CN102761127B - Reactive current compensation method for three-phase grid-connected inverter in situation of unbalanced drop of grid - Google Patents

Abstract

The invention discloses a reactive current compensation method for a three-phase grid-connected inverter when the power grid is unbalanced and drops, and is characterized in that: for grid-connected power generation in which a DC side power source is merged into the power grid through a three-phase grid-connected inverter When the grid voltage drops unbalancedly, reactive power compensation is performed by increasing the reactive current output by the three-phase grid-connected inverter in the phase of the grid-connected grid drop, and at the same time changing the active current of the corresponding phase to offset the reactive current to the three-phase current and is zero. The invention realizes the independent compensation of reactive power under the unbalanced drop condition of the power grid, and avoids the reactive power over-compensation of the non-dropped phase under the unbalanced drop condition.

Description

Reactive current compensation method of three-phase grid-connected inverter when the grid is unbalanced and drops

technical field

The invention relates to a reactive current compensation method for a three-phase grid-connected inverter when the grid is unbalanced and drops. It belongs to the field of power electronics application technology, and mainly relates to the reactive power compensation technology of grid-connected inverters under the condition of unbalanced power grid drop.

Background technique

In recent years, with the increase in the number of grid-connected power generation systems connected to the grid, the capacity of grid-connected power generation systems has also increased. The requirements are getting higher and higher. Reactive power compensation is an important part of low-voltage ride-through control. Immediate and effective reactive power compensation can enhance the low-voltage ride-through capability of the grid-connected system to a certain extent. Reactive power compensation during the low voltage ride-through process of the power generation system puts forward higher requirements. For example, the relevant requirements for reactive power compensation during unbalanced drop given by the German Energy and Water Industry Association are: when the grid voltage occurs unbalanced drop, and The grid power generation system can maintain a certain grid-connected working time and provide reactive power support to the grid. Reactive power compensation is required for the grid voltage drop phase, and the phase without voltage drop does not generate reactive power or generates less reactive power (the voltage must not exceed 110% of the rated voltage). The traditional three-phase grid-connected inverter system has no The power compensation has not considered the unbalanced state of the power grid, and it is easy to cause reactive power over-compensation of the phase where the power grid voltage does not drop in this case.

Contents of the invention

In order to avoid the disadvantages of the above-mentioned prior art, the present invention provides a reactive current compensation method for a three-phase grid-connected inverter when the power grid is unbalanced and drops, so as to realize grid-connected under the condition of unbalanced and dropped power grid The reactive power of the inverter system is independently compensated to avoid reactive power over-compensation of the unfallen phase in the case of unbalanced drop.

The present invention adopts following technical scheme for solving technical problems:

The characteristics of the reactive current compensation method for the three-phase grid-connected inverter of the present invention when the power grid is unbalanced and dropped are: for a grid-connected power generation system that incorporates the DC side power supply into the grid through the three-phase grid-connected inverter, the grid voltage When an unbalanced drop occurs, reactive power compensation is performed by increasing the reactive current output by the three-phase grid-connected inverter in the dropped phase of the grid, and at the same time changing the active current of the corresponding phase to offset the reactive current until the sum of the three-phase currents is zero.

The reactive current compensation method of the three-phase grid-connected inverter of the present invention when the power grid is unbalanced and dropped is also characterized in that the compensation method is performed according to the following steps:

Step 1: In a three-phase grid-connected inverter power generation system, the difference between the active component and reactive component of each phase current after decoupling is 90 degrees. Assuming that the voltage drop occurs in phase A of the grid, it is determined by formula (1) that no Work compensation current:

$\left\{\begin{array}{c}{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; a</span>}<span\; class="notranslate">\; \_</span>\mathrm{<span\; class="notranslate">\; Q</span>}}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; E.</span>}}_{\mathrm{<span\; class="notranslate">\; a</span>}<span\; class="notranslate">\; \_</span>\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; /</span>{\mathrm{<span\; class="notranslate">\; E.</span>}}_{\mathrm{<span\; class="notranslate">\; base</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \&CenterDot;</span>\mathrm{<span\; class="notranslate">\; K</span>}\mathrm{<span\; class="notranslate">\; sin</span>}\mathrm{<span\; class="notranslate">\; \&theta;</span>}<span\; class="notranslate">\; ;</span>\\ {\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; b</span>}<span\; class="notranslate">\; \_</span>\mathrm{<span\; class="notranslate">\; Q</span>}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0</span>}<span\; class="notranslate">\; ;</span>\\ {\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}<span\; class="notranslate">\; \_</span>\mathrm{<span\; class="notranslate">\; Q</span>}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0</span>}<span\; class="notranslate">\; ;</span>\end{array}\right.<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

