Enhancement of Distributed Generation by Using Custom Power Device

Kishor V. Bhadane, M. S. Ballal, and R. M. Moharil

Enhancement of Distributed Generation by Using Custom Power Device

Kishor V. Bhadane, M. S. Ballal, and R. M. Moharil

—Wind energy (WE) has become immensely popular for distributed generation (DG). This case presents the monitoring, modeling, control, and analysis of the two-level three-phase WE based DG system where the electric grid interfacing custom power device (CPD) is controlled to perform the smart exchanging of electric power as per the Indian grid code. WE is connected to DC link of CPD for the grid integration purpose. The CPD based distributed static compensator, i.e. the distributed static synchronous compensator (DSTATCOM), is utilized for injecting the wind power to the point of common coupling (PCC) and also acts against the reactive power demand. The novel indirect current control scheme of DSTATCOM regulates the power import and export between the WE and the electric grid system. It also acts as a compensator and performs both the key features simultaneously. Hence, the penetration of additional generated WE power to the grid is increased by 20% to 25%. The burden of reactive power compensation from grid is reduced by DSTATCOM. The modeling and simulation are done in MATLAB. The results are validated and verified.

Index Terms—Custom power device, distributed generation (DG), distributed static synchronous compensator (DSTATCOM), indirect current control scheme, reactive power.

1. Introduction

The conventional sources for power generation are fossil fuels, nuclear energy, hydro, etc. Due to the use of these energy sources, the environment has been seriously affected. And the fossil fuels cost will increase evidently, which will be exhausted in near future[1]. Due to technology innovation and cost reduction, renewable wind energy is enjoying a rapid growth globally to become an important green electricity source to replace the fossil fuels that are polluting and trending to be exhausted[2]. The integration of distributed energy sources is expected to increase significantly in the near future[3],[4]. Grid-connected renewable wind-photovoltaic systems increased significantly in the last years and are expected to grow significantly until 2015 if governments keep the actual incentives[5]. The integration of embedded power generation systems to existing power systems influences the power quality, and causes voltage quality, over-voltage, reactive power, and safety issues. The widely popular generation resources are wind and the photovoltaic. This can be operated in isolated or grid-connected mode depending upon the requirements[6]. Due to technical issues and uncertainties in power generation from wind and PV systems, these resources need to be integrated along with energy storing devices like batteries, super power capacitors, etc. to enhance the power quality and reliability of the power supply[7],[8]. The efficient and proper operation of a wind energy system depends upon many factors that include the variable wind velocity, power fluctuations, integration to grid challenges, power quality issues, different types of wind turbines, level of penetration of wind power to grid, etc.[9]-[11]. Renewable energy sources are rapidly gaining popularity for sustainable power generation: less polluting and using readily available resources[12]. The development and control of custom power device (CPD) i.e. three level inverter based DSTATCOM was given in [13]. Due to the penetration of renewable energy, the poor power quality arises and these power quality problems have a bad effect on electric systems connected together[14]. A comprehensive literature review of distributed static synchronous compensator (DSTATCOM) and other flexible AC transmission system (FACTS) controllers along with the reactive power generation capability of generators suggests that the present studies and methods are focused on the integration of large distributed energy resources (DER) units within distribution networks[15]-[18]. The DSTATCOM is a superior and faster device for exchanging the reactive power between the WE and grid systems.

The objective of this paper is to present the smart DG by using CPD based DSTATCOM indirect current control scheme. The proposed scheme is created for the utilizationof more renewable energy and contributes for solving the load shedding problem. The motivation of this work is to provide the continuous power in rural areas. It can increase the efficiency and decrease the cost of the two-level three-phase DSTATCOM based WE DG system. The proposed system consists of the hardware of WE connected to DC voltage DSTATCOM. It can supply the real power from WE based DG to the grid system. Under various linear and non-linear loads, it can act as a compensator against reactive power and voltage unbalance conditions. DSTATCOM is the strong mediator of the WE based DG and electric grid system. The penetration of additional generated WE power to the grid is increased by 20% to 25% and the burden of reactive power compensation from the grid is reduced by DSTATCOM.

