Phase-locked loop self-synchronizing unified grid-connected inverter system

Abstract

This invention discloses a phase-locked loop-free self-synchronizing unified grid-connected inverter system, consisting of a circuit module and a control module. The control module obtains various detection signals from the circuit module through measurement, including the voltage and grid-connected current at the inverter's PCC, the grid-side voltage, and the inverter-side inductor current. These signals, along with preset inverter output active and reactive power commands, serve as inputs. The system then passes through a self-synchronization control loop, a reference current generation loop, a compensation current generation loop, a pre-synchronization current generation loop, a fault ride-through control loop, and a voltage amplitude error compensation loop to output a modulated inverter voltage. This achieves phase-locked loop-free self-synchronizing unified control of the grid-connected inverter system under multiple operating conditions. This invention eliminates the need for a dedicated phase-locked loop to establish the inverter output voltage, providing power dispatch and voltage support capabilities. It adapts to various operating conditions, including normal off-grid / grid connection and fault conditions, and operates intelligently and autonomously. It is suitable for grid-connected applications of new energy grid-connected inverters.

Description

Technical Field

[0001] This invention relates to the field of new energy power generation system applications, and more specifically to a phase-locked loop-free self-synchronizing unified grid-connected inverter system for realizing new energy grid connection under multiple operating conditions. Background Technology

[0002] In 2023, global renewable energy generation (including hydropower) is projected to account for 30.2% of total electricity generation. Vigorously developing renewable energy systems to achieve low-carbon and clean power generation has become a global consensus. As a key interface device between uncontrollable renewable energy sources such as wind and solar power and a stable power grid, the efficient and intelligent operation of grid-connected inverters is of great significance to the system.

[0003] The integration of renewable energy systems such as wind and solar power, as well as novel energy storage systems, with the existing power grid primarily relies on power electronic equipment, mainly inverters. In traditional power systems, generator sets achieve synchronous operation through power synchronization mechanisms, while most current renewable energy power generation equipment adopts grid-linked control to integrate with traditional power systems. Grid-linked control typically uses a voltage synchronization mechanism, acquiring the grid connection point voltage and obtaining its phase, amplitude, and other information through a dedicated phase-locked loop (PLL) configured in the system, which serves as a reference voltage for control. Grid-connected inverters using grid-linked control exhibit characteristics of a current source or power source, lacking independent grid-connection capabilities and prone to instability under weak grid conditions. Improving the PLL structure, optimizing PLL control parameters, and reshaping the output impedance of the grid-connected inverter are currently the main solutions to address the potential instability issues of grid-linked control under weak grid conditions. However, this involves more parameter tuning and the need for additional control terms, increasing control complexity.

[0004] Correspondingly, grid-connected inverters employing grid-based control methods have recently garnered increasing attention due to their ability to autonomously establish voltage and thus provide grid support. Currently, typical grid-based control methods mainly fall into two categories: droop control simulating synchronous machine characteristics, virtual synchronous generator control, and virtual oscillator control simulating nonlinear oscillator characteristics. However, inverters using traditional grid-based control still require phase-locked loops (PLLs) for synchronization when reconnecting to the grid. Furthermore, inverters using traditional grid-based control methods are prone to instability under strong grid conditions. Summary of the Invention

[0005] To overcome the shortcomings of the existing technology, this invention provides a phase-locked loop-free self-synchronizing unified grid-connected inverter system. It is a new solution for grid-connected inverter control systems, enabling the inverter output voltage to be established autonomously. It has power scheduling and voltage support capabilities, and can operate intelligently and autonomously under various conditions, including normal off-grid / grid conditions and fault conditions. It is suitable for the control of three-phase grid-connected inverters in grid-connected power generation systems of new energy sources such as photovoltaic power generation, wind power generation, and chemical energy storage.

[0006] The present invention adopts the following technical solution to solve the technical problem:

[0007] The features of the phase-locked loop-free self-synchronizing unified grid-connected inverter system of this invention are:

[0008] The system consists of a circuit module and a control module;

[0009] The circuit module includes a DC power supply, an inverter main circuit, an inverter-side inductor L1 and a grid-side inductor L2, a filter capacitor C, and a grid-side line impedance Z. g Equivalent load Z L , grid-side circuit breaker S1, load-side circuit breaker S2 and grid v gs ;

[0010] The control module obtains various detection signals from the circuit module by measurement, including the voltage v at the inverter PCC. pcc And grid-connected current i2, grid-side voltage v g And the inverter-side inductor current i1; the detection signal is compared with the set inverter output active power P. * and reactive power Q * As input, the inverter modulation voltage is output through the self-synchronization control loop, reference current generation loop, compensation current generation loop, pre-synchronization current generation loop, fault ride-through control loop and voltage amplitude error compensation loop, realizing phase-locked loop-free self-synchronization unified control of the grid-connected inverter system under multiple operating conditions.

