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

Abstract

The invention discloses a phase-locked loop-free self-synchronizing unified grid-connected inverter system, which is composed of a circuit module and a control module, and is characterized in that the control module obtains detection signals in the circuit module through measurement, including inverter PCC voltage and grid-connected current, grid-side power grid voltage and inverter-side inductive current; and outputting an inverter modulation voltage through a self-synchronization control link, a reference current generation link, a compensation current generation link, a pre-synchronization current generation link, a fault ride-through control link and a voltage amplitude error compensation link with set inverter output active power and reactive power instructions serving as input, and outputting the inverter modulation voltage through the self-synchronization control link, the reference current generation link, the compensation current generation link, the pre-synchronization current generation link, the fault ride-through control link and the voltage amplitude error compensation link. And the phase-locked loop-free self-synchronization unified control of the grid-connected inverter system under multiple operation conditions is realized. According to the method, the output voltage of the inverter can be established without using a special phase locking link, and the method has power scheduling and voltage supporting capabilities, adapts to intelligent autonomous operation of multiple working conditions such as a normal off-grid / grid-connected working condition and a fault working condition, and is suitable for a grid-connected application occasion of a new energy grid-connected inverter.

Description

Technical Field

[0001] The present invention relates to the application field of renewable energy power generation systems, and more specifically to a phase-locked loop-free self-synchronous unified grid-connected inverter system for realizing renewable energy grid connection under multiple working conditions. Background Art

[0002] In 2023, the global renewable energy generation (including hydropower) is expected to account for 30.2% of the total power generation. It has become a consensus among countries around the world to vigorously develop renewable energy systems to achieve low-carbon and clean power industry. As a key interface device between uncontrollable wind and solar power and 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 light and new energy storage systems with the existing power grid mainly relies on power electronic equipment mainly based on inverters. In traditional power systems, generator sets achieve synchronous operation through a power synchronization mechanism, while most current new energy power generation equipment uses a grid-following control method to achieve integration with traditional power systems. Grid-following control usually adopts a voltage synchronization mechanism, and collects the grid-connected point voltage and obtains its phase, amplitude and other information through a dedicated phase-locked loop configured by the system, which is used as a reference voltage for control. The grid-connected inverter using the grid-following control method is externally manifested as a current source or power source, does not have the ability to independently build a network, and is prone to instability under weak network conditions. Improving the PLL structure, optimizing the PLL control parameters, and reshaping the output impedance of the grid-connected inverter are currently the main solutions to solve the instability problem that may exist in the grid-following control method when operating in a weak network, but due to the involvement of more parameter settings and the need to add additional control items, the control complexity will increase.

[0004] Correspondingly, grid-connected inverters using grid control methods have attracted more and more attention recently because they can independently establish voltage and provide corresponding grid support functions. At present, typical grid control methods mainly include droop control that simulates synchronous machine characteristics, virtual synchronous generator control, and virtual oscillator control that simulates nonlinear oscillator characteristics. However, when using traditional grid control inverters to reconnect to the grid, a phase-locked loop is still required for synchronization. On the other hand, inverters using traditional grid control methods are prone to instability under strong grid conditions. Summary of the invention

[0005] The present invention is to avoid the deficiencies of the above-mentioned prior art and provide a phase-locked loop-free self-synchronous unified grid-connected inverter system, which is a new solution for the grid-connected inverter control system, realizes the autonomous establishment of the inverter output voltage, has power scheduling and voltage support capabilities, and is adaptable to intelligent and autonomous operation in multiple working conditions such as normal off-grid / grid-connected working conditions and fault working conditions, and is suitable for the control of three-phase grid-connected inverters in new energy grid-connected power generation systems such as photovoltaic power generation, wind power generation, and chemical energy storage.

[0006] The present invention adopts the following technical solutions to solve the technical problems:

[0007] The characteristics of the phase-locked loop-free self-synchronous unified grid-connected inverter system of the present invention are:

[0008] The system is composed 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 the detection signals in the circuit module by measuring: the voltage v at the inverter PCC pcc and grid-connected current i2, grid-side grid voltage v g , and the inverter side inductor current i1; the detection signal and the set inverter output active power P * And reactive power Q * As input, the inverter modulation voltage is output through the self-synchronization control link, the reference current generation link, the compensation current generation link, the pre-synchronization current generation link, the fault ride-through control link and the voltage amplitude error compensation link, so as to realize the phase-locked loop-free self-synchronization unified control of the grid-connected inverter system under multiple operating conditions.

