A power coordination control method for DC microgrids in off-grid environments

Abstract

The present invention discloses a power coordination control method for a DC microgrid group suitable for off-grid operation, specifically: according to the U / I droop characteristics of the underlying distributed micro-source, a suitable voltage reference value is established for the voltage control outer loop; by adjusting the reference voltage of the droop unit twice, the DC bus voltage deviation of the subgrid caused by the underlying droop control is eliminated to achieve smooth connection between subgrids; a power allocation mechanism between mutually-aiding subgrids is established, and the voltage deviation within the allowable range of the busbar is used to complete the preset power transmission between subgrids, thereby reducing the imbalance of the mutual-aiding capacity of the subgrids. This method can quickly respond to sudden changes in load, ensure the reliable operation of each subgrid in the group, and improve the collective operation efficiency of the microgrid group in the off-grid mode.

Description

Technical Field

[0001] The present invention belongs to the technical field of microgrid power coordination and control, and in particular relates to a power coordination control method for a DC microgrid group suitable for off-grid operation. Background Art

[0002] Vigorously developing renewable energy is an important part of my country's energy development strategy. Using microgrids as carriers to absorb distributed renewable energy has become a new trend in power supply and distribution. DC microgrids have attracted widespread attention from researchers because they do not need to consider issues such as reactive power, harmonics, and co-frequency and co-phase. Compared with AC microgrids, DC microgrids reduce the number of voltage level conversions and interface converters, which improves the efficiency of power transmission and reduces the initial investment cost. However, microgrids operating in off-grid mode have defects such as limited capacity, single mode, and poor anti-disturbance ability, which reduces their power supply reliability. Therefore, interconnecting adjacent microgrids into groups can achieve energy mutual assistance and mutual backup between microgrids, which is conducive to the redistribution of energy. Each sub-microgrid in the microgrid group jointly bears the power shortage of the group, absorbs redundant renewable energy, ensures higher power supply quality, and reduces large-scale abandonment of light and wind. Compared with a single microgrid, the structure of a microgrid group is more complex, the power flow is more variable, and the increase in operating conditions makes its coordinated control more difficult.

[0003] The power coordination control methods of microgrids are mainly divided into decentralized, centralized, and distributed control. The typical feature of decentralized control is plug-and-play, without communication, such as traditional droop control and its improved DC bus voltage signal control. Since each unit is decentralized and independent, the operation mode is relatively single, and it is impossible to coordinate the optimal power flow distribution of the microgrid group. Centralized control relies on the communication system to make it easier to predict load and power generation, and can achieve multi-objective global optimization of the system by combining intelligent algorithms. However, when the local line is interrupted or the central processor fails, the system will face instability or even collapse. Distributed control only establishes communication between adjacent control units, without the need for an upper-level central processor, and can also achieve the goal of centralized control through the corresponding algorithm, but the control effect is often affected by communication delay, the algorithm is relatively complex, and the amount of calculation is large. Traditional microgrid power control relies on communication and focuses on the application and improvement of upper-level intelligent algorithms; subnets within the group give priority to autonomous power balance, and only participate in coordinating the power balance of the group when they generate redundant power, which will increase the power burden of the backup energy storage system of the microgrid group, and reduce the clustering benefits, which is not conducive to increasing the penetration rate of renewable energy. Summary of the invention

[0004] The purpose of the present invention is to provide a power coordination control method for a DC microgrid group that is suitable for off-grid operation, thereby solving the problem of low collective efficiency of DC microgrid group operation that exists in the prior art.

[0005] The technical solution adopted by the present invention is a power coordination control method for a DC microgrid group under off-grid conditions, which is specifically implemented according to the following steps:

[0006] Step 1: Establish a suitable voltage reference value for the voltage control outer loop according to the U / I droop characteristics of the underlying distributed micro-sources;

[0007] Step 2: By adjusting the reference voltage of the droop unit for the second time, the DC bus voltage deviation of the subnet caused by the bottom droop control is eliminated to achieve smooth connection between subnets;

[0008] Step 3: Establish a power allocation mechanism between mutually beneficial subgrids, use the voltage deviation within the allowable range of the busbar to complete the preset power transmission between subgrids, and reduce the imbalance of subgrid mutual beneficial capacity.

