INTERNATIONAL CONFERENCE ON CURRENT INNOVATIONS IN ENGINEERING AND TECHNOLOGY
ISBN: 378 - 26 - 138420 - 5
POWER-QUALITY IMPROVEMENT OF GRID INTERCONNECTED WIND ENERGY SOURCE AT THE DISTRIBUTION LEVEL BHARTH KUMAR.D Dr.R.VIJAYA SANTHI PG Scholar Asst.Professor DEPARTMENT OF ELECTRICAL ENGINEERING Andhra University, Visakhapatnam, Andhra Pradesh. ABSTARCT: Utility of power electronic converters are increasingly connected in distribution systems using renewable energy resources. But due to their intermittent nature, they may pose a threat to network in terms of stability, voltage regulation and power quality issues. The present work explains the concept of grid interconnection of Wind Farm using grid interfacing inverter along with the facility of power quality improvement. In this paper it presents control strategy for achieving maximum benefits from grid-interfacing inverter when interconnected in 3-phase 4-wire distribution system is installed. For controlling the performance as a multi-function device which is active power filter functionality inverters are used, using hysteresis current control technique. With such a control, the combination of grid-interfacing inverter and the 3-phase 4-wirelinear/non-linear unbalanced load at point of common coupling appears as balanced linear load to the grid. This new control concept is demonstrated with extensive MATLAB/Simulink simulation study.
energy solution. Since the past decade, there has been an enormous interest in many countries on renewable energy for power generation. The market liberalization and government’s incentives have further accelerated the renewable energy sector growth. A renewable resource is a natural resource which can replenish with the passage of time, either through biological reproduction or other naturally recurring processes. Renewable resources are a part of Earth's natural environment and the largest components of its ecosphere. Renewable resources may be the source of power for renewable energy. Renewable energy is generally defined as energy that comes from resources which are naturally replenished on a human timescale such as sunlight, wind, rain, tides, waves and heat. Renewable energy replaces conventional fuels such as coal, nuclear, natural gas etc. in three distinct areas. The extensive use of power electronics based equipment and non-linear loads at PCC generate harmonic currents, which may deteriorate the quality of power [1], [2]. In [3] an inverter operates as active inductor at a certain frequency to absorb the harmonic current. A similar approach in which a shunt active filter acts as active conductance to damp out the harmonics in distribution network is proposed in [4]. A [5] control strategy
Index Terms—Active power filter (APF), distributed generation (DG), distribution system, grid interconnection, power quality (PQ), renewable energy.
I INTRODUCTION Electric utilities and end users of electric power are becoming increasingly concerned about meeting the growing energy demand. Seventy five percent of total global energy demand is supplied by the burning of fossil fuels. But increasing air pollution, global warming concerns, diminishing fossil fuels and their increasing cost have made it necessary to look towards renewable sources as a future
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for renewable interfacing inverter based on p-q theory is proposed.
ISBN: 378 - 26 - 138420 - 5
(small hydro, modern biomass, wind, solar, geothermal, and bio-fuels) account for another 3% and are growing rapidly. At the national level, at least 30 nations around the world already have renewable energy contributing more than 20% of energy supply. National renewable energy markets are projected to continue to grow strongly in the coming decade and beyond. Wind power, for example, is growing at the rate of 30% annually, with a worldwide installed capacity of 282,482 megawatts (MW) at the end of 2012.
Power generation: Renewable energy provides 19% of electricity generation worldwide. Renewable power generators are spread across many countries, and wind power alone already provides a significant share of electricity in some areas: for example, 14% in the U.S. state of Iowa, 40% in the northern German state of Schleswig-Holstein, and 49% in Denmark. Some countries get most of their power from renewable, including Iceland (100%), Norway (98%), Brazil (86%), Austria (62%) and Sweden (54%).
Renewable energy resources mainly can be used for power generation, heating and transportation purposes. In case of power generation, the generation can take place either as a separate unit to feed a particular area or the renewable energy resources can be interconnected to the grid at the transmission level, subtransmission level and distribution level to enhance load supplying capability of the grid. Large wind farms, concentrated solar power photovoltaic system, bio-power, hydro power, geothermal power are interconnected at the transmission and subtransmission levels. Photovoltaic system, small wind farm, hydro power and fuel cells are interconnected at the distribution level. The resources are connected to grid using grid interfacing inverter by suitable controlling of the inverter switches. But their highly intermittent nature may result in instability and power quality problems; hence an appropriate control circuitry is required.
Heating: Solar hot water makes an important contribution to renewable heat in many countries, most notably in China, which now has 70% of the global total (180 GWth). Most of these systems are installed on multi-family apartment buildings and meet a portion of the hot water needs of an estimated 50–60 million households in China. Worldwide, total installed solar water heating systems meet a portion of the water heating needs of over 70 million households. The use of biomass for heating continues to grow as well. In Sweden, national use of biomass energy has surpassed that of oil. Transport fuels: Renewable biofuels have contributed to a significant decline in oil consumption in the United States since 2006. The 93 billion liters of bio-fuels produced worldwide in 2009 displaced the equivalent of an estimated 68 billion liters of gasoline, equal to about 5% of world gasoline production.
