CN111327042B - Direct-current micro-grid power and voltage adjusting method considering line impedance and local load - Google Patents

Abstract

The invention relates to a method for adjusting the power and voltage of a DC microgrid considering line impedance and local load. The method is used in the island operation mode of a DC microgrid. The DC microgrid contains multiple groups of DGs and different types of loads, each The DG is connected in parallel to the common DC bus through the corresponding converter. This method adopts an improved droop control strategy, so that each DG distributes the load power according to its rated capacity, eliminates the influence of line impedance and local load on power distribution, and maintains the bus voltage. Stablize. Compared with the prior art, the power distribution control and voltage recovery control strategy of the islanded DC microgrid proposed by the present invention can effectively improve the load power distribution accuracy between multiple parallel distributed power sources in the islanded DC microgrid, and improve the performance of the distributed power supply. Efficiency, give full play to the "plug and play" feature of DG. At the same time, maintaining the DC bus voltage enables the microgrid system to operate stably.

Description

Direct-current micro-grid power and voltage adjusting method considering line impedance and local load

Technical Field

The invention relates to output power coordination control and bus voltage stabilization of a direct-current micro-grid connected to a common direct-current bus in parallel, in particular to a direct-current micro-grid power and voltage adjusting method considering line impedance and local load.

Background

In view of the current problems of environmental pollution and shortage of fossil fuels, a Renewable Energy (RES) based distributed power generation technology is proposed and widely used. The access of Distributed Generation (DG) to a power distribution network in the form of a microgrid is generally regarded as one of the effective ways to utilize distributed generation. The traditional micro-grid is divided into a direct current micro-grid and an alternating current micro-grid according to the types of access buses. Compared with an alternating-current micro-grid, the direct-current micro-grid has no problems of phase position, reactive power and the like, so that voltage distribution is unrelated to inductance and capacitance parameters of a line, and voltage control is facilitated.

In an actual direct-current micro-grid system, line impedance inevitably exists between a power electronic converter connected with each DG and a common direct-current bus, and due to the fact that distances between the converters and the bus are different, the line impedance of corresponding branches is necessarily unmatched. Therefore, each group of DGs cannot distribute load power according to a droop coefficient preset by traditional droop control, local loads still exist in the actual microgrid system, the difference further deepens the influence on the power distribution, power distribution deviation is caused, each DG cannot output power according to the rated capacity of the DG, and meanwhile, when the inherent bus voltage deviation caused by the droop control is serious, hidden dangers are brought to the safe and stable operation of the whole microgrid system.

In order to solve the above problems, researchers at home and abroad have conducted extensive research. The documents "Khorsandi A, Ashourlo M, Mokhtari H.A, decentralized control method for a low-voltage DC microrod [ J ]. IEEE Transactions on Energy Conversion,2014,29(4): 793-801", and "Lu X, Sun K, Guerreo J M, et al. State-of-Chage balance using adaptive control for distributed Energy storage systems [ J ]. IEEE Transactions on Industrial Electronics,2013,61 (6): 2804 2815 ″ reduces the influence of the mismatch of line impedance on the load distribution by adaptively correcting the droop coefficient, but the bus voltages all have different deviations. The document Y.Lin and W.Xiao, Novel thread Linear Format of Droop Linear for DC micro Grid J. IEEE Transactions on Smart Grid,2019,10(6):6747-6755 proposes a Piecewise linearized Droop control, but in this method, a balance is sought between current distribution and voltage recovery, and the engineering of multi-segment electrical quantities is difficult to realize. Document "yangjie, jinxinmin, wu schoolwis, etc. an improved current load distribution control strategy applicable to dc microgrid [ J ] chinese electro-mechanical engineering report, 2016, 36 (1): 59-67 "proposes a droop control strategy that replaces voltage with voltage variation rate to achieve accurate distribution of load current, but bus voltage still has large deviation. Document "Jiangwei, Zhao jin, Gaoming, etc. an independent dc microgrid control strategy with voltage self-recovery characteristics study [ J ] grid technology, 2019.1315: 1-9' line impedance information is obtained by a single pulse injection method and is introduced into a droop coefficient in a negative compensation mode to suppress the adverse effect of line impedance inconsistency on current distribution, but the strategy has high requirements on selection of measurement interval time after pulse injection.

