Networking type interface device suitable for energy storage converter and control method

Abstract

The invention discloses a network construction type interface device suitable for an energy storage converter and a control method, and relates to the technical field of power converters. In the network construction type interface device provided by the invention, the voltage of the electric energy output by the energy storage converter is regulated and controlled to a direct-current voltage expected value by arranging the divider resistor and the decoupling inductor in combination with the rectification unit controlled by slow start, and the voltage of the electric energy is maintained near the direct-current voltage expected value by utilizing the pulse width rectification control of the rectification unit; and meanwhile, the increase of the current output by the energy storage converter in the fault ride-through process of the power distribution network is inhibited through the network construction unit with self-synchronization control, so that the energy storage converter does not need to take any current limiting measure in the fault ride-through process of the power distribution network, and the energy storage converter is protected from being burnt by the current increased in the fault ride-through process of the power distribution network.

Description

Technical Field

[0001] The present invention relates to the technical field of power converters, and in particular to a grid-forming interface device and a control method suitable for an energy storage converter. Background Art

[0002] Energy storage converters connected in the form of grid control can provide transient frequency / voltage support for new energy units, increase system rotational inertia, and also help improve the stability of new energy systems. However, unlike synchronous machines, the power module, which is the core of the energy storage converter, has limited design capacity and operating temperature, and the corresponding overload capacity is also limited. The overcurrent capacity of power electronic devices is weak, which can easily lead to the burning of power electronic devices during large disturbances, seriously affecting the reliability of equipment.

[0003] At present, there are three main strategies for suppressing overcurrent during faults: the first is to use power electronic devices with higher power levels to withstand larger current shocks, but this inevitably increases the cost of the converter; the second is to switch to current source mode during the fault and directly control the current, thereby effectively controlling the fault shock current. At this time, the converter externally exhibits current source characteristics, but cannot effectively control the voltage, which will cause voltage stability problems during the fault and fault recovery stages; the third is to adopt a current limiting strategy to achieve fault riding of the grid-type converter without increasing the cost of the converter. However, current limiting, power angle change, and reactive voltage control usually occur simultaneously during transient periods and have mutual influence, which greatly reduces the transient stability of the system.

[0004] Therefore, the power electronic power devices inside the energy storage inverter provided in the prior art have weak overcurrent resistance. A fault in the distribution network will cause a large disturbance in the current of the energy storage inverter, burn the power electronic power devices of the energy storage inverter, and seriously affect the reliability of the operation of the system with the energy storage inverter. Summary of the invention

[0005] In order to solve the problem that the energy storage converter in the prior art will burn the internal power electronic power devices when the distribution network has a fault crossing, the present invention proposes a grid-forming interface device and control method suitable for the energy storage converter. In the grid-forming interface device provided by the present invention, by setting a voltage-dividing resistor, a decoupling inductor and a rectifier unit with a slow start control, the voltage of the electric energy output by the energy storage converter is regulated to the expected value of the DC voltage, and the pulse width rectification control of the rectifier unit is used to maintain the voltage of the electric energy near the expected value of the DC voltage. At the same time, the grid-forming unit with self-synchronous control is used to suppress the increase in the current output by the energy storage converter during the distribution network fault crossing process, so as to ensure that the energy storage converter does not need to take any current limiting measures during the distribution network fault crossing period, and protect the energy storage converter from being burned by the increased current during the distribution network fault crossing process.

[0006] On the one hand, the present invention proposes a meshing interface device suitable for an energy storage converter, the meshing interface device comprising: a voltage-dividing resistor, a decoupling inductor, a rectifier unit, a voltage-stabilizing capacitor, and a meshing unit; the DC bus of the energy storage converter is connected to the rectifier unit through the voltage-dividing resistor and the decoupling inductor, and the rectifier unit is connected to the meshing unit through the voltage-stabilizing capacitor;

[0007] The voltage-dividing resistor divides the electric energy output by the energy storage inverter, and inputs the divided electric energy into the rectifier unit; the decoupling inductor suppresses the noise of the electric energy output by the energy storage inverter; the rectifier unit regulates the voltage of the divided electric energy to an expected DC voltage value through soft start control, and maintains the voltage stability of the electric energy after being regulated to the expected DC voltage value through pulse width modulation rectification control in cooperation with the voltage-stabilizing capacitor, and inputs the electric energy after being regulated to the expected DC voltage value into the networking unit; the networking unit suppresses the current increase of the electric energy in the distribution network through self-synchronization control.

[0008] Optionally, it also includes a filter inductor and a filter capacitor, one side of the filter inductor is connected to the side of the networking unit away from the voltage stabilizing capacitor, and the other side of the filter inductor is connected to the filter capacitor and then connected to the distribution network;

[0009] The filter inductor cooperates with the filter capacitor to filter out ripples in the electric energy input from the networking unit to the distribution network.

[0010] Optionally, a plurality of the filter inductors and the filter capacitors are provided, and each phase output end of the networking unit is connected to each phase line of the distribution network via at least one filter inductor, one side of each filter capacitor is connected to a side of the filter inductor away from the networking unit, and the other side of each filter capacitor is connected to an adjacent filter capacitor through a neutral point.

[0011] Optionally, it further includes a first circuit breaker and a second circuit breaker, wherein the first circuit breaker is connected between the energy storage converter and the voltage-dividing resistor, and the second circuit breaker is connected between the network construction unit and the distribution network;

[0012] The first circuit breaker is used to control the energy storage converter to be connected or disconnected with the rectifier unit;

[0013] The second circuit breaker is used to control the connection or disconnection between the networking unit and the distribution network.

[0014] On the other hand, the present invention proposes a control method for a grid-forming interface device applicable to an energy storage converter, comprising:

[0015] Based on the acquired electric energy output by the energy storage converter, the voltage of the electric energy output by the energy storage converter is regulated to a preset DC voltage expected value by using a rectifier unit through a slow start control;

[0016] The rectifier unit is used to control the rectifier by pulse width modulation to maintain the voltage stability of the electric energy after the DC voltage is adjusted to the desired value;

[0017] Based on the voltage and current of the distribution network obtained in real time, the instantaneous active power and instantaneous reactive power of the distribution network are calculated;

[0018] Based on the instantaneous active power and instantaneous reactive power of the distribution network, the network construction unit suppresses the increase of the current of the electric energy input into the distribution network through self-synchronization control.