In formula (1), I _{a_Q} , I _{b_Q} and I _{c_Q} are the reactive power compensation currents of phase A, phase B and phase C respectively; K is the current compensation coefficient of phase A, E _base is the voltage amplitude before the grid drop of phase A, and E _{a_m} is the voltage amplitude after the phase A grid drops, and θ is the voltage angle of the A phase grid;

Step 2: Use the corresponding active component currents I _{b_P} and I _{c_P} of phase B and phase C obtained by formula (2) to offset the reactive power compensation current of phase A:

In formula (1) and formula (2): I _{a_Q} +I _{b_P} +I _{c_P} ＝0

The reactive current compensation method of the three-phase grid-connected inverter of the present invention when the power grid is unbalanced and dropped is also characterized in that the compensation method is performed according to the following steps:

Step 1, when two-phase unbalanced drop occurs in the power grid, assuming that the voltage drop occurs in phase A and phase B of the power grid, and the drop amplitude is the same, the reactive power compensation currents of phase A, phase B and phase C are obtained by formula (3), respectively are I _{a_Q} , I _{b_Q} and I _{c_Q} ;

Step 2: Offset A, B by the active current components I _{b_P} , I _{c_P} ' and I _{a_P} , I _{c_P} of the two phases B and C and the two phases A and C obtained through formula (4) and formula (5) respectively Two-phase reactive power compensation current, the I _{c_P} ' and I _{c_P} " are respectively the C-phase active current component corresponding to the A-phase reactive power compensation current, and the C-phase active current component corresponding to the B-phase reactive power compensation current;

Then, the three-phase active component current is obtained by formula (6):

_{Ic_P} = _{Ic_P} '+ _{Ic_P} " (6).

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

The grid-connected power generation system of the present invention is a three-phase grid-connected inverter power generation system. When the grid voltage drops unbalancedly, the reactive power compensation current of the three-phase three-wire grid-connected inverter is controlled to realize the power grid unbalanced drop condition. The reactive power of the grid-connected inverter system is compensated independently. Through the control of the grid-connected current, the reactive power of the falling phase of the grid is effectively and independently compensated. Avoid reactive power over-compensation of unsag phase and unsag phase of grid. An effective reactive power compensation algorithm is provided for the grid-connected power generation system under the condition of unbalanced drop. By increasing the active power output of the corresponding phase, the zero-sequence current brought by the reactive power compensation current is offset, so that the sum of the output currents of the three-phase inverters is zero during the grid voltage drop, and the zero-sequence current is avoided. the adverse effects of

Description of drawings

Fig. 1 is a schematic structural diagram of a three-phase grid-connected inverter power generation system of the present invention;

Fig. 2 is a schematic diagram of a single-phase unbalanced drop reactive power compensation method of the present invention;

Fig. 3 is a schematic diagram of the two-phase unbalanced drop reactive power compensation method of the present invention.

Detailed ways

As shown in Figure 1, the structure of the three-phase grid-connected inverter power generation system consists of a three-phase grid-connected inverter 102 and a power grid 104. The function of the three-phase grid-connected inverter 102 is to transfer the energy of the DC The power grid 104 is connected to generate electricity; the grid part usually includes a line impedance L and a transformer 103. Since the transformer 103 is usually a star-delta connection, the zero-sequence current cannot pass through, and the zero-sequence current generated by the inverter will affect the stable operation of the equipment. Magnetic flux leakage and excitation of the iron core will occur in the transformer, and the zero-sequence current needs to be eliminated. The reactive current compensation method of the present invention for the three-phase grid-connected inverter when the power grid is unbalanced and dropped is: for the three-phase grid-connected inverter power generation system, when the grid voltage is unbalanced and dropped, by increasing the three-phase grid-connected inverter The reactive current output by the converter in the power grid drop phase is used for reactive power compensation. The magnitude of the reactive power compensation is proportional to the magnitude of the power grid drop. At the same time, the active current of the corresponding phase is changed to offset the reactive current until the sum of the three-phase current is zero. .

