Attitude towards E-learning Technology -- A study in Kerala

(1)

Volume-5, Issue-1, February-2015

International Journal of Engineering and Management Research

Page Number: 273-278

Performance Analysis of Wind Power Generator through Statcom

Sandeep Kumar1, Sushil Kumar2 1

M.Tech (Student), EE Section, GTBKIET Chhapianwali, Malout, Punjab Technical University, Jalandher, INDIA 2

Assistant Professor, EE Section, GTBKIET Chhapianwali, Malout, Punjab Technical University, Jalandher, INDIA

ABSTRACT

In Present Scenario wind power experiences a tremendous development of its installed capacities. Though, the intermittent of wind generation causes difficulties in the management of power systems. Reactive power management is an important issue in Power system .For this problem, the solution is FACTS device.Wind driven induction generator scheme is used. This paper deals with the application of a Self-Excited Induction Generator SEIG in a small wind power conversion system (WPCS). Such conversion system has capability to supply power demand of the loads with constant voltage and frequency, for which a power managing method is proposed. Induction generator connected with a capacitor bank to supply the necessary reactive power supply. The size of the capacitor bank is just enough to provide the support .Under the transient conditions such as faults or loading, the capacitor bank is not sufficient to supply the reactive power to the system. STATCOM will act under contingency conditions for the selected power system. STATCOM is used to handle power flow between the SEIG and loads Strategy for controlling the voltage and frequency of a wind generation system is presented. A mathematical model of wind generation system and asynchronous generator has been developed and MATLAB model of the integrated system is developed for simulation studies under normal and various speed conditions. Under contingency as describe earlier i.e. in fault, and the loading condition, the reactive power demand has to be met up.

Keywords-– STATCOM, WPSC, SEIG , Power System

I. INTRODUCTION

Wind turbines convert the kinetic energy present in the wind into mechanical energy by means of producing torque. Large scale wind power projects are an attractive alternative to conventional capacity expansion. In the present scenario, most wind turbine manufacturers now equip power generating units by induction generators. They are operated either at fixed speed or variable speed. Generators driven by fixed speed turbines can directly be connected to grid. Variable speed generators need a power electronic converter interface for interconnection with the grid. Variable speed generation is preferred over fixed speed generation. The total amount

of economically extractable power available from the wind is considerably more than present human power use from all sources. Wind turbines convert wind energy to electricity for distribution. Aerodynamic modeling is used to determine the optimum tower height, control systems, number of blades and blade shape. Wind turbines are generally classified depending on their axis of rotation as: -Horizontal axis type,

-Vertical axis type.

The horizontal axis types generally have better performance.

II. BENEFITS OF WIND POWER

Wind power, as an alternative to fossil fuels, is abundant, renewable, widely spread, clean, and produces no greenhouse gas emissions during operation. Wind power is the world‘s rapid growing source of energy. The majority of electricity is generated by burning coal, rather than more eco-friendly methods like hydroelectric power. This use of coal causes untold environmental damage through CO2 and other toxic emissions. The energy sector is by far the biggest source of these emissions, both in the India and globally, and if we are to tackle climate change it is clear we need to move away from burning limited fossil fuel reserves to more sustainable and renewable sources of energy. Wind power has many advantages that make it a lucrative source of power for both utility-scale and small, distributed power generation applications. The beneficial characteristics of wind power include:

• Clean and endless fuel

• Local financial development

• Modular and scalable technology

• Energy price stability

• Reduced dependence on imported fuels

In Other Countries

(2)

source of electrical energy. Total wind power installation in the US was 11,603 MW in 2006 and it increased by 26% in the year 2007 [1]. Figure 1.1 illustrates the total amount of installed wind power in the U.S. power system from the years 2000 to 2007.

In India

In the early 1980s, the government of India established the Ministry of Non-Conventional Energy Sources (MNES) to promote diversification of the country's energy supply and satisfy the ever-increasing energy demand of its rapidly growing economy. In 2006, this ministry was renamed as the Ministry of New and Renewable Energy (MNRE). During the first decade of the 21st century, India emerged as the 2nd leading wind power market in Asia. Currently, its cumulative installed capacity is close to 13 GW, with the market growing at an average rate of over 20% over the past 3 years. More than 2,100 MW wind capacity projects were added in the financial year 2010–11. The installed capacity increased from a modest base of 41.3 MW in 1992 to reach 13,065.78 MW by December 2010.

