EP3286826A1

EP3286826A1 - Dc/dc/ac converter system

Info

Publication number: EP3286826A1
Application number: EP16732784.0A
Authority: EP
Inventors: Marcin LIS
Original assignee: Spirvent Sp Z O O
Current assignee: SPIRVENT SP. Z O. O.
Priority date: 2015-04-24
Filing date: 2016-04-21
Publication date: 2018-02-28
Also published as: PL412104A1; PL229067B1; WO2016171575A1

Abstract

The object of the invention is a DC/DC/AC converter system converting energy from renewable energy sources (1), in particular from photovoltaic panels, comprising an input filter (2) connected with a voltage converter (3) that is in turn connected with the charge storage system (4), and then the system is connected with a transistor bridge (5) followed by a low pass filter (6), an output filter (7) and to the grid connection (8), characterised in that the low pass filter (6) is a magnetically coupled inductor.

Description

DC/DC/AC Converter System

The object of the invention is a DC/DC/AC converter system, converting the energy from Renewable Energy Sources (RES), in particular photovoltaic panels that indicate parasite capacities, to a single-phase or three-phase power grid.

By means of Photovoltaic Panels (PP) solar energy can be converted into electric energy. In this aspect, PP can be treated as the source of uncontrolled DC voltage with limited power. The PV energy can be used for one's own needs in the direct form, without processing (e.g. directly to a heater), or it can be stored by charging batteries, a separated network system can be created, or it can be delivered to the power grid. PP is characterised by a large surface area, which in some cases can lead to creation of capacity between the photovoltaic panels and the earth. The capacity, in the range of 150-750 pF for 3 kW power, does not seem to be large. However, if the phenomena taking place in electric power converting apparatus are taken into account, the existence of such capacity results in closing the flow path for currents of certain frequencies, thus a possibility of electric shock to a person or property damage (which can even lead to unintentional causing of fire) . For the powers higher than 3 kW, the problem still exists, and more sophisticated means of protection against these negative events are required. The block diagram of the classic DC/DC/AC single-phase converter system known in the state of art, converting energy from Renewable Energy Sources (RES) is presented in fig. 1. The existence of leakage capacity causes creation of at least two paths that the so called residual current (leakage current; ii_ckg ) can flow along. The reason for the occurrence of this current is the interference on the DC bus. In an ideal case, when the DClink voltage is constant, no current flows (the PP capacity is treated as a break in the circuit) . However, when the voltage on DClink oscillates, the capacity is the reactance, i.e. it allows the flow of alternating current. Interference resulting from a number of factors occurs in such system. Firstly, ripple of about 100 Hz caused by cooperation with a single-phase grid (300 Hz for three-phase grids) occurs on DClink. Due to transistors switching in DC/DC and DC/AC blocks, high frequency interference is generated. While the high frequency components (higher than 50 kHz) will be eliminated by the EMC (ElectroMagnetic Compatibility) filter, the problem of components resulting directly from switching and from grid ripple still remains.

Currently, there are two trends in solving this issue: with and without a transformer. A (low or high frequency) transformer enables breaking the leakage current paths in the converter. It is however burdened with high loss and relatively low efficiency (of 90-94% magnitude) . Currently, no additional efficiency drop on voltage/current converting is welcome. Solutions without transformers are preferred nowadays since they are characterised with relatively high efficiency (96-98%) and lower cost as compared to transformers. These solutions can be divided into two groups: active and passive ones . European Patent documents EP1626494B1 and EP2290797B1 describe active ways of neutralising leakage current in case of transmitting the energy to a single-phase grid, implemented by a "H5 topology" system. Generally, the presented ways to reduce the leakage current consist in disconnecting the DC bus on transistor switching, which causes reduction of oscillation introduced on the DC bus. In each case, there is a low-pass filter present in the form of inductance LI and L2, which enables controlling the current transferred to the grid. This topology obviously requires the application of an EMC filter as well. Furthermore, using active components is associated with the risk of damaging them, e.g. due to aging. Apart from that, the cost of control (additional PWM signal) and the control system (additional controller with a voltage source for isolated MOSFET transistor) increases.

In three-phase systems, on the other hand, bridges consisting of 12 or more transistors are used. Such topologies (e.g. NPC1, NPC2, i.e. Neutral Point Clamping) require more complex control and cause the reduction of leakage current by reducing oscillation on the DC bus.

