CN107482608B - Direct-current micro-grid bus voltage coordination control method based on demand side power distribution - Google Patents
Direct-current micro-grid bus voltage coordination control method based on demand side power distribution Download PDFInfo
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J1/00—Circuit arrangements for dc mains or dc distribution networks
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J1/00—Circuit arrangements for dc mains or dc distribution networks
- H02J1/14—Balancing the load in a network
Abstract
The invention relates to a direct-current micro-grid bus voltage coordination control method based on demand side power distribution, which is used for prolonging the service life of an energy storage system and reasonably distributing demand side power and comprises the following steps: 1) constructing an independently-operated direct-current micro-grid, wherein the independently-operated direct-current micro-grid comprises a renewable energy system, an energy storage system and a direct-current load, the renewable energy system is respectively connected with a direct-current bus, the energy storage system is composed of a plurality of distributed energy storage units, and the renewable energy system comprises a plurality of photovoltaic power generation units and a wind power generation unit; 2) and reasonably distributing the power of a demand side and adjusting the bus voltage by adopting a direct-current bus voltage stability control strategy. Compared with the prior art, the invention has the advantages of no need of interconnection communication, avoidance of excessive charge and discharge, prolonged service life of the energy storage battery, improved reliability of the energy storage system, ensured supply and demand balance, stable bus voltage, small fluctuation and the like.
Description
Technical Field
The invention relates to a method for coordinating and controlling a direct current micro-grid bus voltage based on demand side power distribution, in particular to a method for coordinating and controlling the direct current micro-grid bus voltage based on demand side power distribution.
Background
With the increasing application of renewable energy Resources (RES) such as photovoltaic and wind power as a distributed power system, a micro-grid becomes a power system with a great development prospect. Because the dc microgrid has the advantages of higher conversion efficiency and power quality, easy access to renewable energy sources, and the like, a dc power system driven by a power electronic converter receives more and more attention than an existing ac power system.
The stable direct current bus voltage can ensure the reliable operation of the direct current micro-grid, and the energy in the micro-grid reaches the supply and demand balance, so that the direct current bus voltage can be kept in a stable range. Power electronic converters provide a flexible and intelligent way for power systems to manage power flow, so in order to maintain a stable dc bus voltage, the rational distribution of demand side power among different converters is one of the important research items. Since the use of micro grids in island operation mode is particularly important in remote areas, and the intermittent and unpredictable load fluctuations of renewable energy sources can cause transient power imbalances, affecting the operation of the micro grids. Therefore, a plurality of Energy Storage Units (ESUs) need to be configured to provide energy support and increase redundancy of the system, so as to ensure reliability of independent operation of the direct current micro grid. When a plurality of energy storage units exist, in order to avoid that part of the energy storage units are over-discharged or deeply charged to stop working due to different charge states, the power of a required side needs to be reasonably distributed according to the charge states of the energy storage units. In addition, when the Energy Storage System (ESS) is withdrawn due to a fault or full charge, the renewable power system should share the power demand of the microgrid according to the respective output capacities, so as to maintain the stability of the bus voltage.
A voltage hierarchical coordination control strategy is provided according to a hybrid energy storage control and system hierarchical coordination control strategy of a direct current microgrid, so that direct current microgrids running under different working conditions have corresponding converters to balance system power, and therefore the voltage of a direct current microgrid bus is stabilized. But its bus voltage cannot be maintained in nominal operation. According to the research on the photovoltaic direct-current microgrid coordinated direct-current voltage control strategy, the working states of all converters are automatically coordinated and controlled according to the variable quantity of the direct-current voltage and the SOC of the storage battery, so that the purposes of stable operation of a system and reduction of voltage fluctuation of a bus are achieved, but the control method has higher communication requirement. The documents Coordinated controlled based on bus-signaling and virtual inertia for island DC microprocessors realize power Coordinated distribution in a direct current microgrid based on a bus voltage signal and a virtual inertia link. But does not take into account the presence of multiple energy storage units. The document distribution control for automatic operation of a three-Port AC/DC/DS hybrid micro proposes to stabilize the bus voltage and reduce the power loss of the system for the energy management of an AC distributed power source, a DC distributed power source and an energy storage system, but this power exchange hierarchical control method adopts droop control to continuously adjust the output power of the power source according to the load power, and cannot utilize the power generation capacity of the system to the maximum extent.
Disclosure of Invention
The invention aims to overcome the defects in the prior art and provide a direct-current micro-grid bus voltage coordination control method based on demand side power distribution, which does not need interconnection communication, avoids excessive charging and discharging, prolongs the service life of an energy storage battery, improves the reliability of an energy storage system, and ensures balanced supply and demand, stable bus voltage and small fluctuation.
