CN110212517B - Distributed unified control method of medium-low voltage direct current power distribution system - Google Patents
Distributed unified control method of medium-low voltage direct current power distribution system Download PDFInfo
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- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
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Abstract
The invention discloses a distributed unified control method of a medium-low voltage direct current power distribution system, which comprises a distributed control method based on a direct current bus voltage signal and a unified control method of a direct current transformer, wherein the distributed control method comprises the following two parts: the distributed control method based on the voltage signal of the direct current bus adjusts respective working modes of photovoltaic units, energy storage units and fuel cell units on the direct current bus according to the voltage interval of the direct current bus voltage, and maintains the voltage stability of the direct current bus together; the unified control method of the direct current transformer is characterized in that a shift control link is added on the basis of a voltage droop control link to obtain a power control function, a medium-voltage direct current bus voltage regulation function and a low-voltage direct current bus voltage regulation function; when the operation scene of the system changes, the direct current transformer does not need to switch the control mode. The invention realizes the integrated control of the medium-low voltage direct current distribution system, has no communication link, is easy to realize, has high reliability and realizes the stable operation of the medium-low voltage direct current distribution system.
Description
The technical field is as follows:
the invention relates to a distributed unified control method for a medium-low voltage direct current power distribution system, and belongs to the field of control in a direct current power distribution technology.
The background art comprises the following steps:
with the development of new energy, direct current load and power electronic technology, direct current power distribution systems are widely applied due to the advantages of flexibility, high efficiency and the like. In consideration of the intermittency of the new energy, the stable operation of the system is guaranteed to be important. The medium-low voltage direct current power distribution system is provided with a plurality of voltage grades, a plurality of direct current buses and a plurality of types of converters, so that the operation mode and the coordination control method of the system are more complicated.
At present, most of researches are focused on a direct-current micro-grid with a single bus structure, and various centralized and distributed control methods are provided. The centralized control method is generally based on a communication system, and transmits electrical information of each module to an energy management system, so as to obtain control instructions of each unit in the system, and realize power balance and optimal operation of the system. The control method has high dependence on communication, numerous devices and higher cost. The distributed control method is represented by distributed control based on a direct current bus voltage signal (DBS), determines the operating mode of each module based on the direct current bus voltage, does not need to communicate with each other, is simple to control, and has a large voltage fluctuation range.
For a DC transformer, it should have functions of power control and medium and low voltage DC bus regulation, but the current unified control method can only realize unified control of single-ended voltage and power of the DC/DC converter, and if the reverse voltage support is to be realized, the control mode must be switched. For medium and low voltage direct current power distribution with a complex structure and high distributed photovoltaic permeability, the operation scenes are numerous and variable, the control mode of the direct current transformer needs to be frequently switched, and once the switching is wrong or not in time, the whole system can be broken down. Therefore, the invention provides a unified control method of the direct current transformer aiming at the problem, unifies the three control functions, and does not need to switch the control modes.
At present, although some documents discuss the topology structure, the control method, part of interface devices, and the stability of the medium-low voltage dc power distribution system, the operation mode and the control method of the medium-low voltage dc power distribution system may not be fully analyzed. The patent 'multi-voltage-level direct-current distribution network distributed control method for accessing distributed power supplies' provides a distributed control mode for a multi-voltage-level direct-current distribution system, a direct-current transformer adopts a constant voltage ratio control method, and each distributed power supply on a direct-current bus adopts droop control, so that the control of the complex direct-current distribution system is greatly simplified. However, all distributed power supplies adopt droop control, and cannot be utilized to the maximum extent. In view of the above problems, the present invention provides a distributed unified control method suitable for a medium-low voltage dc power distribution system, so as to implement integrated control of the system.
The invention content is as follows:
the invention provides a distributed unified control method suitable for a medium-low voltage direct current power distribution system aiming at the operation control problem of the medium-low voltage direct current power distribution system, which can realize the coordinated operation of distributed photovoltaic, energy storage, a direct current transformer and a grid-connected inverter in each state of the system without switching control modes.
The above object of the present invention can be achieved by the following technical solutions:
a distributed unified control method of a medium and low voltage direct current distribution system comprises a distributed control method based on a direct current bus voltage signal and a unified control method of a direct current transformer, wherein the distributed control method comprises the following two parts: the distributed control method based on the direct-current bus voltage signal comprises the following steps:
1.1 the operating range of the direct current bus voltage operation is divided into 4 intervals, namely:
h2 interval, representing [ V ]dcH,Vdcmax]；
H1 interval, representing [ V ]dcN,VdcH]；
Interval L1, representing [ V ]dcL,VdcN]；
Interval L2, representing [ V ]dcmin,VdcL]。
Wherein the content of the first and second substances,
Vdcmaxthe highest voltage allowed by the direct current bus;
VdcHcritical voltage of a droop control mode is adopted for the photovoltaic power generation unit;
VdcNthe rated voltage of the direct current bus;
VdcLa critical voltage at which the fuel cell unit starts to operate;
Vdcminthe lowest voltage allowed by the direct current bus.
