New energy grid-forming control system and method

Abstract

A new energy grid-forming control system and a method. The system comprises a power instruction calculation module, the power instruction calculation module comprising a grid-following control module. The power command calculation module is used for determining actually measured active power and actually measured reactive power. When the control system switches from grid-following control to grid-forming control, the grid-following control module is used for: on the basis of a difference value between reference active power and the actually measured active power and a difference value between reference reactive power and the actually measured reactive power, regulating a current increment of a current loop for grid-following control until the difference value between the reference active power and the actually measured active power and the difference value between the reference reactive power and the actually measured reactive power both meet a third convergence condition and, after the difference value between the reference active power and the actually measured active power and the difference value between the reference reactive power and the actually measured reactive power both have met the third convergence condition, reducing the current increment to 0 on the basis of a third step length.

Description

New energy grid-forming type control system and method TECHNICAL FIELD

[0001] The present application relates to the technical field of new energy, in particular to a new energy grid-forming type control system and method. BACKGROUND

[0002] With the large-scale integration of new energy power generation equipment into the power grid, the inherent physical rotor ratio of synchronous generators gradually decreases, which reduces the system's resistance to disturbances. The absence of the rotor reduces the inertia in the power system, which greatly reduces the ability to resist wideband oscillations and absorb harmonics. Unlike traditional generators, which adjust the electromotive force through excitation and adjust the phase through rotor movement, new energy power electronic generation and consumption equipment rely on multiple control loops, which increases the risk of frequency and voltage fluctuations and flicker. The combination and superposition of different switching devices also worsen the power quality in the system, bringing great crisis and challenges to large-scale new energy grid connection.

[0003] To alleviate the above-mentioned grid connection problems of new energy power generation equipment, a grid-forming control strategy based on grid-forming converters can be applied in the power system. Compared with the follow-grid control strategy represented by vector control, the grid-forming control strategy can greatly improve the stability of the converter. Relying on the virtual rotor equation and excitation unit, the grid-forming control strategy provides configurable inertia and damping for the grid, ensuring the grid adaptability of new energy grid connection, and making it possible for large-scale multi-source heterogeneous new energy converters to operate in the grid.

[0004] However, when switching from follow-grid control to grid-forming control, mutual oscillation between multiple energy storage converters due to different frequencies and the resulting uneven power distribution problems are likely to occur.

[0005] SUMMARY

[0006] To this end, the present application discloses the following technical solutions:

[0007] The first aspect of the present application provides a new energy grid-forming type control system, comprising:

[0008] a mechanical equation module, an excitation equation module, a power instruction calculation module, and a follow-grid control module;

[0009] The power instruction calculation module is configured to determine the measured active power and the measured reactive power.

[0010] The excitation equation module is configured to, when the control system switches from grid-forming control to grid-following control, adjust a voltage increment of the excitation equation module according to a reference voltage and a transformed voltage until a difference between the reference voltage and the transformed voltage meets a first convergence condition, and after the difference between the reference voltage and the transformed voltage meets the first convergence condition, gradually reduce the voltage increment to 0 at a first step length; the transformed voltage is obtained by Park transformation (Park Transformer) from three-phase voltage of a new energy station controlled by the control system.

[0011] The mechanical equation module is configured to, when the control system switches from grid-forming control to grid-following control, adjust an angular velocity increment of the mechanical equation module according to a first phase angle and a second phase angle until a difference between the first phase angle and the second phase angle meets a second convergence condition, and after the difference between the first phase angle and the second phase angle meets the second convergence condition, gradually reduce the angular velocity increment to less than or equal to a second threshold value at a second step length; the first phase angle is a phase angle at an output end of a grid-following control unit, and the second phase angle is a phase angle at an output end of a grid-forming control unit.

[0012] The grid-following control module is configured to, when the control system switches from grid-following control to grid-forming control, adjust a current increment of a current loop for grid-following control according to a difference between a reference active power and a measured active power and a difference between a reference reactive power and a measured reactive power until the difference between the reference active power and the measured active power and the difference between the reference reactive power and the measured reactive power both meet a third convergence condition, and after the difference between the reference active power and the measured active power and the difference between the reference reactive power and the measured reactive power both meet the third convergence condition, reduce the current increment to 0 at a third step length.

[0013] The second aspect of the present application provides a new energy grid-forming control method, comprising:

[0014] When the control system switches from grid-forming control to grid-following control, a voltage increment of the excitation equation module is adjusted according to a reference voltage and a transformed voltage until a difference between the reference voltage and the transformed voltage meets a first convergence condition, and after the difference between the reference voltage and the transformed voltage meets the first convergence condition, the voltage increment is gradually reduced to 0 at a first step length; the transformed voltage is obtained by Park transformation (Park Transformer) from three-phase voltage of a new energy station controlled by the control system.

[0015] When the control system switches from the grid-forming control to the grid-following control, the angular velocity increment of the mechanical equation module is adjusted according to the first phase angle and the second phase angle until the difference between the first phase angle and the second phase angle meets a second convergence condition, and after the difference between the first phase angle and the second phase angle meets the second convergence condition, the angular velocity increment is gradually reduced to be less than or equal to a second threshold value at a second step length; the first phase angle is a phase angle of an output end of the grid-following control unit, and the second phase angle is a phase angle of an output end of the grid-forming control unit;

[0016] When the control system switches from the grid-following control to the grid-forming control, the current increment of the current loop for the grid-following control is adjusted according to the difference between the reference active power and the measured active power and the difference between the reference reactive power and the measured reactive power until the difference between the reference active power and the measured active power and the difference between the reference reactive power and the measured reactive power both meet a third convergence condition, and after the difference between the reference active power and the measured active power and the difference between the reference reactive power and the measured reactive power both meet the third convergence condition, the current increment is reduced to 0 at a third step length.

