An off-grid overload control system and control method for an energy storage inverter

Abstract

The present invention provides an off-grid overload control system and control method for an energy storage inverter, which are applied to the technical field of inverter off-grid control. The system includes an energy storage inverter, and a current compensation module is connected to the energy storage inverter. The energy storage inverter and the current compensation module are connected in parallel to a load. The current compensation module is used to share the load current for the energy storage inverter when the power grid is powered off. The current compensation module includes a super capacitor, a bidirectional DC-DC buck-boost unit, and a bidirectional DC / AC unit. An intermediate capacitor is connected between the bidirectional DC-DC buck-boost unit and the bidirectional DC / AC unit, and the super capacitor is connected to the bidirectional DC-DC buck-boost unit. When the power grid is powered off, the super capacitor boosts the voltage through the bidirectional DC-DC buck-boost unit to provide current compensation for the energy storage inverter. Fast current compensation is performed for the energy storage inverter through the super capacitor, and the stability of the compensation current is improved through effective energy management.

Description

Technical Field

[0001] The present invention belongs to the technical field of off-grid control of inverters, and particularly relates to an off-grid overload control system and control method for an energy storage inverter. Background Art

[0002] An inverter is a converter that converts DC electrical energy into AC electrical energy with a fixed frequency and voltage or a variable frequency and voltage. Off-grid control of an inverter refers to a control method in which the inverter operates independently and provides power to local loads without grid connection. An off-grid inverter usually obtains DC power from a local energy storage device and converts it into AC power to supply local loads. This control method ensures that stable power supply can be continuously provided to the loads when the grid is unavailable or unreliable.

[0003] According to the latest national standard GB / T34120-2023, at the rated voltage, the AC port current of the energy storage converter should continuously operate for no less than 10 minutes at 110% of the rated current; and should continuously operate for no less than 1 minute at 120% of the rated current. Therefore, most energy storage inverter manufacturers design with an overload capacity of 1.2 times or 1.3 times. However, the motor load will report overcurrent at the moment of starting, resulting in continuous start and stop of the energy storage inverter.

[0004] Such as Figure 1 In the existing three-phase energy storage inverter shown, when a grid fault occurs, the STS switch disconnects, and the battery supplies power to the load alone. After the grid returns to normal, the STS switch conducts.

[0005] During off-grid operation, since the off-grid output is at the rated voltage and rated frequency, the off-grid output current is determined by the load. When the energy storage inverter detects that the off-grid output current exceeds 1.3 times the rated current of the energy storage inverter, an overcurrent fault is reported. A common practice is to limit the current wave by wave. When the overcurrent point is triggered, the inverter wave is blocked, and the IGBT does not work. As a result, there is no output voltage from the energy storage inverter at the current time period. However, when the current is large, more time is required to limit the current wave by wave, or it may cause distortion of the inverter output sine wave, and in severe cases, the machine may be damaged. Summary of the Invention

[0006] In view of the above problems in the prior art, the purpose of the present invention is to provide an off-grid overload control method for an energy storage inverter, which performs fast current compensation for the energy storage inverter through a super capacitor and improves the stability of the compensation current through effective energy management.

[0007] An off-grid overload control method for an energy storage inverter, including an energy storage inverter connected to the power grid, a current compensation module is connected to the energy storage inverter, the energy storage inverter and the current compensation module are connected in parallel to the load, and the current compensation module is used to share the load current for the energy storage inverter when the power grid is powered off; the current compensation module includes a super capacitor, a bidirectional DC-DC buck-boost unit, and a bidirectional DC / AC unit, an intermediate capacitor is connected between the bidirectional DC-DC buck-boost unit and the bidirectional DC / AC unit, and the super capacitor is connected to the bidirectional DC-DC buck-boost unit.

[0008] Preferably, when the power grid is powered off, the bidirectional DC-DC buck-boost unit is in the boost mode, the bidirectional DC / AC unit is in the current source mode, the super capacitor is boosted by the bidirectional DC-DC buck-boost unit, the intermediate capacitor is used for voltage stabilization, and the bidirectional DC / AC unit converts the direct current provided by the super capacitor into alternating current to provide current compensation for the energy storage inverter.

[0009] Preferably, when the bidirectional DC / AC unit provides current compensation, the voltage phase of the bidirectional DC / AC unit is set to be the same as the voltage phase of the energy storage inverter.

