2016 Biennial International Conference on Power and Energy Systems:Towards Sustainable Energy (PESTS E)
Cost Effective Design of Bidirectional Energy Management Topology Aftab Alam
Ambuj Sharma
Nirbhar Neogi
Dept. of Electrical & Electronics Engg. Birla Institute of Technology Mesra Ranchi, India Email:
[email protected]
Dept. ofElectrical& Electronics Engg. Birla Institute of Technology Mesra Ranchi, India Email:
[email protected]
Dept. of Electrical & Electronics Engg. Birla Institute of Technology Mesra Ranchi, India Email:
[email protected]
Abstract: In this paper, a cost effective approach for integrating PV based energy resources with grid is proposed. The proposed strategy enables PV based micro grid to function in both islanded mode of operation and grid connected mode of operation. For grid synchronization an op-amp based phase and frequency synchronization has been experimentally developed. A complete experimental set up capable of operating in both modes has been developed and tested. Inductive impedance has been incorporated between inverter output terminal and grid terminal. The bidirectional energy management strategy is also developed using battery as energy storage device.
conditions must be satisfied. These conditions are addressed briefly in this section.
A. Conditionfor Power Dispatch In this section, a case for a lumped parameter loss less voltage bus line is considered. A single phase inverter connected with grid voltage bus is shown in figure I. In this the reactance X includes the reactance of short circuit impedance.
Keywords: Islanded mode, Grid synchronization, Point of common coupling, Bidirectional converter, Photovoltaic sources, Phase lock loop, Energy storage, Distributed energy resources, microgrid.
I.
INTRODUCTION
Recent research trend shows a lot of interest in the field of renewable energy sources based power generation & distribution. Currently popularity of PV based power distribution systems is on the rise. In [1] optimization of energy management system for a smart grid is elaborated. Photovoltaic resources based power distribution system fall in the category of distributed energy resources (DER). In [2] suitability of power electronics based topologies for interfacing with grid is developed. In a PV based system maximum power point tracking, synchronization with grid and islanded mode of operation are main technical challenges. In [3] a control circuit using buck-boost converter has been studied for efficient implementation of maximum power point tracking. In [4, 5, 6] phase lock loop and universal controller based strategies has been developed. A micro grid connected with main grid must be able to operate autonomously during fault at grid side. In [7] a design for power network for scheduled or forced isolation capability is presented. In the islanded mode of operation a strategy for energy management in order to integrate energy storage devices with microgrid is discussed in [8]. For stable operation of micro grid system reactive power management based control strategy is proposed in [9, 10]. II.
BRIEF REVIEW OF WORKING PRINCIPLE
The output voltage of solar panel is DC in nature. In order to integrate solar energy source with utility grid few 978-1-4673-6658-8/16/$31.00 ©20 16 IEEE
Fig. 1. Point of common coupling with short circuit impedance
Let the sending and receiving end voltages be given by equation (1). (1) Vac = Vs = VI.LO, Vg = V2.LO° Then,
VI L5 - V2
VI COS 5 - V2 + jVI sin5
jX
jX
1=---:-::-::-s
(2)
The sending end real & reactive power is given by
Ps + jQs
Ps
= VsI; = VI (cos5 .. VI cos5 + Jsm5) .
+ JQs =
V2 - jVI sin5
(3)
.
-JX
VI V2sin5+ j(V2I - VIV2COS5) X
(4)
Since the line is loss less, the real power dispatched from the sending end is equal to the receiving end. We can therefore write (5) Where Pmax
=
V1 V2
x
is the maximum power that can be
transmitted over the transmission line. The power-angle curve is shown in figure 2. From this
facilitate bidirectional power flow. These both modes of operation will be accomplished using same circuit configuration. However to accommodate both operations different switching strategies are adopted. Po - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - ------ -
-go .0
( dog)
Fig. 2. Power angle curve
figure we can see that for a given power Po , there are two possible values of the bus angle, 15 -150 and I5 max . The angles are given by _
omax omax
=
(6)
Po (--) Pmax 180 0 - 00 .
sm
-1
(7)
B.
