arm microcontroller based svc for power factor correction ... microcontroller based sv… · power...

Copyright © IJIFR 2014. Subject Area: Electronics and Telecommunication Engineering Available Online at: - http://www.ijifr.com/searchjournal.aspx

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www.ijifr.com [email protected] ISSN (Online): 2347-1697

INTERNATIONAL JOURNAL OF INFORMATIVE & FUTURISTIC RESEARCH An Enlightening Online, Open Access, Refereed & Indexed International Journal of Multidisciplinary Research

Volume -1 Issue -12, August 2014

ARM Microcontroller Based SVC for

Power Factor Correction System

Abstract

Electrical distribution systems bearing large losses as the loads are wide spread,

reactive power compensation facilities and improper control of the same. The

comprehensive static VAR compensator consisting of capacitor bank in four binary

sequential steps in conjunction with a thyristor (SCR) or Traic controlled reactor of

smallest step size is employed in the investigative work. Same work deals with the

performance evaluation through analytical studies and practical implementation on an

existing system consisting of a distribution transformer of 1 PH phase 50 Hz, 1KV/230V

capacity. This paper describes development of single phase SCR based Static VAR

Compensate SVC for reactive power compensation and power factor correction using

ARM microcontroller. The ARM microcontroller determines the firing pulse for the

SCR or Traic to compensate excessive reactive components, thus withdraw PF near to

unity. The switching operations obtained are transients free and practically there is no

need to provide inrush current limiting reactors, the TCR size is minimum to providing

small percentages of harmonics, facilitates step less variation of reactive power

depending on load requirement so as maintain power factor near unity always. Keywords— ARM Microcontroller, Power Factor Correction, Reactive Power, Static VAR

Compensator (SVC) Control, Dynamic Control of Reactive Power.

1 Introduction Power factor correction (PFC) circuits were added in power systems to sinusoidal shape the AC line

current and to put it in phase with the ac line voltage. It is necessary to deals with the generation and

transmission of electrical power and consumption of electrical power has an interest in the power

factor of loads because of the dynamic behavior of industrial loads. It is well documented in literature

and through public discussions at various levels that a substantial power loss is taking place in our low

voltage distribution systems on account of poor power factor, due to limited reactive power

compensation facilities and their improper control. Power factors affect cost and efficiencies for both

the electrical power industry and the consumers in addition to the increased operating costs.

PA

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Mr. Pankaj C. Warule 1, Prof. Dr. G. U. Kharat

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1, 2 Dept. of Electronics and Telecommunication Engineering

Sharadchandra Pawar College of Engineering,

Otur, Maharashtra, India

University of Pune.

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Mr. Pankaj C. Warule, Prof. Dr. G. U. Kharat: ARM Microcontroller Based SVC for Power Factor Correction System

www.ijifr.com Email: [email protected] © IJIFR 2014 This paper is available online at - http://www.ijifr.com/searchjournal.aspx

PAPER ID: IJIFR/V1/E12/043

ISSN (Online): 2347-1697

INTERNATIONAL JOURNAL OF INFORMATIVE & FUTURISTIC RESEARCH

Volume -1 Issue -12, August 2014 Author’s Research Area: Power Factor Correction Using ARM Microcontroller

Page No.:204-212

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Therefore, power companies force their customers, especially those which are with the large loads, to

maintain power factors of the supply above a specified amount(usually 0.90 or higher) or be subject

to pay additional charges called low power factor penalty. Electricity utilities thus measure reactive

power used by large industrial customers and charge higher rates as per utilized power factor. Some

consumers install power factor correction schemes at their industry to avoid these higher costs or

penalty. Power factor correction used to adjust the power factor of an AC load or an AC power

transmission system to unity (1.00) through various methods.

