CN108964139A - A kind of hierarchical control micro-grid connection synchronizing frequency control method based on consistency algorithm - Google Patents

Abstract

The invention discloses a layered control method for microgrid grid-connected synchronous frequency control based on a consensus algorithm. According to the state of charge of the energy storage system, an improved droop control strategy is constructed, and the traditional droop control strategy is introduced Considering the state of charge of the energy storage system, it is solved that the load borne by each energy storage unit is proportional to the SOC, and the adjustment ability of the energy storage can be effectively exerted. At the same time, in view of the frequency deviation problem that is prone to occur in the primary control of the microgrid system, a consensus algorithm is designed to realize the non-difference frequency modulation of the primary control. In order to achieve frequency synchronization of microgrid grid-connected, a microgrid grid-connected synchronization control strategy based on secondary control is proposed, which can realize the synchronization of microgrid voltage and main grid voltage and frequency synchronization, so as to realize the transformation of microgrid from island mode to parallel The smooth switching of the grid mode improves the frequency stability and power quality of the large grid and micro grid system.

Description

A Hierarchical Control Microgrid Grid-connected Synchronous Frequency Control Based on Consensus Algorithm method

technical field

The invention relates to the field of grid-connected micro-grids, in particular to a method for applying the hierarchical control of the consistency algorithm to the synchronous frequency control of the grid-connected micro-grids.

Background technique

Because distributed power generation has the advantages of less pollution, high energy utilization rate, and flexible installation location, and compared with centralized power generation, it saves power transmission and distribution resources and operating costs, and reduces the line loss of centralized power transmission. Distributed power generation can reduce the total capacity of the power grid, improve the peak and valley performance of the power grid, and improve the reliability of power supply. It is a powerful supplement and effective support for large power grids. In the past 20 years, most countries have put distributed power generation on the agenda, and people have begun to seriously study the potential benefits of distributed power generation systems. Undoubtedly, distributed generation is one of the development trends of the power system.

Although distributed power generation has outstanding advantages, it has many problems. For example, the cost of single-machine access to distributed power is high, and control is difficult. In addition, distributed power is an uncontrollable source compared to large power grids, so large systems often use restricted and isolated methods to deal with distributed power in order to reduce their impact on large power grids. The active distribution network formed by the mixing of distributed power and users has brought new challenges to the power system. In order to solve the problem of distributed power access, coordinate the contradiction between large power grids and distributed power, and fully exploit distributed power as a power grid and the value and benefits brought by users, at the beginning of this century, scholars put forward the concept of microgrid. The microgrid combines power generation units with a rated power of tens of kilowatts—microsources, loads, energy storage devices, and control devices—to form a single controllable unit that supplies electricity and heat at the same time. The grid adjustment based on the microgrid structure can facilitate the interconnection of large-scale distributed generation (DER) and intervene in the medium and low voltage system, providing a mechanism to make full use of DER units.

In a low-voltage microgrid system, due to the influence of line impedance, traditional droop control is difficult to achieve the distribution of active power and reactive power, and some problems will arise in voltage and frequency regulation. The main performance is the difference in the output voltage amplitude and frequency of the distributed power supply.

There are two modes of operation in the microgrid: island operation and grid-connected operation. When the microgrid is connected to the grid, it is switched from island operation to grid-connected operation mode, which is connected to the main grid through a common connection point. The exchange power output by the inverter is closely related to the measurement accuracy of the voltage amplitude and frequency of the microgrid. Therefore, the amplitude, frequency and phase angle of the voltage at the common connection point of the microgrid must be controlled to be consistent with the main grid. Otherwise, when the grid is connected, it will not only cause a great impact on the main grid, but also affect the stability of the distributed power supply of the microgrid. sex affects.

Contents of the invention

In order to solve the problem of grid-connected microgrid, the present invention proposes a layered control method for synchronous frequency control of microgrid grid-connected based on consensus algorithm, which mainly includes two layers: among them, the primary control consists of improved droop control, power control, voltage control and current control and control system; in view of the frequency deviation problem that is prone to occur in the primary control of the microgrid system, a consensus algorithm is designed to apply the primary control to achieve no frequency difference in the primary control; in order to achieve smooth and parallel microgrid In the secondary control, a synchronous frequency control strategy for grid-connected microgrid is proposed. Through frequency compensation, the frequency difference on both sides of the grid is close to zero at the instant of grid-connected, providing a theoretical basis for grid-connected microgrid.