2. Hardware Renewable Model

The requirement of load demand of domestic consumers is moderated and here light loads are considered such as the resistive lamp load, inductive load of chock coils, capacitive load, LED lighting, etc. Hence, the prototype hardware model is considered according to the wind-solar and load capacities. The model contains the wind-solar system. The wind turbine containing the 4-pole permanent magnet synchronous generator (PMSG) with the capacity of 500 W is specially designed and developed. It can be operated with the cut-in wind speed of 1.5 m/s and the wind speed up to 8.5 m/s. In the north Maharashtra with Jalgaon region, the average wind speed is 2 m/s to 5 m/s. As per the existing wind status, the wind turbine is developed. Similarly, the solar system is designed and developed. Two solar plates are used, each with the 75 W capacity, and the solar total capacity is 150 W. So the overall total capacity of the wind-solar system is 750 W.

The exchanges of electric power, such as to and from the micro grid, to and from the batteries and super capacitor, and to the load, etc. are taken care by DSTATCOM. Both the wind power and solar power are given to the hardware testing kit for testing the power quality at the generation, grid, and load sides. The interface of DSTATCOM is done by using MATLAB.

3. Proposed Model by Using Indirect Current Control Scheme of DSTATCOM

The modeling is done by using instantaneous power theory, by using the Clarke transformation, where Vris r phase voltage, Vyis y phase voltage, and Vbis b phase voltage:

The space vector (sV) is

Fig. 1 indicates the phasor diagram of α-β model.

Fig. 1. Phasor diagram of α-β model.

Resolve Vson d-q,

Resolve sVon α-β,

Put (14) and (15) in (12) and (13):

The 3φ active power is given by,

The 3φ reactive power is given by,

The reactive power is flowing from the higher magnitude of voltage to the lower (i.e. V1＞V2).

The active power is flowing from δ1＞δ2(i.e. the higher δ1to the lower δ2). Fig. 2 indicates the phasor diagram of the d-q model.

Fig. 2. Phasor diagram of d-q model.

4. Role of CPD Based DSTATCOM by Using Indirect Current Control Scheme

A DSTATCOM is a shunt-connected reactive power compensation device that is capable of generating and/or absorbing reactive power and in which the output can be varied to control the specific parameters of an electric power system[19]. One of the biggest advantages of DSTATCOM is compensating current so that it is not dependent on the voltage level at the connection point, that is, the compensating current is not lowered as the voltage drops[20]. The reactive power demand and unbalances seriously affect the wind power system. So the reactive power compensation is required to maintain normal voltage levels in the power system[21], which can be realized by the reactive power compensation device, i.e. DSTATCOM.

The detail model of the indirect current control scheme of DSTATCOM is given in Fig. 3.

The 3φ AC circuits equation of DSTATCOM in r-y-b coordinates is

whererybi means the ir, iy, and ib, which are the 3φ inverter currents; Vdcis the DC battery voltage;rybD is the switching function; l is the inductance; r is the resistance.

According to the Parks transformation the 3φ currents are converted to d-q coordinates.

Through controlling the inverter output voltage, the exchanges of reactive current, i.e. the absorption or generation of reactive power by using DSTATCOM is done. There are different methods for controlling the inverter output voltage, such as the DC link voltage control scheme, phase angle method, and indirect current control method[22].

Fig. 3. Indirect current control scheme.

In this case the indirect current control method is selected because of its flexibility in reactive power compensation by using decoupling of d-q axis currents. The math equations are referred from [13] and [23]. The diagrams and graphs based on MATLAB can refer to Mathwork website and [22]. DSTATCOM for power quality improvement of wind turbine is referred from [24]. For controlling the reactive power or voltage quality, DC voltage, and q and d axises, three PIs (proportional integral) are utilized, which are shown in Fig. 3.

Initially, the 3φ voltages and currents are measured at instantaneous nature. These values are converted to d-q representatives.

where X is the reactance and δ is the angle between DSTATCOM and grid voltage. Equation (32) indicates the reactive power exchange, i.e. the absorption or generation by using DSTATCOM. Similarly, the active power of DSTATCOM is given by (33):

where VGis the grid or source voltage and VDis the DSTATCOM (inverter) voltage. The DC voltage of batteries is controlled by using PI controller as,

The outputs of q and d axises are

The current controllers cross the coupling summing point and PI output indicates the modulation signals in d and q axises as the phase angle of modulation signal:

and the modulation index is given as

There is difference between δ and 1δ,2δ. δ is the phase angle of modulation signal;1δ and2δ are the power angles of active and reactive power. The reactive power management and its enhancement by using CPD, i.e. DSTATCOM under different normal and abnormal conditions are mentioned. WE based DG by using MATLAB is shown in Fig. 4.

