Power Quality Issues and its Improvement in Grid Connected RES

Power Quality Issues and its Improvement in Grid Connected RES

Linimol Ansalam A V

Assistant Professor, Department of EEE, Trinity College of Engineering, Trivandrum, Kerala.

AbstractRenewable energy systems (RES) are being increasingly connected in distribution systems utilizing power electronic converters. This paper presents novel control strategy for achieving maximum benefits from this grid-interfacing inverter using zsource converter when installed in 3-phase 4-wire distribution systems. The inverter is controlled to perform as a multi- function device by incorporating active power filter functionality. The inverter can thus be utilized as: 1) power converter to inject power generated from RES to the grid, and 2) shunt APF to compensate current unbalance, load current harmonics, load reactive power demand and load neutral current. All of these functions may be accomplished either individually or simultaneously. 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 studies and validated through digital signal processor-based laboratory experimental results.

Index Terms Active power filters (APF), distributed generation (DG), distribution system, grid interconnection, power quality (PQ), renewable energy.

1. 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 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 governments incentives have further accelerated the renewable energy sector growth.

   Renewable energy source (RES) integrated at distribution level is termed as distributed generation (DG). The utility is concerned due to the high penetration level of intermittent RES in distribution systems as it may pose a threat to network in terms of stability, voltage regulation and power-quality (PQ) issues. Therefore, the DG systems are required to comply with strict technical and regulatory

   frameworks to ensure safe, reliable and efficient operation of overall network. With the advancement in power electronics and digital control technology, the DG systems can now be actively controlled to enhance the system operation with improved PQ at PCC. However, the extensive use of power electronics based equipment and non-linear loads at PCC generate harmonic currents, which may deteriorate the quality of power.

   Generally, current controlled voltage source inverters are used to interface the intermittent RES in distributed system. Recently, a few control strategies for grid connected inverters incorporating PQ solution have been proposed. In

   [3] an inverter operates as active inductor at a certain frequency to absorb the harmonic current. But the exact calculation of network inductance in real-time is difficult and may deteriorate the control performance. 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]. In [5], a control strategy for renewable interfacing inverter based on theory is proposed. In this strategy both load and inverter current sensing is required to compensate the load current harmonics. The non-linear load current harmonics may result in voltage harmonics and can create a serious PQ problem in the power system network. Active power filters (APF) are extensively used to compensate the load current harmonics and load unbalance at distribution level. This results in an additional hardware cost. Moreover, with adequate control of grid-interfacing inverter, all the four objectives can be accomplished either individually or simultaneously. The PQ constraints at the PCC can therefore be strictly maintained within the utility standards without additional hardware cost.

   The paper is arranged as follows: Section II describes the system under consideration and the controller for grid- interfacing inverter. A digital simulation study is presented in Section III.

2. SYSTEM DESCRIPTION

   The proposed system consists of RES connected to the dc- link of a grid-interfacing 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 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 dc-link [6] [8]. The dc-capacitor decouples the RES from grid and also allows independent control of converters on either side of dc-link.

   A. DC-Link Voltage and Power Control Operation

   Due to the intermittent nature of RES, the generated power is because 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 grid- interfacing inverter. Fig. 2 shows the systematic representation of power transfer from the renewable energy resources to the grid via the dc-link. The current injected by renewable into dc-link at voltage level can be given as IDC= PRES /VDC (1)

   IDC2= (PINV+PLOSS)/VDC (2)

   The current flow on the other side of dc-link can be represented as; Where Pinv, Ploss and Pg 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 Pres = pg.

   B Control of Grid Interfacing Inverter

   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, 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. 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) Pres=0; 2) Pres<total load power; and 3) Pres>total load power. 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 uty 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

   Fig.1. Schematic of proposed renewable based distributed generation system

   Fig. 2.DC-Link equivalent diagram.

