阅读说明：本技术 一种并网换流器并联虚拟电容的控制方法 (Control method for parallel virtual capacitor of grid-connected converter ) 是由 朱晓荣 韩丹慧 于 2018-07-20 设计创作，主要内容包括：本发明公开了一种用于并网换流器的并联虚拟电容的控制方法,包括以下步骤：建立直流微电网系统,信号测量与处理,确定并联虚拟电容的补偿函数,求外环参考电流,电流外环控制,矢量控制,αβ变换和SVPWM调制。本发明具有计算简单,不需增加硬件等优点；在直流微电网系统发生谐振时,本发明可以分别实现抑制直流母线电压振荡和提高直流微电网稳定性的控制目标。(The invention discloses a parallel virtual capacitor control method for a grid-connected converter, which comprises the following steps of establishing a direct current micro-grid system, measuring and processing signals, determining a compensation function of the parallel virtual capacitor, solving an outer ring reference current, performing current outer ring control, performing vector control, αβ transformation and SVPWM (space vector pulse width modulation).)

1. A parallel virtual capacitance control method of a grid-connected converter is characterized by comprising the following steps: the method comprises the following steps:

step 1: establishing a direct-current micro-grid system: the direct-current micro-grid system consists of a direct-current bus, a wind turbine generator, a storage battery, an alternating-current load, an AC-DC converter, a DC-DC converter, a grid-connected converter, an alternating-current power grid, an alternating-current measuring element, a direct-current measuring element, a filter and a control system; the wind turbine generator is connected to a direct current bus through an AC-DC converter, the storage battery is connected to the direct current bus through the DC-DC converter, an alternating current load is connected to the direct current bus through the AC-DC converter, an alternating current power grid is connected to the direct current bus through a grid-connected converter after passing through a filter, the input end of the control system is respectively connected with the output ends of the direct current measuring element and the alternating current measuring element, and the output end of the control system is connected with the input end of the grid-connected inverter;

step 2: signal measurement and processing: measuring the three-phase voltage e of the AC network by means of a voltage sensor_a、e_b、e_cAnd the three-phase voltage u of the grid-connected point of the grid-connected converter_a、u_b、u_cAnd measuring the DC voltage u of the DC bus_dcMeasuring three-phase current i on the AC side of the grid-connected converter through a sum current sensor_d、i_b、i_cDirect-current side current i of grid-connected converter_dcAnd an output current i_o(ii) a Three-phase grid power e measured by the AC measuring element_a、e_b、e_cCalculating a voltage phase angle theta of the power grid through phase-locked loop processing; three-phase voltage e measured by the AC measuring element_a、e_b、e_cCalculating the d-axis component e of the d-axis component in the dq rotation coordinate system through dq transformation_dAnd q-axis component e_q(ii) a Three-phase voltage u of converter grid-connected point measured by the alternating current measuring element_a、u_b、u_cD-axis component u of the d-axis vector in a dq rotating coordinate system is calculated through dq transformation_dAnd q-axis component u_q(ii) a Three-phase grid-connected current i measured by the alternating current measuring element_a、i_b、i_cD-axis component i in dq rotation coordinate system is calculated through dq transformation_dAnd q-axis component i_q；

And step 3: determining a compensation function of the parallel virtual capacitance: considering the dynamic course of the DC bus voltage, DC current i_dcOutput current i_oAnd a DC voltage u_dcThe relationship between them is:

the expression converted into s-domain is:

in practice, the dc voltage is:

therefore, let the feedforward compensation amount be:

let the compensation function be f(s):

wherein: c_virIs a virtual capacitance value; c is a direct-current side capacitor of the grid-connected converter;

and 4, step 4: calculating the reference current i of the outer loop_dc ^*: the outer loop reference current is determined by the droop control and the feedforward compensation amount generated by the parallel virtual capacitor:

wherein: k is a droop coefficient; u. of_NIs a DC bus reference voltage;

and 5: current outer loop control: using constant voltage control, hence i_q ^*The outer loop control is controlled by PI:

wherein: k is a radical of_pl、k_ilProportional integral parameters of the current outer loop PI controller;

and 7: vector control: reference value i_d ^*And i_q ^*Comparing with actual value, performing PI control on the error, and outputting voltage control signal u by voltage feedforward compensation and cross coupling compensation_dAnd u_q：