The photovoltaic, energy storage and fuel cell units on the direct current bus adjust respective working modes according to the voltage interval of the direct current bus voltage, and maintain the direct current bus voltage stable together;
1.2 according to the working state of the converter station, the direct current distribution system has two basic operation modes of grid connection and grid disconnection, the switching between grid connection and grid disconnection does not affect the control method of the photovoltaic and energy storage units, when the system is in grid connection operation, the converter station adopts a fixed direct current voltage control mode, the voltage of a medium-voltage direct current bus is controlled to be a rated value, a direct current transformer controls the per unit value of the voltage deviation of the medium-voltage direct current bus and the low-voltage direct current bus to follow each other, which is equivalent to the control of the voltage of the low-voltage direct current bus to be the rated value, so that when the system is in grid connection operation, the voltage of the direct current bus is the rated value, no voltage deviation exists, the photovoltaic works in a maximum power tracking state, and the energy storage and the fuel cell work in a standby state;
the unified control method of the direct current transformer comprises the following steps:
2.1 adding a shift control link on the basis of a voltage droop control link to obtain a power control function, a medium-voltage direct-current bus voltage regulation function and a low-voltage direct-current bus voltage regulation function;
2.2, calculating to obtain a per unit value of the deviation between the medium-voltage direct-current bus voltage and the low-voltage direct-current bus voltage, and making a difference between the medium-voltage direct-current bus voltage and the low-voltage direct-current bus voltage, wherein the difference value is used for outputting a shift voltage signal through a PI (proportional-integral) controller, and moving a droop characteristic curve of the direct-current transformer, so that the size and the direction of transmission power are automatically adjusted, and the global power balance of the system is finally realized;
2.3 when the system is in off-grid operation, the DC bus voltage is controlled by the photovoltaic, energy storage and fuel cell units, the DC voltage operates in an allowable interval range, and because the DC transformer controls the deviation per unit value of the medium-voltage DC bus voltage and the low-voltage DC bus voltage to be equal, when the voltage of one side changes, the DC transformer transmits the corresponding power to compensate the power shortage of the voltage change side, the bus voltages of the two sides are finally stabilized at a new balance point, and at the moment, the voltage control units on the buses of the two sides jointly inhibit the power fluctuation on the bus of the side or the bus of the other side;
2.4 when the system is in grid-connected operation, the voltage of the medium-voltage direct-current bus is controlled at a rated value by the converter station, and the voltage deviation of the medium-voltage direct-current bus and the voltage of the low-voltage direct-current bus is controlled by the direct-current transformer to be equal in per unit value, so that the voltage of the low-voltage direct-current bus is also the rated value, at the moment, the photovoltaic works in a maximum power tracking state, and the energy storage and fuel cell works in a standby state.
In the distributed unified control method of the medium-low voltage direct current power distribution system, the voltage grade of the medium-voltage direct current bus is 6kV, 10kV or 24 kV; the voltage grade of the low-voltage direct-current bus is 400V, 750V and 1500V.
Has the advantages that:
1. the invention realizes the unified control of the direct current transformer, integrates the functions of power control and medium-voltage and low-voltage direct current bus regulation, and does not need to switch control modes when the system operation mode changes;
2. the invention can realize the integrated control of each module unit in the system based on the direct current bus voltage signal, does not need communication link, and has simple control structure, high reliability and lower cost.
3. The invention can be further expanded, a power dispatching control loop is added on a shifting control loop of the direct current transformer, the system has a dispatching function, and the optimal operation of the system can be realized based on a communication link and an energy management control link.
Description of the drawings:
fig. 1 is a typical topology of a medium-low voltage dc power distribution system.
Fig. 2 is a diagram of a dc bus voltage coordination control method.
Fig. 3 is a bidirectional voltage droop characteristic of the dc transformer.
Fig. 4 is a control block diagram of the dc transformer.
Fig. 5 is a voltage regulation function of the dc transformer.
The specific implementation mode is as follows:
the invention is described in further detail below with reference to the figures and the specific embodiments.
