ISSN Vol.04,Issue.19, December-2016, Pages:

Transcription

1 ISSN Vol.04,Issue.19, December-2016, Pages: Power Quality Enhancement using Distributed Generation Modular Multilevel Inverter with Active Power Control GORLE KALYANI 1, SH SURESH KUMAR BUDI 2 1 PG Scholar, Gokul Group of Institutions, AP, India. 2 Assistant Professor, Gokul Institute of Technology and Sciences, AP, India. Abstract: Due to the wide spread of power electronics equipment in modern electrical systems, the increase of the harmonics disturbance in the ac mains currents has became a major concern due to the adverse effects on all equipment. Power electronic converters are commonly used for interfacing distributed generation (DG) systems to the electrical power network. In DG systems a fast and accurate positive-sequence, fundamental grid voltage frequency and magnitude tracking is required to synchronize grid connected converter systems with the mains. System deals with a single phase inverter for DG systems requiring power quality features, such as harmonics and reactive power compensation for grid-connected operation. The idea is to integrate the DG unit functions with shunt active power filter capabilities. With this approach the inverter controls the active power flow from the renewable energy source to the grid and also performs the nonlinear load current harmonic compensation.instead of using inverter we can use Modular Multilevel Converters in the power system. The Power Quality Enhancement is very good in distribution system because of these regions: This is a Voltage Source Multi-level Converter topology mainly used in high voltage and high power applications. It is an AC to DC topology built by a series connection of (cascaded) halfbridge (or other) modules. This converter exhibits some important advantages with respect to 2- level H-bridge or 3- level (Neutral point clamped, flying capacitor etc., power converters. Keywords: Power, Electronics, Equipment, DG, Grid. I. INTRODUCTION The distributed generation (DG) concept emerged as a way to integrate different power plants, increasing the DG owner s reliability, reducing emissions, and providing additional power quality benefits [1]. Modern electrical systems, due to wide spread of power conversion units and power electronics equipments, causes an increasing harmonics disturbance in the ac mains currents. These harmonics currents causes adverse effects in power systems such as overheating, perturbation of sensitive control and communication equipment, capacitor blowing, motor vibration, excessive neutral currents, resonances with the grid and low power factor. As a result, effective harmonic reduction from the system has become important both to the utilities and to the users. The solution over passive filters for compensating the harmonic distortion and unbalance is the shunt active power filter (APF). The APF is actually an inverter that is connected at the common point of coupling to produce harmonic components which cancel the harmonic components from a group of nonlinear loads to ensure that the resulting total current drawn from the main incoming supply is sinusoidal[2]. Shunt APFs are the most commonly used topology and they are connected in parallel with the AC line. APFs have certain advantages if compared to the passive power filters. They are known to be able to adapt concurrently to changing loads, can be expanded easily and will not affect neighborhood equipments. A. Non-Linear Load Nonlinear loads are loads in which the current waveform does not take the shape of the applied voltage waveform due to a number of reasons, for example, the use of electronic switches that conduct load current only during a fraction of the power frequency period. There are many nonlinear loads drawing non sinusoidal currents from electrical power systems. These non sinusoidal currents pass through different impedances in the power systems and produce voltage harmonics. These voltage harmonics propagate in power systems and affect all of the power system components. [2, 4,5]. B. Harmonics Sources and Effects Generally every device that use power electronic components such as renewable electrical power generation systems, cyclo converters, electronic phase control loads, and pulse width modulation (PWM) drives produce harmonics [2, 5]. Among the most common nonlinear loads in power systems are all types of rectifying devices like those found in power converters, power sources, uninterruptible power supply (UPS) units, and arc devices like electric furnaces and fluorescent lamps. Even linear loads like power transformers can act nonlinear under saturation conditions. These type of non linear loads are the most popular kind which are used in most electrical devices such as mobile cell charger, LCD, PC computer and monitors, and they are used in a very large range in life [5]. The effects of harmonics in power systems and electrical loads are described as the disturbance to Electric and Electronic Devices, and higher losses [5] IJIT. All rights reserved.

