Introducing DC Nano grid as a modular device with efficient Energy Management System

This literature review is for the ongoing research on the DC Nano grid. The aim of this research is to introduce Nano grid as a modular device which has the potential to become a device of choice for consumers not only for off-grid housing but also for grid connected housing and equipment in the future.

Literature Review on Introducing DC Nano grid as a modular device with efficient Energy Management System[edit | edit source]

Sizing of PV array in a DC nano-grid for isolated households after alteration in time of consumption^[1]

Summary:

P. Muthuvel, S. Arul Daniel, and S. K. Paul proposed two approaches in 2017 for sizing the PV array in DC Nano grid.  

• The first method is a simple approach, which directly obtains the size of PV panels based on highest monthly consumption in the year, irradiation and ambient temperature.
• The second method is analytical approach suitable for experts which uses particle swarm optimization (PSO) technique. It is based on detail cost equation with all design variables.
• The size of PV array was determined considering appropriate time shifting of loads.

Results:

• This paper concluded that that alternation of time of consumption of various load profile has resulted in reduced size of PV array as well as the cost of battery storage system significantly
• Again, in case of DC Nano grid design in a rural area, it is adequate to use the simple intuitive method for sizing the PV array.

Bus voltage level choice for standalone residential DC nanogrid^[2]

Summary:

In 2019, S. Moussa, M. J.-B. Ghorbal, and I. Slama-Belkhodja presented a guideline in order to choose appropriate voltage level for standalone residential DC Nano grid to ensure user safely, efficiency and cost saving.

• They have considered power consumption, area of application deployment, cable size and transmission losses as elements to find the suitable bus voltage of DC Nano grid.
• Also, in this paper they have compared results between five suitable voltage levels for DC Nano grid (12V, 24V, 48V, 60V and 100V DC).

Results:

• Most commercialized DC load generally run on 12 or 24V and some on 48V.Hence use of higher voltage level is not suitable as they will require step down converter as interface between bus and the load.
• Using voltage less than 15V is safe for user and for voltage level ranging for 15 to 100V, protection must be implemented only if the power is greater than 1000W only. So in this paper voltage level ranging from 12V to 100V is considered as it requires basic protection.
• 12V DC presents worst performance in terms of voltage drop, power loss and maximum allowed cable length while 100V DC presents the best. The performance increases with the increase of DC voltage level bus except 60V which presents a poorer performance than 48V DC.
• Finally taking the into consideration the cost and performance, they concluded that 48V voltage level is the best choice for their case of study.

A testbed for experimental validation of a low-voltage DC nanogrid for buildings^[3]

Summary:

In 2012, I. Cvetkovic et al presented the testbed for DC Nano grid for its system-level operation analysis with initial experimental results and conclusion.

Testbed Specification:

The testbed comprises 5kW Suntech's Solar Panels, 3.5kW Clean Field Energy's vertical axis wind turbine and 45Ah Saft's Li-ion battery bank, interface to grid (10kW) and Plug in to hybrid vehicle (5kW).

• Total Sourcing Capability of Nano grid is 33.5kW
• Assuming, Source coincidence factor n_SC=80%
• Load Co incidence factor n_LC=50%
• Peak Load Power=13.4kW ≈15kW
• Maximum current for Given Peak Load= 40A
• Bus wire gauge corresponding to current value is AWG 8 (8.37mm²)
• Maximum Voltage drop 7.2 V
• And Maximum Length of bus is 24m.

Converter Specification:

a. The grid interface converter –Energy control center (ECC)

• Two stage, single phase PWM converter.
• Comprises three phase legs: two are used in H bridge configuration to interface with the grid and third one is used as a bidirectional synchronous rectifier dc-dc converter.

b. The battery interface converter:

• The 10kW three phase interleaved, bidirectional buck/boost converter.
• The single, two switch buck/boost converter suffers from relatively large switch turn on, turn off and diode reverse recovery losses. Almost 22% of savings in the total loss can be achieved by three phase interleaved, bidirectional buck/boost converter.
• three phase interleaved, bidirectional buck/boost converter has three times higher frequency compared to single.

c. The PV inverter converter:

• The PV interface converter is a 5kW three phase interleaved boost converter of the same topology as the battery interface converter.
• It features two independent control. One is MPPT loop and another is low bandwidth droop control loop.