[0017] The beneficial effects of the present scheme are as follows:

[0018] When switching from the grid-following control to the grid-forming control, the current increment of the current loop for the grid-following control is adjusted according to the difference between the reference active power and the measured active power and the difference between the reference reactive power and the measured reactive power until the difference between the reference active power and the measured active power and the difference between the reference reactive power and the measured reactive power both meet a third convergence condition, and after the difference between the reference active power and the measured active power and the difference between the reference reactive power and the measured reactive power both meet the third convergence condition, the current increment is reduced to 0 at a third step length. Through this switching mode, smooth switching of the grid-following control to the grid-forming control is realized to meet the requirements of low impact, no disturbance, and consistent power before and after switching. BRIEF DESCRIPTION OF DRAWINGS

[0019] In order to more clearly illustrate the technical solutions in the embodiments or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or the prior art description. Obviously, the drawings in the following description are only embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor on the basis of the provided drawings.

[0020] FIG. 1 is a control strategy block diagram of a conventional grid-forming control system;

[0021] FIG. 2 is a structural schematic diagram of a new energy grid-forming control system according to an embodiment of the present application;

[0022] FIG. 3 is a schematic diagram of a partial module structure according to an embodiment of the present application;

[0023] FIG. 4 is a flowchart of a follow-net type control and a network-constructing type control switching each other according to an embodiment of the present application;

[0024] FIG. 5 is a schematic diagram of a fault identification module according to an embodiment of the present application;

[0025] FIG. 6 is a power angle stability control block diagram before and after a fault according to an embodiment of the present application;

[0026] FIG. 7 is a dynamic admittance adjustment control block diagram of a fault recovery process according to an embodiment of the present application;

[0027] FIG. 8 is a multi-machine parallel control block diagram in a case of no communication among multiple network-constructing type converters according to an embodiment of the present application;

[0028] FIG. 9 is a flowchart of a black start according to an embodiment of the present application;

[0029] FIG. 10 is a flowchart of an anti-islanding detection and protection according to an embodiment of the present application;

[0030] FIG. 11 is a flowchart of a method of a new energy network-constructing type control system according to an embodiment of the present application. DETAILED DESCRIPTION

[0031] The technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, but not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the scope of protection of the present application.

[0032] Please refer to FIG. 1, which is a control strategy block diagram of an existing network-constructing type control system. It can be seen that, compared with a common follow-net type control, the network-constructing type control discards a phase-locked loop part of following the power grid, and instead uses an excitation part of actively adjusting a terminal voltage of a generator and a virtual rotor part of keeping power angle stability. Thus, the control part has autonomous synchronism with the generator, and can also reflect the frequency and voltage adjustment capability of a synchronous generator.

[0033] However, the existing network-constructing type control strategy still has the following defects:

[0034] In a first aspect, in a power system, especially a high proportion of new energy power system, short-circuit fault is the most common and the most severe power grid fault. Similar to synchronous machines, during the moment when the short-circuit fault causes the voltage of the power grid to drop and recover, the rotor and the excitation link of the grid-forming converter will actively maintain the stability of the internal potential. The inherent characteristics of the grid-forming converter make the converter output a large current to maintain the voltage level, and especially the sharp change of the module current at the moment of voltage drop and recovery will cause overcurrent. That is, the grid-forming converter is prone to overcurrent when a short-circuit fault occurs, which in turn causes device damage.

[0035] In a second aspect, in a large-area power outage of the power grid, the power station needs to drive the local load, or in the case of large load fluctuation of the power grid, poor power quality, unstable power grid, and even power grid collapse, etc., the energy storage system needs to be configured to a megawatt (MW) level or even to a larger scale of tens of MW to regulate the power quality, smooth the load fluctuation, and ensure the island operation. When such an isolated power grid has problems such as power grid collapse, the MW-level or even tens of MW energy storage system is required to synchronously and quickly cut into off-grid operation or provide a certain time of auxiliary power to complete the startup of the generator set.

[0036] In a third aspect, the key to the energy storage system entering off-grid operation is to switch from the grid-connected mode to the grid-forming mode, and the current and voltage are smooth during the switching process without obvious impact. In the process of black start of the new energy power generation system, dozens or even hundreds of energy storage converters need to be switched to off-grid operation state or complete zero start-up of the transformer to establish off-grid voltage. This process is prone to mutual oscillation between multiple machines due to different frequencies and uneven power distribution caused thereby.

[0037] In a fourth aspect, in a grid-forming control system, multiple grid-forming converters are prone to systematic oscillation, and the main reason for the oscillation is that the parameters of the grid-forming converters are inconsistent, and the main influencing parameters include inertia coefficient, active droop coefficient, and line reactance, etc. Usually, communication is used to distribute and adjust the control parameters, but this method has great problems in cost and stability.

[0038] In a fifth aspect, island effect refers to the state that the system continues to supply power to a part of the line in the voltage-loss power grid when the power grid loses voltage. Once unplanned islanding occurs, the voltage and frequency in the island will no longer be controlled by the power grid, which may cause damage to user equipment on the power line, interfere with the recovery process of the power grid, and pose a danger to maintenance personnel. Therefore, how to detect and protect against islanding in a grid-forming control system is also extremely important.

[0039] To solve the above problems, the embodiments of the present application provide a new energy grid-forming control system to at least solve the problem of the first aspect.

[0040] Please refer to Fig. 2, which is a structural schematic diagram of a new energy network construction type control system provided by the embodiment.

[0041] The system can include a mechanical equation module, an excitation equation module, a power instruction calculation module, and a network following control module.

[0042] The power instruction calculation module is configured to determine the measured active power and the measured reactive power.

[0043] The excitation equation module is configured to, when the control system is switched from the network construction type control to the network following type control, adjust a voltage increment of the excitation equation module according to the reference voltage and the transformed voltage until a difference between the reference voltage and the transformed voltage meets a first convergence condition, and after the difference between the reference voltage and the transformed voltage meets the first convergence condition, gradually reduce the voltage increment to 0 at a first step length.

[0044] The mechanical equation module is configured to, when the control system is switched from the network construction type control to the network following type control, adjust an angular velocity increment of the mechanical equation module according to a first phase angle and a second phase angle until a difference between the first phase angle and the second phase angle meets a second convergence condition, and after the difference between the first phase angle and the second phase angle meets the second convergence condition, gradually reduce the angular velocity increment to less than or equal to a second threshold value at a second step length; the first phase angle is a phase angle of an output end of the network following type control unit, and the second phase angle is a phase angle of an output end of the network construction type control unit.

