Power Quality Improvement by Using Active Power Conditioner

Power Quality Improvement by using Active Power Conditioner

M.SivakrishnaȦ * and N.Prabhakara RaoȦ

ȦDepartment of Electrical and Electronics Engineering, V.R.Siddhartha Engineering College, Vijayawada, India

Accepted 05 November 2013, Available online 01 December 2013, Vol.3, No.5 (December 2013)

Abstract

This paper presents a three phase active power conditioner to improve power quality. The wide spread increase of non- linear loads now a days, significant amounts of harmonic currents flows through the power system impedance, causing voltage distortion at the harmonic currents frequencies .The distorted voltage waveform causes harmonic currents to be drawn by other loads connected at the point of common coupling. To overcome this drawback the active power conditioner (APC) is used. It acts as an interface between renewable energy sources and the ac bus of the microgrid. The APC inject energy in the microgrid, compensate current harmonics and correct the power factor. The control strategy allows the line current at point of common coupling (PCC) to be balanced and sinusoidal even when the load is unbalanced and the voltage at the PCC is also balanced. The APC has proved to be an important alternative to compensate current and voltage disturbances in power distribution systems.

Keywords: Active power conditioner, Current hysteresis control, Power quality, Renewable energy source.

1. Introduction The active power conditioner introduces current or voltage components which cancel the harmonic components of the 1 Switching power converters and non-linear loads with nonlinear loads or supply lines. large rated power can increase the contamination level in voltages and current waveforms. The harmonic currents 2.1 Shunt active power conditioner and voltages produced by balanced 3-phase non-linear loads such as motor drives, silicon controlled rectifiers These compensate load current harmonics by injecting (SCR), large uninterruptible power supplies (UPS). equal but opposite harmonic compensating current and it The traditional method of current Harmonics reduction acts as a current source injecting the harmonic components involves passive LC filters, which are its simplicity and generated by the load but phase shifted by 1800. low cost. These have several drawbacks such as large size, tuning and risk of resonance problems of current 2.2 Series active power conditioner harmonics. The 4-leg APC can solve problems of current harmonics, reactive power, load current balancing and It operates as a harmonic isolator between the non-linear excessive neutral current. The shunt APC based on voltage load and the utility System. This protects the consumer source inverter is solution to harmonic current problems. from an adequate voltage quality i.e. sags, voltage The APC is pulse width modulated VSI that is unbalances. connected in parallel with the load. It has the capability to inject harmonic current into the AC system with same 2.3 Series-shunt active power conditioner amplitude but opposite phase than that of the load. The principle components of the APC are the VSI, a DC This is combination of series and shunt active power energy storage device that in this case is capacitor, conditioner. Shunt APC located at load side and can be associated control circuits. The performance of an APC used to compensate the load harmonics. Series APC is at depends mainly on the technique used to compute the the source side and can acts as a harmonic blocking filter. reference currents and the control method used to inject An IGBT based Active Power Conditioner is an the desired compensation current into the line. innovative engineering solution, with specific focus on extreme energy efficiency and power quality 2. Active power conditioner enhancement. The core of the active filter system is an IGBT inverter. It has excellent dynamics of less than 200 micro seconds. The conditioner is connected in parallel to *Corresponding author M.Sivakrishna is a M.Tech student and the polluting load, thus protecting the rest of the electrical N.Prabhakara Rao is working as Asst. Prof

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infrastructure. The controller of the power conditioner Ph (t) – Instantaneous harmonic power drawn by the load. analyzes the line currents and cancels out the harmonics generated by the load in real time. 3.1 Estimation of Reference Source Current The fundamental frequency component is removed from the measured current signal. The remaining From the single line diagram shown in fig.3 waveform is then inverted and the controller reference signal drives the IGBT Bridge of the active filter. In the frequency-domain approach each harmonic and its corresponding system characteristics are treated individually and performance can be optimized for the harmonic components in the filtering bandwidth. As a result the same (high) filtering performance can be maintained through the entire filtering bandwidth.

Fig 3: Control Scheme

is (t)  il (t) ic (t) Where, i (t) , i (t) and i (t) are the instantaneous value of source s l c current, load current and the filter current. Fig 1: Active power conditioner And the utility voltage is given by

Vs (t) VmSinwt Where,

Vs (t) – is the instantaneous value of the source voltage,

Vm - is the peak value of the source voltage. If non-linear load is connected then the load current will have a fundamental component and the harmonic components which can be represented as   iL (t)   I n Sin(nwt  n )  I1Sin(wt  1 )   I n Sin(nwt  n ) n1 n2

Instantaneous load power PL (t) is PL (t) Vs (t)*iL (t)   VmSinwt(I1Sin(wt  1 )   In Sin(nwt  n )) n2   (Vm Sinwt* I1 sin(wt  1 ))  ((Vm Sinwt*  I n sin(wt  n )) Fig 2: APC Control Strategy n2