[0011] The phase-locked loop-free self-synchronous unified grid-connected inverter system of the present invention is also characterized in that:

[0012] The control module is composed of various control links, which are: 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-synchronous control link A outputs two mutually orthogonal voltage quantities e and and the modulation voltage v for the inverter main circuit m The parameters are: the measured inverter side inductor current i1, the reference current generating link B, the compensation current generating link C and the pre-synchronization current generating link D output current sum Δi r And the voltage error compensation amount Δv output by the voltage amplitude error compensation link F;

[0014] The reference current generating link B is based on the voltage e and The inverter output active power P is set * And reactive power Q* , fault instruction x f , Low voltage ride through active power reference value and reactive power reference As input, calculate and output the inverter reference current i ref The compensation current generating link C is based on the voltage v at the inverter PCC pcc and the grid-connected current i2, and the voltage e and As input, calculate and obtain the grid-connected output compensation current i m The pre-synchronization current generation link D is based on the grid voltage v g , and the voltage e and As input, calculate and output the pre-synchronization current i pc The sum of the current Δi r is the inverter reference current i ref , compensation current i m and pre-synchronization current i pc sum;

[0015] The fault ride-through control link E is based on the measured voltage v at the inverter PCC pcc , grid-connected current i2 and grid-side grid voltage v g , output fault instruction x f , and output low voltage ride through active power reference value and reactive power reference

[0016] The voltage amplitude error compensation link F is based on the voltage amount e and as well as Reference amplitude under rated conditions As input, the voltage error compensation amount Δv is calculated and output.

[0017] The phase-locked loop-free self-synchronous unified grid-connected inverter system of the present invention is also characterized in that, under normal operating conditions, the inverter system implements unified control according to the following situations:

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

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

[0020] Step 3.1.1: The reference current generating link B calculates the inverter reference current i according to formula (1) ref :

[0021]

[0022] in:

[0023] i refα and i refβ They are the inverter reference current i ref The α and β axis components of

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

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

[0026] The voltages are The α and β axis components of

[0027] T a The inverter start 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] is the voltage The real-time amplitude of , and: V p * yes Reference amplitude under rated conditions; k v is the voltage amplitude error coefficient;

[0032] Step 3.1.3: The self-synchronous control link A calculates the mutually orthogonal voltages e and

[0033]

[0034] in:

[0035] ω0 is the rated frequency, s is the Laplace operator;

[0036] Δi e is the current error, current error Δi e By Δi e =i ref -i1 is obtained by calculation;

[0037] k p , k rare respectively the proportional coefficient and the resonance coefficient of the proportional resonant controller in the self-synchronous control link A;

[0038] k s is the adjustment coefficient, Calculate and obtain;

[0039] Step 3.1.4: Calculate the modulation voltage v of the inverter main circuit using equation (5): m , used for inverter circuit switch control;

[0040] v m =k p Δi e +e (5)

[0041] Normal operating condition 2: Off-grid operation

[0042] The grid-side circuit breaker S1 is disconnected, and the inverter independently carries the equivalent load Z L run;

[0043] The control module automatically starts control according to the following steps:

[0044] Step 3.2.1: Calculate the inverter reference current i according to step 3.1.1 in normal operation condition 1 ref ;

[0045] Step 3.2.2: The compensation current generating link C is calculated according to formula (6) to obtain the compensation current i m :

[0046]

[0047] in:

[0048] i mα and i mβ They are the compensation current i m The α and β axis components of

[0049] P r and Q r are the actual active and reactive powers output from the inverter to the PCC, respectively, and are calculated by formula (7):

[0050]

[0051] in:

[0052] v pccα and v pccβ The voltage v at the inverter PCC obtained by measurement is pcc The α and β axis components of

[0053] i 2αandi 2β They are the α-axis and β-axis components of the inverter grid-connected current i2 obtained by measurement;

[0054] Step 3.2.3: Calculate the voltage error compensation value Δv according to step 3.1.2 in normal operation condition 1;

[0055] Step 3.2.4: Calculate the mutually orthogonal voltages e and

[0056] Current error Δi e By Δi e =i ref -i m -i1 is obtained by calculation;

[0057] Step 3.2.5: Calculate the modulation voltage v of the inverter main circuit according to step 3.1.4 in normal operation condition 1 m , used for inverter circuit switch control;

[0058] Normal operation condition 3: Switching from off-grid to grid-connected

[0059] The control module performs control according to the following steps:

[0060] Step 3.3.1: Calculate the inverter reference current i according to step 3.1.1 in normal operation condition 1 ref ;