[0045] The network following control module is configured to, when the control system is switched from the network following type control to the network construction type control, adjust a current increment of a current loop for the network following type control according to a difference between the reference active power and the measured active power and a difference between the reference reactive power and the measured reactive power until the difference between the reference active power and the measured active power and the difference between the reference reactive power and the measured reactive power both meet a third convergence condition, and after the difference between the reference active power and the measured active power and the difference between the reference reactive power and the measured reactive power both meet the third convergence condition, reduce the current increment to 0 at a third step length.

[0046] Please refer to Fig. 3(1), which is a control structure diagram of the mechanical equation module provided by the embodiment. The structure outside the dashed rectangular frame is the control structure of the mechanical equation in the existing network construction type control system, which will not be described herein again. The structure inside the dashed rectangular frame is the newly added control structure of the embodiment based on the existing mechanical equation.

[0047] As shown in the figure, the mechanical equation module of the embodiment can obtain the phase angle of the output end of a grid-following (GFL) control unit (specifically, a grid-following converter), that is, θ GFL in the figure, and the phase angle of the output end of a grid-forming (GFM) control unit (specifically, a grid-forming converter), that is, θ GFM , processes the two based on a proportional-integral amplifier PI, obtains a phase difference Δω, and takes the phase difference as an angular velocity increment ω_add, that is, assigns the former to the latter, ω_add=Δω, adjusts the phase angle of the output end of the grid-forming control unit based on the angular velocity increment, and stops until the difference between the first phase angle and the second phase angle meets a second convergence condition.

[0048] The second convergence condition can be that the absolute value of the difference between the first phase angle and the second phase angle is less than a certain threshold, for example, less than 0.01, and the corresponding mathematical expression is |θ GFL -θ GFM |<0.01.

[0049] After the difference between the first phase angle and the second phase angle meets the second convergence condition, the mechanical equation module can reduce the angular velocity increment by a second step size to be less than or equal to a second threshold value.

[0050] The second step size can be set to 0.01, and the second threshold value can be 0.0001.

[0051] That is, after the difference between the first phase angle and the second phase angle meets the second convergence condition, the mechanical equation module can repeatedly reduce the angular velocity increment according to ω_add=ω_add-0.01, where the left side is the reduced value and the right side is the reduced value, and after each update, it is determined whether ω_add<=0.0001 is true, if true, the mechanical equation module stops reducing the angular velocity increment, if not, the angular velocity increment is reduced according to the foregoing formula.

[0052] The working principle of the above mechanical equation module can be seen from the flowchart of the grid-forming to grid-following switching shown in (1) of FIG. 4. Wherein ω_add=Δω represents assigning the calculated phase difference to the angular velocity increment ω_add.

[0053] The first phase angle can be obtained by the conventional phase-locked loop module shown in FIG. 2. The conventional phase-locked loop module can have a structure as shown in (2) of FIG. 3.

[0054] Please refer to Fig. 3(3), which is a control block diagram of the excitation equation module of the embodiment. The structure outside the dashed rectangular frame is the control structure of the excitation equation in the existing network-forming type control system, which is not described in detail. The structure inside the dashed rectangular frame is the newly added control structure of the embodiment based on the existing excitation equation.

[0055] As can be seen, the excitation equation module of the embodiment can input the reference voltage Uref and the transformed voltage Ud into the newly added proportional integral amplifier, obtain the voltage difference Δu, and then assign the voltage difference to the voltage increment U_add of the excitation equation module (i.e. U_add = Δu). The reference voltage is adjusted based on the voltage increment until the difference between the reference voltage and the transformed voltage meets the first convergence condition.

[0056] The transformed voltage Ud can be the three-phase voltage U of the new energy station controlled by the control system. abc The voltage value obtained after the conventional phase-locked loop module shown in Fig. 3(2) is processed by the Park transformer.

[0057] The first convergence condition can be that the absolute value of the difference between the reference voltage Uref and the transformed voltage Ud is less than a certain threshold value, for example, less than 1. This convergence condition can be expressed in a mathematical expression as |Uref-Ud|<1.

[0058] The first step size can be set to 0.1, and the first threshold value can be 0.01.

[0059] That is, after the first convergence condition is met, the excitation equation module can repeatedly reduce the voltage increment according to U_add = U_add-0.1, where the left side is the reduced value and the right side is the reduced value. After each update, it is determined whether U_add<=0.01 is true. If true, the excitation equation module stops reducing the voltage increment. At this time, the excitation equation module can directly assign the voltage increment to 0, i.e. U_add = 0. If not, the excitation equation module continues to reduce the voltage increment according to the above formula.

[0060] If there is no need to switch from the network-forming type control to the follow-net type control, the excitation equation control module can directly set the voltage increment to 0, i.e. U_add = 0.

[0061] The working principle of the above excitation equation module can be seen from the flowchart of the network-forming type to follow-net type switching shown in Fig. 4(1).

[0062] The structure of the net-following control module can refer to the structure in the dashed-line rectangular frame of (4) in FIG. 3. When the control system is switched from the net-following type control to the net-constructing type control, the current increment of the current loop used for the net-following type control is adjusted according to the difference between the reference active power Pref and the measured active power P and the difference between the reference reactive power Qref and the measured reactive power Q until the difference between the reference active power and the measured active power and the difference between the reference reactive power and the measured reactive power both satisfy a third convergence condition.

[0063] Specifically, the reference active power Pref and the measured active power P can be calculated by PI to obtain an active power difference ΔP, and the reference reactive power Qref and the measured reactive power Q can be calculated by PI to obtain a reactive power difference ΔQ. The active power difference is assigned to the first current increment ΔId, and the reactive power difference is assigned to the second current increment ΔIq, i.e., ΔId=ΔP and ΔIq=ΔQ.

[0064] After adjusting the current increment in the above manner, the first current Idref and the second current Iqref of the current loop can be correspondingly adjusted, thereby affecting the reference active power Pref and the reference reactive power Qref.

