A Model Predictive Control Strategy for Performance Improvement of Hybrid Energy Storage Systems in DC Microgrid

final year project ppt on Modeling of microgrid

1. A Model Predictive Control Strategy
for Performance Improvement of
Hybrid Energy Storage Systems in
DC Microgrid

2. Applicable Energy Storage Technology for PV
Power Generation
• The lead-acid battery is popular in PV generation system because of
its low cost, but its life cycle is low. Also it will cause environment
problem.
• Other battery like nickel-metal-hydride battery , Li-ion battery
have advantage of high energy density, but they are expansive.
• Supercapacitor is becoming an attractive new technology for energy
storage.

3. Feature of the Supercapacitor
Advantages:
•High life cycle (>500000 cycled times)
•Quickly charge/discharge (>1000A)
•Operating temperature range (-50℃-70 ℃ )
•Environment friendly
Limitations:
•Low energy density
•Linear discharge voltage prevents use of the full
energy spectrum.
•Cells have low voltages
•High self-discharge

4. Executive Summary
Source:
•Photovoltaics is the science of converting sunlight to electrical energy.
Photovoltaic Systems are electrical systems consisting of a PV module or
an array and the Balance of System Components needed to convert
solar energy into usable electricity.
The purpose of this teaching module is to learn about some basic
Photovoltaic System types, their different designs, configurations, and
applications.

5. Stand-Alone Systems
Stand-alone PV systems are sized and designed to provide a specific
electrical load over time using the solar radiation resource at a given
location.
These systems are often installed in remote areas, where there is no
utility grid or it is difficult or even impossible to use any other source of
power supply.

6. Stand Alone components
The typical scheme of a stand-alone PV system

7. Simple Stand Alone systems
Direct-coupled stand-alone system.
 This type of Stand Alone systems is powered by the
array. It is by far the simplest type of stand-alone PV
system; but most complex to design properly.
 It’s designed to operate independent of the electric
utility grid, and is sized to supply certain DC electrical
loads.

8. Common simple DC Stand Alone system
Common applications include water pumps and fans
DC load is matched and directly connected to a PV array
Uses no energy storage
Load only operates when sun is shining, also referred to
as a “day use system”.

9. Stand Alone DC system with batteries
A direct current system that feeds DC appliances with
storage batteries.
The basic components are a PV module, charge
controller, storage batteries, and a load (DC appliance).
 To operate loads at night a system must include a
means of storing energy

10. DC Stand Alone systems serving DC and AC loads
A Stand Alone system can provide power for both AC
and DC loads.
A thorough load analysis considering both factors and
their time of use, will determine the requirements for the
quantity of modules and batteries to put in place.

11. Stand Alone DC system for AC loads
Stand-alone DC systems for AC loads must include an
inverter, which draws DC power from the battery bank and
converts it to AC power for distribution.
The size and cost of any stand alone system is related to
the magnitude and duration of the electrical load and the
solar resource, energy efficiency is critical. For these
reasons, a thorough load analysis is required in the design
and installation of any stand-alone system.

12. Stand Alone Hybrid Systems
A hybrid system provides
energy reliability because there
are two or more charging
systems at work.
Hybrid PV systems are often
the least costly for remote
power applications.

13. Stand Alone Hybrid Systems
Wiring Diagram example of a Hybrid PV system.

14. Critical Load Backup
Bimodal systems are typically used to back up critical
loads, but can also be used to manage the energy supply
for different times of the day in order to reduce electricity
bills.

15. Proposed System

16. Model Predictive Control

17. Cont…
•MPC uses physical system information to predict future evolution and
evaluates a suitable cost function.
• The control signal for the next interval is defined by that sequence which
minimizes future error in the output.
•The prediction is realized using the mathematical model of the dynamic
system.
•The control output is updated at each sampling instant with corrections based
on the new information.
•Prediction of the system future outputs is made using a prediction horizon.
 
•For a prediction horizon
 
N,
 the future outputs yk for k=1 … N are predicted
from known values and future control signals uk for k = 1..N-1.
•The control signals are calculated through an optimization process that
minimizes the future output error.