   Fig. 3. Block diagram representation of grid-interfacing inverter control

   Thus the output of dc-link voltage regulator results in an active current The multiplication of active current component with unity grid voltage vector templates (Ua,Ub andUc ) generates the reference grid currents (Ia*,Ib* andIc* ). The reference grid neutral current is set to zero, being the instantaneous sum of balanced grid currents. The grid synchronizing angle() obtained from phase locked loop (PLL) is used to generate unity vector template as

   Ua=sin() (3)

   Ub=sin(-2/3) (4)

   Uc=sin(+2 /3) (5)

   The actual dc-link voltage is sensed and passed through a first-order low pass filter (LPF) to eliminate the presence of switching ripples on the dc-link voltage and in the generated reference current signals. The difference of this filtered dc-link voltage and reference dc-link voltage is given to a discrete- PI regulator to maintain a constant dc\ link voltage under varying generation and load conditions. Vdcerr = Vdc(n)*-Vdc(n) (6)

   The output of discrete-PI regulator at th sampling instant is expressed as

   Im(n)=Im(n-1)+Kpvdc(Vdoerr(n)-Vdcerr(n- 1)+KIvdc.Vdcerr(n) (7)

   Where Kpvdc=10 and Kivdc= 0.005 are proportional and integral gains of dc-voltage regulator. The instantaneous values of reference three phase grid currents are computed as

   Ia*=Im.Ua (8)

   Ib*=Im.Ub (9)

   Ic*=Im.Uc (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

   In*=0 (11)

   Iaerr=Ia*-Ia (12)

   Iberr=Ib*-Ib (13)

   Icerr=Ic*-Ic (14)

   Inerr=In*-In (15)

   These current errors are given to hysteresis current controller. The hysteresis controller then generates the switching pulses (to) for the gate drives of grid-interfacing inverter. The average model of 4-leg inverter can be obtained by the following state space equations

   =

   (16)

   =

   (17)

   Where Vinva,Vinvb, Vinnvc and Vinvn are the

   three-phase ac switching voltages generated on the output terminal of inverter. These inverter output voltages can be model editors of instantaneous dc bus voltage and switching pulses of the inverter as

   Vinva = {(P1-P4)/2}Vdc (18)

   Vinva = {(P3-P6)/2}Vdc (19)

   Vinva = {(P5-P3)/2}Vdc (20)

   Vinva = {(P7-P8)/2}Vdc (21)

   Similarly the charging currents, and on dc bus due to the each leg of inverter can be expressed as

   Vinvd = Iinva(P1-P4) (22)

   Vinvd = Iinva(P3-P6) (23)

   Vinvd = Iinva(P5-P2) (24)

   Vinvd = Iinva(P7-P8) (25)

3. SIMULATION RESULTS

   In order to verify the proposed control approach to achieve multi-objectives for grid interfaced DG systems connected to a 3-phase 4-wire network, an extensive simulation study is carried out using MATLAB/Simulink. A 4-leg current controlled voltage source inverter is actively controlled to achieve balanced sinusoidal grid currents at unity power factor (UPF) despite of highly unbalanced nonlinear load at PCC under varying renewable generating conditions. A RES with variable output power is connected on the dc-link of grid-interfacing inverter.

   Fig.4.Grid voltage

   An unbalanced 3-phase 4-wire nonlinear load, whose unbalance, harmonics, and reactive power need to be compensated, is connected on PCC.

   Fig.5.Grid Current

   Fig.6. Inverter current

   Fig.7.Load Current

   Fig.8.PQ-Load

   Fig.9. PQ-Inverter

   Fig.10. Neutral Current (In)

4. CONCLUSION

This paper has presented a novel control of an existing grid interfacing inverter to improve the quality of power at PCC for a 3-phase 4-wireDGsystem. It has been shown that the grid-interfacing inverter can be effectively utilized for power conditioning without affecting its normal operation of real power transfer. The grid-interfacing inverter with the proposed approach can be utilized to: i) inject real power generated from RES to the grid, and/or, ii) operate as a shunt Active Power Filter (APF). This approach thus eliminates the need for additional power conditioning equipment to improve the quality of power at PCC.This approach thus eliminates the need for additional power conditioning equipment to improve the quality of power at PCC. Extensive MATLAB/Simulink simulation.

The current unbalance, current harmonics and load reactive power, due to unbalanced and non-linear load connected to the PCC, are compensated effectively such that the grid side currents are always maintained as balanced and sinusoidal at unity power factor. Moreover, the load neutral current is prevented from flowing into the grid side by compensating it locally from the fourth leg of inverter. When the power generated from RES is more than the total load power demand, the grid-interfacing inverter with the proposed control approach not only fulfills the total load active and reactive power demand (with harmonic compensation) but also delivers the excess generated sinusoidal active power to the grid at unity power factor.

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