There are various methods invented for power factor correction. Simplest methods include

switching in or out banks of capacitors. This method for improvement of power factor using switching

in or out capacitor banks is also called as dynamic VAR compensator or dynamic power factor

control. In this method reactive power generation is carried out through switching in or out the

capacitors to obtain a desired power factor at various load conditions hence called as Dynamic VAR

Compensator. In this system switching action is performed by relays, which are unreliable, sluggish,

require frequent maintenance and also introduce switching transients. Another method power factor

correction can be implemented using unloaded synchronous motor connected across the supply. In

this method power factor of the motor is varied by adjusting the field winding excitation and can be

made to behave like a capacitor when over excited. When we compare between these two method, we

can conclude that capacitor bank provides power factor control in discrete steps whereas synchronous

motor provides a smooth control of power factor but they are not fast enough to compensate VAR for

rapid load changes due to large time constant of their field circuit and they have much higher losses.

The above mentioned techniques for power factor correction are very simple but these techniques

having some disadvantages like dynamic VAR compensation, use of mechanical switches and relays,

not fast enough for rapid load changes and higher losses, so more so more reliable technique must be

used to correct for non-linear loads. In this paper an active power factor corrector is used for reactive

power compensation. One of the new technique for active power factor correction is SCR based active

power factor corrector to regulate the reactive power. Same method of active power factor correction

can be addressed by continuous and static VAR control, low losses, and flexibility and provides the

smooth control of flow of reactive power.

The target of this paper is to improve power factor of the supply by using static VAR

compensator. This paper introduces the design, development and implementation is more efficient and

the cost effective Active Power Factor Corrector comprising microcontroller based hardware and

compatible software which will be able to control the power factor of both linear and nonlinear load.

In this paper, Power factor correction scheme is implemented by arranging the thyristor switched

capacitor units in four binary sequential steps. This helps to introduce the reactive power variation

with the least possible resolution. In addition a thyristor controlled reactor of the minimum step size is

operated is conjunction with capacitor bank, so as to achieve continuously variable reactive power

(VAR). The enhancement transformer loading capability the shunt capacitor also improves the feeder

performance Hence reduces voltage drop in the feeder & transformer, better voltage at load end,

improves power factor, improves system security with enhanced utilization of transformer capacity,

gives scope for the additional loading and also increases overall efficiency, saves energy due to

reduced system losses, avoids low power factor penalty and reduces maximum demand charges.


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2 Power Factor

Figure 1: ARM Microcontroller

Power factor is the ratio of watts (true power) to VA (volt-amperes, also called apparent power).

Where the load is resistive only, the power factor is one, or unity, because the voltage waveform and

the current waveform are in phase. Thus, for resistive loads only, true power and VA are the same.

Where the load is reactive, the load stores energy, releasing it during a different part of the cycle. This

shifts the current waveform so that it is offset, or out of phase with the voltage waveform.

Reactive loads can be inductive (electric motors), capacitive, or non-linear (rectifier power

supplies). When the load is inductive, the inductance tends to oppose the flow of current, storing

energy then releasing it later in the cycle. The current waveform lags behind the voltage waveform.

When the load is capacitive, the opposite occurs, and the current waveform leads the voltage

waveform. Normally power system has inductive loads, therefore only lagging power factor occurs

hence capacitors are used to compensate reactive power by producing leading current to the load.










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3 Design Of Static Var Compensator There are various techniques to control reactive power but the static VAR compensators are the

most reliable ones, since they introduces high flexibility in design methodology and reasonable

response amongst fast varying environments. Static VAR compensators are may series or shunt

compensators. Series compensators deal with modification of transmission parameters of the ac

system and shunt compensators decide the load equivalent impedance. Both series & parallel type of

compensators are used to control the reactive power for power factor correction purposes in ac power

systems.

Manually switched capacitors or inductors & Synchronous condensers can be used for power factor

correction purposes, but static VAR compensators using thyristor switched- capacitors and thyristor

controlled reactors are superior, as they are characterized by fast response and high design flexibility.

Now a day, fixed capacitor-thyristor controlled reactor (FC-TCR) compensators are widely used for

power factor correction. They offer the capability of continuous reactive power control. The TCR is

operating at its full capacity in order to absorb the reactive power generated by the fixed capacitor. In

addition they draw large amounts of harmonic current components which increase transmission losses

and disturb the power system network voltage profile. Static VAR compensators using switched-

capacitor banks offer stepping responses in reactive power generation mode and their losses are

proportional to the reactive power demands. FC-TCR and switched capacitor banks static VAR

compensators are usually referred to as conventional static VAR compensators which are basically

characterized by the employment of naturally commutated solid-state switching devices having high

voltage and current ratings. Implementation of this static VAR compensators using Thyristor capable

of generating or absorbing reactive power with fast time response. The recent developments offers

high amount of flexibility in static VAR compensators design for power factor correction and voltage

control purposes.

