NL2009220C2

NL2009220C2 - Electrical power converter.

Info

Publication number: NL2009220C2
Application number: NL2009220A
Authority: NL
Inventors: Jan Abraham Ferreira
Original assignee: Univ Delft Tech
Priority date: 2012-07-23
Filing date: 2012-07-23
Publication date: 2014-01-27

Description

Electrical power converter

The invention relates to an electrical power converter for converting electrical power of a power source connected or 5 connectable at an input to electrical power with a desired amplitude and frequency at an output, wherein between the input and the output a first circuit of submodules is provided, wherein said first circuit of submodules and the power source form a primary power loop and wherein each submodule comprises 10 an energy storage component and each submodule is connected to a controller to drive the submodules in order to arrange that the electrical power at the output is shaped in accordance with preselected values for its amplitude and frequency.

US 7,269,037 discloses a power supply comprising a di-15 rect converter, wherein each phase branch includes a number of identical two-pole networks connected in series, the direct converter being in the form of a bridge circuit whose bridge arms are the phase branches, wherein the bridge circuit is at least one of a two-phase and a three-phase circuit, and wherein 20 the two-pole networks each have: at least one switching state wherein the terminal voltage assumes a positive value and at least one switching stage wherein the terminal voltage assumes a negative value, both voltages irrespective of the magnitude and polarity of the terminal current, and at least one internal 25 energy store.

An electrical power converter in accordance with the preamble is proposed in the article "Circulating Current Control in Modular Multilevel Converters with Fundamental Switching Frequency", by Kalle lives et al, 2012 IEEE seventh Inter-30 national Power Electronics and Motion Control Conference - ECCE Asia, June 2-5 2012, Harbin, China. This article discusses the conversion of DC power to AC power and applies a phase leg with two arms, each arm consisting of a number of series connected halve-bridges, termed submodules, that are equipped with DC ca-35 pacitors acting as energy storage components. The submodules are controlled to keep the voltage across each capacitor close to its nominal value. These capacitor voltages act during operation of the power converter as voltage sources that can be inserted and bypassed in the chain of series connected submod- 2 ules. In this way, an alternating voltage is generated at the output of the power converter by varying the number of submodules that are inserted in each arm. The voltages across the submodule capacitors are varying in time, as the capacitors are 5 charged and discharged due to the arm currents. With high voltages and currents the capacitors need to be large which limits the application possibilities of the power converter of this type. Also the charge and discharge currents of the capacitors restrict the power capabilities of the known power converter.

10 The invention aims to alleviate the problems and re strictions of the prior art power converter.

To this end the power converter of the invention is embodied with the features of one or more of the appended claims .

15 In a first aspect of the invention the first circuit of submodules is provided with a parallel second electrical circuit so as to arrange that the first, circuit and the parallel second circuit form a secondary power loop to enable the flow of local currents between the energy storage components of 20 the submodules. This measure arranges for the possibility to apply smaller capacitors whilst maintaining the power capacity of the power converter.

Beneficially in accordance with the invention the local currents from the secondary power loop flowing between the 25 energy storage components of the submodules are to charge and/or discharge said energy storage components.

Advantageously the secondary power loop operates at a different frequency than the primary power loop. This enables that the respective electrical circuits of the power converter 30 can be tuned to different handling of the voltages and currents in the primary power loop and in the secondary power loop. Preferably to that end the frequency of the voltages and currents in the secondary power loop is at least three times larger than the frequency of the voltages and currents in the pri-35 mary power loop.

Preferably at the converter's output a blocking circuit is provided in order to prevent power of the secondary power flowing to the output.

A benefit of the converter according to the invention 40 is that the average power supplied by a capacitor in a submod- 3 ule to the output can be different from zero. Although the sum of the power in the primary and secondary loop is zero, the measure that the secondary power is blocked or diverted from the output makes it possible to supply an average power differ-5 ent from zero to the output. This means that the power converter of the invention can also be applied to provide DC power at its output. At its input the power converter of the invention may employ either DC or AC power.

There are several possible embodiments to arrange for 10 the parallel second circuit. In one embodiment the parallel second circuit comprises a capacitor and an inductance in series that are connected at the converter's input and having a resonant frequency tuned to the operating frequency of the secondary power loop. Already with this very simple measure it is 15 possible to implement a second circuit for the purpose of distributing the charging and discharging currents amongst the energy storage components within the secondary power loop, whereby the operation of the primary power loop is unaffected by the operation of the secondary power loop. Preferably then the 20 blocking circuit comprises a capacitor and an inductance in parallel having a resonant frequency tuned to the operating frequency of the secondary power loop in order to prevent power of the secondary power flowing to the output.