Disclosure of Invention

The invention aims to overcome the defects of the prior art and provide a method for adjusting the power and the voltage of a direct-current microgrid by considering line impedance and local loads.

The purpose of the invention can be realized by the following technical scheme:

a method for adjusting power and voltage of a direct current microgrid with line impedance and local load considered is used in an island operation mode of the direct current microgrid, the direct current microgrid comprises a plurality of groups of DGs and loads of different types, each DG is connected to a common direct current bus in parallel through a corresponding converter, an improved droop control strategy is adopted in the method, each DG distributes load power according to the rated capacity of the DG, the influence of the line impedance and the local load on power distribution is eliminated, and the bus voltage is kept stable, and the method specifically adopts the following formula:

V_i＝V^*+u_{i_p}+u_{i_v}-m_iP_i

in the formula, V_iAnd P_iOutput voltage and output power, V, of converters connected respectively to the i-th group DG^*For a nominal reference value of the output voltage, u_{i_p}For power control of input u_{i_v}For controlling the input, m, of the bus voltage_iIs the droop coefficient of the ith group DG.

The power control input u_{i_p}For the variation, the value of the current sampling period is calculated according to the value of the previous sampling period, and the calculation method is as follows:

u_{i_p(n+1)}＝u_{i_p(n)}+G_{i_n}

in the formula u_{i_p(n+1)}、u_{i_p(n)}The power control input, G, of the ith group of DGs in the (n + 1) th and nth sampling periods_{i_n}Allocating an adjustment term for the power of the ith group DG in the nth sampling period, wherein alpha is a power adjustment coefficient, P_{i_n}Output power, P, for the ith group of DGs in the nth sampling period^* _{i_n}Desired output power, P, for each DG over n sampling periods_ratediThe rated power of the ith group of DGs, and N is the number of the DG groups.

The bus voltage control input quantity u_{i_v}For the variation, the value of the current sampling period is calculated according to the value of the previous sampling period, and the calculation method is as follows:

in the formula u_{i_v(n+1)}、u_{i_v(n)}Voltage control input quantity H of the ith group DG in the n +1 th and nth sampling periods_{i_n}Bus voltage of ith group DG in nth sampling periodRegulation term, beta is voltage regulation coefficient, Δ V_{i_n}Is the amount of voltage deviation, V_aveThe average value of the output voltages of the DGs is shown, N is the number of groups of the DGs, h is a pinning coefficient, the group of DGs is selected to perform voltage recovery control when h is 1, the group of DGs does not need to perform voltage recovery control when h is 0, h 'is a compensation coefficient, the group of DGs is selected to have a single-point fault when h' is 1, and at the moment, the adjacent alternative DGs are connected to work to serve as a voltage recovery control unit.

The method selects two groups of DGs, wherein one group is an alternative DG, the output voltage stability of each group of DGs is realized by utilizing local output voltage information, and meanwhile, the stability of the system bus voltage is kept.

During the isolated island operation mode, the internal power balance expression of the direct current microgrid is as follows:

in the formula, P_iIs the output power, P, of the i-th group DG_pccIs the power of the common load, P_LocaliPower loss on the local load, P, for the ith group of DGs_lineiThe power loss on the line impedance connected to the corresponding DG converter is represented by N, which is the number of DG groups.

By introducing the sampling holder, the voltage reference value of the droop control is adaptively adjusted according to the real-time output result so as to realize accurate power distribution.

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

(1) by introducing local output power and voltage information, the synchronous signal is used for triggering the sampling holder, the output of the sampling holder performs adaptive vertical intercept compensation on the output voltage of the droop control curve, and the influence of line impedance and local load on power distribution is eliminated. The method has the advantages that global information does not need to be additionally measured, accurate distribution of DG output power can be achieved only by utilizing output local information of each group of DGs through local communication between adjacent DGs, meanwhile, a voltage recovery strategy based on optimized local control is provided, only two groups of DGs (one group of DGs is an alternative DG) need to be selected, output voltage stability of each group of DGs can be achieved by utilizing local output voltage information, and meanwhile, stability of system bus voltage is kept.