[0010] Preferably, when the power grid is powered on, the bidirectional DC / AC unit is in the rectification mode, and the bidirectional DC-DC buck-boost unit is in the buck mode.

[0011] Another object of the present invention is to provide an off-grid overload control method for an energy storage inverter, including the following steps:

[0012] When the power grid is powered off, the energy storage inverter operates in the voltage source mode, the energy storage inverter outputs the rated voltage and the rated frequency. At this time, the bidirectional DC-DC buck-boost unit is in the boost mode, the bidirectional DC / AC unit operates in the current source mode, the super capacitor is boosted by the bidirectional DC-DC buck-boost unit, the voltage is stabilized through the intermediate capacitor, and the bidirectional DC / AC unit converts the direct current provided by the super capacitor into alternating current to provide current compensation for the energy storage inverter;

[0013] After the operation time reaches the set time, it is judged whether the current load current is greater than the threshold value. If it is greater, it means overload, and an overcurrent fault signal is output; if it is not greater, the current loop set value of the bidirectional DC / AC unit is set to 0;

[0014] After the power grid is restored, the energy storage inverter is synchronized with the power grid voltage. After the synchronization is completed, the power grid is powered on. At this time, the energy storage inverter operates in the current source mode and is used for charging and discharging the battery; the bidirectional DC / AC unit is in the rectification mode, and the bidirectional DC-DC buck-boost unit is in the buck mode for charging the super capacitor.

[0015] Preferably, when the bidirectional DC / AC unit operates in the current source mode, the current value given by the current loop of the bidirectional DC / AC unit is Set Value 1, and when the bidirectional DC / AC unit is in the rectification mode, the current value given by the current loop of the bidirectional DC / AC unit is Set Value 2.

[0016] Preferably, Set Value 1 is half of the rated output current of the energy storage inverter, and the range of Set Value 2 is 1 - 2A.

[0017] Preferably, when determining whether the current load current is greater than the threshold, the threshold is set to 1.3 times the rated current of the energy storage inverter.

[0018] Preferably, the bidirectional DC-DC buck-boost unit is modulated by a sine pulse width signal 1, and specifically includes the following steps:

[0019] Calculate the voltage of the intermediate capacitor and the reference voltage to obtain the voltage error of the intermediate capacitor therebetween;

[0020] Input the voltage error signal of the intermediate capacitor into a voltage PI regulator to obtain the current reference value of the intermediate capacitor ;

[0021] Calculate the current of the intermediate capacitor at present and the reference current to obtain the current error of the intermediate capacitor therebetween;

[0022] Input the current error signal of the intermediate capacitor into a current PI regulator 1, and the signal output by the current PI regulator 1 is transmitted to an SPWM module 1 for pulse width modulation, and the signal output by the SPWM module 1 is used to control the bidirectional DC-DC buck-boost unit.

[0023] Preferably, the bidirectional DC / AC unit is modulated by a sine pulse width signal 2, and specifically includes the following steps:

[0024] Limit the current given by the current loop through a current limit and then calculate the current error of the super capacitor between the limited current and the current of the super capacitor at present ;

[0025] Input the current error signal of the super capacitor into a current PI regulator 2 for proportional integral regulation, and the signal output by the current PI regulator 2 is transmitted to an SPWM module 2 for pulse width modulation, and the signal output by the SPWM module 2 is used to control the bidirectional DC / AC unit.

[0026] The beneficial effects of the present invention are as follows: The off-grid overload control system and control method of the energy storage inverter can provide instantaneous current support during load startup by introducing a supercapacitor, achieving rapid current compensation, avoiding frequent start-stop of the energy storage inverter caused by excessive startup current, and improving the stability and reliability of the system. In addition, through the charge and discharge management of the supercapacitor during power grid power failure and restoration, the energy utilization efficiency is improved, and the energy management strategy of the system is optimized.

[0027] Secondly, dynamic current compensation is achieved through the supercapacitor, and whether there is an overload phenomenon is judged in a timely manner, which can effectively reduce the failure rate of the energy storage inverter under overload conditions and extend the service life of the equipment.

[0028] By effectively connecting the supercapacitor, bidirectional DC-DC buck-boost unit, intermediate capacitor, and bidirectional DC / AC unit, an efficient and stable energy management system is constructed. The supercapacitor provides rapid energy response, the bidirectional DC-DC module realizes voltage conversion and bidirectional energy transmission, the intermediate capacitor smooths voltage fluctuations, and the bidirectional DC / AC module converts direct current into alternating current to provide high-quality compensation current for the load or the power grid.

