CN114006403B - Light-storage combined power generation system and multi-mode self-adaptive adjustment operation control algorithm thereof - Google Patents

Abstract

The invention provides a combined photovoltaic and storage power generation system, which includes distributed photovoltaics, a battery energy storage system, a DC/DC converter and a set of grid-side converters; the output of the battery energy storage system is directly connected to the DC bus for Maintain the stability of the DC bus voltage; distributed photovoltaics are connected to the DC bus through the DC converter DC/DC; the DC side of the grid-side converter is connected to the DC bus and the AC side is connected to the grid; on the one hand, the grid-side converter collects sampling points in real time The voltage and current data at 1 and sampling point 2 are used to obtain the operating status information of the power grid and electrical loads; on the other hand, real-time communication is maintained with the DC/DC converter and battery energy storage system to control their operating status. The invention also discloses a multi-mode adaptive adjustment operation control algorithm of the light-storage combined power generation system. The invention can realize the stable operation of the power generation system in the three modes of grid-connected, off-grid and grid-connected and off-grid switching, as well as mutual conversion between modes.

Description

A solar-storage combined power generation system and its multi-mode adaptive adjustment operation control algorithm

Technical field

The invention belongs to the technical field of photovoltaic and energy storage system operation control, and specifically relates to a photovoltaic and storage combined power generation system and its multi-mode adaptive adjustment operation control algorithm.

Background technique

Distributed photovoltaics are easy to install and can make full use of space resources to install on roofs, carport tops, highway service areas and other places. They are currently widely used in Shandong, Jiangsu and other places.

However, distributed photovoltaic power generation is greatly affected by the environment and has strong fluctuations. The impact on the power system after large-scale integration cannot be ignored. The energy storage system can realize two-way flow of energy, and the output power is controllable. The combination of energy storage system and distributed photovoltaics can effectively smooth photovoltaic fluctuations and reduce the impact on the power grid; second, it can improve the utilization rate of distributed photovoltaics; third, It can be used as a backup power supply to supply power to the load when the power grid is abnormal. The advantages of the photovoltaic and storage combined power generation system (hereinafter referred to as the combined power generation system) are obvious, but currently there is research on the control algorithms of the combined power generation system in different modes of grid-connected, off-grid and off-grid switching, as well as how to reduce losses during the operation of the power generation system. There has been no in-depth study.

Contents of the invention

In order to solve the above technical problems, the present invention proposes a combined photovoltaic and storage power generation system and its multi-mode adaptive low-loss adjustment operation control algorithm. First, it can realize the power generation system switching between three modes: grid-connected, off-grid and off-grid. Stable operation under low conditions and mutual conversion between modes; second, it can take into account both autonomous operation and manual operation, and can autonomously switch between the two operating modes according to the actual operating conditions; third, it can reduce power generation during grid-connected operation. The system output fluctuates, reducing the impact on the power grid, while improving the utilization rate of new energy; fourth, it can improve the external power supply capacity of the energy storage system during off-grid operation; fifth, it can reduce its own losses during the operation of the power generation system, further improving energy Utilization.

In order to solve the above technical problems, the technical solutions adopted by the present invention are:

A combined photovoltaic and storage power generation system, including distributed photovoltaics, battery energy storage systems, DC/DC converters and a set of grid-side converters;

The output of the battery energy storage system is directly connected to the DC bus to maintain the stability of the DC bus voltage;

Distributed photovoltaics are connected to the DC bus through the DC/DC converter;

The DC side of the grid-side converter is connected to the DC bus and the AC side is connected to the grid;

On the one hand, the grid-side converter collects the voltage and current data at sampling point 1 and 2 in real time to obtain the operating status information of the power grid and power loads; on the other hand, it interacts with the DC/DC converter and the battery energy storage system. Maintain real-time communication between them and control their operating status.

Sampling point 1 is connected to 380VAC, and sampling point 2 is connected to the electrical load.

The electrical load is connected to the grid through switch QS1, and a grid-connection point switch QS0 is provided on the grid.

