Static synchronous phase modifier based on hybrid supercapacitor

Abstract

The static synchronous phase modifier comprises a DC / AC converter and a controller, the direct current side of the DC / AC converter is connected with an energy storage unit composed of the hybrid supercapacitor, and the alternating current side of the DC / AC converter is provided with an LC filter circuit; the controller adopts a virtual synchronous machine control technology to control the DC / AC converter, and comprises a power calculation part, an active power control part and a reactive power control part; the power calculation part is used for calculating actual active power and actual reactive power according to the current of the filter inductor, the voltage of the filter capacitor and the line current; the active power control part performs active power control according to the actual active power and the reference active power; the reactive power control part performs reactive power control according to the actual reactive power and the reference reactive power. According to the invention, frequency and voltage support can be provided when a short-circuit fault occurs in the line, frequency collapse is prevented, the low-voltage ride-through success rate of the wind turbine generator is improved, and the grid-connected strength is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of electric energy storage, and in particular to a static synchronous phase condenser based on a hybrid supercapacitor. Background Art

[0002] Most of the current renewable energy power generation is connected to the grid through inverters. The inverter-type new energy power generation system does not have the damping and inertia of the synchronous generator. With the increase in the penetration rate of new energy, the rotating standby and rotational inertia in the power system are gradually reduced; the inverter-type new energy power generation system also does not have an excitation link, which leads to a further reduction in the voltage support capacity of the power grid, and then causes problems such as weak grid connection strength of new energy power generation, and is more likely to lose stability during power fluctuations and failures.

[0003] In this context, energy storage devices play an increasingly prominent role in power systems due to their flexible adjustment capabilities of bidirectional charging and discharging. They can provide peak shaving and valley filling in power grids and play an important role in new power systems. The energy storage media commonly used in energy storage systems are mainly energy storage batteries and supercapacitors. There are attempts to connect traditional synchronous phase regulators to low short-circuit ratio wind farms to improve their equivalent short-circuit ratio, improve operational stability, and enhance the grid-connected strength and power delivery capacity of wind farms. Some technical results have been achieved, but the economy and operation and maintenance are still unsatisfactory.

[0004] Energy storage batteries have the characteristic of low power density. If the long-term operation and maintenance is not in place, the energy storage battery will be depleted for a long time. In the event of an accident causing a large impact current, the voltage of the energy storage battery will drop to a state where it cannot operate in a very short time, causing the DC to collapse. Supercapacitors have higher power density than energy storage batteries, which enables supercapacitors to meet the needs of instantaneous impact loads, but the low energy density of supercapacitors cannot meet the regular load requirements of the power system. At the same time, when the supercapacitor releases energy and the charge is low, the terminal voltage will also drop rapidly, resulting in it being unable to meet the minimum voltage requirement on the DC side of the inverter.

[0005] Traditional synchronous condensers can only provide reactive power compensation and voltage support, but cannot perform active power regulation and primary frequency modulation. Moreover, the rotational inertia introduced by them is very limited, which is unable to improve the problem of lack of rotational inertia in new energy stations. In addition, traditional synchronous condensers have slow response speed, high energy consumption and high noise during operation, and as rotating equipment, they require high labor costs for maintenance and repair. Summary of the invention

[0006] The technical problem to be solved by the present invention is to provide a static synchronous phase regulator based on a hybrid supercapacitor, which can coordinate energy storage to ensure that the system can smooth frequency fluctuations, reduce the frequency change rate, and increase the lowest frequency point when wind and solar resources fluctuate and the system switches on and off loads; provide frequency and voltage support when a short circuit fault occurs in the line, prevent frequency collapse, increase the success rate of low voltage ride-through of wind turbines, and improve grid connection strength.

[0007] The technical solution adopted by the present invention to solve its technical problem is: to provide a static synchronous phase condenser based on a hybrid supercapacitor, including a DC / AC converter and a controller, wherein the DC side of the DC / AC converter is connected to an energy storage unit composed of a hybrid supercapacitor, and the AC side is provided with an LC filter circuit; the controller adopts a virtual synchronous machine control technology to control the DC / AC converter, including a power calculation part, an active power control part and a reactive power control part; the power calculation part is used to calculate the actual active power and the actual reactive power according to the current of the filter inductor, the voltage of the filter capacitor and the line current; the active power control part performs active power control according to the actual active power and the reference active power; the reactive power control part performs reactive power control according to the actual reactive power and the reference reactive power.

