the sixth switch S6.
A direct current voltage source VDC (+,−) is coupled to the positive (+) and negative (−) nodes across the 3-level flying capacitor leg 203 and the 2-level fundamental switching leg 205. The VDC voltage source has a value of 2V, while the flying capacitor C1 has an initial voltage charge of V. Before the inverter 201 produces a sinusoidal output voltage, the flying capacitor C1 charges to a voltage V. This takes place during an initialization phase.
The voltage produced between the nodes A and B is the output voltage. The voltage generated by the inverter 201 is a PWM waveform having the following 5 levels: +2V, +V, 0, −V, −2V. While generating these voltage levels, the inverter 201 also maintains the flying capacitor Cl voltage charged to an average value equal to the initial charging voltage V.
Typical unidirectional controlled switches comprise power semiconductors such as Insulated-Gate Bipolar Transistors (IGBTs) with an anti-parallel diode across their emitter-collector junctions. An IGBT is a three-terminal power semiconductor device having an isolated Field Effect Transistor (FET) for the control input (gate (1)) and a bipolar power transistor as a switch (collector (3)-emitter (2)). The power semiconductor devices can also be Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs), Integrated Gate-Commutated Thyristors (IGCTs), Gate Turn-Off Thyristors (GTOs), or other types. The IGBT emitter is equivalent to a MOSFET source, or an IGCT or GTO anode. The IGBT collector is equivalent to a MOSFET drain or an IGCT or GTO cathode. For this disclosure, the unidirectional controlled switches are treated as two terminal (collector-emitter) devices. For the unidirectional controlled switches S1, S2, S3, S4, S5, S6 the anode of the anti-parallel diode is coupled to the emitter.
There are four possible states for the 3-level flying capacitor leg 203 that can be generated with respect to a voltage between node A and a fictitious midpoint O of the VDC supply. If the VDC supply voltage is 2V and the flying capacitor C1 voltage is V, the voltage VAO can have three distinct values as shown in Table I.
In Table I, a value of 1 for switch Sx indicates switch Sx is conducting and a value of 0 indicates switch Sx is not conducting. States 2 and 3 produce the same voltage and have an opposite effect on the state of charge of the flying capacitor C1. Switches S1 and S4 are always switched in opposite—when one is conducting the other one is not. Similarly, switches S2 and S3 are always switched in opposite—when one is conducting the other one is not. In general notation SxSy indicates which switches Sx and Sy are conducting.
A positive current output at node A, state 2 (switches S2 and S4 conduct), will lead to a discharge of the flying capacitor C1 voltage, while state 3 (switches S1 and S3 conduct), will charge the flying capacitor C1. The effect of states 2 and 3 are reversed when a negative current is output at node A. Therefore, alternating between states 2 and 3 when a zero voltage needs to be synthesized can be used to perform capacitor voltage balancing. Because states 2 and 3 generate a zero voltage level, they are referred to as zero states. One prior art PWM principle used for a flying capacitor leg is the Phase-Shifted (PS) method which uses two triangle carriers with a 180 degree phase shift. However, while the PS method provides capacitor voltage balancing, it does not produce an optimum harmonic spectrum and generates transitions similar to a 2-level inverter when the reference sine wave amplitude used for modulation is above 0.5. To obviate this limitation, embodiments use PWM to generate PWM pulses using two triangular carriers that are in-phase with each other. In addition to maintaining the flying capacitor C1 voltage balancing, embodiments minimize the switching performed by S1, S2, S3 and S4 in order to increase inverter 201 efficiency. For an optimum harmonic spectrum, when the output generated by the inverter 201 is positive, the waveform must switch only between 0 and V and between V and 2V levels. The zero states are located at the beginning and end of each of the following switching sequences Seq.1 , Seq.2, Seq.3 and Seq.4
Seq.1=S1S3→S1S2→S1S3,
Seq.2=S1S3→S1S2→S2S4,
Seq.3=S2S4→S1S2→S2S4, and
Seq.4=S2S4→S1S2→S1S3.
Each switching sequence Seq.1, Seq.2, Seq.3 and Seq.4 occupies one cycle (period) of a triangle carrier. One choice would be to only use Seq.2 or Seq.4 as they use both S1S3 and S2S4 states during one switching period and therefore the voltage across the flying capacitor C1 could be easily balanced. However this would create an unwanted situation because if only Seq.4 was used, there would be a case where all four switches will change state at the same time: at the end of one switching period S1 and S3 would conduct and in the next switching period S2 and S4 would conduct. A similar case can be described if only Seq.2 is used. This will lead to increased switching losses as well as increased common-mode voltage being generated with an adverse affect due to increased electromagnetic interference.