Wherein: l is a parasitic resistance of the filter; omega_eIs the synchronous rotation angular frequency; g_i(s)＝k_p+k_i/s，k_p、k_iProportional integral parameters of the current inner loop PI controller;

because the current inner loop control is much faster than the outer loop regulation speed, the inner loop is simplified to a first-order lag link in the control block diagram:

solving the current i on the DC side of the grid-connected converter_dc: according to power balance, active power expressions at the alternating current side and the direct current side of the grid-connected converter are as follows:

wherein: p is active power of alternating current and direct current sides of the grid-connected converter;

step (ii) of8: αβ transformation of the control voltage u to be in dq coordinate system_dAnd u_qThe control voltage u under the αβ coordinate system is obtained after αβ transformation_αAnd u_βWherein u is_αIs an α axis component, u_ββ axis component;

and step 9: SVPWM modulation: will control the voltage u_αAnd u_βObtaining a three-phase modulation signal s of the grid-connected inverter after SVPWM modulation_a、s_b、s_cAnd the voltage is sent to a switching tube of the grid-connected inverter for control.

Technical Field

The invention relates to a control method of a grid-connected converter, in particular to a current feedforward control method of a parallel virtual capacitor of the grid-connected converter, and belongs to the technical field of power supply control.

Background

The micro-grid is an important component of a future intelligent power distribution and utilization system and has important significance for promoting energy conservation and emission reduction and realizing sustainable development of energy. Compared with an alternating-current micro-grid, the direct-current micro-grid can more efficiently and reliably receive distributed renewable energy power generation systems such as wind and light, energy storage units, electric vehicles and other direct-current power loads. A large number of connected distributed power supplies and direct current loads are connected into a direct current microgrid through a DC-DC or AC-DC converter, the direct current microgrid has obvious constant power load characteristics, a multi-converter connection environment is formed, random fluctuation power of the multi-converter connection environment can not only impact direct current bus voltage, but also easily induce system resonance to influence stability of the direct current microgrid.

To improve the stability of the dc microgrid, guo li et al reported in china electro-mechanical engineering, 2016, 36 (04): 927-. Wangxiang et al in power automation equipment, 2017, 37 (05): 92-99 "wind power direct current microgrid small signal stability analysis under island mode" discloses a small signal impedance ratio stability analysis method, and the method is used for analyzing the influence of key parameters such as bus voltage, ride-through frequency, stator resistance and filter capacitance on the output impedance of a direct current end and has a guiding effect on the parameter design of a wind power direct current microgrid system. Huhuheroic et al in grid technology, 2017, 41 (08): 2664-. Sunjianlong et al, report on electrotechnical sciences, 2016, 31 (04): 194-202- 'direct-current peer-to-peer microgrid hybrid energy storage system coordination control strategy' discloses a coordination control strategy based on lithium ion battery and super capacitor hybrid energy storage, which can effectively inhibit direct-current bus voltage impact and fluctuation and remarkably improve system dynamic response.

The control strategy is generally adopted for the DC-DC converter, and is suitable for a DC micro-grid in an island mode, and in a grid-connected mode, the grid-connected converter is used as a main link for adjusting the voltage of a DC bus, and the control strategy is limited.

Disclosure of Invention

The invention aims to provide a parallel virtual capacitor control method of a grid-connected converter.