[0019] Optionally, based on the acquired electric energy output by the energy storage converter, the voltage of the electric energy output by the energy storage converter is regulated to a preset DC voltage expected value by using a rectifier unit through a soft start control, including:

[0020] Based on the control module of the rectifier unit, a DC voltage given curve is established according to a preset DC voltage expected value and a set DC voltage cut-off time;

[0021] According to the given DC voltage curve, the rotation angular frequency of the rectifier unit is set comprehensively to calculate the voltage modulation signal of each phase in the three phases;

[0022] The voltage modulation signal of each phase in the three phases is converted by sinusoidal pulse width modulation, and according to the conversion result, the power device of the rectifier unit adjusts the voltage of the electric energy output by the energy storage converter to the expected DC voltage value.

[0023] Optionally, the DC voltage given curve satisfies the following expression:

[0024]

[0025] In the formula, u x1 、u x2 and u x3 are respectively the first given curve, the second given curve and the third given curve in the DC voltage given curve; T1, T2 and T3 are respectively the DC voltage cut-off time corresponding to the first given curve, the DC voltage cut-off time corresponding to the second given curve and the DC voltage cut-off time corresponding to the third given curve; U dcr is the preset DC voltage expected value, and t is the time recorded by the time counter.

[0026] Optionally, the voltage modulation signal of each phase in the three phases satisfies the following expression:

[0027]

[0028] Among them, u ra 、u rb and u rc are the voltage modulation signals of phase a, phase b and phase c in the three-phase circuit respectively; u f is the given curve of DC voltage, including the first given curve u x1 , the second given curve u x2 and the third given curve u x3 ;ω r is the rotation angular frequency of the rectifier unit, and t is the time recorded by the time counter.

[0029] Optionally, the use of the rectifier unit to control the rectifier through pulse width modulation to maintain the voltage stability of the electric energy after the DC voltage is adjusted to an expected value includes:

[0030] The control module based on the rectifier unit calculates the DC current given value according to the collected DC voltage actual value and the preset DC voltage given value by using the current loop proportional integral regulation method;

[0031] According to the given direct-axis current value, a power factor angle of a preset rectifier unit is calculated to determine a given quadrature-axis current value;

[0032] Decoupling control is performed on the direct-axis current given value and the quadrature-axis current given value to determine a direct-axis voltage modulation signal and a quadrature-axis voltage modulation signal;

[0033] The direct-axis voltage modulation signal and the quadrature-axis voltage modulation signal are converted by space vector pulse width modulation, and according to the conversion result, the power device of the rectifier unit maintains the voltage stability of the electric energy after the DC voltage is regulated to the expected value.

[0034] Optionally, the given value of the direct-axis current satisfies the following expression:

[0035] i dr =K udcp (u dcref -u dct )+∫K udci (u dcref -u dct )dt,

[0036] Among them, i dr is the given value of direct axis current; u dcref is the given value of DC voltage; u dct is the actual value of DC voltage; K udcp K is the DC loop proportional term coefficient; udci is the DC loop integral coefficient; ∫ is the integral sign; t is the time recorded by the time counter.

[0037] Optionally, the quadrature-axis current given value satisfies the following expression:

[0038]

[0039] Among them, i qr is the given value of the quadrature axis current; i dr is the given value of direct axis current; is the power factor angle of the rectifier unit.

[0040] Optionally, the direct-axis voltage modulation signal and the quadrature-axis voltage modulation signal respectively satisfy the following expressions:

[0041]

[0042] Among them, u dm is the direct-axis voltage modulation signal; u qm is the quadrature-axis voltage modulation signal; i qr is the given value of the quadrature axis current; i dr is the given value of direct axis current; R r is the resistance value of the voltage divider resistor; L r is the inductance value of the decoupling inductor; ω r is the rotation angular frequency of the rectifier unit.

[0043] Optionally, the instantaneous active power and the instantaneous reactive power of the distribution network are calculated based on the voltage and current of the distribution network acquired in real time, including:

[0044] Based on the voltage and current of the distribution network acquired in real time, the actual value of the direct-axis voltage, the actual value of the quadrature-axis voltage, the actual value of the direct-axis current and the actual value of the quadrature-axis current of the distribution network are determined by means of coordinate transformation;

[0045] The instantaneous active power and the instantaneous reactive power of the distribution network are calculated according to the actual value of the direct-axis voltage, the actual value of the quadrature-axis voltage, the actual value of the direct-axis current and the actual value of the quadrature-axis current of the distribution network.

[0046] Optionally, the actual value of the direct-axis voltage, the actual value of the quadrature-axis voltage, the actual value of the direct-axis current and the actual value of the quadrature-axis current of the distribution network respectively satisfy the following expressions:

[0047]

[0048] Among them, u d is the actual value of the direct axis voltage; u q is the actual value of the quadrature axis voltage; i d is the actual value of the direct axis current; i q is the actual value of the direct axis current; u a 、u b 、u care the actual voltage values ​​of phase a, phase b and phase c in the three-phase distribution network, respectively. a 、i b 、i c are the actual current values ​​of phase a, phase b and phase c in the three-phase distribution network, ω g is the rotation angular frequency of the network unit; t is the time recorded by the time counter.

[0049] Optionally, the instantaneous active power and instantaneous reactive power of the distribution network satisfy the following expressions respectively:

[0050] p=u d i d +u q i q ,

[0051] q=u q i d -u d i q ,

[0052] Among them, p is the instantaneous active power of the distribution network; q is the instantaneous reactive power of the distribution network; u d is the actual value of the direct axis voltage; u q is the actual value of the quadrature axis voltage; i d is the actual value of the direct axis current; i q is the actual value of the direct axis current.