Frequency Matching & Voltage Regulation The main technical challenge in the field of integrating renewable energy and smart grid with utility grid is frequency matching and voltage regulation at the point of common coupling. There are two different scenarios: one is islanded mode of operation and the other is grid connected operation. If the frequency of inverter output is not identical with the grid frequency then large transient currents may cause damage components at low power carrying side and inject transient current in the grid side [11]. If inverter output voltage is not regulated then reverse power from grid to inverter will begin to flow. As a consequence inverter side circuitry will be damaged as there is no provision for storing the sudden surge of energy due to reverse power. To address this issue an ultra-capacitor may be connected [12]. During islanded mode of operation, there are no such complications.
i-- -'---.--- -: !
POWERn CIRCUIT
i
L__~ ___ ~ ___ ~ ___________________ ~
'----- - ,- - - '
: i
!- '- '- '- '- '- '- '- '- '- '- '-n-'- '- '- '- '- - - .- - - - .- .- - .- .- .- .- .- .- .- .- .- .- .- .- .- .- .- ..:
The first mode of operation is discharge mode. This process involves supplying energy from the battery bank to the connected grid. This is implemented by boosting battery bank voltage to the required level and then supplying to an inverter module which is controlled to produce same frequency as that of grid and the required phase for injecting desired amount of power into the grid. The second mode of operation involves reverse power flow from grid to battery for the charging purpose. This is achieved by converting the grid voltage from AC to DC and controlling charging current. In the power circuit, Battery is the source of energy when power is transferred from inverter to grid side (discharge mode of operation) and it works as energy storage system when power is transferred from grid side to battery ( charging mode of operation). Buck-Boost converter is used in boost mode to increase the DC battery voltage to required DC link voltage level during discharge mode and the converter is used in buck mode in charging mode of operation. H-bridge converter is used as inverter in discharge mode of operation & as rectifier in charge mode of operation as suggested in [13]. Output filter is used for filtering the harmonics present at the output of inverter. Transformer is used for isolation between grid & inverter as well as step up the inverter output voltage to the grid voltage level. The above discussed modes of operation are implemented with the help of a microcontroller unit. In the control circuit, the DC voltage sensor& current sensor sense the input voltage & current of boost converter during discharge mode of operation. It also senses the output voltage & current of buck converter during charge mode of operation. The AC voltage sensor & current sensor sense the output voltage & current ofH-bridge inverter during discharge mode of operation and it senses the input voltage & current of H-bridge converter during charge mode of operation. These sensor signals are fed to micro-controller unit. After this, the control algorithm generates the pulses for Buck-Boost converter & H-bridge inverter to implement discharge mode of operation as well as charge mode of operation enabling intermittent storage of distributed energy resources as proposed in [14]. These control pulses are provided to triggering terminals through a gate drive circuit. The purpose of using gate drive circuit is to provide isolation between controller unit and power circuit. It also maintains proper biasing condition for switching operation of semi-conductor devices such as IGBT and power MOSFET etc.
~ONTROL CIRCUIT
Fig. 3. Block diagram of experimental set-up
C. Block Diagram Block diagram of experimental set-up is shown in figure 3. Here two modes of operation have been developed to
III. EXPERIMENTAL IMPLEMENTATION An experimental setup has been developed for implementation of grid connected inverter operation. This also incorporates bidirectional power flow. The experimental setup
consists of buck-boost converter, single phase inverter, LC filter, synchronization unit.
A. Buck-Boost Converter DC ,------,----,----+'"
+
D2
c D1
Fig. 6. Buck mode of operation
Fig. 4. Buck-Boost converter
Figure 4 shows the circuit diagram of buck-boost converter. The operation of the proposed dc-dc converter in boost mode is explained with the help of electrical equivalent circuit as shown in figure 5. When the power switch S 1 is open in figure 5. The current It flows through capacitor C and the load. The current II decreases during capacitor charging. When the power switch SI is closed in figure 5, the current Ilincreases linearly and diode Dl is reverse biased. During this mode capacitor supplies energy to the load. The capacitors and inductors are to be designed to get the required output voltage. They can be designed based on the boost dc-dc converter specifications.
B. Single Phase Inverter A power MOSFET based single phase inverter circuit is configured as shown in figure 7. The power rating of inverter is limited by power rating of switches. In this case IRFP460 power MOSFET with maximum drain to source voltage of 500V and a continuous drain current of 13A has been used. The switches are fired by a microcontroller based control unit. The controller will alternately the turn on upper and lower switches. The commercially available driver circuit will add a small additional dead time (in the range of 500 to 1000 ns) during the switch transitions in order to avoid accidental switching of upper switch and lower switch simultaneously. The controller will vary the duty cycle of switches. This voltage will be subjected to a LC low-pass filter. The voltage at the output of the filter will follow the waveform and time period of the modulating reference signal.
