Me Control of a Three Phase 4 Wire PWM Inverter With Variable DC Link Voltage and Battery Storage...

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The Discrete Time Control of a Three Phase 4

Wire PWM Inverter with Variable DC Link

Voltage and Battery Storage for PVApplication

Dipl. Ing. Said El-Barbari and Prof. Dr. W. Hofmann

CHEMNITZ UNIVERSITY OF

TECHNOLOGYDepartment of Electrical Machines

and Derives

Abstract

The discrete time control of a three phase 4 wire PWM inverter with variable DC link voltage for

simultaneously supply of three phase and single phase load in transformerless stand alone

photovoltaic application with battery energy storage (BES) and LC output filter is described. The

whole system consists of a photovoltaic array, a battery energy storage, two step up boost converter

and three phase PWM voltage source inverter with a LC output Filter. The first boost converter is

controlled in such a way so that the battery will be always charged at the maximum power point

(MPP) when changes in the insulation or temperature are occurred. The second step up boost

converter is connected in series between the Battery energy storage and the DC link capacitor of the

PWM inverter. It controls the DC link capacitor voltage. The mathematical model of the linearized

system is first obtained. The discretized state space equation of the whole system is derived. A new

control method based on the dead beat control algorithm is implemented to control both the outputvoltage of the LC filter and dc link capacitor voltage so that disturbance of the output voltage due to

load unbalances is eliminated. Simulation results for various operation conditions are presented to

verify the validity of the control method.

Summary

Nowadays more attention is paid to PV system and their related technology for domestic

application as well as in large central power stations. PV systems are advantageous because they areabundant, pollution free and distributed through the earth. The only draw back is that the initial

installation cost is considerably high.
http://www.infotech.tu-chemnitz.de/~ema/staff/el-barbari.htmlhttp://www.infotech.tu-chemnitz.de/~ema/staff/hofmann.htmlhttp://www.tu-chemnitz.de/http://www.tu-chemnitz.de/http://www.infotech.tu-chemnitz.de/~ema/index.htmlhttp://www.infotech.tu-chemnitz.de/~ema/index.htmlhttp://www.infotech.tu-chemnitz.de/~ema/staff/el-barbari.htmlhttp://www.infotech.tu-chemnitz.de/~ema/staff/hofmann.htmlhttp://www.tu-chemnitz.de/http://www.tu-chemnitz.de/http://www.infotech.tu-chemnitz.de/~ema/index.htmlhttp://www.infotech.tu-chemnitz.de/~ema/index.html


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Figure 1 stand alone photovoltaic system with 3 phase 4 wire PWM voltage source inverter

Since the power generated by an array of PV panels is direct-current, it may be transformed, either

into a power with constant voltage for dc applications or into ac power. In both cases it is importantto draw as much energy as possible from the PV panel. The output power of PV generators vary

extensively with the weather conditions such as solar insulation, temperature and cloudy skies. To

obtain the maximum power from such an array under any weather condition it is necessary to

connect the PV array to a converter that can adapt itself to the changing V-I characteristic of the PV

generator (MPPT). In the system illustrated in Figure 1 this is provided by the DC/DC2. In this way

the battery will be always charged at the Maximum Power Point. The goal of the system illustrated

in figure 1 is to supply three as well as single phase loads of any art with constant amplitude

sinusoidal voltage and constant frequency. For this propose the neutral point of the LC output filter

and load is connected to the midpoint of the DC link capacitor bank. Due to load unbalances an

intruding current flows throw the impedance between the neutral point and midpoint and a voltage

drop occurs which distorts the symmetrical output voltage. To solve this problem the followingmeasurements were taken

a zero sequence current and voltage control is implemented

a DC/DC converter is used to control the DC link voltage according to load unbalances

In this way the symmetry of the output voltage is achieved and the linear region of the PWM

modulator of the DC/AC VSI (Voltage Source Inverter) is extended.
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Figure 2 principle of the control method

Since the dead beat control strategy for single phase inverter was discussed in [4], [6] and [8] and

for three phase inverters in [1], [2], [3], [5] and [7], the dead beat control in [1] and [2] is adopted

and extended to mach for three phase 4 wire VSI. The control proposed of the VSI is illustrated in

Figure 2. It contains the current minor loop, voltage major loop, the DC link reference estimation

and the DC link voltage control loop. Here only a brief description of the current and voltage as

well as dc link control is intended since to describe them in details will exceed the limits of the

summary. The control of the VSI is designed in the synchronized dq0 frame. Equation (1) and (2)

describe the discrete system in the synchronized dq0 frame.

(1)

(2)

, ,

, ,


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, , ,

,

Equation (3) and (4) describe the system in vector form

(3)

(4)

Form these equations one can see that the dand the q variables are coupled with each other whereas

the 0 sequence is decoupled. To enhance the performance of the control loop the capacitor voltage

is fed forward as seen in figure 3. To control and separately the coupling elements b and dare

decoupled by the matrixes Fdc and AIdc so that and depend only on a and c. After removing the

couplings, the dead beat controllerGIC is provided.

