International Journal of Electrical and Electronics Engineering Research (IJEEER) ISSN 2250-155X Vol.2, Issue 3 Sep 2012 56-63 Š TJPRC Pvt. Ltd.,
SIMULATION OF PARALLELEDDC CONVERTERSFOR INTEGRATING RENEWABLE SOURCES ABHIMANYU KUMAR YADAV, PANKAJYADAV, BRIJESHTRIPATHI & MAKARAND LOKHANDE School of solar Energy, Pandit Deendayal Petroleum University, Raisonvillage, Gandhinagar, Gujarat-382007, India.
ABSTRACT In recent years, there is rapid development of renewable energy sources and their interconnection to the existing utility grid is increasing day by day.It can create disturbances in grid operation and also may result in poor power quality during transmission. So alternative to this issue can be a DC micro-grids where transmission and utilization of energy is at DC level.With increasing innovations in power electronics (DC converters) and energy storages (ultra-capacitors, flywheel),DC interconnection tomicro-grids could be a better option than AC interconnection grids because of no frequency and phase shift problems need to be considered. Several energy sources are connected in parallel in DC micro-grid which require us to study the parallel operation of converters, their safety and protection.In this paper paralleling of a non-isolatedstep-down and a non-isolatedstep-up converter from a circuit theoretic viewpoint is discussed. In the proposed configuration converters are modeled as voltage sources and their connection in parallel is done to provide voltage regulation across the load. Both converters are modeled as closed loop systems and the simulated result is presented in this paper.
KEYWORDS : Buck Converter, Boost Converter, PID Controller,PSO Algorithm. INTRODUCTION Growing concerns about energy security and climate change have heightened interest in harnessing renewable energy resources as a response to these critical issues. The integration of renewable energy resources cannot be solved in isolation from the other challenges facing modern electricity industries. Electricity industry restructuring processes should now be specifically designed and implemented to accommodate high levels of renewable energy penetration.The paralleling of power converter modules offers a number of advantages over a single, high-power, centralized power supply. The non-isolated converters are used in order to reduce the system cost and to improve the converter efficiency, since the common mode current with the large PV area in these applications can be solved effectively. Paralleling of standardized converter modules is an approach that is used widely in distributed power systems for both front-end and load converters [1,2].The objective of paralleling of converter system is to share the load current equally and stably using various sharing mechanism.If current sharing mechanism is not provided, then it may happen that one or more units can get excessive
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Simulation of Paralleleddc Convertersfor Integrating Renewable Sources
load current.This results in higher thermal stress on that particular units and reduces the system reliability[3,4].Therefore, choice of the paralleling schemes and voltage and current sharing techniques requires a firm understanding of merits and limitations of different paralleling schemes and sharing mechanisms.The parallel processing of load current provides fault tolerance for the system against the failure of a single module [5].In last two decades several approach for paralleling DC-DC converter with various complexity and current sharing performance have been proposed [6,7].The paralleling of converter is basically categorized in two classes, based on their connection styles, current or voltage sharing control structures or feedback functions. In the absence of a current sharing mechanism, even a small variation in module’s output voltages can cause the output currents to be significantly different [4]. Therefore, the engineers must focus on designing a controller that can regulate the output voltage and achieve balanced current distribution. At present in Florida international university, a similar kind of research is being carried out [8].In this research they have considered paralleled wind-based synchronous generators for analysis of DC-bus voltage regulation based on master slave voltage oriented control algorithm for PWM converters.In this paper we have discussed about step-up and step-down converter in parallel configuration and their operation and control. The objective of this is to regulate the voltage at a common point.