[0053] Optionally, the instantaneous active power and instantaneous reactive power of the distribution network are controlled by the network building unit through self-synchronization to suppress the increase of the current of the electric energy input into the distribution network, including:

[0054] Based on the control module of the network unit, the difference between the active power set value and the instantaneous active power is obtained, and the rotation angular frequency is determined through the frequency adjustment of the simulated synchronous machine;

[0055] Integrating the rotation angular frequency based on time to determine a control angle;

[0056] Obtaining the difference between the reactive power given value and the instantaneous reactive power, and determining the control phase by simulating synchronous machine voltage regulation;

[0057] Based on the direct-axis voltage given value, the quadrature-axis voltage given value, the direct-axis voltage actual value and the quadrature-axis voltage actual value, respectively, a direct-axis current given value and a quadrature-axis current given value are determined by using a voltage loop proportional-integral adjustment; based on the direct-axis current given value, the quadrature-axis current given value, the direct-axis current actual value and the quadrature-axis current actual value, a direct-axis voltage modulation signal and a quadrature-axis voltage modulation signal are determined by using a current loop proportional-integral adjustment;

[0058] According to the rotation angular frequency, the control angle, the control phase, the direct-axis voltage modulation signal and the quadrature-axis voltage modulation signal, the power device of the networking unit suppresses the current increase of the electric energy input into the distribution network.

[0059] Optionally, the rotation angular frequency satisfies the following expression:

[0060]

[0061] Among them, ω g is the rotation angular frequency; p ref is the given value of active power; p is the instantaneous active power; K J K is the frequency modulation parameter of the simulated synchronous machine; D is the damping parameter of the simulated synchronous machine; ω g0 is the target value of the rotation angular frequency; t is the time recorded by the time counter; e is a natural constant.

[0062] Optionally, the control angle satisfies the following expression:

[0063] θ g =∫(ω g +ω g0 )dt,

[0064] Among them, θ g is the control angle; ω g is the rotation angular frequency; ω g0 is the target value of the rotation angular frequency; t is the time recorded by the time counter.

[0065] In another aspect, the present invention further provides an electronic device, comprising: at least one processor and a memory; the memory and the processor are connected via a bus;

[0066] The memory is used to store one or more programs;

[0067] When the one or more programs are executed by the at least one processor, the control method of the grid-type interface device applicable to the energy storage converter as described in the above technical solution is implemented.

[0068] On the other hand, the present invention further provides a readable storage medium having an execution program stored thereon, and when the execution program is executed, the control method of the meshing interface device applicable to the energy storage converter as described in the above technical solution is implemented.

[0069] Compared with the prior art, the present invention has the following beneficial effects:

[0070] The present invention provides a networking interface device suitable for an energy storage converter. By setting a voltage-dividing resistor, a decoupling inductor and a rectifier unit with a slow start control, the voltage of the electric energy output by the energy storage converter is regulated to an expected DC voltage value, and the voltage of the electric energy is maintained near the expected DC voltage value by using the pulse width rectification control of the rectifier unit. At the same time, the increase in the current output by the energy storage converter during the distribution network fault ride-through process is suppressed by a networking unit with self-synchronous control, ensuring that the energy storage converter does not need to take any current limiting measures during the distribution network fault ride-through period, protecting the energy storage converter from being burned by the increased current during the distribution network fault ride-through process, and improving the reliability of the use of the energy storage converter.

[0071] The present invention also proposes a control method for a networking interface device suitable for an energy storage inverter. According to a preset expected DC voltage value and an actual DC voltage value of the energy storage inverter collected in real time, the rectifier unit is sequentially controlled to implement soft start control and pulse width rectification. By performing self-synchronization control on the networking unit, the networking unit is utilized to implement active support for the voltage and frequency output by the energy storage inverter, thereby ensuring the normal operation of the energy storage inverter. BRIEF DESCRIPTION OF THE DRAWINGS

[0072] Figure 1 A schematic diagram of the structure of a grid-type interface device applicable to an energy storage converter provided by the present invention;

[0073] Figure 2 It is a fault ride-through waveform diagram of the energy storage converter not equipped with a grid-forming interface device in the present invention;

[0074] Figure 3 A fault ride-through waveform diagram of an energy storage converter equipped with a grid-forming interface device in the present invention;

[0075] Figure 4 A flow chart of a control method for a grid-forming interface device applicable to an energy storage converter provided by the present invention;

[0076] Figure 5 A schematic diagram of a given DC voltage curve provided by the present invention;

[0077] Figure 6 A structural block diagram of pulse width modulation rectification control of a rectifier unit in a control method for a grid interface device applicable to an energy storage converter provided by the present invention;

[0078] Figure 7 A structural block diagram of the self-synchronization control of the networking unit in the control method of the networking interface device applicable to the energy storage converter provided by the present invention;

[0079] Figure 8A structural block diagram of obtaining a direct-axis voltage modulation signal and a quadrature-axis voltage modulation signal of a networking unit in a control method of a networking interface device applicable to an energy storage converter provided by the present invention;

[0080] Fig. 9 This is a schematic structural diagram of an electronic device provided by the present invention. DETAILED DESCRIPTION

[0081] Embodiment 1:

[0082] like Figure 1 As shown, the present invention proposes a meshing interface device suitable for an energy storage converter, the meshing interface device comprising: a voltage-dividing resistor, a decoupling inductor, a rectifier unit, a voltage-stabilizing capacitor and a meshing unit; the DC bus of the energy storage converter is connected to the rectifier unit through the voltage-dividing resistor and the decoupling inductor, and the rectifier unit is connected to the meshing unit through the voltage-stabilizing capacitor;

[0083] The voltage-dividing resistor divides the electric energy output by the energy storage inverter, and inputs the divided electric energy into the rectifier unit; the decoupling inductor suppresses the noise of the electric energy output by the energy storage inverter; the rectifier unit is used to regulate the voltage of the divided electric energy to an expected DC voltage value through soft start control, and the voltage stability of the electric energy after being regulated to the expected DC voltage value is maintained through pulse width modulation rectification control in cooperation with the voltage-stabilizing capacitor, and the electric energy after being regulated to the expected DC voltage value is input into the networking unit; the networking unit suppresses the current increase of the electric energy in the distribution network through self-synchronization control.

[0084] Optionally, it also includes a filter inductor and a filter capacitor, one side of the filter inductor is connected to the side of the networking unit away from the voltage stabilizing capacitor, and the other side of the filter inductor is connected to the filter capacitor and then connected to the distribution network;

[0085] The filter inductor cooperates with the filter capacitor to filter out ripples in the electric energy input from the networking unit to the distribution network.

