AU2018102183A4

AU2018102183A4 - Grid-connected tie-line power control system of micro-grid and method thereof

Info

Publication number: AU2018102183A4
Application number: AU2018102183A
Authority: AU
Inventors: Gang DAI; Jun Pei; Degang SHENG; Dake XU; Yunbing XU
Original assignee: Daqo Group Co Ltd; Nanjing Daqo Electrical Institute Co Ltd
Current assignee: Daqo Group Co Ltd; Nanjing Daqo Electrical Institute Co Ltd
Priority date: 2018-02-01
Filing date: 2018-05-10
Publication date: 2020-10-08
Anticipated expiration: 2026-05-10
Also published as: CN109842147A; WO2019148689A1

Abstract

The invention discloses a control system and method for microgrid grid-connected tie-line power, 5 and the method comprises the following steps: a microgrid central controller collects an electric quantity of each of the microgrid and the tie-line; the relationship between the agreed power Mr and the net load PNot of the microgrid is judged; the agreed power Per is given a predicted value by the energy management system, and if the value of the agreed power is not given by the energy management system, the value of the agreed power r is an average value in the time 10 AIof the net load power Pee of the microgrid. If PAr Nt, a command is issued to the battery management system by the microgrid central controller, and the output of the energy storage unit is adjusted, and the output value of the energy storage unit is P = r -- - ee; pAU! pNet and if , a command is issued to the battery management system by the microgrid central controller to stop the output of the energy storage unit, and the energy storage unit starts to 15 charge. The invention has a fast response, simple scheme and good stability.

Description

Control System and Method for Microgrid Grid-connected Tie-Line Power Flow

Technical Field The invention relates to the technical field of microgrid, in particular to a control system and

method for microgrid grid-connected tie-line power flow.

Background Art In recent years, with the rapid development of the economy, the demand for energy has increased

sharply, and distributed power generation technology has received extensive attention because of

its high utilization rate of energy and small pollution, and microgrid as its technical support has

also received the same attention. The microgrid system combines a distributed power supply, a

load, an energy storage device and a control system to form a small-sized power system. The

microgrid and the large power grid can be mutually supported, and meanwhile, the flexibility of

the microgrid enables the microgrid to operate in a grid-connected mode and an island mode, so

that the grid-connected operation can ensure the safe operation of the microgrid to a great extent.

But the power flow of a tie-line of the microgrid and a power distribution network is usually

limited to a certain extent.

In a microgrid, the output power of distributed power supply such as wind power, solar

photovoltaic and so on has the characteristics of intermittence and randomness, and the load

change often shows certain volatility, which brings great challenges to the stable operation of the

microgrid. The peak-to-valley characteristics of the load and the fluctuation of the distributed

power supply often coexist when the microgrid runs in grid-connected mode. Therefore, after the

power grid is connected with distributed power supply in a large scale, the large power fluctuation

can seriously impact the regional power distribution network and even cause oscillation of the

regional power distribution network. If a series of problems caused by the grid connection of the

microgrid cannot be effectively solved, rapid development and high-efficiency utilization of the

microgrid are hindered. In the prior art, there are also some related schemes for the grid

connection of microgrids, but the schemes are complicated and have insufficient stability.

Chinese patent application No. 201610246209.5, published on July 27, 2016 discloses a method for smoothing power of grid-connected microgrid tie-line, which adopts double-layer optimal

scheduling; the double-layer optimal scheduling comprises upper layer optimization for carrying

out optimal scheduling on the basis of predicted data and lower layer optimization for carrying out