[0009] The present invention is also characterized in that:

[0010] In step 1, specifically:

[0011] Step 1.1: Establish a suitable voltage reference value u for the voltage control outer loop through the U / I droop characteristic equation ref , as shown in formula (1):

[0012]

[0013] Among them, u * MG is the bus voltage given value of the subnet, i o is the converter output current, R d is a virtual resistor;

[0014] Virtual resistance value R for power distribution according to rated capacity of converter d , as shown in formula (2):

[0015]

[0016] Among them, u min is the minimum voltage allowed by the microgrid, I max is the maximum output current of the converter;

[0017] Step 1.2: Divide the working modes according to the different characteristics of each micro-source in the subnet;

[0018] The state of charge SOC of the energy storage unit b,i Set upper and lower limits:

[0019] SOC b,min ≤SOC b,i ≤SOC b,max (3);

[0020] Among them, SOC b,min, SOC b,max They are the upper and lower limits of the SOC of the energy storage unit respectively;

[0021] Bottom layer droop control voltage deviation Δu pri,j as follows:

[0022]

[0023] Among them, u * MG,j is the bus voltage given value of the jth subnet, u MG,j is the actual bus voltage of j subnets.

[0024] In step 2, specifically:

[0025] Step 2.1, when the subgrid is off-grid, the subgrid bus voltage rating is sent to the droop control unit to eliminate the voltage deviation inside the subgrid, and the reference voltage of the droop unit in the subgrid is changed accordingly by vertically moving the droop curve up and down;

[0026]

[0027] Among them, Δu sec,j is the secondary control compensation value of bus voltage under off-grid operation of subgrid, k ps and k is It is the bus voltage secondary adjustment PI control parameter;

[0028] Step 2.2: When the subnets are interconnected, calculate the output voltage u of all subnets using low-bandwidth communication MG,j The root mean square value of the subnet bus voltage u MG,j The data is compared and then sent to all subnet bottom-level controls through the PI regulator;

[0029] Secondary control compensation value Δu′ of bus voltage under sub-grid interconnection sec,j for:

[0030]

[0031] Among them, k′ ps and k′ is is the PI control parameter for secondary adjustment of bus voltage under subgrid interconnection, and n is the number of interconnected sub-microgrids in the group.

[0032] In step 3, specifically:

[0033] Step 3.1, by utilizing the deviation within the allowable range of the bus voltage to complete the preset power transmission, while following the power flow control, the voltage of each subgrid is controlled near the rated value;

[0034] Subnet mutual assistance capability mean ε *MGs as follows:

[0035]

[0036] Where n is the number of interconnected microgrids in the cluster, S N,MG,j represents the capacity of the energy storage unit, α and β are weight coefficients, the sum of which is 1, and m is the number of energy storage batteries in the subnet;

[0037] ε MG,j is the mutual assistance capability value of the subnet, and its imbalance degree δε MG,j The calculation is as follows:

[0038]

[0039] Step 3.2: Subnet mutual assistance capacity imbalance δε MG,j The population voltage correction term Δu is generated by the PI control regulator thi,j , the community collaborative compensation voltage difference is:

[0040]

[0041] Among them, k pt , k it is the PI control parameter of community collaborative control, d v is the voltage regulation constant;

[0042] Step 3.3: The triggering mark of the cluster voltage coordinated control level is when the bus voltages of all sub-grids converge to the root mean square value, which is less than the error margin. And all the interfaces between interconnected subnets are in a closed state, as shown below:

[0043]

[0044] Among them, IC j The subnet interface circuit breaker opening and closing status; Δu thi,j If the value is too large, it will affect the stability of the system and needs to be restricted;

[0045]

[0046] Among them, u dc,j,min 、u dc,j,max are the upper and lower limits of the subnet bus voltage, P DG is the output power of the distributed microgrid, P DG,j,min , P DG,j,max are the upper and lower limits of the distributed micro-source output, SOC m,j is the state of charge of the jth subnet, SOC m,min , SOC m,max It is the upper and lower limits of subnet SOC.