+ ""'---+--,---------, DC
Fig. 5. Boost mode of operation
Output Power P = 100W, Output voltage Vo = 60V, Input voltage Vin = 20 volts, Switching frequency f = 10KHz, Duty cycle (0() = 70 %, Inductor current ripple (t"'h%) =20% (Assumed). Capacitor Voltage (LWL %) = 5 % (Assumed) (8) Inductor (L) = ocxv·m [X(ML%)XIL
. () CapacItor C = - -OCxP --VOxfx(J1VL%)
(9)
The operation of the proposed dc-dc buck (step down) converter is explained with the help of electrical equivalent circuit as shown in figure 6. When the power switch S2 is closed in figure, the current lzincreases linearly and diode Dl is reverse biased input voltage source supplies energy to the load and current across inductor lzrises. When the power switch S2 is open in figure 6, the current II flows through diode Dl and the load, the current lzreduces.
Fig. 7. Single phase inverter
The dead time is very small as compared to switching frequency. The output voltage is represented by equation (10) in which ma denotes the modulation index. The ma should not be greater than 1, because it distorts the output voltage of inverter. (10) This method is used to control the load voltage frequency, wave form and amplitude of the inverter output voltage. The switching frequency should be at least 25 to 100 times of the fundamental desired frequency of the inverter. The PWM signals are produced by Atmel Atmega-16 microcontrollers.
C. LC Filter & Isolation Transformer (Step-Up Transformer)
Fig. 8. LC filter and isolation transformer
A generalized circuit diagram of LC filter is shown in figure 8. The design of an L filter depends on the ripple in current for a specific switching frequency that is present in the PWM output. The design of LC filter is more complicated compared to L filter since the placement of the resonant frequency becomes an important factor which affects the closed loop response as discussed in [15]. The allowable current ripple is once again the criteria for designing L. The capacitor C is constrained by two factors. • The resonant frequency ofthe filter elements • The bandwidth ofthe closed loop system D. Bandwidth Consideration Resonant frequency of LC filter gives the value of capacitance. This resonant frequency is determined by the bandwidth of the closed loop system. This dependency is established keeping in mind those active control methods (which are bandwidth dependent) can be used to implement loss-less resonant damping in higher order filters. Here the maximum possible bandwidth of the system is considered. The maximum bandwidth is not of practical significance but
Fig. 10. Output waveform of synchronization logic
this is a suitable assumption for a first pass iteration. The output voltage of the power converter is dependent on the grid conditions. The input current ii ofthe filter is used as feedback in the control loop. The grid current igis the controlled by varying the inverter output voltage. Hence, the transfer function of output current and input voltage of the filter for zero grid voltage decides the closed loop operation of the filter. In the proposed controller implemented at a modulator. In this regard, there are two delays in the closed loop control system which constrains the bandwidth. l. The Inverter response delay. This is the delay between command given to the inverter and the corresponding change the output of the inverter. The maximum delay is Tsw12 where, Tswis the switching time period. 2. Current data sampling and computational delay. [f the sampling frequency for the current is same as per PWM cycle, then this delay is Tsw. Here sampling frequency is double of the PWM frequency, one sample is taken on the rising half and other sample is taken at falling half of the PWM switching signal, so the delay between the two sample is Tsw12. So the system excluding the filter is essentially modelled as delaye- std , where is td= Tsw. The resonant frequency is decided such that the closed loop system as well as the LC filter provides a phase margin of minimum 45°. E. Design Procedure for An LC Filter
47Kn
Fig. 9. Synchronization logic
1. Selection of Lpubased on switching cycle ripple current consideration. (VDc)pu * D * (1- D) * 1T (II) Lpu = ----'-'--'-~-------'-[sw(pu)
*.,f3 * oirms(pu)
2. Selection of Cpubased on overall bandwidth and resonant frequency. ([2) c = 1 pu
Ctsw(pu))2*L pu
F. Synchronization Logic
In order to synchronize the inverter output with the grid a proper synchronization algorithm must be developed. The synchronization algorithm should perform accurately in the presence of harmonics noise, voltage drop and
commutation spikes. The problem of vanatIOn in grid frequency is addressed in [16]. In large power grids, the variations in line frequency are usually small, but larger variations in frequency occur in small and autonomous grids. An accurate detection of zero voltage crossing critical for the control of grid synchronized converters. Failure to meet these conditions will result poor performance and instability of converter side. The synchronizing logic is implemented shown in figure 9& their corresponding waveform is shown in figurel0 Since Micro-Controller ADCs are not tolerant of negative voltages, so we are using full-wave precision rectifier for generation of SPWM (Sine Pulse Width Modulation) with Operational amplifier as zero crossing detector. The AND output of Operational amplifier and Micro-Controller will fire the four switches of inverter (DC-AC Converter). We are achieving phase and frequency matching between grid voltage and inverter voltage with better accuracy.