CONVERTERS AND ITS CONTROL The step up (buck) and step down (boost) converter consists of a DC input voltage source Vs, inductors ,filter capacitor , controlled switch S , diode D and load resistance R. The buck converter’s output voltage is always lower than input voltage and the output voltage of boost converter is always greater than the input DC voltage. If the switch operates with a Duty ratio D in buck converter, the DC voltage gain of the boost converter is given by is
and in case of boost converter voltage gain
, where Vo is output voltage and D is the Duty cycle of the pulse width modulation
(PWM) signal used to control the Mosfet ON and OFF states.The parameters of DC Buck and Boost converter is tabulated in Table 1 and Table 2respectively. Table1 & 2 Simulation parameter in buck converterSimulation parameter for boost converter
Parameters Input voltage Output voltage Switching frequency
Values 80 V 48V
Inductance Capacitance
1000µH 100µF 0.5 Ω,0.045Ω
ZCL, recon PID gain
100KHz
Kp=41.78,ki=0.3411
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Abhimanyu Kumar Yadav, Pankajyadav, Brijeshtripathi & Makarand Lokhande
Parameters Input voltage Output voltage
Values 30 V
Switching frequency
100KHz
Inductance Capacitance
920µH 20µF
ZCL, recon
5 Ω,0.045Ω
PID gain
Kp=0. 028, ki=20
48V
The closed loop converters tries to control the output voltage overshoot voltage using PID controller and pulse width modulator operating at a fixed frequency of 100 KHz as shown in Fig1. The open loop buck converter is modeled and Equation (1) given below represents a transfer function with cut-off frequency
(1) And the transfer function of boost converter is given below
(2) 2 1.8 1.6
M a g n itu d e
1.4 1.2 1
0.8 0.6 0.4 0.2 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Time(S) Fig1.Pulses from the pulse width modulator
0.9
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Simulation of Paralleleddc Convertersfor Integrating Renewable Sources
PID gain optimization using PSO algorithm: The gain of PID controller used is determined by particle swarm optimization (PSO) algorithm [9].
PSO is a multi-agent parallel search technique where say n flying entities fly through the
multidimensional search space as the algorithm progresses through discrete time steps i.e. t=0, 1,2, ..., while keeping the population size m constant. In the standard PSO algorithm, each particle’s current position Xi (t) = [Xi, 1 (t), Xi, 2 (t),...Xi, n (t)] and its current velocity VI (t) = [VI, 1 (t), VI, 2 (t)... VI, n (t)] , where i=1, 2,...m is considered and accordingly its personal best position Pi(t) and global best position G(t) is found with respect to the origin of search space. Here one position is declared better than another if the former gives a lower value of the objective function than the latter. This function is called the fitness function. Each particle’s initial position vector component Xi, j (0) is picked randomly from a predetermined search range [XLj, XUj] and its velocity components is initialized by choosing at random from interval [-Vjmax, Vjmax]. The initial settings for Pi (t) and G (t) is given below Pi (t) = Xi (0), G (0) = Xk (0) such that f (Xk (0)) ≤ f (Xi (0)) ∀i. The iterative optimization process for the initialized particle begins and the position and velocities of all the particles are updated by the following recursive equations (3), (4). Given equations are for jth dimension of the position and velocity of the ith particle. Vid(t+1) = ωVid(t) + C1Ф1(Pid(t)-Xid(t)) + C2Ф2(Pgd(t)- Xid(t)) Xid (t+1)= Xid (t) + Vid (t+1)
(3) (4)
Where ω: Time-decreasing inertial weight factor designed by Shi and Eberhurt [10]. C1=2. 4, C2=1. 6. Two constant multiplier called self confidence and swarm confidence respectively, Ф1, Ф2. Two uniformly distributed random number.The iteration is fixed for certain number of time steps or until the fitness of the best particle at a certain time step is better than a predefined value is obtained. The fittest vector of the final population upon termination of the algorithm is taken as the possible solution of the problem.