[0011] The features of this invention's phase-locked loop-free self-synchronizing unified grid-connected inverter system also lie in:

[0012] The control module consists of several control links, namely: self-synchronization control link A, reference current generation link B, compensation current generation link C, pre-synchronization current generation link D, fault ride-through control link E, and voltage amplitude error compensation link F, wherein:

[0013] The self-synchronization control loop A outputs two mutually orthogonal voltage quantities e and based on various parameters. And the modulation voltage v used in the inverter main circuit m The parameters are: the measured inverter-side inductor current i1; and the sum of the output currents Δi from the reference current generation stage B, the compensation current generation stage C, and the pre-synchronization current generation stage D. r And the voltage error compensation amount Δv output by the voltage amplitude error compensation circuit F;

[0014] The reference current generation stage B uses the voltage e and The set inverter output active power P * and reactive power Q* Fault command x f Low voltage ride-through active power reference value and reactive power reference value As input, calculate and output the inverter reference current i. ref The compensation current generation stage C uses the voltage v at the inverter PCC. pcc and the grid-connected current i2, and the voltage e and As input, calculate the grid-connected output compensation current i. m The pre-synchronization current generation link D uses the grid-side power grid voltage v g and the voltage quantity e and As input, calculate and output the pre-synchronization current i. pc The total current Δi r Inverter reference current i ref Compensation current i m and pre-synchronization current i pc sum;

[0015] The fault ride-through control element E is based on the measured voltage v at inverter PCC. pcc , grid-connected current i2 and grid-side voltage v g Output fault command x f It also outputs a low-voltage ride-through active power reference value. and reactive power reference value

[0016] The voltage amplitude error compensation link F uses the voltage amount e and as well as Reference amplitude under rated operating conditions The input is used to calculate and output the voltage error compensation amount Δv.

[0017] The characteristic of the phase-locked loop-free self-synchronizing unified grid-connected inverter system of the present invention is that, under normal operating conditions, the inverter system implements unified control as follows:

[0018] Normal operating condition scenario 1: Grid-connected operation

[0019] The reference current generation circuit B, the voltage amplitude error compensation circuit F, and the self-synchronization control circuit A are automatically controlled according to the following steps:

[0020] Step 3.1.1: The reference current generation stage B calculates the inverter reference current i according to equation (1). ref :

[0021]

[0022] in:

[0023] i refα and i refβ These are the inverter reference currents i ref α and β axis components;

[0024] V DC The voltage value of the DC power supply in the circuit module;

[0025] e α ,e β These are the α and β axis components of the voltage quantity e, respectively;

[0026] The voltage quantities are respectively α and β axis components;

[0027] T a This is the inverter startup time;

[0028] Step 3.1.2: The voltage amplitude error compensation link F is calculated according to formula (2) to obtain the voltage error compensation amount Δv:

[0029]

[0030] in:

[0031] It is the voltage quantity The real-time amplitude, and has:

[0032] yes Reference amplitude under rated operating conditions; k v This is the voltage amplitude error coefficient;

[0033] Step 3.1.3: The self-synchronization control loop A calculates the mutually orthogonal voltage quantities e and using equations (3) and (4).

[0034]

[0035] in:

[0036] ω0 is the rated frequency, and s is the Laplace operator;

[0037] Δi e For current error, the current error Δi e From Δi e =i ref -i1 is calculated to obtain;

[0038] k p k rThese are the proportional coefficient and resonant coefficient of the proportional resonant controller in the self-synchronization control loop A, respectively.

[0039] k s The adjustment coefficient is determined by... Calculated;

[0040] Step 3.1.4: Calculate the modulation voltage v of the inverter main circuit using equation (5). m Used for inverter circuit switching control;

[0041] v m =k p Δi e +e (5)

[0042] Normal operating condition scenario 2: Off-grid operation

[0043] When the grid-side circuit breaker S1 is disconnected, the inverter independently drives the equivalent load Z. L run;

[0044] The control module automatically engages control according to the following steps:

[0045] Step 3.2.1: Calculate the inverter reference current i according to step 3.1.1 in normal operating condition 1. ref ;

[0046] Step 3.2.2: The compensation current generation circuit C calculates the compensation current i according to formula (6). m :

[0047]

[0048] in:

[0049] i mα and i mβ The compensation current i are respectively m α and β axis components;

[0050] P r and Q r These are the actual active and reactive power output from the inverter to the PCC, respectively, and are calculated using equation (7):

[0051]

[0052] in:

[0053] v pccα and v pccβ The voltage v at the inverter PCC obtained by measurement are respectively pcc α and β axis components;

[0054] i 2αandi 2β These are the α and β axis components of the measured inverter grid-connected current i2, respectively.