To counteract this, for the case where a “mapped” modulating function (f(M)) is positive, embodiments apply the following optimum order of switching sequences to minimize switching—Seq.1, Seq.2, Seq.3, Seq.4.
However, flying capacitor C1 voltage balancing only takes place during Seq.2 and Seq.4, while during Seq.1 and Seq.3, depending on the current polarity, voltage will either increase or decrease across the flying capacitor C1. To counteract this affect, the order in which Seq.1 and Seq.3 are applied are changed on a fundamental frequency cycle basis. Whatever voltage unbalance results across the flying capacitor C1 during one fundamental frequency cycle as a result of applying Seq.1 and Seq.3, it is counteracted during the next fundamental frequency cycle by reversing the order of switching sequences—Seq.3, Seq.4, Seq.1, Seq.2.
During a “first” fundamental frequency ff cycle, if the mapped modulation function (f(M)) is positive—Seq.1, Seq.2, Seq.3, Seq.4 are used and during a “second” fundamental frequency ff cycle, if the mapped modulation function (f(M)) is positive—Seq.3, Seq.4, Seq.1, Seq.2 are used. This process is continuously repeated and it ensures that when the mapped modulation function f(M) is positive the switching operations for switches S1, S2, S3 and S4 are reduced, because during sequence Seq.1, switches S1 and S4 are not switching, while during sequence Seq.3, switches S2 and S3 are not switching.
For an optimum harmonic spectrum, when the output waveform generated by the inverter 201 is negative, the waveform must switch only between 0 and −V and between −V and −2V levels. The zero states are located in the middle of each of the following switching sequences Seq.5 and Seq.6
Seq.5=S3S4→S1S3→S3S4, and
Seq.6=S3S4→S2S4→S3 S4
Seq.5 and Seq.6 could be modified and split the middle zero state between S1S3 and S2S4 to ensure voltage balancing takes place on each switching period.
However, this would lead to switches S1, S2, S3 and S4 commutating at the same time, in the middle of the switching period with similar adverse affect as described above for the case where the inverter 201 generated waveform was positive.
To counteract this, for the case where the mapped modulation function (f(M)) is negative, embodiments apply the following optimum order of switching sequences to minimize switching—Seq.5, Seq.6.
But since the voltage across the flying capacitor C1 is not balanced during the switching period, a net voltage increase or decrease would appear during each fundamental frequency cycle. The same principle used for the case when the mapped modulation function (f(M)) is positive is applied here and the sequence reverses the order in which the redundant states S1S3 and S2S4 are applied every fundamental cycle. Whatever voltage unbalance results across the flying capacitor C1 during one fundamental frequency cycle as a result of applying Seq.5 and Seq.6 in this order, it is counteracted during the next fundamental frequency cycle by reversing the order of switching sequences—Seq.6, Seq.5.
During the “first” fundamental frequency ff cycle, if the mapped modulation function (f(M)) is negative—Seq.5, Seq.6 are used and during the “second” fundamental frequency ff cycle, if the mapped modulation function (f(M)) is negative—Seq.6, Seq.5 are used. This process is continuously repeated.
The single-phase, 5-level inverter topology 201 operates in a hybrid configuration since the 3-level flying capacitor leg 203 uses PWM to switch and the half-bridge 2-level leg 205 uses a fundamental frequency ff to switch.
A sine wave generator 403 generates a reference sine wave f(t) at a desired fundamental frequency ff, amplitude and phase as
f(y)=M=m sin(ωt+φ), (1)
where −1≦M≦1, m is the sine amplitude, ω=2πff, φ is the initial phase angle of the sine function and t is time. The fundamental frequency ff may be 50 or 60 Hz. For a single-phase inverter 201 application, the initial phase angle φ is 0. The amplitude m varies between 0 and 1 and corresponds to the minimum and maximum voltage that can be produced by the inverter 201. The VDC supply determines the maximum achievable output voltage.
The half-bridge 2-level leg 205 switches at the fundamental frequency ff. f(t) is input to a comparator 405 and compared with zero. If f(t)>0, switch S5 does not conduct (off) and switch S6 conducts (on). Switches S5 and S6 are mutually exclusive. If switch S5 is not off, switch S6 is off.