[0061] Step 3.3.2: Calculate the compensation current i according to step 3.2.2 in normal operation condition 2. m ;

[0062] Step 3.3.3: The pre-synchronization current generation link F is calculated according to formula (8) to obtain the pre-synchronization current i pc :

[0063] i pc =(vv g )Y d (8)

[0064] Where: v is the actual output voltage of the inverter, v g Y is the measured grid voltage. d is the pre-synchronization virtual admittance;

[0065] Step 3.3.4: Calculate the voltage error compensation value Δv according to step 3.1.2 in normal operation condition 1;

[0066] Step 3.3.5: Calculate the mutually orthogonal voltages e and

[0067] Current error Δi e By Δi e =i ref -i m -i pc -i1 is obtained by calculation;

[0068] Step 3.3.6: Calculate the modulation voltage v of the inverter main circuit according to step 3.1.4 in normal operation condition 1 m , used for inverter circuit switch control;

[0069] Normal operation condition 4: Switching from grid-connected to off-grid

[0070] The steps are consistent with those of the normal operating condition 2.

[0071] The phase-locked loop-free self-synchronous unified grid-connected inverter system of the present invention is also characterized in that, under a fault operating condition, the inverter system implements unified control according to the following steps:

[0072] Step 4.1: The fault-through control module E determines and outputs a fault instruction x f , Low Voltage Ride Through Active and Reactive Power Reference and

[0073] Step 4.2: According to the fault instruction x f For different values ​​of , the inverter is controlled according to the following situations:

[0074] Fault operating condition 1: x f =1, the system is operating in a faulty and unexpectedly disconnected condition

[0075] When the measured grid voltage v g and the voltage at PCC v pcc When there is inconsistency, it is judged that the grid-side circuit breaker S1 at the voltage of PCC is accidentally disconnected, and the inverter system enters the off-grid state. The system puts the compensation current generation link C into operation to balance the proximal load according to step 3.2.2 of normal operating condition 2 in claim 3;

[0076] Fault operation condition 2: x f =2, the system is operating in the fault accident closing condition

[0077] The reference current generating link A and the compensation current generating link C are automatically disconnected first, and the reference current generating link A is automatically enabled after the system is stable, and the subsequent control is consistent with the control steps of the system in normal grid-connected operation described in the normal operating condition 1 in claim 3;

[0078] Fault operation condition 3: x f =3, the system operates in low voltage fault ride-through condition

[0079] The voltage amplitude error compensation module F is disconnected, and in order to meet the low-voltage fault ride-through requirements of different types of inverters, different reactive power settings are set according to the degree of grid-side voltage amplitude reduction. And active power given

[0080] Among them, the reactive power is given According to formula (9), we can obtain:

[0081]

[0082] in:

[0083] v pcca 、v pccb 、v pccc is the instantaneous three-phase voltage value at the PCC of the system;

[0084] U n is the amplitude of the grid voltage under rated conditions; S n is the rated apparent power; U pu is the ratio of fault voltage to rated voltage; S F is the apparent power after the fault condition of the power grid, S F It is calculated by formula (10):

[0085]

[0086] Where: U F is the three-phase voltage amplitude at the PCC of the system.

[0087] Active power setting Calculate according to formula (11):

[0088]

[0089] The fault active power is given and reactive power given As the reference power, the reference current i under the fault condition is calculated according to formula (1): refP , and then control is performed in accordance with the normal operating condition 1 in claim 3.

[0090] Compared with the prior art, the present invention has the following beneficial effects:

[0091] 1. Compared with the grid-connected inverter system using the traditional grid-following control method, the method of the present invention does not need to rely on external voltage, does not need to configure a dedicated phase-locked link, and avoids the possible adverse effects of the phase-locked loop on system stability under weak network conditions;

[0092] 2. Compared with the grid-connected inverter system using the traditional grid control method, the method of the present invention does not need to configure a phase-locked loop when reconnecting to the power grid, and can realize flexible plug-and-play of the grid-connected inverter system;

[0093] 3. The method of the present invention can realize the stable operation of the grid-connected inverter under strong and weak grid conditions and the efficient operation 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, voltage amplitude error compensation and other links under different operating conditions, and also has the ride-through control capability under fault conditions.