2 C. Active and Reactive Power Control The integration of APF capability in single-phase inverters needs a particular attention since the control techniques were developed for three phase APFs, and consequently, must be adapted for single-phase systems. The literature presents different solutions to compute the harmonic extraction task for single-phase APFs [6] [7]. The methods are classified in direct and indirect methods in [6]. The direct methods include the Fourier transform method [8], the instantaneous reactive power (IRP) theory [9] [10] and the synchronous reference frame (SRF) theory [11][12]. On the other hand, the indirect methods include the use of enhanced phase-locked loop (EPLL) scheme or a controller such as proportional integral (PI) to find the reference current [6], [7]. Among these solutions, the IRP and the SRF theories are the most addressed ones in the literature [9] [12]. These strategies were originally proposed for threephase systems, but they can be adapted for singlephase systems due to their effectiveness. In threephase systems, both IRP and SRF techniques operate in a reference system with two orthogonal axes (αβ for IRP and dq for SRF). In singlephase systems, since only one-phase variable exists, it is necessary to create one fictitious or imaginary variable in which all frequencies are phase-shifted by 90 electrical degrees with respect to the original variable. With this procedure, a system with two orthogonal variables is created from a single variable, allowing the application of the IRP and SRF theories. However, the computation of the fictitious variable from the existing one is not a simple task since the nonlinear load current has a high harmonic content. The imaginary variables are calculated in [10] by using the Hilbert transform. This method leads to a non causal system and cannot be directly implemented. Some phase delays are introduced in the fictitious variable and implicitly in the inverter current reference [13]. Alternatively, the computation of the reference current can be performed by using a sinusoidal signal integrator (SSI) along with the IRP theory. This approach has been originally proposed by the authors in [14] and allows obtaining without any delay the fictitious variables that are needed to apply the IRP theory or other techniques originally applied for three-phase systems. Moreover, the reference current computation is insensitive to grid voltage distortions. The authors in [14] experimentally validated only the current harmonics compensation features of the strategy since the inverter has been operated only as an active filter. II. POWER QUALITY A. Introduction The paper and the technology on which it is grounded are largely motivated by the power quality issues. The term power quality is rather general concept. Broadly, it may be defined as provision of voltages and system design so that user of electric power can utilized electric energy from the distribution system successfully, without interference on interruption. Power quality is defined in the IEEE 100 Authoritative Dictionary of IEEE Standard Terms as The concept of powering and grounding electronic equipment in a manner that is suitable to the operation of that equipment and compatible with the premise wiring system and other GORLE KALYANI, SH SURESH KUMAR BUDI Volume.04, Issue No.19, December-2016, Pages: connected equipment Utilities may want to define power quality as reliability [8]. From the Power Quality market or industry perspective, it is any product or service that is supplied to users or utilities to measure, treat, remedy, educate engineers or prevent Power Quality issues, problems and related items [6,8,12,13,15,23,28]. This paper critically discusses about the power quality problems, issues and related standards, assessment of power quality issues and methods for its correction with giving a thorough knowledge of harmonics, power quality indices, parameters effecting electric power etc. Fig 1: Block Diagram of Power Quality B. Power Quality Problems & Issues A recent survey of Power Quality experts indicates that 50% of all Power Quality problems are related to grounding, ground bonds, and neutral to ground voltages, ground loops, ground current or other ground associated issues. Electrically operated or connected equipment is affected by Power Quality [9, 10,11, 12, 15, and 16]. Determining the exact problems requires sophisticated electronic test equipment. The following symptoms are indicators of Power Quality problems: Piece of equipment misoperates at the same time of day. Circuit breakers trip without being overloaded. Equipment fails