[0029] When the power grid loses power, it can quickly switch to the off-grid mode, and when the power grid is restored, it can quickly synchronously switch back to the grid-connected mode, ensuring continuous power supply, which is particularly suitable for application scenarios with high requirements for power continuity. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The drawings are used to provide a further understanding of the present invention, and constitute a part of the specification. Together with the embodiments of the present invention, they are used to explain the present invention, and do not constitute a limitation to the present invention. In the drawings:

[0031] Figure 1 is a system block diagram of the prior art related to the present invention;

[0032] Figure 2 is a system block diagram of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0033] Embodiment 1

[0034] As Figure 2 shown, an off-grid overload control system of an energy storage inverter includes an energy storage inverter connected to the power grid. A current compensation module is connected to the energy storage inverter. The energy storage inverter and the current compensation module are connected in parallel to access a load. Among them, the current compensation module is used to share the load current for the energy storage inverter when the power grid loses power.

[0035] Specifically, the current compensation module includes a super capacitor, a bidirectional DC-DC buck-boost unit, and a bidirectional DC / AC unit. An intermediate capacitor is connected between the bidirectional DC-DC buck-boost unit and the bidirectional DC / AC unit, and the super capacitor is connected to the bidirectional DC-DC buck-boost unit.

[0036] When the power grid is powered off, the bidirectional DC-DC buck-boost unit is in the boost mode, the bidirectional DC / AC unit is in the current source mode. The super capacitor is boosted by the bidirectional DC-DC buck-boost unit, the intermediate capacitor is used for voltage stabilization, and the bidirectional DC / AC unit converts the direct current provided by the super capacitor into alternating current to provide current compensation for the energy storage inverter. As Figure 2 shown, the load current is equal to the sum of the output current of the energy storage inverter and the output current of the current compensation module, that is , by providing current compensation for the energy storage inverter through the current compensation module, it is possible to prevent the energy storage inverter from frequently starting and stopping due to excessive starting current of the load, which is beneficial to extending the service life of the energy storage inverter.

[0037] When the power grid is powered on, the bidirectional DC / AC unit is in the rectification mode, and the bidirectional DC-DC buck-boost unit is in the buck mode. The rectification mode of the bidirectional DC / AC unit is used for energy recovery and storage, and can convert the excess energy in the system into direct current and store it in the super capacitor for subsequent use.

[0038] Among them, the super capacitor has a high power density and fast charge and discharge capabilities, and is used to provide fast energy response to achieve current compensation when the power grid is powered off; the bidirectional DC-DC buck-boost unit is used to achieve voltage conversion and bidirectional energy transmission; the bidirectional DC / AC unit converts direct current into alternating current to provide a high-quality power supply for the load.

[0039] Setting an intermediate capacitor between the bidirectional DC-DC buck-boost unit and the bidirectional DC / AC unit can achieve energy storage and buffering. When the load power changes instantaneously, the intermediate capacitor can achieve a faster dynamic response, quickly provide or absorb electric energy, reduce the fluctuations of current and voltage in the system, and improve the stability of the system.

[0040] The intermediate capacitor can also filter out high-frequency harmonics and noise generated by DC-DC conversion and DC / AC conversion, which helps to provide a smooth DC voltage, provides a cleaner input power supply for the bidirectional DC / AC unit, and thus improves the overall performance of the system.

[0041] In addition, the intermediate capacitor is set between the bidirectional DC-DC buck-boost unit and the bidirectional DC / AC unit, which can better support the bidirectional flow of energy. Whether it is supplying power to the load or performing energy feedback through the super capacitor, the intermediate capacitor can play a key role in regulation and stabilization.

[0042] It should be noted that when the bidirectional DC / AC unit provides current compensation, the voltage phase of the bidirectional DC / AC unit is set to be consistent with the voltage phase of the energy storage inverter. In a specific embodiment, the phase-locked loop technology can be used to detect and lock the voltage phase.

[0043] Ensuring that the voltage phases of the bidirectional DC / AC unit and the energy storage inverter are consistent can avoid the impact and loss caused by the phase difference. If the phases of the two power supplies are different, a phase difference will be generated during connection, resulting in an instantaneous large current impact, which may damage the equipment or cause unstable operation. The phase consistency enables smooth current distribution during switching and parallel connection, reducing the fluctuations and impacts during system switching.