The purpose of the invention can be realized by the following technical scheme:
a direct-current micro-grid bus voltage coordination control method based on demand side power distribution is used for prolonging the service life of an energy storage system and reasonably distributing demand side power, and comprises the following steps:
1) constructing an independently-operated direct-current micro-grid, wherein the independently-operated direct-current micro-grid comprises a renewable energy system, an energy storage system and a direct-current load, the renewable energy system is respectively connected with a direct-current bus, the energy storage system is composed of a plurality of distributed energy storage units, and the renewable energy system comprises a plurality of photovoltaic power generation units and a wind power generation unit;
2) and reasonably distributing the power of a demand side and adjusting the bus voltage by adopting a direct-current bus voltage stability control strategy.
In the step 1), the direct-current microgrid comprises the following operation modes:
energy storage unit control mode: in the mode, the renewable energy source system works in a maximum power tracking mode as a constant power source, the energy storage system performs discharge compensation on power lacking on a demand side through self-adaptive droop control or charges and absorbs redundant power on a direct current bus, and the mode comprises two sub-modes of normal work of all energy storage units and quit operation of part of the energy storage units due to fault reasons;
renewable energy control mode: when the energy storage system stops running due to full charge or failure, the voltage of a direct-current bus is increased due to the fact that the energy supply in the direct-current microgrid is larger than the demand, the renewable energy system reduces the output power at the moment, the stability of the voltage of the bus is maintained, and the renewable energy system comprises a secondary mode that renewable energy sources are all put into running and part of renewable energy sources stop running due to failure;
load shedding and throwing mode: in the energy storage unit control mode and the renewable energy control mode, when the load on the demand side is switched, the energy storage system and the renewable energy system automatically distribute the required power, and the sub-modes comprise a load cutting operation sub-mode and a load putting operation sub-mode.
The direct current bus voltage stabilization control strategy comprises energy storage system control, renewable energy system control and bus voltage compensation control.
In the energy storage system control, the energy storage system maintains the stable bus voltage by adopting self-adaptive droop control, and automatically distributes the power on the demand side according to the SOC of each energy storage unit, so that the energy storage unit with the larger SOC has the faster discharging speed and the slower charging speed, the energy storage unit with the smaller SOC has the slower discharging speed and the faster charging speed, and when part of the energy storage units are in failure, the rest energy storage units respond, and the charging and discharging power is adjusted to ensure the balance of the electric energy supply and demand in the system.
The energy storage system control specifically comprises the following steps:
s1 setting initial virtual resistance R of energy storage units of different SOCsv_k(0)；
S2 judging the SOC variation absolute value | delta SOC of the kth energy storage unitkIf | is equal to 1, if yes, go to step S4, if no, go to step S3;
s4 resetting the virtual resistance of the energy storage units of different SOCs;
s3 judging the power P of the kth energy storage unitESU_kIf yes, go to step S5;
s5 obtaining the instantaneous SOC of the main energy storage unitm；
S6, judging whether the charge and discharge state of the kth energy storage unit is changed, if yes, returning to the step S1, and if not, returning to the step S5.
In step S1, the initial virtual resistance R of the energy storage unitv_k(0) The expression of (a) is:
wherein R isv_maxIs the maximum value of the virtual resistance,
SOCmaxand SOCminRespectively, a state-of-charge reference value, a maximum value and a minimum value of the energy storage unit, and when the energy storage system is in a charging state
When the energy storage system is in a discharge state
VerrIs the difference between the demand side bus voltage and the bus reference voltage.
In step S4, the virtual resistance formula of the energy storage units with different SOCs is reset as follows:
wherein R isv_kThe virtual resistance of the energy storage unit after resetting.
The renewable energy system control specifically comprises the following steps:
when the energy storage system stops working due to full charge or failure, the output of the renewable energy system is larger than the total consumption of a demand side, and smooth switching between a maximum power tracking mode and a demand power distribution mode is ensured by adopting power droop control.
The power droop control is as follows:
in the formula, PRES_gAnd PMPP_gRespectively the output power and the maximum power of the g-th renewable energy source, and defining a condition event as P, V and Vdc_loadFor the required side bus voltage, VrefIs the bus reference voltage, b is the droop coefficient, VmaxIs the maximum allowable voltage of the bus.