[0047] The beneficial effects of the present invention are as follows: the method of the present invention makes full use of the power mutual assistance between interconnected subgrids to achieve power balance in the community and reduce the reliance on the community's backup energy storage. The voltage deviation between the subgrid buses is reduced by voltage secondary control to achieve smooth connection between subgrids. The community voltage coordination control further optimizes the bus voltage of each subgrid, and at the same time uses the voltage difference to transmit power between networks to achieve energy mutual assistance. The method can quickly respond to sudden changes in load, ensuring the reliable operation of each subgrid in the group while improving the collective operation efficiency of the microgrid group in off-grid mode. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 It is a topological structure diagram of a DC microgrid group in a power coordination control method of a DC microgrid group under off-grid conditions applicable to the present invention;

[0049] Figure 2 It is a droop curve diagram of distributed micro-sources in the power coordination control method of a DC micro-grid group under off-grid conditions of the present invention;

[0050] Figure 3 It is a block diagram of the switching mode of the underlying converter control strategy in the power coordination control method of the DC microgrid group under off-grid conditions of the present invention;

[0051] Figure 4 It is a block diagram of the bus voltage secondary control and cluster voltage coordinated control strategy in the off-grid DC microgrid group power coordinated control method of the present invention. DETAILED DESCRIPTION

[0052] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0053] The present invention is applicable to the power coordination control method of the DC microgrid group under off-grid conditions. Figure 1 As shown, it includes several sub-microgrids MG and backup energy storage units RES. The sub-microgrid is composed of distributed power sources, energy storage units and loads. The distributed micro sources include photovoltaic power generation PV and wind power generation WT. All units are connected in parallel to the sub-grid bus through DC / DC converters.

[0054] Z i , Z p They are the line impedances from the subgrid and the backup energy storage unit to the group-level bus, and the group-level bus is connected to the external grid through a grid-connected converter.

[0055] The bottom distributed micro-source DG adopts droop control, and its droop control curve is as follows: Figure 2 As shown in the figure, u * MG is the bus voltage given value of the microgrid; u ref is the voltage control outer loop reference value; io1 、i o2 are the output currents of DG1 and DG2 respectively; i o1 ′、i o2 ′ are the output currents of DG1 and DG2 after droop control.

[0056] The underlying converter control strategy mode switching is as follows Figure 3 As shown in the figure, u * bus 、u bus 、u * DG 、u DG are the reference voltage and actual voltage of the subgrid bus and the reference voltage and actual voltage of the distributed micro-source respectively; i DG 、i LDG 、i b 、i Lb are the output current and inductor current of the distributed micro-source and the output current and inductor current of the energy storage unit respectively; R DG , R b are the droop coefficients of the distributed micro-source and the energy storage unit respectively; PI is the proportional integral controller; PWM is the pulse width modulation module.

[0057] The block diagram of subgrid bus voltage secondary control and cluster voltage coordinated control is as follows: Figure 4 As shown in the figure, u MG,j 、u * MG,j are the subnet bus voltage and its reference value, Δu pri,j , Δu sec,j , Δu thi,j are the voltage compensation values ​​of the bottom-level control, bus voltage secondary control, and group-level coordinated control, ε MG,j is the mutual assistance capability value of the subnet, ε * MG,j The mean mutual assistance capacity of the subnet, S N,MG,j represents the capacity of the energy storage unit, α and β are weight coefficients, SOC b,i It is the charging state of the energy storage unit.