real-time control according to collected real-time data. According to the invention, the optimal scheduling mode of double-layer control is applied, so that the control of the power of the tie-line of the grid-connected microgrid can be realized, and the fluctuation of the power of the tie-line is controlled not to exceed a specified range, and the large fluctuation of the power of the tie-line due to the instability of photovoltaic power generation is prevented, and the influence on the stability of the large power grid after the microgrid is connected into the large power grid is reduced. By reducing the fluctuation of the tie-line, the capacity of connecting a large power grid into a photovoltaic power supply can be improved, and the utilization rate of clean energy can be improved. By controlling the electric quantity of an energy storage battery, the battery can be prevented from being overcharged or over-discharged and the service life of the energy storage battery can be prolonged. However, the response speed of this scheme becomes slow, and the fluctuation will activate until it reaches a certain value. In a high-standard power grid, the impact is huge. Chinese patent application No.201310234708.9, published on September 25, 2013, discloses a slide control method and device of new energy synchronization tie-line power, wherein when fluctuation of a grid exists, judgment is performed on a fluctuation frequency band. By utilizing coordination between an energy storage battery and a fuel machine, compensation is performed on fluctuation components of different frequency bands in new energy output power. Existed new energy forms are fully utilized so as to avoid the investment of more devices, the utilization rate of energy is improved, the requirement for power supply of a user is met, and meanwhile, the reconstruction of a large grid is reduced. The cold/hot/electricity combined supply system comprising a light storing system is simple in structure and easy to achieve. In addition, running modes are multiple, the comprehensive treatment effect of power quality is achieved, and improvement of the power quality for remote areas is particularly more obvious. However, in this scheme, the basic filtering control method is adopted in the energy storage system, and the state of charge of the battery is not considered, so on one hand, the service life of the battery can be affected; on the other hand, the power injected into the power grid can fluctuate sharply, and the stability of the power grid can be affected. Any references to prior artshould not be taken as admissions of common general knowledge in the field.

Summary of the Invention Aiming at the problems in the prior art, the invention provides a control system and method for microgrid grid-connected tie-line power, which have the effects of fast response, simple scheme and good stability. The purpose of the invention is achieved through the following technical scheme.

A control system for microgrid grid-connected tie-line power is disclosed, comprising a distributed power supply, a load and an energy storage unit, wherein the distributed power supply, the load and the energy storage unit are respectively connected with a control unit, and the distributed power supply, the load and the energy storage unit are connected with an external power grid through a point of common coupling, and the interface of the distributed power supply and the energy storage unit is an inverter, and the point of common coupling and the inverter are controlled to be on and off through the control unit, and the energy storage unit is connected with a battery management unit. Further, a load controller, a power supply controller and an energy storage controller upload electrical information of the load, the distributed power supply and the energy storage unit to a microgrid central controller through a communication bus; and the microgrid central controller issues corresponding instructions to a lower controller through the communication bus to control the operation of the load, the distributed power supply and the energy storage unit. Further, the distributed power supply includes a photovoltaic power supply and a wind power generation power supply. Further, the point of common coupling and the inverter are on and off by remote control. A control method for grid-connected tie-line power flow of a microgrid system is disclosed, wherein the method comprises the following steps: 1) collecting electric quantity of each of a microgrid and a tie-line by a microgrid central controller, and then executing step 2);

2) judging the relationship between a agreed power Agr and a net load E~et of the microgrid

F according to the electric quantity collected in step 1), wherein the agreed power Ar is given a

predicted value by an energy management system, and if the value is not given by the energy

management system, the value of the agreed power Afr is an average value in the time A of

the net load power SNet of the microgrid, ie. - , = t- to

if Agr > PNET, a command is issued to a battery management system by the microgrid central

controller, and the output of an energy storage unit is adjusted, wherein the output value of the

energy storage unit isP= PAgr - PNe; and

if i -Agr PNEt, a command is issued to the battery management system by the microgrid central

controller to stop the output of the energy storage unit, and the energy storage unit starts to charge, and then returning to step 1). Further, in step 1), the microgrid central controller collects an electric quantity of each of the microgrid and the tie-line, wherein the electric quantity comprises a total active power PDG sent by the distributed power supply, the local load active power Road and the net load power

1 Nut of the microgrid;

PNs1 PLoad DG

a total active power PB that stores the most energy; and the tie-line power Port of the microgrid:

Further, in step 2), the charging and discharging time of the energy storage system is positively

correlated with the value of ,and the charging and discharging frequency of the energy storage

system is negatively correlated with the value ofA , and the fluctuation rate of the output of the

distributed power supply is negatively correlated with the value of t.