II SYSTEM DESCRIPTION: The proposed system consists of RES connected to the dc-link of a gridinterfacing inverter as shown in Fig. 1. The voltage source inverter is a key element of a DG system as it interfaces the renewable energy source to the grid and delivers the
About 16% of global final energy consumption presently comes from renewable resources, with 10% of all energy from traditional biomass, mainly used for heating, and 3.4% from hydroelectricity. New renewable
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generated power. The RES may be a DC source or an AC source with rectifier coupled to dc-link. Usually, the fuel cell and photovoltaic energy sources generate power at variable low dc voltage, while the variable speed wind turbines generate power at variable ac voltage. Thus, the power generated from these renewable sources needs power conditioning (i.e., dc/dc or ac/dc) before connecting on dclink [6]-[8]. The dc-capacitor decouples the RES from grid and also allows independent control of converters on either side of dc-link.
ISBN: 378 - 26 - 138420 - 5
grid via the dc-link. The current injected by renewable into dc-link at voltage level can be given as
Fig. 2. DC-Link equivalent diagram.
1= Where RES.
…………………. (1) is the power generated from
The current flow on the other side of dclink can be represented as,
=
………. (2)
Where , and are total power available at grid-interfacing inverter side, active power supplied to the grid and inverter losses, respectively. If inverter losses are negligible then = B. Control of Grid Interfacing Inverter: Fig.1. Schematic of proposed renewable based distributed generation system A. DC-Link Voltage and Power Control Operation: Due to the intermittent nature of RES, the generated power is of variable nature. The dc-link plays an important role in transferring this variable power from renewable energy source to the grid. RES are represented as current sources connected to the dc-link of a gridinterfacing inverter. Fig. 2 shows the systematic representation of power transfer from the renewable energy resources to the
Fig.3. Block diagram representation of grid-interfacing inverter control.
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The control diagram of grid- interfacing inverter for a 3-phase 4-wire system is shown in Fig. 3. The fourth leg of inverter is used to compensate the neutral current of load. The main aim of proposed approach is to regulate the power at PCC during: 1) = 0; 2) PRES< total load power (PL); and 3) PRES > PL. While performing the power management operation, the inverter is actively controlled in such a way that it always draws/ supplies fundamental active power from/ to the grid. If the load connected to the PCC is non-linear or unbalanced or the combination of both, the given control approach also compensates the harmonics, unbalance, and neutral current. The duty ratio of inverter switches are varied in a power cycle such that the combination of load and inverter injected power appears as balanced resistive load to the grid. The regulation of dc-link voltage carries the information regarding the exchange of active power in between renewable source and grid. Thus the output of dc-link voltage regulator results in an active current Im. The multiplication of active current component (Im) with unity grid voltage vector templates (Ua, Ub and Uc) generates the reference grid currents (Ia*, Ib* and Ic*). The reference grid neutral current (In*) is set to zero, being the instantaneous sum of balanced grid currents. The grid synchronizing angle (θ) obtained from phase locked loop (PLL) is used to generate unity vector template.
( )
…………. (4)
= sin Ө +
…………. (5)
=
∗
( )
−
( ) …..
(6)
The output of discrete-PI regulator at nth sampling instant is expressed as
………….… (7) Where and are proportional and integral gains of dcvoltage regulator. The instantaneous values of reference three phase grid currents are computed as ……………. (8) ………….. (9) …………. (10) The neutral current, present if any, due to the loads connected to the neutral conductor should be compensated by forth leg of grid-interfacing inverter and thus should not be drawn from the grid. In other words, the reference current for the grid neutral current is considered as zero and can be expressed as
= sin(Ө)………………. (3) = sin Ө −
ISBN: 378 - 26 - 138420 - 5
and in the generated reference current signals. The difference of this filtered dclink voltage and reference dc-link voltage (Vdc*) is given to a discrete- PI regulator to maintain a constant dc-link voltage under varying generation and load conditions. The dc-link voltage error (Vdcerr (n)) at nth sampling instant is given as:
…………………..… (11) The reference grid currents (Ia* , Ib* ,Ic* and In*)are compared with actual grid currents (Ia* , Ib* ,Ic* and In*) to compute the current errors as
The actual dc-link voltage (Vdc) is sensed and passed through a second-order low pass filter (LPF) to eliminate the presence of switching ripples on the dc-link voltage
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………...…. (12)
ISBN: 378 - 26 - 138420 - 5
……………(23)
…….……… (13) ………….. (24)
……….……. (14) Similarly ……………. (15)
the
charging
currents
, and on dc bus due to the each leg of inverter can be expressed as
These current errors are given to hysteresis current controller. The hysteresis controller then generates the switching pulses (P1 to Pg ) for the gate drives of grid-interfacing inverter. The average model of 4-leg inverter can be obtained by the following state space equations
………….. (25) ………….. (26) …………… (27)
………... (16)
…………... (28)
…….….. (17)
The switching pattern of each IGBT inside inverter can be formulated On the basis of error between actual and reference current of inverter, which can be explained as:
………….. (18)
If switch
, then upper will be OFF
lower switch will be ON phase “a” leg of
…………. (19)
and in the inverter. If
, then upper switch will be ON
(20)
will be OFF “a” leg of inverter.