Fig. 14. Sinusoidal pulse width modulation firing pulses
G. Point of Common Coupling Logic
Point of common coupling (PCC) connects inverter output terminal with grid through an isolation transformer. A short circuit impedance is also put in series between output of isolation transformer & grid terminal in order to limit short circuit current in case of fault. Point of common coupling is switched only if the conditions of power synchronization are satisfied Isolation
pee
jX
Single Phase H·bndge Inverter I---"-+--,+J
-+-_-,--+-1
Gnd
Fig. II. Point of common coupling configuration for synchronization
IV.
EXPERIMENTAL RESULTS OUTPUT VOLTAGE (6OOST CONVERTER)
00 \\
,........... ;......................... . . . . . . ;.. . . . . . . . . , . . . . . . . . . . . ., . . . . . . . ~
\";:: . '-./7K/=~==========:::::::=j
Fig. 12. Simulation result of Boost Converter
Fig. 13. Hardware result of Boost Converter
Fig. 15. Inverter output before filter
A. Boost Converter Boost converter simulation output has been shown in figure 12 and its hardware output has been shown in figure 13. It follows the simulation result presented in figure. However experimental results show 12% of voltage ripple factor in steady state. The boost converter is fed by 24 volt and average output of 58 volt has been obtained. B. H-Bridge Inverter (Before Synchronization) The inverter output without filter has been shown in figure 15. The inverter output voltage is presented with corresponding switching pulses. The switching pulses for both of the cycles of inverter output voltage has been shown in figure 14. The experimental result closely resembles the simulation result. Figure 16shows inverter output with filter and in this figure grid voltage is also superimposed and grid current is shown in pink colour. C. Inverter Voltage after Synchronization
Figure 16represents synchronized inverter voltage in connection with grid at the inverter based hardware. The light blue colour line shows current waveform of grid. As we can see average value of every half cycle is in reverse phase with grid voltage, implying reverse power flow in grid from inverter side. The current waveform can be improved using better voltage regulation at the inverter output. The charging mode of operation is shown by figure 17. The input of buck converter is 30 volts the average output voltage is 24 volts. The Buck converter acts as the battery charging unit. In the figure 17 light blue colour shows the switching pulses of buck converter, pink colour shows voltage waveform and green colour waveform shows current waveform.
[9]
Alobeidli K, Syed M. H, Moursi M. S, & Zeineldin H. H, " Novel Coordinated Voltage Control for Hybrid Micro-Grid With Islanding Capability", IEEE Transactions on Smart Grid, vol. 6, no. 3, May 2015, pp.1116-1127.
[10] Pouresmaeil E, Mehrasa M, & Catalao 1. P, "A multifunction control strategy for the stable operation of DG units in smart grids", IEEE Transactions on Smart Grid, vol. 6, no. 2, March 2015, pp. 598-607. Fig. 16. Hardware result of inverter and grid waveform after synchronization
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Fig. 17. Hardware result of buck output voltage waveform (pink), output current waveform (green), firing pulse to switch (blue).
V. CONCLUSION A cost effective experimental model of PV based microgrid connected with grid has been successfully developed. Islanded mode of operation and grid connected mode of operation has been tested and corresponding result are presented. A bidirectional energy transfer scheme has been successfully implemented using buck-boost converter. Therefore this proposed scheme has advantage of universal mode of operation as well as energy storage system. There is need of improving the voltage regulation in order to decrease total harmonic distortion in current waveform. REFERENCES
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