18. DC Micro-Grid

19. DC Micro grid (DCMG)
 General model
•Each node can be represented by
a power source (pn) in parallel
with a capacitor (cn) and
admittance (gn).
Equivalent model
•DCMG that integrates renewable and
conventional generation, as well as
energy storage systems (ESS) and
different types of loads.
• The DCMG can connect to the AC
grid to supply or absorb power.
Power converters connect sources,
loads, and energy storage units to the
DC bus.

20. The Key Issues for Supercapacitor Energy
Storage System
• Usually the supercapacitor cell with aqueous electrolyte allows 1V of charge
, Whereas organic electrolyte allows 2.5 volts of charge. To operate at
higher voltages, supercapacitors are connected in series. On a string of
more than three capacitors, voltage balancing is required to prevent
any cell from reaching over-voltage.
• The gravimetric energy density of the supercapacitor is 1 to 10Wh/kg.
Compare to battery, it is lower. Whereas the voltage of the
supercapacitor is linear and drops evenly from full voltage to zero volts.
Because of this, the supercapacitor is unable to deliver the full charge.

21. In China the supercapacitor as product
its technology level:
• energy density: about 4Wh/kg
• life cycle: 100000-500000 cycled times
• operating temperature range (-40℃-70 ℃ )
In order to set up the test system of
supercapacitor energy storage, we have
investigated two kind supercapacitors
from two Chinese companys:
• the supercapacitor cell with organic electrolyte:
2700F, 2.7V, 500000 cycled times
• the supercapacitor cell with aqueous electrolyte:
20000F, 0.7V, 100000 cycled times

22. PV
Array
Buck
Charger
Super-
capacitor
Bank
Boost
DC/DC
Converter
Energy
Management
And
Control
1kW PV Power Generation with Supercapacitor
Energy Storage（300Wh/2kWh）
Load

23. AC Power
Supply
Charger
Super-
Capacitor
Bank
DC/DC
Converter
DC/AC
Inverter
Computer
Test System of Supercapacitor Energy
Storage (500W/60Wh)

24. The Super capacitor block implements
a generic mode

25. Cont….

26. Short circuit Voltage Vs. Short Circuit Current
The SOC for a fully charged supercapacitor is 100% and for an empty
super capacitor is 0%. The SOC is calculated as:

27. Super Capacitor model Block Assumption
•Internal resistance is assumed constant during the charge and the
discharge cycles.
•The model does not take into account temperature effect on the
electrolyte material.
•No aging effect is taken into account.
•Charge redistribution is the same for all values of voltage.
•The block does not model cell balancing.
•Current through the super capacitor is assumed to be continuous.

28. Study of the Efficiency of Battery and
Supercapacitor Bank Charging
• Supercapacitor by constant voltage charging, the efficiency can be up
to 50%
• Supercapacitor by constant current charging, the highest efficiency
can be gotten。Because of the serial and parallel resistance in
Supercapacitor, they will consume electricity when charging. The
amount of charging current will affect the charging efficiency.
• Many times experiments show that the smaller charging current can
lead to higher efficiency.

29. Study of the Efficiency of Battery and
Supercapacitor Energy Storage System
• Voltage balancing can overcome the problem of the capacitance
difference. Each cell can be fully charged.
• Through changing from parallel to serial connected, more energy can
be output. For example, the supercapacitor bank change from parallel to
serial connected for one time while output through DC/DC converter
with 1：0.5 input range, the supercapacitor bank can output 93.75%
stored energy.
• Voltage balancing , changing from parallel to serial connected , high
efficiency electronic circuit including charging and discharging
converter, The total high efficiency of energy storage can be reached.

30. Simulation Implementation

31. Centralized vs. Local Control
Centralized Control Local Control

32. PV Variable Irradiance

33. PV OUTPUT

34. BATTERY OUTPUT

35. SUPERCAPACITOR OUTPUT

36. LOAD OUTPUT

37. Conclusion
•This project proposes a Modern Predictive Control (MPC) of
low computational effort that can achieve voltage or power
regulation for DCMG stabilization.
 
•The bi-directional converter is proposed in this paper as the
DC–DC stage for the supply and DC loads interconnection.
 
•Most distributed energy resources (DER) and energy storage
devices are inherently DC, therein the importance of DCMGs
in modern systems
•The proposed controller is a generalized procedure that can
be extended to larger DCMG independent of the converters
used for the DC–DC conversion.