The bus voltage compensation control specifically comprises the following steps:
when the renewable energy system and the energy storage system adopt a droop control strategy, a bus reference voltage iteration adjustment control strategy based on a fuzzy controller is adopted, the bus voltage deviation is used as fuzzy input, and the bus voltage deviation is output as a bus voltage increment signal to compensate the bus voltage deviation caused by droop control.
Compared with the prior art, the invention has the following advantages:
1) and reasonable distribution of power at a demand side can be realized without interconnection communication among all distributed power supplies in the direct-current micro-grid.
2) The self-adaptive droop control designed by the invention can effectively avoid excessive charging and discharging of part of energy storage units, thereby prolonging the service life of the energy storage battery and improving the reliability of stable operation of the energy storage system.
3) When a part of distributed power supplies in the direct-current micro-grid have faults, the rest distributed power supplies which normally operate can quickly respond, and the direct-current bus voltage is maintained according to the respective operation capacity.
4) When the direct-current micro-grid is in various running states, distributed power supplies are reasonably distributed to the power at the demand side, and the balance of the power supply and demand in the micro-grid is ensured, so that the stability of the direct-current bus voltage is maintained.
5) The designed fuzzy voltage iterative control can effectively compensate the voltage deviation caused by droop control, so that the voltage of the direct current bus runs at a rated value.
6) The fluctuation of the direct current bus voltage is small when disturbance occurs.
Drawings
Fig. 1 is a schematic diagram of a structure of a dc microgrid operating independently.
Fig. 2 is a flow chart of power redistribution control of the kth energy storage unit.
Fig. 3 is a control block diagram of the energy storage system.
Fig. 4 is a renewable energy system control block diagram.
Fig. 5 is a graph of droop control operating characteristics.
FIG. 6 is a block diagram of bus voltage compensation control
Fig. 7 is a diagram of an RTDS-based dc microgrid architecture.
Fig. 8 is a system waveform diagram under real-time condition 1 scene 1, where fig. 8a is a photovoltaic system power waveform diagram, fig. 8b is a load power waveform diagram, fig. 8c is an energy storage system power waveform diagram, and fig. 8d is a dc bus voltage waveform diagram.
Fig. 9 is a waveform diagram of a system adopting conventional droop control in scenario 1, where fig. 9a is a waveform diagram of energy storage system power and fig. 9b is a waveform diagram of dc bus voltage.
Fig. 10 is a system waveform diagram under real-time working condition 1 and scene 2, where fig. 10a is a photovoltaic system power waveform diagram, fig. 10b is a load power waveform diagram, fig. 10c is an energy storage system power waveform diagram, and fig. 10d is a dc bus voltage waveform diagram.
Fig. 11 is a waveform diagram of a system using conventional droop control in scene 2, where fig. 11a is a waveform diagram of energy storage system power and fig. 11b is a waveform diagram of dc bus voltage.
Fig. 12 is a system waveform under real-time condition 2 and scene 3, where fig. 12a is a power waveform diagram of the energy storage system, fig. 12b is a voltage waveform diagram of the dc bus, fig. 12c is a power waveform diagram of the energy storage system, and fig. 12d is a voltage waveform diagram of the dc bus.
Fig. 13 is an operation result when the conventional droop control is employed, in which fig. 13a is a diagram of an energy storage system power waveform, fig. 13b is a diagram of a dc bus voltage waveform, fig. 13c is a diagram of an energy storage system power waveform, and fig. 13d is a diagram of a dc bus voltage waveform.
Fig. 14 is a system waveform diagram under real-time condition 3 and scenario 4, where fig. 14a is a load power waveform diagram, fig. 14b is a photovoltaic system power waveform diagram, and fig. 14c is a dc bus voltage waveform diagram.
Fig. 15 is a voltage waveform diagram without the blur compensation control in scene 4.
Detailed Description
The invention is described in detail below with reference to the figures and specific embodiments.