A multi-mode adaptive adjustment operation control algorithm for a combined photovoltaic and storage power generation system, including the following steps:

Step 1: The grid-side converter reads the voltage information U _{abc_pcc} at sampling point 1, and calculates the grid connection point frequency f _{abc_pcc} based on the voltage information; if it satisfies equation (1), the grid is in normal operation, otherwise the grid is abnormal; the grid is running When normal, the power grid operation flag PCC _flag =1, otherwise the PCC _flag =0;

U _{abc_pcc} ∈ [U _low , U _up ] and f _{abc_pcc} ∈ [f _1ow , f _up ] (1)

Among them, PCC represents the power grid, U _{abc_pcc} represents the three-phase electrical voltage of the grid, f _{abc_pcc} represents the three-phase electrical frequency of the grid, PCC _flag represents the grid operation flag, U _low represents the lowest voltage of the grid, U _up represents the highest voltage of the grid, f _low represents the lowest frequency of the power grid, f _up represents the highest frequency of the power grid;

Step 2: If the power grid is normal, determine the current operating status of the photovoltaic and storage combined power generation system. If it is in the grid-connected operation state, the photovoltaic and storage combined power generation system will enter the grid-connected operation control subroutine, otherwise it will enter the synchronization operation control subroutine;

Step 3: If PCC _flag = 0, turn off the grid connection point switch QS0, and the photovoltaic and storage combined power generation system enters the off-grid operation control subroutine;

Step 4: If PCC _flag = 1, and the photovoltaic and storage combined power generation system is in off-grid operation, the photovoltaic and storage combined power generation system enters the synchronization operation control subroutine.

The grid-connected operation control subroutine of step 2 specifically includes the following steps:

Step 2-1: After entering the grid-connected operation control subroutine, determine the distributed photovoltaic port voltage U _pv . If U _pv > U _{pv_min} , read the current operating status of the DC/DC converter, otherwise determine whether Flag _auto is 1. , if Flag _auto =1, the DC/DC converter stops running, and the DC/DC automatic stop flag of the DC converter is set to 1, that is, PV _flag =1, otherwise the DC/DC converter operates in MPPT mode;

Among them, PV represents distributed photovoltaic, U _{pv_min} represents the minimum voltage of distributed photovoltaic port, Flag _auto represents the automatic operation flag of DC/DC converter, PV _flag represents the distributed photovoltaic operation flag, and MPPT represents the maximum power point tracking solar controller. ;

Step 2-2: If the DC/DC converter is currently running, adjust its operating mode and run in MPPT mode; if the DC/DC converter is not running, read the Flag _auto value, if Flag _auto = 1 Then start the DC/DC converter to run in MPPT mode, otherwise keep the current state;

Step 2-3: Calculate the current load side power P _load (t) based on the voltage U _{abc_load} and current I _{abc_load} data at sampling point 2. Take the direction of current flow to the grid as positive, and read the current distributed power at the same time. Photovoltaic output power P _pv (t);

Step 2-4: Read the current SOC(t) status of the battery energy storage system, and calculate the upper limit of the rechargeable power of the battery energy storage system at the next moment P _{bat_charge_max} (t+1);

P _{bat_charge_max} (t+1)＝SOC _{bat_charge_flag} *P _{bat_charge_set} (2)

Among them, SOC _{bat_charge_flag} is the current battery SOC charging flag, P _{bat_charge_set} is the set battery discharge power, and SOC _max is the upper limit of battery SOC;

Step 2-4: Calculate the grid-side converter output power P _pcs (t+1) at the next moment according to equation (4);

Among them, PCS represents the grid-side converter;

Step 2-5: If the output power value of the grid-side converter at the next moment is less than its own operating loss value, that is, P _pcs (t+1) ≤ P _loss , it will switch to standby operation, otherwise the power will be calculated according to equation (4) The value is grid-connected discharge operation.

The off-grid operation control subroutine of step 3 specifically includes the following steps:

Step 3-1: Set the output voltage U _{pcs_ref} and frequency f _{pcs_ref} of the photovoltaic and storage combined power generation system;

Step 3-2: Read the current power load P _load (t), battery SOC (t), and calculate the maximum output power of distributed photovoltaic power P _{pv_max} (t+1) at the next moment.

P _{pv_max} (t+1)=P _{bat_charge_max} (t+1)+P _load (t) (5);

Step 3-3: Read the distributed photovoltaic port voltage and repeat step 2-1. The DC/DC converter is in power-limited operation, and the maximum output power is P _{pv_max} (t+1).

In step 3-1, the set output voltage and frequency are: U _{pcs_ref} = 380V, f _{pcs_ref} = 50Hz.

The synchronized operation control subroutine of step 4 specifically includes the following steps:

Step 4-1: Adjust the grid-side converter output voltage and frequency according to the grid-connection point voltage;

Step 4-2: Detect the grid-side converter port voltage U _{pcs_real} and phase Phase _{pcs_real} , and determine whether the difference between the voltage U _{abc_pcc} and phase Phase _pcc at the PCC point is within the set threshold range. If |U _pcs-real -U _{pcs_pcc} |＜U _th and |Phase _{pcs_real} -Phase _pcc |＜Phase _th , the grid-connection point switch QS0 is closed, and the grid-side converter switches to standby operation, otherwise it operates according to the given value of equation (6);

Among them, the PCC point is the grid connection point; U _th represents the voltage deviation limit, which is determined based on the PCS's tolerance; Phase _th represents the phase deviation limit, which is determined based on the PCS's tolerance.

Beneficial effects of the present invention:

Compared with the existing technology, the beneficial effects of the present invention are: This patent proposes a multi-mode adaptive adjustment operation control algorithm suitable for integrated photovoltaic and storage power generation systems. This algorithm has the following advantages:

First, it can realize stable operation and automatic conversion of the photovoltaic and storage combined power generation system in different operating modes of grid-connected and off-grid, improving the convenience and operational reliability of the combined power generation system;

Second, it fully considers the operating characteristics of distributed photovoltaic and the operating efficiency of the grid-side converter, and can automatically adjust the operating mode according to the current photovoltaic output characteristics and battery operating status to reduce the overall loss of the system;

Third, the output power of the combined power generation system can be adjusted according to the load to reduce the impact of distributed photovoltaic output fluctuations on the power grid.

Description of the drawings

Figure 1 Access topology of photovoltaic and storage combined power generation system;

Figure 2 The overall operation flow chart of the solar-storage combined power generation system;

Figure 3 The grid-connected operation flow chart of the photovoltaic and storage combined power generation system;

Figure 4 Off-grid operation flow chart of the combined photovoltaic and storage power generation system;

Figure 5 is the synchronized operation flow chart of the solar-storage combined power generation system.

Detailed ways

A solar-storage combined power generation system and its multi-mode adaptive adjustment operation control algorithm of the present invention will be further described in detail below with reference to the accompanying drawings and specific implementation methods.

The topology of the photovoltaic and storage combined power generation system is shown in Figure 1. The main equipment of the power generation system includes distributed photovoltaics, battery energy storage systems, DC converters and a set of grid-side converters. The output of the battery energy storage system is directly connected to the DC bus to maintain the stability of the DC bus voltage. Distributed photovoltaics are connected to the DC bus through DC/DC, the DC side of the grid-side converter is connected to the DC bus, and the AC side is connected to the grid. On the one hand, the grid-side converter collects the voltage and current data at sampling point 1 and 2 in real time to obtain the operating status information of the grid and load; on the other hand, it maintains real-time communication with the DC converter and battery energy storage system. Control its operating status.

After the joint power generation system is started, the operating status of the grid at the grid-connected point must first be determined. When the grid is operating normally, the system as a whole is in grid-connected operation, otherwise the system enters the off-grid operation state.

In the grid-connected operation state, the operating mode of distributed photovoltaics is further determined according to its port voltage. If the voltage is lower than the set threshold, the distributed photovoltaic is controlled to shut down to reduce operating losses. When the voltage is higher than the set threshold, the maximum power is used. Point tracking (MPPT) mode operation. For the grid-side converter, the current operating status is adjusted based on the current state of charge (SOC) of the battery energy storage system, distributed photovoltaic output power, and load power, making full use of distributed photovoltaic to charge the battery, supply power to the load, and improve economy. income.

Under off-grid operation, on the one hand, the grid-side converter operates off-grid to provide stable voltage and frequency support for the load. On the other hand, it needs to adjust the distribution according to the battery energy storage SOC, load power size and the current output power of distributed photovoltaics. Photovoltaic operation with MPPT, power limit or shutdown status.

When the grid-side converter detects that the grid voltage has recovered, it adjusts the voltage and frequency output by the grid-side converter to meet the grid-connection requirements for the same period. After the system grid-connection point switches QS0, the grid-connected operation state is restored.

While realizing the autonomous operation of the optical storage system on and off the grid, manual operation under special circumstances is also taken into consideration. It can switch between autonomous operation and manual operation according to the set operation flag and the current working status.

The photovoltaic and storage combined power generation system of the present invention can adjust its operating mode according to the distributed photovoltaic port voltage to reduce system losses when it is connected to the grid. At the same time, the grid-side converter adjusts the current state of charge (SOC) of the battery energy storage system according to the ), the distributed photovoltaic output power size and the load power size adjust the current operating status, making full use of distributed photovoltaic to charge the battery, supply power to the load, and improve economic benefits. Under off-grid operation, on the one hand, the grid-side converter operates off-grid to provide stable voltage and frequency support for the load. On the other hand, it needs to adjust the distribution according to the battery energy storage SOC, load power and the current output power of distributed photovoltaics. Photovoltaic operation with MPPT, power limit or shutdown status.

As shown in Figures 2 to 5, a multi-mode adaptive adjustment operation control algorithm for a solar-storage combined power generation system includes the following steps:

Step 1: The grid-side converter reads the voltage information U _{abc_pcc} at sampling point 1, and calculates the grid-connection point frequency f _{abc_pcc} based on the voltage information. If it satisfies Equation 1, the power grid is in normal operation, otherwise the power grid is abnormal. When the power grid is running normally, the power grid operation flag PCC _flag = 1, otherwise the PCC _flag = 0;

U _{abc_pcc} ∈ [U _low , U _up ] and f _{abc_pcc} ∈ [f _low , f _up ] (1)

Step 2: If the power grid is normal, further determine the current operating status of the solar-storage combined power generation system. If it is in grid-connected operation, the combined power generation system will enter the grid-connected operation control subroutine, otherwise it will enter the synchronization operation control subroutine;

Step 2-1: After entering the grid-connected operation subroutine, further determine the distributed photovoltaic port voltage U _pv . If U _pv > U _{pv_min} , further read the current operating status of DC/DC. Otherwise, determine whether Flag _auto is 1. If Flag _auto =1, the DC/DC stops running, and the DC/DC automatic shutdown flag is set to 1, that is, PV _flag =1, otherwise the DC/DC operates in MPPT mode;

Step 2-2: If the DC/DC is currently running, adjust its operating mode to operate in MPPT mode. If the DC/DC is not in the running state, further read the Flag _auto value. If Flag _auto = 1, start the DC/DC to run in MPPT mode, otherwise keep the current state;

Step 2-3: Calculate the current load side power P _load (t) based on the voltage U _{abc_load} and current I _{abc_load} data at sampling point 2. Take the direction of current flow to the grid as positive, and read the current distributed photovoltaic output at the same time. Power P _pv (t);

Step 2-4: Read the current SOC(t) status of the energy storage system, and calculate the upper limit of the rechargeable power of the battery energy storage system at the next moment P _{bat_charge_max} (t+1);

P _{bat_charge_max} (t+1)＝SOC _{bat_charge_flag} *P _{bat_charge_set} (2)

Among them, SOC _{bat_charge_flag} is the current battery SOC charging flag, P _{bat_charge_set} is the set battery discharge power, and SOC _max is the upper limit of battery SOC.

Step 2-4: Calculate the grid-side converter output power P _pcs (t+1) at the next moment according to equation (2);

Step 2-5: If the output power value of the grid-side converter at the next moment is less than its own operating loss value, that is, P _pcs (t+1) ≤ P _loss , it will switch to standby operation, otherwise the power will be calculated according to equation (4) The value is grid-connected discharge operation.

Step 3: If PCC _flag = 0, turn off the grid connection point switch QS0, and the combined power generation system enters the off-grid operation control subroutine.

Step 3-1: Set the output voltage and frequency of the combined power generation system, where U _{pcs_ref} = 380V, f _{pcs_ref} = 50Hz;

Step 3-2: Read the current load power P _load (t) and battery SOC (t), and calculate the maximum distributed photovoltaic output power P _{pv_max} (t+1) at the next moment, as shown in Equation (5).

P _{pv_max} (t+1)＝P _{bat_charge_max} (t+1)+P _load (t) (5)

Among them, P _{bat_charge_max} (t+1) is calculated according to formula (2).