[0058] The present invention is applicable to a power coordination control method for a DC microgrid group under off-grid operation, and is specifically implemented according to the following steps:

[0059] Step 1: According to the U / I droop characteristics of the underlying distributed micro-sources, a suitable voltage reference value is established for the voltage control outer loop to achieve local micro-source mode switching under different working conditions in the subnet, specifically:

[0060] Step 1.1: Write down the droop characteristic equation according to the voltage-current relationship between the distributed micro-source and the energy storage unit interface converter in the subnet, and establish a suitable voltage reference value u for the voltage control outer loop through the U / I droop characteristic equation.ref , as shown in formula (1):

[0061]

[0062] Among them, u * MG is the bus voltage given value of the subnet, i o is the converter output current, R d is a virtual resistor;

[0063] The value of the virtual resistance, i.e. the droop coefficient, determines the power distribution between the droop units in the subnet. The virtual resistance value R for power distribution according to the rated capacity of the converter is d , as shown in formula (2):

[0064]

[0065] Among them, u min is the minimum voltage allowed by the microgrid, I max is the maximum output current of the converter;

[0066] The droop control is used to automatically adjust the current injected by the underlying distributed micro-sources into the subgrid bus, so as to realize the distribution of the subgrid load power among the droop units;

[0067] Step 1.2: Divide the working modes according to the different characteristics of each micro-source in the subnet;

[0068] Due to the bidirectional charging and discharging of the energy storage unit, it works in droop mode to stabilize the DC bus voltage; in order to prevent overcharging and over-discharging, the state of charge SOC b,i Set upper and lower limits:

[0069] SOC b,min ≤SOC b,i ≤SOC b,max (3);

[0070] Among them, SOC b,min , SOC b,max They are the upper and lower limits of the SOC of the energy storage unit respectively;

[0071] The SOC calculation formula of the energy storage unit is:

[0072]

[0073] Among them, SOC b,i (0) is the initial SOC of the energy storage unit, η i is the charge / discharge efficiency, S N,b,i is the capacity of the energy storage unit, I i is the battery current;

[0074] The output power of micro-sources such as photovoltaics and wind turbines is not stable and is intermittent. In order to improve the utilization rate and make it work in the maximum power output mode, the photovoltaic output reference voltage is given by the maximum power point tracking controller. When the energy storage unit is in standby operation due to the state of charge exceeding the limit, the micro-source works in the constant voltage mode to maintain the bus voltage stable.

[0075] The control strategy switching of distributed micro-sources is realized through the local controller. The bottom-level droop control realizes the power distribution within the subnet. However, as the droop gain increases, the voltage deviation also increases. The bottom-level droop control voltage deviation value Δu pri,j as follows:

[0076]

[0077] Among them, u * MG,j is the bus voltage given value of the jth subnet, u MG,j is the actual bus voltage of j sub-networks;

[0078] Step 2: By adjusting the reference voltage of the droop unit for the second time, the DC bus voltage deviation of the subnet caused by the bottom droop control is eliminated to achieve smooth connection between subnets. Specifically:

[0079] Step 2.1: In order to suppress the DC bus voltage offset caused by the droop control in step 1 from exceeding the set threshold and causing the power quality to deteriorate, the present invention adopts a bus voltage control suitable for different working conditions, and performs secondary compensation on it to restore it to the allowable range.

[0080] When the subgrid is off-grid, the subgrid bus voltage rating is sent to the droop control unit to eliminate the voltage deviation inside the subgrid, and the reference voltage of the droop unit in the subgrid is changed accordingly by vertically moving the droop curve up and down.

[0081]

[0082] Among them, Δu sec,j is the secondary control compensation value of bus voltage under off-grid operation of subgrid, k ps and k is It is the bus voltage secondary adjustment PI control parameter;

[0083] Step 2.2: When the subnets are interconnected, calculate the output voltage u of all subnets using low-bandwidth communication MG,j The root mean square value of the subnet bus voltage u MG,j The data are compared and then sent to all subnet bottom-level controls via the PI regulator.