Examples
The independently operated direct current micro-grid consists of three parts as shown in figure 1, wherein a renewable energy system containing photovoltaic and wind turbines is connected to a direct current bus through DC-DC or AC-DC, and provides green electric energy for the micro-grid. And the demand sides of alternating current or direct current loads and the like are connected into the microgrid through corresponding converters. An energy storage system formed by the distributed energy storage units performs a charging or discharging process through the bidirectional DC-DC converter, and power between the distributed power generation system and a demand side is balanced. Wherein: pPV_nIs nthPower output by the photovoltaic system; pWTG_mIs mthPower output by the wind power system; pESU_kIs kthPower of energy storage unit, and when PESU_k>0 time represents the power emitted by the energy storage unit, power PESU_k<0 represents the absorbed power of the energy storage unit; pdc_yAnd Pac_zAre each ythDC load and zthPower consumed by the ac load; pRES、PESSAnd PLoadRenewable energy power, energy storage system power and demand side power, respectively, and have:
a typical dc microgrid consists of three parts, namely, renewable energy, an energy storage system, and a dc load. As shown in fig. 1, renewable energy sources include photovoltaic power generation and wind power generation, and the energy storage system is typically a storage battery. Because the application of a microgrid in an island operation mode is particularly important in remote areas, and the coordinated operation of an energy storage system and renewable energy sources in such a mode is a key issue, an important analysis is made here for the island operation mode. The present invention will be further described in terms of working modes, design principles, design methods, validity verification, and the like.
First, DC micro-grid operation mode
Typically, the energy storage system is in either a charging mode or a discharging mode depending on whether the power provided by the renewable energy source can meet the demand-side consumption. However, long term imbalance in supply and demand from the dc microgrid can result in deep charging or over-discharging of the energy storage system. Therefore, in order to prolong the service life of the energy storage system and reasonably distribute the power of the demand side, the operation modes of the direct-current micro-grid can be divided into the following three modes according to the bus voltage stability control strategy provided by the invention:
1. mode 1 (energy storage unit control mode)
In this mode, in order to fully utilize renewable energy, new energy such as wind and light can be regarded as a constant power source when operating in a maximum power tracking mode. The energy storage system adopts self-adaptive droop control to discharge and compensate the power which is deficient on the demand side, or charges and absorbs the redundant power on the direct current bus. Mode 1 can be divided into two modes, mode 1-1 and mode 1-2, depending on the operating state of the energy storage system.
When the energy storage units are all in normal operation, the mode is 1-1, and the power balance relationship in the system is,
PLoad＝PESS+PRES(4)
when part of the energy storage units are out of operation due to faults and the like, the energy storage units are in the modes 1-2, and meanwhile, the rest energy storage units maintain the balance of supply and demand of energy in the microgrid. The power balance relationship in the system becomes,
in the formula, s is the number of the energy storage units which normally run.
2. Mode 2 (renewable energy control mode)
In actual operation, when the energy storage system is out of operation due to full charge or failure, etc., the supply of energy in the system is greater than the demand, which will result in the increase of the bus voltage, so that the renewable energy system is required to reduce its output power, thereby maintaining the stability of the bus voltage. The mode 2 can be classified into two modes, mode 2-1 and mode 2-2, according to the operation state of the renewable energy source.
When the renewable energy source is fully put into operation, the mode is 2-1, and the power balance relation in the system is,
PLoad＝PRES(6)
when part of renewable energy sources quit operation due to faults and are in the mode 2-2, the rest renewable energy sources in normal operation adjust power generation according to respective output capacity, and stability of bus voltage is ensured. The power balance relationship in the system becomes,
wherein r is the amount of renewable energy put into operation.
3. Mode 3 (off-load mode)
In mode 1 and mode 2, when the load on the demand side is switched, the energy storage system and the renewable energy system are required to automatically allocate the required power. The mode 3 can be classified into two modes, a mode 3-1 and a mode 3-2, according to the operation state of the load.
When the load is cut off, the load is in a mode 3-1, at this time, the energy storage system or the renewable energy system needs to adjust respective operation states to redistribute the power on the load side, at this time, the power balance relation in the system is changed,
PLoad＜PRES+PESS(8)
similarly, in mode 3-2 when the load is put into operation, the power balance relationship in the system becomes,
PLoad＞PRES+PESS(9)
direct current bus voltage stabilization control strategy
In the independently operated direct current micro-grid, the supply and demand balance of the source load power can enable the bus voltage to be maintained in a stable range. Therefore, the specific control strategy for properly distributing the demand side power and adjusting the bus voltage is as follows.
1. Energy storage system control
In order to avoid excessive charging and discharging of part of energy storage units, the energy storage system adopts self-adaptive droop control to maintain stable bus voltage, and demand side power is automatically distributed according to the SOC of each energy storage unit. So that the output voltage is such that,
Vdc_k＝Vref-IL_k·Rv_k(10)
in the formula, VrefIs a bus reference voltage; vdc_k、IL_kAnd Rv_kAre each kthThe energy storage unit outputs voltage, inductive current and virtual resistance.
