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Page 1. J SCI IND RES VOL 72 JULY 2013. 446. Journal of Scientific & Industrial Research. Vol. 72, July 2013, pp. 446-453. *Author for correspondence.

446 Journal of Scientific & Industrial Research

J SCI IND RES VOL 72 JULY 2013

Vol. 72, July 2013, pp. 446-453

Photovoltaic Based Three-Phase Three-Wire DSTATCOM to improve Power Quality V. Kamatchi Kannan1 and N. Rengarajan2 1

Department of Electrical and Electronics Engineering, K.S.R College of Engineering, K.S.R,Tiruchengode, Tamilnadu, India. 2 K.S.R. College of Engineering, K.S.R. Kalvi Nagar, Tiruchengode,Tamilnadu, India Received 20 July 2012; revised 28 December 2012; accepted 20 March 2013

Three-phase three-wire Distribution Static Compensator (DSTATCOM) with Photovoltaic (PV) array or battery operated DC-DC boost converter is proposed in this paper. The proposed DSTATCOM consists of a three-leg Voltage Source Converter (VSC) with a DC bus capacitor and it provides continuous reactive power compensation, source harmonic reduction and load compensation throughout the day. With the help of PV array or battery, which is connected to the dc link of VSC via the DC-DC boost converter is used to maintain the desired voltage to the dc bus capacitor for continuous compensation to the load. The Icos F controlling algorithm is proposed for three-phase three-wire DSTATCOM. In this algorithm, the fuzzy logic controller is compared with the conventional PI (Proportional Integral) controller at DC bus to regulate the DC link capacitor voltage. The fuzzy controller is used to maintain the DC link voltage to the reference value. The switching of VSC will occur by Hysteresis based Pulse Width Modulation (PWM) current controller. The simulations are carried out by using MATLAB/simulink software to demonstrate the effectiveness of the proposed scheme. Keywords: Distribution Static Compensator, Photovoltaic Array,Boost Converter, Voltage Source Converter, Icos F Controlling Algorithm, Fuzzy Logic Controller

Introduction Electric utilities and end users of electric power are becoming increasingly concerned about meeting the growing energy demand. Power quality issues are gaining significant attention due to the increase in the number of sensitive loads. The electrical devices like electric motors, transformer s, generators, computer, printer, communication equipment or a house hold appliances. All of these devices and others react adversely to power quality issues, depending on the severity of problems. The power quality problems are mostly related to the source of supply and types of load1. One of the most severe problems of power quality is harmonic distortion and its consequences. Hence, the extensive use of power electronics based equipment and nonlinear loads are creating a growing concern for harmonic distortion in the AC power system. According to the IEEE standard, harmonics in the power system should be limited by two different methods; one is the limit of harmonic current that a user can inject into the utility system at the Point of Common Coupling (PCC) and the other is the limit of harmonic voltage that the utility can supply to any *Author for correspondence E-mail: [email protected]

customer at the PCC2-3. Hence, the current harmonics are often treated as a local problem at least for one feeder in the distribution network. The impedance of the distribution network dampens the harmonic propagation. Therefore, harmonic filtering should be performed nearby to the source of the current harmonics for the best results. If this is done, other equipment will be unaffected by the harmonic producing load. Many researchers have focused on renewable energy source based power quality improvement in the power distribution system 4-5. Custom Power Devices (CPD) has been adopted for the purpose of improving power quality and reliability. Custom power devices like DSTATCOM (Shunt active power filter), DVR (Series active power filter)and UPQC (Combination of series and shunt active power filter) are used for compensating the power quality problems in the current, voltage and both current and voltage respectively 6-7. Among these new devices, special attention has been given to the equipment based on the voltage source converter technology. A representative example of such devices is the Distribution Static Compensator (DSTATCOM) is extensively used to compensate reactive power compensation, source current harmonic reduction and load compensation at distribution level8-9.

KANNAN & RENGARAJAN: PHOTOVOLTAIC BASED THREE PHASE DSTATCOM

447

Fig. 1—Circuit diagram of proposed DSTATCOM

The DSTATCOM controller continuously monitors the load voltages and currents and determines the amount of compensation required by the AC system for a variety of disturbances. The different topologies of DSTATCOM are reported in the literature such as a 4-leg VSC (Voltage Source Converter), three single phases VSC and 3-leg VSC with split capacitor DSTATCOM 10-11 etc. The proposed DSTATCOM consists of three-leg VSC with a dc bus capacitor. For controlling the DSTATCOM and to generate the reference currents, there are number of controllers reported in the literature survey such as instantaneous reactive power theory, adaptive neural network, synchronous frame theory and power balance theory12-18. All the above mentioned algorithms have slow response. Here, the Icos F controlling algorithm is proposed for generating the reference currents 19-20 . Since, after tracking the reference currents with the help of controller and by comparing it with source currents, the switching of VSC will occur with the help of a