The invention adopts the following technical scheme:

a parallel virtual capacitance control method of a grid-connected converter is characterized by comprising the following steps: the method comprises the following steps:

step 1: establishing a direct-current micro-grid system: the direct-current micro-grid system consists of a direct-current bus, a wind turbine generator, a storage battery, an alternating-current load, an AC-DC converter, a DC-DC converter, a grid-connected converter, an alternating-current power grid, an alternating-current measuring element, a direct-current measuring element, a filter and a control system; the wind turbine generator is connected to a direct current bus through an AC-DC converter, the storage battery is connected to the direct current bus through the DC-DC converter, an alternating current load is connected to the direct current bus through the AC-DC converter, an alternating current power grid is connected to the direct current bus through a grid-connected converter after passing through a filter, the input end of the control system is respectively connected with the output ends of the direct current measuring element and the alternating current measuring element, and the output end of the control system is connected with the input end of the grid-connected inverter;

step 2: signal measurement and processing: measuring the three-phase voltage e of the AC network by means of a voltage sensor_a、e_b、e_cAnd the three-phase voltage u of the grid-connected point of the grid-connected converter_a、u_b、u_cAnd measuring the DC voltage u of the DC bus_dcMeasuring three-phase current i on the AC side of the grid-connected converter through a sum current sensor_a、i_b、i_cDirect-current side current i of grid-connected converter_dcAnd an output current i_o(ii) a Three-phase grid power e measured by the AC measuring element_a、e_b、e_cCalculating a voltage phase angle theta of the power grid through phase-locked loop processing; three-phase voltage e measured by the AC measuring element_a、e_b、e_cCalculating the d-axis component e of the d-axis component in the dq rotation coordinate system through dq transformation_dAnd q-axis component e_q(ii) a Three-phase voltage u of converter grid-connected point measured by the alternating current measuring element_a、u_b、u_cD-axis component u of the d-axis vector in a dq rotating coordinate system is calculated through dq transformation_dAnd q-axis component u_q(ii) a Three-phase grid-connected current i measured by the alternating current measuring element_a、i_b、i_cD-axis component i in dq rotation coordinate system is calculated through dq transformation_dAnd q-axis component i_q；

And step 3: determining a compensation function of the parallel virtual capacitance: considering the dynamic course of the DC bus voltage, DC current i_dcOutput current i_oAnd a DC voltage u_dcThe relationship between them is:

the expression converted into s-domain is:

in practice, the dc voltage is:

therefore, let the feedforward compensation amount be:

let the compensation function be f(s):

wherein: c_virIs a virtual capacitance value; c is a direct-current side capacitor of the grid-connected converter;

and 4, step 4: calculating the reference current i of the outer loop_dc ^*: the outer loop reference current is determined by the droop control and the feedforward compensation amount generated by the parallel virtual capacitor:

wherein: k is a droop coefficient; u. of_NIs a DC bus reference voltage;

and 5: current outer loop control: using constant voltage control, hence i_q ^*The outer loop control is controlled by PI:

wherein: k is a radical of_pl、k_ilProportional integral parameters of the current outer loop PI controller;

and 7: vector control: reference value i_d ^*And i_q ^*Comparing with actual value, performing PI control on the error, and outputting voltage control signal u by voltage feedforward compensation and cross coupling compensation_dAnd u_q：

Wherein: l is a parasitic resistance of the filter; omega_eIs the synchronous rotation angular frequency; g_i(s)＝k_p+k_i/s，k_p、k_iProportional integral parameters of the current inner loop PI controller;

because the current inner loop control is much faster than the outer loop regulation speed, the inner loop is simplified to a first-order lag link in the control block diagram:

solving the current i on the DC side of the grid-connected converter_dc: according to power balance, active power expressions at the alternating current side and the direct current side of the grid-connected converter are as follows:

wherein: p is active power of alternating current and direct current sides of the grid-connected converter;

αβ transformation, control voltage u in dq coordinate system_dAnd u_qThe control voltage u under the αβ coordinate system is obtained after αβ transformation_αAnd u_βWherein u is_αIs an α axis component, u_ββ axis component;

and step 9: SVPWM modulation: will control the voltage u_αAnd u_βObtaining a three-phase modulation signal s of the grid-connected inverter after SVPWM modulation_a、s_b、s_cAnd the voltage is sent to a switching tube of the grid-connected inverter for control.