The Static VAR Compensator regulates voltage at its terminals by controlling the amount of reactive

power injected into or absorbed from the power system. When system voltage is low then SVC

generates reactive power (SVC capacitive) and when system voltage is high, it absorbs reactive power

(SVC inductive). Here SVC is used

Fig 1TCR Based SVC

to supplying a varying amount of leading or lagging VAR to the lagging or leading system. The flow

of current through the reactor is varied using phase angle control of Thyristor. Hence the conduction

lagging current, thereby shrink the phase angle distance between the real power and apparent power.

The function of shunt power capacitor is to provide leading (capacitive) kVARs to an electrical

system when and where needed.

4 System Hardware

A. System Model

Basics Components of system consists of following blocks :

Thyristor Switched Capacitor (TSC)

Current Transformer (CT) and potential Transformer (PT)

Signal Conditioning Block For voltage & current measurement

Voltage ZCD and Current ZCD for phase shift measurement

ARM Microcontroller

Isolation and firing circuit of SCR.










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Figure 2. Block Diagram of ARM Based SVC For Power Factor Correction System

B. Phase Shift Measurement

I. Potential Transformer (PT) & Current Transformer (CT)

This is the input stage of the system it actually senses the consumed power by load. Since the

magnitude of voltage and current which can process these high level signals is difficult. So these

signals are transformed into equivalent small level signals 0-230 V range is dropped to 0-6V by

potential transformer and 0-5 A current is dropped to 0-50 mA current

II. Signal Conditioning Block:

This is the second block consist of precision rectifier. The line provided by electricity board carries

ac signals of 50 Hz. After dropping these signals to low levels the actual processing can be

accomplished by transforming this ac signal to an equivalent dc signals. General Purpose Op-amp is

used along with the diodes, resistors and capacitors. A precision rectifier & active filter is very










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accurate in this respect. The dc signal is then applied to the inbuilt ADC of ARM Microcontroller.

AN0 is used for PT and AN1 is used for CT.

III. Zero Crossing Detectors (ZCD)

ZCD block will convert sine wave signal into square wave signal using comparator. Diode

will act as clamper which convert bipolar signal to unipolar. Both the square wave signals for voltage

& current will be given to INT0 & INT1 of ARM controller. When INT0 interrupt will occur then

internal timer will start and it will stop at INT1 interrupt. Time measurement by timer will give the

respective phase shift in terms of Θ.

Figure 3. AC V & I Measurement Signal Conditioning

Figure 4. IZCD & VZCD For Phase Shift Measurement










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Figure 5. IZCD & VZCD Waveform

C. Isolation and Firing Circuit

While developing a firing circuit it is assumed that our firing circuit may handle an anti-parallel

SCR of higher current rating (up to 25A) whose gate drive requirement may be up to 0.5 mA to 1 A.

Just we have to replace anti-parallel SCR of higher rating only to handle a load of higher wattages.

Isolation is provided between power circuit and controller circuit using opto-coupler MCT2E. Trigger

pulse generated from controller applied to opto-coupler. 555 timmer is astable -multivibrator which is

driven by a PWM pulse from the part of micro controller through reset of timer. Timer converts single

pulse into multi pulse. Gate driving capability is improved using Push-Pull pair of transistor.

Capacitor bank will be selected according to the required demand for pf improvement. For respective

bank firing angle is varied to achieve pf close to unity. This provides the VAR compensation for

improving the pf.

Figure 6. Firing Circuit for Traic










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D. ARM 7 Microcontroller

ARM controller performs following steps;

1. Detect rising edge of V and start timer 0.

2. Detect rising edge of I and stop timer 0.

3. Calculate

4. If Δt ≤ 90 then

Convert Δt into degrees and store it.

Store positive sign.