[0055] Step 3.2.3: Calculate the voltage error compensation amount Δv according to step 3.1.2 in normal operating condition 1;

[0056] Step 3.2.4: Calculate the mutually orthogonal voltage quantities e and (as per step 3.1.3 in normal operating condition 1) ;

[0057] Current error Δi e From Δi e =i ref -i m -i1 is calculated to obtain;

[0058] Step 3.2.5: Calculate the modulation voltage v of the inverter main circuit according to step 3.1.4 in normal operating condition 1. m Used for inverter circuit switching control;

[0059] Normal operating condition scenario 3: Off-grid switchover to grid connection

[0060] The control module operates according to the following steps:

[0061] Step 3.3.1: Calculate the inverter reference current i according to step 3.1.1 in normal operating condition 1. ref ;

[0062] Step 3.3.2: Calculate the compensation current i according to step 3.2.2 in normal operating condition 2. m ;

[0063] Step 3.3.3: The pre-synchronization current generation stage F is calculated according to equation (8) to obtain the pre-synchronization current i. pc :

[0064] i pc =(vv g )Y d (8)

[0065] Where: v is the actual output voltage of the inverter, v g To measure the obtained grid-side voltage, Y d For pre-synchronized virtual admittance;

[0066] Step 3.3.4: Calculate the voltage error compensation amount Δv according to step 3.1.2 in normal operating condition 1;

[0067] Step 3.3.5: Calculate the mutually orthogonal voltage quantities e and (as per step 3.1.3 in normal operating condition 1)

[0068] Current error Δi e From Δi e =i ref -i m -i pc -i1 is calculated to obtain;

[0069] Step 3.3.6: Calculate the modulation voltage v of the inverter main circuit according to step 3.1.4 in normal operating condition 1. m Used for inverter circuit switching control;

[0070] Normal operating condition scenario 4: Switching from grid-connected to off-grid

[0071] The steps are consistent with those in the normal operating condition scenario 2.

[0072] The characteristic of the phase-locked loop-free self-synchronizing unified grid-connected inverter system of the present invention is that, under fault operation conditions, the inverter system implements unified control according to the following steps:

[0073] Step 4.1: The fault ride-through control module E determines and outputs a fault command x. f Low voltage ride-through active and reactive power reference and

[0074] Step 4.2: According to the fault command x f The inverter is controlled according to the following conditions depending on the different values ​​of :

[0075] Faulty operating condition 1: x f =1, the system operates under the condition of unexpected network disconnection due to fault.

[0076] When the measured grid-side voltage v of the system is obtained g and the voltage v at PCC pcc If there is a discrepancy, it is determined that the grid-side circuit breaker S1 at the PCC voltage has been accidentally disconnected. At this time, the inverter system enters the off-grid state, and the system engages the compensation current generation link C to balance the near-end load according to step 3.2.2 of the normal operating condition 2.

[0077] Fault operation condition 2: x f =2, the system operates under the condition of unexpected closing due to fault.

[0078] The reference current generation circuit A and the compensation current generation circuit C are automatically disconnected first. After the system stabilizes, the reference current generation circuit A is automatically activated. The subsequent control is the same as the control steps of the system during normal grid connection as described in normal operating condition 1.

[0079] Fault operation condition 3: x f =3, the system operates under low-voltage fault ride-through conditions.

[0080] Disconnect the voltage amplitude error compensation module F, and in order to meet the low-voltage fault ride-through requirements of different types of inverters, set different reactive power inputs according to the degree of grid-side voltage amplitude reduction. and active power given

[0081] Among them, reactive power is given Calculated as shown in equation (9):

[0082]

[0083] in:

[0084] v pcca v pccb v pccc The instantaneous three-phase voltage value at the PCC of the system;

[0085] U n The amplitude of the grid voltage under rated operating conditions; S n U is the rated apparent power. pu S is the ratio of the fault voltage to the rated voltage. F S represents the apparent power after a power grid fault occurs. F Calculated as shown in equation (10):

[0086]

[0087] In the formula: U F The value is the three-phase voltage amplitude at the PCC of the system.

[0088] Active power given Calculated as shown in equation (11):

[0089]

[0090] The active power of the fault is given. and reactive power given The reference current i under fault conditions is obtained by calculating the reference power according to equation (1). refP Then, control is performed in accordance with normal operating condition 1.

[0091] Compared with the prior art, the beneficial effects of the present invention are reflected in:

[0092] 1. Compared with grid-connected inverter systems using traditional grid-connected control methods, the method of the present invention does not rely on external voltage or require a dedicated phase-locked loop, thus avoiding the potential adverse effects of the phase-locked loop on system stability under weak grid conditions.

[0093] 2. Compared with grid-connected inverter systems using traditional grid control methods, the method of the present invention does not require the configuration of a phase-locked loop when reconnecting to the grid, thus enabling the grid-connected inverter system to be flexible and plug-and-play.

[0094] 3. The method of the present invention enables the grid-connected inverter to operate stably under strong and weak grid conditions and operate efficiently under normal grid-connected and off-grid load conditions through the autonomous coordination of self-synchronization control, reference current generation, compensation current generation, pre-synchronization current generation, fault ride-through control, and voltage amplitude error compensation under different operating conditions. It also has the ability to ride through under fault conditions.

[0095] 4. The method of the present invention is applicable to three-phase new energy grid-connected inverter power generation systems and has good versatility. Attached Figure Description

[0096] Figure 1 This is a schematic diagram of the phase-locked loop-free self-synchronizing phase-unified grid-connected inverter system of the present invention;

[0097] Figure 2 This is a structural diagram of the self-synchronization control loop in this invention;

[0098] Figure 3 The waveforms of the PCC point voltage, grid-connected current, and output power when the grid-side line impedance changes during normal grid-connected operation of the system of the present invention.

[0099] Figure 4 The waveforms of the PCC point voltage, grid-connected current, and output power of the system of this invention are shown when it is operating off-grid under normal operating conditions.

[0100] Figure 5 The waveforms of the PCC point voltage, grid-connected current, and output power of the system of this invention when switching from off-grid to grid-connected under normal operating conditions;

[0101] Figure 6 For the system of this invention operating under low-voltage fault ride-through conditions, the waveforms of its PCC point voltage, grid-connected current, and output power are shown. Detailed Implementation

[0102] like Figure 1 As shown, the phase-locked loop-free self-synchronizing unified grid-connected inverter system in this embodiment consists of a circuit module and a control module;

[0103] The circuit module includes a DC power supply, inverter main circuit, inverter-side inductor L1 and grid-side inductor L2, filter capacitor C, and grid-side line impedance Z. g Equivalent load Z L , grid-side circuit breaker S1, load-side circuit breaker S2 and grid v gs In this example, a constant DC voltage source is used as the DC power supply, and its voltage amplitude V DC =650V, which can also be a DC voltage output obtained through other rectifier circuits; the inverter main circuit adopts a three-phase full-bridge topology, which can also be a three-level topology or other three-phase inverter circuits. By selecting the on / off state of the grid-side circuit breaker S1 and the load-side circuit breaker S2 in the circuit module, the system operates under different conditions.

[0104] The control module obtains various detection signals from the circuit module by measurement, including the voltage V at the inverter PCC. pcc And grid-connected current i2, grid-side voltage v g And the inverter-side inductor current i1; the detection signal is compared with the set inverter output active power P. * and reactive power Q * As input, the inverter modulation voltage is output through the self-synchronization control loop, reference current generation loop, compensation current generation loop, pre-synchronization current generation loop, fault ride-through control loop and voltage amplitude error compensation loop, realizing phase-locked loop-free self-synchronization unified control of the grid-connected inverter system under multiple operating conditions.

[0105] In this embodiment, the phase-locked loop-free self-synchronizing grid-connected inverter system is a control system composed of various control links, namely: self-synchronization control link A, reference current generation link B, compensation current generation link C, pre-synchronization current generation link D, fault ride-through control link E, and voltage amplitude error compensation link F, wherein:

[0106] The self-synchronization control element A outputs two mutually orthogonal voltage quantities e and based on various parameters. And the modulation voltage v used in the inverter main circuit m The parameters are as follows: the measured inverter-side inductor current i1, and the sum of the output currents Δi from the reference current generation stage B, the compensation current generation stage C, and the pre-synchronization current generation stage D. r And the voltage error compensation amount Δv output by the voltage amplitude error compensation circuit F.

[0107] Reference current generation element B is based on voltage e and The set inverter output active power P * and reactive power Q * Fault command x f Low voltage ride-through active power reference value and reactive power reference value As input, calculate and output the inverter reference current i. ref The compensation current generation stage C uses the voltage v at the inverter PCC. pcc and grid-connected current i2, and voltage e and As input, calculate the grid-connected output compensation current i. m The pre-synchronous current generation stage D uses the grid-side voltage v. g and voltage e and As input, calculate and output the pre-synchronization current i. pc ; Total current Δi r Inverter reference current i ref Compensation current i m and pre-synchronization current i pc sum.

[0108] The fault ride-through control loop E is based on the measured voltage v at the inverter PCC. pcc , grid-connected current i2 and grid-side voltage v g Output fault command x f It also outputs a low-voltage ride-through active power reference value. and reactive power reference value

[0109] The voltage amplitude error compensation circuit F uses voltage e and as well as Reference amplitude under rated operating conditions The input is used to calculate and output the voltage error compensation amount Δv.

[0110] In this embodiment, the phase-locked loop-free self-synchronizing grid-connected inverter system implements unified control under normal operating conditions as follows:

[0111] Normal operating condition scenario 1: Grid-connected operation

[0112] Under normal operating condition 1, Figure 1 With the grid-side circuit breaker S1 and load-side circuit breaker S2 closed in the circuit module shown, the inverter system operates under normal grid-connected operating conditions. The reference current generation stage B, voltage amplitude error compensation stage F, and self-synchronization control stage A are automatically controlled according to the following steps:

[0113] Step 3.1.1: The reference current generation stage B calculates the inverter reference current i according to equation (1). ref :

[0114]

[0115] in:

[0116] i refα and i refβ These are the inverter reference currents i ref α and β axis components;

[0117] V DC This refers to the voltage value of the DC power supply in the circuit module;

[0118] e α ,e β These are the α and β axis components of the voltage quantity e, respectively;

[0119] They are respectively voltage quantities α and β axis components;

[0120] T a This is the inverter startup time;

[0121] Due to the quadrature output e of the synchronous controller and The initial value is zero, and it is within half a cycle after the inverter starts up, i.e., T. a <t<T a Within +0.01s, the denominator of the reference current expression will be... The value is set to a fixed value of 5000 to ensure a very small reference current in the initial stage, preventing large surges. In a specific embodiment, the inverter output active power P is set under this operating condition. * =6000W and reactive power Q * =0Var. The inverter reference current i is obtained by calculating according to formula (1). ref The α and β axis components.