[0044] In addition, through phase synchronization, the bidirectional DC / AC unit and the energy storage inverter can work together to jointly share the load current, avoiding overloading of a single device. At the same time, the power resources of the energy storage inverter and the supercapacitor are effectively utilized to ensure that the system has good emergency response capabilities while operating efficiently.

[0045] Furthermore, the phase consistency helps to reduce harmonics and power interference, ensuring the quality of power supply. The synchronous operation reduces the interference between modules, improving the overall reliability and operating life of the system.

[0046] Embodiment 2

[0047] The second aspect of the present invention is to propose a method for controlling the off-grid overload of an energy storage inverter, including the following steps:

[0048] When the power grid is powered off, the energy storage inverter operates in the voltage source mode, and the energy storage inverter outputs the rated voltage and rated frequency. At this time, the bidirectional DC-DC buck-boost unit is in the boost mode, the bidirectional DC / AC unit operates in the current source mode, the supercapacitor is boosted by the bidirectional DC-DC buck-boost unit, and the voltage is stabilized through the intermediate capacitor. The bidirectional DC / AC unit converts the direct current provided by the supercapacitor into alternating current to provide current compensation for the energy storage inverter.

[0049] Among them, when the bidirectional DC / AC unit operates in the current source mode, the current value given by the current loop of the bidirectional DC / AC unit is set value one, and set value one is half of the rated current output by the energy storage inverter.

[0050] After the operation time reaches the set time, it is judged whether the current load current is greater than the threshold value. If it is greater, it means overload, and an overcurrent fault signal is output; if it is not greater, the current loop given value of the bidirectional DC / AC unit is set to 0. According to the latest national standard GB / T34120-2023, the threshold value is set to 1.3 times the rated current of the energy storage inverter; the operation time is set according to specific actual requirements. In this embodiment, it is set to 4 sine wave periods of the energy storage inverter.

[0051] After the power grid is restored, the energy storage inverter is synchronized with the grid voltage. After synchronization is completed, the power grid is energized. At this time, the energy storage inverter operates in the current source mode and is used to charge and discharge the battery; the bidirectional DC / AC unit is in the rectification mode. When the bidirectional DC / AC unit is in the rectification mode, the current value given by the current loop of the bidirectional DC / AC unit is the set value two, and the range of the set value two is 1-2A. The bidirectional DC-DC buck-boost unit is in the buck mode and is used to charge the super capacitor to prepare for the next power grid power failure.

[0052] In order to perform buck-boost operation on the super capacitor through the bidirectional DC-DC buck-boost unit, the bidirectional DC-DC buck-boost unit is modulated by the sine pulse width signal one, which specifically includes the following steps:

[0053] Calculate the voltage of the intermediate capacitor And the reference voltage The voltage error of the intermediate capacitor therebetween;

[0054] Input the voltage error signal of the intermediate capacitor into the voltage PI regulator to obtain the current reference value of the intermediate capacitor ;

[0055] Calculate the current of the intermediate capacitor at present And the reference current The current error of the intermediate capacitor therebetween;

[0056] Input the current error signal of the intermediate capacitor into the current PI regulator one, and the signal output by the current PI regulator one is transmitted to the SPWM module one for pulse width modulation. The signal output by the SPWM module one is used to control the bidirectional DC-DC buck-boost unit.

[0057] In order to control the compensation current parameters output by the compensation module, the bidirectional DC / AC unit is modulated by the sine pulse width signal two, which specifically includes the following steps:

[0058] Limit the current given by the current loop through the limiting current Then calculate the current error of the super capacitor between the limited current and the current of the super capacitor at present Therebetween. Among them, the value of the limiting current Is the maximum power of the super capacitor / grid voltage and is related to the selection of the super capacitor.

[0059] Input the current error signal of the super capacitor into the current PI regulator two for proportional integral regulation. The signal output by the current PI regulator two is transmitted to the SPWM module two for pulse width modulation. The signal output by the SPWM module two is used to control the bidirectional DC / AC unit.

[0060] By modulating the bidirectional DC-DC buck-boost unit and the bidirectional DC / AC unit respectively to output accurately controllable compensation current, the stability of the system is improved.