[0008] The energy storage unit includes a plurality of groups of hybrid supercapacitor array structures connected in series, each group of hybrid supercapacitor array structures includes M columns of hybrid supercapacitor arrays connected in parallel, and each column of hybrid supercapacitor arrays includes N hybrid supercapacitors connected in series.

[0009] The active power control part includes: an active-frequency droop control unit, which is used to calculate the intermediate active power according to the reference active power, and the rated frequency and output frequency of the DC / AC converter; and a rotor swing equation unit, which is used to obtain the phase angle according to the intermediate active power and the actual active power.

[0010] The transfer function of the active power-frequency droop control unit is expressed as: m =P ref -k p (ω-ω s ), where P m is the intermediate active power, P ref is the reference active power, k p is the first droop control coefficient, ω is the output frequency of the DC / AC converter, ω s is the rated frequency of the DC / AC converter.

[0011] The transfer function of the rotor swing equation unit is expressed as: Among them, P mis the intermediate active power, P is the actual active power, J is the virtual inertia, ω s is the rated frequency of the DC / AC converter, D is the virtual damping coefficient, and ω is the output frequency of the DC / AC converter.

[0012] The reactive power control part includes: a reactive-voltage droop control unit, which is used to obtain a stable voltage according to a given voltage amplitude, actual reactive power and reference reactive power; and a virtual impedance control unit, which is used to obtain a modulation voltage of the dq axis according to the stable voltage and the output current of the dq axis.

[0013] The transfer function of the reactive power-voltage droop control unit is expressed as: E=E ref -k q (QQ ref ), where E is the stable voltage, E ref is the given voltage amplitude, Q is the actual reactive power, Q ref is the reference reactive power, k q is the second droop control coefficient.

[0014] The transfer function of the virtual impedance control unit is expressed as: Among them, u d is the modulation voltage of the d-axis, u q is the modulation voltage of the q axis, E is the stable voltage, L v is the virtual inductance, i d and i q are the output currents of the d-axis and q-axis respectively, R v is the virtual resistance, ω s is the rated frequency of the DC / AC converter.

[0015] The technical solution adopted by the present invention to solve its technical problem is: to provide a wind power generation system, including a wind turbine set, a machine-side converter and a grid-side converter connected in sequence, and the grid-side converter is connected to the grid connection point of the AC power grid with the above-mentioned static synchronous phase regulator based on the hybrid supercapacitor.

[0016] Beneficial Effects

[0017] Due to the adoption of the above technical solution, the present invention has the following advantages and positive effects compared with the prior art: The static synchronous condenser of the present invention adopts virtual synchronous machine (VSG) control, and there is an active power-frequency droop link in the control link. Therefore, in addition to the ability to achieve voltage support, the present invention can also emit and absorb active power and perform a frequency modulation. At the same time, VSG control also introduces virtual inertia and damping in the active power-frequency control link, which can provide inertia compensation for it after connecting to the new energy power plant, suppress frequency fluctuations, and improve its operating stability and grid-connected strength. Compared with the synchronous condenser, the static synchronous condenser of the present invention has a lower failure rate as a static device, and has obvious advantages in equipment operation, maintenance and repair, which can significantly reduce the work pressure and difficulty of power grid-related departments and save labor costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is a system structure diagram of a static synchronous condenser based on a hybrid supercapacitor in accordance with a first embodiment of the present invention;

[0019] Figure 2 is a structural diagram of an energy storage unit in a first embodiment of the present invention;

[0020] Figure 3 is a charge and discharge voltage curve diagram of a hybrid supercapacitor in the first embodiment of the present invention;

[0021] Figure 4 is a block diagram of a VSG control strategy in a first embodiment of the present invention;

[0022] Figure 5 Schematic diagram of a wind power generation system according to a second embodiment of the present invention. DETAILED DESCRIPTION

[0023] The present invention will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention. In addition, it should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms fall within the scope limited by the appended claims of the application equally.

