DC Microgrid for Commercial Buildings

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International Journal of Advance Electrical and Electronics Engineering (IJAEEE)

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ISSN (Print): 2278-8948, Volume-4 Issue-1, 2015 4

DC Microgrid for Commercial Buildings

J. S. Savier

Department of Electrical and Electronics Engineering

Government Engineering College, Barton Hill Thiruvananthapuram, Kerala, India E-mail: [email protected]

Abstract — Microgrids are power systems in which generation elements are co-located with loads, regardless of the aggregated generation capacity or the grid interconnection. In addition to the generation and loads, these systems generally have storage elements. In this work, a DC microgrid for commercial buildings is proposed. The aim of the proposed system is to enhance the reliability of the microgrid as well as to reduce the electricity bills in commercial buildings. The proposed system is based on the concept that the DC power drawn from the distributed generation systems to be maximized and the storage elements are to be eliminated.

I. INTRODUCTION

Microgrid is a topic extensively researched over the last few years. The following advantages associated with microgrids make it attractive.

• Increased reliability with distributed generation

• Increase efficiency with reduced transmission length

• Easy integration of alternative energy sources

Even though, there are a number of advantages in smart grids, there are a number of technical issues such damping and stability issues, islanding, load sharing, energy management, etc. Even interconnectivity of microgrids to the grid and many other issues are yet to be solved. Another major problem with microgrids is the storage systems which are being incorporated to the systems. In this paper, a new technology based on DC smart grids is proposed for commercial buildings which has many advantages compared to the existing ones which are mainly based on the inverter technologies.

II. METHODOLOGY

Microgrids are mainly based on alternating current (AC) technology, mainly due to the fact that the electrical appliances are working on AC. This introduces a number of energy efficiency issues associated with AC microgrids due to the converters and inverters associated with the microgrid system. Especially, when the generation is mainly due to solar PV in nature, the

general procedure being adopted is to convert the DC voltage obtained from solar PV system into DC and again it is converted into AC for feeding to the loads, which makes the overall power conversion efficiency to lower levels.

The author is of the opinion that if appropriate technologies for commercial buildings, it is highly uneconomical to provide AC microgrids as the converters and inverters associated with the system require periodic maintenance and may require replacement in some cases. The technology behind solar PV cells has been matured and the life of solar panels is of the order of 20-25 years where as the converters and inverters associated with the microgrids have less reliability. Similarly, the battery storage also requires periodic maintenance and /or replacement. In this context, a DC microgrid which uses the mainly solar PV cells has been proposed with a new technology of power sharing between the AC grid and DC microgrid. The proposed method requires only limited conversions between AC and DC.

Solar PV systems 110 V - 250 V

DC generator 110 V DC

Converter 110 V DC bus

Load2 110 V Load1 110 V

Fig. 1. A simple DC micro grid

Figure 1 shows a simple 110 V DC micro grid. The system is incorporated with a 110 V DC generator and a solar PV generation system. The solar PV system voltage ratings are selected in such a way that the sufficient controllability can be achieved. The loads are also rated at 110 V DC. It may be noted the that the voltage level is selected as 110 V DC as such a level would be a safe working voltage in the case of DC. Instead of the converter block shown above, the voltage regulator block can also be incorporated. The system should supply the load demand in the case of sufficient generation is available from the sun. In this case, it is running as

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International Journal of Advance Electrical and Electronics Engineering (IJAEEE)

________________________________________________________________________________________________

ISSN (Print): 2278-8948, Volume-4 Issue-1, 2015 5

equivalent to independent grid and no power is drawn from the AC grid.

However, this may not be the case. During cloudy days, the solar PV system may not be capable of providing sufficient voltage. In this case, a control strategy needs to be developed. The proposed control strategy includes the conversion of AC grid voltage to DC using a rectifier and filter. Then, the voltage at the converter bus of the PV system is measured and the deficiency in voltage is injected using the control logic so as to maintain constant voltage in the load bus. For example if during a cloudy day, the converter bus voltage of the PV system is 85 V DC, the remaining voltage, 25 V DC (110- 85) will be injected from the grid voltage control logic so that the load bus voltage is maintained at 110 V.

Solar PV systems 110 V - 250 V DC generator

110 V DC

Converter 110 V DC bus

Load2 110 V Load1 110 V

AC Grid Voltage 230 V AC

Rectification and Filtering and

control

Converter Bus 0-110 V DC (Variable)

Fig. 2. Modified DC micro grid model

The overall control logic can be explained as follows:

When sufficient solar generation is available, the power demand may be supplied from the solar PV system alone.

When the solar system is not able to provide voltage support, the remaining voltage, i.e., the difference between 110 V DC and converter bus voltage is supplied from the grid. The generation system is designed in such a way that the output voltage is 110 V DC so that no conversion is required. The generator will feed the loads when there are no power from the AC grids is available.

III. ADVANTAGES OF THE PROPOSED METHODOLOGY

The DC microgrid proposed has several advantages, which are listed below:.

a) Synchronization of distributed generators are not necessary.

b) Fluctuation of generated power of distributed generators and load power can be compensated in the DC bus by the addition of voltages from rectified voltage of AC grid.

c) Loads are not affected by voltage sags, voltage swells, three-phase voltage unbalance, and voltage harmonics in the AC system.

d) Power quality is not affected by Inrush current, singlephase loads and single-phase generators.

e) Higher efficiency than AC microgrid.

f) Safe working voltage

IV. CONCLUSIONS

A new topology for DC microgid is proposed in this paper. The proposed topology has many advantages over the commonly used AC microgrid. The proposed methodology is well suited for commercial buiildings and is developed with the aim to reduce investment cost, maintenace and monthly energy bill. The DC microgrid system developed has the higher energy efficiency as compared to the AC microgrid system, as the system has minimum conversions. The proposed system can supply load directly from the solar PV system during sunny days as the loads considered are mainly that of lighting and HVAC loads. In this case, the power drawn from the AC grid is virtually zero. When the solar PV system is not able to supply load demand, the power from the AC grid system is utilised for maintaining DC grid voltage.

When the AC grid system is not capable of maintaining DC grid voltage, DC generator is utilised for maintaning the DC grid voltage.

REFERENCES

[1]. D Ravi Prasad, Dr. B.Rajesh Kamath, K.R Jagadisha and S.K Girish, "Smart DC micro-grid for effective utilization of solar energy", Euro.

Scien.Journal, vol. 8, No.23, pp. 116-124, Oct.

2012.

[2]. P. Savage, R. R. Nordhaus, and S P. Jamieson,

"DC Microgrids: Benefits and Barriers", Report, Yale school of forestry & environmental studies.

[3]. J. H. Jahshan, "Dc microgrids: A direct route to energy efficiency", IEEE EnergyTech 2012 Conference, May 2012.

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