report of Improvement of the Electric Power Quality Using Series Active and Shunt Passive Filters

Improvement of the Electric Power Quality Using

Series Active and Shunt Passive Filters

REVA INSTITUTE OF TECHNOLOGY AND MANAGEMENT Page 1

CHAPTER 1

INTRODUCTION

INTRODUCTION

The increase of nonlinear loads due to the proliferation of electronic equipment causes power

quality in the power system to deteriorate. Harmonic current drawn from a supply by the

nonlinear load results in the distortion of the supply voltage waveform at the point of

common coupling (PCC) due to the source impedance. Both distorted current and voltage

may cause end-user equipment to malfunction, conductors to overheat and may reduce the

efficiency and life expectancy of the equipment connected at the PCC.

Harmonics and reactive power regulation and guidelines are upcoming issues and

increasingly being adopted in distributed power system and industries. Vital use of power

electronic appliances has made power management smart, flexible and efficient. But side by

side they are leading to power pollution due to injection of current and voltage harmonics.

Harmonic pollution creates problems in the integrated power systems. The researchers and

engineers have started giving effort to apply harmonic regulations through guidelines of IEEE

519-1992. Very soon customers have to pay and avail the facility for high performance, high

efficiency, energy saving, reliable, and compact power electronics technology. It is expected

that the continuous efforts by power electronics researchers and engineers will make it

possible to absorb the increased cost for solving the harmonic pollution. The thyristor

controlled reactors of various network configurations are widely used in industries and utility

systems for harmonic mitigation and dynamic power factor correction. These thyristor

controlled reactor operate as a variable reactance in both the inductive and capacitive

domains. By means these two parameters two types of problems are normally encountered.

The first problem is the reactive power (Var) that leads to poor power factor and the

harmonics appears due to presence of power converter devices and nonlinear loads for

example, electrics machines, fluctuating industrial loads, such as electric arc furnaces, rolling

mills, power converters etc. These types of heavy industrial loads are normally concentrated

in one plant and served from one network terminal, and therefore, can be handled best by a

local compensator connected to the same terminal.

The main emphasis of the investigation has been on compactness of configurations,

simplicity in control, reduction in rating of components, thus finally leading to saving in

overall cost. Based on these considerations, a wide range of configurations of power quality

mitigators are developed for providing a detailed exposure to the design engineer in selection

of a particular configuration for a specific application under the given constraints of economy

and the desired performance. Fig 1.1 shows a classical shunt passive filter is connected the




power system through common coupling point (PCC). Because of using non-linear load, the

load current is highly non-linear in nature. The compensating current which is the output of

the shunt passive filter is injected in PCC, by this process the harmonic cancellation take

place and current between the sources is sinusoidal in nature. The passive filter is popular in

cancellation of harmonic current in power system.

Fig.1.1 The Classical Shunt Passive Filter.

To control this process, there are two ways i.e.

(i)Harmonic extraction technique

(ii)Current modulator

1.1.2 Harmonic Extraction

The harmonic extraction is the process in which, reference current is generated by using the

distorted waveform. Many theories have been developed such as p-q theory (instantaneous

reactive power theory), d-q theory, P-I controller, adaptive controller etc. Out of these

theories more than 60% research works have been consider p-q theory and d-q theory due to

their accuracy, robustness and easy calculation.

1.1.3 Current Modulator

Current modulator mainly provides the gate pulse to the ac-dc converter. There may be many

techniques used for giving the gate signals to PWM VSI or CSI such as sinusoidal PWM,

triangular PWM etc. The above described two control techniques (harmonic extraction

technique and current modulator technique) are main research foci of many researchers in

recent years. It may be noted that either harmonics extraction technique or the current

modulator can be used individually or both at a time. Apart from these two techniques, most

of the research works are directed also in dealing with multi-level rectifier control problems.