[0061] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions recorded in the foregoing embodiments or perform equivalent replacements for some of the technical features. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. An off-grid overload control system for an energy storage inverter, comprising an energy storage inverter connected to a power grid, characterized in that: The energy storage inverter is connected to a current compensation module, the energy storage inverter and the current compensation module are connected in parallel to the load, and the current compensation module is used to share the load current for the energy storage inverter when the power grid is off; The current compensation module includes a super capacitor, a bidirectional DC-DC buck-boost unit and a bidirectional DC / AC unit, an intermediate capacitor is connected between the bidirectional DC-DC buck-boost unit and the bidirectional DC / AC unit, and the super capacitor is connected to the bidirectional DC-DC buck-boost unit; The control method of the off-grid overload control system of the energy storage inverter comprises the following steps: When the power grid is off, the energy storage inverter operates in voltage source mode and outputs rated voltage and rated frequency. At this time, the bidirectional DC-DC buck-boost unit is in boost mode, and the bidirectional DC / AC unit operates in current source mode. The supercapacitor is boosted by the bidirectional DC-DC buck-boost unit, and the voltage is stabilized by the intermediate capacitor. The bidirectional DC / AC unit converts the DC power provided by the supercapacitor into AC power to provide current compensation for the energy storage inverter. After the running time reaches the set time, it is determined whether the current load current is greater than the threshold. If it is greater, it indicates overload and an overcurrent fault signal is output; if it is not greater, the current loop setting of the bidirectional DC / AC unit is set to 0; After the grid is restored, the energy storage inverter is synchronized with the grid voltage. After the synchronization is completed, the grid is powered on. At this time, the energy storage inverter operates in the current source mode to charge and discharge the battery; the bidirectional DC / AC unit is in the rectification mode, and the bidirectional DC-DC buck-boost unit is in the buck mode to charge the supercapacitor; When the bidirectional DC / AC unit operates in the current source mode, the current value given by the current loop of the bidirectional DC / AC unit is set value one; when the bidirectional DC / AC unit is in the rectification mode, the current value given by the current loop of the bidirectional DC / AC unit is set value two; The setting value 1 is half of the rated output current of the energy storage inverter, and the setting value 2 is in the range of 1-2A; The bidirectional DC-DC buck-boost unit is modulated by a sinusoidal pulse width signal, and specifically comprises the following steps: Calculate the voltage V of the middle capacitor mid_c With reference voltage The voltage error of the intermediate capacitor between The voltage error signal of the middle capacitor is input into the voltage PI regulator to obtain the current reference value of the middle capacitor. Calculate the current I of the intermediate capacitor mid_c and reference current The current error of the intermediate capacitor between The current error signal of the intermediate capacitor is input into the current PI regulator 1, the signal output by the current PI regulator 1 is transmitted to the SPWM module 1 for pulse width modulation, and the signal output by the SPWM module 1 is used to control the bidirectional DC-DC buck-boost unit; The bidirectional DC / AC unit is modulated by a sinusoidal pulse width signal, specifically comprising the following steps: By limiting the current I ultracap_MAX Limit the given current of the current loop, and then calculate the current after limiting and the current current I of the supercapacitor ultracap The current error of the supercapacitor between The current error signal of the supercapacitor is input into the current PI regulator 2 for proportional integral regulation, and the output signal of the current PI regulator 2 is transmitted to the SPWM module 2 for pulse width modulation. The output signal of the SPWM module 2 is used to control the bidirectional DC / AC unit.

2. The off-grid overload control system for energy storage inverter according to claim 1, characterized in that: When the grid is powered off, the bidirectional DC-DC buck-boost unit is in boost mode, the bidirectional DC / AC unit is in current source mode, the supercapacitor is boosted by the bidirectional DC-DC buck-boost unit, the intermediate capacitor is used for voltage stabilization, and the bidirectional DC / AC unit converts the DC power provided by the supercapacitor into AC power to provide current compensation for the energy storage inverter.

3. The off-grid overload control system for energy storage inverter according to claim 2, characterized in that: When the bidirectional DC / AC unit provides current compensation, the voltage phase of the bidirectional DC / AC unit is set to be consistent with the voltage phase of the energy storage inverter.

4. The off-grid overload control system for energy storage inverter according to claim 1, characterized in that: When the grid is powered on, the bidirectional DC / AC unit is in rectification mode, and the bidirectional DC-DC buck-boost unit is in buck mode.

5. The off-grid overload control system for energy storage inverter according to claim 1, characterized in that: When judging whether the current load current is greater than the threshold, the threshold is set to 1.3 times the rated current of the energy storage inverter.

Images