[0084] Secondary control compensation value Δu′ of bus voltage under sub-grid interconnection sec,j for:

[0085]

[0086] Among them, k′ ps and k′ is is the PI control parameter for secondary adjustment of bus voltage under subgrid interconnection, and n is the number of interconnected sub-microgrids in the group;

[0087] The off-grid subgrid bus voltage is compensated by equation (6), which can realize independent recovery of the bus voltage and ensure power supply stability. The interconnected subgrid bus voltage is compensated by equation (7), so that all bus voltages converge to the same value, realizing smooth connection between subgrids.

[0088] Step 3: Establish a power allocation mechanism between subgrids that can assist each other, use the voltage deviation within the allowable range of the busbar to complete the preset power transmission between subgrids, and reduce the imbalance of subgrid mutual assistance capacity. Specifically:

[0089] Step 3.1: The preset power transmission is completed by utilizing the deviation within the allowable range of the bus voltage, and the voltage of each subgrid is controlled near the rated value while following the power flow control.

[0090] Due to the intermittent and volatile output power of distributed micro-sources and the random changes in load power, it is difficult to predict directly. The proportion of energy storage capacity in the subgrid and its SOC mean can characterize the subgrid's autonomy or mutual assistance capabilities to a certain extent.

[0091] Subnets with high average SOC of energy storage units or high proportion of dispatchable distributed power capacity should bear more power shortages; subnets with low average SOC or low proportion of dispatchable distributed power capacity should absorb more power. The mean mutual assistance capacity of subnets is ε * MGs as follows:

[0092]

[0093] Where n is the number of interconnected microgrids in the cluster, S N,MG,j represents the capacity of the energy storage unit, α and β are weight coefficients, the sum of which is 1, and m is the number of energy storage batteries in the subnet;

[0094] ε MG,j is the mutual assistance capability value of the subnet, and its imbalance degree δε MG,j The calculation is as follows:

[0095]

[0096] Step 3.2: Subnet mutual assistance capacity imbalance δε MG,j The population voltage correction term Δu is generated by the PI control regulator thi,j , in order to reduce the ε between adjacent subnets MG,jThe subgrids adjust the mutual aid power accordingly, and the bus voltage is stabilized. The community collaborative compensation voltage difference is:

[0097]

[0098] Among them, k pt , k it is the PI control parameter of community collaborative control, d v is the voltage regulation constant;

[0099] Step 3.3: The triggering mark of the cluster voltage coordinated control level is when the bus voltages of all sub-grids converge to the root mean square value, which is less than the error margin. And all the interfaces between interconnected subnets are in a closed state, as shown below:

[0100]

[0101] Among them, IC j is the opening and closing state of the subnet interface circuit breaker. It should be noted that Δu thi,j If the value is too large, it will affect the stability of the system and needs to be restricted;

[0102]

[0103] Among them, u dc,j,min 、u dc,j,max are the upper and lower limits of the subnet bus voltage, P DG is the output power of the distributed microgrid, P DG,j,min , P DG,j,max are the upper and lower limits of the distributed micro-source output, SOC m,j is the state of charge of the jth subnet, SOC m,min , SOC m,max It is the upper and lower limits of subnet SOC.