[0086] Optionally, a plurality of the filter inductors and the filter capacitors are provided, and each phase output end of the networking unit is connected to each phase line of the distribution network via at least one filter inductor, one side of each filter capacitor is connected to a side of the filter inductor away from the networking unit, and the other side of each filter capacitor is connected to an adjacent filter capacitor through a neutral point.

[0087] Optionally, it further includes a first circuit breaker and a second circuit breaker, wherein the first circuit breaker is connected between the energy storage converter and the voltage-dividing resistor, and the second circuit breaker is connected between the network construction unit and the distribution network;

[0088] The first circuit breaker is used to control the energy storage converter to be connected or disconnected with the rectifier unit;

[0089] The second circuit breaker is used to control the connection or disconnection between the networking unit and the distribution network.

[0090] The disturbances in the power system include large disturbances and small disturbances. Large disturbances include: generator fault removal, partial or complete removal of the DC transmission system due to fault (or no fault), failure and removal of components such as transformers and lines, and the input or removal of large loads. Small disturbances include plant mode oscillations, inter-machine mode oscillations, and control-related instability. Plant mode oscillations are the rotor angle oscillations of a generator or a single power plant relative to the rest of the system; inter-machine mode oscillations are oscillations between the rotors of several adjacent generators; and control-related instability is caused by improper control adjustments.

[0091] Since any disturbance in the power system may cause abnormality in the distribution network, in order to ensure stable operation of the energy storage converter under disturbance in the distribution network line in the prior art, the present invention provides a networking interface device.

[0092] Exemplarily, the energy storage battery is electrically connected to the energy storage converter, the first circuit breaker is electrically connected to the three-phase lead wire of the energy storage converter, and each output terminal of the first circuit breaker is electrically connected to the rectifier unit after being connected in series with a voltage-dividing resistor and a decoupling inductor. The rectifier unit includes a plurality of power devices and a controller, and the output terminal of the rectifier unit is electrically connected to the network unit after being connected in parallel with a voltage-stabilizing capacitor; each phase output terminal of the network unit is respectively connected to a filter inductor and a filter capacitor; the network unit also includes a plurality of power devices and a controller, and the side of each filter inductor away from the network unit is respectively connected to each input terminal of the second circuit breaker, and each output terminal of the second circuit breaker is respectively connected to each phase line of the distribution network.

[0093] By setting the first circuit breaker and the second circuit breaker, any type of energy storage inverter can be connected to the distribution network. At the same time, through the first circuit breaker and the second circuit breaker, the rectifier unit and the networking unit can be connected between the energy storage inverter and the distribution network. It has the advantages of simple structure, plug and play, and easy maintenance.

[0094] The following example uses the example of connecting the networking interface device provided by the present invention to an energy storage converter with a rated power of 100 kW and a distribution network to verify the effect that the networking interface device provided by the present invention can produce.

[0095] For example, Figure 2 and Figure 3 As shown, Figure 2 and Figure 3 The electrical parameters of the connected energy storage converters are the same, the only difference is Figure 3The grid-forming interface device provided by the present invention is configured in the energy storage inverter, the rated power is 100kW, the DC side input voltage is 650V, the output line voltage is 380V, the switching frequency is 10000Hz, the fundamental frequency is 50Hz, the inductance value of the output filter is 0.1mH, and the capacitance value is 80uF.

[0096] Figure 2 Before the 0.4s moment, the energy storage inverter outputs a power of 100kW. At 0.4s, a three-phase fault occurs in the distribution network, and the voltage of the distribution network drops by 50%. At this time, the instantaneous overload current output by the energy storage inverter reaches 4.7 times the rated output current, which exceeds the tolerance limit of the power electronic power devices in the energy storage inverter. Long-term operation will cause damage to the power electronic power devices of the energy storage inverter.

[0097] Figure 3 Before 0.4s in the figure, the power output of the energy storage converter is 100kW. When it reaches 0.4s, a three-phase fault occurs in the distribution network, and the voltage of the distribution network drops by 50%. At this time, through the suppression effect of the network-forming interface device provided by the present invention, the current amplitude output by the energy storage converter is maintained near the rated current, reducing the probability that the current of the energy storage converter is affected by the faulty distribution network. Figure 2 In terms of protection, the effective protection provided to the power electronic devices of the energy storage converter reduces the probability of damage caused by overload current to the power electronic devices of the energy storage converter.

[0098] In summary, the meshing interface device provided by the present invention utilizes its own voltage-dividing resistor, decoupling inductor, rectifier unit, voltage-stabilizing capacitor and meshing unit to protect the connected energy storage inverter. It is not necessary to add virtual impedance to the energy storage inverter. At the same time, it also ensures the normal operation of the energy storage inverter under large disturbances in the distribution network (for example, fault crossing in the distribution network), thereby ensuring the supporting effect of the energy storage inverter on the voltage and frequency of the distribution network.

[0099] Embodiment 2:

[0100] Reference Figure 4 Based on the meshing interface device disclosed in the above embodiments, the present invention proposes a control method for a meshing interface device suitable for an energy storage converter, including:

[0101] S101, based on the acquired electric energy output by the energy storage converter, using a rectifier unit to control the voltage of the electric energy output by the energy storage converter to a preset DC voltage expected value through a slow start control;

[0102] S102, using the rectifying unit to perform pulse width modulation rectification control to maintain the voltage stability of the electric energy after the DC voltage is regulated to the expected value;

[0103] S103, calculating instantaneous active power and instantaneous reactive power of the distribution network based on the voltage and current of the distribution network acquired in real time;

[0104] S104: Based on the instantaneous active power and instantaneous reactive power of the distribution network, the network construction unit suppresses the increase of the current of the electric energy input into the distribution network through self-synchronization control.

[0105] Exemplarily, when it is detected that the output voltage of the DC bus reaches the expected value of the DC voltage, the power device of the rectifier unit switches from soft start control to pulse width modulation rectifier control (PWM, Pulse Width Modulation), and the rectifier unit maintains the voltage stability of the electric energy output by the energy storage inverter through pulse width modulation rectifier control.

[0106] The present invention controls the voltage of the electric energy output by the energy storage inverter to a preset DC voltage expected value through a rectifier module through a soft start control, and then stabilizes the voltage of the electric energy after position control by pulse width modulation rectification control of the rectifier unit, thereby realizing active support of the voltage and frequency of the distribution network by the energy storage inverter through the self-synchronous control of the networking unit, which can effectively solve the problem that the networking type energy storage inverter is difficult to support during the distribution network failure process, and there is no need to take any current limiting measures or change the control strategy of the energy storage inverter, and it has good transient steady-state performance and good fault ride-through capability.