Modern wind power technology has come a long way in the last two decades, both globally and in India. Improved technology has slowly and steadily improved capacity efficiency. A key trend in the Indian industry is the development of multi megawatt turbines installed at greater hub heights. Larger diameter rotors enable a single wind power generator to capture more energy or power per tower. This allows WTGs to take advantage of higher altitudes with stronger winds and less turbulence (wind speed generally increases with height above the ground). Subsequently larger machines have resulted in a steady increase in the capacity factor on average from 10-12% in 1998 to 20-22% in 2010. For two decades now, global average WTG power ratings have grown almost linearly, with present commercial machines rated on average in the range of 1.5 MW to 2.1 MW.

III. GENERATOR USED IN WIND

POWER

A mechanical interface, consisting of a step –up gear and a suitable coupling transmits the energy to an electrical generator. The output of this generator is connected to the load or system grid. The controller senses the wind direction, wind speed, power output of the generator and other necessary performance quantities of the

system and initiates appropriate control signals to take suitable corrective actions. Several schemes for electrical generation have been developed. These schemes can be broadly classified under these categories:

1. Fixed speed or Constant speed generation system. 2. Variable speed generation system.

Fixed speed or constant speed wind turbines which operate at a nearly constant speed, predetermined by the generator design and gearbox ratio. Fixed-speed

stall-regulated turbines have no options for control input. In

these turbines the turbine blades are designed with fixed pitch to operate near the optimal TSR at a specific wind speed. A variation of the stall regulated concept involves operating the wind turbine at two distinct, constant operating speeds, by either changing the number of poles of the electrical generator or changing the gear ratio. The principal advantage of stall control is its simplicity, but there are significant disadvantages; for instance, the stall regulated wind turbine will not be able to capture wind energy in an efficient manner at wind speeds other than that it is designed for. Fixed-speed pitch-regulated turbines typically use pitch regulation for start-up and after start-up only to control the power above the rated wind speed of the turbine. In a constant speed, stall-controlled wind turbine the turbine output power peaks somewhat higher than the rated limit, then decreases until the cut-out speed is reached. This feature provides an element of passive power output regulation, ensuring that the generator is not overloaded as the wind speed reaches above nominal values.

As constant speed generation system suffers from a number of drawbacks hence variable generation system is preferred.

IV. VARIABLE SPEED WIND ENERGY

CONVERSION SYSTEM

Variable speed pitch-regulated wind turbines have two methods for affecting the turbine operation, namely speed changes and blade pitch changes. In other terms, the control strategies employed in the operation of variable speed wind turbine system are:

(3)

used to control the speed.

2. Power limitation strategy, used above the rated wind speed of the turbine to limit the output power to the rated power by changing the blade pitch to reduce the aerodynamic efficiency, thereby reducing the wind turbine power to acceptable levels.

The regions of the above mentioned control strategies of a variable speed wind turbine system are as shown in the Figure:

In variable speed systems, the turbines rotor absorbs the mechanical power fluctuations by changing its speed.

However, since variable speed operation produces a variable frequency voltage, a power electronic converter must be used to connect to the constant frequency grid It can be achieved by using:

1. WECS with Squirrel cage induction generator. 2. WECS with Wound rotor induction generator. 3. WECS with Doubly-fed induction generator.

V. PROBLEMS ASSOCIATED WITH

WP

A WECS is a complex system converting wind energy to rotational energy and then to electrical energy. The output power or torque of a wind turbine is determined by several factors like wind velocity, size and shape of the turbine, etc. inputs and Outputs of a Wind Turbine. Reactive power

(a). Induction generators used for wind power projects, require reactive power for excitation so substations used in wind power collection systems include substantial capacitor banks for power factor correction.

(b). In AC circuits, energy is stored temporarily in inductive and capacitive elements, the portion of power flow which is temporarily stored in the form of magnetic or

electric fields and flows back and forth in the transmission line due to inductive and capacitive network elements is known as reactive power. This is the unused power which the system has to incur in order to transmit power. Inductors (reactors) are said to store or absorb reactive power, because they store energy in the form of a magnetic field.

(c). Power Generation and Transmission is a complex process, requiring the working of many components of the power system in tandem to maximize the output. One of the main components to form a major part is the reactive power in the system.

(d). It is required to maintain the voltage to deliver the active power through the lines. Loads like motor loads and other loads require reactive power for their operation. To improve the performance of ac power systems, we need to manage this reactive power in an efficient way and this is known as reactive power compensation.