The development of modulating index m in control scheme is shown in Fig. 5.

The producing of phase angle δ of modulating index in the control scheme is shown in Fig. 6.

Fig. 4. WE based DG by using MATLAB.

Fig. 5. DSTATCOM control scheme indicating the modulating index m.

Fig. 6. DSTATCOM control scheme indicating the phase angle delta (δ) of modulating signal.

Fig. 7. DSTATCOM control scheme.

Fig. 8. Experimental set up of proposed model with outside view of wind-solar system.

The implementation of DSTATCOM control scheme is shown in Fig. 7, whose inputs are the combination of modulating index m, phase angle δ of modulation signal and ωt.

Fig. 9. Experimental set up of proposed model with inside view of detail wind-solar system.

The experimental set up consists of the wind-solar system, wind-solar charge controller, DSTATCOM (smart grid tied inverter), power quality testing board, resistive, inductive, and capacitive load, anemometer for wind measurement, energy meter, LED lighting, super power capacitor, 12 V and 26 A battery (02nos), inverter, digital storage oscilloscope, power quality analyzer, current transformer, and potential transformer probes for current and voltage measurement, micro grid PCC connection, digital multimeters, etc. The details of outside and inside view of the entire wind-solar system are shown in Figs. 8 and 9, respectively.

5. Results and Discussions

The smart grid tied inverter (DSTATCOM) by using indirect current control method interfaces with the experimental set up where Idreferenceis entered by external user. By considering the MATLAB based and experimental work, the findings are presented. The charge controller switches off the switch Swafter fully charging the super power capacitor and batteries. The line to ground potential is maintained up to

Fig. 10 indicates the exchange of reactive power between the grid and DSTATCOM is null and at this condition the DC link voltages are shown in Fig. 10. The standard value of DC link voltage is 313 V.

The three phase voltages of this case indicate the null reactive power exchange between the grid and DSTATCOM, which is shown in Fig. 11.

Fig. 12 indicates the findings related to the variation of DC voltages transition from the capacitive mode to the inductive mode of DSTATCOM with respect to Idreference. DC voltages reach the set value in a short period and maintain with small fluctuations. The DC voltages are balanced by the voltage ripple of DC capacitor during the inductive mode, which is less than that of the capacitive mode, because different values of average currents are flowing through the DC capacitor[22].

Figs. 13 and 14 indicate the variations of Iqand Iqreferencealong with their error.

Figs. 15 and 16 show the three-phase voltages and current when DSTATCOM is switched from the capacitive mode to the inductive mode.

Fig. 11. Three phase waveforms in the system.

Fig. 12. DC voltages of capacitor during the transition from capacitive mode to inductive mode of operation.

Fig. 13. Iqreferenceperformance.

Fig. 14. Iqerrorperformance.

Fig. 15. Three-phase voltages of DSTATCOM during capacitive mode to inductive mode.

Fig. 16. Current of DSTATCOM during capacitive mode to inductive mode.

Initially the modulating index m (1 to 0.2) varies rapidly from the period of 0 to 0.05 s and, later on, m is nearly stable with small variations from 0.95 to 0.69 at the time period of 0.06s to 0.1s. This is due to the transition of DSTATCOM from the capacitive mode to the inductive mode. The modulating index is shown in Fig. 17. Similarly, the variation of the phase angle δ of modulating index is shown in Fig. 18. It can be seen from Fig. 17 and Fig. 18, DSTATCOM responds rapidly for any variations in WE base DG. Hence the performance of the power system is enhanced.

Fig. 17. Modulating index m.

Fig. 18. Phase angle delta (δ) of modulating index.

Fig. 19. Combination of m, δ, and tω.

The combination of modulating index m, phase angle of modulating index δ, and tω is shown in Fig. 19. The three-phase voltages of DSTATCOM are indicated by r, y, and b with red, yellow, and blue color waveforms.

Fig. 20. Inverter three-phase Vabcor Vryb.

Fig. 21. Inverter three-phases current.

Fig. 22. Iderror.

Fig. 23. Inverter output voltage performance.

Figs. 20 to 23 indicate the inverter three-phase voltages in the r-y-b form, similarly currents in their respective nature.