System level Tests:

1. First test is with a grid and a battery interface converter connected on the bus and loaded with a 1.8kW resistive load.
2. Second one is battery shut down test.

Control Scheme for a Bidirectional Converter in a Self-Sustaining Low-Voltage DC Nanogrid^[4]

Summary:

In 2015, S. I. Ganesan, D. Pattabiraman, R. K. Govindarajan, M. Rajan, and C. Nagamani proposed the control algorithm for the Energy Management System (EMS) to provide uninterrupted power supply to the DC loads and achieve self – sufficiency by minimizing grid power consumption.

• They have developed a control scheme for a cascaded two stage bidirectional converter interface between the Nano grid and the AC distribution network. The topology they have used is a bidirectional DC-DC converter followed by a three phase inverter. As bidirectional DC-DC converter, Dual Active Bridge (DAB) converter is used which consist of voltage source inverters on either side of a HF transformer.
• An empirical relation for the power flow through converter.
• And optimal link voltage ranges to achieve power transfer between rated limits.

Operation:

• To control the power flow through DAB converter, a simple open loop control is used, by appropriately choosing the capacitor DC link voltage, inherent power flow control between rated limit was achieved. It overcomes circulating current and switching issues without using complex circuit.
• The three phase inverter uses double loop control to regulate the DC capacitor link voltage at the reference value, thereby facilitating power exchange with AC grid.
• The bidirectional converter operates in three modes namely power surplus mode, power deficit mode and idle mode and transition between different modes were achieved based on four level control scheme based on battery voltage.

Result:

• The proposed scheme was compared with existing two mode control method/Power management method and two band control method.
• The proposed system operates idle mode for 17.4 hours while an idle operating for two band control method is 4.1 hour which ensures no power was transferred through DAB during idle mode.
• For a single day the proposed scheme is consumed only 1.83kWhr on the other hand power management control method consumed 2.87kWhr and two mode operation consumed 4.26kWhr.

Model of a hybrid distributed generation system for a DC nano-grid^[5]

Summary:

In 2016, E. Hamatwi, I. E. Davidson, J. Agee, and G. Venayagamoorthy designed, simulated and optimized for a hybrid DC Nano grid, with storage and a backup diesel generator for Umzinyathi district of South Africa.

• Based on simulation using HOMER, the optimal system comprises of a 100kW PV array, thirteen 7.5kW wind turbine, 20kW diesel generator and 96 Trojan T-105 deep cycle batteries.
• The area is estimated to have 80 houses and 15shops. The maximum power consumption for the household and shops is 6.79kWh/day/household and 4.85kWh/day/shop respectively. And total 615.95 kWh/Day power consumed by the DC Nano grid.

Result:

• The net present Cost (NPC) of the system is $459,545, Cost of Energy $0.248/kWh and Payback period of the optimal system obtained is 4 years.
• The PV array, wind turbines and diesel generator generated 50%, 21% and 29% of the total energy respectively where system's renewable fraction is 71%.

DC nanogrids: A low cost PV based solution for livelihood enhancement for rural Bangladesh^[6]

Summary:

In 2014, M. R. Khan and E. D. Brown proposed alternative to diesel based irrigation system in perspective of Bangladesh.

• The solar PV based irrigation system is integrated with DC Nano grid/Rural grid to maximum utilization of the PV power.
• The proposed Nano grid uses battery only to backup for supply of energy during the nigh hours, irrigation can be done during the day time without battery backup.
• The proposed Nano grid system consist of 1.1kW pump with 10 households in cluster. 1.9kW PV system was installed with 500Ah, 12V battery as storage.
• The 1.1kW (1.5HP) irrigation pump with 50% efficiency can run satisfactorily driven by an inverter can pump 90,000 liter of water per day (4hour run time), which can irrigate 67acre of rice field for a whole season.

Feasibility analysis of solar DC Nano grid for off grid rural Bangladesh^[7]

Summary:

In 2015, M. M. H. Sajeeb, A. Rahman, and S. Arif discussed about Nano grid and surveyed an installed DC Nano grid in Kushtia district of Bangladesh.

• The grid was set up to houses clustered in 300m long and 100m wide land.
• The distribution network served 20 households each containing 12V DC fan, light and TV
• 16 batteries of 12V volt 130Ah are used.  The panels used are of 250Wp, 24 volts. The Nano consists of a backup diesel generator.