Step 3-3: Read the photovoltaic port voltage and repeat step 2-1. The DC/DC is in power-limited operation and the maximum output power is P _{pv_max} (t+1).

Step 4: If PCC _flag = 1 and the combined power generation system is in off-grid operation, the combined power generation system enters the synchronization operation control self-program;

Step 4-1: Adjust the output voltage and frequency of the grid-side converter according to the grid-connection point voltage, as shown in equation (6);

Step 4-2: Detect the converter port voltage U _{pcs_real} and phase Phase _{pcs_real} , and determine whether the difference between _the voltage at the PCC point U _{abc_pcc} and the phase Phase _pcc is within the set threshold range. If |U _{pcs_real} -U If _{pcs_pcc} |＜U _th and |Phase _{pcs_real} -Phase _pcc |＜Phase _th , the grid-connected point switch QS0 is closed, and the grid-connected converter switches to standby operation. Otherwise, it operates according to the given value of equation (6).

The above are only the preferred embodiments of the present invention. It should be pointed out that those of ordinary skill in the art can make several improvements and modifications without departing from the principles of the present invention. These improvements and modifications can also be made. should be regarded as the protection scope of the present invention.

Claims (6)

1. The multi-mode self-adaptive regulation operation control algorithm of the light-storage combined power generation system is characterized in that the light-storage combined power generation system comprises a distributed photovoltaic, a battery energy storage system, a direct current converter DC/DC and a set of network-side converters;

the output of the battery energy storage system is directly connected with a direct current bus for maintaining the voltage stability of the direct current bus;

the distributed photovoltaic is connected to a direct current bus through a direct current converter DC/DC;

the direct current side of the grid-side converter is connected with a direct current bus, and the alternating current side is connected with a power grid;

on one hand, the network-side converter collects voltage and current data at the sampling point 1 and the sampling point 2 in real time, and obtains running state information of a power grid and a power load; on the other hand, the real-time communication is kept between the DC/DC converter and the battery energy storage system, and the running state of the battery energy storage system is controlled;

the multi-mode self-adaptive adjustment operation control algorithm comprises the following steps:

step 1: the network side converter reads voltage information U at sampling point 1 _{abc_pcc} And calculates the grid-connected point frequency f according to the voltage information _{abc_pcc} The method comprises the steps of carrying out a first treatment on the surface of the If the power grid meets the formula (1), the power grid is in a normal running state, otherwise, the power grid is abnormal; power grid operation sign PCC when power grid operation is normal _flag =1, otherwise PCC _flag ＝0；

U _{abc_pcc} ∈[U _low ，U _up ]And f _{abc_pcc} ∈[f _low ，f _up ](1)

Wherein PCC represents the power grid, U _{abc_pcc} Representing the three-phase voltage of the network, f _{abc_pcc} Representing the three-phase electrical frequency of the grid, PCC _flag Representing the running sign of the power grid, U _low Representing the lowest voltage of the power network, U _up Representing the highest voltage of the power grid, f _low Representing the lowest frequency of the power grid, f _up Representing the highest frequency of the grid;

step 2: if the power grid is normal, judging the current running state of the optical storage combined power generation system, if the current running state is in the grid-connected running state, performing grid-connected running control subprogram by the optical storage combined power generation system, otherwise, performing synchronous running control subprogram;

step 3: if PCC _flag If the power generation system is=0, the grid-connected point switch QS0 is disconnected, and the optical storage combined power generation system enters an off-grid operation control subroutine;

step 4: if PCC _flag =1, and the optical storage combined power generation system is in an off-grid operation state, the optical storage combined power generation system enters a synchronous operation control subroutine;

the grid-connected operation control subroutine in the step 2 specifically comprises the following steps:

step 2-1: after entering a grid-connected operation control subprogram, judging the voltage U of the distributed photovoltaic port _pv If U _pv ＞U _{pv_min} Reading the current running state of the DC/DC of the direct current converter, otherwise judging Flag _auto Whether or not to be 1, if Flag _auto =1, then the DC converter DC/DC shutdown is performed, and the DC converter DC/DC automatic shutdown flag is set to 1, i.e. PV _flag Otherwise, the direct current converter DC/DC operates in MPPT mode;