(2) The power distribution control and voltage recovery control strategy for the island direct-current microgrid provided by the invention can effectively improve the load power distribution precision among the parallel distributed power supplies in the island direct-current microgrid, improve the efficiency of the distributed power supplies, fully play the plug-and-play characteristic of a DG (distributed generation) and maintain the direct-current bus voltage to ensure that the microgrid system can stably operate.

Drawings

Fig. 1 is a typical dc microgrid structure;

fig. 2 is an equivalent circuit diagram of a microgrid with DG running in parallel;

FIG. 3 is a conventional droop characteristic with local load effects;

fig. 4 is a DG original circuit model and an equivalent circuit model of the present embodiment with a local load;

FIG. 5 is a schematic diagram of power accurate allocation according to the present embodiment;

FIG. 6 is a control block diagram of the system according to the present embodiment;

FIG. 7(a) is a graph showing the DG output power in the first embodiment;

FIG. 7(b) is a graph showing the DG output voltage in the first embodiment;

FIG. 7(c) is a diagram illustrating the DC bus voltage in the first embodiment;

FIG. 8(a) shows the DG output power in the second embodiment;

FIG. 8(b) is a graph showing the DG output voltage in the second embodiment;

FIG. 8(c) is the DC bus voltage in the second embodiment;

FIG. 9(a) shows the DG output power in the third embodiment;

FIG. 9(b) is the DC bus voltage in the third embodiment;

Detailed Description

The invention is described in detail below with reference to the figures and specific embodiments. The present embodiment is implemented on the premise of the technical solution of the present invention, and a detailed implementation manner and a specific operation process are given, but the scope of the present invention is not limited to the following embodiments.

Examples

The present embodiment further describes the present invention in terms of a microgrid overall structure model, analysis of load power distribution errors among DGs under a conventional droop control strategy, analysis of an equivalent circuit model including a local load, a droop-improved power distribution strategy, a voltage recovery control strategy, and verification of examples based on MATLAB under different working conditions.

(1) Micro-grid structure model

A typical dc microgrid architecture is shown in fig. 1, and includes a common dc bus, distributed power sources (including energy storage units) and different types of loads, each unit being connected in parallel to the common dc bus via a respective converter. Meanwhile, the direct current micro-grid can also be connected with an external alternating current power grid through a grid-connected converter.

When an external alternating current power grid fails, a connection switch at the grid-connected converter is disconnected, and the direct current micro-grid works in an island operation mode. At this time, each DG unit in the microgrid coordinates and maintains the power balance and the bus voltage stability in the microgrid. The micro-grid internal power balance expression is as follows

In the formula P_i(i-1, 2, …, N) is the output power of the ith DG unit, P_pccIs the power of the common load, P_LocaliPower loss on the local load for the ith DG unit, P_lineiFor power losses on the line impedance connected to the corresponding DG unit converter.

(2) Power distribution and bus voltage deviation analysis for conventional droop control strategies

The expression of the traditional P-V droop control is

V_i＝V^*-m_iP_i (2)

In the formula, V_iAnd P_iOutput voltage and output power, V, of converters connected respectively to the i-th group DG^*To output voltageReference value, m_iIs the droop coefficient of the ith group DG.

An equivalent circuit diagram of a direct current microgrid island operation mode with N groups of DGs connected in parallel is as follows, and FIG. 2 shows two groups of DGs.

From the equivalent circuit of FIG. 2, the common bus voltage is expressed as

In the formula V_pccIs a common DC bus voltage, R_lineiThe output line impedance values of the converters connected to the i-th group DG are obtained from equations (2) and (3)

Equation (4) shows that introducing droop control when the converter output power is not zero necessarily results in a deviation of the dc bus voltage. From the formula (4)

Equation (5) and equation (6) show that the load power distribution in the microgrid is not only related to the droop coefficient, but also affected by the line impedance. Neglecting the influence of the line impedance, each DG can distribute power according to its rated capacity by a predetermined droop factor, i.e. the power distribution

Wherein P is_ratediAnd P_ratedjThe rated power of the ith group DG and the jth group DG, respectively. Power distribution characteristics under conventional droop control such asAs shown in fig. 3.