Alternatively, systems for passive elimination of leakage current are used. They are systems based on LC and LCL filters. As compared to the active ways of reducing leakage current, they require a larger number of passive elements as well as an additional EMC filter. Controlling the current in such system is more demanding since combination of two variables (e.g. current and output voltage) is required. Besides, overvoltage caused by resonance may occur on LC and LCL filter elements. The resonance can take place for specific conditions occurring in the grid (e.g. cooperation with a thyristor converter, engines, and other unpredictable loads) .

Use of RES allows generating energy for one's own needs. Inverters available on the market do not have the function to limit the produced power, and therefore they cannot be used for managing smart power grids of Smart Grid type. On the market, there are only devices which can "cut off" the output energy with a constant interval of 15-30 seconds by switching on additional heaters or other loads.

It is also worth mentioning that large industrial plants generate large guantities of reactive induction power because of the eguipment they use (motors to a large extent) . In such cases a change of the power coefficient takes place wherein the reactive energy received from the grid is more expensive than the active energy. These plants usually invest in equipment that restore the power coefficient to the appropriate level. It is usually implemented by installing batteries of capacitors.

The technical challenge faced by the present invention is providing such DC/DC/AC converter system that would provide better elimination of the leakage current in the system, will be characterised by a limited number of components, free from active elements, which will have a positive influence upon the economic result, will demonstrate preferred interference characteristics constituting at the same time a durable, reliable and stable solution. Furthermore, it is desired to provide a system that will dynamically react, preferably in less than 1 second, to the changes of the power in the system, and will allow to limit the power of the converter itself, protecting against introduction of too much energy into the grid. Additionally, it is desired that the DC/DC/AC converter system minimize the reactive power consumed by the recipient, positively influencing the economics of the solution. Unexpectedly, the technical problems listed above have been solved by the present invention. The object of the invention is a DC/DC/AC converter system converting energy from renewable energy sources, in particular photovoltaic panels, comprising an input filter connected with a voltage converter that is in turn connected with the charge storage system, and then the system is connected with a transistor bridge followed by a low pass filter, an output filter and to the grid connection, characterised in that the low pass filter is a magnetically coupled inductor. In a preferred embodiment of the invention, bidirectional converter is attached parallel to the voltage converter, to charge the charge storage system of other voltage than the voltage on the DC bus. In another preferred embodiment of the invention, the bidirectional converter is a system of two high-frequency transformers or one high-frequency transformer with an adequate number of transistors on both the primary and secondary side. Preferably, the system also includes a control system functionally coupled with an input filter, a voltage converter, a bidirectional converter, a charge storage system, a transistor bridge and an output filter. The functional coupling of the control system with other components of the converter system means collection of measurement data from the meters included in the output and input filter blocks, controlling the operation of the voltage converter, bidirectional converter, and the transistor bridge. More preferably, the control system is a DSP processor, a processor with an ARM core, a FPGA system with adequate peripherals. In a preferred embodiment of the invention, the input filter and/or the output filter is an EMC filter. In another preferred embodiment of the invention, the voltage converter is a boost type, soft switching boost or interleaved boost converter. In another preferred embodiment of the invention, the charge storage system is an electric component selected from a group including: a capacitor, a supercapacitor , a rechargeable battery, a flow battery, a resonance system or the charge storage system is a system of superconductors storing magnetic energy. Preferably, the transistor bridge is a bridge selected from topologies including: H-bridge, Half- bridge, 3-phase bridge, multilevel matrix converter, multilevel delink converter, NPC1, NPC2, FC (flying capacitor), CI (coupled inductor) . Equally preferably, the grid connection is a single-phase alternating voltage of 230 V and 50 Hz frequency grid connection or three-phase grid connection with alternating phase-to-phase voltage of 400 V and 50 Hz. Introducing modification on the coupled coil from a single-phase to a three-phase one, it is possible to use the system in a three-phase grid with alternating voltage of 400 V phase-to-phase and the frequency of 50 Hz for a higher rated powers of the apparatus. In another preferred embodiment of the invention, external communication devices are connected to the control system, providing collection of data from the external system and transmitting them to the system in order to limit the power and to control the reactive power. In another preferred embodiment of the invention, the connection of the external communication device with the control system is executed as either wired or wireless . The DC/DC/AC converter system according to the present invention converts the energy from the renewable energy sources, in particular in the form of photovoltaic panels, taking into account the leakage capacity and resistance through which the leakage current can flow that negatively influences the effectiveness of transmitting the collected power into a single-phase or a three-phase grid connection. The leakage current has been significantly reduced in this system, increasing the current effectiveness of the whole photovoltaic system. Furthermore, the implementation of magnetically coupled inductor not only enabled elimination of leakage current, but also resulted in the smaller number of DC/DC/AC converter components, which in turn is a solution with a higher durability and reliability, and lower total cost in comparison with the solutions known in the art. Furthermore, the application of the magnetically coupled inductor did not introduce any additional interference into the system, and placing the system in an enclosure reduced the negative influence of the magnetic field generated by the coil upon emission interference. Additionally, the application of an external communication system connected to the control system provided dynamic variation of the system power implemented in a time shorter than 1 second by limiting the power of the DC/DC/AC converter system itself according to the present invention. Furthermore, due to the possibility to indicate the reactive power load on individual phases in real time, the present DC/DC/AC converter system allows generating reactive power with an opposite sign, and a significant reduction of its consumption (assuming there is energy in the photovoltaic panels) .