[0065] After adjusting the current increment each time, it can be judged whether the third convergence condition is satisfied at this time.

[0066] The third convergence condition can be that the active power difference is less than or equal to 10 kilowatts (kW) and the reactive power difference is less than or equal to 10 kW, i.e., Pref-P<=10 kW and Qref-Q<=10 kW.

[0067] If the third convergence condition is not satisfied, the current increment can continue to be adjusted in the foregoing manner, and if the third convergence condition is satisfied, the current increment can be reduced to 0 by a third step size.

[0068] The third step size can be 1, and the manner of reducing the current increment to 0 by the third step size can be:

[0069] The current increment is decremented by the third step size; specifically, the first current increment and the second current increment can be decremented in the manner of ΔId=ΔId-1 and ΔIq=ΔIq-1, where the left side of the formula is the current increment after decrement, and the right side of the formula is the current increment before decrement.

[0070] It is determined whether the value of the current increment after reduction is less than or equal to a third threshold value; the third threshold value corresponding to different current increments can be different, for example, the third threshold value corresponding to the first current increment can be 100 A, and the third threshold value corresponding to the second current increment can be 10 A, so determining whether the value of the current increment after reduction is less than or equal to the third threshold value is equivalent to determining whether ΔId<=100 A and ΔIq<=10 A are true.

[0071] If the value of the current increment after being reduced is less than or equal to the third threshold value, the value of the current increment is set to 0; specifically, the second current increment can be assigned a value of 0, and the voltage increment corresponding to the first current increment is also assigned a value of 0, i.e., ΔIq = 0, ΔVd = 0;

[0072] If the value of the current increment after being reduced is greater than the third threshold value, the step of decrementing the current increment by the third step size is returned to be executed until the value of the current increment after being reduced is less than or equal to the third threshold value.

[0073] Optionally, if it is not necessary to switch from the grid-following control to the grid-forming control, the grid-following control module can directly assign a value of 0 to the second current increment, and assign a value of 0 to the voltage increment corresponding to the first current increment, i.e., ΔIq = 0, ΔVd = 0.

[0074] The working principle of the above grid-following control module can be seen from the flowchart of the switching from the grid-following type to the grid-forming type shown in (2) of FIG. 4.

[0075] Through the control principles of the above modules, the requirements of low impact, no disturbance, and consistent power before and after switching can be achieved in the grid-following-grid-forming switching process.

[0076] Optionally, in order to realize the mutual switching of the grid-following control and the grid-forming control, a small number of grid-following / grid-forming switchable power generation units can be configured in a new energy station in which all power generation units adopt the grid-following control, which can improve the operation ability of the station under a weak grid and avoid instability problems.

[0077] As shown in FIG. 2, the system of the embodiment can further include a fault identification module.

[0078] The fault identification module is configured to identify whether the power grid accessed by the new energy station is in a short-circuit triggering state or a short-circuit recovery state.

[0079] The power instruction calculation module is configured to perform power angle compensation according to the state of the power grid when the power grid is in the short-circuit triggering state or the short-circuit recovery state.

[0080] The fault identification module can output a fault mode signal to indicate whether the power grid accessed by the new energy station is in the short-circuit triggering state or the short-circuit recovery state. Please refer to FIG. 5 for a structural schematic diagram of the fault identification module of the embodiment. LPF in the figure represents a low-pass filter.

[0081] The core of the fault identification module is an SR flip-flop. The SR flip-flop can output different fault mode signals according to the values of the S bit (voltage fault signal bit) and the R bit (current fault signal bit) and in combination with the SR flip-flop truth table shown in Table 1.

[0082] Table 1

[0083] Based on the structure shown in Figure 5, the fault identification module can normalize the voltage error value, and if the voltage error value is less than 7% of the rated voltage value, the voltage fault signal position can be set to 1, and if it is greater than or equal to 7% of the rated voltage value, the voltage fault signal position can be set to 0.

[0084] The fault identification module can normalize the active current error and the reactive current error. If the active current error is greater than 125% of the current rated value, or the reactive current error is greater than 125% of the current rated value, the current fault signal position can be set to 1, and if the active current error is not greater than 125% of the current rated value, and the reactive current error is not greater than 125% of the current rated value, the current fault signal position can be set to 0.

[0085] The rated voltage value U dN , the current rated value I N are all rated values of the grid-forming converter for grid-forming control.

[0086] The power instruction calculation module, when determining that the power grid is in a short-circuit triggering state or in a short-circuit recovery state, can use the fault entry and the fault exit to process the virtual admittance, the reference voltage and the reference power differently to perform power angle compensation and voltage fine-tuning according to the control block diagram shown in Figure 6, so as to not only reduce the current impact limitation controllable when entering the fault, but also ensure the smooth transition of the current when exiting the fault.

[0087] Wherein, P' is determined according to the following formula (1).

[0088] Q' is determined according to the following formula (2).

[0089] Optionally, as shown in Figure 2, the embodiment further comprises:

[0090] The virtual admittance dynamic current limiting module and the dynamic admittance adjusting module;

[0091] The dynamic admittance adjusting module is configured to adjust the admittance value of the virtual admittance dynamic current limiting module when the power grid is in a short-circuit recovery state, so as to suppress the current value output by the grid-forming converter.

[0092] Specifically, the dynamic admittance adjusting module can adjust the admittance value of the virtual admittance dynamic current limiting module according to the control block diagram shown in Figure 7.

[0093] Adjusting the admittance value of the virtual admittance dynamic current limiting module can prevent the power angle hysteresis of the output process from causing the active current to rise and cause overcurrent, and can prevent the sampling deviation during voltage recovery from causing the inertia superposition caused by the voltage difference and phase lag, so as to suppress the large current at this time until the voltage is fully recovered and the mechanical link returns to the steady state through dynamic damping correction.

[0094] Optionally, as shown in FIG. 2, the system of the embodiment further comprises:

[0095] a harmonic (circulating current) extraction module and an auxiliary current loop module.