Droop characteristic curve of DG unit is L₁、L₂When the local load at each DG outlet is not considered, the characteristic curve of the transmission line corresponding to each group of DGs is R₁、R₂Actual output voltage V of DG unit due to line impedance mismatch₁≠V₂At this time, the load power distribution deviation of the system is delta P; the transmission line characteristic curves are shifted to R to the right, respectively, taking into account the local load₁’、R₂'when a new steady state is reached, the load power distribution deviation is Δ P'.

(3) DG local load model equivalent analysis

By equivalently transferring the local loads directly connected with the DGs to the public bus, the equivalent local loads can participate in the power distribution of the whole network as the public loads. The equivalent model is shown in fig. 4.

From the original model and the equivalent model of FIG. 4, the following formula can be derived

It can be deduced from equation (8) that the impedance of the equivalent post-mismatching line satisfies

Based on the equivalent circuit model with the local load in fig. 4, the influence of the unmatched line impedance and the local load on the DG load power distribution can be simplified into the influence of the equivalent unmatched line impedance on the DG load power distribution.

(4) Droop-improved based power allocation control strategy

Based on the equivalent modeling analysis of the direct-current micro-grid containing the local load in the step (3), the invention provides a load power accurate distribution control strategy based on the improved traditional droop control. The specific control strategy is designed as follows

u_{i_p(n+1)}＝u_{i_p(n)}+G_{i_n} (10)

In the formula u_{i_p(n+1)}、u_{i_p(n)}The power control input quantities of the ith group of DGs in the (n + 1) th sampling period and the n-th sampling period are respectively; g_{i_n}A power distribution adjustment item of the ith group DG in the n sampling periods; alpha is a power regulation coefficient; p_{i_n}The output power of the ith group DG in the nth sampling period; p^* _{i_n}Desired output power for each group DG over n sampling periods; the power accurate distribution process is as shown in fig. 5, a sample holder is introduced, and the voltage reference value of droop control is adaptively adjusted according to the real-time output result to realize the power accurate distribution.

As shown in FIG. 5, two groups of DGs with the same rated capacity are taken as an example for analysis, and in this case

The system initially works in a traditional droop control mode, initial working points are A and B, the micro-grid system has power distribution deviation, a local controller sends out a synchronous signal to trigger a sampling holder at the moment, longitudinal intercept compensation is continuously carried out on an original droop output voltage reference value in each sampling period until final steady-state working points fall to C and D, and at the moment, load power distribution does not have deviation.

(5) Voltage recovery control based on optimization constraints

According to the analysis in the step (2), the traditional droop control is poor control, the power distribution is adjusted, and meanwhile, the voltage of a direct current bus may deviate from a rated value, so that the power quality of the system is affected, and in a serious case, the potential safety hazard is even brought to the stable operation of the whole microgrid system. The invention provides a secondary voltage recovery strategy based on optimization containment. And the stable and error-free control of the bus voltage of the whole system can be realized by only using the local output voltage information through a single group of DGs. The specific control process is designed as follows

u_{i_v(n+1)}＝u_{i_v(n)}+H_{i_n} (14)

H_{i_n}＝(h+h')βΔV_{i_n} (15)

h'＝1-h (16)

ΔV_{i_n}＝V^*-V_ave (17)

In the formula: u. of_{i_v(n+1)}、u_{i_v(n)}Voltage control input quantities of the ith group of DGs in the (n + 1) th sampling period and the n-th sampling period are respectively controlled; h_{i_n}A bus voltage regulation item of the ith group DG in the n sampling periods; beta is a voltage regulation coefficient; Δ V_{i_n}Is the voltage deviation amount; v_aveThe average value of the DG output voltage is shown, and N represents the number of DGs; and h is a pinning coefficient, when h is 1, the group of DGs is selected to perform voltage recovery control, and when h is 0, the group of DGs does not need to perform voltage recovery control. h 'is a compensation coefficient, when h' is 1, the group of DGs is selected to have single-point fault, and at the moment, the adjacent alternative DG units are taken over to work as voltage recovery control units, so that the system can be ensured to run safely and stably under any condition.