Compared with the prior art, the invention has the following advantages. 1) The invention relates to a smooth control method of tie-line power of a microgrid containing wind power, solar photovoltaic, energy storage device and other distributed power supplies during grid-connected operation, wherein the control method calculates the fluctuation rate of the tie-line power in the grid-connected operation in real time according to the collected data of each distributed power supply, the local load power and the tie-line power of the microgrid, and accordingly a charging and discharging strategy of an energy storage system is formulated. According to the control method, the fluctuation of the distributed power supply during grid-connected operation of the microgrid can be suppressed, and the reliability and stability of the microgrid are guaranteed; 2) According to the method, under the condition of grid-connected operation, a planned power

Ag" of a tie-line is introduced, and the predicted value is given by the energy management

system (EMS). If the value is not given by the energy management system (EMS), the value is an

average value of net load power ~vot of a microgrid in timeA . Compensation is realized on the

whole microgrid according to the planned power of the tie-line, and charging and discharging are carried out, and smooth change of exchange power between the microgrid and the main network is realized and the fluctuation of the distributed power supply of the microgrid is suppressed, and the purpose of peak clipping and valley filling is achieved; 3) According to the scheme, various distributed power supplies can be provided, including but not limited to wind power generation, photovoltaic power generation and the like so that wind power and photovoltaic power complementation can be realized, and the economical efficiency, environmental friendliness and reliability of power supply are improved; 4) An inverter is adopted as an interface of the distributed power supply and the energy storage system, so that the flexibility and the dynamic performance of the microgrid are effectively improved, and multiple operation modes are involved, and the modes are diversified in operation, and the compatibility is good;

5) In a power utilization peak period, the agreed power Ar is larger than the net load power of

the microgrid, and the energy storage device discharges; in a power utilization valley period, the

agreed power gr is smaller than the net load power of the microgrid, and the energy storage

device is charged to fully play the role of peak clipping and valley filling. the term "comprising" s inclusive unless the context clearly requires otherwise. "comprising"

Brief Description of the Drawings Fig. 1 is a schematic diagram of a microgrid structure of the present invention; Fig. 2 is a schematic diagram of a microgrid central controller; Fig. 3 is a flow diagram of a microgrid control strategy; and Fig. 4 shows the load curve before and after the optimization of the electrical load.

Detailed Description of the Invention The feature of preferred embodiment will now be described in detail with reference to the accompanying drawings and specific embodiments. The feature of preferred embodiment relates to a smooth control method of tie-line power of a microgrid containing wind, light, storage and other distributed power supplies during grid-connected operation, wherein the control method calculates the fluctuation rate of the tie-line power in the grid-connected operation in real time according to the collected data of each distributed power supply, the local load power and the tie-line power of the microgrid, and accordingly a charging and discharging strategy of an energy storage system is formulated. According to the control method, the fluctuation of the distributed power supply during the grid-connected operation of the microgrid can be suppressed, and the reliability and stability of the microgrid are guaranteed. Aiming at the problems existing in grid connection of the existing microgrid, the object of the feature of preferred embodiment is to realize a smooth change of exchange power between the microgrid and the main power grid, smooth fluctuation of distributed power supply in the microgrid and simultaneously realize peak clipping and valley filling under the condition of grid connection operation. As shown in fig.1, the microgrid system includes several distributed power supplies such as wind turbines, solar panels, storage batteries, and an energy storage system, as well as related load. The switches are controlled by PCC (Point of common coupling) and connected to the external power grid. As shown in fig.2, the microgrid system during grid-connected operation comprises a control layer, a monitoring layer and a communication layer, wherein the control layer consists of a microgrid central controller (MGCC) and each lower layer controller, and the lower layer controller comprises a load controller, a power supply controller and an energy storage controller; the monitoring layer consists of monitoring devices such as a current transformer, a voltage transformer and a power meter; the monitoring layer is connected with the control layer, and the lower controller is communicated with the MGCC through the communication layer. The MGCC is connected with the load controller, the power supply controller and the energy storage controller through a communication bus; the load controller, the power supply controller and the energy storage controller upload electric information of the load, the distributed power supply and the energy storage system to the MGCC through a communication bus; and the MGCC sends corresponding instructions to the lower controller through the communication bus to control the running states of the load, the distributed power supply and the energy storage system. As shown in fig.3, the main flow chart of the tie-line power smoothing control method is shown and the specific flow is analyzed as follows: 1) collecting various electrical quantities of a microgrid and a tie-line, comprising: ( current on the tie-line; @bus voltage of a microgrid; @the current of each power supply line; @the current of each load line;