The topological structure of the medium-low voltage direct current power distribution system is shown in fig. 1, and the medium-low voltage direct current power distribution system comprises a medium-voltage direct current bus, a low-voltage direct current bus, a direct current transformer, a photovoltaic power generation unit, an energy storage unit, a fuel cell unit, a grid-connected inverter and various loads. The introduction of the direct current transformer makes the voltage control of the system more flexible and more complex.
The distributed unified control method provided by the invention comprises two parts: the method comprises the steps that firstly, a DBS (direct current bus voltage signal) based distributed control method is used for photovoltaic cells, energy storage cells and fuel cell units; the method is a unified control method for the direct-current transformer. The specific analysis is as follows.
(1) DBS-based distributed control method:
FIG. 2 is a distributed control method of DC bus voltage based on DBS, in which V isdcNAnd VdcRated voltage and actual voltage of the direct current bus; vdcHAnd VdcLThe critical voltage of the photovoltaic power generation unit adopting a droop control mode and the critical voltage of the fuel cell unit starting to work; vdcmaxAnd VdcminThe highest voltage and the lowest voltage allowed by the DC bus, which are allowed by the DC busThe bus voltage range is divided into 4 intervals, namely H2, H1, L1 and L2, which respectively correspond to 4 operation modes.
Under normal conditions, the photovoltaic works in an MPPT (maximum power point tracking) mode to realize the maximum utilization of new energy, but when u is useddc>udcHAnd when the photovoltaic works in a voltage droop control mode, the power reduction operation is carried out. The stored energy works in discharging and charging states in H1 and L1 areas respectively, voltage droop control is adopted, power shortage on a bus is compensated, and bus voltage stability is maintained. The stored output power is limited by the capacity thereof, and the energy storage device respectively works in the maximum power discharge state and the maximum power charge state in H2 and L2 regions. When u isdc<udcLAnd meanwhile, the photovoltaic and the energy storage can not meet the load requirement, and the fuel cell starts to work to maintain the stable bus voltage.
According to the working state of the grid-connected inverter, the direct-current power distribution system has two basic operation states of grid connection and grid disconnection, but the switching of grid connection and grid disconnection does not affect the control methods of photovoltaic units, energy storage units and fuel cell units. When the system is in grid-connected operation, the converter station adopts a constant direct-current voltage control mode, the medium-voltage direct-current bus voltage is controlled to be a rated value, and the direct-current transformer controls the medium-voltage direct-current bus voltage and the low-voltage direct-current bus voltage to follow each other, which is equivalent to the control of the low-voltage direct-current bus voltage to be the rated value. Therefore, when the grid-connected operation is carried out, the voltage of the direct current bus is a rated value, no voltage deviation exists, the photovoltaic works in the MPPT state, and the energy storage and fuel cell works in the standby state.
(2) Unified control method for direct-current transformers
The unified controller of the dc transformer needs to be designed together with the droop controller on the medium and low voltage dc bus. Firstly, defining the voltage deviation value of the direct current bus as the difference value between the real-time voltage and the rated voltage, and performing per-unit processing on the voltage deviation value of the direct current bus as the voltage grades of the medium-voltage direct current bus and the low-voltage direct current bus are different to obtain the voltage deviation value
In the formula, NVMdc、NVLdcThe voltage deviation per unit values of the medium-voltage direct-current bus and the low-voltage direct-current bus are respectively capable of reflecting the deviation degree of the power supply power of the direct-current bus and the load requirement;
rated voltages of the medium voltage and low voltage dc buses respectively,
VMdc、VLdcmedium voltage and low voltage dc bus voltages, respectively; vMmax、VLmaxMaximum voltages allowed by the medium-voltage direct-current bus and the low-voltage direct-current bus are respectively; vMmin、VLminMinimum voltages allowed by the medium-voltage direct-current bus and the low-voltage direct-current bus are respectively; therefore, NVMdc、NVLdcHas a value range of [ -1, 1 [)]。
As shown in fig. 2, the dc bus voltage is divided into a plurality of operation sections, and droop control is performed in a plurality of stages. Respectively and randomly selecting one interval on medium-voltage and low-voltage direct-current buses for analysis, and setting the upper limit of the voltage in the interval as VMup、VLupLower limit of voltage is VMdown、VLdownThen there is
In the formula, kMi、kLjThe droop coefficients of the medium-voltage and low-voltage direct-current bus voltages are respectively; pMdc_i、PLdc_jThe output power of the ith power module and the jth power module on the medium-voltage and low-voltage direct-current buses respectively;
the maximum output power of the ith power module and the jth power module on the medium-voltage direct-current bus and the low-voltage direct-current bus respectively.