In summary, when the microgrid operates in an island mode, stable operation of the microgrid needs to depend on coordination among all internal distributed power supplies, in order to eliminate the influence of output line impedance and local load on power distribution of the distributed power supplies, realize that each group of DGs distributes load power according to the rated capacity of each group of DGs, and ensure stable bus voltage, the overall control strategy provided by the invention is that

V_i＝V^*+u_{i_p}+u_{i_v}-m_iP_i (19)

In the formula u_{i_p}And u_{i_v}For precise distribution of power and bus voltage, respectivelyAnd controlling the input quantity of the secondary control. The overall control block diagram of the system is shown in fig. 6. In fig. 6, S/H is a sample holder, the synchronous trigger signal S is a switching function, when S is 0, the sample holder does not operate, and the system operates in the conventional droop mode; when S is equal to 1, the sampling holder starts to work, and the system works in the improved droop mode. Under the control strategy provided by the invention, the micro-grid system realizes accurate distribution of internal distributed power supply power, and simultaneously, the system voltage can be stabilized at a rated value.

(6) Simulation analysis

To verify the validity and correctness of the proposed strategy, the embodiment builds four groups of Distributed Generators (DG) based on MATLAB₁、DG₂、DG₃、DG₄) The simulation parameters of the island DC micro-grid system are shown in table 1.

TABLE 1 microgrid System parameters

In the embodiment, example verification under three different working conditions is designed for power distribution among parallel distributed power supplies of an island direct-current micro-grid. Firstly, four groups of distributed power supplies with the same rated capacity (10kW) are used for designing an example 1 and an example 2, and the effectiveness of the control strategy provided by the invention is respectively researched under the conditions of not considering local loads and simultaneously considering unmatched line impedance and local loads, and is compared with the traditional droop control strategy; to avoid loss of generality, example 3 sets four groups of distributed power supplies with different rated capacities (DG)₁＝10kW,DG₂＝20kW,DG₃＝30kW,DG₄40kW), the research considers the effectiveness of the load power distribution under the control strategy proposed by the present invention, when the DG output power should satisfy 1:2:3: 4.

The first calculation example: irrespective of local load

The calculation example mainly verifies the effectiveness of the control strategy of the invention when the system does not contain a local load, and according to the result of fig. 7, the system works in the traditional droop control in the period of 0-1s, the DG output power can not be evenly divided (distributed according to the rated capacity) due to the influence of the unmatched line impedance, and meanwhile, the DG output voltage and the bus voltage have deviation. The synchronous signal is switched from 0 to 1 in 1s, the sampling retainer starts to work, the droop curve is adaptively adjusted according to the sampling period, the influence of unmatched line impedance on power distribution is eliminated, each group of DGs outputs power according to the rated capacity of each group of DGs, and meanwhile, the voltage recovery control ensures that the output voltage and the bus voltage of each group of DGs can be maintained near the rated value without large deviation, so that the operation reliability of the whole microgrid is ensured

Example two: taking into account simultaneous effects of local load and line impedance

Example two the effectiveness of the control strategy of the present invention when the local load and the unmatched line impedance exist simultaneously in the system is verified on the basis of example one. The sizes of the four groups of local loads in the second calculation example are 4kW, 5.3kW and 5.3kW respectively. As shown in fig. 8, the system operates in the conventional droop mode during 0-1s, and the output power of the DG cannot be equally divided under the combined action of the local load and the mismatched line impedance. The sampling holder is triggered at 1s, compensation droop control reference voltage is output in each sampling period according to local power and voltage information, no power distribution difference is eliminated, and good voltage quality is guaranteed, and fig. 8(b) and (c) show that both the output voltage of the DG and the DC bus voltage can be stabilized at rated values.

Example three: power distribution with different capacity DGs

Example three without loss of generality, four groups of DG with different rated capacities were simulation verified. The other conditions of the embodiment are the same as the second embodiment, namely, the common influence of the local load and the unmatched line impedance is considered at the same time. At the moment, the output power of the DGs is required to meet 1:2:3:4, the effectiveness of the control strategy is shown in FIG. 9(a), the output power of four groups of DGs is respectively 0.35kW, 0.69kW, 1.04kW and 1.38kW (meeting 1:2:3:4), and meanwhile, as shown in FIG. 9(b), the strategy provided by the invention eliminates the influence of non-matched line resistance and local load on the load power distribution precision, ensures good bus voltage quality, fully exerts the efficiency of the distributed power supply, and improves the reliability and safety of the operation of the microgrid.