[0096] The harmonic (circulating current) extraction module is used to:

[0097] detect the current and voltage of the grid-connected point of the plurality of network-forming type converters in parallel in real time to obtain the Nth harmonic of the current (i.e., i pcc shown in FIG. 8) and the Nth harmonic of the voltage (i.e., u pcc shown in FIG. 8);

[0098] convert the Nth harmonic of the current into a voltage regulation amount (i.e., u hdq shown in FIG. 2) through a PI control loop, and input the voltage regulation amount into the virtual admittance dynamic current limiting module to suppress the voltage loop of the virtual admittance dynamic current limiting module;

[0099] convert the Nth harmonic of the voltage into a current regulation amount (i.e., i hdq shown in FIG. 2) through a PI control loop, and input the current regulation amount into the auxiliary current loop module to suppress the harmonic circulating current through the auxiliary current loop module.

[0100] wherein N is an integer determined according to the modulation mode, the capacitance parameter and the inductance parameter.

[0101] The harmonic (circulating current) extraction module can process the Nth harmonic of the input current and voltage according to the control block diagram shown in FIG. 8 to obtain the corresponding voltage regulation amount and current regulation amount.

[0102] The harmonic (circulating current) extraction module and the auxiliary current loop module are added to:

[0103] In the absence of communication lines, the multi-machine parallel is similar to the voltage source parallel operation, and due to the difficulty in meeting the completely synchronized state, the network converter in the multi-machine parallel condition is most prone to circulating current phenomenon, especially at the moment of power grid fault. When the system has a short-circuit fault, the common connection point voltage drops rapidly, but due to the influence of sampling time, control feedback time and other factors, the system regulation has certain time delay and hysteresis, and the converter output voltage cannot be adjusted rapidly, which leads to the fact that the large pressure difference needs to be borne by the filter and the line impedance, and the converter will bear the transient impact and steady-state overload current containing the non-periodic DC bias component and the periodic fault AC component. The deeper the grid voltage drop, the greater the impact current the system encounters, and it can even reach about seven times the current in normal operation. Unlike the synchronous generator with 6-8 p.u. overcurrent capacity, the fault current of the grid-connected converter should be limited to within 1.5-3 p.u. of the normal grid-connected operation current, especially after the converter is highly power electronic, the ability to withstand overload current is severely limited, and the fault overcurrent will pose a serious threat to the safe and stable operation of the grid.

[0104] And the harmonic (circulating current) extraction module and the auxiliary current loop module can convert the harmonic current into a voltage regulation quantity through a PI control loop, and introduce it into the voltage loop for suppression; convert the harmonic voltage into a current regulation quantity through a PI control loop, and introduce it into the current loop for suppression, so as to suppress the harmonic circulating current and reduce the fault overcurrent.

[0105] Optionally, the system of the embodiment further comprises a voltage accumulation module and a grid phase-locked loop module;

[0106] Referring to FIG. 9, the power instruction calculation module is further configured to:

[0107] After receiving the black start instruction, the DC circuit breaker is closed to perform DC pre-charging;

[0108] After the DC pre-charging is completed, the AC circuit breaker is closed, and the grid phase-locked loop module is started in parallel after the AC circuit breaker is closed to detect the frequency and the phase voltage peak value in real time;

[0109] The mechanical equation module is started at zero active power, and the control phase of the mechanical equation module is set;

[0110] The excitation equation module is started at zero reactive power;

[0111] The voltage accumulation module is configured to accumulate the feedforward voltage of the excitation equation module to the set voltage;

[0112] The power instruction calculation module is further configured to:

[0113] When the frequency deviation of the excitation equation module is less than 0.1 Hz and the voltage peak value of the excitation equation module is greater than or equal to the set voltage, the feedforward voltage of the excitation equation module is fixed as the set voltage to end the black start process. The set voltage can be represented as U o , and the voltage peak value of the excitation equation module can be represented as U d .

[0114] If the frequency deviation of the excitation equation module is not less than 0.1 Hz or the voltage peak value of the excitation equation module is less than the set voltage, the step of starting the mechanical equation module at zero active power and setting the control phase of the mechanical equation module can be returned to be executed until the frequency deviation of the excitation equation module is less than 0.1 Hz and the voltage peak value of the excitation equation module is greater than or equal to the set voltage.

[0115] From the perspective of the inverter controller, the virtual synchronous machine control framework includes a power frequency loop, a reactive voltage loop, and a generator stator voltage model; a zero-voltage-boost parallel black start strategy is proposed, which solves the problems of how to avoid current impact during voltage establishment and how to realize the start of two grid-forming type converters VSG in sequence in the existing parallel black start of grid-forming type converters.

[0116] Optionally, referring to FIG. 2, the system of the embodiment further includes an anti-islanding module, which can perform anti-islanding protection in the following manner, as shown in FIG. 10:

[0117] Detect whether the new energy station meets the anti-islanding detection condition;

[0118] In the case where it is detected that the new energy station meets the anti-islanding detection condition, perform 75 times of reactive harmonic disturbance, and detect in real time whether 75 times of voltage harmonics generated by the reactive harmonic disturbance are greater than 5%;

[0119] If the 75 times of voltage harmonics are greater than 5%, perform anti-islanding protection;

[0120] If the 75 times of voltage harmonics are not greater than 5%, perform 45 times of harmonic disturbance, and detect in real time whether 45 times of voltage harmonics generated by the reactive harmonic disturbance are greater than 5%;

[0121] If the 45 times of voltage harmonics are greater than 5%, perform anti-islanding protection;

[0122] If the 45 times of voltage harmonics are not greater than 5%, exit the anti-islanding detection.

[0123] Optionally, as shown in FIG. 10, the manner in which the anti-islanding module detects whether the new energy station meets the anti-islanding detection condition can be:

[0124] Detect in real time a harmonic change range of the new energy station;

[0125] If the harmonic variation range does not continue to be greater than 2% within the anti-islanding duration, it is determined that the new energy station does not meet the anti-islanding detection condition;

[0126] If the harmonic variation range continues to be greater than 2% within the anti-islanding duration, it is determined whether the new energy station meets the high-low penetration condition;

[0127] If the new energy station meets the high-low penetration condition, it is determined that the new energy station does not meet the anti-islanding detection condition;

[0128] If the new energy station does not meet the high-low penetration condition, it is determined that the new energy station meets the anti-islanding detection condition.