[0107] Optionally, based on the acquired electric energy output by the energy storage converter, the voltage of the electric energy output by the energy storage converter is regulated to a preset DC voltage expected value by using a rectifier unit through a soft start control, including:

[0108] Based on the control module of the rectifier unit, a DC voltage given curve is established according to a preset DC voltage expected value and a set DC voltage cut-off time;

[0109] According to the given DC voltage curve, the rotation angular frequency of the rectifier unit is set comprehensively to calculate the voltage modulation signal of each phase in the three phases;

[0110] The voltage modulation signal of each phase in the three phases is converted by sinusoidal pulse width modulation. According to the conversion result, the power device of the rectifier unit adjusts the voltage of the electric energy output by the energy storage converter to the expected DC voltage value.

[0111] Optional, such as Figure 5 As shown, T1, T2 and T3 are respectively the DC voltage cut-off time corresponding to the first given curve, the DC voltage cut-off time corresponding to the second given curve and the DC voltage cut-off time corresponding to the third given curve; U f1 , U f2 and U f3are respectively the cut-off DC voltage of the first given curve, the cut-off DC voltage of the second given curve and the cut-off DC voltage of the third given curve, and the given DC voltage curves satisfy the following expression:

[0112]

[0113] In the formula, u x1 、u x2 and u x3 are respectively the first given curve, the second given curve and the third given curve in the DC voltage given curve; T1, T2 and T3 are respectively the DC voltage cut-off time corresponding to the first given curve, the DC voltage cut-off time corresponding to the second given curve and the DC voltage cut-off time corresponding to the third given curve; U dcr is the preset DC voltage expected value, and t is the time recorded by the time counter.

[0114] Optionally, the voltage modulation signal of each phase in the three phases satisfies the following expression:

[0115]

[0116] Among them, u ra 、u rb and u rc are the voltage modulation signals of phase a, phase b and phase c in the three-phase circuit respectively; u f is the given curve of DC voltage, including the first given curve u x1 , the second given curve u x2 and the third given curve u x3 ;ω r is the rotation angular frequency of the rectifier unit, and t is the time recorded by the time counter.

[0117] Optional, see Figure 6 , Figure 6 QF1 is the first circuit breaker, U dc is the voltage applied to both ends of the voltage stabilizing capacitor; the rectifier unit is used to control the rectifier by pulse width modulation to maintain the voltage stability of the electric energy after the DC voltage is adjusted to the expected value, including:

[0118] The control module based on the rectifier unit calculates the direct axis (corresponding to the d axis in the dq axis) current given value according to the collected actual DC voltage value and the preset DC voltage given value by using the current loop proportional integral regulation method;

[0119] According to the given direct-axis current value, a calculation is performed in combination with a preset power factor angle of the rectifier unit to determine a given quadrature-axis current value (corresponding to the q-axis in the dq-axis);

[0120] Decoupling control is performed on the direct-axis current given value and the quadrature-axis current given value to determine a direct-axis voltage modulation signal and a quadrature-axis voltage modulation signal;

[0121] The direct-axis voltage modulation signal and the quadrature-axis voltage modulation signal are converted by space vector pulse width modulation (SVPWM). According to the conversion result, the power device of the rectifier unit maintains the voltage stability of the electric energy after the DC voltage is adjusted to the expected value.

[0122] Optionally, the given value of the direct-axis current satisfies the following expression:

[0123] i dr =K udcp (u dcref -u dct )+∫K udci (u dcref -u dct )dt,

[0124] Among them, i dr is the given value of direct axis current; u dcref is the given value of DC voltage; u dct is the actual value of DC voltage; K udcp K is the DC loop proportional term coefficient; udci is the DC loop integral coefficient; ∫ is the integral sign; t is the time recorded by the time counter.

[0125] Optionally, the quadrature-axis current given value satisfies the following expression:

[0126]

[0127] Among them, i qr is the given value of the quadrature axis current; i dr is the given value of direct axis current; is the power factor angle of the rectifier unit.

[0128] Optionally, the direct-axis voltage modulation signal and the quadrature-axis voltage modulation signal respectively satisfy the following expressions:

[0129]

[0130] Among them, u dm is the direct-axis voltage modulation signal; u qm is the quadrature-axis voltage modulation signal; i qr is the given value of the quadrature axis current; i dr is the given value of direct axis current; R r is the resistance value of the voltage divider resistor; L r is the inductance value of the decoupling inductor; ωr is the rotation angular frequency of the rectifier unit.

[0131] Optionally, the instantaneous active power and the instantaneous reactive power of the distribution network are calculated based on the voltage and current of the distribution network acquired in real time, including:

[0132] Based on the voltage and current of the distribution network acquired in real time, the actual value of the direct-axis voltage, the actual value of the quadrature-axis voltage, the actual value of the direct-axis current and the actual value of the quadrature-axis current of the distribution network are determined by means of coordinate transformation;

[0133] The instantaneous active power and the instantaneous reactive power of the distribution network are calculated according to the actual value of the direct-axis voltage, the actual value of the quadrature-axis voltage, the actual value of the direct-axis current and the actual value of the quadrature-axis current of the distribution network.

[0134] Optionally, the actual value of the direct-axis voltage, the actual value of the quadrature-axis voltage, the actual value of the direct-axis current and the actual value of the quadrature-axis current of the distribution network respectively satisfy the following expressions:

[0135]

[0136] Among them, u d is the actual value of the direct axis voltage, that is, the actual value of the d-axis voltage; u q is the actual value of the quadrature axis voltage, that is, the actual value of the q-axis voltage; i d is the actual value of the direct axis current, that is, the actual value of the d-axis voltage; i q is the actual value of the direct axis current, that is, the actual value of the q axis voltage; u a 、u b 、u c are the actual voltage values ​​of phase a, phase b and phase c in the three-phase distribution network, respectively. a 、i b 、i c are the actual current values ​​of phase a, phase b and phase c in the three-phase distribution network, ω g is the rotation angular frequency of the network unit; t is the time recorded by the time counter.