[0094] 4. The method of the present invention is applicable to three-phase new energy grid-connected inverter power generation system and has good versatility. BRIEF DESCRIPTION OF THE DRAWINGS

[0095] Figure 1 A schematic diagram of a self-synchronous phase-unified grid-connected inverter system without a phase-locked loop according to the present invention;

[0096] Figure 2 It is a structural diagram of the self-synchronous control link in the present invention;

[0097] Figure 3 The system of the present invention is connected to the grid under normal operating conditions, and the PCC point voltage, grid-connected current and output power waveforms when the grid-side line impedance changes;

[0098] Figure 4 The PCC point voltage, grid-connected current and output power waveforms of the system of the present invention when it is off-grid under normal operating conditions;

[0099] Figure 5 The PCC point voltage, grid-connected current and output power waveforms of the system of the present invention when it switches from off-grid to grid-connected under normal operating conditions;

[0100] Figure 6 The system of the present invention operates under low voltage fault ride-through conditions, and its PCC point voltage, grid-connected current and output power waveforms are shown in Figure 2. DETAILED DESCRIPTION

[0101] like Figure 1 As shown, the phase-locked loop-free self-synchronous unified grid-connected inverter system in this embodiment is composed of a circuit module and a control module;

[0102] 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 ; In this example, the DC power supply adopts a constant DC voltage source, and its voltage amplitude is V DC =650V, which can also be a DC voltage output obtained by other rectifier circuits; the inverter main circuit adopts a three-phase full-bridge topology, which can also be other three-phase inverter circuits such as a three-level topology. Through the on-off selection of the grid-side circuit breaker S1 and the load-side circuit breaker S2 in the circuit module, the system operates in different working conditions.

[0103] The control module obtains the detection signals in the circuit module by measuring: the voltage v at the inverter PCC pcc and grid-connected current i2, grid-side grid voltage v g , and the inverter side inductor current i1; the detection signal and the set inverter output active power P * And reactive power Q * As input, the inverter modulation voltage is output through the self-synchronization control link, the reference current generation link, the compensation current generation link, the pre-synchronization current generation link, the fault ride-through control link and the voltage amplitude error compensation link, so as to realize the phase-locked loop-free self-synchronization unified control of the grid-connected inverter system under multiple operating conditions.

[0104] In this embodiment, the phase-locked loop-free self-synchronous unified grid-connected inverter system is a control system composed of various control links, and the various control links are: self-synchronous control link A, reference current generation link B, compensation current generation link C, pre-synchronous current generation link D, fault ride-through control link E and voltage amplitude error compensation link F, wherein:

[0105] The self-synchronous control link A outputs two mutually orthogonal voltage quantities e and and the modulation voltage v for the inverter main circuit m ; The parameters are: the measured inverter side inductor current i1, the total output current Δi of the reference current generation link B, the compensation current generation link C and the pre-synchronization current generation link D r And the voltage error compensation amount Δv output by the voltage amplitude error compensation link F.

[0106] The reference current generation link B is based on the voltage e and The inverter output active power P is set * And reactive power Q * , fault instruction x f , Low voltage ride through active power reference value and reactive power reference As input, calculate and output the inverter reference current i ref ; The compensation current generation link C is based on the voltage v at the inverter PCC pcc and the grid-connected current i2, as well as the voltage e and As input, calculate and obtain the grid-connected output compensation current i m ; The pre-synchronous current generation link D is based on the grid voltage v g , and the voltage e and As input, calculate and output the pre-synchronization current i pc ; Total current Δi r is the inverter reference current i ref , compensation current i m and pre-synchronization current i pc sum.

[0107] The fault ride-through control link E obtains the voltage v at the inverter PCC according to the measurement. pcc , grid-connected current i2 and grid-side grid voltage v g , output fault instruction x f , and output low voltage ride through active power reference value and reactive power reference

[0108] The voltage amplitude error compensation link F is based on the voltage quantity e and as well as Reference amplitude under rated conditions As input, the voltage error compensation amount Δv is calculated and output.

[0109] In this embodiment, under normal operating conditions of the phase-locked loop-free self-synchronous unified grid-connected inverter system, the inverter system implements unified control according to the following situations:

[0110] Normal operating condition 1: Grid-connected operation

[0111] Under normal operating condition 1, Figure 1 In the circuit module shown, the grid-side circuit breaker S1 and the load-side circuit breaker S2 are closed, and the inverter system operates in normal grid-connected operation conditions. The reference current generation link B, the voltage amplitude error compensation link F and the self-synchronization control link A are automatically controlled according to the following steps:

[0112] Step 3.1.1: Reference current generation link B calculates the inverter reference current i according to formula (1) ref :

[0113]

[0114] in:

[0115] i refα and i refβ They are the inverter reference current i ref The α and β axis components of

[0116] V DC is the voltage value of the DC power supply in the circuit module;

[0117] e α ,e β They are the α and β axis components of the voltage quantity e respectively;

[0118] The voltage The α and β axis components of

[0119] T a The inverter start time;

[0120] Since the orthogonal outputs e and The initial value is zero. In the half cycle after the inverter is started, that is, T a <t<T a Within +0.01s, the denominator of the reference current expression The fixed value is set to 5000, so that the reference current is very small in the initial stage to prevent a large impact. In the specific embodiment, the inverter output active power P is set under this working condition. * =6000W and reactive power Q * =0Var. Calculate the inverter reference current i according to formula (1): ref The α and β axis components.

