CN117293888A - Weak current network low voltage treatment method based on back-to-back modularized multi-level converter - Google Patents

Abstract

The invention belongs to the technical field of flexible direct current power distribution, and discloses a weak current network low voltage treatment method based on back-to-back modularized multi-level converters. In a weak grid distribution system, the distribution line distance is long, resulting in a large line impedance. The user voltage drops significantly when its user uses electricity. In the invention, a weak grid ac bus delivers ac to MMCs on the distribution grid side. At this time, the MMC works in a rectifying mode, and the direct-current voltage at the output end is higher than the original alternating-current voltage. The MMC distributes direct current to the MMC at the user end on the original alternating current line. The MMC works in an inversion mode, and converts direct current of a direct current bus into alternating current to be transmitted to a user. Finally, the voltage on the distribution line of the weak current network is improved through back-to-back MMC, and the problem of low voltage of the user of the weak current network is solved.

Description

A kind of weak grid low voltage management based on back-to-back modular multi-level converter method

Technical field

The invention relates to the application field of flexible DC power distribution technology, specifically a low-voltage management method for weak power grids based on back-to-back modular multi-level converters.

Background technique

For long-distance low-voltage centralized power supply scenarios for multiple households, there are common characteristics such as large power supply radius, long transmission lines, low power reliability, and poor power quality in Taiwan areas. Affected by the power supply distance, there will be serious loss of active power and reactive power during the power supply process. Therefore, when distributing power to long-distance users, serious voltage drops will occur at the user end, and the power supply reliability will be severe. The supply load voltage is often Below the 200V minimum voltage requirement.

As a new generation of DC transmission technology, flexible DC transmission technology has outstanding advantages such as independent control of active and reactive power, fast and flexible response, and strong scalability. Traditional power transmission technology will produce high losses due to long distances, resulting in significant voltage drops. Traditional two-level converters have high switching frequency, large losses, strong electromagnetic noise, high harmonic content in the output voltage, and expensive high-voltage capacitor banks installed on the DC bus side. These problems limit the use of two-level converters. Applications in voltage applications. Modular multilevel converter, English: modular multilevel converter, abbreviation: MMC. MMC uses low-voltage capacitors and has the characteristics of low sub-module switching frequency, low loss, strong scalability, high conversion efficiency, simple structure and easy maintenance.

Contents of the invention

In order to solve the problem of high losses in long-distance power distribution, which leads to serious voltage drops at the user end, the present invention uses back-to-back modular multi-level converters to realize flexible DC power distribution technology over long distances, while ensuring that voltage requirements are met. Under the premise, the stability of the grid side and the user side should be improved as much as possible to maximize economic benefits.

A low-voltage management method for weak power grids based on back-to-back modular multi-level converters. Includes the following steps:

Step 1: Identify the reasons for low voltage at the user end of the weak power grid;

Step 2: Based on the principle of flexible DC transmission technology, design a back-to-back MMC distribution network model;

Step 3: Use the working principle of MMC to design the control algorithm and circulating current suppression algorithm, and use the nearest level approximation modulation technology to modulate the sub-modules input by MMC, and then determine the working status of the capacitor by sorting the sub-module capacitor voltage to generate PWM signal;

Step 4: The MMC on the distribution side works in rectification mode, using double closed-loop control to ensure the stability of the rectified voltage;

Step 5: The MMC on the user side works in inverter mode and adopts V/f control mode;

Further, the step 1 specifically includes: The step 1 specifically includes: It can be obtained from the voltage balance relationship of the distribution network:

U _grid =U _line +U _load =IZ _line +IZ _load …………(1)

Among them, U _load is the load voltage, U _line is the voltage on the distribution line, U _grid is the grid voltage, I is the current, Z _line is the line impedance, and Z _load is the load impedance. In the distribution network, when the line is long, the Z _line is larger. When U _grid is a fixed value, the larger the Z _line , the larger the U _line , which in turn causes the U _load to decrease.

Further, the step 2 specifically includes: converting the 220V AC phase voltage of the distribution network into a 1000V DC voltage using the two working modes of MMC rectification and inverter. Because the voltage of the distribution network increases from 220V to 1000V, when the distribution power is constant, the higher the voltage, the smaller the current on the line, and the smaller the electric power loss on the line. Finally, the DC bus 1000V DC voltage is converted into 220V AC phase voltage and directly transmitted to the user end to ensure that the user voltage is stable at 220V.