1.2 SERIES ACTIVE POWER FILTER Series APF are operated mainly as a voltage

regulator and a harmonic isolator between the nonlinear load and the utility system. The

series connected filter protects benefit the consumer from an inadequate supply voltage

quality. This type of approach is mostly recommended for compensation of voltage

unbalances and voltage sags from the AC supply and for low applications and represents

economically attractive alternatives to utility power system, since no energy storage is energy

and the overall ratings of the components is smaller. The advantages of series active power

are- automatic compensation for varying loads, resonance free, does not affect power factor

and can be combined with passive filter network. inverter. The output of the series active

power filter is connected to the main lines through series transformers so as to make the load

voltage purely sinusoidal the harmonic voltage is absorbed or injected by the filter.

Fig.1.2 Circuit Configuration of Series active power filter (APF).




2.1 Traditionally, a passive LC power filter is used to eliminate current harmonics when it is

connected in parallel with the load. This compensation equipment has some drawbacks due to

which the passive filter cannot provide a complete solution. These disadvantages are mainly

the following.

The compensation characteristics heavily depend on the system impedance because the filter

impedance has to be smaller than the source impedance in order to eliminate source current

harmonics. Overloads can happen in the passive filter due to the circulation of harmonics

coming from nonlinear loads connected near the connection point of the passive filter. They

are not suitable for variable loads, since, on one hand, they are designed for a specific

reactive power, and on the other hand, the variation of the load impedance can detune the

filter. Series and/or parallel resonances with the rest of the system can appear.

Fig. 2.1 Series active filter and parallel passive filter.

An active power filter, APF, typically consists of a threephase pulsewidth modulation (PWM)

voltage source inverter. When this equipment is connected in series to the ac source

impedance it is possible to improve the compensation characteristics of the passive filters in

parallel connection. This topology is shown in Fig. 2.1, where the active filter is represented

by a controlled source, where Vc is the voltage that the inverter should generate to achieve

the objective of the proposed control algorithm.




Different techniques have been applied to obtain a control signal for the active filter One such

is the generation of a voltage proportional to the source current harmonics. With this control

algorithm, the elimination of series and/or parallel resonances with the rest of the system is

possible. The active filter can prevent the passive filter becoming a harmonics drain on the

close loads. Additionally, it can prevent the compensation features depending on the system

impedance. From the theoretical point of view, the ideal situation would be that the

proportionality constant, k, between the active filter output voltage and source current

harmonics, had a high value. However, at the limit this would be an infinite value and would

mean that the control objective was impossible to achieve. The chosen k value is usually

small so as to avoid high power active filters and instabilities in the system. However, the

choice of the appropriate k value is an unsolved question since it is related to the passive

filter and the source impedance values. Besides, this strategy is not suitable for use in systems

with variable loads because the passive filter reactive power is constant, and therefore, the set

compensation equipment and load has a variable power factor. In another proposed control

technique, the APF generates a voltage waveform similar to the voltage harmonics at the load

side but in opposition. This strategy only prevents the parallel passive filter depending on the

source impedance; the other limitations of the passive filter nevertheless remain.

Fig. 2.2 Transformation from the phase reference system (abc) to the 0αβ system.




Fig. 3.1. Three-phase system.

3.THE DUAL INSTANTANEOUS REACTIVE POWER THEORY

The instantaneous reactive power theory is the most widely used as a control strategy for the

APF. It is mainly applied to Fig. 3. Three-phase system. compensation equipment in parallel

connection. This theory is based on a Clarke coordinate transformation from the phase

coordinates (see Fig. 2.2).