Example 1:

Assuming that there is a voltage drop in phase A of the power grid, the compensation method is carried out as follows:

Step 1: In the three-phase grid-connected inverter power generation system, as shown in Figure 2, A, B, and C are the three-phase active coordinate axes, and A', B', and C' are the three-phase reactive coordinate axes, respectively. The difference between each corresponding active coordinate axis and reactive coordinate axis is 90°. According to the reactive power compensation rule when the grid voltage is unbalanced and drops, when the voltage drops in phase A of the grid, the reactive compensation current is determined by formula (1):

$\left\{\begin{array}{c}{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; a</span>}<span\; class="notranslate">\; \_</span>\mathrm{<span\; class="notranslate">\; Q</span>}}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; E.</span>}}_{\mathrm{<span\; class="notranslate">\; a</span>}<span\; class="notranslate">\; \_</span>\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; /</span>{\mathrm{<span\; class="notranslate">\; E.</span>}}_{\mathrm{<span\; class="notranslate">\; base</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \&CenterDot;</span>\mathrm{<span\; class="notranslate">\; K</span>}\mathrm{<span\; class="notranslate">\; sin</span>}\mathrm{<span\; class="notranslate">\; \&theta;</span>}<span\; class="notranslate">\; ;</span>\\ {\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; b</span>}<span\; class="notranslate">\; \_</span>\mathrm{<span\; class="notranslate">\; Q</span>}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0</span>}<span\; class="notranslate">\; ;</span>\\ {\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}<span\; class="notranslate">\; \_</span>\mathrm{<span\; class="notranslate">\; Q</span>}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0</span>}<span\; class="notranslate">\; ;</span>\end{array}\right.<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

In formula (1), I _{a_Q} , I _{b_Q} and I _{c_Q} are the reactive power compensation currents of phase A, phase B and phase C respectively; K is the current compensation coefficient of phase A (the compensation coefficient is determined by the power grid management department according to the specific power grid), and E _base is the voltage amplitude before the drop of the A-phase grid, E _{a_m} is the voltage amplitude after the drop of the A-phase grid, θ is the voltage angle of the A-phase grid; where (1-E _{a_m} /E _base ) is the calculation formula of the grid voltage drop .

As shown in Figure 2, there is a reactive power compensation current I _{a_Q} on the A-phase reactive coordinate axis after compensation, and the reactive power compensation currents of the B and C phases are 0.

Step 2: As shown in Figure 2, in order to offset the reactive power generated by phase A, two active current components of equal size can be applied on the negative active power axis of phase B and the active power axis of phase C, and the formula (2 ) The corresponding active component currents I _{b_P} and I _{c_P} of phase B and phase C obtained to offset the reactive power compensation current of phase A:

As shown in Figure 2, since the difference between the active coordinate axis and the reactive coordinate axis of each phase is 90°, and the difference between each phase of the three phases is 120°, the vector angle of I _{b_P} is θ-120°-90°, and I _{c_P} The vector angle is θ+120°+90°.

Since the relationship between the reactive power compensation current of phase A and the active current components of phase B and C is:

I _{a_Q} +I _{b_P} +I _{c_P} =0

In formula (1) and formula (2), the active component current can offset the reactive compensation current issued.