PERATION OF PARALLELED CONVERTERS In the proposed type of configuration, each constituent non-isolated converter has its own voltage loop to regulate the output voltage. The need for parallel configuration of converters is to extract maximum power from the existing renewable sources without putting much stress on a particular converter. This is achieved using proper current and voltage sharing mechanism. The circuit diagram of the closed loop paralleled converter configuration is shown in Fig 2.The output impedance of the jthclosed loop converter is given by (5) where
is impedance of closed loop converter and
steady-state current-sharing error is expressed as
is the connecting wire impedance and the
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Abhimanyu Kumar Yadav, Pankajyadav, Brijeshtripathi & Makarand Lokhande
∆I=I1-I2
(6)
Fig2.Parallel connection of closed loop step-up and step down converter This type of configuration can be used with photovoltaic modules as voltage source to regulate the voltage of a bus for DC micro-grid type applications. To accomplish this, here the output voltage is feedback to the comparator where it is compared with a reference voltage and the output is given to PID controller. The PID controller tries to maintain the constant voltage across the load. The gain values of the PID controller is found out using particle swarm optimization technique (PSO) and it is one of the best algorithm which provide accurate gain values. The input voltage for buck and boost converter is varied from 72-80 volts and 20-30 volt randomly to find if there is any difference in output, but in this range no change at output is seen. The broad strips in input voltages of both converter shown in Fig 3 is due wide difference in voltage of 8 and 10 volts. The output voltage across the load is maintained constant at 48 volts and power available at load is 286 watt as shown in Fig 4.
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Simulation of Paralleleddc Convertersfor Integrating Renewable Sources
V in 2 (V )
40
V in 1 (V )
100 80
20
60 0
0.02
0.04
0.06
0.08
0.1
0
0
Time(S) Input voltage of buck converter
0.06
0.08
0.1
30
60
IL 2 ( A )
IL 1 ( A )
20
40
10
20 0
0.02
0.04
0.06
0.08
0.1
0
0
Time(S) Inductor current of buck converter
0.02
0.04
0.06
0.08
0.1
60
V o u t1 ( V )
V o u t2 (V )
Time(S) Inductor current of boost converter
60 40
40
20 0
0.04
Time(S) Input voltage of boost converter
80
0
0.02
20
0
0.02
0.04
0.06
0.08
Time(S) Output voltage of buck converter
0.1
0
0
0.02
0.04
0.06
0.08
Time(S) Output voltage of boost converter
Fig3. Simulated results of the paralleled buck and boost converter
0.1
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v o lta g e a c ro s s lo a d (V )
Abhimanyu Kumar Yadav, Pankajyadav, Brijeshtripathi & Makarand Lokhande
60 40 20 0
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.1
0.07
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O u tp u t p o w e r a t lo a d
Time(S) Voltage across load resistor 300 200 100 0
0
0.01
0.02
0.03
0.04
0.05
0.06
Time(S) Output power at fixed load
Fig4.Output voltage across load RLOAD and power available at load The output power and voltage across the varying load is shown below in Fig5.Here all parameters are kept constant except a load.When the load is increased from 8â„Ś to 10 â„Ś, there is a major fall in the output power from 286 watt to 230 watts but the voltage across the load is maintained constant. The drop in power is due to decrease in square of current in one the converter. Here if current
28
28
27
27
IL 2 (A )
IL 1 (A )
sharing mechanism is used then this drastic drop in output power can be regulated.
26 25
25
0.03
0.04
0.05
0.06
0.07
0.08
0.09
V o lta g e a t lo a d (V )
Time (S) Inductor current of buck converter 60
40
20
0 0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
Time (S) Output voltage across varying load(V)
24 0.02 0.03
0.1
0.1
O u tp u t p o w e r a t lo a d (W )
24 0.02
26
0.04
0.05
0.06
0.07
0.08
0.09
0.1
Time(S) Inductor current of boost converter 300 200 100 0 0.02 0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.1
Time(S) Output power at varying RLOAD
Fig5. Output voltage across load, inductor currents and power available at varying load
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Simulation of Paralleleddc Convertersfor Integrating Renewable Sources
CONCLUSIONS In this paper, acomplete Matlab/Simulink simulation of closed loop paralleled buck and boost converter system is simulated with PID controller based on PSO algorithm.The transfer function of both converters is used to find the appropriate gain value for PID controller using the PSO algorithm. Here both converters are operated with same switching frequency. The variations in output load resistance results in a change of buck inductor current and output power but the output voltageacross the load remains marginally unchanged. The drastic drop in power is due to decrease in square of the inductor current of buck converter while the boost inductor remains unchanged. Keeping the voltage regulated is objective of this paper. Hence this type of converter configuration can beuseful for DC converter dominatedDC micro-grid operation.Moreover, the use of non-isolated DCconverter helps in reducing the cost and also improve the system efficiency.
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