Exemplary embodiments of the present invention have been presented in the drawing, where fig. 1 is a simplified model of a DC/DC/AC converter known in the state of the art, fig. 2 is a block diagram of a first embodiment of the DC/DC/AC converter system according to the present invention, fig. 3 is a block diagram of a second embodiment of the DC/DC/AC converter system according to the present invention, fig. 4 illustrates the noise waveform in the system from fig. 2, fig. 5 illustrates the leakage current waveform in the system according to the state of the art for a first case of photovoltaic panels, fig. 6 illustrates the leakage current curve in the system according to the state of the art for a second case of photovoltaic panels, fig. 7 illustrates the leakage current curve in the system according to the first embodiment of the present invention for a first case of photovoltaic panels, while fig. 8 illustrates the leakage current curve in the system according to the first embodiment of the present invention for a second case of photovoltaic panels .

Example 1

A block diagram of the DC/DC/AC converter system according to the first embodiment of the present invention is illustrated in fig. 2, where consecutive numeral references indicate:

1 - Renewable Energy Source, e.g. photovoltaic panels connected serially-parallel, with uncontrolled DC voltage in 25 - 900 V range, 2 - Input EMC filter,

3 - Boost type converter (one or more channels), regulating the voltage from OZE 1 to DC-Link 4 on a constant level,

4 - DC-Link, large capacitor / supercapacitor/charge storage rechargeable battery, 5 - Transistor bridge, e.g. in the form of a H-Bridge or a NPC1,

6 - Low pass filter implemented by means of a magnetically coupled inductor,

7 - Output EMC filter, 8 - Single-phase 230 VAC 50 Hz grid connection or three-phase, 400 VAC 50 Hz grid connection,

9 - Simplified model of photovoltaic panels capacity,

10 - DSP processor or an FPGA system with peripherals, monitoring the operation of the converter (carrying out measurements, MPPT module, PLL module, control, communication, data archiving) . Renewable Energy Source 1 generates uncontrolled DC voltage. The voltage is converted by converter 3 and stabilised on DC- Link 4. Utilising the properties of the low pass filter 6, transistor bridge 5 generates voltage impulses forcing the flow of current. Filter 6 allows to shape the current by means of adequate control algorithms. The current generated this way is transferred to single-phase grid connection 8. The whole operation of the DC/DC/AC converter is supervised by DSP processor or FPGA system 10 that collects the current /voltage measurements, performs computation of maximum operating point and this way controls converter 3 and bridge 5 for the energy transferred to grid 8 from RES 1 be the maximum and meet all the required standards imposed by power engineering. EMC input filter 2 and EMC output filter 7 filter high frequency wire interference in order to meet the required EMC standards. The capacity of photovoltaic panels 9 requires particular attention. Photovoltaic panels 9 occupy large area. This area causes occurrence of capacity relative to the earth, which, in transformer-less solutions, creates new path for high- frequency current that is unfavourable and creates hazards for personnel working in the vicinity of the panels and the converter. Capacity is estimated to be 150 - 750 pF for single-phase installations with power of up to 3 kW, and is strongly correlated with the atmospheric conditions. The capacity of photovoltaic panels 9 can be represented by means of serial connection of capacity and resistance, e.g. Ci_eak = 330 pF for the first case of photovoltaic panels characterised by an arrangement of 10-12 serially connected mono- and polycrystalline silicon panels with the power of 200-250 W and voltage of 25-35 V each, Ci_eak = 660 pF, Ri_eak = 1 Ω, for the second case of photovoltaic panels characterised by a an arrangement of 20-24 serially connected mono- or polycrystalline silicon panels with the power of 200-250 W and voltage of 25-35 V each.

