CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from the Chinese patent application 2020107226517 filed Jul. 24, 2020, the content of which is incorporated herein in the entirety by reference.

TECHNICAL FIELD

The present invention relates to the technical field of smart grid control, in particular to a photovoltaic power generation virtual inertia compensation system and method based on super capacitor energy storage (hereinafter referred to as SCES).

BACKGROUND OF THE PRESENT INVENTION

At present, with the increasing depletion of traditional fossil energy and environmental pollution, renewable power generation has received more and more attention. Compared with traditional energy sources, photovoltaic power generation systems have broad development and application prospects in the world's energy distribution pattern due to their many advantages. In order to maximize the use of solar resources, photovoltaic power generation systems usually adopt maximum power point tracking control strategies, which cannot respond to power grid frequency changes. Furthermore, photovoltaic power generation systems usually use power electronic inverters as the interface devices with the large power grid. Power electronic devices have the characteristics of fast response speed and high sensitivity, but they lack the inertia and damping characteristics of traditional synchronous generators. Consequently, as the penetration rate of photovoltaics constantly increases, the regulation capability of the power grid is seriously damaged, and the dynamic security of the power grid frequency constantly decreases.

As solutions for the inertia compensation link, the current main solution is to use virtual synchronous generator (VSG) control. The key point is to add a first-order inertia link in the active power control loop so that the grid-connected inverter can mimic the inertia characteristics of traditional synchronous generators. Its energy source is mainly realized by photovoltaic load shedding and arrangement of energy storage batteries on the DC side. The photovoltaic load shedding solution has limited support capacity, and may cause the failed maximum use of solar energy and reduce the economic benefit of the photovoltaic power generation system. The energy storage batteries arranged on the DC side have a slow dynamic response speed and cannot provide the system with inertia response quickly. Meanwhile, the virtual synchronous generator technology is also susceptible to system frequency noise and causing fluctuation of power.

SUMMARY OF THE PRESENT INVENTION

The purpose of the present invention is to overcome the shortcomings of insufficient inertia of traditional photovoltaic power generation systems and provide a photovoltaic power generation virtual inertia compensation system and method based on SCES. The present invention can provide inertia response and suppress fluctuation of frequency when the AC power grid is disturbed, thereby enhancing the regulation capability of the power grid and improving the stability of the power grid.

To solve the technical problems in the prior art, the following technical solution is employed in the present invention. A photovoltaic power generation virtual inertia compensation system based on SCES, comprising a photovoltaic array, a unidirectional DC-DC boost circuit and a grid-connected inverter, which are electrically connected in sequence, the grid-connected inverter outputting alternating current to the power grid, and further comprising a supercapacitor assembly for energy storage, a bidirectional DC-DC converter and a virtual inertia control module; a port on a side of the bidirectional DC-DC converter is electrically connected to an output end of the unidirectional DC-DC boost circuit and an input end of the power grid inverter, and a port on the other side thereof is electrically connected to the supercapacitor assembly; the bidirectional DC-DC converter comprises two power switch devices; the virtual inertia control module comprises a first inverting adder, a virtual inertia converter, a second inverting adder, a first PI controller, a third inverting adder, a second PI controller and a PWM modem, which are electrically connected in sequence; wherein instantaneous frequency and nominal frequency of the power grid are each input to positive and negative input ends of the first inverting adder; the difference between the instantaneous frequency of the power grid and the nominal frequency of the power grid is input to the virtual inertia converter, and the virtual inertia converter outputs a nominal voltage of the supercapacitor assembly; one input end of the positive and negative input ends of the second inverting adder is electrically connected to an output end of a nominal voltage calculator of the supercapacitor assembly, and an instantaneous voltage of the supercapacitor assembly is input to the other input end of the second inverting adder; one input end of the positive and negative input ends of the third inverting adder is electrically connected to an output end of the first PI controller, and an instantaneous current of the supercapacitor assembly is input to the other input end of the third inverting adder; the PWM modem outputs a pulse signal to control the on/off of the power switch device of the bidirectional DC-DC converter.

Further, the virtual inertia control module further comprises a power grid frequency acquisition unit configured to acquire the instantaneous frequency of the power grid, a voltage acquisition unit configured to acquire the instantaneous voltage of the supercapacitor assembly, and a current acquisition unit configured to acquire an instantaneous current of the supercapacitor assembly.