Further, the specific process of step 3 is as follows:

(1) According to the control principle of MMC, the control objectives of MMC are divided into three layers. The outermost layer is overall control, including active power control, reactive power control, and DC voltage control; the middle layer is MMC internal energy balance control, including overall control. Energy balance control, inter-phase energy balance control, inter-bridge arm energy balance control and sub-module energy balance control; the innermost layer is current control, including circulating current suppression, DC current control and AC current control.

(1.1) For the outermost layer, the control object is active power or reactive power; when the MMC works in the rectification mode, double closed-loop control is used to ensure the stability of the rectified voltage; when the MMC works in the inverter mode, the overall control strategy is V/ f control to ensure that the user's voltage V and frequency f meet the standards.

(1.2) For the middle layer, the internal energy balance is ensured by balancing the capacitor voltages of the sub-modules, and comparing and calculating the sampled voltages and currents of the upper and lower bridge arms as well as the number of reference sub-modules of the upper and lower bridge arms. Obtain the PWM signal of the MMC to control the stable operation of the MMC.

(1.3) For the innermost layer, considering the harmonic component of the MMC, the upper arm voltage u _u and the lower arm voltage u _l are summed and sorted to obtain:

in, t is time, U _C is the sub-module capacitor voltage, N is the number of single-bridge arm sub-modules, M is the modulation ratio of MMC, ΔUC _,1 and _ΔUC,2 are the capacitor voltage fluctuation amplitudes of the fundamental frequency and double frequency respectively. value, w is the angular frequency of the output AC voltage,/> is the phase lag angle; since there is a circulating current component inside the MMC, the repeated circulation causes the sub-module capacitor voltage to fluctuate, seriously affecting the power quality. Therefore, a circulating current suppression algorithm is proposed. The circulating current calculation formula is as follows:

Among them, i _{circ_j} is the circulating current of phase j, and i _uj and i _lj are the bridge arm currents of the upper and lower bridge arms. The three-phase circulating currents a, b, c are obtained through dq transformation to obtain their values on the d-axis and q-axis, and compared with the given reference current values of the d-axis and q-axis to obtain the residuals of the two, and then through the comparison The residual is subjected to PIR adjustment to obtain the j-phase reference voltage under circulating current suppression. The transfer function of PIR is:

Among them, s is the independent variable in the complex frequency domain, K _p and K _i are the proportional and integral coefficients respectively, K _r2 and K _r4 are the second and fourth frequency resonant controller gains respectively, and w _c is the bandwidth.

(2) The modulation technology of MMC is the key to the efficient and stable operation of the converter. The implementation process of recent level approach modulation is simple. The effect obtained by recent level approach modulation is to use the step wave of steps U _C to approximate the desired waveform to generate a sine By combining the waveform with the reference voltage of circulating current suppression, the number of reference sub-modules of the j-phase upper and lower bridge arms can be obtained. and

in, is the j-phase voltage reference value obtained by MMC through closed-loop control,/> is the j-phase voltage reference value obtained by the circulating current suppressor, U _dc is the DC side voltage, and round is the downward rounding function.

Further, the specific process of step 4 is as follows:

When the MMC works in the rectification mode, double closed-loop control is used to ensure the stability of the rectified voltage. The active power controlled in the outer loop is the given DC voltage of the DC bus, and the reactive power controlled is the reactive power. After PI adjustment, the d-axis is obtained respectively instruction and q-axis command/> Sample the voltage and current on the AC side and obtain the corresponding current commands i _d0 and i _q0 through dq transformation; find/> with/> The difference; adjust the calculated difference through PI to obtain the voltage reference value of phase j/>

Further, the specific process of step 5 is as follows:

When MMC works in inverter mode, the overall control strategy is V/f control. The active power controlled in the outer loop is frequency, and the reactive power controlled is AC voltage. After PI adjustment, the d-axis command is obtained respectively. and q-axis command/> Sample the voltage and current at the output side of the MMC and obtain the corresponding current instructions i _d1 and i _q1 through dq transformation; find/> with/> The difference; adjust the calculated difference through PI to obtain the voltage reference value of phase j/>

The invention is a method for controlling low voltage in a weak grid based on a back-to-back modular multi-level converter. The AC bus of the weak grid transmits alternating current to the MMC on the distribution grid side. At this time, the MMC is working in the rectification mode, and the MMC will distribute DC power through the original AC line to the MMC at the user end. This MMC works in inverter mode, converting DC power from the DC bus into AC power and delivering it to users. While meeting the above conditions, it will also ensure the stability of the grid side and the user side.