wherein PV represents distributed photovoltaic, U _{pv_min} Representing the minimum voltage of the distributed photovoltaic port, flag _auto Automatic running sign, PV, representing DC/DC of direct current converter _flag Representing a distributed photovoltaic operation sign, wherein MPPT represents a maximum power point tracking solar controller;

step 2-2: if the DC/DC of the direct current converter is in the running state, the running mode of the DC/DC is adjusted, and the DC/DC is operated in an MPPT mode; if the DC/DC of the direct current converter is not in an operation state, the Flag is read _auto Value of Flag _auto Starting a direct current converter DC/DC to operate in an MPPT mode when the power supply is in an operation mode of the MPPT, otherwise, keeping the current state;

step 2-3: according to the read voltage U at the sampling point 2 _{abc_load} Current I _{abc_load} Data calculation of current power consumption load side power size P _load (t) taking the direction of current flowing to the power grid as positive, and simultaneously reading the current distributed photovoltaic output power P _pv (t)；

Step 2-4: reading the current SOC (t) state of the battery energy storage system, and calculating the upper limit value P of chargeable power of the battery energy storage system at the next moment _{bat_charge_max} (t+1)；

P _{bat_charge_max} (t+1)＝SOC _{bat_charge_flag} *P _{bat_charge_set} (2)

Wherein SOC is _{bat_charge_flag} Charge flag for current battery SOC, P _{bat_charge_set} For the set dischargeable power of the battery, SOC _max Is the upper limit value of the battery SOC;

step 2-4: calculating the output power P of the network-side converter at the next moment according to the step (4) _pcs (t+1)；

Wherein PCS stands for network side converter;

step 2-5: if the output power value of the network-side converter at the next moment is smaller than the running loss value of the network-side converter, namely P _pcs (t+1)≤P _loss And (4) switching to standby operation, otherwise, performing grid-connected discharging operation according to the power value calculated in the formula (4).

2. The multi-mode adaptive modulation operation control algorithm of a light-storage cogeneration system according to claim 1, wherein the off-grid operation control subroutine of step 3 specifically comprises the steps of:

step 3-1: setting the output voltage U of the light-storage combined power generation system _{pcs_ref} Frequency f _{pcs_ref} ；

Step 3-2: reading the current power consumption load power P _load (t), calculating the distributed photovoltaic maximum output power P at the next moment by using the battery SOC (t) _{pv_max} (t+1)，

P _{pv_max} (t+1)＝P _{bat_charge_max} (t+1)+P _load (t) (5)；

Step 3-3: reading the voltage of a distributed photovoltaic port, repeating the step 2-1, wherein the DC/DC of the direct current converter is in a power-limited running state, and the maximum output power is P _{pv_max} (t+1)。

3. The multi-mode adaptive regulation operation control algorithm of the light-storage combined power generation system according to claim 2, wherein in step 3-1, the set output voltages and frequencies are respectively: u (U) _{pcs_ref} ＝380V，f _{pcs_ref} ＝50Hz。

4. The multi-mode adaptive modulation operation control algorithm for a combined light and power generation system according to claim 2, wherein the synchronous operation control subroutine of step 4 specifically comprises the steps of:

step 4-1: adjusting the output voltage and frequency of the grid-side converter according to the voltage of the grid-connected point;

step 4-2: detecting the port voltage U of the grid-side converter _{pcs_real} And Phase _{pcs_real} Judging voltage U at PCC point _{abc_pcc} And Phase _pcc If the difference is within the set threshold range, if |U _{pcs_real} -U _{pcs_pcc} |<U _th And |phase _{pcs_real} -Phase _pcc |<Phase _th Closing a grid-connected point switch QS0, converting the grid-side converter into standby operation, and otherwise, operating according to the given value of the formula (6);

wherein the PCC point is a point of combining; u (U) _th Representing a voltage deviation limit value, and taking a value according to the tolerance capacity of the PCS; phase of Phase _th Representing the phase deviation limit, the value is taken according to the tolerance of PCS.

5. The multi-mode adaptive modulation and operation control algorithm for a combined light and power generation system according to claim 1, wherein the sampling point 1 is connected with 380VAC, and the sampling point 2 is connected with an electric load.

6. The multi-mode self-adaptive regulation operation control algorithm of the light-storage combined power generation system according to claim 1, wherein the power load is connected with a power grid through a switch QS1, and a grid-connected point switch QS0 is arranged on the power grid.