during a thunderstorm. Automated systems stop for no apparent reason. Electronic systems fail or fail to operate on a frequent basis. Electronic systems work in one location but not in another location. The commonly used terms those describe the parameters of electrical power that describe or measure power quality are Voltage sags, Voltage variations, Interruptions Swells, Brownouts, Blackouts, Voltage imbalance, Distortion, Harmonics, Harmonic resonance, Interharmonics, Notching, Noise, Impulse, Spikes (Voltage), Ground noise, Common mode noise, Critical load, Crest factor, Electromagnetic compatibility, Dropout, Fault, Flicker, Ground, Raw power, Clean ground, Ground loops, Voltage fluctuations, Transient, Dirty power, Momentary interruption, Over voltage, Under voltage, Nonlinear load, THD, Triplens, Voltage dip, Voltage regulation, Blink, Oscillatory transient etc [4,6,14,18,19]. The issue of electric power quality is gaining importance because of several reasons: The society is becoming increasingly dependent on the electrical supply. A small power outage has a great economical impact on the industrial consumers. A

3 Power Quality Enhancement using Distributed Generation Modular Multilevel Inverter with Active Power Control longer interruption harms practically all operations of a modern society. New equipments are more sensitive to power quality variations. The advent of new power electronic equipment, such as variable speed drives and switched mode power supplies, has brought new disturbances into the supply system. C. Distributed Generator Inverters Distributed generation (or DG) generally refers to smallscale (typically 1 kw 50 MW) electric power generators that produce electricity at a site close to customers or that are tied to an electric distribution system. There are many reasons a customer may choose to install a distributed generator. DG can be used to generate a customer s entire electricity supply; for peak shaving :for standby or emergency generation; as a green power source; or for increased reliability. In some remote locations, DG can be less costly as it eliminates the need for expensive construction of distribution and/or transmission lines. Benefits of Distributed Generating Systems Distributed Generation: ƒ Has a lower capital cost because of the small size of the DG (although the investment cost per kva of a DG can be much higher than that of a large power plant). ƒ May reduce the need for large infrastructure construction or upgrades because the DG can be constructed at the load location. system can solve many typical problems of conventional AC network such as energy security, reduces transmission and high voltage equipment cost etc. However, a small DG has some significant problems of frequency and voltage variation when it is operated in stand-alone mode. Therefore, a small DG should be interconnected with the power system in order to maintain the frequency and the voltage. All forms of active power control in a wind turbine require a reduction in output power, which means a reduction in revenue. This is less of an issue for conventional power stations, where the lost revenue will be compensated, to some extent, by a reduction in fuel cost. Therefore, system operators and energy regulators recognise that a reduction in wind farm output should be used as a last resort. The simplest method is a cap, which means that the wind farm (or a group of wind farms) is instructed to keep its output below a certain level. A more complex version of the cap is to insist that output be kept at a fixed level (delta), below the unconstrained output available from wind. In parallel with a cap, the wind farm may also be instructed to control ramp rate, in other words to limit the rate at which the output power can increase (due to increasing wind speed, or turbines returning to service after some outage). The ramp rate is defined over periods of, for example, one minute or 10 minutes. TABLE I. Simulation Parameters 1. Introduction In general terms, Distributed Generation (DG) is any type of electrical generator or static inverter producing alternating current that (a) has the capability of parallel operation with the utility distribution system, or (b) is designed to operate separately from the utility system and can feed a load that can also be fed by the utility electrical system. A distributed generator is sometimes referred to simply as generator. Distributed generators include induction and synchronous electrical generators as well as any type of electrical inverter capable of producing A/C power. The term Distributed Generation is sometimes used interchangeably