2) judging the relationship between the planned power PAr and the net load oNotof the

microgrid by the MGCC according to the electric quantity collected in step 1), wherein

0 If Ag r Not, a command is issued to a battery management system (BMS) by the MGCC,

and the output of the energy storage system is adjusted, wherein the output value of the energy storage system is PB Pr - Ert; if Agr Not, a command is issued to the battery management system (BMS) by the

MGCC to stop the output of the energy storage system, and the energy storage system starts to charge. 3) returning to step 1) In step 2), the charging and discharging time of the energy storage system is positively correlated

with the value of A t, and the charging and discharging frequency of the energy storage system is

negatively correlated with the value of At, and the fluctuation rate of the output of the distributed

power supply is negatively correlated with the value of A The value of affects the charging

and discharging time and the charging and discharging frequency of the energy storage system, and the value should be considered according to the fluctuation rate of the output of the distributed

power supply. In general, the value of A t can be selected according to the following table.

Table 1 tValue Table

fluctuation rate of distributed power supply ri<10% 10 % Sq<30% 30 %:S 30 20 10

The invention discloses a smooth control method of tie-line power during the grid-connected operation of a microgrid, which comprises the following steps. 1) A microgrid central controller (MGCC) collects electrical quantities of a microgrid and a

tie-line to obtain total active power PDG provided by a distributed power supply, local load active

power Load and net load power Pet of the microgrid.

Fue c Eanad - PDa (1)

2) The microgrid central controller (MGCC) communicates with a battery management system

(BMS) to obtain the total active power "2 of an energy storage system.

3) The exchange power of the microgrid system and the external power grid is obtained, namely

the tie-line power Farfaof the microgrid:

P Ort P"" -a PDG' Pa (2)

4) Due to the intermittence and fluctuation of the distributed power supply and the large peak-to-valley difference characteristics of the local load of the microgrid, the microgrid grid-connected tie-line has large power fluctuation. In order to suppress the fluctuation, the agreed power FAr of the tie-line is introduced, and the predicted value is given by the energy management system (EMS). If the value is not given by the energy management system (EMS), the value is the average value of the net load power rust of the microgrid within a certain time.Taking the daily consumed power load of a certain place on July 1, 2013 as an example, as shown in fig.4, the solid line is the actual load curve on the current day, and the dotted line is the optimized load curve after the strategy is used. As can be seen from the figure, the load fluctuation is large in the afternoon power utilization peak. After the strategy is adopted, the load peak is effectively reduced, and the fluctuation of the power grid is suppressed. In order to further describe the optimization of the original daily load curve by the strategy, points are respectively taken from the original curve and the optimized curve. The point taking method comprises the following steps: starting from point 0, 96 point data are taken every 15 min. The discrete variance and standard deviation of each group of discrete data are evaluated, and the results are shown in the following table: Type of Curve Variance Standard Deviation Original Curve 97.44 9.87 Optimized Curve 96. 15 9.55 Table 1 The variance and standard deviation of the optimized daily load curve are smaller than that of the original daily load curve, so the dispersion degree and fluctuation degree of the optimized daily load curve are smaller than that of the original daily load curve. 5) The control aim of the smooth control method of the tie-line power when the microgrid is in grid-connected operation is to compensate the tie-line power ar~a from the net load PNy of the microgrid system before energy storage is introduced to a preset agreed power Egr.

PAgr = O+Nrt (3)

6) It can be seen from equation (3) that when the agreed power PAgr is greater than the net load

PNet of the microgrid system, the energy storage system adjusts the output PR; when the agreed

power Agr is less than the net load Puer of the microgrid system, the energy storage system is

charged. The microgrid in step 1) comprises a solar photovoltaic (PV), a wind turbine generation (WG) and an energy storage device (Storage Battery, SB). As shown in fig.1, wind and light complementation can be realized, and the economical efficiency, environmental protection and the reliability of power supply are improved. Meanwhile, an inverter is adopted as an interface of the distributed power supply and the energy storage system, such that the flexibility and the dynamic performance of the microgrid are effectively improved. A plurality of operation modes is involved, including a grid-connected operation mode, an isolated island operation mode, a grid-connected to isolated island operation mode, an isolated island to the grid-connected mode and the like. The microgrid central controller (MGCC) in step 1) can carry out the multi-path analog quantity and switching quantity acquisition, and is communicated with an energy management system (EMS) and a battery management system (BMS), and can carry out remote control operation of the switch. Besides, an expert system program is built in to receive, process and forward data in real time as shown in fig.2. According to step 4), given the characteristics of the electric load, the electric peak duration time is about 30 minutes, the fluctuation rate is less than 10%, and takes the value of Nt's 30 min average. In step 6), the charging and discharging state and the output condition of the energy storage system are calculated by the microgrid central controller (MGCC), and sent to a battery management system (BMS). Then the energy storage device is controlled by the battery management system (BMS).