[0121] 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:

[0122]

[0123] in:

[0124] Is the voltage The real-time amplitude of , and:

[0125] V p * yes The reference amplitude under rated working conditions is set as V p * =311V; k v is the voltage amplitude error coefficient. In this embodiment, k is set v =314;

[0126] Step 3.1.3: The self-synchronous control link A calculates the mutually orthogonal voltage quantities e and

[0127]

[0128] in:

[0129] ω0 is the rated frequency, s is the Laplace operator;

[0130] Δi e is the current error, current error Δi e By Δi e =i ref -i1 is obtained by calculation;

[0131] k p , k r are respectively the proportional coefficient and the resonance coefficient of the proportional resonant controller in the self-synchronous control link A; in this embodiment, k is set p =0.078,k r =116.35;

[0132] k s is the adjustment coefficient, Calculated, in this embodiment

[0133] Figure 2 The figure shows the structure of the self-synchronous control link used in this embodiment.

[0134] Step 3.1.4: Calculate the modulation voltage v of the inverter main circuit using equation (5): m , used for inverter circuit switch control;

[0135] v m =k p Δi e +e (5)

[0136] Figure 3 The waveforms of the voltage, grid-connected current and output power at the PCC point when the system of this embodiment is connected to the grid under normal operating conditions and the impedance of the grid-side line changes are shown. Before t = 0.10s, the grid-side line Z is set g =L g =0mH; t = 0.10s, 0.20s and 0.35s, the grid side line Z g Increases to L g =15mH, 20mH and 25mH. Figure 3 As shown, the voltage and grid-connected current at the PCC of the embodiment always remain stable, and the grid-connected current THD% is less than 2%, with good power quality; and the inverter power always follows the given value, with power scheduling capability.

[0137] Normal operating condition 2: Off-grid operation

[0138] Under normal operating condition 2, the modulation voltage v of the inverter main circuit is obtained as follows: m ;

[0139] Under normal operating condition 2, the grid-side circuit breaker S1 is disconnected, and the inverter independently carries the equivalent load Z L In this embodiment, the equivalent load Z is set L The active power P L =3000W, reactive power Q L =0;

[0140] The control module automatically starts control according to the following steps:

[0141] Step 3.2.1: Calculate the inverter reference current i according to step 3.1.1 in normal operation condition 1 ref ;

[0142] Step 3.2.2: Compensation current generation link C calculates the compensation current i according to formula (6) m :

[0143]

[0144] in:

[0145] i mα and i mβ They are the compensation current i m The α and β axis components of

[0146] P r and Q r are the actual active and reactive powers output from the inverter to the PCC, respectively, and are calculated by formula (7):

[0147]

[0148] in:

[0149] v pccα and v pccβ are the measured voltage v at the inverter PCC pcc The α and β axis components of

[0150] i 2α andi 2β They are the α-axis and β-axis components of the inverter grid-connected current i2 obtained by measurement;

[0151] Step 3.2.3: Calculate the voltage error compensation value Δv according to step 3.1.2 in normal operation condition 1;

[0152] Step 3.2.4: Calculate the mutually orthogonal voltages e and

[0153] Current error Δi e By Δi e =i ref -i m -i1 is obtained by calculation;

[0154] Step 3.2.5: Calculate the modulation voltage v of the inverter main circuit according to step 3.1.4 in normal operation condition 1 m , used for inverter circuit switch control;

[0155] Figure 4 The waveforms of the voltage, grid-connected current and output power of the PCC point of the system in this embodiment are shown in the off-grid operation under normal operating conditions. t = 0.01s The inverter system of this embodiment is off-grid and loaded. It can be found that the system of this example can quickly establish the PCC voltage under off-grid conditions and always maintain stability; at this time, the grid-connected current, that is, the current flowing into the PCC, is also always stable, and the THD% is less than 1%, and the power quality is high; the inverter output active power is maintained at around 3000W, and the reactive power is always 0Var, which is consistent with the set load power.