[0122] Step 3.1.2: The voltage amplitude error compensation element F is calculated according to formula (2) to obtain the voltage error compensation amount Δv:

[0123]

[0124] in:

[0125] It is voltage. The real-time amplitude, and has:

[0126] V p * yes In this embodiment, V is set as the reference amplitude under rated operating conditions. p * =311V; k v In this embodiment, k is set as the voltage amplitude error coefficient. v =314;

[0127] Step 3.1.3: The self-synchronization control loop A calculates the mutually orthogonal voltage quantities e and using equations (3) and (4).

[0128]

[0129] in:

[0130] ω0 is the rated frequency, and s is the Laplace operator;

[0131] Δi e For current error, the current error Δi e From Δi e =i ref -i1 is calculated to obtain;

[0132] k p k r These are the proportional coefficient and resonant coefficient of the proportional resonant controller in the self-synchronization control loop A; in this embodiment, k is set to... p =0.078,k r =116.35;

[0133] k s The adjustment coefficient is determined by... Calculated in this embodiment

[0134] Figure 2 The diagram shows the structure of the self-synchronization control loop used in this embodiment.

[0135] Step 3.1.4: Calculate the modulation voltage v of the inverter main circuit using equation (5). m Used for inverter circuit switching control;

[0136] v m =k p Δi e +e (5)

[0137] Figure 3 The diagram illustrates the waveforms of the PCC point voltage, grid-connected current, and output power when the system of this embodiment is operating in grid-connected mode under normal operating conditions and when the grid-side line impedance changes. Before t = 0.10s, the grid-side line Z is set... g =L g =0mH; at t=0.10s, 0.20s and 0.35s, the network-side line Z g Increasing sequentially to L g =15mH, 20mH and 25mH. For example... Figure 3 As shown, in this embodiment, the voltage and grid current at the PCC remain stable, and the grid current THD% is less than 2%, indicating good power quality; moreover, the inverter power always follows the given value, providing power dispatch capability.

[0138] Normal operating condition scenario 2: Off-grid operation

[0139] Under normal operating condition 2, the modulation voltage v of the inverter main circuit is obtained as follows. m ;

[0140] Under normal operating condition 2, the grid-side circuit breaker S1 is open, and the inverter independently drives the equivalent load Z. L In this embodiment, the equivalent load Z is set to run. L active power P L =3000W, reactive power Q L =0;

[0141] The control module automatically engages control according to the following steps:

[0142] Step 3.2.1: Calculate the inverter reference current i according to step 3.1.1 in normal operating condition 1. ref ;

[0143] Step 3.2.2: The compensation current generation circuit C is calculated according to formula (6) to obtain the compensation current i. m :

[0144]

[0145] in:

[0146] i mα and i mβ The compensation current i are respectively m α and β axis components;

[0147] P r and Q r These are the actual active and reactive power output from the inverter to the PCC, respectively, and are calculated using equation (7):

[0148]

[0149] in:

[0150] v pccα and v pccβ The measured voltage v at the inverter PCC is respectively pcc α and β axis components;

[0151] i 2α andi 2β These are the α and β axis components of the measured inverter grid-connected current i2, respectively.

[0152] Step 3.2.3: Calculate the voltage error compensation amount Δv according to step 3.1.2 in normal operating condition 1;

[0153] Step 3.2.4: Calculate the mutually orthogonal voltage quantities e and (as per step 3.1.3 in normal operating condition 1)

[0154] Current error Δi e From Δi e =i ref -i m -i1 is calculated to obtain;

[0155] Step 3.2.5: Calculate the modulation voltage v of the inverter main circuit according to step 3.1.4 in normal operating condition 1. m Used for inverter circuit switching control;

[0156] Figure 4 The diagram shows the waveforms of the PCC point voltage, grid-connected current, and output power of the system in this embodiment under normal off-grid operation. At t=0.01s, the inverter system in this embodiment starts up under off-grid load. It can be observed that the system can quickly establish the PCC voltage under off-grid conditions and maintain stability throughout. At this time, the grid-connected current, i.e., the current flowing into the PCC, is also consistently stable, and the THD% is less than 1%, indicating high power quality. The inverter's output active power is maintained around 3000W, and the reactive power is always 0Var, consistent with the set load power.