In a three-phase system such as that presented in Fig.3, voltage and current vectors can be

defined by

The vector transformations from the phase reference system a-b-c to α-β-0 coordinates can be

obtained, thus

The instantaneous real power in the α-β-0 frame is calculated as follows:




Fig 3.2. Dual instantaneous power theory

In this case, the nonlinear load consists of an uncontrolled three-phase rectifier with an

inductance of 55 mH and a 25 resistor connected in series on the dc side. Fig. 7 shows the

phase “a” load current. The load current total harmonics distortion (THD) is 18.6% and the

power factor 0.947, when the system is not compensated. The 5th and 7th harmonics are the

most important in the current waveform. They are 16.3% and 8.4% of the fundamental

harmonic, respectively. Two LC branches were connected to mitigate the fifth and seventh

harmonics. The source current waveform with the passive filter connected is shown in Fig. 8.

The THD falls to 4.7%. The 5th and 7th harmonics decrease to 3.6% and 0.9%, respectively.




4. SIMULATION RESULTS

The system shown in Fig. 6 has been simulated in the Matlab- Simulink platform to verify the

proposed control. Each power device has been modeled using the SimPowerSystem toolbox

library. The power circuit is a three-phase system supplied by a sinusoidal balanced three-

phase 100-V source with a source inductance of 5.8 mH and a source resistance of 3.6ohm.

The inverter consists of an Insulated Gate Bipolar Transistor (IGBT) bridge. On the dc side,

two 100-V dc sources are connected. An LC filter has been included to eliminate the high

frequency components at the output of the inverter. This set is connected to the power system

by means of three single-phase transformers with a turn ratio of 1:1.

Fig. 4. Series active power and passive filter topology.

The passive filter is constituted by two LC branches tuned to the fifth and seventh harmonics.

Each element value is included in Table I.




CHAPTER 2

RESULTS AND DISCUSSIONS

Here simulation has been carried out for series active, shunt active, shunt passive, filters, by

using MATLAB SIMULINK.FFT analysis is done, THD is observed for various circuits.

Fig 5.FTT analysis of compensating voltage wave form

Fig 6: Representation of each and every harmonic




Fig 7: FFT analysis of compensating current.

. Fig 8: FFT analysis of Compensating voltage wave form at PCC




Simulation

Output waveform




CHAPTER 3

CONCLUSION & REFERENCE

CONCLUSION

A control algorithm for a hybrid power filter constituted by a series active filter and a

passive filter connected in parallel with the load is proposed. The control strategy is

based on the dual vectorial theory of electric power. The new control approach

achieves the following targets. The compensation characteristics of the hybrid

compensator do not depend on the system impedance. The set hybrid filter and load

presents a resistive behavior. This fact eliminates the risk of overload due to the

current harmonics of nonlinear loads close to the compensated system.

This compensator can be applied to loads with random power variation as it is not

affected by changes in the tuning frequency of the passive filter. Furthermore, the

reactive power variation is compensated by the active filter. Series and/or parallel

resonances with the rest of the system are avoided because compensation equipment

and load presents resistive behavior. Therefore, with the proposed control algorithm,

the active filter improves the harmonic compensation features of the passive filter and

the power factor of the load.

REFERENCES

Author(s)

1. P.Salmeron

Electric Engineering Department, Huelva Univ., Huelva, Spain

S. P. Litran

2. F. Z. Peng and D. J. Adams, “Harmonics sources and filtering approaches,in Proc.

Industry Applications Conf., Oct. 1999, vol. 1, pp. 448–455.

3. J. C. Das, “Passive filters-potentialities and limitations,” IEEE Trans. Ind. Appli., vol.

40, no. 1, pp. 232–241, Jan. 2004.

4. H. L. Ginn, III and L. S. Czarnecki, “An optimization based method for selection of

resonant harmonic filter branch parameters,” IEEE Trans.Power Del., vol. 21, no. 3,

pp. 1445–1451, Jul. 2006.

5. ieeexplore.ieee.org/iel5/61/5437451/05361356.pdf?arnumber.

6. www.ijmer.com

Feb. 2004.

http://ieeexplore.ieee.org/search/searchresult.jsp?searchWithin=%22Authors%22:.QT.P.%20Salmeron.QT.&newsearch=true

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