[0156] Normal operation condition 3: Switching from off-grid to grid-connected

[0157] In the normal operation condition 3 of this embodiment, the inverter is set to operate with a rated load of 3000W after off-grid startup, receive a grid-connected instruction for pre-synchronization 0.15s later, and connect to the grid normally after 0.25s of pre-synchronization, and adjust the rated power of the inverter to P * =6000W and Q * =1000Var, the control module performs control as follows:

[0158] Step 3.3.1: Calculate the inverter reference current i according to step 3.1.1 in normal operation condition 1 ref ;

[0159] Step 3.3.2: Calculate the compensation current i according to step 3.2.2 in normal operation condition 2. m ;

[0160] Step 3.3.3: The pre-synchronization current generation link F is calculated according to formula (8) to obtain the pre-synchronization current i pc :

[0161] i pc =(vv g )Yd (8)

[0162] Where: v is the actual output voltage of the inverter, v g Y is the measured grid voltage. d is the pre-synchronization virtual admittance; in this embodiment, Y is set d =0.02;

[0163] Step 3.3.4: Calculate the voltage error compensation value Δv according to step 3.1.2 in normal operation condition 1;

[0164] Step 3.3.5: Calculate the mutually orthogonal voltages e and

[0165] Current error Δi e By Δi e =i ref -i m -i pc -i1 is obtained by calculation;

[0166] Step 3.3.6: Calculate the modulation voltage v of the inverter main circuit according to step 3.1.4 in normal operation condition 1 m , used for inverter circuit switch control;

[0167] Figure 5 The PCC voltage, grid-connected current and output power waveforms of the system in this embodiment are shown when the system is switched from off-grid to grid-connected under normal operating conditions; before t = 0.15s, the system operates in normal off-grid load conditions, and its PCC voltage, grid-connected current and output power waveforms are the same as those of the system in this embodiment. Figure 4 The results are basically the same as shown in Figure 2. 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 The synchronization with the power grid is achieved within two cycles. At t = 0.25s, the system is formally connected to the grid and the pre-synchronization is completed. At this time, the output active power of the embodiment system is adjusted to P = 6000W, and the reactive power is adjusted to Q = 1000Var, which is consistent with the setting.

[0168] Normal operation condition 4: Switching from grid-connected to off-grid

[0169] The steps are the same as those in case 2 of normal operating condition.

[0170] In this embodiment, the phase-locked loop-free self-synchronous unified grid-connected inverter system is operated under fault conditions, and the inverter system implements unified control according to the following steps:

[0171] Step 4.1: The fault-crossing control module E determines and outputs the fault instruction x f, Low Voltage Ride Through Active and Reactive Power Reference and

[0172] Step 4.2: According to the fault instruction x f For different values ​​of , the inverter is controlled according to the following situations:

[0173] Fault operating condition 1: x f =1, the system is operating in the condition of accidental disconnection from the grid

[0174] When the measured grid voltage v g and the voltage at PCC v pcc When there is inconsistency, it is judged that the grid-side circuit breaker S1 at the voltage of PCC is accidentally disconnected. At this time, the inverter system enters the off-grid state, and the system puts into the compensation current generation link C to balance the proximal load according to the normal operating condition 2 step 3.2.2 in claim 3;

[0175] Fault operation condition 2: x f =2, the system is operating in the fault accident closing condition

[0176] The reference current generating link A and the compensation current generating link C are automatically disconnected first, and the reference current generating link A is automatically enabled after the system is stable, and the subsequent control is consistent with the control steps of the normal operation condition 1 system in the normal grid-connected operation in claim 3;

[0177] Fault operation condition 3: x f =3, the system operates in low voltage fault ride-through condition

[0178] The voltage amplitude error compensation module F is disconnected, and in order to meet the low-voltage fault ride-through requirements of different types of inverters, different reactive power settings are set according to the degree of grid-side voltage amplitude reduction. And active power given

[0179] Among them, the reactive power is given According to formula (9), we can obtain:

[0180]

[0181] in:

[0182] v pcca 、v pccb 、v pccc is the instantaneous voltage value of three phases at the system PCC;

[0183] U n is the amplitude of the grid voltage under rated working conditions. In this embodiment, U n =311V; Sn is the rated apparent power; U pu is the ratio of fault voltage to rated voltage;

[0184] S F is the apparent power after the fault condition of the power grid, S F It is calculated by formula (10):

[0185]

[0186] Where: U F is the three-phase voltage amplitude at the system PCC.