Therefore, when the virtual resistance is small, the input or output current of the energy storage unit is large. For a micro-grid system with a small physical scale, the line loss of the units connected in parallel between the same bus can be ignored. So that for a converter accessing the same voltage class system approximately,
Vdc_1≈…≈Vdc_k≈Vdc_Load(11)
in the formula, Vdc_LoadIs the demand side bus voltage.
Therefore, the value range of the virtual resistor can be obtained by solving a voltage equation of a kirchhoff node of the bus voltage column. The deviation of the bus voltage caused by the droop control is,
Verr＝Vdc_Load-Vref(12)
therefore, when Verr>0 time the energy storage system is charged, and V timeerr<And 0, discharging the energy storage system. In general, the SOC of the battery during operation should be maintained between 50% and 100%. And k isthThe SOC of the energy storage unit can be calculated by the following formula,
in the formula, SOCk、SOCk(0) And CbatAre each kthThe energy storage unit instantaneous state of charge, initial state of charge, and capacity.
Thereby k can be adjustedthSOC variation (Δ SOC) of energy storage unitk) The definition is that,
the combined type (10) to (14) can be obtained,
from this, the amount of change in SOC depends on the magnitude of the virtual resistance. Therefore, the initial virtual resistance of the energy storage units of different SOCs is set as follows,
in the formula, Rv_k(0) Is kthAn initial virtual resistance of the energy storage unit; rv_maxThe maximum value of the virtual resistance is obtained;
SOCmaxand SOCminRespectively, a state-of-charge reference value, a maximum value and a minimum value of the energy storage unit, and when the energy storage system is in a charging state
When the energy storage system is in a discharge state
In order to allow the energy storage units with lower states of charge to absorb more power during charging and to provide less power during discharging, demand-side power redistribution is required as shown in the flow chart of fig. 2, where ξ is a constant, ξ is 1 when equation ξ is | Δ SOCkWhen l is true, let kthR of the energy storage unitv_kDecrease to a minimum value (R)v_min). Obviously, it can be known that the virtual resistance of the energy storage unit with smaller SOC is firstly reduced to R during the charging processv_minDuring the discharging process, the virtual resistance of the energy storage unit with larger SOC is firstly reduced to Rv_min. Meanwhile, when the virtual resistance of one energy storage cell in the ESS decreases, the power absorbed or emitted by the energy storage cell increases, and the power absorbed or emitted by the other energy storage cells decreases. Virtual resistance is reduced to Rv_minThe energy storage units can be defined as leading energy storage units, and the rest energy storage units are defined as subordinate energy storage units. Furthermore, the initial state of charge [ SOC ] of the energy storage unit is dominantm(0)]And an initial virtual resistance Rv_m(0)]The following process can be used to calculate:
from equation (15), equation ξ ═ Δ SOCkThe | may be changed to be that,
in the formula, t1Is PESU_kTime of decrease.
Therefore, as can be seen from equation (16), the initial state of charge of the energy storage unit is dominated during the charging process,
similarly, the initial charge state of the energy storage unit is mainly controlled in the discharging process,
in addition, the instantaneous state of charge (SOC) dominating the energy storage can be found by equations (13) and (15)m) In order to realize the purpose,
in particular, fluctuations in the renewable energy system and the demand side can likewise lead to PESU_kDecrease in equation ξ ═ Δ SOCkBefore | is true, whatever fluctuation causes PESU_kReducing, each energy storage unit can calculate a SOC through the above processmAnd the respective virtual resistances are reset as follows,
in addition, under certain conditions the virtual resistance should be reset as follows,
where conditional event A is defined as equation ξ ═ Δ SOCkBefore | is established PESU_kDecrease, conditional event B is defined as ξ>|ΔSOCk∩ is the logical operation AND.
From the above analysis, it can be seen that each energy storage unit can automatically allocate the power of the demand side through adaptive droop control according to the SOC of the energy storage unit. The energy storage unit with the larger SOC has a faster discharging speed and a slower charging speed, and the energy storage unit with the smaller SOC has a slower discharging speed and a faster charging speed. In addition, when part of the energy storage units are in fault, the rest of the energy storage units can respond quickly, and the charging and discharging power is adjusted to ensure that the electric energy supply and demand in the system is balanced, so that the energy storage system has higher reliability. Fig. 3 is a control block diagram of the energy storage system.
2. Renewable energy system control
When the energy storage system is out of operation due to full charge or failure, the output of the renewable energy system is greater than the total consumption of the demand side. To prevent the bus voltage from rising excessively, the renewable energy system should take on the job of distributing the demand side power. Therefore, in order to ensure smooth switching between the maximum power tracking mode and the required power distribution mode, the following power droop control is adopted,
in the formula, PRES_gAnd PMPP_gAre respectively gthThe output power and maximum power of the renewable energy source; defining a condition event definition P as the energy storage system quitting operation; the droop coefficient b is defined as the coefficient,
in the formula, VmaxIs the maximum allowable voltage of the bus.