hysteresis based current controller and hence cancel out the disturbances caused by the nonlinear loads. The main aim of this paper is to maintain the dc link voltage of the three-leg VSC to provide continuous compensation. The photovoltaic (PV) array is used to drive the boost converter to step-up the voltage and maintain the dc link voltage. When continuous compensation is required, the PV array is connected to the boost converter in the day time and during the night time battery acts as a dc source for the boost converter. When excess power is available or compensation is not required the PV array charges the battery. The boost converter presented in this paper utilizes a pulse width modulation technique. By using this technique the boost converter draws constant power from the source. This paper does not discuss the maximum power point algorithm. Generally, PI controller is used to control the dc bus voltage of the VSC. The linear mathematical model of the PI controller is difficult to obtain and also

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fails to perform satisfactorily under nonlinearities and load disturbances. So in this paper fuzzy logic controller is proposed for dc bus voltage control. The proposed system is simulated under MATLAB environment using SIMULINK and simpower system tool boxes to demonstrate the effectiveness of this approach for further reducing the harmonics and to maintain the dc bus voltage.

module. The solar irradiance (G) and temperature (T) were taken as standard test conditions which are 1000 Watt/m2 and 250C respectively. The proposed DSTATCOM has been divided into three operating modes. The modes are (i) Day time excess power mode, (ii) Day time mode (iii) Night time mode. Day time excess power mode

Description of the proposed system Fig. 1 shows the basic circuit diagram of the threephase three-wire system which is used to feed the nonlinear load continuously. The nature of the nonlinear load is to cause distortion in the current. After connecting the nonlinear load, suddenly there will be a distortion in the distribution system. In order to eliminate these distortions, the control of DSTATCOM is achieved by using the Icos F algorithm. The load currents, source voltages and the dc bus voltage are given as an input to the IcosΦ controlling algorithm. The controlling algorithm is used to generate the reference currents. Then the Hysteresis based PWM current controller compares the reference and source currents and gives a switching pulse to the DSTATCOM. The DSTATCOM consists of six Insulated Gate Bipolar Transistor ’s (IGBT) with antiparallel diode based three-leg VSC connected in shunt with the dc bus capacitor. The PV module with the DCDC boost converter is connected with the dc bus capacitor, which is used to give a desired voltage across the capacitor for continuous compensation. According to the gate pulse given, the switching of VSC will occur which injects a currents at the PCC through the interface inductor Lr. PV system model Photovoltaic (PV) is one of the major power sources, becoming more affordable and reliable than utilities 21-22. Photovoltaic is the method of converting solar radiation into direct current electricity which generates an electric power by using semiconductors that exhibit the photovoltaic effect. PV module is a connected assembly of photovoltaic cells. Hence, it will be connected in parallel to produce high current and in series to produce high voltage. It consists of a current source in parallel with a diode which represents the nonlinear impedance of the p-n junction and also a small series and a high parallel intrinsic resistance.To model the PV module in MATLAB-SIMULINK, the parameters are obtained from SHANSHAN ULICAUL-175D photovoltaic

In this mode, the output voltage of the PV array drives the boost converter based DSTATCOM for compensating the source as well as charges the 35V battery. Day time mode

When continuous compensation is required, if the PV output voltage is equal to the requirement of the boost converter input, the PV array can directly connect to the boost converter so as to step-up the voltage and match the dc link voltage of the three-leg VSC. In this mode, the battery is not charged. Night time mode

In this mode, PV output is absent and only the battery supplies the boost converter for providing compensation at the night time. Boost converter to control dc capacitor voltage Boost converter is also called as step-up converter in which the output DC voltage is always greater than its input voltage. The dc boost converter with PV or battery unit is connected to maintain the dc link voltage of the proposed DSTATCOM. It consists of two semiconductor switches and one storage element 23-25. The operation of the boost converter circuit is as follows: When the switch is closed, the inductor gets charged by the PV or battery and stores the energy. The diode blocks the current flowing, so that the load current remains constant which is being supplied due to the discharging of the capacitor. When the switch is open the diode conducts and the energy stored in the inductor discharges and charges the capacitor. Therefore, the load current remains constant throughout the operation23. The boost converter is used to maintain the constant output voltage for all the conditions of temperature and variations in solar irradiance. The 35V of PV or battery voltage is given as an input to the boost converter to get the output voltage of 670V. The switching frequency is chosen to 25 KHz. The inductor and capacitor used in the boost converter is 0.0191 mH and 7000 mF.