Example 2:

Assuming that the voltage drop occurs in phase A and phase B of the power grid, and the drop amplitude is the same, the compensation method is exactly the same as that in embodiment 1. The difference is that the calculation of reactive power compensation current and active component current is performed separately for phase A and phase B. , and finally superimpose the calculated vectors on the same phase, proceed as follows:

Step 1, when two-phase unbalanced drop occurs in the power grid, assuming that the voltage drop occurs in phase A and phase B of the power grid, and the drop amplitude is the same, the reactive compensation currents of phase A and phase B are obtained by formula (3) as I _{a_Q} , I _{b_Q} , as shown in Figure 3:

Step 2: As shown in Figure 3, the active current components I _{b_P} , I _{c_P} ' and I _{a_P} , I _{c_P} of the two phases B and C and the two phases A and C obtained by formula (4) and formula (5) respectively ” to offset the two-phase reactive compensation currents of A and B, I _{c_P} ' and I _{c_P} ” are the active current component of phase C corresponding to the reactive compensation current of phase A, and the active current component of phase C corresponding to the reactive compensation current of phase B :

The magnitudes of the active power compensation components are also:

$\frac{\sqrt{\mathrm{<span\; class="notranslate">\; 3</span>}}}{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; E.</span>}}_{\mathrm{<span\; class="notranslate">\; a</span>}<span\; class="notranslate">\; \_</span>\mathrm{<span\; class="notranslate">\; m</span>}}}{{\mathrm{<span\; class="notranslate">\; E.</span>}}_{\mathrm{<span\; class="notranslate">\; base</span>}}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \&CenterDot;</span>\mathrm{<span\; class="notranslate">\; K</span>}$

Then, the three-phase active component current is obtained by formula (6):

I _{c_P} ＝I _{c_P} ′+I _{c_P} ″ (6)

The reactive power compensation current and active component current calculated by the above formula can also satisfy the mutual cancellation effect, so:

I _{a_Q} +I _{b_Q} +I _{a_P} +I _{b_P} +I _{c_P} =0

Since the output grid-connected current of the grid-connected inverter at this time is an unbalanced current including negative sequence components, large ripples will appear on the DC side of the inverter. However, when the power grid drops, there is no ripple requirement for the DC side voltage of the inverter. Can not be considered.

Claims (2)

1. the reactive-current compensation method of three-phase grid-connected inverter in the time that unbalanced power supply falls, it is characterized in that: for the grid-connected system that DC side power supply is connected to the grid by three-phase grid-connected inverter, in the time that line voltage generation imbalance is fallen, carry out reactive power compensation by increasing the reactive current that three-phase grid-connected inverter falls mutually output at electrical network, the active current that simultaneously changes corresponding phase is offset reactive current to three-phase current and is zero;

Described compensation method is carried out as follows:

Step 1: in three-phase grid-connected inverter electricity generation system, every phase current corresponding real component and idle component after decoupling zero differs 90 degree, supposes to occur as electrical network A that mutually voltage falls, and determines reactive power compensation electric current by formula (1):

In formula (1), I _{a_Q}, I _{b_Q}and I _{c_Q}be respectively A phase, B phase and C phase reactive power compensation electric current; K is A phase current penalty coefficient, E _basefor A phase electrical network falls front voltage magnitude, E _{a_m}for A phase electrical network falls rear voltage magnitude, θ is A phase line voltage angle;

Step 2: by the B phase that obtained by formula (2) and the corresponding real component electric current I of C phase _{b_P}and I _{c_P}offset A phase reactive power compensation electric current:

In formula (1) and formula (2): I _{a_Q}+ I _{b_P}+ I _{c_P}=0.

2. the reactive-current compensation method of three-phase grid-connected inverter according to claim 1 in the time that unbalanced power supply falls, is characterized in that described compensation method carries out as follows:

Step 1, in the time that electrical network generation two-phase imbalance is fallen, suppose to occur with B that mutually voltage falls when electrical network A phase, and it is consistent to fall amplitude, and the reactive power compensation electric current that through type (3) obtains A phase, B phase and C phase is respectively I _{a_Q}, I _{b_Q}and I _{c_Q};

Step 2: B, C two-phase and the A being obtained by through type (4) and formula (5) respectively, the active current I of C two-phase _{b_P}, I _{c_P}' and I _{a_P}, I _{c_P}" offset A, B two-phase reactive power compensation electric current, described I _{c_P}' and I _{c_P}" be respectively the C phase active current of corresponding A phase reactive power compensation electric current and the C phase active current of corresponding B phase reactive power compensation electric current;

Obtaining three-phase real component electric current by formula (6) is again:

I _{c_P}=I _{c_P}′+I _{c_P}″ (6)。