2014 Power and Energy Systems: Towards Sustainable Energy (PESTSE 2014)
Power Sharing Control Scheme for Integrating Various Energy Sources in Smart Grids * # $ Anant Vaibhav , student member, IEEE, Sarthak Jain , student member, IEEE, and Lovely Goyal Dept. of Electrical and Electronics Engineering, Maharaja Agrasen Institute of Technology Delhi - 110086, India * $ #
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Ahstract- The availability of different energy sources, both the conventional and renewable energy sources, urge towards a solution of their application and their accessibility to the consumer. This requires their integration in smart grids on a common platform to deliver clean and uninterrupted power at low cost. The paper proposes to augment these sources on a common DC bus platform with control scheme of power sharing between them by MOSFET switching. This eases the system with the burden of synchronizing the sources as well as prevents it from oscillations and harmonics. Simulation based results demonstrate that proposed method provides effective control of power sharing between the different sources integrated on a common DC bus for aforesaid objectives and situations. The presented results show that degeneration can be easily achieved without induction of any transients in supply and power is made available according to the consumer choice of source. Keywords-MOSFETs; Power sharing; Power quality; Smart grids; I.
INTRODUCTION
Today, Power quality has become a concern to every consumer; A deteriorated power quality may give the unsatisfactory response of the equipment and devices and hamper the power system in many ways [1-2]. One of the major causes of power quality deterioration is use of non linear loads such as computers, drives, converters, etc. , which causes- voltage sag/swell, interruptions, spikes, harmonic distortion, noise, just to mention a few [3-4]. Moreover the depletion of conventional energy sources has paved the way for renewable energy sources, but these sources are not compatible with the already existing grids and their integration again adds to power quality issues [5-7]. Due to the problems discussed above, existing grids are under pressure to deliver the growing demand for power which has suggested the concept of distributed generation systems (DGs) using renewable energy sources increases generation, improves efficiency, improves power quality, cuts energy cost and expenditures, which can be achieved using the concept of smart grids [8]. A smart grid is an electrical grid that uses information and communications technology to gather and act on information, such as information about the behaviors of suppliers and consumer, in an automated fashion to improve the efficiency, reliability, economics, and sustainability of the production and distribution of electricity [9-10]. Smart grid facilitates the
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interconnection of different power generations known as DGs and controls the bidirectional power flow between consumer and generating stations. For implementation of this scheme, common platform is required for the interconnection of different DGs. DGs and load can be integrated on a common platform such as DC bus, which has an advantage of reliability, oscillations obstruction and is free from frequency synchronization [11]. The DC platform has been preferred over AC platform worldwide because it helps in Power transmission and stabilization between unsynchronized AC distribution systems, connection of a remote generating plant to the distribution grid and facilitates interconnection between different terminals generating power at different voltages and/or frequencies [10, II]. With the development and advancement of smart grid technologies, now the power systems have a better ability to adapt to the dynamic behavior of renewable energy resources and the DGs, which offers secure and individual energy options which in turn helps both consumer and utilities to access these resources [11]. Further, the benefits of a smart grid are: (a). Real time energy information, (b).This information empowers smarter energy choices, (c).lt provides timely and affordable solutions [12]. This paper presents a new, efficient, smart and most importantly a consumer friendly way of implementing power sharing in a smart grid, in which the consumer has the freedom of choosing and extracting the proportion of power from DGs available. For this purpose various energy sources are integrated on a common platform of DC bus and are maintained at a constant voltage for this interconnection [11].The deciding factor for power extraction from different energy sources could be based on the tariff and availability, i.e. the consumer can decide the power extraction from the source delivering power at cheaper tariff for a particular span of time. For example, in day time if the consumer feels that the power from renewable source (solar in this case), is economical than the grid supply, then the maximum part of required power could be extracted from the renewable source. Whereas during night time, if extracting power from the renewable source is more expensive due to the intermittent nature of renewable energy sources, consumer has freedom of extracting higher portion of power from the main grid supply.
II.