Compared with the prior art, the invention has the advantages that:

1. the invention does not need to add extra controllers and hardware devices.

2. The invention can realize the control target of improving the stability margin of the system and eliminating the voltage fluctuation of the direct current bus when the direct current micro-grid generates resonance. And the parallel capacitance compensation function of current feedforward is simpler to calculate.

Drawings

FIG. 1 is a flow chart of the present invention;

FIG. 2 is a schematic diagram of a DC microgrid configuration;

FIG. 3 is a schematic diagram of a control strategy after virtual capacitors are connected in parallel;

FIG. 4 is a functional block diagram of the control system after the virtual capacitors are connected in parallel;

Detailed Description

A parallel virtual capacitance control method of a grid-connected converter is shown in figure 1 and is characterized in that: the method comprises the following steps:

step 1: establishing a direct-current micro-grid system: the direct-current micro-grid system consists of a direct-current bus, a wind turbine generator, a storage battery, an alternating-current load, an AC-DC converter, a DC-DC converter, a grid-connected converter, an alternating-current power grid, an alternating-current measuring element, a direct-current measuring element, a filter and a control system; the wind turbine generator is connected to a direct current bus through an AC-DC converter, the storage battery is connected to the direct current bus through the DC-DC converter, an alternating current load is connected to the direct current bus through the AC-DC converter, an alternating current power grid is connected to the direct current bus through a grid-connected converter after passing through a filter, the input end of the control system is respectively connected with the output ends of the direct current measuring element and the alternating current measuring element, and the output end of the control system is connected with the input end of the grid-connected inverter;

step 2: signal measurement and processing: measuring the three-phase voltage of the AC network by means of a voltage sensore_a、e_b、e_cAnd the three-phase voltage u of the grid-connected point of the grid-connected converter_a、u_b、u_cAnd measuring the DC voltage u of the DC bus_dcMeasuring three-phase current i on the AC side of the grid-connected converter through a sum current sensor_a、i_b、i_cDirect-current side current i of grid-connected converter_dcAnd an output current i_o(ii) a Three-phase grid power e measured by the AC measuring element_a、e_b、e_cCalculating a voltage phase angle theta of the power grid through phase-locked loop processing; three-phase voltage e measured by the AC measuring element_a、e_b、e_cCalculating the d-axis component e of the d-axis component in the dq rotation coordinate system through dq transformation_dAnd q-axis component e_q(ii) a Three-phase voltage u of converter grid-connected point measured by the alternating current measuring element_a、u_b、u_cD-axis component u of the d-axis vector in a dq rotating coordinate system is calculated through dq transformation_dAnd q-axis component u_q(ii) a Three-phase grid-connected current i measured by the alternating current measuring element_a、i_b、i_cD-axis component i in dq rotation coordinate system is calculated through dq transformation_dAnd q-axis component i_q；

And step 3: determining a compensation function of the parallel virtual capacitance: considering the dynamic course of the DC bus voltage, DC current i_dcOutput current i_oAnd a DC voltage u_dcThe relationship between them is:

the expression converted into s-domain is:

in practice, the dc voltage is:

therefore, let the feedforward compensation amount be:

let the compensation function be f(s):

wherein: c_virIs a virtual capacitance value; c is a direct-current side capacitor of the grid-connected converter;

and 4, step 4: calculating the reference current i of the outer loop_dc ^*: the outer loop reference current is determined by the droop control and the feedforward compensation amount generated by the parallel virtual capacitor:

wherein: k is a droop coefficient; u. of_NIs a DC bus reference voltage;

and 5: current outer loop control: using constant voltage control, hence i_q ^*The outer loop control is controlled by PI:

wherein: k is a radical of_pl、k_ilProportional integral parameters of the current outer loop PI controller;

and 7: vector control: reference value i_d ^*And i_q ^*Comparing with actual value, performing PI control on the error, and outputting voltage control signal u by voltage feedforward compensation and cross coupling compensation_dAnd u_q：