 Pf (t)  Pr (t)  Ph (t) 3. Basic compensation principle  Pf (t)  Pc (t)

For ideal compensation only the real power supplied by Fig.1 shows the basic compensation principle of shunt the source while all other components should be supplied active power filter. A voltage source inverter (VSI) is used by the active power conditioner. as the shunt active power filter. This is controlled so as to draw or supply a compensating current Ic from or to the The real power drawn from the load is utility, such that it cancels current harmonics on the AC side i.e. this active power filter (APF) generates the P (t) V Sinwt* I sin(wt  ) nonlinearities opposite to the load nonlinearities. Total f m 1 1 instantaneous power drawn by the nonlinear load can be VmSinwt*(I1Sinwt*cos  sin *Coswt) 2 represented as:- Vm I1 * Sin wt *Cos1

PL (t)  Pf (t)  Pr (t)  Ph (t) The peak current supplied by the source I  I  I I  I Cos Where, max sm sL , sm 1 n I Pf (t) - Instantaneous fundamental (real) power absorbed sL is the loss component of current drawn from the by the load, source

Pr (t) : Instantaneous reactive power drawn by the load, Compensation current is I (t)  I (t)  I (t) and c L s

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The 3 phase currents after compensation can be expressed wide between upper and lower band of hysteresis loop as determine of current ripple, the hysteresis control method * and inverter current shape is shown in figure. I a  Pf (t) /Vs (t)  I1 cos1 * Sinwt  I max Sinwt * 0 Ib  I max Sin(wt 120 ) * 0 Ic  I max Sin(wt 120 )

The peak value or the reference current I max is estimated by regulating the DC-bus capacitor voltage of the inverter using PI controller.

4. Control Strategy

4.1 Unit Current Vectors Fig 4: Hysteresis Current Controller The source currents are sensed and converted into the unit sine currents while corresponding phase angles are 4.4 PI-Controller maintained. The unit current vectors templates are represented as The Proportional Integral (PI) control scheme for the active power filter. The DC-bus capacitor voltage is 0 0 ia  Sinwt, ib  Sin(wt 120 ) , ic  Sin(wt 120 ) sensed and compared with a reference voltage and also calculating the error signal. The error signal e =VDC, ref - The amplitude of the sine current is unity in steady state VDC is used as input for PI-controller. The output of PI and the transient condition it may increase or decrease controller has been considered as peak value of the according to the loads. The unit currents should be in reference current. It is further multiplied by the unit sine phase with the source voltages. These unit currents are vectors in phase with the source voltages to obtain the multiplied with amplitude of the estimated reference reference currents. current which is used generate the desired reference The reference currents and actual currents and actual currents. currents are given to a hysteresis based, carrier less PWM converter. The difference of reference current and actual 4.2 Generation of Gating Signals currents decides the operation of switches. The status of the switches is determined according to the error. When The main concept of the control of the APC is to generate the current is increasing and the error exceeds a certain gating signals for the solid state devices of the APC based positive value, the status of the switches changes and the compensating commands variety of approaches, such as current begins to decrease until the error reaches a certain hysteresis based current control, sliding mode of current negative value. Then, the switches status changes again. control, PWM current control fuzzy based current control etc. are implemented either through hardware or software 4.5 Solar Cell to obtain the control signals for the switching devises of the APC. Basically solar cell can be represented as current source in parallel with diode and power loss can be represent with 4.3 Hysteresis Current Controller series, shunt resistors (Rs, Rsh).

The most commonly proposed time domain correction technique present upper and lower tolerance limits are compared to the extracted error signal as long as error is within the tolerance band no switching action occurs whenever the error leaves the tolerance band. In this method, inverter output current is forced to follow the current reference in each phase. Deviation between these two quantities is limited by upper and lower band in a hysteresis loop. If actual current reach upper limit of hysteresis band the inverter leg is switched off so that the current decrease till reach lower band of hysteresis Fig 5: Solar Cell loop. Where the inverter leg is switch on again and actual current increase to upper band. 5. Simulation Results The sine wave shape of reference signal causes inverter switching frequency vary and gives different To validate the proposed control algorithm many current ripples in one fundamental inverter period, gap simulations have been run in various operating conditions

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M.Sivakrishna et al International Journal of Current Engineering and Technology, Vol.3, No.5 (December 2013) using MATLAB. The investigated active power conditioner has been simulated with six IGBTs controlled by the system illustrated in Fig.2. All the parameters are shown in Table.1. During all the simulations, the power coming from the renewable energy sources is considered unvarying.

Table 1: parameters of the APC

Parameters Value

AC voltage Vabc [V] 230

DC-Link voltage (Vdc) [V] 750 Inductor (L) [mH] 3 Fig 7: Load current and source current without APC for Capacitor (C) [µF] 20000 phase A

Hysteresis Band [A] 0.5

The simulation results are grouped and presented according to the following power quality indicators: THD (Total Harmonic Distortion), power factor and unbalanced load.