Claims

1. A power coordination control method for a DC microgrid group under off-grid operation, characterized in that: Follow the steps below to implement it: Step 1: Establish a suitable voltage reference value for the voltage control outer loop according to the U / I droop characteristics of the underlying distributed micro-sources; Step 2: By adjusting the reference voltage of the droop unit for the second time, the DC bus voltage deviation of the subnet caused by the bottom droop control is eliminated to achieve smooth connection between subnets; Step 3: Establish a power allocation mechanism between subgrids that can assist each other, and use the voltage deviation within the allowable range of the busbar to complete the preset power transmission between subgrids, thereby reducing the imbalance of subgrid mutual assistance capacity; Specifically: Step 3.1, by utilizing the deviation within the allowable range of the bus voltage to complete the preset power transmission, while following the power flow control, the voltage of each subgrid is controlled near the rated value; Subnet mutual assistance capacity mean ε * MGs as follows: Where n is the number of interconnected microgrids in the cluster, S N,MG,j represents the capacity of the energy storage unit, α and β are weight coefficients, the sum of which is 1, and m is the number of subnet energy storage batteries; SOC b,i is the charge state of the energy storage unit; ε MG,j is the mutual assistance capability value of the subnet, and its imbalance degree δε MG,j The calculation is as follows: Step 3.2: Subnet mutual assistance capacity imbalance δε MG,j The population voltage correction term Δu is generated by the PI control regulator thi,j , the community collaborative compensation voltage difference is: Among them, k pt , k it is the PI control parameter of community collaborative control, d v is the voltage regulation constant; Step 3.3: The triggering mark of the cluster voltage coordinated control level is when the bus voltages of all sub-grids converge to the root mean square value, which is less than the error margin. And all the interfaces between interconnected subnets are in a closed state, as shown below: Among them, IC j Δu is the opening and closing status of the subnet interface circuit breaker; thi,j If the value is too large, it will affect the stability of the system and needs to be restricted; MG,j is the subnet bus voltage; Among them, u dc,j,min 、u dc,j,max are the upper and lower limits of the subnet bus voltage, P DG，j is the output power of the distributed microgrid, P DG,j,min , P DG,j,max are the upper and lower limits of the distributed micro-source output, SOC m,j is the state of charge of the jth subnet, SOC m,min , SOC m,max It is the upper and lower limits of subnet SOC.

2. The power coordination control method for off-grid DC microgrid group according to claim 1 is characterized in that: In the step 1, specifically: Step 1.1: Establish a suitable voltage reference value u for the voltage control outer loop through the U / I droop characteristic equation ref , as shown in formula (1): Among them, u * MG is the bus voltage given value of the subnet, i o is the converter output current, R d is a virtual resistor; Virtual resistance value R for power distribution according to rated capacity of converter d , as shown in formula (2): Among them, u min is the minimum voltage allowed by the microgrid, I max is the maximum output current of the converter; Step 1.2: Divide the working modes according to the different characteristics of each micro-source in the subnet; The state of charge SOC of the energy storage unit b,i Set upper and lower limits: SOC b,min ≤SOC b,i ≤SOC b,max (3); Among them, SOC b,min , SOC b,max They are the upper and lower limits of the SOC of the energy storage unit respectively; Bottom layer droop control voltage deviation Δu pri,j as follows: Among them, u * MG,j is the bus voltage given value of the jth subnet, u MG,j is the actual bus voltage of j subnets.

3. The power coordination control method for off-grid DC microgrid group according to claim 2 is characterized in that: In the step 2, specifically: Step 2.1, when the subgrid is off-grid, the subgrid bus voltage rating is sent to the droop control unit to eliminate the voltage deviation inside the subgrid, and the reference voltage of the droop unit in the subgrid is changed accordingly by vertically moving the droop curve up and down; Among them, Δu sec,j is the secondary control compensation value of bus voltage under off-grid operation of subgrid, k ps and k is It is the bus voltage secondary adjustment PI control parameter; Step 2.2: When the subnets are interconnected, calculate the output voltage u of all subnets using low-bandwidth communication MG,j The root mean square value of the subnet bus voltage u MG,j The data is compared and then sent to all subnet bottom-level controls through the PI regulator; Secondary control compensation value Δu' of bus voltage under sub-grid interconnection sec,j for: Among them, k' ps and k' is is the PI control parameter for secondary adjustment of bus voltage under subgrid interconnection, and n is the number of interconnected sub-microgrids in the group.

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