Review of Control Strategies for DC Nano-Grid^[8]

Summary:

In 2022, H. Fan, W. Yu, and S. Xia have investigated on differnt control technologies for the DC Nano grid from two aspects: local control and coordinated control.

Local Control: Local control is mainly used for voltage/current control, source side control e.g. output power control of RESs and local power sharing. It has shortest response time. In this paper they have mainly discussed about the following control methods:

1. Current/ Voltage control.
2. Maximum power point tracking control.
3. Active load sharing.
4. Passive load sharing.

Coordinated Control: Coordinate control collects the electrical information of each component, enhance the performance of power sharing and switch operating modes. The following methods have been discussed:

1. Centralized Control
2. Decentralized Control
3. Distributed Control
4. Hybrid control (Centralized/ Distributed)

Result:

• The local control realizes the voltage/ current control method and primary power sharing of the units.
• Droop control has been widely used in local control due to its precise current sharing and excellent voltage regulation abilities.
• ·On the other hand, coordinated control can be used to further improve the accuracies of power sharing and power quality of the grid system.
• The distributed control scheme and communication strategy should be further studied with multi agent based AI algorithms so that these control strategies can operate DCNG stably in all kind of operation mode.

Proposed smart DC nano-grid for green buildings — A reflective view^[9]

Summary:

In 2014, M. H. Shwehdi and S. R. Mohamed proposed a DC Nano grid for smart home which composed of smart loads and home area network with a smart meter. Their proposed green building has the following characteristics:

1. Smart meter, smart load and power flow will be managed by some wireless nodes.
2. There will be coordination between smart loads and smart meter to monitor the power flow, which will be achieved by algorithms.
3. Total load of the building will be managed in such a way that the algorithm can manage the loads in the peak hours.

Conclusion:

1. ZigBee wireless technology will be used for communication of smart meter in DC smart homes. ZigBee is designed for high radio frequency applications which has low data rate, long battery life and secure networking.
2. Standers for DC power distribution is being developed by E-Merge Alliance.
3. Though the proposed DC house is not a direct alternative to AC house but it can bring energy independence with security.

Optimal Integration of a Hybrid Solar-Battery Power Source into Smart Home Nanogrid with Plug-In Electric Vehicle^[10]

In 2017, X. Wu, X. Hu, Y. Teng, S. Qian, and R. Cheng developed an framework for efficient energy management and components sizing of a single smart home with home battery, plug-in vehicle and photovoltaic (PV) arrays.

• The plug-in vehicle system battery allows bidirectional and unidirectional power flow.
• The smart home management system controls power flow between PEV battery, home appliances, PV arrays, home battery, and the utility grid.
• Convex programming (CP) is used for optimal energy management.

Result

• The proposed CP method can effectively solve the optimization problem,
• The total cost in V2H (Vehicle to home/ Bidirectional) mode is 2.6 % lower than that in H2V (Home to vehicle) mode

Control and Operation of a DC Micro grid with Variable Generation and Energy Storage^[11]

Summary:

Lie Xu and Dong Chen discussed about the control and operation of DC Microgrid. They proposed a coordinated strategy of battery system, wind and load management system.

• Under different mode of operation, they classified battery system simply as a standby system or charge/ discharge mode based on the order given by the system or battery management system.

They have considered three modes of operation for secure and reliable power supply

• Mode 1: In this mode of operation the power deficit is adjusted by the supply from AC grid. The wind turbine operates at maximum power, no load shedding required and battery can be charged or discharged accordingly.
• Mode 2: This mode considered the temporary situation when the total power required from grid exceeds its limit or in case of any fault. In this mode the battery switches its role from standby mode to voltage regulation to provide necessary power balancing.
• Mode 3: This mode is considered as islanding mode and AC grid supply is completely unavailable. The DC voltage in now regulated by battery and required power by energy source. But the load shedding may be required in this mode to maintain the grid operation.

Control of Bidirectional DC/DC Converters in Reconfigurable, Modular Battery Systems^[12]

Summary:

M. Muneeb Ur Rehman, and Fan Zhang et. al. proposed a control approach to connect converter modules which are reconfigurable and serve as an interface between battery and DC bus.