with the term Distributed Resources (DR). But DR is intended to encompass non generating technologies such as power storage devices like batteries and flywheels in addition to generators, while DG is limited to small scale (less than 20 MW) electrical generation located close to point of use. D. Active Power Control 1. Introduction The rising concern on a more efficient use of the energy is boosting the interest in expanding electric generating capacities through the use of distributed energy generation (DEG) [1-2]. The main objective of the distributed generation system connected with grid is to control the power that the inverter injects into the grid. According to the grid demands the controller also injected the reactive power. Distributed generation (DG) encompasses a wide range of prime mover technologies, such as internal combustion (IC) engines, gas turbines, micro turbines, photovoltaic, fuel cells and windpower. These distributed generators are characterized mainly by their unplanned location and by a low nominal power rating(less than 1 MW). The integrated DG along with grid This limits the network operator s demands on other forms of generation to change output rapidly. Clearly, it is not possible for wind generation to control automatically the 'negative ramp rate' if the wind drops suddenly. However, with good wind forecasting tools, it is possible to predict a reduction in wind speed in advance; the output of the wind generation can then be gradually reduced in advance of the wind speed reduction, thereby keeping the negative ramp rate at an acceptable level. On systems with relatively high wind penetration, there is often a requirement for frequency response or frequency control. This can take many forms, but the basic principle is that, when instructed, the wind farm reduces its output power by a few percent, and then adjusts it in response to the system frequency. By increasing power when frequency is low, or decreasing power when frequency is high, the wind farm can contribute to controlling the system frequency. reverse power flow modes in the ideal supply voltage conditions are analyzed using the closed loop active power control. The effectiveness of the proposed closed loop control strategy is compared with open loop control under the nonideal supply conditions at the end. The parameters used for simulations are given in Table 1. Volume.04, Issue No.19, November-2016, Pages:

4 GORLE KALYANI, SH SURESH KUMAR BUDI Fig.2. Control Block Diagram for Generation of Switching Pulses for the DG Inverter. Fig.3. Grid Voltage, Grid Currents and DC Link Voltage During Shunt Active Filter Mode of the DG Inverter TABLE II. THD of Grid Currents under Ideal Supply Conditions 2. Shunt Active Filter Mode (Pref=O).When there is no power from the RES the DG inverter operates in shunt active filter mode. The performance of the system with an unbalanced nonlinear load is shown in Fig.3.After the connection of DG at t =0.02 second, the grid currents are balanced and sinusoidal with a total harmonic distortion (THD) of 0.9, 0.89 &0.91 in phases a, b and c. The DC voltage is maintained at the reference value of 800V as shown in Fig Forward Power Flow Mode (Pret< PJ) The DG link DC voltage is assumed constant in this mode in order to evaluate the capability of the proposed control strategy for accurate power tracking. A three phase diode rectifier feeding a load of resistance of 25 n and an inductance of 15mH is connected to the PCC. The nonlinear load currents make the grid currents highly polluted. The DG inverter is connected to the grid at t=0.06 second. The nonlinear part of the load current is supplied by the DG inverter and the grid currents become sinusoidal. Since the load power is greater than the maximum power capacity Pre! of the inverter, the grid also supplies positive power to the load as shown in Fig.4 (a).the grid voltage which is exactly in phase with the grid current is shown in Fig.5 (a), which indicates an improvement in the input power factor. 