In order to achieve the above-mentioned purpose, the technical solution adopted by the present invention is a method for controlling the synchronous frequency of the grid-connected microgrid based on a consensus algorithm, which is characterized in that it includes the following specific steps:

Step (1): Detect the energy storage state of charge SOC of the microgrid and the active power P output by the distributed power supply, and use a sign function related to P and SOC to obtain the dynamic active power droop coefficient b; obtain the dynamic active power droop coefficient b by using the improved droop control through communication The output voltage and frequency of the inverter, calculate the per unit value of the active power of the adjacent inverter, and combine the average consistent iterative algorithm of the weight to control the active power-frequency;

Step (2): During the synchronous grid connection process of the microgrid, decompose the voltage v _odq on the microgrid side and the voltage v _gdq on the main grid side through Park transformation to obtain the dq axis components v _od , v _oq , v _gd , v _gq , and use PI control Synchronous frequency f _sy is represented by functions related to the parameters of the generator and v _od , v _oq , v _gd , v _gq ; the frequency deviation Δf of the microgrid is compensated by the synchronous frequency f _sy , so that when Δf is less than a specified value, it can be synchronously connected to the grid.

Further, the specific process of using a sign function related to P and SOC to obtain the dynamic active power droop coefficient b is: through the use of the sign function, in the case of insufficient active power output, the droop coefficient b is inversely proportional to the SOC, each DG The load borne by the power generation unit is proportional to the SOC; when the active power output is surplus, the situation is the same as the active power output is insufficient;

Where: w _i is the angular frequency of the output voltage, w _iref is the frequency of the rated voltage, k ₀ is the initial droop coefficient, sgn(x) is a sign function, η is the proportional coefficient of the energy storage system capacity, defined as C _ESS,i is the available capacity of energy storage power supply i, min[C _ESS,i ] is the minimum capacity of energy storage power supply i, P _i and P _iref are the actual output active power and rated output active power of the power supply, respectively.

Further, the specific process of active power-frequency control by the average consistent iterative algorithm combined with weights is: as time goes by, the states of all agents in the system can eventually tend to a same value, so that all parallel inverters The per unit value of the active power of the device converges to its mean value:

in: are the per-unit values of the active power of inverters i and j respectively, W _ii and W _ij are the weights of x _i and x _j on node i.

Further, the specific process of using PI controller parameters and functions related to v _od , v _oq , v _gd , and v _gq to express the synchronous frequency f _sy is as follows: the microgrid can be merged into the main grid through a static transfer switch, and the PI controller can Eliminate the frequency deviation, so the synchronous frequency of the microgrid and the main grid can be expressed as:

Among them: f _sy is the synchronization frequency between the microgrid and the main grid, ω _c is the corner frequency, s is the Laplace transform factor, k _psy and k _isy are the proportional parameters and integral parameters of PI in synchronous control, respectively.

Further, the frequency deviation Δf of the microgrid is compensated by the synchronous frequency f _sy :

Among them: k _{p_Δf} and k _{i_Δf} are the control parameters of the secondary control compensator respectively, f _MG is the actual output frequency at the distributed power supply port, Reference frequency at the distributed power port.

Compared with the prior art, the present invention has the following advantages and beneficial effects: the improved droop control strategy proposed by the present invention uses a sign function, and the power supply with sufficient energy storage margin sets a larger droop coefficient, so that its output is relatively large , the power supply with insufficient energy storage margin is set with a small droop coefficient, so that its output is relatively small, and the adjustment ability of energy storage can be effectively played; at the same time, in order to solve the problem of frequency deviation in the primary control, the consistency algorithm It is applied to the primary control to realize the frequency modulation of the microgrid without difference. In order to achieve the final purpose of grid connection, the frequency recovery of the microgrid is proposed in the secondary control to adjust the frequency deviation on both sides, so as to realize the frequency synchronization of the microgrid grid connection.