[0024] The first embodiment of the present invention relates to a static synchronous condenser based on a hybrid supercapacitor, such as Figure 1As shown, it includes a DC / AC converter and a controller, wherein the DC side of the DC / AC converter is connected to an energy storage unit composed of a hybrid supercapacitor, and the AC side is provided with an LC filter circuit; the controller uses a virtual synchronous machine control technology to control the DC / AC converter, including a power calculation part, an active power control part and a reactive power control part; the power calculation part is used to calculate the actual active power and the actual reactive power according to the current of the filter inductor, the voltage of the filter capacitor and the line current; the active power control part performs active power control according to the actual active power and the reference active power; the reactive power control part performs reactive power control according to the actual reactive power and the reference reactive power.

[0025] like Figure 2 As shown, the energy storage unit in this embodiment has ten groups of hybrid supercapacitor array structures connected in series, each group of hybrid supercapacitor array structures includes four columns of hybrid supercapacitor arrays connected in parallel, and each column of hybrid supercapacitor array includes twenty hybrid supercapacitors connected in series. Assuming that the rated voltage of a single hybrid supercapacitor is 4.2V and the capacitance is 20571F, the port voltage of the energy storage unit in this embodiment is 840V and the total capacitance is 410F.

[0026] The electrode material of the hybrid supercapacitor is a battery electrode on one side and a double-layer electrode on the other side. It combines the advantages of supercapacitors and batteries, and has excellent characteristics of high power density and high energy density. It also has a more optimized energy-to-power ratio, higher charge and discharge rate, efficiency, longer service life, and a wider temperature range. The above advantages make it very suitable for power system energy storage occasions.

[0027] The charge capacity and voltage of double-layer supercapacitors maintain a linear relationship as a whole. When the charge is fully released, the voltage drops to zero. The charge and discharge voltage curve of hybrid supercapacitors is different from that of traditional double-layer supercapacitors. Figure 3 As shown, during the charge and discharge process, the voltage variation is smaller and the trend is smoother. When fully discharged, the voltage can be maintained at more than 60% of the rated voltage, which is closer to the characteristics of energy storage batteries.

[0028] This means that the hybrid supercapacitor only needs one stage of DC / AC conversion as an energy storage medium, so it can be directly connected to the DC side of the inverter, and does not require the DC / DC voltage regulation link of traditional supercapacitor energy storage to maintain the voltage level on the DC side of the converter, such as Figure 1 As shown, this not only saves the system construction cost, but also avoids the complexity of the control system, reduces the system failure rate and improves reliability.

[0029] Virtual synchronous machine control is one of the classic network control methods. Figure 1 In, L f and C f They are the inductor and capacitor in the LC filter circuit, R f is the parasitic resistance of the filter inductor, Z line is the line impedance, i labc and u oabc are the current of the filter inductor and the voltage of the filter capacitor, i oabc is the line current, P and P ref are the actual active power and the reference active power, Q and Q ref is the actual reactive power and the reference reactive power, u odq 、i odq and i ldq u oabc 、i oabc and i labc The value obtained after dq transformation, e d and e q are the d-axis and q-axis reference values ​​of the given voltage, E * is a given voltage amplitude, θ ref is the reference value of the phase.

[0030] like Figure 4 As shown, the active power control part in this embodiment includes: an active-frequency droop control unit, which is used to calculate the intermediate active power according to the reference active power, and the rated frequency and output frequency of the DC / AC converter; and a rotor swing equation unit, which is used to obtain the phase angle according to the intermediate active power and the actual active power.

[0031] Among them, the transfer function of the active power-frequency droop control unit is expressed as: P m =P ref -k p (ω-ω s ), where P m is the intermediate active power, P ref is the reference active power, k p is the first droop control coefficient, ω is the output frequency of the DC / AC converter, ω s is the rated frequency of the DC / AC converter.

[0032] The transfer function of the rotor swing equation unit is expressed as: Among them, P m is the intermediate active power, P is the actual active power, J is the virtual inertia, ω s is the rated frequency of the DC / AC converter, D is the virtual damping coefficient, and ω is the output frequency of the DC / AC converter.