[0137] Optionally, the instantaneous active power and instantaneous reactive power of the distribution network satisfy the following expressions respectively:

[0138] p=u d i d +u q i q ,

[0139] q=u q i d -u d i q ,

[0140] Among them, p is the instantaneous active power of the distribution network; q is the instantaneous reactive power of the distribution network; u d is the actual value of the direct axis voltage; u q is the actual value of the quadrature axis voltage; i d is the actual value of the direct axis current; i q is the actual value of the quadrature-axis current.

[0141] Optional, such as Figure 7 As shown in the figure, u abc is the actual voltage value of the distribution network, that is, the voltage of the three phases abc in the distribution network; i abc is the actual current value of the distribution network, that is, the current of the three phases abc in the distribution network; R s and R g are the equivalent resistances of the grid unit side and the distribution network side, respectively, which are used to assist in obtaining the instantaneous active power and instantaneous reactive power of the distribution network; L s and L g The decoupling inductor on the grid unit side and the distribution network side; C is the decoupling capacitor; QF2 is the second circuit breaker.

[0142] The instantaneous active power and instantaneous reactive power of the distribution network are controlled by the network building unit through self-synchronization to suppress the increase of the current of the electric energy input into the distribution network, including:

[0143] Based on the control module of the network unit, the difference between the active power set value and the instantaneous active power is obtained, and the rotation angular frequency is determined through the frequency adjustment of the simulated synchronous machine;

[0144] Integrating the rotation angular frequency based on time to determine a control angle;

[0145] Obtaining the difference between the reactive power given value and the instantaneous reactive power, and determining the control phase by simulating synchronous machine voltage regulation;

[0146] Based on the direct-axis voltage given value, the quadrature-axis voltage given value, the direct-axis voltage actual value and the quadrature-axis voltage actual value, respectively, a direct-axis current given value and a quadrature-axis current given value are determined by using a voltage loop proportional-integral adjustment; based on the direct-axis current given value, the quadrature-axis current given value, the direct-axis current actual value and the quadrature-axis current actual value, a direct-axis voltage modulation signal and a quadrature-axis voltage modulation signal are determined by using a current loop proportional-integral adjustment;

[0147] According to the rotation angular frequency, the control angle, the control phase, the direct-axis voltage modulation signal and the quadrature-axis voltage modulation signal, the power device of the networking unit suppresses the current increase of the electric energy input into the distribution network.

[0148] Optionally, the rotation angular frequency satisfies the following expression:

[0149]

[0150] Among them, ω g is the rotation angular frequency; p ref is the given value of active power; p is the instantaneous active power; K J K is the frequency modulation parameter of the simulated synchronous machine; D is the damping parameter of the simulated synchronous machine; ω g0 is the target value of the rotation angular frequency; t is the time recorded by the time counter; e is a natural constant.

[0151] Optionally, the control angle satisfies the following expression:

[0152] θ g =∫(ω g +ω g0 )dt,

[0153] Among them, θ g is the control angle; ω g is the rotation angular frequency; ω g0 is the target value of the rotation angular frequency; t is the time recorded by the time counter.

[0154] Optionally, the control phase of the networking unit satisfies the following expression:

[0155] u g =∫K Q (q ref -q)dt+u g0 ,

[0156] Among them, u g is the control phase of the network unit; q ref is the given value of reactive power; q is the instantaneous reactive power; K Q is the voltage regulation parameter of the simulated synchronous machine; u g0 is the phase target value of the network unit.

[0157] Optional, such as Figure 8 As shown, K ui / s is the voltage loop integral coefficient K ui Integrate over time; K ii / s is the current loop integral coefficient K ii Integrate over time.

[0158] The direct-axis current given value and the quadrature-axis current given value satisfy the following expressions respectively:

[0159]

[0160] Among them, i dref is the given value of direct axis current; i qrefis the given value of the quadrature axis current; K up is the voltage loop proportional term coefficient; u dref is the given value of direct axis voltage; u qref is the given value of the quadrature axis voltage; u d is the actual value of the direct axis voltage; u q is the actual value of the quadrature axis voltage; K ui is the integral coefficient of the voltage loop; t is the time recorded by the time counter.

[0161] In some embodiments, based on the direct-axis current set value and the direct-axis current actual value, the direct-axis voltage modulation signal is determined by using the current loop proportional-integral regulation; based on the quadrature-axis current set value and the quadrature-axis current actual value, the quadrature-axis voltage modulation signal is determined by using the current loop proportional-integral regulation;

[0162] The direct-axis voltage modulation signal and the quadrature-axis voltage modulation signal respectively satisfy the following expressions:

[0163]

[0164] Among them, u gdm is the direct-axis voltage modulation signal of the network unit; u gqm is the quadrature-axis voltage modulation signal of the network unit; i dref is the given value of direct axis current; i qref is the given value of the quadrature axis current; i d is the actual value of the direct axis current; i q is the actual value of the quadrature axis current; K ip K is the current loop proportional term coefficient; ii is the integral coefficient of the current loop; t is the time recorded by the time counter.

[0165] Embodiment 3:

[0166] like Fig. 9 As shown, the present invention also provides an electronic device, which may be a computer device, a single-chip device, an intelligent mobile device, etc. The electronic device in this embodiment may include a processor, a memory, a transceiver component, etc. The memory, the processor, and the transceiver component are connected via a bus; the memory may be used to store an execution program, and an exemplary execution program may include instructions; the processor is used to execute the instructions stored in the memory. The memory may also be used to store data, which may be called and / or modified when the instructions are executed.

[0167] The processor may be a central processing unit (CPU), or other general-purpose processors, digital signal processors (DSP), or application-specific integrated circuits.

[0168] (Application Specific Integrated Circuit, ASIC), off-the-shelf programmable gate array (Field-Programmable Gate Array, FPGA) or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc., which are the computing core and control core of the terminal, and are suitable for implementing one or more instructions, specifically suitable for loading and executing one or more instructions in the storage medium to implement the corresponding method flow or corresponding function, so as to realize the steps of a control method of a meshing interface device suitable for an energy storage inverter in the above-mentioned embodiment.