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vabc

sin Φ, cos Φ

vdc

iLa

vdcr

iLb

Amplitude Amplitude

PLL

iLc

vtr

vt

Computation of in-phase and quadrature component of load current

vte

vdce Fuzzy Controller

ILa cos Φa

ILa sin Φa

ILb cos Φb

ILc sin Φc

ILc cos Φc

ismd

+

PI Controller

ILb sin Φb

+

+

+

+

1/3

ismq

+

1/3

vabc + + Isd

+ X

X

i* sabcd isa isb isc

-

Unit template generator

uabc

wabc

Isq i*sabcq

i*sabc PWM current generator

Gate pulse to DATATCOM Fig. 2—Block diagram of Icos F controlling algorithm

Control strategy of DSTATCOM The block diagram of IcosΦ controlling algorithm is shown in Fig.2. The source currents(isa, isb and isc) the load currents (iLa, iLb, iLc) the ac terminal voltages (va,vb,vc) and the dc bus voltage (vdc) are sensed and used to extract the reference currents 20, 26. The IcosΦ controlling algorithm is used to generate only the active component of the load currents i.e. Icos F (where I = amplitude of fundamental load current and F = displacement angle of load current). Hence by combining the in-phase and quadrature component, the reference current can be generated. In-Phase Component of Reference Source Currents

The amplitude of active power component of fundamental load current is extracted at zero crossing of the unit template in-phase with PCC voltages20. For a balanced source current, the magnitude of active component of reference source currents can be given as,

Isd = {

| ILa1| cosΦa1+ | ILb1| cosΦb1+ | ILc1| cosΦc1 }+ Ismd 3

… (1) Where, Ismd =output of the dc bus voltage fuzzy logic controller of the VSC The error in dc bus voltage of VSC at nth sampling instant is given as, vdce(n) = vdcr(n) - vdc(n)

… (2)

Where, Vdcr (n) =reference dc bus voltage Vdc (n) = dc link voltage of the VSC To maintain the dc bus voltage a Fuzzy Logic Control (FLC) is implemented27. It consists of two input variable and one output variable. The two inputs to the FLC are error voltage e and the rate of change of error voltage

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Δe. This error voltage is generated by sensed dc bus voltage and compared with the reference voltage.The FLC is used to convert the crisp input variables into linguistic variables using the following five fuzzy sets, which are: NB (negative big), NS (negative small), ZE (zero), PS (positive small) and PB (positive big). The output variable of the fuzzy controller is Ismd. The rule table which is shown in Table 1 contains 25 rules. Here the fuzzy controller is compared with conventional PI controller and the performances of both the controllers are investigated.By using the amplitude of the threephase voltage and unit vector in phase with va, vb and vc the in-phase component of reference source currents are estimated as,

… (3)

i*sad= Isdua; i*sbd= Isdub; i*scd= Isduc

Quadrature Component of Reference Source Currents

The unit vectors (wa, wb and wc) in quadrature with (va, vb and vc) can be calculated using the in-phase unit vectors(ua, ub and uc). The amplitude of reactive power component of fundamental load current is extracted at zero crossing of the unit template quadrature phase of PCC voltages. Table 1—Fuzzy control rule table NB NS ZE PS PB

NB NB NB NB NS ZE

NS NB NB NS ZE PS

ZE NB NS ZE PS PB

PS NS ZE PS PB PB

PB ZE PS PB PB PB

ìï | I | sinΦa1+ | ILb1| sinΦb1+ | ILc1| sinΦc1üï I = í- La1 ý + Ismq sq ï 3 ïþ î

…(4) Where, Ismq = output of the ac terminal voltage PI controller of the VSC The output of the PI controller for maintaining the amplitude of ac terminal voltage at the sampling instant is given as, Ismq(n) = Ismq (n-1) + Kpa {vte(n)-vte(n-1)} + Kiavte(n)

vte (n) and vte (n-1) = error in amplitude of ac terminal voltage at nth and (n-1)th sampling instant Kpa and Kia = proportional and integral gain constants of the ac terminal voltage vte(n) and vte(n-1) = voltage errors in nth and (n-1)th instant The quadrature component of reference source currents are estimated as, i*saq = Isqwa; i*sbq = Isqwb; i*scq = Isqwc ...(6)

30 20 10 0 -10 -20 -30 -40 0.04

0.05

0.06

0.07

0.08

…(5)

Where,

40

Source current (A)

De/ e

For balanced source currents, the magnitude of reactive component of reference source currents can be given as,

0.09 0.1 0.11 Time (s) Fig. 3—Source current waveform without compensation

0.12

0.13

0.14

KANNAN & RENGARAJAN: PHOTOVOLTAIC BASED THREE PHASE DSTATCOM

Injected current (A)

40 20 0 -20 -40 0.04

0.06

0.08

Time (s)

0.1

0.12

0.14

Source current (A)

50

0

Source current (A)