SYSTEM CONFIGURATION
The block diagram of the proposed system is shown in fig. I. It consists of two sources, one is a 3-cD AC source of 415V,
50Hz and the other is a renewable energy source of nov DC. The 3-cD AC source is then rectified using VSC to obtain the DC bus at nov. Thus, a nov DC bus has been developed for the coupling of different sources on a common platform. If the DC source is of different voltage it can be maintained to any reference voltage using DC-DC bucklboost converter. These sources of different nature are connected on the DC bus via MOSFET switches. The switching of MOSFETs is controlled by Op-Amp generated PWM in such a way that the sum of the percentage of the duty cycles provided to both the MOSFETs is 100%. This will ensure the total power availability across the load. Since the load has been rated for 380V, 6kW, the DC bus of nov is being further stepped down to 380V using DC DC buck converter. It has been found that Power Supply Units (PSU) in data centers working with 380V DC that has shown 28% improvement in efficiency than the current average AC distribution efficiency [13]. So, the load has been rated for 380V DC with domestic consumption of around 6kW. III.
CONTROL THEORY
The paper is sectioned with two control schemes- one is used for voltage source converter in current control scheme and the second strategy has been used to generate PWM from op-amps for switching MOSFETs to control the power sharing across the load between different energy sources. A.
DC Bus
HCC controlled VSCAC-DC rectifier (720 V)
supply (415 v, 50 Hz)
3-
Source I Fig. I. System configuration block diagram of proposed system.
carrier signal for comparison with Vref and generates the PWM using the op-amp comparator circuit. The non-inverting terminal of op-amp comparator is fed with the high frequency triangular wave and the inverting terminal of the comparator is connected to a reference voltage. The comparator compares the two voltages at its terminals and generates a PWM wave whose duty cycle depends upon the DC reference voltage. This PWM wave is directly fed to one of the MOSFETs and simultaneously complemented using inverting amplifier of gain 1 and then fed to another MOSFET. The value of adjustable dc voltage (Vref) can be changed to adjust the ratio of power shared by the 2 sources according to the following formulas.
tl
Vref Ts =
Vtri
(1)
4
(2)
maintenance
Fig. 2 represents the control scheme for the generation of pulses for VSC to maintain DC bus at nov. The VSC terminal works as a pump that injects the current required to maintain DC bus voltage at nOV.The error between the reference voltage and actual bus voltage is synthesized by PI controller. The output of PI controller is then further transformed from dq-frame to abc to obtain reference currents for realization of HCC. The HCC switches the VSC in such a manner that it follows the estimated reference current. The d component obtained from PI controller estimates the exact real power required to maintain DC bus voltage. The sin-cos component is realized by PLL for transformation from dq-abc components.
B. PWM generationfor MOSFETswitching Fig. 3 shows the control scheme for the generation of PWM switching signal for MOSFETs. The power sharing is obtained using two MOSFET switches which alternatively switch between different sources across the load. The MOSFET conduction time in a PWM cycle decides the power sharing by the relative source. The pulses for switching are generated using op-amp circuit using the principle of comparator and the technical apparatus of the PWM generation is shown in fig. 4. This circuit consists of a potentiometer, which is used to vary the reference voltage (Vref), and a high frequency generator is used to generate
Duty ratio Cd)
1 =
-
2
[1 - ] Vref
-
Vtri
(3)
Where, Time period of one time cycle of triangular wave. Reference voltage to control duty cycle of PWM. Maximum amplitude of triangular wave (8V). The two sources are connected to the dc link through their MOSFET switches which connects the source to the link at nov. The MOSFETs are switched alternatively (one being ON at one time) which is used to maintain the dc link for the load. The DC link is further bucked to 380V to provide power to the load of 6 kW. The two MOSFETs are switched such that the cumulative output of both the MOSFETs is constant DC current to maintain DC link which is bucked at the load voltage level. A pulsating current is generated in DC circuit by switching the supply on the load, due to which the average power provided by the source reduces and thus the power can be shared using this principle. The MOSFETs are chosen due to their power rating and capability of high frequency switching.
Pulses for
HCC
VSC
Vdc
� �
>
(a) Input reference and TrWnb'111ar
H 5
H �< t"
(b) Generated PWMwave
Fig. 3. (a) depicts the Input reference voltage and Triangular wave and (b) depicts the generated PWM wave. These are the control signals for gates for switching the MOSFETS alternately Complement Using Op-Amp High frequency Vref generator (Variable)
Fig. 4. Control block for PWM switching generation PERFORMANCE EVALUATION
The performance of proposed scheme is simulated in MATLAB environment. The power sharing between the two energy sources of different nature across the load has been analyzed and depicted in fig. 5 and fig. 6. The sources, one grid source of 415V, 50Hz, 3