(5)

Figure 3 principle of the current control loop


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Since the inverter must quickly supply the load current IL to compensate the disturbance of the load,

the prediction of the load current is implemented, as shown in figure 2, so that the predicted load

current is given by ([1], [2])

(6)

In this way the target value of the load current is provided and thus the computation time delay is

compensated. The voltage major loop is constructed in the same manner. The same dead beat

control is also applied to the 0-scequence of currents and voltages except that, in the 0 control loop

no decouplings are needed.

The mathematical model of the DC/DC converter in the continuos conduction mode is established

and linearized ([9], [10]). The digital control is implemented [11] so that the DC link voltage will

follow a certain reference voltage which is given by

(7)

where is given by (8)

and K is a correction factor.


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Figure 4 inverter current and capacitor voltage of the output filter with unbalanced load

(Ru=Rv=20 ,Rw=2000 ) and uncontrolled DC link voltage

Figure 4 shows the simulation results of the dead beat control for unbalanced load with uncontrolled

dc link voltage. The distortion in the inverter currents occur when the control signals exceed the

linear region of the PWM modulator due to load unbalances. As a result, the capacitor voltage of

the output filter is also distorted. Figure 5 shows the simulation results when the DC-link voltage is

controlled. It shows the capacitor voltage of the output filter when the load changes from

(Ru=Rv=60 , Rw=2000 )to (Ru=Rv=20 , Rw=2000 ) as indicated by an arrow in the lift hand

sub diagram. This shows the high dynamic performance of the introduced dead beat control method

as the disturbance of the output voltage is quickly compensated. The distortion of the output voltage

reduces gradually as a result of the controllable dc link voltage as indicated in the right hand sub

diagram.


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Figure 5 capacitor voltage of the output filter with unbalanced load and controlled DC link

voltage
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Figure 6 DC link Voltage

Diagram 6 shows the dc link voltage when the load changes and distortion in the output voltage

occurs. The dc link voltage increases to reduce the output voltage distortion. The oscillations in the

dc link voltage are due to the filter effect in the DC link reference estimation given by equation (8).

However; the oscillation portion is relatively small compared to the dc voltage portion, so that itdoes not affect the output voltage.

References

[1] Takao Kawabata, Takeshi Miyashita and Yushin Yamamoto, "Digital Control of three-Phase

Inverter with LC Filter", IEEE Transactions on Power Electronics, Vol. 6, No. 1, January 1991, pp.

62-72.

[2] Takao Kawabata, Takeshi Miyashita and Yushin Yamamoto, "Dead Beat Control of three-

Phase PWM Inverter", IEEE Transactions on Power Electronics, Vol. 5, No. 1, January 1990, pp.

21-28.

[3] Osman Kkrer, "Deadbeat Control of a Three-Phase Inverter with an Output LC Filter", IEEE

Transactions on Power Electronics, Vol. 11, No. 1, January 1996, pp. 16-23.

[4] Chihchiang Hua and Richard G. Hoft, "Hight Performance Deadbeat Controlled PWM

Inverter using a Current Source Compensator for nonlinear loads", IEEE/PESC 23rd Anual, Toledo,

Spain 1992, pp. 443-450.

[5] Tomoki Yokoyama and Atsuo Kawamura, "Disturbance Observer Based Fully Digital

Controlled PWM Inverter for CVCF Operation", IEEE Transactions on Power Electronics, Vol. 9,

No. 5, September 1994, pp. 473-480.

[6] Atsuo Kawamura and Tomoki Yokoyama, "Comparison of Five Control Methods for Digitally

Feedback Controlled PWM Inverters", EPE Firenze 1991, Vol. 2, pp. 35-40.

[7] Youichi Ito and Shoichi Kawauchi, "Microprocessor-Based Robust Digital Control for UPS

with Three -Phase PWM Inverter", IEEE Transactions on Power Electronics, Vol. 10, No. 2, March

1995, pp. 196-204.

[8] Atsuo Kawamura, Toshimasa Haneyoshi and Richard G. Hoft, "Deadbeat Controlled PWM

Inverter with Parameter Estimation Using Only Voltage Sensor", IEEE Transactions on Power

Electronics, Vol. 3, No. 2, April 1988, pp. 118-125.

[9] P. R. K. Chetty "Current Injected Equivalent Circuit Approach to Modeling and Analysis of

Current Programmed Switching DC-DC Converters (Discontinuous Inductor Conduction Mode)",

IEEE Transactions on Industrial Applications, Vol. IA-18, No. 3, May/June 1982, pp. 295-299.

[10] Francisco Guinjoan, Javier Calvente, Alberto Poveda and Luis Martinez, "Large-Signal

Modeling and Simulation of Switching DC-DC Converter", IEEE Transactions on Power

Electronics, Vol. 12, No. 3, May 1997, pp. 485-494.


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[11] F. Al-Hosini, ABB Corporate Research, Sweden, "An Aproximate Dead-Beat Control

stratigy for the disign of functions regulators in DC/DC Converters", EPE Trondheim 1997, Vol. 3,

pp. 155-160.

Me Control of a Three Phase 4 Wire PWM Inverter With Variable DC Link Voltage and Battery Storage...

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