[0129] If the harmonic variation range continues to be greater than 2% within the anti-islanding duration, it is determined whether the new energy station meets the high-low penetration condition;

[0130] Islanding effect refers to when the power grid fails due to an accident or power maintenance, etc., the grid cannot detect the outage state in time, and a self-sufficient power supply system is formed which is connected to the load alone. Photovoltaic islanding effect can cause a series of hazards to system-related equipment and personnel. A reliable and accurate islanding detection method is the basis for grid-connected inverters to realize anti-islanding function.

[0131] The embodiment can more accurately detect whether the corresponding new energy station has an islanding effect by means of reactive harmonic injection, and can take timely measures when the islanding effect occurs.

[0132] The embodiment of the present application also provides a new energy grid construction type control method, please refer to FIG. 11, the method can include the following steps.

[0133] S1101, when the control system switches from grid construction type control to grid following type control, the voltage increment of the excitation equation module is adjusted according to the reference voltage and the transformed voltage until the difference between the reference voltage and the transformed voltage meets the first convergence condition, and after the difference between the reference voltage and the transformed voltage meets the first convergence condition, the voltage increment is gradually reduced to 0 at a first step length; the transformed voltage is obtained by Park transformation (Park Transformer) from the three-phase voltage of the new energy station controlled by the control system.

[0134] S1102, when the control system switches from the grid-forming control to the grid-following control, adjusting an angular velocity increment of the mechanical equation module according to a first phase angle and a second phase angle until a difference between the first phase angle and the second phase angle meets a second convergence condition, and after the difference between the first phase angle and the second phase angle meets the second convergence condition, gradually reducing the angular velocity increment by a second step length to be less than or equal to a second threshold value; the first phase angle is a phase angle of an output end of the grid-following control unit, and the second phase angle is a phase angle of an output end of the grid-forming control unit.

[0135] S1103, when the control system switches from the grid-following control to the grid-forming control, adjusting a current increment of a current loop for the grid-following control according to a difference between a reference active power and a measured active power and a difference between a reference reactive power and a measured reactive power until the difference between the reference active power and the measured active power and the difference between the reference reactive power and the measured reactive power both meet a third convergence condition, and after the difference between the reference active power and the measured active power and the difference between the reference reactive power and the measured reactive power both meet the third convergence condition, reducing the current increment to 0 by a third step length.

[0136] Optionally, reducing the current increment to 0 by the third step length comprises:

[0137] decrementing the current increment by the third step length;

[0138] determining whether a value of the current increment after being reduced is less than or equal to a third threshold value;

[0139] if the value of the current increment after being reduced is less than or equal to the third threshold value, setting the value of the current increment to 0;

[0140] if the value of the current increment after being reduced is greater than the third threshold value, returning to execute the step of decrementing the current increment by the third step length until the value of the current increment after being reduced is less than or equal to the third threshold value.

[0141] Optionally, the method further comprises:

[0142] identifying whether the power grid to which the new energy station is connected is in a short-circuit triggering state or a short-circuit recovery state;

[0143] when the power grid is in the short-circuit triggering state or in the short-circuit recovery state, performing power angle compensation according to a state of the power grid.

[0144] Optionally, the method further comprises:

[0145] when the power grid is in the short-circuit recovery state, adjusting a conductance value of the virtual conductance dynamic current limiting module to suppress a current value output by the grid-forming converter.

[0146] Optionally, the method further comprises:

[0147] Real-time detection of the current and voltage of the grid-connected point of the multiple grid-forming converters in parallel to obtain the Nth harmonic of the current and the Nth harmonic of the voltage;

[0148] Converting the Nth harmonic of the current into a voltage regulation amount through a PI control loop, and inputting the voltage regulation amount into a virtual admittance dynamic current limiting module to suppress the voltage loop of the virtual admittance dynamic current limiting module;

[0149] Converting the Nth harmonic amount of the voltage into a current regulation amount through a PI control loop, and inputting the current regulation amount into an auxiliary current loop module to suppress the harmonic circulating current through the auxiliary current loop module.

[0150] Optionally, further comprising:

[0151] After receiving the black start instruction, closing the DC circuit breaker to perform DC pre-charging;

[0152] After the DC pre-charging is completed, closing the AC circuit breaker, and starting the grid-connection phase-locked loop module in parallel after closing the AC circuit breaker to real-time detect the frequency and phase voltage peak value;

[0153] Starting the mechanical equation module at zero active power, and setting the control phase of the mechanical equation module;

[0154] Starting the excitation equation module at zero reactive power;

[0155] Accumulating the feedforward voltage of the excitation equation module to the set voltage;

[0156] When the frequency deviation of the excitation equation module is less than 0.1 Hz, and the voltage peak value of the excitation equation module is greater than or equal to the set voltage, fixing the feedforward voltage of the excitation equation module to the set voltage to end the black start process.

[0157] Optionally, further comprising:

[0158] Detecting whether the new energy station meets the anti-islanding detection condition;

[0159] In the case where it is detected that the new energy station meets the anti-islanding detection condition, performing 75 times of reactive harmonic disturbance, and real-time detecting whether the 75th voltage harmonic generated by the reactive harmonic disturbance is greater than 5%;

[0160] If the 75th voltage harmonic is greater than 5%, performing anti-islanding protection;

[0161] If the 75th voltage harmonic is not greater than 5%, performing 45 times of harmonic disturbance, and real-time detecting whether the 45th voltage harmonic generated by the reactive harmonic disturbance is greater than 5%;

[0162] If the 45th voltage harmonic is greater than 5%, performing anti-islanding protection;

[0163] If the 45th voltage harmonic is not greater than 5%, exit the anti-island detection.

[0164] The implementation of each step in the new energy network type control method provided in the embodiments of the present application can refer to the working principle of the new energy network type control system provided in any embodiment of the present application, which will not be described here again.