[0033] like Figure 4As shown, the reactive power control part in this embodiment includes: a reactive-voltage droop control unit, which is used to obtain a stable voltage according to a given voltage amplitude, actual reactive power and reference reactive power, thereby achieving voltage stability of the system; a virtual impedance control unit, which is used to obtain a modulation voltage of the dq axis according to the stable voltage and the output current of the dq axis.

[0034] The transfer function of the reactive power-voltage droop control unit is expressed as: E = E ref -k q (QQ ref ), where E is the stable voltage, E ref is the given voltage amplitude, Q is the actual reactive power, Q ref is the reference reactive power, k q is the second droop control coefficient.

[0035] The principle of the virtual impedance control unit is to control the simulated stator impedance of the virtual synchronous machine, and to obtain the modulation wave voltage u of the dq axis by subtracting the voltage drop on the virtual impedance from the extracted stable voltage. d and u q , its transfer function is expressed as: Among them, u d is the modulation voltage of the d-axis, u q is the modulation voltage of the q axis, E is the stable voltage, L v is the virtual inductance, i d and i q are the output currents of the d-axis and q-axis respectively, R v is the virtual resistance, ω s is the rated frequency of the DC / AC converter.

[0036] It is not difficult to find that the static synchronous condenser of this embodiment adopts virtual synchronous machine control, and there is an active power-frequency droop link in the control link. Therefore, in addition to the ability to achieve voltage support, it can also emit and absorb active power and perform primary frequency modulation. At the same time, VSG control also introduces virtual inertia and damping in the active power-frequency control link, which can provide inertia compensation for it after connecting to the new energy power plant, suppress frequency fluctuations, and improve its operating stability and grid-connected strength. In contrast, traditional synchronous condensers can only provide reactive power compensation and achieve voltage support capabilities, and cannot perform active power regulation and primary frequency modulation. Therefore, the static synchronous condenser of this embodiment has more comprehensive functions, and the above functions are very consistent with the stable operation requirements of new energy power plants, and have high engineering application potential.

[0037] Compared with traditional synchronous phase condensers which are rotating equipment, the static synchronous phase condensers of this embodiment have lower failure rate as static equipment and have obvious advantages in equipment operation and maintenance. In addition, its simple topology that only requires one level of power conversion increases operational reliability and further alleviates operation and maintenance requirements. It can significantly reduce the pressure and difficulty of equipment operation and maintenance for power grid-related work departments, and greatly save manpower and material costs.

[0038] It is worth mentioning that, in addition to the hybrid supercapacitor mentioned above, the energy storage unit of the static synchronous phase regulator proposed in this embodiment can also be replaced by traditional double-layer supercapacitor lead-acid batteries, lithium batteries, etc. However, it should be noted that the voltage charge and discharge curve of traditional supercapacitors is generally linear. When the discharge depth is large, the voltage will be greatly reduced, so that the minimum voltage requirement of the inverter DC power supply cannot be met. Therefore, when using traditional supercapacitors as energy storage units, a DC / DC Boost DC conversion circuit needs to be installed to maintain the DC voltage. When the battery is directly connected to the inverter, there will be a problem of large charging and discharging current ripple, which affects the normal operation of the energy storage system and the cycle life of the battery. Therefore, it is generally considered to install a filter capacitor between the inverter and the battery to reduce the current ripple.

[0039] A second embodiment of the present invention relates to a wind power generation system, such as Figure 5 As shown, it includes a wind turbine set, a machine-side converter and a grid-side converter connected in sequence, and the grid-side converter is connected to the grid-connected point of the AC power grid with a static synchronous phase condenser based on a hybrid supercapacitor of the first embodiment.

[0040] The Matlab / simulink simulation of the application of the hybrid supercapacitor-based static synchronous condenser in a wind farm proposed in this embodiment is as follows:

[0041] Two 1.5MW doubly-fed wind turbines are connected to the synchronous power supply via long lines to create a weak connection, low short-circuit ratio, and low stability wind power operation environment. Interference factors such as load changes, wind speed changes, and ground short-circuit faults are added to this system, and the frequency, voltage, and speed of the wind turbines of the power grid are compared when the 1MW static synchronous condenser system is connected or not, to explore the role of static synchronous condensers in wind farms.