Claims (4)

1. A method for adjusting power and voltage of a direct current microgrid with line impedance and local load considered is characterized in that the method is used in an island operation mode of the direct current microgrid, the direct current microgrid comprises a plurality of groups of DGs and loads of different types, each DG is connected to a common direct current bus in parallel through a corresponding converter, the method adopts an improved droop control strategy, each DG distributes load power according to the rated capacity of the DG, the influence of line impedance and local load on power distribution is eliminated, and the bus voltage is maintained to be stable, and the method specifically adopts the following formula:

V_i＝V^*+u_{i_p}+u_{i_v}-m_iP_i

in the formula, V_iAnd P_iOutput voltage and output power, V, of converters connected respectively to the i-th group DG^*For a nominal reference value of the output voltage, u_{i_p}For power control of input u_{i_v}For controlling the input, m, of the bus voltage_iThe droop coefficient is the ith group DG;

the power control input u_{i_p}Calculating the value of the current sampling period according to the value of the last sampling period as a variable quantity;

the power control input u_{i_p}The calculation method is as follows:

u_{i_p(n+1)}＝u_{i_p(n)}+G_{i_n}

in the formula u_{i_p(n+1)}、u_{i_p(n)}The power control input, G, of the ith group of DGs in the (n + 1) th and nth sampling periods_{i_n}Allocating an adjustment term for the power of the ith group DG in the nth sampling period, wherein alpha is a power adjustment coefficient, P_{i_n}Output power, P, for the ith group of DGs in the nth sampling period^* _{i_n}Desired output power, P, for each DG over n sampling periods_ratediThe rated power of the ith group of DGs is N, and the number of the groups of DGs is N;

the bus voltage control input quantity u_{i_v}Calculating the value of the current sampling period according to the value of the last sampling period as a variable quantity;

the bus voltage control input quantity u_{i_v}The calculation method is as follows:

u_{i_v(n+1)}＝u_{i_v(n)}+H_{i_n}

H_{i_n}＝(h+h′)βΔV_{i_n}

h′＝1-h

ΔV_{i_n}＝V^*-V_ave

in the formula u_{i_v(n+1)}、u_{i_v(n)}Voltage control input quantity H of the ith group DG in the n +1 th and nth sampling periods_{i_n}A bus voltage regulation term of the ith group DG in the nth sampling period, beta is a voltage regulation coefficient, and deltaV_{i_n}Is the amount of voltage deviation, V_aveThe average value of the output voltages of the DGs is shown, N is the number of groups of the DGs, h is a pinning coefficient, the group of DGs is selected to perform voltage recovery control when h is 1, the group of DGs does not need to perform voltage recovery control when h is 0, h 'is a compensation coefficient, the group of DGs is selected to have a single-point fault when h' is 1, and at the moment, the adjacent alternative DGs are connected to work to serve as a voltage recovery control unit.

2. The method for adjusting power and voltage of a direct current microgrid according to claim 1 and taking line impedance and local load into consideration is characterized in that two groups of DGs are selected, one group of DGs is a standby DG, the output voltage of each group of DGs is stabilized by using local output voltage information, and meanwhile, the stability of the system bus voltage is kept.

3. The method for adjusting power and voltage of the direct-current microgrid according to claim 1 and considering line impedance and local load, characterized in that in an island operation mode, the internal power balance expression of the direct-current microgrid is as follows:

in the formula, P_iIs the output power, P, of the i-th group DG_pccIs the power of the common load, P_LocaliPower loss on the local load, P, for the ith group of DGs_lineiThe power loss on the line impedance connected to the corresponding DG converter is represented by N, which is the number of DG groups.

4. The method as claimed in claim 1, wherein the accurate power distribution is achieved by adaptively adjusting the voltage reference value of the droop control according to the real-time output result by introducing a sample-and-hold unit.

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