Further, the power grid frequency acquisition unit is provided with a phase-locked loop configured to detect the instantaneous frequency of the power grid.

Further, the bidirectional DC-DC converter comprises a first fully-controlled power switch device, a second fully-controlled power switch device and an inductor; an emitter of the first fully-controlled power switch device is connected to the positive output end of the unidirectional DC-DC boost circuit; a collector of the second fully-controlled power switch device is connected to the negative output end of the unidirectional DC-DC boost circuit; a collector of the first fully-controlled power switch device is connected to an emitter of the second fully-controlled power switch device and also connected to one end of the inductor, and the other end of the inductor is connected to a anode of the supercapacitor assembly; and the virtual inertia control module outputs pulse signals to correspondingly control the on-duty ratio of the first and second fully-controlled power switch devices.

Further, the power switch devices of the bidirectional DC-DC converter are IGBT, MOSFET or IGCT power switch devices.

Further, the supercapacitor assembly comprises a plurality of paralleled supercapacitor branches; and each supercapacitor branch comprises a plurality of monomer supercapacitors connected in series.

The present invention further provides a photovoltaic power generation virtual inertia compensation method using the photovoltaic power generation virtual inertia compensation system based on SCES. The method comprises the following steps:

step 1: setting a virtual inertia time constant;

step 2: acquiring an instantaneous frequency of a power grid, inputting the acquired instantaneous frequency to an input end of positive and negative input ends of a first inverting adder, and inputting a nominal frequency of the power grid to the other input end, to obtain a difference between the instantaneous frequency and the nominal frequency of the power grid;

step 3: inputting the difference between the instantaneous frequency and the nominal frequency of the power grid to a virtual inertia converter that converts the difference according to the following formula to obtain a nominal voltage of a supercapacitor assembly:

Udc*=Udc⁢⁢02+4⁢Hpv⁢SpvCsup⁢f0⁢(f-f0)

where Udc0represents the rated DC voltage of the supercapacitor assembly, Hpvrepresents the virtual inertia time constant, Spvrepresents the rated capacity of the photovoltaic array, Csuprepresents the capacitance of the supercapacitor assembly, f0represents the nominal frequency of the power grid, and f represents the instantaneous frequency of the power grid;

step 4: acquiring the instantaneous voltage of the supercapacitor assembly, inputting the instantaneous voltage of the supercapacitor assembly to an input end of the positive and negative input ends of the first inverting adder, and inputting the nominal voltage of the supercapacitor assembly to the other input end, to obtain a difference between the instantaneous voltage and the nominal voltage of the supercapacitor assembly;

step 5: inputting the difference between the instantaneous voltage and the nominal voltage of the supercapacitor assembly to the first PI controller that outputs a nominal current of the supercapacitor assembly;

step 6: acquiring the instantaneous current of the supercapacitor assembly, inputting the instantaneous current of the supercapacitor assembly to an input end of positive and negative input ends of the second inverting adder, and inputting the nominal current of the supercapacitor assembly to the other input end, to obtain a difference between the instantaneous current and the nominal current of the supercapacitor assembly; and

step 7: inputting the difference between the instantaneous current and the nominal current of the supercapacitor assembly to the second PI controller, a signal output of which is modulated by the PWM modem and then converted into a pulse signal used to control the on/off of the power switch devices of the bidirectional DC-DC converter.

Further, the virtual inertia time constant is set between 2.5 s to 10 s.

The present invention has following advantages and positive effects:

(1) As a kind of renewable energy, the present invention can achieve friendly gird-connection of photovoltaic power generation systems to improve the frequency regulation capability of the power grid, and enhances the security and stability of the power grid.

(2) As an energy storage device, supercapacitor has the advantages of fast charge and discharge rate, high power density, long cycle life, etc., and when the system frequency is restored to the nominal value, the DC voltage of the supercapacitor is also restored to the rated value, no additional charging equipment is required, and good buffering effect is provided.

(3) Due to the control capability of the bidirectional DC-DC converter, the terminal voltage of the supercapacitor bank and the DC-link voltage may be quite different, so the utilization rate of the supercapacitor can be improved and the installation cost of the supercapacitor can be reduced.

(4) The virtual inertia control module obtains the reference value of the DC voltage of the supercapacitor through the calculation of the instantaneous frequency of the power grid, and directly controls the voltage of the supercapacitor without introducing the derivative frequency with respect to time df/d, thereby avoids the influence of frequency noise on system stability.