According to the definition of per unit in the formula (1), per unit treatment is carried out on the formula (2), per unit droop coefficients of each power module on the direct current bus can be obtained, and the per unit droop coefficients of the direct current bus can be obtained by summing the per unit droop coefficients
In the formula, NM、NLThe number of power supply modules adopting droop control on the medium-voltage direct-current bus and the low-voltage direct-current bus in the voltage interval is respectively; NV (non-volatile memory)Mdc、NVLdcHas a value range of
The divided voltage intervals are processed per unit and then connected to obtain NVMdc、NVLdcHas a value range of [ -1, 1 [)]。
Considering the power P transmitted by a DC transformerTIf the power transmitted from the low-voltage DC bus to the medium-voltage DC bus is in the positive direction, the power is transmitted from the low-voltage DC bus to the medium-voltage DC bus
In the formula, PMload、PLloadThe total load power on the medium-voltage and low-voltage direct-current buses respectively; pMcpc、PLcpcThe total power output by the power supply module is controlled by adopting constant power on the medium-voltage and low-voltage direct-current buses in the voltage interval respectively, and the total power can be regarded as negative load power.
Definition of
In the formula, PM′load、PL′loadThe total load power left after the power supply of the constant power source is removed, so the difference between the per unit values of the voltage deviations of the low-voltage and medium-voltage DC buses is
NVLdc-NVMdc＝(kLPL′load-kMPM′load)+(kL+kM)PT (6)
Definition of
Therefore, the formula (7) can be simplified to
ΔS＝ΔD+NPT (8)
In the formula, Δ S and Δ D represent the degree of difference between the supply and demand between the medium-voltage and low-voltage dc buses, respectively. Therefore, as can be seen from equation (8), the dc transformer transmission power depends on the supply-demand deviation between the medium-voltage and low-voltage dc buses. When Δ S is equal to 0, the difference in power supply between the medium-voltage and low-voltage dc buses is 0, that is, the system achieves global power balance.
FIG. 3 is a droop characteristic curve of a DC transformer, the droop control formula is
In the formula, kTThe droop coefficient of the direct current transformer is obtained;
the maximum output power of the direct current transformer.
As can be taken from fig. 3, the dc transformer operates in medium voltage dc bus voltage regulation mode by regulating the transmission power PTTo maintain the voltage of the medium voltage DC bus stable. Adding a power control link on the voltage droop control loop, switching the direct current transformer from a voltage regulation mode to a power scheduling mode, moving a voltage droop curve from a solid line to a dotted line, and tracking a reference power command PTref. In a centralized control system, the power command is generally given by the energy management system, whereas in a decentralized control system, due to lack of communication, the power command of the dc transformer can only be based on local measurementsAnd (4) determining. A reasonable power exchange command should implement global power balancing (GPS), i.e. changing the conventional power control link into a shift control link, and the working principle thereof can be expressed as
ΔVMdc＝ΔS*GPI (10)
In the formula, GPIIs the transfer function of the PI controller; Δ VMdcIs a voltage shift value. The control loop is targeted by shifting the voltage droop curve by Δ VMdcAnd controlling the voltage deviation per unit value of the medium-voltage direct-current bus to be equal to that of the low-voltage direct-current bus, so as to realize the global power balance of the system.
Therefore, the working principle of the unified control method can be expressed as
Fig. 4 is a control block diagram employing a unified control method. To avoid excessive shifting, the shift term Δ V is requiredMdcThe upper part is added with an amplitude limiting link, and the amplitude limiting range is [ - (V)Mmax-VMmin)，VMmax-VMmin]。
Fig. 5 is a functional diagram of voltage regulation of the dc transformer when the power of the low-voltage dc bus fluctuates. When the power on the low-voltage direct-current bus fluctuates, the voltage of the low-voltage direct-current bus also fluctuates, and the shift control link controls the NV through the PI controllerLdcTracking NVMdcThe transmission power and direction of the direct current transformer are automatically adjusted, and the voltage of the low-voltage direct current bus is maintained, so that the fault supporting capacity and the new energy consumption capacity of the direct current bus are enhanced, and the fault tolerance and the elasticity of a power grid are improved. On the contrary, when the condition of the medium-voltage direct-current bus occurs, the adjusting mode is the same as the low voltage. The unified control method realizes the integration of a power control mode and a medium-voltage and low-voltage direct-current bus voltage control mode, and the three working modes have no obvious distinguishing limit.