In another more preferred embodiment the parallel se-25 cond circuit comprises a further circuit of submodules. This provides a more complex construction of the power converter but is advantageous since the submodules of the first circuit and the parallel second circuit can each be implemented with the same capacity. In comparison with a conventional power convert-30 er of the same capacity, the capacity of the submodules can be halved. This embodiment is further a preferred circuit for constructing a dc-dc converter. The function of blocking the secondary power loop frequency is then fulfilled by a centrally tapped inductor.

35 The beauty of the invention when applied to a three phase power converter that comprises for each phase a separate first circuit of submodules, is that each such first circuit of submodules can act as a parallel second circuit of submodules for at least one of the two neighboring phases. In this way the 40 invention can be implemented with a three phase power converter 4 with comparatively few components. When the submodules form part of a parallel second circuits of one of the three phases, the respective submodules operate preferably at different frequencies to allow independent control of all three phases.

5 The invention will hereinafter be further elucidated with reference to the drawing.

In the drawing: -figure 1 shows a circuit diagram of a conventional power converter; 10 -figure 2 shows a circuit diagram of a first embodi ment of a power converter according to the invention; -figure 3 shows a circuit diagram of a second embodiment of a power converter according to the invention; -figure 4 shows a part of the circuit diagram detail-15 ing the controller that drives a submodule of a power converter according to the invention; and -figure 5 shows the circuit diagram of a three phase power converter according to the invention.

Whenever in the figures the same reference numerals 20 are applied, these numerals refer to the same parts.

Figure 1 shows that a voltage source is connected through an input inductance Lin with a series of submodules SMi, wherein i= 1,...N and N denotes the number of applied submodules. Said submodules are part of a (primary) electrical loop and are 25 distributed between a top section Mtop and a bottom section Mbot normally having the same number of submodules, although this is not a requirement of the invention. The top section Mt0p and the bottom section Mbot connect through an output inductance Lout to the load that is connected to the power converter. The input 30 inductance Lin and the output inductance Lout filter the currents to the load and from the voltage source. Figure 1 shows that each submodule SMi is provided with a capacitor for use as an energy storage component.

Referring to figure 2 the simplest embodiment of the 35 power converter of the invention is shown in which -in addition to the circuit diagram shown in figure 1- a parallel second circuit is applied that forms a secondary power loop for the submodules. This secondary power loop operates at a different frequency than the primary power loop formed by the submodules 40 with the load. Preferably the frequency of the voltages and 5 currents in the secondary power loop is at least three times larger than the frequency of the voltages and currents in the primary power loop in order to properly separate the power flows in the primary current loop and in the secondary current 5 loop. The secondary power loop comprises a capacitor and an inductance in series (indicated with LCseries) that are connected at the converter's input and that operate at a resonant frequency to close the secondary power loop. Further this parallel second circuit includes an inductor Lsec for filtering the cur-10 rents in the secondary power loop. The remainder of the circuit diagram shown in figure 2 is the same as the circuit diagram of figure 1, with one further exception being that preferably at the converter's output a blocking circuit indicated with LCparai-iei is applied consisting of a capacitor and an inductance in 15 parallel, which blocking circuit is tuned to the frequency of the secondary power loop in order to block power of the secondary power loop from flowing to the load.

In figure 3 a preferred embodiment of the power converter of the invention is shown in which parallel to the first 20 circuit of submodules (compare to figure 1), a further circuit of submodules is applied. The two circuits of submodules together form the secondary power loop which is used as a loop for the local currents between the energy storage components of the submodules and to charge and/or discharge said energy stor-25 age components. In comparison with the capacity of the submodules of the conventional power converter shown in figure 1, the submodules used in the power converter of the invention as shown in figure 3 can be half the size. Figure 3 further shows that in this embodiment the output is connected to the two cir-30 cuits of submodules by means of a centrally tapped inductor

Lout· The centrally tapped inductor Lout has a large mutual inductance which in practice will be as large as possible. The leakage inductance Lout is dimensioned to fulfill the filtering function, similar to what is shown in figure 2.