According to the invention, on the basis of the average value of the net load power Exer of the

microgrid within a certain time and the plan value given by the energy management system (EMS), the intermittence and fluctuation of the distributed power supply of the microgrid are effectively suppressed by controlling the output of the energy storage device, such that the reliability of the microgrid is improved and the impact of the microgrid on the grid is reduced. Meanwhile, in a

power utilization peak period, the agreed power er is larger than the net load power of the

microgrid, and the energy storage device is discharged; in a power utilization valley period, the

agreed power Afr is smaller than the net load power of the microgrid, and the energy storage

device is charged to fully play the role of peak clipping and valley filling. And the grid-connected smooth control of the microgrid system is completed.

Claims

Claims

1. A control system for microgrid grid-connected tie-line power, including a distributed power supply, a load and an energy storage unit, wherein the distributed power supply, the load and the energy storage unit are respectively connected with a control unit, and wherein the distributed power supply, the load and the energy storage unit are connected with an external power grid through a point of common coupling, and wherein the interface of the distributed power supply and the energy storage unit is an inverter, and the point of common coupling and the inverter are controlled to be on and off through the control unit, and the energy storage unit is connected with a battery management unit. 2. The control system for microgrid grid-connected tie-line power as recited in claim 1, wherein a load controller, a power supply controller and an energy storage controller upload electrical information of the load, the distributed power supply and the energy storage unit to a microgrid central controller through a communication bus; and the microgrid central controller issues corresponding instructions to a lower controller through the communication bus to control the operation of the load, the distributed power supply and the energy storage unit. 3. The control system for microgrid grid-connected tie-line power as recited in claim 1, wherein the distributed power supply comprises a photovoltaic power supply and a wind power generation power supply. 4. The control system for microgrid grid-connected tie-line power as recited in claim 1, wherein the point of common coupling and the inverter are on and off by remote control. 5. A control method for grid-connected tie-line power of a microgrid system, characterized by including the following steps: 1) collecting electric quantity of each of a microgrid and a tie-line by a microgrid central controller, and then executing step 2);

2) judging the relationship between a agreed power Pgr and a net load PNet of the microgrid

according to the electric quantity collected in step 1), wherein the agreed power Ar is given a

predicted value by an energy management system, and if the value is not given by the energy

management system, the value of the agreed power PAr is an average value in the time 1of

~Ar4 the net load power PNet of the microgrid, i.e. - A, =t - 0 if Aar>a mEt, a command is issued to a battery management system by the microgrid central controller, and the output of an energy storage unit is adjusted, wherein the output value of the

1 energy storage unit is 9 PAgr -NeF; and

if Agr - PNot, a command is issued to the battery management system by the microgrid central

controller to stop the output of the energy storage unit, and the energy storage unit starts to charge, and then returning to step 1). 6. The control method for grid-connected tie-line power of a microgrid system as recited in claim 5, wherein in step 1), the microgrid central controller collects an electric quantity of each of the

microgrid and the tie-line, wherein the electric quantity comprises a total active power PD; sent

by the distributed power supply, a local load active power Pa a and the net load power Pierof

the microgrid;

PNs, I L.oad - DG

total active power of the energy storage system PB; and the tie-line power PCrid of the

microgrid:

Parft -:: PSotzd - PDG - P.0

7. The control method for grid-connected tie-line power of a microgrid system as recited in claim 5, wherein in step 2), the charging and discharging time of the energy storage system is positively

correlated with the value of At, and the charging and discharging frequency of the energy storage

system is negatively correlated with the value ofA , and the fluctuation rate per minute of the

output of the distributed power supply is negatively correlated with the value of .