Fig 8: Load current and source current and APC current for phase A.

5.2 Power factor correction

The second case study shows the ability of the APC to compensate the power factor. The power factor can be controlled with capacitor banks, but distorted conditions the results are poor and also the capacitor life is shorter. For this case study, the load is composed by a three-phase inductor in series with a three-phase resistor and requires about 3kW active power and 4kVAR reactive power. Fig.9 illustrates the source current and the supply voltage respectively at the PCC for each phase. The measured power factor between the source current and supply voltage is 0.9.

Fig 6: Simulink Diagram

5.1 Harmonic compensation

During this case study, the APC is investigated using three-phase diode bridge rectifier with a 60Ω resistor in series with a 0.1mH inductor at the DC side. Fig.7 shows the currents and supply voltage at the PCC. Most of the current required by the load and the balance comes from the source. The current absorbed by the rectifier is not sinusoidal .the frequency noise that can be observed on the APC current waveforms is due to the switching of the Fig 9 .Power factor between Source Current and voltage IGBTs. without APC

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6. Conclusion

In this paper, an APC used to improve power quality. The APC is controlled using an innovative control strategy. The performance of the system improves and the THD is reduced up to very extent. Also, it is seen from the simulation results that the source current and the source voltages are in same phase, the line current at the point of common coupling to be balanced and sinusoidal even when the load is unbalanced.

References Fig 10: Power factor between source current and voltage with APC Vechiu, Ionel; Gurguiatu, Gelu; Rosu, Emil (2010) Advanced active power conditioner to improve power 5.3 Unbalance load quality in microgrids, - IPEC, 2010 Conference Proceedings Digital Object Identifier: 10.1109/ When several single phase loads are unequally distributed IPECON.2010.5697021; Page(s): 728 -733 on a distribution system, the fluctuating power required B. Singh, K. Al-Haddad, A. Chandra (Oct 1999) A Review from each of these loads can cause unbalanced voltage in a of Active Filters for Power Quality Improvement IEEE weak power system. Under unbalanced conditions, the Transactions on Power Electronics, vol. 46, pp. 960-971. distribution system will incur more losses and heating M. Ucara and E. Ozdemir (Jan 2008) Control of a 3-phase 4- effects and will be less stable. leg active power filter under non-ideal mains voltage condition, Electric Power Systems Research, vol. 78, pp. For this case study, the APC is used to compensate the 58-73. unbalance induced by a resistive three-phase load. The M.Montero, E.R. Cadaval, F. Gonzalez, Comparison of phase a of the load is charged with 1050 W, the phase b control strategies for shunt active power filters in three- with 1311 W and the phase c with 1749 W. the Fig.9 phase four-wire systems, IEEE Transactions on Power shows the current and the voltage at the PCC. The Electronics. investigated APC is controlled such that the current M.P. Kazmierkowski, L. Malesani (Oct 1998), Current required by the load is sinusoidal and balanced. The control techniques for three-phase voltage source PWM voltage in this point is also balanced. converters: a survey, IEEE Transactions on Industrial Electronics vol. 45, pp. 691–703. Vikash Anand, Dr.S.K. Srivastava (June 2012), performance investigation of shunt active filter using hysteresis current control method, International Journal Of Engineering Research & Technology (IJERT), Vol.1 Issue 4. A.Zhikang, S. Wenji Z. Ruixiang and F. Chunming Tu (Jan 2009) Development of Hybrid Active Power Filter Based on the Adaptive Fuzzy Dividing Frequency-Control Method, IEEE Transactions on Power Delivery, vol. 24, pp. 424 – 432. B.S. Chen and G. Joos (Nov 2008), Direct Power Control of Active Filters With Averaged Switching Frequency Regulation, IEEE Transactions on Power Electronics, vol. Fig 11: Load current and source current without APC 23, pp. 2729 – 2737. for phase A B.Suresh Kumar, K.Ramesh Reddy and CV.Lalitha PI, fuzzy logic controlled shunt active power filter for three-phase four-wire systems with balanced, unbalanced and variable loads, Journal of Theoretical And Applied Information Technology. P.V. Ram Kumar, M. Surya Kalavathi (Jan 2013), Fuzzy Based Hysteresis Current Controlled Shunt Active Power Filter for Power Conditioning International Journal of Modern Engineering Research (IJMER) Vol.3, Issue.1, PP- 477-485. M. Nejad, S. Pierfederici, J.P. Martin, F. Meibody-Tabar (2007), Study of a hybrid current controller suitable for DC–DC or DC–AC Applications,IEEE Transactions on Power Electronics, vol. 22, pp. 2176–2186. Fig 12: Load current, source current and APC current for phase A

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