5. Unbalanced and Distorted Supply An unbalanced three phase supply can be represented using positive and negative sequence components. The presence of negative sequence components in the voltage causes power control errors which cannot be addressed in an open loop power control. To evaluate the effectiveness of the proposed method, the supply voltages are modified by introducing 10% unbalance with 3% third and fifth harmonics. The performance of the inverter is analyzed using the open loop and proposed closed loop power control strategy. The supply voltage is made unbalanced and distorted at t=0.02sec. The grid currents are balanced and sinusoidal as shown in Fig.6. Figure 7 shows that the closed loop power control strategy is able to track the active power reference with zero steady state errors. With closed loop control, the ripples in the injected fundamental current of DG inverter is reduced as shown in Fig. 8. The distortion in the grid currents is also less than that of open loop control and a comparative of THD is given in Table III. 4. Reverse Power Flow Mode (Pret>PJ) The resistance of the nonlinear load is increased to 50 n in order to reduce the load power than the reference active power of the DG. Figure 4(b) indicates negative grid power, which means that the excess power from the DG is fed back to the grid during this mode. The grid currents are exactly out of phase with the grid voltage as shown in Fig. 5 (b). The THD of the grid currents are well maintained within the IEEE limits as given in Table. II. In all the three modes of operation, the reactive power demand of the load is met by the DG inverter and the reactive power supplied by the grid becomes zero as shown in Fig. 4(c). Fig.4. Grid, DG and Load Active Power in (a) Forward Power Flow Mode, (b) Reverse Power Flow Mode and (c) Reactive Power in Both Modes Volume.04, Issue No.19, December-2016, Pages:

5 Power Quality Enhancement using Distributed Generation Modular Multilevel Inverter with Active Power Control III. CONCLUSION In the Enhancement of the power quality in a grid connected distributed generation modular multilevel system. It has been shown that the DGML inverter can be effectively utilized to inject real power from the RES in the forward and reverse power flow modes and/or operate as a shunt active power filter. The proposed closed loop active power control strategy achieves accurate power tracking with zero steady state errors under ideal and non-ideal supply conditions and can be used as a control technique for integration of DG inverters to the utility grid. Instead of using inverter we implement Modular Multilevel Converters in the power system. The Power Quality Enhancement is very good in distribution system because of these regions: This is a Voltage Source Multi-level Converter topology mainly used in high voltage and high power applications. It is an AC to DC topology built by a series connection of (cascaded) halfbridge (or other) modules. This converter exhibits some important advantages with respect to 2- level H-bridge or 3level (Neutral point clamped, flying capacitor etc.,) power converters. It produces very high quality voltage/current wave forms (i.e. nearly sinusoidal voltage). It converts ac to dc (or opposite) with an efficiency greater than 99% in high power applications. The basic building block (module or cell) is rated for a fraction of the total dc-side voltage IV. REFERENCES [1]Power Quality Enhancement using Distributed Generation Inverters with Active Power Control Preetha K.P. Jayanand B. Reji P. Dept. of Electrical & Electronics Engg. Dept. of Electrical & Electronics Engg.Dept. of Electrical & Electronics Engg. Govt. Engineering College, Thrissur, Kerala, India Govt. Engineering College, Thrissur, Kerala, India Govt. Engineering College, Thrissur, Kerala, India [2]L. R. Limongi, R. Bojoi, A. Tenconi, and L. Clotea, Single-phase inverter with power quality features for distributed generation systems, in Proc. IEEE OPTIM Conf. Rec., 2008, pp generator system," Industrial Electronics, IECON th Annual Conference of IEEE, vol., no., pp , Nov [3]M. Gonzalez,V. Cardenas, and F. Pazos, DQ transformation development for single-phase systems to compensate harmonic distortion and reactive power, in Proc. IEEE CIEP Conf. Rec., 2004, pp [4]S. M. Silva, B. M. Lopes, B. J. C. Filho, R. P. Campana, and W. C. Bosventura, Performance evaluation of PLL algorithms for single-phase grid connected systems, in Proc. IEEE IAS Conf. Rec., 2004, pp [5]Bimal K. Bose, Power Electronics and Motor Drives, Elsevier Inc [6]Ali E., Abdolhosein N. and Stoyan B. Bekiarov, Uninterrutible Power Supplies and Active Filters, CRC Press LLC, [7]L. P. Kunjumuhammed and M. K. Mishra, Comparison of single phase shunt active power filter algorithms, in Proc. IEEE Power India Conf.,2006, pp [8]S. M. Silva, B. M. Lopes, B. J. C. Filho, R. P. Campana, and W. C. Bosventura, Performance evaluation of PLL algorithms for single-phase grid connected systems, in Proc. IEEE IAS Conf. Rec., 2004, pp [9]L. P. Kunjumuhammed and M. K. Mishra, A control algorithm for singlephase active power filter under non-stiff voltage source, IEEE Trans. Power Electron., vol. 21, no. 3, pp , May Volume.04, Issue No.19, November-2016, Pages: