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Stand-Alone Solar PV DC Power System with Battery Backup

This example shows the design of a stand-alone PV DC power system with battery backup and helps you to:

  • Choose the necessary battery rating based on the connected load profile and available solar power.

  • Determine how the panels should be arranged in terms of the number of series-connected strings and the number of panels per string.

  • Select a proper PI controller proportional gain, $(K_v)$, and phase-lead constant, $(T_v)$.

Both solar PV and battery storage support stand-alone loads. The load is connected across the constant DC output. A solar PV system operates in both maximum power point tracking and de-rated voltage control modes. The battery management system uses bi-directional DC-DC converters.

A stand-alone PV system requires six normal operating modes based on the solar irradiance, generated solar power, connected load, state of charge of the battery, maximum battery charging and discharging current limits.

To track the maximum power point (MPP) of solar PV, you can choose between two maximum power point tracking (MPPT) techniques:

  • Incremental conductance

  • Perturbation and observation

You can specify the average daily connected load profile, region's daily available average solar energy (kWhr), solar PV system operating temperature, day of autonomy, battery recharge time, output DC voltage and solar panel specification. solar panel manufacturer data is used to determine number of PV panels required to deliver the specified generation capability.

PI controller of the form $\frac{K_v (sT_v+1)}{sT_v}$ is chosen to control the solar PV and battery management system (BMS).

This example uses:

  • A MATLAB® live script to design the overall standalone PV system.

  • Simulink® to design/simulate the control logic for the system.

  • Simscape™ to simulate the power circuit.

  • Stateflow™ to implement the supervisory control logic.

Stand-Alone PV DC Power System Model

To open a script that designs the standalone PV DC power system, at the Matlab® command line, enter: edit 'ee_solar_standalone_dcsystem_withbatterybackup_data'

The chosen battery and solar PV plant parameters are:

****          For the Given Stand-Alone PV System, Battery Sizing Parameters           ****
*** Calculated amphr of the battery  =  542.91 Ahr 
*** Battery nominal voltage  =  78 V
*** Battery voltage at 80% discharge  =  70.20 V 
*** Number of required battery cell  =  39.00 
*** Average discharge current  =  4.28 A
****          For the Given Solar Panel, PV Plant Parameters           ****
*** Required PV Power rating  =  9.36 kW 
*** Minimum number of panels required per string  =  8 
*** Maximum number of panels connected per string without reaching maximum voltage  =  10 
*** Minimum power rating of the solar PV plant  =  1.80 kW 
*** Maximum power possible per string without reaching maximum DC voltage  =  2.25 kW 
*** Actual number of panels per string  =  8 
*** Number of strings connected in parallel  =  5 
*** Actual solar PV plant power  =  9.01 kW
****                   Battery Charging/Discharging Parameters                             ****
Reference battery charging current = 45.24 A
Maximum battery charging current = 128.29 A
Maximum battery discharging current = 64.14 A
Maximum battery charging Power = 10.01 kW
Maximum battery discharging Power = 5.00 kW

Stand-Alone Solar PV DC Power System Monitoring Panel

This example uses the Simulink® Dashboard feature to display all the real time system parameters. Turn the dashboard knob in the monitoring panel to modify the solar irradiance and the load during the simulation. By changing these parameters, you can observe how the PV system switches between the operating modes.

Solar Plant Subsystem

The solar plant subsystem models a solar plant that contains parallel-connected strings of solar panels. The solar panel is modeled using the Solar Cell block from the Simscape™ Electrical™ library. This example uses the output voltage from the DC bus, open circuit voltage depending on temperature and irradiance to estimate the number of solar panel strings connected in series, and the plant power rating to estimate the number of solar panel strings connected in parallel. Connecting multiple panels can slow the simulation because it increases the number of elements in a model. By assuming uniform irradiance and temperature across all the solar panels, it is possible to reduce the number of solar elements by using the controlled current and voltage sources as shown in the solar panel subsystem.

Maximum Power Point Tracking (MPPT)

Two MPPT techniques are implemented using the variant subsystem. Set the variant variable MPPT to 0 to choose the perturbation and observation MPPT. Set the variable MPPT to 1 to choose incremental conductance.

Intermediate Boost DC-DC Converter

Boost DC-DC converter is used to control the solar PV power. When battery is not fully charged solar PV plant operated in maximum power point. When battery is fully charged and load is smaller than the PV power, solar PV is operated in constant output DC bus voltage control mode.

Battery Management System (BMS)

The battery management system uses a bi-directional DC-DC converter. The battery is charged by the buck converter configuration and it is discharged using the boost converter configuration. To improve battery performance and life cycle, systems with battery backup have limited maximum battery charging and discharging current. This example sets a limit on the maximum amount of power that a battery can supply to the load and absorb from the solar PV source. Here, the maximum charging power is equal to the solar plant capacity at the standard test condition. The chosen maximum charging power should be able to recharge the battery sooner than the battery recharge time specified by the user

Here, separate controller is used for charging and discharging operation. BMS controller have two loops, an outer voltage loop and inner current loop.

Supervisory Control(Mode Control) Parameters

Stand-alone PV system in this example comprises seven operating modes. These modes are selected based on DC bus voltage, solar irradiance and state of charge of the battery. The DC bus voltage level is used as a measure to detect a load imbalance. If the DC bus voltage is greater than $V_{dc-max}$, the system is generating more power than what the load is requiring. If the DC bus voltage is less than $V_{dc-min}$, then the load is requiring more power than what the system is generating.

DC bus voltage level $(V_{dc})$, solar irradiance $(Irrad)$ and the battery state of charge $(SoC)$ are used to decide the suitable operating mode.

Operating modes of the stand-alone PV DC System are:

  • Mode-0 - Start mode (Default simulation starting mode)

  • Mode-1 - PV in output voltage control, battery fully charged and isolated

  • Mode-2 - PV in maximum power point, battery is charging

  • Mode-3 - PV in maximum power point, battery is discharging

  • Mode-4 - Night mode, PV shutdown, battery is discharging

  • Mode-5 - Total system shutdown

  • Mode-6 - PV in maximum power point, battery is charging, load is disconnected

Stateflow mode control diagram

Simulation Output