• The control approach achieves bus voltage regulation and voltage sharing among converters under bidirectional power flow.
• It can interconnect N number converters (modules) in either series or parallel configuration.
• The control strategy balance energy flow by distributing power differentially among converters according to the relative state-of-charge and capacities of the battery.
• The controller estimate and monitor battery SOC and control the sub-module converters.
• The proposed control strategy is verified in hardware experiments using a module composed of two 480 W, 27-37 V series or parallel output dual-active bridge converters, and twelve 25 Ah Li-ion NMC battery cells.

A Distributed Control Strategy Based on DC Bus Signaling for Modular Photovoltaic Generation Systems with Battery Energy Storage^[13]

Summary:

Kai Sun, and Li Zhang et. al. proposed a distributed control strategy for a modular photovoltaic (PV) generation system with battery energy storage elements.

• The modular system is composed of three dc/dc converters for PV arrays, two grid-connected dc/ac converters, and one dc/dc converter for battery and local loads.
• The operation is categorized into four modes: islanding with battery discharging, grid-connected rectification, grid-connected inversion, and islanding with constant voltage (CV) generation.
• The dc bus voltage is used as an information carrier and four operation modes are identified by different dc bus voltage levels.

Control and Optimization of Residential Photovoltaic Power Generation System with High Efficiency Isolated Bidirectional DC–DC Converter^[14]

Summary:

Rui Li and Fangyuan Shi proposed an energy management system to bring stability of the system as well as to increase financial benefits.

• A three-level boost converter extracts maximum power from the PV module.
• A two-stage isolated bidirectional DC-DC converter is used to control the voltage and current of the battery. It also increases DC gain and efficiency. The converter consists of two configuration of isolated bidirectional DC-DC converter in the proposed system. The module is a two-stage isolated bidirectional DC-DC converter, in which a bidirectional LLC or CLLC can be employed in providing electrical isolation. To achieve higher efficiency, the isolated bidirectional DC-DC converter works as a DCX and a buck/boost is added to improve the DC gain.
• The highest efficiency of the isolated bidirectional DC-DC converter achieved was 97.50% under charging mode and 96.03% under discharging mode.
• A three-phase five-level inverter is used which operates in islanding mode or grid connection mode.

Bidirectional DC-DC Converter Topologies and Control Strategies for Interfacing Energy Storage Systems in Micro grids: An Overview^[15]

Summary:

Nisha Kondrath over-viewed three control strategies of bidirectional converters used for microgrid energy storage applications.  

• A bidirectional converter is used to maintain constant DC bus voltage and to regulate charge/discharge of the battery.
• The first control strategy is Current-mode control with two feedback loops, an inner current loop and an outer voltage loop, is a popular control method.
• In power control strategy during the grid-connected mode, battery is controlled based on SOC (State of  Charge). But during the islanded mode, fuzzy control or advanced droop control is used based on the voltage variation.
• And the third strategy to stabilize the dc bus voltage is sliding mode control. This method is robust against system variations as well as input and load variations.

Control Strategy for Power Flow Management in a PV System Supplying DC Loads^[16]

Balasubramanian Indu Rani, and Ganesan Saravana Ilango et.al. developed a power flow management system where operating mode of the bidirectional converter is selected by sensing the battery voltage.

• A dual H-bridge bidirectional converter is used to charge and discharge the battery which is capable of operating in both buck and boost modes. It is used to regulate the dc link voltage also.
• The power management system senses the battery voltage and selects the mode of operation for the bidirectional converter. It also provides the reference current for the hysteresis current controller.
• A hysteresis current controller is used for the current control of an inverter. The reference signal for the hysteresis current controller is generated as a function of the phase of the grid voltage using Phase Locked Loop (PLL).

Power Control of DC Micro Grid Using DC Bus Signaling^[17]

Li Zhang, Tianjin Wu, and Yan Xing et. al. proposed power management scheme for DC micro grid which consists of four operation modes.

• The DC bus voltage is used as an information carrier for mode selection. Operation mode provides the control signal for the PV converters, battery converter and grid-connected converter.
• The switching between different modes and the corresponding changes in control methods for converters is achieved through DC bus voltage without additional communication links.
• The Power management scheme maintains the power balance and stability of DC microgrid under variation of power generation or load.
• Battery DC/DC converter is employed as the grid forming unit and to regulate the DC bus voltage by battery discharging.