The active and reactive power of the wind turbine with and without interaction of DSTATCOM is shown in Fig. 24 (The active power with DSTATCOM is red color waveform, active power without DSTATCOM blue color waveform, reactive power with DSTATCOM violet color waveform, and reactive power without DSTATCOM green color waveform.). The results of hardware, MATLAB simulation, and power quality analyzer testing are presented. To avoid the complexity, the detailed observation table, testing waveforms, huge data are neglected. Hence, Fig. 24 only indicates the active and reactive power exchange and their enhancement between the wind-solar generation and grid sides with the help of CPD.

The enhancement of active power means increasing the active power, and the enhancement of reactive power means the decrease of reactive power with the help of DSTATCOM, which are shown in Fig. 24. It is notable to avoid the confusion of reactive power enhancement, for example, consider the reactive power and its enhancement by using DSTATCOM, which are given by green and violet waveforms, respectively.

The reactive power is decreased from 50 VAR to 40 VAR initially by using DSTATCOM at 9. And later on this difference reaches 20% to 25%. That is, the reactive power is enhanced by 20% to 25%. Hence, the active power fed to the grid from wind-solar system is added 20% to 25% because of the DSTATCOM optimization operation.

Fig. 24. Active and reactive power, with and without DSTATCOM in wind turbine.

Table 1: Active and reactive power exchange by using DSTATCOM in wind turbine

6. Conclusions

This paper has presented the monitoring, modeling, control, and analysis of the two-level three-phase WE based DG system where the electric grid interface DSTATCOM adopting the indirect current control scheme is employed to perform the smart exchange of electric power as per the Indian grid code. The injection of real power from the WE based DG to the grid is done by DSTATCOM. And DSTATCOM also acts as the compensator against the reactive power demand of WE, unbalanced linear and non-linear load. Hence the grid side power is balanced. The findings obtained from the MATLAB simulations and experimental set up show that the DSTATCOM can deliver the required reactive current according to the required reactive power within a short period and also maintain the DC link voltage constant as possible in the case of operation mode variation. Hence, the absorption and delivering of reactive power throughout the three-phase are balanced by using DSTATCOM. In WE based DG, the burden of reactive power compensation from the grid is reduced. The additional real power of 20% to 25% is penetrated to grid.

Acknowledgment

Authors would like to express their sincere thanks to SUZLON wind farm Ltd., Maharashtra Electricity Distribution and Transmission Company Ltd., and Supernova wind farms Ltd. Authors also thank Raisoni Group of Institution Nagpur and Jalgaon for their motivation to the research work.

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Kishor V. Bhadanewas born in Kingaon, India in 1979. He received the B.E. degree from the North Maharashtra University, Jalgaon, India in 2003 and the M.E. degree from the Govt. Engineering Aurangabad, India in 2009. He is currently pursuing the Ph.D. degree

with

the

Departmentof

Electrical Engineering, YCCE, Nagpur under RTM Nagpur. His research interests include renewable energy sources, power quality, grid connected wind energy, custom power device, etc.

Makarand S. Ballalwas born in Nagpur, India in 1972. He received the B.E. degree from the Govt. Engineering College, Aurangabad, India in 1993 and the M.Tech. degree from the VNIT Nagpur, India in 1997, and the Ph.D. degree from VNIT Nagpur in 2007.

His

research

interestsinclude

power quality, condition monitoring of electrical machines, power system protection, HT. TOD metering, etc.

Ravindra M. Moharilwas born in Nagpur, India. He received the B.E. degree from the Govt. Engineering College, Karad and the M.Tech. degree and Ph.D. degree both from the VNIT Nagpur. His research interests include renewable energy sources, power quality, grid connected wind energy, custom power device, etc.

Manuscript received May 26, 2015; revised June 15, 2015.

K. Bhadane is with G. H. Raisoni Institute of Engineering and Managem, Jalgaon 425109, India (Corresponding author e-mail: kishor4293@yahoo.co.in or kishor.bhadane@raisoni.net).

M. S. Ballal and R. M. Moharil are with Electrical Department ,VNIT. Nagpur and YCCE. Nagpur, India (e-mail: msballal1@hotmail.com; rmm_ycce1@gmail.com).

Color versions of one or more of the figures in this paper are available online at http://www.journal.uestc.edu.cn.

Digital Object Identifier: 10.11989/JEST.1674-862X.505262