Where , and are the three-phase ac switching voltages generated on the output terminal of inverter. These inverter output voltages can be modeled in terms of instantaneous dc bus voltage and switching pulses of the inverter as
and lower switch in the phase
Where hb is the width of hysteresis band. On the same principle, the switching pulses for the other remaining three legs can be derived.
……….. (21)
………… (22)
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III SIMULATION RESULTS:
Fig:7 Simulation result of PQ-Grid
Fig:4 Simulation result of Grid Voltages
Fig:8 Simulation result of PQ-Load
Fig:5 Simulation result of Grid Current
Fig:9 Simulation result of Grid Voltages
Fig:6 Simulation result of unbalanced load currents
Fig:10 Simulation result of dc-link Voltages
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IV CONCLUSION
ISBN: 378 - 26 - 138420 - 5
[4] U. Borup, F. Blaabjerg, and P. N. Enjeti, “Sharing of nonlinear load in parallel-connected three-phase converters,” IEEE Trans. Ind. Appl., vol. 37, no. 6, pp. 1817–1823, Nov./Dec. 2001.
Copious literature survey has been done with fruitful discussions about controlling of grid interfacing inverter to improve power quality. From literature survey it is found that hysteresis current control technique is well suited to meet the requirements. The proposed scheme is validated by connecting the controller to the 3-phase 4-wire system. This approach eliminates the need for additional power conditioning equipment to improve the quality of power at PCC.
[5] P. Jintakosonwit, H. Fujita, H. Akagi, and S. Ogasawara, “Implementation and performance of cooperative control of shunt active filters for harmonic damping throughout a power distribution system,” IEEE Trans. Ind. Appl., vol. 39, no. 2, pp. 556–564, Mar./Apr. 2003.
Simulations have been done to validate the performance of the proposed system in MATLAB/Simulink environment. The THD in grid current with grid interfacing inverter comes out to be 1.64% for unbalance non-linear load and 1.67% for unbalanced linear load which is well within of 5 percent limits laid down in IEEE std.
[6] J. P. Pinto, R. Pregitzer, L. F. C. Monteiro, and J. L. Afonso, “3-phase 4wire shunt active power filter with renewable energy interface,” presented at the Conf. IEEE Renewable Energy & Power Quality, Seville, Spain, 2007. [7] F. Blaabjerg, R. Teodorescu, M. Liserre, and A. V. Timbus, “Overview of control and grid synchronization for distributed power generation systems,” IEEE Trans. Ind. Electron., vol. 53, no. 5, pp. 1398–1409, Oct. 2006.
V REFERENCES [1] M. Singh , K. vinod , A. Chandra, and Rajiv K.varma. “ Grid interconnection of renewable energy sources at the distribution level with power-quality improvement features,” IEEE Transactions on power delivery, vol. 26, no. 1, pp. 307-315, January 2011
[8] J. M. Carrasco, L. G. Franquelo, J. T. Bialasiewicz, E. Galván, R. C. P. Guisado, M. Á. M. Prats, J. I. León, and N. M. Alfonso, “Power electronic systems for the grid integration of renewable energy sources: A survey,” IEEE Trans. Ind. Electron., vol. 53, no. 4, pp. 1002–1016,
[2] J. M. Guerrero, L. G. de Vicuna, J. Matas, M. Castilla, and J. Miret, “A wireless controller to enhance dynamic performance of parallel inverters in distributed generation systems,” IEEE Trans. Power Electron., vol. 19, no. 5, pp. 1205–1213, Sep. 2004.
Aug. 2006. [9] B. Renders, K. De Gusseme, W. R. Ryckaert, K. Stockman, L. Vandevelde, and M. H. J. Bollen, “Distributed generation for mitigating voltage dips in low-voltage distribution grids,” IEEE Trans. Power. Del., vol. 23, no. 3, pp. 1581–1588, Jul. 2008.
[3] J. H. R. Enslin and P. J. M. Heskes, “Harmonic interaction between a large number of distributed power inverters and the distribution network,” IEEE Trans. Power Electron., vol. 19, no. 6, pp. 1586– 1593, Nov. 2004.
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