It should be noted that low-pass filter 6 in the form of a magnetically coupled coil of high leakage inductance (e.g. in the 1 do 5 mH range) performs two functions. The first one is the introduction of inductance as the low-pass filters to shape the current transferred to grid connection 8, as described above. It results from the leakage inductance phenomenon that is normally treated as a parasitic phenomenon. The second function consists in using the magnetic coupling for elimination of currents that want to flow through the capacity of photovoltaic panels 9. Since the leakage current always flows through only one wiring of the low-pass filter 6, magnetic coupling causes introduction of significant inductance into its path, forming a low-pass filter of very early cut-off frequency, eliminating it effectively. This solution works irrespective of the method to control transistor bridge 5 (unipolar, bipolar, single-phase or three- phase) . In the DC/DC/AC converter system according to the invention in the single-phase version, measurement were carried out for system noise and leakage current curves for two cases of photovoltaic panels that are characterised, respectively, by leakage capacity Ci_eak of 330 pF and 600 pF, and leakage resistance of Ri_eak of 1 Ω for both cases. The waveforms were generated by observing the current flowing in the branch from Ciea_k and iea_k by means of current probe A622 (measurement on one coil unless stated otherwise) and registered on Tektronix TBS 1102B oscilloscope. The obtained results were compared with analogical measurements in a system known in the state of the art, illustrated in fig. 1. Fig. 4 illustrates the noise characteristics of non-operating system in order to visualise the amount of interference in the system itself, concerning the background. Noise characteristics will allow to compare the leakage current waveform in the system known from the state of the art with the system according to the invention. Comparing the leakage current waveform from fig. 5 (system known from the state of the art) and fig. 7 (system according to the first embodiment of the present invention) , obtained for the first case of a system of photovoltaic panels characterised by leakage capacity Ci_eak = 330 pF, serially connected, with the leakage resistance Ri_eak = 1 Ω, a visible improvement of leakage current elimination can be seen while using the DC/DC/AC converter according to the first embodiment of the present invention. In the system known from the state of the art, the peak-to-peak amplitude of the leakage current reaches ca . 800 mA, wherein in the system according to the first embodiment of the present invention, a reduction of this waveform to the system noise value was observed. Analogically, comparing the leakage current waveform from fig. 6 (system known from the state of the art) and fig. 8 (system according to the first embodiment of the present invention) , obtained for the second case of a system of photovoltaic panels characterised by leakage capacity Ci_eak = 660 pF, serially connected, with the leakage resistance Ri_eak = 1 Ω, a visible improvement of leakage current elimination can be seen while using the DC/DC/AC converter according to the first embodiment of the present invention, which confirm the advantageous operation of the system. Similarly, in the system known from the state of the art, the peak-to-peak amplitude of the leakage current reaches the value in excess of 1500 mA, while in the system according to the first embodiment of the present invention, the leakage current was again reduced to the value below the noise value, so that only the background waveform is visible.

Thanks to the elimination of active elements, the DC/DC/AC converter constitutes a solution of higher durability and reliability, and total lower cost than the solutions known from the state of the art providing elimination of leakage current at a similar level. What's more, after placing the converter in an enclosure, the magnetic field generated by the coil does not escape, thus the negative influence upon generating emission interference has been limited.

Example 2

A block diagram of the DC/DC/AC converter system according to the second embodiment of the present invention is illustrated in fig. 3, where numerical references corresponding to the references used in the block diagram used in the first example of embodiment designate substantially the same devices /system, whereas the additionally applied modules are:

11 - Bidirectional converter that enables connecting of rechargeable batteries or other energy storage of the voltage different than that of the DC bus .

12 - External communication device being responsible for collecting data that are not aggregated by DC/DC/AC converter and communicating and transmitting them do the DC/DC/AC converte . The construction and the operating principle of the DC/DC/AC converter according to the second embodiment of the present invention is convergent with the converter system presented in the first embodiment example. In order to provide additional functionality of the DC/DC/AC converter system, an additional bidirectional converter 11 has been applied, plugged parallel into the converter 3 and functionally connected with control system 10. Preferably, bidirectional converter 11 is a system of two high-frequency transformers or one high-frequency transformer with an adequate number of transistors on both the primary and secondary side. The systems can be implemented by Flyback, Push-pull, Forward, Pull-bridge type converters. Controlling the transistors can take place in an open loop without feedback (signals switching the transistors on are constant in time) bye control system 10 or another control device generating signal with duty cycle of about 50%. In this approach, the control algorithms implemented in control system 10 must be modified. Controlling the transistors can take place in a closed loop, with feedback in the form of voltage and current measurements, control system 10 will generate control signals depending on those measurements, and the algorithms implemented in control system 10 need not to be modified. The primary and secondary wiring must be selected in such a way that, for the preset rated conditions, controlling will enable matching the voltages, mainly do the DC-Link bus. Furthermore bidirectional converter 11 must be equipped with snubbers and have voltage parameters for transistors and diodes so selected that they are not exposed to damage during operation .