[0187] Active power setting Calculate according to formula (11):

[0188]

[0189] The fault active power is given and reactive power given As the reference power, the reference current i under the fault condition is calculated according to formula (1): refP , and then control is performed in accordance with the normal operating condition 1 in claim 3.

[0190] Figure 6 It shows that the system of this embodiment operates in a low voltage fault ride-through condition, and the voltage drops to 0.8U n When the voltage at PCC is v pcc When the voltage drops suddenly, the fault current generated affects the input current error of the inverter self-synchronous control link A, causing the output voltage amplitude and phase to change. However, due to the role of the voltage amplitude error compensation link C, it can achieve balance under a certain current error. Therefore, v pcc Dropped to 0.8U n When the reference current rises, it is still within the inverter overcurrent limit. However, due to the reduction of voltage amplitude, a certain amount of reactive power needs to be absorbed to reach a new steady state. Figure 6 As shown in the figure, before t = 0.1s, the inverter output active power is the rated 6000W, the output current amplitude is about 13A, and after t = 0.1s, the voltage drops to 0.8U n , the inverter output current amplitude rises to 21.5 A. At t=0.3s, the voltage returns to normal voltage, at which time the output active power of the system in this example is adjusted to the rated 6000W again, and the system in the embodiment completes the ride-through control under the fault condition.

Claims

1. A phase-locked loop-free self-synchronous unified grid-connected inverter system, characterized by: The system is composed 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 the detection signals in the circuit module by measuring: the voltage v at the inverter PCC pcc and grid-connected current i2, grid-side grid voltage v g , and the inverter side inductor current i1; the detection signal and the set inverter output active power P * And reactive power Q * As input, the inverter modulation voltage is output through the self-synchronization control link, the reference current generation link, the compensation current generation link, the pre-synchronization current generation link, the fault ride-through control link and the voltage amplitude error compensation link, so as to realize the phase-locked loop-free self-synchronization unified control of the grid-connected inverter system under multiple operating conditions.

2. The phase-locked loop-free self-synchronous unified grid-connected inverter system according to claim 1, characterized in that: The control module is composed of various control links, which are: 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-synchronous control link A outputs two mutually orthogonal voltage quantities e and and the modulation voltage v for the inverter main circuit m The parameters are: the measured inverter side inductor current i1, the reference current generating link B, the compensation current generating link C and the pre-synchronization current generating link D output current sum Δi r And the voltage error compensation amount Δv output by the voltage amplitude error compensation link F; The reference current generating link B is based on the voltage e and The inverter output active power P is set * And reactive power Q * , fault instruction x f , Low voltage ride through active power reference value and reactive power reference As input, calculate and output the inverter reference current i ref The compensation current generating link C is based on the voltage v at the inverter PCC pcc and the grid-connected current i2, and the voltage e and As input, calculate and obtain the grid-connected output compensation current i m The pre-synchronization current generation link D is based on the grid voltage v g , and the voltage e and As input, calculate and output the pre-synchronization current i pc The sum of the current Δi r is the inverter reference current i ref , compensation current i m and pre-synchronization current i pc sum; The fault ride-through control link E is based on the measured voltage v at the inverter PCC pcc , grid-connected current i2 and grid-side grid voltage v g , output fault instruction x f , and output low voltage ride through active power reference value and reactive power reference The voltage amplitude error compensation link F is based on the voltage amount e and as well as Reference amplitude under rated conditions As input, the voltage error compensation amount Δv is calculated and output.