As can be seen from the equation (23), each RES automatically adjusts the output power in accordance with the following equation,
each RES may thus automatically allocate demand side power according to its respective maximum output capacity. FIG. 4 is a block diagram of renewable energy system control, where IRES_gIs gthOutput side current of renewable energy.
3. Bus voltage compensation control
Based on the foregoing analysis, renewable energy systems and storageWhen the droop control strategy is adopted by the system, the power of the demand side can be reasonably distributed under the condition of no need of interconnection communication, and the bus voltage of the direct-current micro-grid is maintained within an allowable range. However, as can be seen from the droop control operating characteristic of fig. 5, when the system is operating at a stable operating point, there is some deviation in the bus voltage. In FIG. 5, Vstedc_LoadFor stabilizing the DC bus voltage, IsteL_kIs at a stable time kthInductive current, P, of the energy storage unitsteRES_gG at the time of stabilizationthOutput power of renewable energy.
Although it is possible to ensure that the voltage has a deviation within the allowable range by limiting the droop coefficient, in some applications the bus voltage is required to be stabilized at a nominal value. Therefore, a fuzzy-based-controller (FIS) based bus reference voltage iterative adjustment control strategy is proposed herein. The bus voltage deviation is input as a fuzzy input and output as a bus voltage increment signal (Δ V), and the rule thereof is formulated as shown in table 1.
TABLE 1 fuzzy inference rules for incremental signals
Wherein: NL, NM and NS are respectively big negative, middle negative and small negative; RZ is zero; PS, PM, and PL were respectively small, medium, and large.
Therefore, the direct current bus reference voltage can be increased or decreased according to the deviation of the bus voltage. Fig. 6 shows a bus voltage iterative compensation control block diagram, where the controller is composed of an FIS and an integration element, and δ V is a bus reference voltage iterative quantity.
Therefore, the formula (10) can be replaced,
Vdc_k＝Vref-IL_k·Rv_k-δV (26)
similarly, the voltage deviation in equation (12) may be replaced with,
Verr＝Vdc_Load-(Vref-δV) (27)
similarly, the RES droop control is adjusted to,
PRES_g＝PMPP_g-b·(Vdc_load-Vref+δV) (28)
third, example analysis
To verify the effectiveness of the control strategy proposed herein, a standalone dc microgrid system as shown in fig. 7 was built in the RTDS hardware platform. In a real-time system, 3 groups of energy storage units form an energy storage system, and two groups of photovoltaic power generation modules are equivalent to a renewable energy system.
1. Real-time operating mode 1
Scene 1: the energy storage system works normally and maintains the bus voltage stable, the charge states of the energy storage units are different, and the photovoltaic power generation system operates in a maximum power tracking mode. The transient working condition is the operation of simulating illumination intensity reduction and load shedding. Fig. 8 is a waveform of operation states of respective units when the control strategy proposed herein is applied, and fig. 9 is an operation result when the conventional droop control is applied.
And at 6s, the output of the photovoltaic system is reduced due to the reduction of the illumination intensity, the energy storage system is switched from a charging state to a discharging state, and the power shortage of the demand side is compensated. And at 12s, the load shedding operation is carried out on the demand side, and the energy storage system adjusts the output to maintain the supply and demand balance in the system. As can be seen from the power fluctuation situation of the energy storage system in fig. 8, when the adaptive droop control strategy proposed herein is adopted, each energy storage unit automatically allocates the power on the demand side according to its respective SOC, so that overcharge and overdischarge of part of the energy storage units can be avoided, and the reliability of the energy storage system is improved. When the conventional droop control is adopted, as can be seen from fig. 9, although the energy storage system can maintain the supply and demand balance of the system, the energy storage units always equally divide the power on the demand side, which is not beneficial to the balance among different SOCs. In addition, comparing the dc bus voltage waveforms in fig. 8 and 9, it can be seen that when the fuzzy compensation control strategy proposed herein is adopted, the bus can be stabilized at 600V, and the response speed is fast and the fluctuation is small, and when the compensation control is not adopted, the bus voltage fluctuation is large and there is a high voltage deviation.
Scene 2: under the working condition of scene 1, the transient working condition is simulated illumination intensity increase and load throwing operation. Fig. 10 is a waveform of operating states of respective units when the control strategy proposed herein is applied, and fig. 11 is an operating result when the conventional droop control is applied.
And (5) increasing the illumination intensity at 6s, so that the energy storage system is switched from a charging state to a discharging state to absorb the redundant power on the direct current bus. And in 12s, the load throwing operation is carried out on the demand side, and the energy storage system automatically adjusts the absorbed power to maintain the balance of supply and demand in the system. Comparing the power fluctuation of the energy storage system in fig. 10 and fig. 11, it can be seen that, when the adaptive droop control strategy proposed herein is adopted, each energy storage unit automatically allocates the power on the demand side according to the respective SOC, so that overcharge and overdischarge of part of the energy storage units can be avoided, and the reliability of the energy storage system is improved. When the traditional droop control is adopted, the energy storage units always equally divide the power of the required side, and the balance among different SOCs is not facilitated. In addition, comparing the dc bus voltage waveforms in fig. 8 and 9, it can be seen that when the fuzzy compensation control strategy proposed herein is adopted, the bus can be stabilized at 600V, and the response speed is fast and the fluctuation is small, and when the compensation control is not adopted, the bus voltage fluctuation is large and there is a high voltage deviation.
2. Real time operating mode 2
Scene 3: the energy storage system maintains the stable bus voltage, the charge states of the energy storage units are different from each other, the photovoltaic power generation system operates in a maximum power tracking mode, and the demand side consumes 7.2 kW. The transient working condition is that a part of energy storage units are simulated to be failed and quit. Fig. 12 is a waveform of the operating states of the energy storage units when the control strategy proposed herein is applied, and fig. 13 is an operating result when the conventional droop control is applied.
When the energy storage system is in a charging process and a discharging process, the output of the photovoltaic system is 11kW and 4kW respectively. And the control of the energy storage unit meets the power redistribution condition in 2s, the energy storage unit with the minimum SOC exits in the charging process in 6s, and the energy storage unit with the maximum SOC exits in the discharging process. As can be seen from the power waveform of the energy storage system in fig. 12, after the power is redistributed on the demand side, the energy storage unit with the smaller SOC absorbs more power in the charging process, and the energy storage unit with the larger SOC emits more power in the opposite direction, and the energy storage unit with the smaller SOC emits more power in the discharging process. And when the energy storage unit bearing the main power supply of the system fails, the rest energy storage units which normally work can quickly respond to automatically distribute the power of the demand side. As can be seen from comparing fig. 12 and fig. 13, when the conventional droop control strategy is adopted, the power of the required side is always equally distributed by each energy storage unit, and overcharge and overdischarge of part of the energy storage units cannot be avoided. It can be seen from the comparison of the dc bus voltage that the use of the fuzzy compensation control herein can ensure that the bus voltage is stabilized at 600V, while when this control is not used, the bus voltage has large fluctuation and voltage deviation.
3. Real time operating mode 3
Scene 4: the energy storage system is quitted in a fault, and the photovoltaic power generation system maintains the stable bus voltage. The transient working condition is simulated load fluctuation and partial photovoltaic unit fault exit. As can be seen from the photovoltaic system power waveform in fig. 14, each photovoltaic power generation unit can perform demand-side power distribution according to its output capacity. And when the load power is reduced in 1s, the output of each photovoltaic power generation unit can be automatically adjusted to maintain the stability of the bus voltage. And when the time is 2s, the group of photovoltaic units quit, and the rest of the photovoltaic units which normally run rapidly improve the output power, so that the normal power supply of the load on the demand side is ensured. As can be seen by comparing the bus voltage waveforms of fig. 14 and 15, when the fuzzy compensation control proposed herein is adopted, the bus voltage deviation caused by the droop control can be effectively reduced.