Compared with the existing technology, the present invention has the following technical advantages:

For long-distance multi-user concentrated power distribution scenarios in weak power grids, there are problems such as large distribution radius, aging lines, and small wire diameters. During the power distribution process, there are problems such as serious line power loss and low user-side voltage. By applying the method of the invention, the distribution line voltage is high, the power loss is small, and the user end voltage is stable without low voltage problems.

Description of drawings

Figure 1 is a schematic diagram of the low voltage compensation of the distribution network of the present invention.

Figure 2 is a topology diagram of the MMC circuit in the present invention.

Figure 3 is a flow chart of the MMC sub-module capacitor voltage balancing method and generating the corresponding PWM signal according to the present invention.

Figure 4 is a low-voltage example diagram of a traditional weak grid user end.

Figure 5 is an example diagram of the distribution network side voltage and the MMC rectifier side voltage when the present invention is used for low voltage compensation.

Figure 6 is a diagram showing an example of the low voltage at the user end of the distribution network being compensated after the implementation of the present invention.

Detailed ways

In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments These are some embodiments of the present invention, but not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without any creative work fall within the scope of protection of the present invention.

Embodiments of the present invention provide a method for managing low voltage in weak grids based on back-to-back modular multi-level converters, which includes the following steps:

Step 1: Analyze the cause of low voltage. Due to long-distance power transmission, the line impedance is large, which will cause serious loss of active power and reactive power, resulting in serious voltage drop at the user end.

As shown in Figure 4, this picture shows the output voltage of the distribution network side and the user-side port voltage in the traditional power distribution mode. This picture selects the voltage of one of the three phases to be 220V, while the user port voltage is only 169V.

Step 2: Based on the analysis of the causes of low voltage, according to the distribution network model shown in Figure 1, the two working modes of MMC rectification and inverter are used to convert the 220V AC phase voltage of the distribution network into 1000V DC voltage. Finally Convert the 1000V DC voltage of the DC bus into 220V AC phase voltage and directly transmit it to the weak grid user end. This invention soon ensured that the user's voltage was stable and up to standard, and also avoided the reactive current loss on the traditional AC distribution network lines.

Step 3: Based on the analysis of the MMC control principle, a circulating current suppression algorithm is proposed. The nearest level approximation modulation technology is used to modulate the MMC. The internal energy balance of the MMC is ensured by balancing the sub-module capacitor voltages, and the sub-module capacitor voltages are sorted. Determine the working status of the capacitor to generate the PWM signal.

According to the MMC topology (shown in Figure 2) control process (shown in Figure 3) and formula (5), n ^* represents the reference value of the number of input sub-modules in the upper (lower) bridge arm. Taking the above bridge arm as an example to illustrate, first sample the sub-module capacitor voltage and bridge arm current, and use the bubble sorting method to sort the sub-module capacitor voltages from small to large. It is the reference value of the number of input sub-modules of the upper arm obtained through the nearest level approximation. If the reference value is less than or equal to 0 or greater than 6 (the number of single-arm sub-modules in this design is 6), all PWM signals will be Set to 0; on the contrary, if the reference value is between 0 and 6, then the PWM signal is further set by the positive and negative of the bridge arm current. If the bridge arm current is positive, then/> The corresponding voltage measurement value will be set to 1, indicating the output of the PWM signal; if the bridge arm current is negative, then the remaining/> The corresponding voltage measurement value is set to 1, indicating that the PWM signal is output. The method of generating the PWM signal of the lower arm is the same as the method of generating the PWM signal of the upper arm. The reference value of the number of input sub-modules of the lower arm/> Also obtained by the nearest level approximation.