[0165] It should be noted that each embodiment in the present specification adopts a progressive manner for description, and each embodiment focuses on the difference from other embodiments, and the same or similar parts between each embodiment can be referred to each other.

[0166] For the convenience of description, the above system or device is described as various modules or units in function respectively. Of course, in the implementation of the present application, the functions of each unit can be implemented in the same or multiple software and / or hardware.

[0167] From the description of the above embodiments, those skilled in the art can clearly understand that the present application can be implemented by means of software and necessary general hardware platform. Based on such understanding, the technical solutions of the present application can be embodied in the form of software product, which can be stored in a storage medium, such as ROM / RAM, magnetic disk, optical disk, etc., including a plurality of instructions to make a computer device (which can be a personal computer, server, or network device, etc.) execute the method described in each embodiment or some parts of the embodiments of the present application.

[0168] Finally, it should be noted that in this paper, relationship terms such as first, second, third and fourth are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between the entities or operations. Moreover, the terms "include", "contain" or any other variants thereof are intended to cover non-exclusive inclusion, so that the process, method, article or device including a series of elements not only includes those elements, but also includes other elements not explicitly listed or inherent to such process, method, article or device. Without more limitation, the element defined by the statement "including a" does not exclude the existence of other identical elements in the process, method, article or device including the element.

[0169] The above is only the preferred embodiment of the present application, and it should be pointed out that for those skilled in the art, without departing from the principle of the present application, some improvements and refinements can be made, which should be regarded as the protection scope of the present application.

Claims

1. A new energy network construction type control system, characterized in that, The method comprises the following steps: a mechanical equation module, an excitation equation module, a power instruction calculation module, the power instruction calculation module comprising a grid-following control module; the power instruction calculation module is used to determine the measured active power and the measured reactive power; the excitation equation module is used to, when the control system is switched from grid-forming control to grid-following control, adjust the voltage increment of the excitation equation module according to the reference voltage and the transformed voltage until the difference between the reference voltage and the transformed voltage meets the first convergence condition, and then gradually reduce the voltage increment to 0 at a first step length after the difference between the reference voltage and the transformed voltage meets the first convergence condition; the transformed voltage is obtained by Park transformation from the three-phase voltage of the new energy station controlled by the control system; the mechanical equation module is used to, when the control system is switched from grid-forming control to grid-following control, adjust the angular velocity increment of the mechanical equation module according to the first phase angle and the second phase angle until the difference between the first phase angle and the second phase angle meets the second convergence condition, and then gradually reduce the angular velocity increment to less than or equal to a second threshold value at a second step length after the difference between the first phase angle and the second phase angle meets the second convergence condition; the first phase angle is the phase angle at the output end of the grid-following control unit, and the second phase angle is the phase angle at the output end of the grid-forming control unit; the grid-following control module is used to, when the control system is switched from grid-following control to grid-forming control, adjust the current increment of the current loop for grid-following control according to the difference between the reference active power and the measured active power and the difference between the reference reactive power and the measured reactive power until the difference between the reference active power and the measured active power and the difference between the reference reactive power and the measured reactive power both meet the third convergence condition, and then reduce the current increment to 0 at a third step length after the difference between the reference active power and the measured active power and the difference between the reference reactive power and the measured reactive power both meet the third convergence condition.

2. The system of claim 1, wherein, when the grid-following control module reduces the current increment to 0 at the third step length, it is used to: decrease the current increment by the third step length; determine whether the value of the current increment after reduction is less than or equal to a third threshold value; if the value of the current increment after reduction is less than or equal to the third threshold value, set the value of the current increment to 0; if the value of the current increment after reduction is greater than the third threshold value, return to execute the step of decreasing the current increment by the third step length until the value of the current increment after reduction is less than or equal to the third threshold value. Further comprising:

3. The system of claim 1, wherein, a fault identification module for identifying whether the power grid accessed by the new energy station is in a short-circuit triggering state or a short-circuit recovery state; the power instruction calculation module is used to perform power angle compensation according to the state of the power grid when the power grid is in the short-circuit triggering state or the short-circuit recovery state. Further comprising:

4. The system of claim 3, wherein, a virtual admittance dynamic current limiting module and a dynamic admittance adjusting module; ​ The dynamic admittance adjusting module is configured to adjust the admittance value of the virtual admittance dynamic current limiting module to suppress the current value output by the grid-connected converter when the power grid is in the short-circuit recovery state.

5. The system of claim 4, wherein, Further comprising: a harmonic (circulating current) extraction module and an auxiliary current loop module; The harmonic (circulating current) extraction module is configured to: real-time detect the current and voltage of the grid-connected point of the plurality of grid-connected converters in parallel to obtain the Nth harmonic of the current and the Nth harmonic of the voltage; convert the Nth harmonic of the current into a voltage regulation amount through a PI control loop, and input the voltage regulation amount into the virtual admittance dynamic current limiting module to suppress the voltage loop of the virtual admittance dynamic current limiting module; convert the Nth harmonic amount of the voltage into a current regulation amount through a PI control loop, and input the current regulation amount into the auxiliary current loop module to suppress the harmonic circulating current through the auxiliary current loop module.

6. The system of claim 1, wherein, Further comprising a voltage accumulation module and a grid-locked phase loop module; The power instruction calculation module is further configured to: after receiving the black start instruction, close the DC circuit breaker to perform DC pre-charging; after the DC pre-charging is completed, close the AC circuit breaker, and after closing the AC circuit breaker, start the grid-locked phase loop module in parallel to real-time detect the frequency and phase voltage peak value; start the mechanical equation module at zero active power and set the control phase of the mechanical equation module; start the excitation equation module at zero reactive power; the voltage accumulation module is configured to accumulate the feedforward voltage of the excitation equation module to a set voltage; The power instruction calculation module is further configured to: when the frequency deviation of the excitation equation module is less than 0.1 Hz and the voltage peak value of the excitation equation module is greater than or equal to the set voltage, fix the feedforward voltage of the excitation equation module to the set voltage to end the black start process.