Description of drawings

Figure 1 is a microgrid structure diagram;

Fig. 2 is a block diagram of improved droop control;

Fig. 3 is a block diagram of microgrid synchronous grid-connected frequency deviation;

Figure 4 is a synchronous control diagram of microgrid grid connection.

Detailed ways

The present invention will be further described below in conjunction with the accompanying drawings.

Figure 1 is a block diagram of the microgrid structure. Each DG power generation unit is connected to the microgrid through a power electronic conversion interface. In order to ensure the stability of the output of the microgrid, an energy storage unit is equipped on the DC side of the inverter.

Step 1: Primary control consists of improved droop control, power control, voltage control, current control and control system using consensus algorithm

Step 1.1, taking the detected energy storage state of charge SOC of the microgrid and the active power P output by the distributed power supply as known local information, and using the improved droop control method to dynamically adjust the active power droop coefficient kp, so that the distributed power supply Bear the user load according to its own power generation margin. The improved droop control method is to adjust the difference by adjusting the droop coefficient of active power. Figure 2 shows the instantaneous active power P=u _d i _d +u _q i _q calculated according to the dqz axis voltage components u _d , u _q and current components i _d , i _q output by the abc-dq transformation, and the average is obtained by filtering Power LPF.

Improve droop control method, specifically:

The traditional frequency droop control strategy is:

w=w _ref +k(PP _ref ) (1)

Where: w is the angular frequency of the output voltage, w _ref is the angular frequency of the rated voltage, k is the droop coefficient, P _ref is the reference active output;

The traditional frequency droop control strategy does not consider the SOC of the energy storage system, and its droop coefficient is a fixed value, which means that when there is a lack of active power or an active power surplus, each energy storage system uses the average distribution of active power, which not only does not consider To the difference of SOC of each energy storage system, but also greatly reduce the utilization rate of energy storage system.

In order to set a larger droop coefficient for a power supply with sufficient energy storage margin, so that its active output is relatively large, and a smaller droop coefficient should be set for a power supply with insufficient energy storage margin, the improved droop adjustment relationship can be obtained as:

Where: w _i is the angular frequency of the output voltage, _wiref is the angular frequency of the rated voltage, k ₀ is the initial droop coefficient, SOC is the state of charge of the energy storage system, sgn(x) is a sign function used to return x For positive and negative cases, η is the proportional coefficient of the energy storage system capacity, defined as Among them, C _ESS,i is the available capacity of energy storage power source i, min[C _ESS,i ] is the minimum capacity of energy storage power source i, P _i and P _iref are the actual output power and rated output power of power source i, respectively.

To improve the droop control, when the active power is surplus, each distributed power supply charges its own energy storage, so that P _i -P _iref <0, that is, the droop coefficient of the corresponding power supply j is:

The energy storage charging power ratio is:

Where: Δw = w _i - w _iref ;

The energy storage elements used in the energy storage system are the same product, so their min[C _ESS,i ] are the same, which can be eliminated by equivalent value in the calculation process.

Therefore, the ratio of the charging power of power source j and power source i to their respective energy storage systems is:

After adopting the improved droop control strategy, the incremental active power output of the power supply is related to the state of charge and total capacity of the energy storage system. When the state of charge SOC _j of the energy storage system of power source j is consistent with the state of charge SOC _i of the energy storage system of power source i, the active power change of the power source is proportional to the energy storage capacity, so that SOC _j and SOC _i decrease at the same rate; When SOC _j > SOC _i , the growth rate of SOC _i will be greater than SOC _j . At this time, SOC _i will gradually approach SOC _j . When the load disturbance time is relatively long and the energy storage capacity meets the requirements, the system adopts With this control strategy, the SOC of the energy storage units of multiple inverter power sources tends to be equal.

The same is true for similar load increases, so multiple distributed power sources using this improved droop control strategy can prevent excessive charging and discharging of energy storage, increase the service life of energy storage, and the SOC of its energy storage units will tend to be equal.

In step 1.2, the improved droop control is adjusted by adjusting the droop coefficient. Each inverter in the microgrid can obtain the information of some adjacent inverters through communication, and combined with the corresponding distributed consensus control algorithm, it can realize the errorless frequency modulation of the microgrid.

