Implementation of Single-Phase Grid Connected Inverter Using

Implementation of Single-Phase Grid Connected Inverter Using TMS320F2812 Krismadinata Chaniago, J. Selvaraj, N.A. Rahim, Centre for Power Electronics, Drives, Automation and Control Department of Electrical Engineering, University of Malaya Malaysia Abstract- This paper describes a digital control method for single-phase grid-connected inverter system with an LCL filter for distributed generation application. Digital PI Current Controller is applied as the control method. The control algorithm is implemented in low cost Digital Signal Processor (DSP) TMS320F2812 eZdsp board. The control method provides robust current regulation and achieve unity power factor. Simulation and experimental results are provided to demonstrate the effectiveness of the design..

I.

INTRODUCTION

Distributed energy offers solutions to many of the nation's most pressing energy and electric power problems, including blackouts and brownouts, energy security concerns, power quality issues, tighter emissions standards, transmission bottlenecks, and the desire for greater control over energy costs[1]. Photovoltaic as one of distributed energies need to convert to AC source in order to connect to the grid. Inverter is used to convert DC to AC. For inverter-based distributed generation systems (DG), the inverters are connected to the existing grid; therefore, the voltage cannot be controlled. Grid-connected system does not require the battery bank and has become the primary PV application for high power applications. The main purpose of the grid-connected system is to transfer maximum solar array energy into grid with a unity power factor [2]. The strategies of current controllers can be classified as: hysteresis controllers, predictive controllers and ramp comparison controllers. The hysteresis controllers utilize some type of hysteresis in the comparison of the currents to the current reference; the predictive controllers calculate the inverter voltages required to force the measured currents to follow the current reference, while the ramp comparison controller compares the current errors to triangle wave to generate the inverter firing signals [3]. Inverter output current or grid current feedback PI control is commonly used for current-controlled inverters, since they perform satisfactorily during transient under limited operating range. Implementation in analog or digital is cheap and straightforward. This paper will present analysis and implementation of digital PI current control for single-phase grid connected inverter. The control algorithm is implemented in DSP TMS320F2812.

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II.

SINGLE-PHASE GRID CONNECTED INVERTER

Single-phase grid connected inverter is similar with single-phase inverters that are widely used in industrial applications such as induction heating, standby power supplies and uninterruptible supplies, however the output of the inverter is directly connected to grid. The inverter consists of four switching devices (represented as ideal switches) connected in the form of a bridge configuration.

S1

Lf

S2

Lg

I grid

Cf

Vdc

Vinv

Vgrid

S4

S3

Fig. 1 Single-phase grid-connected inverter

The single-phase grid connected inverter topology is shown in Fig. 1. It is composed of a dc voltage source (Vdc), four power switches (S1-S4), a filter inductor (Lf), a filter inductor grid (Lg), a filter capacitor (Cf) and utility grid (Vgrid). The switching ripple filter topology selected for the inverter filter was based on a LCL network as shown in Fig. 2

Vgrid

rg

Lg

I grid

rf

Lf

Vinv

I inv

Cf

Fig. 2 LCL Filter topology

The LCL inverter filter equations are as following:

Vinv − Vc f = r f .I inv + L f ⋅

dI inv dt

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(1)

Vc f − Vgrid = rg .I grid + Lg ⋅

(2)

dt

dVc f

and the controller becomes a PI controller. A PI controller has two adjustable or tuning parameter, Kp and Ki.

(3)

dt

Applying Laplace’s transform, the LCL inverter filter can be modeled as shown in Fig. 3. Transfer function for this filter is:

I grid Vinv

triangle wave

∆i0

1 = 3 2 s α + s β + sγ + (r1 + rg )

(4)

 1 K P 1 +  STI PI controller

  

−

1

+

I inv − I grid = C f ⋅

dI grid

0

gate drive

Anti - windup

Fig.4. PI Regulator

Where:

α = ( L f L g C f ); β = (r f Lg C f + rg L f C f ), γ = (r1rg C f + L g + L f )

Fig.4 shows PI current regulator with insertion of an antiwindup to ensure the controlled signal do not saturate. The equation describing a PI controller is as follows: t

Procedure to design LCL filter including example can be found in [4], [5] and [6].