Claims (7)
1. A direct-current micro-grid bus voltage coordination control method based on demand side power distribution is used for prolonging the service life of an energy storage system and reasonably distributing demand side power, and is characterized by comprising the following steps:
1) constructing an independently-operated direct-current micro-grid, wherein the independently-operated direct-current micro-grid comprises a renewable energy system, an energy storage system and a direct-current load, the renewable energy system is respectively connected with a direct-current bus, the energy storage system is composed of a plurality of distributed energy storage units, and the renewable energy system comprises a plurality of photovoltaic power generation units and a wind power generation unit;
2) the method comprises the steps that a direct current bus voltage stability control strategy is adopted to reasonably distribute demand side power and adjust bus voltage, the direct current bus voltage stability control strategy comprises energy storage system control, renewable energy system control and bus voltage compensation control, in the energy storage system control, the energy storage system adopts self-adaptive droop control to maintain bus voltage stability, and demand side power is automatically distributed according to the SOC of each energy storage unit, so that the energy storage unit with the larger SOC has a higher discharging speed and a lower charging speed, the energy storage unit with the smaller SOC has a lower discharging speed and a higher charging speed, and when part of the energy storage units are in fault, the rest energy storage units respond to each other, and the charging and discharging power is adjusted to ensure that the electric energy supply and demand in the system are balanced;
the energy storage system control specifically comprises the following steps:
s1 setting initial virtual resistance R of energy storage units of different SOCsv_k(0)；
S2 judging the SOC variation absolute value | delta SOC of the kth energy storage unitkIf | is equal to 1, if yes, go to step S4, if no, go to step S3;
s4 resetting the virtual resistance of the energy storage units of different SOCs;
s3 judging the power P of the kth energy storage unitESU_kIf yes, go to step S5;
s5 obtaining the instantaneous SOC of the main energy storage unitm；
S6, judging whether the charge and discharge state of the kth energy storage unit is changed, if yes, returning to the step S1, and if not, returning to the step S5.
2. The method for coordinately controlling the bus voltage of the direct current microgrid based on demand side power distribution as claimed in claim 1, wherein in the step 1), the direct current microgrid comprises the following operation modes:
energy storage unit control mode: in the mode, the renewable energy source system works in a maximum power tracking mode as a constant power source, the energy storage system performs discharge compensation on power lacking on a demand side through self-adaptive droop control or charges and absorbs redundant power on a direct current bus, and the mode comprises two sub-modes of normal work of all energy storage units and quit operation of part of the energy storage units due to fault reasons;
renewable energy control mode: when the energy storage system stops running due to full charge or failure, the voltage of a direct-current bus is increased due to the fact that the energy supply in the direct-current microgrid is larger than the demand, the renewable energy system reduces the output power at the moment, the stability of the voltage of the bus is maintained, and the renewable energy system comprises a secondary mode that renewable energy sources are all put into running and part of renewable energy sources stop running due to failure;
load shedding and throwing mode: in the energy storage unit control mode and the renewable energy control mode, when the load on the demand side is switched, the energy storage system and the renewable energy system automatically distribute the required power, and the sub-modes comprise a load cutting operation sub-mode and a load putting operation sub-mode.
3. The method according to claim 1, wherein in step S1, the initial virtual resistance R of the energy storage unit isv_k(0) The expression of (a) is:
wherein R isv_maxIs the maximum value of the virtual resistance,
SOCmaxand SOCminRespectively, a state-of-charge reference value, a maximum value and a minimum value of the energy storage unit, and when the energy storage system is in a charging state
When the energy storage system is in a discharge state
VerrIs the difference between the demand side bus voltage and the bus reference voltage.
4. The method according to claim 1, wherein in step S4, the virtual resistance formulas of the energy storage units with different SOCs are reset as follows:
wherein R isv_kThe virtual resistance of the energy storage unit after resetting.
5. The method according to claim 1, wherein the renewable energy system control specifically comprises:
when the energy storage system stops working due to full charge or failure, the output of the renewable energy system is larger than the total consumption of a demand side, and smooth switching between a maximum power tracking mode and a demand power distribution mode is ensured by adopting power droop control.
6. The method of claim 5, wherein the power droop control is:
in the formula, PRES_gAnd PMPP_gRespectively the output power and the maximum power of the g-th renewable energy source, and defining a condition event as P, V and Vdc_loadFor the required side bus voltage, VrefIs the bus reference voltage, b is the droop coefficient, VmaxIs the maximum allowable voltage of the bus.
7. The method for coordinately controlling the bus voltage of the direct-current microgrid based on demand side power distribution as claimed in claim 1, wherein the bus voltage compensation control specifically comprises:
when the renewable energy system and the energy storage system adopt a droop control strategy, a bus reference voltage iteration adjustment control strategy based on a fuzzy controller is adopted, the bus voltage deviation is used as fuzzy input, and the bus voltage deviation is output as a bus voltage increment signal to compensate the bus voltage deviation caused by droop control.
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| CN109240400A (en) * | 2018-08-06 | 2019-01-18 | 上海大学 | A kind of DC Bus voltage compensatory method based on fuzzy control |
| CN111342530B (en) * | 2020-05-14 | 2020-08-21 | 沈阳微控新能源技术有限公司 | Self-adaptive charge and discharge control method for flywheel energy storage device and energy storage converter |
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