Additionally, external communication device 12 has been connected to control system 10, that enables communication between DSP processor 10 and the external world, and enables: limiting the power transferred to the grid (meeting the condition for some installations where energy transmission to the grid is not allowed) , and controlling both capacitive and inductive reactive power. In particular, external communication device 12 comprises a wireless meter and a communication module. The wireless meter consists of a processor based measurement system with DSP functions with Rogowski coils connected thereto. The measurement system measures voltages and currents in a three-phase system, and based on those data, carries out analyses of, among others, active, reactive, and apparent powers, voltage and current harmonics, and phase shift. These data are sent wireless to the communications module. The communications module consists mainly of a processor with an ARM core, or an FPGA system. It has many communication channels and enables sending/receiving data from the Internet and propagating them within the converter system elements . By receiving data from a wireless meter it is possible to control individual DC/DC/AC converter presets by transmitting them to control system 10. Controlling the reactive power and the active power (energy transferred to the grid) takes place by inputting the presets originating from the external communication device 12 to individual algorithms implemented in the control system 10. Owing to such structure, the functionality of the DC/DC/AC converter is provided as in Example 1, enriching its operation by the possibility to adjust the power transferred to the grid and reactive power consumption, which is particularly advantageous in case of large production plants using RES.

It is worth noting that a certain generalisation has been introduced in the block diagrams presented in figs. 1 and 2. Since these system can be single-phase (solid line between blocks 4-8) or three-phase ones (solid line + dot dashed line) . It does not influence, however, the generality of the above teaching.

Claims

1. A DC/DC/AC converter system converting energy from renewable energy sources (1), in particular from photovoltaic panels, comprising an input filter (2) connected with a voltage converter (3) that is in turn connected with the charge storage system (4), and then the system is connected with a transistor bridge (5) followed by a low pass filter

(6) , an output filter (7) and to the grid connection (8), characterised in that the low pass filter (6) is a magnetically coupled inductor.

2. The system of claim 1, characte ised in that a bidirectional converter (11) is attached parallel to the voltage converter (3), to charge the charge storage system (4) of the voltage different from the voltage on the DC bus.

3. The system of claim 2, characterised in that the bidirectional converter (11) is a system of two high-frequency transformers or one high-frequency transformer with an adequate number of transistors on both the primary and secondary side.

4. The system of any of claims 1 to 3, characterised in that it comprises an additional control system (10) functionally connected with the input filter (2), the voltage converter (3), the bidirectional converter (11), the charge storage system (4), the transistor bridge (5) and the output filter

(7) .

5. The system of claim 4, characterised in that the control system (10) has been selected from a group including a DSP processor, ARM core processor, FPGA system with adequate peripherals .

6. The system of any of claims 1 to 5, characterised in that the input filter (2) and/or the output filter (7) is an EMC filter.

7. The system of any of claims 1 to 6, characterised in that the voltage converter (3) is a boost, soft switching boost, or interleaved boost type converter.

8. The system of any of claims 1 to 7, characterised in that the charge storage system (4) is an electric component selected from a group including: a capacitor, a supercapacitor, a rechargeable battery, a flow battery, a resonance system or the charge storage system (4) is a system of superconductors storing magnetic energy.

9. The system of any of claims 1 to 8, characterised in that the transistor bridge (5) is a bridge selected from topologies including: H-bridge, Half-bridge, 3-phase bridge, multilevel matrix converter, multilevel delink converter, NPC1, NPC2, FC, CI .

10. The system of any of claims 1 to 9, characterised in that the grid connection (8) is a single-phase alternating voltage of 230 V and 50 Hz frequency grid connection or three-phase grid connection with alternating phase-to-phase voltage of 400 V and frequency of 50 Hz.

11. The system of any of claims 3 to 10, characterised in that the external communication device (10) is connected to the control system (10), providing collection of data from the external system and transmitting them to the system in order to limit the power and to control the reactive power.

12. The system of claim 11, characterised in that the connection of the external communication device (12) with the control system (10) is implemented in either wired or wireless way .