[0169] Embodiment 4:

[0170] Based on the same inventive concept, the present invention also provides a readable storage medium, specifically an electronic device readable storage medium (Memory), which is a memory device in an electronic device for storing programs and data. It can be understood that the storage medium here can include both built-in storage media in electronic devices and, of course, extended storage media supported by electronic devices. The storage medium provides a storage space, which stores the operating system of the terminal. In addition, one or more instructions suitable for being loaded and executed by a processor are also stored in the storage space, and these instructions can be one or more execution programs (including program codes). It should be noted that the storage medium here can be a high-speed RAM memory or a non-volatile memory, such as at least one disk memory. The processor loads and executes one or more instructions stored in the storage medium, which can implement the steps of a control method for a network-forming interface device suitable for an energy storage converter in the above embodiment.

[0171] Those skilled in the art will appreciate that embodiments of the present invention may be provided as methods, systems, or computer program products. Therefore, the present invention may take the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware. Moreover, the present invention may take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0172] The present invention is described with reference to flowcharts and / or block diagrams of methods, devices (systems), and computer program products according to embodiments of the present invention. It should be understood that each process and / or block in the flowchart and / or block diagram, as well as the combination of processes and / or blocks in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to produce a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowchart and / or block diagram. Figure 1 A process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0173] These computer program instructions may also be stored in a computer-readable memory capable of directing a computer or other programmable data processing device to operate in a specific manner, so that the instructions stored in the computer-readable memory produce an article of manufacture comprising an instruction device, which implements the process Figure 1 A process or multiple processes and / or boxes Figure 1 A function specified in one or more boxes.

[0174] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operating steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing instructions for implementing the process. Figure 1 A process or multiple processes and / or boxes Figure 1 The steps for the functions specified in one or more boxes.

[0175] The above are merely embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention are included in the scope of the claims of the present invention to be approved.

Claims

1. A network-forming interface device suitable for an energy storage converter, characterized in that: The networking interface device comprises: a voltage-dividing resistor, a decoupling inductor, a rectifier unit, a voltage-stabilizing capacitor and a networking unit; the DC bus of the energy storage converter is connected to the rectifier unit through the voltage-dividing resistor and the decoupling inductor, and the rectifier unit is connected to the networking unit through the voltage-stabilizing capacitor; The voltage-dividing resistor divides the electric energy output by the energy storage inverter, and inputs the divided electric energy into the rectifier unit; the decoupling inductor suppresses the noise of the electric energy output by the energy storage inverter; the rectifier unit regulates the voltage of the divided electric energy to an expected DC voltage value through soft start control, and maintains the voltage stability of the electric energy after being regulated to the expected DC voltage value through pulse width modulation rectification control in cooperation with the voltage-stabilizing capacitor, and inputs the electric energy after being regulated to the expected DC voltage value into the networking unit; the networking unit suppresses the current increase of the electric energy in the distribution network through self-synchronization control.

2. The device according to claim 1, characterized in that It also includes a filter inductor and a filter capacitor, one side of the filter inductor is connected to the side of the network unit away from the voltage stabilizing capacitor, and the other side of the filter inductor is connected to the filter capacitor and then connected to the distribution network; The filter inductor cooperates with the filter capacitor to filter out ripples in the electric energy input from the networking unit to the distribution network.

3. The device according to claim 2, characterized in that A plurality of the filter inductors and filter capacitors are provided, and each phase output end of the networking unit is connected to each phase line of the distribution network via at least one filter inductor, one side of each filter capacitor is connected to a side of the filter inductor away from the networking unit, and the other side of each filter capacitor is connected to an adjacent filter capacitor through a neutral point.

4. The device according to claim 2, characterized in that It also includes a first circuit breaker and a second circuit breaker, wherein the first circuit breaker is connected between the energy storage converter and the voltage dividing resistor, and the second circuit breaker is connected between the network construction unit and the distribution network; The first circuit breaker is used to control the energy storage converter to be connected or disconnected with the rectifier unit; The second circuit breaker is used to control the connection or disconnection between the networking unit and the distribution network.

5. A control method for a grid-type interface device suitable for an energy storage converter, characterized in that: include: Based on the acquired electric energy output by the energy storage converter, the voltage of the electric energy output by the energy storage converter is regulated to a preset DC voltage expected value by using a rectifier unit through a slow start control; The rectifier unit is used to control the rectifier by pulse width modulation to maintain the voltage stability of the electric energy after the DC voltage is adjusted to the desired value; Based on the voltage and current of the distribution network obtained in real time, the instantaneous active power and instantaneous reactive power of the distribution network are calculated; Based on the instantaneous active power and instantaneous reactive power of the distribution network, the network construction unit suppresses the increase of the current of the electric energy input into the distribution network through self-synchronization control.

6. The method according to claim 5, characterized in that Based on the acquired electric energy output by the energy storage converter, the voltage of the electric energy output by the energy storage converter is regulated to a preset DC voltage expected value by using a rectifier unit through a soft start control, including: Based on the control module of the rectifier unit, a DC voltage given curve is established according to a preset DC voltage expected value and a set DC voltage cut-off time; According to the given DC voltage curve, the rotation angular frequency of the rectifier unit is set comprehensively to calculate the voltage modulation signal of each phase in the three phases; The voltage modulation signal of each phase in the three phases is converted by sinusoidal pulse width modulation. According to the conversion result, the power device of the rectifier unit adjusts the voltage of the electric energy output by the energy storage converter to the expected DC voltage value.

7. The method according to claim 6, characterized in that The DC voltage given curve satisfies the following expression: In the formula, u x1 、u x2 and u x3 are respectively the first given curve, the second given curve and the third given curve in the DC voltage given curve; T1, T2 and T3 are respectively the DC voltage cut-off time corresponding to the first given curve, the DC voltage cut-off time corresponding to the second given curve and the DC voltage cut-off time corresponding to the third given curve; U dcr is the preset DC voltage expected value, and t is the time recorded by the time counter.

8. The method according to claim 6, characterized in that The voltage modulation signal of each phase in the three phases satisfies the following expression: Among them, u ra 、u rb and u rc are the voltage modulation signals of phase a, phase b and phase c in the three-phase circuit respectively; u f is the given curve of DC voltage, including the first given curve u x1 , the second given curve u x2 and the third given curve u x3 ;ω r is the rotation angular frequency of the rectifier unit, and t is the time recorded by the time counter.