Consistency means that in a multi-agent system, as time goes by, the states of all agents in the system can eventually tend to the same value, and the consensus algorithm (protocol) refers to the relationship between agents in a complex system. The rules of interaction and consensus theory can be described in combination with graph theory knowledge; therefore, a non-difference frequency modulation control method based on distributed consensus algorithm is proposed.

Where: b is the dynamic droop coefficient in the improved droop control, P _i ^* , are the per-unit values of the active power of inverters i and j respectively, W _ii and W _ij are the weights of x _i and x _j on node i.

It may be advisable to set the rated capacities of inverters i and j as S _i and S _j respectively, then P _i ^* , Satisfy:

P _i ^* =P _i /S _i (8)

Among them, P _i and P _j are the active power of inverter i and j respectively;

According to the local degree distribution weight method of the consensus algorithm, It will make the per unit value of active power of all parallel inverters converge to its mean value, that is, the following formula is established in the steady state of the microgrid.

because It only works in the transient process, and its steady-state value is 0, so the given value of the angular frequency is the expected value w _ref , that is, the zero-difference frequency modulation is realized; at the same time, because the p.u. The state times are equal, so this method can also ensure the reasonable distribution of active power.

The invention not only solves the balanced active power output of the energy storage through the improved droop control of the consistency algorithm, but also realizes the reasonable distribution of the active power of the distributed power supply, greatly improves the utilization rate of the distributed power supply and improves the service life of the energy storage system. Realize the frequency stability of the microgrid and improve the power quality.

Step 2: Realize the frequency synchronization control strategy of the microgrid in the secondary control

In step 2.1, in order to achieve frequency synchronization, the frequency deviation of the microgrid must be reduced to a specified value (determined according to different power systems). Figure 3 is a block diagram of the frequency deviation of the microgrid. From the perspective of quasi-synchronization, the amplitude, frequency and phase angle of the voltage at the common connection point of the microgrid must be controlled to be consistent with the main grid. The shock will also affect the stability of the microgrid distributed power supply.

Among them: Δf is the frequency deviation of the microgrid, k _{p_Δf} and k _{i_Δf} are the control parameters of the secondary control, f _MG is the actual output frequency of the distributed power supply port, Reference frequency at the distributed power port.

The frequency deviation is calculated through the frequency deviation block diagram. In order to achieve the frequency deviation on both sides of the switch at the moment of grid connection is close to zero, the frequency compensation is performed on the microgrid, and the compensation value is the deviation value Δf.

Step 2.2, in order to realize the synchronous control of the grid-connected frequency of the micro-grid, Figure 4 is a synchronous control diagram of the grid-connected micro-grid; the micro-grid can be merged into the main grid through a static transfer switch; before that, there cannot be any energy between the micro-grid and the main grid The exchange, therefore, must be eliminated by the PI controller frequency deviation.

According to the microgrid side voltage v _odq and the main grid side voltage v _gdq output under the abc-dq coordinate transformation, decompose v _odq and v _gdq to obtain the dq axis components v _od , v _oq , v _gd , v _gq ; when the microgrid When the voltage is consistent with the main grid voltage (v _oabc =v _gabc ), v _gq v _od -v _gd v _oq =0, so as to eliminate the voltage deviation.

The synchronous frequency between the microgrid and the main grid can be expressed as:

Where: f _sy is the synchronous frequency between the microgrid and the main grid, ω _c is the corner frequency, s is the Laplace transform factor, v _od and v _oq are the d-axis and q-axis components of v _odq respectively, v _gd , v _gq are the d-axis and q-axis components of v _gdq respectively, and k _psy and k _isy are the proportional parameters and integral parameters of PI in synchronous control, respectively.

In order to make the compensation of synchronous frequency f _sy achieve the purpose of adjusting frequency deviation parameters, in the process of microgrid synchronous grid connection, the expression of frequency deviation is:

In summary, in the process of synchronous grid-connection, the frequency recovery is firstly realized according to the primary control. When the frequency deviation of the microgrid, that is, Δf, is less than a certain value, the frequency synchronization can be realized by using the formula (13), and the microgrid is realized by Smooth switching from island mode to grid-connected mode.