3. The phase-locked loop-free self-synchronous unified grid-connected inverter system according to claim 2, characterized in that Under normal operating conditions, the inverter system implements unified control as follows: Normal operating condition 1: Grid-connected operation The reference current generation link B, the voltage amplitude error compensation link F and the self-synchronous control link A are automatically controlled according to the following steps: Step 3.1.1: The reference current generating link B calculates the inverter reference current i according to formula (1) ref : in: i refα and i refβ They are the inverter reference current i ref The α and β axis components of V DC is the voltage value of the DC power supply in the circuit module; e α ,e β are the α and β axis components of the voltage e respectively; The voltages are The α and β axis components of T a The inverter start 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: is the voltage The real-time amplitude of , and: V p * yes Reference amplitude under rated conditions; k v is the voltage amplitude error coefficient; Step 3.1.3: The self-synchronous control link A calculates the mutually orthogonal voltages e and in: ω0 is the rated frequency, s is the Laplace operator; Δi e is the current error, current error Δi e By Δi e =i ref -i1 is obtained by calculation; k p , k r are respectively the proportional coefficient and the resonance coefficient of the proportional resonant controller in the self-synchronous control link A; k s is the adjustment coefficient, Calculate and obtain; Step 3.1.4: Calculate the modulation voltage v of the inverter main circuit using equation (5): m , used for inverter circuit switch control; v m =k p Δi e +e (5) Normal operating condition 2: Off-grid operation The grid-side circuit breaker S1 is disconnected, and the inverter independently carries the equivalent load Z L run; The control module automatically starts control according to the following steps: Step 3.2.1: Calculate the inverter reference current i according to step 3.1.1 in normal operation condition 1 ref ; Step 3.2.2: The compensation current generating link C is calculated according to formula (6) to obtain the compensation current i m : in: i mα and i mβ They are the compensation current i m The α and β axis components of P r and Q r are the actual active and reactive powers output from the inverter to the PCC, respectively, and are calculated by formula (7): in: v pccα and v pccβ The voltage v at the inverter PCC obtained by measurement is pcc The α and β axis components of i 2α andi 2β They are the α-axis and β-axis components of the inverter grid-connected current i2 obtained by measurement; Step 3.2.3: Calculate the voltage error compensation value Δv according to step 3.1.2 in normal operation condition 1; Step 3.2.4: Calculate the mutually orthogonal voltages e and Current error Δi e By Δi e =i ref -i m -i1 is obtained by calculation; Step 3.2.5: Calculate the modulation voltage v of the inverter main circuit according to step 3.1.4 in normal operation condition 1 m , used for inverter circuit switch control; Normal operation condition 3: Switching from off-grid to grid-connected The control module performs control according to the following steps: Step 3.3.1: Calculate the inverter reference current i according to step 3.1.1 in normal operation condition 1 ref ; Step 3.3.2: Calculate the compensation current i according to step 3.2.2 in normal operation condition 2. m ; Step 3.3.3: The pre-synchronization current generation link F is calculated according to formula (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 grid voltage, Y d is the pre-synchronization virtual admittance; Step 3.3.4: Calculate the voltage error compensation value Δv according to step 3.1.2 in normal operation condition 1; Step 3.3.5: Calculate the mutually orthogonal voltages e and Current error Δi e By Δi e =i ref -i m -i pc -i1 is obtained by calculation; Step 3.3.6: Calculate the modulation voltage v of the inverter main circuit according to step 3.1.4 in normal operation condition 1 m , used for inverter circuit switch control; Normal operation condition 4: Switching from grid-connected to off-grid The steps are consistent with those of the normal operating condition 2.

4. The phase-locked loop-free self-synchronous unified grid-connected inverter system according to claim 2, characterized in that: Under fault operating conditions, the inverter system implements unified control according to the following steps: Step 4.1: The fault-through control module E determines and outputs a fault instruction x f , Low Voltage Ride Through Active and Reactive Power Reference and Step 4.2: According to the fault instruction x f For different values ​​of , the inverter is controlled according to the following situations: Fault operating condition 1: x f =1, the system is operating in a faulty and unexpectedly disconnected condition When the measured grid voltage v g and the voltage at PCC v pcc When there is inconsistency, it is judged that the grid-side circuit breaker S1 at the voltage of PCC is accidentally disconnected, and the inverter system enters the off-grid state. The system puts the compensation current generation link C into operation to balance the proximal load according to step 3.2.2 of normal operating condition 2 in claim 3; Fault operation condition 2: x f =2, the system is operating in the fault accident closing condition The reference current generating link A and the compensation current generating link C are automatically disconnected first, and the reference current generating link A is automatically enabled after the system is stable, and the subsequent control is consistent with the control steps of the system in normal grid-connected operation described in the normal operating condition 1 in claim 3; Fault operation condition 3: x f =3, the system operates in low voltage fault ride-through condition The voltage amplitude error compensation module F is disconnected, and in order to meet the low-voltage fault ride-through requirements of different types of inverters, different reactive power settings are set according to the degree of grid-side voltage amplitude reduction. And active power given Among them, the reactive power is given According to formula (9), we can obtain: in: v pcca 、v pccb 、v pccc is the instantaneous three-phase voltage value at the PCC of the system; U n is the amplitude of the grid voltage under rated conditions; S n is the rated apparent power; U pu is the ratio of fault voltage to rated voltage; S F is the apparent power after the fault condition of the power grid, S F It is calculated by formula (10): Where: U F is the three-phase voltage amplitude at the PCC of the system. Active power setting Calculate according to formula (11): The fault active power is given and reactive power given As the reference power, the reference current i under fault condition is calculated according to formula (1): refP , and then control is performed in accordance with the normal operating condition 1 in claim 3.

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