Step 4: Propose a control algorithm for the modular multi-level rectifier. Using the given DC voltage and reactive power, the d-axis commands are obtained through PI adjustment. and q-axis command/> The voltage and current on the AC side are sampled and the corresponding current commands i _d0 and i _q0 are obtained through dq transformation. Request/> with/> The difference; adjust the calculated difference through PI to obtain the voltage reference value of phase j/> Finally, return to step 3 to obtain the corresponding PWM signal sequence.

As shown in Figure 5, the output phase voltage on the distribution network side is stable at 220V. At 0.4s, the DC voltage on the DC bus is basically stable at the given value of 1000V.

Step 5: Propose a control algorithm for the modular multi-level inverter, and use the given frequency and AC voltage to obtain the d-axis commands through PI adjustment. and q-axis command/> The voltage and current at the output side of the modular multi-level converter are sampled and the corresponding current commands i _d1 and i _q1 are obtained through dq transformation. Request/> with/> The difference; adjust the calculated difference through PI to obtain the voltage reference value of phase j/> Finally, return to step 3 to obtain the corresponding PWM signal sequence.

As shown in Figure 6, the user-side port voltage and current entered a stable state 0.1s ago. During the entire process, the user-side port voltage was always stable at 220V. When a load is added to the user side at 0.3s, the phase voltage stabilizes at 220V, and the current increases and stabilizes after 0.02s. The added load is removed at 0.6s, and the current drops and stabilizes to the original size after 0.02s.

The above are only specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art can easily think of changes or substitutions within the technical scope disclosed in the present invention. All are covered by the protection scope of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scope of the claims.

Claims (6)

1. A weak current network low voltage treatment method based on back-to-back modularized multi-level converter is characterized by comprising the following steps:

step 1: the low voltage cause of the weak current network user side is defined;

step 2: aiming at the low-voltage reason of a weak power grid, based on the principle of a flexible direct current transmission technology, a back-to-back MMC power distribution network model is provided;

step 3: a control algorithm and a circulation suppression algorithm are provided by utilizing the working principle of the MMC, a submodule input by the MMC is modulated by adopting a nearest level approximation modulation technology, and the working state of a capacitor is determined by sequencing the capacitor voltage of the submodule to generate a PWM signal;

step 4: the MMC at the power distribution side works in a rectification mode, and the rectification voltage is ensured to be stable by adopting double closed-loop control;

step 5: the MMC at the user side works in an inversion mode and adopts a V/f control mode.

2. The weak grid low voltage management method based on back-to-back modular multilevel converter according to claim 1, wherein: the step 1 specifically includes: the voltage balance relation of the power distribution network is as follows:

U _grid ＝U _line +U _load =IZ _line +IZ _load

wherein U is _load For the load voltage, U _line For the voltage on the distribution line, U _grid Is the voltage of the power grid, I is the current, Z _line For line impedance, Z _load Is the load impedance; in the power distribution network, when the line is longer, Z _line Larger; when U is _grid At a fixed value, Z _line The larger U _line The larger, thereby resulting in U _load And (3) reducing.

3. The weak grid low voltage management method based on back-to-back modular multilevel converter according to claim 1, wherein: the step 2 specifically includes: converting 220V alternating-current phase voltage of a power distribution network into 1000V direct-current voltage by using two working modes of MMC rectification and inversion, wherein the power distribution network voltage is increased from 220V to 1000V, when the power distribution is constant, the higher the voltage is, the smaller the current on a line is, and the smaller the electric power loss on the line is; and finally converting the 1000V direct current voltage of the direct current bus into 220V alternating current phase voltage and directly transmitting the 220V alternating current phase voltage to a user terminal, so that the user voltage is ensured to be stable at 220V.