Variable speed of wind

Electricity generated from wind power can be highly variable at several different timescales: from hour to hour, daily, and seasonally. Annual variation also exists. Variable load

There are two aspects to the problem of reactive power compensation: load compensation and voltage support. Load compensation consists of improvement in power factor, balancing of real power drawn from the supply, better voltage regulation, etc. of large fluctuating loads. Voltage support consists of reduction of voltage fluctuation at a given terminal of the transmission line. Grid problem

In a wind farm, individual turbines are interconnected with a medium voltage (usually 34.5 kV) power collection system and communications network. At a substation, this medium-voltage electrical current is increased in voltage with a transformer for connection to the high voltage electric power transmission system.

A transmission line is required to bring the generated power to (often remote) markets. For an off-shore plant this may require a submarine cable. Construction of a new high-voltage line may be too costly for the wind resource alone, but wind sites may take advantage of lines installed for conventionally-fueled generation.

Different types of wind turbine generators behave differently during transmission grid disturbances, so extensive modeling of the dynamic electromechanical characteristics of a new wind farm is required by transmission system operators to ensure predictable stable behavior during system faults.

(4)

VI. SOLUTION TO THE PROBLEM

The main reason for reactive power compensation in a system is:

1) the voltage regulation; 2) increased system stability;

3) better utilization of machines connected to the system; 4) reducing losses associated with the system; and 5) to prevent voltage collapse as well as voltage sag.

The impedance of transmission lines and the need for lagging VAR by most machines in a generating system results in the consumption of reactive power, thus affecting the stability limits of the system as well as transmission lines. Unnecessary voltage drops lead to increased losses which needs to be supplied by the source and in turn leading to outages in the line due to increased stress on the system to carry this imaginary power. Thus we can infer that the compensation of reactive power not only mitigates all these effects but also helps in better transient response to faults and disturbances. In recent times there has been an increased focus on the techniques used for the compensation and with better devices included in the technology, the compensation is made more effective. It is very much required that the lines be relieved of the obligation to carry the reactive power, which is better provided near the generators or the loads. Shunt compensation can be installed near the load, in a distribution substation or transmission substation

Facts devices

Flexible AC transmission system or FACTS devices used are:

1) VAR generators.

a) Fixed or mechanically switched capacitors. b) Synchronous condensers.

c) Thyristorized VAR compensators. (i) Thyristors switched capacitors (TSCs). (ii) Thyristor controlled reactor (TCRs). (iii) Combined TSC and TCR.

(iv) Thyristor controlled series capacitor (TCSC). 2) Self Commutated VAR compensators.

a) Static synchronous compensators (STATCOMs). b) Static synchronous series compensators (SSSCs). c) Unified power flow controllers (UPFCs). d) Dynamic voltage restorers (DVRs). STATCOM

A STATCOM is comparable to a Synchronous Condenser (or Compensator) which can supply variable reactive power and regulate the voltage of the bus where it is connected.

• A STATCOM 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.

• STATCOM can supply the required reactive power under various operating conditions, to control the network voltage actively and thus,

improve the steady state stability of the network.

The equivalent circuit of a Synchronous Condenser (SC) is shown in Fig. which shows a variable AC voltage source (E) whose magnitude is controlled by adjusting the field current. Neglecting losses, the phase angle (δ) difference between the generated voltage (E) and the bus voltage (V) can be assumed to be zero.

By varying the magnitude of E, the reactive current supplied by SC can be varied. When E = V, the reactive current output is zero. When E > V, the SC acts as a capacitor, whereas when E < V, the SC acts as an

inductor. When δ= 0, the reactive current drawn (Ir

Properties of STATCOM

This shunt connected static compensator was developed as an advanced static VAR compensator where a voltage source convertor (VSC) is used in-stead of the controllable reactors and switched capacitors. Although VSCs require self-commutated power semiconductor devices such as GTO, IGBT, IGCT, MCT, etc. (with higher costs and losses) unlike in the case of variable impedance type SVC which use thyristor devices, there are many technical advantages of a STATCOM over a SVC. These are primarily:

(a) Faster response

(b) Requires less space as bulky passive components (such as reactors) are eliminated

(c) Inherently modular and re-locatable

(d) It can be interfaced with real power sources such as battery, fuel cell or SMES (superconducting magnetic energy storage)

Why STATCOM

A STATCOM has superior performance during low voltage condition as the reactive current can be maintained constant (In a SVC, the capacitive reactive current drops linearly with the voltage at the limit (of capacitive susceptance). It is even possible to increase the reactive current in a STATCOM under transient conditions if the devices are rated for the transient overload. In a SVC, the maximum reactive current is determined by the rating of the passive components – reactors and capacitors. Further the justification regarding the use of STATCOM is as follows:

(5)

• Voltage regulated by the wind generators equipped with only fixed capacitors can become higher than the voltage limit of 1.05 p.u.