35 Each of the submodules requires to be driven by a con troller in order to implement the currents in the primary power loop and the secondary power loop. This is schematically shown in figure 4 depicting one single submodule. This submodule is driven by a pulse width modulation generator PWM which derives 40 its modulation for the submodule from two separate controllers 6 for the primary power loop and the secondary power loop, respectively. The requirements of the primary power loop are reflected by a voltage controller vprim controller operating at a primary frequency for the primary power loop that provides a 5 setpoint vprim. The requirements of the secondary power loop handling the power exchange between the top and bottom submodules and operating at a second frequency, are reflected by a power loop controller indicated with Psec controller. The Psec controller provides a setpoint vsec resulting eventually into a 10 current isec that is superposed to a current iprim resulting from the .operation of the primary power loop. As figure 4 clearly shows the setpoints of the two controllers combined provide the setpoint that drives the pulse width modulator generator PWM for the individual submodule in relation to the primary power 15 loop (Vprim) and the secondary power loop (vsec) , resulting in the required synthesized output voltage and output current of this module.

Finally reference is made to figure 5 in which the circuit diagram is shown of a three phase power converter ac-20 cording to the invention. The figure shows that for each phase a separate first circuit of submodules is present, and that each such first circuit of submodules acts as a parallel second circuit a, b, c of submodules for at least one of the two neighboring phases. Independent control of all three phases can 25 be achieved by selecting different frequencies when these circuits of submodules operate as a parallel second circuit of a neighboring circuit of submodules.

It is explicitly pointed out that the foregoing description is not limiting as to the appended claims, but merely 30 serves to elucidate these claims and to remove any possible ambiguity of these claims. The scope of protection that merits the invention is however exclusively determined by the appended claims, without being limited to what is explained in the foregoing with reference to the drawing.

Claims

An electrical voltage converter for converting electrical power from a voltage source connected or connectable to an input to electrical power with a desired amplitude and frequency at an output, wherein a first circuit of sub-modules is provided between the input and the output, wherein said first circuit of sub-modules and the voltage source form a first power loop and wherein each sub-module has an energy storage component and each sub-module is connected to a controller for controlling the sub-modules to cause the electrical power at the output to be formed in according to preselected values for the amplitude and frequency, characterized in that the first circuit of sub-modules is provided with a parallel second electrical circuit in order to cause the first circuit and the parallel second circuit to form a secondary power loop for the enable that local currents flow between the energy storage components of the sub-modules.

Electric power inverter according to claim 1, characterized in that the local currents of the secondary power loop that flow between the energy storage components of the sub-modules serve for charging and / or discharging said energy storage components.

3. Electric power converter as claimed in claim 1 or 2, characterized in that the secondary power loop operates on a frequency that deviates from the frequency of the primary power loop.

4. Electric power converter according to claim 3, characterized in that the frequency of the voltages and currents in the secondary power loop is at least three times greater than the frequency of the voltages and currents in the primary power loop.

5. Electric power converter as claimed in any of the claims 1-4, characterized in that a blocking circuit is provided at the output of the inverter.

6. An electric power converter according to any one of claims 1-5, characterized in that the parallel second circuit comprises a capacitor and a coil in series which are connected to the input of the inverter and have a resonance frequency which is tuned to the operating frequency of the secondary power loop.

7. Electric power converter according to claim 6, characterized in that a blocking circuit is provided at the output of the converter with a capacitor and a coil 10 which are connected in parallel and have a resonance frequency which is tuned to the operating frequency of the secondary power loop.

8. Electric power converter as claimed in any of the claims 1-7, characterized in that it has a coil which connects one or more of the sub-modules to the output.

9. An electrical power converter as claimed in any one of claims 1-8, characterized in that the parallel second circuit comprises a further circuit of sub-modules.

10. Electric power converter according to claims 8 and 9, characterized in that the coil connecting one or more of the sub-modules to the output is centrally branched and is connected from this central branch to the output.

11. Electric power converter as claimed in any of the claims 1-5 or 8-10, characterized in that the sub-modules of the first circuit and the parallel second circuit each have the same capacity.

12. Electric power converter as claimed in any of the claims 1-5 or 8-11, characterized in that it is a three-phase power converter which has a separate first circuit of sub-modules for each phase, in which each such first circuit of sub-modules operates as parallel second circuit of submodules for at least one of the two neighboring phases.