9. The method according to claim 5, characterized in that The method of utilizing the rectifying unit to control the rectification through pulse width modulation to maintain the voltage stability of the electric energy after the DC voltage is adjusted to the expected value includes: The control module based on the rectifier unit calculates the DC current given value according to the collected DC voltage actual value and the preset DC voltage given value by using the current loop proportional integral regulation method; According to the given direct-axis current value, a power factor angle of a preset rectifier unit is calculated to determine a given quadrature-axis current value; Decoupling control is performed on the direct-axis current given value and the quadrature-axis current given value to determine a direct-axis voltage modulation signal and a quadrature-axis voltage modulation signal; The direct-axis voltage modulation signal and the quadrature-axis voltage modulation signal are converted by space vector pulse width modulation, and according to the conversion result, the power device of the rectifier unit maintains the voltage stability of the electric energy after the DC voltage is regulated to the expected value.

10. The method according to claim 9, characterized in that The direct-axis current given value satisfies the following expression: and dr =K udcp (in dcref -in dct )+∫K udci (in dcref -in dct )dt, Among them, i dr is the given value of direct axis current; u dcref is the given value of DC voltage; u dct is the actual value of DC voltage; K udcp K is the DC loop proportional term coefficient; udci is the DC loop integral coefficient; ∫ is the integral sign; t is the time recorded by the time counter.

11. The method according to claim 9, characterized in that The quadrature-axis current given value satisfies the following expression: Among them, i qr is the given value of the quadrature axis current; i dr is the given value of direct axis current; is the power factor angle of the rectifier unit.

12. The method according to claim 9, characterized in that The direct-axis voltage modulation signal and the quadrature-axis voltage modulation signal satisfy the following expressions respectively: Among them, u dm is the direct-axis voltage modulation signal; u qm is the quadrature-axis voltage modulation signal; i qr is the given value of the quadrature axis current; i dr is the given value of direct axis current; R r is the resistance value of the voltage divider resistor; L r is the inductance value of the decoupling inductor; ω r is the rotation angular frequency of the rectifier unit.

13. The method according to claim 5, characterized in that The instantaneous active power and the instantaneous reactive power of the distribution network are calculated based on the voltage and current of the distribution network obtained in real time, including: Based on the voltage and current of the distribution network acquired in real time, the actual value of the direct-axis voltage, the actual value of the quadrature-axis voltage, the actual value of the direct-axis current and the actual value of the quadrature-axis current of the distribution network are determined by means of coordinate transformation; The instantaneous active power and the instantaneous reactive power of the distribution network are calculated according to the actual value of the direct-axis voltage, the actual value of the quadrature-axis voltage, the actual value of the direct-axis current and the actual value of the quadrature-axis current of the distribution network.

14. The method according to claim 13, characterized in that The actual value of the direct-axis voltage, the actual value of the quadrature-axis voltage, the actual value of the direct-axis current and the actual value of the quadrature-axis current of the distribution network respectively satisfy the following expressions: Among them, u d is the actual value of the direct-axis voltage; u q is the actual value of the quadrature axis voltage; i d is the actual value of the direct axis current; i q is the actual value of the direct axis current; u a 、u b 、u c are the actual voltage values ​​of phase a, phase b and phase c in the three-phase distribution network, respectively. a 、i b 、i c are the actual current values ​​of phase a, phase b and phase c in the three-phase distribution network, ω g is the rotation angular frequency of the network unit; t is the time recorded by the time counter.

15. The method according to claim 13, characterized in that The instantaneous active power and instantaneous reactive power of the distribution network satisfy the following expressions respectively: p=u d and d +in q and q , q=u q i d -u d i q , Among them, p is the instantaneous active power of the distribution network; q is the instantaneous reactive power of the distribution network; u d is the actual value of the direct-axis voltage; u q is the actual value of the quadrature axis voltage; i d is the actual value of the direct axis current; i q is the actual value of the direct axis current.

16. The method according to claim 5, characterized in that The instantaneous active power and instantaneous reactive power of the distribution network are controlled by the network building unit through self-synchronization to suppress the increase of the current of the electric energy input into the distribution network, including: Based on the control module of the network unit, the difference between the active power set value and the instantaneous active power is obtained, and the rotation angular frequency is determined through the frequency adjustment of the simulated synchronous machine; Integrating the rotation angular frequency based on time to determine a control angle; Obtaining the difference between the reactive power given value and the instantaneous reactive power, and determining the control phase by simulating synchronous machine voltage regulation; Based on the direct-axis voltage given value, the quadrature-axis voltage given value, the direct-axis voltage actual value and the quadrature-axis voltage actual value, respectively, the direct-axis current given value and the quadrature-axis current given value are determined by using the voltage loop proportional integral regulation; based on the direct-axis current given value, the quadrature-axis current given value, the direct-axis current actual value and the quadrature-axis current actual value, the direct-axis voltage modulation signal and the quadrature-axis voltage modulation signal are determined by using the current loop proportional integral regulation; According to the rotation angular frequency, the control angle, the control phase, the direct-axis voltage modulation signal and the quadrature-axis voltage modulation signal, the power device of the networking unit suppresses the current increase of the electric energy input into the distribution network.

17. The method according to claim 16, characterized in that The rotation angular frequency satisfies the following expression: Among them, ω g is the rotation angular frequency; p ref is the given value of active power; p is the instantaneous active power; K J is the frequency modulation parameter of the simulated synchronous machine; K D is the damping parameter of the simulated synchronous machine; ω g0 is the target value of the rotation angular frequency; t is the time recorded by the time counter; e is a natural constant.

18. The method according to claim 16, characterized in that The control angle satisfies the following expression: i g =∫(ω g +oh g0 )dt, Among them, θ g is the control angle; ω g is the rotation angular frequency; ω g0 is the target value of the rotation angular frequency; t is the time recorded by the time counter.

19. An electronic device, characterized in that: include: at least one processor and memory; The memory and the processor are connected via a bus; The memory is used to store one or more programs; When the one or more programs are executed by the at least one processor, the control method of the grid-type interface device applicable to the energy storage converter as claimed in any one of claims 5 to 18 is implemented.

20. A readable storage medium, characterized in that: An execution program is stored thereon, and when the execution program is executed, a control method for a grid-type interface device suitable for an energy storage converter as claimed in any one of claims 5 to 18 is implemented.

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