[0157] Normal operating condition scenario 3: Off-grid switchover to grid connection

[0158] In this embodiment, under normal operating condition 3, the inverter is set to operate with a rated load of 3000W after off-grid startup. It receives a grid connection command at 0.15s for pre-synchronization, and after 0.25s of pre-synchronization, it operates normally in grid-connected mode, adjusting the inverter's rated power to P. * =6000W and Q * =1000Var, the control module operates according to the following steps:

[0159] Step 3.3.1: Calculate the inverter reference current i according to step 3.1.1 in normal operating condition 1. ref ;

[0160] Step 3.3.2: Calculate the compensation current i according to step 3.2.2 in normal operating condition 2. m ;

[0161] Step 3.3.3: The pre-synchronization current generation stage F is calculated according to equation (8) to obtain the pre-synchronization current i. pc :

[0162] i pc =(vv g )Yd (8)

[0163] Where: v is the actual output voltage of the inverter, v g To measure the obtained grid-side voltage, Y d For pre-synchronized virtual admittance; in this embodiment, Y is set d =0.02;

[0164] Step 3.3.4: Calculate the voltage error compensation amount Δv according to step 3.1.2 in normal operating condition 1;

[0165] Step 3.3.5: Calculate the mutually orthogonal voltage quantities e and (as per step 3.1.3 in normal operating condition 1)

[0166] Current error Δi e From Δi e =i ref -i m -i pc -i1 is calculated to obtain;

[0167] Step 3.3.6: Calculate the modulation voltage v of the inverter main circuit according to step 3.1.4 in normal operating condition 1. m Used for inverter circuit switching control;

[0168] Figure 5 The diagram shows the waveforms of the PCC point voltage, grid-connected current, and output power of the system in this embodiment when switching from off-grid to grid-connected operation under normal operating conditions. Before t = 0.15s, the system operates under normal off-grid load conditions, and the waveforms of its PCC point voltage, grid-connected current, and output power are shown below. Figure 4 The results are basically consistent. After the grid connection command is issued at t=0.15s, the pre-synchronization current link D takes effect, and the output of the self-synchronization controller link A is... Synchronization with the power grid is achieved within two cycles. Formal grid connection is completed at t = 0.25s, marking the completion of pre-synchronization. At this point, in this embodiment, the system output active power is adjusted to P = 6000W, and reactive power is adjusted to Q = 1000Va, consistent with the settings.

[0169] Normal operating condition scenario 4: Switching from grid-connected to off-grid

[0170] The steps are the same as in normal operating condition 2.

[0171] In this embodiment, under fault operation conditions, the phase-locked loop-free self-synchronizing unified grid-connected inverter system implements unified control according to the following steps:

[0172] Step 4.1: The fault ride-through control module E determines and outputs a fault command x. fLow voltage ride-through active and reactive power reference and

[0173] Step 4.2: According to the fault command x f The inverter is controlled according to the following conditions depending on the different values ​​of :

[0174] Faulty operating condition 1: x f =1, the system is operating under conditions of unexpected failure and disconnection from the network.

[0175] When the measured system grid-side voltage v g and the voltage v at PCC pcc If there is a discrepancy, it is determined that the grid-side circuit breaker S1 at the PCC voltage has been accidentally disconnected. At this time, the inverter system enters the off-grid state, and the system engages the compensation current generation link C to balance the near-end load according to the normal operating condition 2 step 3.2.2.

[0176] Fault operation condition 2: x f =2, the system is operating under the condition of unexpected closing due to a fault.

[0177] The reference current generation circuit A and the compensation current generation circuit C are automatically disconnected first. After the system stabilizes, the reference current generation circuit A is automatically activated. The subsequent control is the same as the control steps when the system is running normally in the grid-connected operation mode 1.

[0178] Fault operation condition 3: x f =3, the system is operating under low-voltage fault ride-through conditions.

[0179] Disconnect the voltage amplitude error compensation module F, and in order to meet the low-voltage fault ride-through requirements of different types of inverters, set different reactive power inputs according to the degree of grid-side voltage amplitude reduction. and active power given

[0180] Among them, reactive power is given Calculated as shown in equation (9):

[0181]

[0182] in:

[0183] v pcca v pccb v pccc This represents the instantaneous three-phase voltage at the system's PCC.

[0184] U n In this embodiment, U is set to the amplitude under the rated operating conditions of the power grid voltage. n =311V;S nU is the rated apparent power. pu This is the ratio of the fault voltage to the rated voltage.

[0185] S F S represents the apparent power after a power grid fault occurs. F Calculated as shown in equation (10):

[0186]

[0187] In the formula: U F This represents the three-phase voltage amplitude at the PCC point of the system.

[0188] Active power given P F * Calculated as shown in equation (11):

[0189]

[0190] Give the fault active power and reactive power given The reference current i under fault conditions is obtained by calculating the reference power according to equation (1). refP Then, control is performed in accordance with normal operating condition 1.