In conclusion, the distributed unified control method can realize integrated control of the system based on DBS, does not need a communication link, has a simple control structure, high reliability and lower cost, and can be expanded on the basis of the existing control loop if upper-layer energy management is needed.
It should be understood that the above examples are only for illustrating the specific embodiments of the technical solutions of the present invention, and are not intended to limit the scope of the present invention. Various equivalent modifications and alterations of this invention will occur to those skilled in the art after reading this disclosure, and it is intended to cover such alternatives and modifications as fall within the scope of the invention as defined by the appended claims.
Claims (2)
1. A distributed unified control method of a medium and low voltage direct current distribution system is characterized by comprising a distributed control method based on a direct current bus voltage signal and a unified control method of a direct current transformer, wherein the distributed control method comprises the following two parts: the distributed control method based on the direct-current bus voltage signal comprises the following steps:
1.1 the operating range of the direct current bus voltage operation is divided into 4 intervals, namely:
h2 interval, representing [ V ]dcH,Vdcmax]；
H1 interval, representing [ V ]dcN,VdcH]；
Interval L1, representing [ V ]dcL,VdcN]；
Interval L2, representing [ V ]dcmin,VdcL]；
Wherein the content of the first and second substances,
Vdcmaxthe highest voltage allowed by the direct current bus;
VdcHcritical voltage of a droop control mode is adopted for the photovoltaic power generation unit;
VdcNthe rated voltage of the direct current bus;
VdcLa critical voltage at which the fuel cell unit starts to operate;
Vdcminthe lowest voltage allowed by the direct current bus;
the photovoltaic, energy storage and fuel cell units on the direct current bus adjust respective working modes according to the voltage interval of the direct current bus voltage, and maintain the direct current bus voltage stable together;
1.2 according to the working state of the converter station, the direct current distribution system has two basic operation modes of grid connection and grid disconnection, the switching between grid connection and grid disconnection does not affect the control method of the photovoltaic and energy storage units, when the system is in grid connection operation, the converter station adopts a fixed direct current voltage control mode, the voltage of a medium-voltage direct current bus is controlled to be a rated value, a direct current transformer controls the per unit value of the voltage deviation of the medium-voltage direct current bus and the low-voltage direct current bus to follow each other, which is equivalent to the control of the voltage of the low-voltage direct current bus to be the rated value, so that when the system is in grid connection operation, the voltage of the direct current bus is the rated value, no voltage deviation exists, the photovoltaic works in a maximum power tracking state, and the energy storage and the fuel cell work in a standby state;
the unified control method of the direct current transformer comprises the following steps:
2.1 adding a shift control link on the basis of a voltage droop control link to obtain a unified control method of the direct current transformer, and integrating the functions of power control, medium-voltage direct current bus voltage regulation and low-voltage direct current bus voltage regulation;
2.2, calculating to obtain a per unit value of the deviation between the medium-voltage direct-current bus voltage and the low-voltage direct-current bus voltage, and making a difference between the medium-voltage direct-current bus voltage and the low-voltage direct-current bus voltage, wherein the difference value is used for outputting a shift voltage signal through a PI (proportional-integral) controller, and moving a droop characteristic curve of the direct-current transformer, so that the size and the direction of transmission power are automatically adjusted, and the global power balance of the system is finally realized;
2.3 when the system is in off-grid operation, the DC bus voltage is controlled by the photovoltaic, energy storage and fuel cell units, the DC voltage operates in an allowable interval range, and because the DC transformer controls the deviation per unit value of the medium-voltage DC bus voltage and the low-voltage DC bus voltage to be equal, when the voltage of one side changes, the DC transformer transmits the corresponding power to compensate the power shortage of the voltage change side, the bus voltages of the two sides are finally stabilized at a new balance point, and at the moment, the voltage control units on the buses of the two sides jointly inhibit the power fluctuation on the bus of the side or the bus of the other side;
2.4 when the system is in grid-connected operation, the voltage of the medium-voltage direct-current bus is controlled at a rated value by the converter station, and the voltage deviation of the medium-voltage direct-current bus and the voltage of the low-voltage direct-current bus is controlled by the direct-current transformer to be equal in per unit value, so that the voltage of the low-voltage direct-current bus is also the rated value, at the moment, the photovoltaic works in a maximum power tracking state, and the energy storage and fuel cell works in a standby state.
2. The distributed unified control method for the medium-low voltage dc power distribution system according to claim 1, wherein the medium-voltage dc bus voltage level is 6kV, 10kV, 24 kV; the voltage grade of the low-voltage direct-current bus is 400V, 750V and 1500V.
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