4. The weak grid low voltage management method based on back-to-back modular multilevel converter according to claim 1, wherein: the step 3 specifically includes:

(1) According to the control principle of the MMC, the control targets of the MMC are divided into three layers, and the outermost layer is integrally controlled, including active power control, reactive power control and direct current voltage control; the middle layer is MMC internal energy balance control, including overall energy balance control, interphase energy balance control, inter-bridge arm energy balance control and submodule energy balance control; the innermost layer is current control including circulation suppression, direct current control and alternating current control;

(1.1) for the outermost layer, the object to be controlled is an active or inactive amount; when the MMC works in a rectification mode, double closed-loop control is adopted to ensure the stability of rectification voltage; when the MMC works in the inversion mode, the overall control strategy is V/f control, so that the voltage V and the frequency f of the user are ensured to reach the standard;

(1.2) for the middle layer, the balance of internal energy is ensured by balancing the capacitance voltage of the sub-modules, and PWM signals of the MMC are obtained by comparing and calculating the sampled voltages and currents of the upper and lower bridge arms and the number of the reference sub-modules of the upper and lower bridge arms, so that the stable operation of the MMC is controlled;

(1.3) regarding the innermost layer, taking into account the harmonic component of MMC, the upper arm voltage u _u And lower bridge arm voltage u _l And (3) after summation and finishing, obtaining:

wherein,t is time, U _C For the capacitance voltage of the submodules, N is Shan Qiaobei submodules, M is the modulation ratio of MMC, and DeltaU _C,1 And DeltaU _C,2 The fluctuation amplitude of the capacitor voltage of fundamental frequency and double frequency respectively, w is the angular frequency of the output alternating voltage,/and%>Is DeltaU _C,1 And DeltaU _C,2 Phase lag angle; the circulation component exists in the MMC, the circulation makes the capacitance voltage of the submodule generate fluctuation, the electric energy quality is seriously influenced, and a circulation suppression algorithm is adopted, and a circulation calculation formula is as follows:

wherein i is _{circ_j} Is the circulation of j phase, i _uj And i _lj Bridge arm currents of an upper bridge arm and a lower bridge arm respectively; the a, b, c three-phase circulation is converted by dq to obtain the values of the circulation on d axis and q axis, and the values are matched with the parameters of the given d axis and q axisComparing the current values to obtain residual errors, and performing PIR (phase-of-information) adjustment on the residual errors to obtain j-phase reference voltage under loop current inhibitionThe transfer function of PIR is:

wherein s is complex frequency domain independent variable, K _p And K is equal to _i Respectively is a proportion and an integral coefficient, K _r2 And K is equal to _r4 Gain, w of the frequency doubling and quadrupling resonant controller respectively _c Is the bandwidth; w is the angular frequency of the output alternating voltage;

(2) The MMC modulation technology is the key of efficient and stable operation of the converter, the implementation process of the recent level approximation modulation is simple, and the effect obtained by the recent level approximation modulation is that a step U is used _C The step wave of the upper bridge arm and the lower bridge arm of the j phase can be obtained by combining the reference voltage with the reference voltage of loop current inhibitionAnd (3) with

Wherein,for the voltage reference value of the j phase of MMC through closed loop control, < >>Is circulation inhibitionVoltage reference value of j phase obtained by the generator, U _dc As the direct-current side voltage, round is a downward rounding function; n is the number of Shan Qiaobei sub-modules.

5. The weak grid low voltage management method based on back-to-back modular multilevel converter according to claim 1, wherein: the step 4 specifically includes: when the MMC works in the rectification mode, double closed-loop control is adopted to ensure stable rectification voltage, the controlled active quantity of the outer ring is given direct-current voltage of the direct-current bus, the controlled inactive quantity is reactive power, and d-axis instructions are respectively obtained through PI regulationAnd q-axis instruction>Sampling the voltage and current of the alternating current side and obtaining a corresponding current instruction i through dq conversion _d0 And i _q0 The method comprises the steps of carrying out a first treatment on the surface of the Ask for->And->Is the difference between (1); the obtained difference is subjected to PI regulation to obtain a voltage reference value of j phases>

6. The weak grid low voltage management method based on back-to-back modular multilevel converter according to claim 1, wherein: the step 5 specifically includes: when the MMC works in the inversion mode, the overall control strategy is V/f control, the controlled active quantity of the outer ring is frequency, the controlled inactive quantity is alternating voltage, and d-axis instructions are respectively obtained through PI regulationAnd q-axis instructionSampling the voltage and current of MMC output side and obtaining corresponding current instruction i through dq conversion _d1 And i _q1 The method comprises the steps of carrying out a first treatment on the surface of the Solving forAnd->Is the difference between (1); the obtained difference is subjected to PI regulation to obtain a voltage reference value of j phases>