[0191] Figure 6 This embodiment demonstrates that the system operates under low-voltage fault ride-through conditions, with the voltage dropping to 0.8U. n At that time, the waveforms of the voltage at point PCC, grid-connected current, and output power are shown. When the voltage at point PCC is v... pcc During a sudden voltage drop, the resulting fault current affects the input current error of the inverter's self-synchronization control loop A, causing changes in the output voltage amplitude and phase. However, due to the effect of the voltage amplitude error compensation loop C, a balance can be achieved under a certain current error. Therefore, v pcc It plummeted to 0.8U n At this time, the rise in reference current remains within the inverter's overcurrent limit. However, due to the decrease in voltage amplitude, a certain amount of reactive power needs to be absorbed to reach a new steady state. For example... Figure 6 As shown, before t=0.1s, the inverter output active power is the rated 6000W, and the output current amplitude is approximately 13A. After t=0.1s, the voltage drops to 0.8U. n The inverter output current amplitude rises to 21.5A. At t=0.3s, the voltage returns to normal, and the active power output of the system in this example is adjusted back to the rated 6000W. The system in this example completes the ride-through control under fault conditions.

Claims

1. A phase-locked loop-free, self-synchronizing, unified grid-connected inverter system, characterized in that: The system consists of a circuit module and a control module; The circuit module includes a DC power supply, an inverter main circuit, an inverter-side inductor L1 and a grid-side inductor L2, a filter capacitor C, and a grid-side line impedance Z. g Equivalent load Z L , grid-side circuit breaker S1, load-side circuit breaker S2 and grid v gs ; The control module obtains various detection signals from the circuit module by measurement, including the voltage v at the inverter PCC. pcc And grid-connected current i2, grid-side voltage v g And the inverter-side inductor current i1; the detection signal is compared with the set inverter output active power P. * and reactive power Q * As input, the inverter modulation voltage is output through the self-synchronization control loop, reference current generation loop, compensation current generation loop, pre-synchronization current generation loop, fault ride-through control loop and voltage amplitude error compensation loop, so as to realize the phase-locked loop-free self-synchronization unified control of the grid-connected inverter system under multiple operating conditions. The control module consists of several control links, namely: self-synchronization control link A, reference current generation link B, compensation current generation link C, pre-synchronization current generation link D, fault ride-through control link E, and voltage amplitude error compensation link F, wherein: The self-synchronization control loop A outputs two mutually orthogonal voltage quantities e and based on various parameters. And the modulation voltage v used in the inverter main circuit m The parameters are: the measured inverter-side inductor current i1, and the sum of the output currents Δi of the reference current generation stage B, the compensation current generation stage C, and the pre-synchronization current generation stage D. r And the voltage error compensation amount Δv output by the voltage amplitude error compensation circuit F; The reference current generation stage B uses the voltage e and The set inverter output active power P * and reactive power Q * Fault command x f Low voltage ride-through active power reference value and reactive power reference value As input, calculate and output the inverter reference current i. ref The compensation current generation stage C uses the voltage v at the inverter PCC. pcc and the grid-connected current i2, and the voltage e and As input, calculate the grid-connected output compensation current i. m The pre-synchronization current generation link D uses the grid-side power grid voltage v g and the voltage quantity e and As input, calculate and output the pre-synchronization current i. pc The total current Δi r Inverter reference current i ref Compensation current i m and pre-synchronization current i pc sum; The fault ride-through control element E is based on the measured voltage v at inverter PCC. pcc , grid-connected current i2 and grid-side voltage v g Output fault command x f It also outputs a low-voltage ride-through active power reference value. and reactive power reference value The voltage amplitude error compensation link F uses the voltage amount e and as well as Reference amplitude under rated operating conditions As input, calculate and output the voltage error compensation amount Δv. It is the voltage quantity The real-time amplitude.