5. If Δt ˃ 90 then

Calculate

Convert Δta into degrees and store.

Store negative sign.

6. Process V sample.

7. Process I sample.

8. Process Φ sample.

9. The ARM controller process each and every samples of V, I and Φ; accordingly it display the value

of V, I, phase angle (Φ), power factor

(cosΦ), sinΦ, active power (VIcosΦ), reactive power (VIsinΦ).

10. After reading the each sample of reactive power, ARM controller takes the further control action

as per Lookup Table.

5 Algorithm 1) Measure voltage current and frequency for connected load resistive or reactive.

2) Measure phase shift of I and V from ZCD nature using timer and interrupt of ARM from phase

shift calculate PF.

3) We may display active and reactive power on LCD using PF without compensation.

4) For compensation we have to add equal and opposite reactance in parallel with load.

5) By controlling firing angle of capacitor (Traic) we can compensate pf upto unity.

6) Capacitor may be 2,3,4,5,... as per requirement.

7) After compensation we may observe PF which will be improved.

8) From required pf & actual measured pf error calculation is done in software. Firing angle is

calculated to compensate pf upto unity. Control action implemented will be proportional or PI.

6 . Software Design The programming can be done in C language. C Programming makes the program

development cycle short, enables use of the modular programming approach. Readily available

modules in C compilers for embedded system & library codes that can directly port into the system

programmer codes. MPLAB software is used for compiling program.

7 Results & Conclusion

The implemented power factor corrector has been tested on variable resistor in series with fixed

inductive coil. Investigations were carried out at five load conditions by varying the values of load

resistance. It is clear that the system is able to adjust the power factor from its low initial value to an

almost unity power factor. Analysis of the waveforms for different values of load power factors










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showed that the correction of the power factor did not take any observable time. It was also observed

that the ARM microcontroller based switching did not introduce any distortions in the output

waveform while correcting the power factor. In conclusion, the proposed system is able to adjust the

power factor to a desirable value of approximately 1 and hence the system may be very useful for

power control application where the load power factor changes abruptly. It can be observed from the

waveforms there is no phase angle displacement between main input voltage and current with SVC.

As the values of the impedance is decreased i.e. variation in the value of inductance from Henry (H)

to mH, the uncorrected power factor value rises giving minimum and maximum values of 0.35 and

0.99 respectively. The value of corrected power factor is almost near to unity. The system is able to

adjust the power factor from its low initial value to an almost unity power factor. Analysis of the

waveforms for different values of impedance power factors showed that the correction of the power

factor did not take any observable time. It was also observed that the ARM microcontroller based

switching did not introduce any distortions in the output waveform.

8 References

1. “The Experimental Studies of Transient Free Digital SVC Controller with Thyristor Binary Compensator

at 125 KVA Distribution Transformer”, Dadgonda R. Patil and Uppala Gudaru, Proceedings of the World

Congress on Engineering 2012 Vol II, WCE 2012, July 4 - 6, 2012, London, U.K.

2. “Thyristorised Real Time Power Factor Correction (TRTPFC)”, Sanjay N. Patel, International Journal of

Engineering Research & Technology (IJERT) Vol. 2 Issue 3, March – 2013, ISSN: 2278-0181

3. “An Innovative Transient Free Adaptive SVC in Stepless Mode of Control”, U. Gudaru and D. R. Patil,

World Academy of Science, Engineering and Technology 53 2011.

4. “Power Factor Correction of Non-Linear Loads Employing a Single Phase Active Power Filter: Control

Strategy, Design Methodology and Experimentation”,Fabiana Pottker and Ivo Barbi, Federal University of

Santa Catarina,Department of Electrical Engineering

5. “Understanding power factor and input current harmonics in switched mode power supplies”,TDI Power,

Whitepaper-TW0062,Feb 2009,Alan Gobbi.

6. “Power Factor Correction in Audio Applications, White paper,Extron.

7. “Op-Amps and Linear Integrated Circuits”,Ramakant A. Gaykwad,Fourth Edition,Eastern Economy

Edition.




arm microcontroller based svc for power factor correction ... microcontroller based sv… · power...

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