Power Balance Modes and Dynamic Grid Power Flow in Solar PV and Battery Storage Experimental DC-Link Micro grid^[18]

Rupak Kanti Dhar, and Adel Merabet et. al. proposed an energy management system for a DC-link micro grid  based on different power balance modes and dynamic grid power flow.

• The control system includes the local control units for the PV system, the battery storage system, and the voltage source inverter,
• In Battery Storage System a buck-boost converter is operated for the purpose of controlling the battery current to track a reference. Its local control includes a current controller based on a proportional-integral (PI) controller. The energy management system provides the reference current for the battery in order to charge-discharge the battery to meet the demand.
• The inverter control unit regulates the DC link voltage to maintain it constant also controls the inverter AC current.
• Based on the available power from the PV source, the battery SOC, and the grid power, the microgrid is operated under a power balance mode to meet the demand while optimizing the use of the battery storage.

A Unified Control and Power Management Scheme for PV-Battery-Based Hybrid Micro grids for Both Grid-Connected and Islanded Modes^[19]

Zhehan Yi, and Wanxin Dong et. al. proposes a comprehensive control and power management system (CAPMS) for PV-battery-based hybrid micro grids with both ac and dc buses, for both grid-connected and islanded modes.

• CAPMS is a centralized power management system consisting of a monitoring modules that senses the real-time parameters from PV system, battery system and all the power converters.
• After sensing the parameters, CAPMS selects specific control method to be applied to the converters to provide reliable power.
• Depending on the PV output power, SoC and power limit of the battery, DC and AC loads, and the grid demand, CAPMS decides the operation modes of the PV array and the battery  and provides proper reference values to the controllers.
• As the DC bus voltage is controlled, DC loads of certain voltage level can be connected to the DC bus without additional converters.

Energy Management Strategy of Islanded Micro grid Based on Power Flow Control^[20]

Ye Zhang and Hong Jie Jia et. al. proposes a new energy management strategy of islanded microgrid based on power flow control.

• Battery storage system keeps the DC bus voltage constant, PV array as a current source is connected to the DC bus and a main DC/AC inverter in VF mode is used to provide the voltage and frequency references for the AC bus.
• Dual-loop control is applied in the DC/DC converter connected with the battery. The outer voltage loop keeps the DC bus voltage constant and the inner loop improves the dynamic behavior.

A Management of power flow for DC Micro grid with Solar and Wind Energy Sources^[21]

Gowtham.K,and Hariprasath.P et..al. presented a control strategy for Management of power flow in DC micro grid with solar and wind energy sources.

• DC link voltage is regulated by the battery circuit while maximum power is extracted from Solar and Wind to feed the loads connected at the DC bus
• A ceaseless power is provided in the DC side with the help of Management of power flow algorithm.

Low Voltage Direct Current(LVDC) Nano grid for Home Application^[22]

Sigi C Joseph, and Dr. Ashok S et. al. Investigated power and control architectures for the purpose of implementing Low Voltage Direct Current (LVDC) Nanogrids for residential applications.

• A Nano grid controller (NSC) is used which is apparently the central controller for the system. It controls and manages all the modules within the LVDC.
• LVDC Nano grid has intelligent power switch and outlet which has the ability to measure and control electrical devices connected in this system.
• The NSC communicates with the other nodes in LVDC network, remotely controls and monitors nodes such as solar PV controller, battery charge controller and LVDC power socket.
• The NSC make decisions such as selection mode of operation, management of power socket and switch and controlling the battery.
• The LVDC Nano grid allow plug and play operation of the system where the new systems can be added or removed from the nanogrids network seamlessly.

Power Management Strategy Based on Virtual Inertia for DC Micro grids^[23]

Pedro José dos Santos Neto and João Pedro Carvalho Silveira et. al. proposed a power management system where virtual inertia concept is applied with a state of charge to regulate the charging and discharging process of battery.

• The microgrid becomes vulnerable due to low inertia during the transients.
• In this virtual inertia technique, the ESS transient response is controlled by increasing the inertia of the DC grid system. Thus, high-rate peaks of power are avoided, which improves the ESS life cycle.
• A SOC-based function, ψ(soc), is applied to manage the ESS charge and discharge process.
• The proposed power management technique is a centralized control.
• Also the proposed strategy simplifies the communication link between the grid inverter and the ESS (Since VSC power is the only information exchanged).

References[edit | edit source]