2. The phase-locked loop-free self-synchronizing unified grid-connected inverter system according to claim 1, characterized in that, Under normal operating conditions, the inverter system is uniformly controlled as follows: Normal operating condition scenario 1: Grid-connected operation The reference current generation circuit B, the voltage amplitude error compensation circuit F, and the self-synchronization control circuit A are automatically controlled according to the following steps: Step 3.1.1: The reference current generation stage B calculates the inverter reference current i according to equation (1). ref : in: i refα and i refβ These are the inverter reference currents i ref α and β axis components; V DC The voltage value of the DC power supply in the circuit module; e α ,e β These are the α and β axis components of the voltage quantity e, respectively; The voltage quantities are respectively α and β axis components; T a This is the inverter startup time; Step 3.1.2: The voltage amplitude error compensation link F is calculated according to formula (2) to obtain the voltage error compensation amount Δv: in: V p * yes Reference amplitude under rated operating conditions; k v This is the voltage amplitude error coefficient; Step 3.1.3: The self-synchronization control loop A calculates the mutually orthogonal voltage quantities e and using equations (3) and (4). in: ω0 is the rated frequency, and s is the Laplace operator; Δi e For current error, the current error Δi e From Δi e =i ref -i1 is calculated to obtain; k p k r These are the proportional coefficient and resonant coefficient of the proportional resonant controller in the self-synchronization control loop A, respectively. k s The adjustment coefficient is determined by... Calculated; Step 3.1.4: Calculate the modulation voltage v of the inverter main circuit using equation (5). m Used for inverter circuit switching control; v m =k p Δi e +e (5) Normal operating condition scenario 2: Off-grid operation When the grid-side circuit breaker S1 is disconnected, the inverter independently drives the equivalent load Z. L run; The control module automatically engages control according to the following steps: Step 3.2.1: Calculate the inverter reference current i according to step 3.1.1 in normal operating condition 1. ref ; Step 3.2.2: The compensation current generation circuit C calculates the compensation current i according to formula (6). m : in: i mα and i mβ The compensation current i are respectively m α and β axis components; P r and Q r These are the actual active and reactive power output from the inverter to the PCC, respectively, and are calculated using equation (7): in: v pccα and v pccβ The voltage v at the inverter PCC obtained by measurement are respectively pcc α and β axis components; i 2α andi 2β These are the α and β axis components of the measured inverter grid-connected current i2, respectively. Step 3.2.3: Calculate the voltage error compensation amount Δv according to step 3.1.2 in normal operating condition 1; Step 3.2.4: Calculate the mutually orthogonal voltage quantities e and (as per step 3.1.3 in normal operating condition 1) Current error Δi e From Δi e =i ref -i m -i1 is calculated to obtain; Step 3.2.5: Calculate the modulation voltage v of the inverter main circuit according to step 3.1.4 in normal operating condition 1. m Used for inverter circuit switching control; Normal operating condition scenario 3: Off-grid switchover to grid connection The control module operates according to the following steps: Step 3.3.1: Calculate the inverter reference current i according to step 3.1.1 in normal operating condition 1. ref ; Step 3.3.2: Calculate the compensation current i according to step 3.2.2 in normal operating condition 2. m ; Step 3.3.3: The pre-synchronization current generation stage D is calculated according to equation (8) to obtain the pre-synchronization current i. pc : i pc =(vv g )Y d (8) Where: v is the actual output voltage of the inverter, v g To measure the obtained grid-side voltage, Y d For pre-synchronized virtual admittance; Step 3.3.4: Calculate the voltage error compensation amount Δv according to step 3.1.2 in normal operating condition 1; Step 3.3.5: Calculate the mutually orthogonal voltage quantities e and (as per step 3.1.3 in normal operating condition 1) Current error Δi e From Δi e =i ref -i m -i pc -i1 is calculated to obtain; Step 3.3.6: Calculate the modulation voltage v of the inverter main circuit according to step 3.1.4 in normal operating condition 1. m Used for inverter circuit switching control; Normal operating condition scenario 4: Switching from grid-connected to off-grid The steps are consistent with those in the normal operating condition scenario 2.

3. The phase-locked loop-free self-synchronizing unified grid-connected inverter system according to claim 1, characterized in that, Under fault operation conditions, the inverter system implements unified control according to the following steps: Step 4.1: The fault passage control loop E determines and outputs a fault command x. f Low voltage ride-through active and reactive power reference and Step 4.2: According to the fault command x f The inverter is controlled according to the following conditions depending on the different values ​​of : Faulty operating condition 1: x f =1, the system operates under the condition of unexpected network disconnection due to fault. When the measured system grid-side voltage v g and the voltage v at PCC pcc If there is a discrepancy, it is determined that the grid-side circuit breaker S1 at the PCC voltage has been accidentally disconnected. At this time, the inverter system enters the off-grid state, and the system engages the compensation current generation link C to balance the near-end load according to step 3.2.2 of the normal operating condition 2. Fault operation condition 2: x f =2, the system operates under the condition of unexpected closing due to a fault; The reference current generation circuit B and the compensation current generation circuit C are automatically disconnected first. After the system stabilizes, the reference current generation circuit B is automatically activated. The subsequent control is the same as the control steps of the system during normal grid connection as described in normal operating condition 1. Fault operation condition 3: x f =3, the system operates under low-voltage fault ride-through conditions. The voltage amplitude error compensation circuit F is disconnected, and to meet the low-voltage fault ride-through requirements of different types of inverters, different reactive power inputs are set according to the degree of voltage amplitude reduction on the grid side. and active power given Among them, reactive power is given Calculated as shown in equation (9): in: v pcca v pccb v pccc The instantaneous three-phase voltage value at the PCC of the system; U n The amplitude of the grid voltage under rated operating conditions; S n U is the rated apparent power. pu This is the ratio of the fault voltage to the rated voltage. S F S represents the apparent power after a power grid fault occurs. F Calculated as shown in equation (10): In the formula: U F The three-phase voltage amplitude at the PCC of the system; Active power given Calculated as shown in equation (11): Give the fault active power and reactive power given The reference current i under fault conditions is obtained by calculating the reference power according to equation (1). refP Then, control is performed in accordance with normal operating condition 1.

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