7. The system of claim 1, wherein, Further comprising an anti-islanding module configured to: detect whether the new energy station meets the anti-islanding detection condition; if it is detected that the new energy station meets the anti-islanding detection condition, perform 75 times of reactive harmonic disturbance, and real-time detect whether the 75th voltage harmonic generated by the reactive harmonic disturbance is greater than 5%; if the 75th voltage harmonic is greater than 5%, perform anti-islanding protection; if the 75th voltage harmonic is not greater than 5%, perform 45 times of harmonic disturbance, and real-time detect whether the 45th voltage harmonic generated by the reactive harmonic disturbance is greater than 5%; if the 45th voltage harmonic is greater than 5%, perform anti-islanding protection; if the 45th voltage harmonic is not greater than 5%, exit the anti-islanding detection.

8. The system of claim 7, wherein, When the anti-islanding module detects whether the new energy station meets the anti-islanding detection condition, it is configured to: real-time detect the harmonic variation range of the new energy station; if the harmonic variation range does not continuously exceed 2% within the anti-islanding duration, determine that the new energy station does not meet the anti-islanding detection condition; if the harmonic variation range continuously exceeds 2% within the anti-islanding duration, judge whether the new energy station meets the high-low penetration condition; if the new energy station meets the high-low penetration condition, determine that the new energy station does not meet the anti-islanding detection condition; If the new energy station does not satisfy the high-low penetration condition, it is determined that the new energy station satisfies the anti-islanding detection condition.

9. A new energy network construction type control method, characterized in that, Comprise: When the control system switches from grid-forming control to grid-following control, the voltage increment of the excitation equation module is adjusted according to the reference voltage and the transformed voltage until the difference between the reference voltage and the transformed voltage satisfies a first convergence condition, and after the difference between the reference voltage and the transformed voltage satisfies the first convergence condition, the voltage increment is gradually reduced to 0 by a first step size; the transformed voltage is obtained by Park transformation of the three-phase voltage of the new energy station controlled by the control system; When the control system switches from grid-forming control to grid-following control, the angular velocity increment of the mechanical equation module is adjusted according to the first phase angle And the second phase angle until the difference between the first phase angle and the second phase angle satisfies a second convergence condition, and after the difference between the first phase angle and the second phase angle satisfies the second convergence condition, the angular velocity increment is gradually reduced to less than or equal to a second threshold value by a second step size; the first phase angle is the phase angle of the output end of the grid-following control unit, and the second phase angle is the phase angle of the output end of the grid-forming control unit; When the control system switches from grid-following control to grid-forming control, the current increment of the current loop for grid-following control is adjusted according to the difference between the reference active power and the measured active power, and the difference between the reference reactive power and the measured reactive power until the difference between the reference active power and the measured active power, and the difference between the reference reactive power and the measured reactive power all satisfy a third convergence condition, and after the difference between the reference active power and the measured active power, and the difference between the reference reactive power and the measured reactive power all satisfy the third convergence condition, the current increment is reduced to 0 by a third step size.

10. The method of claim 9, wherein, The current increment is reduced to 0 by a third step size, comprising: The current increment is reduced by a third step size; Determine whether the value of the current increment after reduction is less than or equal to a third threshold value; If the value of the current increment after reduction is less than or equal to the third threshold value, set the value of the current increment to 0; If the value of the current increment after reduction is greater than the third threshold value, return to execute the step of reducing the current increment by a third step size until the value of the current increment after reduction is less than or equal to the third threshold value.

11. The method of claim 9, wherein, Further comprising: Identify whether the power grid to which the new energy station is connected is in a short-circuit triggering state or a short-circuit recovery state; When the power grid is in the short-circuit triggering state or the short-circuit recovery state, perform power angle compensation according to the state of the power grid.

12. The method of claim 11, wherein, Further comprising: When the power grid is in the short-circuit recovery state, adjust the admittance value of the virtual admittance dynamic current limiting module to suppress the current value output by the grid-forming converter.

13. The method of claim 12, wherein, Further comprising: Real-time detect the current and voltage of the grid-connected point of the plurality of grid-forming converters connected in parallel to obtain the Nth harmonic of the current and the Nth harmonic of the voltage; The Nth harmonic of the current is converted into a voltage regulation quantity through a PI control loop, and the voltage regulation quantity is input into the virtual admittance dynamic current limiting module to suppress the voltage loop of the virtual admittance dynamic current limiting module The Nth harmonic of the voltage is converted into a current regulation quantity through a PI control loop, and the current regulation quantity is input into the auxiliary current loop module to suppress the harmonic circulating current through the auxiliary current loop module. Further comprising:

14. The method of claim 9, wherein, After receiving the black start instruction, the DC circuit breaker is closed for DC pre-charging; After the DC pre-charging is completed, the AC circuit breaker is closed, and the network phase-locked loop module is started in parallel after the AC circuit breaker is closed to detect the frequency and the peak value of the phase voltage in real time; The mechanical equation module is started at zero active power, and the control phase of the mechanical equation module is set; The excitation equation module is started at zero reactive power; The feedforward voltage of the excitation equation module is accumulated to the set voltage; When the frequency deviation of the excitation equation module is less than 0.1 Hz and the voltage peak value of the excitation equation module is greater than or equal to the set voltage, the feedforward voltage of the excitation equation module is fixed to the set voltage to end the black start process. Further comprising:

15. The method of claim 9, wherein, Detecting whether the new energy station meets the anti-islanding detection condition; If it is detected that the new energy station meets the anti-islanding detection condition, performing 75 times of reactive harmonic disturbance and detecting in real time whether the 75 times of voltage harmonics generated by the reactive harmonic disturbance are greater than 5%; If the 75 times of voltage harmonics are greater than 5%, anti-islanding protection is performed; If the 75 times of voltage harmonics are not greater than 5%, performing 45 times of harmonic disturbance and detecting in real time whether the 45 times of voltage harmonics generated by the reactive harmonic disturbance are greater than 5%; If the 45 times of voltage harmonics are greater than 5%, anti-islanding protection is performed; If the 45 times of voltage harmonics are not greater than 5%, the anti-islanding detection is exited. ​

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