Vinv

+−

1 rf + SL f

I inv + −

1 rg + SL g

Fig.3. LCL equivalent block diagram

In this case, 1kW inverter were designed with Vdc = 200V, m = 0.9, and an LCL filter having a resonance frequency of 1.84 kHz. L1=3mH, L2=1mH, r1=0.2Ω, r2=0.1Ω and Cf = 10µF. II.

(5)

Where: u(t) is the control signal e(t) is the error signal t is the continuous-time domain time variable τ is the calculus variable of integration Kp is the proportional mode control gain Ki is the integral mode control gain

I grid −+

∫ e(τ )dτ

τ =0

1 SC f Vgrid

u (t ) = K p e(t ) + K i

A digital PI model can be obtained from conversion of the analog model or continuous to discrete. Various methods of discretization can be seen in [4]. The purpose of the digital PI controller is that the current error e(k) can be kept to a minimum value. The current error e(k) is defined as e(k) = Iref(k) - Io(k) Where Iref(k) is the specified sinusoidal current command and Io(k) is the measured output current. A digital PI controller is used to generate the required control current Iref(k) according to e(k). As follows:

DIGITAL PI CURRENT CONTROL

u ( z ) = K p e( z ) +

In inverter-based DG, the output voltage from inverter must be higher than the Vgrid. It is required to assure power flow to grid. Since Vgrid is uncontrollable, the only way of controlling the operation of the system is by controlling the current that is following into the grid. Approximately 75% of feedback controllers in the process industry are PI controllers; most of the balances are PID controller. Some applications require only P, or PD controllers, but these are few. [7] A proportional controller has only one adjustable or tuning parameter, so this controller have no integration property and will always exhibit error at steady-state in the presence of disturbances and changes in set-point. To remove the steady-state offset in the controlled variable of the process, the integral or reset action is added

Ki e( z ) 1 − z −1

(6)

And the incremental form of the algorithm can be derived as

u (k ) = u (k − 1) + ∆u (k ) ∆u (k ) = K P ⋅ [e(k ) − e(k − 1)] + K i ⋅ e(k )

(7)

(8) Relationship between continuous-time and discrete-time domain of the PI algorithm can be expressed as: P term K p e( t ) = K p e ( k ) (9) t

I term

Ki

∫ τ

=0

k

e(τ )dτ ≅ K i ∑ i =0

h [e(i) + e(i − 1)] 2

Time relationship: t=k*h Where: h is the sampling period

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(10) (11)

k is the discrete-time index: k = 0, 1, 2, …

I ref (k )

+− I 0 (k )

e(k )

Kp

+ +

Ki '

u (k )

I 0 (t )

inverter

T Fig.5 Diagram of digital PI current controller for inverter Fig.7. Simulation result Grid voltage

For simplification purposes, it is convenient to define new controller gains as:

K i ' = Ki

h 2

(12) k

u ( k ) = K p e( k ) + K i ∑ i =0

h [e(i) + e(i − 1)] 2

(13)

To eliminate the need to calculate the full summation each time step (which would require an ever increasing amount of computation as time goes on), the summation is expressed as a running sum: sum(k ) = sum(k − 1) + e(k ) + e(k − 1) (14)

[

u (k ) = K p e(k ) + K i ' sum (k )

]

(15) Fig. 8 Waveform of injected current to grid Iload

IV.

SIMULATION RESULT

Simulations have been conducted for a single-phase gridconnected inverter using PSIM power electronic simulation software. In the simulations, DC voltage source is 200V, the grid voltage is 120V and the grid frequency is 50Hz; Fig 6 shows PWM pattern for unfiltered inverter output voltage.