The output of comparator 405 is input to a frequency divider 435 that divides the compared reference sine wave f(t) frequency by two and generates a signal SQF.
A mapping function f(M) ensures that with the constraint of having one half-bridge 2-level leg 205 clamped to either the positive or the negative VDC (+,−) source, the Vac output waveform (between nodes A and B) will only have zero to positive (0 to V or V to 2V), or zero to negative (0 to −V or −V to −2V) transitions during the positive and negative fundamental frequency ff half-cycles. Without mapping the reference sine wave f(t), unwanted transitions would occur. For example, during the positive (negative) fundamental frequency ff half-cycle, there would be transitions between zero and negative (positive) levels. This would have a detrimental affect on the Vac output voltage waveform quality.
The reference sine wave f(t) is input to a mapping function 407. The mapping function 407 output f(M) is defined as
f(M)=2M−1, if M>0, or
f(M)=2M+1, if M≦0. (2)
The mapping function 407 maps the reference sine wave generator f(t) 403 output by multiplying it by 2 and adding or subtracting 1 based on the reference sine wave f(t) polarity.
The output of the mapping function 407 is input to a comparator 411 which compares f(M) with a positive triangle carrier TC1415. The output of the mapping function block 407 is also input to a comparator 413 which compares f(M) with a negative triangle carrier TC2417.
The positive triangle carrier wave TC1 and the negative triangle carrier wave TC2 are generated each having a period T
where fs is the frequency of triangle carriers TC1 and TC2.
The carriers TC1 and TC2 are used to synthesize the switch signal waveforms S1PULSE, S2PULSE, S3PULSE and S4PULSE that control the 3-level flying capacitor leg 203 switches S1, S2, S3 and S4. Carrier TC1 is a periodic positive triangle waveform and carrier TC2 is a periodic negative triangle waveform. The relationship between the triangle carriers TC1 and TC2 frequency fs and the fundamental frequency ff is
T
f
=T
S
·N, (4)
where N is an integer. N can be chosen based on the power level of the inverter and the cooling available for the semiconductors. Typically, in a low power range (1-3 kilowatts), the switching frequency could be very high (20-40 kHz, N equal to 400-800) because the IGBT switching losses are small. For high power (hundreds of kilowatts), IGBT switching losses are very high so the switching frequency is 1-3 kHz, which means N could be as low as 20-60. The choice of N relates to tolerable losses for a given application.
The positive triangle waveform generator 415 generates carrier TC1 defined as
and the negative triangle waveform generator 417 generates carrier TC2 defined as
The comparison 411 of carrier TC1(t) with f(M) outputs a signal VAOP that is 1 when f(M)>TC1(t) and 0 when f(M) is not greater than TC1(t). The comparison 413 of carrier TC2(t) with f(M) outputs a signal VAON that is 1 when f(M)>TC2(t) and 0 when f(M) is not greater than TC2(t).
A signal SQP with a frequency fs is generated 423 based on the triangle carrier period Ts
A signal SQN with a frequency fs is generated 425 based on the triangle carrier period Ts
The signal SQP is input to a frequency divider 427 that divides the signal SQP frequency by two. The output signal SQPO2 with a frequency
is defined as
SQPO2=1, when 0<t<TS, or
=0, when TS<t<2·TS. (9)
The signal SQN is input to a frequency divider 429 that divides the signal SQN frequency by two. The output signal SQNO2 with a frequency
is defined as
SQNO2=0, when 0<t<TS, or
=1, when TS<t<2·TS. (10)
The signal SQPO2 is input to a frequency divider 431 that divides the signal SQPO2 frequency by two. The output signal SQPO4 with a frequency
is defined as
SQPO4=1, when 0<t<2·TS, or
=0, when 2·TS<t<4·TS. (11)
The signal SQNO2 is input to a frequency divider 433 that divides the signal SQNO2 frequency by two. The output signal SQNO4 with a frequency
is defined as
SQNO4=0, when 0<t<2·TS, or
=1, when 2·TS<t<4·TS. (12)
Embodiments use the generated signals VAOP, SQP, SQPO2 and SQPO4, and VAON, SQN, SQNO2 and SQNO4 to further generate the switch signal waveforms S1PULSE, S2PULSE, S3PULSE and S4PULSE.