[0042] The simulation effect of the static synchronous condenser is as follows:

[0043] 1) When the system has load switching, wind speed changes, or short circuit faults, the static synchronous condenser of this embodiment can provide active power support through primary frequency modulation. In addition, the effects of virtual inertia and damping can effectively smooth out system frequency fluctuations, reduce the frequency change rate, and increase the lowest frequency point. In extreme cases, it can avoid frequency collapse of the wind power system.

[0044] 2) When a short circuit occurs in the system, reactive power can be generated in time to support the voltage at the outlet of the wind farm, helping the wind turbine to successfully complete low voltage riding and prevent the wind turbine from being disconnected from the grid due to low voltage.

Claims

1. A static synchronous condenser based on a hybrid supercapacitor, characterized in that: It comprises a DC / AC converter and a controller, wherein the DC side of the DC / AC converter is connected to an energy storage unit composed of a hybrid supercapacitor, and the AC side is provided with an LC filter circuit; the controller uses a virtual synchronous machine control technology to control the DC / AC converter, and comprises a power calculation part, an active power control part and a reactive power control part; the power calculation part is used to calculate the actual active power and the actual reactive power according to the current of the filter inductor, the voltage of the filter capacitor and the line current; The active power control part performs active power control according to the actual active power and the reference active power; the reactive power control part performs reactive power control according to the actual reactive power and the reference reactive power.

2. The static synchronous condenser based on a hybrid supercapacitor according to claim 1, characterized in that: The energy storage unit includes several groups of hybrid supercapacitor array structures connected in series, each group of hybrid supercapacitor array structures includes M columns of hybrid supercapacitor arrays connected in parallel, and each column of hybrid supercapacitor array includes N hybrid supercapacitors connected in series.

3. The static synchronous condenser based on a hybrid supercapacitor according to claim 1, characterized in that: The active power control part includes: an active-frequency droop control unit, which is used to calculate the intermediate active power according to the reference active power, and the rated frequency and output frequency of the DC / AC converter; and a rotor swing equation unit, which is used to obtain the phase angle according to the intermediate active power and the actual active power.

4. The static synchronous condenser based on a hybrid supercapacitor according to claim 3, characterized in that: The transfer function of the active power-frequency droop control unit is expressed as: m =P ref -k p (ω-ω s ), where P m is the intermediate active power, P ref is the reference active power, k p is the first droop control coefficient, ω is the output frequency of the DC / AC converter, ω s is the rated frequency of the DC / AC converter.

5. The static synchronous condenser based on a hybrid supercapacitor according to claim 3, characterized in that: The transfer function of the rotor swing equation unit is expressed as: Among them, P m is the intermediate active power, P is the actual active power, J is the virtual inertia, ω s is the rated frequency of the DC / AC converter, D is the virtual damping coefficient, and ω is the output frequency of the DC / AC converter.

6. The static synchronous condenser based on a hybrid supercapacitor according to claim 1, characterized in that: The reactive power control part includes: a reactive-voltage droop control unit, which is used to obtain a stable voltage according to a given voltage amplitude, actual reactive power and reference reactive power; and a virtual impedance control unit, which is used to obtain a modulation voltage of the dq axis according to the stable voltage and the output current of the dq axis.

7. The static synchronous condenser based on a hybrid supercapacitor according to claim 6, characterized in that: The transfer function of the reactive power-voltage droop control unit is expressed as: E=E ref -k q (QQ ref ), where E is the stable voltage, E ref is the given voltage amplitude, Q is the actual reactive power, Q ref is the reference reactive power, k q is the second droop control coefficient.

8. The static synchronous condenser based on a hybrid supercapacitor according to claim 6, characterized in that: The transfer function of the virtual impedance control unit is expressed as: Among them, u d is the modulation voltage of the d-axis, u q is the modulation voltage of the q axis, E is the stable voltage, L v is the virtual inductance, i d and i q are the output currents of the d-axis and q-axis respectively, R v is the virtual resistance, ω s is the rated frequency of the DC / AC converter.

9. A wind power generation system, comprising a wind turbine, a machine-side converter and a grid-side converter connected in sequence, characterized in that: The grid-side converter and the AC grid connection point are connected to a static synchronous phase condenser based on a hybrid supercapacitor as described in any one of claims 1 to 8.

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