Fig. 9 Simulation result Grid voltage and Grid current at unity power factor V. EXPERIMENT RESULT

Fig. 6 Simulation result of Inverter output voltage before inductor

Fig. 7 shows the waveform of Vgrid and Fig. 8 shows waveform of injected current to grid Iload. Waveform of produced current is at unity power factor with grid voltage. This can be shown in Fig. 9.

The control algorithm discussed so far has been implemented and tested experimentally on a single-phase grid connected inverter. The inverter’s parameters are equal to parameters in the simulation. Control algorithm of this inverter is implemented in fix-point TMS320F2812.Experiment setup of this digital signal processing for this inverter as shown in Fig. 10, while hardware setup can be seen in Fig.11.

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In Fig.13, and 14 shows output voltage of inverter and current that fed to grid. Fig. 15 shows the combined waveform of the current and grid voltage to show that the system is operating at near unity power factor. The Current waveform is the same phase as voltage waveform, while in Fig.16 shown a graphic harmonic distortion.

1>

Fig. 10. Block diagram of experiment setup

Fig. 13 Experiment result of Grid voltage (100V/div, 5ms/div)

2>

Fig.11. Inverter hardware setup

Fig. 14 Experiment result of Grid current (10A/div, 5ms/div)

1 >

Fig. 12 Experiment result of Inverter output voltage before inductor (100V/div, 5ms/div)

Fig. 12 shows experiment result of output inverter before fed to filter. In this figure, is shown unipolar PWM pattern. To avoid short circuit in one of inverter leg, a dead band of 4 µs is implemented.

Fig.15. Experiment result Grid voltage and current at unity power factor (100V/div,10A/div, 5ms/div)

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3.4% 3.1% 2.7% 2.4% 2.0% 1.7% 1.4% 1.0% 0.7% 0.3% 0.0%

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50

Fig.16. Harmonic magnitude as a % of the fundamentalamplitude, THD=4.82%

Generally, the experiment results are similar with the simulation result to show that the PI current control can be implemented with grid connected inverter. VI.

CONCLUSION

This paper presents a digital feedback PI current controller for single-phase grid connected inverter. The effectiveness of the control scheme has been verified both by simulation and experimentally. The current produced by this inverter is in phase with grid voltage and also achieve near unity power factor. This method is robust and effective than conventional reference signal generation by the controller and matching it with the grid voltage at later stage. ACKNOWLEDGMENT This work has been supported by United Nations Development Programme-Global Environment Facility, Pusat Tenaga Malaysia and University of Malaya, for Malaysia Building Integrated Photovoltaic Technology Application Project, The authors would like to thank all the people who are concern. REFERENCES [1]

www.eere.energy.gov/de/

[2]

Il-Song Kim,” Sliding mode controller for the single-phase gridconnected photovoltaic system”. Applied Energy 2006, Elsevier

[3]

Kojabadi, et. al, "A Novel DSP-Based Current-Controlled PWM Strategy for Single Phase Grid Connected Inverters", IEEE Trans. on Power Electronics, Vol. 21, No.4, July 2000, pp. 985-993

[4]

M. Liserre, F. Blaabjerg, and A. Dell’Aquila, “Step-by-step design procedure for a grid-connected three-phase PWM Voltage Source Converter,” Int. J. Electron., vol. 91, no. 8, pp. 445–460, Aug. 2004.

[5]

M. Bojrup, “Advanced control of active filters in a battery charger application,” Ph.D. dissertation, Lund Univ. Technology., Lund, Sweden, 1999.

[6]

M. Lindgren, “Modeling and control of voltage source converters connected to the grid,” Ph.D. dissertation, Chalmers Univ. Technol., Göteborg, Sweden, 1998.

[7]

M. Gopal, “Digital Control and State variable methods,” Second Edition, Singapore Mc Graw Hill, 2004

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