• Hence, a fixed capacitor cannot serve as the only source of reactive power compensation.

• STATCOM has the ability to compensate both the reactive power flow while providing voltage regulation support to the connected power system by injecting current in phase with the line voltage

• Reactive power compensation is required to maintain normal voltage levels in the power system. Reactive power imbalances, which can seriously affect the power system, can be minimized by reactive power compensation devices such as the STATCOM.

• The STATCOM can also contribute to the low voltage ride through requirement because it can operate at full capacity even at lower voltages STATCOM’s basic diagram

Basically STATCOM is a FACTS device which is also known as electronic generator of reactive power. It consists of a VSC, a DC energy storage device (Capacitor), and a coupling transformer which connects the VSC in shunt to the power network as shown in Figure.

Single line diagram of the STATCOM connected to power grid.

VII. SIMULATION RESULTS

With STATCOM simulation results:

Rotor And Stator Current

Fig:-Active and Reactive Generator power, Active and Reactive Grid power, Active and Reactive Load power

Without STATCOM:-

VIII. CONCLUSIONS

• A control Strategy using STATCOM for voltage and frequency control is proposed for isolated wind turbine system

• The isolated WT is represented using a IG

• STATCOM provides additional reactive power support to the system

(6)

• The STATCOM senses the abnormal conditions such as fault and variant load conditions generates control signal in reference to the frequency change and the voltage change

• Controls the current generated by the VSC thus regulating the whole power system

The proposed wind power generating system can be used in the isolated rural, ships and mountainous areas far from the conventional source

REFERENCES

[1] “STATCOM for Improved Dynamic Performance of Wind Farms in Power Grid”G. Elsady, Y. A. Mobarak, and A-R Youssef, Proceedings of the 14th International Middle East Power Systems Conference (MEPCON’10), Cairo University, Egypt, December 19-21, 2010, Paper ID 207.

[2] “Dynamic Response Of Wind Power Genarators Using Statcom” M.Mohammadha Hussaini1, Dr. R. Anita2, International Journal of Engineering and Technology Vol.2 (4), 2010, 297-304.

[3] “Power Quality Improvement of Grid Connected Wind Energy System by Statcom for Balanced and Unbalanced Linear and Nonlinear Loads” Ganesh.Harimanikyam1, S.V.R. Lakshmi Kumari2, International Journal of Engineering Research and Development e-ISSN: 2278-067X, p-ISSN: 2278-800X, www.ijerd.com Volume 3, Issue 1 (August 2012), PP. 09-17.

[4] “Study of Induction Generator based Wind Turbine Systems” Y. HU Senior Member, IEEE, Zhe CHEN, Senior Member, IEEE, The International Conference on Electrical Engineering 2009.

[5] “Fault Ride through Capability Improvement of Wind Turbine Based DFIG Considering an Optimized Crowbar Along with STATCOM under Grid Fault Condition” Alireza Zohoori, Ahad Kazemi and Rouhollah Shafaie, Research Journal of Applied Sciences, Engineering and Technology 5(7): 2297-2302, 2013 ISSN: 2040-7459; e-ISSN: 2040-746.

[6] “ Active and Reactive Power Control for Grid Connected Wind Energy System with Statcom” G. V. S. Babu1, N. Sri Hareesh, CH. Rambabu, International Journal of Engineering Research & Technology (IJERT) Vol. 1 Issue 9, November- 2012 ISSN: 2278-0181. [7] “FACTS Devices for Reactive Power Compensation of Wind Energy Conversion System” Amarjeet Singh1*and Amit Tiwari2, S-JPSET : ISSN : 2229-7111, Vol. 2, Issue 1.

[8] “STATCOM-Based Voltage Regulator for Self-Excited Induction Generator Feeding Nonlinear Loads” Bhim Singh, Senior Member, IEEE, S. S. Murthy, Senior Member, IEEE, and Sushma Gupta, IEEE Transactions On Industrial Electronics, Vol. 53, No. 5, October 2006. [9] “SmartPark as a Virtual STATCOM” Pinaki Mitra, Member, IEEE, Ganesh Kumar Venayagamoorthy, Senior Member, IEEE, and Keith A. Corzine, Senior Member, IEEE, IEEE TRANSACTIONS ON SMART GRID, VOL. 2, NO. 3, SEPTEMBER 2011.

[10] “Grid Integration of Large DFIG-Based Wind Farms Using VSC Transmission” Lie Xu, Senior Member, IEEE, Liangzhong Yao, and Christian Sasse, Manuscript received August 29, 2006; revised January 22, 2007. Paper no. TPWRS-00561-2006.