-50 0.04

0.06

0.08

0.1 0.12 Time (s) Fig. 4—Injected current and source current waveform

0.14

40 20 0 -2 0 -4 0 0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

35

40

100 80 60 40 20

Source current (A)

Mag (% of Fundamental)

Time (s) Fundamental (50 Hz)= 29.67, THD= 23.98 %

0 40 20 0 -2 0 -4 0 0.04

5

0.06

10

0.08

15 20 Harmonic order (a)

0.1

0.12

25

0.14

30

0.16

0.18

0.2

30

35

40

Mag (% of Fundamental)

Time (s) Fundamental (50 Hz)= 31.2, THD= 1.43 % 100 80 60 40 20 0 0

5

10

15 20 Harmonic order (b)

25

Fig. 5—Current harmonics and its THD waveform (a) without compensation (b) with compensation

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Table 2—Comparison of THD values of DSTATCOM before and after compensation After compensation Phases Phase A Phase B Phase C

Before compensation 23.98 23.98 23.98

IcosΦ using PI controller 2.28 2.24 2.22

IcosΦ using fuzzy controller 1.43 1.38 1.39

Fig. 6—DC bus capacitor voltage Reference Source Currents

The reference source currents can be extracted by the sum of in-phase and quadrature components of the reference source current. Then the reference source currents are compared with the source currents in hysteresis based PWM current controller for generating gate signals for IGBT switches of VSC based DSTATCOM. Simulation results The three-phase three-wire distribution system is assumed to be connected permanently to the bridge rectifier nonlinear load. Due to the existence of this nonlinear load, the three-phase distribution system source current is highly distorted. The analysis of the proposed boost converter operated DSTATCOM for a three-phase three-wire system has been done using MATLAB software using SIMULINK and Power System Blockset (PSB) toolboxes. The simulation waveforms for fuzzy controller operated IcosΦ controlling algorithm are follows. The resulting distorted three phase source current waveform is shown in Fig. 3 for the simulation time interval between 0.04 second and 0.14 second. The obtained THD for all the three phases are same and it is 23.98%. The highly distorted source currents can be made sinusoidal when the proposed DSTATCOM system

injects three phase current with appropriate amplitude. The injected current of the proposed DSTATCOM for all the three phases and the obtained three phase source current waveform after compensation is shown in Fig. 4. The phase A source current waveform with its harmonic spectrum for without compensation and with compensation is shown in Fig. 5. It is observed from the waveform that the phase A source current THD after compensation is reduced from 23.98% to 1.43%. The obtained THD for phase B source current is reduced from 23.98% to 1.38%. Similarly, the THD obtained for phase C is also reduced from 23.98% to 1.39%. The comparison of THD values of Icos F with PI and with fuzzy controller is shown in Table 2.The distorted current caused by the nonlinear load is compensated and the source current was made sinusoidal. The DC bus capacitor voltage waveform for IcosΦ with PI and IcosΦ with fuzzy controller is shown in Fig. 6.The obtained THD values for all the three phases are less than 5 % which meets the requirements of the IEEE-519-1992 standards. The main drawback of the proposed scheme is that, the solar irradiation level for simulation is considered to be constant at 1000 w/m2 throughout the day, which is not true in practice. Hence, the PV output voltage varies with change in irradiation level which affects the output voltage of the dc-dc boost converter. Therefore, the

KANNAN & RENGARAJAN: PHOTOVOLTAIC BASED THREE PHASE DSTATCOM

dc-link voltage is not maintained constant which influences the operation of continuous compensation.

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Conclusion The simulation of the Photovoltaic (PV) array or battery operated DC-DC boost converter fed three-leg VSC based DSTATCOM has been carried out for reactive power compensation, source harmonic reduction and load current compensation in the distribution system. The DSTATCOM was controlled by Icos F algorithm in which fuzzy controller is employed at dc bus voltage. The purpose of boost converter is to step up the voltage so as to match the dc link voltage of the three-leg VSC based DSTATCOM for continuous compensation. In IcosΦ algorithm, the PI controller is compared with fuzzy controller at dc bus voltage, the Icos F with fuzzy controller has a less distortion in the source current waveform and hence the THD value has been further reduced. The THD value is below the permissible limit of 5% (IEEE-519-1992). The MATLAB software with its simulink and Power System Block set (PSB) toolboxes has been used to validate the proposed system. Appendix AC line voltage: 415 V, 50Hz Non-linear load: Three phase bridge rectifier with R = 20W AC inductor: 2.5 mH DC bus capacitance of DSTATCOM, Cdc: 7000 μF DC bus voltage of DSTATCOM: 670 V DC voltage PI controller: Kpd = 0.1, Kid = 1 PCC voltage PI controller: Kpq =0.1, Kiq =1 References 1 2 3 4

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