The above embodiments are only used to illustrate the design concept and characteristics of the present invention, and its purpose is to enable those skilled in the art to understand the content of the present invention and implement it accordingly. The protection scope of the present invention is not limited to the above embodiments. Therefore, all equivalent changes or modifications based on the principles and design ideas disclosed in the present invention are within the protection scope of the present invention.

Claims (5)

1. a kind of hierarchical control micro-grid connection synchronizing frequency control method based on consistency algorithm, which is characterized in that including Following specific steps:

Step (1): detect micro-capacitance sensor energy storage charge state SOC and distributed generation resource output active-power P, using one with P, the relevant sign function of SOC obtains the active sagging coefficient b of dynamic；It is obtained by communication using the inverter for improving sagging control Output voltage and frequency, calculate adjacent inverter active power per unit value, in conjunction with weight average homogeneity iterative algorithm carry out The control of active-frequency；

Step (2): micro-capacitance sensor synchronize it is grid-connected during, by Park transformation by micro- voltage on line side v_odqAnd main voltage on line side v_gdq Decomposition obtains dq axis component v_od、v_oq、v_gd、v_gq, utilize PI controller parameter and v_od、v_oq、v_gd、v_gqRelevant function representation is same Synchronizing frequency f_sy；The synchronized frequency f of micro-capacitance sensor frequency deviation Δ f_syCompensation can synchronize when so that Δ f being less than some specified value It is grid-connected.

2. a kind of hierarchical control micro-grid connection synchronizing frequency controlling party based on consistency algorithm according to claim 1 Method, which is characterized in that described to obtain the specific mistake of the active sagging coefficient b of dynamic using a sign function relevant to P, SOC Journey are as follows: by the use to sign function, in the active insufficient situation of output, sagging coefficient b and SOC is inversely proportional, each DG hair The load that electric unit is undertaken is directly proportional to SOC；In the case that active output is more than needed, situation is insufficient with active output；

Wherein: w_iFor the angular frequency of output voltage, w_irefFor the frequency of voltage rating, k₀For initial sagging coefficient, sgn (x) is symbol Number function, η is the proportionality coefficient of energy storage system capacity, is defined asC_ESS,iFor the available appearance of accumulation power supply i Amount, min [C_ESS,i] be accumulation power supply i minimum capacity, P_iAnd P_irefIt is power supply reality output active power respectively and specified defeated Active power out.

3. a kind of hierarchical control micro-grid connection synchronizing frequency control based on consistency algorithm according to claim 1 or 2 Method processed, which is characterized in that the average homogeneity iterative algorithm of the combination weight carries out the detailed process of the control of active-frequency Are as follows: over time, the state of all intelligent bodies can finally tend to an identical value in system, so that all parallel connections The active power per unit value of inverter converges on its mean value:

Wherein: P_i ^*,The per unit value of the active power of respectively inverter i, j, W_ii,W_ijFor x_i,x_jWeight in node i.

4. a kind of hierarchical control micro-grid connection synchronizing frequency controlling party based on consistency algorithm according to claim 1 Method, which is characterized in that described to utilize PI controller parameter and v_od、v_oq、v_gd、v_gqRelevant function representation synchronizing frequency f_syTool Body process are as follows: micro-capacitance sensor can pass through static transfer switch and become owner of power grid, eliminate frequency departure, therefore micro- electricity by PI controller The synchronizing frequency of net and main power grid can indicate are as follows:

Wherein: f_syFor the synchronizing frequency between micro-capacitance sensor and main power grid, ω_cFor corner frequency, s is the Laplace transform factor, k_psyAnd k_isyThe scale parameter and integral parameter of PI respectively in synchronously control.

5. a kind of hierarchical control micro-grid connection synchronizing frequency controlling party based on consistency algorithm according to claim 1 Method, which is characterized in that the synchronized frequency f of micro-capacitance sensor frequency deviation Δ f_syCompensation:

Wherein: k_{p_Δf}And k_{i_Δf}The respectively control parameter of Two-stage control compensator, f_MGIt is to be exported in fact at distributed electrical source port Border frequency,Reference frequency at distributed electrical source port.