Wherein: l is a parasitic resistance of the filter; omega_eIs the same asRotating angular frequency; g_i(s)＝k_p+k_i/s，k_p、k_iProportional integral parameters of the current inner loop PI controller;

because the current inner loop control is much faster than the outer loop regulation speed, the inner loop is simplified to a first-order lag link in the control block diagram:

solving the current i on the DC side of the grid-connected converter_dc: according to power balance, active power expressions at the alternating current side and the direct current side of the grid-connected converter are as follows:

wherein: p is active power of alternating current and direct current sides of the grid-connected converter;

αβ transformation, control voltage u in dq coordinate system_dAnd u_qThe control voltage u under the αβ coordinate system is obtained after αβ transformation_αAnd u_βWherein u is_αIs an α axis component, u_ββ axis component;

and step 9: SVPWM modulation: will control the voltage u_αAnd u_βObtaining a three-phase modulation signal s of the grid-connected inverter after SVPWM modulation_a、s_b、s_cAnd the voltage is sent to a switching tube of the grid-connected inverter for control.

The present embodiment employs a voltage source grid-connected inverter. The dc voltage reference value is generally a dc-side rated voltage, which in this example is 400V.

The whole direct-current micro-grid system consists of a wind turbine generator, a storage battery, a constant-power load, an AC-DC converter, a DC-DC converter, a direct-current bus, a grid-connected converter, a filter and a power grid; the control of an AC-DC converter connected with the wind turbine generator mainly realizes maximum power tracking; the control of a DC-DC converter connected with the storage battery mainly realizes constant power control; the control of the grid-connected converter mainly realizes the control of direct-current voltage; the invention mainly aims at the control of a grid-connected converter, namely a control strategy of serially connecting virtual impedance when a direct current micro-grid is unstable by a virtual inertia control system.

Considering the dynamic course of the DC bus voltage, DC current i_dcOutput current i_oAnd a DC voltage u_dcThe relationship between them is:

the expression converted into s-domain is:

in practice, the dc voltage is:

therefore, let the feedforward compensation amount be:

let the compensation function be f(s):

wherein: c_virIs a virtual capacitance value; c is a direct-current side capacitor of the grid-connected converter;

the outer loop reference current is determined by the droop control and the feedforward compensation amount generated by the parallel virtual capacitor:

wherein: k is a droop coefficient; u. of_NIs a DC bus reference voltage;

using constant voltage control, hence i_q ^*The outer loop control is controlled by PI:

wherein: k is a radical of_pl、k_ilProportional integral parameters of the current outer loop PI controller;

reference value i_d ^*And i_q ^*Comparing with actual value, performing PI control on the error, and outputting voltage control signal u by voltage feedforward compensation and cross coupling compensation_dAnd u_q：

Wherein: l is a parasitic resistance of the filter; omega_eIs the synchronous rotation angular frequency; g_i(s)＝k_p+k_i/s，k_p、k_iProportional integral parameters of the current inner loop PI controller;

control voltage u to be in dq coordinate system_dAnd u_qThe control voltage u under the αβ coordinate system is obtained after αβ transformation_αAnd u_βWherein u is_αIs an α axis component, u_ββ axis component;

will control the voltage u_αAnd u_βObtaining a three-phase modulation signal s of the grid-connected inverter after SVPWM modulation_a、s_b、s_cAnd the voltage is sent to a switching tube of the grid-connected inverter for control.

Fig. 3 gives a schematic diagram of the control strategy and fig. 4 gives a control block diagram of the control principle. The dashed box shows the current feedforward compensation part after the virtual capacitor is connected in parallel.