Signals S1P, S2P, S1N and S2N are generated using logic combinations of the previously generated signals
S1P=SQPO4(SQPO2+SQP+VAOP)+SQNO4[VAOPSQPO2+SQNO2(SQN+VAOP)], (13)
S2P=SQNO4(SQPO2+SQP+VAOP)+SQPO4[VAOPSQPO2+SQNO2(SQN+VAOP)], (14)
S1N=VAONSQPO2, and (15)
S2N=VAONSQNO2. (16)
Based on the polarity of the mapped function f(M), signals SX and SY are generated from signals S1P and S1N, and, S2P and S2N. The mapped function f(M) is input to a comparator 437 which outputs a logic 1 when f(M)÷0 and 0 when f(M)<0. A signal selector 439 chooses either signal S1P or signal S1N as signal SX depending on the mapped function f(M) polarity. Similarly, a signal selector 441 chooses either signal S2P or signal S2N as signal SY depending on the mapped function f(M) polarity.
SX=S1P, when f(M)>0, or
=S1N, when f(M)<0, and (17)
SY=S2P, when f(M)>0, or
=S2N, when f(M)<0. (18)
The switch signal waveform S1PULSE is generated from
S1PULSE=SX
The switch signal waveform S2PULSE is generated from
S2PULSE=SY
The switch signal waveform S3PULSE is generated from
S3PULSE=
The switch signal waveform S4PULSE is generated from
S4PULSE=S1PULSE (22)
For the three-phase inverter 301 shown in
The control for the three-phase inverter 301 is based on the control 401 for the single-phase inverter 201. Three control systems 401A, 401B, 401C (not shown) are used to control phases A, B and C. Each reference sine wave fA(t), fB(t), fC(t) is displaced by
(120 degrees). This is performed by selecting the initial phase angle φ of each reference sine wave generator 403A, 403B, 403C (not shown) as follows: for phase A, the reference sine wave fA(t) initial phase angle φ is 0, for phase B the reference sine wave fB(t) initial phase angle φ is
(120 degrees) and for phase C the reference sine wave fC(t) initial phase angle φ is
(240 degrees)
The reference sine waves fA(t), fB(t), fC(t) may include Common Mode Offset (CMO) which is added to produce more line-line voltage from a given direct current voltage source VDC. The addition of the CMO term has no adverse effect on the line-line voltage of a balanced three-phase system. CMO allows the inverter 301 to produce approximately 15.5% more voltage by increasing the reference sine wave amplitudes m to 1.155.
The three-phase inverter 301 uses 18 switches (phase A—S1A, S2A, S3A, S4A, S5A, S6A, phase B—S1B, S2B, S3B, S4B, S5B, S6B and phase C S1C, S2C, S3C, S4C, S5C, S6C) and three flying capacitors C1A, C1B, C1C. Of the 18 switches, six devices switch at the desired fundamental frequency ff. By comparison, a prior art 5-level diode-clamped inverter uses 24 switches, all PWM switching, and 12 diodes. A prior art flying capacitor topology requires 24 switching devices, all PWM switching, and 9 flying capacitors.
Embodiments reduce the cost of a three-phase 5-level inverter and produce an optimum output voltage spectrum with flying capacitor voltage balancing. The advantages of the three-phase inverter 301 are: 1) a three-phase 5-level topology can be obtained using only 12 PWM switching devices and 6 fundamental switching devices, 2) only 3 flying capacitors are used and voltage balancing is performed, 3) due to the fixed fundamental frequency ff (50/60 Hz) in grid and UPS applications, the flying capacitors C1A, C1B, C1C do not suffer from the limitations encountered in large motor drives operating at low fundamental frequencies, 4) for an LCL-type filter, an inverter 301 side inductor can be the transformer leakage inductance while the grid side inductance operates on the five-level voltage thereby reducing losses, 5) the inverter 301 provides a neutral connection that can be used for a three-phase four-wire system, and 6) the inverter 301 is modular and uses half-bridge modules.
The three-phase 5-level inverter 301 was simulated in Matlab/Simulink.
With RL loads placed on the outputs of each phase 303A, 303B, 303C,
One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.
This application claims the benefit of U.S. Provisional Application No. 61/447,168, filed on Feb. 28, 2011, the disclosure which is incorporated herein by reference in its entirety.
Number | Date | Country | |
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61447168 | Feb 2011 | US |